Chip design

Abstract

A method of producing a biosensor chip comprises; applying a working electrode, counter electrode and a reference electrode to a substrate 3161; the working electrode is applied to the substrate by applying an adhesive layer 3151 to the substrate, then a conductive layer 3121 to the adhesive layer, then immobilising aptamers on the conductive layer to form an aptamer layer 3141, wherein the conductive layer is applied by sputtering. Another chip comprises a conductive layer with specified grain size, surface roughness or purity. Another method of producing a chip comprises a working electrode comprising a conductive layer which is applied by electrodeposition onto a PCB substrate, and an aptamer layer thereupon.

Description

Field of the Invention The present invention provides chips for a biosensor and methods for producing such chips, in particular to chips which comprise electrodes which are sputtered or electroplated. Background to the Invention Biosensors are analytical devices which are capable of providing a quantitative or semi-quantitative signal by the detection of a biological or chemical analyte. Biosensors typically comprise: a recognition element, such as a biochemical receptor, which selectively binds to the target analyte; a physicochemical transducer on which the biorecognition element may be immobilized, that converts the binding of an analyte into a quantifiable signal; and a substrate which supports the recognition element and transducer. In brief, a biosensor functions based on the biorecognition event of the immobilized bioreceptor with the target analyte, which typically results in some form of physicochemical alteration within the biorecognition layer. A transducer can then convert this into a signal proportional to the concentration of the analyte in the sample. In certain biosensors, the biochemical receptors are aptamers. Aptamers are oligonucleotides or peptide molecules which are able to bind to specific target molecules, depending on their sequence. In the absence of a target molecule ( / .e. in its native state), an aptamer will tend to adopt a particular secondary and / or tertiary structure, which may be complementary to the target molecule. Upon binding to the target molecule, an aptamer may exhibit a conformational change which alters its structure. Aptamers may be conjugated to signalling molecules in order to provide a signal which can be modulated by this conformational change. Electrochemical biosensors are a type of biosensor which transduce the detection of an analyte into a current and / or a voltage. In such biosensors, the recognition element may be an aptamer which is conjugated to a redox reporter, positioned on an electrode. Redox reporters, such as methylene blue (M B), are molecules which are able to transfer electrons between themselves and a conductive electrode, generating a faradaic current in the electrode. Typically, the distance between the redox reporter and the electrode surface will influence the speed of electron transfer, and therefore influence the faradaic current which can be measured (for example by a potentiostat). The general process underlying the operation of the electrodes in these electrochemical biosensors is shown in Figure 1 (which is taken from WO 2020 / 104933 A1). Specifically, the electrode has a conductive surface (3121), on which is immobilised an aptamer (3122) conjugated to a redox reporter (3123). When the target molecule (3124) binds to the aptamer (3122), the conformational change this induces causes the proximity of the redox reporter (3123) to the conductive surface (3121) to be altered. As a result, the rate of electron transfer between the redox reporter (3123) and the conductive surface (3121) is modified, resulting in a change in the faradaic current. In Figure 1, the redox reporter (3123) is brought closer to the conductive surface (3121), which would increase the speed of electron transfer and therefore increase the measurable faradaic current, which can be transduced into a signal. One way to provide an electrochemical biosensor comprising an aptamer layer, as described in WO 2020 / 104933 A1, is to use printed circuit board (PCB) substrates onto which a gold layer can be deposited using electroless nickel immersion gold techniques. However, prior art electrodes utilising aptamer layers can suffer from low sensitivity, as well as undesirable levels of variation in electrode sensitivity between sensors. Accordingly, there remains a need for improved electrode-bearing chips for application in biosensors. Summary of the Invention The present invention is based on discoveries which can enhance the sensitivity and reliability of chips for applications in biosensors. In particular, it has surprisingly been found that working electrodes which comprise aptamers immobilised onto a conductive layer exhibit greater sensitivity and reliability when the conductive layer is formed by sputtering. It has also been found that PCB electrodes with an electroplated conductive layer perform better and have greater sensitivity than PCB electrodes made using other techniques, such as electroless nickel immersion gold (ENIG) plating. Thus, in one aspect, the present invention provides a method for producing a chip for a biosensor, the method comprising applying a working electrode, counter electrode and a reference electrode to a substrate. The working electrode is applied to the substrate by applying an adhesive layer to a substrate; applying a conductive layer to the adhesive layer by sputtering; and immobilising aptamers on the conductive layer to form an aptamer layer. In another aspect the present invention provides a chip for a biosensor, the chip comprising a substrate on which a working electrode, a counter electrode, and a reference electrode are disposed. The working electrode is a sputtered electrode and comprises a conductive layer; an adhesive layer disposed between the substrate and the conductive layer; and an aptamer layer in contact with the conductive layer. The aptamer layer comprising aptamers immobilised onto the conductive layer. Also provided is a chip for a biosensor, the chip comprising a substrate on which a working electrode, a counter electrode, and a reference electrode are disposed. The working electrode comprises a conductive layer; an adhesive layer disposed between the substrate and the conductive layer; and an aptamer layer in contact with the conductive layer. The aptamer layer comprising aptamers immobilised onto the conductive layer. The conductive layer has: a grain size in the range of from 5 nm to 60 nm, preferably from 10 nm to 50 nm, and more preferably from 20 nm to 40 nm; a roughness RMS value in the range of from 0.1 nm to 10 nm, preferably from 0.2 nm to 5 nm, and more preferably from 0.3 nm to 2 nm; and / or a purity of up to 99.9%, preferably up to 99.95%, and more preferably up to 99.99%. In a further aspect, the present invention provides a method for producing a chip for a biosensor, the method comprising applying a working electrode, counter electrode and a reference electrode to a PCB substrate. The working electrode is applied to the PCB substrate by applying a conductive layer to the PCB substrate by electrodeposition; and immobilising aptamers on the conductive layer to form an aptamer layer. In yet another aspect, the present invention provides a chip for a biosensor, the chip comprising a PCB substrate on which a working electrode, a counter electrode, and a reference electrode are disposed. The working electrode is an electroplated electrode and comprises a conductive layer; and an aptamer layer in contact with the conductive layer, the aptamer layer comprising aptamers immobilised onto the conductive layer. In a yet further aspect, the present invention provides a cartridge for a biosensor comprising a chip of the present invention. Also provided is a biosensor comprising said cartridge or a chip of the present invention, wherein the biosensor is for detecting the presence, and preferably the concentration, of an analyte in a biological fluid, such as blood. Description of the Figures Figure 1 is a schematic representation taken from WO 2020 / 104933 A1 showing the inducement of a conformational change upon aptamer binding to a target, the aptamer comprising a redox reporter. Figure 2 is a cross-sectional view of a working electrode showing a substrate, an adhesive layer, a conductive layer and an aptamer layer. Figure 3 is a chart comparing the sensitivity of electrodes obtained by sputtering and electroless nickel immersion gold (ENIG) PCB methods. Figure 4 is a chart comparing the sensitivity of electrodes obtained by electroplating and electroless nickel immersion gold (ENIG) PCB methods. Figure 5 is a graph comparing the relative standard deviations of electrodes obtained by sputtering, electroplated PCB and electroless nickel immersion gold (ENIG) PCB methods. Figure 6 is a graph comparing the blank signals of MB (methylene blue) conjugatedaptamer immobilized electrodes obtained by sputtering, electroplated PCB and electroless nickel immersion gold (ENIG) PCB methods. Figure 7 is a top perspective view of a chip design containing working electrodes which radially surround a reference electrode. Figure 8 is a top perspective view of a chip design containing three working electrodes arranged in a row, with reference and counter electrodes positioned above the row. Figure 9 is an exploded view of a cartridge for a biosensor, comprising a chip as described herein. Detailed Description The present invention relates to chips and cartridges for biosensors, as well as to methods for producing such chips. A chip, as used herein, refers to a structure designed to receive a sample and be able to provide a signal, such as an electric current, in order to indicate the presence of a target analyte. As described above, a biosensor is an analytical device which is capable of providing a quantitative or semi-quantitative signal by the detection of a biological or chemical substance. General chip structure The chips of the present invention comprise a substrate on which a working electrode, a counter electrode and a reference electrode are disposed. A working electrode refers to an electrode which is able to generate measurable signals in response to the presence of a target analyte, as will be discussed below. A reference electrode provides a fixed, known redox potential against which the redox potential (and any changes therein) of the working electrode can be measured. It will be appreciated that any suitable reference electrode composition may be used, provided that it maintains a substantially constant potential throughout when exposed to the samples in question. For instance, a silver chloride (Ag / AgCI) electrode may be suitable. The counter electrode acts as the corresponding cathode or anode to the working electrode, in order to complete the electrical circuit of which the working electrode forms part. Any suitable counter electrode composition may be used, such as a Pt electrode. However, it is preferred that at least one, and preferably both, of the counter electrode and the reference electrode comprises the same conductive and adhesive layer as the working electrodes (discussed below), but without an aptamer layer immobilised onto the conductive layer. Accordingly, it is particularly preferred that at least one, and preferably both, of the counter electrode and the reference electrode comprises a conductive layer comprising Au, and an adhesive layer comprising Cr disposed between the conductive layer and the substrate. A gold reference electrode provides a reference potential which is relatively stable, if not as a stable as silver chloride, such that any slight potential shift arising from the use of gold electrode is trivial in determining the sensor output response. Sputtered electrode The working electrode is a sputtered electrode, i.e. at least the conductive layer and preferably the adhesive layer are provided onto the substrate using sputtering. The reference electrode and / or counter electrode of the chip may also be sputtered electrodes. Preferably, all electrodes present on the chip are sputtered. The present invention includes methods of producing chips for a biosensor. In one aspect, these chips have a sputtered electrode. As used herein, a sputtered electrode refers to an electrode whereby at least the conductive layer is applied by sputtering. Sputtering is a type of physical vapor deposition which may be used to form films on substrates, in which a plasma of high energy ions or particles cause ejection of atoms from the surface layer of a solid target. Generally, the process involves ejecting the atoms from a target (which contains the material desired to be deposited) by bombarding the target with these energetic ions. The ejected atoms are released from the target in the vapor phase, and are deposited onto a substrate placed in front of the target upon contacting it. This results in the formation of a layer of target atoms on the substrate. A sputtering gas of an inert material, typically argon, is often used as the source of these energetic ions; however it will be appreciated that the specific gas used will depend on the atomic weight of the target atoms, since a similar atomic weight is preferred for effective momentum transfer. The argon will be ionised by application of a strong electrical field applied between the target and the grounded chamber in which the reaction is occurring, resulting in the formation of a plasma. Sputtering can be carried out using magnetron sputtering, radio frequency sputtering, ion-assisted deposition, or pulsed-laser deposition, any of which may be suitably employed for the purposes of the methods and chips of the present invention. Particularly suitable is DC magnetron sputtering. The process of sputtering may also involve application of a mask layer over the substrate onto which the target material is intended to be sputtered. Such mask layers leave regions of the substrate surface exposed, through which the exposed surface can be sputtered. The masks are removed from the substrate once sputtering is completed. This allows for sputtering of conductive and adhesive layers on selective areas of the substrate surface. Thus, electrode designs may be efficiently provided on the substrate surface, avoiding complete coverage of the substrate. It is desirable that the substrate on which the working, counter and reference electrodes is disposed is a good insulator, i.e. it does not conduct electrical current, or that it comprises an insulating layer. It will be appreciated, however, that this is primarily the case for electrochemical biosensors - other types of biosensor which rely less on transmission of electrical current will be less concerned with the conductivity of the substrate. The substrate is preferably a hydrophobic material, or at least has a hydrophobic surface. This prevents the spreading of biochemical reagents (e.g. aptamer solution) onto the substrate surface. Such spreading can cause non-uniform concentrations of aptamer when being immobilised onto the conductive layer. Compatibility of the substrate with sputtering processes are also important. Specifically, they should be sufficiently mechanically stable to withstand any mechanical stresses that may occur during the sputtering process. Furthermore, the substrate should be chemically inert to both the sputtered material and the process gases used as part of the sputtering process. Another desirable feature of the substrate is the smoothness of its surface, as irregularities in this surface may ultimately confer a similarly uneven surface to the electrode. As such, the substrate may have an Ra (Roughness Average) value of measured peaks and valleys of at least 0.01 nm, preferably at least 0.025 nm, and more preferably 0.05 nm. The Ra value may be up to 0.5 nm, preferably up to 0.4 nm, and more preferably up to 0.3 nm. Thus, the Ra value may be in the range of from 0.01 nm to 0.5 nm, preferably from 0.025 nm to 0.4 nm, and more preferably from 0.05 nm to 0.3 nm. Ra may be measured according to the standards set out in ASME B46.1. Although some substrates naturally have a smooth surface, others may be polished to achieve a smoother surface, for instance by using chemical mechanical polishing (CMP). The substrate may be selected from sapphire, glass, ceramic, SiO2 or plastic substrates. Preferably, the substrate is selected from sapphire, SiO2 or plastic substrates. Suitable plastic substrates may comprise polyvinyl chlorides, polycarbonates, polyimides, polystyrenes, polypropylenes or polyethylenes, or any combinations thereof. It is particularly preferred that the substrate is a sapphire substrate. As used herein, sapphire refers to the specific crystalline form of aluminium oxide (a-AhOs), but it will be appreciated that similar crystal forms of aluminium oxide may equally be used. Sapphire substrates are able to provide an appropriately smooth surface, whilst also being durable enough to withstand the manufacturing processes (described below), such as drilling and the sputtering of adhesive and conductive layers. The surface of the substrate onto which the electrodes are provided may have an area of up to 400 mm2, preferably up to 300 mm2, and more preferably up to 250 mm2. The surface of the substrate may have an area of at least 25 mm2, preferably at least 50 mm2, and more preferably at least 100 mm2. Thus, the surface of the substrate may have an area in the range of from 25 mm2 to 400 mm2, preferably from 50 mm2 to 300 mm2, and more preferably from 100 mm2 to 250 mm2. For instance, in some embodiments the area will be about 100 mm2,121 mm2, 144 mm2, 169 mm2, or 225 mm2. For instance, this surface of the substrate may take the form of a square, preferably with dimensions of 10mm x 10mm, or 13mm x 13mm, although other substrate geometries (such as substantially rectangular or circular surfaces) may equally be used. The substrate may have a thickness of up to 1 mm, preferably up to 750 pm, and more preferably up to 650 pm. The substrate may have a thickness of at least 150 pm, preferably at least 300 pm, and more preferably at least 500 pm. Thus, the substrate may have a thickness in the range of from 150 pm to 1 mm, preferably from 300 pm to 750 pm, and more preferably from 500 pm to 650 pm. It will be appreciated that the thicknesses of layers referred to herein represent the minimum and maximum thicknesses observed in these layers, i.e. a thickness of from Ato B means that the minimum thickness of the layer is at least A and the maximum thickness of the layer is up to B. The thickness of the substrate may be measured using a digital micrometer, while the thickness of the adhesive and conductive layers may be measured more effectively using scanning electron microscopy. Specifically, scanning electron microscopy may be used to generate images of these layers, with the thickness preferably determined using a computer program. As the size of the chip is primarily determined by the substrate, the dimensions of the chip will follow those of the substrate. However, it will be appreciated that the design of the chip according to the present invention places no significant limitation on the surface area or thickness of the substrate used. Sputtered electrodes comprise an adhesive layer disposed between the substrate and a conductive layer. Thus, methods in which a sputtered electrode is applied to a substrate involve a step of applying a conductive layer to an adhesive layer. Accordingly, chips for biosensors comprise a conductive layer which is provided adjacent to, such as on top of, the adhesive layer by sputtering. As used herein, a conductive layer refers to a layer which comprises material able to conduct electrical current, and which is responsive to changes in electric fields. Although the adhesive layer may also be made of a metal (as discussed below) which might exhibit some degree of conductivity, it will be appreciated that the conductive layer and the adhesive layer are provided as discrete layers, for instance with each layer having a different composition. The conductive layer will typically be in direct contact with the adhesive layer. Alternatively, other layers may be disposed between the adhesive layer and the conductive layer. The conductive layer may comprise, or consist of, a metal. In particular, the metal may be selected from Au, Ag, Pt, Cu, and Ni, or any combinations thereof. It is particularly preferred that the conductive layer comprises Au, due to its favourable conductive properties, biocompatibility, non-corrodibility, its passivity in various operating environments and the number of chemical reactions available for immobilising aptamers onto Au particles. In one particularly preferred embodiment, the conductive layer is made of Au. The conductive layer is sputtered onto the adhesive layer which is disposed on the substrate. When sputtering the conductive layer, the deposition time, i.e. the time during which the conductive layer is actively being sputtered onto the substrate, may be up to 90 minutes, preferably up to 60 minutes and more preferably up to 30 minutes. The deposition time may be at least 10 minutes, preferably at least 12.5 minutes, and more preferably at least 15 minutes. Thus, the deposition time may be from 10 to 90 minutes, preferably from 12.5 to 60 minutes, and more preferably from 15 to 30 minutes. Slower deposition, and therefore longer deposition times, may be used as this can result in a better quality conductive layer. The conductive layer may have a thickness of up to 200 nm, preferably up to 170 nm and more preferably up to 150 nm. The conductive layer may have a thickness of at least 50 nm, preferably at least 70 nm and more preferably at least 80 nm. Thus, the conductive layer may have a thickness in the range of from 50 nm to 200 nm, preferably from 70 nm to 170 nm, and more preferably from 80 to 150 nm. However, it will be appreciated that there is no functional limitation on the thickness of the conductive layer for the electrode to be effective - it is primarily the cost and scalability of the resulting chips which renders this a concern. When the conductive layer is in direct contact with the adhesive layer, the materials of the adhesive and conductive layers may be interdiffused to some extent, i.e. a sublayer may be formed at the interface of the adhesive and conductive layers which comprises the materials of both layers. For example, in an embodiment where the conductive layer comprises Au and is disposed on an adhesive layer which comprises Cr, an Au-Cr alloy may be formed at their interface. In such embodiments, the formation of this alloy indicates superior electrical and physical contact between the two layers. Sputtering the conductive layer can impart it with particular properties distinguishable from conductive layers formed by other methods, such as electroplating or electroless nickel gold immersion. These properties include: roughness, grain size and purity. Roughness of the conductive layer refers to the uniformity of the surface texture, and can be defined and measured according to the standards set out in ISO 21920-2:2021. Sputtered conductive layers, such as those comprising Au, of sputtered working electrodes according to the present invention may have an RMS (Root Mean Square of the valleys and peaks of a surface) value of up to 10 nm, preferably up to 5 nm, and more preferably up to 2 nm. The RMS value may be at least 0.1 nm, preferably at least 0.2 nm, and more preferably at least 0.3 nm. Thus, the RMS value may be in the range of from 0.1 nm to 10 nm, preferably from 0.2 nm to 5 nm, and more preferably from 0.3 nm to 2 nm. In contrast, the RMS value of a conductive layer formed by electroless nickel immersion gold processes typically falls in the range of 100-200 nm. Roughness may be measured using atomic force microscopy (AFM) and software for image analysis and profile extraction (such as Gwyddion software). The grain size of the conductive layer refers to the mean diameter of a grain of the conductive material. Conductive layers, such as those comprising Au, of sputtered working electrodes according to the present invention may have a grain size of up to 60 nm, preferably up to 50 nm, and more preferably up to 40 nm. The grain size may be at least 5 nm, preferably at least 10 nm, and more preferably at least 20 nm. Thus, the grain size may be in the range of from 5 nm to 60 nm, preferably from 10 nm to 50 nm, and more preferably from 20 nm to 40 nm. In contrast, the grain size of a conductive layer formed by electroless nickel immersion gold processes typically falls in the range of 1-2 pm. Grain size may be measured according to the standards set out in ASTM E112-13, for instance following the planimetric procedure. The grain height of the conductive layer refers to the mean distance between the lowest vertical point and highest vertical point of the grains of the conductive material. Conductive layers, such as those comprising Au, of sputtered working electrodes according to the present invention may have a grain height of up to 15 nm, preferably up to 12 nm, and preferably up to 10 nm. The grain height may be at least 2 nm, preferably at least 4 nm, and more preferably at least 5 nm. Thus, the grain height may be in the range of from 2 nm to 15 nm, preferably from 4 nm to 12 nm, and more preferably from 5 nm to 10 nm. Grain height may be determined using scanning tunnelling microscopy. The conductive layer is preferably highly pure, i.e. it does not contain significant amounts of other materials. For instance, the conductive layer, such as those comprising Au, of sputtered working electrodes according to the present invention may have a purity of up to 99.9%, preferably up to 99.95%, and more preferably up to 99.99%. As mentioned above, sputtered electrodes comprise an adhesive layer disposed between the substrate and a conductive layer. Thus, methods in which a sputtered electrode is applied to a substrate involve a step of applying an adhesive layer to a substrate. It will be appreciated that the term ‘adhesive layer’ does not denote a junction at which two materials are directly bonded together (e.g. by a bonding technique like plasma bonding), but rather requires an adhesive material to be present so that the two materials are indirectly bonded together and separated by the adhesive layer. Many metals used as conductive layers for electrodes (such as Au, Ag and Pt) can adhere poorly when provided directly onto a substrate, potentially leading to peeling, delamination or deterioration of the electrode. A particular problem which may arise is that the conductive layer may be scraped off or otherwise removed when soldering electrical connections, such as contact points, to the electrode. An adhesive layer serves to promote adhesion of the conductive layer to the substrate, improving the integrity of the working electrode. This is beneficial for providing more stable contact between the conductive layer and the electrical connection. The adhesive layer will typically be in direct contact with the substrate. Though less preferred, other layers may be present between the substrate and the adhesive layer, such as further conductive layers, further adhesive layers or combinations thereof. The adhesive layer may comprise, or consist of, a metal. This metal is preferably selected from Cr, Ti, Ni, Ta, W or Nb, or any combinations thereof. It is particularly preferred that the adhesive layer comprises, or consists of, Cr. This is because it has been found that Cr is a particularly effective adhesive layer which is able to provide a stable conductive layer for the working electrode, especially when used with Au (which is the preferred material for the conductive layer, as discussed below). The adhesive layer may have a thickness of up to 50 nm, preferably up to 30 nm and more preferably up to 20 nm. The adhesive layer may have a thickness of at least 3 nm, preferably at least 5 nm and more preferably at least 8 nm. Thus, the adhesive layer may have a thickness in the range of from 3 nm to 50 nm, preferably from 5 nm to 30 nm, and more preferably from 8 to 20 nm. Adhesive layers of this thickness are able to promote excellent adherence of the conductive layer to the substrate, without interfering with the electrical properties of the electrode. Thickness of the adhesive layer may be measured using the techniques as described above. It is preferred that the adhesive layer is applied to the substrate by sputtering, of the kinds as previously described. This imparts properties to the adhesive layer, such as surface roughness and hardness values, which can distinguish it from other application methods. For instance, the roughness (as defined above) of the sputtered adhesive layer, such as those comprising Cr, of sputtered electrodes according to the present invention may have an RMS value of up to 10 nm, preferably up to 5 nm, and more preferably up to 2 nm. The RMS value may be at least 0.1 nm, preferably at least 0.2 nm, and more preferably at least 0.3 nm. Thus, the RMS value may be in the range of from 0.1 nm to 10 nm, preferably from 0.2 nm to 5 nm, and more preferably from 0.3 nm to 2 nm. RMS may be determined using a method as described above. The deposition time, i.e. the time during which the adhesive is actively being sputtered onto the substrate, for the sputtering of the adhesive may be up to 10 minutes, preferably up to 6 minutes and more preferably up to 4 minutes. The deposition time may be at least 30 seconds, preferably at least 45 seconds and more preferably at least 1 minute. Thus, the deposition time may be in the range of from 30 second to 10 minutes, preferably from 45 seconds to 6 minutes, and more preferably from 1 to 4 minutes. Electroplated electrode In another aspect, the methods of the present invention involve a step of applying a conductive layer to a PCB (printed circuit board) substrate, wherein the conductive layer is applied by electrodeposition (also referred to as electroplating). Accordingly, in these aspects chips for biosensors comprise an electroplated conductive layer. The reference electrode and / or counter electrode of the chip may also be electroplated electrodes. Preferably, all electrodes present on the chip are electroplated. Electroplating is the process of using electrodeposition to coat an object in a layer of conductive material. During the process, an electric current is passed through a solution that contains ions of the material that will form the conductive layer, resulting in atoms of the conductive layer being subsequently deposited onto the substrate to form the conductive layer. More specifically, electrodeposition occurs through the reduction of cations of a conductive material by means of a direct electric current. The part to be coated, in this case the substrate, acts as the cathode of an electrolytic cell. The electrolyte is a solution of a salt of the conductive material to be coated, and the anode is usually either a block of the conductive material, or of some other inert conductive material. In practice, the salt of the conductive material in the electrolyte is reduced at the cathode, resulting in deposition of the conductive material upon it. Further cations of the conductive material can be released into solution by oxidation at the anode (if the anode is made of the conductive material), these cations then being able to be reduced at, and thereby plate, the cathode. Any PCB of the sort known in the art is suitable as a substrate for forming an electroplated electrode according to the present invention. A PCB refers to a structure made up of alternating layers of conductive metal (typically copper) and layers of non-conductive material for insulation purposes. It is preferred that the substrate comprises a glass-reinforced epoxy laminate material such as an FR4 material preferably in accordance with NEMA LI 1-1998 (R2011). For instance, a PCB may comprise: a base substrate made of e.g. FR4, metal or ceramic material; a copper layer disposed upon the substrate; and an insulating solder mask disposed upon the copper layer. It will be appreciated that copper etching of the PCB is carried out to expose copper regions of the PCB before electroplating. A PCB can be single sided or double sided, i.e. it contains a substrate disposed between two conductive layers. The same features regarding the substrate surface area and the chip size as described for the sputtered electrode substrate above apply equally to this PCB substrate as well. The conductive layer may be disposed on the PCB substrate such that the conductive layer and the PCB substrate are in direct contact. Typically, the conductive layer will be disposed on a copper layer of the PCB which has been exposed by etching. Further layers may be present between the conductive layer and the PCB substrate. For instance, an underlayer may sit between the conductive layer and a copper layer of the PCB substrate such that the underlayer is preferably in direct contact with both the copper layer and the conductive layer. Underlayers are described in greater detail below. The same materials as described above (in relation to the sputtered electrode conductive layer) are suitable for providing the electroplated conductive layer. It is particularly preferred that the electroplated conductive layer comprises, or consists of, Au. The electroplated conductive layer may have a thickness of up to 200 nm, preferably up to 170 nm and more preferably up to 150 nm. The conductive layer may have a thickness of at least 50 nm, preferably at least 70 nm and more preferably at least 80 nm. Thus, the conductive layer may have a thickness in the range of from 50 nm to 200 nm, preferably from 70 nm to 170 nm, and more preferably from 80 to 150 nm. As mentioned, in these aspects the conductive layer is applied by electrodeposition of the conductive layer onto the substrate. Electroplating the conductive layer can impart it with particular properties distinguishable from conductive layers formed by other methods, such as sputtering or electroless nickel gold immersion. These properties include: roughness, grain size and hardness. Roughness and grain size, along with methods for measuring the same, are described above. Where the electroplated conductive layer comprises Au, hard gold electroplating is preferably used. In hard gold electroplating, the gold may be deposited as an alloy with a second metal, such as iron, nickel or cobalt. However, in preferred embodiments, the conductive layer consists of gold. An underlayer preferably sits between the PCB substrate (e.g. an exposed copper layer of the PCB substrate) and the conductive layer of gold alloy. Thus, in some embodiments, an underlayer is applied to a PCB substrate, and the conductive layer is applied to the underlayer. The underlayer may comprise, and preferably consists of, a metal selected from iron, nickel or cobalt. Nickel is particularly preferred. In preferred embodiments, the underlayer is applied by electroplating and is therefore an electroplated underlayer. The underlayer typically has a thickness of at least 0.5 pm, preferably at least 1 pm, and more preferably at least 2 pm. The underlayer typically has a thickness of up to 4 pm, preferably up to 3.5 pm, and more up to 3.5 pm. Thus, the underlayer typically has a thickness in the range of from 0.5 pm to 4 pm, preferably from 1 pm to 3.5 pm, and more preferably from 2 pm to 3.5 pm. Although less preferred, soft gold electroplating can also be used, in which pure gold is deposited onto the substrate. Roughness of the conductive layer may differ if it is provided by electrodeposition. Conductive layers, such as those comprising Au, of electroplated working electrodes according to the present invention may have an RMS value of up to 30 nm, preferably up to 25 nm, and more preferably up to 20 nm. The RMS value may be at least 10 nm, preferably at least 12.5 nm, and more preferably at least 15 nm. Thus, the RMS value may be in the range of from 10 nm to 30 nm, preferably from 12.5 nm to 25 nm, and more preferably from 15 nm to 20 nm. In contrast, and as mentioned above, the RMS value of a conductive layer formed by electroless nickel immersion gold processes typically falls in the range of 100-200 nm. RMS may be determined using a method as described above. Conductive layers, such as those comprising Au, of electroplated working electrodes according to the present invention may have a grain size of up to 50 nm, preferably up to 40 nm, and more preferably up to 30 nm. The grain size may be at least 10 nm, preferably at least 15 nm, and more preferably at least 20 nm. Thus, the grain size may be in the range of from 10 nm to 50 nm, preferably from 15 nm to 40 nm, and more preferably from 20 nm to 30 nm. These grain sizes are typically obtained using hard gold electroplating. In contrast, the grain size of a conductive layer formed by electroless nickel immersion gold processes or by soft gold plating typically falls in the range of 1-2 pm. Grain size may be measured according to the protocols set out above. The hardness of the conductive layer refers to its ability to resist penetration by a more solid body. Conductive layers, such as those comprising Au formed by hard gold electroplating, of electroplated working electrodes according to the present invention may have a hardness of up to 300 HK25, preferably up to 250 HK25 and more preferably up to 200 HK25. The hardness may be at least 100 HK25, preferably at least 115 HK25, and more preferably at least 130 HK25. Thus, the hardness may be in the range of from 100 to 300 HK25, preferably from 115 to 250 HK25, and more preferably from 130 to 200 HK25. Hardness can be measured using the protocols set out in ASTM E92-23. Where the conductive layer comprises Au and is formed by soft gold electroplating, the conductive layer may have a hardness of up to 120 HK25, preferably up to 100 HK25 and more preferably up to 90 HK25. The hardness may be at least 10 HK25, preferably at least 15 HK25, and more preferably at least 20 HK25. Thus, the hardness may be in the range of from 10 to 120 HK25, preferably from 15 to 100 HK25, and more preferably from 20 to 90 HK25. Aptamer layer Methods of the present invention, irrespective of whether the electrode is a sputtered or an electroplated electrode, include a step of immobilising aptamers on the conductive layer to form an aptamer layer. Accordingly, a working electrode on chips according to the present invention comprises an aptamer layer in contact with the conductive layer, wherein the aptamer layer comprises aptamers immobilised onto the conductive layer. As described above, aptamers are oligonucleotides or peptide molecules which are able to bind to a target analyte, depending on their sequence. The aptamers present in the aptamer layer exhibit a conformational change when the aptamer binds to the target analyte. The aptamers may comprise deoxyribonucleic acid (DNA), ribose nucleic acid (RNA), nucleic acid analogues (XNA) and peptides. For instance, the aptamer may comprise single stranded (ss-)DNA, ss-RNAorss-XNA, and preferably comprises ss-DNA. However, it will be appreciated that the appropriate choice of aptamer will depend on the target analyte intended to be detected. The aptamers may be immobilized onto the conductive layer by covalent bonding of the aptamer to the surface layer with an active group such as thiol (-SH), carboxyl (-COOH), hydroxyl (-OH) at an end, thus forming the aptamer layer. For instance, in preferred embodiments where the aptamer is an oligonucleotide and the conductive layer is Au, a 5’ or a 3’ end of the oligonucleotide may be covalently bonded to the Au layer. Modification of the conductive layer surface can be achieved via biochemical coupling reactions such as NHS / EDC primary amine group modification or thiol-gold modification based on different materials of the electrode. A blocking agent such as L-cysteine, 6-Mercapto-l-hexanol can be used to back-fill the space between aptamers to produce a more uniform distribution of aptamers and prevent non-specific surface absorption of molecules in the test sample to the conductive layer, resulting in slightly larger signal ‘background noise’. Typically, the aptamers are labelled with a redox reporter, which are molecules which are able to transfer electrons between themselves and a conductive electrode, generating a faradaic current in the electrode. The redox reporter may be selected from methylene blue, ferrocene, thionine, anthraquinone, Nile Blue, dabcyl, 2,6-dichlorophenal-indophenol, gallocyanine, ROX, Neutral Red or derivatives thereof. Preferably, the redox reporter is methylene blue or ferrocene, and more preferably is methylene blue, as this often provides a more stable electrochemical performance with regard to repeated electrochemical interrogations, repeated sensing / regeneration iterations, and usage in complex sample matrices such as blood serum, and long-term storage. The redox reporter may be attached substantially anywhere on an aptamer. Preferably, the redox reporter is attached closer to a distal end of the aptamer relative to the end of the aptamer which is bonded to the conductive layer, and more preferably is attached to a distal end of the aptamer. It will be appreciated that an ‘end’ of an aptamer is intended to refer to a section of the aptamer terminating with the last atom of the aptamer, rather than solely the last atom itself (although this meaning is also included). The redox reporter can be connected to the aptamer via a linking group. One end of the linking group is bonded to the aptamer and the other end of the linking group is bonded to the redox reporter. Suitable linking groups include alkanediyl groups such as a C5-20 alkanediyl group (e.g. a C12 group). The alkanediyl groups are preferably unsubstituted. Although less preferred, the redox reporter can alternatively be connected directly to the aptamer. The aptamers are preferably labelled with a redox reporter and exhibit a conformational change upon binding to an analyte. As mentioned, the proximity of the redox reporter to the conductive layer depends upon whether the aptamer is in a bound state or unbound state. In the example shown in Figure 1 (which is taken from WO 2020 / 104933 A1), the aptamer 3122 folds towards the conductive layer 3121 upon binding of the analyte, but in different embodiments the aptamer may open away from the conductive layer or undergo different conformational changes. The conformational change results in greater faradic discharge from the redox reporter to the conductive layer 3121 if the redox reporter is brought in closer proximity to the conductive layer 3121, which can be sampled by the transduction methods described. The transduction of the resulting altered charge transfer rate may be performed by electrochemical methods such as square wave voltammetry (SWV), differential pulse voltammetry (DPV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), amperometry, cyclic voltammetry (CV), or alternating-current (ACV). The first two methods are particularly well suited for monitoring surface-confined reactions, as they reduce non-faradaic background charging currents. Thus, in preferred embodiments the aptamers exhibit a conformational change which increases the proximity of the redox reporter to the conductive layer when the aptamer binds to an analyte. However, it will be appreciated that aptamers which exhibit conformational changes which move the redox reporter further away from the conductive layer when binding to an analyte may also be used. The aptamers for target analyte may be developed using SELEX (Systematic Evolution of Ligands by Exponential enrichment) method, where they are selected for high binding affinity to target. The aptamer sequence may be further modified to produce a greater structural change by: 1) truncating the aptamer to make it shorter by conducting experiments to approximate the region on the aptamer that binds to target and then removing DNA bases that are not in the binding region; and 2) mismatching bases in the aptamer’s structure to make it less stable, whereby when the aptamer binds to a target the conformational change is more significant. The process of developing and optimising the structure of an aptamer may involve several rounds of testing to identify an aptamer which shows a high affinity for the target. Barrier layer Typically, a chip comprising aptamers must be stored in a wet state or, if kept in a dry state, it must be used within a few hours before degradation occurs. Therefore, in some embodiments of the present invention, the chip includes a barrier layer over the aptamer layer, the barrier layer preferably comprising a hydrogel. The barrier layer forms a physical barrier between the aptamers and oxidants in the atmosphere. The barrier layer is preferably a gel, so it is physically stable and does not dissolve, and moreover is porous thereby allowing only smaller molecules through to the aptamer layer of the chip during use. Providing such a barrier layer has the advantage of improving the long-term stability of the chip, and reduces the requirement for refrigeration of the chip. Chip designs In order to optimise the performance of the chips according to the present invention, the electrodes may be provided onto the substrate in specific designs. In addition, further features may be present to enable the chip to integrate with a device. For instance, the chip may comprise electrical contact points for connecting the chip to a handheld device that forms part of a biosensor. These contact points are in electrical connection with the conductive layer of the working electrode and with the reference and counter electrodes. Preferably, each electrode is connected to a dedicated contact point. When the contact points of the chip make contact with, for example, corresponding pins on a handheld device that forms part of a biosensor, a potentiostatic circuit is formed, such that electrodes become operable to be activated and sampled. For instance, using SWV, the faradic discharge resulting from conformational change of the redox-labelled aptamer can be sampled, whereby a peak current can be acquired and converted to digital signal. For chips with sputtered electrodes, and in particular for chips where the substrate is sapphire, it is preferred that the electrical contact points are on the same surface of the chip as the working electrode, counter electrode and reference electrode. As such, these chips may be connected to corresponding pins in a device in a horizontal fashion. For chips with electroplated electrodes, the electrical contact points may be present on the same surface of the chip as the working, counter and reference electrodes. Thus, the chip may be connected to corresponding pins in a device in a horizontal fashion similar to sputtered electrodes. However, with electroplated electrodes, the electrical contact points may alternatively be located on the reverse side of the chip to the side on which the electrodes are located. These contact points are in operable connection with the electrodes through vias passing through the PCB. Thus, the chip may be connected to corresponding pins in a device in a vertical fashion. Any suitable electrical connection means known in the art may be used to provide these electrical contact points. An electrical trace connects each electrode to the electrical contact point for said electrode. The electrical trace may be made of any suitably electrically conductive material. For chips comprising sputtered electrodes, the electrical connection means and electrical traces are preferably provided by sputtering. However, for chips comprising electroplated electrodes, the electrical connection means and electrical traces are preferably provided by printing. In some embodiments, the reference electrode may be positioned closer to the working electrode than the counter electrode, i.e. the shortest distance between the working electrode and the reference electrode is smaller than the shortest distance between the working electrode and the counter electrode. However, the chips of the present invention are compact enough that this arrangement is not necessary in all cases. Thus, in other embodiments, it may be the counter electrode which is positioned closer to the working electrode, rather than the reference electrode. For instance, the shortest distance between the working and counter electrode could be up to 3 mm, preferably up to 2 mm, and more preferably up to 1 mm. The shortest distance between the working and counter electrode may be at least 0.5 mm, preferably at least 0.6 mm, and more preferably at least 0.75 mm. Thus, the shortest distance between the working and counter electrode may be in the range of from 0.5 to 3 mm, preferably from 0.6 mm to 2 mm, and more preferably from 0.75 mm to 1 mm. These same distances are applicable to the distance between the working and reference electrodes. The chip can include a single working electrode, but it is preferred that there are more than one working electrodes. This allows for multiple readings, one from each working electrode, to be taken and an average to be taken from across these readings, thereby providing a more statistically reliable reading. For instance, the chip may comprise two to seven working electrodes, preferably three to five working electrodes, and more preferably three working electrodes. This arrangement provides sufficient working electrode surface area on the chip for the detection of a target analyte, whilst also allowing sufficient space for the reference and counter electrodes, and still retaining a conveniently small chip. At least three working electrodes is particularly preferred, as this allows the ability to determine if a reading from one electrode is defective, allowing such a reading to be discounted. The chip preferably comprises no more than one counter electrode and / or no more than one reference electrode. This, while still providing a functional chip, provides more surface area for the placement of working electrodes to better detect the presence of an analyte in a sample. However, the chip may alternatively comprise more than one counter electrode and / or more than one reference electrode. As depicted in Figure 7, more than one working electrode (701) may radially surround the reference electrode (702), and the counter electrode (703) is preferably positioned radially outwards of the more than one working electrodes (701). For instance, the reference electrode (702) may be located centrally relative to the other electrodes, with the working electrodes (701) spaced equidistantly around its perimeter. In Figure 7, the counter electrode (703) is depicted as following the contours of the working electrodes (701) which it surrounds, such that a substantially uniform distance between the working electrode (701) and the counter electrode (703) is maintained along its length. However, it will be appreciated that this is an exemplary embodiment only and that other configurations are possible. The chip design depicted in Figure 7 may be applied to any of the chips as described herein, however it is preferred that it is used for an electroplated chip. The reference (702) and working (701) electrodes may be of a teardrop shape, i.e. having a globular form at a top end, tapering to a thinner section at a bottom end. The thinner section of these electrodes is preferably connected to the electrical connection means, such as by an electrical trace. It has been found that such a configuration helps to smooth current flow and reduce ohmic iR drop. The chip may also include a boundary section (704), which encloses a surface area of the chip which contains at least a portion, and preferably the majority, of the working, reference and counter electrodes. When a sample is applied to the surface of the chip, this boundary section (704) is able to retain the sample on the surfaces of the electrodes. The boundary section (704) can comprise the same material as the conductive layer of the electrodes, and can be formed using the same methods, i.e. by sputtering for a sputtered chip, or by electroplating for an electroplated chip. The boundary section (704) may substantially define a circle, an oval, or a trapezoid. However, it will be appreciated that the boundary section can define any shape, provided that it is able to enclose the electrodes. In other embodiments, as depicted in Figure 8, the more than one working electrode (801) are arranged in a row, and the reference (802) and counter (803) electrodes are preferably positioned above the row, and typically extend from the first working electrode to the last working electrode in the row. The chip design depicted in Figure 8 may be applied to any of the chips as described herein, however the compact design makes it particularly advantageous for a sputtered chip. Although the working electrodes (801) in Figure 8 are shown as circular, it will be appreciated that they may be any other suitable shape, such as a tear drop as described above. The chips depicted in Figures 7 and 8 also include electrical traces (705, 805) in electrical connection with the working (701, 801), reference (702, 802) and counter (703, 803) electrodes. These electrical traces (705, 805) connect the electrodes to a contact point (706, 806), and can be thinner than the connection terminals (706, 806) as shown in Figure 7, or the same thickness as the connection terminals (706, 806) as shown in Figure 8. As such, it will be appreciated that the electrical trace need not be thinner than the connection terminal. In chips according to the present invention, the electrodes may be shaped such that a uniform distance is maintained between at least a portion of the reference and counter electrodes. A uniform distance may also be maintained between the working and reference electrodes, and / or between the working and counter electrodes. The length in a first dimension, i.e. the longest distance from one edge to another edge, of the electrodes, in particular the working electrodes, may be up to 5 mm, preferably up to 3 mm, and more preferably up to 2 mm. The diameter may be at least 0.1 mm, preferably at least 0.5 mm, and more preferably at least 1 mm. Thus, the length in a first dimension of the electrodes may be in the range of from 0.1 to 5 mm, preferably from 0.5 to 3 mm, and more preferably from 1 to 2 mm. The length in a second dimension (perpendicular to the first dimension of the electrodes, in particular the working electrodes, may be up to 5 mm, preferably up to 3 mm, and more preferably up to 2 mm. The diameter may be at least 0.1 mm, preferably at least 0.5 mm, and more preferably at least 1 mm. Thus, the length in a second dimension of the electrodes may be in the range of from 0.1 to 5 mm, preferably from 0.5 to 3 mm, and more preferably from 1 to 2 mm. Cartridge and Biosensor comprising the chip The sputtered or electroplated chips of the present invention may be used to detect the presence of a target analyte in a sample, by placing said sample in contact with the electrodes on the chip. Any sample may be suitable provided it can interact with the aptamer layer of the electrodes on the chip. For instance, the sample may be a biological fluid such as blood, saliva or milk, and it is preferred that the sample is blood. The chips of the present invention may be incorporated into a cartridge, as shown in Figure 9. Thus, in some embodiments, the invention provides a cartridge comprising a chip of the present invention. The cartridge (901) is for accepting a fluid sample. The cartridge (901) can comprise a cover (907) which comprises a sample collector (902), which is for receiving the fluid sample and directing it towards the chip (903). The cartridge accepts a chip (903) which is located directly under the sample collector (902), the chip (903) being mounted in a base (906). A seal (905) may be present between the cover (907) and the chip (903), in order to surround the electrodes of the chip and prevent leakage of the fluid sample into the rest of the cartridge. The cover (907) may be fastened to the base (906) with an interference fit. For instance, the base (906) and cover (907) may be fastened using protrusions (such as pegs) in one which fit into notches (such as holes) in the other. The cartridge (901) can be attached to a reading device comprising a circuit board, operable to cause the chip to be activated and to transmit resulting measurements of analyte concentrations. The fluid sample flows downward through an opening (904) in the base of the sample collector (902) and over the chip (903) for testing. The cartridge may also comprise a lid to cover the sample collector, which ensures a closed system. Ensuring that the chip operates in a closed system while the reader device analyses the sample (or sends the information generated by the chip to be analysed elsewhere) may help to prevent external contaminants entering the cartridge and reacting with the chip, resulting in greater noise. The lid may be a separate component or formed of one piece with the sample collector, for instance with a living hinge. The reading device accepts a cartridge, and may perform the computing aspects required to interpret the signal received from the chip. However, it will be appreciated that computing aspects may also be carried out elsewhere. For instance, the reading device may send a signal to an external computing means which performs the analysis of the signal received from the chip. The sample collector (902) accepts the sample, which flows downwards over the surface of the chip (903), which is in fluid connection with the sample collector (902). The shape of the sample collector (902) shown in Figure 9 is substantially cuboid, however it will be appreciated that the sample collector (902) may come in a variety of shapes and sizes, provided it still enables fluid communication of the sample with the chip (903). Different collectors may be designed for all types of fluid samples, such as blood, saliva, or even non-bodily fluids such as infant formula. The cartridge may comprise a visible volume indicator within the sample collector to indicate to the user the required volume of fluid sample they must drop into the cartridge. The sample collector may comprise a filter and / or other pre-treatment mechanisms such as active charcoal carbon filters, which may serve to remove materials that have a high risk of causing false positives and false negative test results. The sample collector may be constructed of any material, or of several parts and materials, including durable materials that can be washed and reused, so that for instance, the chip can be disposed of after use and a new chip can be reinserted into the test cartridge, thereby reducing waste. However, it will be understood that the chips of the present invention may beneficially be reused for multiple tests (provided they are washed to remove the prior sample and unbind any analyte from the aptamer layer). Once used, the entire cartridge may be disposed of and replaced within a reader device. Alternatively, only the chip may be removed from the cartridge and replaced. The present invention therefore provides a device comprising a cartridge as described above, and / or a chip as described herein. The present invention also provides a method of analysing a sample using a chip as described herein. The method comprises the steps of: contacting a sample with a chip as described herein; and determining at least the presence or absence of a target analyte within the sample based on a signal received from the chip. In such methods of analysing a sample, the chip may be a sputtered chip or an electroplated chip, of any sort as described herein. Examples Manufacture of a sputtered chip A sapphire substrate with a thickness of around 500 pm was cleaned using ultrasonication in water for 5 minutes. It was subsequently cleaned using acetone and isopropyl alcohol for 3-5 minutes, after which it was rinsed in deionised water and dried under nitrogen gas. UV-ozone cleaning was then performed on the sapphire substrate. A sputtering mask was then placed onto the substrate, and the mask-covered substrate placed into a DC magnetron sputtering chamber. Before sputtering was performed, the main chamber was evacuated to 5 x 10-6 Torr. For the sputtering of both the adhesive and conductive layers, argon gas was supplied as the sputtering gas at a sputtering pressure of 5 mTorr and the chamber was maintained at a temperature of 25.2 °C. First, the adhesive layer of Cr was sputtered onto the mask-covered substrate located over the anode of the chamber. A power of 50W was applied to the cathode (below the Cr target). The deposition time was 240 seconds, resulting in a Cr adhesive layer 10 nm thick disposed upon the substrate. The Au conductive layer was then sputtered onto the adhesive layer, using an Au target and a power of 20W, with a deposition time of 3600 seconds. The thickness of the resulting Au conductive layer, disposed on the adhesive layer, was 120 nm. Thiolated aptamers, labelled with the redox reporter methylene blue, were then immobilised onto the surface of the Au conductive layer to form the aptamer layer. Figure 2 depicts a cross-sectional view of one embodiment of a sputtered electrode chip according to the present invention. The chip comprises a substrate (3161) and a conductive layer (3121), between which is disposed an adhesive layer (3151). Immobilised onto the conductive layer (3121) is an aptamer layer (3141). Covering the upper surface of the aptamer layer (3141) is a barrier layer (3131). Electrode performance The performance of sputtered electrodes and electroplated electrodes against prior art electrodes (formed from ENIG plating on a PCB substrate) was compared. For each of these chips, the conductive layers of the working electrodes were made of Au. For the sputtered chip, the substrate was sapphire and the adhesive layer was Or. For the electroplated electrode, the substrate was an FR4 PCB and the underlayer was Ni. Vancomycin (a tricyclic glycopeptide antibiotic) was used as the target analyte, and correspondingly the aptamers of the aptamer layer were selected to bind to vancomycin. The aptamers were also labelled with the redox reporter methylene blue. Two different concentrations of vancomycin (15 mg / l and 100 mg / l) were tested on each of the three electrode types, and the signal gain at 360 Hz was measured. As can be seen from Figure 3, the sputtered chip performed better than the ENIG PCB chip at both concentrations, providing a signal gain of 33.32% vs 10.00% at 15 mg / l vancomycin, and 60.18% vs 18.00% at 100 mg / l vancomycin. Similarly, as can be seen from Figure 4, the electroplated chip performed better than the ENIG PCB chip at both concentrations, providing a signal gain of 25.24% vs 10.00% at 15 mg / l, and 45.69% vs 18.00% at 100 mg / l. This data indicates that both the sputtered chip and the electroplated chip exhibit greater sensitivity to the presence of an analyte than the ENIG PCB chip. In these tests, the relative standard deviations of the signals acquired from these various chips were also examined for absolute currents at 360 Hz. The results are shown in Figure 5, which demonstrates that the sputtered chip exhibited a consistently low variation of around 7.6% across the blank (i.e. a sample solution without vancomycin), 15 mg / l and 100 mg / l samples. The electroplated chip, while having a higher variation for the blank sample (at around 12.4%), exhibited even lower variation than the sputtered chip for both the 15 mg / l (around 6%) and 100 mg / l (around 5.2%) samples, with this decreasing as the amount of target analyte increased. Contrastingly, the ENIG PCB chip had a lower blank variation (at around 7.2%), but exhibited higher variation than both the sputtered and electroplated chips for both 15 mg / l and 100 mg / l at around 12.2% and 15.8% respectively, with the variation increasing with higher concentrations of analyte. The blank signal (i.e. the signal arising from the addition of a sample without a target analyte present) for each of these chips was also examined, with the results being shown in Figure 6, which is a graph depicting the absolute peak current over multiple repeats at 360 Hz. All types of electrodes showed appreciable stability over 4 different repeats of blank sample addition; there was no signal drift or loss, indicating that aptamer immobilisation is stable for each of the tested chip types. The sputtered chip gave the highest signal, while the electroplated and ENIG chips demonstrated similar signal 5 strengths and stabilities.

Claims

1. A method of producing a chip for a biosensor, the method comprising applying a working electrode, counter electrode and a reference electrode to a substrate, wherein the working electrode is applied to the substrate by the following steps: applying an adhesive layer to a substrate;applying a conductive layer to the adhesive layer; andimmobilising aptamers on the conductive layer to form an aptamer layer; wherein the conductive layer is applied by sputtering.

2. The method of Claim 1, wherein each of the working electrode, counter electrode and reference electrode are sputtered electrodes.

3. The method of any preceding claim, wherein the substrate is selected from silica, sapphire, glass or ceramic substrates, and preferably is a sapphire substrate.

4. The method of any preceding claim, wherein:the conductive layer comprises: a metal preferably selected from Au, Ag, Pt, Cu and Ni; C; or a conductive polymer, and preferably comprises Au; and / or the adhesive layer comprises a metal preferably selected from Cr, Ti and Ni, and preferably comprises Cr.

5. The method of any preceding claim, wherein the conductive layer and the adhesive layer are interdiffused.

6. The method of any preceding claim, wherein the aptamers in the aptamer layer comprise a redox reporter, preferably selected from methylene blue and ferrocene.

7. The method of any preceding claim, wherein the conductive layer is from 50 to 200 nm, preferably from between 70 to 170 nm, and more preferably from 80 to 150 nm in thickness.

8. The method of any preceding claim, wherein the adhesive layer is from 3 nm to 50 nm, preferably from 5 nm to 30 nm, and more preferably from 8 to 20 nm in thickness.

9. The method of any preceding claim, wherein the chip comprises a barrier layer over the aptamer layer, the barrier layer preferably comprising a hydrogel.

10. The method of any preceding claim, wherein the chip further comprises electrical contact points for connecting the chip to a handheld device that forms part of a biosensor, the electrical contacts preferably being on the same surface of the chip as the working electrode, counter electrode and reference electrode.

11. The method of any preceding claim, wherein the chip has a size of in the range of from 25 mm2 to 400 mm2, preferably from 50 mm2 to 300 mm2, and more preferably from 100 mm2 to 250 mm2.

12. The method of any preceding claim, wherein the reference electrode is positioned closer to the working electrode than the counter electrode.

13. The method of any preceding claim, wherein the chip comprises more than one working electrode, such as two to seven and preferably three to five working electrodes, and more preferably wherein the chip comprises three working electrodes.

14. The method of Claim 13, wherein the more than one working electrodes radially surround the reference electrode, and the counter electrode is preferably positioned radially outwards of the more than one working electrode.

15. The method of Claim 13, wherein the more than one working electrodes are arranged in a row, and the reference and counter electrodes are preferably positioned above the row, typically extending from the first working electrode to the last working electrode in the row.

16. The method of any preceding claim, wherein the chip comprises no more than one counter electrode and / or no more than one reference electrode.

17. The method of any preceding claim, wherein the working electrode is shaped as a tear drop.

18. The method of any preceding claim, wherein:the deposition time for sputtering of the conductive layer is from 10 to 90 minutes, preferably from 12.5 to 60 minutes, and more preferably from 15 to 30 minutes; and / orthe adhesive layer is applied by sputtering, and the deposition time for the sputtering of the adhesive layer is in the range of from 30 second to 10 minutes, preferably from 45 seconds to 6 minutes, and more preferably from 1 to 4 minutes.

19. A chip for a biosensor, the chip comprising a substrate on which a working electrode, a counter electrode, and a reference electrode are disposed, wherein the working electrode comprises:a conductive layer;an adhesive layer disposed between the substrate and the conductive layer; andan aptamer layer in contact with the conductive layer, the aptamer layer comprising aptamers immobilised onto the conductive layer, wherein the working electrode is a sputtered electrode.

20. A chip for a biosensor, the chip comprising a substrate on which a working electrode, a counter electrode, and a reference electrode are disposed, wherein the working electrode comprises:a conductive layer;an adhesive layer disposed between the substrate and the conductive layer; andan aptamer layer in contact with the conductive layer, the aptamer layer comprising aptamers immobilised onto the conductive layer,wherein the conductive layer has at least one of:a grain size in the range of from 5 nm to 60 nm, preferably from 10 nm to 50 nm, and more preferably from 20 nm to 40 nm;a roughness RMS value in the range of from 0.1 nm to 10 nm, preferably from 0.2 nm to 5 nm, and more preferably from 0.3 nm to 2 nm; anda purity of up to 99.9%, preferably up to 99.95%, and more preferably up to 99.99%.

21. The chip of Claim 19 or Claim 20, wherein the chip is as defined in any of Claims 2 to 17.

22. A method of producing a chip for a biosensor, the method comprising applying a working electrode, counter electrode and a reference electrode to a substrate, wherein the working electrode is applied to the substrate by the following steps: applying a conductive layer to the substrate; and immobilising aptamers on the conductive layer to form an aptamer layer;wherein the substrate is a PCB and the conductive layer is applied by electrodeposition.

23. A chip for a biosensor, the chip comprising a substrate on which a working electrode, a counter electrode, and a reference electrode are disposed, wherein the working electrode comprises:a conductive layer; andan aptamer layer in contact with the conductive layer, the aptamer layer comprising aptamers immobilised onto the conductive layer,wherein the substrate is a PCB and the working electrode is an electroplated electrode.

24. A cartridge for a biosensor comprising the chip of any of Claims 19 to 21 or Claim 23.

25. A biosensor comprising the chip of any of Claims 19 to 21 or Claim 23, or the cartridge of Claim 24, wherein the biosensor is for detecting the presence, and preferably the concentration, of an analyte in a biological fluid, such as blood.

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