Zinc oxide nanotube arrays for detecting gases and volatile compounds

Abstract

Described herein is an array of zinc oxide nanostructures, particularly nanotubes, and methods for using the nanostructures to detect an analyte in a gas mixture sample. Also described herein are methods for manufacturing an array of zinc oxide nanostructures. Another embodiment described herein is an array of zinc oxide nanostructures, particularly nanotubes, and methods for using the nanostructures to detect ppb-levels of hydrogen at room temperature.

Description

[0001] ZINC OXIDE NANOTUBE ARRAYS FOR DETECTING GASES AND VOLATILE COMPOUNDS CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No.63 / 560,108, filed on March 1, 2024 and U.S. Provisional Patent Application No. 63 / 703,371 filed on October 4, 2024, each of which is incorporated by reference herein in its entirety. BACKGROUND Nanostructures exhibit unique properties due to the high surface area to volume ratio and the constraints imposed by the low dimensions. Nano-porous films and arrays of one-dimensional materials provide a large roughness factor, defined as the ratio of the geometric surface area to the flat area that the film occupies. Such materials are highly useful for applications utilizing the surface properties of materials because higher surface area provides a large number of active sites on the surface. Among the nanostructures, the nanotube arrays are distinct because they possess exceptional optical, electrical, chemical, and mechanical properties due not only to high surface area but also to nanoscale walls enabling the charge carriers to stay always in the vicinity of the surface. For this reason, nanotube arrays are highly sought after for applications ranging from catalysis to biomedical devices. Electrochemical anodization is one of the oldest commercial techniques for fabricating abrasion and corrosion resistant oxide coatings on metals. It is a low-cost process requiring a simple experimental set up. Anodization is carried out in a vessel with an electrolyte into which two electrodes, one an electrocatalyst (e.g., platinum) and the other the metal to be oxidized, are inserted. A positive potential is applied at the metal to be oxidized (anode) and a negative potential at the electrocatalyst (cathode). Compact or nanostructured oxide layers are then formed under the action of the electric field depending upon the process parameters (e.g., electrolyte composition, voltage, and temperature) applied. Titanium dioxide (TiO2) was the first material in which the successful fabrication of nanotube array was demonstrated using anodic oxidation. The attention received by titania nanotube arrays triggered the research on the development of a similar structure using many other materials; however, efforts have only successfully produced aluminum oxide, tantalum oxide, hafnium oxide, zirconium oxide, vanadium oxide, niobium pentoxide and iron oxide. Zinc oxide (ZnO) is a direct wide band gap (~3.37 eV) semiconductor with numerousattractive properties, for instance high electron mobility (205–1000 cm2 V 1 s 1), large excitonbinding energy (60 meV), biocompatibility, among others. Additionally, ZnO can be easily produced in nanostructured form, which has attracted research efforts in many applications including photocatalysis, solar cells, biomedicine, optoelectronics, and gas sensing. For most applications, the goal of creating nanostructures is to increase the surface area to volume ratio and provide facile pathways for the charge carriers. Numerous studies have shown that nanotube architecture is highly efficient for this purpose. Titanium dioxide (TiO2) nanotube array fabricated using electrochemical anodization is one of the most explored materials due to its unique properties appropriate for a spectrum of applications. Extended performance of these materials has been achieved through the creation of heterostructures and doping with appropriate compounds. On the other hand, the fabrication of self-organized ZnO nanotubes via anodization has appeared to be an impossible task for researchers in the field and this limited the application scope of this semiconductor material. Nevertheless, we have solved this puzzle. We have developed a method to grow well-defined nanotube arrays of ZnO using anodization. Recent studies have shown that semiconductor metal oxides (SMOX) are potentially useful for sensing hydrogen and volatile organic compounds to diagnose the disease state of the body. However, current SMOX sensors still suffer from challenges like poor selectivity and low sensitivity to concentrations in the ppm, ppb and ppt level. Many studies explore fabrication of nanostructured forms to solve the sensitivity issue and doping with other materials to create synergistic reactions that can result in better selectivity to certain volatile organic compounds (VOCs). The nanostructured architecture additionally enhances potential to use these sensors at room temperature. In addition, data processing techniques like principal component analysis (PCA) and machine learning are utilized to achieve better selectivity to the VOCs. The fabrication of self-organized anodic ZnO nanotubes provides an opportunity to obtain a nanostructure that can potentially respond differently to gases and VOCs. For instance, the hydrothermally fabricated ZnO nanotubes provide lesser degrees of freedom, only the thickness and diameter of the nanorods can be varied, whereas the anodic nanotubes provide an opportunity to vary the pore size, wall thickness, length, spacing between the tubes, barrier layer and at times, the nanotubes can be fabricated in form of semi periodic rings. All these structures provide active sites for the gas to interact with the material. Varghese et al., showed that titania nanotubes portray unprecedented hydrogen gas sensing, with the ability to tune the response based on the wall thickness. The low wall thickness (~10 nm) in titania nanotubes (an n-type semiconductor) triggers quantum enhancement, depleting all grains of electrons when kept in oxygen atmosphere (e.g., in air) and flooding the grains with electrons when exposed to reducing gases, which increases the response. The fabrication of ZnO nanotubes with tunable wall thickness opens pathways for using ZnO nanotubes in many of these applications with performance projected to be higher than that of presently used nanotubes. In our initial effort to fabricate ZnO nanotube arrays, we obtained for the first time a mixture of self-organized ZnO nanotube / nanowire mixture and illustrated its superior performance to detect breast cancer related VOCs. A simple aqueous electrolyte consisting of sodium carbonate and sodium bicarbonate was used, resulting in a very long (tens of microns) nanotube and nanowire combination. A challenge with this approach was that the obtained nanotubes were not mechanically stable. They are usually powder-like and fragile for device applications. We continued the efforts and now discovered the necessary experimental conditions to fabricate stable self-ordered ZnO nanotube arrays using anodic oxidation. The nanotube parameters include pore size, nanotube wall thickness, length of the nanotubes, and separation between the nanotubes. We also demonstrated the potential application of the ZnO nanotubes in hydrogen gas sensing. Driven by the global push for net-zero emissions, hydrogen is gaining wide acceptance as an alternative fuel. The vision of a hydrogen economy is getting closer to reality with the emergence of many recent technological innovations. One of the most attractive features of hydrogen is that it is not a greenhouse gas. Nevertheless, it poses environmental risks as an indirect greenhouse gas, primarily by extending the atmospheric lifetime of methane. As hydrogen is becoming a cornerstone of clean energy initiatives, the real-time detection of hydrogen leaks is critical for both safety and environmental protection. ZnO nanostructures are also desired as hydrogen gas sensors. A common challenge in these studies with the present ZnO nanostructures, just like many other studies, is either the high temperature requirement or the need for doping to obtain reasonable responses to the desired gas. What is needed are arrays of zinc oxide nanotubes in a vertical arrangement that enhances surface area, methods for fabricating said zinc oxide nanotubes, and methods of using said zinc nanotubes to detect analytes, specifically hydrogen gas at the ppm level at room temperature. SUMMARY One embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0– 60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the sample; and detecting a signal generated in response to the sample. In one aspect, the analyte comprises a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl- ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. In another aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, a change in the signal indicates that an analyte is present in the sample and a lack of change in the signal indicates that an analyte is absent. In another aspect, a lack of change indicates that the analyte is present at a concentration below a detection limit. In another aspect, the sample comprises the respiration or exhalation of a subject. In another aspect, a presence or absence of one or more analytes indicates the subject has a disease or disorder. In another aspect, the disease or disorder is cancer, diabetes, COVID-19, Alzheimer’s, neonatal enterocolitis, liver disease, kidney disease, pulmonary diseases, asthma, tuberculosis, autoimmune diseases, or combinations thereof. In another aspect, the cancer is breast cancer, lung cancer, colorectal cancer, pancreatic cancer, thyroid cancer, liver cancer, brain cancer, blood cancer, melanoma, oral and oropharyngeal cancer, kidney cancer and prostate cancer. Another embodiment described herein is a method for manufacturing a zinc oxide nanostructure array described herein, the method comprising: providing a zinc substrate and a cathode in an electrolyte, wherein the zinc substrate comprises a first planar surface and a second planar surface, wherein the electrolyte comprises a base, water, and an organic solvent comprising ethylene glycol, diethylene glycol, polyethylene glycol, formamide, dimethyl sulfoxide, acetic acid, glycerol, or combinations thereof; applying a voltage to the zinc substrate and the cathode thereby reducing the cathode and oxidizing said zinc substrate on the first and second planar surfaces to form a plurality of zinc oxide nanostructures on the first and second planar surface of the zinc substrate; enlarging the zinc oxide nanostructures by continuing to apply the voltage; and fully crystallizing the zinc oxide nanostructures with a heat treatment. In one aspect, enlarging the zinc oxide nanostructures comprises increasing a thickness of said nanostructures. In another aspect, enlarging the zinc oxide nanostructures comprises increasing diameter and a thickness of said nanostructures. In another aspect, continuing to apply the voltage comprises increasing the voltage. In another aspect, enlarging the zinc nanostructures comprises heating from about 1–50 °C. In another aspect, the cathode comprises platinum, graphite, stainless steel or nickel or its alloys. In another aspect, the voltage can be varied from 5–100 V. In another aspect, the base comprises sodium hydroxide, potassium hydroxide, ammonium hydroxide, calcium hydroxide, lithium hydroxide, cesium hydroxide or a combination thereof. In another aspect, the electrolyte further comprises urea. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, or phosphoric acid. In another aspect, the electrolyte further comprises a salt. In another aspect, the salt comprises potassium fluoride, sodium fluoride, lithium fluoride, ammonium fluoride, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium carbonate, calcium carbonate, sodium nitrate, or potassium nitrate, or combinations thereof. In another aspect, the electrolyte comprises: 0.25–3.0 M of the base; 10–300 mM of potassium fluoride; 1–80 vol. % of water; and 20 to 99 vol. % of the organic solvent. In another aspect, the base comprises potassium hydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide, cesium hydroxide, or combinations thereof. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, nitric acid, and perchloric acid. In another aspect, the method further comprises ultrasonicating the zinc substrate and the cathode material in at least one solvent before applying them respectively as anode and cathode in an electrochemical anodization cell. In another aspect, the at least one solvent is isopropanol, acetone, or water. In another aspect, the heat treatment is performed at 100–420 °C. Another embodiment described herein is an array of zinc oxide nanostructure array obtained by the methods described herein. In one aspect, each zinc oxide nanostructure comprises an average outer diameter of about 20–500 nm. In another aspect, each zinc oxide nanostructure comprises an average height of about 50 nm to 1000 μm. In another aspect, each zinc oxide nanostructure comprises a vertical alignment angle of about 0–60 degrees off the vertical direction. In another aspect, each zinc oxide nanostructure comprises an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3. In another aspect, each zinc oxide nanostructure comprises an average roughness factor of about 10–3,50,000. In another aspect, the array of zinc oxide nanostructures has an electrical conductivity of about 0.001–10 m. Another embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting hydrogen in a gas mixture sample, the method comprising: providing a sensor comprising a nanostructure described herein; contacting the sensor with the sample; and detecting a signal generated in response to hydrogen in the sample. In another aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. In another aspect, a lack of change indicates that the hydrogen is present at a concentration below a detection limit. In another aspect, the sample comprises gas or air in the vicinity of equipment or apparata. In another aspect, the method is for detecting hydrogen gas leaks. Another embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the sample; and detecting a signal generated in response to the sample. In one aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, the detection is performed at room temperature. In another aspect, the detection is performed at temperatures from 15–300 °C. In another aspect, the analyte is a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. Another embodiment described herein is a method for detecting hydrogen in ambient air, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the ambient air; and detecting a signal generated in response to hydrogen in the sample. In one aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, the detection is performed at room temperature. In another aspect, the detection is performed at temperatures from 15–300 °C. In another aspect, a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. In another aspect, a lack of change indicates that the hydrogen is present at a concentration below a detection limit. In another aspect, the detection limit in air is about 100 ppb (0.0000001%) at room temperature. In another aspect, the sample comprises gas or air in the vicinity of equipment or apparata. In another aspect, in the method is for detecting hydrogen gas leaks. DESCRIPTION OF THE DRAWINGS FIG. 1A–C show scanning electron microscope (SEM) images (top view) of zinc oxide nanostructures anodized at 20 V in chloride containing aqueous electrolytes. FIG.1A shows nanostructures formed after 30 minutes in a chloride containing aqueous electrolytes further comprising 20 mM hydrochloric acid. FIG.1B shows nanostructures formed after 10 minutes in a chloride containing aqueous electrolytes further comprising 20 mM NH4Cl. FIG 1C shows nanostructures formed after 20 minutes in a chloride containing aqueous electrolyte further comprising 20 mM NH4Cl. FIG. 2A–B show scanning electron microscope (SEM) images (top view) of the morphology of zinc oxide nanostructures anodized at 20 V in aqueous nitrate containing electrolyte. FIG. 2A shows nanostructures formed after 30 minutes in a chloride containing aqueous electrolytes further comprising 50 mM nitric acid. FIG.2B shows nanostructures formed after 30 minutes in a chloride containing aqueous electrolytes further comprising 50 mM NH4NO3. FIG. 3A–C show scanning electron microscope (SEM) images of the structure formed when Zn was anodized in 5.7 mM sodium carbonate and 6.5 mM sodium bicarbonate aqueous electrolyte at 20 V for 20 min. FIG.3A shows the top view perspective of the resulting zinc oxide structure. FIG.3B shows a magnified view of a rod. FIG.3C shows a cross-sectional of a film of zinc oxide wires. FIG.4A–C show scanning electron microscope (SEM) images of the effect of anodization time on ZnO nanostructure formation in a 5.7 mM sodium carbonate and 6.5 mM sodium bicarbonate aqueous electrolyte at 20 V. FIG.4A shows the ZnO nanostructures formed after 60 seconds. FIG.4B shows the ZnO nanostructures formed after 360 seconds. FIG.4C shows the ZnO nanostructures formed after 840 seconds. FIG. 5A–D show scanning electron microscope (SEM) images of ZnO nanostructures formed by anodizing Zn in 5.7 mM sodium carbonate and 6.5 mM sodium bicarbonate electrolyte at 20 V for 13–14 minutes. FIG. 5A shows large diameter zinc oxide rods. FIG. 5B shows a mixture of zinc oxide rods and tubes. FIG.5C shows zinc oxide wires covering the surface of zinc oxide rods and tubes. FIG.5D shows the hollowing of a zinc oxide rod into a zinc oxide tube. FIG. 6A–D show scanning electron microscope (SEM) images of ZnO nanostructures formed by anodizing Zn in an aqueous electrolyte containing 6 mM of potassium bicarbonate and 6 mM of sodium bicarbonate at 10 V for 13–14 minutes without and with 6 mM urea in the electrolyte. FIG. 6A and 6B show surface images showing wires / rods fabricated in electrolyte with no urea. FIG.6C and 6D shows surface images showing nanotubes grown in the electrolyte with urea. FIG. 7A–D show scanning electron microscope (SEM) images of ZnO produced by anodizing Zn at 20 V for 13–14 min in an electrolyte consisting of 6 mM sodium bicarbonate and 6 mM calcium carbonate stirred at 500 rpm. FIG. 7A shows low magnification surface image exhibiting tubular flowers and rods. FIG.7B shows higher magnification images of the flower like pattern consisting of tubular structures. FIG. 7C shows low magnification showing rods in the produced ZnO sample. FIG.7D shows higher magnification images of the hexagonal rods with signs of dissolution at the center indicating transition from rod to tube. FIG. 8A–F shows cross-sectional SEM images of ZnO nanostructures formed in an aqueous electrolyte containing 50 mM HF and 2 M KOH exposed to different anodization voltages and times. FIG.8A shows ZnO nanostructures formed at 10 V and 3 minutes. FIG.8B shows ZnO nanostructures formed at 20 V and 3 minutes. FIG.8C shows ZnO nanostructures formed at 30 V and 3 minutes. FIG.8D shows ZnO nanostructures formed at 40 V and 3 minutes. FIG. 8E shows ZnO nanostructures formed at 50 V and 3 minutes. FIG.8F shows ZnO nanostructures formed at 60 V and 2 minutes. FIG.9A–F surface SEM images of ZnO nanostructures formed in an aqueous electrolyte containing 50 mM HF and 2 M KOH exposed to different anodization voltages and times. FIG. 9A shows ZnO nanostructures formed at 10 V and 3 minutes. FIG.9B shows ZnO nanostructures formed at 20 V and 3 minutes. FIG.9C shows ZnO nanostructures formed at 30 V and 3 minutes. FIG. 9D shows ZnO nanostructures formed at 40 V and 3 minutes. FIG. 9E shows ZnO nanostructures formed at 50 V and 3 minutes. FIG.9F shows ZnO nanostructures formed at 60 V and 2 minutes. FIG.10A–B show the morphology of ZnO nanostructures formed in 0.5 M HF, 2 M NH4OH, and ethylene glycol (EG) at 50 V for 1 hour at RT (24 °C). FIG 10A shows the surface of ZnO nanostructures. FIG.10 B shows the cross-sectional view. FIG.11A–D show SEM images of ZnO nanostructures on zinc anodized in an electrolyte containing EG, 40 vol. % water, and 1 M NaOH at 55 V and 5 °C. FIG.11A shows the surface SEM image of ZnO nanostructures formed in the absence of HF (0 mM HF) and after 1 hour anodization. FIG. 11B shows the cross-sectional SEM image of ZnO nanostructures formed in the absence of HF (0 mM HF) and after 1 hour anodization. FIG.11C shows the surface SEM image of ZnO nanostructures formed in the presence of 10 mM HF and after a 2-hour anodization. FIG.11D shows the cross-sectional SEM image of ZnO nanostructures formed in the presence of 10 mM HF and after a 2-hour anodization. FIG.12A–B show SEM images of ZnO nanostructures on anodized zinc in an electrolyte containing DEG, 30 vol. % water, and 1 M KOH at 30 V and 5 °C. FIG.12A shows the surface SEM image of ZnO nanostructures formed. FIG.12B shows the cross-sectional SEM image of ZnO nanostructure formed. FIG.13 shows the cross-sectional view of an 8-micron thick nanotube array film (before cleaning the surface) prepared in 1 M KOH, 40 vol.% H2O, and 60 mM KF in EG at 30 V and 5°C. FIG. 14 shows a 1-micron thick nanotube array film (before cleaning) prepared in 1 M NaOH, 40 vol.% H2O, and 60 mM KF in EG at 30 V and RT. FIG.15 shows the top surface (i.e., view of pores) of nanotubes (after cleaning) prepared in 1.1 NaOH, 40 vol.% H2O, and 30 mM KF in EG at 30 V and RT. FIG.16A–B show SEM images of ZnO nanotubes prepared in 0.83 M sodium hydroxide, 30 vol. % H2O, and 60 mM potassium fluoride in ethylene glycol (EG) at 2 °C and 30 V. FIG.16A shows the cross-sectional view of the ZnO nanotubes. FIG.16B shows the top surface (i.e., view of pores) of the ZnO nanotubes. FIG.17A–B show SEM images of ZnO nanotubes prepared in 1.5 M potassium hydroxide, 30 vol. % H2O, and 36 mM potassium fluoride in EG anodized for 1 hour. FIG.17A shows the cross-sectional view of the ZnO nanotubes. FIG.17B shows the top surface (i.e., view of pores) of the ZnO nanotubes. FIG.18A–B show SEM images of ZnO nanotubes prepared in 1.5 M potassium hydroxide, 30 vol. % H2O, and 36 mM potassium fluoride in EG anodized for 2 hours. FIG.18A shows the cross-sectional view of the ZnO nanotubes. FIG.18B shows the top surface (i.e., view of pores) of the ZnO nanotubes. FIG. 19A–B show transmission electron microscopy (TEM) images of ZnO nanotubes prepared in 1.1 NaOH, 40 vol% H2O, and 30 mM KF in EG at 30 V and RT. FIG.20 shows the top surface (i.e., view of pores) of the ZnO nanotube film prepared in 1 M NaOH, 40 vol.% H2O, 60 mM KF, and 25 mM acetic acid in EG and anodized for 40 minutes at 25 V and 10 °C. FIG. 21 XRD pattern of a zinc oxide nanotube film heat-treated at 400 °C and that of a zinc substrate. ZnO (ZO) and Zn (Z) represent the peaks arising from zinc oxide and zinc crystal phases. XRD showed that the zinc oxide films crystallized in wurtzite phase. The zinc oxide sample was prepared using an electrolyte consisting of 1 M NaOH, 40 vol.% H2O, 60 mM KF, and 60 vol. % ethylene glycol. The anodization was done at 30 V for 60 minutes at room temperature (25 °C). FIG. 22A–B show the resistance of nanotubes exposed alternately to pure air and hydrogen diluted with argon. The resistance decreased when the atmosphere was switched from air to hydrogen (in argon) and increased to the original value when the atmosphere was changed back to air. FIG.22A shows the resistance variation in two similar nanotube samples. The film for Sensor 1 was prepared by anodizing zinc in an electrolyte consisting of 1M NaOH, 30 vol % H2O, 60mM KF and 70 vol% EG at 30V and 10 °C for 20 minutes. The film for Sensor 2 was prepared in 1M NaOH, 30 vol % H2O, 30 mM KF and 70 vol. % EG and the anodization was conducted at 30V and 10 °C for 20 minutes. The sensors were exposed to 400 ppm hydrogen in argon. FIG. 22B shows the resistance variation of three sensors prepared under different conditions. The film for the sensors were prepared under the following conditions. Sensor 1: 1 M NaOH, 30 vol % H2O, 60 mM KF and 70 vol% EG at 30 V and 10 °C for 20 minutes. Sensor 2: 1 M NaOH, 30 vol % H2O, 30 mM KF and 70 vol% EG at 30 V and 10 °C for 20 minutes. Sensor 3: 1 M NaOH 30 vol % H2O and 70 vol% EG at 30 V and 25 °C for 20 minutes. The sensors were exposed to 40 ppm, 200 ppm and 400 ppm of hydrogen in argon. FIG.23 shows the PCA scatter plot showing distinct cluster of breast cancer related VOCs after exposure to a three-sensor array labelled as Sensor 1, Sensor 2 and Sensor 3 with nanotubes fabricated under different conditions. The films were prepared in 1 M NaOH, 30 vol % H2O, 60 mM KF, and 70 vol % EG electrolyte at 25 °C. The voltages used were 20 V, 30 V and 40 V for Sensor 1, Sensor 2, and Sensor 3, respectively. FIG. 24 shows corresponding variations in resistance after exposing the sensors (mentioned in FIG.23) to carbon dioxide (CO2), heptanal, ethanol, and 2-propanol in air. FIG.25 shows response of the ZnO nanotube samples to 45 ppm 2-propanol (in air) in a three-sensor array in which nanotubes were formed in the following conditions. Sensor 1: 2 M potassium hydroxide, 40 vol. % water, 60 mM potassium fluoride in EG (60 vol.%) at 15 V and 2 °C for 60 min. Sensor 2: 2 M potassium hydroxide, 40 vol. % water, 20 mM potassium fluoride in EG (60 vo. %) at 15 V and 2 °C for 60 min. Sensor 3: 1 M potassium hydroxide, 40 vol. % water, 60 mM potassium fluoride in EG (60 vol. %) at 15 V and 25 °C for 60 min. FIG.26A–B show response of a three-sensor array to 15, 45, 80 and 140 ppm 2-propanol in air. The films were prepared in 1 M sodium hydroxide, 60 mM potassium fluoride in EG at 30 V and 25 °C with different water contents. Water content in the electrolyte was 30, 40, and 50 vol. % for Sensor 1, Sensor 2, and Sensor 3, respectively. FIG. 26A shows the change in resistance in these three sensors in response to 2-propanol. The films for Sensor 1 were annealed at 200 °C and those for Sensor 2 and Sensor 3 were annealed at 400 °C. FIG. 26B shows sensitivity versus 2-propanol concentration plot for the three sensors mentioned in FIG. 26A. FIG.27A–B show ZnO nanotube samples XRD and TEM analysis. FIG.27A shows XRD peaks showing the crystalline nature of the unannealed (as-prepared) ZnO nanotube samples (middle curve) compared to the annealed sample at 400 °C ZnO (top curve), and Zn (Z) foil (bottom curve) characteristic peaks. ZnO characteristic peaks are observed in both the annealed and unannealed samples. FIG.27B shows TEM images of the ZnO nanotubes showing partially porous nanotubes. Further TEM analysis also showed that some of the tubes still have a zinc complex insoluble in the electrolyte during the anodization. FIG.28 shows SEM images illustrating the morphology of ZnO nanostructures in various electrolytes. The left column shows samples at room temperature containing 30 vol. % water, 1 M KOH, 60 mM KF, and EG at 45 V. The middle column shows samples at room temperature containing 40 vol % water, 1.1 M NaOH, 30 mM KF and EG at 30 V. The right column shows samples at 2 °C containing 40 vol. % water, 2 M KOH, 60 mM KF, and EG at 15 V. FIG.29 shows a schematic of the four-sensor array used for hydrogen gas sensing. The sensor array eliminates changes in responses due to slight undesired changes in fabrication conditions. FIG.30A–D show typical responses of the 200 °C annealed ZnO nanotubes of different thickness (l). FIG. 30A shows a four-sensor array showing a consistent four order drop in resistance when exposed to 400 ppm of hydrogen in argon and a quick recovery. FIG.30B shows a magnified view of the response, showing response time of ~50 s and recovery time of ~5 s. FIG. 30C shows variation of response for different concentration (400–1600 ppm). FIG. 30D shows the estimation of limit of detection for these samples. FIG.31A–B shows nanotube length experiments. FIG.31A shows the effect of length of nanotubes. FIG. 31B shows the response of the 5 m nanotube sample showing increased response and recovery times. FIG. 32A–D show data for nanotube sensors. FIG. 32A shows the sensitivity of the unannealed (as prepared) samples of different nanotube lengths. FIG.32B shows a magnified view of the response for unannealed (as prepared) samples. FIG.32C shows the sensitivity of the ZnO sensor at 150 °C in oxygen ambient. FIG.32D shows the determination of LOD for the 200 °C annealed sensor in ambient oxygen. FIG. 33A–D show palladium coated ZnO sensors. FIG. 33A shows the response of palladium coated ZnO samples to hydrogen in oxygen ambient at room temperature. FIG.33B and FIG.33C show SEM micrographs of the sample anodized in glycerol showing cross section and top surface. FIG.33D shows a TEM image illustrating the open ZnO nanotubes. FIG. 34 shows a palladium coated ZnO sensor in 50% relative humidity placed 10 cm away from a hydrogen leak source. FIG.35 shows the gas sensing mechanism for nanotubes. FIG.36A–B shows the response of the ZnO nanotubes to low and high concentrations of hydrogen. FIG.36A shows the response of the ZnO nanotubes to 80 ppm hydrogen in air at room temperature after annealing the samples in hydrogen at 300 °C. FIG.36B shows the response of this sensor toward higher hydrogen concentrations. FIG. 37A–C show an electrodeposited Zn film on FTO glass. FIG. 37A shows a photograph and FIG. 37B shows an SEM image of an electrodeposited Zn film on FTO glass. FIG. 37C shows a typical anodization curve for the electrodeposited Zn film with a standardresistor (1 k ) connected in series with the anodization cell.DETAILED DESCRIPTION Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. For example, any nomenclatures used in connection with, and techniques of materials science and chemistry, described herein are well known and commonly used in the art. In case of conflict, the present disclosure, including definitions, will control. Exemplary methods and materials are described below, although methods and materials similar or equivalent to those described herein can be used in practice or testing of the embodiments and aspects described herein. As used herein, the terms such as “include,” “including,” “contain,” “containing,” “having,” and the like mean “comprising.” The disclosure also contemplates other embodiments “comprising,” “consisting of,” and “consisting essentially of,” the embodiments or elements presented herein, whether explicitly set forth or not. As used herein, the term “a,” “an,” “the” and similar terms used in the context of the disclosure (especially in the context of the claims) are to be construed to cover both the singular and plural unless otherwise indicated herein or clearly contradicted by the context. In addition, “a,” “an,” or “the” means “one or more” unless otherwise specified. As used herein, the term “or” can be conjunctive or disjunctive. As used herein, the term “substantially” means to a great or significant extent, but not completely. As used herein, the term “about” or “approximately” as applied to one or more values of interest, refers to a value that is similar to a stated reference value, or within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, such as the limitations of the measurement system. In one aspect, the term “about” refers to any values, including both integers and fractional components that are within a variation of up to ± 10% of the value modified by the term “about.” Alternatively, “about” can mean within 3 or more standard deviations, per the practice in the art. Alternatively, such as with respect to biological systems or processes, the term “about” can mean within an order of magnitude, in some embodiments within 5-fold, and in some embodiments within 2-fold, of a value. As used herein, the symbol means “about” or “approximately.” All ranges disclosed herein include both end points as discrete values as well as all integers and fractions specified within the range. For example, a range of 0.1–2.0 includes 0.1, 0.2, 0.3, 0.4. . . 2.0. If the end points are modified by the term “about,” the range specified is expanded by a variation of up to ±10% of any value within the range or within 3 or more standard deviations, including the end points. As used herein, the terms “active ingredient” or “active pharmaceutical ingredient” refer to a pharmaceutical agent, active ingredient, compound, or substance, compositions, or mixtures thereof, that provide a pharmacological, often beneficial, effect. As used herein, the terms “control,” or “reference” are used herein interchangeably. A “reference” or “control” level may be a predetermined value or range, which is employed as a baseline or benchmark against which to assess a measured result. “Control” also refers to control experiments. As used herein, the terms “room temperature,” “RT,” or “ambient temperature” refer to the typical temperature in an indoor laboratory setting. In one aspect, the laboratory setting is climate controlled to maintain the temperature at a substantially uniform temperature or with a specific range of temperatures. In one aspect, “room temperature” refers a temperature of about 15– 30 °C, including all integers and endpoints within the specified range. In another aspect, “room temperature” refers a temperature of about 15–35 °C; about 20–32 °C; about 22–30 °C; about 25–30 °C; about 27–30 °C; about 15–22 °C; about 15–25 °C; about 15–27 °C; about 20–22 °C; about 20–25 °C; about 20–27 °C; about 22–25 °C; about 22–27 °C; about 25–27 °C; about 15 °C ± 10%; about 20 °C ± 10%; about 22 °C ± 10%; about 25 °C ± 10%; about 27 °C ± 10%; ~20 °C, ~22 °C, ~25 °C, or ~27 °C, at standard atmospheric pressure. As used herein, the terms “anodized film” also refers to as anodized coating, anodic oxide, anodic oxide coating, anodic film, anodic layer, anodic coating, oxide film, oxide layer, oxide coating, etc., can be used interchangeably and can refer to suitable metal oxide materials, specifically zinc oxide materials. As used herein, anodized film means the film fabricated using electrochemical anodization. The metal oxide material described comprise a plurality of interconnected fragments of the described metal material, such as zinc oxide, that form ordered and disordered networks of the metal. As described herein an “ordered network” refers to networks that result in uniform or unidirectional formations of metal oxide. In one aspect, “ordered network” refers to the vertically aligned nanostructures such as nanotube arrays. As described herein and a “disordered network” refers to networks that comprise a mixture of formations of the metal oxide material. “Formations of metal oxide material” refer to the formations that the metal oxide material can be configured. Formations include, but are not limited to wire-like, flake-like, needle-like, rod-like, flower-like, bouquet-like, strip-like, sponge-like, tube-like, or combinations thereof. As described herein, the term “wire-like” refers to refers to an elongated structure having a length that is substantially larger than its width, i.e., a structure having dimensions in a longitudinal direction that are substantially larger than its dimensions in a transverse direction with a full (filled) wire, such as e.g., a microwire or a nanowire, a pillar, such as e.g., a micropillar or a nanopillar or a rod (nanorod or microrod). Contrastingly, the term “tubular” or “tube-like” refers to an elongated structure having a length that is substantially larger than its width, i.e., a structure having dimensions in a longitudinal direction that are substantially larger than its dimensions in a transverse direction with a full substantially hollow tube, such as e.g., a microtube or a nanotube. As described herein, “outer diameter” refers to the outer diameter of a nanostructure. The outer diameter of the nanotube further refers to the outer diameter that may contact other formations of metal oxide material. In one embodiment the outer diameter of a nanostructure comprises from about 20 to 500 nanometers (nm). In one aspect, the outer diameter of the nanotubes comprise about 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 120 nm, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm, 320 nm, 340 nm, 360 nm, 380 nm, 400 nm, 420 nm, 440 nm, 460 nm, 480 nm, 500 nm, or greater. In another aspect, the outer diameter of a nanostructures comprises about 20–50 nm, 20–75 nm, 20–100 nm, 20–125 nm, 20–150 nm, 20–175 nm, 20–200 nm, 20–225 nm, 20–250 nm, 20–275 nm, 20–300 nm, 20–325 nm, 20–350 nm, 20–375 nm, 20–400 nm, 20–425 nm, 20–450 nm, 20–475 nm, 20–500 nm, 30–100 nm, 30–150 nm, 30–200 nm, 30–250 nm, 30– 300 nm, 30–350 nm, 30–400 nm, 30–450 nm, 30–500 nm, 40–100 nm, 40–150 nm, 40–200 nm, 40–250 nm, 40–300 nm, 40–350 nm, 40–400 nm, 40–450 nm, 40–500 nm, 50–100 nm, 50–150 nm, 50–200 nm, 50–250 nm, 50–300 nm, 50–350 nm, 50–400 nm, 50–450 nm, 50–500 nm, 100– 200 nm, 100–250 nm, 100–300 nm, 100–350 nm, 100–400 nm, 100–450 nm, or 100–500 nm, including all endpoints, integers and subranges within the disclosed ranges. As described herein, “height” of the nanostructures refers to the longitudinal length of the nanostructures. An “average height” refers to the mode, median, or mean of the heights of the generated nanotubes. In one embodiment the average height of the nanostructures comprises from about 0.5 to 1000 micrometers (μm). In one aspect, the average height of the nanotubes comprises 0.05 μm, 0.075 μm, 0.1 μm, 0.125 μm, 0.15 μm, 0.175 μm, 0.2 μm, 0.225 μm, 0.25 μm, 0.275 μm, 0.3 μm, 0.325 μm, 0.35 μm, 0.375 μm, 0.4 μm, 0.425 μm, 0.45 μm, 0.475 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 350 μm, 400 μm, 450 μm, 500 μm, 550 μm, 600 μm, 650 μm, 700 μm, 750 μm, 800 μm, 850 μm, 900 μm, 950 μm, 1000 μm, or greater. In another aspect, the average height of the nanotubes comprises 0.05–0.5 μm, 0.05–1 μm, 0.05–5 μm, 0.05–10 μm, 0.05–15 μm, 0.05–20 μm, 0.05–25 μm, 0.05–30 μm, 0.05–35 μm, 0.05–40 μm, 0.05–45 μm, 0.05–50 μm, 0.05–75 μm, 0.05–100 μm, 0.05–200 μm, 0.05–300 μm, 0.05–400 μm, 0.05–500 μm, 0.5–1 μm, 0.5–5 μm, 0.5–10 μm, 0.5–15 μm, 0.5–20 μm, 0.5–25 μm, 0.5–30 μm, 0.5–35 μm, 0.5–40 μm, 0.5–45 μm, 0.5–50 μm, 0.5–75 μm, 0.5–100 μm, 0.5–200 μm, 0.5–300 μm, 0.5–400 μm, 0.5–500 μm, 0.5–600 μm, 0.5–700 μm, 0.5–800 μm, 0.5–900 μm, 0.5–1000 μm, 1–5 μm, 1–10 μm, 1–15 μm, 1–20 μm, 1–25 μm, 1–30 μm, 1–35 μm, 1–40 μm, 1–45 μm, 1–50 μm, 1–75 μm, 1–100 μm, 1–200 μm, 1–300 μm, 1–400 μm, 1–500 μm, 1–600 μm, 1–700 μm, 1–800 μm, 1–900 μm, 1– 1000 μm, 5–10 μm, 5–20 μm, 5–30 μm, 5–40 μm, 5–50 μm, 5–75 μm, 5–100 μm, 5–200 μm, 5– 300 μm, 5–400 μm, 5–500 μm, 5–600 μm, 5–700 μm, 5–800 μm, 5–900 μm, 5–1000 μm, 10–50 μm, 10–75 μm, 10–100 μm, 10–200 μm, 10–300 μm, 10–400 μm, 10–500 μm, 10–600 μm, 10– 700 μm, 10–800 μm, 10–900 μm, 10–1000 μm, 50–100 μm, 50–200 μm, 50–300 μm, 50–400 μm, 50–500 μm, 50–600 μm, 50–700 μm, 50–800 μm, 50–900 μm, or 50–1000 μm, including all endpoints, integers and subranges within the disclosed ranges. As described herein, “roughness factor” refers to a ratio of a real surface area of the nanostructured film to the area of the substrate on which it is coated (this is the area of the flat surface). In one embodiment, the roughness factor of the nanostructures comprises from about 10 to about 3,50,000. In one aspect, the roughness factor comprises 10, 25, 50, 75, 100, 250, 500, 1000, 1250, 1500, 1750, 2000, 2250, 2500, 2750, 3000, 3250, 3500, 3750, 4000, 4250, 4500, 4750, 5000, 5250, 5500, 5750, 6000, 6250, 6500, 6750, 7000, 7250, 7500, 7750, 8000, 8250, 8500, 8750, 9000, 9250, 9500, 9750, 10000, or greater. In another aspect, the roughness factor comprises 10–100, 10–500, 10–1000, 10–1500, 10–2000, 10–2500, 10–3000, 10–3500, 10–4000, 10–4500, 10–5000, 10–5500, 10–6000, 10–6500, 10–7000, 10–7500, 10–8000, 10– 8500, 10–9000, 10–9500, 10–10000, 100–1000, 100–1500, 100–2000, 100–2500, 100–3000, 100–3500, 100–4000, 100–4500, 100–5000, 100–5500, 100–6000, 100–6500, 100–7000, 100– 7500, 100–8000, 100–8500, 100–9000, 100–9500, 100–10000, 500–1000, 500–1500, 500–2000, 500–2500, 500–3000, 500–3500, 500–4000, 500–4500, 500–5000, 500–5500, 500–6000, 500– 6500, 500–7000, 500–7500, 500–8000, 500–8500, 500–9000, 500–9500, 500–10000, 1000– 1500, 1000–2000, 1000–2500, 1000–3000, 1000–3500, 1000–4000, 1000–4500, 1000–5000, 1000–5500, 1000–6000, 1000–6500, 1000–7000, 1000–7500, 1000–8000, 1000–8500, 1000– 9000, 1000–9500, 1000–10000, 5000–6000, 5000–7000, 5000–8000, 5000–9000, or 5000– 10000, including all endpoints, integers and subranges within the disclosed ranges. In some embodiments, a nanostructure will adjacently border at least one other nanostructure. As described herein, “adjacent bordering” refers to the lateral wall to wall spacing between adjacent nanostructures. In one embodiment, the lateral wall to wall spacing between adjacent nanostructures comprises from about 0 to about 100 nm or greater. In one aspect, the lateral wall to wall spacing between adjacent nanostructures comprises 0 nm, 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, or greater. In another aspect, the lateral wall to wall spacing between adjacent nanostructures comprises 0– 5 nm, 0–10 nm, 0–15 nm, 0–20 nm, 0–25 nm, 0–30 nm, 0–35 nm, 0–40 nm, 0–45 nm, 0–50 nm, 0–55 nm, 0–60 nm, 0–65 nm, 0–70 nm, 0–75 nm, 0–80 nm, 0–85 nm, 0–90 nm, 0–95 nm, 0–100 nm, 0.5–5 nm, 0.5–10 nm, 0.5–15 nm, 0.5–20 nm, 0.5–25 nm, 0.5–30 nm, 0.5–35 nm, 0.5–40 nm, 0.5–45 nm, 0.5–50 nm, 0.5–55 nm, 0.5–60 nm, 0.5–65 nm, 0.5–70 nm, 0.5–75 nm, 0.5–80 nm, 0.5–85 nm, 0.5–90 nm, 0.5–95 nm, 0.5–100 nm, 1–5 nm, 1–10 nm, 1–15 nm, 1–20 nm, 1– 25 nm, 1–30 nm, 1–35 nm, 1–40 nm, 1–45 nm, 1–50 nm, 1–55 nm, 1–60 nm, 1–65 nm, 1–70 nm, 1–75 nm, 1–80 nm, 1–85 nm, 1–90 nm, 1–95 nm, 1–100 nm, 2–5 nm, 2–10 nm, 2–15 nm, 2–20 nm, 2–25 nm, 2–30 nm, 2–35 nm, 2–40 nm, 2–45 nm, 2–50 nm, 2–55 nm, 2–60 nm, 2–65 nm, 2–70 nm, 2–75 nm, 2–80 nm, 2–85 nm, 2–90 nm, 2–95 nm, 2–100 nm, 5–10 nm, 5–15 nm, 5–20 nm, 5–25 nm, 5–30 nm, 5–35 nm, 5–40 nm, 5–45 nm, 5–50 nm, 5–55 nm, 5–60 nm, 5–65 nm, 5–70 nm, 5–75 nm, 5–80 nm, 5–85 nm, 5–90 nm, 5–95 nm, 5–100 nm, 10–15 nm, 10–20 nm, 10–25 nm, 10–30 nm, 10–35 nm, 10–40 nm, 10–45 nm, 10–50 nm, 10–55 nm, 10–60 nm, 10–65 nm, 10–70 nm, 10–75 nm, 10–80 nm, 10–85 nm, 10–90 nm, 10–95 nm, 10–100 nm, 25–50 nm, 25–55 nm, 25–60 nm, 25–65 nm, 25–70 nm, 25–75 nm, 25–80 nm, 25–85 nm, 25–90 nm, 25–95 nm, 25–100 nm, 50–75 nm, 50–80 nm, 50–85 nm, 50–90 nm, 50–95 nm, or 50–100 nm, including all endpoints, integers and subranges within the disclosed ranges. As described herein, “nanostructure density” refers to the space nanostructures occupy and relate to the mass per unit of volume. In one embodiment, the nanostructure density comprises about 100 to about 5100 kg / m3. In one aspect, the nanostructure density comprises , the nanostructure density comprises 100 kg / m3, 200 kg / m3, 300 kg / m3, 400 kg / m3, 500 kg / m3, 600 kg / m3, 700 kg / m3, 800 kg / m3, 900 kg / m3, 1000 kg / m3, 1100 kg / m3, 1200 kg / m3, 1300 kg / m3, 1400 kg / m3, 1500 kg / m3, 1600 kg / m3, 1700 kg / m3, 1800 kg / m3, 1900 kg / m3, 2000 kg / m3, 2100 kg / m3, 2200 kg / m3, 2300 kg / m3, 2400 kg / m3, 2500 kg / m3, 2600 kg / m3, 2700 kg / m3, 2800 kg / m3, 2900 kg / m3, 3000 kg / m3, 3100 kg / m3, 3200 kg / m3, 3300 kg / m3, 3400 kg / m3, 3500 kg / m3, 3600 kg / m3, 3700 kg / m3, 3800 kg / m3, 3900 kg / m3, 4000 kg / m3, 4100 kg / m3, 4200 kg / m3, 4300 kg / m3, 4400 kg / m3, 4500 kg / m3, 4600 kg / m3, 4700 kg / m3, 4800 kg / m3, 4900 kg / m3, 5000 kg / m3, 5100 kg / m3, or greater. In another aspect, 100–250 kg / m3, 100–500 kg / m3, 100–750 kg / m3, 100–1000 kg / m3, 100–1250 kg / m3, 100–1500 kg / m3, 100–1750 kg / m3, 100–2000 kg / m3, 100–2250 kg / m3, 100– 2500 kg / m3, 100–2750 kg / m3, 100–3000 kg / m3, 100–3250 kg / m3, 100–3500 kg / m3, 100–3750 kg / m3, 100–4000 kg / m3, 100–4250 kg / m3, 100–4500 kg / m3, 100–4750 kg / m3, 100–5000 kg / m3, 100–5100 kg / m3, 250–500 kg / m3, 250–750 kg / m3, 250–1000 kg / m3, 250–1250 kg / m3, 250–1500 kg / m3, 250–1750 kg / m3, 250–2000 kg / m3, 250–2250 kg / m3, 250–2500 kg / m3, 250–2750 kg / m3, 250–3000 kg / m3, 250–3250 kg / m3, 250–3500 kg / m3, 250–3750 kg / m3, 250–4000 kg / m3, 250– 4250 kg / m3, 250–4500 kg / m3, 250–4750 kg / m3, 250–5000 kg / m3, 250–5100 kg / m3,1000–1500 kg / m3, 1000–1750 kg / m3, 1000–2000 kg / m3, 1000–2250 kg / m3, 1000–2500 kg / m3, 1000–2750 kg / m3, 1000–3000 kg / m3, 1000–3250 kg / m3, 1000–3500 kg / m3, 1000–3750 kg / m3, 1000–4000 kg / m3, 1000–4250 kg / m3, 1000–4500 kg / m3, 1000–4750 kg / m3, 1000–5000 kg / m3, 1000–5100 kg / m3,2500–3000 kg / m3, 2500–3250 kg / m3, 2500–3500 kg / m3, 2500–3750 kg / m3, 2500–4000 kg / m3, 2500–4250 kg / m3, 2500–4500 kg / m3, 2500–4750 kg / m3, 2500–5000 kg / m3,3500–4000 kg / m3, 3500–4250 kg / m3, 3500–4500 kg / m3, 3500–4750 kg / m3, 3500–5000 kg / m3, or 3500–5100 kg / m3, including all endpoints, integers and subranges within the disclosed ranges. In some embodiments, the nanostructure has a porosity. As described herein, “porosity” refers to an amount of void regions of the film per unit total volume of the film. For example, if the porosity of a material is 60% then the material has 60% vacant regions and 40% solid regions. “Average porosity” refers to the mode, median, or mean of the porosity of the generated array of nanostructures. In one embodiment, the average porosity comprises about 5% to about 90%. In one aspect, 5 %, 10 %, 15 %, 20 %, 25 %, 30 %, 35 %, 40 %, 45 %, 50 %, 55 %, 60 %, 65 %, 70 %, 75 %, 80 %, 85 %, 90 %, or greater. In another aspect, the average porosity comprises 5–10 %, 5–20 %, 5–30 %, 5–40 %, 5–50 %, 5–60 %, 5–70 %, 5–80 %, 5–90 %, 10–20 %, 10–30 %, 10–40 %, 10–50 %, 10–60 %, 10–70 %, 10–80 %, 10–90 %, 15–20 %, 15–30 %, 15–40 %, 15– 50 %, 15–60 %, 15–70 %, 15–80 %, 15–90 %, 20–30 %, 20–40 %, 20–50 %, 20–60 %, 20–70 %, 20–80 %, 20–90 %, 35–40 %, 35–50 %, 35–60 %, 35–70 %, 35–80 %, 35–90 %, 40–50 %, 40–60 %, 40–70 %, 40–80 %, 40–90 %, 50–60 %, 50–70 %, 50–80 %, 50–90 %, 60–70 %, 60– 80 %, 60–90 %, 70–80 %, 70–90 %, or 80–90 %, including all endpoints, integers and subranges within the disclosed ranges. As described herein, “volumetric surface area” refers to the surface area of the nanostructures per unit volume. In one embodiment, the volumetric surface area comprises about 5 × 106m2 / m3to about 500 × 106m2 / m3. In one aspect, the volumetric surface area (in m2) of the nanostructure array contained in unit volume (in m3) comprises 5 × 106m2 / m3, 10 × 106m2 / m3, 15 × 106m2 / m3, 20 × 106m2 / m3, 25 × 106m2 / m3, 30 × 106m2 / m3, 35 × 106m2 / m3, 40 × 106m2 / m3, 45 × 106m2 / m3, 50 × 106m2 / m3, 75 × 106m2 / m3, 100 × 106m2 / m3, 125 × 106m2 / m3, 150 × 106m2 / m3, 175 × 106m2 / m3, 200 × 106m2 / m3, 225 × 106m2 / m3, 250 × 106m2 / m3, 275 × 106m2 / m3, 300 × 106m2 / m3, 325 × 106m2 / m3, 350 × 106m2 / m3, 375 × 106m2 / m3, 400 × 106m2 / m3, 425 × 106m2 / m3, 450 × 106m2 / m3, 475 × 106m2 / m3, 500 × 106m2 / m3, or greater. In another aspect, the volumetric surface area of the nanostructures comprises 5 × 106to 25 × 106m2 / m3, 5 × 106to 50 × 106m2 / m3, 5 × 106to 75 × 106m2 / m3, 5 × 106to 100 × 106m2 / m3, 5 × 106to 125 × 106m2 / m3, 5 × 106to 150 × 106m2 / m3, 5 × 106to 175 × 106m2 / m3, 5 × 106to 200 × 106m2 / m3, 5 × 106to 225 × 106m2 / m3, 5 × 106to 250 × 106m2 / m3, 5 × 106to 275 × 106m2 / m3, 5 × 106to 300 × 106m2 / m3, 5 × 106to 325 × 106m2 / m3, 5 × 106to 350 × 106m2 / m3, 5 × 106to 375 × 106m2 / m3, 5 × 106to 400 × 106m2 / m3, 5 × 106to 425 × 106m2 / m3, 5 × 106to 450 × 106m2 / m3, 5 × 106to 475 × 106m2 / m3, 5 × 106to 500 × 106m2 / m3, 25 × 106to 50 × 106m2 / m3, 25 × 106to 75 × 106m2 / m3, 25 × 106to 100 × 106m2 / m3, 25 × 106to 150 × 106m2 / m3, 25 × 106to 200 × 106m2 / m3, 25 × 106to 250 × 106m2 / m3, 25 × 106to 300 × 106m2 / m3, 25 × 106to 350 × 106m2 / m3, 25 × 106to 400 × 106m2 / m3, 25 × 106to 450 × 106m2 / m3, 25 × 106to 500 × 106m2 / m3, 50 × 106to 100 × 106m2 / m3, 50 × 106to 200 × 106m2 / m3, 50 × 106to 300 × 106m2 / m3, 50 × 106to 400 × 106m2 / m3, or 50 × 106to 500 × 106m2 / m3, including all endpoints, integers and subranges within the disclosed ranges. In some embodiments the array of nanostructures may have an electrical resistivity. As described herein, “electrical resistivity” or “resistivity” refers to the extent at which an array of nanostructures opposes the flow of electric current. It would be recognized by a person of ordinary skill in the art that resistivity is the inverse of conductivity. In one embodiment, the array of nanostructures has a resistivity of about 0.001 to about 10 m. In one aspect, resistivity of the array of nanostructures comprises 0.001 m, 0.002 m, 0.003 m, 0.004 m, 0.005 m, 0.006 m, 0.007 m, 0.008 m, 0.009 m, 0.01 m, 0.02 m, 0.03 m, 0.04 m, 0.05 m, 0.06 m, 0.07 m, 0.08 m, 0.09 m, 0.1 m, 0.15 m, 0.2 m, 0.25 m, 0.3 m, 0.35 m, 0.4 m, 0.45 m, 0.5 m, 0.55 m, 0.6 m, 0.65 m, 0.7 m, 0.75 m, 0.8 m, 0.85 m, 0.9 m, 0.95 m, 1 m, 2 m, 3 m, 4 m, 5 m, 6 m, 7 m, 8 m, 9 m, or 10 m. In another aspect, the resistivity of the array of nanostructures comprises 0.001– 0.002 mm, 0.001–0.003 mm, 0.001–0.004 mm, 0.001–0.005 mm, 0.001–0.006 mm, 0.001–0.007 mm, 0.001–0.008 mm, 0.001–0.009 mm, 0.001–0.01 mm, 0.001–0.02 mm, 0.001–0.03 mm, 0.001–0.04 mm, 0.001–0.05 mm, 0.001–0.06 mm, 0.001–0.07 mm, 0.001–0.08 mm, 0.001–0.09 mm, 0.001–0.1 mm, 0.001–0.2 mm, 0.001–0.3 mm, 0.001–0.4 mm, 0.001–0.5 mm, 0.001–0.6 mm, 0.001–0.7 mm, 0.001–0.8 mm, 0.001– 0.9 mm, 0.001–1 mm, 0.001–2 mm, 0.001–3 mm, 0.001–4 mm, 0.001–5 mm, 0.001– 6 mm, 0.001–7 mm, 0.001–8 mm, 0.001–9 mm, 0.002–0.003 mm, 0.002–0.004 mm, 0.002–0.005 mm, 0.002–0.006 mm, 0.002–0.007 mm, 0.002–0.008 mm, 0.002– 0.009 mm, 0.002–0.01 mm, 0.002–0.05 mm, 0.002–0.1 mm, 0.002–0.2 mm, 0.002– 0.3 mm, 0.002–0.4 mm, 0.002–0.5 mm, 0.002–0.6 mm, 0.002–0.7 mm, 0.002–0.8 mm, 0.002–0.9 mm, 0.002–1 mm, 0.002–2 mm, 0.002–3 mm, 0.002–4 mm, 0.002–5 mm, 0.002–6 mm, 0.002–7 mm, 0.002–8 mm, 0.002–9 mm, 0.003–0.004 mm, 0.003–0.005 mm, 0.003–0.006 mm, 0.003–0.007 mm, 0.003–0.008 mm, 0.003–0.009 mm, 0.003–0.01 mm, 0.003–0.05 mm, 0.003–0.1 mm, 0.003–0.2 mm, 0.003–0.3 mm, 0.003–0.4 mm, 0.003–0.5 mm, 0.003–0.6 mm, 0.003–0.7 mm, 0.003–0.8 mm, 0.003–0.9 mm, 0.003–1 mm, 0.003–2 mm, 0.003–3 mm, 0.003–4 mm, 0.003–5 mm, 0.003–6 mm, 0.003–7 mm, 0.003–8 mm, 0.003–9 mm, 0.004–0.005 mm, 0.004– 0.006 mm, 0.004–0.007 mm, 0.004–0.008 mm, 0.004–0.009 mm, 0.004–0.01 mm, 0.004–0.05 mm, 0.004–0.1 mm, 0.004–0.2 mm, 0.004–0.3 mm, 0.004–0.4 mm, 0.004– 0.5 mm, 0.004–0.6 mm, 0.004–0.7 mm, 0.004–0.8 mm, 0.004–0.9 mm, 0.004–1 mm, 0.004–2 mm, 0.004–3 mm, 0.004–4 mm, 0.004–5 mm, 0.004–6 mm, 0.004–7 mm, 0.004–8 mm, or 0.004–9 mm, including all endpoints, integers and subranges within the disclosed ranges. In some embodiments, the nanostructure is a nanotube. In some embodiment, the nanotube comprises a pore. As described herein, a “pore” refers to an opening of at least one end of the nanotube that extends longitudinally along the nanotube. In some embodiments, the pore of a nanotube may substantially fully extend the length of the nanotube. In some embodiments, a nanotube may have a pore on the distal and proximal end of the nanotube. In some embodiments, the pore on the distal and proximal end of the nanotube will be connected. As described herein, a “pore diameter” is the diameter of an opening at the distal and / or proximal end of the nanotube. In one embodiment, the pore diameter comprises from about 10 nm to about 400 nm. In one aspect, the pore diameter comprises 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 125 nm, 150 nm, 175 nm, 200 nm, 225 nm, 250 nm, 275 nm, 300 nm, 325 nm, 350 nm, 375 nm, 400 nm, or greater. In another aspect, the pore diameter comprises 10–50 nm, 10–75 nm, 10–100 nm, 10–150 nm, 10–200 nm, 10–250 nm, 10–300 nm, 10–350 nm, 10–400 nm, 25– 50 nm, 25–75 nm, 25–100 nm, 25–125 nm, 25–150 nm, 25–175 nm, 25–200 nm, 25–225 nm, 25–250 nm, 25–275 nm, 25–300 nm, 25–325 nm, 25–350 nm, 25–375 nm, 25–400 nm, 35–50 nm, 35–75 nm, 35–100 nm, 35–125 nm, 35–150 nm, 35–175 nm, 35–200 nm, 35–225 nm, 35– 250 nm, 35–275 nm, 35–300 nm, 35–325 nm, 35–350 nm, 35–375 nm, 35–400 nm, 50–75 nm, 50–100 nm, 50–125 nm, 50–150 nm, 50–175 nm, 50–200 nm, 50–225 nm, 50–250 nm, 50–275 nm, 50–300 nm, 50–325 nm, 50–350 nm, 50–375 nm, 50–400 nm, 75–100 nm, 75–125 nm, 75– 150 nm, 75–175 nm, 75–200 nm, 75–225 nm, 75–250 nm, 75–275 nm, 75–300 nm, 75–325 nm, 75–350 nm, 75–375 nm, 75–400 nm, including all endpoints, integers and subranges within the disclosed ranges. Described herein is a method for detecting analytes in a gas mixture sample. In some embodiments, the analytes in the gas mixture are permanent gases. The method comprises the contacting of a sensor comprising continuous and discontinuous regions of semiconducting metal oxide nanostructures with direct breath (exhalation) or a breath sample and detecting a signal generated by the material due to its interaction with a permanent gas or a volatile organic compound. As described herein “permanent gas” refers to a gas that primarily exists in a gaseous state and may not be liquefied by pressure alone at ambient temperatures. Non-limiting examples of permanent gases, include but are not limited to, hydrogen. In further embodiments, the gaseous mixture of the permanent gases originates from a sample of the atmosphere or existing air in a particular location or from exhaled gas. In some embodiments, the analytes in the gas mixture are volatile organic compounds, also referred to herein as “VOC.” Non-limiting examples of VOC, include but are not limited to, heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. It is to be understood that the VOC to be detected comprise non-polar organic molecules as well as polar organic molecules, and combinations of polar and non-polar organic molecules. In specific embodiments, the sample comprising VOC is an exhalation, respiration, or breath sample. The collection of an exhalation, respiration, or breath sample, can be performed in any manner known to a person of ordinary skill in the art. In some embodiments, a plurality of signals can be detected, each signal being collected at different applied voltages. The detected signal can be resistance, conductance, impedance, or a combination thereof. Described herein is the exhalation, respiration, or breath sample analysis which is directed to the diagnosis of various diseases or disorders. One embodiment described herein is a method for detecting diseases or disorders in a subject in need thereof by contacting a breath sample from the subject to a sensor comprising continuous and discontinuous regions of semiconducting metal oxide nanostructures and detecting a signal comprising resistance, conductance, impedance, or a combination thereof. Non-limiting examples of diseases or disorders comprise cancer (different types), COVID-19, Alzheimers, neonatal enterocolitis, liver disease, kidney disease, pulmonary diseases such as asthma and tuberculosis, or autoimmune disease. Encompassed by the information disclosed herein is the detection or diagnosis of cancer by the detection of VOC indicative of cancer. One embodiment described herein is a method for detecting cancer in a subject in need thereof by contacting a breath sample from the subject to a sensor comprising continuous and discontinuous regions of semiconducting metal oxide nanostructures and detecting a signal comprising resistance, conductance, impedance, or a combination thereof. In some embodiments, the cancer to be diagnosed includes, but is not limited to, lung cancer, colorectal cancer, pancreatic cancer, thyroid cancer, liver cancer, brain cancer, blood cancer, melanoma, oral and oropharyngeal cancer, kidney cancer and prostate cancer. Described herein is a ZnO sensor that is responsive to hydrogen in air. Earlier versions of the sensor had high sensitivity only to hydrogen diluted with nitrogen. The sensor was responsive to hydrogen mixed with air only when the hydrogen concentration was high (in percent levels). The sensor described herein has an ultrathin coating of palladium on the sensor surface and uses a low thickness zinc foil substrate for sensor fabrication. These steps permit the sensors to respond to parts per million (ppm) levels of hydrogen in air. This is a significant improvement as it facilitates the development of practically useful devices. Although the sensor can detect hydrogen (in air) down to about 100 ppm (0.01%) at room temperature, the sensitivity is highest when the sensor temperature is about 150 °C. The sensor can be operated at different temperatures depending upon the application and environment with a sensitivity calibration based on temperature of the sensor. High humidity can affect the sensitivity; however, the sensors do not stop functioning unless there is water condensation on the surface. FIG.34 shows the feasibility of using the sensor for hydrogen leak detection in air at room temperature. Described herein are steps for the ZnO nanotube array synthesis. A glycerol-based electrolyte provides well-defined nanotubes with open pores. Resistance was connected in series to the electrodes in the circuit for anodization. This made both voltage and current vary with time and gave better control over the anodization parameters for obtaining the desired quality of the nanotube array films. It also allowed the anodization to be performed at room temperature (25 °C). Another embodiment described herein is a method for making transparent nanotube arrays of ZnO. This is done by melting zinc and applying it on glass and then anodizing the film. This permits the fabrication of ZnO nanotube arrays on practically any substrate. This method enables fabrication of sensors on flexible substrates. If this is integrated with flexible electronics, wearable devices can be fabricated. Earlier work was performed using nanotubes heat treated at elevated temperatures (up to 400 °C). The new as-fabricated material was crystalline, and these were also responsive to hydrogen. While a short annealing at about 100 °C is preferred to remove water and organics from the film, a heat treatment step is not essential for the sensor functioning. This is important for commercial applications as it reduces the energy and cost involved in the device fabrication. In an open or a closed industrial environment where hydrogen is handled (e.g., hydrogen pipelines), the sensors can be dispersed, and hydrogen leaks can be monitored by analyzing the sensor responses with a central computer using wireless communication. The sensors need very low power for operation, and these can be fabricated on sticky tapes, which can be applied near pipe joints and other places. One embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0– 60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 25–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting hydrogen in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the sample; and detecting a signal generated in response to hydrogen in the sample. In another aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. In another aspect, a lack of change indicates that the hydrogen is present at a concentration below a detection limit. In another aspect, the sample comprises gas or air in the vicinity of equipment or apparata. In another aspect, the method is for detecting hydrogen gas leaks. Another embodiment described herein is a method for manufacturing a zinc oxide nanostructure array, the method comprising: providing a zinc substrate and a cathode in an electrolyte, wherein the zinc substrate comprises a first surface and a second surface, wherein the electrolyte comprises a base, water, and an organic solvent comprising ethylene glycol, diethylene glycol, polyethylene glycol, formamide, dimethyl sulfoxide, acetic acid, glycerol, or combinations thereof; applying a voltage to the zinc substrate and the cathode thereby reducing the cathode and oxidizing said zinc substrate on the first and second surfaces to form a plurality of zinc oxide nanostructures on the first and second surface of the zinc substrate; enlarging the zinc oxide nanostructures by continuing to apply the voltage; and fully crystallizing the zinc oxide nanostructures with a heat treatment. In one aspect, enlarging the zinc oxide nanostructures comprises increasing a thickness of said nanostructures. In another aspect, enlarging the zinc oxide nanostructures comprises increasing diameter and a thickness of said nanostructures. In another aspect, continuing to apply the voltage comprises increasing the voltage. In another aspect, enlarging the zinc nanostructures comprises heating from about 1–50 °C. In another aspect, the cathode comprises platinum, graphite, stainless steel or nickel or its alloys. In another aspect, the voltage can be varied from 5–100 V. In another aspect, the base comprises sodium hydroxide, potassium hydroxide, ammonium hydroxide, calcium hydroxide, lithium hydroxide, cesium hydroxide or a combination thereof. In another aspect, the electrolyte further comprises urea. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, or phosphoric acid. In another aspect, the electrolyte further comprises a salt. In another aspect, the salt comprises potassium fluoride, sodium fluoride, lithium fluoride, ammonium fluoride, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium carbonate, calcium carbonate, sodium nitrate, or potassium nitrate, or combinations thereof. In another aspect, the electrolyte comprises: 0.25–3.0 M of the base; 10–300 mM of potassium fluoride; 1–80 vol. % of water; and 20 to 99 vol. % of the organic solvent. In another aspect, the base comprises potassium hydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide, cesium hydroxide, or combinations thereof. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, nitric acid, and perchloric acid. In another aspect, the method further comprises ultrasonicating the zinc substrate and the cathode material in at least one solvent before applying them respectively as anode and cathode in an electrochemical anodization cell. In another aspect, the at least one solvent is isopropanol, acetone, or water. In another aspect, the heat treatment is performed at 100–420 °C. Another embodiment described herein is an array of zinc oxide nanostructure obtained by the method of manufacturing. In one aspect, each zinc oxide nanostructure comprises an average outer diameter of about 20–500 nm. In another aspect, each zinc oxide nanostructure comprises an average height of about 50 nm to 1000 μm. In another aspect, each zinc oxide nanostructure comprises a vertical alignment angle of about 0–60 degrees off the vertical direction. In another aspect, each zinc oxide nanostructure comprises an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3. In another aspect, each zinc oxide nanostructure comprises an average roughness factor of about 10–3,50,000, In another aspect, the array of zinc oxide nanostructures has an electrical conductivity of about 0.001–10 m. Another embodiment described herein is a method for detecting analytes in a gas mixture sample. In some embodiments, the analytes in the gas mixture are permanent gases. The method comprises the exposure of a sensor comprising continuous and discontinuous regions of semiconducting metal oxide nanostructures to direct breath or a breath sample and detecting a signal generated by a permanent gas or a volatile organic compound. As described herein “permanent gas” refers to a gas that primarily exists in a gaseous state and may not be liquefied by pressure alone at ambient temperatures. Non-limiting examples of permanent gases, include but are not limited to, hydrogen, carbon dioxide, nitrogen, argon, ammonia, and hydrogen sulfide. In further embodiments, the gaseous mixture of the permanent gases originates from a sample of the atmosphere or existing in air a particular location or in exhaled gas. One embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0– 60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the sample; and detecting a signal generated in response to the sample. In one aspect, the analyte comprises a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl- ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. In another aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, a change in the signal indicates that an analyte is present in the sample and a lack of change in the signal indicates that an analyte is absent. In another aspect, a lack of change indicates that the analyte is present at a concentration below a detection limit. In another aspect, the sample comprises the respiration or exhalation of a subject. In another aspect, a presence or absence of one or more analytes indicates the subject has a disease or disorder. In another aspect, the disease or disorder is cancer, diabetes, COVID-19, Alzheimer’s, neonatal enterocolitis, liver disease, kidney disease, pulmonary diseases, asthma, tuberculosis, autoimmune diseases, or combinations thereof. In another aspect, the cancer is breast cancer, lung cancer, colorectal cancer, pancreatic cancer, thyroid cancer, liver cancer, brain cancer, blood cancer, melanoma, oral and oropharyngeal cancer, kidney cancer and prostate cancer. Another embodiment described herein is a method for manufacturing a zinc oxide nanostructure array described herein, the method comprising: providing a zinc substrate and a cathode in an electrolyte, wherein the zinc substrate comprises a first planar surface and a second planar surface, wherein the electrolyte comprises a base, water, and an organic solvent comprising ethylene glycol, diethylene glycol, polyethylene glycol, formamide, dimethyl sulfoxide, acetic acid, glycerol, or combinations thereof; applying a voltage to the zinc substrate and the cathode thereby reducing the cathode and oxidizing said zinc substrate on the first and second planar surfaces to form a plurality of zinc oxide nanostructures on the first and second planar surface of the zinc substrate; enlarging the zinc oxide nanostructures by continuing to apply the voltage; and fully crystallizing the zinc oxide nanostructures with a heat treatment. In one aspect, enlarging the zinc oxide nanostructures comprises increasing a thickness of said nanostructures. In another aspect, enlarging the zinc oxide nanostructures comprises increasing diameter and a thickness of said nanostructures. In another aspect, continuing to apply the voltage comprises increasing the voltage. In another aspect, enlarging the zinc nanostructures comprises heating from about 1–50 °C. In another aspect, the cathode comprises platinum, graphite, stainless steel or nickel or its alloys. In another aspect, the voltage can be varied from 5–100 V. In another aspect, the base comprises sodium hydroxide, potassium hydroxide, ammonium hydroxide, calcium hydroxide, lithium hydroxide, cesium hydroxide or a combination thereof. In another aspect, the electrolyte further comprises urea. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, or phosphoric acid. In another aspect, the electrolyte further comprises a salt. In another aspect, the salt comprises potassium fluoride, sodium fluoride, lithium fluoride, ammonium fluoride, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium carbonate, calcium carbonate, sodium nitrate, or potassium nitrate, or combinations thereof. In another aspect, the electrolyte comprises:0.25–3.0 M of the base; 10–300 mM of potassium fluoride; 1–80 vol. % of water; and 20 to 99 vol. % of the organic solvent. In another aspect, the base comprises potassium hydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide, cesium hydroxide, or combinations thereof. In another aspect, the electrolyte further comprises an acid. In another aspect, the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, nitric acid, and perchloric acid. In another aspect, the method further comprises ultrasonicating the zinc substrate and the cathode material in at least one solvent before applying them respectively as anode and cathode in an electrochemical anodization cell. In another aspect, the at least one solvent is isopropanol, acetone, or water. In another aspect, heat treatment is performed at 100–420 °C. Another embodiment described herein is an array of zinc oxide nanostructure array obtained by the methods described herein. In one aspect, each zinc oxide nanostructure comprises an average outer diameter of about 20–500 nm. In another aspect, each zinc oxide nanostructure comprises an average height of about 50 nm to 1000 μm. In another aspect, each zinc oxide nanostructure comprises a vertical alignment angle of about 0–60 degrees off the vertical direction. In another aspect, each zinc oxide nanostructure comprises an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3. In another aspect, each zinc oxide nanostructure comprises an average roughness factor of about 10–3,50,000. In another aspect, the array of zinc oxide nanostructures has an electrical conductivity of about 0.001–10 m. Another embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting hydrogen in a gas mixture sample, the method comprising: providing a sensor comprising a nanostructure described herein; contacting the sensor with the sample; and detecting a signal generated in response to hydrogen in the sample. In another aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. In another aspect, a lack of change indicates that the hydrogen is present at a concentration below a detection limit. In another aspect, the sample comprises gas or air in the vicinity of equipment or apparata. In another aspect, the method is for detecting hydrogen gas leaks. Another embodiment described herein is a nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. In one aspect, the nanostructures are nanotubes. In another aspect, the nanotubes comprise a pore diameter of about 10–400 nm. In another aspect, the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. In another aspect, the array has an electrical conductivity of about 0.001–10 m. In another aspect, the nanostructures further comprise a density of 100–5100 kg / m3. In another aspect, the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Another embodiment described herein is a method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the sample; and detecting a signal generated in response to the sample. In one aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, the detection is performed at room temperature. In another aspect, the detection is performed at temperatures from 15–300 °C. In another aspect, the analyte is a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. Another embodiment described herein is a method for detecting hydrogen in ambient air, the method comprising: providing a sensor comprising the nanostructure array described herein; contacting the sensor with the ambient air; and detecting a signal generated in response to hydrogen in the sample. In one aspect, the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. In another aspect, the detection is performed at room temperature. In another aspect, the detection is performed at temperatures from 15–300 °C. In another aspect, a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. In another aspect, a lack of change indicates that the hydrogen is present at a concentration below a detection limit. In another aspect, the detection limit in air is about 100 ppb (0.0000001%) at room temperature. In another aspect, the sample comprises gas or air in the vicinity of equipment or apparata. In another aspect, in the method is for detecting hydrogen gas leaks. It will be apparent to one of ordinary skill in the relevant art that suitable modifications and adaptations to the compositions, formulations, methods, processes, apparata, assemblies, and applications described herein can be made without departing from the scope of any embodiments or aspects thereof. The compositions, apparata, assemblies, and methods provided are exemplary and are not intended to limit the scope of any of the disclosed embodiments. All the various embodiments, aspects, and options disclosed herein can be combined in any variations or iterations. The scope of the compositions, formulations, methods, apparata, assemblies, and processes described herein include all actual or potential combinations of embodiments, aspects, options, examples, and preferences described herein. The compositions, formulations, apparata, assemblies, or methods described herein may omit any component or step, substitute any component or step disclosed herein, or include any component or step disclosed elsewhere herein. The ratios of the mass of any component of any of the compositions or formulations disclosed herein to the mass of any other component in the formulation or to the total mass of the other components in the formulation are hereby disclosed as if they were expressly disclosed. Should the meaning of any terms in any of the patents or publications incorporated by reference conflict with the meaning of the terms used in this disclosure, the meanings of the terms or phrases in this disclosure are controlling. All patents and publications cited herein are incorporated by reference herein for the specific teachings thereof. Various embodiments and aspects of the inventions described herein are summarized by the following clauses: Clause 1. A nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. Clause 2. The array of clause 1, wherein the nanostructures are nanotubes. Clause 3. The array of clause 1 or 2, wherein the nanotubes comprise a pore diameter of about 10–400 nm. Clause 4. The array of any one of clauses 1–3, wherein the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. Clause 5. The array of any one of clauses 1–4, wherein the array has an electrical conductivity of about 0.001–10 m. Clause 6. The array of any one of clauses 1–5, wherein the nanostructures further comprise a density of 100–5100 kg / m3. Clause 7. The array of any one of clauses 1–6, wherein the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Clause 8. A method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array of any one of clauses 1–7; contacting the sensor with the sample; and detecting a signal generated in response to the sample. Clause 9. The method of clause 8, wherein the analyte comprises a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3- dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4- dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. Clause 10. The method of clause 8 or 9, wherein the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. Clause 11. The method of any one of clauses 8–10, wherein a change in the signal indicates that an analyte is present in the sample and a lack of change in the signal indicates that an analyte is absent. Clause 12. The method of any one of clauses 8–11, wherein a lack of change indicates that the analyte is present at a concentration below a detection limit. Clause 13. The method of any one of clauses 8–12, wherein the sample comprises the respiration or exhalation of a subject. Clause 14. The method of any one of clauses 8–13, wherein a presence or absence of one or more analytes indicates the subject has a disease or disorder. Clause 15. The method of any one of clauses 8–14, wherein the disease or disorder is cancer, diabetes, COVID-19, Alzheimer’s, neonatal enterocolitis, liver disease, kidney disease, pulmonary diseases, asthma, tuberculosis, autoimmune diseases, or combinations thereof. Clause 16. The method of any one of clauses 8–15, wherein the cancer is breast cancer, lung cancer, colorectal cancer, pancreatic cancer, thyroid cancer, liver cancer, brain cancer, blood cancer, melanoma, oral and oropharyngeal cancer, kidney cancer and prostate cancer. Clause 17. A method for manufacturing a zinc oxide nanostructure array according to any one of clauses 1–7, the method comprising: providing a zinc substrate and a cathode in an electrolyte, wherein the zinc substrate comprises a first planar surface and a second planar surface, wherein the electrolyte comprises a base, water, and an organic solvent comprising ethylene glycol, diethylene glycol, polyethylene glycol, formamide, dimethyl sulfoxide, acetic acid, glycerol, or combinations thereof; applying a voltage to the zinc substrate and the cathode thereby reducing the cathode and oxidizing said zinc substrate on the first and second planar surfaces to form a plurality of zinc oxide nanostructures on the first and second planar surface of the zinc substrate; enlarging the zinc oxide nanostructures by continuing to apply the voltage; and fully crystallizing the zinc oxide nanostructures with a heat treatment. Clause 18. The method of clause 17, wherein enlarging the zinc oxide nanostructures comprises increasing a thickness of said nanostructures. Clause 19. The method of clause 17 or 18, wherein enlarging the zinc oxide nanostructures comprises increasing diameter and a thickness of said nanostructures. Clause 20. The method of any one of clauses 17–19, wherein continuing to apply the voltage comprises increasing the voltage. Clause 21. The method of any one of clauses 17–20, wherein enlarging the zinc nanostructures comprises heating from about 1–50 °C. Clause 22. The method of any one of clauses 17–21, wherein the cathode comprises platinum, graphite, stainless steel or nickel or its alloys. Clause 23. The method of any one of clauses 17–22, wherein the voltage can be varied from 5–100 V. Clause 24. The method of any one of clauses 17–23, wherein the base comprises sodium hydroxide, potassium hydroxide, ammonium hydroxide, calcium hydroxide, lithium hydroxide, cesium hydroxide or a combination thereof. Clause 25. The method of any one of clauses 17–24, wherein the electrolyte further comprises urea. Clause 26. The method of any one of clauses 17–25, wherein the electrolyte further comprises an acid. Clause 27. The method of any one of clauses 17–26, wherein the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, sulfuric acid, nitric acid, perchloric acid, or phosphoric acid. Clause 28. The method of any one of clauses 17–27, wherein the electrolyte further comprises a salt. Clause 29. The method of any one of clauses 17–28, wherein the salt comprises potassium fluoride, sodium fluoride, lithium fluoride, ammonium fluoride, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium carbonate, calcium carbonate, sodium nitrate, or potassium nitrate, or combinations thereof. Clause 30. The method of any one of clauses 17–29, wherein the electrolyte comprises:0.25– 3.0 M of the base; 10–300 mM of potassium fluoride; 1–80 vol. % of water; and 20 to 99 vol. % of the organic solvent. Clause 31. The method of any one of clauses 17–30, wherein the base comprises potassium hydroxide, sodium hydroxide, ammonium hydroxide, lithium hydroxide, cesium hydroxide, or combinations thereof. Clause 32. The method of any one of clauses 17–31, wherein the electrolyte further comprises an acid. Clause 33. The method of any one of clauses 17–32, wherein the acid is selected from the group consisting of hydrofluoric acid, acetic acid, hydrochloric acid, nitric acid, and perchloric acid. Clause 34. The method of any one of clauses 17–33, wherein the method further comprises ultrasonicating the zinc substrate and the cathode material in at least one solvent before applying them respectively as anode and cathode in an electrochemical anodization cell. Clause 35. The method of any one of clauses 17–34, wherein the at least one solvent is isopropanol, acetone, or water. Clause 36. The method of any one of clauses 17–35, wherein the heat treatment is performed at 100–420 °C. Clause 37. An array of zinc oxide nanostructure array obtained by any one of clauses 17–36. Clause 38. The zinc oxide nanostructure array of clause 37, wherein each zinc oxide nanostructure comprises an average outer diameter of about 20–500 nm. Clause 39. The zinc oxide nanostructure array of clause 37 or 38, wherein each zinc oxide nanostructure comprises an average height of about 50 nm to 1000 μm. Clause 40. The zinc oxide nanostructure array of any one of clauses 37–39, wherein each zinc oxide nanostructure comprises a vertical alignment angle of about 0–60 degrees off the vertical direction. Clause 41. The zinc oxide nanostructure array of any one of clauses 37–40, wherein each zinc oxide nanostructure comprises an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3. Clause 42. The zinc oxide nanostructure array of any one of clauses 37–41, wherein each zinc oxide nanostructure comprises an average roughness factor of about 10–3,50,000. Clause 43. The zinc oxide nanostructure array of any one of clauses 37–42, wherein the array of zinc oxide nanostructures has an electrical conductivity of about 0.001–10 m. Clause 44. A nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. Clause 45. The array of clause 44, wherein the nanostructures are nanotubes. Clause 46. The array of clause 44 or 45, wherein the nanotubes comprise a pore diameter of about 10–400 nm. Clause 47. The array of any one of clauses 44–46, wherein the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. Clause 48. The array of any one of clauses 44–47, wherein the array has an electrical conductivity of about 0.001–10 m. Clause 49. The array of any one of clauses 44–48, wherein the nanostructures further comprise a density of 100–5100 kg / m3. Clause 50. The array of any one of clauses 44–49, wherein the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Clause 51. A method for detecting hydrogen in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array of any one of clauses 44–50; contacting the sensor with the sample; and detecting a signal generated in response to hydrogen in the sample. Clause 52. The method of clause 51, wherein the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. Clause 53. The method of clause 51 or 52, wherein a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. Clause 54. The method of any one of clauses 51–53, wherein a lack of change indicates that the hydrogen is present at a concentration below a detection limit. Clause 55. The method of any one of clauses 51–54, wherein the sample comprises gas or air in the vicinity of equipment or apparata. Clause 56. The method of any one of clauses 51–55, wherein the method is for detecting hydrogen gas leaks. Clause 57. A nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%. Clause 58. The array of clause 57, wherein the nanostructures are nanotubes. Clause 59. The array of clause 57 or 58, wherein the nanotubes comprise a pore diameter of about 10–400 nm. Clause 60. The array of any one of clauses 57–59, wherein the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures. Clause 61. The array of any one of clauses 57–60, wherein the array has an electrical conductivity of about 0.001–10 m. Clause 62. The array of any one of clauses 57–61, wherein the nanostructures further comprise a density of 100–5100 kg / m3. Clause 63. The array of any one of clauses 57–62, wherein the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon. Clause 64. A method for detecting an analyte in a gas mixture sample, the method comprising: providing a sensor comprising the nanostructure array of any one of clauses 57–63; contacting the sensor with the sample; and detecting a signal generated in response to the sample. Clause 65. A method for detecting hydrogen in ambient air, the method comprising: providing a sensor comprising the nanostructure array of any one of clauses 57–63; contacting the sensor with the ambient air; and detecting a signal generated in response to hydrogen in the sample. Clause 66. The method of clause 64 or 65, wherein the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof. Clause 67. The method of any one of clauses 64 or 65–66, wherein the detection is performed at room temperature. Clause 68. The method of any one of clauses 64 or 65–67, wherein the detection is performed at temperatures from 15–300 °C. Clause 69. The method of clause 64, wherein the analyte is a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl- 4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3- butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof. Clause 70. The method of clause 65, wherein a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample. Clause 71. The method of clause 65 or 70, wherein a lack of change indicates that the hydrogen is present at a concentration below a detection limit. Clause 72. The method of any one of clauses 65 or 70–71, wherein the detection limit in air is about 100 ppb (0.0000001%) at room temperature. Clause 73. The method of any one of clauses 65 or 70–72, wherein the sample comprises gas or air in the vicinity of equipment or apparata. Clause 74. The method of any one of clauses 65 or 70–73, wherein the method is for detecting hydrogen gas leaks. EXAMPLES Example 1 Materials Isopropanol (Sigma Aldrich), acetone (Sigma Aldrich), diethylene glycol (99% pure, Sigma Aldrich), and ethylene Glycol (99% pure, Sigma Aldrich), zinc metal foils (99% pure, Thermo Fisher Scientific), sodium hydroxide pellets (Sigma Aldrich), potassium fluoride (Sigma Aldrich), hydrogen (99% pure, Matheson Tri Gas), Argon gas (99% pure, Matheson Tri Gas) and Air (Matheson Tri Gas) were used without any further purification. Volatile organic compounds such as 2-propanol, heptanal, ethanol and methanol were procured from Sigma Aldrich. Distilled water was used to achieve required molarity. Synthesis of ZnO Nanotubes and Arrays of ZnO Nanotubes Zinc oxide (ZnO) nanotube arrays were obtained by anodizing cleaned zinc metal foils in an appropriate electrolyte. The zinc foils were first cleaned by ultrasonicating for the foils for 5 minutes in acetone then isopropanol and finally in water. For growing anodic nanostructures, zinc was used as the anode and platinum as the cathode in an electrochemical anodization cell. Anodization was conducted at voltages ranging from 15–60 V. Table 1 displays the different anodization conditions to fabricate the nanostructures of zinc oxide. In some instance, the electrolyte contained a glycol. In these instances, the amount of glycol added was the “rest” of the mixture. “Rest” refers to a remaining quantity of the total electrolyte volume and the volumes of hydrochloric acid, sodium / potassium hydroxide and water first. The glycol volume was the total volume minus the volumes of the above-mentioned components. The as-prepared samples were annealed at different temperatures (up to 400 °C) for different durations (typically 2 hours). Briefly, the process of annealing included loading the samples in a tube furnace where oxygen was passed through the furnace tube. The temperature was increased at a rate of ~7 °C / min to the desired temperature (100 °C to 420 °C), where the temperature was kept constant for 2 hours. Then the samples were cooled at a rate of ~7 °C per minute to room temperature. The annealing process produced fully crystalline structures of the fabricated ZnO nanotubes. The crystal structure of the fabricated ZnO nanotube was wurtzite. Table 1. Anodization conditions used to fabricate nanostructures of zinc oxide Condition Electrolyte Water Voltage (V) Time (min) Temperature 20 mM–1 M Room 1 hydrochloric acid in Yes 20–40 1–60 temperature water (RT; ~25 °C) 220 mM ammoniumchloride in waterYes 20 10-20 RT350 mM nitric acid inwater Yes 20 30 RT450 mM ammoniumnitrate in water Yes 20 30 RT50 mM sodium sulfate, 50 mM sodium 5 bisulfate, 20 mM Yes 20 30 RT hydrofluoric acid, and water 5.7 mM sodium carbonate, 6.8 mM 6 sodium bicarbonate, Yes 10 20 RT and water 6 mM sodium bicarbonate, 6 mM 7 potassium Yes 10 3–14 RT bicarbonate, 6 mM urea, and water 6 mM sodium 8 bicarbonate, 6 mM calcium carbonate, Yes 20 13–14 RT and water 2 M potassium 9 hydroxide, 50 mM hydrofluoric acid, and Yes 10–60 2–5 RT water 1–2 M ammonium hydroxide, 0.5 M 10 hydrofluoric acid, Yes 20–50 60 RT water, and ethylene glycol (rest) 0.5–2 M sodium hydroxide, 50 mM 11 hydrofluoric acid, 7-40 vol.% 20–40 60 RT water, and ethylene glycol (rest) 1 M potassium hydroxide, 50 mM 12 potassium fluoride, 30 vol.% 30 300 5 °C water, and diethylene glycol (rest) 0.5–2 M potassium hydroxide, 10–60 mM 13 potassium, fluoride, 20–60 25–180 2–40 °Cwater, and ethylene glycol (rest) 0.5–1.5 M sodium hydroxide, 10–60 mM 30-50 14 potassium fluoride,20–60 25–180 2–40 °Cwater, and ethylenevol.%glycol (rest) 1 M sodium hydroxide, 60 mM potassium 15 fluoride, 5–50 mM 40 vol.% 10–50 25–180 2–25 °C acetic acid, water, and ethylene glycol (rest) Following the synthesis of the zinc oxide nanostructures, samples were prepared for SEM imaging. To prepare a sample of zinc oxide nanostructure, a small piece is cut from the sample (film on the substrate) and mounted on an SEM stub using a double-sided carbon tape. The sides of the sample are coated with silver paste to minimize surface charging effects during SEM measurements. Without this conducting coating on the sides, charging of the samples takes place and it will be difficult to take clear images. The samples are then put in an oven at 100 °C for 2 hours before loading them in the SEM chamber. Chloride Electrolytes Zinc foil was anodized in various conditions containing chloride ions with the optimal ones listed in Table 1 as conditions 1 and 2. Anodization in HCl electrolytes (HCl concentration in the range 20 mM to 1 M) did not give rise to any nanostructures (FIG. 1A) except the formation of some microplate-like structures when anodization was done at 40 V in 0.1 M and 1 M HCl respectively for 60 s and 3600 s. Use of 20 mM ammonium chloride (NH4Cl) in place of HCl yielded petal-like structures as shown in FIG.1B–1C when anodization was done for 10 min and 20 min, respectively. Longer anodization time increased the thickness, but films became non- uniform. Nitrate Electrolytes After several experiments to find stable conditions for nitrate-based electrolytes, stable nanostructures were achieved with 20 V anodization in a nitrate-based electrolytes for 30 minutes. The pH of the electrolyte was 5. FIG.2A–2B show the structures obtained from these conditions. Irregular particle-like structures were formed in both electrolytes. At higher anodization voltage, the formed oxide layer was etched leaving the bare zinc. Sulfate Electrolytes Sulfate-based electrolytes were used to form nanotubes of ZnO. In these experiments, equimolar (50 mM) sodium sulfate and sodium bisulfate were mixed with 20 mM hydrofluoric acid in water. The zinc foil was anodized at 20 V for 30 minutes at room temperature. Scanning electron microscopy (SEM) analysis showed 2D petal-like structures with irregular channels. These films were powdery and unstable for device use. Sodium Bicarbonate and Sodium Carbonate Electrolytes Experiments revealed that zinc oxide is stable at high pH values. An electrolyte solution including 5.7 mM sodium carbonate and 6.8 mM sodium bicarbonate exposed to higher 20 V for 20 minutes. The SEM images of the resulting ZnO nanotubes revealed distinct nanowires with large diameter range (typical diameter ~300 nm) as shown in FIG.3A. The length of the resulting ZnO nanotubes was over 60 μm (FIG.3B). The goal was to fabricate films with only nanotubes with high mechanical strength and structural integrity suitable for chemical sensing. To improve the architecture, the effect of anodization time in response an electrolyte solution including sodium bicarbonate and sodium carbonate were studied. The result nanotubes over time are shown in FIG.4A–C. It was observed that the oxidation rates decrease, and chemical etching dominates resulting in tubular structures. Further studies showed that after 14 minutes of anodization, the surface had wires, rods and nanotubes as shown in FIG. 5A–D. The wires bundle together as they grow to several microns. FIG. 5A shows fibers that grew into hexagonal faceted rods. Rods / tubes with larger diameter (300–900 nm) were observed. FIG 5B shows rods of different sizes in which dissolution occurred to form tubes. At some regions on the surface, thin wires of diameter less than 50 nm grew into a length of tens of microns. The electrochemical metal ion migration and oxidation slow down as length of the nanostructure increases. In the case of larger diameter rods, chemical etching became dominant at this stage. Small pits formed on the top surface of the rods, and it burrows through converting the rods into tubes as obvious from FIG.5C–D. Sodium Bicarbonate and Potassium Bicarbonate with Urea Electrolytes An electrolyte including 6 mM of sodium bicarbonate and 6 mM of potassium bicarbonate was used to anodize zinc at 10 and 20 V for 13–14 minutes. With this electrolyte the pH was lowered slightly to about 8–9 compared to the previous electrolyte. In order to lower the pH, urea was added. As in the case of condition 6, the nanostructures formed at 10 V with this electrolyte had wires, rods, and tubes with an average diameter of 200 nm as shown in FIG.6A–B. Addition of urea and anodizing at 20 V resulted in more continuous nanotubes of average diameter of 250 nm as shown in FIG.6C–D. This shows that the addition of urea in the electrolyte may increase population of nanotubes in a film. Sodium Bicarbonate and Calcium Carbonate Electrolytes To find the effect of the cation on the structure of the films formed, sodium carbonate was replaced with calcium carbonate. An equimolar mixture of sodium bicarbonate and calcium carbonate in distilled (DI) water was used as the electrolyte. Anodization was performed at 20 V for 13–14 minutes with stirring at 500 rotations per minute. FIG.7A–D shows the SEM images of the formed structures. The surface showed more open nanotubes and less debris than the NaHCO3–NaCO3system, but the morphology remained the same. The flower-like region had tubular structures with the majority having open pores resembling straws. The surrounding regions had hexagonal rod structures with major diagonal length in the range of 100–800 nm, with an average of 300 nm. Stirring made the anodized nanostructures cleaner with less broken rods and wires. Nevertheless, the mechanical stability of these structures was lower than that obtained with the previous carbonate-based electrolytes. Potassium Hydroxide and Hydrofluoric Acid Electrolytes To obtain more mechanically stable films suitable for chemical sensing, zinc foil was anodized in high pH electrolytes. In these experiments, 2 M KOH and 50 mM HF in DI water was used. The anodization was performed at voltages ranging 10–60 V. The anodization current was very high even for small voltages due to the high reactivity of zinc. HF was added to this electrolyte to control the high oxidation rate of zinc, which made the current manageable. FIG. 8A–F shows the obtained nanostructures. At 10 V, small tubular structures with outer dimensions of 80–90 nm and 1 μm length were obtained. However, the pores of the nanotubes were not very open. Increasing the voltage to 40 V, resulted in nanotubes with a length of 2.7 μm and a pore diameter of 110 nm as shown in FIG. 9A–F. Beyond 40 V, the chemical reactions became vigorous, and the solution became hot, which resulted in breaking of the tubes to a shorter length of about 1.2 m. This opened pores at irregular regions of the film. Even though this electrolyte resulted in less open tubes, it produced a more stable films that are suitable for gas sensor measurements. The stability of the films was determined to be roughly over 80% as determined by a peeling test. Ammonium Hydroxide (NH4OH), Hydrofluoric Acid (HF), and Ethylene Glycol (EG) Electrolytes In the electrolyte solutions, water was replaced with Ethylene Glycol (EG). Ammonium hydroxide was also used to increase the pH for the electrolyte, which favorably growth of vertically aligned nanotubular structures. A solution containing 0.5 M HF and 1 M NH4OH dissolved in EG was used for the anodization at 20 V for 2 hours. This resulted in a porous structure with thickness about 1.2 μm. The structures were disordered and non-homogenous due to low field assisted dissolution. Increasing the voltage to 50 V gave porous structure with interconnected pores with a reduced thickness (200 nm). To increase the porosity of the structure, the hydroxide molarity was increased to 2 M, which increased the thickness of the structure to 340 nm. FIG. 10A–B shows structure. Sodium Hydroxide (NaOH), Hydrofluoric Acid (HF), and Ethylene Glycol (EG) Electrolytes Hydroxide-based electrolytes were further explored using NaOH and KOH interchangeably at different concentrations. Both NaOH and KOH produced similar structures. In this system we also explored the effect of each component of the electrolyte and how it affects the nanostructures. FIG. 11A–D show the typical SEM images of structures obtained from hydroxide-based systems. As evident from FIG. 11A–D, nanotubes were formed in this electrolyte. Nevertheless, pores of many of the nanotubes were filled with material and resembled a rod. Potassium Hydroxide (KOH), Potassium Fluoride (KF), and Diethylene Glycol (DEG) Electrolytes Anodization of zinc foil in DEG-based electrolyte was explored to determine the solvent had an effect on the opening of the nanotubes. The anodization was performed at 30 V in an electrolyte consisting of 1 M potassium hydroxide, 30% water, 50 mM potassium fluoride, and diethylene glycol at 5 °C. The SEM images (FIG. 12A–B) showed nanotube structure in films formed in this electrolyte also. The DEG based sample had the top dense surface detached from the nanotube array, and this helped in reducing the cleaning efforts after fabrication and before SEM analysis. During anodization, it was observed that the current density was lower in this electrolyte and resulted into a much shorter cross section (~1 μm) compared to similar EG based samples. Potassium or Sodium Hydroxide, Potassium Fluoride (KF), and Ethylene Glycol (EG) Electrolytes In this electrolyte, we replaced DEG with EG. Both NaOH and KOH were used as the hydroxide. Well-defined nanotubes with open pores were obtained with these conditions (FIG. 13–15). It was observed that reducing temperature resulted in more ordered tubular structures (FIG.16A–B). This may be due to the reduction in the activity of the electrolyte. This observation was consistent for both sodium hydroxide and potassium hydroxide containing electrolytes. Similarly, low voltage ranges (20–30 V), resulted in thinner more ordered nanotubular structures, but less open, compared to higher voltage (30–50 V) ranges. High water content (30–50 vol. %) showed a tendency of faster growth rate, which resulted in bending of the nanotubes if let to run for a long time. Potassium or Sodium Hydroxide, Potassium Fluoride (KF), Acetic Acid, and Ethylene Glycol (EG) Electrolytes In an effort to obtain more distinct open tubes, buffer agents such as, acetic acid, were used to minimize local fluctuations in the pH of the electrolyte. With the inclusion of a small quantity of acetic acid (5–50 mM), we observed a smooth current-time curve similar to the standard curves recorded during the formation of anodic titanium dioxide nanotubes. SEM images of the samples with acetic acid show more ordered structures with higher spacing between the tubes (FIG.17–20). The tubes were smaller (typical diameter 100 nm) than those formed under conditions 13 and 14. The acetic acid was found etched to the top surface selectively. Nevertheless, it was not clear if tubes were open from top to bottom. Sensor Development The properties of the nanotube samples were tailored to make them sensitive to gases and organic compounds of significance to medical diagnosis. As-prepared samples showed weak crystallinity. These samples did not exhibit appropriate sensing characteristics. The samples were annealed at different temperatures up to 400 °C in oxygen for obtaining the desired sensing properties. The x-ray diffraction (XRD) patterns of a zinc substrate and a heat treated ZnO nanotube sample are shown in FIG.21. The samples annealed at 400 °C show crystalline ZnO in wurtzite phase. Transmission electron microscope (TEM) images of the samples are shown in FIG.19A and 19B. The tubular nature of ZnO is evident from this figure. Platinum contacts were sputtered onto the sample surface for measuring the changes in resistance upon exposure to hydrogen gas. A chamber of volume 0.2 liter equipped with micro- heaters was used for the tests. The chamber had one inlet for the gas and an exhaust to remove the gas. The test gas was introduced into the chamber using a mass flow controller (MKS instruments) in continuous mode. The test was diluted with argon to obtain various concentrations in the parts per million (ppm) range. At the end of this cycle, air was used to remove the test gas from the chamber and restore the resistance. A picoammeter was used to measure the resistance at the surface of the samples with time. The variation of resistance with time and concentration were plotted and analyzed to understand the efficacy of the sensors. Fabricated Arrays of ZnO Nanotubes as Room Temperature Gas Sensors FIG.22A–B shows the sensor resistance when exposed to hydrogen repeatedly at room temperature. The sensors fabricated using the nanotubes heat treated at 200 °C showed the highest sensitivity to hydrogen. The nanotube preparation conditions are given in the figure caption. FIG. 22A shows the responses of two separate sensors prepared under similar conditions. The identical responses of the sensors indicate repeatability of the material characteristics. It is evident from the figure that the resistance changed almost four orders of magnitude when the sensors were exposed to 400 ppm hydrogen in argon. FIG.22B shows the resistance variation in three other sensors measured at H2concentrations 40, 200 and 400 ppm. The sensors were fabricated in three different conditions as given in the figure caption. The highest response was achieved by increasing the surface area (porous nature), reducing the wall thickness, and optimizing the thickness. The obtained response was superior to other low dimensional structures. Fabricated Arrays of ZnO Nanotubes as Volatile Organic Compound (VOC) Cancer Sensors Detection of volatile organic compounds (VOCs) in exhaled breath is an emerging noninvasive technique to effectively diagnose cancer. VOCs are created due to the peroxidation of the cell membrane species due to tumor growth. Recent research efforts have been focused on the design of highly selective and sensitive sensors to aid the detection of breast cancer related VOCs in human breath. Conventional breath analysis techniques such as gas chromatography are considered reliable, however, require skilled personnel to operate and long processing times. Fabricated ZnO nanotubes were used to detect breast cancer related VOCs such as ethanol, 2-propanaol and heptanal in simulated human breath. Principal Component Analysis was then applied to this choice of VOCs to discriminate the response. A machine learning algorithm (k Nearest Neighbors) was developed to predict the nature of VOC based on the response. FIG. 23 and FIG. 24 describes the effect of four different volatile organic compounds (VOCs) related to breast cancer, and their effect on two dimensionality reduction techniques (Principal Component Analysis). The experimental data was obtained with the sensors operating at 240 °C. The sensors were exposed to 60 ppm of each respective VOC. The sensitivity wascomputed using the equation, where Rais the resistance of sensor in air (baseline) and Rgis the resistance of sensor in target gas. The first (PC1) and second principal component (PC2) are plotted in FIG.23 and the sensor resistance variations are shown in FIG.24. A three-sensor array providing three responses was used and therefore, the maximum principal component obtained was three. The explained variance was used to decide the number of principal components to be involved in the analysis. The total explained variance was 99.6% for the two principal components, with PC1 contributing 98.1% and PC2 contributing 0.5%. As evident from FIG.23, the VOCs formed distinct clusters. The separation between CO2and heptanal cluster was small. This result may be in response to the two gases having close responses from the sensor array, indicating that the detection may overlap. From the PCA, a supervised algorithm using the machine learning algorithm named k- nearest neighbors (kNN) was designed. The normalized sensor response was split into training (70%) and testing sets (30%) for each separate VOC. The accuracy of the model depends on the size of the dataset. A small dataset giving 100% accuracy for the classifier was used. The confusion matrix (Table 2) shows the result of the classification procedure. The rows represent the actual gases: CO2, heptanal, ethanol, and 2-Propanol, whereas the columns represent the predicted gases by the kNN classifier. For instance, the first row and first column, CO2vs CO2contains the count of instances (1) where CO2was correctly classified as CO2. All the values along the diagonal represent current predictions, meaning the model correctly classified those instances into their actual classes. Values outside the main diagonal would represent misclassifications (i.e., false positives and true negatives). In the experiment discussed here, there was no misclassifications because we used a small dataset, which gave 100% accuracy. Table 2. Confusion Matrix Showing Outcome of kNN CO2 Heptanal Ethanol 2-Propanol CO2 1 0 0 0 Heptanal 0 4 0 0 Ethanol 0 0 3 0 2-Propanol 0 0 0 1 The PCA plot gives a visual representation of the separate VOCs exposed to the sensor array. The main objective of the sensor array is to predict the nature of VOC exposed to it. To achieve this, a generalized model based on the known data (training data) was generated. Subsequently, the model was tested on unknown data obtained from sensor response. In this way, the sensor was able to predict the presence of even very low concentrations of breast cancer VOCs. In particular, the amount of isopropanol was detected as low as 15 ppm. FIG.25 shows response of the ZnO nanotube samples to 9 ppm 2-propanol (in air) in a three-sensor array in which nanotubes were formed in different conditions. Sensor 1 was prepared in 2 M potassium hydroxide, 40 vol. % water, 60 mM potassium fluoride in EG (60 vol.%) at 15 V and 2 °C for 60 min. Sensor 2 was prepared in 2 M potassium hydroxide, 40 vol. % water, 20 mM potassium fluoride in EG (60 vo. %) at 15 V and 2 °C for 60 min. Sensor 3 was prepared in 1 M potassium hydroxide, 40 vol. % water, 60 mM potassium fluoride in EG (60 vol. %) at 15 V and 25 °C for 60 min. All sensors showed a consistent response upon cycling the environment between air with 9 ppm isopropanol and pure air. FIG. 26A–B show response of the samples to different concentrations (3, 9, 16 and 28 ppm) of 2-propanol in a three-sensor array. The films were prepared in 1 M sodium hydroxide, 60 mM potassium fluoride in EG at 30 V and 25 °C with water contents 30, 40, and 50 vol. % respectively for Sensor 1, Sensor 2, and Sensor 3. FIG.26A shows the change in resistance in the three sensors in response to 2-propanol. All sensors showed an increase in the magnitude of resistance change with 2-propanol concentration. The film in Sensor 1 was annealed at 200 °C and the films in Sensor 2 and Sensor 3 were annealed at 400 °C. FIG. 26B shows the sensitivity versus 2-propanol concentration plot for the three sensors mentioned in FIG.26A. The sensitivities of all sensors were linear in the range 15 to 140 ppm. Example 2 Synthesis of ZnO Nanotubes For hydrogen gas sensing it was hypothesized that nanotubes with thin walls (< Debye length) were required. Thin walled ZnO nanotubes would increase the depletion region and surface area to volume ratio (SVR) several times fold. This would result in improved gas sensing responses to desired gases. To achieve this, the cleaned zinc metal foils were anodized in an electrolyte containing potassium hydroxide (KOH), water, potassium fluoride (KF) and ethylene glycol (EG). The concentrations of these constituents were changed to achieve certain morphologies as shown in FIG.28. The zinc (99.9% pure, Sigma Aldrich) foils were first cleaned by ultrasonicating in acetone, isopropanol, and then water for 5 minutes in each. During the anodization, zinc foil was used as the anode and platinum as the cathode in an electrolyte. The metal foils were anodized at various voltages ranging from 15–60 V depending on the desired structure. The as-prepared samples were annealed at 100 °C, 200 °C, or 400 °C for 2 hours to study the effect of crystallinity. Room Temperature Hydrogen Gas Sensing The superiority of the ZnO nanotubes was tested using hydrogen gas sensing and compared to the literature. For this purpose, nanotubes of different lengths were used. Some samples were annealed at 100 °C, 200 °C, or 400 °C for 2 hours in oxygen atmosphere. Platinum contacts were made on the surface of the annealed samples to facilitate measurements of conductivity change when exposed to different gases. A chamber of volume 0.2 liter, loaded with four samples – sensor array, with ceramic heaters underneath the sample. The chamber has one inlet for the gas and an exhaust to remove the gas (see FIG.29). 4% hydrogen in nitrogen was introduced into the chamber, with nitrogen as the carrier gas using a mass flow controller (MKSinstruments) in continuous mode. In this way, a flow rate of 10 cm3 min 1 amounted to 400 ppmof hydrogen gas in the chamber at standard temperature and pressure. At the end of this cycle, air was used to clean the chamber and restore the samples to their initial state (baseline resistance). A picoammeter was used to measure the resistance at the surface of the samples during this exposure time. The resistance was plotted and analyzed for sensitivity, response time, recovery time and selectivity to hydrogen gas. ZnO Characterization FIG. 28 shows the SEM images of the as prepared samples. FIG. 28A–B represent samples prepared in low water electrolyte (30 vol. %) at 45 V and at 2°C (low temperature). We observed that these samples had larger pores (> 100 nm) and were less porous. Additionally, these samples had a thick ZnO layer at the top (~20 nm) and bottom (~50 nm). These two layers encapsulate the formed structure and control the rate of growth and porosity of the structures. The bottom layer at the Zn–ZnO interface limits both the movement of zinc ions into and out of the formed structures. The top layer at the interface between the ZnO–electrolyte interface limits the penetration of the electrolyte into the grown structures which limits the dissolution of the nanotubes. These two layers in this low water condition ensures a quick equilibration of the oxidation-dissolution processes, limiting the thickness of the fabricated structures to within 1 m. The top layer was cleaned off by chemical etching, which makes it challenging to control the final thickness of the obtained structures. This morphology was not ideal for devices because of the low SVR, and the presence of bottom layer would increase scattering of carriers causing noisy baselines and requirement of higher operating voltages. After extensive experimentation, we observed that by increasing the water in the electrolyte, the number of open pores increased after cleaning. FIG. 28C–D shows a sample fabricated in an electrolyte with 40 vol. % water at room temperature (~25 °C). SEM analysis showed that the as-prepared sample still had the bottom and top ZnO layer, but both were significantly reduced. This is partially attributed to the increased ion motion in the structures due to the increased temperature and water. This growth rate in this sample was also higher, which necessitated a reduction in anodization time. This is because over anodization produces nanostructures that are thinner at the top than at the bottom. Despite the slight increase in porosity of these structures, and a reduction of top and bottom layer, these samples still possessed the challenges of their predecessors, i.e., the nanotube walls were still thick (~100 nm). To solve this problem, we fabricated the samples in a solution containing high water (40 vol. %) at a higher pH (~14) environment at 2 °C (low temperature). FIG.28D–E shows the SEM images of these films. In this environment, we observed that the electrochemical etching was increased — with the top and bottom layer disappearing after anodization. The top layer became porous and was easily cleaned off ultrasonically. The removal of the bottom layer would hypothetically give more stable devices by reducing electron scattering and improving the gas dynamics within the nanotube array. This would also alternatively improve the response and recovery time for the sensor. This structure was more porous, and the nanotubes were thinner in size (~60 nm), which guarantees a reduction in wall thickness. FIG.27A–B shows the XRD patterns and TEM images of these nanotubes, respectively. The XRD was performed on the unannealed (as prepared) and annealed samples. XRD of the zinc foil used as a substrate for the fabricated nanotubes was also measured to be used as a reference. Annealing of the ZnO samples in oxygen ambient was only done up to a maximum temperature of 400 °C because the zinc metal substrate would melt at temperatures beyond 400 °C. FIG.27A shows that the annealed samples had all the peaks corresponding to hexagonal wurtzite phase of ZnO. There is only one zinc peak (101) which originates from the substrate. Additionally, we observed that the unannealed (as prepared) samples as well possessed both zinc and zinc oxide peaks. This is a desirable attribute that is not commonly obtained via anodization process. We set out to test the unannealed samples, with the idea that even a slight response to hydrogen would open pathways for utilizing ZnO nanotubes in fabrication of flexible electronic devices for gas monitoring. On the other hand, TEM micrographs of these samples show a porous interior, confirming a nanotubular nature. However, some of the tubes showed a clogged interior, indicating that the obtained structures are not completely etched. It was also observed that only samples fabricated with the high water and pH conditions showed a partial nanotubular nature. This is attributed to the reduction in the nanotube wall thickness and overall size of the tube. The samples obtained from other conditions had walls too thick (~100 nm) to obtain any information about the nature of the interior of the fabricated structures. Further TEM analysis using the selected area electron diffraction (SAED) also supported the appearance of ZnO wurtzite phase in the as prepared sample. Room Temperature Hydrogen Gas Sensing The nanotubes fabricated using the third condition (high water + high pH) gave the most consistent response to hydrogen gas at room temperature. The superiority of the response for this condition originates from reduced nanotube wall thickness (< 20 nm), increased surface area, and elimination of top and bottom layers. FIG.29 shows a schematic of the chamber supporting the four ZnO sensors used in the array. We used four sensors to eliminate any errors due to undesirable changes in the conditions. Initially we used nanotube samples annealed at 400 °C in oxygen ambient. We observed over several runs that these sensors suffered from unstable baselines and long recovery times. This is not ideal for a practical hydrogen gas sensor. To alleviate this, we lowered the annealing temperature to 200 °C, and these sensors boosted a highly stable baseline and faster recovery time (~5 s). A possible explanation for this behavior is the difference in surface energy between the two samples. The high temperature annealed samples potentially have high surface energies – increasing response and recovery time. A high surface energy would imply stronger bonding between the nanotube surface and the analyte (hydrogen) whereas low surface energies form weaker bonds with the analyte that are easily broken at room temperature. FIG. 30 shows the typical improved responses for samples annealed at 200 °C in oxygen ambient. We observed that these samples had a response time of about 50 s, and a recovery time of about 5 s. Additionally, we estimated the limit of detection as 100 ppm using the linear region of the sensor. This limit can still be extended further to ppb level by improving the crystallinity and limiting the surface defects on the sample. In this study, we observed that shorter nanotubes (~1 m) gave a higher response compared to longer nanotubes (2 m). Effect of Nanotube Length To further understand the role of the nanotube length, we fabricated three sensors of variable length based on the anodization time (20, 90, and 180 minutes) for the same electrolyte. SEM analysis performed for the different samples to verify the lengths of the nanotube for each anodization time. Exposing these sensors to 400 ppm of hydrogen showed that the nanotubes of length, 1 m, showed a higher sensitivity (104) compared to the longer nanotubes (3 m and 5 m). Contrary to our expectation, the 5 m nanotube showed a higher response compared to the 3 m nanotube. The longer nanotubes also had a longer response and recovery time. This is expected as increased anodization usually led to a reduction in surface area to volume ratio (SVR). Additionally, ZnO usually shows a pyramid structure for lengthy anodization – with complete rod like nature at the top, and reduction in porosity. The nature of the response of the 5 m sample in FIG.31B also provides more insights – showing two saturation regions. The first region would correspond to a response from the outer surface without any penetration into the pores. Eventually, the hydrogen would penetrate the pores hence the second drop in resistance. This would also explain why the 5 m nanotube has a response greater than the 3 m nanotubes. Response from the Unannealed Samples The improved performance of the samples annealed at 200 °C inspired testing of the unannealed (as-prepared) sensors to 400 ppm of hydrogen gas at room temperature. The obtained sensitivity (~400) is lower than that annealed at 200 °C (FIG 32A–B). XRD analysis (FIG.27) of these unannealed samples showed peaks for both Zn and ZnO. The low response is attributed to the low crystallinity in the samples and presence of impurities on the surface. Regardless, the fact that we can obtain a response to hydrogen in the unannealed samples outweighs the reduction in the response because of the potential applications. The as prepared samples retain the desirable properties of fast response (recovery) times i.e., 50 (5) s. Additionally, the shorter nanotube samples (1 m) have a higher sensitivity compared to the longer nanotubes (2 m). Response of ZnO Sensor in Ambient Oxygen The operation of ZnO in ambient oxygen is crucial for practical sensors in many fields. Many studies acknowledge this challenge with many studies performed with nitrogen as the carrier gas, or a mixture of nitrogen and oxygen. The performance of ZnO in oxygen is reduced because oxygen blocks the active sites preventing the adsorption of hydrogen onto the surface of the sensor. Different studies in literature have shown that the use of metal catalysts, for instance platinum, palladium improves performance in oxygen environments. This is because these catalysts have the potential to dissociate hydrogen molecules into atomic hydrogen, or oxygen into atomic oxygen which are presumed to be more reactive hence potentially improves the performance of the sensor at room temperature. In our study, we examined the performance of the undoped ZnO nanostructures using oxygen as a carrier gas. We obtained that the sensitivity reduces in presence of oxygen, and we must trade off room temperature operation to obtain a response. At 150 °C, we obtain a response in oxygen ambient with an increased limit of detection (LOD) of 900 ppm. FIG.32C–D shows the sensitivity with oxygen as carrier gas for different concentrations of hydrogen. With distribution of palladium particles on the surface and further refinement of the crystallinity of the ZnO nanotubes, there was scope further to reduce the limit of detection and improve performance in oxygen environment at room temperature. Table 3 shows the comparison of our sensor with previous studies of hydrogen gas sensing using different materials. Table 3. Comparison of Hydrogen Gas SensorsDetectionOperating Response / Material Year RangeTemp (°C) Recovery Selectivity Ref. Time (s) N2, CH4, This work 2024 4–100% RT 50 / 5 ethanol, 2- propanol WO3-x 2024 100–300 ppm RT 1.6 / 2.8NH3, ethanol,acetone Pt-SnO2 2024 50–500 ppm NA 400-900NH3, CO, NO2,H2S Schottky type 201100–400,000 PD / porous Si2ppm [3]Pd / B-C / n-Si 2012 0–40,000ppm RT 170 / 325 NA [4] NiO:Pd 2014300 ppb–150ppm 145 660 / NA NA [5]Pd / Mg thin films 2015 0.5–4 bar 100 1 / 60 NA [6] Pd / MgPd alloy 2015 2 bar RT 3 / 3 NA [7] PMMA / Pd / Graphene 2015 0.025–2% RT 108 / 331 CH4 / CO / NO2 [8] Pt / Graphene 2015 0–20,000diethyl ether, aerogelppm 320 0.97 / 0.72n-pentane[9]Pd / SnO2 thin films 2017 100–2000 ppm 175 1 / 512 NA

[0010] Pd / Graphene 2017 0.25–1% RT 40 / 490 NA

[0011] CO2, Amorphous Pd / ZnO 2018 0.5–6% 80 156 / 61 methanol, ethanol, acetone Pd NP / IZO 2024 100 ppb –1% 250 20 / 51 NA

[0013] Pd / HfO2 / GaOx / GaN 2018 5–10,000 ppm RT 36 / 35 NA

[0014] n2O3 201810 pNH3, CO, ethyl Ipb – 500ppm 260 1.7 / 1.5acetate,

[0015] styrene 1000-10CO, HCHO, Pd / M-WO3 2019000ppm RT 79 / 8NO2, ethanol,

[0016] acetone 1. Yadav et al., Int. J. Hydrogen Energy 50: 878-888 (2024). 2. Duoc et al., Int. J. Hydrogen Energy 61: 774-782 (2024). 3. Razi et al., Sens. Actuators B Chem.146(1): 53-60 (2010). 4. Li et al., Sens. Actuators B Chem.161(1): 1102-1107 (2012). 5. Kandyla et al., Mater. Lett.119: 51-55 (2014). 6. Gautam et al., Sens. Actuators B Chem.176: 453-459 (2013). 7. Gautam et al., Int. J. Hydrogen Energy 40(45): 15549-15555 (2015). 8. Hong et al., ACS Appl. Mater. interfaces 7(6): 3554-3561 (2015). 9. Harley-Trochimczyk et al., Sens. Actuators B Chem.206: 399-406 (2015). 10. Deivasegamani et al., Microchimica Acta 184: 4765-4773 (2017). 11. Alfano et al., Sens. Actuators B Chem.253: 1163-1169 (2017). 12. Kim et al., Sens. Actuators B Chem.262: 460-468 (2018). 13. Chiu et al., Sens. Actuators B Chem.415: 136015 (2024). 14. Chang et al. Int. J. Hydrogen Energy 43 (42): 19816-19824 (2018). 15. Li et al., Int. J. Hydrogen Energy 43 (50): 22746-22755 (2018). 16. Wu et al., J. Alloys Compd.776: 965-973 (2019) Effect of Palladium on Room Temperature Hydrogen Gas Sensing To improve the response of the ZnO sensors in oxygen ambient, we altered the electrolyte – replacing the glycol-based solvents with glycerol and maintaining other anodization parameters that gave the best structures. We observed that with the use of glycerol, the top surface layer was chemically etched during the anodization, resulting into open well-spaced nanotubes with thickness of about 1 m. FIG.33B shows the SEM image of the glycerol-based nanostructures with more open tubes and no surface layer. This suggests a clear increase in surface area to volume ratio in this structure. We explore the performance of these structures in the hydrogen gas sensing in oxygen ambient at room temperature. To this end, we observed an immediate improvement in the sensitivity of the sensor to hydrogen in oxygen ambient at room temperature. The sensor gave a sensitivity of about 10 for 160 ppm of hydrogen and showed an increase in sensitivity with increase in concentration. To increase the performance of the sensor to hydrogen at room temperature in oxygen ambient, we also deposited palladium on the surface. We observed an improvement in the performance of the sensor at room temperature (FIG 33A) and at 50 °C. Fabrication of ZnO Nanotubes on Glass and Other Substrates We developed a method to fabricate ZnO nanotubes on any substrate. For such purposes, typically a ZnO film should be first deposited on the substrate using physical vapor deposition or thin film deposition techniques (glass, for example). Nevertheless, we could eliminate this time consuming and energy intensive process. We melted zinc using a hot iron and distributed the liquid on the substrate to form a film. This film was then anodized to get transparent ZnO nanotubes. The ability to fabricate transparent ZnO on glass and other substrates of different shapes and sizes could broaden the application scope of the material. Improvement in the Structure of the Nanotubes We applied a constant voltage across the anode (zinc) and cathode (platinum) during our regular anodization. In a new development, we connected a resistor in series with the electrodes and obtained more distinct nanotubes. The resistor made enables variation of voltage and current simultaneously. The current was found to oscillate significantly depending upon the size of the samples. The nanotubes pulled out from the anodization bath during the initial oscillations showed a compact surface layer. Nevertheless, this layer was absent and open pores were clearly visible in samples taken out at a later time. Gas Sensing Mechanism The commonly accepted mechanism of semiconductor metal oxides is based on the adsorption / desorption of oxygen gas onto the surfaces of oxide. The adsorption of oxygen results in trapping of electrons at the surface creating a depletion region (red) hence a reduction in conductivity (baseline). Exposure to a reducing gas (hydrogen) is followed by a reaction with the adsorbed oxygen to produce water vapor, hence freeing the electrons and reducing the depletion region. This leads to an increase in conductivity. FIG. 35A–B (Schematic) shows the two extremes of nanotube wall thickness. FIG. 33A shows a nanotube with a wall thickness larger than the Debye length and the corresponding barrier height controlled by grain boundaries between the grains. FIG. 33B) demonstrates that reducing the wall thickness to less than the Debye length, we enter the flat band regime where quantum confinement effects become relevant. This leads to complete depletion of the grains and an improvement in the response. FIG.35C–D shows that the expected sensitivity to hydrogen is higher in nanotubes with a small wall thickness. In this study we reduced the wall thickness to minimum of 20 nm, which is almost comparable to twice the Debye length of ZnO. Further reduction of the wall thickness is necessary to achieve greater performance at room temperature. We also tested the gas sensor potential to detect hydrogen gas leaks by placing it 10 cm away from a hydrogen source. We observe that in this case the sensor shows a high sensitivity to hydrogen gas in this environment. FIG.34 shows this response. In this study, we have demonstrated the successful fabrication of ZnO nanotubes via anodic oxidation of zinc metal. The superiority of ZnO nanotubes is portrayed through the unprecedented hydrogen gas sensing sensitivity of 104. The high sensitivity of the nanotubes is attributed to the ability to reduce the wall thickness to 20 nm, while increasing the surface area to volume ratio. In addition, we discovered that the sensitivity to hydrogen is not proportional to the length of the nanotubes. In summary, this study opens a pathway for the successful use of ZnO nanotubes in many fields. Example 3 Detection of Low Concentrations of Hydrogen The room temperature sensitivity of ZnO nanotubes to low concentrations of hydrogen was further examined. This was achieved by coating a layer of palladium on top of the nanotubes. The sensitivity was highest at 150 °C. In an effort to enhance the sensitivity of the ZnO nanotubes at room temperature, we annealed the samples in hydrogen environment at 300 °C for 2 hours. Hydrogen annealing was thought to have four possible effects on the sample: (1) creation ofoxygen vacancies according to the reaction: Olattice + 2 e VÖ + 2 O2 (g), (2) modification of the surface adsorption characteristics via passivation of dangling bonds with hydrogen, (3) reduction in the Schottky barrier between metal contact and ZnO semiconductor, which makes the changes in the electrical resistance (sensor response) faster, and (4) reduction in the size of the grain boundaries leading to better charge transport due to diminished scattering. All these effects can enhance the performance of the hydrogen sensor at room temperature. FIG.36A–B shows that the sensor is sensitive to even lower concentrations of hydrogen at room temperature with no palladium coating atop the nanotubes. The concentration range for detection can go below 80 ppm (the limit of the present measurement setup) to above 8000 ppm (0.8%). We observed that the hydrogen annealing resulted a lower and stable baseline compared to the oxygen annealing (used previously). This reduction in the baseline noise is a key step in lowering the limit of detection. Low noise would potentially help the sensor in detecting hydrogen in ppb (parts per billion) levels. Development of ZnO Nanotubes on Glass The fabrication of zinc films on glass substrates by melting zinc directly on the substrate was demonstrated in experiments above. With this unconventional approach, we could fabricate thick films of Zn on glass. The development of this method was possible due to the low melting point (419.5 °C) of Zn. The Zn films were anodized to form nanostructured ZnO films. The technology to fabricate ZnO nanotubes on different substrates would open numerous opportunities for utilizing the material for device applications, including gas sensing. For instance, the low melting point (419.5 °C) of zinc prevents heat treatment of ZnO nanotube films on Zn substrates (e.g., Zn foil) at a temperature above 419.5 °C while with ZnO nanotube films on glass substrates, we will have the freedom to anneal the ZnO to temperatures higher than 400 °C, potentially refining the microstructure for better response. Although the melting technique is cost effective, scalable, and saves time compared to standard thin film coating techniques such as thermal evaporation, it has the limitation that the temperature of the substrate surface should be raised above 419.5 °C to obtain a zinc film. This prevents the use of low melting point substrates such as plastic for flexible devices. An electrodeposition process was developed as an alternative cost-effective, scalable approach to deposit the Zn films on conducting glass (e.g., fluorine doped tin oxide (FTO) film coated glass) and low melting point substrates such as conducting plastic films. A goal was to fabricate thick and uniform zinc films with a robust Zn / glass interface that enable high voltage (> 30 V) anodization to yield uniform ZnO nanotube films. To achieve this, two steps were performed: (1) deposition of a thin Zn seed layer using a Zn containing electrolyte, a Zn anode and an FTO glass substrate as the cathode and (2) deposition of a thicker Zn layer on this seed layer after replacing zinc anode with platinum. The seed layer was deposited for less than 10 s at 3.8 V in an electrolyte consisting of zinc chloride (ZnCl2) in water (pH ~6). For the thicker layer deposition, the same electrolyte and cathode (FTO) were used; however, the anodization was conducted for a longer duration (typically less than 20 minutes) with Pt as anode. FIG.37A shows a photograph of the Zn film on FTO glass. FIG.37B shows the SEM image of the film indicating large grains (size ~1 μm). The Zn film was dried at 100 °C after deposition. The electrodeposited Zn films were then anodized by connecting a resistance (1 k ) inseries with the anodization cell as discussed in the previous disclosure. The series resistance would prevent destructive currents in the circuit and facilitate the supply of a voltage just necessary to grow ZnO nanotubes on the Zn film coated glass substrates. FIG.37C shows the anodization curve obtained for a film. The curve has a shape similar to the typical current curves obtained during the anodization of zinc foil to form ZnO nanotubes, which shows that the anodization processes in Zn foils and electrodeposited Zn films take identical pathways. The FTO glass-electrodeposited zinc film interface was stronger as evident from the fact that the anodization proceeded without the film getting dissolved into the electrolyte. Uniform robust ZnO nanotubes are being prepared using these zinc films. This room temperature process facilitates fabrication of ZnO nanotube films on flexible substrates such as plastic and integrate them with flexible electronics.

Claims

CLAIMS What is claimed:

1. A nanostructure array comprising: a plurality of zinc oxide nanostructures, wherein each nanostructure comprises: an average outer diameter of about 20–500 nm; an average height of about 0.050–1000 μm; a vertical alignment angle of about 0–60 degrees off the vertical direction; and an average volumetric surface area of about 5 × 106to 500 × 106m2 / m3; an average roughness factor of about 10–3,50,000; and an average porosity of 5–90%.

2. The array of claim 1, wherein the nanostructures are nanotubes.

3. The array of claim 2, wherein the nanotubes comprise a pore diameter of about 10–400 nm.

4. The array of claim 1, wherein the nanostructures further comprise about 0–100 nm of lateral wall to wall spacing between adjacent nanostructures.

5. The array of claim 1, wherein the array has an electrical conductivity of about 0.001–10 m.

6. The array of claim 1, wherein the nanostructures further comprise a density of 100–5100 kg / m3.

7. The array of claim 1, wherein the array comprises vertically aligned zinc oxide nanostructures on a zinc substrate; a zinc film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; a fluorine doped tin oxide (FTO) film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon; or any conducting film-coated substrate comprising glass, quartz, plastic or related compounds, metal, or silicon.

8. A method for detecting an analyte in a gas mixture sample, the method comprising:providing a sensor comprising the nanostructure array of claim 1; contacting the sensor with the sample; and detecting a signal generated in response to the sample.

9. A method for detecting hydrogen in ambient air, the method comprising: providing a sensor comprising the nanostructure array of claim 1; contacting the sensor with the ambient air; and detecting a signal generated in response to hydrogen in the sample.

10. The method of claim 8 or 9, wherein the detected signal is electrical resistance, electrical conductance, electrical impedance, or a combination thereof.

11. The method of claim 8 or 9, wherein the detection is performed at room temperature.

12. The method of claim 8 or 9, wherein the detection is performed at temperatures from 15– 300 °C.

13. The method of claim 8, wherein the analyte is a volatile organic compound (VOC) selected from one or more of heptanal, ethanol, 2-propanol, acetophenone, acetone, methanol, isopropyl myristate, nonanal, hexane, methane, pentane, ethane, dimethyl / trimethyl amine, isoprene, methyl mercaptan, pentene, 2,3-dihydro-1-phenyl-4(1H)-quinazolinone, 1-phenyl-ethanone, 2,5,6-trimethyloctane, 1,4-dimethoxy-2, 3-butnediol, cyclohexanone, 3-methylhexane, decene, caryophyllene, naphthalene, trichloroethylene, hydrogen, oxygen, carbon dioxide, carbon monoxide, nitrogen, oxides of nitrogen, argon, ammonia, hydrogen sulfide, or combinations thereof.

14. The method of claim 9, wherein a change in the signal indicates that hydrogen is present in the sample and a lack of change in the signal indicates that hydrogen is absent from the sample.

15. The method of claim 9, wherein a lack of change indicates that the hydrogen is present at a concentration below a detection limit.

16. The method of claim 9, wherein the detection limit in air is about 100 ppb (0.0000001%) at room temperature.

17. The method of claim 9, wherein the sample comprises gas or air in the vicinity of equipment or apparata.

18. The method of claim 9, wherein the method is for detecting hydrogen gas leaks.

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