Electrochemical sensor

Abstract

A voltammetric pH sensor, especially for characterising wellbore fluids, comprises a plurality of electrodes with a redox active organic compound attached to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, wherein the compound has at least one substituent group which promotes hydrogen bonding at a said functional group and thereby increases the reaction rate of proton transfer. The substituent group may form an internal hydrogen bond with a redox-convertible group or may enhance polarity to promote electrostatic interaction with water molecules and reduce activation energy. Typical examples include alizarin or 1,2-dihydroxy-anthraquinone (RH=72-48-0), quinizarin or 1,4-dihydroxy-anthraquinone (RN=81-64-1), 2-acetoxy-benzoquinone (RN=1125-55-9), chloranil or 2,3,4,5-tetrachloro-benzoquinone (RN=118-75-2) and 1,4-diamino-2,3-dichloro-anthraquinone (RN=81-42-5) deposited on a glassy carbon electrode. In this way, anomalous measurements at low ionic strength and low concentrations of pH buffering species can be overcome.

1 - 18 . (canceled) 
     
     
         19 . An electrochemical sensor comprising a redox active organic compound immobilized to an electrode and having a functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase,
 wherein the compound has at least one substituent group containing oxygen or nitrogen and positioned to participate in internal hydrogen bonding at the functional group.   
     
     
         20 . A sensor according to  claim 19 , wherein said redox active organic compound incorporates a partial structure 
       
         
           
           
               
               
           
         
       
       wherein the carbon atoms are joined by conjugated bonds. 
     
     
         21 . A sensor according to  claim 19 , wherein the redox active organic compound comprises at least two fused aromatic rings with oxygen or nitrogen-containing substituents, convertible between reduced and oxidized forms, on adjacent fused aromatic rings in positions allowing formation of an internal hydrogen bond between the reduced form of one said substituent and the oxidized form of another said substituent. 
     
     
         22 . A sensor according to  claim 21 , wherein the redox active organic compound has two oxygen-containing substituents, convertible between hydroxyl and keto forms, on a first aromatic ring and at least two oxygen-containing substituents, convertible between hydroxyl and keto forms, on a second aromatic ring fused with the first said ring, with at least two of said substituents in positions allowing formation of an internal hydrogen bond between the hydroxyl form of one said substituent on one ring and the keto form of another said substituent on the other said ring. 
     
     
         23 . A sensor according to  claim 19 , wherein the redox active organic compound is selected from 2-acetylbenzoquinone, 2,3-diacetylbenzoquinone, 1,2-dihydroxynaphthoquinone, 1,4-dihydroxynaphthoquinone and homologues and analogues of these compounds which have additional substituents or have additional rings fused with the benzoquinone or naphthoquinone rings, and wherein the keto groups of the benzoquinone or napthoquinone may be in ortho or para positions relative to each other. 
     
     
         24 . A sensor according to  claim 19 , comprising a plurality of electrodes with the redox active organic compound immobilized on one of the electrodes. 
     
     
         25 . A sensor according to  claim 19 , wherein oxidation and/or reduction of said redox active compound is sensitive to pH and the sensor further comprises a second redox active compound as a reference, immobilized to the same or another electrode, the oxidation and reduction of the second redox active compound are substantially insensitive to pH. 
     
     
         26 . A sensor according to  claim 19 , wherein the redox active organic compound has a structure with at least one substituent group configured such that the electron and proton transfer are concerted. 
     
     
         27 . Measuring apparatus comprising a sensor according to  claim 19 , together with means to apply variable voltage to the sensor and measure current while the voltage is systematically varied. 
     
     
         28 . A downhole tool for measuring characteristic parameters of wellbore fluids, comprising an electrochemical sensor in accordance with  claim 19 . 
     
     
         29 . A method of measuring the pH of water or other aqueous liquid comprising exposing the water or other aqueous liquid to a sensor according to  claim 19 , applying variable potential to the electrode with the redox-active organic compound immobilized thereon and determining the applied potential at a maximum current for redox reaction of the compound. 
     
     
         30 . A method of measuring the pH of unbuffered aqueous liquid comprising
 exposing the aqueous liquid to a sensor comprising a redox active compound immobilized to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous liquid, the electrochemical conversion being sensitive to the pH of the aqueous liquid;   wherein the compound has at least one substituent group which increases the reaction rate of proton transfer by reducing the activation energy for transfer of a proton and/or promoting hydrogen bonding at a said functional group;   applying variable potential to the electrode with the redox-active compound immobilized thereon; and   determining the applied potential at a maximum current for redox reaction of the compound.   
     
     
         31 . A method according to  claim 30 , wherein said at least one substituent group is a substituent group containing oxygen or nitrogen and positioned to participate in internal hydrogen bonding. 
     
     
         32 . A method according to  claim 30 , wherein said at least one substituent group is an electron withdrawing group. 
     
     
         33 . A method according to  claim 31 , wherein said redox active organic compound incorporates a partial structure 
       
         
           
           
               
               
           
         
       
       wherein the carbon atoms are joined by conjugated bonds. 
     
     
         34 . A method according to  claim 30 , wherein the redox active organic compound comprises at least two fused aromatic rings with oxygen or nitrogen-containing substituents, convertible between reduced and oxidized forms, on adjacent fused aromatic rings in positions allowing formation of an internal hydrogen bond between the reduced form of one said substituent and the oxidized form of another said substituent. 
     
     
         35 . A sensor according to  claim 34 , wherein the redox active organic compound has two oxygen-containing substituents, convertible between hydroxyl and keto forms, on a first aromatic ring and at least two oxygen-containing substituents, also convertible between hydroxyl and keto forms, on a second aromatic ring fused with the first said ring, with at least two of said substituents in positions allowing formation of an internal hydrogen bond between the hydroxyl form of one said substituent on one ring and the keto form of another said substituent on the other said ring. 
     
     
         36 . A sensor according to  claim 34 , wherein the redox active organic compound comprises at least two fused aromatic rings with two oxygen-containing substituents, convertible between reduced (hydroxyl) and oxidized (keto) forms, on a first aromatic ring and at least two nitrogen containing substituents, also convertible between reduced and oxidized forms, on a second aromatic ring fused with the first said ring, with at least two of said substituents in positions allowing formation of an internal hydrogen bond between the reduced form of a nitrogen-containing substituent on one ring and the keto form of an oxygen-containing substituent on the other said ring, the compound further having electron withdrawing substituents reducing the basicity of the nitrogen-containing substituents. 
     
     
         37 . An electrochemical sensor according to  claim 31 , wherein the redox active organic compound comprises at least one aromatic ring with at least one oxygen-containing substituent thereon, convertible between hydroxyl and keto forms and at least two substituents are located on said ring or connected thereto through conjugated bonds and contain atoms other than carbon or hydrogen and increase the polarity of the molecule. 
     
     
         38 . A sensor according to  claim 37 , wherein the said substituents are halogen atoms. 
     
     
         39 . A sensor according to  claim 37 , wherein the redox active organic compound comprises at least one aromatic ring with two oxygen-containing substituents thereon, convertible between hydroxyl and keto forms, and at least three halogen atoms as substituents on said ring or on one or more adjacent aromatic rings fused with the first said ring. 
     
     
         40 . A method according to  claim 30 , wherein the redox active organic compound is selected from trihalobenzoquinone, tetrahalobenzoquinone, 1,4-diamino-2,3-dihalo-naphthoquinone, 2-acetylbenzoquinone, 2,3-diacetylbenzoquinone, 1,2-dihydroxynaphthoquinone, 1,4-dihydroxynaphthoquinone and homolgues and analogues of these compounds which have additional substituents or have additional rings fused with the benzoquinone or naphthoquinone rings, and wherein the keto groups of the benzoquinone or napthoquinone are in ortho or para positions relative to each other. 
     
     
         41 . A method according to  claim 30 , wherein the sensor further comprises a second redox active compound as a reference, immobilized to the same or another electrode, the oxidation and reduction of the second redox active compound being substantially insensitive to pH of the aqueous liquid.