Abstract:
An electrode assembly that may be used, for example, for electrochemically analysing a sample to determine the presence (or otherwise) of a species having biomembrane activity comprises at least one working electrode comprised of a conductive carrier substrate having a surface coated with mercury immobilised on the surface of the substrate. The surface of the mercury remote from said substrate is coated with a phospholipid layer. The preferred carrier substrate is platinum. The electrode assembly may be incorporated in a flow cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. Ser. No. 12/671,585 filed Feb. 1, 2010, which was the national stage of International Application No. PCT/GB08/02591 filed Jul. 30, 2008, which claims foreign priority to GB 0714866.1 filed Jul. 31, 2007, which are hereby incorporated by reference. 
     
    
       [0002]    The present invention relates to an electrode assembly having an electrode incorporating a phospholipid layer simulating a biomembrane, to an electrochemical biosensor incorporating such an assembly, and to the use of the biosensor. The sensor may be used, for example, for electrochemically analysing a sample to determine the presence (or otherwise) of a species having biomembrane activity (e.g. a toxin) or for investigating whether a species (e.g. a potential pharmaceutical) has biomembrane activity. 
         [0003]    Surface layers on electrodes have been studied for some time as membrane models for electrochemical interrogation&#39;. One method has exploited the fact that long chain alkanethiols can be bound to metallic surfaces such as gold or silver using the thiol linkage to form self assembled layers (SAM) 2 . The system is employed as a biosensor, for example by looking at analyte specific binding reactions on the film surface 3  or by investigating coupled enzyme reactions with faradaic species in solution 4 . The advantages of this approach for sensing applications are the stability of the system. A disadvantage is that the bound layers are not fluid and do not entirely resemble a cell membrane. Because of this disadvantage, the system has been elaborated by deposition of phospholipid layers non-covalently on the tethered layer to make an asymmetrical bilayer 5 . However, the complexity of the resulting model membrane system leads away from the simplicity required for a robust analytical device. An alternative strategy for the study of surface membrane like layers on electrodes has been the non-covalent deposition of surfactant/phospholipid films on electrode surfaces 6-19 . An advantage of this approach is that the layers are mobile, undergo reversible phase transitions as a function of applied potential and desorb at extreme potentials (&gt;1 V) 10,17,18  These processes underlie the mechanisms involved in the electroporation of bilayers and biological membranes 19 . 
         [0004]    Relevant in this regard is GB-A-2 193 326 (Natural Environment Research Council) which discloses a biosensor based on the use of a hanging drop mercury electrode (HDME) in which each successive mercury drop is formed with a phospholipid layer for the purpose of a measurement to be made with the electrode. More specifically, the HMDE (which functions as a working electrode in an electrochemical cell further comprising an auxiliary electrode and a reference electrode) has a capillary tip (at which successive mercury drops may be formed) which is reciprocally moveable into and out of an electrolyte liquid having on its surface a film of a phospholipid (e.g. dioleoyl lecithin or egg lecithin). A species to be investigated by the biosensor is included in the electrolyte liquid. In one cycle of operation, a mercury drop is formed at the tip whilst it is within the liquid and below the lipid film. The tip is then withdrawn upwardly through the film and then downwardly back into the liquid. In this way, there is formed on the surface of the mercury drop a lipid film which has a very similar structure and properties to half a biological membrane. Measurements to investigate the species in solution may then be made by monitoring the phospholipid layer on the mercury electrode immersed in the electrolyte liquid by rapid cyclic voltammetry using a saw tooth waveform at a rapid ramp rate (e.g. 40 V s −1 ). At low voltages, the resulting current is proportional to the capacitance of the surface of the electrode at high voltages, the capacitance shows sharp peaks, representing phase changes of the phospholipid layer which correspond to its fluidity and (given that the electrolyte liquid does not contain any biomembrance active components) is very characteristic of the pure phospholipid. These peaks are shown for dioleoyl phosphatidylcholine (DOPC) in accompanying  FIG. 6   a  (see below). However the presence of a species having biomembrane activity changes the fluidity and affects the phase changes, thus influencing the form of the peaks. The biosensor may therefore be used for detecting the presence of biomembrane active components in a sample. The detection limit depends on the nature of the particular compound in the sample but for some polyaromatic compounds is in the region of 1 ppb in water. 
         [0005]    The system of phospholipid monolayers non covalently deposited on liquid mercury thus represents a unique and biologically relevant case due to the compatibility of the fluid phospholipid with the liquid mercury. Three features emerge from this property: 
         [0006]    (i) The phase transitions representing the ingression of water and subsequent orientational changes are sharp and are represented by distinct capacitative peaks in a cyclic and ac voltammetry plot 9 . 
         [0007]    (ii) The phospholipid layer on mercury is sensitive to interaction with biological membrane active species which change the structure and fluidity or organisation of the layer in a selective manner. The interactions influence the nature of the phase transitions and the impedance properties of the layer 14 . These interactions are different from those commonly exploited in membrane based biosensors which rely on a binding reaction between an analyte species and moeities attached to the membrane or monolayer/bilayer 16 . 
         [0008]    Recently we showed that that the phospholipid-Hg system could be also used to screen peptides in solution and that it responded selectively to the functionality of short peptides 14 . Initially the interaction between the layers and gramicidin derivatives was examined 11,12 , later studies looked at the modification of the monolayer system by an anti-microbial peptide 21,22  and finally the relationship between the structure and the monolayer activity of short chain eleven residue β-sheet self assembling peptides was examined 14 . In all cases the interactions mirrored very closely the biological membrane activity of the peptides. 
         [0009]    In spite of the distinct advantages discussed above, the phospholipid/hanging Hg drop electrode (HMDE) system has numerous severe drawbacks. In particular the HMDE is fragile so that the HMDE system is most suitable for use in a laboratory so that application “in the field” is limited. Additionally, the HMDE requires the use of relatively large amounts of liquid Hg with consequential toxicity factors to be taken into consideration. Moreover the HMDE can only be imaged with difficulty 24 . 
         [0010]    It is therefore an object of the present invention to obviate or mitigate the above mentioned disadvantage. 
         [0011]    According to a first aspect of the present invention there is provided an electrode assembly comprising at least one working electrode comprised of a conductive carrier substrate having a surface coated with mercury immobilised on the surface of the substrate, wherein the surface of the mercury remote from said metal is coated with a phospholipid layer. 
         [0012]    According to a second aspect of the invention there is provided a biosensor comprising
       (i) an electrode assembly as defined for the first aspect of the invention,   (ii) at least one counter electrode for the working electrodes,   (iii) a reference electrode   (iv) means for applying a periodically varying voltage to the at least one working electrode, and   (v) means for determining variations in the differential capacitance of the phospholipid against the counter electrode.       
 
         [0018]    Therefore in the electrode assembly of the first aspect of the invention, the working electrode is comprised of metallic (liquid) mercury (on which the phospholipid layer is provided) immobilised on a conductive substrate. Electrodes comprised of metallic mercury deposited on a conductive substrate are known in the art and are often referred to as “thin film mercury electrodes” and sometimes as “amalgam electrodes”. Thus, for example, mercury-on-iridium electrodes are known and are disclosed, for example, in U.S. Pat. No. 5,378,343 (Kounaves). This prior specification discloses an electrode assembly format comprising an array of ultramicroelectrodes arranged on a substrate, each ultramicroelectrode comprising mercury-on-iridium. The electrodes disclosed in U.S. Pat. No. 5,378,343 are proposed for use in detecting various heavy metals in water by means of anodic stripping voltammetry in which, initially, a negative potential is applied to the mercury so that metal ions in solution are electrochemically reduced and concentrated into the mercury and, subsequently, the applied potential is scanned slowly in the positive direction which results in a peak current at the oxidation potential of each metal proportional to its concentration. There is however no disclosure in U.S. Pat. No. 5,378,343 of coating the mercury with a phospholipid for the purpose of investigating species having biomembrane activity. 
         [0019]    Electrode assemblies in accordance with the first aspect of the invention have a number of advantages. In particular, the electrode assembly is robust (in contrast to the somewhat fragile nature of the hanging mercury drop electrode) thus allowing a biosensor incorporating such an electrode assembly to have a wide range of uses outside the laboratory. The immobilised phospholipid layer can give rise to sharper peaks than for a hanging drop mercury electrode system, the sharper peaks being better for analytical purposes. Additionally the amount of mercury required for an electrode can be significantly decreased as compared to a hanging drop mercury electrode, thus significantly reducing toxicity. Furthermore, there are advantages (as compared to a hanging mercury drop electrode) in relation to the stability of the phospholipid layer which is dependent on the ratio of edge to surface area. In the case of an electrode assembly in accordance with the invention, the ratio of edge to surface area of the working electrode can be very much larger than in the case of a mercury drop so that the stability of the phospholipid layer on the surface will be higher. In this regard we have demonstrated (see Example 6 below) that electrode assemblies in accordance with the invention (i.e. with phospholipid deposited on the mercury coating) may be subjected to repeated interrogation by cyclic voltammetry and produce reproducible results over significant periods of time. Additionally, the phospholipid layer may be cleaned off the mercury coating and a fresh layer applied. In a series of tests (see also Example 6 below) we have established that (for a particular composite electrode comprised of the conducting substrate with mercury coating) the successively deposited phospholipid layers provide a high degree of reproducibility in terms of the results obtained by interrogating the layers by cyclic voltammetry. 
         [0020]    Conveniently, the cleaning of the phospholipid from the mercury surface may be effected by scanning the working electrode in a cathodic direction (e.g. over the range (−0.2 V to −2.625 V) @ 97 Vs −1 ) with the electrode being immersed in electrolyte so as to desorb any contaminating organic material into the bulk solution. Surprisingly we have found that similar scan conditions when applied for much shortly periods of time than used for cleaning can be used to deposit the phospholipid layer on the mercury film. For the purposes of this deposition, the electrode will be immersed in electrolyte which incorporates the phospholipid to be deposited. The phospholipid may be added to the aqueous electrolyte in the form of a dispersion prepared by agitation (e.g. using sonication) of the phospholipid in an aqueous medium. During the deposition procedure, the mercury electrode may be monitored by cyclic voltammetry and deposition of the layer may be detected by the appearance in a cyclic voltammogram of a trace characteristic of the phospholipid. With the appearance of this trace, the deposition procedure will be complete. 
         [0021]    Electrode assemblies in accordance with the invention provide results very similar to those obtained for the Hanging Mercury Drop Electrode of the prior art. Consequently much of the extensively recorded data for the HMDE may be applied to electrode assemblies in accordance with the invention. However in contrast to the HMDE, the mercury surface of the composite electrode (i.e. the electrode comprised of the conductive substrate and the mercury coating) may be re-used in a repetitive cycle of steps (i) to (iii) below:
       (i) depositing a fresh phospholipid layer;   (ii) effecting a measurement on a sample; and   (iii) cleaning the electrode to remove the “used” phospholipid.       
 
         [0025]    Consequently there is no need to regenerate the mercury coating for each measurement, cf the HMDE for which a mercury drop is generated in situ for each successive measurement. 
         [0026]    Significantly we have established that the mercury layer (even though it is in the form of a liquid immobilised on the surface) has sufficient adhesion to the conductive substrate on which it is deposited to allow the electrode assembly to be used in a flow cell as described more fully herein. In combination with the features described above, such a flow cell may be operated in a repeated series of steps which comprise:
       (i) deposition of phospholipid on to the mercury coating (with electrolyte flowing through the cell);   (ii) effecting a measurement on a sample passed through the cell; and   (iii) cleaning the phospholipid layer from the mercury coating.       
 
         [0030]    It will be appreciated that operation of such a flow cell may be easily automated and provides a very convenient measurement technique. 
         [0031]    The electrode assembly in accordance with the invention is conveniently provided on a “chip” assembly which also incorporates a working electrode and a reference electrode. Such a chip assembly is eminently suitable for use in the above described flow cell. Embodiments of the invention are therefore able to provide a “lab-on-a-chip” functionally suitable for on-line measurement purposes. 
         [0032]    Conductive substrates for use in the invention (i.e. the substrate on which the mercury is deposited) preferably have a resistivity of less than 1 ohm metre, more preferably less than 1×10 −2  ohm metre and ideally less than 1×10 −3  ohm metre. 
         [0033]    The carrier substrate may be a metal selected from the group consisting of iridium, platinum, palladium and tantalum which are selected as “carriers” because of their refractory inert properties and their low solubility in mercury and because the mercury can be deposited on such metals (e.g. by electrodeposition) to give a uniform film. The smooth mercury surface allows for defect-free formations of the phospholipid layer. A further conductive carrier substrate that may be employed in the invention is carbon, e.g. in the form of graphite or glassy carbon. 
         [0034]    Iridium is an appropriate carrier metal for use in accordance with the invention because of its low solubility in mercury and also because the difference between the metal&#39;s work function and that of mercury is relatively high thereby ensuring maximum wettability of mercury on the metal. 
         [0035]    For example (and without wishing to be bound by theory) we believe platinum has a particularly appropriate solubility with respect to mercury to allow production of an “amalgam-like” joint which holds the mercury relatively strongly on the platinum whilst providing a good surface for the mercury to allow phospholipid deposition and provide good membrane activity. Furthermore, in construction of electrode assemblies in accordance with the invention on a “chip” (e.g. based on a silicon wafer) the use of platinum as the conductive carrier substrate allows a reference electrode to be incorporated readily on the same chip. 
         [0036]    The biosensor of the second aspect of the invention functions by monitoring the lipid layer and in particular the modification thereof due to the presence in a sample under investigation of a species having biomembrane activity. Measurements are made by voltammetry to determine variations in the differential capacitance of the phospholipid as a function of voltage against the reference electrode, in a similar manner to that disclosed in GB-B 2 193 326. Most preferably measurement is by means of rapid cyclic voltammetry, preferably using a sawtooth waveform with a ramp rate of &gt;1 Vs −1  (e.g. 40-100 Vs −1 ). The voltage excursion used in rapid cyclic voltammetry may be from −0.4 V to −1.2 V vs Ag/AgCl 3.5 M KCl. The output current (i) is proportional to the differential capacitance (C d ) as indicated by the equation: 
         [0000]        C   d   =i /( v×A ) where  A  is the electrode area and  v  is the ramp rate. 
         [0037]    The experimental set-up for RCV involves the application of the sawtooth wave form using a function generator with input to a potentiostat which applies the waveform to the working electrode. The resulting current response is recorded via an acquisition board and plotted against the applied waveform. 
         [0038]    DC cyclic voltammetry provides rapid assessment of the layer&#39;s capacity over a defined potential window specific to the phospholipid monolayer&#39;s structure and environment. 
         [0039]    Alternatively measurement may be made by ac voltammetry using, for example, a voltage ramp of about 5 mV s −1  with a superimposed sinusoidal voltage of frequency, f, about 75 Hertz and of amplitude, ΔE, about 0.005 V. The output ac current is separated into both in phase and out of phase components. The out of phase current (i″) is proportional to the differential capacitance (C d ) as expressed by the equation: 
         [0000]        C   d   =i″/( 2π× f×ΔE×A )
 
         [0040]    The experimental set-up for ac voltammetry involves adding the sinusoidal waveform to the above voltage ramp and inputting to a potentiostat which applies the resulting waveform to the working electrode. The ac current response is fed into a lock-in amplifier where the in phase and out of phase components with the applied ac waveform of the current are separated and recorded on a data acquisition system. The out of phase current is plotted against the ramp voltage. 
         [0041]    The electrode assembly of the first aspect of the invention may comprise a single working electrode but more preferably comprises a plurality of the working electrodes. For the or each working electrode there should be no exposed free conductive carrier substrate surface. This ensures that instability issues associated with hydrogen gas production via water reduction during voltammetry measurements with the working electrode in contact with an aqueous media are circumvented by the large hydrogen overpotential provided by the mercury layer. 
         [0042]    The working electrode may for example be circular and/or have a maximum surface dimension in any direction of 2 μm to 1 mm, although dimensions outside this range are not precluded. The Examples described below utilise a circular electrode having a diameter of about 960 μm. Alternatively, the working electrode may be a microelectrode and preferably sized such that the mercury has a maximum surface dimensions of 2 μm to 10 μm in any direction. The or each working electrode may, for example, be circular with a diameter of 2 μm to 10 μm. Such dimensions serve to maximise the edge-to area and thereby enhance stability of the phospholipid layer. 
         [0043]    Preferably the electrode assembly comprises a layer of a conductive carrier substrate selected from the group consisting of platinum, palladium, tantalum and iridium sandwiched between first and second insulating substrate layers (preferably silica), one of which (the “first” substrate layer) is penetrated by at least one through aperture which, in effect, defines a well for which said carrier metal provides a basal surface, the well incorporating the mercury coating (for said carrier metal) on which the phospholipid layer is provided, thereby forming a said working electrode. In the case of a microelectrode, the or each well may, for example, be circular and have a selected diameter in the range 2 μm to 10 μm and (given that the mercury layer is hemispherical) may have a depth of about 1 μm which corresponds therefore to the wall thickness. 
         [0044]    The mercury layer and its phospholipid coating should occupy the full cross-section of the well (so no carrier substrate is exposed). This total coverage of the carrier substrate at the basal surface of the well suppresses water reduction by carrier substrate during the voltammetry measurement. Ideally also the configuration of the mercury layer and its phospholipid coating are such that the latter has a planar surface flush with the surface of the first insulating substrate layer. 
         [0045]    A preferred microelectrode array in accordance with the invention comprises a plurality of said wells formed in the first insulator layer, the carrier substrate at the base of each of said wells being coated with mercury which in turn is provided with the lipid layer. Such a microelectrode array thus comprises a carrier metal layer provided between the insulating substrates with portions of the carrier substrate providing respective basal surfaces for the individual wells. 
         [0046]    Conveniently the electrode assembly may incorporate a conducting layer (e.g. gold) sandwiched between the second insulating substrate and the carrier metal layer with which the conducting layer is in electrically conducting relationship, thereby improving the conductivity of the device. 
         [0047]    Microelectrode arrays in accordance with the invention may incorporate electrodes additional to the “mercury-on-carrier metal” electrodes. Thus, for example, the array may incorporate the reference electrode (e.g. Ag/AgCl)/pseudo-reference (e.g. Pt) and/or the counter electrode (e.g Pt) provided on an exterior surface of the first insulting substrate. 
         [0048]    Preferred microelectrode arrays in accordance with the invention comprise:
       (i) first and second insulating layers,   (ii) a layer of a carrier metal selected from the group consisting of platinum, palladium, tantalum and iridium provided between said insulating layers,   (iii) a plurality of wells formed in the first layer such that the carrier metal layer provides respective basal surfaces for the wells, said discrete portions each forming part of a working electrode comprised of said discrete portion, a mercury coating therefor and a phospholipid layer on the surface of the mercury,   (iv) optionally a conducting layer provided between said carrier metal layer and the second substrate and being in electrically conducting relationship therewith,   (v) a counter electrode provided on the first insulating layer, and   (vi) a reference electrode provided on the first insulating layer.       
 
         [0055]    The first insulating layer may, for example, be silica or silicon nitride. 
         [0056]    The second insulating may be silica and may be formed on a silicon wafer. 
         [0057]    In a preferred microelectrode array in accordance with the invention, the carrier metal is platinum which, in addition to providing the surface on which the mercury is deposited, also serves to provide conductive traces connecting the electrode of the invention to electrical contact means provided on the array. In this embodiment, the separate conducting layer (iv) may be omitted. 
         [0058]    Preferably the wells are circular, e.g. with a diameter of 2-10 μm although diameters outside this range may be used, the reference electrode is Ag/Ag Cl and/or the counter electrode is platinum. 
         [0059]    A microelectrode array comprised of (i)-(vi) defined above is able to provide a so-called “lab-on-a-chip” functionality suitable for on-line use. 
         [0060]    Electrode assemblies in accordance with the invention may be used in a static cell or a flow-cell. Such a flow cell may comprise a “measurement cell” (e.g. in the form of a chamber) in which the working electrodes are exposed. The flow-cell will further comprise inlet and outlet channels communicating with the “measurement cell”. In use of the flow-cell electrolyte and analyte will be supplied via the inlet channel(s) to the “measurement cell” where they flow over the working electrode and then out through the outlet channel(s). There may be more than one type of working electrode (of the invention) in the “measurement cell”, with the various electrodes being distinguished by the particular phospholipid deposited on the mercury surface. Each electrode of the flow cell may be connected (e.g. by an appropriate mechanical or electronic switching arrangement) potentiostat such that each working electrode may be addressed individually. Thus individual results can be obtained for the various different phospholipids. Alternatively the combined signal from the various electrode assemblies (with their different phospholipids) may be recorded. In these ways, the effect of the same analyte on a plurality of different phospholipids may be determined and provide a “fingerprint” for that analyte. 
         [0061]    The phospholipid provided as a layer on the surface of the mercury may be saturated or may have a degree of unsaturation. Examples of suitable phospholipids include dimyristoyl, phosphatidyl choline (DMPC—saturated), dioleoyl phosphhtidyl choline (DOPC—unsaturated) and egg lecithin (egg PC—saturated/unsaturated) Such lipids incorporate choline-based head groups and other lipids incorporating this head group may be used. However the head group may be amine based as in dioleoyl phosphatidylethanolamine (DOPE) or hydroxyl based as in 1,2-dioleoyl-sn-glycero-3-phospho(ethylene glycol) (sodium salt) or contain a combination of chemical functional groups as in 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(succinyl) (sodium salt). These functional groups may be present in ‘natural’ lipids or may be synthesized to confer a desired chemical surface. Each lipid present as a homogeneous monolayer or as a fraction of a heterogeneous monolayer will confer unique properties to that layer that may be observed through the layers capacitance over the interrogation potential window. Modification of the phospholipid head group may be used and be important in the sensor&#39;s operation due to this being the functionalised surface that is presented directly to the electrolyte. The layers properties may be tuned by changing the length of the carbon chains and their saturation or through the modification of the head groups. 
         [0062]    More specifically, phospholipids with two 9-cis-octadecenoic chains are fluid at room temperature and capable of forming impermeable monolayers on mercury. They exhibit sharp pseudo capacitative phase transitions within the potential range interrogated by rapid cyclic voltammetry. By comparison, dimyristoyl lipids (incorporating C 14  saturated chains) have the advantage of being less susceptible to oxidation than dioleoyl lipids but exhibit less prominent phase transitions. 
         [0063]    If desired the phospholipid layer may be associated with additional components for modifying the properties of that layer. These components may be incorporated within the layer or covalently tethered thereto and examples include peptides (to form ion channels), oligonucleotides or molecules complementary to the target molecule. 
         [0064]    Microelectrode arrays in accordance with the invention may be produced by successively depositing on to an insulating substrate (which ultimately provides the aforementioned second insulating substrate) an optional conducting layer (e.g. gold) and a carrier metal layer (e.g. both by E-beam evaporation) prior to deposition of a further insulating layer (the aforementioned first layer) to overlie the carrier metal. The second insulating layer, may, for example, be a silicon wafer with a SiO 2  surface layer. Deposition of the optional conducting layer (e.g. gold) and carrier metal layer may for example be E-beam evaporation. If desired, titanium adhesion layers may be deposited between (i) the second substrate and the conducting layer, (ii) between the conducting layer and the carrier metal layer, and (iii) on to the carrier metal layer. The first insulating layer may, for example be SiO 2  or silicon nitride and deposited by low temperature plasma enhanced chemical vapour deposition (PECVD). Subsequently the first insulating layer may be etched through to the carrier metal so that the aforementioned wells are formed with the iridium layer providing a basal surface for the wells. 
         [0065]    If desired, reference and/or counter electrodes may be formed on the first layer prior to the next step of the fabrication procedure in which mercury is deposited on the carrier metal. 
         [0066]    The mercury layer may be formed by electrodeposition. The amount of mercury deposited should be sufficient to give a continuous film of the mercury on the conductive carrier substrate. Increasing the thickness of the mercury layer will enhance the stability of that layer and allow repeated the mercury layer to allow repeated cycles of phospholipid layer deposition, sample measurement and removal of the phospholipid layer so that one electrode may be used many times without disruption of the mercury. Additionally thickness of mercury layer will be a consideration for use of the electrode assembly in a flow cell where the mercury layer is required to withstand forces associated with liquid flows through the cell. The amount of charge required for the electrodeposition process will depend on the required thickness of the mercury layer and this will in turn depend on factors such as the surface area of the conductive substrate (on to which the mercury is to be electrodeposited) and the concentration of mercury ions in the mercury deposition electrolyte. By way of example, Example 4 uses one Coulomb of charge to deposit a satisfactory mercury layer on to a circular platinum substrate having a diameter of 960 μm at the particular mercury concentration in the deposition electrolyte. Example 7 below (illustrating use of a flow cell) employs an electrode formed by using two Coulombs of charge to deposit mercury on to a circular platinum substrate having a diameter of 960 μm. For the same concentration of solution, lesser amounts of charge will be required for platinum substrates of smaller diameter. The converse is true for substrates of larger diameter. 
         [0067]    Controlled electrodeposition of mercury from a solution containing Hg 2+  ions (e.g. provided by Hg(NO 3 ) 2 ) may be effected within an electrochemical cell. The mercury deposition electrolyte may contain HClO 4  which prevents oxidation of the Hg species. However the lipid interrogation will not take place in this acidic solution and will instead be in a more benign electrolyte such as 0.1M KCl. Alternatively electrodeposition of mercury can be effected by pipetting a drop of the base electrolyte (e.g. HClO 4 +Hg(II)) onto the surface of the first insulator. The electrodeposition may be facilitated by applying a potential of ˜−0.4V vs. 3.5 M KCl Ag/AgCl to the working electrode surface in the deposition solution and recording the charge passed which is directly proportional to the mass of mercury deposited the deposition can be stopped by breaking the circuit. This allows precise control over the amount of Hg deposited. 
         [0068]    Once the mercury layer has been deposited, the assembly can be transferred to a neutral electrolyte such as 0.1M KCl and the electrode surface electrochemically cleaned by applying an extreme negative potential&lt;−2V evolving hydrogen gas to ‘scrub’ the surface free from organics. 
         [0069]    Subsequently the phospholipid may be deposited on the mercury. This may be effected in a number of ways, for example: 
         [0070]    (a) The deposition can be carried out as has been done previously with the HMDE by transferring the phospholipid on to the array from an excess of the phospholipid provided as a film on a liquid. Both vertical and horizontal insertion through the solution gas interface can be effected. This procedure leads to formation of the phospholipid as a monolayer on the mercury surface. 
         [0071]    (b) A controlled deposition can be carried out in a Langmuir Blodgett trough by inserting the array vertically through a phospholipid monolayer of precise coverage at the solution gas interface. Advantage is taken of the controlled deposition by calibrating the properties of the phospholipid layers with the phospholipid coverage at the gas-solution interface. As with (a), a monolayer of the phospholipid is formed on the mercury surface. 
         [0072]    (c) A phospholipid layer may be deposited onto the array surface by allowing the evaporation of a concentrated pentane solution containing the phospholipid of interest subsequent to the array being immersed in the solution. The layer may then be made homogeneous by submerging the array in an aqueous electrolyte such as 0.1M KCl and rapidly cycling the potential from −0.2 to −1.2V continuously. This method provides for formation of a phospholipid monolayer by facilitating self-assembly through applying a rapid potential ramp which causes the phospholipids on the surface to continuously rearrange themselves into the lowest energy configuration (ie: a monolayer at −0.4V vs 3.5M KCl Ag/AgCl which is close to the Potential of Zero Charge (PZC) of the mercury). 
         [0073]    (d) A drop of phospholipid can be evaporated on the silicon wafer. When free of solvent the wafer is transferred to an electrochemical cell. In this case a monolayer is obtained on the electrode array by continued scanning using RCV. This allows the phospholipids to anneal into a monolayer. 
         [0074]    (e) A vesicle or phospholipid dispersion can be allowed to adsorb during flow inside a flow cell. As with (d), a monolayer is obtained by continued scanning using RCV, whilst the phospholipid solution is flowed over the electrode array. 
         [0075]    Using the above methods of deposition the configuration of the phospholipid on the surface can be evaluated using RCV, ACV and impedance together with atomic force microscopy (AFM) studies. In RCV and ACV the characteristics of the two capacitance peaks, corresponding to the phospholipid phase transitions determine the configuration and coverage of the phospholipid layer on the electrode 20 . The voltammograms of the deposited phospholipid can be checked to gauge if the voltammometric peaks representing the phospholipid phase transitions are influenced by electrode size. Indeed both unsaturated (eg DOPC), saturated (eg DMPC) and saturated/unsaturated (eg egg lecithin (egg PC)) phospholipids can be tested, to evaluate their stability in the presence of air. The phospholipid layer may be removed by desorption from the electrode surface at the extreme potential of −2.0 vs 3.5M KCl Ag/AgCl. The mercury surface will be routinely cleaned in this manner. 
         [0076]    The biosensor of the invention may be used, for example, for determining (i) the presence or otherwise in a sample of a species known to have biomembrane activity or (ii) whether or not a particular substance has biomembrane activity. The measurements may be made by voltammetry, (e.g. rapid cyclic voltammetry or ac-voltammetry effected in conventional manner such as along the lines disclosed in GB-A-2 193 326 (see for example  FIG. 1  thereof). RCV will be used for rapid interrogation of the sensor surface and in particular to determine the effect of the interaction of the two capacitance of the phospholipid which are particular sensitive to interactions. 
         [0077]    Generally the determination procedure will involve an aqueous electrolyte liquid in which the electrode assembly in an electrochemical cell will be immersed, if necessary, electrolytes (e.g. KCl) can be added to the liquid. It is however also contemplated that one or more drops of the electrolyte liquid could be applied to electrode assembly for the purposes of the measurement without the need for locating the assembly in an electrochemical cell. In this case it will be necessary to use phospholipids which are stable in the presence of air. Saturated dimyristoyl phosphatidylcholine, (DMPC) has been shown to be suitable for this and it also displays potential induced phase transitions which can be used for analytical applications. Also within the scope of the invention is the analysis of gas or vapour samples for the presence therein of a species with biomembrane activity. In this case it would be necessary for an electrolyte liquid to be provided on the working electrode. 
         [0078]    The invention has applications in a wide number of fields. These include:
       (a) routine testing of drinking water for the presence of biomembrane active compounds which may be toxic agents;   (b) testing of potential pharmaceutical products for their biomembrane activity;   (c) detection of toxic gases and explosive vapours which (provided they interact with the monolayer surface) may be diagnosed using a multivariate analysis approach depending on the strength of the interaction with monolayers of varying chemical functionality; and   (d) environmental applications such as the in situ analysis of natural and marine waters for biomembrane active compounds such as (i) toxic biomembrane active peptides produced by blooms of cyanobacteria and dinoflagelates or (ii) pollutants.       
 
         [0083]    It will be appreciated that the system may be calibrated for sensor application with a series of compounds eg polycyclic aromatic hydrocarbons 7 , phenothiazine drugs 8  and anti-microbial peptides 23  which are known to modify the structure of phospholipid layers on electrodes. In each case the disruption/modification of the phospholipid layer has been well characterised electrochemically. 
         [0084]    In the case of a microelectrode array, the individual working electrodes may incorporate phospholipids varying in the functionality of lipid head groups that respond in a specific, but different, fashion to target analytes. The chemical nature of the target analyte can be dissected by multi-variant analysis of the magnitudes of interactions with the array monolayer surface presented functional groups. Phospholipids with two 9-cis-octadecenoic chains are 18 carbon atoms long, unsaturated and fluid at room temperature capable of forming semi-permeable monolayers on mercury. They exhibit sharp pseudo capacitative phase transitions within the potential region interrogated by rapid CV. Also of interest are dimyristoyl lipids which are 14 carbon atom saturated chains that has the advantage of being less susceptible to oxidation than di-oleoyl lipids but exhibit less prominent phase transitions. The sensor can employ a combination of these lipids with varying functional head groups so that each array possesses a single homogenous population of phospholipids presenting a functionalised surface with which the target analyte can interact. The phospholipids acting as a transducer to the interaction allow for further incorporation of selective elements, either peptide based such as immunoglobins, oligo-nucleotides or complementary molecules to the target molecule either incorporated with-in the layer or covalently tethered to the layer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0085]    The invention will be further described by way of example only with reference to the accompanying drawings, in which: 
           [0086]      FIG. 1  is a schematic cross-section (to a much enlarged scale) of one embodiment of electrode assembly in accordance with the invention; 
           [0087]      FIG. 2  is a perspective view of an intermediate structure for the fabrication of an electrode assembly in accordance with the invention; 
           [0088]      FIG. 3  ( a )-( i ) illustrate steps in the production of a microelectrode array of the type for which  FIG. 2  shows an intermediate structure; 
           [0089]      FIG. 4  illustrates a further embodiment of microelectrode array in accordance with the invention; 
           [0090]      FIG. 5  schematically illustrates the principal components of a measurement system incorporating the microelectrode array of  FIG. 4 ; 
           [0091]      FIG. 6  demonstrates the results of Example 1, more particularly experimental results for a biosensor in accordance with the invention in comparison with those for a hanging drop mercury electrode arrangement of the prior art; 
           [0092]      FIG. 7  demonstrates the results of Example 2, more particularly experimental results for a microelectrode array in accordance with the invention; 
           [0093]      FIG. 8  is a schematic cross-section (to a much enlarged scale) of a further embodiment of electrode assembly in accordance with the invention; 
           [0094]      FIG. 9  is a schematic perspective view (to a much enlarged scale) of a further embodiment of electrode assembly in accordance with the invention; 
           [0095]      FIGS. 10(   a )-( c ) are (to a much enlarged scale) schematic sectional, end and plan views respectively of one embodiment of flow cell in accordance with the invention; 
           [0096]      FIGS. 11(   a ) and  11 ( b ) are cyclic voltammetry scans obtained for a composite electrode comprising platinum with a mercury coating; 
           [0097]      FIG. 13  is a cyclic voltammetry scan for a Hanging Mercury Drop Electrode; 
           [0098]      FIG. 14  illustrates the chemical formulae of various phospholipids; 
           [0099]      FIGS. 15(   a )-( e ) are specific capacitance plots for electrode assemblies in accordance with the invention incorporating the phospholipids illustrated in  FIG. 14 ; 
           [0100]      FIG. 16  ( a )-( c ) are specific capacitance plots demonstrating stability of electrode assemblies in accordance with the invention as carried out in Example 6; 
           [0101]      FIGS. 17 and 18  illustrate results obtained using a flow cell in accordance with the procedure of Example 7; 
           [0102]      FIGS. 19(   a ) and ( b ) are specific capacitance plots for a Hanging Mercury Drop Electrode and Pt/Hg electrode as obtained following the procedure of Example 8; 
           [0103]      FIGS. 20(   a ) and ( b ) show forward and reverse scans comparing respectively a HMDE electrode and an electrode in accordance with the invention, both incorporating a monolayer film of DOPC; and 
           [0104]      FIGS. 21 to 28  illustrate the results obtained in Examples 10 to 17 respectively each of which compares results obtained for a HMDE and an electrode assembly in accordance with the invention for analysis of specific analytes and/or amounts thereof. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0105]    Referring firstly to  FIG. 1 , an electrode assembly  1  in accordance with the invention comprises a working electrode  2  which is comprised of the combination of an iridium layer  3  having a surface coating of mercury  4  on which is deposited a phospholipid monolayer  5 . 
         [0106]    As illustrated in  FIG. 1 , the mercury layer  4  is located within a well structure (for which iridium layer  3  provides a basal surface) formed in an upper silica layer  6 , the well being circular with a selected diameter in the range 2 μm to 10 μm and having a depth of 0.5 μm. The mercury occupies the full cross-section of the well structure whereby there is no exposed free iridium. 
         [0107]    Additional principal features of the illustrated electrode assembly are a lower silicon substrate  7  with a silica surface and a gold conducting layer  8  which is a electrically conducting relationship with iridium layer  3 . Provided between the silicon/silica layer  7  and gold layer  8  is a titanium adhesion layer  9  and similar adhesion layers  10  and  11  are provided respectively
       (i) between the iridium layer  3  and gold layer  8 , and   (ii) between iridium layer  3  and upper insulating layer  6 .       
 
         [0110]    Reference is now made to  FIG. 2  which is a perspective view of a microelectrode array based on the structure illustrated in  FIG. 1  but with the titanium adhesion layers  9 - 11  omitted for the purposes of clarity and the well structures (referenced in  FIG. 2  as  12 ) being shown as “empty” (i.e. without the mercury layer  4  and its associated phospholipid monolayer  5 ).  FIG. 2  does however also illustrate circular and part-circular reference electrodes  13  provided on the upper insulating layer  6 , each electrode  13  having a radius of 50 μm and each being centred at a respective well  12 . 
         [0111]      FIG. 3  illustrates a step wise procedure for producing an electrode assembly of the type illustrated in  FIG. 2 . 
         [0112]    The illustrated procedure starts with a 10 cm×10 cm silicon wafer with a 90 nm SiO 2  surface layer (e.g. as available from IDB Technologies). This provides layer  7  for the structure illustrated in  FIGS. 1 and 2 . 
         [0113]    There is then deposited in succession on to the SiO 2  surface layer (a) the titanium adhesion layer  9  (30 nm), (b) the gold conduction layer  8  (100 nm), (c) the titanium adhesion layer  10  (30 nm), (d) the iridium layer  3  (30 nm) and (e) the titanium adhesion layer  11  (30 nm). All of these layers may be deposited by E-beam evaporation. It should be noted that, at this stage, the titanium adhesion layer  11  is deposited as a continuous layer. The resulting structure is depicted in  FIG. 3   a.    
         [0114]    In the next step of the process, a 500 nm SiO 2  layer is deposited on titanium adhesion layer  11  by means of low-temperature plasma enhanced chemical vapour deposition (PECVD) to provide layer  6  (but not having, at this stage, the wells  12  formed therein). The product at this stage is shown in  FIG. 3   b.    
         [0115]    For the next step of the process, a positive resist material is spun on to the SiO 2  layer  6  then baked at 150° C. and washed in chlorobenzene to provide a “hardened” surface resist layer depicted by reference numeral  14  (see  FIG. 3   c ). 
         [0116]    A mask  15  incorporating the pattern for the silver rings  13  is positioned over resist layer  14  which is then patterned using photo/E-beam lithography ( FIG. 3(   d )). 
         [0117]    Mask  15  is now removed and next resist layer  14  is chemically etched down to the SiO 2  layer  6  ( FIG. 3   e ). 
         [0118]    In the step illustrated in  FIG. 3(   f ) a 100 nm silver layer  16  is evaporated on to the surface of the resist layer  14  but, more importantly, also within the channels of the pattern of circles and arcs defined therein. 
         [0119]    Subsequently resist layer  14  is removed to leave a structure as illustrated in  FIG. 3(   g ) in which the upper surface has circular and part circular traces  17 . 
         [0120]    Although not illustrated in the drawings, procedures generally along the lines described for  FIGS. 3  ( c )-( f ) may be repeated to provide a platinum counter electrode on the silica layer  6 . 
         [0121]    A further resist layer  18  capable of withstanding plasma etching is now applied to silica layer  6  (and overlays the silver traces  17 ) and plasma etched to form a pattern of circular apertures  19 , that expose the iridium layer ( FIG. 3(   h ). In other words, the etching is through the SiO 2  layer  6  and also through the titanium adhesion layer  11 . 
         [0122]    If desired, a 10 nm gold wetting layer may be deposited at this stage to the exposed iridium surfaces at the bases of the apertures  19 . 
         [0123]    Removal of the resist  18  produces the structure illustrated in  FIG. 3(   i ) in which wells  12  (corresponding to those in  FIG. 2)  have been formed. 
         [0124]    The silver trace  12  may be anodised in a chloride rich solution to produce a stable Ag/Ag/Cl reference electrode with fast kinetics and a stable reference potential. 
         [0125]    The layer of mercury  4  (described with reference to  FIG. 1 ) may be electro-deposited (as described above) on to those portions of the iridium layer  3  exposed at the base of the wells. Subsequently the phospholipid layer may be deposited, again using techniques as described above. 
         [0126]      FIG. 4  illustrates a particular embodiment of microelectrode array in accordance with the invention. 
         [0127]    The illustrated electrode assembly  100  comprises a 10×10 array of working electrodes  101  produced in the manner described more fully above with reference to  FIG. 3  and thus comprising mercury coated iridium provided with a phospholipid layer on the mercury surface, the electrodes  101  being associated with a conducting trace  102 . 
         [0128]    The assembly further comprises a Ag/Ag/Cl reference electrode (not individually referenced but similar to that shown in  FIG. 2 ) associated with a conducting trace  103 . A platinum electrode  104  associated with a conducting trace  105  is also provided. 
         [0129]    Conducting traces  102 ,  103  and  105  are associated with respective gold contact pads  106 ,  107  and  108  respectively for connection to electronic control/measurement systems, e.g. as shown schematically in  FIG. 5 . 
         [0130]    Although the construction of the sensor has been described with specific reference to iridium as the carrier metal, it will be appreciated that the same principles of construction may be employed for the other carrier metals that may be used in accordance with the invention (i.e. palladium, platinum and tantalum). 
         [0131]    Reference is made to  FIG. 8  which is a schematic cross-sectional view to a much enlarged scale of a composite Pt/Hg electrode assembly  200  on which a phospholipid layer may be deposited. 
         [0132]    Electrode assembly  200  comprises a silicon wafer  201  on which is deposited a layer  202  of silicon nitride (e.g. having a thickness of 500 nm). Formed in the silicon nitride is at least one well on the base of which is an adhesion layer (e.g. 30 nm) of titanium  203  on which is deposited a 100 nm thick layer of platinum  204 . Filling the well is a layer of mercury  205  provided on its upper surface (as viewed in  FIG. 8 ) with a monolayer  206  of a phospholipid. Reference numeral  207  in  FIG. 8  represents electrolyte solution in which the electrode would, in use, be immersed. Although not illustrated in  FIG. 8 , platinum layer  204  is associated with a conductive trace for connection to a potentiostat. 
         [0133]      FIG. 9  illustrates an electrode assembly  300  constructed in accordance with the general principles shown in, and described with reference to,  FIG. 8  above. 
         [0134]    The electrode assembly  300  is generally rectangular and is formed towards one end thereof with two working electrodes  301  and  302  each within a well of a silicon nitride layer  303 . Towards the opposite end of electrode assembly  301  are two contact pads  304  and  305 . Electrode  301  is connected by a conductive trace  306  to contact pad  304  whereas electrode  302  is connected by conductive trace  307  to contact pad  305 . 
         [0135]    Referring now to  FIGS. 10(   a )-( c ) there are illustrated schematic views of one embodiment of flow cell  400  constructed for the purposes of proof-of principle” (see Example 7 below) and incorporating an electrode assembly  300  of the type described with reference to  FIG. 9 . 
         [0136]    The schematic drawings of  10 ( a )-( c ) respectively illustrate side, end and plan views of the flow cell  400  which (particularly from  FIGS. 10(   a ) and ( b )) will be seen to comprise a base portion  401  and a top portion  402  both formed in the manner described more fully below. 
         [0137]    Upper surface of base portion  401  is formed with a rebate such that electrode assembly  300  may sit therein so that its end provided with the contact pads  304  and  305  projects from the flow cell (see particularly  FIG. 10(   b )). It should be understand from  FIG. 10(   b ) that, in the illustrated position of the electrode assembly  300 , the electrodes  301  and  302  as well as the contact pads  304  and  305  are uppermost. 
         [0138]    The under surface of upper portion  402  has a central recess which (with upper portion  401  and lower portion  402  assembled together in the manner illustrated in  FIG. 10(   a )) forms a “measurement cell”  403  into which the electrodes  301  and  302  face. Leading to the left from measurement cell  403  (as viewed in Fig(a) is an electrolyte entry channel  404  and leading to the right is an electrolyte outlet channel  405 . Communicating with electrolyte inlet channel  404  is an injection port  406  for a sample to be analysed whereas leading from electrolyte outlet channel  404  are bores  407  and  408 , one for incorporating a Ag/AgCl reference electrode and the other a Pt auxiliary electrode (neither shown) separately from the electrode assembly  401 . 
         [0139]    An annular groove is formed in the under surface of upper portion  402  of flow cell  400  and receives a sealing O-ring  409 . 
         [0140]    Although not illustrated in the drawings, screw holes are provided for receiving screws to assemble upper and lower portions  41  and  42  together. 
         [0141]    Each of the electrodes  301  and  302  may have a different phospholipid deposited therein. 
         [0142]    For measurement purposes the working electrodes  301  and  302  as well as the auxiliary and reference electrodes are connected to a potentiostat in the manner illustrated in  FIG. 5 . The arrangement will be such that electrodes  301  and  302  can be individually addressed. 
         [0143]    In use of the flow cell, electrolyte is passed into inlet  404  and allowed to flow through “measurement well”  403  and then through outlet channel  405 . Sample to be analysed may be injected into part  406 . Individual measurements may then be made for the effect of the sample on different phospholipids on electrodes  301  and  302 . 
         [0144]    The invention is illustrated by the following non-limiting Examples. 
       Example 1 
       [0145]    To establish proof of principle, mercury was electrodeposited on Ir circular discs surrounded by glass as an insulator. The electrodes were washed with deionised water and a phospholipid was then deposited on the mercury layer by passing the mercury coated electrode through a film of a solution of dioleoyl phosphatidycholine (DOPC) in pentane at an electrolyte-gas interface. Subsequently the pentane was evaporated to leave the DOPC on the mercury surface. 
         [0146]    The phospholipid layers were monitored by rapid cyclic voltammetry (RCV) at 80 V s −1 . 
         [0147]    A comparative experiment was conducted using DOPC on a hanging drop mercury electrode (prior art). 
         [0148]    The results are shown in  FIG. 6  ( a ) for which the lighter trace shows the results for the mercury coated iridium electrode whereas the darker trace is for the HMDE electrode. It will be seen from  FIG. 6  ( a ) that the two traces are virtually identical and both demonstrate the characteristic peaks ( 1  and  2 ). 
         [0149]      FIG. 6  ( b ) shows the results for the mercury coated iridium electrode scanned at 100 V s −1 . Once again the two characteristic peaks ( 1  and  2 ) can clearly be seen. 
         [0150]    The results show that the mercury coated electrode shows sharper voltammetric peaks at a rapid scan rate. This finding indicates that the occurrence of the phase transitions is a function of electrode size. Sharper voltammetric peaks are more suitable for analytical purposes and favour the microelectrode as a support for phospholipids. We have shown that removal of phospholipid from the microelectrode surface can be achieved by applying an extreme potential of −3.0 V vs Ag/AgCl. The voltammetric peaks displayed in the RCV s in  FIGS. 6  ( a ) and ( b ) can be used for the selective analytical recognition of a large number of dissolved organic species at very low (nano to μmol dm −3 ) level. These results show that stable ordered phospholipid layers can be deposited on to Hg/Ir microelectrodes. 
       Example 2 
       [0151]    Mercury was electrodeposited on a close packed hexagonal array of 1800 platinum micro electrodes (on a base of Pt of 2 mm diameter), the microelectrodes being of dimension 10 μm diameter with a 20 μm spacing centre-to-centre. Phospholipid DOPC was deposited on this array of microelectrodes by evaporating a solution of phospholipid on the surface of the array. 
         [0152]    The array was introduced into an electrolyte solution where it was voltammetrically cycled between potentials of −0.2 and −2.0 V vs. Ag/AgCl. This annealed the DOPC monolayer to form a stable organised layer as demonstrated by  FIG. 7  which is a cyclic voltammetry plot at 30 V s −1  from −0.2 V to −1.0 V. This plot displays the characteristic phase transitions. These layers have been shown to be stable for at least one hour. 
       Example 3 
       [0153]    Electrode assemblies of the type illustrated in  FIG. 9  was prepared using the procedure set out below to produce an assembly in which the wells in the silicon nitride layer  303  at a diameter of 960 μm. 
         [0154]    A 100 mm thick silicon wafer substrate was cleaned with piranha solution (a 2:1 (v:v) mixture of sulphuric acid and hydrogen peroxide) and subjected to thermal oxidation to grow a layer of dense oxide on the wafer surface. Standard UV photolithography techniques were used to produce a plurality of identical resist patterns on the wafer each corresponding to one electrode assembly. The individual resist patterns had developed positive resist (absent resist) at the regions corresponding to the working electrodes  301  and  302 , the contact pads  304  and  305  and the conductive traces  306  and  307 . 
         [0155]    Using conventional thermal evaporation techniques a 30 nm thick titanium adhesion layer and then a 100 nm thick platinum layer were applied to the substrate (see layers  203  and  204  in  FIG. 8 ). 
         [0156]    The pattern was revealed using the standard practice of “metal lift-off” by dissolving the photo-resist in acetone. 
         [0157]    A layer of silicon nitride approximately 500 nm thick was then deposited using Plasma Enhanced Chemical Vapour Deposition (PECVD) (see layer  202  in  FIG. 8 ). 
         [0158]    Further UV photolithography was then used to pattern the electrode regions  301  and  302  and the contact pads  304  and  305  (resist developed selectively in these regions) using a second photo mask (etch mask). The underlying silicon nitride was then etched using a hydrofluoric acid based wet etch down to the surface of the platinum layer, so the latter was exposed at the base of wells in the silicon nitride and to provide the contact pads  304  and  305 . 
         [0159]    The remaining resist was then removed and the device cleaned with piranha solution. 
         [0160]    Individual electrode assemblies were subsequently isolated from the others formed on the wafer by dicing the wafer using a wafer saw. 
         [0161]    Electrodeposition of mercury onto platinum disc working electrodes was performed in a standard three electrode cell containing a double junction reference electrode (3.5 mol dm −3  KCl, Ag/AgCl inner filling, 0.1 mol dm −3  perchloric acid outer filling) and a platinum bar auxiliary electrode both supplied by Metrohm. The working electrodes were introduced into the cell by means of a micromanipulator and connected via crocodile clips attached to platinum bond pads. The potential at the surface of the working electrodes was set using a PGSTAT12 (Ecochemie, Utrecht, The Netherlands)potentiostat controlled by AUTOLAB software. The silicon wafer based working electrodes were cleaned prior to electrodeposition in a hot solution of sulphuric acid (Fisher Scientific) and 30% hydrogen peroxide (Fluka) in a ratio of approximately 3:1 before drying under nitrogen. Electrodeposition was performed at −0.4V vs. the 3.5 mol dm −3  KCL Ag/AgCl reference and monitored by means of chronocoulometry. The deposition was terminated by opening the circuit and immediately removing of the electrode from the deposition solution once 1 Coulomb of charge had passed. 
         [0162]    The liquid mercury deposited on the platinum was in the form of a “flattened hemisphere”. The mercury was immobilised sufficiently on the platinum to allow the electrode assembly to be turned “upside down” without loss of mercury. 
       Example 4 
       [0163]    This Example demonstrates the stability of an electrode assembly produced in accordance with the procedure of Example 3. 
         [0164]    The electrode assembly was tested in a three electrode cell which was temperature controlled at 25° C. and which contained 0.1 mol-dm −3  KCl phosphate buffered at pH 7.4. The solution was deaerated prior to introduction of the electrode assembly by bubbling argon gas through the stirred solution. The electrode assembly was lowered into the cell using a micro manipulator and connected to the external potentiostat by crocodile clips attached to one of the electrode contact pads. The working electrode potential was set relative to a Ag/AgCl 3.5 mol·dm −3  KCl reference electrode separated from the cell by a porous glass frit. A platinum bar counter electrode completed the circuit. Rapid cyclic voltammetry measurements were carried out using an ACM research potentiostat (ACM instruments, Cumbria, UK) interfaced to a Powerlab 4/25 signal generator and ADC (AD Instruments, Oxfordshire, UK) controlled by Scope™ software. 
         [0165]    The electrode assembly was washed with a jet of Mili Q water before introduction into the cell. It was then electrochemically cleaned for 30 seconds using the procedure described in the following paragraph. 
         [0166]    For the cleaning procedure, the potential of the working electrode was scanned in a cathodic direction rapidly desorbing any contaminating organic material into the bulk solution using the following conditions:
       Range (−0.2 V to −2.625 V) @ 97V·s −1  
           The following UV curve was then recorded:   
           Range (−0.2V to −2V) @ 40V·s −1  
           (5 single consecutive sweeps recorded+a sweep recorded after repetitive cycling once the trace is visibly stable—after ≈2 seconds)   
               
 
         [0171]    Subsequently the above described cleaning procedure was operated continuously for 30 minutes. 
         [0172]    The following UV curve was then again recorded:
       Range (−0.2V to −2V) @ 40V·s- 1  
           (5 single consecutive sweeps recorded+a sweep recorded after repetitive cycling once the trace is visibly stable—after ≈2 seconds)   
               
 
         [0175]    The results are presented in  FIGS. 11(   a ) and  11 ( b ) which respectively show the curves obtained after the 30 seconds initial cleaning and after the 30 minutes cleaning. 
         [0176]    For the purposes of comparison,  FIG. 12  shows an IN curve obtained under the same conditions as those described above for a platinum electrode (more specifically an electrode assembly of the type produced in accordance with Example 3 above but without deposition of mercury).  FIG. 13  shows an IN curve obtained using the conditions described above for a Hanging Mercury Drop Electrode normalised by surface area. 
         [0177]    Referring firstly to  FIG. 13 , it will be seen that the IN curve for the HMDE displays a characteristic “water hump” (see left hand part of the curve illustrated in  FIG. 13 ). This “water hump” is not seen in the IN curve for platinum illustrated in  FIG. 12 . 
         [0178]    Referring now to  FIGS. 11(   a ) and  11 ( b ), it will firstly be noted that both curves display the characteristic “water hump” displayed by mercury (cf  FIG. 13) . Thus the electrode assembly produced in accordance with Example 3 displays the characteristics of a mercury electrode rather than a platinum electrode. Moreover a comparison of  FIGS. 11(   a ) and  11 ( b ) which are respectively before and after the 30 minute cleaning period demonstrates that the curves are identical indicating there is no significant loss in surface area and the surface character remains the same. 
         [0179]    The surface area of the electrode can be calculated accurately in accordance with Equation (1) from the capacitance current using the value of specific capacitance for mercury measured for the water hump at ˜−0.3V as ˜40 μF cm- 2  [27]). 
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         [0180]    For the Pe/Hg composite electrode used in the above experiments this yielded a value for surface area of the electrode as ˜0.744 mm 2 . The surface area of the platinum disc electrode (prior to mercury deposition) was ˜0.724 mm 2  measured using an optical microscope. The calculated value for the composite electrode (i.e. mercury deposited on the platinum) was slightly higher than that for the flat disc providing a reasonable result for a flattened hemisphere because it lies between the bounds of a perfectly flat film and a hemisphere. Values for surface area calculated in this fashion were used to produce the specific capacitance plots for composite electrodes coated with phospholipid monolayers in subsequent Examples. 
       Example 5 
       [0181]    This Example demonstrates the ability of an electrode assembly produced in accordance with the procedure of Example 3 to support phospholipid monolayers. 
         [0182]    The following 5 phospholipids were selected and varied only in the chemical functionality of their head group region:
   (a) 1,2-Dioleoyl-sn-Glycero-3-Phosphocholine (DOPC)   (b) 1,2 Dioleoyl-sn-Glycero-3-Phosphoethanolamine (DOPE)   (c) 1,2-Dioleoyl-sn-Glycero-3-[Phospho-rac-(1-glycerol)] (Sodium Salt) DOPG)   (d) 1,2-Dioleoyl-sn-Glycero-3-[Phospho-L-Serine] (Sodium Salt) (DOPS)   (e) 1,2-Dioleoyl-sn-Glycero-3-Phospho(Ethylene Glycol) (Sodium Salt) (DOPEG)   
 
         [0188]    The chemical structural formulae of the phospholipids (a)-(e) are shown in  FIG. 12 . 
         [0189]    Solutions of each lipid of weight concentration 2 mg·ml −1  dissolved in 1:4 chloroform:pentane were prepared separately yielding molar concentrations in the range 2.5 mmol·dm −3  to 3 mmol·dm −3 . The electrochemical cell was set-up as described in Example 4, containing 0.1 mol·dm- 3  KCl phosphate buffered at a concentration of 0.01 mol·dm 3  to pH 7.4 and deaerated with argon for 30 minutes prior to introduction of the working electrode. 12.5 μl of each lipid solution was added by syringe to the electrolyte and the working electrode was then lowered through the solution/argon interface. The electrode was cleaned for 30 seconds using the in-situ electrochemical cleaning method described in Example 4 before lifting the electrode through the solution interface briefly and re-submerging it to form an evenly coated layer. 
         [0190]    No discernible differences were observed in repetitive scans at 40 V·s −1  over the potential range −0.2 V to −1.2 V for the electrode coated at open circuit, with the potential held at a constant −0.2 V or while repetitively cycled over the above range (data not shown). 
         [0191]    Cathodic scans of cyclic voltammograms recorded at 40 V s −1  were converted to specific capacitance plots for the phospholipids (a) DOPC, (b) DOPE, (c) DOPG, (d) DOPS and (e) DOPEG. The results are plotted in  FIGS. 15(   a )-( e ) respectively for which the thick line is the specific capacitance plot and the thin line is for an electrochemically cleaned electrode assembly of the type produced in Example 3 (i.e. no deposited phospholipid). 
         [0192]    It will be seen from the data presented in  FIG. 15  that the phospholipids act as variable dielectrics over the potential range between the potential of zero charge (p.z.c.) for mercury and the layer&#39;s desorption potential. The phospholipids impart selective chemical functionality to the surface and greatly affect the surface potential and capacitance profile producing unique finger print peaks relating to complex phase transitions of the absorbed layers. 
       Example 6 
       [0193]    This Example demonstrates the stability of the phospholipid monolayers. 
         [0194]    Using the procedure of Example 5, the mercury surface of electrode assemblies produced in accordance with Example 4 were separately coated with the phospholipids DOPC, DOPS and DOPG. 
         [0195]    For the DOPC coated electrode, a cyclic voltammogram was recorded at 40 V·s −1  over the potential range −0.2 V to −1.2 V. The electrode was then electrochemically cleaned in-situ for 30 seconds using the cleaning technique described in Example 4 and the procedure repeated six times. The voltammograms were converted to specific capacitance plots and the average trace plotted. The results are shown in  FIG. 16(   a ) in which the narrow error bars (error bars=±1 SD) give a good indication as to the high level of measurement reproducibility between coatings. Thus the phospholipid monolayer deposition exhibited a high degree of similarity between experiments. 
         [0196]    For the DOPS coated electrode, an initial scan was recorded at 40 V·s −1  of the potential range −0.2V to −1.2V. The scan was repeated and recorded every 1 minute time interval for 30 consecutive minutes. The cyclic voltammograms were converted to specific capacitance plots and the average trace plotted. The results are shown in  FIG. 16(   b ) in which the narrow error bars (±1 SD) indicate that the capacitance minimum and current peak remain stable over the course of the experiment thus demonstrating the monolayer integrity similarly remains stable and reproducible. 
         [0197]    To test stability and reproducibility in the case of continually cycling the potential between measurements, the DOPG coated electrode was continuously cycled at 40 V·s −1  over the potential range −0.2 V to −1.2 V resulting in interrogation≈15 times per second, the cycle lasting 50 ms for the ramp+40 ms software forced delay. The scans were recorded initially and after 5, 15 and 25 minutes. The cyclic voltammograms were converted to specific capacitance plots and the traces overlaid to see any significant changes in the capacitance of the monolayer. The results are shown in  FIG. 16(   c ) from which it can be seen that the traces overlay almost exactly indicating that monolayer integrity is sufficiently stable over the time period measured. 
       Example 7 
       [0198]    This Example demonstrates use of a prototype flow cell  400  of the type illustrated in  FIG. 10  which was constructed to establish “proof-of-principle” for use of an electrode assembly in accordance with the invention in a flow cell. The flow cell had an overall length of 5 cm, a height of 3 cm (each of lower and upper portions  401  and  402  having a height of 1.5 cm) and a width of 2 cm. Injection port  406  as well as entry and exit channels  404  and  405  were of 4 mm diameter. Bores  407  and  408  had a diameter of 2 mm and were angled at 45 degrees to exit channel  405 . The electrode assembly  300  was produced in accordance with the procedure of Example 3 but passing 2 C of charge (rather than 1 C). Electrode assembly  300  had a length of 1 cm, a width of 5 mm and a depth of 0.5 mm. 
         [0199]    A Ag/AgCl microelectrode was provided in bore  407  and platinum counter electrode in bore  408 . 
         [0200]    Contact pads  304  and  305  on the electrode assembly were individually connected to a potentionstat and could be addressed individually by means of a two way switch. 
         [0201]    Lipid vesicle deposition dispersions of DOPC and DOPS were prepared by dissolving 25 mg of powdered pure phospholipid in 50:50 chloroform/methanol and rotary evaporating in a glass round bottomed flask at 25° C. under light vacuum until dry. The residue was then re-suspended in 12.5 ml of phosphate buffered saline (0.1 mol·dm −3  KCl, pH 7.4) to produce a 2 mg·ml −1  dispersion which was then tip sonicated for ≈20 minutes to produce vesicles. 
         [0202]    The lipid layers were deposited by injecting ˜100 μL of 2 mg·mL −1  DOPC or DOPS vesicle dispersions into the injection port  406  upstream of the electrodes while electrolyte (0.1 m KCl phosphate buffered (10 mM) to pH 7.4) composition, flow rate?) was passed into and along inlet channel  404 , through measurement cell  403  and out through channel  405  at a rate of 5 ml per minute. 
         [0203]    It was found that the DOPC layers could be deposited using the same conditions as adopted for the cleaning procedure described in Example 4 but applied for ca 2 seconds. The potential cycling was stopped by opening the circuit when over-covered layer thinned sufficiently to exhibit the sharp phase transition peaks shown in  FIG. 17  which is rapid cyclic voltammogram at 36 V s −1  of the electrode coated with DOPC (thick line) measured with the experimental flow cell. For the purposes of comparison,  FIG. 17  also incorporates the corresponding trace for the Pt/Hg electrode (thin line) in the absence of lipid. 
         [0204]    The DOPS layers were deposited under different potential conditions from DOPC due to the DOPS layers spreading rapidly at potentials&lt;−1.4 V. Initial trials suggest that DOPS can be deposited over a lower potential range sweep with a cathodic apex of −1.1 V. The DOPS layers were allowed slowly to build with time and successive additions to produce an IN curve (thick line) as seen in  FIG. 18  which is a rapid cyclic voltammogram (thick line) at 38 V s −1  of the electrode coated with DOPS measured within the experimental flow cell. For the purposes of comparison,  FIG. 18  also incorporates the trace (thin line) for the rapid cyclic voltammogram at 36 V s −1  of the Pt/Hg electrode without lipid. 
         [0205]    Both lipid layers (DOPC and DOPS) on the electrodes proved to be stable over a period of &gt;10 minutes with electrolyte flowing at ˜4.5 mL·min −1  and each could be deposited with a reasonable degree of reproducibility after cleaning the electrode in-situ and repeating the procedure. 
         [0206]    A feature of the prototype flow cell was the instability of the micro-reference electrode potential which was measured as +260 mV when compared with the Ag/AgCl (3.5 mol·dm −3 ) reference electrode used in the static cell. Thus all potentials quoted from data within the flow cell are vs. a drifting Ag/AgCl reference. The exact drift can be evaluated by comparing the positions of the first or second phase transition peaks of DOPC which occur at defined potentials on the Hg surface. 
         [0207]    From  FIGS. 17 and 18  a clear slant to the traces can be observed as well as broadening of the current peaks. This can be attributed to three contributing factors. Firstly the cell was found not to be water tight due to an imperfect seal with the O-ring. (Any small electrolyte leaks can produce interfering faradaic currents when it comes into contact with the crocodile clips and contact pads.) Secondly, the electrolyte contains a larger quantity of dissolved oxygen than the static cell which is more efficient at de-aeration. Thirdly, the distance of the reference electrode from the working electrodes is slightly greater and there is a higher solution resistance in the flow cell due to the smaller reference electrode fritt that may become more easily blocked by phospholipid flows, the greater solution resistance influences the potential applied to the working electrodes through the phenomena of Ohmic drop. 
         [0208]    In spite of the “deficiencies” of the prototype flow cell, this Example clearly demonstrates a number of significant points. Firstly, the mercury layers are stable in the electrolyte flow. Secondly the phospholipids can be deposited on to the mercury layers and (when interrogated by cyclic voltammetry) give peaks corresponding with those obtained in a static cell, allowing for the “deficiencies” of the prototype flow cell. Thirdly the lipid layers are stable in the electrolyte flow. Fourthly the lipid layers can conveniently be deposited from the electrolyte flow by applying the appropriate potential to the electrode assemblies  301  and  302  of the electrode assembly  300 . 
       Example 8 
       [0209]    This Example demonstrates similarity in properties of a Hanging Mercury Drop Electrode (HMDE) and Pt/Hg electrode as produced by the procedure of Example 3. 
         [0210]    The HMDE was based on using a capillary with a diameter of 0.1 mm so as to provide a surface area for the mercury drop about the same as the surface area of the mercury in the electrode in accordance with the invention. 
         [0211]    The HMDE and the electrode of the invention were compared side-by-side in a static cell configuration. All experiments were carried out in 0.1 M potassium chloride solution. Measurements were taken at 75 Hz between −0.4 to −1.15 V with 4.95 mV rms at a scan rate of 5 mV s −1 . 
         [0212]    Capacitance—potential scans for the HMDE and the composite Pt/Hg electrode are shown in  FIGS. 19(   a ) and ( b ) respectively. Although there is a slight difference in the capacitance values shown in  FIGS. 19(   a ) and ( b ), the shape of the plots indicates close similarity in the properties of the two electrodes. 
       Example 9 
       [0213]    This Example compares a HMDE electrode with an electrode as produced in accordance with Example 3, both coated with a monolayer film of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), new layers of which were deposited between consecutive scans. Experiments were performed in 0.1M Kcl. A negative going ‘forward’ potential scan was performed between −0.4 V and −1.15 V, and then followed immediately by a positive going ‘reverse’ scan from −1.15 V to −0.4 V with 4.95 mV rms at a scan rate of 5 mVs −1 . The results for the HMDE and the electrode of the invention are shown in  FIGS. 20(   a ) and ( b ) respectively. 
         [0214]    As with the scans of the “bare” electrodes in the electrolyte solution (results shown in  FIG. 19 ), both electrodes show comparable results. The ‘reverse’ potential scans show that the electrode of the invention is slightly more stable than the HMDE electrode. 
       Example 10 
       [0215]    The procedure of Example 9 was repeated but with the incorporation in the electrolyte of chlorpromazine at a concentration of 0.5 μmol dm −3 . The structure of chlorpromazine is as shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0216]    The results for HMDE and the electrode of the invention are represented by the dark lines in  FIGS. 21(   a ) and ( b ) respectively. For the purposes of comparison, these Figures also incorporate the results obtained in Example 9. 
         [0217]    Once again, both electrodes show very similar capacitance—potential scans. 
       Example 11 
       [0218]    The procedure of Example 9 was repeated but with the incorporation in the electrolyte of promethazine at a concentration of 0.5 μmol dm −3 . The structure of promethazine is shown below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0219]    The results for HMDE and the electrode of the invention are represented by the dark lines in  FIGS. 22(   a ) and  22 ( b ) respectively. For the purposes of comparison, these Figures also incorporate the results obtained in Example 9. 
         [0220]    In this case, both electrodes show response to the test compound but with the electrode of the invention displaying the stronger response (greater depression of the peaks) than the HMDE. 
       Example 12 
       [0221]    The procedure of Example 9 was repeated but with the incorporation in the electrolyte of H16 at a concentration of 0.5 μmol dm −3 . The structure of H16 is shown below. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0222]    The results for HMDE and the electrode of the invention are represented by the dark lines in  FIGS. 23(   a ) and  23 ( b ) respectively. For the purposes of comparison, these Figures also incorporate the results obtained in Example 9. 
         [0223]    In this case, the electrode of the invention shows a far greater response than that seen with the HMDE, thus indicating greater sensitivity of the former than the latter. 
       Example 13 
       [0224]    Example 12 was repeated but using 5 μmol dm −3  of H16. 
         [0225]    The results for HMDE and the electrode of the invention are represented by the dark lines in  FIGS. 24(   a ) and ( b ) respectively. For the purposes of comparison, these Figures also incorporate the results obtained in Example 9. 
         [0226]      FIGS. 24(   a ) and ( b ) demonstrate a large response to the test compound that is easily visible on both electrodes, with the electrode of the invention still showing the slightly larger response. 
       Example 14 
       [0227]    Following the procedure of Example 9, Pt/Hg composite electrodes produced in accordance with the procedure of Example 3 and provided individually with monolayers of DOPC, DOPE and DOPS were evaluated. 
         [0228]    The results are shown in  FIGS. 25(   a )-( c ) which show the results for DOPC, DOPE and DOPS respectively (note the difference in vertical scale for DOPS compared to the other two lipids shown). 
         [0229]    The results show that monolayers of all three types of phospholipid can be formed on the electrodes and that each produces a different characteristic trace in the AC Voltammetry experiments. 
       Example 15 
       [0230]    The procedure of Example 14 was repeated but incorporating 0.5 μmol dm −3  chlorpromazine in the electrolyte solution. 
         [0231]    The results were illustrated by the dark lines in  FIGS. 26(   a )-( c ) which show the results for DOPC, DOPE and DOPS respectively. For the purposes of comparison,  FIGS. 26(   a )-( c ) also incorporate the results shown in  FIG. 25 . 
         [0232]      FIG. 26  clearly demonstrates interaction of the chlorpromazine with the lipid monolayers (cf the superimposed results from  FIG. 25  for the “non-interacting” monolayers). 
       Example 16 
       [0233]    The procedure of Example 10 was repeated for electrodes produced in accordance with Example 3 and incorporating a DOPC monolayer to evaluate the system for the following compounds all provided in the electrolyte at a concentration of 0.5 μmol dm −3 , save for (0 limonene which was used at a concentration of 5 μmol dm −3 :
       (a) Chlorpromazine   (b) Promethazine   (c) H16   (d) MB 327   (e) Fluoranthene   (f) Limonene       
 
         [0240]    The results are represented by the dark lines shown in  FIGS. 27(   a )-( f ) respectively which also incorporate the structural formulae of the compounds tested and the results obtained for the DOPC coated electrode without test compound in the electrolyte. 
         [0241]    The results show that all six of the compounds are detectable by the system. Also, that it is possible to differentiate between them at comparable concentrations. 
       Example 17 
       [0242]    The procedure of Example 10 was repeated using chlorpromazine concentrations of:
       (a) 0.05 μmol dm −3      (b) 0.1 μmol dm −3      (c) 0.2 μmol dm −3      (d) 0.5 μmol dm −3          
 
         [0247]    The results are represented by the dark lines shown in  FIGS. 28(   a )-( d ) respectively which also incorporate results obtained for the DOPC electrode without chlorpromazine incorporated in the electrolyte. 
         [0248]    As can be seen from  FIGS. 28(   a )-( d ) there was a response from the system, even at the lowest chlorpromazine concentration tested, i.e. 0.05 μmol dm −3 . 
         [0249]    Using the formula for ppb of: 
         [0000]    
       
         
           
             
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         [0000]    and assuming a density for the solution used as 1 g per ml, the sensitivity of 0.05 μmol dm  −3  converts to approximately 18 ppb. 
       REFERENCES 
       [0000]    
       
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