WO2024040275A1

WO2024040275A1 - An electrochemical device and detection method

Info

Publication number: WO2024040275A1
Application number: PCT/ZA2023/050047
Authority: WO
Inventors: Willem Jacobus PEROLD; Gerhard Walzl; Stephan Petri SCHOEMAN
Original assignee: Stellenbosch University
Priority date: 2022-08-19
Filing date: 2023-08-18
Publication date: 2024-02-22
Also published as: GB2621635A; GB202212139D0

Abstract

An electrochemical device for detecting a target analyte in a fluid sample and a method of using the device. The electrochemical device comprises a porous working electrode with a selected receptor immobilised thereon which is configured to detect the target analyte. A porous separation membrane is located between the working electrode and a porous counter electrode. The separation membrane is configured to separate the working electrode and the counter electrode whilst allowing for the passage of fluid sample therethrough. The working electrode, separation membrane and counter electrode are superimposed.

Description

AN ELECTROCHEMICAL DEVICE AND DETECTION METHOD

FIELD OF THE INVENTION

This invention relates to an electrochemical device. In particular, it relates to an electrochemical test device which includes conductive electrodes configured to detect and quantify target analytes.

BACKGROUND TO THE INVENTION

Electrochemical sensors are commonly used for diagnosing or monitoring disease and detecting pollutants or contaminants in food or the environment. An electrochemical sensor generally includes a receptor which binds a target analyte and a transducer or electrode, the surface of which is used as a site for the binding reaction. The electrode converts the binding reaction into a measurable electric signal. The electrode may either oxidize or reduce an analyte of interest and current is produced from the reaction which is used to calculate the amount or concentration of the analyte. Depending on which electrochemical method is used, the current measured by the electrode or the change in potential caused by the redox reaction occurring on the electrode, is directly proportional to the concentration of the target analyte in a given sample.

Electrochemical sensors are divided into different classes based on how electrochemical measurements are carried out. The configuration of the device may vary according to whether voltametric or amperometric measurements are carried out. A typical configuration consists of two to four conductive electrodes. An electrochemical cell requires a working electrode and a counter electrode, whilst a further reference electrode is optional. The reference electrode is used to monitor the potential of the working electrode. The cell is completed by an electrolytic liquid, connecting all three electrodes. The counter electrode can act as both the counter and reference electrode in a two-electrode system. A two-electrode sensor is subject to potential drift and may only provide meaningful results in a low current system.

Electrochemical biosensors detect biomolecules and require selective biomolecule-receptors immobilised onto or associated with the working electrode surface where the binding reaction occurs. Electrochemical biosensors incorporate electro- and bioanalytical methods to determine a specific biomolecule’s concentration or chemical reactivity in a solution. Electrochemical biosensors are useful in clinical practice due to their portability, quantitative measurements, speed and sensitivity. Screen printed electrode electrochemical sensors, for example, are commercially available devices with three electrodes. It includes a working electrode which is the electrode where the potential is controlled and where the current is measured. The working electrode serves as a surface on which the electrochemical reaction to be sensed takes place. The surface may be modified as may be required for a particular sensing application. A reference electrode is used to measure the working electrode potential. A counter or auxiliary electrode is a conductor that completes the circuit of the three-electrode cell and allows passage of current. It enables analysis of processes in which electronic transfer takes place. The screen-printed electrode has a connection system arranged so that each of the three electrodes can be connected to an electronic instrument or measuring system, such as a potentiostat.

Lateral flow assays (LFAs), on the other hand, are simple, user-friendly, and low-cost diagnostic techniques for detecting the presence of a target substance in a liquid sample. LFA devices have a wide range of applications and can test a variety of samples like urine, blood, saliva, sweat, serum, and other fluids. A pregnancy test, for example, is an LFA detecting a specific hormone in a urine sample. LFAs comprise membranes, strips or pads of materials capable of transporting liquid sample assembled on a solid, non-absorptive backing material for reinforcement. LFAs typically include a control line to confirm that the test is working properly, along with one or more target or test lines.

A typical lateral flow test comprises of a sample application pad, a conjugate pad, a transportation membrane commonly made from nitrocellulose, and an absorbent pad or wick. The sample application pad acts as a sponge and holds an excess of sample fluid. Once soaked, the fluid flows to the second conjugate pad which has conjugates (usually antibodies labeled with a visual tag such as gold nanoparticles) in a salt-sugar matrix. The conjugate pad contains all the reagents required for a chemical binding reaction between the target molecule (e.g., an antigen) and its receptor (e.g., the labeled antibody) provided on the pad's surface. The conjugate pad effectively labels target molecules with a visual tag as they pass through the pad and continue across the transportation membrane to the test and control lines. The test line shows a signal, often a color, when the labeled target molecule binds to its receptor immobilized at the test line. The control line usually has affinity ligands for the conjugates which indicate that the sample has flowed through and the biomolecules in the conjugate pad are active. After passing through the test and control lines, the sample fluid enters the final porous material, the absorbent pad or wick, which simply acts as a waste container.

The assay in a lateral flow device may be a direct assay (sandwich assay) used for larger analytes with multiple binding sites, or a competitive assay used for small molecules with limited binding sites. In a direct or sandwich assay, binding of the labelled target molecule to receptors on the test line indicate a positive result. In a competitive assay, a target analyte blocks a conjugate binding to the test line, yielding no visual band when the analyte is present. In other words, the absence of a visual test line indicates a positive result.

Lateral flow assays can be incorporated in user-friendly point-of-care (POC) sensing devices to provide rapid results in resource-limited settings. They find use in assays for detecting a range of different types of analytes of varying size, from small molecules to macromolecules. Lateral flow immunoassays employ biorecognition agents such as pathogens or biomarkers to detect disease. Tuberculosis (TB), for example, is generally still being diagnosed via molecular assays, radiological methods, and immunological assays. These diagnostic methods are bound to clinical or laboratory environments. Screening for potential cases in decentralised settings coincides with resource-scarce environments, driving the need for point-of-care (POC) devices.

A disadvantage of lateral flow assays is that the results are either qualitative or semi-quantitative. Lateral flow devices, albeit a rapid and straightforward testing platform, are subject to falsenegative results in conditions where the target analyte is present in low concentrations. Researchers have attempted to integrate electrochemical biosensing and lateral flow assays to obtain the advantages of each in a single device (A. Perju and N. Wongkaew, Analytical and Bioanalytical Chemistry, vol. 413, no. 22, pp. 5535-5549, 2021 ; G. Ruiz-Vega, M. Kitsara, M. A. Pellitero, E. Baldrich, and F. J. del Campo, ChemElectroChem, vol. 4, no. 4, pp. 880-889; P. D. Sinawang, V. Rai, R. E. lonescu, and R. S. Marks, Biosensors and Bioelectronics, vol. 77, pp. 400-408, 2016; D. Du, J. Wang, L. Wang, D. Lu, and Y. Lin, Analytical Chemistry, vol. 84, no. 3, pp. 1380-1385, 02 2012; and G. Ruiz, M. Kitsara, M. Pellitero, E. Baldrich, and F. del Campo, ChemElectroChem, vol. 4, 022017). However, there is a need for an improved and more sensitive electrochemical device which is easy to manufacture and use.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention there is provided an electrochemical device for detecting a target analyte in a fluid sample comprising a porous working electrode with a selected receptor immobilised thereon which is configured to detect the target analyte; a porous counter electrode; and a porous separation membrane between the working electrode and the counter electrode which is configured to separate the working electrode and the counter electrode whilst allowing for the passage of fluid sample therethrough; wherein the working electrode, separation membrane and counter electrode are superimposed, i.e., layered or stacked on top of one another.

The electrochemical device may be a lateral flow device that includes a lateral flow transport membrane abutting the working electrode and configured to transport the fluid sample onto and through the porous working electrode, separation membrane and counter electrode in use. The working electrode, separation membrane and counter electrode may be superimposed or stacked between a first lateral flow transport membrane abutting an operatively upper surface of the working electrode and a second lateral flow transport membrane abutting an operatively lower surface of the counter electrode.

The electrochemical device may further include a porous control electrode on an opposite side of the counter electrode to the working electrode, with a second porous separation membrane between the control electrode and the counter electrode, the second porous separation membrane being configured to separate the control electrode and the counter electrode whilst allowing for the passage of fluid sample therethrough. The control electrode may be configured to detect whether transport of the fluid sample from the lateral flow transport membrane onto and through the superimposed working electrode, separation membrane, counter electrode, second separation membrane and control electrode occurred. The control electrode may have a second selected receptor immobilised thereon which is configured to detect a second target analyte in use. The second receptor may be configured to detect a second target analyte known to be present in the fluid sample, thereby serving as a control. The first target analyte may be a biomolecule and the receptor on the working electrode may be a bioreceptor configured to bind the biomolecule. The second target analyte may be a bioreceptor, optionally conjugated to a nanoparticle label and derived from a conjugate pad of the electrochemical lateral flow device.

In an embodiment which includes a control electrode, the working electrode, separation membrane, counter electrode, second separation membrane and control electrode may be superimposed or stacked between a first lateral flow transport membrane abutting an operatively upper surface of the working electrode and a second lateral flow transport membrane abutting an operatively lower surface of the control electrode. An end portion of the first lateral flow transport membrane may extend at least partially over the operatively upper surface of the working electrode. The first lateral flow transport membrane may include a sample receiving zone at or near its opposite, free end. The sample receiving zone may be an absorbent sample application pad which abuts the first lateral flow transport membrane. An end portion of the second lateral flow transport membrane may extend at least partially over an operatively lower surface of the counter electrode or an operatively lower surface of the control electrode of an embodiment including a control electrode. The second lateral flow transport membrane may include an absorbent pad at or near its opposite, free end.

The electrochemical device may include a reference electrode configured to sample the potential applied to the electrodes. The reference electrode may be a silver electrode with a silver chloride layer.

The electrochemical device may include a non-conductive support or backing with a channel defined therein which is configured to receive the lateral flow transport membrane. The support may also include locating formations extending transverse to the channel which are configured to receive and locate the electrodes relative to each other and relative to the lateral flow transport membrane. The locating formations may be configured to locate the electrodes in a superimposed or stacked configuration. An insulating material may be provided within the locating formations to seal connection portions of the electrodes. The locating formations may include grooves configured to receive the inactive, non-working, connection portions of the electrodes and an insulating material may be provided within the grooves and over the connection portions of the electrodes to seal the connection portions from the working or active portions of the electrodes which transport fluid sample in use.

In a different embodiment, the electrochemical device may include a working electrode which is provided with a non-conductive support or backing, a counter electrode provided with a non- conductive support or backing, and a control electrode provided with a non-conductive support or backing. The device may include separation layers which have a gap therein such that when assembled, a well is formed around the electrodes, the well configured to receive a fluid sample therein.

The electrochemical device may include a measurement system configured to apply a potential to the working electrode and monitor current flowing between the working electrode and counter electrode. The measurement system may include a second, independent circuit configured to apply a potential to the control electrode and monitor current flowing between the control electrode and counter electrode.

The measurement system may include input voltage amplification means and processing means configured to independently monitor and record changes in current flowing between the working electrode and the counter electrode and between the control electrode and the counter electrode, when the device includes a control electrode. The measurement system may be a potentiostat.

The electrodes may be carbon nanofiber sheets. The lateral flow transport membrane may be made of nitrocellulose, polyvinylidene fluoride or paper. The separation membrane may be made of nitrocellulose, polyvinylidene fluoride or paper. The separation membrane may have a thickness of between about 50 to 200 pm, preferably between about 100 and 200 pm, more preferably between about 1 10 and 150 pm, most preferably a thickness of about 135 pm. The lateral flow transport membrane and optionally also the separation membrane may be modified to include a redox probe. The modification may comprise impregnating the lateral flow transport membrane and optionally the separation membrane with a redox probe solution, preferably a buffered solution of a 1 :1 mixture of potassium ferrocyanide to potassium ferricyanide. Alternatively, the redox probe solution may be added onto the lateral flow transport membrane during use.

In accordance with a second aspect of the invention, there is provided a method of determining an amount of a target analyte present in a fluid sample using the electrochemical device as described above, the method comprising adding the fluid sample onto the device where it is then received at the working electrode; optionally adding a redox probe solution onto the device; applying a potential to the working electrode; and measuring a current between the working electrode and the counter electrode to detect changes in current which signify binding of the target analyte to the receptor.

When a redox probe solution is to be added onto the device, the redox probe solution may be added onto the working electrode or onto the lateral flow transport membrane.

Square wave voltammetry may be used to measure and monitor changes in peak current. The changes in current measured may be used to calculate the amount of target analyte present in the fluid. The method may further include applying a potential to the control electrode and measuring the current between the control electrode and the counter electrode to detect changes in current which signify binding of the second target analyte to the second receptor immobilised on the control electrode.

The first and/or second target analytes may be biomolecules and the first and/or second receptor may be bioreceptors. The bioreceptor on the working electrode may be an anti-/W. tuberculosis antibody or a combination of anti-/W. tuberculosis antibodies and the target biomolecule may be an antigen associated with M. tuberculosis disease.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

Figure 1 is an exploded three-dimensional view of an embodiment of an electrochemical device in the form of a lateral flow device;

Figure 2 is a three-dimensional view of the embodiment of Figure 1 in an assembled condition;

Figure 3 is a three-dimensional view the embodiment of Figure 1 in an assembled condition with the reference electrode not shown;

Figure 4 is a three-dimensional view of the embodiment of Figure 1 connected to a potentiostat measuring system;

Figure 5 is a flow diagram that illustrates a method of determining an amount of a target biomolecule present in a fluid sample using the electrochemical lateral flow device;

Figure 6 is a plot of the current measured with cyclic voltammetry experiments carried out on a DropSens screen printed electrode and an electrochemical lateral flow device (sensor R) with two working electrodes (W1 and W2); Figure 7 is a plot of the square wave voltammetry current response of the electrochemical lateral flow device (sensor R) working electrodes with varying pulse sizes at a 10 Hz sweep frequency;

Figure 8 is a plot of the square wave voltammetry current response of the electrochemical lateral flow device (sensor R) working electrodes with varying pulse sizes at a 20 Hz sweep frequency;

Figure 9 is a plot of the square wave voltammetry current response of the electrochemical lateral flow device (sensor R) working electrodes with varying pulse sizes at a 50 Hz sweep frequency;

Figure 10 are plots of the average and standard deviation of square wave voltammetry current peak responses at 10, 20 and 50 Hz sweep frequencies and different pulse sizes of 0.001 , 0.002, 0.005, 0.01 , 0.02, 0.05, 0.1 , 0.15, 0.2, 0.25 V measured with (a) the DropSens screen printed electrode (n=10) and (b) the electrochemical lateral flow device (sensor R) (n=10);

Figure 11 is an exploded three-dimensional view of another embodiment of an electrochemical device; and

Figure 12 is a three-dimensional view of the embodiment of Figure 1 1 in an assembled condition.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

An electrochemical device for detecting a target analyte in a fluid sample is provided. The device includes porous, conductive working and counter electrodes which allow for the passage of the fluid sample through the electrodes, with an electrolytic liquid if required. A porous separation membrane between the working electrode and counter electrode separates the working electrode from the counter electrode whilst allowing for the passage of the fluid sample therethrough. The working electrode, separation membrane and counter electrode are superimposed. The device integrates electrode-based sensing in a test device or assay. The porous and conductive working and counter electrodes may be in the form of flat sheets separated by the separation membrane located between the flat electrodes. The separation membrane is porous to allow for the passage of the fluid sample from the working electrode to the counter electrode thereby completing the electrolytic electrochemical cell. The separation membrane accordingly physically separates the working electrode and the counter electrode from each other by a very small distance, i.e., its thickness, whilst allowing for the passage of fluid sample, which may include target analytes such as biomolecules and electrolytic fluid (a redox probe solution), therethrough. The separation membrane prevents a short circuit and simultaneously provides a transport matrix for the fluid sample.

The working electrode may be made from conductive nanofibers having inherent porosity. The conductive nanofibers may be formed by electrospinning. Electrospun nanofibers possess intrinsic high surface-to-volume ratio and high porosity (up to 90%) with interconnected voids. The conductive nanofibers may be carbon nanofibers or nanofibers formed from conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT), poly(pyrrole) (PPy) or poly(aniline) (PANI). Alternatively, the working electrodes may comprise synthetic polymers such as poly-amide (PA), poly(lactic acid) (PLA), poly(vinyl alcohol) (PVA), poly(vinylpyrrolidone) (PVP) or natural macromolecules such as chitosan, silk fibroin, and collagen or any of their derivatives which have been blended with conductive polymers to provide them with suitable electrical, electrochemical and electromechanical properties. The working electrode may also be made from a conductive and porous textile or paper which has been modified to be conductive, whilst retaining porosity. When the device is to be used in biosensing applications, the working electrode material should be biocompatible.

The working electrode has a selected receptor immobilised thereon which is specific to a target analyte so that it can detect the presence of the target analyte in the sample fluid by selectively binding it in a binding reaction which results in a detectable and quantifiable electric signal i.e., a measurable change in current flowing between the working electrode and counter electrode because of the binding reaction affecting the surface area of the working electrode that is available for electron transfer. A working electrode comprising conductive nanofibers provides a large surface area for receptor immobilisation and peak current transfer in use. The target analyte is preferably a biomolecule and the receptor a bioreceptor so that the device can be used in biosensing applications. In alternative applications, the target analyte may be a metal and the device used to detect and quantify trace metals in a fluid sample to detect contamination, for example.

At least one lateral flow transport membrane may abut the working electrode so that it can transport fluid sample onto and through the porous working electrode, separation membrane and counter electrode by capillary action. The lateral flow transport membrane preferably covers the entire surface of the flat working electrode so that fluid sample can be transported onto and through the working electrode, separation membrane and counter electrode which are superimposed, i.e., layered or stacked, on top of each other. In this manner the device provides for both lateral and vertical flow of fluid sample through the superimposed electrodes in use. When the electrodes are stacked vertically relative to the lateral flow transport membrane, gravity may transport the fluid sample through the electrodes. Gravity may be the main driving force of fluid transportation in stacked electrodes and separation membranes, depending on the type of porous material the electrodes and separation membranes are made of, its pore size and pore size uniformity.

The electrochemical lateral flow device has improved sensitivity due to the nature and configuration of the electrodes. An entire electrode, including its internal surfaces due to its porosity, is in contact with the fluid sample when it is passed onto and through the porous electrode. Each electrode effectively gets the fluid sample, and thus potentially target molecules, onto its surfaces from all directions. Advantageously, more of the receptor can be immobilised onto the surface of the porous working electrode since the entire electrode matrix in three- dimensional space will be exposed to fluid sample during transport of the fluid through the working electrode in use. In addition, the thin separation membrane between the working and counter electrode results in a very small separation distance between the electrodes that enables more sensitive current readings.

The optional lateral flow transport membrane provides a steady transport means of fluid sample onto the electrodes. Depending on the application of the device, the lateral flow transport membrane may be configured to transport specific types of fluid samples and target analytes. In biosensing applications, the lateral flow transport membrane may be configured to transport fluid samples, such as blood, saliva, plasma, serum, urine or other bodily fluids, and different types or sizes of target biomolecules. The lateral flow transport membrane may include a sample receiving zone which may be in the form of a sample application pad. The lateral flow transport membrane or the sample application pad may be configured to filter out undesirable molecules from the fluid sample that may hinder the ability of the device to effectively detect and quantify the target analyte. The sample application pad may, for example include a filter that selectively allows a target biomolecule and other similar sized molecules in the fluid sample to be transported along the lateral flow transport membrane, whilst larger biomolecules are filtered off and retained on the sample application pad by the filter.

The device preferably has a configuration in which the lateral flow transport membrane is separated into two distinct parts, a first and second lateral flow transport membrane, on either side of the stacked electrodes. Accordingly, the superimposed working electrode, separation membrane and counter electrode are between a first lateral flow transport membrane which abuts or contacts an operatively upper surface of the working electrode and a second lateral flow transport membrane which abuts or contacts an operatively lower surface of the counter electrode. The first lateral flow transport membrane is configured to receive the sample fluid, optionally with an electrolytic liquid, and transport it onto and through the stacked electrodes via capillary action, whereas the second lateral flow transport membrane transports fluid sample away from the stacked electrodes, thereby ensuring that most of the fluid sample and thus the target analyte get transported through the electrodes so that the maximum amount of target analyte can bind the receptor within a given contact time to allow for measurement of the full extent of binding thereafter.

The electrochemical lateral flow device preferably includes a third conductive and porous electrode, which effectively serves as a second working electrode and is referred to herein as a control electrode based on its most likely function or use in the device. In a stacked configuration of the electrodes, the control electrode will be on an opposite side of the counter electrode to the working electrode, with a second porous separation membrane between the control electrode and the counter electrode. The second porous separation membrane separates the control electrode and the counter electrode to prevent an electrical short circuit, whilst allowing for the passage of fluid sample through its porous matrix.

In embodiments which include a control electrode, the working electrode, separation membrane, counter electrode, second separation membrane and control electrode are all preferably superimposed or stacked between the first lateral flow transport membrane abutting or contacting the operatively upper surface of the working electrode and a second lateral flow transport membrane abutting or contacting an operatively lower surface of the control electrode. The superimposed or stacked configuration of the working, counter and control electrode allows for this second working electrode, i.e., the control electrode, to be incorporated in the device without the need for a second counter electrode or a second reference electrode, making the entire device more compact and capable of miniaturisation for use in a POC device. Indeed, the flat electrodes (which may or may not include a control electrode) stacked on top of each other are already in a compact configuration in comparison to other existing devices such as screen-printed electrodes in which the conductive electrodes are laterally spaced apart along a flat non-conductive support or other electrochemical lateral flow devices in which electrodes are spaced laterally along the length of a lateral flow strip.

The control electrode may be configured to detect whether successful transport of the fluid sample from the lateral flow transport membrane onto and through the superimposed working electrode, separation membrane, counter electrode, second separation membrane and control electrode occurred and/or whether particular biomolecules that may be used in biosensing were active and capable of biorecognition and binding reactions. The control electrode may, for example, have a second selected receptor immobilised thereon which is configured to detect a second target analyte in use. The second target analyte may be known to be present in the fluid sample at the time that the sample passes through the control electrode, thereby serving as a control. The second target analyte may be a biomolecule. The biomolecule may be originally present in the fluid sample or added thereto, optionally from a control pad with biomolecules having an affinity or binding site for the second bioreceptor. These biomolecules must be releasably attached to the control pad so that they release from the control pad when the sample fluid passes therethrough. The control pad may be provided on the first lateral flow membrane downstream of the sample receiving zone or sample application pad of the lateral flow membrane. The control pad releases the biomolecule when the sample flows through the pad so that the biomolecule on the pad flows with the original sample fluid along the lateral flow transport membrane and through the stacked electrodes where the biomolecule from the pad recognises and binds the second bioreceptor immobilised on the control electrode. The recognition or binding event results in a measurable peak current change at the control electrode, particularly in the presence of an electrolytic fluid, which indicates that the biomolecule was active and able to bind to the second bioreceptor and that the sample fluid had successfully been transported through all the stacked electrodes. The amount or concentration of biomolecule that was detected by the control electrode can be quantified from the change in peak current measured at the control electrode, thereby allowing for better control in terms of the information provided about the test conditions whilst measuring the amount or concentration of the first target molecule at the working electrode.

The second target biomolecule may be a biorecognition agent or conjugate for the first target biomolecule, optionally labelled with a suitable nanoparticle if colorimetric detection is preferred over electrolytic sensing on the control electrode in simpler, less expensive embodiments of the device. Such conjugates may be derived from a so-called conjugate pad of the electrochemical lateral flow device as are typical of lateral flow assays.

Alternatively, the receptor on the second working electrode or control electrode may simply be configured to detect a second target analyte which is not known to be present in the sample, but which is tested for in addition to the first target analyte. In this manner more than one target analyte or biomolecule, such as more than one biomarker or more than one type of antigen can be detected with a single electrochemical lateral flow device without substantially affecting the size of the device. The device may even include multiple working electrodes capable of detecting multiple different target molecules in a fluid sample, provided that there is a counter electrode opposite each working electrode in a stacked configuration of the electrodes.

For the most effective transport of fluid sample onto the stacked electrodes, an end portion of the first lateral flow transport membrane should extend over the entire operatively upper surface of the working electrode thereby covering its upper surface. The first lateral flow transport membrane may include the sample receiving zone at or near its opposite, free end and a control or conjugate pad may be intermediate the sample receiving zone and the portion of the lateral flow transport membrane extending over the working electrode. The sample receiving zone may be an absorbent sample application pad which is provided adjacent to or is layered on top of the first lateral flow transport membrane. An end portion of the second lateral flow transport membrane may extend at least partially under, but preferably under the entire operatively lower surface of the counter electrode, or an operatively lower surface of the control electrode of an embodiment including a control electrode. An absorbent pad or wick may be provided adjacent to or on the opposite, free end of the second lateral flow transport membrane. The absorbent pad or wick is configured to absorb excess fluid sample after transport thereof through the lateral flow membrane.

The electrochemical lateral flow device may include a reference electrode configured to sample the potential applied to the electrodes. The reference electrode may be a silver electrode with a surface modified with chloride anions.

The control electrode or a third working electrode included in the device may be configured to monitor temperature during measurements since electron transferability is higher at higher temperatures. Monitoring the temperature during measurements in this manner will allow for temperature corrections to be made when quantifying the amount of target analyte present in the fluid sample.

The lateral flow transport membrane and optionally also the separation membrane may be modified to include a redox probe. The modification may comprise impregnating the lateral flow transport membrane and optionally the separation membrane with a redox probe solution, preferably a buffered solution of a 1 :1 mixture of potassium ferrocyanide/ferricyanide. Alternatively, the redox probe solution may be added during use, either prior to addition of the fluid sample or simultaneously or concurrently with the fluid sample. The redox probe may also be mixed in the fluid sample prior to adding it onto the device. An embodiment of an electrochemical device (100) is shown in Figures 1 to 4. In this embodiment, the electrochemical device is in the form of lateral flow device which includes a first lateral flow transport membrane (101 ), in this embodiment a nitrocellulose membrane, which has a sample application pad (103) at or near its one end (105) that is configured to receive the sample fluid and transport it towards the lateral flow transport membrane and in the direction of the electrodes. A control or conjugate pad (107), in this embodiment a glass fibre pad, that holds and preserves detection reagents such as unbound biomolecules (109) or conjugates is located adjacent the sample application pad (103) and between the sample application pad and the electrodes so that it is downstream from the sample application pad (103) in use. As the sample is added to the sample application pad, it is transported through the conjugate pad (107), at which point the conjugates (109) are released into the sample fluid and are able to bind target biomolecules (11 1 ) in the sample fluid, if present. The sample fluid and conjugates, some of which may be bound to the target biomolecule (111 ), then flow together towards and along the first lateral flow transport membrane (101 ). The sample application pad and conjugate pad may be configured for specific sample types, sample volumes and sizes of biomolecules by having a selected thicknesses and water absorption ability. The wicking rate and conjugate release rate of the conjugate pad may also be optimised as may be required for a specific sensing application.

The opposite end portion (1 13) of the first lateral flow transport membrane (101 ) extends over a porous and conductive working electrode (115) of the device. In the assembled condition of the device the lateral flow transport membrane (101 ) abuts or is contiguous with an operatively upper surface (1 17) of the working electrode (1 15). The flat, sheet-like working electrode (1 15) has a selected bioreceptor (119) immobilised thereon which is configured to detect the target biomolecule (1 11 ). The working electrode consists of conductive nanofibers (121 ), in this embodiment, carbon nanofibers, which have a high surface area to volume ratio to maximise the amount of bioreceptor (119) that can be immobilised on the working electrode nanofibers (121 ) for a selected working electrode volume. Any suitable highly conductive, non-soluble, and biocompatible nanofibrous material that can be manufactured into electrode strips can be used. The selected bioreceptor (119) has a binding site arranged to bind the target biomolecule (1 11 ), which in turn may be bound to a conjugate (109). After the binding reaction or event takes place, a measurable and quantifiable electric signal is generated at the working electrode in the presence of a redox probe solution, which may be in the form of an increase or decrease in peak current measured between the working electrode (1 15) and a porous and conductive counter electrode (123) arranged to be opposite the working electrode (115) and separated by a porous, non-conductive separation membrane (125). To quantify the amount of target analyte most accurately from a measured change in current, the current measurement may be done a selected time after the sample fluid has been transported along and through the lateral flow transport membrane and electrodes so that sufficient contact time is allowed for and all target biomolecule in the sample is able bind to the bioreceptor immobilised on the working electrode surface. The change in current measured at the working electrode may be an increase or decrease in peak current depending on various factors, including the pH of the buffered redox probe solution and the pK_a of the target biomolecule.

The flat and sheet-like counter electrode (123) is, in this embodiment, formed from the same type of conductive nanofibers (122) as the working electrode (1 15), preferably carbon nanofibers. The separation membrane (125) is a flat sheet or membrane consisting of a substantially non- conductive and porous material, in this embodiment nitrocellulose with a thickness of 135 pm ± 15 pm, and it is held captive between the working electrode (115) and the counter electrode (123) so that it physically separates the working electrode (1 15) and the counter electrode (123) whilst allowing for the passage of fluid sample though its pores in use. The separation membrane (125) may be made of any non-conductive porous material that can be shaped into a thin membrane or sheet such as nitrocellulose, polyvinylidene fluoride or paper and may have a thickness ranging between about 50 to 200 pm, preferably between about 100 and 200 pm, more preferably between about 1 10 and 150 pm, most preferably a thickness of about 135 pm.

The embodiment shown in Figures 1 and 2 include a second porous and conductive working electrode or control electrode (127) on an opposite side of the counter electrode (123) to the working electrode (115) with a second porous and non-conductive separation membrane (129) held captive between the control electrode (127) and the counter electrode (123). The second porous and non-conductive membrane (129) is made of nitrocellulose with a thickness of 135 pm ± 15 pm in this embodiment, but could also be formed from polyvinylidene fluoride or paper with a thickness of between about 50 to 200 pm, for example. The control electrode (127) has a second bioreceptor (131 ) immobilised on its conductive nanofibers (124), preferably carbon nanofibers. The second bioreceptor (131 ) may be configured to bind the biomolecules or conjugates derived from the control or conjugate pad (107) to demonstrate that the biomolecules are conjugates are active and have been successfully transported together with the sample fluid from the sample application pad (103) through the control pad (107) and first lateral flow transport membrane (101 ) and onto and through the working electrode (115), first separation membrane (125), counter electrode (123), second separation membrane (129) and finally onto and through the control electrode (127) where the second binding reaction between the second target biomolecule and second bioreceptor (131 ) occurs. The second binding reaction results in an electric signal in the form of a quantifiable change in peak current flowing between the control electrode (127) and counter electrode (123). In the embodiment shown in Figures 1 and 2, the second target biomolecule is a conjugate (109) from a control or conjugate pad (107). A second lateral flow transport membrane (133), in this embodiment a nitrocellulose membrane, is provided operatively below the control electrode (127) with an end portion (135) of the second lateral flow transport membrane abutting or contacting an operatively lower surface (137) of the control electrode (127) in the assembled condition of the device. The end portion (135) extends under the control electrode (127) so as to cover the operatively lower surface of the control electrode (127). An absorbent pad or wick (139), in this embodiment a nonwoven cellulose fibre pad, is provided at or near the opposite, free end (141 ) of the second lateral flow transport membrane (133) and is in contact with it. The absorbent pad (139) is thicker than the lateral flow transport membranes (101 , 133) and is configured to absorb excess sample fluid in use. Both the first and second lateral flow transport membranes (101 , 133) may be nitrocellulose, polyvinylidene fluoride or paper membranes. However, any suitable non-conductive, porous and solid material that can be formed into a lateral flow strip can be used.

The electrochemical lateral flow device (100) includes a non-conductive support (143) or backing with a channel (145) defined therein which is configured to receive the lateral flow transport membrane (101 , 133). The support (143) is preferably formed from a plastics material, more preferably a thermoplastic such as polymethyl methacrylate (PMMA) so that it can easily and inexpensively be printed with a three-dimensional printer, formed by an injection moulding process, or etched with a carbon dioxide (CO2) laser system from PMMA sheets. The channel is stepped and includes a shoulder (147) to define two channel portions (149, 151 ) of different depths. The step or shoulder (147) height is selected to accommodate the superimposed electrodes (115, 123, 127) and separation membranes (125, 129) between the lateral flow membranes (101 , 133). The first lateral flow transport membrane (101 ) is supported in the first, shallow channel portion (149) so that it is positioned operatively above the upper surface (1 17) of the working electrode (115) and in contact with it. The second lateral flow transport membrane (133) is supported in the second, deeper channel portion (151 ) so that it is operatively below and in contact with the operatively lower surface (137) of the control electrode (127).

The support (143) has locating formations (153) extending from the channel (145) and transverse to the channel (145) which are configured to receive and locate the electrodes (1 15, 123, 127) relative to each other and relative to the lateral flow transport membranes (101 , 133) in the channel (145). The locating formations (153) are configured to locate the electrodes in their superimposed or stacked configuration. In particular, the locating formations (153) are recesses (155, 157, 159) defined in the support (143) and configured to receive the respective electrodes (115, 123, 127) of the device (100). The recesses (155, 157, 159) each have a different, selected depth that ensures that the electrodes (1 15, 123, 127) are supported at the appropriate height relative to the lateral flow transport membranes (101 , 133) and in an arrangement in which the counter electrode (123) and the separation membranes (125, 129) on either side of the counter electrode (123) are between the working electrode (115) and the control electrode (127). In other words, the first recess (155) which receives and supports the working electrode (115) has the smallest depth (is the shallowest), the second recess (157) which receives and supports the counter electrode (123) has an intermediate depth relative to the other recesses (155, 159) and the third recess (159) which receives and supports the control electrode (127) is the deepest.

The locating formations (153) each have a groove (161 , 163, 165) in which an insulating material (167), which is shown in Figure 2 only, is provided. In this embodiment, the insulating material (167) is petroleum jelly, but it could be another hydrophobic substance such as paraffin wax. The insulating material (167) is provided over inactive, connection portions (169, 171 , 173) of the respective electrodes (115, 123, 127) received in the grooves (161 , 163, 165) of the support (143) to seal the connection portions (169, 171 , 173) of the electrodes (115, 123, 127) from the active, working portions of the electrodes arranged in a stack (179), shown in Figure 3, and which participate in electrolytic detection and transport fluid sample in use.

The embodiment of the electrochemical lateral flow device (100) includes a reference electrode (175), shown in Figures 1 , 2 and 4, which, in this embodiment, is a silver electrode coated in silver chloride. The reference electrode (175) is used to sample the potentials applied between the working electrode (1 15) and counter electrode (123) and between the control electrode (127) and counter electrode (123). The reference electrode (175) provides a stable and well-defined electrochemical potential against which the potentials of the working electrode (115) and control electrode (127) can be controlled and measured. The reference electrode (175) should be formed from a highly conductive material so that it has the minimum impedance. Some embodiments of the electrochemical lateral flow device do not include a reference electrode (175), as it does not carry out an essential function in relation to detecting and measuring the concentration of one or more target biomolecules. The support (143) includes a receiving formation for the reference electrode, in this embodiment slots (177) on either side of the channel (145), that are configured to receive and hold the reference electrode near the stacked electrodes. An insulating material such as petroleum jelly may be added between the reference electrode and the absorbent pad to prevent egress of the redox probe solution that was added onto the working electrode away from the electrodes.

The electrochemical device may be a disposable electrochemical measurement device to be connected to a separate measurement system such as a potentiostat (201 ) as shown in Figure 4. Alternatively, the electrochemical lateral flow device may by a standalone, portable POC device which includes a power source and a suitable current measurement system or components.

The electrochemical lateral flow device itself or a separate measurement system should have means to control the electric potential of the working electrode relative to the counter electrode. The device or measurement system should also include means to control the electric potential of the control electrode or further working electrodes relative to its counter electrode(s) if such electrodes form part of the device. The device or measurement system should also include means of measuring current between the working electrode and counter electrode. In embodiments that include a control electrode, the device or measuring system may include independent means of measuring changes in current between the working electrode and counter electrode and between the control electrode or any other working electrodes and one or more further counter electrodes.

The measurement system may include input voltage amplification means and processing means configured to independently calculate, monitor and record changes in current passing between the working electrode and the counter electrode and between the control electrode and the counter electrode. A potentiostat which includes operational amplifiers and further customary electronic components such as voltage sources, electrometers, voltmeters, function generators, analog to digital converters, and digital to analog converters, is a suitable measurement system as it is configured to apply a potential to the working electrode relative to the counter electrode, sample the potential of the working electrode via the reference electrode, and measure the current flowing between the working electrode and counter electrode. A single potentiostat with an electronic multiplexer and control circuit or a bipotentiostat may be used as a measurement system with or in an electrochemical lateral flow device which includes a second working electrode or a control electrode in addition to the first working electrode. Multichannel potentiostats are suitable for multiple working electrodes incorporated in a single electrochemical lateral flow device.

In use, a potential is applied to the working electrode and the potentiostat is configured to control the potential and measure current flowing between the working electrode and counter electrode. The potentiostat includes a second, independent circuit via which a potential is applied between the control electrode and counter electrode and the potentiostat is further configured to control the potential and measure current flowing between the control electrode and counter electrode.

The electrochemical flow device itself or a separate measurement system should include or be connectable to suitable processing means such as a computing device configured to monitor and record changes in current flowing between the working electrode and the counter electrode, and optionally also between the control electrode and the counter electrode when the device includes a control electrode. The processing means or computing device may have machine-readable instructions installed thereon for monitoring and recording changes in current and the test conditions during use of the device. The machine-readable instructions may further include for processing the measured current and for calculating and quantifying target biomolecule concentrations from the changes in current measured. Such processing means and the machine- readable instructions on it may form part of the electrochemical flow device itself when it is configured to be a point-of-care device with a user-interface that is configured to display the result of the target biomolecule detection and quantification processes.

A method of determining an amount of a target biomolecule present in a fluid sample using the electrochemical lateral flow device as described above is also provided. The electrochemical lateral flow device may include a measurement system to carry out some of the steps of the method or may be connected to the measurement system to carry out those steps of the method. An embodiment of a method (300) of determining an amount of a target biomolecule present in a fluid sample using the electrochemical lateral flow device is shown schematically in Figure 5 and comprises the steps of adding the fluid sample onto the lateral flow transport membrane (301 ); adding a redox probe solution onto the device (303), in particular onto the working electrode and/or the lateral flow transport membrane at least partially covering it; applying a controlled potential to the working electrode (305); and measuring a current between the working electrode and the counter electrode (307) to detect changes in current which signify binding of the target biomolecule to the bioreceptor.

The addition of the redox probe solution is optional, particularly in embodiments of the device which include a redox probe in the lateral flow transport membranes and/or separation membranes. When a redox probe solution is required, the steps of adding the redox probe solution onto the lateral flow transport membrane (301 ) and adding the fluid sample onto the lateral flow transport membrane (303) may happen sequentially or simultaneously. The redox probe solution and fluid sample may, for example, be premixed and added together onto the lateral flow transport membrane. Alternatively, the redox probe solution may be added after the fluid sample has been added onto the device. The redox probe solution may be added through an aperture defined in the support (143) of the electrochemical lateral flow device which is operatively above the working electrode of the device. The aperture is then configured to receive the redox probe solution and pass it onto the device, particularly onto the working electrode, or the portion of the lateral flow membrane at least partially or wholly covering the upper surface of the working electrode. The sample fluid may be added onto a sample receiving zone or sample application pad of the electrochemical lateral flow device. In some embodiments, the redox probe may also be added to the sample application pad from which it gets transported towards the electrodes.

The redox probe solution or electrolytic liquid may be prepared to be provided in a kit with the lateral flow electrochemical sensor device. The redox probe solution may be a buffered solution of a 1 :1 mixture of potassium ferrocyanide/ferricyanide.

The method may further include the steps of applying a potential to the control electrode and measuring the current between the control electrode and the counter electrode to detect changes in peak current which signify binding of the second target biomolecule to the second bioreceptor immobilised on the control electrode.

Voltametric or amperometric methods may be used in applying a controlled potential to the working electrode (and control electrode) and measure and record changes in current which signify binding of the target biomolecules to their corresponding or complimentary bioreceptors immobilised on the electrodes. The current changes may be processed to quantify the amount of target biomolecules in the sample in terms of a concentration of the target biomolecule in the sample. Square wave voltammetry is preferably used to measure and monitor changes in peak current. These changes may also be recorded and used to calculate the target biomolecule concentration.

The bioreceptor immobilised on the working electrode may be one or more antibodies configured to detect one or more target antigens or biomarkers. Conversely, the bioreceptor may be one or more antigens or biomarkers configured to detect one or more target antibodies. Another alternative is for the bioreceptor to be one or more antigens or biomarkers and the target biomolecule to be the same or a similar antigen or biomarker, but in the presence of a fixed concentration of antibody added to the working electrode in a competitive assay. The competitive assay may also be conversely configured to have an antibody as the bioreceptor and for the assay to be configured to detect the presence of an antibody in the presence of a fixed concentration of antigen.

The bioreceptor on the control electrode may be an antibody or antigen that is not specific to the first target biomolecule. The antibody or antigen may be specific for its corresponding antigen/antibody binding partner, which may optionally be labelled with colloidal gold, gold nanoparticles or horseradish peroxidase for colorimetric detection, if desired. In one application of the device, the bioreceptor immobilised onto the working electrode may be an anti-/W. tuberculosis antibody or a combination of anti-/W. tuberculosis antibodies and the target biomolecule in the sample fluid may be an antigen or biomarker associated with M. tuberculosis disease. Conversely, the M. tuberculosis antigen or biomarker may be immobilised on the working electrode and be configured to detect antibodies. The device may be configured to carry out a direct or indirect assay for diagnosing tuberculosis.

EXAMPLES

Methods and materials

An electrochemical lateral flow device was manufactured by cutting the various components with a laser cutter (TS4040 40W CO2 laser) and assembling them under a microscope with 10x magnification. The support was laser cut from PMMA sheets and has a size of approximately 40 x 50 x 5 mm. The device has two working electrodes (one of which may serve as a control) and one counter electrode. All three electrodes (the working electrode, counter electrode and control electrode) have a working area of 4 x 4 mm and a surface area of 20-30 m²/gm and were cut with the laser from carbon nanofiber sheets (PR-19-XT-LHT) from Pyrograf (Cedarville, United States). The electrodes have a thickness of about 100 pm. The fibres were cut at a 90% power setting and 10 mm/s speed. Paraffin wax was melted on a hotplate and added into the grooves containing the connection portions of the electrodes to seal the electrode connection points from the potentiostat interface. Nitrocellulose sheets (HiFlow Plus 135) were cut into separation membranes of the same shape and size as the working areas of the electrodes (4 x 4 mm). A silver reference electrode was cut from a sheet of fine silver. The reference electrode was soaked in 3% sodium hypochlorite for 10 minutes to coat it in a silver chloride layer. The connection point of the reference electrode was sanded to remove the silver chloride layer.

The electrodes were assembled on the PMMA support by stacking the nitrocellulose membranes and the electrodes. The PMMA support was inspected under the 10x microscope for inconsistencies, and the edges of the etched areas were smoothed with a scalpel to prevent the PMMA support from damaging the layered components. The grooves in the PMMA support were shaped to house the fibres at different heights based on the thickness of the nitrocellulose and electrode layers. The grooves were filled with paraffin wax to level the PMMA support and create an internal well for the applied samples. PMMA well and the Ag/AgCI reference electrode were set on top of the device working area. Ethanol was used to clean the device.

Potassium ferricyanide added to a phosphate buffered saline (PBS) buffer (10 mM, pH 7.4) was used for the preparation of the redox probe solution (electrolytic liquid). A PalmSens4 potentiostat was used for all electrochemical measurements (Palmsense, Netherlands), and the data was captured using the PSTrace interface designed for the PalmSens4.

The electrochemical lateral flow device’s potential sensing capabilities were tested by comparing sensing results with a commercially available DropSens carbon nanofibre modified screen printed electrode (SPE) (DRP-110CNF).

Cyclic Voltammetry

The sensor electrochemical reactivity and structural integrity were validated by conducting cyclic voltammetry (CV) tests. 50 pl of 10 mM potassium ferricyanide dissolved in deionised water as electrolytic liquid was applied to the working area. The device was connected to the Palmsens potentiostat via the exposed contact points. The Dropsens SPE were connected with a Dropsens SPE cable to the potentiostat. The parameters of the applied CV sweep were four cycles at 100 mV per second, 5 mV steps, upper vertex +0.6 V, and lower vertex -0.4 V. The sensors were removed from the connection system and inspected for electrolytic liquid seeping. The sensors were washed in deionised water four times and set to dry at room temperature. CV was used to screen the manufactured devices for inconsistencies such as seeping of the electrolytic liquid through the seals to react with the copper connections.

Square Wave Voltammetry

The sensors were characterised further with square wave voltammetry (SWV), and 50 pl of the electrolytic liquid was applied onto the working area. The effects of the change in SWV step size were investigated by experimenting on the DropSens SPE with a variable step size (0.001 , 0.002, 0.005, 0.01 , 0.02, 0.05). Three different frequencies (10 Hz, 20 Hz, 50 Hz) were selected for each of the different step sizes, and the pulse size (0.001 , 0.002, 0.005, 0.01 , 0.02, 0.05, 0.1 , 0.15, 0.2, 0.25) at each frequency. Sensor R was tested at a step size of 0.05 V and a range of frequencies (10 Hz, 20 Hz, 50 Hz) with variable pulse sizes (0.001 , 0.002,0.005, 0.01 , 0.02, 0.05, 0.1 , 0.15, 0.2, 0.25).

An automated baseline algorithm was developed that determines the relative inflexion points of the measurements. A second order Butterworth filter was employed to filter out frequencies above 1 Hz to remove unwanted noise on the signal. The Butterworth filter provides a maximum flat response for the passband frequencies, ensuring little to no signal attenuation in the passband range. The nature of the current response of the experiments allows for a static selection of the cut off frequency. The algorithm uses the filtered signal to determine left and righthand indices for a linear baseline for a given dataset. The algorithm firstly selects the righthand index for the baseline by evaluating the progression of the measurement starting at the highest potential. The algorithm selects an index at the local minimum of the first derivative of the filtered signal closest to the first data point. If the point selected is the first point in the dataset, it could be possible that the initial sampled current has not yet reached a steady state. The algorithm searches for an arbitrary derivative change in a non-steady-state sample. The algorithm finds a local maximum of the filtered signal if the selection is out of range for a sensible righthand index selection. A sensible selection would mean that the selected index is within the first ten per cent of the dataset’s data points. Failing all of the criteria forces the algorithm to inspect the rate of change of the filtered signal. The algorithm finds a saddle point and selects this index as the righthand coordinate for the baseline. The algorithm returns the first point in the dataset if all the cases fail to provide an index. The methodology to select a lefthand index differs from the righthand selection by the perspective of the evaluation. The algorithm generates the righthand index relative to the characteristics of the filtered signal, whereas the lefthand value is calculated relative to the selected righthand index. The algorithm determines the lefthand index by evaluating the gradient of the local data points and the gradient of the hypothetical baseline. The absolute value of the gradient of the hypothetical baseline should be larger than or equal to the comparison value to generate a line tangent to the signal. The algorithm validates that the hypothetical baseline does not intersect with the signal at any given point. The signal might offer multiple indexes that can generate tangent lines, hence the rule to prevent intersecting lines. The algorithm returns the last point in the data set if it fails to calculate a tangent line.

Results

Cyclic Voltammetry

Figure 6 is a plot of the current measurements of the CV experiments of both the DropSens SPE and the electrochemical lateral flow device (sensor R) with two working electrodes (W1 and W2). The experiments were conducted with a 10 mM ferricyanide electrolytic liquid concentration at 100 mV per second and 5 mV steps. The upper vertex of the sweep was set to +0.6 V and the lower vertex at -0.4 V. The half-wave reaction potential of the SPE corresponds with that of W1 and W2 of Sensor R. The measured current of the electrochemical lateral flow device, i.e., sensor R proved to be consistently higher than that of the DropSens SPE. As shown in Figure 6, the reduction peak for the DropSens sensor is -133.15 pA. The measured current (reduction peak) for W1 is -940.5 pA and -1053.79 pA for W2. Square Wave Voltammetry

The results of the 10, 20, and 50 Hz experiments with the manufactured electrochemical lateral flow device (sensor R) are shown in Figures 7 to 9. Figures 7 to 9 show the SWV current response of sensor R working electrodes with varying pulse sizes at 10, 20, and 50 Hz sweep frequencies, respectively. The pulse amplitudes were 0.001 , 0.002, 0.005, 0.01 , 0.02, 0.05, 0.1 , 0.15, 0.2, 0.25 V. The potential range of the SWV sweep was 0.4 V to -0.3 V. The measurements shown are for both working electrodes of the device, normalised with the above-described baseline algorithm. The results show a half-wave reduction potential of around 0,1 V. The form of the SWV measurements proved to be consistent throughout the experiments with sensor R meaning that the paraffin wax successfully prevented copper contamination at the potentiostat interface.

The comparative results of the SWV measurements of the DropSens SPE and sensor R electrodes are shown in Figure 10. The larger applied pulses and a slower scanning rate increased the peak current response. The effect of the pulse sizes can be seen in both the sensors. However, the DropSens SPE seems to be less affected by the change in frequency on the peak current. The 20 Hz measurements trended higher than the 10 and 50 Hz measurements. The peak currents of Sensor R in Figure 8b is shown with three distinct trends for the different frequencies. The 10 Hz experiments yielded the highest peak currents at the different pulse sizes, followed by 20 and 50 Hz. The slower scanning rate results may be susceptible to bulk electrolyte diffusion affecting the measurements due to the prolonged time the electrodes have to be kept at a constant potential. Table 1 and 2 below include the average, variance, and percentage variance of the peak currents of the DropSens SPE and sensor R, respectively.

Table 1 . SWV measurements of the DropSens SPE.

Table 2. SWV measurements of Sensor R.

Sensor R proved to have substantial absolute variance compared to the DropSens SPE in all experiments. Ideally, the variance should be negligible to provide consistent results. It can be noted that the variance could probably be reduced if the sensor was manufactured at industry standards. When the relative variance between the sensors are compared, sensor R has a lower variance for all experiments conducted at 10 and 20 Hz, except at 0,002 V pulse and 20 Hz sampling frequency. The magnitude of the peak currents of sensor R in all cases were recorded to be higher than that of the DropSens SPE, which inherently would mean that a more significant variance would have less of an effect on the accuracy of the measurements. As an example, choosing a standard frequency and pulse size that might be employed in biosensing, 20 Hz and 0.05 V, the DropSens absolute peak measurement is 112,98 pA with a variance of 29,47 pA (26,08%). The absolute peak current for Sensor R is 1214,69 pA with a variance of 158,14 pA (13,02%). Even though sensor R has a greater variance, the relative range of the variance is less than that of the DropSens SPE in this specific case.

The magnitude of the currents of sensor R differed from the DropSens SPE at a minimum of 7,4 times at 50 Hz and pulse size of 0,1 V. The maximum difference is 54,3 times at 10 Hz and pulse size 0.001 V. The average peak size of all the measurements for sensor R is 16.2 times higher than that recorded with the DropSens sensors. The magnitude increase is significant in the realm of biosensors because expensive and low-noise electronic equipment is required to measure low- current systems, making more feasible to utilise as a POC device.

The above example demonstrates that carbon nanofibre electrodes can be integrated into a lateral flow device to provide a lateral flow electrochemical biosensor. The nitrocellulose membrane separation of the electrodes proved to be successful, simultaneously providing insulation between the electrodes and acting as a transport medium for the electrolyte. The manufactured electrochemical lateral flow device, sensor R, delivers measurable currents due to the high surface area of the carbon nanofibre electrodes, improving the viability of utilising the sensor as a point-of-care device. The sensor showed a competitive or improved level of variance for a wide range of experiments compared to carbon SPE. Sensor R yielded consistent results at low to medium frequencies, whilst the SPE was more consistent at the high frequencies. The variance in the measured peak current can be improved by introducing stricter manufacturing protocols and scaled production.

The electrochemical lateral flow device tested yielded consistently higher current readings. The magnitude increase is significant in the realm of biosensors because expensive and low-noise electronic equipment is required to measure low-current systems, making the device more suitable for use as a point-of-care device. Immobilisation of bioreceptors, in particular antibodies, on the electrodes can be achieved by functionalisation of the electrodes via electrografting and providing carboxyl groups on the surface which are configured to covalently bind antibodies, for example.

Advantageously, the electrochemical lateral flow device described herein merges lateral flow device technology and electrochemical device technology by using porous electrodes that are capable of transporting electrolytic liquid and fluid sample. As a result, a fully quantitative lateral flow device is provided. The device embodies the combined features of the simplicity of a lateral flow device and the sensitivity of an electrochemical device for quantification. The device is simple and user friendly and can be employed as a POC device that is relatively inexpensive to manufacture. The high surface area of the porous carbon nanofiber electrodes results in a device with improved sensitivity during electrochemical sensing. The sensitivity is further improved due to the fluid sample being in contact with all surfaces of the electrode as the sample gets transported through the electrodes due to their porous nature. The carbon nanofibre electrodes provide a large surface area for electron transfer due to their porous nature and their stacked configuration on porous separation membranes in terms of which each surface of the electrode can contact sample fluid to sense a target analyte. As a results, sensing events yield higher currents which are easier to measure, require less sensitive equipment and are less affected by environmental factors. Furthermore, the electrodes are in a more robust configuration for use in a point-of-care device. A further advantage is the ability to include a second working electrode, optionally serving as a control electrode to validate sensing results.

Another embodiment of an electrochemical device (1000) is shown in Figures 11 and 12. In this embodiment, the device (1000) consists of a series of non-conductive films, each shaped to hold electrodes in place equidistant from each other. Reference numerals that relate to features that correspond to the features of the embodiment of Figures 1 -4 have the same first three digits but are multiplied by 10 - i.e., feature 11 10 of this embodiment corresponds to feature 1 11 of the previous embodiment (both being target molecules), and so on.

The electrodes include a working electrode (1150), counter electrode (1230), reference electrode (1750) and control electrode (1270). The device includes a first porous separation membrane (1240) above the working electrode (1150), a second porous separation membrane (1250) between the working electrode (1150) and the counter electrode (1230), and a third porous separation membrane (1290) between the counter electrode (1230) and the control electrode (1270).

Each electrode is placed on or within its own non-conductive backing film, which in this example may be a thin non-conductive film made from a material such as polyvinyl chloride (PVC) or polymethyl methacrylate (PMMA). The control electrode (1270) is thus placed on a first backing film (1520) and secured in place by a separation layer (1460). The separation layer is also a nonconducting film that adheres to the control electrode backing film (1520) with a rubber or an acrylic adhesive layer. Petroleum jelly (1670) seals off the working area of the electrode from its electrode connection (1730). A gap (1500) is formed in the separation layer (1460) and all subsequent layers together form a well (1020, shown in Figure 12) wherein the conductive electrodes are separated by the second porous separation membranes (1240, 1250, 1290). The third porous separation membrane (1290) is placed on to the control electrode (1270) before a counter electrode backing film (1540) is adhered to the already assembled separation layer (1460). All layers are assembled in this order and a final layer (1440) secures the working electrode (1 150) to the assembly.

The device (1000) follows the same procedure for testing for an antigen as with the described lateral flow device of the previous embodiment, but in this embodiment the samples are manually added into and removed from the well (1020) instead of utilising the lateral flow delivery and sample pretreatment as described with reference to the embodiment of Figures 1 to 4.

The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. The working electrode size (surface area) and separation membrane thickness may for example be optimized to achieve the desired minimum sensitivity. The configuration of the device may also be optimized according to the type of analyte to be measured.

The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. Finally, throughout the specification and accompanying claims, unless the context requires otherwise, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

CLAIMS:

1. An electrochemical device (100, 1000) for detecting a target analyte in a fluid sample comprising: a porous working electrode (115, 1 150) with a selected receptor (119, 1190) immobilised thereon which is configured to detect the target analyte; a porous counter electrode (123, 1230); and a porous separation membrane (125, 1290) between the working electrode and the counter electrode which is configured to separate the working electrode and the counter electrode whilst allowing for the passage of the fluid sample therethrough; wherein the working electrode, separation membrane and counter electrode are superimposed.

2. The electrochemical device as claimed in claim 1 , wherein the device is a lateral flow device that includes a lateral flow transport membrane (101 ) abutting the working electrode and configured to transport the fluid sample onto and through the porous working electrode, separation membrane and counter electrode in use.

3. The electrochemical device as claimed in claim 2, wherein the working electrode, separation membrane and counter electrode are superimposed between a first lateral flow transport membrane (101 ) abutting an operatively upper surface (1 17) of the working electrode and a second lateral flow transport membrane (133) abutting an operatively lower surface of the counter electrode.

4. The electrochemical device as claimed in any one of claims 1 to 3, further including a porous control electrode (127) on an opposite side of the counter electrode to the working electrode, with a second porous separation membrane (129) between the control electrode and the counter electrode which is configured to separate the control electrode and the counter electrode whilst allowing for the passage of fluid sample therethrough, the control electrode having a second selected receptor (131 ) immobilised thereon which is configured to detect a second target analyte in use.

5. The electrochemical device as claimed in claim 4, wherein the second receptor is configured to detect a second target analyte known to be present in the fluid sample when it passes through the control electrode and serves as a control.

6. The electrochemical device as claimed in claim 4 or claim 5, wherein the working electrode, separation membrane, counter electrode, second separation membrane and control electrode are superimposed between a first lateral flow transport membrane (101 ) abutting an operatively upper surface (1 17) of the working electrode and a second lateral flow transport membrane (133) abutting an operatively lower surface (137) of the control electrode.

7. The electrochemical device as claimed in claim 2 or claim 6, wherein an end portion (113) of the first lateral flow transport membrane (101 ) extends at least partially over the operatively upper surface of the working electrode and wherein the first lateral flow transport membrane includes a sample receiving zone at or near its opposite, free end.

8. The electrochemical device as claimed in claim 2 or claim 6, wherein an end portion (135) of the second lateral flow transport membrane extends at least partially over the operatively lower surface of the counter electrode or control electrode and wherein the second lateral flow membrane includes an absorbent pad (139) at or near its opposite, free end.

9. The electrochemical device as claimed in any one of claims 1 to 8, which includes a reference electrode (175) configured to sample the potential applied to the electrodes.

10. The electrochemical device as claimed in any one of claims 1 to 9, which includes a non- conductive support (143) having a channel (145) defined therein which is configured to receive a lateral flow transport membrane and having locating formations (153) extending transverse to the channel, the locating formations being configured to receive and locate the electrodes relative to each other and relative to the lateral flow transport membrane in the channel, and wherein an insulating material (167) is provided within the locating formations to seal connection portions of the electrodes.

1 1. The electrochemical device (1000) as claimed in claim 1 , wherein the working electrode (1150) is provided with a non-conductive support or backing (1430), the counter electrode (1230) is provided with a non-conductive support or backing (1540) and the control electrode (1270) is provided with a non-conductive support or backing (1520).

12. The electrochemical device as claimed in claim 1 1 , wherein the device includes separation layers (1460) and wherein the separation layers are provided with a gap (1500) therein such than when assembled, a well (1020) is formed around the electrodes, the well configured to receive the fluid sample therein.

13. The electrochemical device as claimed in any one of claims 1 to 12, which includes a measurement system (201 ) configured to apply a potential to the working electrode and monitor current flowing between the working electrode and counter electrode.

14. The electrochemical device as claimed in claim 13, wherein the measurement system has a second, independent electronic circuit configured to apply a potential to a control electrode and monitor current flowing between the control electrode and counter electrode.

15. The electrochemical device as claimed in claim 14, wherein the measurement system includes input voltage amplification means and processing means configured to independently monitor and record changes in current passing between the working electrode and the counter electrode and between the control electrode and the counter electrode.

16. The electrochemical device as claimed in any one of claims 1 to 15, wherein the electrodes are carbon nanofiber sheets.

17. The electrochemical device as claimed in claim 2 or claim 6, wherein the lateral flow transport membrane is made of nitrocellulose, polyvinylidene fluoride or paper.

18. The electrochemical device as claimed in any one of claims 1 to 17, wherein the separation membrane is made of nitrocellulose, polyvinylidene fluoride or paper.

19. The electrochemical device as claimed in any one of claims 1 to 18, wherein the separation membrane has a thickness of 50 to 200 pm.

20. The electrochemical device as claimed in claim 2 or claim 6, wherein the lateral flow transport membrane and optionally the separation membrane are modified to include a redox probe.

21. The electrochemical device as claimed in any one of claims 1 to 20, wherein the target analyte is a biomolecule, and the receptor is a bioreceptor configured to bind the biomolecule. A method of determining an amount of a target analyte present in a fluid sample using the electrochemical device as claimed in any one of claims 1 to 21 comprising adding the fluid sample onto the device to be received at the working electrode; optionally adding a redox probe solution onto the device; applying a potential to the working electrode; and measuring a current between the working electrode and the counter electrode to detect changes in the current which signify binding of the target analyte to the receptor. The method as claimed in claim 22, wherein square wave voltammetry is used to measure and monitor changes in peak current. The method as claimed in claim 22 or claim 23, wherein the changes in current measured are used to calculate the amount of target analyte present in the fluid. The method as claimed in any one of claims 22 to 24, further including the steps of applying a potential to the control electrode and measuring the current between the control electrode and the counter electrode to detect changes in current which signify binding of the second target analyte to the second receptor immobilised on the control electrode. The method as claimed in any one of claims 22 to 25, wherein the receptor on the working electrode is an anti-/W. tuberculosis antibody or a combination of anti-/W. tuberculosis antibodies and the target analyte is an antigen associated with M. tuberculosis disease.