SENSOR COMPONENT AND PROCESS FOR PRODUCING SENSOR COMPONENT USING ELECTROPOL YMERIZATION

Abstract

A process for producing a sensor component for detecting an analyte; a sensor component producible by the process; a process for detecting an analyte; and a device comprising the sensor component. The process comprises electrochemically growing a plurality of conducting polymer molecules from a monomer electrolyte solution to provide a percolation network. The plurality of conducting polymer molecules are grown on the surface of an insulating substrate to connect a first electrode to a second electrode and are capable of displaying a change in an electrical property in response to interaction with an analyte A plurality of conductive nodes may be disposed on a surface of the insulating substrate. A potentiostatic method or a galvanostatic method may be employed to grow the plurality of conducting polymers. Chronoamperometry may be employed to electrochemically grow the plurality of conducting polymers. Cyclic voltammetry is not employed to grow the plurality of conducting polymers.

Claims

1. A process for producing a sensor component for detecting an analyte, the process comprising: providing an insulating substrate disposed between a first electrode and a second electrode; providing a monomer electrolyte solution, the monomer electrolyte solution comprising a plurality of monomer molecules capable of electrochemical growth to form a plurality of conducting polymer molecules; electrochemically growing a plurality of conducting polymer molecules from the monomer electrolyte solution, the plurality of conducting polymer molecules being grown on the surface of the insulating substrate to connect the first electrode to the second electrode and being capable of displaying a change in an electrical property in response to interaction with an analyte; and ceasing electrochemical growth of the conductive polymer molecules to provide a percolation network; wherein cyclic voltammetry is not employed to grow the plurality of conducting polymers.

2. The process of claim 1, wherein a plurality of conductive nodes are disposed on a surface of the insulating substrate.

3. The process of claim 1, wherein a potentiostatic method or a galvanostatic method is employed to grow the plurality of conducting polymers or wherein chronoamperometry is employed to electrochemically grow the plurality of conducting polymers.

4. (canceled)

5. The process of claim 1, wherein an average potential of at least +0.5V is applied to the first electrode and/or the second electrode to effect electrochemical growth of the plurality of conducting polymer molecules.

6. The process of claim 1, wherein the plurality of monomer molecules comprise a phenylene, a vinylene, an acetylene, an azulene, a naphthalene, a pyrene, an aniline, a fluorene, a pyrrole, a thiophene, a carbazole, an indole, and/or an azepine or wherein the plurality of conducting polymer molecules are selected from a polyphenylene, a polyparaphenylene, a polyparaphenylene vinylene, a polyparaphenylene acetylene, a polyazulene, a polynaphthalene, a polypyrene, a polyaniline, a polyparaphenylene sulphide, a polyfluorene, a polypyrrole, a polythiphene, a polycarbazole, a polyindole, and/or a polyazepine.

7. (canceled)

8. The process of claim 1, wherein the percolation network is doped by exposure to an electrolyte solution that does not comprise a plurality of monomer molecules capable of electrochemical growth to form a plurality of conducting polymer molecules.

9. A sensor component producible by the process of claim 1.

10. A sensor component for detecting an analyte comprising: a first electrode and a second electrode; an insulating substrate disposed between the first electrode and the second electrode; a plurality of conducting polymer molecules, the conducting polymer molecules being capable of displaying a change in an electrical property in response to interaction with an analyte; the plurality of conducting polymer molecules forming a percolation network that electrically connects the first electrode to the second electrode; wherein at least 90% of the plurality of conducting polymer molecules are electrically connected to the first electrode and/or the second electrode.

11. The sensor component of claim 10, wherein a plurality of conductive nodes are disposed on a surface of the insulating substrate and at least some of the plurality of conducting polymer molecules are bonded to one or more nodes, and thereby electrical junctions are formed between separate nodes or between a node and an electrode, thereby forming the percolation network that electrically connects the first electrode to the second electrode.

12. The sensor component of claim 10, wherein at least 5% of the plurality of conducting polymer molecules are electrically connected to both the first and the second electrodes.

13. The process of claim 1, wherein the first electrode and the second electrode independently comprise a metal selected from groups 3 to 16 of the periodic table of the elements, graphite, a conducting oxide, a conducting nitride, a conducting carbide or a mixture thereof.

14. The process of claim 13, wherein the first electrode and the second electrode independently comprise platinum, palladium, copper, gold, silver, zinc, indium tin oxide, graphite or a mixture thereof.

15. The process of claim 1, wherein the first electrode and the second electrode are interdigitated.

16. The process of claim 1 additionally comprising one or more further electrodes.

17. The process of claim 1, wherein the insulating substrate comprises magnesium oxide, strontium titanate, beryllium oxide, aluminium oxide, aluminium nitride, silicon oxide or a mixture thereof.

18. A process for detecting an analyte, the process comprising: exposing a sample to a sensor component, wherein the sensor component comprises: a first electrode and a second electrode; an insulating substrate disposed between the first electrode and the second electrode; and a plurality of conducting polymer molecules, the conducting polymer molecules being capable of displaying a change in an electrical property in response to interaction with an analyte; wherein the plurality of conducting polymer molecules forms a percolation network that electrically connects the first electrode to the second electrode and at least 90% of the plurality of conducting polymer molecules are electrically connected to the first electrode and/or the second electrode; and measuring a change in an electrical property of the percolation network.

19. The process of claim 18, wherein the electrical property is conductance.

20. The process of claim 18, wherein the analyte is ammonia.

21. A device comprising the sensor component of claim 9, and detection means capable of detecting a change in an electrical property of the sensor component due to the interaction of an analyte with the plurality of conducting polymers.

22. A device comprising the sensor component of claim 8, and detection means capable of detecting a change in an electrical property of the sensor component due to the interaction of an analyte with the plurality of conducting polymers.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0082] FIG. 1 is a diagram containing conceptual graphs for a percolation network showing the relationship between electrical property of the network and the number of conducting elements in the network.

[0083] FIG. 2 is a schematic diagram of the stages of electrochemical growth of a conducting polymer from the monomer solution. In (a) the polymer growth fronts advance towards each other and a few strands meet at the percolation threshold (panel 5). In (b) gold nanoparticles act as nucleation centres and the percolation threshold has been reached in panel 3.

[0084] FIG. 3 is a linear plot of conductance as a function of chronoamperometric transient time for plain substrates between the electrodes (circles) and substrates between the electrodes that have been decorated with gold nanoparticles (squares).

[0085] FIG. 4 is a logarithmic plot of conductance as a function of chronoamperometric transient time for plain substrates between the electrodes (circles) and substrates between the electrodes that have been decorated with gold nanoparticles (squares).

[0086] FIG. 5 shows chemiresistive responses of doped PPy percolation networks to ammonia in a dry nitrogen carrier gas. In (a) a gold-decorated substrate between the electrodes is used with a starting resistance of 5.5 kΩ, and in (b) a plain substrate between the electrodes is used with a starting resistance of 20.9 kΩ. The gradients of the curves in (a, b) are shown in (c, d), respectively.

[0087] FIG. 6 shows the rate of change of the signal to noise ratio responses to different concentrations of ammonia of percolation networks with different levels of connectivity for (a) substrates decorated with gold nanoparticles between the electrodes and (b) for plain substrates between the electrodes.

[0088] FIG. 7 is a graph showing the limit of detection (LOD) for ammonia gas versus PPy percolation network resistance for nanoparticle-decorated substrates between the electrodes (squares) and plain substrates between the electrodes (circles). The curves are drawn as a guide to the eye in the semi-logarithmic plot.

[0089] FIG. 8 is a schematic layout of the ammonia gas sensing rig used for ammonia gas sensing experiments.

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLES

[0090] Pt interdigitated electrodes on glass substrates were purchased from Micrux (Spain). Each interdigitated electrode contained 180 pairs of 5 μm wide electrodes separated by a gap of 5 μm. Interdigitated electrodes were cleaned with concentrated nitric acid (HNO.sub.3, 90%) then sonicated in methanol (CH.sub.3OH, 99.9%), ethanol (C.sub.2H.sub.5OH, 99.8%) and acetone (C.sub.3H.sub.6O 99.8%) for 10 minutes each prior to use. All solyents were purchased from Sigma-Aldrich UK.

[0091] Au films of 5 nm were deposited at a rate of 0.2 Å/s using a Knudsen-cell in an ultra-high vacuum chamber with a base pressure of 10.sup.−8 Pa. Average layer thickness was continuously monitored throughout deposition using a quartz crystal microbalance. The layer was annealed at 200° C. for 1 h. This thermal evaporation in ultra high vacuum (UHV) followed by an annealing step that causes dewetting results in dispersed nanoparticles on the glass substrate (Lefferts et al. Appl. Phys. Lett. 112, 251602 (2018)).

[0092] All electrochemical experiments were performed using a PGSTAT204 Autolab potentiostat (Methrohm, UK) interfaced to a PC with NOVA version 1.11 software. Pyrrole (Py, 98%), lithium perchlorate (LiClO.sub.4, 95%) and acetonitrile (CH.sub.3CN, 99.8%) were purchased from Sigma-Aldrich UK. A three electrode cell was employed with a Pt coil (BASi, USA) as the counter electrode. An Ag/AgCl (CH Instruments, USA) reference electrode was used. The electrodes of the interdigitated electrode were connected and used as the working electrode.

[0093] For the electrochemical growth of polypyrrole, 0.1 M pyrrole was prepared in 0.1 M LiClO.sub.4/acetonitrile and a potentiostatic method was used. The potential was stepped up to 1 V from 0 V and held for various periods. After deposition the substrates were put back in monomer-less solution (0.1 M LiClO.sub.4/acetonitrile) and held at 1 V for 60 s to p-dope the polypyrrole.

[0094] After p-doping, the substrates were allowed to dry in air after washing with acetonitrile. DC resistance measurements were made between the interdigitated electrodes by applying a voltage of 1 V and measuring the resulting current.

[0095] FIG. 8 shows a schematic of the gas sensing rig used for the ammonia gas sensing experiments. Ammonia gas (10 ppm, nitrogen fill) and N.sub.2 gas (for further dilution of NH.sub.3) were purchased from BOC gases UK. The flow rates from the respective gas cylinders were controlled by digital mass flow controllers (Alicat, USA) which flow into a Swagelok T-joint to ensure mixing of the gases before entering the inlet of the gas chamber. The percolation sensors are placed on a sample stage in the chamber with electrical connections running from inside the chamber to a multimeter and power supply outside. The sensing chamber is first purged with nitrogen gas for 45 minutes to remove any impurities present in the chamber or in the sensing layer on the interdigitated electrodes. Then a voltage of 1 V is applied to the two electrodes and the current is monitored as a function of time on a computer equipped with Benchvue software. Once a stable baseline is reached different concentrations of ammonia gas are introduced into the system. The ppb concentration of ammonia gas that was introduced into the sensing chamber was calculated by the relative flowrates of the two mass flow controllers while always maintaining a constant flow rate of 500 standard cubic centimetres per minute (sccm).

Example1—Chronoamperometric Growth of Percolation Networks

[0096] To create the CP networks shown in FIG. 2 chronoamperometric growth for transient times between 5 s and 70 s, followed by p-type doping, was carried out for plain electrodes and electrodes decorated with gold nanoparticles.

[0097] The electrical conductance was determined by applying a dc potential of 1V between the interdigitated electrodes and measuring the current (FIG. 3). It will be understood that the resistance (R) of an object is defined as the ratio of voltage across it (V) to current through it (I), i.e. R=V/I while the conductance (G) is the reciprocal, i.e. G=1/R.

[0098] For the plain electrodes (FIG. 3 and FIG. 4, circles) it can be seen that there is no appreciable increase in conductivity until 40 s, which is equivalent to the beginning of the percolation region in FIG. 1 and FIG. 2a (panel 5). As more CP is grown, so the conductivity between the electrodes increases until a continuous thin film is created.

[0099] Conversely, for the nanoparticle-decorated electrodes (FIG. 3, squares) a jump in conductivity already occurs at 15 s, corresponding to panel 3 in FIG. 2b, and there is a further significant increase at 20 s. Beyond 20 s there is no appreciable increase in conductivity, presumably because once all the nanoparticles have been electrically connected a thick film of CP is required for any further substantial impact.

Example 2—Ammonia Gas Sensing

[0100] The polypyrrole percolation networks of Example 1, having various degrees of connectivity, were tested for their performance in the gas sensing rig (FIG. 8). Ammonia was used as the analyte.

[0101] Typical sensing responses from 700 parts per billion (ppb) to 100 ppb are shown in FIG. 5 for a 5.5 kΩ network with Au particle-decorated electrodes (FIG. 5a), and a 20.9 kΩ network with plain electrodes (FIG. 5b). The effect of 100 ppb ammonia exposure can be seen in both the response curves. Numerous sensors with a broad range of electrical resistances were created and these were evaluated using a testing protocol of 5 minutes of ammonia exposure in a dry nitrogen carrier gas, followed by dry nitrogen exposure until the sensor baseline was restored, which was typically in less than 30 mins, but was substantially shorter for low concentrations of ammonia.

[0102] The maximum rate of change of the sensor as a function of analyte exposure is multiplied by 3, representing 3 standard deviations, or a 99% confidence interval, to arrive at a number for the limit of detection (LOD).

[0103] The gradients of the sensor response curves are shown in FIG. 5c,d with the peaks of the spikes corresponding to the greatest rates of change. These peaks are unambiguously associated with sensor performance as they do not depend on the duration of analyte exposure.

[0104] Each sensor with a different level of network connectivity will have a different maximum rate of change response, with the networks at the percolation threshold showing the greatest sensitivity (FIG. 1b). However the percolation threshold networks will also suffer from the greatest noise levels. Ultimately, we are interested in optimising the signal to noise ratio of the sensor (FIG. 1c). So rather than plotting the spike heights in FIG. 4c,d, we divide these heights by the root mean square of the noise measured for the individual sensors. These results are shown in FIG. 6 as a function of analyte concentration.

[0105] FIG. 6a shows that the sensor with the steepest gradient, and hence the optimum performance, is the 5.5 kΩ network. For lower network connectivity the 25.5 kΩ and 119.4 kΩ sensors have higher sensitivity, but greater noise levels. Conversely, the networks with greater connectivity (1.1 kΩ and 49.1 Ω) have lower noise levels, but also lower sensitivities. The 49.1 Ω sensor is in effect operating at maximum connectivity, is not a percolation network, and can be thought of as a thin film device.

[0106] Data from the chemiresistive percolation sensors with plain electrodes is shown in FIG. 6b, which also shows that the optimum sensor with the highest signal to noise ratio response lies in a region of the percolation curve that is a little beyond the percolation threshold (box in FIG. 1a).

[0107] The LODs can be calculated straightforwardly from the linear fits in FIG. 6 and are defined as the point where the signal is a factor of 3 greater than the noise. The LOD for each sensor is plotted against resistance in FIG. 7. This figure shows that a more effective percolation device is created when using an Au nanoparticle scaffold (minimum LOD of 18±2 ppb in FIG. 7 curve) than without (minimum LOD of 27±5 ppb in FIG. 7 curve).

CONCLUSION

[0108] The results demonstrate that the inventors have electrochemically grown doped PPy percolation networks between interdigitated electrodes on glass substrates, and that these networks can be used for high sensitivity ammonia sensing. The strategy of pre-patterning the glass with Au nanoparticles improves the sensitivity and it is proposed that the nanoparticles act as fresh nucleation centres, increasing the ability of the network to spread. The ideal level of network connectivity for high sensitivity gas sensing is just beyond the percolation threshold where the SNR is optimised. In this region the LOD of 18±2 ppb is better by a factor of 20 compared with thin film devices made with the same CP.