GB2296975A

GB2296975A - Electrochemical sensor with electrode oscillating relative to the analyte solution

Info

Publication number: GB2296975A
Application number: GB9600467A
Authority: GB
Inventors: Arthur Mcnaughtan
Original assignee: University of Glasgow; Glasgow Caledonian University
Current assignee: University of Glasgow; Glasgow Caledonian University
Priority date: 1995-01-11
Filing date: 1996-01-10
Publication date: 1996-07-17
Anticipated expiration: 2016-01-10
Also published as: GB2296975B; GB9500485D0; GB9600467D0

Abstract

An electrochemical system for determining the concentration of an analyte in a solution has a first electrode 6 for immersion in the solution 10 and transducer means 12 for oscillating the solution in a vertical direction relative to the first electrode 6. A bias voltage is applied between the first electrode 6 and a second electrode 13 which is also immersed in the solution. Phase sensitive detection means is provided and comprises a current to voltage converter coupled in series with the first and second electrodes. The output voltage provided by the converter is coupled to an input of a lock-in amplifier which also receives an input signal corresponding to an oscillating signal used to drive the transducer means 12. The lock-in amplifier provides an output signal which is proportional to the faradaic component flowing through that portion of the first electrode which is oscillating into and out of the solution.

Description

ELECTROCHEMICAL SENSORS The present invention relates to electrochemical sensors used for determining the concentration of an analyte present in a solution. The invention is applicable particularly, but not necessarily, to sensors which use microelectrodes.

Electrochemical sensors are widely used in analytical investigations in order to identify an analyte, or to determine the concentration of an analyte, in solution. Electrochemical sensors rely upon the basic processes of oxidation and reduction of a species at an electrode and the associated gain or loss of electrons there. Amperometric methods use a working electrode and a reference electrode immersed in a solution containing the species of interest and measure the current flowing between the electrodes when an external voltage is applied between the electrodes. Provided the mass transport of the species under investigation to the working electrode is controlled, the measured faradaic current will be proportional to the concentration of the species in the solution.

Sensor systems which use a microelectrode (typically less than 50m in diameter) offer a number of advantages over systems which use conventional sized electrodes (typically 1 to 5mm) in electroanalytical studies. In particular, the ohmic drop in a solution of interest is reduced, charging currents are reduced, steady state diffusion limited conditions are rapidly achieved and the small physical size of the electrodes enables them to be used in applications where larger conventional sized electrodes cannot be used, e.g. where only a small volume -of solution is available.

Whilst a wide range of microelectrode geometries including disc, micro-ring, hemispherical, band, cylindrical, disc arrays and interdigitated arrays, have been reported in the literature, the most commonly used microelectrode comprises a fine platinum wire sealed within a fine glass capillary tube, one end of the tube being polished to expose a circular area of the platinum wire for exposure to the solution. Such a microelectrode 1 is shown in Figure 1 where the upper end of the microelectrode is attached to a printed circuit board 4 to enable an external electrical connection to be made to the platinum wire.

Whilst the lower exposed area of the platinum wire 2 of the microelectrode 1 shown in Figure 1 can be controlled with a high degree accuracy, it is more difficult to ensure that the seal between the glass 3 and the platinum wire 2 is sufficient to prevent electrolyte from penetrating into the gap between the electrode and the capillary. This penetration has the effect of increasing the area of the platinum electrode exposed to the solution and can negate the aforesaid advantages which microelectrodes are designed to achieve. In addition, significant leakage of current can occur across the glass capillary tube where the tube is immersed in the electrolyte.

It will also be appreciated that electrodes of the type shown in Figure 1 are difficult to manufacture and, whilst insulating and electrode materials having better electrical properties than glass and platinum are available, they cannot practically be used to produce devices of the type shown in Figure 1 because of the difficulty in manufacturing the electrode with a good seal. Also, some electrode materials such as gold have proved difficult to seal in glass due to different thermal coefficients of expansion.

It is a first object of the present invention to overcome or at least mitigate the disadvantages of known electrochemical sensors.

It is a second object of the invention to provide a microelectrode, no insulated part of which is required to be immersed in solution when the microelectrode is in use.

These and other objects are achieved by providing an apparatus and a method for varying the level of immersion of an electrode in a solution by a relatively small amount to provide a small active sensing area which is frequently wetted in order to simulate the use of a microelectrode.

According to a first aspect of the present invention there is provided an electrochemical sensor system for determining the concentration of an analyte in a solution, the system comprising: a first electrode for immersion in the solution; transducer means for varying the level of immersion of the first electrode in the solution in an oscillating manner at-a given frequency; a second reference electrode for immersion in the solution; means for applying a bias signal across the first and second electrodes to cause a current to flow therebetween; and phase sensitive detection means for determining the component of said current which is in-phase with the oscillation frequency of the first electrode.

The above first aspect of the present invention may comprise means for automatically calculating the concentration of the analyte from said in-phase component of the current, e.g. using a look-up table specific to that analyte.

The present invention enables a very small surface area of the electrode to be exposed intermittently to the solution. The system is thus able to simulate the use of very small diameter microelectrodes because the phase sensitive detector allows current, which flows through the continuously immersed portion of the first electrode, to be rejected in favour of the current which flows intermittently through the small surface area of the electrode which moves into and out of the solution. The latter small surface area is referred to hereinafter as the "auxiliary" area.

Preferably, the oscillation frequency of the first electrode with respect to the solution is slow relative to the rate at which the faradaic current flowing across the auxiliary area of the first electrode reaches a steady state condition, for example an order of magnitude slower or preferably several orders of magnitude slower.

Preferably, the oscillation rate is less than 100Hz. The oscillation frequency may be fixed or may vary over time.

Preferably, the phase sensitive detection means comprises a current to voltage convertor coupled in series with the first and second electrodes and which provides an output voltage which is proportional to the current flowing between the electrodes. This output is coupled to an input of a lock-in amplifier which is also arranged to receive an input signal corresponding to an oscillating signal used to drive the transducer means.

The lock-in amplifier is arranged to provide an output signal which is proportional to said faradaic component through the auxiliary area.

The varying level of immersion of the first electrode in the solution may be achieved by providing means for varying the position of the electrode, e.g. a piezoelectric transducer, and maintaining the position of the solution fixed, or by maintaining the position of the electrode fixed and providing means for varying the position of the solution.

A preferred embodiment of the present invention adopts the latter approach and achieves oscillating movement of the surface of the solution by creating an oscillating pressure change above the solution. For example, the electrode may be contained within a bell jar arrangement, a lower region of which is immersed in the solution so as to leave a gas pocket trapped in the bell jar above the surface of the solution where the electrode enters the solution. The pressure in the bell jar may be varied in a number of ways, for example using a piezoelectric transducer or a loud speaker contained within the bell jar.

In use the bell jar is preferably filled with an inert gas, for example nitrogen, to reduce the chances of oxygen, or some other reactive gas, reacting with the electrode or the solution. A vent hole may be provided in the bell jar to allow the space inside the bell jar, and above the surface of the solution, which is usually filled with air, to be purged with nitrogen prior to conducting measurements.

The bias voltage used to drive the faradaic reaction may be a constant voltage or may be a signal which varies slowly relative to the immersion oscillation frequency, for example a linear ramp or stepped ramp.

The level of immersion of the first electrode in the solution may be arranged to vary sinusoidally but preferably varies substantially in the form of a square wave.

The first electrode preferably comprises a metal wire, for example platinum, extending through, and projecting from, one end of a glass tube. The length of the projection may be large relative to the amplitude of the immersion oscillation. In use, that portion of the electrode which is surrounded by the glass tube is not immersed in the solution.

The immersed end region of the first electrode may be insulated to prevent the flow of current through the end region and thereby to prevent a significant volume of the solution, beneath the electrode, from being depleted of analyte.

In one embodiment, the end region of the electrode is coated with a suitable insulating material, e.g. a dielectric polymer. In an alternative embodiment, the system comprises a capillary tube having an opening directly beneath the tip of the first electrode and being filled with an inert gas so that the tip can rest upon or project into the inert gas.

According to a second aspect of the present invention there is provided a sensor for use as part of an electrochemical detection system in determining the concentration of an analyte in a solution, the sensor comprising an electrode having an active sensing area for partial immersion in the solution, transducer means for varying the level of immersion of the electrode sensing area in the solution, and means for electrically coupling the electrode to a reference electrode of the system.

According to a third aspect of the present invention there is provided a method of determining the concentration of an analyte in a solution, the method comprising the steps of: partially immersing a first electrode in the solution and immersing a second reference electrode in the solution; varying the level of immersion of a sensing area of the first electrode in the solution in an oscillating manner at an oscillation frequency so that a predetermined sensor area of said first electrode is intermittently immersed in said solution; applying a dc bias signal across the first and second electrodes so as to cause a current to flow therebetween; determining the ac component of said current corresponding to said predetermined sensing area which is in-phase with the oscillation frequency of the first electrode using phase sensitive detection means; and calculating from said in-phase component the concentration of the analyte.

In an embodiment of the inventive method, current flow in through the immersed lower end of the first electrode is inhibited by insulating an end region of that electrode. This reduces the depletion of the analyte in the solution beneath the electrode, increasing the sensitivity of the measurement. The end region of the first electrode may be insulated from the solution by coating the end with an insulating material.

Alternatively, the method may comprise mounting the first electrode in the solution so that its end region rests upon, or penetrates into, an inert gas. This may be acheived by providing an inert gas filled capillary tube beneath the first electrode so that the electrode end penetrates through an open end of the capillary tube into the gas.

For a better understanding of the present invention and in order to show how the same may be carried into effect an embodiment of the invention will now be described by way of example, with reference to the accompanying drawings, in which: Figures la and b show a conventional microelectrode sensor for use in an electrochemical detection system; Figure 2 shows an embodiment of a microelectrode sensor in accordance with one aspect of the invention and having means for varying the surface level of a solution in which an electrode of the sensor is immersed; Figure 3 shows, in schematic form, control and detection circuitry of an electrochemical detection system using the sensor of Figure 2; Figure 4 shows a typical voltammetric response of the system of Figure 3; Figure 5 shows a depleted volume of solution which may form around an electrode of the system of Figure 2;; Figure 6 shows a modification to the electrode of Figure 2 and the resulting depletion layer; and Figure 7 shows a modification to the system of Figure 2 arranged to overcome the problem of the depletion layer shown in Figure 5.

There is shown in Figure 2 a sensor 5 for use in an electrochemical detection system. The sensor 5 comprises a platinum wire 6 which extends through a glass tube 7, the glass tube being heat sealed around the platinum wire to provide mechanical support for the wire, and isolation from the solution, for the wire. A small end portion 8 of the platinum wire 6 protrudes from the bottom of the glass tube for a length of approximately 1cam. The upper portion of the microelectrode, where it is surrounded by the glass tubing, has a cross-sectional which is identical to that shown in Figure l(b).

A cylindrical housing 9 extends circumferentially around the lower region of the electrode and is sealed in an airtight manner around a portion of the glass tubing, the lower end of the housing 9 being open. When the electrode and housing are lowered into the solution 10, an air pocket is trapped in the chamber 11 formed between the upper wall of the housing 9 and the surface of the solution.

In order to enable the pressure of the gas pocket to be varied, an annular electromechanical diaphragm 12 is attached inside the housing 9 to its upper wall. The electromechanical diaphragm 12 may be a piezoceramic transducer or may be a loudspeaker. The diaphragm is driven by an electrical signal having the form of a square wave, the electrical signal being fed through a wall of the housing 9 (not shown in the Figures).

A vent hole 12a is provided in the upper wall of the housing 9 and provides a closeable escape hole for trapped air when nitrogen is bubbled into the chamber 11 from beneath the surface of the solution prior to conducting an electrochemical measurement. Purging of the chamber in this way is carried out to remove oxygen which may react with the solution or with the electrode giving rise to measurement errors.

When an oscillating drive signal is fed to the electromechanical diaphragm, providing that the amplitude of oscillation of the diaphragm is sufficient, the level of the solution within the chamber will correspondingly move up and down between the broken lines shown in Figure 2. For a constant amplitude of drive signal fed to the diaphragm, the amplitude of variation of the solution level will be constant. The area of the electrode exposed to the solution will vary depending on the amplitude variations and may be small for a relatively small amplitude signal fed to the diaphragm.

Figure 3 shows, in schematic form, a system for controlling and monitoring the sensor of Figure 2. In addition to placing the sensor 5 into the solution 10 containing the analyte whose concentration is to be measured, a reference electrode 13 is also immersed in the solution. The surface area of the reference electrode exposed to the solution is significantly greater than the exposed surface area of the microelectrode. The reference electrode is of the silver/silver-chloride type which proves stable over long periods of time.

Coupled to the reference electrode 13 is a linear ramp generator 14 which applies a triangular voltage waveform between the reference electrode and a ground line. The amplitude of the linear ramp voltage is typically -0.5V having a period of 100 seconds. The platinum wire 6 of the microelectrode is coupled via a lead to the negative input of an operational amplifier 15 which forms, in combination with a feedback resistor 16, a current to voltage convertor. The positive input of the operational amplifier is coupled to a common ground with the linear ramp generator 14. The output of the current voltage to convertor is a voltage Vsjgi which is proportional to the current flowing in solution between the microelectrode 6 and the reference electrode 13.

A square wave generator 17 for generating a square wave having a period of 14Hz is coupled to the electromechanical diaphragm 12 contained within the housing 9 of the microelectrode sensor. As discussed above, the application of this square wave to the electromechanical diaphragm 12 induces switching of the pressure in the chamber 11 between a relatively high level and a relatively low level. This variation in turn causes a corresponding small alternating switch in the level of faradaic current flowing through the microelectrode. The oscillation frequency is chosen to be slow enough such that the faradaic current can reach a steady state condition soon after a step change in pressure occurs in the chamber but before the pressure switches again.

In order to extract that part of the signal V, which results from the current flowing across the auxiliary portion of the microelectrode, the output from the current to voltage convertor is applied to an input of a lock-in amplifier 18. The lock-in amplifier 18 receives at a second reference input the same square wave signal Vreference which is fed to the electromechanical diaphragm 12 of the microelectrode sensors.

The lock-in amplifier 18 acts as an ac amplifier, eliminating the dc component of the input signal, and provides a dc output voltage which is proportional to both the amplitude of Vsjgni and the relative phase difference between V, and Vreference. For a square wave reference signal the output of the lock-in amplifier is given by: Vout = (2/#) |Vsignal| x Vreference| COS # where # is the phase difference between the two signals.

The output voltage of the linear ramp generator is supplied to drive the x- axis of an xy chart recorder 19, the y- input of the chart recorder being provided by the output voltage Vx, of the lock-in amplifier 18. Figure 4 shows a typical trace for one period of the ramp where the solution comprises 16ml of 1 molar potassium chloride plus 0.lml of 10-2 molar potassium ferrocyanide. The yaxis has been calibrated to convert V into a corresponding electrode current. On the rising side of the ramp, where the microelectrode bias voltage rises from 0 to 0.5 volts relative to the reference electrode, the electrode current remains constant at a relatively low level until a threshold bias voltage is reached at about 0.25 volts.This threshold bias voltage corresponds to the potential at which faradaic current begins to flow across the auxiliary active area of the microelectrode. The electrode current levels out at about 0.3V and remains substantially constant until the bias voltage peaks at 0.5V. On the falling side of the bias voltage, the electrode current substantially follows the upward rising trace back to the starting point at oV.

Under certain circumstances, during the measurement process a significant volume of the solution surrounding the electrode end 8 will become depleted of the analyte.

Most importantly, the depleted volume extends below the end of the immersed electrode 8. This is illustrated in Figure 5 where the depleted volume 20 is shown shaded.

It will be appreciated that if the electrode is displaced relative to the solution during analysis by a distance D which is not sufficient to penetrate into the nondepleted solution, Little change in the measured electrode current will result from the displacement.

However, if the displacement D is made large enough to allow the electrode to penetrate through the depletion layer, the change in the immersed electrode area, the auxiliary portion, is excessively large and the above noted advantages achieved by microelectrodes are lost.

A solution to this problem is illustrated in Figure 6, where the tip of the electrode 8 is coated with a thin insulating layer 21 which prevents the passage of current through the tip. Thus, no significant volume of solution beneath the electrode is depleted of analyte. As the electrode is displaced vertically relative to the solution, the electrode tip is repeatedly moved into and out of a substantially non-depleted volume of solution.

An alternative solution is shown in Figure 7, where the electrode tip is located on top of an open end 22 of a capillary tube 23. The tube is sealed at its other end 24 and is filled with nitrogen so that the electrode tip penetrates into the nitrogen at the tube end 22. The bubble effectively insulates the electrode tip so that as the solution level is moved up and down, non-depleted solution is drawn to the electrode.

It will be appreciated by the skilled person that various modifications may be made to the above described embodiment within the scope of the present invention.

For example, the level of the solution may be maintained fixed with the electrode being raised and lowered in an oscillating manner with the solution remaining fixed. To achieve this, the microelectrode could be attached to a piezoceramic transducer which produces oscillations in the vertical direction.

Claims

1. An electrochemical sensor system for determining the concentration of an analyte in a solution, the system comprising: a first electrode for immersion in the solution; transducer means for varying the level of immersion of the first electrode in the solution in an oscillating manner at a given frequency; a second reference electrode for immersion in the solution; means for applying a bias signal across the first and second electrodes to cause a current to flow therebetween; and phase sensitive detection means for determining the component of said current which is in-phase with the oscillation frequency of the first electrode.

2. A system according to claim 1 and comprising computer means for automatically calculating the concentration of the analyte from the in-phase component of the current.

3. A system according to claim 1 or 2, wherein the oscillation frequency of the first electrode with respect to the solution is arranged to be slower than the rate at which the faradaic current flowing across the auxiliary area of the first electrode reaches a steady state condition.

4. A system according to claim 3, wherein said oscillation frequency is arranged to be at least an order of magnitude slower.

5. A system according to any one of the preceding -claims, wherein the phase sensitive detection means comprises a current to voltage convertor coupled in series with the first and second electrodes and which is arranged to provide an output voltage which is proportional to the current flowing between the electrodes, said output voltage being coupled to an input of a lock-in amplifier which is also arranged to receive an input signal corresponding to the oscillating signal used to drive the transducer means.

6. A system according to any one of the preceding claims and comprising means for varying the position of the electrode, whilst maintaining the position of the solution substantially fixed.

7. A system according to any one of claims 1 to 5, wherein the electrode is contained within a bell jar arrangement, a lower region of which is immersed in the solution in use so as to leave a gas pocket trapped in the bell jar above the surface of the solution where the electrode enters the solution, the bell jar arrangement comprising means for varying the pressure in the bell jar to vary the level of the solution.

8. A system according te claim 7, wherein in use the bell jar is filled with an inert gas to reduce the chances of oxygen, or some other reactive gas, reacting with the electrode or the solution.

9. A system according to any one of the preceding claims, werein the first electrode comprises a metal wire extending through, and projecting from, one end of a glass tube and wherein in use that portion of the electrode which is surrounded by the glass tube is not immersed in the solution.

10. A system according to any one of the preceding claims, wherein the immersed end region of the first electrode is insulated so that in use the flow of current through the end region is prevented thereby preventing a significant volume of the solution, beneath the electrode, from being depleted of analyte.

11. A system according to any one of claims 1 to 9, wherein the system comprises a capillary tube for immersion in a solution, the tube having an opening arranged directly beneath the tip of the first electrode and being filled with an inert gas so that the tip can rest upon or project into the inert gas.

12. An electrochemical sensor system substantially as hereinbefore described with reference to Figure 2 to 4 of the accompanying drawings or that system as modified by Figure 6 or 7.

13. A sensor for use as part of an electrochemical detection system in determining the concentration of an analyte in a solution the sensor comprising an electrode having an active sensing area for partial immersion in the solution, transducer means for varying the level of immersion of the electrode sensing area in the solution, and means for electrically coupling the electrode to a reference electrode of the system.

14. A method of determining the concentration of an -analyte in a solution, the method comprising the steps of: partially immersing a first electrode in the solution and immersing a second reference electrode in the solution; varying the level of immersion of a sensing area of the first electrode in the solution in an oscillating manner at an oscillation frequency so that a predetermined sensor area of said first electrode is intermittently immersed in said solution; applying a dc bias signal across the first and second electrodes so as to cause a current to flow therebetween; determining the ac component of said current corresponding to said predetermined sensing area which is in-phase with the oscillation frequency of the first electrode using phase sensitive detection means; and calculating from said in-phase component the concentration of the analyte.

15. A method of determining the concentration of an analyte in a solution substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings or that method as modified by Figure 6 or 7.