GB2581988A

GB2581988A - Assay device

Info

Publication number: GB2581988A
Application number: GB1903009.7A
Authority: GB
Inventors: Nikolaenko Andrey
Original assignee: Sumitomo Chemical Co Ltd
Current assignee: Sumitomo Chemical Co Ltd
Priority date: 2019-03-06
Filing date: 2019-03-06
Publication date: 2020-09-09
Also published as: WO2020178557A1; GB201903009D0

Abstract

An assay device 1 for determining the presence/concentration of a target analyte within a liquid transport path 2 is adapted to receive a path 2 having a first end 3, second end 4 and sample receiving portion 5. The transport path transports received sample 6 towards the second end. The device includes one or more light sources 8 arranged so that when the transport path is received by the device the light sources illuminate one or more portions 10 of the path located between the receiving portion 5 and the second end. The device also includes one or more photodiodes 11 arranged so that when the transport path is received by the device the photodiodes receive light 9 transmitted through an illuminated portion 10. The assay device also includes one or more photocurrent processing channels 12, each channel is configured to receive a photocurrent 13 from a corresponding photodiode and to provide a corresponding output signal 14. The minimum detectable photocurrent is less than or equal to 1.5 pA for each processing channel.

Description

ASSAY DEVICE

BACKGROUND

Embodiments of the present disclosure relate to assay devices. More particularly, but not by way of limitation, some embodiments of the present disclosure relate to assay devices for reading the results of lateral flow immunoassay tests and/or microfluidic assays.

Biological testing for the presence and/or concentration of an analyte may be conducted for a variety of reasons including, amongst other applications, preliminary diagnosis, screening samples for presence of controlled substances and management of long term health conditions.

Lateral flow devices (also known as "lateral flow immunoassays") are one variety of biological testing. Lateral flow devices may be used to test a liquid sample, such as saliva, blood or urine, for the presence of an analyte Examples of lateral flow devices include home pregnancy tests, home ovulation tests, tests for other hormones, tests for specific pathogens and tests for specific drugs. For example, EP 0 291 194 M describes a lateral flow device for performing a pregnancy test.

In a typical lateral flow testing strip, a liquid sample is introduced at one end of a porous strip which is then drawn along the strip by capillary action (or "wicking"). A portion of the lateral flow strip is pre-treated with labelling particles which are activated with a reagent which binds to the analyte to form a complex if the analyte is present in the sample. The bound complexes and also unreacted labelling particles continue to propagate along the strip before reaching a testing region which is pre-treated with an immobilised binding reagent which binds bound complexes of analyte and labelling particles and does not bind unreacted labelling particles. The labelling particles have a distinctive colour, or other detectable optical or non-optical property, and the development of a concentration of labelling particles in the test regions provides an observable indication that the analyte has been detected. Lateral flow test strips may be based on, for example, colorimetric labelling using gold or latex nanoparticles, fluorescent marker molecules or magnetic labelling particles.

Another variety of biological testing involves assays conducted in a microfluidic device, or one or more channels thereof Liquid assays may be measured based on colorimetry or fluorescence. An advantage of some liquid based assays is that they may allow tests to be conducted using very small (e.g. picolitre) volumes Some immunoassays may be conducted in a microfluidic device.

Sometimes, merely determining the presence or absence of an analyte is desired, i.e. a qualitative test. In other applications, an accurate concentration of the analyte may be desired, i.e. a quantitative test. For example, WO 2008/101732 Al describes an optical measuring instrument and measuring device. The optical measuring instrument includes at least one source for providing at least one electromagnetic beam to irradiate a sample and to interact with the specimen within the sample, at least one sensor for detecting an output of the interaction between the specimen and the electromagnetic beam, an integrally formed mechanical bench for the optical and electronic components and a sample holder for holding the sample. The at least one source, the at least one sensor, and the mechanical bench are integrated in one monolithic optoelectronic module and the sample holder can be connected to this module.

SUMMARY

In some embodiments there is provided an assay device for determining the presence and/or concentration of a target analyte within a liquid transport path. The assay device is adapted to at least partly receive a liquid transport path having a first end, a second end and a sample receiving portion proximate to the first end. The liquid transport path is adapted to transport a liquid sample received in the sample receiving portion towards the second end. The assay device includes one or more light sources. The one or more light sources are arranged so that when the liquid transport path is received by the assay device, the one or more light sources illuminate one or more illuminated portions of the liquid transport path which are located between the sample receiving portion and the second end The assay device also includes one or more photodiodes. The one or more photodiodes are arranged so that when the liquid transport path is received by the assay device, the one or more photodiodes receive light transmitted through an illuminated portion of the liquid transport path. The assay device also includes one or more photocurrent processing channels. Each photocurrent processing channel is configured and arranged to receive a photocurrent from a corresponding photodiode, and to provide a corresponding output signal The minimum detectable photocurrent is less than or equal to 1.5 pA for each photocurrent processing channel In some embodiments, the assay device may be a reader device.

In some embodiments, some of the illuminated portions may correspond to regions of interest, and other illuminated portions may correspond to reference regions. In some embodiments, output signals corresponding to reference regions may be used for performing background corrections for output signals corresponding to regions of interest. In some embodiments, output signals may be digital signals In some embodiments, a minimum detectable change in the output signal may be limited by dark current of the photocurrent processing channels. In some embodiments, the minimum detectable photocurrent being less than or equal to 1.5 pA may mean that the standard error of each photocurrent processing channel is less than or equal to 1.5 pA. In some embodiments, the minimum detectable photocurrent being less than or equal to 1.5 pA may mean that the standard error of each photocurrent processing channel is less than or equal to one third of 1.5 pA. In other words, the minimum detectable photocurrent may be three times the standard error of the photocurrent processing channel(s). In some embodiments, the minimum detectable photocurrent being less than or equal to 1.5 pA may mean that the standard error of each photocurrent processing channel is less than or equal to one fifth of 1.5 pA. In other words, the minimum detectable photocurrent may be five times the standard error of the photocurrent processing channel(s).

In some embodiments, the minimum detectable photocurrent may be less than or equal to 1.0 pA. In some embodiments, the minimum detectable photocurrent may be less than or equal to 0.5 pA. In some embodiments, the minimum detectable photocurrent may be less than or equal to 0.1 pA.

In some embodiments, each illuminated portion may correspond to a single light source. In some embodiments, each illuminated portion may correspond to a single photodiode. In some embodiments, each illuminated portion may correspond to a single light source and a single photodiode. In some embodiments, each illuminated portion may correspond to two or more light sources having differing emission spectra. In some embodiments, each illuminated portion may correspond to two or more photodiodes having different spectral sensitivities. In some embodiments, each illuminated portion may correspond to two or more light sources having differing emission spectra, and to two or more photodiodes having different spectral sensitivities.

In some embodiments, each photocurrent processing channel may include an amplifier and an analog-to-digital converter. In some embodiments, the one or more photocurrent processing channels may comprise a mi crocontrol 1 er, and the m i crocontrol 1 er may provide one or more analog-to-digital converters. In some embodiments, each analogue-to-digital convertor may have a precision greater than 8-bits, for example, each analogue-to-digital convertor may have a precision of 10-bits, 12-bits or 16-bits. In some embodiments, the amplifier may be a trans-impedance amplifier. In some embodiments, the amplifier may include an operational amplifier In some embodiments, an optical path between the one or more light sources and the one or more photodiodes may include no monochromator(s) In some embodiments, the optical path may include no beamsplitter(s) between the one or more light sources and the one or more photodiodes. In some embodiments, the optical path may include no fibre couplers and/or fibre splitters between the one or more light sources and the one or more photodiodes.

In some embodiments, the light sources may take the form of one or more light emitting diodes, organic light emitting diodes, lasers, laser diodes, tungsten filament bulbs, halogen bulbs, fluorescent tubes and/or compact fluorescent bulbs. In some embodiments, organic light emitting diodes may be solution processed In some embodiments, photodiodes may be inorganic photodiodes or organic photodiodes. In some embodiments, organic photodiodes may be solution processed.

In some embodiments, the assay device may include a plurality of photodiodes arranged in an array. In some embodiments, the array may include more photodiodes in a first direction than in a second, perpendicular direction. In some embodiments, the assay device may include a single photodiode corresponding to each illuminated portion and having an area substantially corresponding to an area of the illuminated portion.

In some embodiments, a slit or aperture may be included in the optical path between the one or more light sources and the illuminated portion of the liquid transport path. In some embodiments, a slit or aperture may be included in the optical path between the illuminated portion of the liquid transport path and the one or more photodiodes. In some embodiments, each slit may have adjustable width. In some embodiments, each slit may have a width of greater than or equal to 1 mm. In some embodiments, each slit may have a width of up to 2.2 mm. In some embodiments, each slit may have a width between 100 um and 1 mm inclusive. In some embodiments, each slit may have a width between 300 um and 500 jtm inclusive.

In some embodiments, a diffuser may be included in the optical path between the one or more light sources and the illuminated portion of the liquid transport path In some embodiments, a diffuser may be included in the optical path between the illuminated portion of the liquid transport path and the one or more photodiodes.

In some embodiments, the one or more processing channels may include a filtering module configured to perform averaging across a period corresponding to one or more cycles of a mains electricity signal, such that any component of interference from the mains electricity signal may be reduced or removed from the output signals In some embodiments, the mains electricity signal may have a frequency within the range between and including 50 Hz to 60 Hz. In some embodiments, each processing channel may obtain one thousand samples during one period of the mains electricity signal, and the output signal may be based on an average across said one thousand samples. In some embodiments samples may be obtained at regular sampling intervals. In some embodiments samples may be obtained at irregular sampling intervals. When samples are obtained at irregular sampling intervals, samples may be interpolated onto regularly spaced time points before calculating an average of the interpolated values. In some embodiments, the filtering module may be implemented by a processor of a microcontroller executing a compiled computer program stored in a memory of the microcontroller. In some embodiments, the filtering module may be implemented using a suitably configured field-programmable gate array. In some embodiments, the filtering module may be implemented using one or more digital electronic microprocessors executing a compiled computer program. In some embodiments, the filtering module may be implemented by a microcontroller which implements the one or more analog-to-digital converters. In some embodiments, the filtering module may be implemented using a time-averaging circuit.

In some embodiments, the assay device may be adapted to at least partly receive a liquid transport path having an effective transmittance 71, of less than 10%, wherein the effective transmittance le is given by the ratio: = SIS0 in which S is a measured output signal from a processing channel when the liquid transport path is received in the assay device and So is a measured output signal from the same processing channel when the liquid transport path is not received in the assay device In some embodiments, the effective transmittance L; may be less than or equal to 8%. In some embodiments, the effective transmittance T may be less than or equal to 5%. In some embodiments, the effective transmittance i may be less than or equal to 3%. In some embodiments, the effective transmittance Te may be less than or equal to 1%.

In sonic embodiments, when an illuminated portion is defined by an aperture having a width of 1 mm in the direction between the first and second ends, the corresponding effective transmittance le of the liquid transport path may be less than or equal to 2.2+0.1%. In some embodiments, when an illuminated portion is defined by an aperture having a width of 2.2 mm in the direction between the first and second ends, the effective transmittance 7; of the liquid transport path may be less than or equal to 4.2+0.1%.

In some embodiments, the liquid transport path may include a porous strip.

In some embodiments, the porous strip may include fibrous material. In some embodiments, the porous strip may include nitrocellulose. In some embodiments, the porous strip may take the form of a lateral flow immunoassay test strip. In some embodiments, the porous strip may be supported on a substrate. In some embodiments, the substrate may be opaque. In some embodiments, opaque may correspond to an effective transmittance Te for the substrate of less than or equal to 109:O. In some embodiments, the effective transmittance Ye of the substrate may be less than or equal to 8%. In some embodiments, the effective transmittance Te of the substrate may be less than or equal to 5%. In some embodiments, the effective transmittance Te of the substrate may be less than or equal to 3% In some embodiments, the effective transmittance 71, of the substrate may be less than or equal to 1% In some embodiments, the liquid transport path may include one or more channels of a m crofluidic device.

In some embodiments, each light source may be an organic light emitting diode having an external quantum efficiency of less than or equal to 0.5%. In some embodiments, each organic light emitting diode may have an external quantum efficiency of less than or equal to 0.1%.

In some embodiments, the assay device may further include a determination module configured to determine, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path.

In some embodiments, the determination module may be implemented by a processor of a microcontroller executing a compiled computer program stored in a memory of the microcontroller. In some embodiments, the determination module may be implemented using a suitably configured field-programmable gate array. In some embodiments, the determination module may be implemented using one or more digital electronic processors executing a compiled computer program. In some embodiments, the determination module and the filtering module may be implemented by the same microcontroller, by the same field-programmable gate array, or by the same one or more digital electronic processors.

In some embodiments, the assay device may further include a communications interface configured to provide the output signals to an external processing device In some embodiments, the communications interface may be implemented by a microcontroller which implements the filtering module and/or the determination module In some embodiments, the communications interface may be implemented by a field-programmable gate array which implements the filtering module and/or the determination module In some embodiments, the external processing device may be a desktop computer, a laptop computer, a tablet computer, a mobile telephone, a handheld purpose specific computing device, and so forth.

In some embodiments, an assay test may include the assay device and a liquid transport path at least partly received by the assay device In some embodiments, the assay device and liquid transport path may be contained within, or integrated with, a casing, enclosure or package.

In some embodiments, a system may include the assay device, a liquid transport path at least partly received by the assay device, and an external processing device configured to determine, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path In some embodiments, the external processing device may determine the presence and/or concentration of a target analyte within the liquid transport path using one or more digital electronic processors executing a compiled computer program. In some embodiments, the external processing device may implement the filtering module.

In some embodiments, the liquid transport path may include a porous strip supported on an opaque substrate. In some embodiments, the liquid transport path may be arranged so that the one or more light sources illuminate the porous strip through the opaque substrate.

In some embodiments, a testing kit may include the assay device and a liquid transport path adapted to be received by the assay device.

In some embodiments, there is provided a method of using the assay device, the assay test, the system or the testing kit. The method includes applying a quantity of liquid sample to the liquid transport path. The method includes waiting for a duration at least long enough for the liquid sample to propagate through at least one illuminated portion of the liquid transport path. The method includes obtaining output signals from the one or more photocurrent processing channels. The method includes determining, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path.

In some embodiments, there is provided a method of determining the presence and/or concentration of a target analyte within a liquid transport path having a first end, a second end and a sample receiving portion proximate to the first end. The liquid transport path is adapted to transport a liquid sample received in the sample receiving portion towards the second end. The method includes illuminating, using one or more light sources, one or more illuminated portions of the liquid transport path which are located between the sample receiving portion and the second end The method includes receiving, using one or more photodiodes, light transmitted through the one or more illuminated portions of the liquid transport path. The method includes generating, using one or more photocurrent processing channels which receive photocurrent from corresponding photodiodes, an output signal, wherein the resolution for detecting received photocurrent is less than 1.5 pA.

In some embodiments, generating each output signal may include obtaining an average across a period corresponding to one or more cycles of a mains electricity signal, such that any contribution of interference from the mains electricity signal may be reduced or removed from the output signal.

In some embodiments, the liquid transport path may have an effective transmittance re of less than 10%, wherein the effective transmittance 1; may be given by the ratio: 2 = S/So in which S is a measured output signal from a processing channel when the liquid transport path is received between one or more light sources and one or more photodiodes, and So is a measured output signal from the same processing channel when the liquid transport path is not received between one or more light sources and one or more photodiodes.

DESCRIPTION OF THE DRAWINGS

The disclosed technology and accompanying figures describe some implementations of the disclosed technology.

Figure I_ illustrates an example of an assay device; Figure 2 illustrates a portion of a lateral flow test strip; Figure 3 illustrates a photocurrent processing channel; Figure 4 illustrates dark-currents measured using photocurrent processing channels; Figure 5 illustrates a circuit for a photocurrent processing channel; Figure 6 illustrates a portion of a second example of an assay device, Figure 7 illustrates a portion of a third example of an assay device; Figure 8 illustrates a first method; Figure 9 illustrates a second method; Figure 10 illustrates a self-contained lateral flow assay device; Figures 11A and 11B illustrate a reader device for reading a lateral flow test strip; Figure 12 presents measurements obtained using an assay device according to the present specification; Figure 13 presents comparative measurements obtained using an assay device which is not an embodiment according to the present specification; Figure 14 illustrates an assay device reading a channel of a microfluidic device; and Figure 15 illustrates using an assay device to obtain background corrected measurement.

The drawings are not drawn to scale and have various viewpoints and perspectives. The drawings are some implementations and examples. Additionally, some components and/or operations may be separated into different blocks or combined into a single block for the purposes of discussion of some of the embodiments of the disclosed technology. Moreover, while the technology is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the technology to the particular implementations described. On the contrary, the technology is intended to cover all modifications, equivalents, and alternatives falling within the scope of the technology as defined by the appended claims.

DETAILED DESCRIPTION

Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise," "comprising," and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to." As used herein, the terms "connected," "coupled," or any variant thereof means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, electromagnetic, or a combination thereof Additionally, the words "herein," "above," "below," and words of similar import, when used in this application, refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word "or," in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. As used herein, by a material "over" a layer is meant that the material is in direct contact with the layer or is spaced apart therefrom by one or more intervening layers. As used herein, by a material "on" a layer is meant that the material is in direct contact with that layer. A layer "between" two other layers as described herein may be in direct contact with each of the two layers it is between or may be spaced apart from one or both of the two other layers by one or more intervening layers.

The teachings of the technology provided herein can be applied to other systems, not necessarily the system described below. The elements and acts of the various examples described below can be combined to provide further implementations of the technology. Some alternative implementations of the technology may include not only additional elements to those implementations noted below, but also may include fewer elements.

These and other changes can be made to the technology in light of the following detailed description. While the description describes certain examples of the technology, and describes the best mode contemplated, no matter how detailed the description appears, the technology can be practiced in many ways. Details of the system may vary considerably in its specific implementation, while still being encompassed by the technology disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the technology should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the technology with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the technology to the specific examples disclosed in the specification, unless the Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the technology encompasses not only the disclosed examples, but also all equivalent ways of practicing or implementing the technology under the claims.

To reduce the number of claims, certain aspects of the technology are presented below in certain claim forms, but the applicant contemplates the various aspects of the technology in any number of claim forms. For example, while some aspect of the technology may be recited as a computer-readable medium claim, other aspects may likewise be embodied as a computer-readable medium claim, or in other forms, such as being embodied in a meansplus-function claim.

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of implementations of the disclosed technology. It will be apparent, however, to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

The techniques introduced here can be embodied as special-purpose hardware (e.g., circuitry), as programmable circuitry appropriately programmed with software and/or firmware, or as a combination of special-purpose and programmable circuitry. Hence, embodiments may include a machine-readable medium having stored thereon instructions which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), magneto-optical disks, ROMs, random access memories (RAMs), erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), magnetic or optical cards, flash memory, or other type of media / machine-readable medium suitable for storing electronic instructions. The machine-readable medium includes non-transitory medium, where non-transitory excludes propagation signals. For example, a processor can be connected to a non-transitory computer-readable medium that stores instructions for executing instructions by the processor.

Quantitative detectors for biological testing methods may require optical components such as beamsplitters, lenses, monochromators, filters etc. Such components may be complex, expensive and/or bulky, and may have properties which vary considerably with the wavelength of light Such factors have been problematic for the integration of quantitative detectors into compact, single use disposable tests for use at home or at point-of-care. Similarly, such factors have been problematic for the integration of quantitative detectors into compact, low cost and low power handheld reading devices for assay tests Additionally, lateral flow immunoassays are typically prepared using nitrocellulose strips supported on opaque substrates. Consequently, quantitative readers for lateral flow immunoassays may need to either operate in a reflection mode, or use specially customized nitrocellulose strips supported on transparent substrates for a transmission mode. Operation in reflection mode adds bulk to a reader, as a reflection geometry typically needs more clearance to illuminate and read the lateral flow immunoassays compared to a transmission mode. Use of specially customized nitrocellulose strips supported on transparent substrates is impractical for a general purpose reader, as many commonly produced lateral flow immunoassays may be incompatible.

Quantitative detectors for biological testing methods are typically configured with the assumption that the concentration of a target analyte or labels bound to said target analyte will be the limiting factor on resolution, rather than the intensity of light reaching a light detector. Consequently, the minimum detection threshold of incident light has not been considered a priority.

The present inventors have found that by utilising a relatively high sensitivity signal processing path (high sensitivity compared to conventional assay devices), an assay device may be improved. For example, a signal processing path having improved sensitivity may improve the resolution for detecting changes in optical density, and may also improve a minimum detectable concentration of a target analyte. A signal processing path having improved sensitivity may also facilitate new features. For example, a transmission mode geometry may be used even when the assay to be read is substantially opaque. This may avoid the need for specially customised transparent lateral flow test strips for integrated single use devices, and may improve the inter-compatibility of a general purpose reader. Additionally or alternatively, cheap, low grade organic light emitting diodes (i.e. low external quantum efficiency) may be enabled as a light source. This insight, as embodied in the examples of assay devices and method described in this specification, permits that an assay device for quantitative readout of an assay may be more compact, cheaper, and/or capable of reading any standard lateral flow immunoassay. The inventors have further realised, surprisingly, that reading an opaque assay in a transmission geometry using assay devices having a high sensitivity signal processing path may improve a minimum detectable concentration of a target analyte by facilitating a two-colour background correction method. Assay devices specifically adapted to make use of this surprising realisation are also described (see Figure 15). Assay devices according to this specification may be of particular use for single use home or point-of-care testing kits, and/or for compact, general purpose handheld readers.

Figure 1 is a schematic illustration of an assay device 1 according to some embodiments of the present disclosure.

The assay device 1 is intended for use in determining the presence and/or concentration of at least one target analyte within a liquid transport path 2. The assay device 1 is adapted to at least partly receive a liquid transport path 2. The liquid transport path 2 has a first end 3, a second end 4 and a sample receiving portion 5 arranged proximate to the first end 2. The liquid transport path 2 is adapted to transport a liquid sample 6 received in the sample receiving portion 5 in a flow direction 7 towards the second end 4.

The assay device 1 includes one or more light sources 8. When a liquid transport path 2 is received by the assay device 1, the light sources 8 are arranged with respect to the received liquid transport path 2 so that light 9 emitted by the light sources is incident on one or more illuminated portions 10 of the liquid transport path 2 located between the sample receiving portion 5 and the second end 4. The assay device 1 also includes one or more photodiodes 11 arranged opposite (transmission geometry) to the light sources 8. When a liquid transport path 2 is received by the assay device 1, the photodiodes 11 receive light 9 transmitted through the illuminated portion 10 of the liquid transport path 2. The assay device 1 also includes a photocurrent processing channel 12 corresponding to each photodiode 11. Each photocurrent processing channel 12 receives a photocurrent 13 from a respective photodiode 11, and processes the photocurrent 13 to provide a corresponding output signal 14. Photocurrent processing channels 12 according to embodiments of the present specification are configured to provide a minimum detectable photocurrent which is less than or equal to 1.5 pA.

This minimum detectable photocurrent of less than or equal to 1.5 pA may also be referred to as "ultra-high" sensitivity in the context of assay devices 1 for reading the results of biological testing assays. As mentioned hereinbefore, the sensitivity of a reader has not typically been considered to be an important or limiting factor, because it has conventionally been considered that the sensitivity of biological testing assays was limited by the assays themselves, rather than by a reader used to quantify the results. In some examples, the output signals 14 may take the form of digital signals 29 (Figure 3).

Each illuminated portion 10 may correspond to a single light source 8, or to two or more light sources 8, for example arranged to form an array. Each light source 8 may take the form of one or more light emitting diodes, organic light emitting diodes, lasers, laser diodes, tungsten filament bulbs, halogen bulbs, fluorescent tubes and/or compact fluorescent bulbs. Each illuminated portion 10 may correspond to a single photodiode 11, or to two or more photodiodes 11, for example arranged to form an array. Each photodiode llmay be an inorganic photodiode or an organic photodiode. In some embodiments, each illuminated portion 10 corresponds to a single light source 9 in the form of an organic light emitting diode, and to a single photodiode 11 in the form of an organic photodiode. Organic light emitting diodes and /or organic photodiodes may be solution processed. The assay device 1 may include a single light source 8 and/or a single photodiode 11 having active areas substantially corresponding to the illuminated portion 10.

The assay device I may include an integrated liquid transport path 2 (Figure 10).

Alternatively, the assay device 1 may be configured to receive a separate liquid transport path 2 in the form of a porous strip 15 (Figure 2) of a lateral flow test strip 16 (Figure 2), or one or more channels 17 (Figure 14) of a microfluidic device 18 (Figure 14). In some embodiments the liquid transport path 2 is configured to perform a colorimetric assay test such as, for example, an immunoassay test. The assay device 1 may be configured to read the results of the colorimetric assay test using the pairing of one or more light sources 8 and one or more photodiodes 11. In the present specification, the term "colorimetric assay test" includes assays conducted using non-visible wavelengths of light 9, for example infrared or ultraviolet light.

A flow front 19 separates a wet portion of the liquid transport path 2 from a dry portion. The flow front 19 moves towards the second end 4 in the flow direction 7. The liquid transport path 2 transports the liquid sample 6 in the flow direction 7 by wetting/capillary action, or similar mechanisms. For example, the liquid transport path 2 may take the form of a porous medium such as, for example, a porous strip 15 (Figure 2). A porous medium forming the liquid transport path 2 may include nitrocellulose or other fibrous materials capable of transporting an aqueous liquid by capillary action, whether inherently or following appropriate surface treatments. Micro-fluidic channels 17 (Figure 14) are sufficiently thin in at least one dimension that capillary forces may act to draw the liquid sample 6 in and along the flow direction 7.

In embodiments where the assay device 1 includes an integrated liquid transport path 2, the one or more light sources 8 and corresponding photodiodes 11 may be arranged relative to the illuminated portion 10 during fabrication/assembly of the assay device 1. In embodiments where the assay device 1 is configured to receive a separate liquid transport path 2, either the assay device 1, the liquid transport path 2, or both, may be configured with features to enable accurate and reproducible alignment of one or more regions of interest 23 (Figure 2) of the liquid transport path 2 with the one or more light sources 8 and the corresponding photodiodes 11. For example, in some embodiments either or both of the assay device 1 and the liquid transport path 2 may include registration indicia. In some embodiments, the assay device 1 may include a slot 60 (Figure 11A) having a specific length such that a liquid transport path 2 may be positioned correctly by simply being placed with the first or second end 3, 4 abutting a closed end 61 (Figure 11B) of the slot 60 (Figure 11A) In some embodiments, one or more slits or apertures 53 (Figure 10) may be included in the optical path between the one or more light sources 9 and the one or more photodiodes 11, in order to collimate the light 9. When slits or apertures 53 (Figure 10) are included, each may have adjustable width. When slits or apertures 53 (Figure 10) are included, each may have a width of greater than or equal to 1 mm, between 100 p.m and 1 mm inclusive, or between 300 gm and 500 pm inclusive. In some embodiments, a diffuser may be included in the optical path between the one or more light sources 9 and the one or more photodiodes 11. (6

An optical path between the one or more light sources 9 and the one or more photodiodes 11 may include no monochromator(s), no beamsplitter(s) and no fibre couplers and/or fibre splitters.

As described here nbefore, lateral flow immunoassay test strips are typically fabricated using opaque substrates as standard Consequently, readers typically operate in a reflection mode, since the transmission through an opaque substrate will provide a very low signal.

The ultra-high (<1.5 pA) sensitivity of a photocurrent processing channel 12 according to assay devices 1 of the present specification enables measurements in transmission using an opaque liquid transport path 2.

For example, in some embodiments the assay device 1 may include, or be adapted to at least partly receive, a liquid transport path 2 having an effective transmittance /1 of less than 10%. The parameter of effective transmittance 1/1 may be defined as the ratio: -S So in which S is a measured output signal 14 from a processing channel 12 when the liquid transport path 2 is between the light sources(s) 8 and the photodiode(s) 11, and So is a measured output signal 14 from the same processing channel 12 when the liquid transport path 2 is not between the light sources(s) 8 and the photodiode(s) 11. In this way, since it may be measured in-situ using the assay device 1 itself, the effective transmittance re represents a convenient, reliable and reproducible parameter.

The ultra-high (<1.5 pA) sensitivity of the assay device 1 permits that in some embodiments, the effective transmittance Te of the liquid transport path 2 may be lower than 10%. For example, in various embodiments the assay device 1 may include, or be adapted to at least partly receive, a liquid transport path 2 having an effective transmittance Te. of less than 8%, less than 5%, less than 3% or less than 1°,10. Such sensitivity provides the capability to read lateral flow immunoassays produced using standard opaque substrates in a transmission mode. This avoids the need for customised transparent substrates, and may enable production of compact (compared to reflection mode devices) assays devices 1 which may be compatible with any type of lateral flow immunoassay test.

In one example, an illuminated portion 10 was defined by an aperture having a width of 1 mm in the direction x between the first and second ends 3, 4, and the corresponding effective transmittance le of a standard, opaque lateral flow strip was measured as 2.2%. The corresponding effective transmittance Ye for a customized transparent lateral flow strip was measured as 25%. For illuminated portions 10 having a small width, for example less than about 2 mm, the effective transmittance Te exhibits some dependence on the width of the illuminated portion 10. This effect is believed to be a result of scattering in the liquid flow path 2, and plateaus for wider illuminated portions 10. The measurements were repeated using an illuminated portion 10 defined by an aperture having a width of 2.2 mm, and the effective transmittance T e of the standard, opaque lateral flow strip was measured as 4.2%, as compared to 41% for the customized transparent lateral flow strip. In practice the width of the illuminated portion 10 along the flow direction 7 is typically fixed for a given assay device I. Another advantage of using an assay device 1 having ultra-high (<1.5 pA) sensitivity may be that cheap, low grade solution processed organic light-emitting diodes may provide the light source(s) 8. For example, each light source 8 may be an organic light emitting diode having an external quantum efficiency of less than or equal to 0.5%, or less than or equal to 0.1%. Such low grade organic light-emitting diodes may not provide sufficient light 9 in many conventional readers, but may be used for the assay devices 1 according to this specification. Cheap, low grade solution processed organic light-emitting diodes may provide a cost advantage for disposable, single use assay tests intended for home use or use at point-of-care.

Figure 2 is a schematic illustration of a lateral flow test strip 16 which may provide a liquid transport path 2 according to some embodiments of the present disclosure.

The liquid transport path 2 may take the form of a lateral flow test strip 16, and may be integrated into the assay device 1 (Figure 10), or received into the assay device 1 (Figures 11A, 11B) Lateral flow test strips 16 (also known as "lateral flow immunoassays") are a variety of biological testing kit. Lateral flow test strips 16 may be used to test a liquid sample 6, such as saliva, blood or urine, for the presence of a target analyte. Examples of lateral flow test strips 16 include home pregnancy tests, home ovulation tests, tests for other hormones, tests for specific pathogens and tests for specific drugs. Lateral flow test strips 16 may also be used for testing food and/or drink products to determine the presence or concentration of impurities and so forth.

A typical lateral flow test strip 16 includes a porous strip 15 supported on a substrate 20 which, as mentioned here nbefore, is typically opaque, or at least not selected for the purpose of providing transparency. Portions of the porous strip 15 are treated with reagents to define a test region 21 and, optionally, a control region 22.

To use a typical lateral flow test strip 16, a liquid sample 6 is introduced to the sample receiving portion 5 proximate to a first end 3 of the porous strip 15, and the liquid sample 6 is then drawn along the lateral flow test strip 16 towards the second end 4 by capillary action (or "wicking"). A conjugate pad 44 (Figure 10) of the lateral flow strip 16 is pre-treated with labelling particles (not shown) which are activated with a reagent which binds to the target analyte to form a complex if the target analyte is present in the liquid sample 6. The bound complexes, and also unreacted labelling particles continue to propagate along the lateral flow test strip 16 in the flow direction 7 before reaching a test region 21 which is pre-treated with an immobilised binding reagent which binds complexes of analyte bound to labelling particles and does not bind unreacted labelling particles. The labelling particles have a distinctive colour, or otherwise absorb one or more ranges of ultraviolet, infrared or visible light. The development of a concentration of labelling particles in the test region 20 may be measured and quantified using the assay device 1, as described herein. The assay device 1 may perform measurements on developed lateral flow test strips 16, i.e. the liquid sample 6 has been left for a pre-set period to be drawn along the test strip 16. Alternatively, the assay device 1 may perform kinetic, i.e. dynamic time resolved measurements of the absorbance (sometimes also referred to as "optical density") of the test region 21. The control region 22 is treated with an immobilised binding reagent which binds unreacted labelling particles, and optionally also complexes of analyte bound to labelling particles not bound in the test region 21. A change in absorbance of the control region 22 provides an indication that the assay has operated correctly, and protects against some types of false negative results.

The porous strip 15 may include or be formed of nitrocellulose, or other fibrous materials capable of transporting an aqueous liquid by capillary action, whether inherently or following appropriate surface treatments. In some examples, the substrate 20 may be opaque. For example, the substrate 20, or the overall lateral flow test strip 16, may correspond to an effective transmittance Te for the substrate of less than or equal to 10%, less than or equal to 8°A, less than or equal to 5%, less than or equal to 3%, or less than or equal to 1%.

As assay device 1 may be configured to illuminate more than one portion 10 of a liquid transport path 2 received or receivable by the assay device 1, Each illuminated portion 10 may, in use, be illuminated by one or more light sources 8 and may correspond to one or more photodiodes 11. At least one illuminated portion 10 will correspond to a region of interest 23 of the liquid transport path 2. Optionally, one or more further illuminated portions 10 may correspond to reference regions 24 of the liquid transport path 2. Output signals 14 corresponding to reference regions 24 may be used for performing background corrections for output signals 14 corresponding to regions of interest 23.

For example, for a liquid transport path 2 in the form of a lateral flow test strip 16, as schematically illustrated in Figure 2, first and second regions of interest 23a, 23b may correspond to the test region 21 and control region 22 respectively. Additionally, first, second and third reference regions 24a, 24b, 24c may be defined before the test region 21, between the test region 21 and the control region 22, and after the control region 22 (with respect to the flow direction 7, x). The reference regions 24a, 24b, 24c may be used to correct absorbance values for the regions of interest 23a, 23b for a background absorbance of the porous strip 15 and substrate 20. One reference region 24 could be used. However, it may be useful to obtain values for three or more reference regions to account for any variable background signal corresponding to, for example, unbound labelling particles and/or coloured substances present in the liquid sample 6. Regions of interest 23 and/or reference regions 24 need not have equal widths parallel to the flow direction 7.

The preceding discussion of regions of interest 23 and reference regions 24 has been illustrated with reference to a liquid transport path 2 in the form of a lateral flow test strip 16. However, regions of interest 23 and reference regions 24 are equally applicable to a liquid transport path 2 in the form of a channel 17 of a microfluidic device 18 (Figure 14).

Figure 3 is a schematic illustration of an example of a photocurrent processing channel 25 which may be used in assay devices 1 according to some embodiments of the present disclosure.

The photocurrent processing channel 25 includes an amplifier 26 and an analog-to-digital convertor (ADC). The amplifier 26 receives and amplifies the photocurrent 13, and provides an amplified photocurrent 28 to the ADC 27. The amplifier 26 may take the form of a trans-impedance amplifier, for example an operational amplifier (Figure 5). The ADC 27 converts the amplified photocurrent 28 into a digital signal 29. In some examples, the digital signal 29 may correspond to the output signal 14. In some examples, one or more photocurrent processing channels 12 may include a microcontroller having an integrated ADC. Each ADC 27 preferably has a precision greater than 8-bits, for example, each ADC 27 may have a precision of 10-bits, 12-bits or 16-bits. In some examples, the dynamic range of the ADC 27 is set to a level such that a quantisation step of the ADC 27 (minimum detectable shift in input) is less than, or significantly less than, an amplitude of a noise signal in the amplified photocurrent 28.

Optionally, the photocurrent processing channel 25 may also include a filtering module 30. The ADC 27 may oversample the amplified photocurrent 28. In other words, the ADC 27 may sample the amplified photocurrent 28 at a sample rate which is very much higher than the typical timescale for changes in the absorbance of the test or control regions 21, 22. For example, the ADC 27 may use a sampling rate of hundreds, thousands or preferably tens of thousands of Hz. By contrast, the absorbance of the test or control regions 21, 22 will typically exhibit noticeable changes on timescales of the order of at least seconds or tens of seconds. Provided that a quantisation step of the ADC 27 is less than a typical amplitude of a noise signal in the amplified photocurrent 28, the filtering module 30 may apply an averaging process to the digital signal 29 in order to effectively reduce the noise in the output signal 14. This will tend to improve the signal-to-noise ratio of the output signal 14 by reducing the error by a factor of (N-1)-'5, where N is the number of samples in the digital signal 29 averaged to produce a corresponding sample of the output signal 14.

Additionally, the period used by the filtering module 30 for obtaining an average may be set specifically to average out any interference (also sometimes called "pick-up") corresponding to a mains electricity supply. By setting an appropriate averaging period, corresponding to one or more cycles of a mains electricity signal, any component of interference from the mains electricity signal would be expected to be reduced or removed from the output signal 14. A mains electricity signal may have a frequency within the range between and including 50 Hz to 60 Hz. In some examples, the ADC 27 may obtain one thousand samples during one period of the mains electricity signal, and a corresponding sample in the output signal 14 may be based on an average across said one thousand samples. The filtering module 30 may be implemented by a processor of a microcontroller executing a compiled computer program stored in a memory of the microcontroller, a suitably configured field-programmable gate array, one or more digital electronic processors executing a compiled computer program, and so forth. The amplifier 26, ADC 27 and/or filtering module 30 may be integrated as a single chip, for example an application specific integrated circuit.

In some examples, the filtering module 30 may instead be implemented in the analog domain before digitisation by the ADC 27, for example using a time-averaging circuit Detemiining a minimum detectable photocurrent Figure 4 presents measurements of dark current corresponding to a pair of photocurrent processing channels 12 of an assay device 1 according to some embodiments of the present disclosure.

The solid black line corresponds to a first photocurrent processing channel 12, 25, and the dashed line corresponds to a second photocurrent processing channel 12, 25. In total, dark currents were measured for five separate photocurrent processing channels 12, 25, over a total duration of about 90 seconds. Parameters of the measurements are presented in Table 1:

Table 1

Photocurrent processing channel Standard error (pA) 1 0.474 2 0.451 3 0.455 4 0.518 0.435 Average error: 0.467 It may be observed that the standard error was, on average, 0.467 pA, and consequently applying a standard assumption that a signal of at least three times the standard error may be reliably detected, the minimum detectable photocurrent based on these data is about 1.4 pA. In other examples, if even greater reliability is necessary, the minimum detectable photocurrent may be taken as five times the standard error of the photocurrent processing channel 12 dark current.

The minimum detectable photocurrent may be reduced further by increasing the gain of the amplifier 26, the oversampling rate of the ADC 27, the precision of the ADC (e.g. increasing the number of bits) and/or by using a higher quality photodiode I. Example circuit for the photocurrent processing channel Figure 5 schematically illustrates a non-limiting example of a circuit 35 for providing all or part of a photocurrent processing channel 12, 25 of an assay device 1 according to some embodiments of the present disclosure The amplifier 26 may be provided by a first operational amplifier Al, having an inverting input connected to a first node 31, a non-inverting input connected to a second node 32, and an output connected to a third node 33. A feedback network controls the gain of the first operational amplifier Al and includes a capacitor (1/ and a resistor/2f; connected in parallel between the first and third nodes 31, 33. A corresponding photodiode 11 is connected between the first and second nodes 31, 32. The ADC 27 has a resolution of 16 bits, and has a positive input connected to the third node 33 and a negative input connected to the second node 32. A second operational amplifier A2 has an inverting input and an output both connected to the second node 32, and a non-inverting input connected to a fourth node 34. The fourth node 34 corresponds to the mid-point of a voltage divider formed from a pair of resistances Ru. One resistance Rd connects between the fourth node 34 and ground, whilst the other resistance 16 connects between the fourth node 34 and a reference voltage terminal of the ADC 27.

The circuit 35 may be particularly useful when multiple photodiodes I I are configured with a common cathode. The circuit 35 may have an effect of applying half the ADC 27 reference voltage Vref to the non-inverting input of the first operational amplifier Al and the photodiode 11 cathode, and to measure a voltage difference between the output of the first operational amplifier Al and Vref. This may sometimes be described as using the ADC 27 in a "pseudo-differential" mode. In some examples, the amplifier 26, Al may configured with a gain of 1/107, and a response/integration time constant of RC = 2.2 ms (bandwidth -450 Hz). Note that oversampling by the ADC 27 at higher rates that the amplifier 26 bandwidth may still be useful to remove noise and/or mains electrical pickup in the amplified signal 28 output by the amplifier 26.

The ADC 27 output Vow may provide the output signal 14. Alternatively, when a filtering module 30 is used, the ADC 27 output V./ may provide the digital signal 29 to the filtering module 30.

The date presented in Figure 4 and Table I were obtained using the circuit 35 with an ADC sampling rate of 50 kHz and subsequent processing by a filtering module 30 to obtain averages over 20 ms periods, i.e. over 1,000 samples.

Second assay device Figure 6 is a schematic illustration of a second assay device 36 according to some embodiments of the present disclosure.

The second assay device 36 is the same as the assay device 1, except that the second assay device 36 further includes a determination module 37. Only part of the second assay device 36 is schematically illustrated in Figure 6 in the interests of brevity. The determination module 37 is configured to determine, based on the received output signals 14, the presence and/or concentration of a target analyte within the liquid transport path 2 In some examples, an absorbance measured using the output signals 14 may be converted to an estimate of a concentration of the target analyte using an empirical relationship established based on calibration experiments performed using known concentrations of the target analyte The determination module 37 may be programmed with one or more suitable empirical relationships. The determination module 37 may provide an output 38 in the form of one or more concentration values for the target analyte and/or a flag denoting the presence or absence of the target analyte.

The determination module 37 may be implemented by a processor of a microcontroller executing a compiled computer program stored in a memory of the microcontroller, using a suitably configured field-programmable gate array, using one or more digital electronic processors executing a compiled computer program and so forth. The one or more photocurrent processing channels 12 and the determination module 37 may be integrated in a single unit, for example an application specific integrated circuit.

Third assay device Figure 7 is a schematic illustration of a third assay device 39 and an external processing device 40, according to some embodiments of the present disclosure.

The third assay device 39 is the same as the assay device 1, except that the third assay device 39 further includes a communications interface 41. Only part of the third assay device 39 is schematically illustrated in Figure 7 in the interests of brevity. The communications interface 41 is configured to provide the output signals 14 to the external processing device 40. The communications interface 41 may operate according to any suitable wired or wireless data transmission protocol, for example a wired connection via universal serial bus or a wireless connection via Bluetooth (RTM). The external processing device 40 may be a desktop computer, a laptop computer, a tablet computer, a mobile telephone, a handheld purpose specific computing device, and so forth.

The external processing device 40 is configured to determine, based on the received output signals 14, the presence and/or concentration of a target analyte within the liquid transport path 2. In some examples, an absorbance measured using the output signals 14 may be converted to an estimate of a concentration of the target analyte using an empirical relationship established from calibration experiments performed using known concentrations of the target analyte. The external processing device 40 may be programmed with one or more suitable empirical relationships. The external processing device 40 may provide the output 38 in the form of one or more concentration values for the target analyte and/or a flag denoting the presence or absence of the target analyte.

The external processing device 40 may be implemented by a processor of a microcontroller executing a compiled computer program stored in a memory of the microcontroller, using a suitably configured field-programmable gate array, using one or more digital electronic processors executing a compiled computer program and so forth.

First method Figure 8 is a process flow diagram which schematically illustrates a first method of using an assay device 1, 36, 39 according to some embodiments of the present disclosure.

A quantity of liquid sample 6 is applied to the sample receiving portion 5 of the liquid transport path 2 (step SO.

The liquid sample 6 is left for a duration at least long enough for the liquid sample 6 to propagate through at least one illuminated portion 13 of the liquid transport path 2 (step S2). The propagation period may be a pre-set interval, for example, 5 minutes, 10 minutes, as determined from calibration experiments.

Output signals 14 are obtained from the one or more photocunent processing channels 12 of the assay device 1(step S3). Output signals 14 may be obtained over a second duration long enough to ensure that the flow front 19 has passed through the liquid transport path 2, and that the assay has had sufficient development time. In some examples, dynamic measurements may be obtained to permit tracking the rate of change of absorbance in each region of interest 23, as well as a final value.

Based on the output signals 14, the presence and/or concentration of a target analyte within the liquid transport path 2 is determined (step S4). In some examples, an absorbance measured using the output signals 14 may be converted to an estimate of a concentration of the target analyte using an empirical relationship established from calibration experiments performed using known concentrations of the target analyte. The determination of the presence and/or concentration of a target analyte within the liquid transport path 2 may be carried out by the determination module 37 forming part of the assay device 1, 36, or by an external processing device 40 which is separate from the assay device 1, 39.

If further measurements are to be conducted, the first method is repeated for a new liquid transport path 2 and/or assay device I, 36, 39 (step S5).

The first method is applicable to assay devices 1, 36, 39 which incorporate an integral liquid transport path 2 The first method is also applicable to assay devices 1, 36, 39 into which the liquid transport path 2 is received after application of the liquid sample 6.

Second method Figure 9 is a process flow diagram which schematically illustrates a second method of using an assay device 1, 36, 39 according to some embodiments of the present disclosure In examples where the liquid transport path 2 is separate from the assay device 1, 36, 39, the liquid transport path 2 may be received into the assay device 1, 36, 39(step S6a). In examples in which the liquid transport path 2 is integrated as part of the assay device 1, 36, 39, this step is redundant If a liquid sample 6 has not previously been introduced to the liquid transport path 2, then the liquid sample 6 is applied to the sample receiving portion (step S6I3). A delay to permit propagation of the liquid sample 6 may be necessary. Alternatively, the second method may be commenced with a liquid transport path 2 already emplaced between the light source(s) 8 and photodiode(s) 11, and having allowed time for the assay to develop.

The one or more light sources 8 are illuminated to direct light 9 through one or more illuminated portions 10 of the liquid transport path 2, each illuminated portion I 0 being located between the sample receiving portion 5 and the second end 4 (step S7).

The one of more photodiodes 11 receive the light 9 transmitted through the one or more illuminated portions 10 of the liquid transport path 2 (step 58). Each photodiode 11 supplies a photocurrent 13 to a corresponding photocurrent processing channel 12.

The one or more photocurrent processing channels 12 receive photocurrent(s) 13 from corresponding photodiodes 11, and each photocurrent processing channel 12 processes the received photocurrent 13 to generate a corresponding an output signal (step S9). The resolution of each photocurrent processing channel 12 for detecting received photocurrent 13 is less than 1.5 pA. Generating each output signal 14 may include obtaining an average across a period corresponding to one or more cycles of a mains electricity signal using the filtering module 30, such that any contribution of interference/pick-up from the mains electricity signal is reduced or removed from the output signal 14.

Optionally, the presence and/or concentration of a target analyte within the liquid transport path 2 may be determined by the determination module 37 or the external processing device 40 (step 510).

If further measurements are to be conducted, the second method is repeated for a new liquid transport path 2 and/or assay device 1, 36, 39 (step Si!).

The second method is applicable to assay devices 1, 36, 39 which incorporate an integral liquid transport path 2. The second method is also applicable to assay devices I, 36, 39 into which the liquid transport path 2 is received after application of the liquid sample 6.

Self-contained assay device Figure 10 is a schematic illustration of a self-contained assay device 42 including an integral liquid transport path 2, according to some embodiments of the present disclosure.

The assay device 1, 36, 39 may take the form of a self-contained lateral flow testing device 42.

The lateral flow testing device 42 includes a liquid transport path 2 in the form of a porous strip 15 divided into a sample pad 43, a conjugate pad 44, a test pad 45 and a wick pad 46. The porous strip 15 is supported by a substrate 20, and the lateral flow test strip 16 is received into a base 47. A lid 48 is attached to the base 47 to secure the lateral flow test strip 16 and cover parts of the lateral flow test strip 16 which do not require exposure. The lid 48 includes a sample receiving window 49 which exposes part of the sample pad 43 to define the sample receiving portion 5 The lid 48 and base 47 are made from a polymer such as, for example, polycarbonate, polystyrene, polypropylene or similar materials The base 47 includes a recess 50 into which a pair of light sources 8 are received (for example organic light-emitting diodes). The lid 48 includes a recess 51 into which a pair of photodiodes 11 are received. One pair of a light source 8 and a corresponding photodiode 11 are arranged on opposite sides of a test region 21 formed in the test pad 45 of the porous strip 15. A second pair of a light source 8 and a corresponding photodiode 11 are arranged on opposite sides of a control region 22 formed in the test pad 45 of the porous strip 15. Slit members 52 separate the light sources 8 from the porous strip 15 to define narrow slits 53 with widths typically in the range between 300 um to 500 pm inclusive. The slit members 52 define slits 53 which extend transversely across the width of the porous strip 15. For example, if the porous strip 15 extends in a first direction r and has a thickness in a third direction z, then the slits 53 extend in a second direction y. Further slit members 52 define slits 53 which separate the photodiodes II from the porous strip 15. The slits 53 may be covered by a thin layer of transparent material to prevent moisture entering into the recesses 50, 51. Material may be considered to be transparent to a particular wavelength A, or range of wavelengths 41, if it transmits more than 50%, more than 75%, more than 85%, more than 90% or more than 95% of the light at that wavelength A, or within the range of wavelengths Al. A diffuser (not shown) may optionally be included between each light source 8 and the corresponding slit 53.

A controller 54 is housed within the base 47, and integrates the functions of a pair of photocurrent processing channels 12 and the determination module 37. Alternatively, the controller 54 may be housed within the lid 48. One or more output devices 55 are housed in the lid 48. The one or more output devices 55 may be provided elsewhere in the lateral flow testing device 42 provided that the output devices 55 are visible and/or accessible to a user when the lateral flow testing device 42 is resting on a flat surface with the sample receiving window 49 facing up. The output device(s) 55 may display an indication of the output 38 in the form of a presence or concentration of the target analyte in the liquid sample 6. The output devices 55 may display the results of comparing a measured concentration of the target analyte against a threshold. For example, the output devices 55 may provide indications in the form of "positive", or "negative" and so forth. The output devices 55 may include organic or inorganic light-emitting diodes, a liquid crystal display, a buzzer or sounder, and so forth Additionally or alternatively, the output devices 55 may also include a communications interface for outputting the results 38 via a wired or wireless connection. The controller 54 is connected to the light sources 8, photodiodes 11, output devices 56 and a battery (not shown) by suitable conductors (not shown). In other examples, the self-contained lateral flow testing device 42 may not need a battery (not shown) and may instead be powered via a communications interface such as, for example, a universal serial bus (USB) connection.

A liquid sample 6 suspected of containing a target analyte is introduced to the sample receiving portion 5 through the sample receiving window 49 using, for example, a dropper 56 or similar implement. In other examples a liquid sample 6 may be introduced by dipping the sample receiving window 49 in a container holding liquid sample 6, or by placing the sample receiving window 49 so as to intersect a flow of liquid sample 6, and so forth. The liquid sample 6 is transported along the liquid transport path 2 towards the second end 4 by a capillary, or wicking, action of the porosity of the porous strip 43, 44, 45, 46. The sample pad 43 of the porous strip 15 is typically made from fibrous cellulose filter material.

The conjugate pad 44 has been pre-treated with at least one particulate labelled binding reagent for binding an analyte which is being tested for, to form a labelled-particle-analyte complex (not shown). A particulate labelled binding reagent is typically, for example, a nanometre-or micrometre-sized label particle which has been sensitised to specifically bind to the analyte, for example, using antibodies or antigens. The particles provide a detectable response, which is usually a visible optical response such as a particular colour, but may take other forms. For example, particles may be used which are visible under infrared or ultraviolet light, and so forth. Typically, the conjugate pad 44 will be treated with one type of particulate labelled binding reagent to test for the presence of one type of analyte in the liquid sample 6. However, lateral flow devices 42 may be produced which test for two or more analytes using two or more particulate labelled binding reagents concurrently (for example multiple test regions 21). The conjugate pad 44 is typically made from fibrous glass, cellulose or surface modified polyester materials.

As the flow front 19 moves into the test pad 45, labelled-particle-analyte complexes and unbound label particles are carried along towards the second end 4. The test pad 45 includes one or more test regions 21 and control regions 22, each of which are monitored by a corresponding light source 8 and photodiode 11 pair. A test region 21 is pre-treated with an immobilised binding reagent which specifically binds the labelled-particle-analyte complexes and which does not bind the unreacted label particles. As the labelled-particle-analyte complexes are bound in the test region 21, the concentration of the label particles in the test region 21 increases. The concentration increase may be monitored by measuring the absorbance of the test region 21 using the corresponding light source 8 and photodiode 11 as described hereinbefore. The absorbance of the test region 21 may be measured once a set duration has expired since the liquid sample 6 was added. Alternatively, the absorbance of the test region 21 may be measured continuously or at regular intervals as the lateral flow strip 16 is developed.

To provide distinction between a negative test and a test which has simply not functioned correctly, a control region 22 is often provided between the test region 21 and the second end 4. The control region 22 is pre-treated with a second immobilised binding reagent which specifically binds unbound label particles and which does not bind the labelled-particleanalyte complexes. Alternatively, the control region 22 may be pre-treated with a nonspecific immobilised binding reagent which binds either unbound label particles or labelledparticle-analyte complexes. In this way, if the lateral flow testing device 42 has functioned correctly and the liquid sample 6 has passed through the conjugate pad 44 and the test pad 45, the control region 22 will exhibit a change in absorbance. The absorbance of the control region 22 may be measured by the second pair of a light source 8 and a photodetector 11, in the same way as the test region 21.

The test pad 45 is typically made from fibrous nitrocellulose, polyvinyl dene fluoride, polyethersulfone (PES) or charge modified nylon materials.

The wick pad 46 provided proximate to the second end 4 soaks up liquid sample 6 which has passed through the test pad 45 and helps to maintain through-flow of the liquid sample 6. The wick pad 46 is typically made from fibrous cellulose filter material Assay reader device Figures 11A and 11B are schematic illustrations of an assay device 1 in the form of a reading device 57 for receiving lateral flow test strips 16, according to some embodiments of the present disclosure.

Referring in particular to Figure 11A, the reading device 57 includes first and second light sources 8a, 8b (for example organic light emitting diodes), first and second photodiodes Ila, I I b, a controller 54, one or more output devices 55 and a battery 58, all contained within a casing 59 formed of a durable material such as plastic or metal. The casing 59 may be a single integrally formed piece, or may be formed from two or more pieces clipped or fastened together using conventional means. The reading device 57 also includes a slot 60 for receiving a liquid transport path 2. The slot 60 may be formed as a feature of the casing 59. The slot 60 has a first end open at an exterior of the reading device 57, and a second, closed end 61.

The first light source 8a faces the first photodiode lla across the slot 60 to form a first pair 8a, 11 a. The illuminated section 10 is delimited by slits 53 formed on either side of the slot 60. The slits 53 may be integrally formed as features of the casing 59. Similarly, at a distance spaced along the slot 60 from the first pair 8a, 11a, the second light source 8b faces the second photodiode 1lb across the slot 60 to form a second pair 8b, 1 lb. Referring in particular to Figure 11B, a liquid transport path 2 in the form of a lateral flow test strip 16 is received into the slot 60 and pressed into the reading device 57 until the second end 4 of the lateral flow test strip 16 abuts the closed end 61 of the slot 60. When the second end 4 of the lateral flow test strip 16 abuts the closed end 61 of the slot 60, the test region 21 is aligned within the illuminated portion 10 corresponding to the first light source 8a and the control region 22 is aligned with the illuminated portion 10 corresponding to the second light source 8b. In this way, an absorbance of the test region 21 may be measured using the first pair 8a, 11 a and an absorbance of the control region 22 may be measured using the second pair 8b, 1 lb. The controller 54 determines the presence and/or concentration of the target analyte in the test region 21, and may use the control region 22 to verify that the assay has been performed correctly. The results are output via the one or more output devices 55 as described hereinbefore, for example either directly to a user using one or more light emitters and/or displays, or via a wired or wireless communications interface.

Although two pairs of light sources 8a, 8b and 11 a, 1 lb are shown, the reading device 57 may include a pair of a light source 8 and a photodiode 11 corresponding to each region of interest 23 and optionally each reference region 24 of lateral flow test strips 16 to be measured Alternatively, instead of receiving a lateral flow test strip 16 in a predetermined position with respect to the light sources 8 and photodiodes 11, the reading device 57 may include a single light source 8 and photodiode I I pair, and a lateral flow test strip 16 may be passed through the illuminated portion 10 to scan the whole or a part of the length of the lateral flow test strip 16. In this way, multiple regions of interest 23 and/or reference regions 24 may be measured without requiring multiple light sources 8 and corresponding photodiodes 11.

Although Figure 11B illustrates a free-standing lateral flow test strip 16 being directly received into the reading device 57, in other examples the reading device 57 may be configured to receive lateral flow test strips 16 which are packaged within containers or cassettes (not shown). In some examples, lateral flow test strips 16 may be packaged with opaque (as previously defined) containers or cassettes and the reading device 57 may perform measurements through the opaque container or cassette. In other examples, the reading device 57 may include a moveable sample receiving stage (not shown) on or within which a packaged or free-standing lateral flow test strip 16 may be placed and/or secured. The sample receiving stage (not shown) may be moveable between a first position in which a lateral flow test strip 16 may be placed and/or secured, and one or more further positions in which one or more regions of interest 23 and/or reference regions 24 are arranged within corresponding illuminated portions (s) 10 of the reading device 57. When the reading device 57 is configured to scan a lateral flow test strip 16 through an illuminated portion 10, the sample receiving stage (not shown) may be motorised and controlled by the controller 54.

Measurements may be triggered automatically when a lateral flow test strip 16 is received into the reading device 57. For example, a micro-switch or light gate may determine when the second end 4 of the lateral flow test strip 16 abuts the closed end 61 of the slot 60. Alternatively, the reading device 57 may include an input device such as a switch or button which a user may actuate to trigger the controller 54 to obtain a measurement once a lateral flow test strip 16 has been loaded.

Illustrative experimental data The preceding discussion may be better understood with reference to illustrative experimental data obtained using an example of an assay device 1. The assay devices, 1, 36, 39, 42 and readers 57 described herein are not limited to the specific conditions and samples used to obtain the illustrative experimental data presented hereinafter.

Figure 12 presents measurements of changes in optical density (absorbance) of a test region 21 of a lateral flow test strip 16 over a duration following introduction of a liquid sample 5, and obtained using an example of an assay device 1 according to some embodiments of the present disclosure.

The data for Figure 12 were obtained using an assay device 1 having a photocurrent processing channel 12 which included the exemplary circuit 35 and also a filtering module 30. The data were obtained using an ADC 27 sampling rate of 50 kHz. Subsequent processing by the filtering module 30 obtained averages over 20 ms periods, i.e. over 1,000 samples.

Figure 13 presents measurements of changes in optical density (absorbance) ) of a test region 21 of a lateral flow test strip 16 over the duration following introduction of a liquid sample 5, and obtained using a comparative assay device (not shown) which lacked the sensitivity according to embodiments of the present specification.

The data for Figure 13 were obtained using a comparative assay device (not shown) which included an amplifier 26 having the same gain and bandwidth as the exemplary circuit 35, but having a different ADC. Unlike the circuit 35, the ADC used for comparative assay device was 12-bit, was operated at a sampling rate of 12.5 kHz, and was not configured to permit effective filtering of mains electrical 50Hz noise.

Comparing Figures 12 and 13, it may be observed that assay devices 1, 36, 39, 42 and readers 57 according to the present specification may exhibit substantially improved sensitivity, for example in terms of improved signal-to-noise which permits detecting smaller signals.

MODIFICATIONS

The present invention is not limited to the disclosed embodiments. It will be appreciated that many modifications may be made to the embodiments hereinbefore described Such modifications may involve equivalent and other features which are already known in the design, manufacture and use of assay devices and which may be used instead of or in addition to features already described herein. Features of one embodiment may be replaced or supplemented by features of another embodiment.

Some examples according to the present specification may take the form of testing kits including an assay device I, 36, 39 or reader 57 and a liquid transport path 2 adapted to be received by the assay device 1, 36, 39 or reader 57 Examples have been described in relation to liquid transport paths 2 in the form of lateral flow test strips 16 However, other types of liquid transport path 2 may be used such as, for example, one or more channels 17 (Figure 14) of a microfluidic device 18 (Figure 14).

Figures 14 is a schematic illustration of a fourth assay device 62 according to some embodiments of the present disclosure.

The fourth assay device 62 is similar to the assay devices 1, 36, 39 described hereinbefore, except that the liquid transport path 2 takes the form of one or more channels 17 of a microfluidic device 18. Only the portions of the fourth assay device 62 which differ from previous examples are shown. The one or more channels 17 are formed within a microfluidic structure 63 such as, for example, a section 64 defining the one or more channels 17, sealed with a top plate 65, for example a glass slide or a plastic sheet. In some examples, the section 64 may be moulded, milled, cast, and so forth. First and second ports 66, 67 may be used to inject and/or extract liquid sample 5 from the channel 17. Liquid sample 5 may be flowed continuously through the channel 17, for example from the first port 66 to the second port 67. Alternatively, liquid sample 5 may be introduced into the channel 17 and left for a predetermined duration. For example, an assay may be an enzyme amplified assay (ELISA), in which a target analyte may be bound to an enzyme and also bound to the channel 17 walls. In such an example, the liquid sample 5 may comprise a substrate which is converted into a labelling substance by the enzyme bound to the target analyte.

The use of ultra-sensitive (<1 5 pA) photocurrent processing channels 12 may enable improvements in sensitivity, use of opaque materials for the microfluidic structure 63 and/or use of low grade (EQE < 0.5%) organic light-emitting diodes, in a similar way as for the first, second or third assays devices 1, 36, 39, the self-contained assay device 42 and/or the reader 57 Two-colour background correction using an opaque substrate Figures 15 is a schematic illustration of fifth assay device 68 according to some embodiments of the present disclosure.

The fifth assay device 68 is the same as the assay devices 1, 36, 39, 42 and/or reader 57 described hereinbefore, except that the fifth assay device 68 includes one or more first light sources 69 and one or more second light sources 70 corresponding to each illuminated portion 10. Only the portions of the fifth assay device 68 which differ from previous examples are shown.

Each first light source 69 emits first light 71 having a first spectral bandwidth 411 centred at a first wavelength Ai, and each second light source 70 emits second light 72 having a second spectral bandwidth 422 centred at a second wavelength 22. The first spectral bandwidth 411 corresponds to light 71which is relatively strongly absorbed by a target analyte, or labelling particles or molecules bound to or associated with the target analyte. The second spectral bandwidth 422 corresponds to light 72 which does not significantly interact with a target analyte, or with labelling particles or molecules bound to, or associated, with the target analyte.

A lateral flow test strip 16 including a substrate 20 and a porous strip 15 is between, or may be received between, the light sources 69, 70 and photodiode(s) 11, such that the substrate 20 is between the light sources 69, 70 and the porous strip 15. The substrate is opaque, and strongly scatters light 71, 72 across a wide range of wavelengths. In this way, the substrate 20 may act as a diffuser, so that light sources 69, 70 which are placed side-by-side, or interdigitated, may provide relatively uniform illumination of the porous strip 15 within the illuminated portion 10.

The porous strip 15 is typically made of fibres (not shown) which scatter and/or absorb light across a broad range of wavelengths in an approximately similar way. For example, the proportion of light 71 at the first wavelength Al which is scattered or absorbed by the porous strip 15 is approximately the same as the proportion of light 72 at the second wavelength 22 which is scattered or absorbed by the porous strip 15. However, the porous strip 15 is not physically uniform, and the density of fibres may vary from point to point along the porous strip 15. Such background variations of absorbance, which are due to inhomogeneity of the porous strip 15, may limit the sensitivity of a measurement.

The fifth assay device 68 may compensate for such background variations of absorbance due to the inhomogeneity of the porous strip 15 (or other structure(s) providing the liquid transport path 2), provided that the target analyte, or labelling particles or molecules bound to or associated with the target analyte, have an absorbance spectrum which exhibits significant differences between the first and second wavelengths AI, 22. In particular the first spectral bandwidth Ail/ corresponds to first light '71 which is relatively strongly absorbed by a target analyte, or labelling particles or molecules bound to or associated with the target analyte, whereas the second light 72 having second spectral bandwidth 422 is relatively weakly absorbed.

The first and second light sources 69, 70 are illuminated alternately, and corresponding output signals 14 are recorded. The output signals 14 may be converted into a first absorbance Ai measured using the first spectral bandwidth 42i, and a second absorbance A2 measured using the second spectral bandwidth 4).2. The first absorbance Al includes contributions from the porous strip 15 and also from the target analyte, or labelling particles or molecules bound to, or associated with, the target analyte. There is also an additional contribution due to an absorbance of the substrate 20. By contrast, the second absorbance Az only includes a significant contribution from the porous strip 15, in addition to a contribution from the substrate 20. By obtaining the difference A7 -A2, or the weighted difference Ai -a,42 with a being a weighting factor, the relative contribution from the porous strip 15 (and the substrate) may be reduced or removed. In this way, the sensitivity of the fifth assay device 68 may be further improved.

The use of the opaque substrate 20 as a diffuser in order to implement a two-colour background correction is enabled by the ultra-sensitive (<1.5 pA) resolution of photocurrent processing channels 12 of assay devices 1, 36, 39, 42, 62, 68 and readers 57 according to the present specification.

The fifth assay device 68 has been described with a pair of light sources 69, 70 and a single photodiode 11. However, in an alternative configuration, a single light source 8 which emits light 71, 72 spanning, or at least including, both spectral bandwidths A),I, 422 may be used instead. In such an example, a first photodiode 11 may have a filter (not shown) which blocks the second spectral bandwidth 422 and a second photodiode 11 may have a filter which blocks the first spectral bandwidth AA/. For filtering on the photodiode 11 side, the lateral flow strip 16 is preferably oriented with the substrate 20 closest to the pair of filtered photodiodes 11. In this way, the substrate 20 may act as a diffuser for light 71, 72 which has passed through the porous strip 15, before the light 71, 72 reaches the first and second photodiodes 11.

The use of ultra-sensitive (<1.5 pA) photocurrent processing channels 12 may provide further flexibility in the design of assay devices 1, 36, 39, 42, 62, 68 and/or readers 57, enabling functional modifications which might decrease photocurrent signals below a detection threshold of a less sensitive devices.

For example, one or more reference regions 24 of the lateral flow test strip 16 may be made relatively narrower (for example <1 mm wide) compared to one or more regions of interest 23. The potential advantage from making reference regions 24 relatively narrower may originate from using regions of interest 23 which are at least wide enough to accommodate variability in the positions of test and control regions 21, 22 along the flow direction 7. In an example of a lateral flow test strip 16 in which an average distance between test and control regions 21, 22 is fixed, the width of regions of interest 23 can only be increased at the expense of decreasing the width of adjacent reference regions 24. Narrowing of the reference regions 24 will decrease a photocurrent 13 measured by a corresponding photodiode 11. However, using ultra-sensitive (<1.5 pA) photocurrent processing channels 12 according to the present specification may enable reference regions 24 to be relatively narrowed, whilst retaining sufficient resolution for background correction purposes. In this way, a width available for regions of interest 23 may be increased. Increasing the width of regions of interest 23 may permit relaxation of the tolerances for forming and/or positioning test and control regions 21, 22 with respect to an assay device 1, 36, 39, 42, 62, 68 or reader 57.

The alternative fifth assay device 68 described hereinbefore may employ a single light source 8 which emits light 71, 72 in combination with a pair of first and second photodiodes 11 having respective filters (not shown) which block light in the first or second spectral bandwidths 421, 422. The filters (not shown) may be printed on the top of photodiodes 11. Ideal filters would provide 100% transmission for a desired pass-band, for example the first spectral bandwidth 42i, and 0% transmission outside the desired pass-band, for example the second spectral bandwidth 422 (complete blocking). In practice, a printed filter may absorb a significant amount of light even in the desired pass-band, which decreases a photocurrent 13 measured by a corresponding photodiode 11. Ultra-sensitive (<1.5 pA) photocurrent processing channels 12 according to the present specification may enable filters used for two-colour background subtraction to be made relatively thicker in order to block more undesired light, without a corresponding decrease in the intensity of the transmitted light in the desired pass-band from becoming limiting.

The fifth assay device 68 (and the alternative using a pair of photodiodes) which implement a two-colour background correction works best when there is an efficient spatial intermixing of the two colours of light 71, 72 transmitted to, or received from, the liquid transport path 2 As an alternative to the use of a diffuser (not shown) or an opaque substrate 20 acting as diffuser, improved intermixing may be achieved by increasing a distance between the liquid transport path 2 and the photodiode 11 (for examples using a single light source 8 and pair of photodiodes 11), or between the liquid transport path 2 and the first and second light sources 69, 70 (for examples using two light sources 69, 70 and a single photodiode). Increasing the separation between a divergent light source 8, 69, 70 (and most light sources 8, 69, 70 are divergent to a greater or lesser extent) and a corresponding photodiode 11 acts to reduce corresponding photocurrent(s) 13. Consequently, use of ultra-sensitive (<1.5 pA) photocurrent processing channels 12 according to the present specification may enable increasing the separation between optical components whilst maintaining a sufficient resolution to perform measurements of an assay.

Some or all of the above modifications may be combined. For example a separation between a pair of light sources 69, 70 may be increased in combination with a decreased width of a reference region 24. This may compound a decrease in the photocurrent 13 corresponding to the reference region 24. In another example, relatively narrowed reference regions 24 may be combined with relatively thickened printed colour filters for a two-colour background correction. In such examples, the use of ultra-sensitive (<1.5 pA) photocurrent processing channels 12 according to the present specification may provide sufficient resolution for viable measurements of an assay to be obtained. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combination of features disclosed herein either explicitly or implicitly or any generalization thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

Claims 1. An assay device for determining the presence and/or concentration of a target analyte within a liquid transport path; wherein the assay device is adapted to at least partly receive a liquid transport path having a first end, a second end and a sample receiving portion proximate to the first end, the liquid transport path being adapted to transport a liquid sample received in the sample receiving portion towards the second end; the assay device comprising: one or more light sources configured to, when the liquid transport path is received by the assay device, illuminate one or more illuminated portions of the liquid transport path which are located between the sample receiving portion and the second end; one or more photodiodes configured to, when the liquid transport path is received by the assay device, receive light transmitted through an illuminated portion of the liquid transport path; one or more photocurrent processing channels, each photocurrent processing channel being configured to receive a photocurrent from a photodiode and to provide a corresponding output signal, wherein the minimum detectable photocurrent is less than or equal to 1.5 pA.
2. An assay device according to claim I., wherein the one or more processing channels comprise a filtering module configured to perform averaging across a period corresponding to one or more cycles of a mains electricity signal, such that any component of interference from the mains electricity signal is reduced or removed from the output signals.
3. An assay device according to claim I, wherein the assay device adapted to at least partly receive a liquid transport path having an effective transmittance Te of less than 10% wherein the effective transmittance lie is given by the ratio: 1 = S/So in which S is a measured output signal from a processing channel when the liquid transport path is received in the assay device and So is a measured output signal from the same processing channel when the liquid transport path is not received in the assay device
4. An assay device according to any one of claims I to 3, wherein the liquid transport path comprises a porous strip.
5. An assay device according to any one of claims I to 3, wherein the liquid transport path comprises one or more channels of a microfluidic device.
6. An assay device according to claim I, wherein each light source is an organic light emitting diode having an external quantum efficiency of less than or equal to 0.5%.
7. An assay device according to any one of claims 1_ to 6, further comprising a determination module configured to determine, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path.
8. An assay device according to any one of claims 1_ to 6, further comprising a communications interface configured to provide the output signals to an external processing device.
An assay test comprising: an assay device according to any one of claims 1 to 8, and a liquid transport path at least partly received by the assay device.
10. A system comprising: an assay device according to claim 8; a liquid transport path at least partly received by the assay device; and an external processing device configured to determine, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path.
11 An assay test according to claim 9 or a system according to claim 10, wherein the liquid transport path comprises a porous strip supported on an opaque substrate, wherein the liquid transport path is arranged so that the one or more light sources illuminate the porous strip through the opaque substrate
12. A testing kit comprising: an assay device according to any one of claims 1 to 8; and a liquid transport path adapted to be received by the assay device.
13. A method of using an assay device, assay test, system or testing kit according to any one of claims Ito 12, the method comprising: applying a quantity of liquid sample to the liquid transport path; waiting for a duration at least long enough for the liquid sample to propagate through at least one illuminated portion of the liquid transport path; obtaining output signals from the one or more photocurrent processing channels; and determining, based on the output signals, the presence and/or concentration of a target analyte within the liquid transport path.
14 A method of determining the presence and/or concentration of a target analyte within a liquid transport path having a first end, a second end and a sample receiving portion proximate to the first end, the liquid transport path being adapted to transport a liquid sample received in the sample receiving portion towards the second end, the method comprising: illuminating, using one or more light sources, one or more illuminated portions of the liquid transport path which are located between the sample receiving portion and the second end; receiving, using one of more photodiodes, light transmitted through the one or more illuminated portions of the liquid transport path; generating, using one or more photocurrent processing channels which receive photocurrent from corresponding photodiodes, an output signal, wherein the resolution for detecting received photocurrent is less than 1 5 pA
15. A method according to claim 14, wherein generating each output signal comprises obtaining an average across a period corresponding to one or more cycles of a mains electricity signal, such that any contribution of interference from the mains electricity signal is reduced or removed from the output signal.
16. A method according to claims 14 or 15, wherein the liquid transport path has an effective transmittance To of less than 10%, wherein the effective transmittance To is given by the ratio: = S/So in which S is a measured output signal from a processing channel when the liquid transport path is received between one or more light sources and one or more photodiodes, and So is a measured output signal from the same processing channel when the liquid transport path is not received between one or more light sources and one or more photodiodes.