GB2593724A

GB2593724A - Gas sensor

Info

Publication number: GB2593724A
Application number: GB2004692.6A
Authority: GB
Inventors: Cachelin Pascal; Tobjörk Daniel
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2020-03-31
Filing date: 2020-03-31
Publication date: 2021-10-06
Also published as: GB202004692D0

Abstract

A method of monitoring a concentration (C) of a constituent of a fluid using a field effect transistor or an electrochemical transistor comprises; applying a measurement drain voltage across the transistor, and applying a measurement gate voltage to a gate of the transistor; periodically estimating a threshold voltage (Vest) of the transistor by measuring, for a fixed drain voltage, drain currents (ID(VG)) corresponding to two or more gate voltages (VG) within a range between a first gate voltage (V1) and a second gate voltage (V2), wherein each of the first gate voltage (V1) and the second gate voltage (V2) is within 25% of the measurement gate voltage (VDm); calculating the estimated threshold voltage (Vest) for the transistor based on the measured drain currents (ID(VG)) and corresponding gate voltages (VG); and calculating a concentration (C) of the constituent based on the estimated threshold voltage (Vest).

Description

GAS SENSOR

BACKGROUND

Embodiments of the present disclosure relate to methods of estimating the threshold voltage of semiconductor devices such as field-effect transistors and/or electrochemical transistors. Some embodiments of the present disclosure relate to use of semiconductor devices such as field-effect transistors and/or electrochemical transistors for detecting concentrations of a constituent of a fluid (gas or liquid). Some embodiments of the present disclosure relate to detection of gases, in particular alkenes.

The use of thin film transistors as sensors is disclosed in, for example, Feng et al. tnencapsulated Air-stable Organic Field Effect Transistor by All Solution Processes for Low Power Vapor Sensing" Scientific Reports 6:20671 DOI: 10.1038/srep20671 and Besar et al, "Printable ammonia sensor based on organic field effect transistor", Organic Electronics, Volume 15, Issue 11, November 2014, Pages 3221-3230.

Ethylene produced by plants can accelerate ripening of climatcric fruit, the opening of flowers, and the shedding of plant leaves. The compound 1-methylcyclopropene (1-MCP) is known for use in inhibiting such processes.

WO 2019/063493 Al describes a gas sensor system made up of a first gas sensor that is sensitive to both a target gas and a secondary gas and a second sensor that is only sensitive to the target gas. The response of the two gas sensors is processed to detect a presence of or a concentration of the target gas.

Use of such devices as sensors relies on having a substantially invariable threshold (or "switch-on") voltage, or at least on being able to measure variation and/or drift in the threshold voltage in order to permit correction of measurements. For example, US 2017/0234816 Al describes systems and methods for measuring a temperature dependency of a threshold voltage.

Changes in the environment of a sensor, for example concentrations of non-target gasses, may also cause variations in the threshold voltage of a device being used as a sensor. For example, The impact of 02 concentration on threshold voltages has previously been reported, see Chong, E.-G., Own, Y.-S., Kim, S.-H., & Lee, S.-Y. (2011). Effect of oxygen on the threshold voltage of a-IGZO TFT. Journal of Electrical Engineering and Technology, 6(4), 539-542.

Gas sensors based on monitoring threshold voltages have also been proposed, see Eberle, S., Roman, C., & Hierold, C. (2018). Effect of varying gate distance on the threshold voltage shift in carbon nanotube field effect transistor gas sensors. Microelectronic Engineering, 193, 86-90. ht tps://doi.org/10.10161j. mee.2018.02.027.

SUMMARY

In some embodiments there is provided a method including monitoring a concentration of a constituent of a fluid using a transistor selected from a field effect transistor or an electrochemical transistor. The method includes applying a measurement drain voltage across the transistor, and applying a measurement gate voltage to a gate of the transistor. The method also includes periodically estimating a threshold voltage of the transistor by measuring, for a fixed drain voltage, drain currents corresponding to two or more gate voltages within a range between a first gate voltage and a second gate voltage. Each of the first gate voltage and the second gate voltage is within 25% of the measurement gate voltage. Periodically estimating the threshold voltage of the transistor also includes calculating the estimated threshold voltage for the transistor based on the measured drain currents and corresponding gate voltages. The method also includes calculating the concentration of the constituent based on the estimated threshold voltage.

Herein, any references to -drain current" may be replaced by "source current" without modification. The threshold voltage may be estimated according to a pre-determined or dynamically determined schedule. The threshold voltage may be estimated in response to an output of another sensor. The threshold voltage may be estimated in response to a user input.

The first and second gate voltages may not span a threshold voltage of the transistor. Calculating an estimated threshold voltage for the transistor may be based only on measured drain currents and corresponding gate voltages within the range between the first and second gate voltages. The measurement drain voltage, the measurement gate voltage, the fixed drain voltage, the first gate voltage and the second gate voltage may all correspond, for at least a desired range of concentrations of the constituent, to a regime in which the drain current varies approximately in proportion to the square of the gate voltage. In other words, the measurement drain voltage, the measurement gate voltage, the fixed drain voltage, the first gate voltage and the second gate voltage may be calibrated (or selected during a calibration process) for the transistor based on the range of concentrations of the constituent which it is desired to measure. A larger separation of the first and second voltages may provide a more robust measurement, but may only be useable for a narrower range of target concentrations.

The measurement drain voltage, the measurement gate voltage, the fixed drain voltage, the first gate voltage and the second gate voltage may all be calibrated (or selected during a calibration process) for a desired range of use temperatures. The measurement drain voltage, the measurement gate voltage, the fixed drain voltage, the first gate voltage and the second gate voltage may all be calibrated (or selected during a calibration process) for a desired range of relative humidity. The measurement drain voltage, the measurement gate voltage, the fixed drain voltage, the first gate voltage and the second gate voltage may all be calibrated (or selected during a calibration process) to correspond to a regime in which the drain current varies in proportion to the square of the gate voltage, within a desired range of concentrations of the constituent, in an atmosphere having a composition including 78±0.5% nitrogen, 21±0.5% Oxygen, less than 1.5% Argon and less than 1% carbon dioxide, at a temperature of 20 degrees Celsius, a pressure of one atmosphere (101 kPa), and a relative humidity between 10% and 90%.

The second gate voltage may differ from the first gate voltage by at least 5%, at least 10%, at least 15% or at least 20% of the first gate voltage. The second gate voltage may be less than or equal to 90% of the first gate voltage.

Measuring drain currents corresponding to two or more gate voltages may include measuring a first drain current corresponding to the first gate voltage, and measuring a second drain current corresponding to the second gate voltage.

An estimated threshold voltage for the transistor may be calculated according to: v2.1W1 -Vest -r r vI41 -v 1121 in which Vest is the estimated threshold voltage, Vi is the first gate voltage, V2 is the second gate voltage, b is the first drain current and /2 is the second drain current.

The first gate voltage may be equal to the measurement gate voltage. The fixed drain voltage may be equal to the measurement drain voltage.

The method may also include measuring, using the fixed drain voltage, a measurement drain current corresponding to the measurement gate voltage. Calculating the concentration of the constituent may he additionally based on the measurement drain current. Calculating the concentration of the constituent may be additionally based on a change in the measurement drain current relative to an earlier time and/or a reference device not exposed to the fluid. The measurement drain current may be one of the drain currents measured during the periodic estimation of the threshold voltage of the transistor.

The method may also include, for a second transistor of the same type as the first transistor and configured to be insensitive to the concentration of the constituent or insensitive to the concentration of a background constituent, in response to estimating the threshold voltage of the transistor, estimating a second threshold voltage (V"i) of the second transistor.

Estimating the second threshold voltage of the second transistor may include measuring, for the fixed drain voltage applied to the second transistor, second drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage. Estimating the second threshold voltage of the second transistor may include calculating a second estimated threshold voltage for the second transistor based on the measured second drain currents and corresponding gate voltages. Calculating the concentration of the constituent may be additionally based on the second estimated threshold voltage.

The second transistor may be of the same type as the first transistor. The second transistor may be formed of the same materials as the first transistor. The second transistor may be separated from the fluid by one or more filters, semi-permeable membranes, molecular sieves, absorbent materials, and so forth. The junction between a source electrode and a semiconductor region of the second transistor may include a blocking layer which prevents diffusion and/or adsorption of the constituent or diffusion and/or adsorption of the background constituent. The junction between a drain electrode and the semiconductor region of the second transistor may include a blocking layer which prevents diffusion of the constituent or diffusion of the background constituent.

The transistor may he exposed to a test fluid during the estimation of the threshold voltage. The method may also include exposing the transistor to a filtered fluid. The filtered fluid may have been treated to reduce or remove the concentration of the constituent. The method also includes estimating a third threshold voltage (17,-0 of the transistor by measuring, for the fixed drain voltage applied to the transistor, third drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage. Estimating the third threshold voltage of the transistor may also include calculating a third estimated threshold voltage for the transistor based on the measured third drain currents and corresponding gate voltages. Calculating the concentration of the constituent may be additionally based on the third estimated threshold voltage.

The constituent may be a liquid or a solute dissolved in a liquid.

The constituent may be a gas. The concentration of the constituent may take the form of a partial pressure of the gas. The gas may be 1-methylcyclopropene or ethylene. The background constituent may be ethylene or 1-methylcyclopropene.

The transistor may be an organic field effect transistor. The transistor may be an organic thin-film transistor.

The transistor may be an inorganic field effect transistor. An inorganic field effect transistor may be a metal-oxide field effect transistor.

The transistor may be an electrochemical transistor.

According to a second aspect of the invention there is provided apparatus for measuring a concentration of a constituent of a fluid. The apparatus includes a transistor in the form of a field effect transistor or an electrochemical transistor. The apparatus also includes a controller configured to monitor a concentration of a constituent of a fluid using the transistor. Monitoring the concentration of the constituent includes applying a measurement drain voltage across the transistor, and applying a measurement gate voltage to a gate of the transistor. The controller is also configured to periodically estimate a threshold voltage of the transistor by measuring, for a fixed drain voltage, drain currents corresponding to two or more gate voltages within a range between a first gate voltage and a second gate voltage. Each of the first gate voltage and the second gate voltage is within 25% of the measurement gate voltage. The apparatus is also configured to calculate an estimated threshold voltage for the transistor based on the measured drain currents and corresponding gate voltages. The apparatus is also configured to calculate the concentration of the constituent based on the estimated threshold voltage.

The apparatus may include features corresponding to any features of the method.

Calculating the estimated threshold voltage may be based only on the first and second drain currents and the first. and second gate voltages.

The transistor and the controller may be housed in a detector. The estimated threshold voltage may be calculated by the controller and the concentration of the constituent may be calculated by the controller.

The transistor and the controller may be housed in a detector. The apparatus may also include a monitoring device in communication with the detector. The estimated threshold voltage may be calculated by the controller and the concentration of the constituent may be calculated by the monitoring device.

The monitoring device may include a processor and volatile memory. The processor may take the form of one or more general purpose digital electronic central processing units (CPUs). The monitoring device may communicate with the detector using a wired or wireless link. The monitoring device may communicate with two or more detectors concurrently. The monitoring device may communicate with two or more detectors according to a predetermined or dynamically determined sequence.

The transistor and the controller may be housed in a detector. The apparatus may also include a monitoring device in communication with the detector. The estimated threshold voltage may be calculated by the monitoring device and the concentration of the constituent may be calculated by the monitoring device.

Measuring drain currents corresponding to two or more gate voltages may include measuring a first drain cut-rent corresponding to the first gate voltage, and measuring a second drain current corresponding to the second gate voltage.

The controller may be further configured to measure, at the fixed drain voltage, a measurement drain current corresponding to the measurement gate voltage. Calculating the concentration of the constituent may be additionally based on the measurement drain current.

The first gate voltage may be equal to the measurement gate voltage. The fixed drain voltage may be equal to the measurement drain voltage. The measurement drain current may be one of the drain currents measured during the periodic estimation of the threshold voltage of the transistor.

The apparatus may also include a second transistor of the same type as the first transistor and configured to be insensitive to the concentration of the constituent or insensitive to the concentration of a background constituent. The controller may be further configured, in response to estimating the threshold voltage of the transistor, to estimate a second threshold voltage (V,4) of the second transistor. Estimating the second threshold voltage of the second transistor may include measuring, for the fixed drain voltage applied to the second transistor, second drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage. The apparatus may he configured to calculate a second estimated threshold voltage for the second transistor based on the measured second drain currents and corresponding gate voltages. Calculating the concentration of the constituent may be additionally based on the second estimated threshold voltage.

The second transistor may be of the same type as the first transistor. The second transistor may be formed of the same materials as the first transistor. The second transistor may be housed in the detector. The second estimated threshold voltage may be calculated by the controller. The second estimated threshold voltage may be calculated by the monitoring device.

The apparatus may be configured to expose the transistor to a test fluid during the estimation of the threshold voltage. The apparatus may be further configured to expose the transistor to a filtered fluid. The filtered fluid may have been treated to reduce or remove the constituent. The apparatus may also be configured to estimate a third threshold voltage (V,J) of the transistor by measuring, for the fixed drain voltage applied to the transistor, third drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage. The apparatus may also be configured to calculate a third estimated threshold voltage for the transistor based on the measured third drain currents and corresponding gate voltages. Calculating the concentration of the constituent may be additionally based on the third estimated threshold voltage.

The detector may be connected to a monitored environment via a first fluid flow path including a first valve or via a second fluid flow path including a second valve and a filter configured to reduce or remove the constituent. A return path from the detector to the monitored environment may include a valve configured to prevent back-flow into the detector. The second fluid flow path may include a re-hydration source. The transistor may be exposed to the test fluid by opening the first valve and closing the second valve. The transistor may he exposed to the filtered fluid by closing the first valve and opening the second valve.

The constituent may be a liquid. The constituent may be a gas. The gas may be 1-methylcyclopropene.

The transistor may be an organic field effect transistor or an inorganic field effect transistor.

DESCRIPTION OF THE DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology.

Figure 1 schematically illustrates the structure of a first example of an organic semiconductor transistor; Figure 2 schematically illustrates the structure of a second example of an organic semiconductor transistor; Figure 3 schematically illustrates the structure of a third example of an organic semiconductor transistor; Figure 4 is a block diagram of a monitoring system; Figure 5 is a process flow diagram of a method using the monitoring system shown in Figure 4; Figure 6 schematically illustrates a transfer sweep; Figure 7 schematically illustrates determining a threshold voltage using transfer sweep data; Figure 8 schematically illustrates gate voltage as a function of time for the method shown in Figure 5; Figure 9 is a process flow diagram of an improved method of estimating a threshold voltage of a transistor; Figure 10 schematically illustrates determining an estimated threshold voltage using the improved method shown in Figure 9; Figure 11 schematically illustrates gate voltage as a function of time for the improved method shown in Figure 9; Figure 12 is a block diagram of a second detector for use in the monitoring system shown in Figure 4; Figure 13 schematically illustrates the structure of a first specific transistor; Figure 14 schematically illustrates the structure of a second specific transistor; Figure 15 schematically illustrates the structure of a fourth specific transistor; Figures 16 and 17 schematically illustrate calculating a concentration of a target constituent in the presence of a secondary (background) constituent which may affect the measurement.

Figure 18 is a block diagram of a portion of a second monitoring system; Figure 19 presents an experimentally measured transfer sweep for an organic thin-film transistor; Figure 20 presents the experimentally measured transfer sweep shown in Figure 19 plotted using the square root of drain currents; Figure 21 presents results of measurements of drain current and estimated threshold voltage in atmospheres having differing levels of Oxygen, 02; and Figure 22 presents estimated threshold voltage and measured drain current as a function of time, spanning repeated exposures to 1-MCP at a concentration of 1 part-per-million (ppm).

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of die embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof. Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. As used herein, by a material -over" a layer is meant that the material is in direct contact with the layer or is spaced apart therefrom by one or more intervening layers. As used herein, by a material "on" a layer is meant that the material is in direct contact with that layer. A layer "between" two other layers as described herein may be in direct contact with each of the two layers it is between or may be spaced apart from one or both of the two other layers by one or more intervening layers.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may likewise be embodied as a computer-readable medium claim or in other forms, such as being embodied in a meansplus-function claim.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

The techniques introduced here can be embodied as special-purpose hardware (e.g., circuitry), as programmable circuitry appropriately programmed with software and/or firmware, or as a combination of special-purpose and programmable circuitry. Hence, embodiments may include a machine-readable medium having stored thereon instructions which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), magneto-optical disks. ROMs, random access memories (R A M s), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media / machine-readable medium suitable for storing electronic instructions. The machine-readable medium includes non-transitory medium, where non-transitory excludes propagation signals. For example, a processor can be connected to a non-transitory computer-readable medium that stores instructions for executing instructions by the processor.

Transistors such as field-effect transistors and/or electrochemical transistors may be used to monitor/measure concentrations of a constituent of a fluid. For example, a concentration of dissolved solute in a liquid, or a partial pressure of a particular target gas in an atmosphere. Typically, this involves applying a measurement drain voltage VD"i across the transistor (between source and drain electrodes) and applying a measurement gate voltage VG", to a gate of the transistor (between gate and source electrodes). A measurement drain current /Dn, changes in response to interactions of the constitucnt with the transistor. The source current IS could be measured instead of the drain current ID, and in this specification any reference to a drain current ID should be read as referring to a drain current and/or source current /s. For example, the constituent may interact with and modify properties of a semiconductor material forming the channel, and/or interact with the boundaries between electrodes and the semiconductor material to modify charge injection characteristics. The measurement drain current. /Dm may be converted into a measured concentration C of the constituent by comparison against, calibration measurements performed using a range of known constituent concentrations C. However, in most cases the transistor will not interact exclusively with the constituent, and other ambient fluids (liquid or gas) or other factors may also vary the characteristics of the transistor being used as a sensor. For example, in a gas sensor, oxygen concentration, humidity and other environmental factors may cause changes in the response of a sensor to a particular concentration of the constituent.

In one particular example of measuring a concentration C of 1-methylcyclopropene (1-MCP) in a gaseous atmosphere of a fruit storage unit which also contains concentrations of ethylene, the inventors have found that moving from a high background oxygen, 02, concentration to a low background 02 concentration may increase the relative response of transistors used as sensors for 1-MCP, when measured as a function of the decrease in a measurement drain current tom.

Such variations in the environment, which are not correlated with desired measurements of constituent concentration, require additional calibration of sensors. Returning to the example of measuring 1-MCP in a fruit storage unit, low oxygen environments may be employed in such applications.

Another example when monitoring/measuring of threshold voltages Vth may be useful is in order to compensate for ageing or bias-stress decay in of a transistor.

The requirement for additional calibrations may be mitigated by monitoring the threshold voltage Vth of a transistor used as a sensor. Conventionally, threshold voltages Vth are measured by conducting "transfer sweep" measurements of transistor transfer characteristics. This involves varying the applied gate voltage VG across a wide range spanning the threshold voltage Vth, and using the corresponding drain currents ID to extrapolate the threshold voltage Vth.

Conventional transfer sweep measurements may have several drawbacks. Firstly, obtaining a transfer sweep is relatively time consuming, requiring that the gate voltage VG he varied widely. A maximum rate (V/s) at which a gate voltage VG may be ramped may be limited by factors such as, but not limited to, gate capacitance CG, semiconductor material response time, dwell time needed to obtain a single measurement in combination with a relatively large number of measurements, and so forth. As measurements of the constituent concentration cannot be obtained during such transfer sweep measurements (except perhaps at a time when VG is equal to the measurement gate voltage Vum), transfer sweeps may decrease the responsivity of a transistor used as a sensor. This may be especially detrimental when a sensor is to be used as part of a feedback loop for controlling constituent concentration C in a particular monitored environment. Returning to the example of 1-MCP monitoring, this gas is used to suppress ripening of fruit. If the concentration C is to be adjusted by pumping gasses in and/or out of a monitored environment, slow responsivity could lead to overshooting a target concentration.

Secondly, transfer sweeps typically require the gate voltage VG to be varied between extrema' points -i.e. relatively high biasing voltages. This can lead to increased power consumption. For permanently wired sensors this may he less of an issue. However, for battery powered sensors deployed to monitor an environment (for example reporting using low-power wireless links), power consumption is a significant consideration. Returning to the example of 1-MCP monitoring in fruit storage, once a storage area is sealed, it is not desirable to have to re-open it to replace batteries.

Application of high biasing voltages during transfer sweeps also applies so called "bias-stress" to transistors used as sensors. In particular for organic field-effect transistors and/or organic electrochemical transistors, the organic semiconductors used may undergo material degradation when exposed to high biasing voltages (large electric fields). Such degradation will cause a sensor to drift from calibrated values and become unreliable. Consequently, reducing the occurrence and duration of high biasing voltages may extend the useful lifetime of transistors used as sensors The present specification is based, at least in part, on the inventors' surprising discovery that an adequate estimate Ve,st of the threshold voltage Vth of a transistor may be obtained without performing a conventional transfer sweep. Instead, the inventors have established that the gate voltage VG may be varied across a much narrower range of voltages, often close to (or starting from) a measurement gate voltage VGht used for normal operation as a sensor. Importantly, the reduced range of voltages does not span (or even approach) the threshold voltage Va.. The resulting estimate Vesr of the threshold voltage Vth has been demonstrated to be useable directly for estimating the concentration of a target constituent, or to compensate measurement drain currents lErm for changes in the ambient environment or in concentrations of background constituents of the fluid. At minimum, the inventors have found that only two measurements of drain current /D(V/), /D(V2) at two different gate voltages VI, V2 are required.

Amongst other advantages, the methods of the present specification may enable shorter delays between measurements of a concentration C (improved responsivity), reduced power consumption and/or reduced bias-stressing of a field-effect transistor or electrochemical transistor used as a sensor.

The methods of the present specification are applicable to a wide range of different applications, including monitoring of 1-MCP in fruit storage units with reduced sensitivity to changes in background oxygen concentration and/or humidity (concentration of water vapor).

Referring also to Figure 1, a first transistor 1 is shown.

The first transistor 1 is an example of a bottom contact, bottom-gate organic field-effect transistor suitable for use as a sensor for measuring a concentration C of a target constituent in a fluid -for example a gas sensor for sensing concentration C of a gas. The first transistor 1 includes a gate electrode 2 over a substrate 3, source and drain electrodes 4. 5, a dielectric layer 6 between the gate electrode 2 and the source and drain electrodes 4, 5, and an organic semiconductor layer 7 in contact with the source and drain electrodes 4, 5. The organic semiconductor layer 7 may at least partially cover, or may completely cover, the source and drain electrodes 4, 5.

Referring also to Figure 2, a second transistor 8 is shown.

The second transistor 8 is an example of a top-contact, bottom gate organic field-effect transistor suitable for use as a sensor for measuring a concentration C of a constituent in a 1.5 fluid. The second transistor 8 is the same as the first transistor 1, except that the organic semiconductor layer 7 is between the dielectric layer 6 and the source and drain electrodes 4, 5.

Referring also to Figure 3, a third transistor 9 is shown.

The third transistor 9 is an example of a top gate organic field-effect transistor suitable for use as a sensor for measuring a concentration of a constituent in a fluid as described hereinafter. The third transistor 9 comprises source and drain electrodes 4, 5, an organic semiconductor layer 7 contacting the source and drain electrodes 4, 5, and a dielectric layer 6 between the gate electrode 2 and the organic semiconductor layer 7. The dielectric layer 6 of the third transistor 9 is preferably a fluid-permeable material, preferably an organic material, which allows permeation of the fluid (e.g. gas) through the dielectric layer 6 to the organic semiconducting layer 7. Bottom gate transistors such as the first and second transistors 1, 8 may be preferable to top gate transistors such as the third transistor 9, because a fluid to be measured may more readily access the organic semiconductor material 7 of a bottom gate device.

The source and drain electrodes 4, 5 can be formed from a wide range of conducting materials for example a metal (e.g. gold), metal alloy, metal compound (e.g. indium tin oxide) or conductive polymer. The choice of materials for the source and drain electrodes 4, 5 may be based on providing suitable charge injection to a material of the organic semiconducting layer 7. Similarly, the gate electrodes 2 may be formed from any conducting material, for example a metal (e.g. aluminium), a metal alloy, a conductive metal compound (e.g. a conductive metal oxide such as indium tin oxide) or a conductive polymer.

The length of a channel defined between the source and drain electrodes 4, 5 of the first, second or third transistors 1, 8, 9 may be up to 500 microns, but preferably the length is less than 200 microns, more preferably less than 100 microns.

Although the first to third transistors 1. 8. 9 have been described as including organic semiconductors, it should be appreciated that an inorganic semiconductor may be used in place of an organic semiconductor as described anywhere herein. For example, the organic semiconductor layers 7 of the first to third transistors 1, 8, 9 could be replaced with semiconducting metal oxides, or any other semiconducting material which experiences a change in either hulk and/or charge injection properties upon exposure to a concentration C of the target constituent.

Organic semiconductors as described herein may be selected from conjugated non-polymeric semiconductors; polymers comprising conjugated groups in a main chain or in a side group thereof; and carbon semiconductors such as graphene and carbon nanotubes.

An organic semiconductor layer 7 of any of the first to third transistors 1, 8, 9 may include or consist of a semiconducting polymer and / or a non-polymeric organic semiconductor. The organic semiconductor layer 7 may comprise a blend of a non-polymeric organic semiconductor and a polymer. Exemplary organic semiconductors are disclosed in WO 2016/001095, the contents of which are incorporated herein by reference.

The organic semiconductor layers 7 of the first and second transistors 1, 8 (bottom gate) preferably include or consist of only one type of organic semiconductor. The organic semiconductor layer of the third transistor 9 (top gate) is preferably a mixture of a non-polymeric and polymeric organic semiconductor.

Organic semiconducting layers 7 may be deposited by any suitable technique, including evaporation and deposition from a solution including or consisting of one or more organic semiconducting materials and at least one solvent. Exemplary solvents include benzenes with one or more alkyl substituents, preferably one or more C1-10 alkyl substituents, such as toluene and xylene; tetralin; and chloroform. Solution deposition techniques include coating and printing methods, for example spin coating dip-coating, slot-die coating, ink jet printing, gravure printing, flexographic printing and screen printing.

Optionally, the organic semiconducting layer 7 of an organic field-effect transistor 1, 8, 9 has a thickness in the range of about 10 um to 200 nm.

Exemplary inorganic semiconductors which could be substituted for the organic semiconducting layer 7 include, without limitation, n-doped silicon; p-doped silicon; compound semiconductors, for example III-V semiconductors such as GaAs or InGaAs; doped or undoped metal oxides; doped or undoped metal sulfides; doped or undoped metal selenides; or doped or undoped metal tellurides.

The dielectric layer 6 is formed of a dielectric material (insulating material). Preferably, the dielectric constant, k, of the dielectric material is at least 2 or at least 3. The dielectric material may be organic, inorganic or a mixture thereof. Preferred inorganic materials include Sift, SiN, and spin-on-glass (SOG). Preferred organic materials are polymers and include insulating polymers such as poly vinylalcohol (PVA), polyvinylpyrrolidine (PVP), acrylates such as polymethylmethacrylate (PMM A) and benzocyclobutanes (BCBs), poly(vinyl phenol) (PVPh), poly(vinyl cinnamate) P(VCn), poly(vinylidene fluoride-cohexafluoropropylene) P(VIDE-1-1FP), P(VDF-TrFE-CTFE), and self-assembled monolayers, e.g. silanes, on oxide. The polymer may be crosslinkable. The dielectric layer 6 may be formed from a blend of materials or comprise a multi-layered structure. In the case of a bottom-gate transistor 1, 8, the gate electrode 2 may be reacted, for example oxidised, to form the dielectric layer 6.

The dielectric layer 6 may be deposited by thermal evaporation, vacuum processing or lamination techniques as are known in the art. Alternatively, the dielectric layer 6 may be deposited from solution using, for example, spin coating or ink jet printing techniques and/or other solution deposition techniques discussed above. In the case of a bottom gate transistor 1, 8, the dielectric layer should not be dissolved if an organic semiconductor layer 7 is deposited onto it from solution. In the case of a top-gate transistor 9, the organic semiconductor layer 7 should not be dissolved if the dielectric layer 6 is deposited from solution.

Techniques to avoid such dissolution include: use of orthogonal solvents for example use of a solvent for deposition of the organic semiconducting layer 7 that does not dissolve the dielectric layer 6 in the case of a bottom gate transistor 1, 8 or vice versa in the case of a top gate transistor 9; cross linking of the dielectric layer 6 before deposition of the organic semiconductor layer 7 in the case of a bottom gate transistor 1, 8; or deposition from solution of a blend of the dielectric material and the organic semiconductor followed by vertical phase separation as disclosed in, for example, L. Qiu, et al., Adv. Mater. 2008, 20, 1141.

The thickness of the dielectric layer 6 is preferably less than 2 micrometres, more preferably less than 500 nm.

The substrate 3 of a transistor 1, 8, 9 as described herein may be any insulating substrate, for example glass or plastic.

Referring also Figure 4, a monitoring system 10 is shown.

The monitoring system 10 monitors a monitored environment 11. The monitored environment 11 contains, or is otherwise filled or partially filled with a fluid (not shown). For example, the monitored environment 11 may be a room or a storage container filled with a gas, or at least partly filled with a liquid, a pipe, and so forth. Referring again to the example of 1-MCP monitoring, the monitored environment 11 may be a room or other container for storing fruit, cut flowers and so forth.

A detector 12 is also contained within the monitored environment 11. The monitored environment 11 contains at least one detector 12, but may optionally include up to a number N of detectors 12. The detector(s) 12 communicate with a monitoring module 13 outside the monitored environment 11 via respective links 14. The links 14 may be wired or wireless links, but in either case should preferably not compromise the integlity of the monitored environment 11 to contain the fluid (gas or liquid). For example, when the links 14 are wired, the wires/cables may pass through one or more seals. Similarly, if the links 14 are wireless, an exterior of the monitored environment should not block the transmissions. Wireless links may use radio frequency, microwave, infra-red or visible radiation. Wireless links need not use electromagnetic radiation, and may use acoustic or ultrasonic vibrations for transmission across a boundary of the monitored environment 11.

In other examples (not shown) the monitoring module 13 may be within the monitored environment 11 instead. A further wired or wireless link (not shown) may be used to communicate between the monitoring module 13 and the outside of the monitored environment 11. In some examples, one or more detectors 12 may be outside of a main portion of the monitored environment 11, and may be connected to the monitored environment 11 by one or more fluid-flow paths to allow sampling.

Each detector 12 includes a transistor 15, a controller 16 and a power supply 17. The transistor 15 takes the form of a field-effect transistor (including thin-film transistors) or an electrochemical transistor which is sensitive to the concentration C of a target constituent which it is desired to measure/monitor. The transistor 15 is housed in or supported by the detector 12, and the detector 12 will need to he configured to allow the fluid to access the transistor 15. Depending on the type of fluid monitored (liquid or gas, conductive or not), other components of the detector 12 such as the controller 16 and power supply 17 may be sealed off from the monitored environment 11. The transistor 15 may correspond to, for example, any of the first to third transistors 1, 8, 9 (Figures Ito 3). When the fluid contained by the monitored environment 11 is a liquid, the target constituent may correspond to a liquid mixed with one or more other liquids, or the target constituent may correspond to a dissolved solute having a concentration in, for example kg.m-3, molsn-3 and so forth. When the fluid contained by the monitored environment 11 is a gaseous atmosphere, the target constituent may correspond to one gas in that atmosphere having a concentration in, for example, partsper-million (ppm), a partial pressure, and so forth. All detectors 12 may be sensitive to the same target constituent, however, in some examples different types of detector 12 may be included to sense concentrations C corresponding to multiple target constituents. In other examples, each detector 12 may include multiple different types of transistor 15, each configured to detect concentrations of a different target constituent.

The controller 16 may be any device suitable for controlling the transistor 15 to carry out the methods described herein. For example, the controller 16 may take the form of a suitably programmed microcontroller, a field programmable gate array, or one or more general purpose digital electronic central processing units (CPUs) in combination with volatile memory. The controller 16 may also provide communications via the link 14. In some examples, communications over the link 14 may be provided by a separate communications module/interface (not shown), under control of the controller 16. The power supply 17 may be a wired supply, for example a mains electricity supply or a supply provided along a link 14 in the form of a cable. Alternatively, the power supply 17 may be a single-use or rechargeable battery.

A detector 12 using a wireless link 14 and including a power supply 17 in the form of a battery may be particularly useful when the monitored environment is large, because lengths of cables running across/around the monitored environment 11 may he avoided.

Optionally, each detector 12 may include non-volatile storage 18. The detector 12 may store measurements made using the transistor 15 to the storage 18, for example every measurement or only periodically. The storage 18 may provide a local backup in the event of failure of the monitoring device 13 and/or the link 14 The monitoring device 13 may have links 14 to one, some or all of the N detectors (N? 1). Although a single monitoring device 13 is shown in Figure 4, in some examples two or more identical monitoring devices 13 may be used to monitor a large number N of detectors 12. Each monitoring device 13 includes a processor 19, volatile memory 20, and non-volatile storage 21. Optionally. a monitoring device 13 may include a communications interface 22. The processor 19 may take the form of one or more general purpose digital electronic central processing units (CPUs). Examples of suitable monitoring devices include, without being limited to, a laptop or desktop personal computer, or similar data processing devices The monitoring device 13 may communicate with two or more detectors 12 concurrently using the links 14, or may communicate with two or more detectors 12 according to a predetermined or dynamically determined sequence.

In some examples the monitoring device 13 may be permanently installed relative to the monitored environment 11. However, the monitoring device 13 may be mobile, and may establish links 14 with the detectors 12 in response to being brought into proximity.

For example, a number of detectors 12 may be installed adjacent to valves, joints, inspection hatches and so forth in a chemical plant, and the transistors 15 may be configured to detect leaks. The controller 16 may periodically wake each detector 12 to take a measurement and store it to on-board storage 18. The monitoring device 13 may be a portable device, for example a mobile phone or tablet computer, which when close enough establishes a link to a particular detector 12 to read out the measurements from the storage 18. For example, a maintenance engineer may carry the monitoring device 13 around an inspection route visiting each detector 12 in turn. This may allow an intermittent leak to be detected, even if the communications range of the link 14 for each detector 12 is short (e.g. RFID, Bluetooth RTM or similar) When present, the communications interface 22 may enable the monitoring device 13 to communicate, for example to send detection alerts, across one or more wired or wireless networks. For example, the communications interface 22 may be capable of communicating across mobile phone networks to send text alerts if a concentration C of a target constituent exceeding a threshold level is detected.

The function of the monitoring system 10 may become further apparent in relation to a specific example application and target constituent. For example, the monitoring system 10 may be installed for a monitored environment 11 in which alkenes may be present in the atmosphere, for example a warehouse in which harvested climateric fruits and/or cut flowers are stored and in which ethylene may be generated.

The presence and / or concentration of ethylene may be determined using the transistors 15 of one or more detectors 12. If the monitoring device 13 determined that the ethylene concentration C reaches or exceeds a predetermined threshold value, which may be any value greater than zero, then the monitoring device 13 may control a 1-MCP source to release 1-MCP to retard the effect of the ethylene, such as ripening of fruit or opening of flowers in the environment.

Additionally or alternatively, the monitoring device 13 may control a 1-MCP source to release 1-MCP into the monitored environment if a 1-MCP concentration C measured using transistors 15 of one or more detectors 12 falls to or below a threshold 1-MCP concentration C value. The threshold 1-MCP concentration C value may be zero or a positive value.

The monitored environment may be divided into a plurality of regions, and a detector 12 may be installed in each region.

Although a specific application of the monitoring system 10 has been described with reference to 1-MCP and ethylene, it will be appreciated that the monitoring system 10 may be used in detection of strained alkenes generally, optionally compounds comprising a cyclopropene or cyclobutene group, of which alkylpropenes such as 1-MCP are examples; in detection of aliphatic alkenes, optionally ethylene, propene, 1-butene or 2-butene; and / or in detection of compounds with a dipole moment, such as hydrocarbons which do not have a mirror plane bisecting a carbon-carbon bond of the hydrocarbon. Preferably, compounds with a dipole moment as described herein have a dipole moment of greater than 0.2 Debyes optionally greater than 0.3 or 0.4 Debyes. The monitoring system 10 may also be used in the detection of aromatic alkenes.

The monitoring system 10 is not limited to detection of alkenes, and may be used to measure/monitor concentrations C any target constituent of a fluid (gas or liquid) which a transistor 15 is responsive to or may be configured to be responsive to.

The monitoring system 10 may include one or more control sensors (not shown). to provide a baseline for measurements of the detectors 12 to take into account variables such as one or more of humidity, temperature, pressure, and concentrations of constituents other than a target constituent or target constituents in the monitored environment 11. One or more control sensors (not shown) may be isolated from the monitored environment 11, for example by encapsulation of the, or each, control sensor (not shown), or may be exposed to the monitored environment 11, depending on the parameter sensed.

Referring also to Figure 5, a method of using the monitoring system 10 shall be described.

The monitoring device 13 addresses the first detector 12 across the respective link 14 (step Si).

The controller 16 of the addressed detector 12 conducts a transfer sweep of the transistor 15 (step 52). For example, a measurement drain voltage Vd", is applied across the transistor 15 whilst the gate voltage VG is ramped between a minimum sweep voltage V"pi" and a maximum sweep voltage V.,.

Referring also to Figure 6, a schematic illustration of a transfer sweep 23 of a transistor 15 is shown.

The transfer sweep 23 corresponds to measurements of the drain current ID as a function of gate voltage VG, i.e. /D(VG) for Vinin < VG < Vmax. Depending on a DC offset, the sign of the drain current In may or may not change during the transfer sweep 23, but this detail is neither essential nor problematic for determining the threshold voltage Vth.

Using the results of the transfer sweep (step 52), for example transfer sweep 23, the controller 16 determines the threshold voltage Va, of the transistor 15 (step S3).

Referring also to Figure 7, determination oldie threshold voltage Vih is illustrated.

The threshold voltage Vii, is determined by extrapolating a quadratic region AV",,,d of the transfer sweep 23 for which the drain current //) varies approximately in proportion with the square of the gate voltage VG. The quadratic region AVq,""/ may be visualised by plotting square root of the (modulus) of the drain current 111)105 against the gate voltage VG. In this plot, for example as shown in Figure 7, the quadratic region AV,,"thi is approximately linear.

A straight line fit to within the quadratic region AVq",,d is extrapolated to in=0, and the intercept is the threshold voltage VII1.

A measurement drain current ID,,, is obtained whilst applying a measurement gate voltage VG", to the transistor 15 gate, with the bias across the transistor 15 equal to the measurement drain voltage VD", (step S4). This step may be optional if the range V,"i" to includes the measurement gate voltage VGtn, since in this case the measurement drain current /Dm may be extracted from the transfer sweep 23 data. Even if the range V",i" to V""" includes the measurement gate voltage VG,,,, it may be preferable to measure the measurement drain current lbw separately from the transfer sweep 23, for example to permit a longer dwell time at the measurement gate voltage VG", for time averaging of noise.

The concentration C of the constituent is estimated (or equivalently calculated) based on the measurement drain current 'an and the threshold voltage Vth (step S5). For example, values of measurement drain current /b,, and/or threshold voltage Vil, corresponding to known calibration concentrations C may be measured when calibrating a detector 15. In operation, the calibrated values may be looked-up and/or interpolated/extrapolated to determine a concentration C corresponding to a measurement drain current /D,,,.

If there are further detectors 12 connected to the monitoring device 13 (step S6IYes), then the next detector 12 is addressed (step S7) and the measurement process repeated (steps S2 through S5).

The method has been explained assuming that the monitoring device 13 addresses the connected detectors 12 sequentially. Alternatively, the monitoring device 13 may carry out steps S2 through S5 concurrently for two or more detectors 12. In another variation, steps S2 through S5 may be carried out internally to each detector 12, for example by the controller 16, and only the estimated concentrations C may be sent to the monitoring device 13 across the links 14.

Referring also to Figure 8, a schematic plot of gate voltage VG versus time t is shown for the method using a transfer sweep 23.

Starting at a time ti, the transfer sweep 23 is started at gate voltage VG = 14,u. Between time ti and the end of the transfer sweep. the gate voltage VG is linearly ramped from the minimum value Vmh to the maximum value VG= The total duration 12-11 of the transfer sweep 23 may typically last in the region of 15 to 60 seconds. Following the transfer sweep 23, the gate voltage VG is set to the measurement gate voltage VG", for a period t3-t2. Although the transfer sweep 23 encompassed the measurements gate voltage VG,, obtaining a measurement of the measurement drain current /Din with a longer dwell time allows time averaging to reduce any influence of noise. Alternatively, the period between times t2 and H may be omitted, and the measurement drain current /Dm may be extracted from the transfer sweep 23.

As explained hereinbefore, obtaining a transfer sweep 23 is relatively time consuming and requires that the gate voltage VG be varied widely between V,"i" and V",. The time taken to obtain a transfer sweep 23 may be further compounded in applications where the measurement device 13 time multiplexes readout of two or more detectors. The time needed to complete the transfer sweep 23 and determine the threshold voltage Vth may decrease the responsivity of the detector 12. Secondly, transfer sweeps 23 spanning between relatively high biasing voltages may have relatively high power consumption. Furthermore, application of relatively high biasing voltages Vnii", V,""," during a transfer sweep 23 may also increase bias-stress of a transistor 15, shortening an effective lifetime of the corresponding detector 12.

Improved method of estimating the threshold voltage.

The inventors of the present specification have developed an improved method of estimating the threshold voltage Vth. The improved method may enable shorter delays between measurements (improved responsivity), reduced power consumption and/or reduced bias-stressing of a field-effect transistor or electrochemical transistor used providing transistor 15.

Referring also to Figure 9, the improved method of estimating the threshold voltage is illustrated.

The improved method is based on monitoring a concentration C of a target constituent of a fluid using a transistor 15 selected from a field effect transistor or an electrochemical transistor. The monitoring involves, at least some of the time, applying a measurement drain voltage VD," across the transistor 15 (between source and drain electrodes 4, 5, whilst a measurement gate voltage VG," is applied to the gate of the transistor 15 and a resulting measurement drain current ID, is measured.

For each detector 12 in the monitoring system 10, the threshold voltage Vii, of the corresponding transistor 15 is periodically estimated using the improved method illustrated in Figure 9.

A first detector 12 connected to the monitoring device 13 is addressed (step S8).

The drain voltage VD applied across the transistor 5 is held to a fixed drain voltage vpf throughout the following measurements. Preferably, the fixed drain voltage Vij is equal to the measurement drain voltage Vim,. However, the fixed drain voltage VDf may differ from the measurement drain voltage VD", in some examples.

The gate voltage Vu applied to the transistor 15 is set equal to a first gate voltage Vu = Vi (step S9). The corresponding drain current /D(Vu)=1D(Vi) is measured (step S10). If VG is not equal to a second gate voltage V2 (step SHINo), then the gate voltage VG is incremented by a gate voltage step 617 to VG+ bV (step 512) and the next drain current /D(VG) is measured (step 510). In general if a number M of measurements are to be taken within the range between the first gate voltage VI and the second gate voltage V2, the gate voltage step (5V will be set to I1/2-V21/(M-1). At minimum, M=2 so that the gate voltage step is 6V =1177-V21, and a pair of drain currents!D(17i) and JD(V2) are measured Once the gate voltage VG is equal to (or greater than) the second gate voltage V2 (step SHINo), then an estimated gate voltage Ve is calculated based on the measured drain currents /D(1/2), /D(VrEbv), Th(V2-611), ID(V2) and the corresponding gate voltages V], 1/2-F6V, ..., V2-6V, 172 (step SI3). The first gate voltage 1/2 and the second gate voltage V2 are both within 25% of the measurement gate voltage VG,". This condition helps to ensure that the first and second gate voltages VI, V2 are sufficiently different to the actual threshold voltage Vth as to lie within (or at least close to) the quadratic region,z1V0"act in order for an estimate threshold voltage Vest to provide a meaningful estimate of the actual threshold voltage V (which may always be determined with reasonable accuracy for calibration purposes using a full transfer sweep 23).

The second gate voltage 12 may differ from the first gate voltage Vi by at least 5%, at least 10%, at least 15% or at least 20% of the first gate voltage 1/2. The precise difference I 1/2-V21 may depend on the range of concentrations C of the target constituent which a detector 12 is intended to measure.

Referring also to Figure 10, calculation of the estimated threshold voltage Vest is illustrated.

In the simplest case, M=2 and the estimated threshold voltage Vest is calculated based on a first drain current NV?) corresponding to the first gate voltage V/ and a second drain current in(V2) corresponding to the second gate voltage V2. In this case, the estimated threshold voltage Ve,/ may be calculated based on the intercept of a straight line drawn through the two points (1/1(1/2)1°.5, 1/2) and (I/D(V2)1°.5, V2): -liirng V5 v2t -Ilinwo -IIID(v2)1 (1) Although simple and computationally efficient at run-time, the method using a single pair of measurements (M=2) may be more vulnerable to noise. Therefore, a larger number of measurements of drain current ID may be obtained within the narrow (relative to a transfer sweep 23) range of voltages between and including the first and second gate voltages Vi, 172. For a number M>2 of measured drain currents ID, a linear regression line is calculated using the square roots of the measured drain currents lip(1/2)1° 5, liD( V/±601 0.5, ..., IID(V2-6V)I 0.5, 1/D(72)105 and the corresponding gate voltages 1/2, 1/2+6V, V2-617, V2 (the latter being the independent variable). The linear regression line is extrapolated to the intercept JD(Vi) = 0 to find the estimated threshold value Vesl. Even for a relatively large number of measurements M and correspondingly small gate voltage step (51/ = IVJ-V21/(14-1), the improved method differs from conventional transfer sweep 23 analysis in two ways. Firstly, the range 11/2-1/21 is less than, or much less than the range IV,"-Vn1i"1 of a transfer sweep 23. Secondly, the range 117max-Vmul of a transfer sweep 23 spans the actual threshold voltage Vih, so that Vinin<Viti<Vn, whereas the range IVi-V21 of the improved method does not span the actual threshold voltage Vth so that either the condition Vi<Vti, and 1.72<Vth holds or else V2>Vth and V2>Vih (depending on whether the transistor 15 is n-type or p-type for example).

If the estimated voltage threshold Vect which is calculated ever lies between the first and second gate voltages V?. V2, this would represent an error condition -for example that a detector 12 is being used outside of a calibrated range of operating conditions (see further discussion of operating ranges hereinafter). Equally, if either of the differences IVi-Vestl or I V2-Vesil ever drops below a threshold amount Alicar then this may indicate that a detector 12 is operating close to, or outside, its calibrated operating conditions, and that the estimated threshold voltage 17,31 may diverge significantly from the actual threshold voltage Vaz.

Optionally, a measurement drain current /Dm is measured whilst applying the measurement drain voltage VDm and measurement gate voltage VG," (step S14). Preferably, the fixed drain voltage Vpf is equal to the measurement drain voltage VD", and at least one of the applied gate voltages V 1, V2-6V, V2 is equal to the measurement gate voltage VG,,, so that a separate measurement (step S14) of the measurement drain current Ion, may be omitted. For example, Vi or V2 may be equal to the measurement gate voltage Vu,,,.

The concentration C of the target constituent in the monitored fluid is estimated based on at least the estimated threshold voltage Vest (step S15). For example, values of measurement drain current /Dm and/or threshold voltage Vih corresponding to known calibration concentrations C may be measured when calibrating a detector 15. In operation, the calibrated values may be looked-up and/or interpolated/extrapolated to determine a concentration C corresponding to an estimated threshold voltage V,,t and optionally also a measurement drain current Ian. The concentration C of the constituent may be based on a number of additional variables in addition to the estimated threshold voltage including but not limited to, the measurement drain current /Dm, one or more further estimated threshold values V"a, V9t3, Vref corresponding to different transistors and/or the same transistor 15 when exposed to a different environment. A further example of estimating the concentration C of a target constituent in the presence of a concentration Cm of a potentially interfering secondary (background) constituent is discussed hereinafter in relation to Figures 16 and 17.

If there are further detectors 12 connected to the monitoring device 13 (step 16IYes), then the next detector 12 is addressed (step S17) and the measurement process repeated (steps S9 through SI6).

The improved method has been explained assuming that the monitoring device 13 addresses the connected detectors 12 sequentially. Alternatively, the monitoring device 13 may carry out steps S9 through S15 concurrently for two or more detectors 12. In another variation, steps 59 through SI5 may be carried out internally to each detector 12, for example by the controller 16, and only the estimated concentrations C of target constituent may be sent to the monitoring device 13 across the links 14.

Referring also to Figure 11, a schematic plot of gate voltage VG versus time t is shown for the improved method.

At time ti, the first gate voltage 17G=1/2 is applied for a dwell time t4-0 whilst the corresponding first drain current /D(1/2) is measured. After a delay t5-0, the second gate voltage V6=1/2 is applied for a dwell time /6-ts whilst the corresponding second drain current /D(V2) is measured. If only two drain current In measurements are obtained each time a detector 12 makes a measurement of concentration C, then the delay ts-t4 may be zero (or as close as is practical, i.e. switching speed limited by self-capacitance/inductance and so forth). Preferably the dwell times /4-ti, /645 are substantially identical. If the number of drain current in measurements is more than two, 4/>2, then one or more additional dwell times, for example ta-t7 corresponding to intermediate gate voltage(s), for example VG= VI-F6V = V2-617, may occur between the end ti of the first dwell time 14-ri and the start rs of the final dwell time 1-6-13. As already mentioned, preferably at least one of the measured drain currents /D(1/2), /D(1/2-Ebv), /D(V2-617), /D(V2) provides the measurement drain current /Thn.

The total duration t6-ti needed to obtain enough data to estimate the concentration C of the target constituent may be significantly reduced compared to the duration t3-// of a full transfer sweep 23 approach. For example, from a typical time of 15 to 60 second for the transfer sweep 23 measurement, the improved method may reduce the time to estimate the concentration C to several seconds or even less than a second. The improvement in response times will be multiplied in applications where the measurement device 13 time-multiplexes addressing of two of more detectors 12. Additionally, the improved method may reduce bias stress and/or power consumption.

The improved method described with reference to Figures 9 to 11 is equally applicable to measuring/monitoring a concentration C of a target constituent in liquid or gaseous fluids. For example, the monitored environment 11 may contain a fluid in the form of a liquid, and the target constituent may be a target liquid mixed with the liquid fluid, or a target solute dissolved in the liquid fluid. Alternatively, the monitored environment 11 may contain a fluid in the form of a gas, and the target constituent may he a target gas mixed with the gaseous fluid.

The transistor 15 used in a detector 12 may he of any suitable type, for example the transistor 15 may be an organic field effect transistor (OFET) such as an organic thin-film transistor (OTFT), for example one of the first to third transistors 1, 8, 9. Alternatively, the transistor 15 need not be an organic field effect transistor, and may be of any other type which exhibits a detectable response to a concentration C of a target constituent. For example, the transistor 15 may be an inorganic field effect transistor (for example a metal-oxide field effect transistor), or an electrochemical transistor.

The improved method to estimate the threshold voltage (Figure 9) may be conducted according to a pre-determined schedule. For example, each detector 12 may obtain an estimated threshold voltage Vest at intervals of milliseconds, seconds, minutes or days, depending on the application. Additionally or alternatively, the timing of obtaining estimated threshold voltages Vest may he dynamically determined, for example in response to output of a related sensor such as a pressure sensor (not shown) and/or a temperature sensor (not shown), and/or in response to a user input. For example, a user may actuate a valve or other mechanism enabling a greater or lesser degree of interaction between die interior and exterior of the monitored environment 11, and this may trigger one or more detectors 12.

The measurement drain voltage VD,", the measurement gate voltage VG",, the fixed drain voltage Vph the first gate voltage Vi and the second gate voltage V2 should all correspond to the quadratic region /117",t in which the drain current 11) varies approximately in proportion to the square of the gate voltage VG. Such correspondence should be calibrated for at least a desired range of concentrations C of the target constituent. In practice, this means that calibration of a detector 12 may typically involve performing full transfer sweeps 23 corresponding to a range of known concentrations C of the target constituent which correspond to the intended operating range the detector 12 is intended to monitor/measure. Such calibrations may also need to be repeated spanning the range of variability in background parameters which the detector 12 is intended to be rated for operation in. Background parameters may include, without limitation, an operating temperature range, an operating pressure range, an operating range of humidity (e.g. for a fluid in the form of a gaseous mixture), an operating range of concentrations of constituents other than a target constituent. Once such calibrations have been performed and the range of actual threshold voltages VII, and quadratic regions AVq""d which the detector 12 could experience in use have been mapped out, suitable values for the measurement drain voltage Vo"" the measurement gate voltage VG,, the fixed drain voltage VD]; the first gate voltage Vi and the second gate voltage V2 may be selected. A larger separation of the first and second voltages V7, V2 may provide a more robust value of estimated threshold voltage Vest, but may only be useable for a narrower range of concentrations C of the target constituent. Since transistors 15 may be manufactured with substantially reproducible properties, extensive calibration will only be necessary for a small batch of transistors 15 and the incorporating detectors 12. Once determined, the values VG,", VG", Vpf, VI and V2 may be used for all subsequently produced detectors 12, re-calibrating only, for example, between different production batches, when the design is changed, and so forth.

As explained hereinafter, the improved method may enable reduction of the number and/or range of environmental conditions in which calibration is required, because the parameter of estimated threshold voltage V5, has been found to be less sensitive to environment changes than a measurement drain current /D", (see Figures 19 to 22 and corresponding description).

In other words, the measurement drain voltage VD,n, the measurement gate voltage VG,, the fixed drain voltage Viij, the first gate voltage Vi and the second gate voltage V2 are all calibrated based on the range of concentrations C of the target constituent which it is desired to measure, as well as the ambient conditions in which the transistor 15 (and detector 12) are intended for use. In this respect, the detector 12 is the same as any other type of measurement device, all of which are only rated across a given range of a measured parameter and for use in a given range of operating environments.

For applications in which small concentrations of a target constituent are to be detected in an otherwise relatively stable environment, a single calibration environment may be sufficient. For example, for sensing in air, the measurement drain voltage VD,, the measurement gate voltage VG,,, the fixed drain voltage VI* the first gate voltage V7 and the second gate voltage V2 may be calibrated for a range of concentrations C of the constituent in an atmosphere having a composition including 78±0.5% nitrogen, 21±0.5% Oxygen, less than 1.5% Argon and less than 1% carbon dioxide, at a temperature of 20 degrees Celsius, a pressure of one atmosphere (101 kPa), and a relative humidity between 30% and 70%.

The processes of the improved method may be distributed between the detector 12 and the monitoring device 13 in a number of different ways. For example, both the estimated threshold voltage V1 and the concentration C of the target constituent may be calculated by the controller 16 of a detector 12 incorporating the transistor 15. In other examples, the estimated threshold voltage V, may be calculated by the controller 16 of a detector 12 incorporating the transistor 15, whilst the concentration C of the target constituent may he calculated by the processor 19 of the monitoring device 13. A concentration C may be calculated corresponding to each detector 12, or the monitoring device 13 may calculate a single concentration C based on information received from two or more detectors 12. In still other examples, each detector 12 may transmit raw measurements (e.g. of drain current M(VG) at a particular gate voltage VG) across the respective link 14, and the processor 19 of the monitoring device 13 may calculate both the estimate threshold voltages Vt.,/ and the concentration(s) C. The precise division of functions between detector(s) 12 and monitoring device 13 may depend on a variety of factors including, hut not limited to, the bandwidth of links 14, the need to minimise power consumption of the detectors, and so forth.

The improved method may be applied to the hereinbefore described example of monitoring concentrations of 1-methylcyclopropene (1-MCP) in the context of storing fruit, cut flowers and other products which may be sensitive to alkenes.

The operating conditions for detectors 12 including transistors 15 configured for sensing 1-MCP gas can be challenging, with a dynamically changing background atmosphere including variations in the amount of both oxygen 01 and water vapour 1110 present. In particular, the inventors have noted that 01 levels may impact the response of transistors 15 in the form of gas sensing organic thin-film transistors. The inventors have also noted that. such variations cannot simply he corrected for by using conventional reference device arrangements. As explained hereinafter, the inventors have discovered that the improved method for obtaining estimated threshold voltages Vest may provide reduced sensitivity to changing 02 levels in 1-MCP sensing applications. This may allow monitoring systems 10 for 1-MCP, or other gases present in a gaseous atmosphere having varying compositions, to use the improved method to obtain improved performance (compared to a full transfer sweep 23) without the need to change the hardware by adding further Oi sensors or devices.

In some example, the monitoring device 13 may feed estimated threshold voltages VIII and measurement drain currents b", into a machine learning process. This could be used to train detectors 12 dining an initial calibration, and/or be continued in operation to continuously improve accuracy.

First method of measuring concentration of a target constituent A first method of measuring a concentration C of a target constituent using the improved method shall be described hereinafter.

Referring also to Figure 12, a second detector 24 is shown.

The second detector 24 is similar to the detector 12, except that instead of the single transistor 15, the second detector includes a specific transistor 25 and a generic transistor 26. The generic transistor 26 is sensitive to both a target constituent and a secondary (or background) constituent which may affect the measurements, and may be a field-effect transistor (organic or inorganic) or an electrochemical transistor. For example the target constituent may be 1-MCP and the secondary constituent may be ethylene, or vice versa. The specific transistor 25 is sensitive to either the target constituent or the secondary constituent, but not both. Which of the target constituent or the secondary constituent is not important, as either option will allow deconvolution of the effects of the target constituent from the secondary constituent. For example, the specific transistor 25 may be sensitive to 1-MCP and insensitive to ethylene, or may be insensitive to 1-MCP and sensitive to ethylene. The specific transistor 25 may be a field-effect transistor (organic or inorganic) or an electrochemical transistor. The generic transistor 26 and the specific transistor 25 should be of the same type, and in terms of structure and materials the specific transistor 25 and the generic transistor 26 should differ only so far as is required to provide the required sensitivities. One or more second detectors 24 may be substituted for detectors 12 in the monitoring system 10.

In order to provide specificity, the specific transistor 25 may be separated from the fluid being measured/monitored by one or more fillers, semi-permeable membranes, molecular sieves and so forth, for blocking the target constituent or the background constituent from reaching the semiconductor channel and/or junctions with the source and drain electrodes 4, 5. Additionally or alternatively, a junction between a source and/or drain electrode 4, 5 and a semiconductor region (e.g. organic semi-conductor layer 7) of the specific transistor 25 may include a blocking layer 28 (Figure 13) which prevents diffusion of the target constituent or the secondary (background) constituent.

The improved method may be extended using the second detector 24. The genetic transistor 26 may be used as previously described in relation to the transistor 15 to obtain data for calculating the estimated threshold voltage V. Additionally, in response to, or concurrently with, calculating the estimated threshold voltage lint, a second estimated threshold V"sf (or reference threshold) is also calculated corresponding to the specific transistor 25. The second estimated threshold Vwf is obtained using the improved method, in exactly the same way and using the same gate voltages Vi, 1/2-FoV, V2-6V, V2 as for obtaining the estimated threshold voltage V",. The second (reference) estimated threshold voltage V"f may be calculated by the controller 16 of the second detector 24 or by the processor 19 of the monitoring device 13, depending on implementation choices regarding the division of calculations between detector(s) 12 and monitoring device(s) 13.

The controller 16 or processor 19 (depending on implementation) then calculates the concentration C of the target constituent based on the estimated threshold voltage Vest, the reference (second) threshold voltage V"f, and optionally also the measurement drain current Lam Any one of the first, second or third transistors 1, 8, 9 described hereinbefore may be used to provide the generic transistor 26.

Referring also to Figure 13, a first exemplary specific transistor 27 is shown.

The first specific transistor 27 is the same as the first transistor 1, except that an additional blocking layer 28 separates the source and drain electrodes 4, 5 from the organic semiconductor layer 7.

The blocking layer 28 at least partially covers a surface of the source and drain electrodes 4, 5 which would otherwise be in contact with the organic semiconductor layer 7. A first portion 28a covers the source electrode 4, and a second portion 28b covers the drain electrode 5. The blocking layer portions 28a, 28b may be physically separate, or a single blocking layer 28 may be deposited covering the source electrode 4, the drain electrode 5 and the intervening surface of the dielectric layer 6. The blocking layer 28 is in direct contact with the organic semiconducting layer 7 or, if present, the organic charge-transporting layer (not shown), thereby forming an interface between the source and drain electrodes 4, 5 and the organic layer semiconducting layer 7. The blocking layer 28 of the first specific transistor 27 may partially or completely prevent either the target constituent or a secondary (background) constituent from binding to the surface of the source and drain electrodes 4, 5 on which the blocking layer 28 has been formed. For example, for 1-MCP measurements the blocking layer 28 may block ethylene or may block l -MCP (either option will enable differentiation between 1-MCP and ethylene).

it may be possible for the target constituent or a secondary (background) constituent to bind to an outer surface of the source and/or drain electrodes 4, 5 if they are not completely covered with the blocking layer 28, however such outer surfaces are not at an interface with the organic semiconducting layer 7, so that the target constituent or a secondary (background) constituent binding to an uncovered, outer surface of the source or drain electrodes 4, 5 may have little or no effect on charge injection into the organic semiconducting layer 7.

The generic transistor 26, for example the first transistor 1, does not have a blocking layer 28, and so both the target constituent and the secondary (background) constituent coming into contact with source and drain electrodes 4, 5 at the electrode 4, 5 / organic semiconductor layer 7 interface are not blocked from binding to the surface of the source and drain electrodes 4, 5.

A constituent contacting an electrode 4, 5 surface, such as a constituent having a dipole moment, may result in a change in work function at the electrode 4, 5 surface, for example as a result of binding of the constituent to the electrode 4, 5 surface. Schottky current dependence on work function may mean that even a relatively small change in work function can have a large effect on injected currents at the source and drain 4, 5. The generic transistor 26 may be affected in this way (or via other mechanisms) by one or more background constituents, in addition to a target constituent. The specific transistor 25, for example the first specific transistor 27, is configured to only be affected by one of the target constituent and the background constituent (either one will work.) A source or drain electrode 4, 5 having a blocking layer 28 thereon may undergo a smaller change in work function upon exposure to the target constituent and/or the secondary (background) constituent than the same source or drain electrode 4, 5 without a blocking layer thereon.

Use of specific and generic transistors 25, 26 with and without blocking layers 28 as described herein may provide improved measurements of a concentration C of the target constituent in a fluid which also contains a secondary (background) constituent. Such improvements may be in addition to the reduced sensitivity of estimated threshold voltages Vest to background variations, as compared to drain currents ID, which the inventors have noted (see Figures 19 to 22 and corresponding description) The blocking layer 28 of the first specific transistor 27 is preferably a monolayer formed on a surface of the source and drain electrodes 4, 5. A blocking layer 28 may be formed from a binding compound of formula (I):

R-X (I)

wherein R is an organic residue and X is a binding group for binding to the surface of the source and drain electrodes. The binding group X may bind to the source and drain electrodes 4, 5 to form a self-assembled monolayer.

X may be selected according to the material of the source and drain electrodes 4, 5. Preferably, X is a thiol or a silane group. A thiol group X is particularly preferred in the case where the source and drain electrodes 4,5 are formed of gold.

Preferably. R is a C1_30 hydrocarbyl group which may be unsubstituted or substituted with one or more substituents. Exemplary C1_30 hydrocarbyl groups are: C6_20 aromatic groups, preferably phenyl, phenyl with one or more C1_20 alkyl groups; and phenyl-C1_20 alkyl which may be substituted with one or more Ci_no alkyl groups.

A preferred substituent of the Ci_30 hydrocarbyl group is fluorine, and one or more H atoms of the in hydrocarbyl group may be replaced with fluorine.

Exemplary compounds of formula (I) include, but are not limited to, 4-fluorobenzenethiol (4-FBT), Pentafluorobenzenethiol (PFBT), 1-Octadecanethiol (ODT) and 2-Phenylethanethiol (PET).

The blocking layer 28 may alter the work function of the source and drain electrodes 4, 5 it is formed on. The blocking layer 28 may be selected according to the effect, if any, of the blocking layer 28 on the work function of the source and / or drain electrodes 4, 5 and the required charge injection requirements of the first specific transistor 27 such as the work function -organic semiconductor layer 7 highest occupied molecular orbital (HOMO) gap in the case of a p-type bottom-gate organic thin-film transistor BO-OTFT or the work function -organic semiconductor layer 7 lowest unoccupied molecular orbital (LUMO) gap in the case of a n-type BG-OTFT.

Preferably the work function of the source and drain electrodes of a p-type OTFT is increased following treatment, preferably to 5.0 or more. The HOMO of a p-type semiconducting material is preferably at least 5.0 eV, optionally 5.0-5.5 eV.

A blocking layer 28 in the form of a monolayer may be formed on the source and/or drain electrodes 4, 5, by depositing a binding compound on the electrode or electrodes 4, 5, for example from a solution of the binding compound in one or more solvents. The binding compound may be selectively deposited onto the source and drain electrodes 4, 5 only, or may be deposited by a non-selective process such as spin-coating or dip-coating.

A bottom-contact, bottom gate transistor may he formed by depositing the binding compound onto the source and drain electrodes 4, 5 over a dielectric layer 6 and then depositing the organic semiconducting layer 7. Binding compound which is not bound to the source and drain electrodes 4, 5, for example binding compound on the dielectric layer 6 following a non-selective deposition process, may be removed by washing.

The blocking layer 28 may be a material which is absorbed onto the surface of the source and/or drain electrodes 4, 5.

Referring also to Figure 14, a second example of a specific transistor 29 is shown.

The second specific transistor 29 is the same as the second transistor 8, except that a blocking layer 28, 28a is added between the source electrode 4 and the organic semiconducting layer 7 and the blocking layer 28, 28b is added between the drain electrode 5 and the organic semiconducting layer 7.

Referring also to Figure 15, a third example of a specific transistor 30 is shown.

The third specific transistor 30 is the same as the third transistor 9, except that the blocking layer 28, 28a is added between the source electrode 4 and the organic semiconducting layer 7 and the blocking layer 28, 28b is added between the drain electrode 5 and the organic semiconducting layer 7.

Further details of devices and methods using blocking layers may be found in WO/2019/063493 Al, the contents of which am incorporated herein by reference.

Referring also to Figures 16 and 17, an example method of calculating a concentration C of the target constituent in the presence of a secondary (background) constituent affecting the measurement shall be explained.

In the following explanation, the generic transistor 26 presumed is sensitive to the concentration C of the target constituent and also to the concentration Cm, of a secondary (background) constituent also present in the fluid, as described hereinbefore. For brevity, the specific transistor 25 shall be presumed insensitive to the concentration C of the target constituent and sensitive to the concentration Cbck of the secondary (background) constituent. However, the method is readily adapted for the situation that the specific transistor 25 is sensitive to the concentration C of the target constituent and insensitive to the concentration Cbck of the secondary (background) constituent.

Referring in particular to Figure 16, the situation is illustrated for a measurement obtained using the specific transistor 25, which is insensitive to the concentration C of the target constituent.

A baseline slope 31 (chained line) shows the slope and intercept Vhcbe expected in the absence of a concentration C of the target constituent and a concentration Cbck of the secondary (background) constituent. The baseline slope 31 is pre-calibrated when neither the target constituent nor the secondary constituent are present. The second (reference) estimated threshold voltage Vref is obtained by extrapolating a specific measurement slope 32 from the pair of points (ID( VA Vi) and (/D(V2), V2). The specific measurement slope 32 is shifted from the baseline slope 31 as a result of the concentration Cbck of the secondary (background) constituent. At the intercept, i.e. /D=0, the shift is equal to a background change VI"k = Vhels, -Vcck. The specific measurement slope 32 may be approximated as the baseline slope 31 translated by the background change 4Vbck, however, in practice the gradient may also be slightly varied by the concentration Chrk of the secondary (background) constituent.

Referring in particular to Figure 17, the situation is illustrated for a measurement obtained using the generic transistor 26.

The estimated threshold voltage Vesi is obtained by extrapolating a generic measurement slope 33 from the points Th(V/). VI and ID(112), V2. The generic measurement slope 33 is shifted from the baseline slope 31 as a result of the concentration C of the target constituent and the concentration Cbek of the secondary (background) constituent. At the intercept (/D=0), the total change is AVol = Vbase -Vest. The generic measurement slope 33 may be approximated as the baseline slope 31 translated by the total change AV, however, in practice the gradient may also be slightly varied by the concentrations C. Cbek of the target and secondary (background) constituents. The total change LIVtut may be modelled as a sum of the changes due to the target and secondary (background) constituents, i.e. z1V,,,, = AVsig + AVbck, with AVsig representing the signal resulting from the concentration C of the target constituent. Since the background change z1Vbek is obtainable from the estimation of the second threshold voltage Verf, this may be subtracted from the total change AVt",. to leave the signal AVsig due to the concentration C of the target constituent. The signal AV sig due to the target constituent can be converted to a corresponding concentration C based on predetermined empirical calibration data obtained used known concentrations C of the target constituent in the absence of the secondary (background) constituent. In this way, calibration is only required based on the concentration C of the target constituent, because the effects of the secondary (background) constituent may be compensated at the time of measurement as desciibed.

Although in principle a similar correction could be performed using drain current ID values directly, the inventors have discovered that making the correction based on the estimated voltage thresholds Vest, Veci may he more reliable. This is believed to be because for the estimated voltage thresholds 11,51, V,, the impact of changes in the background environment, including concentrations of one or more background constituents, is lessened. Using the estimated voltage thresholds Vest, Vref may also mean that the impact of changes in charge carrier mobility on the transistor 15, 25, 26 may be ignored. For example a background gas that. acts as a charge trap would impact the drain current ID response, but not the threshold voltage 17th response determined as an estimated voltage threshold Vest, Vrei. (see Figures 19 to 22 and corresponding description).

Although only a pair Th(1.7]), /D(1.72) of drain current measurements are shown, the method described hereinbefore is equally applicable when more than two (M>2) gate voltages are applied.

It shall be appreciated that when there is more than one background constituent present or possibly present, superior performance will be obtained if the specific transistor 25 is made insensitive to the target constituent.

Second method of measuring concentration of a target constituent A second method of measuring a concentration C of a target constituent using the improved method shall he described hereinafter.

Referring also to Figure 18, a portion of a second monitoring system 34 is shown.

The second monitoring system 34 includes a monitored environment 11 and a detector 12 arranged for sampling the monitored environment 11. Although not. shown in Figure 18, the second monitoring system 34 includes a monitoring device 13 which is coupled to the detector 12 by a corresponding link 14. The second monitoring system 34 may include additional detectors 12, each of which may be configured identically to the illustrated detector 12.

The first method of measuring a concentration C of a target constituent in the presence of a secondary (background) constituent used a pair of a specific transistor 25 and a generic transistor 26 to obtain the first and second estimated voltage thresholds V,, Vref. In contrast to this, the second method of measuring a concentration C of a target constituent in the presence of a secondary (background) constituent obtains the first and second estimated voltage thresholds Vet, Vuf using the same transistor 15 (which is the same as a generic transistor 26), but at different times and by filtering the fluid when obtaining the second estimated voltage threshold Fluid (liquid or gas) is drawn from the monitored environment 11 through a first path 35 which subsequently divides into a second path 36 and a third path 37. Flow along the second and third paths 36, 37 is controlled by first and second valves 38, 39. The first valve 38 may be opened to permit fluid flow along the second path 36, or the second valve 39 may be opened to permit fluid flow along the third path 37, but the first and second valves 38, 39 are not both opened at the same time. Operation of the first and second valves 38, 39 is controlled by primary valve control signals 40. The control signals 40 may be supplied from the detector 12, or from the monitoring device 13, depending for example on the application and on the data processing resources available on the detector 12 and monitoring device 13.

The second path 36 flows directly to the transistor 15 of the detector 12. The third path 37 leads to a filter 41 which is configured to remove, or at least significantly reduce, the concentration C of the target constituent or the concentration Cbek of a background constituent, but not both. Whether the target constituent or the background constituent is removed (or reduced) is not critical, as either option will enable the corresponding contributions to the measurement to be differentiated. From the filter 41, a fourth path 42 allows the filtered fluid to flow to the transistor 15 of the detector 12.

The filter 41 may be any device or chemical suitable for selectively removing either the target constituent or the background constituent. For example, the filter 41 may be a porous material through which some constituents may pass but not others, a molecular sieve, a reagent, a liquid through which a fluid in the form of a gas is bubbled, an absorbent such as clay, and so forth.

From the detector 12, the fluid or filtered fluid is returned to the monitored environment 11 via a fifth path 43 (or return path). The fifth path 43 includes a pump 44 which operates to draw fluid through the detector 12. The pump 44 only needs to operate when a measurement of concentration C is required, and may be switched on and off using pump control signals 45 provided by the detector 12 or the monitoring device 13 depending for example on the application and on the data processing resources available on the detector 12 and monitoring device 13.

Optionally, a third valve 46 in the form of a one-way valve may be included in the fifth path 43 to prevent back-flow along the fifth path 43 to the detector 12. A third valve 46 may be unnecessary, depending on the type of pump 44 used (e.g. a peristaltic pump would prevent back-flow).

When the fluid is a gas, and depending on the type of filter used, the filtered gas may become dehydrated with respect to the monitored environment 11 and gas which is sampled using the second path 36. Optionally, a re-hydration unit 47 may be included in the third path 42 to restore the humidity level of the filtered gas to that of the monitored environment 11. For example, filtered gas may be bubbled through, or passed over, water in a re-hydration unit 47.

Optionally, a flushing system may be provided using a fourth valve 48 and a fifth valve 49. The fourth valve 48 connects to one side of the detector 12, between the first and second valves 38, 39 and the detector 12, and the fifth valve 49 connects to the fifth path 43 between the pump 44 and the monitored environment 11 (between the pump 44 and the third valve 46 if present). With the first and second valves 38, 39 closed, the fourth and fifth valves 48, 49 may be opened to allow flowing of a flushing fluid 50 (liquid or gas) through the detector 50. The flushing fluid 50 does not include the target constituent or the background constituent, and may be used to remove traces of the target constituent and/or the background constituent from the detector 12 in between measurements of the fluid and filtered fluid (using first and second valves 38, 39). For example, the flushing fluid 50 may be dry, nitrogen N2. The fourth and fifth valves 48, 49 may be opened and closed using secondary valve control signals 51 supplied by the detector 12 or the monitoring device 13 depending for example on the application and on the data processing resources available on the detector 12 and monitoring device 13.

In some examples it may be desirable to avoid returning fluid to the monitored environment 11, for example to avoid contaminating the monitored environment with oil from a pump 44. In such examples, the fifth path 43 may be omitted and the fifth valve 49 may be used to vent the fluid passing through the detector 12 outside of the monitored environment 11.

According to the second method of measuring a concentration C of the target constituent, the estimated voltage threshold Vest is calculated according to the improved method during a first period in which the first valve 38 is opened and the second valve 39 is closed. Consequently, the estimated voltage threshold V, is derived from exposure to the concentration C of the target constituent and the concentration Cbck of the secondary (background) constituent, and allows the total change 4V = AVsig + AVhd, to be obtained. The second estimated threshold voltage 17,-,/ is then calculated during a second period in which the first valve 38 is closed and the second valve 39 opened so that the detector 12 is exposed to the filtered fluid. Consequently, if the filter 41 removes the target constituent, the background change 417b,d, may be obtained from the second estimated voltage threshold Vref, and subtracted from the total change AV0, to obtain the desired signal AVsig.

EXPERIMENTAL DATA

Referring also to Figure 19, an experimentally measured transfer sweep 52 for an organic thin film transistor is shown.

The transistor used to obtain the experimentally measured transfer sweep 52 was a top gate bottom contact organic thin-film transistor having the basic structure of the third transistor 9. The source and drain electrodes 4, 5 were formed of gold and the gate electrode 2 was formed of aluminium. The dielectric layer 6 was formed of polytetralluoroethylene (PTFE), speci fi call y pol y14,5-difluoro-2,2-bis(tri uoromethyl)-1,3-dio xole-co-tetrall uoroethylenei: Ox F3C CF3

F F F F

which is a material used for its exceptionally large gas permeability (due to large free volume) and which is also suitable as gate dielectric material due to relatively low dielectric constant and orthogonal solvent compatibility The organic semiconductor layer 7 was formed of a polymer semiconductor having the structure: The channel length between source and drain electrodes 4, 5 was 125 pm. A total of thirty two transistors were produced on each substrate, using the fabrication procedure explained hereinafter and measurements were obtained by measuring from four transistors simultaneously.

The substrates 3 used were borosilicate glass substrates with dimensions 50 mm x 50 mm sized and a thickness of 0.3 mm The source and drain electrode 4, 5 were deposited by thermal evaporation of 40 nm thick gold, Au (with 3 nm thick Chromium, Cr, underneath) through shadow masks with a channel length of 125 pm and a channel width of 4 mm. The organic semiconductor layer 7 was formed of polymer organic semiconductor (see structure hereinbefore) spin coated from a 1%w/v solution in 1,2,4-trimethylbenzene to a thickness of 40 nm and (hied at 100°C for 1 minute in air. The dielectric layer 6 formed of poly[4,5-dilluoro-2,2-bis(trilluoromethyl)-1, 3-dioxole-co-tetrafluoroethylene] was spin coated from a 2.5% w/v solution in a 50:50 blend of perfluorinated solvents, to a 300 nm thickness and dried at 80°C for 10 minutes (after 5 minutes of an initial drying phase while spinning in order to ensure a uniform thickness). The pair of perfluorinated solvents used were fluorinert FC-43 and FC-770, having the respective structures: The gate electrodes 2 were formed as 125 pm wide fingers separated by 125 pm gaps, each oriented across the channel. The gate electrodes were deposited by thermal evaporation of 200 nm thick aluminium, Al (with 3 nm thick chromium, Cr underneath) through a shadow mask.

Referring also to Figure 20, the experimentally measured transfer sweep 52 shown in Figure 19 is plotted using the square root of drain currents In instead.

A fitted straight line 53 is also shown, extrapolating the quadratic region 17,1"ad to a threshold voltage of Vat 7-t; -2.4 V. Based on the measured transfer sweep 52, the value of the measurement drain voltage VD", the fixed drain voltage Vpf, the measurement gate voltage VG", and the first gate voltage Vi were all set to VD,,,=Vtr1/4.;,,,=V/=-6 V. whilst the second gate voltage was set to V2=-5.5 V. These values were used to calculate estimated threshold voltages V, using a two-point (M=2) implementation of the improved method, applied to detection of constituent in the form of 1-MCP.

Referring also to Figure 21, results of measurements in atmospheres having differing levels of Oxygen, On, are presented.

Unshaded columns 54 show the measured changes in estimated threshold voltage Vest upon exposure to 1-MCP at a concentration C of 1 part-per-million (ppm) on the primary, left-side axis. Hatched columns 55 show the corresponding measured % changes in drain current ID upon exposure to I-MCP on the secondary, right-side axis.

Three atmospheres were used, air (-21% 0/, 78% N), 1% 0/ in Nn balance, and 1% 02, 3% CO, in N, balance (also referred to herein as "CA"). The concentration of 1-MCP to which the test transistors were exposed was the same for all ambient atmospheric compositions. Comparing air against the two low 02 atmospheres, it may be observed that the relative change in estimated threshold voltage V", upon reduction in 02 concentration was in both cases noticeably less than the %change in drain current /D. This demonstrates that using the improved method to obtain estimate threshold voltages Vest, instead of relying only on drain currents /D, may provide a measurement of 1-MCP which is more robust against changes in a background concentration of oxygen On. In applications such as storage of fruit or cut flowers, this may be useful because 02 concentrations may vary considerably.

Referring also to Figure 22, the estimated threshold voltage Vect and the measurement drain current Min are shown as a function of time, spanning repeated exposures to 1-MCP at a concentration of 1 part-per-million (ppm) It may be observed that the estimated threshold voltage Vest responds in a similar fashion to the drain current, here the measurement drain current /D", with similar rates of response upon exposure to the 1-MCP.

Using the improved method to obtain estimated threshold voltages Vest, the response of some OTFTs (all having the same structure described in relation to Figure 19) to different atmospheres shows a significantly decreased impact of changing 02 levels: Atmosphere Estimated threshold Measurement drain voltage Vest response (.4 Vsig) current /um response (%) Air 0.26 -10 1% 02,3% CO2 in balance 0.33 -33 N2 Table 1: Impact of changing atmosphere compositions on estimated threshold voltages V"t and measurement drain currents Mtn.

MODIFICATIONS

The present invention is not limited to the disclosed embodiments. It will he appreciated that many modifications may he made to the embodiments hereinbefore described. Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of gas sensors, and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

The improved method used to calculate estimated threshold voltages V es,,V ref of a transistor 1, 8, 9, 15, 25, 26 could also be used to estimate a value of the transconductance g," of a transistor 1, 8, 9, 15, 25, 26. The transconductance gm is is defined as: -,/ ( v2)1 ( 14) gm Where the presence of the target constituent has the impact of changing the mobility p or capacitance Ct"", of the transistor 1, 8, 9, 15, 25, 26, the concentration C of the target constituent can be calculated by monitoring changes in the estimated transconductance g,,, and comparing with known calibration values, using the expression: gm -gCtrans Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. V2

Claims

Claims 1. A method comprising: monitoring a concentration of a constituent of a fluid using a transistor selected from a field effect transistor or an electrochemical transistor, comprising applying a measurement drain voltage across the transistor, and applying a measurement gate voltage to a gate of the transistor; periodically estimating a threshold voltage of the transistor by: measuring, for a fixed drain voltage, drain currents corresponding to two or more gate voltages within a range between a first gate voltage and a second gate voltage, wherein each of first gate voltage and the second gate voltage is within 25% of the measurement gate voltage; calculating an estimated threshold voltage for the transistor based on the measured drain currents and corresponding gate voltages; calculating the concentration of die constituent based on the estimated threshold voltage.
2. The method according to claim 1, wherein measuring drain currents corresponding to two or more gate voltages comprises: measuring a first drain current corresponding to the first gate voltage; and measuring a second drain current corresponding to the second gate voltage.
3. A method according to claims 1 or 2, wherein the first gate voltage is equal to the measurement gate voltage.
4. A method according to any one of claims 1 to 3, wherein the fixed drain voltage is equal to the measurement drain voltage.
5. A method according to any one of claims 1 to 4, further comprising: measuring, using the fixed drain voltage, a measurement drain current corresponding to the measurement gate voltage; wherein calculating the concentration of the constituent is additionally based on the measurement drain current.
6. A method according to any one of claims 1 to 5, further comprising, for a second transistor of the same type as the first transistor and configured to be insensitive to the concentration of the constituent or insensitive to the concentration of a background constituent: in response to estimating the threshold voltage of the transistor, estimating a second threshold voltage of the second transistor by: measuring, for the fixed drain voltage applied to the second transistor, second drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage; calculating a second estimated threshold voltage for the second transistor based on the measured second drain cun-ents and corresponding gate voltages; wherein calculating the concentration of the constituent is additionally based on the second estimated threshold voltage.
7. A method according to any one of claims 1 to 5, wherein the transistor is exposed to a test fluid during the estimation of the threshold voltage, and wherein the method further comprises: exposing the transistor to a filtered fluid, wherein the filtered fluid has been treated to reduce or remove the concentration of the constituent; estimating a third threshold voltage of the transistor by: measuring, for the fixed drain voltage applied to the transistor, third drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage; calculating a third estimated threshold voltage for the transistor based on the measured third drain currents and corresponding gate voltages; wherein calculating the concentration of the constituent is additionally based on the third estimated threshold voltage.
8. A method according to any one of claims 1 to 7, wherein the constituent is a liquid or a solute dissolved in a liquid.
9. A method according to any one of claims 1 to 7, wherein the constituent is a gas.
10. A method according to claim 9, wherein the gas is 1-methylcyclopropene or ethylene.
11. A method according to any one of claims 1 to 10, wherein the transistor is an organic field effect transistor.
12. A method according to any one of claims 1 to 10, wherein the transistor is an inorganic field effect transistor.
13. A method according to any one of claims 1 to 10, wherein the transistor is an electrochemical transistor.
14. Apparatus for measuring a concentration of a constituent of a fluid, comprising: a transistor in the form of a field effect transistor or an electrochemical transistor; a controller configured to: monitor a concentration of a constituent of a fluid using the transistor, comprising applying a measurement drain voltage across the transistor, and applying a measurement gate voltage to a gate of the transistor; periodically estimate a threshold voltage of the transistor by: measuring, for a fixed drain voltage, drain currents corresponding to two or more gate voltages within a range between a first gate voltage and a second gate voltage, wherein each of the first gate voltage and the second gate voltage is within 25% of the measurement gate voltage; the apparatus configured to: calculate an estimated threshold voltage for the transistor based on the measured drain currents and corresponding gate voltages; calculate the concentration of the constituent based on the estimated threshold voltage.
15. Apparatus according to claim 14, wherein the transistor and the controller are housed in a detector; wherein die estimated threshold voltage is calculated by the controller and the concentration of the constituent is calculated by the controller.
16. Apparatus according to claim 14, wherein the transistor and the controller are housed in a detector, the apparatus further comprising: a monitoring device in communication with the detector; wherein the estimated threshold voltage is calculated by the controller and the concentration of the constituent is calculated by the monitoring device.
17. Apparatus according to claim 14, wherein the transistor and the controller are housed in a detector, the apparatus further comprising: a monitoring device in communication with the detector; wherein the estimated threshold voltage is calculated by the monitoring device and the concentration of the constituent is calculated by the monitoring device.
18. Apparatus according to any one of claims 14 to 17, wherein measuring drain currents corresponding to two or more gate voltages comprises: measuring a first drain current corresponding to the first gate voltage; and measuring a second drain current corresponding to the second gate voltage.
19. Apparatus according to any one of claims 14 to 17, wherein the controller is further configured to measure, at the fixed drain voltage, a measurement drain current corresponding to the measurement gate voltage; wherein calculating the concentration of the constituent is additionally based on the measurement drain current.
20. Apparatus according to any one of claims 14 to 19, further comprising a second transistor of the same type as the first transistor and configured to be insensitive to the concentration of the constituent or insensitive to the concentration of a background constituent; wherein the controller is further configured, in response to estimating the threshold voltage of the transistor, to estimate a second threshold voltage of the second transistor by: measuring, for the fixed drain voltage applied to the second transistor, second drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage; the apparatus further configured calculate a second estimated threshold voltage for the second transistor based on the measured second drain currents and corresponding gate voltages; wherein calculating the concentration of the constituent is additionally based on the second estimated threshold voltage.
21. Apparatus according to any one of claims 14 to 19, wherein the apparatus is configured to expose the transistor to a test fluid during the estimation of the threshold voltage, and wherein the apparatus is further configured: to expose the transistor to a filtered fluid, wherein the filtered fluid has been treated to reduce or remove the constituent; to estimate a third threshold voltage of the transistor by measuring, for the fixed drain voltage applied to the transistor, third drain currents corresponding to two or more gate voltages within the range between the first gate voltage and the second gate voltage; to calculate a third estimated threshold voltage for the transistor based on the measured third drain currents and corresponding gate voltages; wherein calculating the concentration of the constituent is additionally based on the third estimated threshold voltage.
22. Apparatus according to any one of claims 14 to 21, wherein the constituent is a liquid.
23. Apparatus according to any one of claims 14 to 21, wherein the constituent is a gas.
24. Apparatus according to claim 23, wherein the gas is 1-methylcyclopropene.
25. A method according to any one of claims 10 to 19, wherein the transistor is an organic field effect transistor or an inorganic field effect transistor.