Electrochemical sensor

Abstract

This invention relates to an electrochemical sensor for determining the presence or amount of an analyte (e.g., free chlorine) in a sample.

Description

[0001] This invention relates to an electrochemical sensor for determining the presence or amount of an analyte (e.g., free chlorine) in a sample.

[0002] The detection of free chlorine is usually performed by colorimetry. The most widely used colorimetric method uses N,N-diethyl-p-phenylenediamine (DPD). Other methods use tetramethylbenzidine (TMB), the diammonium salt of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), o-toluidine, or eugenol azobenzidine.

[0003] These methods may be limited in scope due to the mechanism by which chromophores are generated in the presence of free chlorine. At high free chlorine concentrations, the method may suffer from color “fading” or “bleaching.” Bleaching, for example, with DPD is generally believed to occur in samples with free chlorine concentrations of 10 mg / L or higher.

[0004] As an example of this phenomenon, DPD undergoes a two-step oxidation process. In the first oxidation step, DPD reacts with a small amount of chlorine at a near-neutral pH to produce Wurster dye as the main oxidation product. At higher oxidation levels (typically a free chlorine concentration of 10 mg / L, which corresponds to a free chlorine:DPD ratio of approximately 0.40:1), the second oxidation step favors the formation of unstable colorless imines, to which the bleaching visual effect can be attributed. Figure 4 Table 1 shows the product concentrations of the reaction between HOCl (free chlorine) and DPD at different molar ratios. (DPD, DPD) + and DPD 2+ These refer to the starting reagent, the first electron oxidation product, and the second electron oxidation product, respectively. It can be seen that at a [HOCl] / [DPD] ratio greater than approximately 0.50:1, DPD... + The concentration of DPD decreases. Therefore, bleaching with DPD limits its use in defining low concentration ranges of free chlorine.

[0005] Table 1

[0006]

[0007] The single-electron oxidation of TMB produces a TMB cation radical. This radical is then balanced with a blue charge-transfer complex of TMB. Further oxidation produces a bright yellow TMB diimine. Figure 5 Table 2 shows the product concentrations of the reaction between HOCl (free chlorine) and TMB at different molar ratios. TMB, TMB --- TMB 2+ and TMB 2+These refer to the starting reagent, the first electron oxidation product, and the second electron oxidation product, respectively. It can be seen that at a [HOCl] / [TMB] ratio greater than approximately 0.5, TMB---TMB 2+ The concentration decreased.

[0008] Table 2

[0009]

[0010] The first oxidation step of ABTS forms the free radical cation ABTS. + And the azo-substituted ABTS produced by peroxidation that is unstable in aqueous solution. 2+ Depending on the amount of ABTS present, the characteristic blue color from the indicator can be retained at free chlorine concentrations up to 250 mg / L. In bulk solution, color loss occurs in the presence of free chlorine when the free chlorine:ABTS ratio is >5:1.

[0011] The molecular extinction coefficient is a measure of the intensity of light absorbed by a compound at a specific wavelength. In the case of ABTS, the molar extinction coefficient is high (approximately 28,500 M at 405 nm). -1 cm -1 This enables colorimetric detection of free chlorine at low levels. However, the high molar extinction coefficient hinders the use of ABTS for colorimetric analysis at high chlorine concentrations. The linear calibration range for chlorine was determined to be 0.07 mg / L. -1 Up to 0.7 mg / L -1 .

[0012] Table 3

[0013]

[0014] Figure 2 Table 3 shows the product concentrations of the reaction between HOCl (free chlorine) and ABTS at different molar ratios. (ABTS, ABTS) + and ABTS 2+ These refer to the starting reagent, the first electron oxidation product, and the second electron oxidation product, respectively. It can be seen that at [HOCl] / [ABTS] ratios of 2 to 5, ABTS... + The concentration of [something] increases and stabilizes.

[0015] This invention is based on the understanding that the size of the molecule and the presence of sulfonic acid groups capable of extensive electron delocalization can endow radical cations, such as ABTS, with the ability to possess free radical cations. + The relative stability of such a substance at high free chlorine concentrations is advantageous for the electrochemical detection of high concentrations of free chlorine, at which the use of such a substance in colorimetric methods is excluded.

[0016] Therefore, from a first aspect, the present invention provides an electrochemical sensor for determining the presence or quantity of an analyte in a sample, comprising:

[0017] An elongated substrate layer having a first end opposite to the second end;

[0018] A first conductive track, a second conductive track, and a third conductive track are axially deposited on a substrate in a substantially parallel and mutually spaced relationship. The first conductive track constitutes a reference electrode. The second conductive track has a conductive deposit near its second end on the substrate, which constitutes a counter electrode. The third conductive track has a conductive deposit near its second end on the substrate, which constitutes a working electrode. Each of the first, second, and third conductive tracks terminates at an electrical contact near its first end on the substrate.

[0019] A non-conductive layer is deposited on a first conductive track, a second conductive track, and a third conductive track, wherein the non-conductive layer is configured to fully expose each electrical contact near a first end of the substrate, expose conductive deposits near a second end of the second conductive track near the substrate, expose the first conductive track near a second end of the substrate, and expose one or more discrete working areas of the conductive deposits near a second end of the third conductive track near the substrate; and

[0020] A reagent formulation of a salt of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) or a salt of N,N'-bis(2,4-di-sulfobenzyl)bitoluidine (SBT), said reagent formulation being deposited on or near one or more discrete working regions of a conductive deposit of a third conductive trajectory.

[0021] In a preferred embodiment, the electrochemical sensor comprises a reagent preparation of an ABTS salt. At the surface of the sensor, the ABTS salt is present at a high local concentration, which makes the azobiscation ABTS... 2+ The formation in the second oxidation step is advantageously less likely than in the bulk solution.

[0022] Preferably, the salt of ABTS is the diammonium salt of ABTS.

[0023] Preferably, the salt of SBT is the tetrasodium salt of SBT.

[0024] The reagent formulation may also contain one or more additives, such as buffers, gelling agents, thickeners, viscosity modifiers, wetting agents, or stabilizers. Typical additives are one or more of the group consisting of ethanol, sodium phosphate, potassium phthalate, sodium carbonate, disodium EDTA, hydroxyethyl cellulose, carboxymethyl cellulose (CMC), and polyvinylpyrrolidone.

[0025] Preferably, the reagent formulation contains a buffer that maintains the pH in the range of 5 to 6 during use. This is advantageously used to limit interference from monochloramine in the measurement. Preferably, the buffer is an acidic salt (e.g., sodium bisulfate or sodium maleate).

[0026] The reagent formulation can be in the form of a reagent layer. The reagent formulation can be deposited on or near one or more discrete working areas of a conductive deposit and dried to form a reagent layer.

[0027] The reagent layer may include a porous matrix. The reagent layer may include a porous matrix impregnated with reagents. The porous matrix may contain polyvinylpyrrolidone and / or hydroxyethyl cellulose. The reagents may be impregnated into the porous matrix by printing or microdosing.

[0028] The reagent formulation can be in the form of multiple reagent doses. The reagent formulation can be discretely deposited on or near one or more working areas of a conductive deposit and dried to form reagent doses.

[0029] Electrochemical sensors may also include a porous membrane mounted on or adjacent to a non-conductive layer, thereby covering the reagent preparation. Alternatively, electrochemical sensors may be membrane-free.

[0030] Preferably, the second conductive trajectory is located between the first conductive trajectory and the third conductive trajectory.

[0031] The third conductive trajectory can be located between the first conductive trajectory and the second conductive trajectory.

[0032] In a preferred embodiment of the present invention, the electrochemical sensor further includes:

[0033] A fourth conductive track is axially deposited onto a substrate, wherein the first, second, third, and fourth conductive tracks are substantially parallel and spaced apart from each other, wherein a conductive deposit is present at the second end of the fourth conductive track near the substrate, thereby forming a working electrode pair with the third and fourth conductive tracks, wherein each of the first, second, third, and fourth conductive tracks terminates near a first end of the substrate at an electrical contact, wherein a non-conductive layer is deposited on the first, second, third, and fourth conductive tracks and is configured to fully expose each electrical contact near the first end of the substrate, expose the conductive deposit at the second end of the second conductive track near the substrate, expose the first conductive track near the second end of the substrate, expose one or more discrete working regions of the conductive deposit of the third conductive track, and expose one or more discrete working regions of the conductive deposit of the fourth conductive track, wherein additional reagent preparations are deposited on or near one or more discrete working regions of the conductive deposit of the fourth conductive track.

[0034] The third and fourth conductive traces can be located on the sides (flanked) of the first and second conductive traces.

[0035] The second and fourth conductive traces can be located to the sides of the first and third conductive traces.

[0036] Other reagent formulations may also contain one or more additives, such as buffers, gelling agents, thickeners, viscosity modifiers, wetting agents, or stabilizers. Typical additives are one or more of the group consisting of ethanol, sodium phosphate, potassium phthalate, sodium carbonate, disodium EDTA, hydroxyethyl cellulose, carboxymethyl cellulose (CMC), and polyvinylpyrrolidone.

[0037] Additional reagent formulations can be in the form of reagent layers. Additional reagent formulations can be deposited on or near one or more discrete working areas of a conductive deposit and dried to form reagent layers.

[0038] The reagent layer may include a porous matrix. The reagent layer may include a porous matrix impregnated with reagents. The porous matrix may contain polyvinylpyrrolidone and / or hydroxyethyl cellulose. The reagents may be impregnated into the porous matrix by printing or micro-dosing.

[0039] Additional reagent formulations can be prepared in the form of multiple additional reagent doses. Additional reagent formulations can be discretely deposited on or near one or more working areas of the conductive deposit and dried to form additional reagent doses.

[0040] The additional reagent formulation may be a formulation of TMB, DPD, o-toluidine, or syringaldehyde azide. Preferably, the additional reagent formulation is an additional reagent formulation of TMB.

[0041] In a preferred embodiment, the reagent preparation is a salt of ABTS, and the additional reagent preparation is a TMB preparation. This embodiment constitutes a dual sensor that can advantageously operate at a single potential under high and low concentrations (e.g.) of free chlorine.

[0042] Preferably, the discrete working areas of the conductive deposits of the third conductive trajectory (and (if present) the fourth conductive trajectory) are exposed through a window. Particularly preferably, the window exposes the conductive deposits on the second conductive trajectory near the second end of the substrate and the first conductive trajectory near the second end of the substrate.

[0043] Preferably, multiple discrete working regions of the conductive deposits of the third conductive trajectory (and (if present) the fourth conductive trajectory) are exposed through a hole array.

[0044] Electrochemical sensors may also include: a deposit of supporting electrolyte deposited on a non-conductive layer.

[0045] Electrolyte-supporting deposits can take the form of multiple doses deposited in a relationship that is substantially parallel to and spaced apart from each other by the first, second, and third conductive trajectories (and, if present, a fourth conductive trajectories).

[0046] Electrolyte-supporting deposits can be deposited in the region between the first and third conductive tracks of the non-conductive layer (e.g., only between them).

[0047] Electrolyte-supporting deposits can be deposited in the region of the non-conductive layer between any of the conductive deposits on the second, third, first, and fourth conductive tracks.

[0048] The supporting electrolyte can be KCl, KBr, or KNO3.

[0049] Hole arrays can be fabricated in a non-conductive layer using mechanical, chemical, or physical removal techniques such as ablation (e.g., photoablation) or etching. Hole arrays can also be fabricated in a non-conductive layer via stencil printing.

[0050] Each hole can have a basically regular shape. Typically, the holes are uniformly shaped. Each hole can be basically circular or non-circular (e.g., rectangular or square).

[0051] Each aperture can be elongated (e.g., linear). Each elongated aperture can be substantially parallel to (e.g., perpendicular to) the first, second, third, and fourth conductive traces.

[0052] Preferably, each elongated hole is substantially perpendicular to the first conductive trajectory, the second conductive trajectory, the third conductive trajectory, and the fourth conductive trajectory (e.g., horizontal).

[0053] Preferably, each hole is basically circular.

[0054] The array can take any suitable pattern (e.g., cube or rectangle). The array can contain 10 to 500 holes, preferably 50 to 200 holes, more preferably 80 to 120 holes, and most preferably about 95 holes.

[0055] Preferably, the size (e.g., diameter) of each hole is in the range of 50 μm to 400 μm (e.g., about 350 μm).

[0056] Each aperture can be elongated (e.g., linear). Each elongated aperture can be substantially parallel to (e.g., perpendicular to) the first, second, and third conductive traces.

[0057] Preferably, each elongated hole is substantially perpendicular to the first conductive trajectory, the second conductive trajectory, and the third conductive trajectory (e.g., horizontal).

[0058] In a preferred embodiment, each aperture in the aperture array is substantially rectangular (e.g., a microstrip). For example, each aperture may be microscopic in width (e.g., about 50 micrometers) and macroscopic in length.

[0059] In a preferred embodiment, the hole array is a basic rectangular array.

[0060] The non-conductive layer can be fabricated using known deposition or growth techniques such as printing (e.g., screen printing, inkjet printing, or thick film printing), casting, spinning, sputtering, photolithography, vapor deposition, spraying, or vacuum deposition. Preferably, the non-conductive layer is fabricated by screen printing. The non-conductive layer can be composed of non-conductive ink.

[0061] The base layer can be a sheet or a strip. The base layer is typically made of an insulating polymer. It can be made of polyester, polycarbonate, or polyvinyl chloride.

[0062] Each conductive trace can be fabricated using known deposition or growth techniques such as printing (e.g., screen printing, silkscreen printing, or thick film printing), casting, spinning, sputtering, photolithography, vapor deposition, spraying, ablation, or vacuum deposition. Each conductive trace can be made of an inert metal such as gold, silver, or platinum. Each conductive trace can be made of conductive ink such as silver or silver / silver chloride ink. The conductive ink can be printable.

[0063] The conductive deposits can be gold, platinum, silver, or carbon deposits.

[0064] Carbon deposits on each conductive track can be deposited using known techniques such as printing (e.g., screen printing, inkjet printing, or thick film printing), sputtering, photolithography, vapor deposition, spraying, or vacuum deposition. The carbon deposits can consist of inert carbon such as graphite, glassy carbon, pyrolytic carbon, carbon nanotubes, graphene, or carbon modified with metal nanoparticles or electrochemical activators.

[0065] Typically, electrochemical sensors are connected to instruments in the system (preferably portable field instruments), which facilitates the ampere operation of the electrochemical sensors.

[0066] Electrochemical sensors can be integrated into online systems. Alternatively, they can be portable. Electrochemical sensors can also be single-use (e.g., disposable). Typically, the amount of analyte is its concentration.

[0067] Typically, the analyte is the target oxidant. The analyte can be one or more of the following groups: chlorine dioxide, chlorine, chlorite, hypochlorite, free chlorine, total chlorine, ozone, peracetic acid, hydrogen peroxide, bromine, and monochloramine.

[0068] In a preferred embodiment, the analyte is free chlorine.

[0069] The analyte can be present in aqueous or non-aqueous solutions (e.g., alcohols or hydrocarbons). Aqueous solutions can be drinking water, recreational water, specially processed water, or wastewater (e.g., industrial wastewater). Drinking water is preferred.

[0070] The analyte can be brought into contact with the working electrode by immersing the working electrode in the sample or by loading the sample onto the working electrode.

[0071] The invention will now be described in a non-limiting manner with reference to the accompanying drawings, in which:

[0072] Figure 1 A commercial electrochemical sensor based on TMB (Chlorosense) is shown. TM Current response relative to free chlorine (HOCl) concentration and current response according to one embodiment of the present invention;

[0073] Figure 2 The reaction of ABTS diammonium salt with HOCl at various concentrations is shown;

[0074] Figure 3 An embodiment of the electrochemical sensor of the present invention (dual sensor) is schematically illustrated;

[0075] Figure 4 The reaction of DPD with HOCl at various concentrations is shown;

[0076] Figure 5 The reactions of TMB with various concentrations of HOCl are shown; and

[0077] Figure 6 The current response of ABTS diammonium salt to free chlorine (top) and monochloramine (bottom) is shown.

[0078] Figure 1 A commercial electrochemical sensor based on TMB (Chlorosense) is shown. TM The current response at 20°C relative to free chlorine (HOCl) concentration and the current response of one embodiment of the invention are described. The high concentration of TMB at the electrode surface allows TMB to operate effectively up to a maximum of 25 mg / L. Above 25 mg / L, much greater variability in the results is observed, and no enhancement of the electrochemical signal is observed between 25 mg / L and 50 mg / L. The signal response of the ABTS diammonium salt increases consistently throughout the concentration range, thus allowing the detection of free chlorine concentrations up to 100 mg / L.

[0079] Figure 3 An embodiment of the electrochemical sensor 1 of the present invention is schematically illustrated. The electrochemical sensor 1 includes a substrate in the form of a polymer tape, on which a continuous layer is gradually deposited by screen printing. The first continuous layer consists of four parallel, spaced-apart silver conductive tracks. Each of the conductive tracks terminates near a first end 11 of the tape with an electrical contact 10. The first of the four conductive tracks constitutes a reference electrode 4. Carbon is deposited on the second of the four conductive tracks near the second end 13 of the tape to form a counter electrode 5. Carbon is deposited on the third of the four conductive tracks near the second end 13 of the tape to form a first working electrode 6. Carbon is deposited on the fourth of the four conductive tracks near the second end 13 of the tape to form a second working electrode 7. The working electrodes 6 and 7 are located on the sides of the reference electrode 4 and the counter electrode 5.

[0080] An insulating layer of non-conductive ink is printed on the top of each electrode 4, 5, 6, and 7 on the mesh screen. Figure 3 (Partially removed as shown in the image). During screen printing, the screen used to deposit non-conductive ink exposes electrical contacts 10 and electrodes 4, 5, 6, and 7 through windows.

[0081] A dry reagent preparation (see below) for measuring low concentrations of free chlorine TMB is deposited on the first working electrode 6.

[0082] concentration unit TMB 1.46 mM maleate buffer 100 mM Polyvinylpyrrolidone 0.05 % (w / v) ethanol 35 % (v / V) Calcium sulfate 5.5 mM

[0083] A layer of dried reagent preparation of ABTS diammonium salt for measuring high concentrations of free chlorine is deposited on the second working electrode 7. The preparation of ABTS diammonium salt is as follows:

[0084]

[0085] A deposit 15 is deposited on the non-conductive ink between the reference electrode 4 and the counter electrode 5 to serve as a dose of potassium chloride to support the electrolyte.

[0086] Sodium maleate was used to buffer the ABTS diammonium salt formulation to pH 6 to allow the sensor to operate in chlorine solutions within the pH range of 4 to 9. At low pH (approximately 2), the ABTS diammonium salt reacts with chlorine and monochloramine, thus high monochloramine interference is observed. At high pH (approximately 6), the ABTS diammonium salt reacts slowly with monochloramine, and the interference is reduced. Figure 6 The current response of ABTS diammonium salt to free chlorine (top) and monochloramine (bottom) is shown.

[0087] Figure 1 The different operating concentration ranges shown for TMB and ABTS diammonium salts make... Figure 3 The embodiment of the invention shown can be used as a dual sensor with a wide operating concentration range. The detection of free chlorine is optimized to achieve high-precision detection of low concentrations of free chlorine on the first working electrode 6 and high-precision detection of high concentrations of free chlorine on the second working electrode 7. A potentiostat measures both the first working electrode 6 and the second working electrode 7 at the same potential. The analysis time interval between the first and second working electrodes is 45 to 55 seconds, and the applied potential is -200 mV.

Claims

1. An electrochemical sensor for determining the presence or amount of an analyte in a sample, comprising: an elongated base layer having a first end opposite a second end; a first conductive trace, a second conductive trace, and a third conductive trace axially deposited onto the base layer in substantially parallel, mutually spaced apart relation, wherein the first conductive trace constitutes a reference electrode, wherein the second conductive trace is conductively deposited proximate the second end of the base layer thereby constituting a counter electrode, and the third conductive trace is conductively deposited proximate the second end of the base layer thereby constituting a working electrode, wherein each of the first, second, and third conductive traces terminates in an electrical contact proximate the first end of the base layer; a non-conductive layer deposited over the first, second, and third conductive traces, wherein the non-conductive layer is fabricated to fully expose the electrical contacts proximate the first end of the base layer, to expose the conductive deposit on the second conductive trace proximate the second end of the base layer, to expose the first conductive trace proximate the second end of the base layer, and to expose one or more discrete working areas of the conductive deposit of the third conductive trace proximate the second end of the base layer; and a reagent formulation of a salt of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) deposited on or proximate the one or more discrete working areas of the conductive deposit of the third conductive trace; a fourth conductive trace axially deposited onto the base layer, wherein the first, second, third, and fourth conductive traces are in substantially parallel, mutually spaced apart relation, wherein the fourth conductive trace is conductively deposited proximate the second end of the base layer whereby the third and fourth conductive traces constitute a working electrode pair, wherein the fourth conductive trace terminates in an electrical contact proximate the first end of the base layer, wherein the non-conductive layer is deposited over the fourth conductive trace and fully exposes one or more discrete working areas of the conductive deposit of the fourth conductive trace, wherein an additional reagent formulation is deposited on or proximate the one or more discrete working areas of the conductive deposit of the fourth conductive trace; wherein the additional reagent formulation is an additional reagent formulation of TMB, DPD, o-tolidine, or syringaldazine; and wherein the reagent formulation of a salt of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) is deposited only on the third conductive trace, and the additional reagent is deposited only on the fourth conductive trace.

2. The electrochemical sensor of claim 1, wherein the reagent formulation further comprises a buffer that, in use, maintains a pH in the range of 5 to 6.

3. The electrochemical sensor of claim 2, wherein the buffer is an acidic salt.

4. The electrochemical sensor of any one of claims 1 to 3, wherein the third conductive track and the fourth conductive track are located lateral to the first conductive track and the second conductive track.

5. The electrochemical sensor of any one of claims 1 to 3, wherein the further reagent formulation is a further reagent formulation of TMB.

6. The electrochemical sensor of any one of claims 1 to 3, wherein the salt of 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) is a diammonium salt of 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid).

7. The electrochemical sensor of any one of claims 1 to 3, wherein the analyte is free chlorine.

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