ELECTROCHEMICAL SENSOR

Abstract: The invention provides an electrochemical sensor of the type having a sensing electrode (12), a reference electrode (14) and a counter electrode (16) all in contact with an electrochemical medium, an amplifier (18) capable in operation of maintaining a first potential between the sensing electrode and the reference electrode, and means for detecting a current flowing at the sensing electrode or at the counter electrode in response to an electrochemical reaction at the sensing electrode due to the presence of a material of the type being monitored. According to the invention, there is an array of the sensing electrodes and a corresponding array of one of the reference electrodes and the counter electrodes, and means (30, 32) are provided for cycling an applied potential between the different electrodes in at least one of the arrays whereby each sensing electrode is in turn subjected to (a) a sensing potential at which detection can take place, and (b) a refreshing potential at which a discarge current flows between the associated counter electrode and sensing electrode to clean the surface of the sensing electrode.

Description: ELECTROCHEMICAL SENSOR The present invention relates to electrochemical sensors. Electrochemical sensors are known for use in the determination of species in aqueous solutions and for use in the detection of gas in an atmosphere being monitored. In particular, electrochemical sensors employing microelectrodes having small physical dimensions including a diameter of the order of 10 μm are presently employed in solid state gas detectors. Such sensors, in which the electrodes are effectively point electrodes, have the advantage over other conventional sensors in that the current obtainable is much higher than would normally be expected for their surface area because the molecular flux per unit area is higher. Electrochemical sensors of the type employed in solid state gas detectors and in electrolytic gas detectors have a sensing (or working) electrode which is in communication with the atmosphere being sensed, a counter-electrode and a reference electrode and all three electrodes are in contact with an electrochemical medium within the sensor and are connected via respective terminals to the circuitry within the detector. The potential difference between the sensing electrode and the reference electrode may be controlled and in some sensors this is done by connecting these two electrodes to the inputs of an operational amplifier either directly or through a resistor, see e.g. U.K. Patent Specification Nos. 1 101 101. U.S. Patent Specification No. 3 776 832, European Patent Application No.O 220 896 and International Patent Application No. W 90/12315. A circuit generally in accordance with the above patents is shown in Figure 1 of the accompanying drawings, in which the sensor is indicated by the general reference number 10 and includes an electrolyte (sulphuric acid). A sensing electrode 12, a reference electrode 14 and a counter electrode 16 are all in contact with the electrolyte. The sensing electrode 12 and the reference electrode 14 are joined via terminals 12a and 14a to respective inputs of an operational amplifier 18 whose output is connected to the counter electrode 16 via a terminal 16a. A resistor 20 is present between the sensing electrode 12 and its input to the operational amplifier 18, and is connected via a line 22 to a ground or other fixed reference potential. The sensing electrode 12 is in contact with an atmosphere that is being monitored and when the atmosphere contains a gas of the type being detected, this gas undergoes an electrochemical reaction which depolarises the sensing electrode 12, causing the potential of that electrode to alter. This causes an imbalance between the potential of the sensing electrode 12 and the reference electrode 14 and hence between the inputs of the operational amplifier 18. The potential difference between these inputs causes the operational amplifier to supply current through its output to the counter electrode 16 and hence causes a current to flow in the sensor cell 10 between the counter electrode 16 and the sensing electrode 12 (however substantially no current flows between the reference electrode and the sensing electrode). The current flowing through the sensor cell, which is directly related to the amount of gas in the atmosphere, can be measured, for example, by including a resistor between the amplifier output and the counter electrode 16 and measuring the voltage drop across the resistor (see U.S. Patent No. 3 776 832); alternatively, the current flowing through the cell may be measured by a current follower connected to the line 22 connected to the resistor 20 or by measuring the potential difference across a resistor between the line 22 and a ground or other fixed potential (see European Patent Application No.O 220 896); alternatively, the voltage drop across the resistor 20 may be measured (see U.K. Patent No.1 101 101). The resistor 20 is included between the sensing electrode and the operational amplifier in order to slow the response time of the sensor and thus provide immunity from electronic noise and fluctuations in the potential of the sensing electrode; the value of the resistor 20 is generally chosen to be between 0 and 500 ohms. In sensors of the type described with reference to Figure 1 , the electrochemical action at the surface of the sensing electrode 12 tends to result in oxidation and eventual deterioration of the efficiency of the sensor 10 over time, which is a significant problem in applications requiring a precisely accurate output at all times. Periodic electrochemical cleaning and refreshing of the electrode surface are thus required, which means an interruption and introduces a further problem in applications requiring a continuous output. The present invention seeks to overcome the above problems and to provide an electrochemical sensor in which electrode cleaning can take place without affecting the sensor output. According to the present invention, there is provided an electrochemical sensor sensor having a sensing electrode, a reference electrode and a counter electrode all in contact with an electrochemical medium, means capable in operation of maintaining a first potential between the sensing and the reference electrodes, means for detecting a current flowing at the sensing electrode or at the counter electrode in response to an electrochemical reaction at the sensing electrode due to the presence of a material of the type being monitored, characterised in that there is an array of the sensing electrodes and a corresponding array of one of the reference electrodes and the counter electrodes, and means are provided for cycling an applied potential between different electrodes in at least one of said arrays whereby each sensing electrode is in turn subjected to a sensing potential at which detection can take place. An advantage of this arrangement is that each sensing electrode and counter electrode in the associated array is subjected in turn to a sensing or a measurement potential at which detection can take place. As the measurement potential is cycled round the electrodes, the electrodes not at the required measurement potential can undergo cleaning and refreshing to provide an accurate output and, if required, the output from the array can be continuous. Preferably, the cycling means comprise an electronic gating arrangement, for example a multiplexer arrangement. The present invention is described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a known circuit for a gas detector; Figure 2 is a circuit diagram of a portion of an electrochemical sensor according to the present invention illustrating the connections between a single sensing electrode, a reference electrode, a single counter electrode and an operational amplifier; Figure 3 is a circuit diagram showing further portions of the electrochemical sensor of Figure 2. which has a 3 x 3 array of sensing electrodes and a corresponding array of counter electrodes; Figure 4 is a signal diagram showing how the current at each of the sensing electrodes or alternatively each of the counter electrodes in the electrochemical sensor of Figure 3 varies with the applied voltage and, at a particular time, has a different value; Figure 5 is a circuit diagram showing a variation on the circuitry of Figures 2 and 3; Figure 6 is a circuit diagram showing how the electrochemical sensor of Figures 2 to 5 can be modified such that the output from the operational amplifier can be cycled for an array of n by m counter electrodes, and Figure 7 is a diagrammatic view illustrating an array of n by m counter electrodes. Figure 1 relates to the prior art and Figures 2 to 6 illustrate embodiments of the present invention. The same reference numbers will be used throughout, however, to indicate comparable components. Referring initially to Figure 2, the circuit shown in this Figure is similar to the circuit of Figure 1 except in the following key respects: Figure 2 shows only one of each of the sensing electrode 12, the reference electrode 14 and the counter electrode 16 but in fact there are an array of the sensing electrodes 12 and a corresponding array of the counter electrodes 16 with a single reference electrode 14 all connected to one operational amplifier 18. The output of the operational amplifier 18 is connected to the illustrated counter electrode terminal 16a. and thence to the illustrated counter electrode 16, by way of a multiplexer 30 to be described in greater detail below. Essentially, the multiplexer 30 enables the output from the operational amplifier 18 to be cycled round all of the counter electrodes 16, although only one is shown in Figure 2 for the sake of simplicity. Another difference over the Figure 1 circuit lies in the provision of a capacitor 32 connected to the illustrated terminal 16a so that a potential applied by way of the multiplexer 30 to the illustrated counter electrode 16 can be stored, and indeed a respective capacitor 32 is connected to each of the terminals 16a for storing the potential applied to the associated counter electrode 16. As shown, the illustrated sensing electrode 12 is connected by way of the resistor 20 to the line 22, and each of the other sensing electrodes in the array of such electrodes is connected by way of a respective further resistor 20 to the line 22. A measuring device is connected across the resistor 20, and either respective measuring arrangements are associated with each of the resistors 20 or a further multiplexing arrangement synchronised with the multiplexer 30 allows the voltage drop across each of the resistors 20 to be measured in turn. Finally, a reference diode 34 is connected between the illustrated resistor 20 and the input of the operational amplifier 18 associated with the sensing electrode 12 in order to provide a predetermined offset voltage between the two inputs of the operational amplifier 18 for generating a predetermined potential V, at its output. In place of the reference diode 34, an electronic circuit providing a defined potential or a digital to an analogue converter driven from a microprocessor could equally well be used. The operation of the circuitry shown in Figure 2 is essentially the same as the operation of the circuitry shown in Figure 1 apart from the cycling of the potential generated at the output of the operational amplifier 18, round the array of counter electrodes 16. This will now be described further with reference to Figure 3. Figure 3 shows a specific example of the present invention having a 3 X 3 array of the counter electrodes 16 and a 3 X 3 array of the sensing electrodes 12. As shown in Figure 3, the output of the operational amplifier 18 is connected to each of the counter electrode terminals 16a, .,, 16a, 2 ...16a 3 3 and thence to a respective counter electrode 16,.,, 16, 2 ...16 3 3 by way of the multiplexing arrangement 30. Each of the counter electrode terminals 16a, , 16a 3 3 is also connected to an associated capacitor 32 as shown. Corresponding to each counter electrode 16 is a respective sensing electrode 12, and the sensing electrodes 12, , 12 3 3 are connected respectively to sensing electrode terminals 12a, , 12a, 3 . The sensing electrode terminals 12a, , 12a 3 3 are connected by way of respective resistors 20 to the line 22 and thence through the reference diode 34 to the operational amplifier 18 as mentioned above. It will be noticed that there is only a single reference electrode 14 connected directly to the operational amplifier 18, although there is both an array of sensing electrodes 12 and an array of the counter electrodes 16. The multiplexing arrangement 30 comprises a first multiplexer 40 having a single input, which is provided by the output of the operational amplifier 18 and is controlled by a control bus X. The multiplexer 40 has three outputs each connected to the input of a respective further multiplexer 42, 44, 46 also controlled by way of the common control bus X. Again, each of the multiplexers 42, 44, 46 has three outputs connected to a respective one of the terminals 16a. By way of example, the outputs of the multiplexer 42 are connected to the terminals 16a, .,, 16a, 2 and 16a, 3 . By virtue of this arrangement, the potential V, obtained at the output of the operational amplifier 18 is supplied by way of the multiplexer 40 first to the multiplexer 42, which in turn first applies the potential V, to the counter electrode terminal 16a, , and begins to charge up the associated capacitor 32. When the capacitor 32 associated with the terminal 16a, , is partially charged, the multiplexer 40 switches the signal under the control of the control bus X to the multiplexer 44 which applies the potential V, to the counter electrode terminal 16a 2 ., to start charging the associated capacitor 32. Next, the potential V, is applied by way of the multiplexer 46 to the terminal 16^ , for charging the capacitor 32 associated with this terminal. The signal is switched subsequently to the terminals 16a 1 2 , 16a 2.2 and 16a 3 2 and then to the terminals 16a 3 .,, 16a 3 2 and 16a 3 3 . In this way. the potential V, at the output of the amplifier 18 is repeatedly cycled round the counter-electrode terminals 16a, . .....16a 3 3 . and the capacitors 32 are charged successively. At any one moment, each of the counter electrodes 16 will be at a different potential V according to the charge stored on the associated capacitor 32. Only one of the counter-electrode 16 is at a potential V m which is a sensing or measurement potential at which detection can take place. When detection occurs in this situation, the current at the respective opposed sensing electrode 12 is picked up and the potential difference across the respective resistor 20 connected thereto is measured.. The potential V, at the output of the operational amplifier 18 is selected to be such that when the charge stored on any given capacitor 32 reaches a level to bring the associated counter electrode 16 also to the potential V,. then a cleaning or refreshing peak current flows through the sensor between that counter electrode 16 and the opposed sensing electrode 12 to clean the associated sensing electrode 12. This results in a discharge of the relevant capacitor 32. following which charging begins of this capacitor 30 anew in the next cycling of the potential generated at the output of the amplifier 18 applied by the multiplexers 40 to 46. Figure 4 shows the relationship between current I and voltage V in each corresponding pair of sensing and counter-electrodes 12, 16 and also shows how the potential attained at each counter-electrode 16 varies according to the position of that counter electrode within the array of such electrodes. At a given time Tl, the potential of each counter electrode 16 is represented by x and at a given time T2, the potential at each counter-electrode 16 is represented by o. As can be seen, at any particular time, only one of the counter-electrodes 16 will be at the measurement potential V B . Further, as can be seen, the counter-electrodes 16 successively attain the potential V, at which a discharge current flows and cleaning of the opposed sensing electrode 12 occurs. Figure 5 shows a 3 x 3 array of the counter electrodes 16 and a 3 x 3 array of the sensing electrodes 12, like Figure 3. In the present instance, however, a respective capacitor 32 is connected between the terminals 16a and 12a of each associated pair of counter and sensing electrodes 16, 12 for storing potential, and a respective resistor 20 is connected between each sensing electrode terminal 12a and the line 22. A multiplexing arrangement 50 is connected to all of the terminals 12a for cycling round the sensing electrodes 12 for the pick-up of currents for detecting purposes, and is connected by way of an ammeter 52 to the line 22 for measuring the current. Referring now to Figure 6, this figure shows how the same principle can be applied to an embodiment of electrochemical sensor having an array of n by m counter electrodes 16 and an array of n by m sensing electrodes 12. In this case, the output of the operational amplifier 18 is connected to a first multiplexer 140 controlled by a control bus X and having a set of m outputs each connected to a respective further multiplexer 142,, 142 m . The further multiplexers are again controlled by the common control bus X and each has a further n outputs connected to a respective counter electrode terminal 16a, ......16a_- fI1 and to a respective capacitor 132. The array of sensing electrodes 12 are connected by way of sensing electrode terminals 12a, , 12a_- n through respective resistors 20 and the reference diode 34 to the operational amplifier 18. As before, there is a single reference electrode 14. The array of counter electrodes 16 is illustrated in Figure 7, which shows how the outputs of a each of the multiplexers 142 are connected respectively to a column of the counter electrode terminals 16a ...16a, tD and how each counter electrode terminal 16a,. , to 16a, , _ in the respective column being connected to the respective capacitor 132. In the described circuitry, the voltage drop across the respective resistor 20 is in each case measured for detecting the current flowing through the associated pair of counter and sensing electrodes in sensor cell 10. Of course, an alternative possibility is to insert respective resistors in the connections leading to the counter-electrodes 16 and to measure the voltage drop there. Various other modifications are also possible in the circuitry described. In particular, the capacitor 32 is optional if a storing function and electrochemical cleaning is not required. The multiplexers ' may of course be replaced by an alternative electronic gating arrangement or by an appropriate central processing unit and software. Further, the surface or surfaces of individual or plural electrodes in the array may be modified by techniques such as ion bombardment or electroplating or by coating techniques such as screen or ink jet printing to give respective electrodes different activity or specifity to chemical species. It will readily be perceived that the advantage of the present invention is in the provision at all times of a pair of sensing and counter electrodes at an appropriate potential for measuring the electrochemical parameter for which the sensor is being employed, whilst at the same time allowing other pairs of sensing and counter electrodes to reach an appropriate applied potential for cleaning purposes. The present invention is primarily applicable to sensors in which the sensing electrodes are microelectrodes; a microelectrode includes a small exposed area of an inert conductive material, for example platinum or some other said electrocatalytic material. Microelectrodes are known that consist of a platinum wire that is enclosed within a glass sheath that has been drawn, cut to size and the exposed cut end polished. The diameter of the exposed platinum wire will generally be 15 - 30 μm but smaller diameters are known and indeed diameters up to 1mm are also known. Naturally, the cross section of the exposed area need not be circular and any cross section of comparable area could be used. Although the present invention has been discussed in terms both of the sensing of both gaseous and dissolved species, it is primarily concerned with the sensing of dissolved species, particularly dissolved gases, eg. oxygen.

Claims: CLAIMS 1. An electrochemical sensor having a sensing electrode (12), a reference electrode (14) and a counter electrode (16) all in contact with an electrochemical medium, means (18) capable in operation of maintaining a first potential between the sensing and the reference electrodes, means for detecting a current flowing at the sensing electrode or at the counter electrode in response to an electrochemical reaction at the sensing electrode due to the presence of a material of the type being monitored, characterised in that there is an array of the sensing electrodes (12) and a corresponding array of one of the reference electrodes (14) and the counter electrodes (16), and means (30) are provided for cycling an applied potential between different electrodes in at least one of said arrays whereby each sensing electrode (12) is in turn subjected to a sensing potential at which detection can take place. 2. An electrochemical sensor according to claim 1 characterised by means (32) arranged to cooperate with the cycling means so as to subject each sensing electrode in turn to (a) the sensing potential, and (b) a refreshing potential at which a discharge current flows between the associated counter electrode and sensing electrode to clean the surface of the sensing electrode. 3. An electrochemical sensor according to claim 2 characterised in that said cooperating means comprise storing means associated respectively with each of the electrodes in said array. 4. An electrochemical sensor according to any preceding claim characterised in that the cycling means comprise an electronic gating arrangement. 5. An electrochemical sensor according to claim 4 characterised in that the gating arrangement comprises a multiplexing arrangement. 6. An electrochemical sensor according to any preceding claim characterised in that the two arrays constitute an array of the sensing electrodes and an array of the counter electrodes, and in that the cycling means are arranged to cycle the applied potential round different electrodes in the array of counter electrodes. 7. An electrochemical sensor according to any preceding claim characterised in that the means capable in operation of maintaining said first potential comprise an amplifier.

Inventors: IREDALE PETER JULIAN, IREDALE, PETER, JULIAN
Applicants: IREDALE PETER JULIAN, NEOTRONICS LTD, IREDALE, PETER, JULIAN, NEOTRONICS LIMITED
Application year: 1995
Main classification: G01N27
IPC codes: G01N27