WO2015099546A1

WO2015099546A1 - Systems and methods for sensing ascorbate in liquid samples

Info

Publication number: WO2015099546A1
Application number: PCT/NZ2014/050027
Authority: WO
Inventors: Michael George FITZPATRICK; David Jackson PERRY; Graham Ralph PRONK-JONES
Original assignee: Feedback Research Limited
Priority date: 2013-12-23
Filing date: 2014-12-23
Publication date: 2015-07-02

Abstract

An electrochemical sensor apparatus (2) is provided for facilitating an electrochemical reaction of the liquid sample. The apparatus (2) comprises; a sensor base (3); and an electrode system (3a) formed on said sensor base wherein at least one electrode in the electrode system comprises graphene.

Description

SYSTEMS AND METHODS FOR SENSING ASCORBATE IN LIQUID SAMPLES TECHNICAL FIELD OF THE INVENTION

The present invention relates to systems and methods for sensing ascorbate in liquid samples. in particular, though not solely, the present invention is directed to a sensing apparatus including a disposable electrochemical sensor for quantifiable detection of ascorbate concentration in liquid samples. The Liquid samples may be biological samples, such as a blood sample.

BACKGROUND OF THE INVENTION

Vitamin C or ascorbic acid (or simply ascorbate) plays a vital role in the human body. It helps the human body to fight infections such as the common cold.

Further, it acts as an antioxidant that can neutralize harmful free radicals and toxins. Ascorbate is also able to regenerate other antioxidants such as vitamin E and is required for the synthesis of collagen, the intercellular "cement" substance which gives structure to muscles, vascular tissues, bones, tendons and ligaments. Generally, monitoring of ascorbate in biological samples such as blood or urine, and beverages such as juices or wine, is done in a laboratory using sophisticated devices. There are no devices or systems available thus far that can dependably monitor ascorbate concentration outside the laboratory environment, for example at home or at a doctor's clinic.

One difficulty with measuring ascorbate concentration is that careful sample handling is required due to rapid degradation of ascorbate in ambient conditions. Also, biological fluids and beverages contain compounds that interfere with various methods of ascorbate analysis. These factors make reliable measurement challenging outside the laboratory environment

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common genera! knowledge in the art.

It is an object of the present invention to provide improved systems and methods for sensing ascorbate in liquid samples or to overcome one or more of the above shortcomings or address the above desiderata, or to at least provide the public with a useful choice.

Another object of the invention is to provide an apparatus or method for measuring ascorbate concentration in a biological sample.

Another object of the invention is to provide an apparatus or method for measuring ascorbate concentration using a disposable sensor.

BRIEF DESCRIPTION OF THE INVENTION

In a first aspect the present invention may be said to broadly consist in apparatus for measuring ascorbate concentration in a liquid sample, comprising

electrochemical sensor apparatus for facilitating an electrochemical reaction of the liquid sample.

Preferably said electrochemical sensor apparatus comprises,

a sensor base;

an electrode system formed on said sensor base wherein at least one electrode in the electrode system comprises graphene.

Preferably said electrode system consists of a working electrode, a reference electrode and a counter electrode.

Preferably said working electrode is formed of a material having high conductivity. Preferably the working electrode comprises graphene. Preferably the material used for making said working electrode consists of dried graphene ink.

Preferably said reference electrode comprises silver chloride.

Preferably said counter electrode comprises carbon.

Preferably said counter electrode is made from a material consisting of dried carbon ink.

Preferably the electrode system and/or conductors associated therewith are disposed on said sensor base using one or more of: printing, vapour deposition, vacuum deposition, moulding. Preferably the electrochemical sensor apparatus includes a filter means.

Preferably said filter is made from cellulose paper that allows the flow of said liquid sample to said electrode system. Preferably said filter also allows, or is adapted to allow, said liquid sample to evenly distribute over said electrode system.

Preferably said electrochemical sensor apparatus has a proximal end where the liquid sample is applied and a distal end having electrical contacts for electrically connecting the electrode system to measuring circuitry.

Preferably the electrode system is disposed on said proximal end and said electrical contacts are disposed on said distal end. Preferably said proximal end and said distal end are shaped differently, or at least one end includes an identifier or indicia, for identification of the proximal end and/or the distal end.

Preferably the electrochemical sensor apparatus is provided as a sensor strip. Preferably the sensor strip is disposable.

In a further aspect the invention broadly provides apparatus for measuring ascorbate concentration in a liquid sample, the apparatus comprising:

means for taking a plurality of measurements of current flow between a working electrode and a counter electrode of an electrochemical sensor, and; means for selecting one of the measurements as a measurement representative of the ascorbate concentration.

Preferably the means for selecting include one or more of; means for determining stability of a measurement circuit used to derive the current; means for determining whether the current is in a range; means for determining whether an input or inputs to the measurement circuit are stable.

means for detecting current flow between a working electrode and a counter electrode of an electrochemical sensor over a plurality of ranges of current flow, and;

means for selecting one of the ranges as a range for obtaining a measurement representative of the ascorbate concentration.

In a further aspect the invention broadly provides a method for measuring ascorbate concentration in a liquid sample, the apparatus comprising:

taking a plurality of measurements of current flow between a working electrode and a counter electrode of an electrochemical sensor, and;

selecting one of the measurements as a measurement representative of the ascorbate concentration.

detecting current flow between a working electrode and a counter electrode of an electrochemical sensor over a plurality of ranges of current flow, and; selecting one of the ranges as a range for obtaining a measurement representative of the ascorbate concentration.

In a further aspect the invention may broadly be said to consist in apparatus for measuring ascorbate concentration in a liquid sample, the apparatus comprising: means for making electrical contact with electrical contacts on an electrochemical sensor on which the liquid sample is placed;

means for applying voltage across a working electrode and a reference electrode of the electrochemical sensor,

means for measuring current flowing between said working electrode and a counter electrode of the electrochemical sensor; and

means for determining the concentration of ascorbate in said liquid sample based on the measured current. Preferably said apparatus applies a scaling factor to the measured current to determine concentration of ascorbate in said liquid sample.

Preferably said apparatus comprises a potentiostat circuit for determining the value of the ascorbate in said liquid sample.

Preferably said apparatus has a plurality of measurement ranges.

Preferably said apparatus measures the ascorbate concentration for three different range settings, high, medium and low range settings.

Preferably said apparatus has one or more switches to connect to high range circuit when measuring for high ranges.

Preferably said apparatus has one or more switches to connect to medium range circuit when measuring for medium ranges.

Preferably said apparatus has one or more switches to connect to low range circuit when measuring for low ranges. Preferably said apparatus has means for determining whether the potentiostat circuit was balanced during the measurement period.

Preferably said apparatus checks whether the potentiostat circuit remains balanced when measuring high range, medium range and low range values.

Preferably said apparatus has a calibration resistor for calibrating the potentiostat circuit before beginning the actual measurement process. Preferably said apparatus has means for applying a temperature correction to the obtained concentration of the ascorbate to give the actual concentration of ascorbate.

Preferably said apparatus has a display panel for displaying the ascorbate concentration.

Preferably said apparatus has a memory for storing the ascorbate concentration.

Preferably said last stored values calibration parameters, temperature correction factor are stored in said memory.

Alternatively said apparatus further comprises a means for receiving one or more removable storage for providing one or more of; last stored values calibration parameters, temperature correction factor.

Preferably said apparatus has one or more temperature sensors for measuring the temperature during the test process.

In another aspect the invention broadly consists in a method for determining the concentration of ascorbate in a liquid sample, the method including

calibrating apparatus for measuring ascorbate concentration in the liquid sample;

conducting electrochemical test and measurement on the liquid sample; and reporting concentration of the ascorbate.

Preferably calibrating said apparatus comprises:

setting up a calibration voltage;

determining whether said apparatus is reading as expected;

connecting a calibration resistor;

measuring any offsets; and

calibrating said apparatus based on measured offsets. Preferably said calibrating process is repeated for high range circuit, medium range circuit, and low range circuit measuring high range, medium range and low range concentrations.

Preferably parameters for calibrating said apparatus are stored in internal memory of the apparatus.

Alternatively said parameters may also be obtained from an external memory connected to the apparatus. Preferably conducting electrochemical test on the liquid sample comprises:

setting up a test voltage in said apparatus;

applying said test voltage to a sensor strip using the high range circuit; obtaining at least one value of the current flowing through the high range circuit;

applying said test voltage to said sensor strip using the medium range circuit;

obtaining at least one value of the current flowing through the medium range circuit;

applying said test voltage to said sensor strip using low range circuit; and obtaining at least one value of the current flowing through the low range circuit.

Preferably the method further comprises determining whether said high range circuit, medium range circuit and low range circuit remain balanced for the period when said electrochemical test was conducted.

Preferably said high range, medium range and low range current values are stored for reporting purposes.

Preferably reporting, concentration of the ascorbate in the liquid sample, comprises:

determining whether the low range current values are within a predefined limit and the low range circuit remains balanced;

using the low range current values if values remain within the predefined limit and the low range circuit was balanced;

if said low range current value did not remain within the predefined limit and the low range circuit did not remain balanced, determining whether the medium range current values are within a predefined limit and the medium range circuit remains balanced;

using the medium range current values if said medium range current values remain within the predefined limit and the medium range circuit was balanced; using high range current value if said medium range current values did not remain within the predefined limit and the medium range circuit did not remain balanced,

applying scaling factor to either one of the high range, medium range, and low range current value to convert the current value to concentration of the ascorbate; and

reporting said ascorbate concentration value.

Preferably the reporting further comprises applying a temperature correction to the obtained ascorbate concentration value to obtain the accurate concentration of the ascorbate adjusted to the test temperature. . In another aspect the invention broadly provides apparatus for determining an ascorbate concentration in a liquid sample, the apparatus comprising a circuit having means to select a plurality of ranges of ascorbate concentrations,

Preferably the apparatus includes balance detection means. Preferably the apparatus includes calibration means.

Preferably the apparatus includes one or more switching means to connect said sensor strip to a potentiostat circuit during the electrochemical test on the liquid sample.

Preferably said switching means disconnect said sensor strip during the calibration process.

In another aspect the present invention may be said to broadly consist in apparatus for measuring ascorbate concentration in a liquid sample as described herein with reference to any one or more of the accompanying drawings. In another aspect the present invention may be said to broadly consist in a potentiostat circuit for measuring ascorbate concentration in a liquid sample as described herein with reference to any one or more of the accompanying drawings. In another aspect the present invention may be said to broadly consist in a method for determining the concentration of ascorbate in a liquid sample as described herein with reference to any one or more of the accompanying drawings,

In another aspect the present invention may be said to broadly consist in a method of calibrating said apparatus for determining the concentration of ascorbate in a liquid sample as described herein with reference to any one or more of the accompanying drawings.

In another aspect the present invention may be said to broadly consist in a method of conducting electrochemical test on the liquid sample as described herein with reference to any one or more of the accompanying drawings.

In another aspect the present invention may be said to broadly consist in a reporting the concentration of ascorbate in liquid sample as described herein with reference to any one or more of the accompanying drawings.

As used herein the term "and/or" means "and" or "or", or both. As used herein "(s)" following a noun means the plural and/or singular forms of the noun.

The term "comprising" as used in this specification means "consisting at least in part of". When interpreting statements in this specification which include that term, the features, prefaced by that term in each statement, all need to be present, but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner. it is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example, 1 , 1.1 , 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7).

The entire disclosures of all applications, patents and publications, cited above and below, if any, are hereby incorporated by reference.

This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements and features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will now be described with reference to the accompanying drawings in which; Shows a diagrammatic illustration of an apparatus for measuring ascorbate concentration comprising an electrochemical sensor apparatus and a meter;

Shows a sensor strip for measuring ascorbate concentrations;

Shows an exploded view of the sensor strip, as shown in figure 2, showing different parts of the sensor strip;

Shows a potentiostat circuit for measuring ascorbate concentration;

Shows a flow chart of an overall process for determining ascorbate concentration;

Shows a flow chart for a calibration process for calibrating the electrochemical sensor;

Shows a flow chart for a test process depicting the steps to measure ascorbate concentration; and

Shows a flow chart for a reporting process for reporting the concentration of ascorbate to the user.

DETAILED DESCRIPTION OF THE INVENTION

Selected embodiments will now be described with reference to figures 1 through 7. Figure 1 shows an apparatus 1 for measuring ascorbate concentration comprising an electrochemical sensor 2 and a meter A. In one implementation

electrochemical sensor 2 is a disposable sensor strip. Electrochemical sensor 2 is referred as a sensor strip throughout the description of the invention for purposes of convenience. In a preferred embodiment, sensor strip 2 is generally a flat strip and is shaped such that one end of the sensor strip fits within a slot B of the meter A. Preferably the shape of the sensor strip is rectangular as it is easy to handle, however a variety of other shapes may be used.

As shown diagrammatically in figure 1 , the meter A may also include a display C which may provide reading of ascorbate concentrations and/or direction for a user. Buttons D may also be provided to allow user to interact with the device. Port E may be provided to allow the device to exchange information with or interface with another device. Referring to Figure 2, the sensor strip 2 has a proximal end 2a and a distal end 2b The liquid sample being measured, such as blood sample, urine sample, saliva, juice, wine is applied at the proximal end 2a and the distal end 2b is shaped to fit inside the slot B of the meter A. The proximal end 2a has an electrode system 4 where the liquid sample is applied to measure the concentration of ascorbate in the liquid sample. In one embodiment, the width/shape of the proximal end 2a and distal end 2b may vary to distinguish between the proximal end 2a and distal end 2b. In other embodiments, some kind of indicia or identifier is provided on the sensor strip to differentiate between the proximal end 2a and the distal end 2b.

Figure 2a shows an exploded view of the sensor strip 2 showing different parts or the layers of the sensor strip 2 in detail. The sensor strip 2 comprises a sensor base 3, the electrode system 4, a dielectric 5, a spacer 6, a filter 7 and a cover 8. The sensor base 3 is used as a substrate to carry electrically conductive and/or electrochemically reactive regions which may be applied by a printing process for example. The conductive or reactive regions, in use, form part of an electric circuit that is used to measure the concentration of ascorbate in the liquid sample as will be described further below. Preferably, the sensor base 3 is made up of inert material, for example polycarbonates. The electronic circuit is made up of electrically conductive tracks or paths path 3a, such as silver ink. The printed electronic circuit on the sensor base 3 defines the position of electrode system 4 and their respective electrical contacts 3b. The electrode system 4 is disposed on the sensor base 2 at the proximal end 2a of the sensor strip 2 and the electrical contacts 3b are placed at distal ends 2b. The electrode system 4 and electrical contacts are connected by a plurality of conductive tracks 3a.

The electrode system 4 consists of a working electrode 4a, a reference electrode 4b, and a counter electrode 4c. The working electrode 4a, the reference electrode 4b and the counter electrode 4c are connected to the electrical contacts 3b through conductive tracks 3a. The electrical contacts 3a are connected to corresponding electrical contacts (not shown) in meter A when end 2b of the sensor strip is inserted in slot B of the meter A. Working electrode 4a may be made from any conducting material. In a preferred embodiment of the invention the working electrode 4a is made from dried graphene ink. Particularly, graphene helps in elimination of interferences by oxidisable species. The invention may thus allow improved accuracy for use with samples of a biological or natural origin which may have other oxidisable species, as opposed to samples from artificial or manufactured solutions or compositions. The reference electrode 4b may be made from silver chloride, such as CI-4008 from Engineering Conductive Materials. Preferably no current flows through the reference electrode 4b. The counter electrode 4c is made up of dried carbon ink.

The electrode system 4 may be deposited on the sensor base 3 by different methods, such as vapour deposition, printing, vacuum deposition, or moulding.

The dielectric insulator 5 is positioned above the electrode system 4. Dielectric insulator 5 provides an insulation layer such that only the electrode system is exposed to the liquid sample, while the insulator covers the remainder of the strip apart from end 2b. The electric contacts 3b on distal end 2b are also not covered by dielectric insulator 5 so that they are exposed for electrical contact with the circuitry within the meter A. The dielectric layer 5 also helps to prevent short circuits.

The spacer 6 of the sensor strip 2 provides a housing for the filter 7, particularly a hydrophilic filter. Particularly, the spacer 6 engages with the filter 7 in a manner that prevents filter 7 from falling. The spacer 6 may be made from suitable inert material such as polycarbonates. The hydrophilic filter 7 is preferably made up of a material that causes a liquid sample to move to the electrode system 4, such as cellulose paper. The hydrophilic filter 7 restricts the entry of solid material to the electrode system 4. The filter 7 also allows the sample to evenly spread over the electrode system 4. In a preferred embodiment the pH of the hydrophilic filter is less than 7.

The cover 8 provides an upper housing for the hydrophilic filter 7. The cover 8 is a liquid impermeable membrane and includes a small aperture to allow access of the liquid sample to the hydrophilic filter 7. In a preferred embodiment cover 8 is also made from suitable inert materials such as polycarbonate.

The ascorbate concentrations are measured by connecting the sensor strip 2 to the meter A. In one embodiment the meter A is shaped to be suitable held in a user's hand. The meter A has a measurement circuit for performing an

electrochemical measurement on the sensor strip 2 to determine the concentration of ascorbate. Preferably said circuitry is a potentiostat system and is explained in further detail with reference to figure 3 below.

In one embodiment meter A has a display panel C to display the reading of the ascorbate concentration to the user. The meter A may also have buttons D to control the meter, or a bare minimum a power or simple activation switch. The meter A has a slot B into which the sensor strip 2 is inserted. Sensor strip 2 may also come in contact with the meter A by other appropriate means, for example placing sensor strip 2 on meter A.

Meter A may also have a storage memory or means, such as port E or possibly a wireless communication link to connect to external storage that stores the past readings. In one implementation, the past readings may be used to calibrate the electrochemical sensor 1. Those skilled in the art will appreciate that the meter may take a variety of forms, and in some embodiments may interact with other apparatus. For example the meter apparatus may interact with a smart phone, tablet or other computational or processing device.

In one embodiment the meter may simply comprise a housing for receiving the sensor strip, and may have means to connect to another device such as a smartphone. In this way the smart phone, for example, may provide an interface for the user to initiate and/or view a test and/or test results. Furthermore, the connectively provided to data networks by modern computer apparatus such as smartphones, tablet computers and personal computers allows the results of tests to be stored remotely and to be sent to other parties such as doctors or caregivers. In this manner, further information such as time and date of the test can be reliably gathered. Moreover, in some embodiments applications running on another device may also beneficially provide further user data such as heart rate, blood pressure or body temperature. In yet a further embodiment, use of the meter with apparatus that has the ability to communicate with remote apparatus or devices enables another party such as a doctor or caregiver to re-program the meter or change some operating parameters remotely.

The electrochemical sensor 1, when operating under the appropriate condition, is ascorbate specific and largely eliminates interference by oxidisable species, such as uric acid.

Fig. 3 illustrates a potentiostat system for determining the concentration of ascorbate in a liquid sample. The potentiostat uses the working electrode 4a, the reference electrode 4b and the counter electrode 4c to determine the

concentration of ascorbate in a liquid sample.

In one embodiment the present invention uses three range settings in the system; high range, medium range and low range based on the assumption that the concentration of ascorbate can be high, medium or low in the liquid sample. The liquid sample is tested in all the three ranges and a large number of readings are taken for all the three ranges. This helps to accurately determine concentration of ascorbate in the liquid sample and also minimizes any error that may arise.

In use, the distal end 2b of the sensor strip 2 is inserted into slot B of the meter A. Subsequently the liquid sample is applied to the sensor strip 2. For example, if a blood sample is to be applied to the sensor strip 2 the user's skin is pierced by a needle or a lancing device and the blood is dabbed to the sensor strip at its proximal end 2a. Similarly, other liquid samples can be applied to the sensor strip 2 using a dropper or a syringe. The sensor strip 2 is connected such that the electrical contacts 3b on the sensor strip 2 are in electrical connection with corresponding contacts of the circuitry in the meter device A.

Meter A then applies a test potential across the working electrode 4a and reference electrode 4b and the current is measured at the applied potential. The meter A must be capable of applying potential of a value that facilitates flow of electrons between the working electrode and the counter electrode. Ideally no current passes through the reference electrode 4b.

The potentiostat has an operational amplifier 10, which receives at the inverting input the potential of the reference electrode. The set reference potential is received at the non-inverting input, and the amplifier generates an error/correction signal which is sent via feedback loop to the counter electrode 4c. The counter electrode 4c based on the correction signal, supplies energy such that the reference potential is applied to the reference electrode i.e. the required potential difference is applied between working electrode 4a and reference electrode 4b. The reaction at the counter electrode 4c is equal and opposite to the redox reaction occurring at the working electrode 4a.

Subsequently, the potentiostat circuit switches between resistors 12, 14, 16 to measure the three different ranges of current, high range, medium range, and low range respectively by changing gain settings. Different gain settings correspond to different ranges of current high range, medium range and low range. A low gain setting corresponds to high current range, a medium gain setting corresponds to medium current range and low gain setting corresponds to high current. When measuring for high current (low gain setting), switch 8 connects resistor 2 to the feedback loop and other resistors 14 and 16 are out of the feedback loop. Similarly resistors 14 and 16 are connected by switch 18 to the feedback loop to measure medium and low current ranges. In use, a set working electrode voltage is applied at the non-inverting input and the reaction potential is received at the inverting input of an operational amplifier 20. The differential output obtained based on different gain setting is fed to the non- inverting input of the operational amplifier 22. The current is obtained at the output of the operational amplifier 22. The obtained current corresponds to concentration of ascorbate in the liquid sample and is then selected during the reporting process as illustrated in figure 6.

In an embodiment of the present invention, the operational amplifier 20 as shown in figure 3b is also used determine whether the circuit is balanced or not. If the inputs of the amplifier 20 have the same voltage then the circuit is balanced or is in linear controlled mode. If the inputs of the amplifier 20 are not same then the electrochemical sensor gives an indication that the system is unbalanced, i.e., the feedback loop is not holding the circuit in balance. For example the current being measured by the potentiostat is too high for the selected ranges. Then the readings for the next higher ranges are considered. Also the circuit is checked to determine whether the circuit remains balanced for next higher range.

Before determining concentration of ascorbate in a liquid sample the sensor undergoes a calibration process to determine whether the electrochemical sensor is operating as expected. In order to implement this process the electrochemical sensor uses a calibration resistor to make a determination whether the sensor is performing as expected. In one implementation, calibration parameters may be coded into the firmware of the potentiostat system. In another implementation, calibration parameters may be stored in an internal memory of meter A. In yet another implementation, calibration parameters may be obtained from an external memory device connected to the meter A.

Temperature correction may be applied to the obtained ascorbate concentration to accurately determine the ascorbate concentration in the liquid sample at the temperature of the test. The temperature correction parameters are stored in the memory of the meter and are applied to the algorithm that is used to calculate ascorbate concentration based on the measurement of actual temperature made by the meter's temperature sensors.

Figure 4 illustrates the three stages used in determining the level of ascorbate in the liquid sample. The electrochemical sensor 2 Is connected to the meter A and the liquid sample is applied to the sensor strip. Subsequently, the electrochemical sensor 1 is calibrated in step 40 to check whether the system is reading as expected. This step is illustrated in more detail in figure 5. The next step 42 is the actual test process when the electrochemical reactions occur at the electrode system 4 to determine the concentration of ascorbate for all the three current ranges. The test process is illustrated in figure 6. The last stage is the reporting process in step 44 and is show in figure 7. During this stage the actual value of the ascorbate is provided to the user and also the temperature correction are applied at this stage to eliminate the errors obtained due to temperature fluctuation. In one implementation, the potentiostat circuit has switching means (not shown) that disconnects the sensor strip 2, internally within the meter A without actually removing the sensor strip, from a measuring circuit when calibration process takes place and connects the sensor strip 2 when the test process takes place.

Figure 5 illustrates the calibration process for determining whether the

electrochemical sensor Is reading as expected. In use, the sensor strip 2 is provided to the meter A. The liquid sample to be tested is applied to the sensor strip 2 and then the calibration process takes place. In one implementation, the liquid sample can be applied first and then the sensor strip 2 can be connected with the meter A.

The process begins at step 50 with setting up calibration voltage and then in step 52 making a determination whether the analog to digital converter (ADCs) is reading as expected. Subsequently in step 54 the calibration resistor is connected to the potentiostat circuit to measure the variations or offsets in step 56 for the three ranges, high, medium and low. Particularly the low range circuit is connected first in step 58 and the readings are taken for this circuit. The readings obtained are compared with a standard value to measure the offsets. Based on the measured offsets the circuit is calibrated to measure in the expected ranges. Similarly the medium and high range circuits (in steps 60 and 62) are checked to measure the offset and the device is calibrated accordingly to read in expected ranges. The sensor strip 2 is disconnected internally for the time when the calibration resistor is connected, i.e., no reaction occurs on the sensor strip during the calibration process

The standard or expected values for the three ranges are either hard coded into the firmware or obtained from the internal/external memory. These values may be based on the tests conducted in the past.

Upon calibrating the electrochemical sensor 1 , the actual test process begins and is depicted in figure 6. The test process begins at step 70, a test voltage is set that is to be applied through the meter A. The test voltage is chosen which facilitates flow of electrons from or to the working electrode 4a. Subsequently by using the switching means the calibration resistor is

disconnected and the sensor strip with the liquid sample is connected to the potentiostat circuitry within the meter A in step 72. For example, the electrical contacts 3b on sensor strip 2 are electrically connected to electrical contacts inside the meter A which then connects the sensor strip to potentiostat circuit through the switching means.

In step 74, the test voltage is applied across the working electrode 4a and reference electrode 4b through the meter A. The readings across the high range circuit are taken first in step 76 which is connected for a pre-defined length of time. Many readings are taken using high range circuit. Subsequently the circuit is connected to medium range circuit in step 78 and low range circuit in step 80 for a set time and a number of readings are taken for medium and low range circuit. The readings are stored in the internal memory of the meter A and are then used for reporting purposes.

Also when the circuit is connected to high, medium, and low ranges for taking the readings of the ascorbate level, the inputs of the operational amplifier 20 are also checked to determine whether the circuit is balanced, i.e., operating in a linear controlled mode as explained earlier. If the inputs of the operational amplifier have the same voltage then the circuit remains balanced.

Figure 7 depicts the reporting process for reporting the value of the ascorbate concentration to the user in a manner that can be understood by the user operating the electrochemical sensor 1.

The process begins at step 82 with checking the result obtained when the low range circuit was connected. Firstly it is determined whether the low range limit are within the limits, i.e., whether the readings obtained are in predefined limit.

Secondly it is determined whether the circuit remained balanced (as explained above) for low ranges. If the circuit remained balanced and the readings are within the limits, the low range result is selected for reporting to the user in step 84. If the readings are not within the limit and the circuit did not remain balanced it can be concluded that the current being measured by the low range circuit is too high for the low range circuit.

In this situation reading of the medium range circuit is taken for similar analysis in step 86, If the reading of the medium range circuit is within the set range and the circuit remains balanced, this reading is used for reporting to the user in step 88. If the medium range result were not within the limit and the circuit did not remain balanced during the test process then the result is not considered for reporting, as the current being measured for the medium range circuit is too high for this circuit. Then the results of high range circuit are taken for reporting purposes in step 90. Subsequently, a scaling factor is applied to the selected result in step 92 (one of the high range, medium range and low range value). The scaling factor converts the current measured to the concentration of ascorbate in the liquid. For example, the reading indicates the concentration of ascorbate in the liquid sample. Temperature correction is also applied to the obtained ascorbate level in step 94 to account for the effect of temperature variations caused during the test procedure. The electrochemical sensor may have temperature sensors to measure the temperature throughout the test procedure. Based on stored and captured temperature value appropriate temperature correction is applied to the value of the ascorbate concentration.

The obtained ascorbate value is displayed to the user in step 96. In one implementation the value is also stored for future references. It will be seen that the invention allows a convenient, low cost and reliable system for measuring ascorbate concentration in a liquid sample, particularly a biological sample such as a blood sample. The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the appended claims.

Claims

WHAT WE CLAIM IS:

1. An apparatus for measuring ascorbate concentration in a liquid sample,

comprising: an electrochemical sensor apparatus for facilitating an

electrochemical reaction of the liquid sample.

2. The apparatus as claimed in claim 1, wherein the electrochemical sensor apparatus comprises,

a sensor base; and

3. The apparatus as claimed in claim 2, wherein the electrode system consists of a working electrode, a reference electrode and a counter electrode.

4. The apparatus as claimed in claim 3, wherein the working electrode is formed of a material having high conductivity.

5. The apparatus as claimed in claim 4, wherein the working electrode comprises graphene.

6. The apparatus as claimed in any one of claim 4 or 5, wherein the material used for making said working electrode consists of dried graphene ink.

7. The apparatus as claimed in claim 3, wherein the reference electrode

comprises silver chloride.

8. The apparatus as claimed in claim 3, wherein the counter electrode comprises carbon.

9. The apparatus as claimed in claim 8, wherein the counter electrode is made from a material consisting of dried carbon ink.

10. The apparatus as claimed in any one of claims 2 to 9, wherein the electrode system and/or conductors associated therewith are disposed on said sensor base using one or more of: printing, vapour deposition, vacuum deposition, moulding.

11. The apparatus as claimed in any one of claims 1 to 10, wherein the

electrochemical sensor apparatus includes a filter means.

12. The apparatus as claimed in claim 11, wherein the filter is made from a

cellulose paper that allows the flow of said liquid sample to said electrode system,

13. The apparatus as claimed in claim 11 or 12, wherein the filter allows, or is adapted to allow, said liquid sample to evenly distribute over said electrode system.

14. The apparatus as claimed in any one of claims 1 to 13, wherein the

electrochemical sensor apparatus has a proximal end where the liquid sample is applied and a distal end having electrical contacts for electrically connecting t e electrode system to measuring circuitry.

15. The apparatus as claimed in claims 14, wherein the electrode system is

disposed on said proximal end and the electrical contacts are disposed on said distal end.

16. The apparatus as claimed in claim 14 or 15, wherein the proximal end and the distal end are shaped differently, or at least one end includes an identifier or indicia, for identification of the proximal end and/or the distal end.

17. The apparatus as claimed in any one of claims 17, wherein the

electrochemical sensor apparatus is disposable.

18. An apparatus for measuring ascorbate concentration in a liquid sample, the apparatus comprising:

means for taking a plurality of measurements of current flow between a working electrode and a counter electrode of an electrochemical sensor; and means for selecting one of the measurements as a measurement representative of the ascorbate concentration.

19. An apparatus for measuring ascorbate concentration in a liquid sample, the apparatus comprising:

means for detecting current flow between a working electrode and a counter electrode of an electrochemical sensor over a plurality of ranges of current flow; and

means for selecting one of the ranges as a range for obtaining a

measurement representative of the ascorbate concentration.

20. A method for measuring ascorbate concentration in a liquid sample, the

apparatus comprising:

taking a plurality of measurements of current flow between a working electrode and a counter electrode of an electrochemical sensor; and

21.A method or apparatus for measuring ascorbate concentration substantially as herein described.