WO2017165907A1 - Humidity sensors and uses thereof - Google Patents

Abstract

A moisture sensitive device comprising a substrate and a moisture sensitive coating on at least part of the substrate is disclosed. The thickness of the moisture sensitive coating is dependent on the amount of moisture in the atmosphere adjacent the coating. The coating and/or substrate interfere(s) with light incident upon the coating resulting in a light output which varies according to the thickness of the coating.

Description

HUMIDITY SENSORS AND USES THEREOF

PRIORITY DOCUMENT

[0001 ] The present application claims priority from Australian Provisional Patent Application No.

2016901170 titled "HUMIDITY SENSORS AND USES THEREOF" and filed on 30 March 2016, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to sensors that provide an optical output in response to moisture levels in an atmosphere in contact with the sensors. In a particular form, the present disclosure relates to sensors that provide a colorimetric output that is indicative of the relative humidity of the air adjacent the sensors. The disclosure also relates to methods for making the sensors.

BACKGROUND

[0003 ] The amount of moisture in the air (i.e. the relative humidity) is an important consideration in many industries and applications, including food processing, packaging, hospitals, museums, cleanrooms, green houses, domestic appliances, and the like.

[0004 ] Devices for measuring relative humidity ("humidity sensors") are typically one of two types. Resistivity based sensors measure changes in the resistivity of humidity sensitive polymers as a function of relative humidity. Capacitance based sensors employ a capacitor with an air dielectric and changes in the relative humidity between the capacitor plates changes the dielectric, and, hence, the capacitance of the sensor.

[0005] In known polymer based resistivity sensors the absorption of water into a sensor structure causes a change in resistance and a change in dielectric constant of the polymer. These physical changes can be transformed into electrical signals which are related to the water concentration in the polymer and which in ton are related to the relative humidity in the air surrounding the polymer. However, these resistivity based sensors suffer from the drawback that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the sensors necessary to make a resistance measurement. The result is erroneous readings, among other problems. Known capacitance based sensors tend to be relatively complex structures incorporating a moisture-insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, moisture sensitive material overlaying the electrodes. Capacitance based sensors also tend to be inaccurate at high relative humidity values. [0006] There is thus a need to provide a humidity sensor that addresses at least one of the problems with known humidity sensors. Alternatively, or in addition, there is a need to provide a humidity sensor that is relatively simple to manufacture and use. Alternatively, or in addition, there is a need to for a durable and/or compact and/or efficient humidity sensor that can be used to measure moisture content in gaseous atmospheres.

SUMMARY

[0007] According to a first aspect of the present disclosure, there is provided a moisture sensitive device comprising a substrate and a moisture sensitive coating on at least part of the substrate, wherein the thickness of the moisture sensitive coating is dependent on the amount of moisture in the atmosphere adjacent the coating and wherein the coating and/or substrate interfere(s) with light incident upon the coating resulting in a light output which varies according to the thickness of the coating.

[0008] As used herein, the term "light" is intended to encompass infrared, visible (i.e. light) and ultraviolet radiation. Thus, the term encompasses visible light (i.e. radiation having wavelengths in the range of 400 to 700 nm), as well as infrared radiation (with longer wavelengths) and ultraviolet radiation (with shorter wavelengths).

[0009] It will be evident that the light output is affected by the thickness of the coating which, in turn, is effected by the relatively humidity in the atmosphere adjacent the coating. Thus, the light output is or can be correlated with the relative humidity in the atmosphere adjacent the coating and, in this way, the device can be used to as a humidity sensor to measure or sense relative humidity.

[0010] Thus, according to a second aspect of the present disclosure, there is provided a humidity sensor comprising a substrate and a moisture sensitive coating on at least part of said substrate, wherein the thickness of the moisture sensitive coating is dependent on the amount of moisture in the atmosphere adjacent the coating and wherein the coating and/or substrate interfere(s) with light incident upon the coating resulting in a light output which varies according to the thickness of the coating

[001 1 ] Advantageously, the changes in thickness of the coating are reversible which means that the device and sensor of the first and second aspects can be used to continuously measure or monitor relative humidity.

[0012] Whist it is contemplated that the coating can be any material, such as a polymer, whose thickness is dependent on the moisture content of the atmosphere in contact with the coating, polyelectrolyte multilayer(s) (PEMs) are particularly suitable for this purpose. Polyelectrolyte multilayers are known in the art and comprise a polycationic polymer layer and a polyanionic polymer layer. Electrostatic interactions between the polycationic polymer layer and the polyanionic polymer layer create alternating layers of sequentially adsorbed polyions. Thus, a polyelectrolyte layer pair includes two alternating polyelectrolytes of complementary charge, positive and negative, coupled by the interaction of those charges, the coupled pair of polyelectrolytes forming one layer pair of the multilayer. The coating of the present disclosure may comprise from 1 to about 1000 PEM bilayers.

[00131 The polycationic polymer may be selected from one or more polyelectrolytes having a quaternary ammonium group, such as poly(diallyldimethylammonium chloride)(PDDA), poly(vinylbenzyltrimethyl- ammonium)(PVBTA), ionenes, poly(acryloxyethyltrimethyl ammonium chloride), poly(methacryloxy(2- hydroxy)propyltrimethyl ammonium chloride), and copolymers thereof; polyelectrolytes having a pyridinium group, such as poly(N-methylvinylpyridine) (PMVP), other poly(N-alkylvinylpyridines), and copolymers thereof; protonated polyamines, such as poly(allylamine hydrochloride) (PAH) and polyethyleneimmine (PEI); and polypeptides, such as poly(L-lysine) (PLL).

[0014] The polyanionic polymer may be selected from one or more polyelectrolytes having a sulfonate group (S0₃ ² ), such as poly(styrenesulfonate)(PSS), poly(vinylsulfonic acid) (PVS), poly(2-acrylamido-2- methyl-1 -propane sulfonic acid)(PAMPS), sulfonated poly(ether ether ketone) (SPEE ), sulfonated lignin, poly(ethylenesulfonic acid), poly(methacryloxyethylsulfonic acid), their salts, and copolymers thereof; polycarboxylates, such as poly(acrylic acid)(PAA), poly(methacrylic acid), hyaluronic acid (HA), poly (L-3,4-Dihydroxyphenylalanine) (PDopa), their salts, and copolymers thereof; and polyphosphonic acids, such as poly(vinylphosphonic acid) (PVPA); DNA, their salts, and copolymers thereof.

100151 Without intending to be bound by any specific theory on how the device and sensor of the present disclosure work, it will generally be the case that the coating and the substrate will have different refractive indices. Some of the light incident upon the coating (i.e. "incident light") is reflected and some is refracted by the coating. The refracted light experiences absorption (i.e. optical loss) as it propagates through the coating. Further reflection at the coating-substrate interface returns a fraction of the remaining light back, where it interferes (i.e. coherently combines) with the light of the first reflection. As a result, the total reflection by the coating is strengthened at certain wavelengths but weakened at others, resulting in an observable change in the optical output. The wavelengths that are strengthened and weakened are dependent on the thickness of the coating. Thus, the coating is an optical interference coating, the material and thickness of which are chosen so that the coating has particular known, spectral reflectance and spectral absorption characteristics.

10016] Thus, according to a third aspect of the present disclosure, there is provided a humidity sensor comprising: a substrate and one or more humidity sensitive polyelectrolyte multilayer(s) on the substrate, wherein said polyelectrolyte multilayer(s) provide an optical signal output and wherein a value of the optical signal output varies depending on the relative humidity in the atmosphere adjacent the polyelectrolyte multilayer(s).

[0017] According to a fourth aspect of the present disclosure, there is provided a humidity sensor comprising a substrate and a plurality of humidity sensitive polyelectrolyte multilayer coatings on the substrate, wherein at least two of the polyelectrolyte multilayer coatings have different numbers of polyelectrolyte bilayers and wherein said polyelectrolyte multilayer coatings provide an optical signal output and wherein a value of the optical signal output varies depending on the relative humidity in the atmosphere adjacent the polyelectrolyte multilayer coatings. In certain of these embodiments, the sensor is in the form of a colorimetric indicator strip.

[0018 ] According to a fifth aspect of the present disclosure, there is provided a humidity sensing apparatus comprising a humidity sensor of any of the second to fourth aspects, a light source operatively associated with the sensor and configured to deliver light incident upon the coating, and a light detector operatively associated with the sensor and configured to detect output light from the sensor.

[0019] The light source may be an infrared (IR) source, a visible light source or an ultraviolet (UV) light source. The light source can be monochromic, such as a laser, or it can be source of light with a spectrum spanning a broad range.

[0020] The detector may be a photodetector. The photodetector may produce an output electrical signal that is dependent on the intensity and/or wavelength of the output light detected by the photodetector.

[0021 ] According to a sixth aspect of the present disclosure, there is provided a humidity sensor comprising a curved, tapered optical fiber having a moisture sensitive coating that is dependent on the amount of moisture in the atmosphere adjacent the coating, a light source configured to feed light into one end of the fiber and a light detector configured to collect modulated light from the other end and wherein a change in the coating thickness results in a change in the refractive index which causes an absorption- based loss due to the leakage of light from the fiber and resulting in a modulated light output which varies according to the thickness of the coating.

[0022 ] According to a seventh aspect of the present disclosure, there is provided a method of making a device or sensor according to the first to sixth aspects, the method comprising providing a substrate, and coating the substrate with a moisture sensitive coating.

[0023 ] According to an eighth aspect of the present disclosure, there is provided a method of measuring rel ative h umidity, the method comprising contacting a device or sensor of the first to sixth aspects with an atmosphere having a humidity to be measured, measuring an optical output from the device or sensor, and determining relative humidity based on the measured optical output.

BRIEF DESCRIPTION OF FIGURES

10024] Embodiments of the present disclosure will be discussed with reference to the accompanying figures wherein:

[0025] Figure 1 : (a) is a photograph of substrates having from 5.0 to 10.5 PEM bilayers coatings at ambient conditions; (b) is a plot of wavelength (nm) vs normalized reflectance for substrates having 7.5 (black line), 8.0 (red line), 8.5 (blue line) and 9.0 (green line) PEM bilayers; (c) is a plot of the number of PEM bilayers vs thickness (nm; black line) and peak wavelength (nm; red line); and (d) is a schematic demonstrating the underlying light interference mechanism;

[0026] Figure 2: (a) is a photograph of substrates having 8.0 PEM bilayers (top row) and 8.5 PEM bilayers (bottom row) at different relative humidity values; (b) is a plot of wavelength (nm) vs normalized reflectance for a sensor having 8.0 PEM bilayers; (c) is a plot of relative humidity (%) vs peak wavelength (nm) for substrates having 8.0 (red line) and 8.5 (blue line) PEM bilayers; (d) is a plot of relative humidity (%) vs thickness (nm) for substrates having 8.0 (red line) and 8.5 (blue line) PEM bilayers; and (e) is a plot of relative humidity (%) vs thickness (μηι) for substrates having 20.0 PEM bilayers;

[0027] Figure 3: (a) is a schematic diagram of a humidity sensor apparatus of embodiments of the disclosure; (b) is a plot of relative humidity (%) vs normalized voltage (V) for substrates having 8.0 (red line) and 8.5 (blue line) PEM bilayers; (c) is a plot of time (s) vs voltage (a.u.) when the relative humidity exposed to the coating is switched between 50% and 90%; and (d) shows an expanded area of the plot at (c);

[0028] Figure 4: (a) is a photograph of a substrate having 8.0 PEM bilayers before (left) and after (right) the substrate is breathed on; (b) is a photograph of a substrate having 8.0 PEM bilayers before (left) and after (right) a finger is placed in close proximity to the coating; (c) is a schematic diagram of a touchless control apparatus of embodiments of the disclosure; (d) is a plot of time (s) vs voltage (a.u.) obtained by pulsed breathing on the coating; (e) is a plot of time (s) vs voltage (a.u.) obtained by pulsed finger tapping on the coating; (f) is a plot of the threshold as a percentage of Vmax (%) vs relative humidity (%) based on Figure 4(d); and (g) is a plot of the threshold as a percentage of Vmax (%) vs relative humidity (%) based on Figure 4(e); [0029 ] Figure 5 is a schematic diagram of an optical fiber based humidity sensor according to embodiments of the disclosure; and

[0030] Figure 6 is a schematic diagram of a distributed humidity sensor according to embodiments of the disclosure.

DESCRIPTION OF EMBODIMENTS

[0031 ] As discussed, disclosed herein is a moisture sensitive device. The device comprises a substrate and a moisture sensitive coating on at least part of the substrate. The thickness of the moisture sensitive coating is dependent on the amount of moisture in the atmosphere adjacent the coating. The coating and/or substrate interfere(s) with light incident upon the coating resulting in a light output which varies according to the thickness of the coating. Thus, the device can be used to measure an amount of moisture in the atmosphere that is in contact with the coating.

[0032] In certain embodiments, the device is suitable for use as a humidity sensor.

[0033] In certain embodiments, the optical output from the device is a colour (i.e. an output in the visible spectrum). For reasons discussed in more detail later, the colour output is dependent on the thickness of the coating which, in turn, is dependent on the moisture content of the atmosphere in contact with the coating. Thus, the coating will have a colour that is dependent on the moisture content of the atmosphere (i.e. the relative humidity) and different moisture contents give rise to different colours. This then provides a simple, cost-effective device that can be used to give a visual indication of the moisture content of the atmosphere without the need for a source of power or sophisticated electronics as may be needed for capacitance based humidity sensors.

[0034] In other embodiments, the light source may be an IR or UV source and the optical output may be IR or UV radiation that has been modulated by the coating. Commercially available IR and UV lamps are suitable IR and UV sources.

[0035] The substrate may be any rigid, semi-rigid or flexible material on which the coating can be formed. In certain embodiments, the substrate is an opaque or reflective material. The substrate can be glass, plastic, ceramic, metal, metalloid or alloy. Suitable substrates include, but are not limited to, polished silicon wafers, black or dark and reflective plastics, and CdTe substrates.

[0036] The substrate can take any suitable form, such as a plate, chip, etc. The substrate has at least one surface which can be coated with the coating. The coating may cover all or part of the surface. In certain embodiments, the substrate is a strip. This then provides a colorimetric indicator strip that can be used to measure relative humidity.

[0037] The surface, or part thereof, of the substrate to be coated may be treated prior to coating. The surface may be treated physically, chemically and/or electrochemically for better adherence of the coating. For example, the surface may be abraded or etched to enhance adherence of the coating.

Alternatively, or in addition, the surface may be treated with an adherent to enhance adherence of the coating. The adherent may form an intermediary coating between the substrate and the coating.

[0038 ] In certain embodiments, the coating is a polymer. In certain specific embodiments, the polymer comprises one or more polyelectrolyte multilayer(s) (PEMs). PEMs are known and have previously been used in electroluminescent LEDs, conducting polymer composites, assembly of proteins and metal nanoparticle systems, thin film optoelectronic devices, and nanostructured thin film coatings.

[0039 ] The polyelectrolyte multilayers may comprise a polycationic polymer layer comprising monomer units that are positively charged and a polyanionic polymer layer or comprising monomer units that are negatively charged. Electrostatic interactions between the polycationic polymer layer and the polyanionic polymer layer create alternating layers of sequentially adsorbed polyions. As used herein, the combination of a polycationic polymer layer and a polyanionic polymer layer may be referred to as a "PEM bilayer". The coating may comprise from 1 to about 1000 PEM bilayers. More specifically, the coating may comprise, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 1 1, 1 1.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20.5, 21, 21.5, 22, 22.5, 23, 23.5, 24, 24.5, 25, 25.5, 26, 26.5, 27, 27.5, 28, 28.5, 29, 29.5, 30, 30.5, 31, 3 1.5, 32, 32.5, 33, 33.5, 34, 34.5, 35, 35.5, 36, 36.5, 37, 37.5, 38, 38.5, 39, 39.5, 40, 40.5, 41, 41.5, 42, 42.5, 43, 43.5, 44, 44.5, 45, 45.5, 46, 46.5, 47, 47.5, 48, 48.5, 49, 49.5 or 50 PEM bilayers. The term "x.5" in this regard means that the coating comprises one unpaired polyelectrolyte layer, and that can be either a polycationic polymer layer and a polyanionic polymer layer.

[0040 ] In certain embodiments, the coating may comprise one or more additional components in addition to the PEM coating, provided that the other components do not affect the humidity-sensitive properties of the coating. The coating could be in the form of a sandwich structure. For example, it can be stacked as A/PEM/B/PEM/C/PEM/A/B/C/PEM, where the sequence of the different layers can be changed.

[0041 ] In certain embodiments, the substrate comprises more than one coating. In these embodiments, two or more coatings may be formed on the substrate with each coating having a different number of PEM bilayers. At any given relative humidity value the colour output is different for different PEM bilayer numbers. Thus, in these embodiments, two or more optical outputs can be obtained from a single sensor. The combined optical outputs may provide a more accurate reading of the relative humidity when compared to a single optical output.

[0042] Thus, in some embodiments, provided herein is a humidity sensor comprising a substrate and a plurality of humidity sensitive polyelectrolyte multilayer coatings on the substrate, wherein at least two of the polyelectrolyte multilayer coatings have different numbers of polyelectrolyte bilayers and wherein said polyelectrolyte multilayer coatings provide an optical signal output and wherein a value of the optical signal output varies depending on the relative humidity in the atmosphere adjacent the polyelectrolyte multilayer coatings. In certain of these embodiments, the sensor is in the form of a colorimetric indicator strip.

[0043 ] The PEM bilayers can be formed using a layer-by-layer assembly technique by repetitive, alternating treatment of the substrate with a solution containing the polycationic polymer and a solution containing the polyanionic polymer. Electrostatic attraction between the polyelectrolyte in solution and the oppositely charged polyelectrolyte already adsorbed to the substrate binds the

solution polyelectrolyte to the adsorbed polyelectrolyte at the film solution interface. The process cycle is driven by steric effects that prevent complete neutralization of the charge on the

incoming polyelectrolyte during adsorption, leading to a surface charge reversal that facilitates adsorption of the next (oppositely charged) polyelectrolyte layer.

[0044] Polyelectrolytes may be linear or branched and may be synthetic, naturally occurring or synthetically modified naturally occurring macromolecules. The polyelectrolytes may be copolymers that have a combination of charged and/or neutral monomers (e.g. positive and neutral; negative and neutral; positive and negative; or positive, negative and neutral).

[0045 ] The molecular weight of the polyelectrolyte polymers is typically about 100 to about 10,000,000 grams/mole. Each polyelectrolyte solution typically comprises about 0.001 % to about 60% by weight of a polyelectrolyte , such as about 0.1% to about 10% by weight.

[0046] Suitable polyanionic polymers include polyelectrolytes having a sulfonate group (S0₃ ² ), such as poly(styrenesulfonate)(PSS), poly(vinylsulfonic acid) (PVS), poly(2-acrylamido-2-methyl-l -propane sulfonic acid)(PAMPS), sulfonated poly(ether ether ketone) (SPEE ), sulfonated lignin,

poly(ethylenesulfonic acid), poly(methacryloxyethylsulfonic acid), their salts, and copolymers thereof; and polycarboxylates, such as poly(acrylic acid)(PAA) and poly(methacrylic acid), hyaluronic acid (HA), poly (L-3,4-dihydroxyphenylalanine) (PDopa), their salts, and copolymers thereof; polyphosphonic acids, such as poly(vinylphosphonic acid) (PVPA); and DNA, their salts, and copolymers thereof. [0047] Suitable polycationic polymers include polyelectrolytes having a quaternary ammonium group, such as poly(diallyldimethylammonium chloride)(PDDA), poly(vinylbenzyltrimethyl- ammonium)(PVBTA), ionenes, poly(acryloxyethyltrimethyl ammonium chloride), poly(methacryloxy(2- hydroxy)propyltrimethyl ammonium chloride), and copolymers thereof; polyelectrolytes having a pyridinium group, such as poly(N-methylvinylpyridine) (PMVP), other poly(N-alkylvinylpyridines), and copolymers thereof; protonated polyamines, such as poly(allylamine hydrochloride) (PAH) and polyethyleneimmine (PEI); and polypeptides, such as poly(L-lysine) (PLL).

[0048] In certain embodiments, the polyanionic polymer is a polysulfonate polymer. In certain specific embodiments, the polyanionic polymer is poly(styrenesulfonate) (PSS). In other or specific embodiments, the polyanionic polymer is poly (L-3,4-dihydroxyphenylalanine) (PDopa).

[0049] In certain embodiments, the polycationic polymer is a poly ammonium polymer. In certain specific embodiments, the polycationic polymer is poly(diallyldimethylammonium chloride) (PDDA). In other specific embodiments, the polycationic polymer is poly(allylamine hydrochloride) (PAH). In still other specific embodiments, the polycationic polymer is polyethyleneimmine (PEI).

[0050] The polycationic polymer and the polyanionic polymer are water and/or organic soluble, or dispersed in water and/or organic solvent. Organic solvents that can be used include methanol, ethanol, dimethylformamide, acctonitrile, carbon tetrachloride, and methylene chloride.

[0051 ] Solutions containing the polyelectrolyte may also contain one or more salts that dissociate to stable ions (e.g. sodium chloride). A salt can be included in the polyelectrolyte solutions to control the thickness of the layers. Generally, salts in the polyelectrolyte solution increase the thickness of the adsorbed polyelectrolyte layer. The amount of salt added to the polyelectrolyte solution typically depends on the specific combinations of polycationic and polyanionic polymers used as well as the salt used. Using NaCl as an example, for PDDA/PSS multilayers, the NaCl concentration can be varied from 0.05-5 M, such as 0.1-3M, 0.2-2.5M, or 0.5-2 M; for PDDA/PDopa multilayers, the NaCl concentration can be varied from 0.05- 1 M, such as 0.1-0.8, 0.2-0.6M, 0.3-0.5 M.

[0052] Polyelectrolyte solutions and/or a polyelectrolyte complex solutions

or polyelectrolyte dispersions may be deposited on the substrate by any appropriate method, such as dip coating, casting, soaking, spin coating, sedimenting and/or spraying. Dip coating is particularly suitable when applying the coating using alternating exposure of oppositely charged polyelectrolyte solutions.

[0053 ] The polyelectrolyte deposition process is governed by a variety of factors, including but not limited to polyelectrolyte solution pH, ionic strength, temperature, polyelectrolyte molecular weight, the nature of the polyelectrolyte chemical functional groups, and the deposition method. [0054 ] Polyelectrolyte layers are deposited onto the substrate or the growing multilayer as separate layers, with roughly equivalent amounts of polycationic polymer and polyanionic polymer deposited in each deposition cycle.

[0055] The duration in which a polyelectrolyte solution is typically in contact with the surface (i.e. the contact time) can vary from a few seconds to several minutes or hours to achieve a preferred thickness. Generally, a contact time of about 20 minutes using solutions having polyanionic polymer and polycationic polymer concentrations of about 1 mg/mL provides a satisfactory thickness. A washing step may be carried out between consecutive depositions. A drying step may be applied between depositions.

[0056 ] A variety of additives can be incorporated into a coating as it is formed. Additives that may be incorporated into PEMs include inorganic materials, such as metallic oxide particles (e.g. silicon dioxide, aluminium oxide, titanium dioxide, iron oxide, zirconium oxide and vanadium oxide), metals, biocides, antibacterial agents, and anticorrosion agents.

[0057] As mentioned, the light output of the device varies according to the thickness of the

coating. Figure 1 (d) shows an example of a plane parallel coated substrate. Note that the coating thickness (h) is not represented true to scale in the figure since the coating is much thinner than the substrate whose thickness will generally be in the order of millimetres. The light ray incident upon the coating is partly reflected at the air-coating and coating-substrate interfaces. The light that is reflected at the coating-substrate interface is also refracted by the coating which has a refractive index (nl) which is different to the refractive index of air (nl) and the substrate (n3). The refracted portion of light experiences absorption (i.e. optical loss) as it propagates through the coating. The second reflection at the coating-substrate interface returns a fraction of the remaining light back, where it interferes (i.e.

coherently combines) with the light of the first reflection. As a result, the total reflection by the coating is strengthened at certain wavelengths but weakened at others, resulting in an observable colour. The observable colour is determined by the optical path length which, in turn, is determined by the thickness ( ?) of the coating. If the optical path length changes due to variation of the thickness of the coating, the wavelength of the maximum intensity light observed also changes, resulting in a colour change.

[0058] The light incident upon the coating comes from a light source. As used herein, the term light is intended to include not only visible light but also ultraviolet light and infrared light. The light source may be a natural light source or an artificial light source. The light produced by the light source and incident upon the coating may be light of two or more wavelengths, such as visible light, or it may be

monochromatic, such laser light of a single wavelength.

[0059] A ray from the light source is incident on the coating at an angle. The angle may be from 1° to 90° from normal. [0060] The reflected light provides an optical output signal which can be detected and/or measured using a suitable detector. In some embodiments, the detector is the human eye. In other embodiments, the reflected light impinges on the receiving surface of a photosensitive detector (photodetector). The photodetector may then produce an electrical signal and the photodetector may be operatively associated with a measuring instrument or a recording device for recording the intensity curve of the reflected light.

[0061 1 It is contemplated that transmitted light could be used. For example, the coating may be formed on a transparent or translucent substrate and a light source may be associated with the device so that light is incident on the coating and a photodetector or other detector may detect light transmitted through the coating and the substrate.

[0062] The device or sensor described herein may form part of a humidity sensing apparatus comprising the device or sensor, a light source operatively associated with the device or sensor and configured to deliver light incident upon the coating, and a light detector operatively associated with the device or sensor and configured to detect output light from the sensor.

[0063 ] The light source of the humidity sensing apparatus may be an infrared (IR) source, a visible light source or an ultraviolet (UV) light source. The light source can be monochromic, such as a laser, or it can be source of light with a spectrum spanning a broad range.

[0064] The detector may be a photodetector. A range of photodetector s are available commercially and can be used. For example, a Thorlabs PDB450C photodetector is suitable. The photodetector produces an output electrical signal that is dependent on the intensity and/or wavelength of the output l ight detected by the photodetector. The output electrical signal may be used to produce a visual signal (e.g. using an oscilloscope), it may be fed into a comparator circuit to power a light source (e.g. a light-emitting diode) or it may be fed into a digital acquisition module to produce a digital signal. In the latter case, the humidity sensing apparatus may form part of a touch control apparatus wherein humidity caused by a user's breath or by a body part (e.g. a finger) being placed adjacent the coating causes a change in humidity which then results in an optical output signal which is eventually converted into a digital signal which can be used to perform a function under the control of a microcontroller. For example, a user's breath may be used to switch an electrical apparatus, such as a light, on or off, or implement commands through a computer, such as play music, or make a phone call.

[0065] In other embodiments, the substrate may be an optical fiber. Specifically, a tapered optical fiber may be dip coated as previously described to form a coating fiber. At least part of the optical fiber may be curved, such as U-shaped. Light from a laser source can be fed into one end of the fiber and the modulated light is collected from the other end by a photodetector. When exposed to humidity, changes in the coating thickness result in a change in the refractive index which causes an absorption-based loss due to the leakage of light from the fiber into the coating. The change in the detected optical power provides a measure of the relative humidity.

[0066] A coated optical fiber may be used as part of a distributed humidity sensor. The optical fiber coated with the humidity-sensitive coating can be interrogated with an optical reflectometry technique such as optical frequency-domain reflectometry.

[0067] The moisture sensitive device or humidity sensor described herein can be used in a variety of applications including, but not limited to, assembly lines (food processing factories, coating factories), packaging (packaging factories), air conditioners (hospitals, museums, cleanrooms, homes, cars), water locators (geologists, water industries), greenhouses (farmers), meteorology stations (weather forecasters), dryers (domestic appliances industries), engine test beds (automotive industries), heat-treating furnaces (glass industries, construction industries), liquid-cooled electronics (electronics industries), planetary- exploration rovers (space programs), and smart clothing (athletes, patients).

EXAMPLES

[00681 Example 1 - Formation of moisture sensitive devices

[0069] Poly(diallyldimethylammonium) (PDDA) was used as a polycationic polymer and

poly(styrenesulfonate) (PSS) was used as a polyanionic polymer. Polished Si wafers or black/dark and reflective plastics were used as substrates.

[0070] PDDA and PSS aqueous solutions containing PDDA and PSS with concentrations of 1 mg/mL, respectively, and NaCl with concentrations of 1.0 M were prepared. (PDDA/PSS)n PEMs, where n represents the bilayer numbers, were prepared on freshly cleaned substrates by alternating immersion of the substrates into aqueous solutions of PDDA and PSS for 20 min until the desired layer number was reached. Each immersion step was followed by thorough rinsing with water. After the desired layer number was reached, the resulting PEMs were thoroughly rinsed with water and gently dried with N₂ flow.

[0071 ] Coating characterisation

[0072] Substrates were coated with from 5.0 to 10.5 bilayers according to the method described above. As shown in Figure 1(a), depending on the number of bilayers and the illumination/observation angle, the coating exhibits a different colour at ambient conditions, ranging from shades of blue, yellow, red and green. The colour of each coating thickness results from the interference between same-wavelength light across the range of visible wavelengths shown in Figure 1 (b). Figure 1(c) shows that the number of bilayers is proportional to the thickness of the coating, as well as the peak wavelength of the first order (i.e. count from right side of observed range).

[0073] The underlying mechanism (Figure I d) is that rays of light incident upon the air-coating interface from a specific angle, with a fraction reflected and the rest refracted. The refracted portion experiences absorption (i.e. optical loss) as it propagates through the medium. The second reflection at the coating- substrate interface returns a fraction of the remaining light back, where it interferes (i.e. coherently combines) with the light of the first reflection. As a result, the total reflection by the coating is strengthened at certain wavelengths but weakened at others, resulting in an observable colour.

[0074] Humidity sensitivity

[0075 ] As the coating is exposed to increased levels of relative humidity, its colour changes between shades of blue, green, yellow and red. As shown in Figure 2(a), a different colour map exists for different number of bilayers (i.e. top row is 8.0 bilayers, bottom row is 7.5 bilayers). If used as a colorimetric indicator strip, their combined reading provides a more accurate feedback of the relative humidity. The corresponding reflection spectra at each relative humidity value are shown in Figure 2(b). The peak wavelength and thickness as a function of relative humidity for two different thicknesses are shown in Figures 2(c) and (d). Figure 2(e) shows a 56% maximum change in the thickness of a coating with 20.0 bilayers as it absorbs moisture and it is clearly evident that wavelength shifts are primarily caused by thickness changes rather than refractive index changes.

10076] Example 2 - Laser based humidity sensor

[0077] To monitor a fast-changing transmission spectrum under the influence of different levels of relative humidity, a narrow-linewidth laser (Thorlabs HNLS008R-EC) centred around 633 nm wavelength was used to probe the coating of the device of Example 1 at 55^° from its normal (i.e. large angle enables closer interaction without blocking the beam path). The reflected beam was measured with a photodetector (Thorlabs PDB450C), and displayed on an oscilloscope (Rigol DS6104), as shown in Figure 3(a).

[0078 ] The coating was exposed to different lev els of relative humidity by blowing nitrogen through a 5 mm diameter glass tube positioned at 3 mm from the surface of the coating. The nitrogen was controlled by mixing dry and wet (i.e. passed through 50 ^"C water) nitrogen through two flow meters at the initial stage, which yielded a total air flow of 10 litres per minute. The ratio and thus relative humidity were calibrated using a commercial humidity sensor (RisePro, 1% resolution) at the intermediate stage. The insertion of a three-way tube switch before the output at the final stage ensured that the system pressure was kept constant despite switching between the ambient and a specific relative humidity. The photodetector converts the optical signal into an electrical signal, and the measured voltage as a function of the relative humidity for a coating with 8.0 bilayers is shown as the familiar sinusoidal waveform in Figure 3(b). The gradients are steeper with higher levels of relative humidity, because the wavelength shifts are larger. Figure 3(c) shows the temporal behaviour of the measured signal when the relative humidity exposed to the coating is switched between 50% and 90%. The response and recovery times based on the transition time between 90% and 10% of the maximum voltage were measured to be 35 ms and 950 ms respectively (Figure 3(d)), which are faster than the previous reports in the literature.

[0079 ] Example 3 - Touchless control apparatus

[0080 ] As described in the preceding example, a photodetector can be used to convert the output optical signal into an electrical signal, and the measured voltage as a function of the relative humidity. The electrical signal generated can then be used in various applications. For the application of touchless control, a user can either use breath or a finger to induce a response from the coating. Figure 4(a) shows the colour of a coating with 8.0 bilayers before and after a breath exposure. Figure 4(b) shows the distance-dependent colour change when a finger is placed in close proximity of the coating. It was found in both cases that the coating recovers from dew point (i.e. relative humidity reaches or exceeds 100%), which suggests good repeatability for practical applications. The setup for two touchless-control demonstrations, namely LED and computer control (described later), are shown in Figure 4(c). The signal from the photodetector not only feeds into the oscilloscope but also to: (A) a comparator circuit (i.e. with hysteresis to suppress noise) powering a light-emitting diode, and (B) a digital acquisition module (i.e. converts analogue to digital) before a computer. For the LED, both breath and finger tests were performed with the threshold (i.e. Vref) set to 90% of the maximum voltage. Figures 4(d) and 4(e) show that the voltage measured across the LED (i.e. indicates brightness) closely follow the measured signal from the coating. The response of the touchless control in both cases can be optimized for the target environment by tailoring the threshold, and thus the critical relative humidity or finger distance, as shown in Figures 4(f) and (g). Note that longer finger distances exhibit larger uncertainties, due to the greater chance of the ambient air flow changing the moisture transfer.

[0081 ] Example 4 - Humidity sensor based on tapered optical fiber

[0082] A highly sensitive fiber-optic humidity sensor can be made by dip coating along a U-shaped optical microfiber made by tapering an optical fiber (e.g. more than 100 microns in diameter) into a narrow uniform waist (e.g. less than 20 microns in diameter) with biconical transitions, as shown in Figure 5. Light from a laser source is launched into one end and the modulated light is collected from the other end by a photodetector. When exposed to humidity, changes in the coating thickness (e.g. initially ranging from hundreds of nanometers to a few micrometers thick) should not strain the microfiber due to the freedom to expand or shrink on both sides. Likewise, the change of refractive index (e.g. initially between 1.3 to 1.5) should not cause significant optical loss due to the leakage of light from the core (i.e. microfiber) into the cladding (i.e. coating). However, the increase of water-absorption-induced optical loss will cause a considerable optical loss. The change in the detected optical power provides a measure of the relative humidity. The sensor head (i.e. coated microfiber plus transitions) can be packaged by a protective casing with holes to allow humid air to flow in and out. The detection system (i.e. laser diode and photodetector) can be placed remotely and connected to the sensor head via optical fibers.

[0083 ] Example 5 - Distributed humidity sensor based on optical reflectometry

[0084 ] A distributed humidity sensor can be made with the sensing fiber coated with the humidity- sensitive coating, and interrogated with an optical reflectometry technique such as optical frequency- domain reflectometry (Figure 6). This operates by sweeping the wavelength of the laser source that feeds into a Mach-Zehnder interferometer. The sensing arm contains a circulator that is connected to the sensing fiber, which continuously back-reflects with a cumulative sequence of wavelengths exhibiting phases proportional to the distances that the light has travelled. A shorter reference arm operates in transmission mode. These two paths of light coherently recombine at a coupler. Sweeping a short wavelength range with a single back-reflection creates a single set of interference fringes with a single beat frequency of narrow linewidth. For mul tiple back-reflections, multiple sets of fringes with scal ed free spectral ranges are superimposed, forming multiple beat frequencies. The beat frequencies correspond to the distance along the FUT. A photodetector or balanced photodetector with an oscilloscope captures the interference fringes, and performing FFT resolves the beat frequencies that are related to the distances. A reference Mach-Zehnder interferometer with a known path imbalance can simplify frequency-distance conversion. It can also be used to resample the signals to correct for sweep- wavelength irregularities. This is achieved by taking the time-varying reference signal, converting into a complex signal via the Hilbert transform, extracting the phase info, creating a regular phase grid, interpolating the complex signal with the previous two sets of data, converting back into a time-varying signal via the inverse Hilbert transform, and interpolating the complex signal of the sensing signal in the same fashion.

[0085 ] Example 6 - Formation of alternative moisture sensitive devices

10086] Poly(diallyldimethylammonium) (PDDA) was used as a polycationic polymer and poly(L-3,4- dihydroxyphenylalanine) (PDopa) was used as a polyanionic polymer. Polished Si wafers, black/dark and reflective plastics were used as substrates. Poly(L-3,4-dihydroxyphenylalanine) was prepared via self- polymerization of L-3,4-dihydroxyphenylalanine (L-Dopa). Typically, L-Dopa (0.197 g) was dissolved in 100 mL of water. The pH of the resulting aqueous solution of L-Dopa was adjusted to 8.5 using Tris powder, the resulting aqueous solution of L-Dopa was held at 80 °C for 12 h under stirring to accelerate the self-polymerization of L-Dopa. After being cooled to room temperature, the reaction solution was stored under ambient conditions for more than 5 days, and a stable, black aqueous solution of Poly(L-3,4- dihydroxyphenylalanine) was thus obtained.

[0087] PDDA and PDopa aqueous solutions containing PDDA and PSS with concentrations of 1 mg/mL, respectively, and NaCl with concentrations of 0.5 M were prepared. (PDDA/PDopa)n PEMs, where n represents the bilayer numbers, were prepared on freshly cleaned substrates by alternating immersion of the substrates into aqueous solutions of PDDA and PDopa for 20 min until the desired layer number was reached. Each immersion step was followed by thorough rinsing with water. After the desired layer number was reached, the resulting PEMs were thoroughly rinsed with water and gently dried with N₂ flow.

[0088] Results

[0089] The (PDDA/PDopa)n coatings on substrates with bilayer numbers of 15, 20, 30, 35, 40, 45, 50, 55, 60 exhibit different colors at ambient conditions, ranging from shades of blue, yellow, red and green. All the above-mentioned (PDDA/PDopa)n coatings can exhibit different colors at different relative humidity. For example, the (PDDAZPDopa)n coating with 30 bilayers exhibits color changes between shades of blue, green, yellow and red with the relative humidity increasing from 0% to 100%.

[0090] The (PDDA/PDopa)n coatings can also be used for laser-based humidity sensors, touchless control apparatus, humidity sensors based on tapered optical fiber, and distributed humidity sensors based on optical reflectometry by using the setups shown in Figure 3c, Figure 4c, Figure 5 and Figure 6, respectively.

10091 ] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

10092] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

10093] It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims

1. A moisture sensitive device comprising a substrate and a moisture sensitive coating on at least part of the substrate, wherein the thickness of the moisture sensitive coating is dependent on the amount of moisture in the atmosphere adjacent the coating and wherein the coating and/or substrate interfere(s) with light incident upon the coating resulting in a light output which varies according to the thickness of the coating.

2. The moisture sensitive device according to claim 1 , wherein the light output comprises light reflected from the device.

3. The moisture sensitive device according to claim 1 , wherein the light output comprises light transmitted through the device.

4. The moisture sensitive device according to any one of claims 1 to 3, wherein the coating comprises a polyelectrolyte multilayer (PEMs).

5. The moisture sensitive device according to claim 4, wherein the PEM comprises polyelectrolyte layer pairs comprising a polycationic polymer layer and a polyanionic polymer layer.

6. The moisture sensitive device according to claim 5, wherein the coating comprises from 1 to about 1000 PEM bilayers, with each PEM bilayer comprising a polycationic polymer layer and a polyanionic polymer layer.

7. The moisture sensitive device according to either claim 5 or claim 6, wherein the polycationic polymer is selected from one or more of the group consisting of: polyelectrolytes having a quaternary ammonium group, such as poly(diallyldimethylammonium chloride)(PDDA), poly(vinylbenzyltrimethyl- ammonium)(PVBTA), ionenes, poly(acryloxyethyltrimethyl ammonium chloride), poly(methacryloxy(2- hydroxy)propyltrimethyl ammonium chloride), and copolymers thereof; polyelectrolytes having a pyridinium group, such as poly(N-methylvinylpyridine) (PMVP), other poly(N-alkylvinylpyridines), and copolymers thereof; protonated polyamines, such as poly(allylamine hydrochloride) (PAH) and

polyethyleneimmine (PEI); and polypeptides, such as poly(L-lysine) (PLL).

8. The moisture sensitive device according to claim 7, wherein the polycationic polymer is selected from one or more of the group consisting of poly(diallyldimethylammonium) (PDDA), poly(allylamine hydrochloride) (PAH), and polyethyleneimmine (PEI).

9. The moisture sensitive device according to any one of claims 5 to 8, wherein the polyanionic polymer is selected from one or more of the group consisting of: polyelectrolytes having a sulfonate group (S0₃ ² ), such as poly(styrenesulfonate)(PSS), poly( vinyl sulfonic acid) (PVS), poly(2-acrylamido-2-methyl- 1 -propane sulfonic acid)(PAMPS), sulfonated poly(ether ether ketone) (SPEEK), sulfonated lignin, poly(ethylenesulfonic acid), poly(methacryloxyethylsulfonic acid), their salts, and copolymers thereof; polycarboxylates, such as poly(acrylic acid)(PAA), poly(methacrylic acid), hyaluronic acid (HA), poly (L- 3,4-Dihydroxyphenylalanine) (PDopa), their salts, and copolymers thereof; and polyphosphonic acids, such as poly(vinylphosphonic acid) (PVPA); DNA, their salts, and copolymers thereof.

10. The moisture sensitive device according to claim 9, wherein the polyanionic polymer is selected from one or more of the group consisting of poly(styrenesulfonate) (PSS), and poly (L-3,4- d ihy droxyphenyl alan ine) (PDopa).

11. The moisture sensitive device according to any one of claims 4 to 10, wherein the substrate has two or more separate moisture sensitive coatings and at least two of the two or more coatings have a different number of PEM bilayers.

12. The moisture sensitive device according to any one of claims 1 to 1 1 , wherein the substrate is an opaque or reflective material.

13. The moisture sensitive device according to any one of claims 1 to 12, wherein the substrate is glass, plastic, ceramic, metal, metalloid or alloy.

14. The moisture sensitive device according to claim 13, wherein the substrate is selected from the group consisting of a polished silicon wafer, a black or dark and reflective plastic, and a CdTe substrate.

15. The moisture sensitive device according to any one of claims 1 to 14, wherein the substrate is a strip.

16. A humidity sensor comprising the moisture sensitive device of any one of claims 1 to 15.

17. The humidity sensor according to claim 16, wherein the changes in thickness of the coating are reversible.

18. A humidity sensor comprising: a substrate and one or more humidity sensitive polyelectrolyte multilayer coatings on the substrate, wherein said polyelectrolyte multilayer coatings provide an optical signal output and wherein a value of the optical signal output varies depending on the relative humidity in the atmosphere adjacent the polyelectrolyte multilayer coatings.

19. The humidity sensor according to claim 18, wherein the coating comprises from 1 to about 1000 polyelectrolyte bilayers, with each polyelectrolyte bilayer comprising a polycationic polymer layer and a polyanionic polymer layer.

20. The humidity sensor according to either claim 18 or claim 19, wherein the polycationic polymer is selected from one or more of the group consisting of: polyelectrolytes having a quaternary ammonium group, such as poly(diallyldimethylammonium chloride)(PDDA), poly(vinylbenzyltrimethyl-ammonium)(PVBTA), ionenes, poly(acryloxyethyltrimethyl ammonium chloride), poly(methacryloxy(2-hydroxy)propyltrimethyl ammonium chloride), and copolymers thereof; polyelectrolytes having a pyridinium group, such as poly(N- methylvinylpyridine) (PMVP), other poly(N-alkylvinylpyridines), and copolymers thereof; protonated polyamines, such as poly(allylamine hydrochloride) (PAH) and polyethyleneimmine (PEI); and

polypeptides, such as poly(L-lysine) (PLL).

21. The humidity sensor according to claim 20, wherein the polycationic polymer is selected from one or more of the group consisting of poly(diallyldimethylammonium) (PDDA), poly(allylamine hydrochloride) (PAH), and polyethyleneimmine (PEI).

22. The humidity sensor according to any one of claims 19 to 21, wherein the polyanionic polymer is selected from one or more of the group consisting of: polyelectrolytes having a sulfonate group (S0₃ ² ), such as poly(styrenesulfonate)(PSS), poly(vinylsulfonic acid) (PVS), poly(2-acrylamido-2-methyl- 1 -propane sulfonic acid)(PAMPS), sulfonated poly(ether ether ketone) (SPEE ), sulfonated lignin,

poly(ethylenesulfonic acid), poly(methacryloxyethylsulfonic acid), their salts, and copolymers thereof; polycarboxylates, such as poly(acrylic acid)(PAA), poly(methacrylic acid), hyaluronic acid (HA), poly (L- 3,4-DihydtOxyphenylalanine) (PDopa), their salts, and copolymers thereof; and polyphosphonic acids, such as poly(vinylphosphonic acid) (PVPA); DNA, their salts, and copolymers thereof.

23. The humidity sensor according to claim 22, wherein the polyanionic polymer is selected from one or more of the group consisting of poly(styrenesulfonate) (PSS), and poly (L-3,4-dihydroxyphenylalanine) (PDopa).

24. The humidity sensor according to any one of claims 1 8 to 23, wherein the substrate is an opaque or reflective material.

25. The humidity sensor according to any one of claims 18 to 24, wherein the substrate is glass, plastic, ceramic, metal, metalloid or alloy.

26. The humidity sensor according to claim 25, wherein the substrate is selected from the group consisting of a polished silicon wafer, a black or dark and reflective plastic, and a CdTe substrate.

27. The humidity sensor according to any one of claims 18 to 26, wherein the substrate is a strip.

28. The humidity sensor according to any one of claims 18 to 27 comprising two or more separate humidi ty sensitive polyelectrolyte multilayer coatings with at least two of the two or more coatings have a different number of polyelectrolyte bilayers.

29. A humidity sensing apparatus comprising a humidity sensor of any one of claims 16 to 28, a light source operatively associated with the sensor and configured to deliver light incident upon the coating, and a light detector operatively associated with the sensor and configured to detect output light from the sensor.

30. The humidity sensing apparatus according to claim 29, wherein the detector is a photodetector.

31. The humidity sensing apparatus according to claim 30, wherein the photodetector produces an output electrical signal that is dependent on the intensity and/or wavelength of the output light detected by the photodetector.

32. A humidity sensor comprising a curved, tapered optical fiber having a moisture sensitive coating that is dependent on the amount of moisture in the atmosphere adjacent the coating, a light source configured to feed light into one end of the fiber and a light detector configured to collect modulated light from the other end and wherein a change in the coating thickness results in a change in the refractive index which causes an absorption-based loss due to the leakage of light from the fiber and resulting in a modulated light output which varies according to the thickness of the coating.

33. A method of making a device or sensor according to the any one of claims 1 to 32, the method comprising providing a substrate, and coating the substrate with a moisture sensitive coating.

34. A method of measuring relative humidi ty, the method comprising contacting a device or sensor of any one of claims 1 to 32 with an atmosphere having a humidity to be measured, measuring an optical output from the device or sensor, and determining relative humidity based on the measured optical output.