WO2015193645A1

WO2015193645A1 - Gas sensors and gas sensor arrays

Info

Publication number: WO2015193645A1
Application number: PCT/GB2015/051716
Authority: WO
Inventors: Victor Higgs
Original assignee: Applied Nanodetectors Ltd.
Priority date: 2014-06-19
Filing date: 2015-06-11
Publication date: 2015-12-23
Also published as: GB2527340A; GB201410931D0

Abstract

The present invention relates to gas sensors and gas sensor arrays, and methods of their formation. The gas sensors and gas sensor arrays according to the present invention comprise nanomaterials.

Description

Title: Gas sensors and gas sensor arrays

Description of Invention The present invention relates to gas sensors and gas sensor arrays. More particularly, the present invention relates to gas sensors and gas sensor arrays for detecting volatile organic compounds. The present invention also relates to a method of forming gas sensors and gas sensor arrays. Chemical analysis of the human breath is a technique that could be used as a non-invasive tool for early disease detection and diagnosis. The detection of volatile organic compounds (VOCs) in the breath is of particular interest for clinical diagnostics and the self-management of chronic respiratory conditions. There are approximately more than 200 different compounds that have been detected in human breath, some of which have been correlated, either on their own or in combination with other compounds, to various diseases. Laboratory based analysis of exhaled air is a complex, expensive and time consuming process and thus is not in wide spread use in occupational medicine. Currently, metal oxide based gas sensors are used to monitor different VOCs due to their response to different types of gases and low cost. Such metal oxide based gas sensors do not include nanomaterials. Metal doping of gas sensors has been used to enhance metal oxides catalytic activity towards a particular gas or gases. However, these types of gas sensors have a number of limitations in terms of poor sensitivity and selectivity. In addition, these types of gas sensors must be heated to temperatures up to 400°C to obtain optimum performance, which can involve relatively high electrical power consumption (for example 200 mW); this limits their application in portable devices. There is a need for low power and low cost gas sensors which can be used in hand-held portable devices, enabling near patient testing and home monitoring using breath analysis. According to a first aspect of the present invention, there is provided a method of forming a gas sensor, the method comprising:

providing a first solution of nanowires, the solution comprising a solvent and nanowires;

placing the first solution of nanowires on a first set of at least two electrodes;

applying an AC (alternating current) voltage signal to the first set of at least two electrodes; and,

permitting the solvent to evaporate. In another aspect of the present invention, there is provided a method of forming a gas sensor array, the method comprising the steps above and further comprising:

providing a second solution of nanowires, the solution comprising a solvent and nanowires;

placing the second solution of nanowires on a second set of at least two electrodes;

applying an AC (alternating current) voltage signal to the second set of at least two electrodes; and,

permitting the solvent to evaporate.

Preferably, further comprising:

repeating the steps of claim 1 or claim 2 for any one of 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 different solutions of nanowires, each repeat on different sets of at least two electrodes. Further preferably, further comprising the step of:

prior to placing the first solution of nanowires and the second solution of nanowires on the first set of at least two electrodes, the second set of at least two electrodes or the first and second set of at least two electrodes, placing a barrier layer around the electrodes to prevent the solutions of nanowires on each set of electrodes from mixing.

Advantageously, wherein the barrier layer has a void around the area of the electrodes to form each gas sensor.

Preferably, wherein the barrier layer is formed of perspex.

Further preferably, further comprising the step of removing the barrier layer when the solvent has evaporated.

Advantageously, wherein the nanowires have: a length (longest dimension) of from 20 nm to 15 pm, preferably from 1 pm to 15 pm; and, a width (shortest dimension) of from 1 nm to 300 nm, preferably from 1 nm to 100 nm, 50 nm or 10 nm; and, an aspect ratio (length divided by width) of from 20 to 50, preferably to 40 or 30.

Preferably, wherein the nanowires are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂0₅ and GaN, or combinations thereof.

Further preferably, wherein the solutions of nanowires further comprise nanoparticles.

Advantageously, wherein the nanoparticles have: at least one dimension from 1 nm to 100 nm, preferably from 1 nm to 10 nm; and, an aspect ratio (length divided by width) of from 1 to 4, preferably from 1 to 2. Preferably, wherein the nanoparticles are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂0₅ or GaN, or combinations thereof. Further preferably, wherein the solution of nanowires and nanoparticles is mixed using an ultrasonic bath.

Advantageously, wherein the at least two electrodes, in each case, are individually electrically isolatable and metallic; and/or wherein the electrodes are spaced by from 1 pm to 20 pm; optionally from 1 pm to 2 pm, 5 pm or 10 pm.

Preferably, wherein each electrode is formed of 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 prongs.

Further preferably, wherein two or more of the prongs of each electrode interdigitate with two or more of the prongs of another electrode. Advantageously, wherein the solvent is water, ethanol or isopropanol; and/or wherein the electrodes rest on a substrate; optionally wherein the substrate is formed of glass, silicon or plastic; optionally wherein the plastic is polyethylene terephthalate or polyethylene naphthalate. In another aspect of the present invention, there is provided a gas sensor or a gas sensor array obtainable by a method according to any one of the above methods.

In another aspect of the present invention, there is provided a gas sensor comprising:

at least two electrodes, and, nanowires arranged between the at least two electrodes, wherein the nanowires have a density of from 10⁶ nanowires/cm³ up to and including 10¹⁴ nanowires/cm³ Preferably, wherein the nanowires have a density of from 10⁶ nanowires/cm³, optionally greater than 10⁷ nanowires/cm³, optionally greater than 10⁸ nanowires/cm³, optionally greater than 10⁹ nanowires/cm³, optionally greater than 10¹⁰ nanowires/cm³, optionally greater than 10¹¹ nanowires/cm³, optionally greater than 10¹² nanowires/cm³, optionally greater than 10¹³ nanowires/cm³, up to and including 10¹⁴ nanowires/cm³.

Further preferably, wherein the nanowires are arranged in an open nanowire network. Advantageously, wherein the nanowires have: a length (longest dimension) of from 20 nm to 15 pm, preferably from 1 pm to 15 pm; and, a width (shortest dimension) of from 1 nm to 300 nm, preferably from 1 nm to 100 nm, 50 nm or 10 nm; and, an aspect ratio (length divided by width) of from 20 to 50, preferably to 40 or 30.

Preferably, wherein the nanowires are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂0₅ and GaN, or combinations thereof. Further preferably, wherein the gas sensor further comprises nanoparticles mixed with the nanowires.

Advantageously, wherein the nanoparticles have: at least one dimension from 1 nm to 100 nm, preferably from 1 nm to 10 nm; and, an aspect ratio (length divided by width) of from 1 to 4, preferably from 1 to 2. Preferably, wherein the nanoparticles are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂0₅ or GaN, or combinations thereof. Further preferably, wherein the at least two electrodes, in each case, are individually electrically isolatable and metallic; and/or wherein the electrodes are spaced by from 1 pm to 20 pm; optionally from 1 pm to 2 pm, 5 pm or 10 pm. Advantageously, wherein each electrode is formed of 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 prongs. Preferably, wherein two or more of the prongs of each electrode interdigitate with two or more of the prongs of another electrode; and/or wherein the electrodes rest on a substrate; optionally wherein the substrate is formed of glass, silicon or plastic; optionally wherein the plastic is polyethylene terephthalate or polyethylene naphthalate.

In another aspect of the present invention, there is provided a gas sensor array comprising two or more gas sensors according to any one of the above.

Preferably, wherein the gas sensor array comprises 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 different gas sensors.

In another aspect of the present invention, there is provided a method of measuring the presence of a volatile organic compound, comprising:

exposing a gas sensor or a gas sensor array according to any one of the above to one or more volatile organic compounds; and, measuring the change in conductance of the gas sensor or gas sensor array.

Preferably, further comprising the step of:

prior to exposing the gas sensor or gas sensor array to one or more volatile organic compounds, exposing the gas sensor or gas sensor array to UV light.

Further preferably, wherein the UV light has a wavelength greater than the band gap of the nanomaterial on each particular gas sensor.

Embodiments of the invention are described below with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an array of gas sensors according to the present invention.

Figure 2 is a schematic representation of two alternative sets of electrodes prior to forming arrays of gas sensors according to the present invention.

Figure 3 is a top-plan view of an array of four gas sensors according to the present invention.

Figure 4 is a reflected light optical micrograph of four alternative ZnO nanowire gas sensors, each of Figures 4a-4d having different densities of nanowires.

Figure 5 shows the response of gas sensors according to the present invention to different volatile organic compounds. Figure 6 shows the response of another gas sensor according to the present invention to different volatile organic compounds. Figure 7 shows the response of other gas sensors according to the present invention to different volatile organic compounds.

Figure 8 shows the response of other gas sensor arrays according to the present invention to different volatile organic compounds after performing a principal component analysis to the response data.

Figure 9 shows the change in a sensor's response for different power densities.

Some of the components of the gas sensors of the present invention, together with their sources, are set out below.

Nanomaterials

Nanomaterials are materials which are sized in at least one dimension from 1 to 300 nm, preferably from 1 to 100 nm. Examples of nanomaterials include, but are not limited to, carbon nanotubes, buckminsterfullerene, nanowires, nanoparticles, nanopowders, nanobelts and nanocrystals. Compared to larger scale materials, nanomaterials have a high ratio of surface area to volume; this results in nanomaterials having different properties compared to larger scale materials.

Nanowires

Nanowires are defined in this specification as nanomaterials which have a length (longest dimension) of from 20 nm to 15 pm, preferably from 1 pm to 15 pm, and a width (shortest dimension) of from 1 nm to 300 nm, preferably from 1 nm to 100 nm, 50 nm or 10 nm, and an aspect ratio (length divided by width) of from 20 to 50, preferably to 40 or 30. Preferred nanowires are generally cylindrical, although they can have different geometric shapes, for example rectangular. If the nanowires are not cylindrical, the aspect ratio is calculated as the length divided by the greatest dimension across a cross-section of the nanowire.

Nanowires used in the present invention can be metallic (for example Au, Pt, Cu, Ag, Co, Fe and Ni) or semiconducting (for example Si, SnO₂, WO3, ZnO, TiO₂, InP, CuO, Cu₂O, NiO, MnO₂, V₂O₅ and GaN), but not insulating.

Nanoparticles Nanoparticles are defined in this specification as particles having at least one dimension from 1 nm to 100 nm, preferably from 1 nm to 10 nm, and an aspect ratio (length divided by width) of from 1 to 4, preferably from 1 to 2. Nanoparticles used in the present invention can be metallic (for example Au, Pt, Ag, Fe, Pd, Rh, ZnS and CdSe) or semiconducting (for example Si, SnO₂, WO3, ZnO, TiO₂, InP, CuO, Cu₂O, NiO, MnO₂, V₂O₅ and GaN), but not insulating.

Open nanowire network Open nanowire networks are arrangements of nanowires which are physically and electrically connected to one or more other nanowires in the network to form an open, highly branched, porous, intertwined structure. Open nanowire networks are discussed, for example, on page 404, in Advances in Nanotechnology Research and Application: 2013 Edition, Edited by Q. Ashton Acton, PhD, published by ScholarlyEditions™, Atlanta, Georgia, 2013.

The present invention concerns including nanowires in gas sensors. The inventors found that including nanowires in gas sensors provides the sensors with useful properties. Without wishing to be bound by theory, some of the properties of the nanowires are thought to arise from the relatively large surface to volume ratio (compared with conventional, larger scale materials). Sensors according to the present invention generate enough electrical power to self-heat themselves when exposed to UV light, due to photo-desorption of surface molecules and do not require external heaters.

Another aspect of including nanowires in gas sensors is their fast response and recovery after sensing any particular gas; this is linked to the large nanowire surface area. This is a direct advantage over porous metal oxide based sensors which have diffusion limited gas reactions, affecting their response and recovery. Although simple single nanowire sensor devices have been demonstrated it is difficult and expensive to make them in a scalable way.

The present inventors fabricated gas sensor arrays which are composed of different metal oxides (for example ZnO and T1O2) nanowires, optionally doped with Au and Ag nanoparticles (<10 nm). These arrays may be optically excited using UV light (365 nm) to use the photocatalytic action of the metal doped metal oxides to detect and differentiate between different VOCs. Alternatively, the gas sensor arrays of the present invention may be excited by the application of heat.

Gas sensors according to the present invention undergo the photocatalytic effect when exposed to UV light. When a photocatalytic material is illuminated by light greater than its band gap energy (hv≥Eg), the photocatalyst generates electron/hole pairs which diffuse out to the surface of the photocatalytic material, and start a number of chemical reactions. According to certain embodiments of the present invention, a UV LED bulb producing a wavelength of 365 nm (hv=3.39eV) is used; the band gap of Ti0₂=3.2eV, and ZnO=3.2eV. If other materials are used with different band gaps, alternative wavelengths of UV or visible light may be used to produce the photocatalytic effect. In certain embodiments, T1O2 and ZnO are used as semiconductor photocatalysts because of their high photosensitivity, photochemical stability, large band gap, strong oxidizing power and low cost. A wide range of oxide materials have been studied for photoelectrochemical properties, including WO3 (band gap 2.7eV), Fe₂0₃ (band gap 2.2eV), CuO (band gap 1 .6 eV) and Cu₂O (band gap 2.3 eV). Gas sensor preparation

Figure 1 shows an array 10 of four gas sensors A, B, C and D according to the present invention. The array 10 comprises an array of electrodes 1 ,2,3,4,5,6,7,8. The electrodes 1 ,2,3,4,5,6,7,8 are metallic, preferably formed of Pt or Au. The electrodes 1 ,2,3,4,5,6,7,8 are connected to a voltage source 50, such that a voltage can be applied between two or more electrodes. Any one or more of electrodes 1 ,2,3,4,5,6,7,8 may be formed of a plurality of prongs, which interdigitate with another electrode having a plurality of prongs (shown in more detail with reference to Figure 2B).

Figure 1 shows the array 10 once formed, that is each sensor A, B, C and D comprises a nanomaterial, which can be a different nanomaterial for each sensor, or the same for two or more of the sensors, the nanomaterial forming an electrical link between a pair of electrodes. In Figure 1 , sensor A comprises electrodes 1 and 2 with nanomaterial 30 forming an electrical link between electrodes 1 and 2. Sensor B comprises electrodes 3 and 4 with a nanomaterial 20 (which can be a different nanomaterial to nanomaterial 30) forming an electrical link between electrodes 3 and 4. Sensor C comprises electrodes 5 and 6 with a nanomaterial (which can be a different nanomaterial to nanomaterials 20 and 30) forming an electrical link between electrodes 5 and 6. Sensor D comprises electrodes 7 and 8 with a nanomaterial (which can be a different nanomaterial to nanomaterials 20 and 30) forming an electrical link between electrodes 7 and 8. In this embodiment, the sets of electrodes rest on a generally inert substrate 40, which can be composed of glass. Alternatively, the substrate 40 can be composed of a silicon substrate or a plastics substrate, for example polyethylene terephthalate (PET) or polyethylene naphthalate (PEN).

In this embodiment there are four sensors A, B, C and D in one array. In other embodiments, there can be two, three, five, six, seven, eight, nine, ten or more sensors in any one array. In other embodiments, each sensor A, B, C and D may comprise more than one prong of each electrode, for example 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 prongs of each electrode.

Figure 2A shows the array 10 of Figure 1 , prior to placing nanomaterials between the pairs of electrodes. In Figure 2A, a nozzle 200 is located above the array 10. The nozzle 200 can provide a desired amount of a nanomaterial or nanomaterials, optionally in a solvent. The solvent is preferably water, ethanol or isopropanol.

Figure 2B shows an alternative array 101 , prior to placing nanomaterials between the pairs of electrodes, which is similar to the array shown in Figure 2A, with each electrode 1 1 , 21 , 31 , 41 , 51 , 61 , 71 , 81 formed of a plurality of interdigitated prongs. Each gas sensor A1 , B1 , C1 and D1 , once formed, of Figure 2B comprises two electrodes. In figure 2B, each electrode is formed of four prongs which interdigitate with the four prongs of a corresponding electrode (for example the four prongs of electrode 1 1 interdigitate with the four prongs of electrode 21 ). Nozzle 200 can provide a desired amount of nanomaterial to each pair of electrodes to form gas sensors A1 , B1 , C1 and D1 .

In Figure 3, a gas sensor array 102 is shown in plan view. The array comprises gas sensors A2, B2, C2 and D2. For clarity, only electrodes 12 and 22 of sensor A2 are numbered in Figure 3. The electrodes 12 and 22, in this embodiment, each comprises 31 prongs (not shown). Each gas sensor A2, B2, C2 and D2 comprises a pair of electrodes, each electrode being individually electrically isolatable and each electrode comprising a plurality of prongs interdigitated with another electrode. The electrode prongs may be separated by any nanometre scale distance. In the embodiment of Figure 3, the electrode prongs are each 5pm in width and are separated from an interdigitated electrode prong by 5pm.

Figure 3 shows the dimensions of the different components of the array 102, in this embodiment. Around sensor A2 is a circle 300. The circle 300 shows a thin layer of organic polymer, for example perspex, (a non-limiting example of a barrier layer) which is present on the majority of the area of the array 102 but not present within each of the circles 300 et al. This void within the sensor structure permits the nozzle 200 to place a solution of nanomaterial onto any particular set of electrodes without it contaminating any other gas sensor being formed on the same array 102. The organic polymer can be peeled away after the solvent has evaporated from each sensor.

Nanowires (for example ZnO or T1O2) were deposited at each sensor (A,B,C,D or A1 ,B1 ,C1 ,D1 or A2,B2,C2,D2) as required. In certain non-limiting embodiments, the nanowires were dispersed in water, ethanol or isopropanol before being deposited at each sensor through the nozzle 200.

In one embodiment, the dispersions were produced by placing the desired nanowires into a container. Solvent was then added to the container and the mixture was optionally placed in an ultrasonic bath for 5 minutes. The dispersions were decanted and filtered using a PTFE filter and then centrifuged to remove any particles with dimensions greater than 15 pm, optionally greater than 1 pm, from the dispersions. When using ZnO or TiO₂ nanowires in water, according to one embodiment, the solution was well dispersed and almost clear. The concentration of nanowires in one exemplary embodiment ranged from 0.01 mg/ml to 100 mg/ml, preferably from 0.1 mg/ml to 10 mg/ml. If nanoparticles were to be added to a solution of nanowires to prepare a particular gas sensor, nanoparticles (for example, Au or Ag nanoparticles) were dispersed in water or ethanol. The Ag or Au nanoparticles solutions were added to the nanowire dispersions and mixed using an ultrasonic bath, for approximately 5 minutes. This method permitted the nanowires to be mixed with a uniform density of nanoparticles. When a sensor comprised a mixture of nanowires and nanoparticles, this was found to enhance the photocatalytic activity of the particular sensor.

The resulting solution (nanowires solution, or, nanowires and nanoparticles solution) was then drop-cast onto interdigitated electrodes from a nozzle 200 (as shown in Figure 2). In some embodiments, the electrodes had a fixed metal electrode width and spacing, for example an electrode width and spacing from 1 pm to 20 pm, preferably from 1 pm to 2 pm, 5 pm or 10 pm.

When the droplet was placed onto the electrodes, the electrodes were biased with an AC (alternating current) voltage signal (1 -100 kHz) and 1 -10 V amplitude, which was optimized depending on the nanomaterial type to provide order to the arrangement of the nanowires between the electrodes. Applying an AC voltage generated an alternating electrostatic force on the nanowires in the solution, which could be altered to vary the alignment of the nanowires between the electrodes. The application of an AC voltage signal to nanowires in a solvent was described in J. Suehiro, Nanotechnology 17 (2006) 2567-2573, and is referred to as dielectrophoresis.

In the embodiment described with reference to Figure 1 , each sensor A, B, C, D is composed of three dimensional arrangements of metal oxide nanowires (20 and 30, for example) which are arranged on a glass substrate 40. Each substrate (10x6 mm in the embodiment shown in Figure 3) contains four individually isolatable interdigitated electrodes (referred to as 1 ,2,3,4,5,6,7,8 in Figure 1 ). The resistance across each pair of electrodes (1 and 2, 3 and 4, 5 and 6, 7 and 8) can be measured when the array 10 is exposed to a sample of gas to be measured. Depending on the particular nanomaterial present between each pair of electrodes, different readings of resistance are measured depending on the test substance. On this basis, the readings from each sensor A, B, C and D can be measured to ascertain which substances are present in any given sample.

To create a three dimensional arrangement of nanowires which have an open nanowire network, the gap between electrodes is greater than the average nanowire length. During application of the AC (alternating current) voltage signal, the nanowires are attracted to the AC field around the electrode and align to the metal electrode in an open nanowire network. By increasing the density of nanowires in the dispersion a two dimensional arrangement develops that is angular to the metal electrodes.

Reflected light optical micrographs of a ZnO nanowire sensor structure are shown in Figures 4a-d. The micrographs of Figures 4a-d show sensor structures with 5 pm electrode with and 5 pm gap. The samples were placed on the optical stage of a metallurgical microscope and illuminated using a standard tungsten light source. The images were obtained using a GXML3003, Trinocular reflected light metallurgical microscope, GX Microscopes, and captured using a GXCAM-3 Digital microscope camera (Resolution 2048x1536). The density of nanowires used in the dispersion was increased, starting with a low density in Figure 4a to the highest density in Figure 4d (Number of nanowires in each starting solution: A=1 mg/cm³=4.86x10¹² nanowires per cm³, B=2mg/cm³=9.71x10¹² nanowires per cm³, C=4mg/ cm³=1.94x10¹³ nanowires per cm³, D=5mg/cm³ =2.43x10¹³ nanowires per cm³), to determine optimum sensor performance. Metal oxide nanowires cannot usually be imaged in an optical microscope because the nanowires are below the optical resolution (<1 pm) and the features observed in the micrographs are interference related and do not accurately relate to the true density. However, the images of Figure 4 can be used to understand alignment of the nanowires. The optical features observed in the microscope change in alignment and density (two-dimensional density of the nanowires after solvent evaporated Fig 4a=1 .23x10⁶/cm², 4b=5.15x10⁶/cm² 4c=1 .84x10⁷/cm² 4d greater than 10⁷/cm²).

In the embodiment of Figure 4, the micrographs were taken from sensors with different coverage of nanowires. The metal electrode spacing was 5 pm and the average length of ZnO nanowires was 3 pm with a diameter of 70 nm. At low concentrations (Figure 4a) of ZnO nanowires the nanowires connected to the metal electrodes at an angle and start to bridge each other. As the density of nanowires increases two dimensional arrangements formed. Sensors containing different densities of nanowires were then exposed to different volatile organic compounds to determine the optimum sensor structure. The most sensitive array sensors contained a high density (three-dimensional density of from 10⁶ nanowires/cm³, optionally greater than 10⁷ nanowires/cm³, optionally greater than 10⁸ nanowires/cm³, optionally greater than 10⁹ nanowires/cm³, optionally greater than 10¹⁰ nanowires/cm³, optionally greater than 10¹¹ nanowires/cm³, optionally greater than 10¹² nanowires/cm³, optionally greater than 10¹³ nanowires/cm³, up to and including 10¹⁴ nanowires/cm³), of crossed nanowires having an open nanowire network as shown in Figure 4d. Without wishing to be bound by theory, it is thought that the ordering of the nanowires by application of an AC (alternating current) voltage signal coupled with a high density of nanowires, provides the greatest surface area for volatile organic compounds to adsorb to, therefore providing high sensitivity of the gas sensors. The order of the nanowires is described as an open nanowire network.

Gas exposure tests

A gas sensor array was loaded into a stainless steel gas chamber. A fixed voltage (from 1 to 5V) was applied to the sensor electrodes to measure the nanomaterial conductance change during gas exposure. The gas sensor array included, in different experiments, two or more gas sensors, each gas sensor as described with reference to Figure 3 and having (in all cases at a three- dimensional density of from 4.86x10⁸ to 2.45x10¹⁰ nanowires/cm³ or (nanowires + nanoparticles)/cm³, respectively): ZnO nanowires; ZnO nanowires doped with Au nanoparticles; ZnO nanowires doped with Ag nanoparticles; TiO₂ nanowires doped with Au nanoparticles; and/or, TiO₂ nanowires doped with Ag nanoparticles. The sensors were excited using an ultraviolet light emitting diode (365 nm wavelength) which had a controlled light intensity output and the gas sensing measurements were made at room temperature. The light intensity was measured using a commercially available silicon carbide photodetector, namely, a sglux GmbH TOCON A6.

The sensors were then exposed to pure nitrogen or zero air (zero air being ambient air filtered to contain less than 0.1 parts per million of total hydrocarbons) to produce a background gas measurement for the sensing experiments. Different gas mixtures were produced for gas sensor testing each mixture contained VOCs or inorganic gases in nitrogen. The gas sensors were then exposed to a sample gas mixture and their response recorded. The response of each sensor element was recorded and the experiment was repeated five times to calculate their average response. At a molecular level, the gas sample caused a change in conductivity of any particular gas sensor on adsorption to the sensor.

The sensor response was calculated using the following equation.

S=(G1 -G0)/G1

Where G1 and GO represent the conductance under and before the exposure to different gas mixtures.

Figure 5 shows a comparison between two gas sensors, one comprising nanomaterial including ZnO nanowires, and the other including ZnO nanowires doped with Ag nanoparticles. In this example, the gas sensors included: ZnO nanowires; and, ZnO nanowires doped with Ag nanoparticles (at 4.86x10 to 2.45x10¹⁰ nanowires/cm³ or (nanowires + nanoparticles)/cm³, respectively). Figure 5 shows that peaks are observed when each of ethanol, isopropyl alcohol (IPA) and acetone are introduced.

Figure 6 shows a comparison between two gas sensors, both comprising nanomaterial ZnO nanowires doped with Au nanoparticles. In this example, the gas sensors included: ZnO nanowires doped with Au nanoparticles (at 4.86x10⁸ to 2.45x10¹⁰ nanowires/cm³ or (nanowires + nanoparticles)/cm³, respectively). Figure 6 shows that peaks are observed when each of ethanol, formaldehyde, acetone and isopropyl alcohol (IPA) are introduced. The peaks are qualitatively the same for both gas sensors. The recovery and responses are similar for each sensor and the sensor recovery is very different for each chemical. This transient recovery can yield further information useful in understanding the sensed chemical's interactions with the sensor.

Figure 7 shows a comparison between two gas sensors, one comprising nanomaterial T1O2 nanowires, and the other including T1O2 nanowires doped with Au nanoparticles. In this example, the gas sensors included: T1O2 nanowires; and, TiO₂ nanowires doped with Au nanoparticles (at 4.86x10⁸ to 2.45x10¹⁰ nanowires/cm³ or (nanowires + nanoparticles)/cm³, respectively). Figure 7 shows that peaks are observed when each of ethanol, acetone and isopropyl alcohol (IPA) are introduced. Figure 8 was produced by providing an array having four gas sensors; namely, a gas sensor comprising nanomaterial ZnO nanowires doped with Au nanoparticles; a gas sensor comprising nanomaterial T1O2 nanowires doped with Ag nanoparticles; a gas sensor comprising nanomaterial TiO₂ nanowires doped with Au nanoparticles; and, a gas sensor comprising nanomaterial ZnO nanowires (without nanoparticles) (all at 4.86x10⁸ to 2.45x10¹⁰ nanowires/cm³ or (nanowires + nanoparticles)/cm³, respectively). Each gas sensor was different and therefore responded to different chemicals in different ways. The array was exposed to the same concentration (1 ppm plus or minus 20%) of different gases: acetone, ethanol, isopropyl alcohol and formaldehyde, and then the sensor array response recorded. Data analysis of the sensor response to the VOC chemicals was analysed using principal component analysis (PCA). The analysis process is summarized below:

• The sensor response was calculated using the observed change in its conductivity in relation to the background signal. As shown in the equation: Sensor Response=AG=(G1 - G0)/G0

Where GO is the conductance of the sensor before gas exposure and G1 is the conductance of the sensor after gas exposure.

• The sensor responses were normalized to eliminate any effects caused by external factors.

· Then the data was auto-scaled to unit variance; to eliminate any arbitrary weighting factors.

• The results were then analysed using a commercial software package ("Solo", as sold by Eigenvector Reseach Inc.) to determine the variance in the data.

Figure 8 is a PCA Biplot showing that the sensor array can discriminate between ethanol, formaldehyde, acetone and isopropanol (IPA) and the total variance of PC1 and PC2 is 83.82 %, which confirms the creditability of the data set. Further considerations

The gas sensors of the present invention are different to other nanowire gas sensors at least because the nanowires are angularly arranged causing them to cross each other. In other words, the nanowires are more ordered that being randomly distributed but not all parallel. This is in contrast to any single nanowire device or devices having multiple parallel nanowires, which do not cross. Random networks of nanowires are useful for some applications because the variable properties observed for individual nanowires become averaged, which produces devices with more uniform properties. However, very large dense random nanowire networks are needed to overcome the variations caused by single nanowires and to produce reliable consistent devices with uniform characteristics. Charge transport in random networks of nanowires is affected by the wire contact resistance when nanowires cross. These crossed and bridged nanowires degrade the conductivity and mobility of random networks compared with single nanowires devices.

The present inventors have shown that a high density of crossed nanowires improves the sensor performance, in terms of sensitivity and recovery after gas exposure. Without wishing to be bound by theory, this is due to a number of different reasons caused by an electronic effect at the interface the crossed nanowires. In these crossed wires there is an electrical junction formed at the intersection of the nanowires and a subsequent potential barrier. When the nanowires are exposed to different VOCs, the conduction changes at the junction will be much greater than the surface of individual nanowires. This is shown by gas sensor measurements on randomly arranged nanowires which have a much lower sensitivity and slower recovery after gas exposure than arrays that have crossed nanowires and an angular arrangement.

It is believed that using the dielectrophoresis technique during sensor fabrication increases the number of crossed nanowires (compared to a random distribution) and forms an open nanowire network. The resultant angular nanowires structure is more active than a random network because gases can easily enter and leave the structure freely. The AC signal applied during dielectrophoresis improves the electrical pathways through the network and provides greater ohmic connections between the nanowires and metal electrodes. Also the open nanowire network is believed to enhance electron mobility creating more active sensors. The metal doping method, when using nanowires doped with nanoparticles, produces highly active photo-catalyst sensors. Most approaches to forming photo-catalyst sensors use metal evaporation and then thermal annealing. It is believed that by using reagent free metal nanoparticles there is an optimal interaction of clean Au or Ag nanoparticles, for example, with the metallic nanowires, producing strong metal particle-nanowire interactions.

The ultrasonic process used during metal doping of nanowires produces a chemical interaction between the nanoparticles and the nanowires. Defect formation within the nanowires may occur during ultrasonic agitation to facilitate metal particle deposition, which also enhances the sensor performance.

UV light has been used to optically activate nanowires gas sensors rather than using external heaters on the sensor structure. However there is no discussion of the photo-catalytic effect in metal-doped metal oxide nanowires for gas sensors. The present inventors utilised UV excitation power to obtain the photo-catalytic effect (from 0.2 to 10 mWs). The present inventors noticed that the excitation power could affect the gas sensors selectivity to different VOC gasses. Increasing the UV LED power changed the sensor response to different VOCs. At fixed power density differences in sensor response (% change, response and recovery) are numerically analysable to differentiate between different VOCs. Increasing the power density also changed sensor response and this provides another way of differentiating between VOCs.

Figure 9 shows the change in the response of a sensor for Ethanol, IPA and Acetone at two different UV LED power densities. Figure 9 shows a comparison between one gas sensor exposed to different power densities, the gas sensor comprising nanomaterial including ZnO nanowires doped with Ag nanoparticles. In this example, the gas sensor included: ZnO nanowires doped with Ag nanoparticles (at 4.86x10⁸ to 2.45x10¹⁰ (nanowires + nanoparticles)/cm³). The sensor response at 2.0 mW excitation is greater for Ethanol and I PA, compared to Acetone, at increased power density (increasing from 0.5mW for the higher plot to 2mW for the lower plot).

The presently disclosed Au and Ag nanoparticles doped gas sensors show a unique property when reacting to different VOCs; after gas exposure the sensors show a decrease in conductivity; while the disclosed ZnO nanowire (not doped with nanoparticles) sensors exhibit an increase in conductivity while reacting with VOCs. ZnO sensors for VOC detection show an increase in conductivity after reaction with VOCs (i.e. reducing gases). In general, for n-type semiconductor metal oxides such as ZnO, the conductance increases when interacting with reducing gases. This can be explained by the following surface reaction, after oxygen adsorption, it adsorbs electrons from the conduction band of the metal oxide to form a new species, O^". Then this oxygen species reacts with reducing gases (for example ethanol) to produce electrons which increase the conductance of the n-type semiconductor (for example ZnO and T1O2). This effect is shown below in the equations below. O₂ (gas) + 2e→20^"(ads)

C2H5OH (gas)+6O^" (ads)→ 2CO₂ (gas) +3H₂O (gas)+6e^~

In general, the addition of Au or Ag nanoparticles to ZnO metal oxide nanowire sensors enhances this effect, (a bigger increase in conductivity is observed after VOC exposure as compared metal free ZnO sensors) this is due to enhanced surface adsorption producing enhanced surface reactivity.

Whereas Au or Ag nanoparticle doped ZnO sensors under UV excitation show a decrease in conductivity after VOC exposure. Without wishing to be bound by theory, this behaviour is believed to be due to the unique formation of very small nanoparticles (<10 nm) that have an intimate interaction with the ZnO nanowires and act as efficient photo-catalysts. Under UV excitation these sensors form electron hole pairs which produce new species that are involved in the oxidation and reduction reactions on the sensor surface.

ZnO + hv→ e + h⁺

0₂ (ad) + e^"→0₂ ^"

H₂0→OH- + H⁺

The surface oxidation can occur through the formation of surface hydroxyl OH^' radicals which trap a hole, these radicals are very reactive to adsorbed VOC molecules, and produce a number intermediate organic compounds before complete reaction to form CO2 and H₂O and mineral acid.

H₂O + h⁺→ OH- + h⁺

R-H + OH^"→ R- + H₂O

R- + h⁺→ R⁺→CO₂ + H₂O

During the UV excitation and VOC gas exposure on these sensors, there will be a number compounds produced during photo-oxidation. These reactions consume electron and hole pairs and therefore decrease the sensor conductivity.

Intermediate chemicals have been detected in the photochemical reactions of different VOCs, for example for alcohols the oxidation reaction leads to dehydrogenation and the formation of aldehydes. For ethanol photo-oxidation on ZnO and Au-ZnO, the following reaction occurs to produce acetlyaldehyde.

C2H5OH (gas) → C2H5OH (ad) → C₂H₅O (ad) + H (ad)

Then the adsorbed C2H5O species becomes oxidised by photoelectrons

C2H5O (ad) + e^"→ C2H5O- (ad) → CH₃CHO (ad) + H (ad)→ CH₃CHO (gas) The consumption of photoelectrons in this reaction leads to a lower concentration of electrons which could also reduce the number of electrons returning the ZnO conduction band and therefore it leads to a reduction of its conductivity. Similar types of reaction could occur on the metal doped Ti0₂ nanowire sensors. The improved photo-catalytic effect in the presently disclosed metal doped catalyst is attributed to the charge separation due to the Au and Ag nanoparticles. This in effect leads to more electrons and holes being available for photo-oxidation. As can be seen from the above results, gas sensors according to the present invention have beneficial properties.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1 . A method of forming a gas sensor, the method comprising:

permitting the solvent to evaporate.

2. A method of forming a gas sensor array, the method comprising the steps of claim 1 and further comprising:

permitting the solvent to evaporate.

3. The method of claim 2, further comprising:

repeating the steps of claim 1 or claim 2 for any one of 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 different solutions of nanowires, each repeat on different sets of at least two electrodes.

4. The method of any one of claims 2 or 3, further comprising the step of: prior to placing the first solution of nanowires and the second solution of nanowires on the first set of at least two electrodes, the second set of at least two electrodes or the first and second set of at least two electrodes, placing a barrier layer around the electrodes to prevent the solutions of nanowires on each set of electrodes from mixing.

5. The method of claim 4, wherein the barrier layer has a void around the area of the electrodes to form each gas sensor.

6. The method of claim 5, wherein the barrier layer is formed of perspex.

7. The method of any one of claims 4-6, further comprising the step of removing the barrier layer when the solvent has evaporated.

8. A method of any one of claims 1 -7, wherein the nanowires have: a length (longest dimension) of from 20 nm to 15 pm, preferably from 1 pm to 15 pm; and, a width (shortest dimension) of from 1 nm to 300 nm, preferably from 1 nm to 100 nm, 50 nm or 10 nm; and, an aspect ratio (length divided by width) of from 20 to 50, preferably to 40 or 30.

9. The method of claim 8, wherein the nanowires are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂0₅ and GaN, or combinations thereof.

10. The method of any one of the previous claims, wherein the solutions of nanowires further comprise nanoparticles.

1 1 . The method of claim 10, wherein the nanoparticles have: at least one dimension from 1 nm to 100 nm, preferably from 1 nm to 10 nm; and, an aspect ratio (length divided by width) of from 1 to 4, preferably from 1 to 2.

12. The method of claim 10 or claim 1 1 , wherein the nanoparticles are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, TiO₂, InP, CuO, Cu₂O, NiO, MnO₂, V₂Os or GaN, or combinations thereof.

13. The method of any one of claims 10-12, wherein the solution of nanowires and nanoparticles is mixed using an ultrasonic bath.

14. The method of any one of the previous claims, wherein the at least two electrodes, in each case, are individually electrically isolatable and metallic; and/or wherein the electrodes are spaced by from 1 pm to 20 pm; optionally from 1 pm to 2 m, 5 pm or 10 pm.

15. The method of any one of the previous claims, wherein each electrode is formed of 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 ,

22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 prongs.

16. The method of claim 15, wherein two or more of the prongs of each electrode interdigitate with two or more of the prongs of another electrode.

17. The method of any one of claims 1 -16, wherein the solvent is water, ethanol or isopropanol; and/or wherein the electrodes rest on a substrate, optionally wherein the substrate is formed of glass, silicon or plastic; optionally wherein the plastic is polyethylene terephthalate or polyethylene naphthalate.

18. A gas sensor or a gas sensor array obtainable by a method according to any one of claims 1 -17.

19. A gas sensor comprising:

at least two electrodes, and,

nanowires arranged between the at least two electrodes, wherein the nanowires have a density of from 10⁶ nanowires/cm³ up to and including 10¹⁴ nanowires/cm³.

20. The gas sensor of claim 19, wherein the nanowires have a density of from 10⁶ nanowires/cm³, optionally greater than 10⁷ nanowires/cm³, optionally greater than 10⁸ nanowires/cm³, optionally greater than 10⁹ nanowires/cm³, optionally greater than 10¹⁰ nanowires/cm³, optionally greater than 10¹¹ nanowires/cm³, optionally greater than 10¹² nanowires/cm³, optionally greater than 10¹³ nanowires/cm³, up to and including 10¹⁴ nanowires/cm³.

21 . A gas sensor according to any one of claims 19 or 20, wherein the nanowires are arranged in an open nanowire network.

22. A gas sensor according to any one of claims 19-21 , wherein the nanowires have: a length (longest dimension) of from 20 nm to 15 pm, preferably from 1 pm to 15 pm; and, a width (shortest dimension) of from 1 nm to 300 nm, preferably from 1 nm to 100 nm, 50 nm or 10 nm; and, an aspect ratio (length divided by width) of from 20 to 50, preferably to 40 or 30.

23. The gas sensor of any one of claims 19-22, wherein the nanowires are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, Ti0₂, InP, CuO, Cu₂0, NiO, Mn0₂, V₂Os and GaN, or combinations thereof.

24. The gas sensor of any one of claims 19-23, wherein the gas sensor further comprises nanoparticles mixed with the nanowires.

25. The gas sensor of claim 24, wherein the nanoparticles have: at least one dimension from 1 nm to 100 nm, preferably from 1 nm to 10 nm; and, an aspect ratio (length divided by width) of from 1 to 4, preferably from 1 to 2.

26. The gas sensor of claim 24 or claim 25, wherein the nanoparticles are formed of Au, Pt, Cu, Ag, Co, Fe, Ni, Si, Sn0₂, W0₃, ZnO, TiO₂, InP, CuO, Cu₂O, NiO, MnO₂, V₂O₅ or GaN, or combinations thereof.

27. The gas sensor of any one of claims 19-26, wherein the at least two electrodes, in each case, are individually electrically isolatable and metallic; and/or wherein the electrodes are spaced by from 1 pm to 20 μητι; optionally from 1 pm to 2 pm, 5 pm or 10 pm.

28. The gas sensor of claim 27, wherein each electrode is formed of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26,

27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 prongs.

29. The gas sensor of claim 28, wherein two or more of the prongs of each electrode interdigitate with two or more of the prongs of another electrode; and/or wherein the electrodes rest on a substrate; optionally wherein the substrate is formed of glass, silicon or plastic; optionally wherein the plastic is polyethylene terephthalate or polyethylene naphthalate.

30. A gas sensor array comprising two or more gas sensors according to any one of claims 19-29.

31. A gas sensor array according to claim 30, wherein the gas sensor array comprises 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 , 22, 23, 24, 25, 26, 27, 28, 29, 30, 31 , 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 , 42, 43, 44, 45, 46, 47, 48, 49 or 50 different gas sensors.

32. A method of measuring the presence of a volatile organic compound, comprising:

exposing a gas sensor or a gas sensor array according to any one of claims 18-31 to one or more volatile organic compounds; and,

measuring the change in conductance of the gas sensor or gas sensor array.

33. The method of claim 32, further comprising the step of: prior to exposing the gas sensor or gas sensor array to one or more volatile organic compounds, exposing the gas sensor or gas sensor array to UV light or visible light.

34. The method of claim 33, wherein the UV light or visible light has a wavelength greater than the band gap of the nanomaterial on each particular gas sensor.

35. A gas sensor or a gas sensor array as hereinbefore described, with reference to Figures 1 , 2, 3 or 4.

36. Any novel feature or combination of features disclosed herein.