Abstract:
A potentiostat is provided for a biosensor circuit and permits sequential and simultaneous measurements to be performed at different cells across a matrix of biosensing devices. Accordingly, a potentiostat comprises a first differential amplifier for receiving a scanning voltage at a first input terminal and a voltage at the reference electrode at a second input terminal and for generating an output voltage at an output terminal to be applied to the working electrode, wherein, when in use, a feedback loop of the potentiostat circuit is closed between the reference electrode and the working electrode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Several aspects of the present invention relate, in general, to a potentiostat and a biosensor circuit employing the potentiostat. The invention provides a multi-channel biosensor circuit having a potentiostat that enables sequential and simultaneous measurements to be performed at different cells across an array of biosensing devices. 
         [0002]    1. Field of the Invention 
         [0003]    A potentiostat is a measurement apparatus commonly used in the electrochemical field for executing voltammetric techniques. Cyclic voltammetry is a particularly useful voltammetric technique and aids the study of reversible reduction-oxidation (redox) reactions. When such techniques are applied to the biosensing field, a reversible redox reaction can take place at the interface between the surface of a biosensor selective receptive membrane (a bioreceptor) and an electrolyte. 
         [0004]    2. Description of the Related Art 
         [0005]    Three examples of known potentiostat circuits are illustrated in  FIGS. 1   a  to  1   c .  FIG. 2  is a cross-sectional schematic diagram of a sample undergoing cyclic voltammetric measurement using a potentiostat circuit of  FIG. 1 . Referring to  FIG. 1 , a potentiostat circuit  1000  is connected to a counter electrode CE, a reference electrode RE and a working electrode WE. As best shown in  FIG. 2 , the circuit is completed by a buffer solution containing samples under test, hereunder referred to as an analyte or electrolyte  1001 , which is represented by a resistor  1020  in  FIGS. 1   a  to  1   c  and a bioreceptor layer  1015 , which is represented as a capacitor and resistor  1025  connected in parallel in  FIGS. 1   a  to  1   c . The reference electrode is separated from the working electrode WE by an insulating layer  1030  of, for example, Si 3 N 4  in  FIG. 2 . The reference electrode RE draws no current and is positioned in close proximity to the working electrode WE in order to measure the voltage at the surface of the bioreceptor layer  1015  and close the feedback loop for a first operational amplifier A 1 . Once the feedback loop is closed, the high gain at the first operational amplifier A 1  ensures that the voltage at the reference electrode, and hence the voltage at the surface of the bioreceptor layer  1015 , with respect to the voltage at the working electrode equals V SCAN , as shown in  FIGS. 1   a  and  1   b , or —(R 2 /R 1 ) V SCAN , as shown in  FIG. 1   c.    
         [0006]    The bioreceptor layer  1015  may be a phage or enzyme that binds with a predetermined DNA or RNA strand, a peptide or another biological molecule, thereby changing the resistance and/or the capacitance of the circuit in which the electrodes are connected. Commonly the bioreceptor layer  1015  is known as a probe and the selective biological molecule it interacts with as a target. The characteristics of the capacitance and resistance as functions of applied voltage across them (in this case V SCAN ) vary if the target biological molecule bonds or reacts with the phage or the enzyme provided as the bioreceptor layer  1015 , allowing the presence of the biological molecule to be detected. Specifically, the potentiostat circuit measures the current at the working electrode WE while a time varying voltage (reflected as the voltage at the reference electrode RE) and equal to V SCAN  in the examples as shown in  FIGS. 1   a  and  1   b  is applied at the surface of the bioreceptor layer . 
         [0007]    A time varying periodic voltage signal is typically used as V SCAN  in  FIGS. 1   a  to  1   c . This is beneficial when the potentiostat circuit is being used to measure a redox reaction, since the use of a DC voltage may affect the reaction. As V SCAN  oscillates, the first half cycle promotes a reduction/oxidation reaction at the interface and the second half cycle promotes an oxidation/reduction reaction at the interface. As V SCAN  varies, the rate of the reduction/oxidation reaction varies. At some points the reduction and oxidation reaction rates are at their maximum due to a diffusion of charge carrying species occurring due to the concentration gradient. The required charge transfer is supplied/removed at the working electrode. In operation, the reduction and oxidation reaction cycles are observed as a time varying current flow, I OUT (t) measured at the working electrode WE versus a cycle of scanning voltage, V SCAN (t). The voltage at the working electrode can be held at any DC reference level. The output of the circuit is typically a hysteresis loop with peaks pointing in opposite directions, as shown in  FIG. 3 . The shape of the hysteresis loop indicates whether a particular redox reaction has taken place, whether it is reversible and enables a user to study the rate of reaction by adjusting the scan rate dependency. The locations and magnitudes of the peaks are results of the reactants in equilibrium during the charge transfer that occurs in a redox reaction, and hence can be employed to extract the concentration of species in the analyte. 
         [0008]    A multi-channel potentiostat circuit is required for the analysis of a matrix of samples or to perform analysis a sample using a, matrix of bioreceptors. A known multi-channel potentiostat circuit is illustrated in  FIG. 4 . Referring to  FIG. 4 , a number of different matrix elements cell  1 , cell  2 , cell N, each share a common analyte or electrolyte. With such an arrangement, the counter electrodes CE of each matrix element cell  1 , cell  2 , . . . , cell N are connected together, meaning that effectively a single counter electrode CE is used. Each matrix element comprises a reference electrode RE disposed close to the surface of a bioreceptor layer (not shown in  FIG. 4 ) and a working electrode WE with specific properties such as current sensitivity and dynamic range for the bioreceptor layer. Since each matrix element shares the same feedback loop between the counter electrode and each reference electrode, only one matrix element at a time may be used if the reference electrode is to be held at a required cyclic voltage V SCAN . The matrix elements must therefore be selected and measured sequentially. 
         [0009]    The cyclic voltage V SCAN  that is applied to a cell typically requires measurement time of seconds to hundreds of seconds per cell. In the case of a DNA chip, the number of cells can increase to well over a thousand resulting in an integration time of hours as each cell is sequentially measured and read out. Also, in order to obtain a repeatable output, the number of cycles is large, which further increases measurement time. Furthermore, some samples may degrade or disintegrate during such long measurement periods, and expensive measures are required to keep them in a stable condition. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of the present invention, there is provided a potentiostat comprising a first differential amplifier for receiving a scanning voltage at a first input terminal and a voltage of a reference electrode at a second input terminal and for generating an output voltage at an output terminal to be reflected at a working electrode, wherein, when in use, a feedback loop of the potentiostat circuit is closed between the reference electrode and the working electrode. 
         [0011]    Preferably, the output voltage of the first differential amplifier is applied to a first input terminal of a first operational amplifier having a negative feedback between an output terminal and a second input terminal, wherein the working electrode is connected to the second input terminal of the first operational amplifier and held at the voltage of the first input terminal of the first operational amplifier. 
         [0012]    More preferably, the first input terminal is a non-inverting input terminal and the second input terminal is an inverting input terminal. 
         [0013]    The potentiostat may further comprise a second differential amplifier having a first input terminal connected to the output terminal of the first operational amplifier; and a second input terminal connected to the first input terminal of the first operational amplifier. 
         [0014]    In that case, the first input terminal of the second differential amplifier may be an inverting input terminal and the second input terminal of the second differential amplifier may be a non-inverting input terminal. 
         [0015]    The potentiostat may further comprise: a second operational amplifier having a first input terminal connected to the reference electrode; and an output terminal connected to the second input terminal of the first differential amplifier. 
         [0016]    In another aspect, the present invention provides a single-channel biosensor circuit comprising: a counter electrode, a reference electrode, a working electrode, and a potentiostat circuit as described above. 
         [0017]    In yet another aspect, the present invention provides a multi-channel biosensor circuit including a potentiostat as described above and further comprising: an array of counter electrodes, reference electrodes and working electrodes, each reference electrode in the array of reference electrodes being connected to a respective first terminal of a first switch; and each working electrode in the array of working electrodes being connected to a respective first terminal of a second switch, the first switch and the second switch each having a second terminal being connected to the potentiostat circuit, wherein outputs of a shift register unit are connected to a gate terminal of each respective first switch and each respective second switch and thereby adapted to select a counter electrode, reference electrode and working electrode in the array of for sequential readout by the potentiostat circuit. 
         [0018]    The biosensor circuits may be capable of being read out simultaneously. Preferably, an input line is shared by each respective first differential amplifier of each respective potentiostat for receiving the scanning voltage. Alternatively, each first differential amplifier of each respective potentiostat may be connected to a respective input line for receiving the scanning voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0020]      FIGS. 1   a ,  1   b  and  1   c  are schematic circuit diagrams of known potentiostats; 
           [0021]      FIG. 2  is a schematic cross-sectional diagram of an electrochemical cell showing an interface between a sample solution and a bioreceptive layer undergoing cyclic voltammetric measurement; 
           [0022]      FIG. 3  is a schematic diagram of a theoretical measurement result of a sample undergoing cyclic voltammetric measurement; 
           [0023]      FIG. 4  is a schematic circuit diagram of a known multi-channel potentiostat; 
           [0024]      FIG. 5   a  is a schematic circuit diagram of a biosensor including a potentiostat suitable for use in a multi-channel biosensor according to a first embodiment of the present invention; 
           [0025]      FIG. 5   b  is a schematic block diagram of a potentiostat suitable for use in a multi-channel biosensor according to the first embodiment of the present invention; 
           [0026]      FIG. 6  is a schematic circuit diagram of a biosensor including a potentiostat suitable for use in a multi-channel biosensor according to a second embodiment of the present invention; 
           [0027]      FIG. 7  is a schematic circuit diagram of a biosensor including a potentiostat suitable for use in a multi-channel biosensor according to a third embodiment of the present invention; 
           [0028]      FIG. 8   a  is a schematic circuit diagram of a multi-channel biosensor employed for sequential readout according to a fourth embodiment of the present invention; 
           [0029]      FIG. 8   b  is a schematic circuit diagram of a multi-channel biosensor employed for simultaneous readout according to a fifth embodiment of the present invention; and 
           [0030]      FIG. 8   c  is a schematic circuit diagram of a multi-channel biosensor employed for simultaneous readout according to a sixth embodiment of the present invention. 
       
    
    
       [0031]    Throughout the following description, like reference numerals shall be used to identify like parts. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Referring to  FIG. 5   a , a first embodiment of the present invention comprises a biosensor circuit  1  consisting of a three terminal electrochemical cell, differential amplifiers and a current-to-voltage converter, with a feedback voltage signal path formed between the working electrode WE and the reference electrode RE. The three terminal electrochemical cell is formed using the counter electrode CE, the reference electrode RE and the working electrode WE. The counter electrode CE is connected to a first voltage supply rail  2  connected to a bias voltage V BIAS    3 . A first resistor  4  is connected between the counter electrode CE and a reference electrode RE and represents an electrolyte comprising at least one target biological molecule. In a similar manner to that illustrated in  FIG. 2 , the reference electrode RE is disposed upon an insulator (not shown in  FIG. 5 ) in close proximity to a working electrode WE, which is disposed upon a bioreceptive layer modeled in  FIG. 5   a  by a parallel resistor and capacitor arrangement  6 . 
         [0033]    The reference electrode RE is connected to a potentiostat circuit  5 . The reference electrode RE, which draws very little or zero current, is connected to a non-inverting input terminal  8  of a first differential amplifier (or difference amplifier) DA 1  with a gain of unity. The inverting input terminal of the first differential amplifier DA 1  is connected to a second voltage supply rail  18  for providing a cyclic voltage V SCAN . The output terminal  34  of the first differential amplifier DA 1  is connected to a non-inverting input terminal of a first operational amplifier OA 1  configured as a current-voltage converter. 
         [0034]    The inverting input of the first operational amplifier OA 1  is connected to the working electrode WE. The output terminal of the first operational amplifier OA 1  is connected to a first terminal of a second resistor  26  configured as a feedback resistor in the current-voltage converter. The second terminal of the second resistor  26  is connected to a fourth node  28  located between the working electrode WE and the inverting input of the first operational amplifier OA 1 . 
         [0035]    The output terminal of the first operational amplifier OA 1  is also connected to an inverting input terminal of a second differential amplifier DA 2 . The non-inverting input terminal of the second differential amplifier DA 2  is connected to the output terminal  34  of the first differential amplifier DA 1  and a non-inverting input terminal of the first operational amplifier OA 1 . An output terminal of the second differential amplifier is connected to an output stage (not shown in  FIG. 5 ). 
         [0036]    Referring to  FIG. 5   b , the potentiostat circuit  5  is represented as a schematic block diagram having inputs: V SCAN , the voltage of the reference electrode RE and the current i drawn by the working electrode WE. The potentiostat also comprises an output terminal in order to generate an output voltage, V OUT . 
         [0037]    Referring to  FIG. 6 , a second embodiment of the present invention comprises of a biosensor circuit  1 , consisting of a three terminal electrochemical cell, differential amplifiers and a current-to-voltage converter, with a feedback voltage signal path formed between the working electrode WE and the reference electrode RE. The three terminal electrochemical cell is formed using the counter electrode CE, the reference electrode RE and the working electrode WE. The counter electrode CE is connected to the first voltage supply rail  2  connected to a bias voltage V BIAS    3  equal to the system ground. A first resistor  4  is connected between the counter electrode CE and a reference electrode RE and represents an electrolyte comprising at least one target biological molecule. In a similar manner to that illustrated in  FIG. 2 , the reference electrode RE is disposed upon an insulator (not shown in  FIG. 6 ) in close proximity to a working electrode WE, which is disposed upon a bioreceptive layer represented in  FIG. 6  by a parallel resistor and capacitor arrangement  6 . 
         [0038]    The reference electrode RE and working electrode WE are connected to a potentiostat circuit  5 . The reference electrode RE is connected to the non-inverting input of the differential amplifier DA 1  comprising operational amplifiers A 1 , A 2  and A 5 , and first, second, third and fourth impedance loads. The non-inverting input of the first differential amplifier DA 1  is formed by the non-inverting input of an operational amplifier A 1  with a high open-loop gain. The operational amplifier A 1  is configured to operate as a unity gain voltage follower by connecting its inverting input to its output terminal. It is preferable to provide an operational amplifier A 5 , identical to the operational amplifier A 1 , with the output of the operational connected to the inverting input to compensate the input offset voltage of the operational amplifier A 1 . The non-inverting input of the operational amplifier A 5  forms the inverting input of the first differential amplifier DA 1  and is connected to a supply rail node  18  that provides a cyclic voltage V SCAN . The non-inverting input  12  of the operational amplifier A 2  is connected to the first terminal of the first impedance load  10  and the first terminal of the second impedance load  14 . The second terminal of the first impedance load is connected to the output terminal of the operational amplifier A 1 . The second terminal of the second impedance load is connected to the system ground  16 . The inverting input  22  of the operational amplifier A 2  is connected to the first terminal of the third impedance load  20  and the first terminal of the fourth impedance load  24 . The second terminal of the third impedance load  20  is connected to the output terminal of the operational amplifier A 5 , effectively connected to the scanning voltage V SCAN  at node  18 . The second terminal of the fourth impedance load is connected to the output terminal of the operational amplifier A 2 . It is preferable to provide the same values for the first to the fourth impedance loads and obtain a voltage V 2  equals V RE −V SCAN  at node  34 . 
         [0039]    The inverting input  28  of the operational amplifier A 3  with a very high open-loop gain is connected to the working electrode WE. A feedback resistor R FB    26  is connected between the inverting input  28  and the output terminal  29  of the operational amplifier A 3 , forming a current-to-voltage converter. The high open-loop gain of A 3  forces the voltages at the inverting and non-inverting inputs to be equal. As a result, the local voltage difference between the reference electrode RE and working electrode WE, V RE −V WE≈V   SCAN , induces a current i caused by the redox reaction at the bioreceptor interface. This current i flows from the working electrode  28 , through the feedback resistor R FB    26  to node  29  and generate a voltage at node  29  equals V WE -i R FB . 
         [0040]    The second differential amplifier DA 2  comprises an operational amplifier A 4  with its non-inverting input  40  connected to the first terminal of the fifth impedance load  32  and the first terminal of the sixth impedance load  36 . The second terminal of the fifth impedance load  32  is connected to node  34  that forms the non-inverting input of the operational amplifier OA 1 . The second terminal of the sixth impedance load  36  is connected to the system ground  38 . The inverting input  44  of the operational amplifier A 4  is connected to the first terminal of the seventh impedance load  30  and the first terminal of the eighth impedance load  42 . The second terminal of the seventh impedance load  30  is connected to node  29  that forms the output of the operational amplifier OA 1 . The second terminal of the eighth impedance load  42  connects to the output of the operational amplifier A 4 . It is preferable to provide all impedance loads equal in value and obtain an output voltage at the operational amplifier A 4  equal to i R FB . 
         [0041]    The implementation of DA 1  and DA 2  are not limited to the example embodiment shown in  FIG. 6 . Both differential amplifiers DA 1  and DA 2  are required to provide an output equal to the difference of their inputs with unity gain. The differential amplifier DA 1  is required to draw minimal current from the reference electrode RE and the differential amplifier DA 2  is required to provide sufficient output current for the subsequent stage. To simplify the design, DA 1  consists of DA 2  with additional operational amplifiers A 1  and A 5  configured as voltage followers at both inverting and non-inverting inputs to minimise the input current. Operational amplifier A 5  is optional, but its presence is preferable because a matched pair of A 1  and A 5  will allow the cancellation of their inherited offset voltages and hence minimize the output error at operational amplifier A 2  configured as differential amplifiers. 
         [0042]    Referring to  FIG. 7 , a third embodiment of the present invention is the same as the second embodiment, except that all impedance loads are implemented with capacitive loads. It is preferable to have all capacitive loads equal to C. 
         [0043]    Referring to  FIG. 8   a , a multi-channel biosensor circuit  1  is arranged for sequential readout according to an embodiment of the present invention and comprises an array of counter electrodes CE, reference electrodes RE, working electrodes WE, electrolytes and bioreceptor layers. 
         [0044]    The counter electrodes CE are connected to a first voltage supply rail  2  connected to ground or a DC voltage source  3 . Each counter electrode CE is connected to a first terminal of a first resistor  4  having a second terminal connected to the reference electrode RE. The first resistor  4  represents the electrolyte comprising at least one target biological molecule. In a similar manner to that illustrated in  FIG. 3 , the reference electrode RE is disposed upon an insulator (not shown in  FIG. 8   a ) in close proximity to a working electrode WE, which is disposed adjacent a bioreceptor layer represented in  FIG. 8   a  by a parallel resistor and capacitor arrangement  6 . 
         [0045]    Each reference electrode RE is connected to a first terminal of a respective first transistor  50  having a gate terminal connected to a shift register  52  and a second terminal connected a first input terminal of a potentiostat circuit  5 . Each working electrode WE is connected to a first terminal of a respective second transistor  54  having a gate terminal connected to the shift register  52  and a third terminal connected to a second input terminal of the potentiostat circuit  5 . The potentiostat circuit  5  has a third input terminal connected to a second voltage supply rail  18  to receive a voltage V SCAN , which is also applied to an input terminal of the shift register  52 . The potentiostat circuit  5  comprises an output terminal for outputting a generated voltage V OUT . 
         [0046]    Referring to  FIG. 8   b , a multi-channel biosensor circuit l is arranged for simultaneous readout according to a third embodiment of the present invention and comprises an array of counter electrodes CE, reference electrodes RE, working electrodes WE, electrolytes, bioreceptor layers and potentiostat circuits  5 . 
         [0047]    The counter electrodes CE are connected to a first voltage supply rail  2  connected to ground or a DC voltage source  3 . Each counter electrode CE is connected to a first terminal of a first resistor  4  having a second terminal connected to the reference electrode RE. The first resistor  4  represents the electrolyte comprising at least one target biological molecule. In a similar manner to that illustrated in  FIG. 3 , the reference electrode RE is disposed upon an insulator  30  (not shown in  FIG. 8   b ) in close proximity to a working electrode WE, which is disposed adjacent a bioreceptor layer represented in  FIG. 8   b  by a parallel resistor and capacitor arrangement  6 . 
         [0048]    Each reference electrode RE is connected to a first input terminal of a respective Nth potentiostat circuit  5  and each working electrode is connected to a second input terminal of the respective N th  potentiostat circuit  5 . Each Nth potentiostat circuit  5  comprises a third input terminal to receive a voltage V SCAN  and an output terminal for outputting a generated voltage V OUT . 
         [0049]    In operation of the multi-channel biosensor circuit  1  according to this embodiment of the present invention, the counter electrode CE is held at a fixed, ground potential. The circuit is completed by an analyte or electrolyte represented by the resistor  4 , and a bioreceptor layer represented as the parallel resistor and capacitor arrangement  6 . As current flows through the electrochemical cell, the reference electrode RE is self-adjusted to a voltage V RE . Without drawing any current, this voltage is detected by the non-inverting input terminal of the first amplifier A 1  which is configured as a voltage follower, and copied to its output terminal in order to avoid upsetting the operation of the electrochemical cell. The output voltage is then applied to the non-inverting input terminal of the second amplifier A 2  (in  FIGS. 6 and 7 ). 
         [0050]    A cyclic scanning voltage V SCAN  is applied to the inverting input terminal of the second amplifier A 2 . The second amplifier A 2  and its corresponding impedance loads  10 ,  14 ,  20  and  24 , form a differential amplifier with unity gain, giving an output voltage V 2 =(V RE −V SCAN ), which is applied to the non-inverting input terminal of the third amplifier A 3 . 
         [0051]    The voltage of the working electrode WE is controlled and adjusted by the output voltage V 2  of the second amplifier A 2  by applying the Virtual Earth Theory to the third amplifier A 3 . Due to a high open-loop gain in the third amplifier A 3 , and the use of a second resistor  26  for negative feedback, the voltages at the inverting and non-inverting inputs of the third amplifier A 3  are effectively equal. Therefore, the voltage of the working electrode WE equal to V 2 =(V RE −V SCAN ). The current drawn at the working electrode WE is converted to an output voltage=(V 2 - i R FB ) by the third amplifier A 3  configured as a current-voltage converter. This output voltage is applied to the inverting input  29  of the differential amplifier formed by the fourth amplifier A 4  and corresponding impedance loads  30 ,  32 ,  42  and  36 . The non-inverting input of this differential amplifier, node  34 , is connected to the output of the second amplifier A 2 . As a result, the output voltage V OUT , given by V OUT =i R FB  is generated. 
         [0052]    Of course, it will be understood that the embodiment illustrated in  FIG. 5  operates in a corresponding manner. 
         [0053]    In operation of the multi-channel biosensor of  FIG. 8   a  employed for sequential readout in the present invention, the circuit of  FIG. 8   a  is characterised by the presence of the shift register  52 , the plurality of switching transistors  50 ,  54  and also the potentiostat circuit  5 . The counter electrode CE can be at a fixed ground potential or at an arbitrary DC voltage. The potentiostat circuit  5  described above with reference to  FIGS. 5 to 7  is represented by a block diagram having inputs V SCAN , the voltage of the reference electrode V RE  and the current i drawn by the working electrode WE in order to generate an output voltage, V OUT . 
         [0054]    In order to sequentially readout an individual cell, the reference electrode RE and the working electrode WE of the individual cell are selected by the shift register  52  by applying a signal to the gate terminal of the respective first and second transistors  50 ,  54 . A voltage the same as the voltage of the reference electrode V RE  is established at the surface of the bioreceptor layer and the voltage of the working electrode V WE  can be controlled such that V RE −V WE =V SCAN  through the virtual earth of the operational amplifier (OA 1  or A 3 ) responsible for current-to-voltage conversion, as discussed above in connection with the description of  FIGS. 5 to 7 . Successive individual cells can be sequentially selected by the shift register  50 . 
         [0055]    In operation of the multi-channel biosensor of  FIG. 8   b  employed for simultaneous readout in the present invention, the circuit of  FIG. 8   b  is characterised by the presence of N-number of potentiostat circuits  5 . The common counter electrodes  2  are held at a fixed ground potential or at an arbitrary DC voltage  3 . Since each potentiostat circuit  5  comprises an individual, closed feedback loop, the output voltage V OUT  can be read out for each cell simultaneously. The arrangement enables the dropping of a biosensor circuit  1  within a testing vessel comprising an analyte and subsequently the reading out of the generated output voltages V OUT  for each of the potentiostat circuits simultaneously. Chemical crosstalk of reaction materials between adjacent cells can be prevented by providing sufficient physical spacing. 
         [0056]    The foregoing description has been given by way of example only and it will be appreciated by a person skilled in the art that modifications can be made without departing from the scope of the present invention. 
         [0057]    The voltage V SCAN  applied to each potentiostat  5  illustrated in  FIG. 8   b  can be applied separately to each potentiostat  5  thereby enabling a different value of V SCAN  to be applied to each potentiostat  5 , as shown in  FIG. 8   c.    
         [0058]    In addition,  FIG. 5  is a simplified schematic of the potentiostat circuit  5  and reset switches may be used where appropriate. One counter electrode can be employed for the potentiostat circuits  5  illustrated in  FIGS. 8   a  to  8   c . The biosensor circuit  1  of  FIGS. 8   a  to  8   c  can be located on a single chip or substrate and since the cell area is large, space remains to incorporate additional circuits to include auto-zero amplifiers, peak detection and discrimination and digitisation means. 
         [0059]    The shape of the input voltage V SCAN  is not limited to a triangular waveform. It can be of sawtooth waveform, sinusoidal waveform, square waveform, or pulses including varying mark-space ratios. The input voltage V SCAN  is not limited to a cyclic AC voltage. V SCAN  can be a constant voltage applied to a DC differential amplifier in place of the second amplifier A 2 . In this embodiment, the first capacitor  10 , the second capacitor  14 , the third capacitor  20  and the fourth capacitor  24  are replaced by resistors or other impedance loads as is known in the art. A DC differential amplifier can also be used in place of the fourth amplifier A 4 .