Chemical sensor

Abstract

A transistor device (10) is disclosed comprising a source electrode (14) a drain electrode (12) and an enzyme (31) for facilitating generation of a charge carrier from an analyte. The transistor device also comprises a polymer layer (30) for retaining the enzyme (31), the polymer layer (30) being conductive to the charge carrier. The device also comprises an ohmic conductor (32) in contact with said polymer layer (30) for applying a gate voltage to said polymer layer (30). The device also comprises an organic semiconducting layer (18) connecting the source electrode (14) to the drain electrode (12). Also disclosed is a method of making and using the device (10).

Claims

1. A process for detecting and/or determining a concentration or an amount of an analyte in a sample, the process comprising: (a) preparing a sample comprising an analyte; (b) contacting the sample with a transistor device comprising: a source electrode; a drain electrode; an enzyme for facilitating generation of a charge carrier from an analyte; a polymer layer for retaining the enzyme, the polymer layer being conductive to the charge carrier; an ohmic conductor in contact with said polymer layer for applying a gate voltage to said polymer layer; and an organic semiconducting layer in contact with said polymer layer, the organic semiconducting layer connecting the source electrode to the drain electrode, (c) using the device to: detect the analyte; and/or determine the concentration or the amount of the analyte based on an electrical parameter of the device.

2. The process of claim 1, wherein the process further comprises applying a voltage to the drain electrode with respect to the source electrode.

3. The process method of claim 1, wherein the process further comprises applying a voltage to the ohmic conductor with respect to the source electrode.

4. The process method of claim 1, wherein the process further comprises applying a voltage to the drain electrode with respect to the source electrode and applying a voltage to the ohmic conductor with respect to the source electrode.

5. The process of claim 4, wherein the voltage applied to the ohmic conductor and the voltage applied to the drain electrode have the same polarity with respect to the source electrode.

6. The process of claim 3, wherein the voltage applied to the ohmic conductor is greater than that required to liberate H+ from H2O2, and lower than that required to cause electrolysis of water.

7. The process of claim 4, wherein the voltage applied to the drain electrode is greater than that required to liberate H+ from H2O2, and lower than that required to cause electrolysis of water.

8. The process of claim 3, wherein the voltage applied to the ohmic conductor is between about 0 V and −2 V.

9. The process of claim 3, wherein the voltage applied to the ohmic conductor is about −1 V.

10. The process of claim 4, wherein the voltage applied to the drain electrode is between about 0 V and −2 V.

11. The process of claim 4, wherein the voltage applied to the drain electrode is about −1 V.

12. The process of claim 1, further comprising detecting drain current through the device.

13. The process of claim 12, wherein the concentration or amount of the compound is determined based on a magnitude of the drain current.

14. The process of claim 1, wherein the compound is glucose and the enzyme is glucose oxidase.

15. The process of claim 1, wherein the sample is bodily fluid.

16. The process of claim 15, wherein the bodily fluid is saliva.

17. The process of claim 1, wherein the sample is contacted with the polymer layer of the device.

18. The process of claim 1, wherein a layer of the enzyme is formed on a surface of the polymer layer of the device.

19. The process of claim 1, wherein the organic semiconductor comprises, consists, or consists essentially of at least one organic compound that has semiconducting properties, the at least one organic compound being any one or more of: polyacetylenes, porphyrins, phthalocyanins, fullerenes, polyparaphenylenes, polyphenylenevinylenes, polyfluorenes, polythiophenes, polypyrroles, polypyridines, polycarbazoles, polypyridinevinylenes, polyarylvinylenes, poly (p-phenylmethylvinylenes), including derivatives and co-polymers thereof, and further including mixtures thereof.

20. The process of claim 1, wherein the polymer layer comprises, consists, or consists essentially of a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

(2) FIG. 1 shows the structure of a device in accordance with one embodiment of the invention;

(3) FIG. 2 shows the a perspective view (not to scale) modelling a fabrication of the device of FIG. 1;

(4) FIG. 3A shows a measure of drain current (I.sub.D) vs time for the device according to FIGS. 1 and 2, with various thicknesses between the polymer layer and the drain and source, showing a fast decay followed by a slow rise in the measure of drain current;

(5) FIG. 3B is an expanded view of the fast decay shown in FIG. 3A;

(6) FIG. 3C is an expanded view of the slow rise shown in FIG. 3A;

(7) FIG. 4A shows a ratio A.sub.fast/A.sub.slow, A being a constant that is related to diffusion distance and an effective diffusion constant, for the device according to FIGS. 1 and 2, with various thicknesses between the polymer layer and the drain and source;

(8) FIG. 4B illustrates a variation in the measure of drain current as a function of time for two OTFT devices, both without a PVP dielectric, and having a P3HT organic semiconducting layer having a thickness of about 22 nm and about 9 nm, respectively;

(9) FIG. 4C depicts schematic diagrams showing the location of proposed doping region in the case of a thick P3HT layer and the case of a critical thickness P3HT layer;

(10) FIG. 5A shows a measure of drain current (I.sub.D) vs time for the device according to FIGS. 1 and 2 for two different voltages between (i) ohmic conductor for applying a voltage to the polymer layer and (ii) the drain electrode of the device;

(11) FIG. 5B shows gate current (ie current into the ohmic conductor) as a function of time for the two voltages between the ohmic conductor and drain electrode of the device shown in FIG. 5A;

(12) FIG. 5C includes schematic illustrations showing the electric forces act within the device serving to either retard or enhance protonic doping of a semiconductor channel of the device, for the two voltages between the ohmic conductor and drain electrode of the device shown in FIGS. 5A and 5B;

(13) FIG. 6 shows calibration curves for inkjet-printed OTFT sensor devices with P3HT thicknesses varying from 22 nm to 390 nm;

(14) FIG. 7A to 7C a measure of drain current (I.sub.D) vs time for the device according to FIGS. 1 and 2, comparing the case of having a PVP dielectric layer with not having a PVP dielectric layer;

(15) FIG. 8A shows profilometry for the device according to FIGS. 1 and 2 comparing layers made from (a) inkjet-printed GOX on spin coated Nafion and (b) drop-cast Nafion:GOX; and

(16) FIGS. 8B-8E shows microscopy for the device according to FIGS. 1 and 2 layers made from (a) inkjet-printed GOX on spin coated Nafion and (b) drop-cast Nafion:GOX;

(17) FIG. 9 shows the response of devices with a spin-coated Nafion film and inkjet-printed GOX to range of glucose analyte concentrations (0 to 100 mM); and

(18) FIG. 10 shows measured OTFT output characteristics for the device according to FIGS. 1 and 2, comparing the case of having a PVP dielectric layer with not having a PVP dielectric layer.

DEFINITIONS

(19) The following are some definitions that may be helpful in understanding the description of the present invention. These are intended as general definitions only and should in no way limit the scope of the present invention to those terms alone, but are put forth for a better understanding of the following description.

(20) Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated step or element or integer or group of steps or elements or integers, but not the exclusion of any other step or element or integer or group of elements or integers. Thus, in the context of this specification, the term “comprising” means “including principally, but not necessarily solely”.

(21) In the context of this specification, the term “about” is understood to refer to a range of numbers that a person of skill in the art would consider equivalent to the recited value in the context of achieving the same function or result.

(22) In the context of this specification, the terms “a” and “an” refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.

(23) In the context of this specification, the term “bodily fluid” is understood to include any liquid which originates within a human or animal body, including fluids that are secreted or excreted. Non-limiting examples of bodily fluids include: blood, saliva, sweat, urine, breast milk, bile and peritoneal fluid.

(24) In the context of this specification, the term “top” means farthest away from the substrate, and the term “bottom” means closest to the substrate. Where a first layer is described as “disposed above” a second layer, the first layer is disposed farther away from the substrate. Furthermore, where a first layer is described as being “disposed above” a second layer, additional intermediate layers may be present in between the first and second layers, unless it is specified that the first layer is contact with (ie physically contacting) the second layer.

(25) As used herein, like reference numerals in different figures are intended to refer to the same features.

(26) The invention will now be described in more detail, by way of illustration only, with respect to the following examples. The examples are intended to serve to illustrate this invention and should in no way be construed as limiting the generality of the disclosure of the description throughout this specification.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(27) An exemplary transistor device 10 in accordance with one embodiment of the invention is illustrated in FIG. 1, which includes a conceptual representation of the device's structure. The device 10 includes a drain electrode 12 and source electrode 14 on a substrate 16. A two-layered film comprised of an organic semiconducting layer 18 and a hygroscopic dielectric layer 20 covers a portion of the drain and source electrodes, with the organic semiconducting layer 18 extending between the source and drain electrodes. A polymer layer 30 is, located at the top of the device 10. An ohmic conductor 32 is in contact with the polymer layer 30 to enable a gate voltage to be applied to the polymer layer, The polymer layer 30 is disposed above and is in contact with the dielectric layer 20. The dielectric layer 20 is disposed above and is in contact with the organic semiconducting layer 18. The organic semiconducting layer 18 is disposed above and in between the source electrode 14 and the drain electrode 12. The organic semiconducting layer 18 is also in contact with the source electrode 14 and the drain electrode 12. The source electrode 14 and the drain electrode 12 are disposed above, and are in contact with, the substrate 16. The substrate 16 is located at the bottom of the device 10.

(28) The polymer layer 30 retains an enzyme 31 such that the enzyme is embedded within and/or retained on a surface of the polymer layer. In the embodiment described herein, the polymer layer 30 is a porous layer of a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (eg Nafion). The enzyme, which may be glucose oxidase (GOX) for example, is distributed throughout the layer or is at least partially contained within the layer.

(29) The enzyme is selected to facilitate generation of charge carriers when an analyte contacts the device and a minimum electric potential (ie gate voltage or potential) is applied to the ohmic conductor 32, the voltage being selected relative to at least one of the drain and source electrodes. The charge carriers comprise hydrogen ions and electrons. The ohmic conductor 32 applies the gate voltage to the polymer layer 30, resulting in a substantial electric field component in a vertical plane (ie in a plane perpendicular to the top surface of the semiconductor layer). The organic semiconductor is configured to enable flow of electrical current between the source electrode and the drain electrode as a result of the generation of said charge carriers.

(30) In the devices of the present invention, it is believed that the gate potential controls the doping and de-doping of the semiconducting compound(s) via ion migration from the site of ion generation to the active channel in the organic semiconductor. In use, a gate voltage V.sub.G and a drain voltage V.sub.D are applied to the device 10 (the voltages being with respect to the source 14, as shown in FIG. 1), and a sample comprising glucose, for example a bodily fluid such as saliva, is contacted with the polymer layer 30. Glucose in the sample is degraded via an enzymatic reaction with GOX thereby producing H.sub.2O.sub.2. The gate voltage and drain voltage applied cause electrolysis of H.sub.2O.sub.2 thereby liberating H.sup.+ ions. The H.sup.+ ions are conducted though the polymer layer (eg Nafion) and, if present, the dielectric (eg PVP), to the organic semiconducting layer. This results in doping of the semiconductor, and consequentially, current between the drain and the source electrodes. Thus, the increase in H.sup.+ ions results in an increase in drain current, such that a relationship is established between the amount of glucose present in the sample and the magnitude of the drain current.

(31) The gate voltage V.sub.G and drain voltage V.sub.D provide a sufficiently strong electric field to liberate H.sup.+ from H.sub.2O.sub.2, but not enough to cause electrolysis of water, as electrolysis of water may lead to a decrease in the signal-to-noise ratio of the sensor. In at least one embodiment, the gate voltage and drain voltage applied are between about 0 V and −2 V, eg about −1 V.

(32) An exemplary device of the prior art is disclosed in International patent application PCT/AU2013/000207, filed 5 Mar. 2013. That patent application provides an example of organic transistor with a Nafion polymer layer and a hygroscopic dielectric layer.

(33) However, the present inventors have identified that the device can operate effectively without a hygroscopic dielectric layer between the polymer layer and the organic semiconductor layer.

(34) Further the inventors of the present patent application have identified advantageous device behaviour by incorporating the enzyme into the device after depositing the polymer layer. Advantageously, the enzyme may be deposited by ink-jet printing, potentially improving manufacturing costs, at least for manufacturing setup.

(35) Further the inventors of the present invention have identified that the device operates at least for an organic semiconductor layer thickness of less than about 390 nm. Further, there is advantageous device behaviour when the organic semiconducting layer has a thickness in the range of about 75 nm and about 100 nm, between the polymer layer 30 and the source and drain electrodes. The advantage is that in this range the inventors have identified that the device has a calibration curve that has a one-to-one correspondence between a calibration parameter and glucose concentration for concentrations between 0.1 mM and 100 mM. Further their results have shown the calibration curve as being is essentially linear over that range.

(36) Further, there is also advantageous device behaviour when the organic semiconducting layer instead has a thickness in the range of about 36 nm or less, between the polymer layer 30 and the source and drain electrodes. The advantage in this case is a faster response time for the device.

(37) A perspective view (not to scale) of a device 10 fabricated in accordance with the present invention is illustrated in FIG. 2. In this Figure, the source electrode 14, drain electrode 12 and ohmic conductor 32 are provided as pre-patterned ITO on the substrate 16, which is a glass slide (Kintec). A channel length of 20 μm and a width of 3 mm is provided between and covering part of the source electrode 14 and the drain electrode 12. A layer of poly (3-hexyl-thiophene) (P3HT) (Lumtec) with 100 nm thickness is spin-coated on top of the ITO as the organic semiconducting layer 18. Poly (4-vinylphenol) (Aldrich) is spin-coated on top of the P3HT layer with a thickness of approximately 36 nm to form the dielectric layer 20. However, in other embodiments, the Poly (4-vinylphenol) layer is thinner, and in at least one embodiment, there is no Poly (4-vinylphenol) layer. A Nafion solution (Product Number 274704-Sigma-Aldrich Pty. Ltd., Castle Hill NSW, Australia) is spin coated onto the film that the includes the semiconductor (and dielectric, in embodiments in which a dielectric is present) to form the polymer layer. The enzyme, which in this embodiment is glucose oxidise, is ink-jet printed onto the polymer layer. In some embodiments a layer of the enzyme 31 may be formed on the surface of the polymer layer 30 (as depicted in FIG. 1). However, in other embodiments, the enzyme may be additionally or alternatively absorbed at least partly into the polymer layer—hence for simplicity in FIG. 2 the polymer layer and enzyme are illustrated as a single component 30. It will therefore be appreciated, however, that the fabricated device as shown in FIG. 2 may, in some embodiments, include a layer of the enzyme 31 formed on the surface of the polymer layer 30.

(38) The ohmic conductor 32 is laterally offset from the source and drain electrodes, as is part of the polymer layer. In use, a sample 34 (eg a person's bodily fluid) is deposited onto polymer layer 30. GOX breaks down glucose into H.sub.2O.sub.2, amongst other by-products, and the electrolysis of H.sub.2O.sub.2 occurs at a voltage magnitude of 0.7 V, which liberates H.sup.+ ions. Accordingly, in order to bias the device appropriately, the voltage sufficient to liberate ions from the H.sub.2O.sub.2 should be available, but the voltage should preferably not exceed the potential difference required to cause electrolysis of water (1.23 V) which would decrease the signal-to-noise ratio of the sensor.

(39) For glucose sensing measurements, the device was biased at V.sub.D=V.sub.G=−1 V and 10 μL of sample of the analyte in solution (various glucose concentrations in water) was dropped on top of the device (i.e. onto the polymer layer 30 which comprises the GOX) whilst I.sub.D was measured as a function of time. The H.sub.2O.sub.2 liberated by the enzymatic reaction on glucose and its subsequent electrolysis leads to additional ions in the system which has a similar effect to increasing the level of V.sub.G to achieve a higher I.sub.D.

(40) The characteristic response of a device having a prior sensor architecture—comprising ITO source drain electrodes, a P3HT organic semiconducting layer, a poly (4-vinylphenol) (PVP) dielectric layer, and a layer comprising the Nafion membrane pre-mixed with GOX—to glucose analyte solutions was disclosed in PCT/AU2013/000207.

(41) Subsequent investigations by the present inventors have shown that removing or reducing the thickness the dielectric layer can improve the response time of the sensor.

EXAMPLES

(42) Examples illustrating various features of the present invention will now be described for a device manufactured in accordance with FIG. 2, in which the no dielectric layer is present between the polymer layer (in this case Nafion) and the organic semiconductor layer (in this case P3HT).

(43) FIGS. 3A-3B shows the effect of varying the thickness of the organic semiconducting layer from 108 nm to 22 nm upon the rate of sensor response, represented by drain current (I.sub.D) as a function of time. FIG. 3A shows variation of I.sub.D with time for the P3HT/Nafion:GOX OTFT architecture with a P3HT layer having a thickness of 108 nm (indicated by 44), 36 nm (indicated by 46), and 22 nm (indicated by 48). The glucose solution is added at t=0 and a fast decay and a slower rise in I.sub.D is observed for all P3HT layer thicknesses. FIG. 3B shows an expanded view of the fast decay process for the P3HT/Nafion:GOX OTFT architecture for the three different thicknesses for the semiconductor layer. FIG. 3C shows an expanded view of the slow rise process for the P3HT/Nafion:GOX OTFT architecture for the three different thicknesses for the semiconductor layer.

(44) For the sake of data clarity, I.sub.D is presented as a ratio of its stabilized level prior to addition of sample (glucose-containing saliva) minus the minimum value of this ratio (which occurs soon after sample addition)—a quantity referred to hereafter as “adjusted I.sub.D”.

(45) As the thickness of the P3HT channel is reduced, it appears that the response time of a fast de-doping process 40 and a slow doping process 42 both reduce, consistent with diffusion processes that traverse a reduced layer thickness. Both processes can be modelled by one dimensional solutions to Fick's second law of diffusion, n(x,t)=n.sub.0 erfc{x/[2/(D.sub.eff t).sup.0.5]}=n.sub.0 erfc(A/t), where A (comprising x(diffusion distance) and Deff (effective diffusion constant)) and n.sub.0 (initial concentration) are treated as fitting constants with the fit solution shown as dashed lines in FIGS. 3A and 3B. Fitting the fast (A.sub.fast) and slow (A.sub.slow) responses to Ficks law provides an estimate for the value of A for each process where A=x/[2(D.sub.eff t).sup.0.5].

(46) The fitted values of A are 0.62 (for 108 nm P3HT), 0.48 (for 36 nm P3HT), and 0.44 (for 22 nm P3HT) for the fast decay process. The fitted values of A are 10.3 (for 108 nm P3HT), 9.4 (for 36 nm P3HT), and 3.6 (for 22 nm P3HT) for the slow decay process.

(47) It is the understanding of the present inventors that the fast dedoping and slow doping processes have to diffuse across the same layer thickness for a given OTFT architecture. As such, A.sub.fast/A.sub.slow=[D.sub.slow/D.sub.fast].sup.0.5, where A.sub.slow and A.sub.fast are the fitting parameters, and D.sub.slow and D.sub.fast are the effective diffusion constants, for the slow and fast processes, respectively. Thus, the ratio A.sub.fast/A.sub.slow as a function of changing layer thickness should only depend upon the ratio of the diffusion constants of the two processes.

(48) FIG. 4A shows the variation of A.sub.fast/A.sub.slow as a function of the different device layer thicknesses and includes the data for both the devices with PVP (layer thickness 400 nm), as data point 50, and the devices of varying P3HT thickness without a PVP layer, as data points 52, 54, and 56. The A.sub.fast/A.sub.slow ratio is relatively invariant across both the OTFT device with a PVP layer and the devices without a PVP layer but with P3HT layers that are at least 36 nm thick.

(49) This observation supports the assertion that, for each OTFT architecture, the fast dedoping and slow doping process diffuse across the same effective distance and indicates that the ratio of the corresponding diffusion constants is invariant.

(50) However, when the P3HT thickness drops below 36 nm, there is an abrupt increase in the A.sub.fast A.sub.slow ratio. The fitted values for A (for both the slow and fast processes) decrease with decreasing P3HT layer thickness, consistent with more rapid diffusive transport. Moreover, the decrease in A.sub.slow for the very thinnest P3HT layer is more dramatic than the corresponding decrease in A.sub.fast, indicating that there is a difference in the two processes for P3HT thicknesses below about 36 nm.

(51) FIG. 4B illustrates the variation of I.sub.D as a function of time for non-PVP OTFT devices with a P3HT layer having a thickness of 22 nm (the upper dotted line in the figure) and about 9 nm (the lower dotted line in the figure). An examination of FIG. 4B reveals that this abrupt increase is dominated by a critical change in the effective diffusion constant for the slow doping process for P3HT thicknesses below 36 nm.

(52) The data in FIG. 4A are consistent with the presence of some thickness of P3HT which only serves to slow the diffusion rate of protons to the active (doping) region of the channel. As such, when the P3HT thickness drops below a critical value (about 36 nm) then there is no diffusive barrier to protons accessing the doping region of the device. Indeed, this suggests that any further reduction of the P3HT layer thickness should not affect the diffusion rate of either the fast or slow process (since there is no diffusive barrier) and reducing the P3HT thickness now merely alters the absolute number of doping sites and therefore the current in the channel.

(53) To confirm this, the P3HT layer thickness was further reduced from 22 nm by decreasing the concentration of the P3HT solution (from 5 mg mL.sup.−1 to 2 mg mL.sup.−1). The resulting P3HT layers exhibited regions of incomplete coverage and hence the layer thickness (about 9 nm) could only be estimated from the P3HT loading. Despite this, functional devices could be prepared. FIG. 4B compares I.sub.D as a function of time for two devices. respectively with a 22 nm and a 9 nm thick P3HT layer. FIG. 4B shows that the device response is lower for the thinner P3HT layer and that I.sub.D at saturation has reduced from a value of about 1.2 to a value of about 0.5 on this scale, corresponding to a 42% reduction in current that is quantitatively consistent with the reduced P3HT thickness. Fitting the current to the Fick's law reveals that the only fitted parameter that has to be changed is that of n.sub.0 (which governs the absolute magnitude of the response), whereas the fitted value of A is constant for both fast and slow processes for both of these P3HT thicknesses. Consequently, the data are consistent with a doping region which, for thicker P3HT layers, does not lie at the interface between the P3HT and the Nafion layer but instead lies at some small distance from the source and drain electrodes and is overlayed by undoped P3HT through which protons must diffuse (or conducted in some other way).

(54) Based on this, FIG. 4C depicts schematic diagrams showing the location of the identified doping region (the hatched area 58 in the diagram) for a thick P3HT layer (left hand diagram) and critical thickness P3HT layer (right hand diagram). As the P3HT layer is reduced, it reaches a critical thickness at which the Nafion layer interface is coincident with the doping region and subsequent decreases in P3HT thickness serve only to decrease the size of the doping region and hence the observed current.

(55) FIGS. 5A-5D shows the effect of changing gate voltage (V.sub.GS) upon device performance after addition of 30 mM glucose solution. FIG. 5A shows variation of I.sub.D as a function of time for V.sub.GS=−1.0V (lower dotted line) and V.sub.GS=−0.3V (upper dotted line). This figure shows that as the V.sub.GS is made more positive (changed from −1.0V to −0.3 V) so the value of adjusted I.sub.D at saturation increases (from about 0.6 to about 1.75 on this scale). FIG. 5B shows variation of gate current as a function of time for V.sub.GS=−1.0V (lower dotted line) and V.sub.GS=−0.3V (upper dotted line). This figure indicates that this rise in I.sub.D is associated with a change in the polarity of the net current flowing from gate to source. These results are consistent with a change in the net electric field experienced by the charge carriers (protons). When V.sub.GS=−1.0 V, there is a net electric field from source to gate (since both ohmic conductor 32 and drain electrode 12 are held at −1.0V relative to the source electrode 14), whereas when V.sub.GS=−0.3 V, there is a net electric field from gate to drain. These electric forces act in addition to the diffusion gradient within the device serving to either retard (V.sub.GS=−1.0 V) or enhance (V.sub.GS=−0.3 V) protonic doping of the channel, as shown schematically in FIG. 5C, which indicates the gate current flow and net electric field (unfilled arrows) for V.sub.GS=−1.0V and −0.3 V. This change in V.sub.GS increases the sensitivity of the device response and consequently subsequent measurements discussed hereinafter have been conducted with a gate bias voltage of −0.3 V.

(56) The present have identified that to improve consistency of the GOX thickness and to reduce aggregation GOX, the polymer (Nafion) of the polymer layer can be may be spin coated as a first step, and the enzyme (GOX) can be subsequently inkjet-printed onto the polymer.

(57) The inventors attribute this improvement to both the higher solubility of GOX in water compared to a solvent mixture in which GOX is pre-mixed with Nafion and drop-cast together. Additionally or alternatively, the improvement may be due to a slower deposition rate. In devices with inkjet-printed GOX, the enzymatic activity of the devices remained intact, and the response time of the devices improved (conceivably due to diffusion through the thinner spin-coated Nafion layer being a faster process than through the thicker, drop-cast layer, as well as due to the enzyme being more readily available to the analyte, since it is more evenly dispersed). For such devices with a spin-coated Nafion layer and inkjet-printed enzyme, a calibration parameter, X, was calculated from t=0 to 500 s. The parameter X was defined as follows:

(58) $\chi =\frac{{\int }_{t=0}^{t=500}{I}_d\left(t\right)\mathrm{dt}}{{\int }_{t=0}^{t=500}{I}_d\left(0\right)\mathrm{dt}}.$

(59) FIG. 6 shows the calibration curves (average calibration parameter, X, as a function of glucose concentration) for inkjet-printed OTFT sensor devices with P3HT thicknesses varying from 22 nm to 390 nm. In particular FIG. 6 shows calibration curves for P3HT thicknesses of: 22 nm (curve a), 36 nm (curve b), 74 nm (curve c), 108 nm (curve d), and 390 nm (curve e).

(60) Although the thinner P3HT devices have a faster response, the results shown in FIG. 6 indicate that variability in the device response to the glucose analyte in solution also increases for thinner P3HT devices, resulting in a reduced linearity of the calibration curve. For embodiments where linearity of the calibration curve is important, the results show that a P3HT thickness of about 74 nm to about 108 nm (eg 75-100 nm) as providing a good balance between glucose sensitivity and reproducibility. As illustrated in curves c and d of FIG. 6, an approximately linear response can be expected between glucose concentration and X for glucose concentrations between 100 μM and 100 mM.

(61) Examples illustrating that the device 10 can operate without a hygroscopic dielectric layer 20 between the polymer layer 30 and semiconductor layer 18 will now be described.

I. Experimental Procedure

(62) Pre-patterned ITO-on-glass substrates (15 Ω□.sup.−1 ITO, Xin Yan Technology) were used for the substrate, the source and drain electrodes and ohmic conductor 32 of the fabricated devices. Poly-3-hexylthiophene (P3HT) (MW ˜20 000, synthesised in the labs) was dissolved in CHCl.sub.3 (Sigma-Aldrich) at various concentrations and sonicated for ˜1 hour or until the material was entirely dissolved. Poly-4-vinylphenol (PVP) (Sigma-Aldrich) was dissolved in ethanol (Sigma-Aldrich) at a concentration of 80 mg mL.sup.−1 and sonicated for ˜1 hour or until the material was entirely dissolved. Nafion solution (5% by weight in lower aliphatic alcohols and water, Sigma-Aldrich) was used as received. Glucose oxidase (GOX) (Sigma) was either mixed with the as received Nafion solution at a concentration of 20 mg mL.sup.−1 or dissolved in purified water (Milli-Q purification system, Millipore) at a concentration of 50 mg mL.sup.−1 prior to processing. Glucose (Sigma-Aldrich) was dissolved in purified water at various concentrations.

(63) The pre-patterned ITO-on-glass substrates were first cleaned with methanol and purified water. P3HT solution in CHCl.sub.3 was spin-coated onto the substrates at 2000 rpm for 60 seconds. P3HT solutions of 5 mg mL.sup.−1, 10 mg mL.sup.−1, 15 mg mL.sup.−1, 20 mg mL.sup.−1, and 40 mg mL.sup.−1 were prepared, with average thicknesses of films spun from these concentrations of P3HT were 22 nm, 36 nm, 74 nm, 108 nm and 390 nm respectively. The P3HT layer was patterned and then left to dry for 15 minutes at 40° C. For devices with a PVP layer, PVP solution was then spun on top of the P3HT layer at 2000 rpm for 60 seconds (film thickness ˜400 nm), then patterned and dried. For these PVP-containing devices, the Nafion:GOX mixture was then drop-cast above the source-drain channel area and connected to the ITO gate pad of the substrate and dried for approximately 30 minutes. This drop-cast Nafion:GOX layer allows protonic conduction. For the first devices in which the Nafion and GOX were deposited independently, Nafion solution was first spin-coated at 500 rpm for 120 seconds. Subsequently, the aqueous GOX solution was either drop-cast or inkjet-printed above the source-drain channel area to enable a comparison between these two different methods of device preparation.

(64) Inkjet-printed GOX was deposited using a Fujifilm Dimatix DMP 2800 piezoelectric inkjet-printer. 2 mL of the aqueous GOX solution was injected into a cartridge (DMC 11610, Fujifilm Dimatix). GOX solution was printed onto a ˜7 mm.sup.2 area over the channel of each device, and was then dried on a hotplate at 40° C. The printing conditions were: 20 μm drop spacing, 10 layers, 28° C. platen heating, ˜25 V drive voltage, jetting frequency 2 kHz.

(65) For measurements of drain current (I.sub.D) and gate current (I.sub.G) versus time for various glucose concentrations, two Keithley 2400 source meters were used to collect the data with the source electrode considered as the common electrode (0 V) and the drain voltage (V.sub.DS) held at −1 V. Gate voltage (V.sub.GS) was held at either −0.3 V or −1 V (see discussion below). After time to allow I.sub.D to stabilise, 5 μL of an aqueous glucose solution was dropped on top of each device, immediately above its source-drain channel. Glucose concentrations between 100 μM and 100 mM were used in this study, with I.sub.D and I.sub.G being recorded for a further 10 minutes after addition of the analyte solution. Film thickness measurements were taken using a Tencor Alpha-Step 500 surface profilometer.

III. Characterization of Sensors with and without PVP Layer

(66) FIG. 7A shows the I.sub.D versus time characteristics of devices prepared with the standard architecture (P3HT/PVP/Nafion:GOX), indicated by curve 60, and without the PVP layer (P3HT/Nafion:GOX), indicated by curve 62, to a drop of 30 mM glucose analyte solution; revealing operation og the two time dependent processes discussed above. Specifically, there is firstly a rapid drop in I.sub.D upon addition of the analyte solution to the Nafion gate electrode at time t=0 (best seen in FIG. 7B). This process is independent of glucose concentration and arises from dedoping of the P3HT channel since it also occurs upon the addition of deionised water alone. Second, there is a much slower rise in drain current (best seen in FIG. 7C) that is correlated with glucose concentration and thus defines the functional response time of the sensor. This slower process arises from protonic diffusion and doping of the P3HT channel. As discussed above, both processes can be modelled by one dimensional solutions to Fick's second law of diffusion, n(x,t)=n.sub.0 erfc{x/[2(D.sub.eff t).sup.0.5]}=n.sub.0 erfc(A/t.sup.0.5), where A (comprising x (diffusion distance) and D.sub.eff (effective diffusion constant)) and n.sub.0 (initial concentration), are treated as fitting constants with the fit solution shown as dashed lines in FIG. 7B and FIG. 7C. The fitted values of A are: 0.9 (with PVP) and 0.62 (without PVP) for the fast decay process. The fitted values of A are: 18 (with PVP) and 10.3 (without PVP) for the slow decay process. Elimination of the PVP layer reduces the response time of both processes, consistent with diffusion processes that traverse a reduced layer thickness. The data highlights that the PVP layer is not required for device function and serves only to slow the device response.

III. Optical Microscopy and Profilometry of Spin Coated and Inkjet-Printed GOX-Containing Layers

(67) FIGS. 8A-E shows profilometry and microscopy of both the inkjet-printed and drop-cast Nafion:GOX layers. FIG. 8A shows profilometry of regions of both inkjet-printed GOX on spin-coated Nafion and drop-cast GOX:Nafion mixture. The profilometry in FIG. 8A reveals a dramatic decrease in the surface roughness of the inkjet-printed GOX layer compared to the drop cast films suggesting a much more even distribution of the enzyme. FIGS. 8B and 8C show optical micrographs of a region of GOX inkjet-printed on a spin-cast Nafion layer (scale bars are 1 mm and 100 μm respectively). FIGS. 8D and 8E show optical micrographs of a region of drop-cast Nafion:GOX mixture (scale bars are 1 mm and 100 μm respectively). The microscopy results in FIGS. 8B to 8E confirm that the inkjet-printed devices are much more uniform across the entire area of the film.

IV. Example Data—ID Vs Time for a Range of Glucose Concentrations

(68) FIG. 9 shows the response of devices with a spin-coated Nafion film and inkjet-printed GOX to range of glucose analyte concentrations (0 to 100 mM). The figure shows adjusted drain current as a function of time for OTFT devices with different glucose analyte concentrations: 100 mM (curve A), 10 mM (curve B), 1 mM (curve C), 0.1 mM (curve D) and 0 mM (curve E). FIG. 9 reveals two key features. First, it is clear that the activity of the GOX has remained after inkjet-printing; demonstrating that the enzyme activity is retained even after the fabrication process. Second, the rise time of the devices is now much faster than observed for drop-cast devices 13 and thus much more responsive devices have been fabricated using the inkjet-printing approach. It is also possible that the reoxidation of the reduced GOX may not be very efficient and that this process is limiting the response of the devices. As such, further improvements in sensor response may be possible by increasing oxygen accessibility in the device.

V. Example Data—OTFT Output Characteristics

(69) In FIG. 10, sets of curves labelled b and c show the output characteristics for the PVP-containing sensor device and the PVP-free sensor device prior to inkjet-printing of the enzyme, respectively. In each of curve sets, the lowest curve represents V.sub.G=4V, and the highest curve represents V.sub.G=−1.4V. As can be seen from these figures, each of these devices exhibit drain current modulation with changes in gate voltage.

(70) It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.