Enhanced conductive polymers, conductive polymer-based gas sensors, and methods of making and using the same

Abstract

There are disclosed chemiresistive gas sensors having one or more enhanced analyte detection properties for the detection of one or more target analyte. The gas sensors are characterized by a sensing region formed from a conductive polymer, wherein the conductive polymer is electropolymerized discontinuously to provide an enhanced percolation region characterized by a highly abrupt change in conductance in a plot of conductance vs. polymer grown. In addition, there are disclosed processes for making the chemiresistive gas sensors and to processes for enhancing one or more analyte detection properties of a conductive polymer.

Description

[0001] ENHANCED CONDUCTIVE POLYMERS, CONDUCTIVE POLYMER-BASED GAS

[0002] SENSORS, AND METHODS OF MAKING AND USING THE SAME

[0003] FIELD

[0004] The present invention relates to conductive polymers having one or more enhanced analyte detection properties and to chemiresistive gas sensors comprising the conductive polymers. In addition, the present invention is directed to processes for making the chemiresistive gas sensors and to processes for enhancing one or more analyte detection properties of a conductive polymer.

[0005] BACKGROUND

[0006] Chemiresistive gas sensors are devices that detect the presence of gases in a surrounding environment by measuring changes in the electrical resistance of a sensing material. These sensors typically comprise a sensing material made of a metal oxide, polymer, or other material having a large surface area that can interact with gas molecules, such as target analyte(s), in a gas. In known methods, the sensing material is deposited onto a substrate, such as a silicon wafer or a glass, ceramic, or polymer substrate. The sensing material is then patterned into a specific shape using a suitable technique, such as lithography techniques. Typically, also, the sensing material is in contact with electrodes that are used to measure an electrical property, e.g., resistance, of the sensing material. In operation, the sensor is exposed to a gas suspected of containing the target analyte(s) and an electrical potential or current is applied across the sensing material. As gas molecules interact with the sensing material, such as by absorption and / or adsorption, an electrical property, e.g., resistance, of the sensing material changes, which is measured as a change in electrical potential or current at the electrodes. From the measured values, the concentration of the target analyte(s) may be determined.

[0007] Chemiresistive gas sensors comprising conductive polymers as the sensing material have been extensively studied, due to their relatively high sensitivity and selectivity towards various target analytes. To prepare conductive polymers for chemiresistive sensors, electropolymerization methods may be used. Electropolymerization refers a process that involves depositing a conductive polymer onto an electrode. This is achieved by applying an electrical potential or current to the electrode while it is immersed in a polymerization solution. The polymerization solution contains both the monomers needed to form the conductive polymer and a supporting electrolyte. Through this method, the conductive polymer gradually forms on the surface of the electrode and extends towards a suitable substrate surrounding the electrode. During that growth process, it is theorized that conductive polymer networks develop and a region (referred to as a “percolation region”) forms on the substrate where there is an increase in electrical conductance as the conductive polymer grows and more conductive polymer networks connect (shown by reference numeral 4 in Figure 1). This percolation region 4 follows an initial insulating region 2 (shown by reference numeral 2 in Figure 1) where the conductive polymer formed has not yet bridged the gaps between the electrodes.

[0008] Following the percolation region 4, when enough conductive polymer networks converge, the conductive polymer transitions to a thin film region 6 (shown by reference numeral 6 in Figure 1) where the conductance values start to plateau. This behavior, which suggests the existence of an insulating region, percolation region, and thin film region as a function of deposited conductive polymer is illustrated in Figure 1. Conventional methods of growing conductive polymers having the above characteristics, as illustrated by the growth curve of Figure 1, may be referred to as classical percolation growth, and the percolation region thereof may be referred to as a classical percolation region. An example of a process of growing conductive polymers is disclosed, for example, in US 20230031121. In comparison to the thin film region 6, the influence of gas molecules on electrical conductance is enhanced in the percolation region 4. This is because the interacting species, e.g., target analyte(s), can rapidly disrupt electrical connections through the conductive polymer percolation networks.

[0009] Despite the advancements in the field, it would be desirable if conventional methods of forming conductive polymers were improved to provide conductive polymer percolation networks that have a more abrupt change in electrical properties, e.g., resistance and conductance, in the percolation region 4. A more abrupt transition in the percolation region 4 would theoretically result in a sensor material having enhanced analyte detection properties, such as increased sensitivity, decreased response time, decreased recovery time, decreased baseline drift, lower limits of detection, and / or increased selectivity for one or more target analytes. Improved methods for reliably and efficiently providing conductive polymers having such enhanced analyte detection properties are thus desired. SUMMARY

[0010] This summary may introduce a selection of concepts in a simplified form, which may be described in further detail below. This summary is not intended to necessarily identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0011] As used herein, the terms “at least one”, “one or more”, and “and / or” are understood to be open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B, and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and / or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together.

[0012] As used in the present application, the singular forms “a,” “an,” and “the” include plural referents unless otherwise specified.

[0013] Unless otherwise stated herein or clear from the context, any percentages referred to herein are expressed as percent by weight based on a total weight of the subject composition.

[0014] The present inventors have developed improved conductive polymers having greatly enhanced analyte detection properties (relative to known conductive polymer); chemiresistive gas sensors comprising the improved conductive polymers; methods for making the improved conductive polymers and chemiresistive gas sensors; and methods for modifying / enhancing the analyte detection properties of a conductive polymer.

[0015] In one aspect, the present inventors have surprisingly found that improved conductive polymers having enhanced conductive polymer networks and chemiresistive gas sensors may be formed by electropolymerization from a polymerization solution comprising a plurality of monomers of the conductive polymer and a supporting electrolyte in a discontinuous manner. In particular, the present inventors have found that the discontinuous growth of conductive polymer produces conductive polymer networks having a percolation region with a notably more abrupt (steeper) change in conductance compared to the more gradual change in conductance for conductive polymer networks formed by the conventional continuous growth (classical percolation growth) processes. This percolation region having a more abrupt conductance change relative to those formed by classical percolation growth may be referred to as an “explosive percolation region” herein and its formation may be referred to as “explosive percolation growth.” As a result of the more abrupt change in conductance of the explosive percolation region, the conductive polymers and chemiresistive gas sensors described herein provide analyte detection properties not yet seen in the chemiresistive gas sensor field, e.g., increased sensitivity, decreased response time (thus fast response), decreased recovery time, decreased baseline drift (stable baseline), decreased resistance upon exposure to samples and / or target analyte(s) with electron donating properties (e.g. ammonia), lower limits of detection, and / or increased selectivity for target analytes compared to known materials and sensors. In one aspect, even assuming use of the same materials as in known methods for making conductive polymers and chemiresistive gas sensors, the novel and inventive features in the methods of making conductive polymers and chemiresistive gas sensors described herein produce conductive polymers and chemiresistive gas sensors having these enhanced analyte detection properties.

[0016] In accordance with an aspect of the present invention, there is provided a method for making a chemiresistive gas sensor comprising: providing a substrate having spaced-apart electrodes thereon; and electropolymerizing a conductive polymer on the electrodes and the substrate between the electrodes from a polymerization solution comprising a plurality of monomers of the conductive polymer and a supporting electrolyte, wherein the electropolymerizing comprises applying a plurality of non-overlapping electrical pulses to the electrodes in the polymerization solution to form the conductive polymer, the conductive polymer comprising a plurality of conductive polymer networks.

[0017] In an embodiment, one or more analyte detection properties are selected from the group consisting of increased sensitivity, decreased response time, decreased recovery time, decreased baseline drift, lower limits of detection, and increased selectivity for target analytes. In an embodiment, the analyte detection properties of conductive polymer networks formed by electropolymerizing the conductive polymer discontinuously and / or with spaced apart pulses as described herein are enhanced relative to conductive polymer networks formed by electropolymerizing the same conductive polymer continuously.

[0018] In accordance with another aspect, there is disclosed a process for making a chemiresistive gas sensor comprising: electropolymerizing a conductive polymer on a substrate from a polymerization solution comprising a plurality of monomers of the conductive polymer and a supporting electrolyte, wherein the electropolymerizing comprises providing an electrical potential or current discontinuously to the electrodes in the polymerization solution to form the conductive polymer, the conductive polymer comprising a plurality of conductive polymer networks.

[0019] As used herein, the term “discontinuous” is meant that during electropolymerization of the conductive polymer, an electrical potential or current is not provided in a continuous manner to the electrodes. Instead, between intervals of electrical pulses, there are periods where no amount of electrical potential or current is provided to the electrodes or where the electrical potential or current provided to the electrodes is decreased by at least 50% relative to the electrical potential or current of the most recent pulse. In certain embodiments, a plurality of electrical pulses are provided to the electrodes discontinuously by way of spaced apart pulses, wherein each pulse is for a predetermined duration and comprises an electrical potential or current provided in a similar range (± 10%) or at an identical value to the other pulses of the plurality of spaced apart pulses.

[0020] In accordance with another aspect of the present invention, there is disclosed a process for detecting an analyte of interest with a chemiresistive gas sensor comprising: forming a chemiresistive gas sensor according to a method as described herein; and detecting the presence of a target analyte by exposing the chemiresistive gas sensor to a gas suspected of comprising the target analyte, wherein, in the presence of the target analyte, the conductive polymer displays a change in an electrical property in response to an interaction with the target analyte.

[0021] In accordance with another aspect of the present invention, there is disclosed a chemiresistive gas sensor comprising: spaced-apart electrodes on a substrate; and a sensing region between the electrodes, wherein the sensing region comprises a conductive polymer comprising a plurality of conductive polymer networks that electrically bridge the electrodes, and wherein the conductive polymer comprises an explosive percolation region in a plot of conductance vs. amount of conductive polymer formed.

[0022] In accordance with yet another aspect, there is provided a chemiresistive gas sensor comprising: spaced-apart electrodes on a substrate; a sensing region between the electrodes, wherein the sensing region comprises a conductive polymer comprising a plurality of conductive polymer networks that bridge the electrodes, and wherein the conductive polymer networks comprise one or more enhanced analyte detection properties due to being electropolymerized discontinuously vs. being electropolymerized continuously. In accordance with yet another aspect, there is provided a conductive polymer comprising a plurality of conductive polymer networks, wherein the conductive polymer comprises an explosive percolation region in a plot of conductance vs. amount of conductive polymer formed.

[0023] In accordance with yet another aspect, there is provided a conductive polymer formed by a method of forming a conductive polymer as described herein.

[0024] In accordance with yet another aspect, there is provided a chemiresistive sensor comprising a conductive polymer formed by a method of forming a conductive polymer as described herein.

[0025] In some embodiments, the conductive polymer as described herein is characterized by having a value for a derivative of the change in conductance (AC) / change in charge (AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is greater than a value for a derivative for the same plot (derivative of AC / AQ vs. amount of charge (LIC ) ) for a conductive polymer formed continuously. In some embodiments, the value for the derivative is at least 3x, 5x, lOx, or 20x greater than the value of the derivative for the same plot for a conductive polymer formed continuously. In some embodiments, the value of the derivative is a maximum value for each plot.

[0026] In some embodiments, the conductive polymer (18) is characterized by having a sensor response according to the formula (Rg-Ra) / Rt for an analyte, e.g., a target analyte, which is 5000x greater than a sensor response of a conductive polymer formed continuously. In some embodiments, the target analyte is an electron donating analyte, e.g., ammonia.

[0027] BRIEF DESCRIPTION OF THE FIGURES

[0028] The accompanying drawings, which are included to provide a further understanding of the disclosure and constitute a part of this specification, illustrate exemplary embodiments, and together with the description help to explain the principles of the disclosure. In the drawings:

[0029] Figure 1 is a Prior Art graph that illustrates the conductance of a conductive polymer formed by continuous electropolymerization as a function of the amount of deposited polymer.

[0030] Figure 2 illustrates a chemiresistive gas sensor in accordance with an aspect of the present invention.

[0031] Figure 3 is a cross-sectional view of the chemiresistive gas sensor of Figure 2. Figure 4 illustrates a comparison between the percolation region of a conductive polymer formed according to aspects of the present invention (explosive percolation) vs. conventional processes (classical percolation).

[0032] Figures 5A-F show a process of producing explosive percolation networks in accordance with an aspect of the present invention.

[0033] Figure 6 is a graph showing the conductance of a conductive polymer formed by discontinuous electropolymerization in accordance with an aspect of the present invention as a function of deposited conductive polymer vs. that of conventional conductive polymers formed continuously.

[0034] Figure 7 is a graph showing a derivative of the change in conductance vs. the change in charge (AC / AQ) from the gas sensors shown in Figure 6.

[0035] Figure 8 shows the sensor response of a gas sensor according to aspects of the present invention (explosive percolation) to ammonia vapor.

[0036] Figure 9 shows the response of a gas sensor made by conventional processes (classical percolation) to ammonia vapor.

[0037] Figure 10 shows the response (Rg- Ra) / Rt) of gas sensors made according to aspects of the present invention (ExPl -3) vs. gas sensors made by conventional processes, e.g., CsPl-3).

[0038] Figure 11 shows a response-recovery time curve of a gas sensor according to aspects of the present invention to ammonia.

[0039] DETAILED DESCRIPTION

[0040] Aspects of the present invention relate to conductive polymers comprising conductive polymer networks for chemiresistive gas sensors for detecting one or more target analytes (analytes of interest) and, in particular, to conductive polymer-based chemiresistive gas sensors having the conductive polymers. The conductive polymers and gas sensors described herein provide one or more enhanced analyte detection properties relative to known sensors. The present invention is further directed to processes for making the conductive polymer-based chemiresistive gas sensors and processes for enhancing one or more analyte detection properties of conductive polymers.

[0041] Referring again to the Figures, there is shown in Figure 2 a chemiresistive gas sensor 10 (hereinafter “gas sensor 10” or “sensor 10” for ease of reference) in accordance with an aspect of the present invention. The sensor 10 comprises at least a pair of spaced-apart electrodes 12 on a substrate 14. A sensing region 16 is provided on at least a portion of the substrate 14 in between the electrodes 12. As will be explained in further detail below, the sensing region 16 comprises a conductive polymer 18 having conductive polymer networks 20 that electrically bridge the pair of electrodes 12. Figure 3 illustrates a cross-sectional view of the sensor 10 shown in Figure 2, but without the conductive polymer (before electropolymerization).

[0042] The pair of electrodes 12 of the sensor 10 may comprise any suitable conductive material. In an embodiment, the electrodes 12 comprise a conductive metal, a semiconductor, glassy carbon, graphite or a conductive polymer material. In certain embodiments, the electrodes 12 independently comprise a noble metal material. In a particular embodiment, the electrodes 12 comprise an electrically conductive material selected from the group consisting of a platinum group metal, gold, and carbon-based materials. Alternatively, the electrically conductive material may comprise any other suitable material. In certain embodiments, one or more of the electrodes 12 comprise platinum. In addition, in certain embodiments, the electrodes 12 may be interdigitated with respect to one another, and thus define one or more interdigitated electrodes. In certain embodiments, the sensor 10 comprises a plurality of interdigitated electrodes.

[0043] The substrate 14 may comprise any suitable material having the desired properties for the substrate, e.g., an electrically insulating material. In an embodiment, the substrate 14 comprises an electrically insulating substrate, such as glass, a fiberglass-epoxy laminate, a ceramic material, an electrically insulating polymer material, a silicon wafer, or any other suitable material. In particular embodiments, the substrate 14 comprises an electrically insulating polymer selected from the group consisting of polyethylene (PE), polyethylene terephthalate (PET), polyvinylchloride (PVC), polypropylene (PP), polyamide (PA), polystyrene (PS), polyimide (PI), and like polymers.

[0044] The sensing region 16 may comprise any conductive polymer 18 having the desired analyte detection properties described herein. In an embodiment, the conductive polymer 18 comprises any suitable conductive polymer capable of being electropolymerized from a polymerization solution comprising a plurality of monomers of the conductive polymer to be formed and a supporting electrolyte. In certain embodiments, the conductive polymer 18 may comprise a member selected from the group consisting of polythiophenes, such as poly(3,4- ethylenedioxythiophene)(PEDOT); and other conductive polymers, such as polyaniline (PANI), polypyrrole (PPy), polycarbazole, and their derivatives. The conductive polymer 18 is typically deposited about an entirety of the electrodes 12 and then the conductive polymer 18 grows gradually through the spaces between the electrodes 12. As the conductive polymer 18 grows, the conductive polymer networks 20 of the conductive polymer 18 increasingly form between the electrodes 12.

[0045] In operation, an electrical potential difference (voltage) may be provided to the electrodes 12 and the sensing region 16. In the presence of one or more target analytes, an electrical property, e.g., resistance, of the conductive polymer networks 20 of the sensing region 16 may change, which is indicative of the amount of the analyte(s) being detected. As will be explained in further detail below, the conductive polymer networks described herein are characterized by an extremely rapid and reversible change in an electrical property, e.g., resistance, upon the interaction with one or more target analytes (also referred to as “an analyte of interest” or “analytes of interest”). As a result of the ability of the sensor 10 to rapidly and reversibly change one or more electrical properties in the presence of one or more target analytes, the conductive polymer networks 20, conductive polymers 18 including the same, and sensors 10 incorporating such conductive polymer networks 20 provide enhanced analyte detection properties relative to conductive polymer networks, conductive polymers, and sensors heretofore known and disclosed.

[0046] In an embodiment, the enhanced analyte detection properties for the detection of one or more target analytes comprise one or more of the following: increased sensitivity, decreased response time (thus, fast response), decreased recovery time, decreased baseline drift (stable baseline), decreased resistance upon exposure to target analytes with electron donating properties (e.g., ammonia), lower limits of detection, and / or increased selectivity for one or more target analytes relative to conductive polymers grown continuously vs. discontinuously as described herein.

[0047] The sensing region 16 may be of any suitable thickness. In a particular embodiment, the sensing region 16 has a thickness of from 0.5 nm to 1000 nm, such as from 1 to 900 nm, 2 to 750 nm, 5 to 500 nm, or any suitable range between 0.5 and 1000 nm. The electrodes 12 may be separated from one another by any suitable distance, such as 100 nm or more, 500 nm or more, 1 micron or more, 10 micron or more, 20 micron or more, 50 micron or more, or 100 micron or more. The distance between the electrodes 12 may further include any numeric ranges between these values.

[0048] The electrodes 12 are typically in electrical communication with a suitable power source for providing and / or measuring an electrical potential or current to the electrodes 12. In addition, the electrodes 12 may be in electrical communication with one or more computing devices having a processor specially programmed to collect and analyze data from each sensor 10. In certain embodiments, the one or more computer devices may comprise computer-readable instructions for determining the amount of one or more target analytes in a gas sample from the sensor data.

[0049] The gas sensors 10 described herein are capable of the detection of one or more target analytes due to the detection of a change in an electrical property of the conductive polymer networks 20 of the sensing region 16, such as resistance. Without limitation, the gas sensors 10 described herein may be suitable for the detection of one or more target analytes selected from the group consisting of CO2.NO, N2O, NO2, N2O4, SO2, O2, O3, NH3, C2H2, H2, N2, H2S, aromatics, hazardous gases, explosive compounds, volatile biomarkers, volatile organic compounds, combinations thereof, and any other volatile substances capable of being detected by the gas sensors described herein. In particular embodiments, the gas sensors 10 described herein are suitable for and capable of the detection of ammonia in gas. In other embodiments, the sensors 10 may be suitable for and capable of the detection of air pollutants, e.g., volatile organic compounds (VOCs); toxic and / or explosive and flammable materials; indoor and outdoor air quality; health monitoring; clinical diagnostics; monitoring of food quality or storage; or any other suitable application.

[0050] The present inventors have surprisingly found that the conductive polymers and gas sensors 10 formed according to the present invention have particularly exceptional analyte detection properties. Without limitation, the enhanced analyte detection properties include one or more of increased sensitivity, decreased response time, decreased recovery time, decreased baseline drift, lower limits of detection, and / or increased selectivity for target analytes relative to known conductive polymers and sensors comprising the same.

[0051] In certain embodiments, the gas sensor 10 has a response time of less than 100 seconds, less than 50 seconds, less than 10 seconds less than 5 seconds, less than 1 second, and even less than 0.5 seconds. In an embodiment, the gas sensor has a response time of from 1-100 seconds.

[0052] In certain embodiments, the gas sensor 10 has a recovery time of less than 1000 seconds, less than 500 seconds, less than 100 seconds, less than 50 seconds, less than 20 seconds, less than 15 seconds, less than 10 seconds, or even less than 5 seconds. The recovery time is particularly critical for gas sensors as they allow the sensors to reset and analyze further gas samples to provide more robust data or additional analysis. In an embodiment, the gas sensor 10 has a recovery time of 10-1000 seconds. Critically, in some embodiments, the sensing region 16, conductive polymer 18, conductive polymer networks 20, and sensors 10 disclosed herein are characterized by the presence of an extremely rapid increase in conductance in a growth curve of the conductive polymer as the conductive polymer is electrochemically deposited.

[0053] Referring now to Figure 4, Figure 4 represents a plot of conductance vs. amount of polymer formed. As shown, as conductive polymer is formed on surface(s) of the substrate 14, the conductive polymer initially comprises an insulating region 22 where conductance changes minimally, if at all. This describes the situation where the conductive polymer is formed on the electrodes 12 surfaces towards the space between the electrodes 12, but not yet bridges the space. Thereafter, the conductance of the formed conductive polymer gradually increases as a function of the polymer formed as illustrated by reference numeral 24 in Figure 3. Thereafter, the conductance value then again plateaus as further conductive polymer is formed which is shown as thin film region 26. The conventional increase in conductance as a function of conductive polymer formed between the insulating region 22 and the thin film region 26 may be referred to as a “classical percolation region” and is shown by reference numeral 24.

[0054] In contrast to conventional conductive polymer materials, the region having a much more abrupt and dramatic change in conductance as a function of conductive polymer 18 formed between the insulating region 22 and the thin film region 26 may be referred hereinafter to as an “explosive percolation region 28” in the conductive polymer 18 formed according to aspects of the present invention.

[0055] As discussed herein, the explosive percolation region is formed by a method as described herein where the conductive polymer is formed discontinuously vs. continuously as in conventional methods. Critically, in aspects of the present invention, the explosive percolation region of a conductive polymer formed as described herein has a value for a derivative of the change in conductance / change in charge (AC / AQ) on a plot of the derivative of AC / AQ vs. amount of charge (pC) that is greater than a value for a derivative of the change in conductance / change in charge (AC / AQ) for the same plot (derivative of AC / AQ vs. amount of charge (pC)) for a conductive polymer is formed continuously. See also, for example, Figures 6-7, discussed in further detail below.

[0056] In some embodiments, the explosive percolation region of a conductive polymer formed as described herein may be characterized as having a value for a derivative of AC / AQ on a plot of the derivative of AC / AQ vs. amount of charge (pC) that is at least 3x, at least 5x, at least lOx, or even at least 20x greater than a value for a derivative of the change in conductance / change in charge (AC / AQ) for the same plot for a conductive polymer, e.g., the same conductive polymer, formed continuously. In some embodiments, the value is greater than 3x. In some embodiments, the value is greater than 5x. In some embodiments, the value is greater than lOx. In some embodiments, the value is greater than 20x. In some embodiments, the value of the derivative is a maximum value for each plot of derivative of AC / AQ vs. amount of charge (pC).

[0057] In some embodiments, the explosive percolation region 28 is characterized by a conductance change of at least 500,000x, l,000,000x, 10,000,000x, 20,000,000x, 40,000,000x in the explosive percolation region 28 upon reaching a threshold value in the amount of conductive polymer formed. The “threshold value” is thus a value of conductive polymer formed at which this very sudden change in resistance abruptly occurs.

[0058] In some embodiments, the conductive polymer (18) is characterized by having a sensor response according to the formula (Rg-Ra) / Rt for an analyte, e.g., target analyte, which is at least 5000x greater than a sensor response of a conductive polymer formed continuously. In some embodiments, the target analyte is an electron donating analyte, e.g., ammonia.

[0059] The present inventors have found that conductive polymers having the above-described explosive percolation region 28 having enhanced analyte detection properties may be formed by electropolymerizing the conductive polymer 18 in a different manner from the conventional formation of conductive polymers. In particular, in certain embodiments, the conductive polymers according to the present invention are formed by providing electrical potential or current in a discontinuous manner to the electrodes in the polymerization solution comprising the monomers for the conductive polymer being formed and a supporting electrolyte. The present inventors have further surprisingly found that the discontinuous electropolymerization of the conductive polymer prolongs the emergence of the conductive polymer networks 20 that allow charge to pass. This discontinuous and prolonged growth provides a notably steeper change in resistance and / or conductance within the explosive percolation region 28 relative to known materials. In certain embodiments, the conductive polymer provides at least a two-orders of magnitude change in resistance upon contact with a target analyte relative to a conductive polymer grown continuously. In further embodiments, the conductive polymer provides at least a three-orders of magnitude change in resistance upon contact with a target analyte relative to a conductive polymer grown continuously In accordance with one aspect, there are disclosed methods for modifying one or more electrical properties of a conductive polymer and / or increasing one or more analyte detection properties of the conductive polymer. In particular, the analyte detection capabilities of a known conductive polymer can be enhanced by forming the conductive polymer via the discontinuous electropolymerization methods described herein, which result in the conductive polymer having one or more enhanced analyte detection properties. In other embodiments, there are disclosed methods for forming the gas sensors 10 described herein comprising conductive polymers 18 with a plurality of conductive polymer networks 20 having the enhanced one or more analyte detection properties.

[0060] As stated previously, the method of forming a conductive polymer as described herein comprises electropolymerizing the conductive polymer on the electrodes 12 and on the substrate 14 between the electrodes 12 from a polymerization solution comprising a plurality of monomers of the conductive polymer and a supporting electrolyte, wherein the electropolymerizing comprises discontinuously applying electrical pulses to the electrodes 12.

[0061] In particular embodiments, the method comprises electropolymerizing the conductive polymer 18 on the electrodes 12 and on the substrate 14 between the electrodes 12 from a polymerization solution comprising a plurality of monomers of the conductive polymer 18 and a supporting electrolyte, wherein the electropolymerizing comprises providing a plurality of nonoverlapping electrical pulses to the electrodes 12 in the polymerization solution to form the conductive polymer 18 with a plurality of conductive polymer networks 20.

[0062] Any suitable number of non-overlapping pulses (hereinafter also “pulses” for ease of reference) may be provided during the electropolymerization process to form the conductive polymer. In the process, it is appreciated that any two given pulses are at least separated from one another by an interval such that they are “non- overlapping.” By “interval” as used herein, it is thus meant a period of time where the electrical potential or current is reduced by at least 50% of a minimum electrical potential or current of the closest (most adjacent) prior pulse. In some embodiments, the electrical potential or current is reduced by at least 55%, 60%, 65%, 70%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or more of a minimum electrical potential or current of the closest (most adjacent) prior pulse.

[0063] In certain embodiments, any two adjacent pulses may be separated from one another by an interval defining a complete cessation of electrical potential or current to the electrode, e.g., 0 volts (V) or 0 amperes (A). The intervals may likewise last for any suitable duration, such as at least 0.001, at least 0.01, at least 0.1, or at least 1 second. In certain embodiments, the interval between any two pulses has a duration of from 0.001 to 10 seconds, such as from 0.01 to 1 second.

[0064] In an embodiment, the process comprises delivering at least 2, 5, 10, 20, 50, 100, 500 or more pulses to the electrode in the polymerization solution. In certain embodiments, the method comprises delivering from 10 to 1000 pulses, such as 5 to 500, 5 to 50, or 5 to 15 electrical pulses to electrodes. The electrical pulses may be provided by delivering a predetermined electrical potential or current to the electrodes 12 from a suitable source. The pulses may also be for any suitable duration to provide the desired effect. In an embodiment, each of the pulses have a duration of at least 0.001, at least 0.01, at least 0.1, or at least 1 second. In certain embodiments, the pulses each have a duration of from 0.001 to 10 seconds, such as from 0.01 to 1 second.

[0065] In certain embodiments, each of the pulses is provided to the electrodes 12 for the same duration. In other embodiments, the duration of at least one pulse may be different with respect to at least one other pulse. In certain embodiments, the pulses comprise an equal amount of electrical potential or current. In other embodiments, the amount of electrical potential or current within at least one pulse may be different with respect to at least one other pulse.

[0066] In certain embodiments, the sum of the charge of each of the pulses adds up to a total charge applied to the electrodes 12. In other words, in some embodiments, the total charge is divided by the number of pulses to be provided. For example, if a total charge of 1000 pC is to be provided to the electrode in 10 pulses, the amount of charge per pulse may be set to 100 pC. The total amount of charge may be any suitable amount to grow the desired conductive polymer with the desired properties. In certain embodiments, the amount of charge is proportional to the electrode area. In an embodiment, the total charge may be at least 1 pC, at least 10 pC, at least 100 pC, at least 500 pC, at least 1000 pC, at least 5000 pC or any other suitable value.

[0067] In some embodiments, an electrical potential provided to the electrodes 12 is from 1.0 V to 1.5 V, such as 1.3 V vs. an Ag / AgCl reference electrode. It is appreciated that the total charge and / or electrical potential need to have a value that is high enough for the conductive polymer 18 to form on the electrodes 12 and on the substrate 14 between the electrodes 12. In particular, in some embodiments, the applied potential needs to be sufficiently large (positive) to oxidize the selected monomers such that they will couple together and form the desired conductive polymer. In other embodiments, the selected monomers may also be electropolymerized by reduction at a sufficiently negative potential. Hence, it is appreciated that the applied electrical potential may be dependent on the monomer(s) used.

[0068] In certain embodiments, an electrical property, e.g., resistance, of the forming conductive polymer 18 is measured during one or more of the intervals. In this way, a growth curve that monitors the progress and properties of the forming conductive polymer can be viewed. By monitoring the growth of the conductive polymer 18, the process can be stopped once the explosive percolation region 28 has been formed, but prior to or just after the formation of a substantial amount of conductive polymer networks 20. In general, the process of forming a gas sensor 10 as described herein can be halted at any point once the conductive polymer enters the percolation region 28. In an embodiment, the process may be specifically terminated at approximately 400 uC to ensure the formation of an adequate amount of conductive polymer 18. Alternatively, one could choose to halt the process closer to the onset of the percolation region 28. This approach may be preferred particularly when creating gas sensors intended for detecting lower concentrations of gases. Ultimately, the skilled artisan would appreciate that the decision as to when to terminate the process may depend on the specific parameters that need to be optimized and the intended application. Specific parameters that may influence the decision when to terminate the electropolymerization process may include the electrode area, electrode material, distance between electrodes, type of monomer and its concentration, type of electrolyte and its concentration, type of solvent, temperature, and hydrodynamic effects (stirring or agitation of the solution) during the electropolymerization process.

[0069] During the processes described herein, the electrodes 12 and substrate 14 are in contact with a polymerization solution. In the method, the polymerization solution comprises a plurality of monomers of the conductive polymer to be formed, e.g., 3,4-ethylenedioxythiophene (EDOT) monomers when the conductive polymer is poly(3,4-ethylenedioxythiophene (PEDOT), and a supporting electrolyte, e.g., tetrabutylammonium perchlorate (TBACIO4). The monomer concentration may be any suitable amount, such as from 0.01 to 10 M, such as 0.1 to 5 M, 0.1 to 1 M, or any range between 0.01 to 10 M.

[0070] The polymerization solution further comprises any suitable solution for dissolving the monomers therein and for providing the function of a supporting electrolyte (e.g., provide a desired degree of conductivity to the solution) in the electropolymerization reaction. In one embodiment, the polymerization solution comprises a solvent for at least dissolving the plurality of monomers. The solvent may comprise any suitable organic or inorganic solvent. In an embodiment, the solvent comprises water. In another embodiment, the solvent comprises an organic protic or aprotic solvent. In certain embodiments, the solvent comprises acetonitrile. In other embodiments, the solvent comprises an ionic liquid as set forth below.

[0071] The supporting electrolyte may comprise any suitable material for its intended purpose. In certain embodiments, the supporting electrolyte comprises an acid, base, or salt compound. In certain embodiments, the supporting electrolyte may comprise one or more of sodium chloride, poly(sodium 4- styrenesulfonate), potassium chloride, hydrochloric acid, sulfuric acid, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, tetrabutylammonium perchlorate, tetrabutylammonium tetrafluoroborate, tetrabutylammonium hexafluorophosphate, or mixtures thereof. In a particular embodiment, the supporting electrolyte comprises tetrabutylammonium perchlorate.

[0072] In another embodiment, the polymerization solution comprises a solvent along with an ionic liquid, which collectively dissolves the plurality of monomers and acts as a supporting electrolyte. Thus, in this embodiment, the supporting electrolyte at least comprises an ionic liquid. The solvent may comprise any suitable inorganic or organic solvent as described above. In an embodiment, the ionic liquid is one that is a liquid at temperatures of 100° C or less. In certain embodiments, the ionic liquid is formed from: cations, such as those selected from the group consisting of: 1-ethyl- 3-methylimidazolium (EMIM), l-butyl-3-methylimidazolium (BMIM), and 1 -butyl- 1- methylpyrrolidinium (BMPyrr); and anions, such as those selected from the group consisting of: acetate (OAc), octanoate (OOc), methylcarbonate (MC), trifluoromethanesulfonate (OTf), methanesulfonate (McSOfl, tetrafluoroborate (BF4), bis(trifluoromethylsulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and propionate (OPr). In this embodiment, the plurality of monomers are dissolved in a combination of the solvent and the ionic liquid.

[0073] In still another embodiment, the polymerization solution comprises primarily (e.g., greater than 50%, 60%, 70%, 80%, or 90 wt %) or completely of an ionic liquid. In this embodiment, the plurality of monomers can not only be dissolved in the ionic liquid, but the ionic liquid also acts a supporting electrolyte for the electropolymerization reaction. Thus, in this embodiment, the supporting electrolyte comprises or consists of an ionic liquid. In certain embodiments, the ionic liquid is formed from: cations, such as those selected from the group consisting of: l-ethyl-3- methylimidazolium (EMIM), l-butyl-3-methylimidazolium (BMIM), and 1 -butyl- 1- methylpyrrolidinium (BMPyrr); and anions, such as those selected from the group consisting of: acetate (OAc), octanoate (OOc), methylcarbonate (MC), trifluoromethanesulfonate (OTf), methanesulfonate (McSOfl, tetrafluoroborate (BF4), bis(trifluoromethylsulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and propionate (OPr).

[0074] Referring again to the Figures, Figures 5A-F illustrate the production of a conductive polymer according to the present invention vs. conventional (classical percolation) processes. Referring to block 5A, a substrate 14 is provided having spaced apart electrodes 12. The discontinuous elctropolymerization process then begins as described herein and percolation networks progressively begin to form as shown in blocks 5B, 5C, and 5D. As can be seen, in block 5E, sufficient conductive polymer networks have formed to electrically bridge the electrodes and allow a current to pass through the conductive polymer networks 20. In contrast to the orderly formation of the conductive polymer networks 20 by discontinuously electropolymerizing the conductive polymer according to aspects of the present invention, continuous electropolymerization produces a more random orientation of conductive polymer networks. Block 5F represents the formation of a thin film region as previously described herein wherein conductance values plateau.

[0075] In particular embodiments, the present inventors have found one or more analyte detection properties of the sensor 10 may be further improved by electrochemically conditioning the formed conductive polymer as described herein. The electrochemical conditioning may be performed by providing an electrical potential or current to the electrodes 12 as described above, but without the monomers of the conductive polymer 18. While not wishing to be bound by theory, it is believed that such an electrochemical conditioning process inserts charge carriers into the conjugated polymer chain or removes charge carriers from the conjugated polymer chain (depending on the applied electrical potential or current). In particular, neutral polymer chains, such as PANI, PPy, and PEDOT, may be oxidized (p-doped) or reduced (n-doped) by providing a deficit or an excess of pi-electrons into the polymer lattice.

[0076] The following are simplified reaction mechanisms of the electrochemical conditioning process for a p-doped conductive polymer, wherein CP+is the conductive polymer in the oxidized (p-doped) form, CP’ is the conductive polymer in the reduced form (n-doped form), and CP is the conductive polymer in the neutral (undoped) form, and A’ and D+are the conditioning ions (doping ions): p-doping

[0077] CP + A’ - CP+A’ + e~ undoping

[0078] Analogous reactions can be given for n-doping: n-doping

[0079] CP + D++ e’ <- CP’ D+undoping

[0080] In an embodiment, the electrochemical conditioning comprises p-doping or undoping the conductive polymer. The p-doping may be performed in the presence of any material comprising a suitable amount of anions, such as Cl’, CIOT, PFe’, or PSS’, including salts comprising such anions. The n-doping may be performed in the presence of any material comprising a suitable amount of cations, such as Li+, Na+, K+, or TBA+, including salts comprising such cations.

[0081] The electrochemical conditioning may be done for any suitable period of time at a suitable, electrical potential or current. In an embodiment, the electrochemical conditioning is done by applying an electrical potential from 0.1 to 1.0 V (vs. Ag / AgCl) to the electrodes in the solution without monomers for a period of 30 seconds to 5 minutes, such as from 1 -2 minutes. In a particular embodiment, the electrochemical conditioning is done by subjecting the conductive polymer to 0.1 V electrical potential for 60 seconds in 0.1 M TBACIO4 in acetonitrile. The electrochemical conditioning step described herein may be performed during or after the formation of the conductive polymer. In certain embodiments, the electrochemical conditioning is done after the electropolymerization step, i.e., after the desired amount of conductive polymer is formed.

[0082] In accordance with another aspect of the present invention, there is provided a process for making a chemiresistive gas sensor with the conductive polymer formed above. The method comprises providing a pair of electrodes on a substrate as described herein. A sensing region comprising the conductive polymer is then formed on the electrodes and the space between the electrodes 12 on the substrate 14 by electropolymerization as described above.

[0083] In accordance with another aspect of the present invention, there is further disclosed a process for detecting an analyte of interest with a chemiresistive gas sensor comprising: forming a chemiresistive gas sensor as described herein; and detecting the presence of one or more target analytes by exposing the chemiresistive gas sensor to a gas suspected of comprising one or more target analytes, wherein, in the presence of the analyte of interest, the conductive polymer displays a change in an electrical property in response to an interaction with the analyte of interest. The function and advantages of the above-described and other embodiments of the present invention will be further understood from the following examples. These examples are intended to be illustrative in nature and are not considered to be limiting to the scope of the invention.

[0084] EXAMPLES

[0085] Sensor Fabrication Experiments

[0086] 1.1 Materials and components

[0087] 3,4-Ethylenedioxythiophene (EDOT, C6H6O2S, >97%), Acetonitrile (MeCN, C2H3N anhydrous, 99.8%), and tetrabutylammonium perchlorate (TBACIO4, N(C4H9)4C1O4, >99.0%) were obtained from Sigma-Aldrich. An ammonia solution (NH3, 25%) was acquired from Merck.

[0088] Pt interdigitated electrodes (IDEs) consisting of 180 pairs of 5 pm wide electrodes with 5 pm gaps on a glass substrate (10 x 6 x 0.75 mm) were purchased from Micrux, Spain. Before electrochemical and electropolymerization experiments, the IDEs were cleaned for 3 minutes with a steady flow of compressed air of ISO 8573-1:2021 Class 1.2.1 standard.

[0089] All electrochemical, electropolymerization, resistance, and doping experiments were performed using a potentiostat (Ivium CompactStat.h standard, Ivium Technology, The Netherlands), connected to a computer equipped with a software (IviumSoft 4.1018). The Ag / AgCl part of a screen-printed electrode (DRP-C1 IL, DropSens, Metrohm) was utilized as the reference electrode. Meanwhile, a platinum rod (3. Id x 49.71 mm) was used as the counter electrode. The two pads of the Pt IDE were connected to a single alligator clip and together served as the working electrode. All experiments were carried out in a 30 mL glass vessel containing 10 mL of the polymerization solutions. A PTFE lid with vacant holes for the potentiostat connections was used to cover the glass vessel.

[0090] Solutions were deaerated first with nitrogen for 15 minutes to remove dissolved gases that may contribute to the redox reactions. The IDEs were then carefully placed so that only the circular region, where the digits are located, was submerged in the solution. During testing, the nitrogen gas was continuously fed on the space above the solution to serve as a gas blanket.

[0091] 1.2 Preparation of polymerization solution

[0092] A polymerization solution of 0.01 M EDOT monomer and 0.1 M TBACIO4 in acetonitrile was prepared for sensor fabrication experiments. EDOT was chosen as the monomer for its polymer’s excellent stability, processability, and high conductivity. Meanwhile, TBACIO4, whose anion has been used as a dopant for various polymer gas sensors, was selected as the supporting electrolyte. The concentrations of the monomer and the supporting electrolyte were decided upon by consideration of the size and geometry of the IDE. Acetonitrile was chosen as the solvent because it easily dissolves both EDOT and TBACIO4. Furthermore, it has a wide electrochemical stability window, as well as a high relative permittivity, allowing for excellent electrolyte dissociation and, thus, a good ionic conductivity.

[0093] 1.3 Electropolymerization, resistance, and doping experiments

[0094] A series of polymerization experiments were performed to generate classical percolation (CsP) and explosive percolation (ExP) growth curves. A crucial modification in the manner of polymerization was made to achieve ExP growth, and this will be discussed shortly. The optimal polymerization conditions for sensor fabrication were then determined according to the generated CsP and ExP growth curves. Galvanostatic polymerization was also utilized in this study for finer control of the polymerization process.

[0095] Potentiodynamic polymerizations of the solution were conducted first to obtain cyclic voltammograms (CVs), which were used to characterize the behavior of the polymerization solution in the three-electrode system. To accomplish this, the potential was scanned at a rate of 100 mV / s from -1.0 to 2.0 V (vs. Ag / AgCl). Thereafter, a preliminary percolation growth curve was then generated. From the obtained CVs obtained, an electropolymerization potential (EEP) was selected. The potential between the IDEs and the reference electrode was kept at the EEP for 1-10 s.

[0096] 1.4 Discontinuous polymerization

[0097] Critically, rather than using multiple IDEs, a single IDE was employed for the entire polymerization interval. In this way, polymerizations were done in a stepwise and discontinuous manner using a single IDE in accordance with the present invention (“discontinuous method”). One round of polymerization for continuous 10 seconds corresponds to 10 rounds of successive polymerization, each for one second, using the discontinuous method described herein.

[0098] 1.5 Resistance measurements

[0099] After each round of polymer growth, the IDEs were thoroughly rinsed with acetonitrile and left to dry in the air. Resistances in the air were then measured by applying a DC potential of 1.0 V for 60 s between the two pads of the IDE. The final current reading was noted, and a resistance value was computed. The reciprocals of the resistance (conductance) were then used to construct a percolation growth curve in a graph of Conductance vs. Time.

[0100] 1.6 Galvanostatic polymerizations

[0101] Galvanostatic polymerizations were performed afterwards in order to make finer percolation growth curves. Using the chronoamperograms acquired from the previous potentiostatic polymerizations as a reference, the total amount of charge (Q) required to arrive at the thin film region was determined through calculation (Qthinfiim = I x dt), where I = current and t = time. The total amount of charge was then partitioned to obtain a certain charge per round of polymerization (QEP) that would result in a better-defined percolation region in a reasonable time. The current between the IDEs and the counter electrode was then kept constant so that the calculated QEP would be applied per round of polymerization.

[0102] From the galvanostatic polymerizations, two sets of percolation curves were made. The first set was created using the discontinuous method described herein, while the second set was obtained by applying multiples of the calculated QEP in separate and fresh IDEs. Rinsing and resistance measurements also followed. However, this time the percolation growth curves were plotted in Conductance vs. Amount of Charge Applied graphs.

[0103] ExP and CsP gas sensors were fabricated using the optimal conditions that were obtained from the percolation growth curves. After a specific amount of charge to be applied was selected, galvanostatic polymerizations were performed. Following the polymer growth, the percolation gas sensors then underwent a doping- dedoping process in the presence of an anion. This was accomplished by immersing the sensors in 0.1 M TBAClO4 / acetonitrile solution for 60 seconds while applying a 0.1 V potential. After the doping-dedoping process, the sensors were rinsed with acetonitrile and allowed to dry in the air before measuring the resistance. All the fabricated gas sensors were stored in clean Petri dishes when not in use. All measurements were done at ambient temperature.

[0104] 1.7 Results

[0105] Figures 6-7 show the percolation growth from galvanostatic polymerization of 0.01 M EDOT / 0.1 M TBACIO4 in MeCN, WE: Pt IDE, RE: Ag / AgCl, CE: Pt Rod, QEP: 100 pC, I: 20 pA. The explosive percolation points were obtained from the median of measurements from 3 IDEs as more polymer were deposited each round. The classical percolation points were obtained from the median of measurements from 3 IDE replicates that were applied with the same amount of charge and all using fresh IDEs.

[0106] As can be seen from Figure 6, the two generated growth curves belong to separate percolation variants. The most notable of the differences is the protracted and significantly steeper increase in the conductances of the discontinuous method’s curve at 0 to 800 pC of applied charge. During this period, the conventional (continuous) method’s conductances were already steadily rising. Another feature that sets them apart is the conventional method’s larger amount of dispersion in the conductance values. Further, as can be seen, there is a longer period where no change in conductance occurs in the formed conductive polymer in the case of explosive percolation.

[0107] Figure 7 is a graph showing a derivative of the change in conductance vs. the change in charge (AC / AQ) vs. amount of charge for the conductive polymers shown in Figure 6. This graph, done in logarithmic scale, shows the stark differences between explosive and percolation growth (and resulting explosive and classical percolation regions). As shown in Figure 7, the explosive growth (discontinuous growth of a conductive polymer as described herein) has a markedly steeper curve for this plot vs. conventional growth (continuous growth). As can be seen in Figure 7, the explosive percolation curve has a value for the derivative of AC (conductance) / AQ charge (AC / AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is at least greater, and particularly, at least 3x greater than a value for the derivative of AC (conductance) / AQ charge (AC / AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) on the classical percolation curve (conventional continuous growth of the conductive polymer).

[0108] This was to be expected, given the steady and uninterrupted polymerization which gave rise to more random network growth. Meanwhile, the discontinuous growth according to aspects of the present invention introduced some form of orderliness in the way the polymer grew, which explains why the conductances there have lesser amounts of dispersion.

[0109] 1.8 Ammonia gas analysis

[0110] The fabricated ExP sensors were placed in a flow-through chamber and were left under a steady flow of air until a stable resistance was reached. Ammonia was chosen as the analyte for the initial characterization of ExP sensors. The CsP sensors’ response to ammonia was also evaluated for comparison. The ammonia content in the headspace was estimated to be 28% wt. The term “sensor response (S)” can be defined in various ways depending on the kind of measurement. In one embodiment, the sensor response is defined as the ratio of the sensor’s resistance in the air (Ra) to its resistance in the presence of an analyte gas (Rg). For the purposes of this disclosure, a definition: (Rg-Ra) / Rt is provided, wherein (Ra) is the subject sensor’s resistance in the air (Ra), (Rg) is the subject sensor’s resistance in the presence of an analyte gas (Rg), and wherein RL represents the lower resistance value, whether in air or in the presence of an analyte gas. This definition accurately reflects the direction of resistance changes, ensuring the consistency of the sensor responses. In Table 1 below, the responses of ExP and CsP sensors to replicate exposures of ammonia gas are presented.

[0111] Table 1. Percolation sensor responses Response (Rg-Ra) / Rt to ammonia gas exposures. The mean and standard deviation values were calculated from five (5) separate ammonia exposures.

[0112] As can be seen from Table 1, chemiresistive sensors formed according to a method as described herein may provide a significant different sensor response to an analyte, e.g., an electron donating analyte, such as ammonia. In particular, as shown in Table 1, in some embodiments, the chemiresistive sensors described herein may provide a sensor response, as determined by the formula (Rg-Ra) / Rt, which is 5000x greater than the sensor response of conventional chemiresistive sensors, i.e., those having a conductive polymer formed continuously vs. discontinuously. In some embodiments, the value is 7500x greater or lOOOOx greater. Figure 8 shows these results in graphical form. In this study, the Rgvalues were obtained from the average ten points before the analyte ingress while the Ra values were obtained from the average ten points before the start of the air purge cycle. In particular, Figure 8 shows the sensor response of ExP gas sensor 1, which has a starting resistance of 3.36 G , to a 5-time exposure of ammonia vapor. The analyte ingress is represented by the dashed circles, whereas the start of purge cycle is represented by the solid circles.

[0113] Figure 9 shows the sensor response of CsP gas sensor 1, which has a starting resistance of 44.9 kQ, to a 5-time exposure of ammonia vapor. The analyte ingress is represented by the dashed circles, whereas the start of purge cycle is represented by the solid circles.

[0114] Figure 10 shows a bar graph of sensor responses (Rg / Ra) to ammonia gas on different scales. In particular, Figure 10 shows sensor responses (Rg-Ra) / Rt, where: Rg / Ra are sensor resistance in gas(sample) and RL is the lower resistance (air or sample) of chemiresistive gas sensors to ammonia vapor, reported as averages of three replicates, in logarithmic scale. Error bars were obtained from the standard deviation of the same replicates. ExP = Explosive percolation sensors, CsP = Classical percolation sensors.

[0115] In the bar graphs of Figure 10, one can observe that ExP sensors have a much greater sensor response, when compared to CsP sensors. It is noted that the Y-axis includes a scale break since the response values differ significantly by order of magnitude between the ExP and CsP sensors. This can be attributed to how the networks were connected in the sharper percolation region of ExP. Meanwhile, the same response pattern, but with some variations in magnitude, was observed for all ExP sensors. These variations are reasonable since sensors fabricated in the percolation region display considerable variability in the way the polymer networks were connected, even though they were made with the same method and parameters. These small differences in the polymer network have a significant influence on how the network responds to gases as a whole, resulting in the mentioned difference in magnitude between sensors of the same type.

[0116] Two other important parameters in chemiresistive sensors are the “response time” and the “recovery time.” Respectively, they are defined as the time required for the resistance to achieve 90% of the complete response (minimum or maximum) following the analyte gas exposure and the time it takes for a sensor to recover to 90% of the original baseline signal upon the removal of analyte gas. The ExP gas sensor 1 was utilized in evaluating these parameters since it has the most stable baseline and lowest drift in responses among the fabricated sensors. As a guide in the calculation, the response-recovery time curve of the third exposure of ExP gas sensor 1 to ammonia is illustrated in Figure 11.

[0117] According to the aforementioned definitions and calculations, the response time and recovery time of ExP gas sensor 1 to ammonia were 3.0 ± 1.7 s and 15 ± 2 s, respectively. These values were influenced by the gas concentration, operating temperature of the sensor, and environmental factors including humidity. Generally, when the analyte gas concentration is high and when the operating temperature is low, the response and recovery times for chemiresistive gas sensors are longer. This was not the case with the ExP gas sensors. Compared to other gas sensors, particularly CsP gas sensors, the response and recovery times were found to be shorter, even though the ammonia gas concentration tested was around 28% wt and the sensor was operated at ambient temperature.

[0118] Embodiments of the disclosure described above do not limit the scope of the invention, since these embodiments are merely examples of the embodiments of the invention, which is defined by the appended claims and their legal equivalents. Any equivalent embodiments are intended to be within the scope of this invention. Indeed, various modifications of the disclosure, in addition to those shown and described herein, such as alternative useful combinations of the elements described, may become apparent to those skilled in the art from the description. Such modifications and embodiments are also intended to fall within the scope of the appended claims.

[0119] It is to be understood that the configurations and / or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible.

Claims

CLAIMS1. A process for forming a chemiresistive gas sensor (10) comprising: providing a substrate (14) having spaced-apart electrodes (12) thereon; and electropolymerizing a conductive polymer (18) on the electrodes and the substrate (14) between the electrodes (12) from polymerization solution comprising a plurality of monomers of the conductive polymer (18) and a supporting electrolyte, wherein the electropolymerizing comprises applying a plurality of non-overlapping electrical pulses to the electrodes (12) in the polymerization solution to form the conductive polymer (18), the conductive polymer (18) comprising a plurality of conductive polymer networks (20).

2. The process of claim 1, wherein the plurality of non-overlapping electrical pulses provides a total charge to the electrode in the polymerization solution of at least 10 uC, at least 100 uC, at least 500 pC, at least 1000 pC, or at least 5000 pC, such as from 500 pC to 1500 pC.

3. The process of any one of the preceding claims, wherein each of the plurality of nonoverlapping pulses have a duration of at least 0.001 second, such as from 0.01 to 10 seconds per intervals.

4. The process of any one of the preceding claims, wherein the plurality of non-overlapping electrical pulses provides a total charge to the electrodes in the polymerization solution, wherein the total charge is divided amongst the plurality of pulses, and wherein the total charge is then applied to the electrodes (12) in the polymerization solution amongst the plurality of pulses.

5. The process of any one of the preceding claims, wherein adjacent pulses of the plurality of non- overlapping electrical pulses are separated by a complete cessation of electrical potential or current to the electrodes (12) in the polymerization solution.

6. The process of any one of the preceding claims, wherein the conductive polymer (18) is characterized as having a value for a derivative of the change in conductance (AC) / change in charge (AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is greater than a value for a derivative for the same plot for a conductive polymer formed continuously.

7. The process of any one of the preceding claims, wherein the conductive polymer (18) is characterized as having a value for a derivative of the change in conductance (AC) / change in charge (AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is at least 3x greater than a value for a derivative for the same plot for a conductive polymer formed continuously.

8. The process of any one of the preceding claims, wherein the value is a maximum for each plot.

9. The process of any one of the preceding claims, wherein the conductive polymer (18) is characterized by having a sensor response according to the formula (Rg-Ra) / Rt for an analyte which is 5000x greater than a sensor response of a conductive polymer formed continuously.

10. The process of claim 8, wherein the analyte is an electron donating analyte.

11. The process of any one of the preceding claims, wherein the conductive polymer (18) provides at least a two-order of magnitude change in resistance upon contact with an analyte of interest relative to a conductive polymer grown continuously.

12. The process of any one of the preceding claims, wherein the electropolymerizing is stopped prior to the formation of a thin film (26).

13. The process of any one of the preceding claims, further comprising electrochemically conditioning the conductive polymer (18) by providing an electrical potential or current to the conductive polymer (18) in the presence of the polymerization solution without the plurality of monomers.

14. The process of claim 13, wherein the electrochemical conditioning is done after the electropolymerization.

15. The process of any one of the preceding claims, wherein the polymerization solution further comprises a solvent for dissolving at least the plurality of monomers.

16. The process of any one of the preceding claims, wherein the supporting electrolyte comprises an ionic liquid.

17. A process for making a chemiresistive gas sensor (10) comprising: electropolymerizing a conductive polymer (18) on a substrate (14) from a polymerization solution comprising a plurality of monomers of the conductive polymer (18) and a supporting electrolyte, wherein the electropolymerizing comprises providing an electrical potential or current discontinuously to the electrode in the polymerization solution to form the conductive polymer (18), the conductive polymer (18) comprising a plurality of conductive polymer networks (20).

18. A process for detecting a target analyte with a chemiresistive gas sensor comprising: forming a chemiresistive gas sensor (10) according a method of any one of claims 1 to 17; and detecting the presence of the target analyte by exposing the chemiresistive gas sensor to a gas suspected of comprising the target analyte, wherein, in the presence of the target analyte, the conductive polymer displays a change in an electrical property in response to an interaction with the target analyte.

19. A conductive polymer formed by a process of any one of claims 1 to 17.

20. A chemiresistive sensor comprising a conductive polymer formed by a process of any one of claims 1 to 17.

21. A chemiresistive gas sensor (10) comprising: spaced-apart electrodes on a substrate (14); and a sensing region (16) between the electrodes (12), wherein the sensing region (16) comprises a conductive polymer (18) comprising a plurality of conductive polymer networks (20) that electrically bridge the electrodes (12), and wherein the conductive polymer (18) comprises an explosive percolation region (28) in a plot of conductance vs. amount of conductive polymer (18) formed.

22. The chemiresistive gas sensor (10) of claim 21, wherein the conductive polymer is characterized as having a value for the derivative of the change in conductance (AC) / change in charge (AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is at least greater than a value for a derivative for the same plot for a conductive polymer formed continuously.

23. The chemiresistive gas sensor (10) of any one of claims 21 to 22, wherein the conductive polymer is characterized as having a value for a derivative of the change in conductance (AC) / change in charge (AQ) on a plot of the derivative of AC / AQ vs. amount of charge (uC) that is at least 3x greater than a value for a derivative for the same plot for a conductive polymer formed continuously.

24. The chemiresistive gas sensor (10) of any one of claims 21 to 23, wherein the conductive polymer (18) is characterized by having a sensor response according to the formula (Rg-Ra) / Rt for an analyte which is 5000x greater than a sensor response of a conductive polymer formed continuously.

25. The process of claim 24, wherein the analyte is an electron donating analyte.

26. The chemiresistive gas sensor (10) of any one of claims 21 to 25, wherein the conductive polymer (18) is grown by applying a plurality of non-overlapping electrical pulses to the electrodes (12) in a polymerization solution comprising monomers of the conductive polymer (18), and wherein the conductive polymer (18) provides at least a two-orders of magnitude change in resistance upon contact with an analyte of interest relative to a conductive polymer grown continuously.

27. The chemiresistive gas sensor (10) of any one of claims 21 to 26, wherein the conductive polymer (18) is selected from the group consisting of polythiophenes, such as poly(3,4- ethylenedioxythiophene) (PEDOT), and conductive polymers, such as polyaniline (PANI), polypyrrole (PPy), and polycarbazole, and their derivatives.

28. The chemiresistive gas sensor (10) of any one of claims 21 to 27, wherein the conductive polymer (18) is doped and / or neutral (undoped).

29. The chemiresistive gas sensor (10) of any one of claims 21 to 28, wherein the gas sensor (10) is capable of detection of an analyte selected from the group consisting of CO2, NO, N2O, NO2, N2O4, SO2, O2, O3, NH3, C2H2, H2, N2, H2S, aromatics, hazardous gases, explosive compounds, volatile biomarkers, volatile organic compounds, and other volatile substances30. The chemiresistive gas sensor (10) of any one of claims 21 to 29, wherein the gas sensor (10) has a response time of less than 1-100 seconds and a recovery time of 10-1000 seconds.

31. The chemiresistive gas sensor (10) of any one of claims 21 to 30, wherein, upon contact with an analyte of interest, the conductive polymer networks (20) exhibit a change in resistance.

32. Use of the chemiresistive gas sensor (10) according to any one of claims 20 to 31 for the detection of one or more target analytes selected from the group consisting of CO2.NO, N2O, NO2, N2O4, SO2, O2, O3, NH3, C2H2, H2, N2, H2S, aromatics, hazardous gases, explosive compounds, volatile biomarkers, volatile organic compounds, and combinations thereof.

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