Acetone sensor and method

Abstract

A sensor for measuring acetone includes: an array of nanowires extending from a substrate, each of the nanowires having a corresponding surface substantially composed of a compound semiconductor and a diameter of less than 100 nm; an electrical contact material disposed on a first portion of the surface of each nanowire to form a Schottky barrier contact with the first portion; an amine-functionalized polymer disposed on the nanowires to functionalize a second portion of the surface of each nanowire; and an electrical contact with the substrate for measuring a current flowing between the electrical contact and the Schottky barrier contact on the nanowire when the nanowire is exposed to light or biased, such that when the nanowire is exposed to a mixture of molecules including acetone, the measured current characterizes the concentration of acetone in the mixture.

Description

Technical Field

[0001] This invention relates to sensing, and more particularly to a sensor for measuring acetone and a method for manufacturing a sensor for measuring acetone. Background Technology

[0002] Diabetes is a life-threatening disease affecting a large segment of the population. Currently, people with diabetes need to monitor their blood sugar levels at least twice a day by pricking a finger to produce a drop of blood and placing that drop on a test strip from a device such as an electronic blood glucose meter to detect dangerously high levels of blood sugar. This procedure is inconvenient, invasive, and unpleasant. However, low levels of insulin can lead to a dangerous condition called diabetic ketoacidosis (“DKA”), in which the body breaks down fat instead of glucose, producing high levels of ketones that can damage the body's organs. These ketones can be detected in urine, blood, and breath. In particular, recent research has shown a correlation between blood ketone levels and breath acetone levels, suggesting that measuring breath acetone may be a viable alternative to routine blood tests.

[0003] For example, the presence of acetone in exhaled breath is a unique biomarker for people with diabetes (type 1 or type 2) or those on a "ketogenic" diet. However, previous attempts to develop acetone breath sensors have faced numerous challenges, particularly poor performance, such as poor sensitivity, selectivity, and dynamic range, as well as high operating temperatures.

[0004] The aim is to alleviate one or more difficulties of the existing technology or at least provide a useful alternative. Summary of the Invention

[0005] According to some embodiments of the present invention, a sensor for measuring acetone is provided, the sensor comprising:

[0006] An array of nanowires extending from a substrate, each of which has a corresponding surface composed primarily of compound semiconductors and a diameter of less than 100 nm.

[0007] An electrical contact material is disposed on a first portion of the surface of each nanowire to form a Schottky barrier contact with the first portion;

[0008] An amine-functionalized polymer disposed on nanowires to functionalize a second portion of the surface of each nanowire; and

[0009] Electrical contacts with a substrate, which are used to enable the measurement of the current flowing between the electrical contacts and the Schottky barrier contacts on the nanowire when the nanowire is exposed to light or biased, such that when the nanowire is exposed to a mixture of molecules including acetone, the measured current characterizes the concentration of acetone in the mixture.

[0010] In some implementations, each of the nanowires has a diameter of about 50 nm to 60 nm.

[0011] In some embodiments, the amine-functionalized polymer is chitosan.

[0012] In some embodiments, the electrical contact material is essentially composed of a metal selected from the group consisting of Pt, Pd, Ni, and Au. In some embodiments, the electrical contact material is essentially composed of Pt.

[0013] In some implementations, the sensor is self-powered when exposed to light.

[0014] In some embodiments, the compound semiconductor is selected from the group consisting of InP, InAs, GaP, and GaAs. In some embodiments, the compound semiconductor is undoped InP.

[0015] In some implementations, the sensor has a response time of < 20 seconds and a detection limit of < 0.1 ppm.

[0016] In some implementations, the sensor is flexible and is a component of a wearable sensing device. The molecular mixture can be in the gas or liquid phase.

[0017] According to some embodiments of the present invention, a handheld acetone measuring device is provided, which includes any of the sensors described above. In some embodiments, the handheld acetone measuring device includes a light-emitting diode configured to emit light toward a sensor to cause current to flow between an electrical contact and a Schottky barrier contact. The device can be configured to diagnose diabetes and ketoacidosis based on human respiration.

[0018] According to some embodiments of the present invention, a method for producing a sensor for measuring acetone is provided, the method comprising the following steps:

[0019] An array of nanowires extending from a substrate is provided, each of the nanowires having a corresponding surface composed essentially of compound semiconductors and a diameter of less than 100 nm;

[0020] Electrical contact material is deposited on a first portion of the surface of each nanowire to form a Schottky barrier contact with the first portion;

[0021] Amine-functionalized polymers were deposited onto a second portion of the surface of each nanowire; and

[0022] Electrical contacts are formed on the substrate, which are used to enable the measurement of the current flowing between the electrical contacts and the Schottky barrier contacts on the nanowire when the nanowire is exposed to light or biased.

[0023] In this study, when nanowires are exposed to a mixture of molecules including acetone, the measured current characterizes the concentration of acetone in the mixture.

[0024] In some embodiments, the step of providing an array of nanowires includes growing an array of nanowires from a substrate using an addition bottom-up growth process.

[0025] In some other embodiments, the step of providing an array of nanowires includes forming the array of nanowires by a top-down etching process.

[0026] In some embodiments, the step of depositing amine-functionalized polymers includes drop-casting chitosan onto nanowires.

[0027] In some embodiments, the step of depositing electrical contact material on a first portion of the surface of each nanowire includes depositing the electrical contact material onto the nanowire at an angle such that the electrical contact material is deposited on only one side of each nanowire.

[0028] In some implementations, the electrical contact material is essentially composed of Pt.

[0029] In some implementations, each of the nanowires has a diameter of about 50 nm to 60 nm.

[0030] In some embodiments, the compound semiconductor is selected from the group consisting of InP, InAs, GaP, and GaAs. In some embodiments, the compound semiconductor is undoped InP.

[0031] In some embodiments, the electrical contact material is essentially composed of a metal selected from the group consisting of Pt, Pd, Ni and Au. Attached Figure Description

[0032] Some embodiments of the invention are described below by way of example only with reference to the accompanying drawings, in which:

[0033] Figure 1(a) A schematic diagram of an acetone sensor according to an embodiment of the invention, comprising an array of InP nanowires (“NW”) functionalized with chitosan and used to measure acetone in the exhaled breath of a human subject by measuring current using a digital electrometer and displaying a graph of the measurement results as a function of time on a computer display; a scanning electron microscope (“SEM”) image of the sensor is also shown.

[0034] Figure 1 Figures (b) to (f) are a set of schematic diagrams and accompanying SEM images illustrating a method for producing an acetone sensor according to an embodiment of the present invention;

[0035] Figure 2 This is a schematic diagram showing the growth of InP nanowires from bottom to top on an InP wafer substrate through openings in a patterned SiO2 mask layer.

[0036] Figure 3 (a) is a schematic diagram showing the test of the acetone sensor;

[0037] Figure 3 (b) through (h) are graphs showing measurements taken by the acetone sensor using the apparatus shown in (a);

[0038] Figure 4 Includes two graphs showing the response time and recovery time of the acetone sensor based on acetone concentration for acetone concentration ranges of 0 ppb to 10 ppb and 100 ppb to 1000 ppb, respectively.

[0039] Figure 5 Includes graphs showing the stability and reliability of the acetone sensor over time and for acetone concentrations of 0.1 ppm, 2 ppm, and 10 ppm;

[0040] Figure 6 and Figure 7 This is a set of graphs showing the performance of the acetone sensor under various conditions, including other ketone groups and a range of relative humidity.

[0041] Figure 8 Including schematic images and SEM images of the acetone sensor during production using a top-down production method;

[0042] Figure 9 This includes a schematic diagram of an acetone sensor produced using a top-down manufacturing process, as well as graphs showing the performance of the resulting sensor.

[0043] Figure 10 This is a circuit diagram showing the components of a portable handheld electronic acetone sensing device for sensing acetone exhaled in human respiration and including an acetone sensor; and

[0044] Figure 11 This paper demonstrates the use of a portable, handheld electronic acetone sensor to detect acetone exhaled in human respiration and to measure its properties. Detailed Implementation

[0045] To address the shortcomings of existing acetone sensors, the inventors have developed a miniature acetone sensor exhibiting high sensitivity and selectivity for acetone sensing at room temperature. Effective breath detection for diabetes monitoring requires the detection of acetone concentrations in the range of 0.3 ppm to 1.8 ppm. As described below, embodiments of the present invention can detect acetone concentrations from <1 ppb to >10 ppm. 5 The sensor detects acetone in respiration over an extremely wide concentration range of ppm, demonstrating its ability to detect acetone levels ranging from characteristic levels of low-level diabetes to those characteristic of high-risk DKA, where respirable acetone concentrations can exceed 75 ppm, with high selectivity and a short response time of approximately 5 seconds. The sensor can also be transferred to a flexible substrate to facilitate integration into wearable devices. In the described embodiment, the sensor is also self-powered, enabling continuous monitoring without the need for a power supply.

[0046] like Figure 1 As shown, the acetone sensor comprises an array of compound semiconductor nanowires extending from a substrate, each nanowire having a diameter of less than 100 nm. Electrical contact material disposed on a first portion of the surface of each nanowire forms a Schottky barrier (or Schottky junction) contact, and an amine-containing polymer is deposited on the nanowires to functionalize a second portion of the surface of each nanowire. By forming electrical contacts with a common substrate, the treated nanowires effectively constitute a set of parallel-connected photovoltaic cells, and current flows between the substrate contacts and the Schottky barrier contacts on the nanowires when the nanowires are exposed to light or when the Schottky junctions are biased. Under light irradiation, the current forms the baseline of the sensor signal. Due to the amine-functionalized nanowire surface, the current changes upon exposure to acetone. In particular, when the nanowires are exposed to a molecular mixture including acetone, the measured change in current is linearly correlated with the concentration of acetone in the mixture, and therefore can be calibrated to quantitatively measure this concentration.

[0047] In the described embodiments, the nanowires are substantially composed of the compound semiconductor indium phosphide (InP); however, in other embodiments, the nanowires may be composed, for example, other compound semiconductors such as GaAs, InAs, or GaN. The inventors have found that the sensor exhibits particularly high sensitivity and selectivity to acetone when the electrical contact material is platinum, the nanowires are composed of undoped InP, the amine-functionalized polymer is chitosan, and the nanowires are self-powered by being exposed to light (rather than being biased by an external power source). However, as described below, in other embodiments, any or all of these parameters may be varied.

[0048] Figure 1 This is a combined diagram illustrating an acetone sensor and a method for producing an acetone sensor according to various embodiments of the present invention. The top portion (a) of the figure shows a schematic representation of the sensor exposed to light and acetone from the breath of a human subject. In the described embodiment, the amine-functionalized polymer is chitosan, whose chemical structure is shown on the left. However, it will be apparent to those skilled in the art that other amine-functionalized polymers may be used in other embodiments. The two scanning electron microscope (SEM) images on the right show the entire nanowire array in a planar view (and before the addition of chitosan) and a portion of the array at a higher magnification and a tilt angle of 30°.

[0049] The acetone sensor described herein can be manufactured using the production method described below, and as follows: Figure 1 As shown in (b) through (e), although it will be apparent to those skilled in the art that the acetone sensor described herein can also be manufactured by alternative production methods. In embodiments of the described production method, an array of InP nanowires with a diameter less than 100 nm is fabricated. In the described embodiments, this is achieved through bottom-up addition processing steps, specifically involving selected-region epitaxial growth of InP nanowires on an InP substrate, on mutually spaced regions of an InP wafer exposed by corresponding openings in an electron beam patterned SiO2 mask layer deposited on the InP wafer, via metal-organic chemical vapor deposition (“MOVPE” or “MOCVD”), such as... Figure 2 As shown schematically in the diagram.

[0050] Besides forming corresponding small openings in the mask layer (e.g., using electron beam lithography), the key to growing thin (<100 nm) high aspect ratio nanowires is reducing radial growth, and this can be achieved by growing nanowires with a wurtzite (“WZ”) crystal structure on a single-crystal substrate wafer, which produces predominantly axial growth and negligible radial growth, as well as defect-free and taper-free nanowires. In the described embodiment, a (111)A-oriented Si-doped InP wafer is used.

[0051] In prior work, the inventors investigated the effect of nanowire diameter on the sensitivity of the resulting sensor and found that sensitivity was poor for nanowire diameters greater than 100 nm. Therefore, in the described embodiments, nanowire diameters of 50 nm to 60 nm are produced because these diameters provide high sensitivity, as further described below. The spatial dimensions of the nanowire array are 400 µm × 400 µm.

[0052] Once arrays of nanowires are produced, such as Figure 1 As shown in (a), a photoresist is deposited on an InP substrate to mask it from the subsequent metal contact deposition step. The photoresist also has the benefit of providing mechanical stability to the nanowires. In the described embodiment, the photoresist is SU8-5. Since the photoresist also covers the nanowires, a controlled etching step is subsequently used to remove the upper portion of the photoresist layer, thereby exposing the corresponding (and tunable) portion of the nanowires, leaving the remaining portion of the nanowires and the photoresist-coated substrate, as shown. Figure 1 As shown in (c). In the described embodiment, this is achieved by plasma etching. However, alternative etching methods can be used in other embodiments.

[0053] Then, metal contacts are formed on only a portion of each exposed nanowire portion to form Schottky electrical contacts on a portion of the nanowire, while leaving the remaining exposed portion of the nanowire for subsequent functionalization. In the described embodiment, such electrical contacts are achieved by oblique shadow deposition of appropriate contact metal. That is, the contact metal is deposited at an angle relative to the substrate, wherein the angle is chosen such that the contact metal is deposited only on one side of each nanowire, while the other side of the nanowire is shaded during deposition to avoid metal deposition. The contact metal is also deposited on the exposed planar surface of the remaining photoresist layer, interconnecting the nanowires and creating… Figure 1 The structure shown in (d) is an example. As can be seen from the SEM image, the deposition of metal on only one side of each nanowire causes the nanowire to be slightly bent.

[0054] In prior work, the inventors investigated various contact metals, including platinum (Pt), aluminum (Al), and gold (Au), for forming a Schottky barrier with InP nanowires. However, it was found that the choice of metal significantly affected the sensitivity of the resulting sensor, with platinum providing the best performance. Therefore, in the described embodiment, platinum is used as the contact metal.

[0055] The amine-functionalized polymer (chitosan in the described embodiment) was then deposited onto the nanowire array and Schottky contacts. The chitosan was diluted in a 2.5% aqueous acetic acid solution (1:10 dilution from the saturated solution, concentration 28%). In the described embodiment, deposition was achieved by drop-casting, followed by baking in an electric furnace at 50°C to remove the solvent; the inventors found this to be more effective than spin coating. However, alternative deposition methods, dilution mixtures, and concentrations can be used in other embodiments.

[0056] Such as SEM images and Figure 1 As illustrated in (e), a film-like chitosan layer is formed by drop casting over a nanowire array, wherein the chitosan layer covers or is embedded in the upper portion of the nanowires. In some embodiments, this porous heterostructure is also configured to enhance the sensor's performance in response to gas exposure; for example, by finely tuning the molecular weight and degree of acetylation of the chitosan, by adjusting the thickness of the chitosan layer via alternative deposition methods such as spin coating, and / or by functionalizing it using alternative chemicals containing -NH2 functional groups as substitutes for chitosan.

[0057] like Figure 1 As shown in (f), a second electrical contact (Ti / Au in the described embodiment) is formed on an InP wafer substrate, and when the Schottky barrier contact on the nanowire is exposed to light, current flows in the circuit between the second contact and the Schottky barrier contact on the nanowire, since the nanowire array is a form of photovoltaic cell. While this “self-powered” operating mode is particularly convenient and provides optimal sensing performance, the sensor can also operate in the dark by applying a bias voltage across the contacts, although at the cost of reduced sensitivity to acetone concentration as described below.

[0058] Whether operating under light or in darkness, when the nanowires are exposed to acetone and a mixture simulating air or breath, the interaction between acetone and the chitosan and InP nanowires causes a change in current depending on the concentration of acetone in the gas mixture. Therefore, the resulting measurement of current change represents the concentration of acetone, and the measurement can be calibrated to provide a direct quantitative measurement of the concentration of acetone in the gas mixture. In the context of diabetes monitoring, the sensor can be used to detect different levels of acetone in respiration, including dangerous levels found in the respiration of diabetic patients.

[0059] Because nanowires are grown incrementally from the substrate, the fabrication method described above is a type of method known in the art as a "bottom-up" approach. In an alternative embodiment, the top-down fabrication method forms nanowires by mask etching of the InP substrate (e.g., using an inductively coupled plasma (ICP) etching tool) to remove selected portions of the substrate to form high aspect ratio InP nanowires from the remaining unetched areas. The advantage of the top-down (subtractive) fabrication method is that it may be less expensive than the bottom-up (additive) fabrication method based on epitaxial growth.

[0060] Although embodiments and examples of the invention are described primarily in the context of human respiratory sensing, the described sensor is equally applicable to, for example, detecting acetone in the breath of other animals, including pets such as dogs, cats, and cows, in which diabetes is a serious problem. Furthermore, the described sensor can also be used to detect acetone in fluids, including acetone in sweat.

[0061] Example I

[0062] The following describes the manufacturing process and the resulting sensor, as well as examples of measurements of acetone sensing performance and other sensor characteristics.

[0063] Substrate preparation

[0064] In one example, n-type (Si-doped, 10⁻⁶) silicon nanoparticles were deposited at 300°C via plasma-enhanced chemical vapor deposition (PECVD). 18 cm -3 Up to 10 19 cm -3 Resistivity 0.6 x 10 -3 Ω-cm to 6 x 10 -3A 30 nm SiO2 layer was deposited on an Inp substrate (Ω-cm) (111); the thickness was measured by elliptic polarization. A negative photoresist AR6200.09 was spin-coated onto the SiO2 layer (Step 1: 500 rpm for 5 seconds; Step 2: 2000 rpm for 60 seconds) and baked in an electric furnace at 150°C for 1 minute. The baked photoresist was patterned using a Raith 150 electron beam lithography (EBL) system. The pattern for the desired nanowire layout was defined as a square array of 400 µm × 400 µm hexagonal dots with a diameter of 40 nm and a spacing of 600 nm. The areas of the electron beam modified photoresist layer were removed by the appropriate photoresist developer, and any residue was removed from these areas using oxygen plasma (300 W, 2 minutes, 300 sccm O2 flow rate). The exposed areas of the SiO2 layer were then removed from the InP substrate by reactive ion etching (RIE) (20 sccm CHF3, RF power: 20 W, 4.5 min) to pattern the SiO2 layer, thereby defining a mask layer for subsequent nanowire growth.

[0065] Nanowire growth

[0066] To provide a clean and smooth exposed InP surface for nanowire growth, the newly exposed areas of the InP surface were oxidized for 2 minutes using a 10% H2O2 solution, and the resulting oxide layer was stripped for 2 minutes using a 10% H3PO4 solution. These two steps were repeated sequentially 5 times, and immediately after the final etching, the sample was transferred to an MOVPE reactor to epitaxially grow nanowires from the areas of the InP substrate exposed through a patterned SiO2 mask layer.

[0067] InP nanowires were grown using an AIXTRON 200 / 4 MOVPE reactor operating at a base pressure of 100 mbar, with H2 as the carrier gas at a total flow rate of 14.5 L / min. Trimethylindium (TMIn) and phosphine (PH3) were used as precursors for Group III (In) and Group V (P), respectively. The molar fractions of TMIn and PH3 were set to 9.38 × 10⁻⁶. -6 and 7.59×10 -4 This corresponds to a V / III ratio of 80. The sample was baked at 750°C for 10 minutes under a protective current of pH 3, and the InP nanowires were grown at 730°C for 4 minutes. Under these MOVPE growth conditions, even without the introduction of any dopant, nominally undoped InP nanowires (“undoped InP”) are known to be slightly n-type due to background doping.

[0068] Acetone sensor manufacturing

[0069] As described above, sensors are produced from nanowire arrays by forming Schottky barrier contacts on a first portion of each nanowire and functionalizing a second portion of the nanowire.

[0070] To achieve the above objectives, SU8-5 photoresist was spin-coated onto the sample to completely cover the nanowire array. To protect the thin nanowires from damage during the fabrication process, a low spin speed of 1000 rpm was applied, followed by a two-step soft baking process at 65°C and 95°C, each for two minutes, to remove the solvent from the photoresist film.

[0071] The SU8-5 film was then partially removed by etching using a barrel etcher (PVA TeplaGigabatch 310M) with an O2 flow rate of 300 sccm and a power of 500 W to expose the top ~500 nm of the nanowires. The sample was then fully exposed to UV irradiation and baked at 150°C to cure the remaining photoresist. The remaining photoresist isolates the InP substrate from the subsequent metal deposition and also provides mechanical support for the nanowires.

[0072] Then, during evaporation in an electron beam evaporator, a 60 nm Pt layer was deposited on only one side of each nanowire by mounting the sample on a tilted sample holder (to provide a deposition angle of approximately ~45°). Pt was also deposited on top of the remaining photoresist layer, forming a continuous conductive film that electrically interconnects all the parallel nanowires, thus forming a single “top” contact of the sensor. Since the contact layer is deposited on only one side of each nanowire, the other side of the nanowire remains exposed and can be used to sense the analyte.

[0073] The bottom contacts (Ti / Au: 10 nm / 100 nm) are formed after the top contacts by removing the polymer from the portion of the sample substrate near the nanowire array and depositing metal on the exposed substrate. Alternatively, other conductive metals such as Al can be used.

[0074] In the case of forming bottom contacts, the array is functionalized by drop casting an aqueous chitosan-acetic acid solution onto the array and then baking it on an electric furnace at 50°C to remove the solvent.

[0075] Electrical characteristics

[0076] The electrical characteristics of the sensor were measured using a Keysight B2900 precision source measurement unit and a solar simulator, such as... Figure 3 (a) Schematic illustration. During operation, a solar simulator was used with 1x sunlight at AM1.5 spectrum (i.e., a standard air quality factor of 1.5) and a concentration of 42.3 mW / cm². 2 Illumination sensor, such as Figure 3As shown in (b). Figure 3 (c) is a graph showing the current-voltage (IV) curves of the sensor under dark and light conditions, respectively. The two curves illustrate the typical rectification behavior of the Schottky barrier contact, which in this case is due to the high work function of the Pt deposited on the nanowires and the surface band bending of the InP beneath the deposited Pt. The photovoltaic effect was observed using light irradiation from a solar simulator, where the short-circuit current (IV) is... SC The open-circuit voltage is 136 nA, and the open-circuit voltage (V) is 136 OC The voltage is 80 mV, which makes self-powered sensing operation possible. In particular, the short-circuit current constitutes the sensing signal under constant light illumination.

[0077] Acetone sensing measurement

[0078] Using an FTIR600 environmentally sealed sample stage (from Linkham Scientific Instruments Ltd.), the gas sensing performance of the sensor was measured using Au probes for contacting the sensor contacts / electrodes, a mass flow controller (MFC) from Bronkhorst, a solar simulator, and gas cylinders. Figure 3 (a) is shown schematically. For gas sensing measurements, the carrier gas simulates a N2 to O2 volume ratio of 4 (V / L). N2 / V O2 = ~4, N2 and O2, BOC gas) of air. For ppm level concentrations, the total gas flow rate is maintained at 1 L / min, and for sub-ppm level concentration measurements, the total gas flow rate is maintained at 0.5 L / min. For analyte gas measurements, the target volatile organic compounds (VOCs) (ethanol, 9.91 ppm (N2), Coregas; NO2, 10.1 ppm (N2), Coregas; methanol, 10 ppm (N2), BOC gas; acetone, 10 ppm (N2), BOC gas) are diluted to the desired concentration using simulated air before purging the sample stage chamber.

[0079] Figure 3 (d) is the sensor response (shown as a percentage change in short-circuit current) over time, under illumination conditions (no bias voltage) and darkness (0.5 V bias voltage) conditions, the concentration of acetone was repeatedly cycled against continuous target acetone concentrations of 2 ppm, 4 ppm, 6 ppm, 8 ppm and 10 ppm: rising from zero to the target concentration and returning to zero. Figure 3 (f) Similarly, the sensor response to irradiation at cyclic concentrations of acetone of 100 ppb, 200 ppb, 400 ppb, 600 ppb, 800 ppb, and 1000 ppb is shown, and Figure 6(b) For higher concentrations from 4500 ppm to >10%. As can be clearly seen from the graphs, the irradiated sensor is effective in detecting the presence of acetone in gas mixtures and provides a response signal that appears to be approximately linear with acetone concentration within its applicable range. These results confirm that the sensor provides a suitable response for detecting various acetone levels caused by DKA, diabetes, and weight loss under a ketogenic diet.

[0080] exist Figure 3 The sensor performance at lower acetone concentrations from 0.4 ppb to 10 ppb is shown in (g) and (h). The sensor still provides a meaningful response (0.7%) at 0.4 ppb and exhibits a linear response over this concentration range, thereby estimating the acetone detection limit (“LOD”) to be 0.18 ppb. This is a good result, demonstrating that the sensor’s operating range extends down to acetone concentrations as low as parts per trillion.

[0081] Depend on Figure 3 The data summarized in (e) confirm a clear linear sensor response, comprising graphs of the peak sensor response R against acetone concentration for two acetone concentration ranges: from 100 ppb to 1000 ppb (main graph) and from 0.4 ppm to 10 ppm (inset). Each graph also includes linear regression results for the concentration range, where the slope defines the sensor sensitivity S = R / C ( / ppb). For the higher concentration range (100 ppb to 1000 ppb), the sensor sensitivity is 0.032% / ppb, significantly less than the 0.48% / ppb sensitivity for the lower concentration range (0.4 ppb to 10 ppb).

[0082] like Figure 4 As shown, for acetone levels from 0.1 ppm to 1 ppm, the sensor exhibits a rapid response (T0). res (~25 seconds) and recovery (T) rec The response time is approximately 39 seconds (although about 5 times slower than sensors produced using the top-down manufacturing method described below). In practical applications, such a fast response time is crucial for near real-time diagnostics. Figure 5 In summary, the sensor provided a consistent response signal over 16 acetone sensing cycles, demonstrating good stability and reproducibility.

[0083] In addition to high sensitivity, high selectivity for acetone is also important in the context of acetone breath sensing, as human breath typically includes many other VOCs at concentrations ranging from ppt to ppm. Therefore, selective sensors are needed for diabetes monitoring to distinguish acetone from these interfering VOCs.

[0084] Figure 6 (a) is a graph showing the sensor's measurement response to various VOCs and atmospheric gases at a concentration of 1 ppm. The various VOCs include acetone (CH3COCH3), methyl nitride (CH3NO2), ethanol (C2H5OH), propane (C6H6), and carbon dioxide (CO2). Clearly, the sensor exhibits high selectivity for acetone relative to these other VOCs, producing a response of less than 5% for all other VOCs, at least an order of magnitude higher for acetone, compared to a response of 49 ± 2% for acetone.

[0085] In further measurements, a bubbler device was used to evaporate higher concentrations of acetone and 2-butanone solvents, respectively. Figure 6 (b) is a graph showing the sensor's response to each of these solvent vapors over time during cycling at solvent concentrations ranging from 0.45% (4500 ppm) to 13.1% (131,000 ppm), as illustrated. Clearly, the sensor maintains a positive sensing response correlation at least within this concentration range, which extends to... Figure 3 The sensor can detect concentrations more than 10,000 times higher, demonstrating consistent operation over an extremely wide concentration range. Although both acetone and 2-butanone are small volatile ketones, the sensor exhibits high sensitivity only for acetone. This property is significant for clinical breath analysis because 2-butanone is a smaller ketone (like acetone) and is a biomarker that can be present in respiration (e.g., in lung cancer patients), so the ability to distinguish it from acetone can greatly reduce false positives.

[0086] Finally, a major challenge in breath analysis is the presence of water vapor in the breath, which can degrade sensing performance by saturating the sensor and thus reducing its sensitivity to the target gas. The effect of humidity on acetone sensing was investigated using the same bubbler apparatus described above. Figure 6 As shown in (c), compared to sensing in dry air, the sensor's response to acetone decreased by 10 ± 2%, 30 ± 6%, and 50 ± 12% for relative humidity ("RH") values ​​of 20%, 50%, and 65%, respectively. However, Figure 6 (d) shows that the sensor maintained a consistent sensitivity largely independent of humidity, indicating that the sensor is not overwhelmed by humid environments. The sensor's response to potential interfering molecules ethanol and CO2 in exhaled breath was also measured at different relative humidities, and the results are... Figure 6 To summarize in (e). It is evident that even at high relative humidity, the sensor maintains its high selectivity for acetone, with the acetone response being more than 10 times higher than its response to interfering gases.

[0087] The sensor described in this article can also be used in the dark by applying a bias voltage across the sensor, but at the cost of sacrificing sensitivity to acetone concentration, such as... Figure 7 As shown in (b). A similar result occurs when the sensor is made of n-doped InP nanowires instead of undoped InP nanowires. Figure 7 (a) The sensor’s response to cyclic acetone is compared to that of a similar sensor using undoped, non-n-type InP nanowires. Undoped InP nanowire sensors typically provide a higher response to acetone because doping in the nanowires only increases the current value, not the current change.

[0088] Example II

[0089] In another example, the acetone sensor is manufactured using the top-down manufacturing method described above, such as... Figure 8 (a) is shown schematically. A 200 nm SiO2 layer is deposited on an undoped InP wafer by PECVD. An ARP6200.09 photoresist layer is deposited over the SiO2 layer and patterned using electron beam lithography (EBL) to form openings in the photoresist that typically correspond to the desired nanowire positions and diameters. A 70 nm Ni layer is deposited over the patterned photoresist layer by electron beam evaporation, and the photoresist layer is removed to transfer the pattern to the Ni layer by lift-off, forming... Figure 8 (a) The leftmost diagram and Figure 8 (b) shows the structure schematically illustrated in the SEM image.

[0090] Then, the exposed areas of the SiO2 layer are removed using ICP-RIE with fluorine gas, leaving the Ni / SiO2 bilayer as a mask, such as... Figure 8 (a) in the second image and Figure 8 (c) is schematically shown in a 30° tilted view of the SEM image, to form the subsequent InP etching using ICP-RIE with chlorine gas. Figure 8 The structure shown in the third image of (a), and after removing the Ni / SiO2 double mask, leaving Figure 8 In the fourth and rightmost images of (a) and Figure 8 The structure is schematically shown in the SEM image of (d).

[0091] The nanowire diameter is ultimately determined by the mask and ICP etching process, with the minimum achievable mask size being approximately 40 nm to 50 nm via EBL and Ni deposition. Figure 8The SEM image in (d) shows that the resulting nanowires are tapered due to the etching of the exposed sidewalls of the nanowires during deep etching of the substrate, with the diameter of each nanowire increasing from ~50 nm at the top to ~130 nm at the base. Another limitation of the top-down fabrication method is the small nanowire length due to the limited etch selectivity of the mask and the InP substrate, resulting in slow mask removal during the etching process. The average length of the nanowires produced by the top-down fabrication method is about 2 μm, much shorter than the nanowires grown by the bottom-up fabrication method described above, which are longer than 4 μm.

[0092] Once the nanowires are formed as described above, the steps for forming the electrical contacts and the chitosan drop casting are the same as those described above for the bottom-up production method. Figure 8 (e) is a SEM image showing the nanowires after the tilted angle deposition of Pt contacts, and Figure 8 (f) and (g) both show the nanowires after chitosan drop casting.

[0093] Figure 9 (a) is a schematic image of the obtained sensor, and Figure 9 (b) through (h) show measurements taken by sensors produced using a top-down manufacturing process. Figure 9 (b) is a graph showing the IV characteristics of the sensor using a 1.5 AM solar simulator under both dark and light conditions.

[0094] Figure 9 (c) and (e) show the time-dependent responses of the irradiated sensor to pulsed acetone concentrations ranging from 1 ppm to 0.1 ppm and from 10 ppm to 2 ppm, respectively. Figure 9 (d) shows the light intensity from 42.3 mW•cm. -2 Reduced to 4.2 mW•cm -2 The graph below shows the sensing response of acetone at 1 ppm. Notably, with a decrease in light intensity by an order of magnitude, the acetone response only slightly decreased from 6.5% to 4.7%, demonstrating its applicability in real-world environmental settings where ambient light intensity can vary significantly.

[0095] Figure 9 (g) confirmed that the sensing signal has a linear relationship with the acetone concentration. Figure 9 (h) is a graph comparing the sensor’s response to each of 10 ppm concentrations of acetone, 2-butanone, ethylbenzene, ethanol, propane, NO2 and 10% CO2, confirming that the top-down manufactured sensor also has excellent selectivity for acetone. Figure 9(f) shows the time-dependent response of the sensor to acetone concentrations ranging from 2 ppm to 10 ppm at relative humidity (RH) levels of 0%, 20%, 50%, and 65%.

[0096] from Figure 9 The data shown clearly demonstrates that acetone sensors produced using top-down manufacturing methods generally exhibit similar characteristics to those produced using bottom-up methods. However, as summarized in Table 1 below, significant performance differences exist. For example, the acetone sensing response of the top-down sensor (6.5%@1 ppm) is significantly shorter than that of the bottom-up sensor (49%@1 ppm), and the LOD (4.5 ppb) of the top-down sensor is also greater. Conversely, the response and recovery times of the top-down sensor are approximately five times shorter than those of the bottom-up sensor. Nevertheless, within the relevant diabetic acetone concentration range of 0.1 ppm to 10 ppm, the top-down sensor exhibits a high sensitivity to acetone at 6.61% / ppm.

[0097] Table 1: Performance Comparison of Sensors Produced Using Bottom-Up and Top-Down Manufacturing Methods

[0098] Production methods R (1 ppm) S (% / ppm) <![CDATA[T res / T rec (s)]]> LOD (ppb) bottom up 49 % 480 ~ 25 / 39 0.33 Top to bottom 6.5 % 6.61 ~ 5 / 6 36

[0099] Example III

[0100] The top-down acetone sensor described above is also used as the sensing component in portable handheld electronic acetone sensing devices, also referred to herein as "ketone whistles," suitable for respiratory sensing in point-of-care applications. The top-down acetone sensor was chosen for this application because of its response time T... res (~5 seconds) and recovery time T rec (~7 seconds) is relatively short and easy to manufacture, while exhibiting moisture resistance, insensitivity to light intensity and acetone selectivity, although its sensitivity is slightly worse.

[0101] Figure 10 This is a block diagram showing the electronic components of the device, where sensor 1002 is illuminated (and thus powered) by a red light-emitting diode (LED) 1004. The device also includes a commercial CO2 sensor 1006 (to ensure sufficient breath depth to accommodate acetone), a signal processing circuitry system 1008, and an OLED screen 1010 for displaying acetone and CO2 measurements. The red LED 1004 is positioned directly on top of the sensor to eliminate any interference from ambient light fluctuations.

[0102] Considering the wide range of acetone concentrations corresponding to various physiological states, such as Figure 11As shown in (b), the apparatus used for measuring acetone respiration needs to be calibrated. This apparatus is calibrated using the laboratory gas system described above at a fixed acetone concentration and relative humidity. Figure 11 The calibration curve shown in (c) indicates that the sensitivity decreases with decreasing acetone concentration from 6.11 mV / ppm (0.1 ppm to 10 ppm) to 0.013 mV / ppm (50 ppm to 1000 ppm), which is consistent with... Figure 3 The laboratory measurements shown in (e) are consistent. The near-linear correlation between the output voltage signal and acetone concentration over several orders of magnitude confirms that the device is suitable for diagnosing diabetes, assisting with ketogenic diets, and even monitoring ketoacidosis (DKA), which, if undetected, can lead to dehydration, confusion, loss of consciousness, and even death if left untreated.

[0103] For the breath test, VOC sampling was performed using a Tedra bag (PVDF 0.6 L, Sigma-Aldrich), and breath samples were collected from healthy subjects and simulated diabetic breaths (see [link to study]). Figure 11 (a) The breath of diabetic patients typically contains acetone gas at concentrations ranging from 1.8 ppm to 20 ppm (e.g., Figure 11 (b) shows that the concentration of simulated breath in diabetic patients was adjusted to ~5 ppm by filling the Tedra bag with ~50% healthy exhaled air and ~50% simulated air containing 10 ppm acetone. Principal component analysis (PCA) was applied to analyze the output voltage-dependent respiratory test measurements, and the results were presented in Figure 11 As shown in (e), the differences between 10 simulated diabetic breathing samples and 10 healthy subjects were clearly classified into two distinguishable clusters with no overlap. This confirms that the device can effectively diagnose diabetes in human subjects.

[0104] Many modifications will be apparent to those skilled in the art without departing from the scope of this invention.

Claims

1. A sensor for measuring acetone, comprising: An array of nanowires extending from a substrate, each of which has a corresponding surface composed primarily of compound semiconductors and a diameter of less than 100 nm; An electrical contact material is disposed on a first portion of the surface of each nanowire to form a Schottky barrier contact with the first portion; An amine-functionalized polymer is disposed on the nanowires to functionalize a second portion of the surface of each nanowire. as well as Electrical contacts with the substrate, the electrical contacts being configured to enable measurement of the current flowing between the electrical contacts and the Schottky barrier contacts on the nanowire when the nanowire is exposed to light or biased, such that when the nanowire is exposed to a mixture of molecules including acetone, the measured current characterizes the concentration of acetone in the mixture.

2. The sensor according to claim 1, wherein, Each of the nanowires has a diameter of approximately 50 nm to 60 nm.

3. The sensor according to claim 1 or 2, wherein, The amine-functionalized polymer is chitosan.

4. The sensor according to any one of claims 1 to 3, wherein, The electrical contact material is essentially composed of metals selected from the group consisting of Pt, Pd, Ni and Au.

5. The sensor according to claim 4, wherein, The electrical contact material is primarily composed of Pt.

6. The sensor according to any one of claims 1 to 5, wherein, The sensor is self-powered when exposed to light.

7. The sensor according to any one of claims 1 to 6, wherein, The compound semiconductor is selected from the group consisting of InP, InAs, GaP and GaAs.

8. The sensor according to any one of claims 1 to 7, wherein, The compound semiconductor is undoped InP.

9. The sensor according to any one of claims 1 to 8, wherein, The sensor has a response time of < 20 seconds and a detection limit of < 0.1 ppm.

10. The sensor according to any one of claims 1 to 9, wherein, The sensor is flexible and is a component of a wearable sensing device.

11. The sensor according to any one of claims 1 to 10, wherein, The mixture of molecules is in the gas phase.

12. A handheld acetone measuring device, comprising a sensor according to any one of claims 1 to 11.

13. The handheld acetone measuring device of claim 12, comprising a light-emitting diode configured to emit light toward the sensor such that current flows between the electrical contact and the Schottky barrier contact.

14. The handheld acetone measuring device according to claim 12 or 13, wherein, The device is configured to diagnose diabetes and ketoacidosis based on human respiration.

15. A method for producing a sensor for measuring acetone, the method comprising the steps of: An array of nanowires extending from a substrate is provided, each of the nanowires having a corresponding surface composed substantially of a compound semiconductor and a diameter of less than 100 nm; Electrical contact material is deposited on a first portion of the surface of each nanowire to form a Schottky barrier contact with the first portion; An amine-functionalized polymer is deposited onto a second portion of the surface of each nanowire; as well as An electrical contact is formed with the substrate, the electrical contact being configured to enable measurement of the current flowing between the electrical contact and the Schottky barrier contact on the nanowire when the nanowire is exposed to light or biased. When the nanowires are exposed to a mixture of molecules including acetone, the measured current characterizes the concentration of acetone in the mixture.

16. The method according to claim 15, wherein, Providing the array of nanowires includes growing the array of nanowires from the substrate using an addition bottom-up growth process.

17. The method according to claim 15, wherein, Providing the array of nanowires includes forming the array of nanowires by a top-down etching process.

18. The method according to any one of claims 15 to 17, wherein, The step of depositing amine-functionalized polymers includes: drop-casting chitosan onto the nanowires.

19. The method according to any one of claims 15 to 18, wherein, The step of depositing electrical contact material on a first portion of the surface of each nanowire includes: depositing the electrical contact material onto the nanowire at an angle such that the electrical contact material is deposited on only one side of each nanowire.

20. The method according to any one of claims 15 to 19, wherein, The electrical contact material is primarily composed of Pt.

21. The method according to any one of claims 15 to 20, wherein, Each of the nanowires has a diameter of approximately 50 nm to 60 nm.

22. The method according to any one of claims 15 to 21, wherein, The compound semiconductor is selected from the group consisting of InP, InAs, GaP and GaAs.

23. The method according to any one of claims 15 to 22, wherein, The compound semiconductor is undoped InP.

24. The method according to any one of claims 15 to 23, wherein, The electrical contact material is essentially composed of metals selected from the group consisting of Pt, Pd, Ni and Au.

25. The sensor according to any one of claims 1 to 11, or the device according to any one of claims 12 to 14, or the method according to any one of claims 15 to 24, wherein, Mixtures containing molecules such as acetone are sweat or breath.

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