Calibration electrode

Abstract

A calibration electrode for calibrating a reference system of an electrochemical sensor, such as a potentiometric sensor or an ion selective electrode. The calibration electrode has an active surface comprising redox functionalities. The redox functionalities set the pH of a reference solution proximal to the calibration electrode. A voltammetric signal is applied to the calibration electrode to produce a response that is determined by the set pH. The response of the calibration electrode to the voltammetric signal is used to calibrate/adjust a reference potential produced by a reference electrode of the reference system of the electrochemical sensor. This calibration corrects the detrimental effect of reference electrode drift.

Claims

1. A calibration system for an electrochemical sensor, comprising: a fluid reservoir comprising an opening and containing a reference solution; a porous frit disposed in the opening and configured to contain the reference solution in the fluid reservoir and to contact an analyte being sensed by a potentiometric sensor; a reference electrode disposed in the fluid reservoir such that it is immersed in the reference solution and configured to generate a reference signal; a calibration electrode comprising an active surface disposed in the fluid reservoir such that it is immersed in the reference solution, wherein the active surface is configured to set a pH of the reference solution proximal to the calibration electrode; a voltammetric source configured to apply a voltammetric signal to the calibration electrode; and a processor configured to receive a voltammetric response generated by the calibration electrode in response to the applied voltammetric signal and to use the voltammetric response to calibrate the reference signal.

2. The calibration system of claim 1, wherein the calibration electrode comprises carbon.

3. The calibration system of claim 1, wherein the calibration electrode comprises carbon black, graphite, glassy carbon, carbon fibers, carbon nanotubes, multi-walled carbon nanotubes, edge plane carbon, graphene, and/or boron doped diamond.

4. The calibration system of claim 1, wherein the active surface comprises one or more of any of: a chemically modified carbon surface comprising oxygen and/or nitrogen containing functional groups; covalently bonded redox functionalities; physical adsorbed redox functionalities; a film on the active surface; one or more electrodes on the active surface; and/or a carbon composite.

5. The calibration system of claim 1, wherein the active surface is produced by one of thermal, chemical, mechanical, plasma, or laser treatment of the calibration electrode.

6. The calibration system of claim 1, wherein the active surface comprises a metal.

7. A method for calibrating a reference system of an electrochemical sensor, the reference system comprising a fluid reservoir with an opening covered by a porous fit, the method comprising: contacting a calibration electrode comprising redox active functionalities with a reference solution in the fluid reservoir; using the redox active functionalities to set a pH of the reference solution local to the calibration electrode; applying a voltammetric sweep to the calibration electrode; measuring a voltammetric response to the applied voltammetric sweep, wherein the voltammetric response is generated by an oxidation/reduction of the redox active functionalities and the voltammetric response is determined by the pH set by the redox active functionalities; and using the voltammetric response to adjust a reference potential of the reference system.

8. The method of claim 7, wherein a measured potential of the redox active functionalities is used as a calibration constant.

9. The method according claim 7, wherein a potential in the voltammetric response is a one of a potential of a maximum of an oxidation current produced by the redox active functionalities in response to the voltammetric sweep and the pH or a maximum of a reduction current produced by the redox active functionalities in response to the voltammetric sweep and the pH.

10. The method according to claim 7, comprising controlling a local environment of the reference solution by controlling a hydrogen ion concentration of the reference solution proximal to the calibration electrode.

11. The method according to claim 7, wherein the voltammetric sweep is applied periodically to the calibration electrode.

12. The method according to any claim 7, wherein the electrochemical sensor is calibrated at the same time as it is being used to sense properties of a fluid.

13. A calibration system for an electrochemical sensor, comprising: a reference chamber containing a reference solution; a reference electrode disposed in the reference chamber and at least partially submerged in the reference solution and configured to produce a reference potential; a calibration electrode disposed in the reference chamber and at least partially submerged in the reference solution; a voltammetric device configured to communicate a voltammetric signal to the calibration electrode and receive a voltammetric response from the calibration electrode; a processor configured to process the voltammetric response to calibrate the reference potential; and a counter electrode disposed in the reference chamber and at least partially submerged in the reference solution.

14. The calibration system of claim 13, wherein the voltammetric device comprises a potentiostat.

15. The calibration system of claim 13, wherein the reference solution comprises a metallic salt.

16. The calibration system of claim 13, wherein the reference solution comprises one of KCl, NaCl, Na.sub.2SO.sub.4, or K.sub.2SO.sub.4.

17. The calibration system of claim 13, wherein the reference chamber comprises a porous frit configured to contact a solution being analyzed by the electrochemical sensor.

18. The calibration system of claim 13, wherein the calibration system is for calibration of an electrochemical sensor comprising one of an ion selective sensor, a voltammetric sensor, an amperometric sensor, or a glass electrode.

19. The calibration system of claim 13, wherein the voltammetric signal comprises one of a square wave, a ramped wave, or a pulsed wave, or wherein the voltammetric signal is swept across the calibration electrode with respect to the reference electrode.

20. The calibration system of claim 13, wherein the calibration electrode comprises at least one of a microelectrode or a microelectrode array.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

(2) FIG. 1 illustrates a glass electrode pH sensor with a reference electrode.

(3) FIG. 2 illustrates an electrochemical sensor system with a calibration system, in accordance with some embodiments of the present disclosure.

(4) FIG. 3 depicts a schematic representation of a sensor output from an electrochemical sensor comprising a calibration system, in accordance with some embodiments of the present disclosure, using various voltammetric sweep profiles.

(5) FIG. 4 illustrates an electrochemical sensor comprising a calibration system, according to some embodiments of the present disclosure.

(6) FIG. 5 depicts a schematic representation of a sensor output from an electrochemical sensor comprising a calibration system, in accordance with some embodiments of the present disclosure.

DESCRIPTION

(7) The ensuing description provides some embodiment(s) of the invention, and is not intended to limit the scope, applicability or configuration of the invention or inventions. Various changes may be made in the function and arrangement of elements without departing from the scope of the invention as set forth herein. Some embodiments may be practiced without all the specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments.

(8) Some embodiments may be described as a process which is depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed, but could have additional steps not included in the figure and may start or end at any step or block. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function.

(9) Moreover, as disclosed herein, the term “storage medium” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “computer-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data.

(10) Furthermore, embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium such as storage medium. A processor(s) may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class or any combination of instructions, data structures or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc.

(11) Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings and figures. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the subject matter herein. However, it will be apparent to one of ordinary skill in the art that the subject matter may be practiced without these specific details. In other instances, well known methods, procedures, components, and systems have not been described in detail so as not to unnecessarily obscure features of the embodiments. In the following description, it should be understood that features of one embodiment may be used in combination with features from another embodiment where the features of the different embodiment are not incompatible.

(12) It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first object or step could be termed a second object or step, and, similarly, a second object or step could be termed a first object or step. The first object or step, and the second object or step, are both objects or steps, respectively, but they are not to be considered the same object or step.

(13) The terminology used in the description of the disclosure herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the subject matter. As used in this description and the appended claims, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “includes,” “including,” “comprises,” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

(14) As used herein, the term “if” may be construed to mean “when” or “upon” or “in response to determining” or “in response to detecting”, depending on the context. Similarly, the phrase “if it is determined” or “if [a stated condition or event] is detected” may be construed to mean “upon determining” or “in response to determining” or “upon detecting [the stated condition or event]” or “in response to detecting [the stated condition or event],” depending on the context.

(15) The following description provides reference to use of a calibration electrode in accordance with the present disclosure with a glass electrode. This combination is made merely as an example of the operation of the calibration electrode, since glass electrodes are probably the most ubiquitous of electrochemical sensors, and as persons of skill in the art will understand, the calibration electrode may be used in the same manner with any other electrochemical sensor using a reference electrode/system.

(16) Redox active functionalities are functionalities that may be oxidized and reduced, and redox activity may refer to either or both of those processes.

(17) Redox functionalities and/or redox-active functionalities comprise functionalities that are tailored to be sensitive to the presence or concentration of an analyte in a solution. For example, when a voltammetric signal is applied to the tailored redox active functionality, a redox current/potential generated by the redox-active functionalities will depend upon presence or concentration of the analyte in the solution. The solution may comprise of hydrogen ions and in such cases the redox active functionalities are sensitive to the pH of the solution. For most electrochemical sensors, the redox active functionalities can undergo a reversible electrochemical redox reaction dependent upon the concentration of analyte (hydrogen ions for a pH meter; other analytes for other analyte sensing devices). For a conventional pH sensor, such as the glass electrode, the hydrogen ions associate with the glass membrane causing a change in the measured potential at the electrode housed behind the glass membrane with respect to the reference. In the conventional glass pH electrode, in essence, two Ag/AgCl electrodes are used; one behind the pH sensitive glass and one behind a conventional frit. As such, in some embodiments, therefore the calibration electrode described herein may be used to determine conditions in the chamber behind the glass membrane and/or to calibrate the Ag/AgCl electrode behind the glass membrane.

(18) In a sample solution with an applied electrical potential, for example, where there is a high concentration of hydrogen ions present in a sample solution, the redox reaction occurs at a lower potential. Conversely, where there is a low concentration of hydrogen ions present in a sample solution, the redox reaction occurs at a higher potential. The relationship between these characteristic potentials and the sample solution pH is a function of the chemical identity of the redox active functionality. An algorithm converts electrical potential to pH value to provide a means of determining the pH of an unknown sample.

(19) For some embodiments, redox active functionalities may comprise pH sensitive redox active functionalities such as for example: anthraquinone (AQ), phenanthrenequinone (PAQ), N,N′-diphenyl-p-phenylenediamine (DPPD), anthracene, naphthaquinone, para-benzoquinone, diazo-containing compounds, porphyrins, nicotinamides, including NADH, NAD and N-methylnicotinamide, quinone thiol, monoquaternized N-alkyl-4,4′-bipyridinium, RuO, and Ni(OH).sub.2, and derivatives of those compounds; CO-sensitive ASMs: ferrocenyl ferraazetine disulfide; alkaline metal cation sensitive ASMs: 1,1′-(1,4,10,13-tetraoxa-7,1-diazacyclooctadecane-7,16-diyl dimethyl), ferrocenyl thiol, other ferrocene derivatives containing covalently attached cryptands, and certain metal complexes with Fe.sup.2+/Fe3.sup.+, Co2.sup.+/Co3.sup.+, Cu.sup.+/Cu2.sup.+, ferrocenyl ferraazetine and ferrocenyl cryptands, 1-hydro-1′-(6(pyrrol-1-yl)hexyl-4,4′-bipyridinium bis(hexafluoro-phosphate) and or the like. Further, redox active functionalities that may be activated on a surface of the calibration electrode such as C—O functionalities are described herein.

(20) A reference electrode is an electrode that may be used to establish the potential difference applied to a working electrode of an electrochemical sensor. Generally, reference electrodes comprise a fixed chemical composition and, therefore generate a fixed electrochemical potential. This fixed electrochemical potential provides for measurement of the potential difference applied to the working electrode of the electrochemical sensor. It is imperative that the composition of the reference electrode remains constant, and hence almost no current should be passed through the reference electrode to prevent electrolysis. To prevent passing current through the reference electrode, electrochemical sensors generally comprise a counter electrode to complete the circuit. However, two-electrode electrochemical sensors may be used where the working electrode is a microelectrode. This is possible because the currents passed at the electrode are small.

(21) A working electrode of the electrochemical sensor is the electrode at which the electrochemical process for detecting an analyte of interest occurs. In a sensor, the working electrode may be sensitive to one or more analyte(s) in a test solution, or it may be chemically modified with analyte sensitive species/materials or surface modified so that a surface of the electrode comprises redox active functionalities. The electrochemical response of the working electrode is measured after a perturbation is applied. The perturbation may comprise an electric signal, voltammetric signal and/or the like, for example, application of a potential difference to the working electrode induces electron transfer to occur, and the resulting current at the working electrode can be recorded as a function of the applied potential.

(22) FIG. 1 illustrates a glass electrode pH sensor with a reference electrode. The glass electrode is essentially a combination electrode, which combines both a glass electrode and a reference electrodes. The glass electrode consists of a sensing part of the glass electrode, often referred to as the sensing electrode which comprises a bulb 1 made from a specific glass, an internal electrode 2 and an internal solution 3. Merely by way of example, the internal electrode may comprise a silver chloride electrode, a calomel electrode or the like. The internal solution 3 may comprise a pH=7 buffered solution of 0.1 mol/L KCl for pH electrodes, 0.1 mol/L XCl for pX electrodes or the like, where X is the ion under determination. In a glass electrode using a silver chloride electrode, a small amount of AgCl precipitate 4 may form inside the glass electrode.

(23) The reference system 5, often simply referred to as the reference electrode, comprises a reference electrode 9 immersed in a reference internal solution 6. The reference electrode may comprise a silver chloride electrode, a calomel electrode or the like. The reference internal solution 6 may comprise a salt such as a silver chloride, potassium chloride or the like. For example, the reference internal solution may comprise 0.1 mol/L KCl or the like.

(24) The glass electrode further comprises a junction 7 that provides for communication between the internal parts of the glass electrode and the solution being analyzed by the glass electrode. The junction 7 may comprise a ceramic, a capillary containing asbestos or quartz fiber and/or the like. The glass electrode comprises a housing 8 housing the components of the glass electrode. The housing 8 may be made from non-conductive glass, plastic and/or the like.

(25) The bottom of the glass electrode balloons out into a round thin glass bulb. The pH electrode is best thought of as a tube within a tube. The inner tube contains an unchanging solution. Also inside the inner tube is the cathode terminus of the reference probe. The anodic terminus wraps itself around the outside of the inner tube and ends with the same sort of reference probe as was on the inside of the inner tube. It is filled with a reference solution and has contact with the solution on the outside of the pH probe by way of a porous plug that serves as a salt bridge.

(26) FIG. 2 illustrates a calibration electrode in accordance with some embodiments of the present disclosure. The calibration electrode comprises an electrode body 20. The electrode body may comprise carbon, a metal, a metal ion and/or the like. Carbon based electrodes and metallic compounds are commonly used in electrochemistry because of their good electrical conductivity. Carbon materials also have low density, low thermal expansion, and low elasticity and are cost effective, readily available, and suitable for modification. The electrode body in some embodiments may comprise carbon black, graphite, glassy carbon, carbon fibers, carbon nanotubes, multi-walled carbon nanotubes, edge plane carbon, graphene and or the like. Graphene provides a strong electrode with good conductivity and advantageous chemical properties.

(27) The electrode body may comprise a substrate or the like that is connected to an electrical connection 27. The electrical connection 27 may comprise a wire or the like that is configured to communicate electrical signals to and from the calibration electrode.

(28) The calibration electrode comprises an active surface 23. The active surface 23 comprises a surface that is configured to both set the pH of a low capacity buffer solution proximal to the active surface and respond to the set pH proximal to the active surface.

(29) It is known that carbon materials respond to pH without activation. This response is due to the presence of chemisorbed oxygen on the graphite surface that cause the formation of C═O groups. Methods have been developed to modify the carbon materials to improve or tune their electrocatalytic, redox, or ion-sensing properties. These methods of activating the carbon material to provide for the immobilization of pH-sensing substances onto the active surface can be divided into five main groups: (1) chemical modification of the carbon surface with oxygen and/or nitrogen containing functional groups; (2) covalent bonding of pH-sensing compounds onto the active surface initiated either by chemical or electrochemical activation; (3) physical adsorption of pH sensing compounds onto the active surface; (4) fabrication of film electrodes on the active surface; and (5) fabrication of carbon composite electrodes where the surface of the carbon composite comprises pH sensing substances.

(30) FIG. 3 illustrates the active surface 23 formed on the electrode body 20, where the electrode body 20 comprises a carbon material. In some embodiments, the electrode body 20 and the active surface 23 comprise a single substrate and the active surface 23 simply comprises a surface of the electrode body 20 that is configured in use to be in contact with a low buffering capacity solution and/or a reference solution in the reference chamber of the electrochemical sensor. Merely by way of example, the electrode body 20 may comprise a composite electrode and the active surface 23 is simply a surface of the electrode body 20 that is configured to be contacted with the low buffering capacity/reference solution.

(31) In some embodiments, the active surface 23 is configured to be contacted with a low buffering capacity solution. Such solutions, because of low proton transfer rates resulting from their low buffer capacity proximal to the active surface 23, comprise a pH value proximal to the active surface that is set by the properties of the active surface. In some embodiments, the properties of the active surface 23 are configured so that the active surface 23 sets a pH value of a reference solution proximal to the active surface 23. For example, the reference solution may comprise KCl, HCl and/or another salt configured to work with the reference electrode material. In some embodiments, the active surface 23 may have acidic or basic properties so as to set a local pH in the reference solution to a pH above or below a pH of seven (7). In some embodiments, redox active functionalities of the active surface 23 may set the local pH of the low buffering capacity/reference solution.

(32) In embodiments of the present disclosure, the active surface 23 may be metallic. Like carbon, a metal may be treated so that substances are formed on a surface of the metal and these substances may comprise substances that will undergo oxidation/reduction in the presence of protons/hydrogen ions such that their electrical response in their presence depends upon a concentration of hydrogen ions, and is therefore indicative of pH. As with the case of a carbon electrode, the electrode body 20 may comprise the metal and the active surface 23 may simply comprise a surface of the electrode body 20 that is configured to be contacted with a low buffering capacity/reference solution.

(33) In some embodiments, the active surface 23 may be formed by a metallic ion disposed in the low buffering capacity solution, where the low buffering capacity solution is a saline solution and then metal ion may produce a redox couple in the saline solution which provides for redox activity at a surface of the electrode body 20.

(34) Boron doped diamond has also been found to be capable of being used to measure pH of a solution. In some embodiments of the present disclosure the active surface 23 may comprise boron doped diamond. When a current is passed through the active surface 23 in the presence of the low buffering capacity/reference solution, a response of the active surface 23 is determined by a hydrogen evolution reaction on the active surface 23, where the reaction is provided by:
2H.sup.++2e−>H.sub.2
and, as such is indicative of pH.

(35) The active surface technologies described have been developed to provide an active surface that can be used to measure pH without the need to deposit reagents onto the sensing electrode, i.e., the sensing electrode itself comprises the redox active functionalities. Such technology has been pursued as it removes the issue of coupling reagents with the electrode and to increase the robustness of the sensing electrode. As such, a main drive of the technology has been to increase the redox response of the active surface to increase sensitivity.

(36) As with electrochemical pH sensors using reagents coupled with the sensing electrode, the active surface, like the reagents, will set the pH of a low buffering capacity solution in a region of the low buffering capacity solution proximal to the sensing electrode. Moreover, Applicants have found that the active surface, like the reagents coupled with the sensing electrode, can be configured to set the pH local to the active surface when the active surface is contacted with a reference solution of a reference electrode system. As with electrochemical pH sensors using reagents, this effect has been seen as detrimental to using active surface type electrodes to sense pH of low buffering capacity solutions, such as water, seawater, saline solutions and the like. To avoid this issue, active surfaces have been developed with low concentrations of redox active functionalities so that the pH setting effect of the functionalities is swamped by the pH of the low buffering capacity solution.

(37) In embodiments of the present disclosure, the active surface 23 may comprise a carbon, carbon derivative, boron doped diamond or the like comprising redox active functionalities that are configured to set the pH proximal to the active surface 23 such that when an electronic signal is applied to the active surface 23 a redox response of the active surface 23 is determined by/identifies the pH set by the redox active functionalities.

(38) In embodiments of the present disclosure, the calibration electrode is chemically or physically activated to generate redox active functionalities at the active surface 23.

(39) These redox active functionalities are configured to control the local environment of the electrode surface either directly through the introduction of acidic or basic moieties or indirectly through the introduction of redox active moieties, which when oxidized or reduced, take-up or release protons, thus locally changing the pH at the electrode surface.

(40) It has recently shown that mechanical, laser, heat and chemical activation of carbon surfaces (BDD, graphite, carbon nanotube, graphene, glassy carbon, basal plane carbon and edge plane carbon in solid or printed formats) produce electrochemical active sites on the surface of the carbon.

(41) ##STR00001##

(42) The carbon chemistry depicted above illustrates the types of functional groups that can be produced on a carbon surface. Research has shown that carbon surface functionalities can provide good pH responses across the entire pH range. In the presence of low buffering capacity media, such as those found within an ion selective electrode reference chamber, which typically contain a known concentration of inert electrolyte (KCl, NaCl, NaClO.sub.3.sup.−, Na.sub.2SO.sub.4 etc.), the pH sensing capabilities of these systems fail and rather than measuring the pH of the low buffering capacity media, a sensor comprising a sensing electrode including carbon surface functionalities will measure the pH set by the carbon surface functionalities. This is because the carbon surface functionalities control the pH local to the electrode surface, due to the intake (or release) of protons by the carbon surface functionalities, making the local environment acidic or alkali.

(43) FIG. 4 illustrates an electrochemical sensor comprising a calibration system, according to some embodiments of the present disclosure. In FIG. 4, a novel means of calibrating the electrochemical potential of the reference electrode is provided by the introduction of an additional voltammetric electrochemical measurement.

(44) In FIG. 4, a calibration electrode 43 is placed inside a reference electrode chamber of an electrochemical sensor. A reference electrode for the electrochemical sensor, which is being calibrated, may also be used as a reference electrode for the calibration system, i.e., in the calibration system a voltammetric response of the calibration electrode may be processed against a sweep applied between the calibration electrode, the reference electrode and/or a counter electrode. For example, in some embodiments, the calibration system of the present disclosure may comprise a two or three electrode system. In some embodiments, the reference electrode of the calibration system may comprise the reference electrode of the electrochemical sensor. The two or three electrode system may be used in a voltammetric system to produce a peak potential corresponding to a pH of a solution in contact with the electrodes. Which pH is set by the properties of the calibration electrode.

(45) In some embodiment, prior to deployment of the electrochemical sensor, a voltammetric scan can be taken between the reference electrode and the calibration electrode and a potentiometric measurement recoded by the sensor in a known solution. This initializing scan sets the parameters of the electrochemical sensor taking into account how the particular electrochemical sensor is behaving. In essence, this is a one-time initial calibration for the particular electrochemical sensor. For example, in an ISE, the calibration takes into account how the ion-selective layer responds with respect to the reference electrode, and the voltammetric sweep measures the potential of the reference electrode with respect to the calibration electrode.

(46) In embodiments of the present disclosure, the calibration electrode is chemically or physically activated to generate redox active functionalities on the surface of the electrode. These redox active functionalities are configured to control the pH of the local environment of the electrode surface, either: (a) directly through the introduction of acidic or basic moieties; or (b) indirectly through the introduction of redox active moieties, which when oxidized or reduced, uptake or release proton thus locally changing the pH at the electrode surface. In some embodiments, the redox active functionalities may comprise a combination of those in group (a) and those in group (b).

(47) In some embodiments, the calibration system comprises a counter electrode. However, in some embodiments, when the calibration electrode passes sufficiently low current, the counter electrode may not be required.

(48) In some embodiments of the present disclosure, the calibration electrode may comprise an active surface produced by mechanical, laser, heat and/or chemical activation of a carbon surface, such as boron doped diamond, graphite, carbon nanotube, graphene, glassy carbon, basal plane carbon and edge plane carbon in solid or printed formats.

(49) In embodiments of the present disclosure, the active surface sets the local pH of a low buffering capacity solution and measures this pH to generate a calibration signal that is used to correct drift in the reference potential, this may be or is by means of measuring a voltage and then determining the pH by means of a calibration plot. In embodiments of the present disclosure, the redox active functional groups present on the active surface produce pH local to the active surface that is independent of the ion concentration within the reference electrode chamber.

(50) In In embodiments of the present disclosure, a potentiometer or the like may be used to apply a voltammetric signal to the electrochemical sensor and to the calibration system. The electrochemical sensor's response to the voltammetric signal may be used to determine properties, such as concentration, of the selected ion. The calibration electrodes response to the voltammetric signal may be used as a calibration signal to correct for any drift in the reference potential of the electrochemical sensor.

(51) In embodiments of the present disclosure, the calibration system may be used periodically by running a voltammetric sweep against the calibration electrode, which is housed in the reference system behind a porous frit. The response of the calibration electrode to an applied voltammetric signal may take the form of a voltammogram. In some embodiments, a potential of the redox active species, such as a peak potential comprising of a peak in the reduction/oxidation current of the redox active species, is used to correct any drift occurring in the reference electrode. A frequency of voltammetric sweep may be selected in accordance with the application for which the electrochemical sensor is being used. In some embodiments, the electrochemical sensor may be run using a modified bi-potentiostat system.

(52) FIG. 5 depicts a schematic representation of a sensor output from an electrochemical sensor comprising a calibration system, in accordance with some embodiments of the present disclosure. In FIG. 5, the electrochemical sensor generates output A at times t.sub.1 through t.sub.4. The outputs correspond to a selected ion being measured and are determined from a response of the sensing electrode determined using a reference potential of the reference electrode.

(53) Output B illustrates a comparison of the reference potential to a potential obtained from the calibration electrode in response to a voltammetric signal, such as a voltammetric sweep. The potential obtained from the calibration electrode may comprise a potential of a peak oxidation/reduction current produced by the calibration electrode, the redox functionalities on the active surface of the calibration electrode. Since the calibration electrode is maintained in contact with a low buffering capacity solution in the reference system, the calibration electrode sets the pH of the low buffering capacity solution local to the calibration electrode and the potential of the peak reduction/oxidation currents will be constant despite any changes to the low buffering capacity solution.

(54) In general, electrochemical sensor's use reference systems that comprise low buffering capacity solutions such as KCl solution and/or the like so the standard reference solution does not need to be changed for use with the calibration system. While the potential of the oxidation/reduction currents is described for calibration, any reference point in the voltammetric response of the calibration electrode may be used, such as turning points, point of maximum or minimum change in slope of the voltammogram or the like. Further, signal processing methods such wavelet interpretation and or the like may be used to identify reference points in the voltammogram and their corresponding potential for use for calibration of the reference potential. Advantageously, the peak redox/reduction potential of the calibration electrode response may be determined by testing of the electrochemical sensor when built, which may be used for batches of the electrochemical sensor's, or calculated empirically, and this data may be used in the voltammogram analysis. In some embodiments, a potential for either the peak reduction or oxidation current is used to correct the reference potential for drift.

(55) In FIG. 5, a position of the reference potential relative to a peak potential in the voltammetric response of the calibration electrode at time t.sub.4 is shown to have shifted with respect to the relative potentials at t.sub.1, t.sub.2 and t.sub.3. Without the calibration system of the present disclosure, this change in the reference potential would have produced an inaccurate measurement at t.sub.4. With the calibration system of the present disclosure, however, the drift of the reference electrode can be corrected and the correct measurement of the selected ion processed.

(56) In embodiments of the present disclosure, the calibration system can be used to provide QA/QC of the electrochemical sensor's reference system. As provided in FIG. 4, the electrochemical sensor includes an integrated electrochemical cell/chamber and a potentiostat, and the system uses a voltammetric signal from the potentiostat to produce a sweep between the reference electrode and the calibration electrode in the integrated electrochemical cell/chamber to generate a potential that may be used for QA/QC of the reference electrode during operation of the sensor. The sensor may in some embodiments run using a modified bi-potentiostat system. Such a set-up obviates the need for reference sensor calibration prior to deployment as is the case with current commercial electrochemical sensor's. In embodiments of the present disclosure, prior to deployment, the internal circuit can be measured and this may be used to set the parameters for the reference system.

(57) While the principles of the disclosure have been described above in connection with specific apparatuses and methods, it is to be clearly understood that this description is made only by way of example and not as limitation on the scope of the invention.