Oxygen sensor

Abstract

A sensor configured to detect 02 as target analyte in the presence of visible light, providing a reversible response to 02 concentration changes, wherein the sensor comprises: two electrodes; and a sensing composite arranged between the two electrodes; wherein the sensing composite comprises: one or more carbon-based nanostructures having an outer surface; one or more metal-oxides functionalized to the outer surface of the one or more carbon-based nanostructures; and one or more photosensitizer capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbon-based nanostructure, wherein said photosensitizers are functionalized to the outer surface of the one or more carbon-based nanostructures and / or the one or more metal-oxides.

Description

[0001] OXYGEN SENSOR

[0002] Various exemplary embodiments relate to a sensor for detecting a target analyte O2 in the presence of visible light, and more specifically relate to a sensor that includes a sensing composite, which includes carbon-based nanostructures, metal-oxides functionalized to the carbon-based nanostructures, and photosensitizers.

[0003] Accurately measuring oxygen concentration and issuing warnings about abnormal conditions is crucial to ensure personnel safety in industrial safety settings, such as mines, oil production facilities, chemical plants, and so on. In addition to safety considerations, environmental concerns also play a significant role in driving the demand for oxygen gas sensors. Dissolved oxygen is essential for maintaining balanced aquatic ecosystems, while toxic gases such as ammonia, ozone, and chlorine in wastewater treatment plants underscore the need for accurate oxygen gas sensors. Furthermore, in the automotive industry, accurate oxygen level assessment is critical for adjusting air-fuel ratios in engines and industrial processes. In the biomedical sciences, measuring oxygen levels is key for evaluating cell health and disease progression. Finally, the timely detection of packaging leaks via accurate oxygen level monitoring is essential for preventing the spoilage of sensitive goods in the food and pharmaceutical industries.

[0004] Given the wide range of settings and conditions where oxygen sensors would be useful, there is a continuing need to develop versatile oxygen sensors that can be miniaturized, manufactured at lower cost, distributed in the field while requiring low power to operate. Currently available oxygen gas sensing technologies are not ideal for applications that require real-time analysis at ambient conditions using compact, low-cost devices with low power requirements. Oxygen sensing technology that is available for this purpose features some combination of humidity intolerance, limited selectivity, cumbersome sample preparation, short device lifetime, and / or the need to apply energy-intensive treatments such as heat, vacuum, or UV light for signal recovery and sensor re-use.

[0005] The three most commonly used classes of O2 sensing technologies are high-temperature resistive sensors, electrochemical sensors, and optical sensors. Resistive sensors are based on metal oxides operating at high temperatures and may provide a commercially established means to detect O2. Although the principle of operation is simple, and compact devices can be manufactured, resistive sensors have major disadvantages that stem from their high temperature of operation. This requires a heating element and hence constant power input, and precludes any remote monitoring application. Further, metal oxides cross- react with a wide range of gases and volatile organic compounds at high temperatures. Hence, the sensing response with resistive sensors is not selective for oxygen. Alternatively, metal oxides that function at room temperature are typically not tolerant to humidity.

[0006] Electrochemical sensors identify oxygen by a redox reaction that takes place in an electrochemical cell that contains a working, reference, and counter electrodes. Although they represent a selective means of oxygen detection, electrochemical sensors are considered more complex than resistors owing to a three- electrode configuration that necessitates the use of a potentiostat for signal collection. In addition, the use of a reference electrode means that electrochemical sensors require periodic calibration by trained personnel, which significantly raises operational cost. As an alternative, single-use solid-state electrochemical sensors have been developed that feature inherently limited operational lifetimes. Hence, a very clear tradeoff exists with electrochemical sensors between accuracy and sensor lifetime. Owing to the simpler 2-electrode setup, the chemiresistive sensors do not require a reference electrode and are therefore simpler to operate, without sacrificing selectivity / stability.

[0007] Optical sensors include colorimetric probes and luminescent probes. While colorimetric probes undergo a color change in response to oxygen exposure, luminescent probes show a change in luminescence properties in the presence of oxygen. Although such technologies are highly sensitive to oxygen, they have unique drawbacks. For instance, colorimetric probes cannot provide real-time information about oxygen levels and are typically suitable for a single measurement. Luminescent probes on the other hand rely on relatively complex and hence costly device architectures and electronics that are needed to accurately quantify changes in the luminescence profile of a material (i.e. fiber optics probes, photodiodes, etc.).

[0008] XIA Yl ET AL: "A room-temperature methane sensor based on Pd-decorated ZnO / rGO hybrids enhanced by visible light photocatalysis”, SENSORS AND ACTUATORS B: CHEMICAL, ELSEVIER BV, NL, vol. 304, 31 October 2019, discloses a methane sensor based on Pd-decorated ZnO / rGO (reduced graphene oxide) hybrids whose performance is enhanced under visible light illumination. The sensor enables highly sensitive CH4detection at room temperature, achieving a maximum response of 63.4% at 1 % CH4 concentration and fast response and recovery times. Pd nanoparticles function as photocatalytic promoters under visible light, rGO nanosheets contribute to charge transfer, and the presence of multiple heterojunctions within the hybrid structure assists in charge separation. The sensor performance is associated with photocatalytic oxidation of methane under visible light. It was observed that ZnO / Pd and rGO / ZnO / Pd samples show higher photocatalytic activity than ZnO and rGO / ZnO alone, indicating that the Pd nanoparticles function as photocatalytic promoters for CH4oxidation at room temperature. Thus, Xia et al describe a light-induced oxidation process.

[0009] ZHOU YONG ET AL: "UV Illumination-Enhanced Molecular Ammonia Detection Based On a Ternary- Reduced Graphene Oxide-Titanium Dioxide-Au Composite Film at Room Temperature", ANALYTICAL CHEMISTRY, [Online], vol. 91 , no. 5, 5 March 2019, pages 3311-3318, disclose a chemiresistive ammonia (NH3) sensor based on a ternary composite of reduced graphene oxide (rGO), titanium dioxide (Ti02), and gold nanoparticles. The sensor operates at room temperature and its signal is characterized by a decrease in resistance upon exposure to NH3. NH3detection is explained by the oxidation of NH3molecules after surface-bound oxygen species are desorbed from the Ti02under UV illumination. This desorption clears the depletion region on the Ti 02surface, making it accessible for NH3adsorption. The sensor demonstrates good selectivity specifically towards reducing gases that can adsorb on Ti02, with NH3showing the strongest response among tested gases. This selectivity is attributed to three main factors, namely the surface acidity of Ti02which favors NH3adsorption, strong interactions between NH3and surface hydroxyl groups on Ti02, and the relatively high electron-donor ability of NH3compared to other analytes such as H2S. Gold has a signal-enhancing effect due to a more effective charge separation during electron-hole pair formation (see Figure 6b of Zhou), as the sensor still produces a signal in the absence of gold.

[0010] Thus, similar to Xia et al., Zhou et al. describe a light-induced oxidation process. In both cases, the analyte is oxidized at the surface of the metal oxide, yet the nature of the light absorber and the specific gas target differ.

[0011] YUN JUMI ET AL: "Improvement of NO Gas Sensing Properties of Polyaniline / MWCNT Composite by Photocatalytic Effect of TiO2”, JOURNAL OF NANOMATERIALS, [Online], vol. 2013, no. 1 , 21 October 2013, discloses a NO gas sensor based on a composite of polyaniline, multi-walled carbon nanotubes (MWCNT), and Ti02. The sensor shows enhanced sensitivity and faster response under UV illumination due to the photocatalytic activity of Ti02, which oxidizes NO to acidic species such as HNO3, N02, and HN02. These products are adsorbed on the polyaniline-coated MWCNTs, leading to a decrease in electrical resistance characteristic of p-type semiconductors.

[0012] Sanchez et al (Sensors and Actuators B 140 (2009), 17-23) disclose a gas sensor based on multi-walled carbon nanotube and Ti02composites, designed for the detection of ammonia. The sensor operates at room temperature and shows a decrease in electrical resistance upon exposure to NH3, consistent with the behavior of p-type materials interacting with reducing gases. The composite is fabricated by functionalizing multi-walled carbon nanotubes via acid treatment and embedding them into a Ti02matrix prepared through a sol-gel process. The carbon nanotubes show an increased resistance upon ammonia adsorption which is typical of p-type semiconductors in the presence of a reducing gas (i.e., holes undergo recombination with the electrons donated by ammonia).

[0013] Although unfunctionalized single-walled carbon nanotubes (SWCNTs) exhibit some O2 sensitivity, their extended ir-sidewalls are indiscriminate adsorption sites and thus yield nonspecific sensing signals. The incorporation of a receptor is thus necessary with SWCNTs to translate a molecular interaction with O2 into a selective sensing response. Various functionalized SWCNTs have been investigated, but each approach features some combination of humidity intolerance, limited selectivity, cumbersome sample preparation, short device lifetime and / or the need to apply energy-intensive treatments such as heat, vacuum, or UV light for signal recovery and sensor re-use. An unmet need therefore exists to develop sensitive, selective, and robust O2 detection schemes using carbon-based nanostructures in chemiresistors.

[0014] The problem of the present invention is therefore providing a sensitive, selective, and robust sensor for O2 with a user-friendly design.

[0015] The problem is solved by the sensor according to claim 1. Further preferred embodiments are subject of dependent claims 2 to 15.

[0016] The sensor of the present invention is configured to detect O2 as target analyte in the presence of visible light providing a reversible response to O2 concentration changes, wherein the sensor comprises: two electrodes; and a sensing composite arranged between the two electrodes; wherein the sensing composite comprises: one or more carbon-based nanostructures having an outer surface; one or more metal-oxides functionalized to the outer surface of the one or more carbon-based nanostructures; and one or more photosensitizer capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbonbased nanostructure, wherein said photosensitizers are functionalized to the outer surface of the one or more carbon-based nanostructures and / or the one or more metal-oxides.

[0017] The function of the sensor according to the present invention is based on an optically triggered charge transfer process that is modulated by the presence of 02. A photosensitizer absorbs visible light and injects electrons into the underlying carbon-based nanostructure that is functionalized with one or more metal- oxides. 02acts as an excited state quencher and / or electron trap, interrupting or influencing this charge transfer. This means that 02quenches photoexcited states, i.e. in the presence of oxygen photoexcited electrons are not even formed and / or photoexcited electrons return to the electronic ground state, affecting the expected flow of electrical charge. The sensor thus mainly operates via trapping or quenching of photoexcited states, rather than chemical conversion of the analyte O2.

[0018] The sensor according to the present invention allows for exceptional selectivity, as the response is governed by a specific interaction with 02at the electronic level, rather than broad chemical reactivity. This ensures minimal interference from other gases and provides reliable, targeted detection. Furthermore, it is inherently reversible and stable, as no irreversible chemical reactions take place on the surface. This leads to excellent long-term performance and minimal signal drift, even after repeated exposure to the analyte. Finally, the sensor works efficiently at room temperature and under visible light, enabling low-power operation and integration into compact, portable, and user-friendly systems.

[0019] Within the context of the present invention the term “electrode” refers to an electrically conductive structure configured to establish electrical contact with the sensing composite and to enable the measurement of its electrical resistance or conductivity. The electrodes may include one or more layers of a conductor preferably selected from the group consisting of gold (Au), chromium (Cr), platinum (Pt), silver (Ag), copper (Cu), and carbon (C) and combinations thereof. The gap between the two electrodes may range from 5 m to 1 mm, or about 200 pm.

[0020] In the context of the present invention, the term “a reversible response to O2 concentration changes” refers to the ability of the sensor to return to its original baseline conditions (e.g., electrical resistance) after exposure to 02and subsequent removal of the analyte. Specifically, the sensor consistently recovers to at least 90% of its baseline without undergoing permanent structural or functional changes. This capability enables repeated measurement cycles while maintaining optimal performance.

[0021] In various embodiments, a sensor is used to detect O2 in the presence of visible light. The sensor may include two electrodes and a sensing region arranged between the two electrodes. The sensing region may have a sensing composite that includes one or more carbon-based nanostructures having an outer surface, one or more metal-oxides functionalized to the outer surface of the carbon-based nanostructures, and one or more photosensitizers capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbon-based nanostructure, wherein said photosensitizers are functionalized to the outer surface of the carbon-based nanostructure(s) and / or the one or more metal-oxide(s).

[0022] In one embodiment of the present invention the sensing composite of the sensor described in the present invention preferably comprises single-walled carbon nanotubes (SWCNTs) as carbon-based nanostructures having an outer surface. These single-walled carbon nanotubes are preferably functionalized with titanium dioxide on their outer surface, along with a metal-organic complex that acts as a photosensitizer and is capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbon-based nanostructure. This combination creates a functionally integrated and electronically synergistic composite. TiO2 effectively accepts electrons injected from the photoexcited metal-organic complex. Meanwhile, the single-walled carbon nanotubes provide a highly conductive pathway for charge transport, facilitating the direct electrical readout of photoinduced charge transfer events. Upon excitation by visible light, the metal-organic complex undergoes metal-to-ligand charge transfer (MLCT), injecting an electron into the conduction band of the TiO2. This electron is then transferred to the carbon nanotubes, resulting in a measurable change in resistance. In this way, the single-walled carbon nanotubes serve not only as efficient charge collectors but also as the active transducers that convert photoinduced electronic changes into an electrical signal. Notably, exceptional results can be achieved with this sensing composite when the metal-organic complex utilized is 4,4,-[P(O)(OH)2]2-2,2,-bipyridine)(CO)3Re(Br) (=(pbpy)(CO)3Re(Br).

[0023] In various embodiments, one or more of the sensors may be arranged within a vehicle.

[0024] In various embodiments, lightweight chemiresistors based on carbon nanomaterials, e.g. single-walled carbon nanotubes (SWCNTs), represent an attractive detection platform owing to room-temperature operation, and a possibility of incorporation into ultra-small microelectronic devices.

[0025] In the drawings:

[0026] FIG. 1 depicts an electrode pattern having a SWCNT-TiO2-Re chemiresistor film arranged thereon;

[0027] FIG. 2 depicts a manufacturing process for obtaining the sensor of various embodiments described herein;

[0028] FIG. 3 depicts an analyte sensor that is enabled by light harvesting in various embodiments; FIG. 4 is a graph depicting an averaged resistance trace of SWCNT-TiO2-Re in response to three repeated 2-minute O2 exposures (995 ppm);

[0029] FIG. 5 is a graph depicting the sensing response of SWCNT-TiO2-Re to a 2-minute O2 exposure (995 ppm) compared to control sensors with various omitted components;

[0030] FIG. 6 is a graph depicting a relative alignment of SWCNT, TiO2, and Re photosensitizer electronic bands in SWCNT-TiO2-Re;

[0031] FIG. 7 is a set of graphs depicting an averaged resistance trace of SWCNT-TiO2-Re in response to a 1 - minute O2 exposure of various concentrations (50-500 ppm);

[0032] FIG. 8 is a graph depicting the sensing response of SWCNT-TiO2-Re to a 1 -minute O2 exposure of various concentrations (20-18000 ppm); and

[0033] FIG. 9 is a graph depicting the sensing response of SWCNT-TiO2-Re to various gases compared to the sensing response to O2 before and after interferant exposure. Shaded areas and error bars represent standard deviations (N = 4 chemiresistors).

[0034] FIG. 10 is a graph that extends the data presented in FIG. 9, illustrating the sensing response of SWCNT- TiO2-Re to various gases. It includes a comparison of the sensing response to O2 before and after exposure to interferents, along with the additional data for ammonia (NH3). The shaded areas and error bars represent standard deviations (N = 4 chemiresistors).

[0035] Fig. 11 shows the stability of the sensor over an extended period.

[0036] Fig. 12 presents comparative experiments in which the components of the chemiresistor have been systematically omitted.

[0037] Fig. 13 shows normalized and drift-corrected resistance trace for 2 min exposure of “SWCNT-TiO2-organic sensitizer 5” to 1000 ppm O2. The shaded area represents standard deviation (N = 3 chemiresistors); the data was collected under red light irradiation (628 nm) at room temperature using dry nitrogen carrier gas (R.H. = 0%). All data for Figs. 4-12 was collected under green light irradiation at room temperature using dry nitrogen carrier gas (RH = 0%).

[0038] Elements that are identical, similar or have the same effect are given the same reference signs in the Figures. The Figures and the proportions of the elements shown in the Figures are not to be regarded as true to scale. Rather, individual elements can be shown exaggeratedly large for better representability and / or for better comprehensibility.

[0039] In the following detailed description, reference is made to the accompanying drawings which form part thereof and in which specific embodiments are shown for illustrative purposes in which the invention may be practiced. Since components of embodiments may be positioned in a number of different orientations, the directional terminology is for illustrative purposes and is not limiting in any way. It is to be understood that other embodiments may be used and structural or logical changes may be made without departing from the scope of protection of the present invention. It is to be understood that the features of the various exemplary embodiments described herein may be combined with each other, unless specifically stated otherwise. The following detailed description is therefore not to be understood in a limiting sense, and the scope of protection of the present invention is defined by the appended claims.

[0040] Surprisingly, a sensor has been discovered that may allow for rapid sensing of analyte levels of oxygen (O2) in a non-limiting embodiment, at room temperature under visible light illumination. The sensor may include two electrodes and a sensing composite arranged between the two electrodes. The sensing composite may include one or more carbon-based nanostructures having an outer surface, one or more metal-oxides functionalized to the outer surface of the one or more carbon-based nanostructures, and one or more photosensitizers functionalized to the outer surface of the one or more carbon-based nanostructures and / or the one or more metal-oxides.

[0041] Such a sensor undergoes photoinduced charge transfer that is sensitive to ppm levels of the analyte oxygen (O2), which yields an incredibly sensitive response at room temperature. The sensor also has a stable shelf life at room temperature, of up to at least 8 weeks but potentially up to 260 weeks, or even longer (see Figure 11). The sensor may be configured to detect the target analyte based on a change in resistance, conductance, capacitance, voltage, or current, preferably resistance. The change is measurable and detected with very simple equipment, such as a handheld digital multimeter, a source meter, a potentiostat, a data logger and so on. The target analyte is the gas O2.

[0042] In various embodiments, the sensor is configured to detect the target analyte at a temperature ranging from -50 °C to 200 °C, preferably from 18 °C to 40 °C. The sensor may be configured to detect the analyte at a concentration of up to 1 ,000,000 ppm, alternatively from 0 ppm to 500,000 ppm, or from 1 ppm to 200,000 ppm.

[0043] In various embodiments, the sensor is configured to detect the analyte via photosensitization using the photosensitizer(s).

[0044] The photosensitizer is capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the carbon-based nanostructure as depicted in FIG. 3. Given that the dynamics of photoexcited states in metal complexes are sensitive to the presence of O2, O2 modulates the photo-induced electron injection process in the sensing composite, which leads to a measurable bulk conductance change.

[0045] The sensing composite may be arranged as a film over the sensing region where the carbon-based nanostructures, having the one or more metal-oxide nanoparticles and the photosensitizer(s) functionalized thereto, are substantially uniformly dispersed within the film.

[0046] The sensor may also be used for various purposes in a vehicle, e.g. for measuring automobile exhaust, water and air quality monitoring, food spoilage detection, industrial quality control, and so on.

[0047] As depicted in FIG. 2, the sensor 200 has a base layer 100, which may be or include a ceramic, such as but not limited to silicon dioxide (SiO2), aluminum oxide (AI2O3), calcium oxide (CaO), potassium oxide (K2O), zirconium oxide (ZrO2), magnesium oxide (MgO), silicon nitride (SisN4), and combinations thereof. The base layer may also be other materials, such as glass, pure silicon, or a flexible polymer, such as but not limited to polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI), and combinations thereof. Depending on the application of the sensor 200, the skilled person would understand what material to use for the base layer 100. The base layer may have a positive electrode 102A and a negative electrode 102B arranged thereon. The electrodes 102A, 102B may include one or more layers of a conductor, such as but not limited to gold (Au), chromium (Cr), platinum (Pt), silver (Ag), copper (Cu), carbon (C) and combinations thereof. In various embodiments, the gap between electrodes 102A, 102B may range from 5 pm to 1 mm, or about 200 pm. The sensing composite 104 may be arranged between the electrodes 102A, 102B and on the base layer 100; this is the sensing region of the sensor 200. The sensing composite 104 may optionally contact the electrodes 102A, 102B. As mentioned above, the sensing composite 104 may include carbon-based nanostructures 104A, but not be limited to, single-walled carbon nanotubes, graphene, fullerenes, heterofullerenes, carbon nanodots, carbon nanoparticles, graphite, multi-walled carbon nanotubes, or combinations thereof, preferably single-walled carbon nanotubes. The size of the carbon-based nanostructures may range from 1 nm to 100 pm, preferably from 50 nm to 500 nm. The conductive carbon nanostructures allow the measurements to be made at room temperature.

[0048] A metal-oxide 104B is functionalized to the outer surface of the carbon-based nanostructures 104A to form functionalized nanostructures 104A, 104B. The metal-oxide 104B may be or include, but is not limited to, titanium dioxide (TiC ), tin oxide (SnC ), zinc oxide (ZnO), zinc tin oxide (ZnxSnxOx), nickel oxide (NiO), copper oxide (CuxOx), cobalt oxide (CoxOx), iron oxide (FexOx), zinc magnesium oxide (Zni-xMgxO), magnesium oxide (MgO), vanadium oxide (VxOx), niobium oxide (NbxOx), tantalum oxide (TaxOx), molybdenum oxide (MoxOx), tungsten oxide (WxOx), indium oxide (lnxOx), gallium oxide (GaxOx), palladium oxide (PdO), zirconium oxide (ZrC ), aluminum oxide (AI2O3), strontium oxide (SrO), strontium titanium oxide (SrTiOs), lanthanum oxide (La2O3), cerium oxide (CexOx), praseodymium oxide (PrxOx), promethium oxide (Pr Os), samarium oxide (Snr^Os), europium oxide (EU2O3), gadolinium oxide (Gd20s), terbium oxide (TbxOx), dysprosium oxide (Dy20s), holmium oxide (HO2O3), erbium oxide (Er20s), thulium oxide (Tn s), ytterbium oxide (Yb2O3), lutetium oxide (LU2O3), and combinations thereof.

[0049] The functionalized nanostructures 104A, 104B may have a core-shell structure where the carbon-based nanostructure 104A is arranged as the core, and where the metal-oxide 104B is arranged as a layer at least partially around the carbon-based nanostructure 104A, or alternatively fully around or optionally encapsulating the carbon-based nanostructure 104A. The layer of the metal-oxide 104B arranged around the carbon-based nanostructure 104A may have a thickness ranging from 5 nm to 500 nm, alternatively from 10 nm to 100 nm.

[0050] The functionalized nanostructures 104A, 104B may be arranged on the base layer 100 via spray coating, bar coating, blade coating, slot die coating, dip coating, spin coating, drop casting, and combinations thereof. The arranging of the functionalized nanostructures 104A, 104B onto the base layer 100 may occur at a temperature ranging from 23 °C to 200 °C, preferably 140°C. The photosensitizers 104C may then be functionalized to the outer surface of the carbon-based nanostructures 104A and / or the metal-oxides 104B. The one or more photosensitizer(s) may include, but not be limited to, a metal, such as rhenium, manganese, zirconium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, cerium, or combinations thereof, preferably rhenium; an organic functional group, such as pyridines, bipyridines, terpyridines, imines, diimines, phenyls, biphenyls, terphenyls, tri-arylamines, indolines, coumarins, tetrahydroquinolines, porphyrins, phthalocyanines, anthracenes, heteroanthracenes, carbazoles, imidazoles and other heterocyclic amines, carbenes, carbodiimides, amides, amines, N,N- dialkylanilines, guanidinates, hemicyanines, merocyanines, squaraines, perylenes, boron- dipyrromethenes, alkynes, alkenes, thiophenes, carboxylic acids, phosphonic acids, sulfonic acids, halogens, carbonyls, cyanides, isocyanides, isothiocyanides, phosphines, arsines, and combinations thereof; or combinations including the metal(s) and organic functional group(s). The skilled person would understand which metal(s), functional group(s), or combinations thereof to use depending on the desired application of the sensor.

[0051] Within the context of the present invention, a photosensitizer is defined as a molecular sensitizer capable of absorbing visible light and injecting electrons into the underlying metal oxide and / or into the carbon based nanostructure. It can be either an organic or a metal-organic compound. The photosensitizer typically exhibits strong absorption in the visible spectrum (380 to 800 nm) and has a suitable HOMO-LUMO gap to ensure efficient excitation.

[0052] Preferably, the photosensitizer is a metal-organic complex. Typically, they comprise a central metal ion coordinated to a variety of ligands. At least one of these ligands is designed to exhibit strong photoabsorption characteristics, thus, it comprises a conjugated ir-system. Examples for such ligands are porphyrins, phthalocyanines, anthracenes and other polycyclic aromatic hydrocarbons, carbazoles, terpyridines, bipyridines, squaraines, merocyanines and heteroanthracenes. Furthermore, one or more of these ligands can comprise a linker allowing to form a strong bond, typically a covalent bond, between the metal-organic complex and the underlying metal oxide. This strong bond allows for efficient metal-to-ligand charge transfer (MLCT) and photoinduced charge injection into the conduction band. The linkers typically comprise functional groups such as carboxylic acids (carboxylates) or phosphonic acids (phosphonates). Alternatively, such linkers may be absent, in which case the photosenitizer physioabsorbs on the metal oxide. However, better performance can be typically obtained in the presence of a linker. Non-limiting examples of metal-organic photosensitizers include Re and Mn carbonyl complexes, Ru and Os polypyridine complexes, Or, Mo, and W isocyanide and carbonyl complexes, cyclometalated Rh and Ir polypyridine complexes, Fe polypyridine complexes, Cu polypyridine complexes, Zn porphyrin complexes, and phthalocyanine complexes (Zn, Ru, Ti). Each of these complexes, if not already present in the list above, contains at least one additional ligand exhibiting strong photoabsorption characteristics, which are characterized by their conjugated ir-systems and optionally include a linker such as carboxylic acids or phosphonic acids. For example, in the case of W carbonyl complexes, an additional ligand exhibiting strong photoabsorption characteristics is necessary, whereas for Zn porphyrin complexes, this is not the case. Best results could be obtained with (pbpy)(CO)3Re(Br), a Re carbonyl complex comprising a bipyridine ligand that exhibits strong photoabsorption characteristics; this bipyridine includes phosphonate linker groups that form a strong bond with the underlying metal oxide.

[0053] The photosensitizer can also be an organic compound that is able to absorb visible light. To be effective, it must possess a conjugated ir-system that allows strong light absorption in the visible spectrum. Furthermore, the organic compound can comprise a linker allowing to form a strong bond between the photosensitizer and the underlying metal oxide. These linkers typically comprise functional groups such as carboxylic acids (carboxylates) or phosphonic acids (phosphonates). Alternatively, such linkers may be absent, in which case the photosenitizer physioabsorbs on the metal oxide. Examples are squaraines, porphyrins, phthalocyanines, cyanines, hemicyanines, merocyanines, perylenes, optionally comprising a linker group such as carboxylic acids or phosphonic acids.

[0054] In various embodiments, the photosensitizers preferably include a metal organic complex comprising the metal, such as rhenium, manganese, zirconium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, cerium or combinations thereof, preferably rhenium, as well as various functional groups for purposes of functionalizing the metal to the outer surface of the one or more carbon-based nanostructures and / or the one or more metal-oxides of the sensor. Such functional groups may be or include, but are not limited to, pyridines, bipyridines, terpyridines, imines, diimines, phenyls, biphenyls, terphenyls, tri-aryl amines, indolines, coumarins, tetrahydroquinolines, porphyrins, phthalocyanines, anthracenes, heteroanthracenes, carbazoles, imidazoles and other heterocyclic amines, carbenes, carbodiimides, amides, amines, N,N- dialkylanilines, guanidinates, hemicyanines, merocyanines, squaraines, perylenes, boron- dipyrromethenes, alkynes, alkenes, thiophenes, carboxylic acids, phosphonic acids, sulfonic acids, halogens, carbonyls, cyanides, isocyanides, isothiocyanides, phosphines, arsines, and combinations thereof. A non-limiting example of the photosensitizer may be or include (pbpy)(CO)3Re(Br). One skilled in the art would understand which functional groups could be used in combinations with the metal to form the metal organic complex. Said metal organic complexes typically form strong coordinative or covalent bonds with the metal oxide surface, allowing for efficient metal-to-ligand charge transfer (MLCT) and photoinduced charge injection into the conduction band.

[0055] In various embodiments, the photosensitizer 104C may be functionalized to the functionalized nanostructures 104A, 104B by soaking the device in a DMSO solution of the photosensitizer, e.g. (pbpy)(CO)3Re(Br) in a non-limiting embodiment, for a time period ranging from 1 h to 72 h, such as from 16 h to 18 h. Together, the carbon-based nanostructures, the metal-oxide particles functionalized to the carbon-based nanostructures, and the photosensitizers functionalized to the carbon-based nanostructures and / or the metal-oxide particles is referred to herein as the sensing composite 104 within the sensor 200. The size of the sensing composite 104 may range from 1 m to 1 mm in diameter, preferably from 50 pm to 500 pm.

[0056] Alternative methods of functionalizing the photosensitizer 104C to the functionalized nanostructures 104A, 104B may be or include, but not be limited to the direct deposition of the photosensitizer via spray coating, bar coating, blade coating, slot die coating, spin coating, drop casting, and combinations thereof.

[0057] As depicted in FIG. 4, the sensor was tested and examined under visible light irradiation in the presence and absence of oxygen (O2). A mixture of O2 in N2 in a ratio of 1 O2to 1000 N2 was delivered to a gas-tight enclosure housing four sensors at a flow rate of 1 L min1. The enclosure had a glass window that allowed for controlled visible light irradiation of the sensors without disturbance of the gas flow. Green light, having a wavelength of about 516 nm, was selected for the experiment. Exposure of SWCNT-TiO2-Re to 0.1 % (1000 ppm) of oxygen (O2) for 120 seconds at room temperature under the green light resulted in a significant sensor response, taken as the normalized change in device resistance [AR / Ro (%) = (R-Ro) / Ro x 100% where Ro = initial resistance] of -30.5 ± 4.3% after 120 seconds (FIG. 4).

[0058] The sensor response magnitude was found to be consistent after repeated exposures and was reversible where nearly full baseline recovery was achieved after 500 seconds of purging with N2 between O2 exposures. Such rapid O2 response and recovery times suggests that SWCNT-TiO2-Re is a viable sensing composite for reversible, room temperature O2 detection under visible light radiation.

[0059] FIG. 5 illustrates data from control sensors that was fabricated to determine whether all sensor components are needed for the observed O2 response. Devices having only SWCNTs exhibited a negligible O2 response, as did those devices fabricated with the exclusion of TiO2. Further, omission of SWCNTs yielded a composite (TiO2-Re) with resistance values too high to be monitored using the same data acquisition unit as that for FIG. 4. Exclusion of the rhenium sensitizer in SWCNT-TiO2 led to a significantly smaller, but importantly, irreversible signal. Lastly, exposing SWCNT-TiO2-Re to O2 without green light illumination did not produce a sensing response. Therefore, the data establishes that SWCNTs, TiO2, and the Re sensitizer and visible light irradiation are all crucial for the highly sensitive and reversible chemiresistive response to O2.

[0060] As depicted in FIG. 6, experiments were also conducted to corroborate the origin of the O2 sensing response in SWCNT-TiO2-Re under visible light. Previous studies on SWCNT-TiO2 composites under UV-light irradiation have shown photo-induced electron injection that occurred from TiO2 to the conduction band of SWCNTs. Given that semiconducting SWCNTs exhibit p-type behavior under ambient conditions, electron injection from TiO2 was shown to result in a reduction of carrier (hole) density in SWCNTs, thus producing an increased device resistance. While UV light is necessary to trigger this process in bare SWCNT-TiO2, it was hypothesized that the introduction of Re would sensitize TiO2 and enable electron injection under visible light irradiation as shown in FIG. 6. The relevant electronic absorption band of (pbpy)(CO)3Re(Br) at 389 nm (E =3274 M1cm1) is known to bear a metal-to-ligand-charge-transfer (MLCT) character, dominated by a transition to the TiO2-bound bipyridyl fragment from Re-centered orbitals. Further, (pbpy)(CO)3Re(Br) exhibits a HOMO-LUMO gap at 2.52 eV and excited-state oxidation potential (E* / E+) of -0.93 V (vs NHE), as determined by a combination of UV-Vis, and cyclic voltammetry experiments, and is thus well-suited for photoinduced electron injection into SWCNTs via TiO2 (FIG. 6). Consistent with this view, exposure of SWCNT-TiO2-Re to green light (about 516 nm, ca. 1.0 mW / cm2) resulted in a sharp increase in sensor resistance while SWCNT-TiO2 exhibited a significantly smaller photoinduced change in sensor resistance under the same conditions. Irradiation of SWCNT-TiO2-Re in the presence of oxygen (O2) resulted in a markedly smaller drop in sensor resistance. Taken together, these results suggest that the presence of O2 hinders photoinduced electron injection likely by trapping photoexcited electrons in SWCNT-TiO2-Re, and thus gives rise to a sensing signal through a decrease in sensor resistance.

[0061] As shown in Figures 9 and 10, the sensor according to the present invention demonstrates pronounced selectivity for O2 over a range of oxidizing and reducing gases, including nitrous oxide (N2O), carbon dioxide (CO2), hydrogen (H2), methane (CH4), and ammonia (NH3). This selectivity is attributed to an optically triggered charge transfer process modulated by the presence of O2. Consequently, the sensor utilizes visible light to excite a sensitizer, while O2 influences the electronic dynamics (quenching and trapping) of the excited state without resulting in a chemical reaction with the analyte itself. A novel class of sensors, such as chemiresistive oxygen sensors in various embodiments, may function at room-temperature, and feature low power requirements with the possibility of incorporation into ultrasmall microelectronic devices.

[0062] The sensors may provide a rapid and reversible response to ppm level oxygen changes across a wide concentration range, such as from 0.01 ppm to 500,000 ppm, or from 1.1 ppm to 200,000 ppm. The sensors feature long term stability, operate under various humidity levels, and are selective for various gases over interferant gases.

[0063] As shown in Figure 10, the sensor according to the present invention was found to exhibit clear selectivity for O2 over oxidizing and reducing gases such as nitrous oxide (N2O), carbon dioxide (CO2), hydrogen (H2) and methane (CFU). The NH3 experiments presented in Figure 10 demonstrate that the sensor can detect NH3, though with an inverse signal, indicating a distinct detection mechanism compared to that for oxygen. Additional experiments have shown that NH3 can be detected by the SWCNT-TiO2 system even in the absence of a photosensitizer and regardless of light presence. In contrast, the sensor according to the present invention for detecting O2 operates exclusively in the presence of light and requires the presence of a photosensitizer capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide. As illustrated in Figure 5, the absence of the photosensitizer led to a signal that was, importantly, irreversible and significantly smaller.

[0064] In a non-limiting embodiment, the sensor may have an active sensing element that includes an inexpensive composite comprising a carbon nanotube-titanium dioxide (SWCNT-TiC ) with a small amount of a rhenium (Re) photosensitizer functionalized to the carbon nanotube and / or the titanium dioxide. The rhenium photosensitizer is capable of harvesting visible light, which induces a resistance change in the underlying SWCNT-TiO2 network due to electron injection. Given that the dynamics of photoexcited states in metal complexes are sensitive to the presence of oxygen, the photo-induced electron injection process in the composite is influenced by the presence of oxygen, which leads to a measurable bulk resistance change. The resistance change is measurable with very simple equipment (such as a handheld digital multimeter) and may be the basis of the sensor response.

[0065] The SWCNT-TiO2 composite may be synthesized using commercially-available materials, and it may be spray-coated onto a commercially available interdigitated electrode pattern. The electrode pattern may be soaked in a solution containing the rhenium photosensitizer . Once the electrode pattern is dried, it can be connected to a data-acquisition unit that applies a small potential, e.g. from 0.01 to 1 V, preferably 0.1 V, across the electrodes to measure its resistance. This setup is exceedingly simple and can be incorporated into a miniaturized resistor platform and can allow for a low power, low-cost, and distributable sensing of oxygen.

[0066] Under green LED-illumination, the resistance across the electrode pattern may be monitored, and shows very sensitive changes to various oxygen concentrations.

[0067] The conductive carbon nanotube-titanium dioxide (SWCNT-TiC ) composite and rhenium photosensitizer give a standout performance in oxygen sensing due to the unique alignment of the electronic bands, ability to harvest visible light, and robustness with respect to environmental conditions. This combination of features allows the sensor to work with minimal power input, and yields sensitive and selective response to oxygen, even in the presence of other interferant gases and / or humidity. The sensors, in various embodiments herein, offer an advantage, in that they are able to provide real-time information about oxygen levels with simple form factors at potentially lower unit cost.

[0068] Various examples are described below that relate to what is described above and shown in the figures.

[0069] Example 1 is a sensor for detecting O2 as target analyte in the presence of visible light providing a reversible response to O2 concentration changes. The sensor may include two electrodes and a sensing region arranged between the two electrodes. The sensing region may have a sensing material that includes one or more carbon-based nanostructures having an outer surface, one or more metal-oxides functionalized to the outer surface of the carbon-based nanostructures, and one or more photosensitizer capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbon-based nanostructure, wherein said photosensitizers are functionalized to the outer surface of the carbon-based nanostructure(s) and / or the one or more metal- oxide(s).

[0070] Example 2 is set up according to example 1 , wherein one or more carbon-based nanostructures comprises single-walled carbon nanotubes, graphene, fullerenes, heterofullerenes, carbon nanodots, carbon nanoparticles, graphite, multi-walled carbon nanotubes, or combinations thereof, preferably single-walled carbon nanotubes. Example 3 is set up according to example 1 or 2, wherein the sensor is configured to detect the target analyte based on a change in resistance, conductance, capacitance, voltage, or current, preferably resistance.

[0071] Example 4 is set up according to one of examples 1 to 3, wherein the visible light has a wavelength ranging from 380 nm to 800 nm, preferably 500 nm to 570 nm.

[0072] Example 5 is set up according to one of examples 1 to 4, wherein the one or more metal-oxides comprises one or more of titanium dioxide (TiC ), tin oxide (SnC ), zinc oxide (ZnO), zinc tin oxide (ZnxSnxOx), nickel oxide (NiO), copper oxide (CuxOx), cobalt oxide (CoxOx), iron oxide (FexOx), zinc magnesium oxide (Zm- xMgxO), magnesium oxide (MgO), vanadium oxide (VxOx), niobium oxide (NbxOx), tantalum oxide (TaxOx), molybdenum oxide (MoxOx), tungsten oxide (WxOx), indium oxide (lnxOx), gallium oxide (GaxOx), palladium oxide (PdO), zirconium oxide (ZrC ), aluminum oxide (AI2O3), strontium oxide (SrO), strontium titanium oxide (SrTiOs), lanthanum oxide (La2O3), cerium oxide (CexOx), praseodymium oxide (PrxOx), promethium oxide (Pr Os), samarium oxide (Snr^Os), europium oxide (EU2O3), gadolinium oxide (Gd20s), terbium oxide (TbxOx), dysprosium oxide (Dy20s), holmium oxide (HO2O3), erbium oxide (Er20s), thulium oxide (Tn s), ytterbium oxide (Yb2O3), lutetium oxide (LU2O3), and combinations thereof.

[0073] Example 6 is set up according to one of examples 1 to 5, wherein the sensor is configured to detect O2 at a temperature ranging from -50 °C to 200 °C, preferably from 18 °C to 40 °C.

[0074] Example 7 is set up according to one of examples 1 to 6, wherein the sensor is configured to detect O2 at a concentration of up to 100% (1 ,000,000 ppm), alternatively from 1 ppm to 200,000 ppm.

[0075] Example 8 is set up according to examples 1 to 7, wherein the sensor is configured to detect O2 via photosensitization.

[0076] Example 9 is set up according to one of examples 1 to 8, wherein the one or more photosensitizers comprises a metal selected from the group comprising rhenium, manganese, zirconium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, cerium or combinations thereof, preferably rhenium; an organic functional group comprising a pyridine, a bipyridine, a terpyridine, an imine, a diimine, a phenyl, a biphenyl, a terphenyl, a tri-arylamine, an indoline, a coumarin, a tetrahydroquinoline, a porphyrin, a phthalocyanine, an anthracene, a heteroanthracene, a carbazole, a heterocyclic amine, a carbene, a carbodiimide, an amide, an amine, a N,N-dialkylaniline, a guanidinate, a hemicyanine, a merocyanine, a squaraine, a perylene, a boron-dipyrromethene, an alkyne, an alkene, a thiophene, a carboxylic acid, a phosphonic acid, a sulfonic acid, a halogen, a carbonyl, a cyanide, an isocyanide, an isothiocyanide, a phosphine, an arsine, and combinations thereof; or combinations including the metal(s) and the organic functional group(s) thereof.

[0077] Example 10 is set up according to one of examples 1 to 9, wherein the sensing composite is arranged as a film over the sensing region; wherein the one or more carbon-based nanostructures having the one or more metal-oxide nanoparticles and the one or more photosensitizers functionalized thereto are substantially uniformly dispersed within the film. Metal-oxide nanoparticles are preferred because their high surface area enhances interfacial contact, enabling more efficient charge transfer and interaction with O2

[0078] Example 11 is set up according to one of examples 1 to 10, wherein the size of the one or more carbonbased nanostructures ranges from 1 nm to 100 pm, preferably from 100 nm to 500 nm.

[0079] In example 12, one or more sensors according to one of examples 1 to 11 , is arranged in a vehicle, water quality monitoring system, air quality monitoring system, soil respiration analysis, food spoilage detection system, industrial quality and process control, respiratory protective equipment, rebreathers, medical equipment, and combinations thereof.

[0080] Experimental part

[0081] Materials: Unless otherwise stated, reagents were used as supplied from commercial sources without any further purification. Ti(O'Pr)4 (99.999% trace metal basis) and [Re(CO)sBr] were purchased from Sigma- Aldrich and stored in an argon-filled glovebox at -35 °C. SWCNTs (P3-SWNT, > 90% carbonaceous purity, 5-7 wt% metal content, 500 nm-1.5 pm bundle length, 4-5 nm bundle diameter, 1.55 ± 0.1 nm individual tube diameter, lot # 03-A036) were purchased from Carbon Solutions, Inc and dried under high vacuum before storage in an argon-filled glovebox. Triethylamine (EtaN), [Pd(PPh3)4], and indium-tin oxide (ITO) coated glass slides were purchased from Sigma-Aldrich. Triphenylphosphane (PPhs) was purchased from Apollo. 4,4’-Dibromo-2,2’-bipyridine was purchased from abcr. [(pbpy)(CO)3ReBr] (Pbpy = 4,4’-[P(O)(OH)2]2- 2,2’-bipyridine; Re) was synthesized by modification of reported procedures (see synthesis below). The ruthenium dyes (cbpy)2Ru(NCS)2 (cbpy = 4,4’-(C(O)OH)2-2,2’-bipyridine; N3) and [(n- Bu)4N]2[(c2bpy)2Ru(NCS)2] (c2bpy = 4-(C(O)OH)-4’-(C(O)O)-2,2’-bipyridine; N719) were purchased from Solaronix and purified by recrystallization. Solvents were dried and de-gassed using an argon-connected MBraun InertGas SPS-7 solvent purification system. The residual water content in solvents was determined using a Mettler-Toledo C30 coulometric Karl Fischer titrator. Following purification, solvents were stored over 3 A or 4 A molecular sieves in an argon-filled glovebox. Gases were purchased from Pangas or Air Liquide and equipped with gas-flow regulators: N2O (5.0 grade), synthetic air (N2: 80% / O2: 20%), H2 (5.0 grade), CO2 (4.5 grade), CPU (5.5 grade), C2H4 (3.0 grade), CO (4.7 grade). Interdigitated gold electrodes on ceramic substrate were purchased from DropSens (IDEAU200). All air- and moisture-sensitive manipulations were carried out using standard vacuum line Schlenk technique or in an argon-filled MBraun LabMaster Pro glovebox. Reactions were carried out in oven- or flame-dried glassware equipped with a magnetic stir bar. Microwave synthesis was performed using a Biotage® I nitiator+ instrument equipped with Robot Eight.

[0082] Cyclic Voltammetry (CV) was performed in an argon-filled MBraun UniLab glovebox with a Gamry Interface 1010E potentiostat / galvanostat / ZRA using a three-electrode electrochemical cell. If not stated otherwise, a Ag / AgNOs (0.010 M in [(n-Bu)4N][PFe] in MeCN, BASi), glassy carbon disk (1.6 mm2, eDAQ), and Pt-wire were used as reference, working and counter electrodes respectively. The working electrode was polished before each experiment on a pad using an alox-slurry (0.050 mm) and rinsed sequentially with millipore water, isopropanol ('PrOH), and acetone. Experiments were conducted using 0.10 M [(n- Bu)4N][PFe] supporting electrolyte in DMF or MeCN. First, a background scan of a blank sample containing electrolyte and solvent was performed to determine the solvent window and ensure a stable potential and the absence of contaminants. Next, the respective compound (1.0 mM) was dissolved in the blank sample and three scans of the full window starting at the open circuit potential (OCP) were recorded. The first scan is reported. CVs were internally referenced against the Fc / Fc+couple. If necessary, data smoothening using the Savitzky-Golay method was applied.

[0083] Flash column chromatography was performed on a Biotage® Isolera™ One system with Star columns using technical grade solvents.

[0084] Statistical Analysis: Chemiresistive sensing data was pre-processed by normalization. A drift correction was applied in cases where the absolute response was of interest using linear regression fitting of the baseline. Data of repeated measurements under same conditions are presented as mean ± standard deviation. The sample sizes (n) for each statistical analysis are indicated. Statistical analyses were performed using the software OriginPro 2024 (OriginLab Corporation, Massachusetts, USA). Synthesis of Tetraethyl [2,2’-Bipyridine]-4,4’-diylbis(phosphonate)

[0085] The synthesis was adapted from a literature report (M. Ding, D. C. Sorescu, A. Star, J. Am. Chem. Soc. 2013, 135, 9015). In an oven-dried 250 mL two-neck round-bottom flask equipped with a stirring bar, 4,4'- dibromo-2,2'-bipyridine (2.00 g, 6.37 mmol, 1.0 equiv), PPhs (8.35 g, 31.9 mmol, 5.0 equiv), diethyl phosphite (1.90 mL, 14.7 mmol, 2.3 equiv), [Pd(PPhs)4] (0.37 mg, 0.32 mmol, 0.05 equiv), and EtsN (2.00 mL, 14.7 mmol, 2.3 equiv) were refluxed in dry toluene (60 mL) at 110 °C for 15 h under an argon atmosphere. After cooling to room temperature, the yellow reaction mixture was extracted with aqueous ammonia. The colorless organic phase was washed with water and brine, then dried over MgSCU, filtered through Celite, and concentrated under reduced pressure. The crude product was purified by silica gel flash column chromatography (0-4% MeOH in CH2CI2). PPhs eluted at 100% CH2CI2. The product partly coeluted with triphenyl phosphine oxide. Pure fractions were selected by TLC spotting and afforded the product as a white solid (1.20 g, 44%).

[0086] Synthesis of [(pbpy)(C0)3ReBr]

[0087] The synthesis was adapted from a literature report (C. D. Windle, E. Pastor, A. Reynal, A. C. Whitwood, Y. Vaynzof, J. R. Durrant, R. N. Perutz, E. Reisner, Chem. Eur. J. 2015, 21 , 3746.). In the glovebox, tetraethyl [2,2’-bipyridine]-4,4’-diylbis(phosphonate) (200 mg, 0.47 mmol, 1.00 equiv) and [Re(CO)sBr] (191 mg, 0.47mmol, 1.00 equiv) were combined in an oven-dried microwave vial equipped with a stirring bar and dissolved in anhydrous toluene (50 mL). The reaction mixture was heated to 80 °C for 19 h in a microwave reactor. The solvent was then evaporated under reduced pressure. Precipitation from CHbCb / hexane afforded the intermediate product the as an orange solid (340 mg, 94%). The intermediate (300 mg, 0.39 mmol) was then dried overnight in vacuo in a two-neck 50 mL round-bottom flask equipped with a stirring bar and a reflux condenser. Under an argon atmosphere, anhydrous CHCh (15 mL) and bromotri methylsilane (3.5 mL, 26.7 mmol, 68.0 equiv) were added. The reaction mixture was refluxed at 65 °C for 24 h. The volatiles were evaporated in vacuo after cooling to room temperature. Dry MeOH (3 mL) was added to the residue, the mixture was stirred for 3 h and then dried in vacuo. The crude was purified by precipitation from MeOH / EteO affording [(pbpy)(CO)3ReBr] as a bright orange solid (166 mg, 65%).

[0088] Preparation of SWCNT-Tit -Re

[0089] Fabrication of the SWCNT-TiC Chemiresistor Platform

[0090] The SWCNT-TiO2 hybrid material was prepared according to a modified literature protocol (M. Ding, D. C. Sorescu, A. Star, J. Am. Chem. Soc. 2013, 135, 9015). Under air, a 20 mL crimp vial equipped with a magnetic stir bar was charged with SWCNT (0.5 mL of 1.0 mg / mL in EtOH, freshly sonicated), dry EtOH (16.5 mL), and deionized H2O (50 pL) (remark: It was found that adding 0.3% H2O to the reaction mixture was beneficial for the sensing performance of SWCNT-TiO2-Re.) The vial was sealed with a crimp cap, the dispersion was sonicated for 5 minutes, and the reaction vessel was brought into an argon-filled glovebox. Under vigorous stirring, Ti(O'Pr)4 (50 pL) was added dropwise to the dispersion. The reaction mixture was then removed from the glovebox and further stirred for 15 minutes at 1000 rpm. Next, H2O (3.0 mL) was continuously added to the mixture over the course of 50 minutes using a syringe pump (rate of addition = 0.06 mL / min). The reaction mixture was then further stirred for 15 minutes, and the product was isolated on a nylon membrane filter (0.2 pm), washed with EtOH (3 x 15 mL), and dried under high vacuum. Typically, around 15 mg of SWCNT-TiO2 were obtained after drying under high vacuum. The product (SWCNT-TiO2) was then removed from the nylon filter and dispersed in deionized H2O (20 mL) in a crimp vial. This stock dispersion was stored and used to deposit SWCNT-TiO2 films onto electrodes (Figure S1 B). Prior to each use, the SWCNT-TiO2 dispersion was sonicated for at least 30 minutes. In a typical procedure, the dispersion was loaded into an airbrush (Revolution BR, Iwata) and manually spray-coated onto the gaps of the interdigitated electrode pattern that was placed on a heating plate set to 150 °C (Figure S1A). Aluminum foil was used to mask the electrode pattern and confine the SWCNT-TiO2 film deposition to the desired electrode gap region. The dispersion was sprayed intermittently in short (ca. 0.2 sec) bursts at a distance of about 5 cm from the substrate surface. Pressurized air (2 bar) was used as carrier gas. Successful thin film deposition could be observed by momentary wetting of the IDE pattern. Typically, ca. 20 short bursts of the SWCNT-TiO2 dispersion were sufficient to attain the desired device resistance of 1- 5 kQ. The devices were dried at 180 °C under air overnight.

[0091] It was noticed that extended storage (4 weeks) of SWCNT-TiO2 in the aqueous dispersion led to improved photosensing performance of the resulting active sensing material. This effect may be attributed to a morphology change due to Ostwald ripening. Sensitizer Immobilization on SWCNT-TiC

[0092] Electrodes bearing the SWCNT-TiO2 film were soaked in 2.0 mL of a DMSO solution (0.2 mM) of the sensitizer at room temperature in the dark for 16 h. The devices were then washed by soaking in DMSO (2 x 1 minute) and in deionized water (1 x 10 sec) at room temperature to remove unbound sensitizer, then dried under high vacuum for 90 minutes. When not in use, the electrodes were stored in the dark.

[0093] Synthesis of Organic Sensitizer (5)

[0094] 3-methyl-2-butanone (1.20 equiv),

[0095] The synthesis was adapted from a literature report (Tomasulo, M.; Kaanumal, S. L.; Sortino, S.; Raymo, F. M„ J. Org. Chem. 2007, 72, 595-605.)

[0096] In a 250 mL round-bottom flask equipped with a magnetic stir bar and a reflux condenser, 4- hydrazinobenzoic acid (3.60 g, 23.7 mmol, I .OO equiv), 3-methyl-2-butanone (3.04 mL, 28.4 mmol, 1 .20 equiv), and H2SO4 (0.63 mL, 11 .8 mmol, 0.50 equiv) were dissolved in ethanol (120 mL). The reaction mixture was heated under reflux for 16 h. After cooling to room temperature, the reaction mixture was filtered. A saturated aqueous solution of K2CO3 was added, until pH 4 was reached. The product was then extracted using CH2CI2 (3 x 100 mL) and the combined organic phases were washed using brine, dried using MgSC , and concentrated. 1 was obtained as a beige powder after precipitation from acetone / pentane (3.56 g, 74%).

[0097] The synthesis was adapted from a literature report (Barbero, N.; Magistris, C.; Park, J.; Saccone, D.; Quagliotto, P.; Buscaino, R.; Medana, C.; Barolo, C.; Viscardi, G., Org. Lett. 2015, 17, 3306-3309).

[0098] In a 20 mL crimp vial equipped with a magnetic stir bar, 1 (478 mg, 2.35 mmol, I .OO equiv) and 1- iodooctane (1.70 mL, 9.41 mmol, 4.00 equiv) were dissolved in anhydrous acetonitrile (8.00 mL). The vial was sealed with a crimp cap and the reaction mixture was heated to 155 °C for 45 min in the microwave reactor. Diethyl ether was added to the reaction mixture, until precipitation was observed. The precipitate was filtered off and recrystallized from acetone / diethyl ether. 2 was obtained as a beige solid (279 mg, 27%).

[0099] The synthesis was adapted from a literature report (Barbero, N.; Magistris, C.; Park, J.; Saccone, D.; Quagliotto, P.; Buscaino, R.; Medana, C.; Barolo, C.; Viscardi, G., Org. Lett. 2015, 17, 3306-3309).

[0100] To a 5 mL crimp vial equipped with a magnetic stir bar were added 2,3,3-trimethylindolenine (1.01 mL, 6.28 mmol, 1 .00 equiv) and iodoethane (2.03 mL, 25.1 mmol, 4.00 equiv). The vial was sealed with a crimp cap and the reaction mixture was heated to 155 °C for 45 min in the microwave reactor. The solid was washed with diethyl ether (3 x 10 mL), filtered, and recrystallized from acetone / diethyl ether. 3 was obtained as beige solid (1.51 g, 76%).

[0101] The synthesis was adapted from a literature report (Barbero, N.; Magistris, C.; Park, J.; Saccone, D.; Quagliotto, P.; Buscaino, R.; Medana, C.; Barolo, C.; Viscardi, G., Org. Lett. 2015, 17, 3306-3309).

[0102] In a 20 mL crimp vial equipped with a magnetic stir bar, 2 (1.20 g, 2.71 mmol, 1.00 equiv) and 3,4- diethoxycyclobut-3-ene-1, 2-dione (0.80 mL, 5.41 mmol, 2.00 equiv) were dissolved in ethanol (17 mL) and EtsN (0.68 mL). The vial was sealed with a crimp cap and the reaction mixture was heated to 90 °C for 45 min in the microwave reactor. The reaction mixture was concentrated and purified by silica gel flash column chromatography (0-4% MeOH in CH2CI2). 4 was obtained as a yellow-golden solid (813 mg, 68%).

[0103] The synthesis was adapted from a literature report (Barbero, N.; Magistris, C.; Park, J.; Saccone, D.; Quagliotto, P.; Buscaino, R.; Medana, C.; Barolo, C.; Viscardi, G., Org. Lett. 2015, 17, 3306-3309). In a 20 mL crimp vial equipped with a magnetic stir bar, 4 (100 mg, 0.23 mmol, 1.00 equiv) and 3 (72 mg, 0.23 mmol, 1.00 equiv) were dissolved in toluene (2.50 mL) and 1 -butanol (2.50 mL). The vial was sealed with a crimp cap and the reaction mixture was heated to 160 °C for 25 min in the microwave reactor. The reaction mixture was concentrated and purified by silica gel flash column chromatography (0-4% MeOH in CH2CI2), followed by recrystallization from acetone / diethyl ether. Organic sensitizer 5 was obtained as a blue sparkly solid (55 mg, 42%).

[0104] Preparation of SWCNT-TiC - organic sensitizer 5

[0105] The introduction of the organic sensitizer to the SWCNT-TiO2 composite is in analogy to SWCNT-TiO2-Re.

[0106] O2 Detection Measurements

[0107] Chemiresistive sensing measurements were carried out using a custom-made sensing platform. Interdigitated electrodes bearing the SWCNT-TiO2-Re film were inserted into a custom-made sensing board containing a 2 * 17 pin edge connector (TE Connectivity: CONN SEC II 17 POS 100C / L, Figure S16A). The edge connector containing the electrodes was then sealed inside of a custom-made PTFE chamber featuring a sealed glass window and openings for gas inlet and exhaust. The sensing board was connected to an Agilent Keysight DAQ970A data collection unit equipped with a DAQM901A 20 channel multiplexer (2 / 4 wire) module using a ribbon cable. Using Agilent BenchLink Data Logger 3 software, resistance values were recorded during the sensing measurements. The scan rate was set to 1 scan per second.

[0108] Gases were introduced to the PTFE chamber by connection to a gas flow-controlling platform via stainless steel tubing (Swagelok). Gas flow and concentration were adjusted using mass-flow controllers (MFCs, Alicat Scientific). Three MFCs (MC-5SLPM-D / 5V) were used to deliver the carrier gas (N2) at controlled relative humidity with a total flow rate of 1.00 L / min. Relative humidity was adjusted by controlling the relative ratios of dry and water-saturated carrier gas streams. A humidity sensor (Sensirion: SEK-SHT45- AD1 B-SENSORS) was used to calibrate relative humidity. The analyte gas concentration was controlled using a finely adjustable Alicat Scientific MFC (MC-100SCCM-D / 5V).

[0109] The electrodes were irradiated by an LED through the sensing chamber glass window from a distance of 2.0 cm. The LED was purchased from LCFOCUS (LC-10RGB-C30). The LED current was set to 0.08 A for all measurements using a VOLTCRAFT LPS 1305 power supply, which corresponds to an irradiance of approximately 200 mW / mm2.

[0110] The change in device resistance resulting from O2 exposure was converted to the normalized change in resistance [DR / Ro = (RRo) / Ro 100%; Ro = initial resistance]. All sensing data is reported as an average of at least 3 separate devices and is depicted with shaded areas (for sensing traces) and error bars (for bar graphs) to show standard deviations.

[0111] Prior to the start of sensing experiments, the devices were equilibrated under 1.00 SLM carrier gas flow and light irradiation for 15 minutes to ensure a stable baseline. A drift correction was applied in cases where the absolute response was of interest using linear regression fitting of the baseline.

[0112] Control Experiments

[0113] NH3Detection Measurements

[0114] Chemiresistive NH3 detection measurements were carried out in analogy to O2 with the exception that analyte concentration was controlled using a finely adjustable corrosive-resistant Alicat Scientific MFC (MCS-100SCCM-D-DB9M-PCV03 / 5M).

[0115] The NH3 experiments illustrated in Figure 10 reveal that the sensor is capable of detecting NH3, albeit with an inverted signal, indicating a different detection mechanism from that used for oxygen. Further investigations have confirmed that the SWCNT-TiO2 system can identify NH3 even without a photosensitizer and independent of light conditions.

[0116] Systematic Omission of Chemiresistor Components

[0117] The O2 detection performance of the active sensing material SWCNT-TiO2-Re under green light illumination was compared to modified versions that omitted one or more components. Application of TiO2 (without SWCNT), prepared by the sol-gel method described above (“Preparation of SWCNT-TiO2-Re”) leads to no measurable conductance, and the same holds true for TiO2-Re. Therefore, no chemiresistive sensing experiment could be performed. In the case of SWCNT-TiO2, a response to 1000 ppm of O2 is observed. However, this response does not show any reversibility, meaning that once the sensor has reacted to a certain level of the substance, it does not return to its original state after the exposure is removed. Instead, the sensor provides a cumulative measure of exposure rather than a reversible detection capability. SWCNT-Re refers to electrodes bearing SWCNTs that were soaked in a DMSO solution of [(pbpy)(CO)3ReBr] overnight, then washed, and dried according to the method described above (“Preparation of SWCNT-TiO2-Re”). Figure 12A shows the normalized and drift-corrected resistance traces for 2 min exposure of SWCNT-TiO2-Re to 1000 ppm O2 compared to the control devices. Figure 12B shows data represented as bar graphs. Shaded areas and error bars represent standard deviations (N = 4 chemiresistors); all data were collected under green light irradiation at room temperature using dry nitrogen carrier gas (R.H. = 0%).

Claims

Patent Claims1 . A sensor configured to detect O2 as target analyte in the presence of visible light, providing a reversible response to O2 concentration changes, wherein the sensor comprises: two electrodes; and a sensing composite arranged between the two electrodes; wherein the sensing composite comprises: one or more carbon-based nanostructures having an outer surface; one or more metal-oxides functionalized to the outer surface of the one or more carbon-based nanostructures; and one or more photosensitizer capable of harvesting visible light, thereby inducing a photoconductance change in the underlying metal-oxide functionalized to the one or more carbonbased nanostructure, wherein said photosensitizers are functionalized to the outer surface of the one or more carbon-based nanostructures and / or the one or more metal-oxides.

2. The sensor according to claim 1 , wherein the photosensitizer comprises a metal organic complex.

3. The sensor according to any of the preceding claims, wherein the one or more carbon-based nanostructures comprises single-walled carbon nanotubes, graphene, fullerenes, heterofullerenes, carbon nanodots, carbon nanoparticles, graphite, multi-walled carbon nanotubes, or combinations thereof, preferably single-walled carbon nanotubes.

4. The sensor according to any of the preceding claims, wherein the sensor is configured to detect the target analyte based on a change in resistance, conductance, capacitance, voltage, or current, preferably resistance.

5. The sensor of claim 4, wherein the visible light has a wavelength ranging from 380 nm to 800 nm, preferably 500 nm to 570 nm.

6. The sensor of any of the preceding claims, wherein the one or more metal-oxides comprises one or more of titanium dioxide (TiC ), tin oxide (SnO2), zinc oxide (ZnO), zinc tin oxide (ZnxSnxOx), nickel oxide (NiO), copper oxide (CuxOx), cobalt oxide (CoxOx), iron oxide (FexOx), zinc magnesium oxide (Zni-xMgxO), magnesium oxide (MgO), vanadium oxide (VxOx), niobium oxide (NbxOx), tantalum oxide (TaxOx), molybdenum oxide (MoxOx), tungsten oxide (WxOx), indium oxide (lnxOx), gallium oxide (GaxOx), palladium oxide (PdO), zirconium oxide (ZrO2), aluminum oxide (AI2O3), strontium oxide (SrO), strontium titanium oxide (SrTiOs), lanthanum oxide (La2O3), cerium oxide (CexOx),praseodymium oxide (PrxOx), promethium oxide (P1TI2O3), samarium oxide (SrrteOs), europium oxide (EU2O3), gadolinium oxide (Gd20s), terbium oxide (TbxOx), dysprosium oxide (Dy20s), holmium oxide (HO2O3), erbium oxide (Er20s), thulium oxide (T1TI2O3), ytterbium oxide (Yb2O3), lutetium oxide (LU2O3), and combinations thereof.

7. The sensor of any of the preceding claims, wherein the sensor is configured to detect O2 at a temperature ranging from -50 °C to 200 °C, preferably from 18 °C to 40 °C.

8. The sensor of any of the preceding claims, wherein the sensor is configured to detect O2 at a concentration of up to 1’000’000 ppm, alternatively from 1 ppm to 200’000 ppm.

9. The sensor of any of the preceding claims, wherein the sensor is configured to detect O2 via photosensitization.

10. The sensor of any of the preceding claims, wherein the one or more photosensitizers comprises a metal-organic complex comprising a metal selected from the group comprising rhenium, manganese, zirconium, vanadium, chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, cerium or combinations thereof, preferably rhenium; and an organic functional group comprising a pyridine, a bipyridine, a terpyridine, an imine, a diimine, a phenyl, a biphenyl, a terphenyl, a tri-arylamine, an indoline, a coumarin, a tetrahydroquinoline, a porphyrin, a phthalocyanine, an anthracene, a heteroanthracene, a carbazole, a heterocyclic amine, a carbene, a carbodiimide, an amide, an amine, a N, N-dialkylaniline, a guanidinate, a hemicyanine, a merocyanine, a squaraine, a perylene, a boron-dipyrromethene, an alkyne, an alkene, a thiophene, a carboxylic acid, a phosphonic acid, a sulfonic acid, a halogen, a carbonyl, a cyanide, an isocyanide, an isothiocyanide, a phosphine, an arsine, and combinations thereof; or combinations including the one or more metals and the one or more organic functional groups thereof.11 . The sensor of any of the preceding claims, wherein the sensing composite is arranged as a film over the sensing region; wherein the one or more carbon-based nanostructures having the one or more metal-oxide nanoparticles and the one or more photosensitizers functionalized thereto are substantially uniformly dispersed within the film.

12. The sensor of any of the preceding claims, wherein the size of the one or more carbon-based nanostructures ranges from 1 nm to 100 pm, preferably from 100 nm to 500 nm.

13. The sensor according to any of the preceding claims, wherein the sensing composite comprises a single-walled carbon nanotubes (SWCNT), TiO2 functionalized to the outer surface of the single-walled carbon nanotubes; and a metal organic complex as photosensitizer.

14. The sensor according to claim 13, wherein the metal organic complex is (4,4’-[P(O)(OH)2]2-2,2’- bipyridine)(CO)3Re(Br).

15. The use of the sensor of any of the preceding claims in a vehicle, water quality monitoring system, air quality monitoring system, soil respiration analysis, food spoilage detection system, industrial quality and process control, respiratory protective equipment, rebreathers, medical equipment, and combinations thereof.

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