Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/327—Biochemical electrodes, e.g. electrical or mechanical details for in vitro measurements
  • G01N27/3275—Sensing specific biomolecules, e.g. nucleic acid strands, based on an electrode surface reaction
  • G01N27/3277—Sensing specific biomolecules, e.g. nucleic acid strands, based on an electrode surface reaction being a redox reaction, e.g. detection by cyclic voltammetry
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/40—Semi-permeable membranes or partitions
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/327—Biochemical electrodes, e.g. electrical or mechanical details for in vitro measurements
  • G01N27/3271—Amperometric enzyme electrodes for analytes in body fluids, e.g. glucose in blood
  • G01N27/3272—Test elements therefor, i.e. disposable laminated substrates with electrodes, reagent and channels

Definitions

• the present invention pertains to the medical field, particularly to the detection of hydrogen peroxide or phosphate related compounds in test or clinical samples by using potentiometric sensors. More particularly, the present invention solves the problem of providing a potentiometric sensor devices for the detection of chemical species in solution with a novel configuration.
• the mixed potential (MP) of the Pt electrode is used to explain the potentiometric response to hydrogen peroxide or to phosphate related compounds, and the use of specific polyelectrolyte coatings is herein presented as a way to control the kinetic processes that lead to the resting potential of the electrode (Baez et al. Analytica Chimica Acta 1097 (2020) 204-213). It is well known that -like with many other metals- the resting potential of a Pt electrode in an electrolyte solution is not under equilibrium, but under a mixed potential (MP) regime, where oxidation (electron-generating) and reduction (electron scavenging) reactions create anodic (i a ) and cathodic (4) exchange currents.
• MP mixed potential
• n k is the number of electrons exchanged in the rate-limiting step.
• the driving force of the reaction -the overpotential (%)- is the difference between the resting potential of the electrode (E) and the equilibrium potential of the reaction (E k ) under study, and F, R and T are the Faraday's constant, universal gas constant and the absolute temperature, respectively.
• OCP Open Circuit Potential
• OCP Open Circuit Potential
• E a and E c are the electrode potential calculated for the anodic or cathodic exchange currents
• K a and K c include a series of parameters and constant terms for each reaction
• b a and /? c are the Tafel slopes for the anodic and cathodic processes.
• expected Tafel slopes are 120 and 60 mV for one-electron and 2-electron transfer, respectively.
• additional factors that lead to more complex reaction mechanisms require corrections to the Tafel equation that result in a wider range of slopes.
• Tafel slopes are key to understand the potentiometric response of these MP systems, since the OCP is linked to changes of the exchange currents. Any perturbation that leads to the readjustment of the exchange currents will result in a change of the electrode potential according to their Tafel plots, such as the exposure of the WE (working electrode) to the solution containing the chemical entity to be tested. This is the reason why the OCP of many metal electrodes -including Pt- respond to the dissolved oxygen concentration.
• the two major factors to consider when analyzing the MP are the cathodic and anodic exchange current, from one side, and the surface condition, from the other.
• the cathodic process the strong dependence of the OCP with the concentration of dissolved O evidences the major contribution of the ORR to the i c .
• the ORR acts removing electrons.
• a decrease of the ORR rate is reflected as drop of the electrode potential.
• increasing the rate of this reaction drives the potential to higher values.
• the anodic reaction is usually associated with the oxidation of Pt on the surface of the electrode.
• Tafel slopes of the ORR will play a key role determining the sensitivity of the OCP. What is more relevant about the anodic process, though, is that it generates a layer of PtO, which inhibits the ORR, as it will be discussed below.
• the ORR on Pt has received significant attention during the last decades because it is the limiting factor in the improvement of the performance of fuel cells (I Katsounaros, WB Schneider, JC Meier, U Benedikt, PU Biedermann, AA Auer, KJJ Mayrhofer, Phys. Chem. Chem. Phys., 2012, 14, 7384).
• the actual mechanism of the reaction is not fully elucidated, and the evidence suggest that different reaction pathways may occur depending on the conditions.
• the ORR on Pt is sluggish and does not proceed in a reversible way.
• the ORR includes a number of elementary steps with different intermediaries (NM markovic, PN Ross Jr, Surface Science Reports 2002, 45, 117). Oxygen may be directly reduced to water (the 4-electron pathway) or be first reduced to hydrogen peroxide, which in turn may undergo different reactions.
• Oxygen may be directly reduced to water (the 4-electron pathway) or be first reduced to hydrogen peroxide, which in turn may undergo different reactions.
• the most recent models and experimental evidence suggest that the mechanism of the ORR involves a first step of adsorption of O2 onto Pt. Then a one-electron transfer is the rate-limiting step. Under these conditions, a Tafel slope of 120mV/decades (Markovic et al.) can be expected. Nevertheless, a number of factors may affect the reaction kinetics. Therefore, Tafel slopes of 60 and 120 mV/decade are typically reported, but values ranging from 40 to 80 mV/decade can be also found.
• the surface condition of the Pt electrode has a strong influence on the MP.
• Competitive adsorption of species on Pt has been studied by Markovic and Ross, who evaluated the ORR in the presence of sulphate and chloride anions. The effect of this spectator species on Pt has been also recently reviewed.
• the adsorption of anions on the surface of the metal blocks the adsorption of oxygen, decreasing the ORR, and therefore affecting the MP.
• this is a severe limitation for the use of bare Pt electrodes in solution, since the matrix will produce a significant and variable degree of interference.
• the OCP of a Pt electrode (or a gold or cobalt electrode) is produced by a MP mechanism, where the ORR plays a fundamental role.
• the kinetics of the ORR is controlled by the oxygen adsorption on free Pt sites available (and/or the change on the energy for adsorption of reaction intermediaries) followed by one electron transfer yield a Tafel slope of 120 mV.
• surface effect will play an important role inhibiting the ORR.
• a potential-dependent surface oxide coverage will have a critical influence, since the total quantity and nature of the surface oxide species being reduced (degree of surface oxidation) has influence on the measured potential. Oxide- coverage factors have also been used to explain values of Tafel slopes below 120 mV/decades.
• Last, but not least, a characteristic of the MP is that there is a continuous generation and consumption of chemical species at the surface of the electrode, which make these systems highly dependent on the mass transport phenomena. Any analytically useful approach would optionally improve by stabilizing the concentration of gradients on the surface of the electrode and minimize the interference effect of foreign species.
• the mixed potential (MP) of the Pt, gold or cobalt electrode (or any mixture thereof) to provide a potentiometric response to hydrogen peroxide or to phosphate related compounds by using a non-traditional potentiometric approach.
• MP mixed potential
• two electrodes must be in contact with the solution, in particular the working electrode and the reference electrode must be in contact with the solution. Such requirement makes these systems very dependent on the correct selection of the working and the reference electrodes used for the system to efficiently work.
• the results presented herein confirm that a potentiometric sensor or cell can use a polyelectrolyte bridge, such as a Nafion polyelectrolyte bridge, to connect the WE and RE instead of the electrolyte solution that is tested, and that such polyelectrolyte bridge effectively closes the circuit and allows detecting the chemical species, such as H O , or phosphate related compounds in said solution.
• a polyelectrolyte bridge such as a Nafion polyelectrolyte bridge
• the present invention confronts the problem that for the detection of chemical species in solution by using potentiometric sensors or cells, two electrodes must be in contact with the solution, in particular the working electrode and the reference electrode must be in contact with the solution. Such requirement makes these systems very dependent on the correct selection of the working and the reference electrodes used for the system to efficiently work.
• the present invention resolves this problem by connecting each of the electrodes of a potentiometric sensor or cell, by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit.
• By effectively closing the potentiometric circuit by using the polyelectrolyte bridge instead of the solution such potentiometric sensors or cells are not as dependent on the selection of a very specific reference, and also working, electrode and can be easily miniaturize.
• potentiometric measurements require the use of two electrodes (a measuring (working) and a reference electrode) which are connected by an electrolyte solution; in the present invention, such measurements require the use of two electrodes (a measuring (working) and a reference electrode) which are connected by a polyelectrolyte bridge instead of the electrolyte solution to be tested.
• a voltmeter will then usually measure the electrical potential difference between the two electrodes, and the voltage will be related to the concentration of the analyte, which presence and/or concentration is being determined.
• a first aspect of the present invention refers to an in vitro use of a potentiometric sensor or cell to, preferably selectively and directly, determine the concentration of hydrogen peroxide in an aqueous solution, wherein the potentiometric sensor or cell comprises:
• a reference electrode comprising a support which in turn comprises a conductive material
• a hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit; and wherein the use is characterized in that the aqueous solution is in direct contact with the working electrode, and in that instead of using the aqueous solution to close the potentiometric circuit between the two electrodes, the hydrogen peroxide selective electrode and the reference electrode (RE), the potentiometric cell uses the polyelectrolyte bridge to connect the electrodes.
• the aqueous solution is in direct contact with the working electrode, and in that instead of using the aqueous solution to close the potentiometric circuit between the two electrodes, the hydrogen peroxide selective electrode and the reference electrode (RE), the potentiometric cell uses the polyelectrolyt
• the hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold
• a phosphate (or phosphate related compounds) selective electrode comprising a porous redox sensitive surface consisting of cobalt.
• phosphate related compounds is herein understood as inorganic (P04, H2P04, HP04) and organic, such as adenosine 5-triphosphate (ATP), adenosine 5-diphosphates (ADP) and higher molecular weight nucleotides, phosphate compounds.
• aqueous solution or "electrolyte solution” is preferably understood as a biological fluid such as whole blood, preferably undiluted whole blood, intracellular fluids, saliva, cerebrospinal fluid, blood sera, blood plasma, sweat and urine. More preferably, said solution is whole blood, in particular undiluted whole blood.
• the use is preferably performed in a single drop, of preferably about 50 pL, reducing the volume 2 orders of magnitude. Noteworthy further reduction of volume down to the single pL is possible based on the geometry optimization. The drop use for the detection does not have to create a bridge between the two electrodes, however, it must be in contact with at least the working electrode.
• the hydrogen peroxide selective electrode is positioned directly above the reference electrode.
• the hydrogen peroxide selective electrode comprises a porous redox sensitive surface consisting of platinum.
• the polyelectrolyte bridge connecting the electrodes comprises or is made of perfluorosulfonic acid ionomers.
• the polyelectrolyte bridge connecting the electrodes comprises or is made of Nafion or Aquivion, preferably Nafion.
• the reference electrode comprises a support which in turn comprises a conductive material selected from any of the following list consisting of: silver, platinum, gold, nickel, zinc, copper, aluminum and carbon.
• the reference electrode and the hydrogen peroxide selective electrode are made of the same material, preferably gold or platinum.
• the polyelectrolyte bridge connecting the electrodes comprises, consists of or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers.
• Preferred polyelectrolytes are those selected from the list consisting of Nafion, Polyethylenimine (PEI) or polyaziridine, wherein polyaziridine is a polymer with repeating units composed of the amine group and two carbon aliphatic CH2CH2 spacer.
• PFSA Perfluorosulfonic acid
• PFSA Perfluorosulfonic acid
• a preferred class of perfluorosulfonic acid ionomers are PFSA-polytetrafluoroethylene copolymers of Formula (II),
• the number of repeat units x and y are such that there are less than 15 X units for each y and the value of m and n are integers between 0 and 5.
• the hydrogen peroxide selective electrode comprises a porous redox sensitive surface consisting of platinum or gold and at least one layer of a proton exchange membrane on said redox sensitive surface, wherein said layer of a proton exchange membrane comprises a copolymer of Formula (II),
• said layer of a proton exchange membrane further comprises a glucose oxidase entrapped therein or any oxidase with hydrogen peroxide as its product.
• a yet further preferred embodiment of this first aspect of the invention refers to the in vitro use of the previous aspect of the invention, wherein said potentiometric sensor or cell preferably comprises a support made of paper which in turn comprises the selective electrode and the reference electrode, thus providing a paper-based sensor.
• said paper-based sensors can be, for example, used to selectively and directly determine the concentration of hydrogen peroxide in an aqueous solution, wherein preferably said aqueous solution is a biological fluid such as whole blood, preferably undiluted whole blood, intracellular fluids, saliva blood sera and urine.
• said selective and direct determination of the concentration of hydrogen peroxide in an aqueous solution in turn determines the concentration of glucose, galactose, cholesterol, uric acid, lactic acid and amino acid in said solution.
• a second aspect of the present invention refers to an in vitro method to, preferably selectively and directhy determine the concentration of hydrogen peroxide in an aqueous solution, the method comprising; a. providing the aqueous solution in contact with the hydrogen peroxide selective electrode of a potentiometric sensor or cell comprising: i. A reference electrode comprising a support which in turn comprises a conductive material; and ii.
• a hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein the hydrogen peroxide selective electrode is preferably positioned directly above the reference electrode, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit; and wherein the polyelectrolyte bridge connecting the electrodes comprises or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers; and b. measuring a potential difference between the reference electrode and the hydrogen peroxide selective electrode; and c.
• the potentiometric cell uses the polyelectrolyte bridge to connect the electrodes instead of the solution.
• the hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, could be optionally replace by a phosphate (or phosphate related compounds) selective electrode comprising a porous redox sensitive surface consisting of cobalt.
• the hydrogen peroxide selective electrode comprises a porous redox sensitive surface consisting of platinum.
• the polyelectrolyte bridge connecting the electrodes comprises or is made of perfluorosulfonic acid ionomers. In another preferred embodiment of this second aspect of the invention, the polyelectrolyte bridge connecting the electrodes comprises or is made of Nafion or Aquivion, preferably Nafion.
• the reference electrode comprises a support which in turn comprises a conductive material selected from any of the following list consisting of: silver, platinum, gold, nickel, zinc, copper, aluminum and carbon.
• the reference electrode and the hydrogen peroxide selective electrode are made of the same material, preferably gold or platinum.
• said layer of a proton exchange membrane further comprises a glucose oxidase entrapped therein or any oxidase with hydrogen peroxide as its product.
• a yet further preferred embodiment of this second aspect of the invention refers to the in vitro method of the previous aspect of the invention, wherein said potentiometric sensor or cell preferably comprises a support made of paper which in turn comprises the selective electrode and the reference electrode, thus providing a paper-based sensor.
• said paper-based sensors can be, for example, used to selectively and directly determine the concentration of hydrogen peroxide in an aqueous solution, wherein preferably said aqueous solution is a biological fluid such as whole blood, preferably undiluted whole blood, intracellular fluids, saliva blood sera and urine.
• said selective and direct determination of the concentration of hydrogen peroxide in an aqueous solution in turn determines the concentration of glucose, galactose, cholesterol, uric acid, lactic acid and amino acid in said solution.
• a third aspect of the invention refers to a potentiometric sensor or cell suitable for selectively measuring chemical species such as hydrogen peroxide or phosphate related compounds in solution, which comprises: a.
• a reference electrode comprising a support which in turn comprises a conductive material such as silver, platinum, gold, nickel, zinc, copper, aluminum or carbon; and b.
• a hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, or a phosphate (or phosphate related compounds) selective electrode comprising a porous redox sensitive surface consisting of cobalt; wherein the electrodes are preferably positioned directly above the other, and wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes.
• the polyelectrolyte bridge connecting the electrodes comprises, consists of or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers.
• Preferred polyelectrolytes are those selected from the list consisting of Nafion, Polyethylenimine (PEI) or polyaziridine, wherein polyaziridine is a polymer with repeating units composed of the amine group and two carbon aliphatic CH2CH2 spacer.
• PFSA Perfluorosulfonic acid
• a preferred class of perfluorosulfonic acid ionomers are PFSA- polytetrafluoroethylene copolymers of Formula (II), where x, y, m and n represent the numbers of repeat units x and y are the numbers for tetrafluoroethylene and perfluorosulfonic acid repeat units respectively and m and n are the repeat units in the side chains of perfluorosulfonic acid blocks x and y are equivalent weight dependent.
• a preferred embodiment of the third aspect of the invention refers to a potentiometric sensor or cell suitable for selectively measuring hydrogen peroxide or phosphate related compounds in solution, which comprises: a.
• a reference electrode comprising a support which in turn comprises a conductive material such as silver, platinum, gold, nickel, zinc, copper, aluminum or carbon; and b.
• a hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold and at least one layer of a proton exchange membrane on said redox sensitive surface; or a phosphate related compounds selective electrode comprising a porous redox sensitive surface consisting cobalt and at least one layer of a proton exchange membrane on said redox sensitive surface, wherein the electrodes are preferably positioned directly above the other, and wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes; and wherein said layer of a proton exchange membrane comprises a copolymer of Formula (II),
• the layer/s of a proton exchange membrane further comprises a glucose oxidase entrapped therein or any oxidase with hydrogen peroxide as its product.
• a still further preferred embodiment of the third aspect of the invention refers to a potentiometric sensor or cell capable of selectively measuring hydrogen peroxide in an aqueous solution comprising a selective electrode and a reference electrode, wherein the selective electrode comprises a redox sensitive surface consisting of platinum or gold and optionally at least one layer of a proton exchange membrane on said redox sensitive surface as defined in the different embodiments of this aspect of the invention.
• said hydrogen peroxide potentiometric cell comprises: a.
• a reference electrode comprising a conductive material such as Ag/AgCI onto which a membrane comprising sodium chloride and PVB dissolved in an appropriate solvent such as methanol is deposited; and b.
• a hydrogen peroxide selective electrode comprising a redox sensitive surface consisting of platinum or gold and at least one layer of a proton exchange membrane on said redox sensitive surface as defined in the first aspect of the invention or in any of its preferred embodiments.
• PVB or Butvar B-98 is understood as polyvinyl butyryl having a molecular weight between 40000-70000 g/mol with butyryl content between 78 and 80% weight per total weight of the polyvinyl butyryl (w/w), hydroxyl content between 18 and 20% (w/w) and acetate less than 2.5%, preferably between 1.5 and 2.5% (w/w).
• a more particular alternative embodiment of this third aspect of the invention refers to a potentiometric sensor or cell to, preferably selectively and directly, determine the concentration of hydrogen peroxide in an aqueous solution, wherein the potentiometric sensor or cell comprises: i. A reference electrode comprising a support which in turn comprises a conductive material; and ii.
• a hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the potentiometric circuit, so that instead of usingthe aqueous solution to close the potentiometric circuit between the two electrodes, the hydrogen peroxide selective electrode and the reference electrode (RE), the potentiometric cell uses the polyelectrolyte bridge to connect the electrodes.
• the potentiometric cell is configured so that the aqueous solution is in contact with the hydrogen peroxide selective electrode and does not close the potentiometric circuit between the two electrodes. See figure 6, wherein the aqueous solution is primarily in contact with the hydrogen peroxide selective electrode.
• the hydrogen peroxide selective electrode comprises a porous redox sensitive surface consisting of platinum.
• the polyelectrolyte bridge connecting the electrodes comprises or is made of perfluorosulfonic acid ionomers.
• the polyelectrolyte bridge connecting the electrodes comprises or is made of Nafion or Aquivion, preferably Nafion.
• the reference electrode comprises a support which in turn comprises a conductive material selected from any of the following list consisting of: silver, platinum, gold, nickel, zinc, copper, aluminum and carbon.
• the reference electrode and the hydrogen peroxide selective electrode are made of the same material, preferably gold or platinum.
• said layer of a proton exchange membrane further comprises a glucose oxidase entrapped therein or any oxidase with hydrogen peroxide as its product.
• this third aspect of the invention refers to the potentiometric sensor or cell of the previous aspect of the invention, wherein said potentiometric sensor or cell preferably comprises a support made of paper which in turn comprises the selective electrode and the reference electrode, thus providing a paper-based sensor.
• paper-based sensors can be, for example, used to selectively and directly determine the concentration of hydrogen peroxide in an aqueous solution, wherein preferably said aqueous solution is a biological fluid such as whole blood, preferably undiluted whole blood, intracellular fluids, saliva blood sera and urine.
• said selective and direct determination of the concentration of hydrogen peroxide in an aqueous solution in turn determines the concentration of glucose, galactose, cholesterol, uric acid, lactic acid and amino acid in said solution.
• FIG. 1 Paper-based sensor configuration for hydrogen peroxide detection (right) and for glucose detection (left) a. Nafion ® 5% membrane, b. Pt sputtered paper strip, c. Nafion ® 10% d. Conductive paper strip, e. Plastic mask. f. GOx enzyme. In the left are the components of the sensor, and in the right the assembled sensor.
• Figure 2 Response time of a sensor to concentration of hydrogen peroxide (logarithmic units) in PBS: left) time 2500 to 4000s refers to a sensor in a cell of 5 mL volume and right) time 4500 to 6500s refers to a single drop of 50 pL volume.
• Figure 3 calibration curve of a sensor (corresponding to data from Figure 2) blue data obtained in solution (PBS 5 mL) and in drop (PBS 50 uL)
• Figure 4 a) Time trace of glucose sensor and b) corresponding calibration curve in PBS (5 mL).
• Figure 5 Traditional approach and for the detection of chemical species in solution by using potentiometric sensors.
• a new cell geometry using metal-sputtered papers in working and reference electrodes, and a Nafion membrane as a conductive media is applied in the construction of a paper- based potentiometric all-solid-state sensor for hydrogen peroxide detection, i.e. the biomarker of the oxidase enzyme reaction as well as, for glucose as a model biomarker.
• Nafion ® 117 solution (10% in a mixture of lower aliphatic alcohols and water); glucose oxidase (GOx) (from Aspergillus niger type X-S, lyophilized powder (100,000 - 250.000 units/g) D-glucose); hydrogen peroxide (30 wt. % in water) and D-Glucose (Glu) were purchased from Sigma-Aldrich.
• GOx glucose oxidase
• Glu D-Glucose
• Paper sensor construction ( Figure 1). two conductive paper strips are used, namely: the upper strip (made with a Pt sputtered paper) acts as WE (working electrode); the lower strip is a conducting paper that acts as reference electrode (RE). WE and RE are glued using a drop of Nafion ® 10% (sandwiched between the plastic masks). Finally, a drop of Nafion ® 5 % is located over the electrochemically active window, covering the exposed area of the WE. In all cases the conductive paper strips were cut with a width of 0.4 cm. Results
• FIG 2 shows the response and calibration curve for the novel sensors to the addition of FI202 in PBS (5 mL cell).
• the bridge indeed connects the working to the reference electrode (Nafion solution).
• the additions give a negative response (from -7 to -1) with a sensitivity of -90 mV per decade in the linear range from -5 to -3 (Figure 3 reports the corresponding calibration curve).
• Figure 3 reports the corresponding calibration curve.
• these results confirm that the proposed potentiometriccell -using Nafion to connect WE and RE instead of the solution- effectively closes the circuit and allows detecting FI202 in solution.
• this is the first report for this kind of configuration for the detection of chemical species in solution.
• the novel configuration allows detection in reduced volume such as shown in Figure 2 right.
• the detection was performed in a single drop of 50 pL reducing the volume 2 order of magnitude. Noteworthy further reduction of volume down to the single pL is possible based on the geometry optimization. Analytical figures are comparable to the ones in solution although with a shifted linear range. The sensitivity here was 100 mV/dec in the -4 -2 linear range. Furthermore, the linear range corresponds to the clinical range of blood glucose and it is therefore of utmost importance.
• a glucose sensor was constructed: GOx enzyme was entrapped in the first layer of Nafion so that it catalyzes the oxidation of glucose added to the solution producing H202, which is detected by the working electrode.
• Figure 4a shows the time trace obtained in artificial serum (AS).
• AS artificial serum
• the measured EMF decreases with the increasing concentration of glucose (sensitivity of -53 mV/dec in the linear range -3.5 -2.5).
• a potentiometric cell suitable for selectively measuring hydrogen peroxide in solution which comprises: a. A reference electrode comprising a support which in turn comprises a conductive material; and b. A hydrogen peroxide selective electrode comprising a porous redox sensitive surface consisting of platinum or gold, wherein the hydrogen peroxide selective electrode is positioned directly above the reference electrode, wherein each of the electrodes is connected to the other by a polyelectrolyte bridge provided between the electrodes, and wherein the surfaces of each of the electrodes that contact the polyelectrolyte bridge provided between the electrodes permits to effectively close the circuit allowing the detection of the chemical species in solution; and wherein the polyelectrolyte bridge connecting the electrodes comprises or is made of polyelectrolytes such as perfluorosulfonic acid ionomers or polyammonium ionomers.
• the reference electrode comprises a support which in turn comprises a conductive material selected from any of the following list consisting of: silver, platinum, gold, nickel, zinc, copper, aluminum and carbon.
• the hydrogen peroxide selective electrode comprises a porous redox sensitive surface consisting of platinum or gold and at least one layer of a proton exchange membrane on said redox sensitive surface, wherein said layer of a proton exchange membrane comprises a copolymer of Formula (II),
• aqueous solution is a biological fluid such as whole blood, preferably undiluted whole blood, intracellular fluids, saliva, cerebrospinal fluid, blood sera, blood plasma, sweat and urine.

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