Electrode systems for electrochemical sensors

Abstract

The present invention relates generally to systems and methods for improved electrochemical measurement of analytes. The preferred embodiments employ electrode systems including an analyte-measuring electrode for measuring the analyte or the product of an enzyme reaction with the analyte and an auxiliary electrode configured to generate oxygen and/or reduce electrochemical interferants. Oxygen generation by the auxiliary electrode advantageously improves oxygen availability to the enzyme and/or counter electrode; thereby enabling the electrochemical sensors of the preferred embodiments to function even during ischemic conditions. Interferant modification by the auxiliary electrode advantageously renders them substantially non-reactive at the analyte-measuring electrode, thereby reducing or eliminating inaccuracies in the analyte signal due to electrochemical interferants.

1. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme, wherein the enzyme reacts with the analyte; 
 an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and 
 an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material that is directly impermeable to glucose but is permeable to oxygen. 
 
     
     
       2. The electrochemical sensor of  claim 1 , wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       3. The electrochemical sensor of  claim 1 , wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       4. The electrochemical sensor of  claim 1 , wherein the polymer comprises a material that is permeable to interfering species. 
     
     
       5. The electrochemical sensor of  claim 4 , wherein the polymer comprises a material having a molecular weight that allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       6. The electrochemical sensor of  claim 1 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       7. The electrochemical sensor of  claim 1 , wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferant interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       8. The electrochemical sensor of  claim 7 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       9. The electrochemical sensor of  claim 1 , configured for measuring a concentration of glucose in a fluid. 
     
     
       10. The electrochemical sensor of  claim 1 , configured for insertion into a subcutaneous tissue of a host. 
     
     
       11. The electrochemical sensor of  claim 1 , configured for implantation into a subcutaneous tissue of a host. 
     
     
       12. The electrochemical sensor of  claim 1 , configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       13. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme, wherein the enzyme reacts with the analyte; 
 an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and 
 an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is directly situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material that is impermeable to glucose but is permeable to oxygen and permeable to interfering species. 
 
     
     
       14. The electrochemical sensor of  claim 13 , wherein the polymer comprises a material having a molecular weight that blocks glucose and allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       15. The electrochemical sensor of  claim 13 , wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       16. The electrochemical sensor of  claim 13 , wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       17. The electrochemical sensor of  claim 13 , wherein the polymer comprises a material having a molecular weight that allows transport therethrough of urate, ascorbate, and acetaminophen. 
     
     
       18. The electrochemical sensor of  claim 13 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       19. The electrochemical sensor of  claim 13 , wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferant interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       20. The electrochemical sensor of  claim 19 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       21. The electrochemical sensor of  claim 13 , configured for measuring a concentration of glucose in a fluid. 
     
     
       22. The electrochemical sensor of  claim 13 , configured for insertion into a subcutaneous tissue of a host. 
     
     
       23. The electrochemical sensor of  claim 13 , configured for implantation into a subcutaneous tissue of a host. 
     
     
       24. The electrochemical sensor of  claim 13 , configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       25. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme, wherein the enzyme reacts with the analyte; 
 an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and 
 an auxiliary electrode comprising a conductive material and configured to modify an electrochemical interferant interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material that is impermeable to glucose but is permeable to oxygen. 
 
     
     
       26. The electrochemical sensor of  claim 25 , wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       27. The electrochemical sensor of  claim 25 , wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       28. The electrochemical sensor of  claim 25 , wherein the polymer comprises a material that is permeable to an electrochemical interferant interferent. 
     
     
       29. The electrochemical sensor of  claim 25 , wherein the polymer comprises a material that is impermeable to glucose but is permeable to oxygen and interferants interferents. 
     
     
       30. The electrochemical sensor of  claim 25 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1V. 
     
     
       31. The electrochemical sensor of  claim 25 , wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       32. The electrochemical sensor of  claim 31 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       33. The electrochemical sensor of  claim 25 , configured for measuring a concentration of glucose in a fluid. 
     
     
       34. The electrochemical sensor of  claim 25 , configured for insertion into a subcutaneous tissue of a host. 
     
     
       35. The electrochemical sensor of  claim 25 , configured for implantation into a subcutaneous tissue of a host. 
     
     
       36. The electrochemical sensor of  claim 25 , configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       37. The electrochemical sensor of  claim 25 , wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       38. The electrochemical sensor of  claim 37 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       39. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme, wherein the enzyme reacts with the analyte; 
 an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and 
 an auxiliary electrode comprising a conductive material and configured to modify an electrochemical interferant interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material having a molecular weight that blocks glucose and allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
 
     
     
       40. The electrochemical sensor of  claim 39 , wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       41. The electrochemical sensor of  claim 39 , wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       42. The electrochemical sensor of  claim 39 , wherein the polymer comprises a material that is permeable to an electrochemical interferant interferent. 
     
     
       43. The electrochemical sensor of  claim 39 , wherein the polymer comprises a material that is permeable to interferants interferents. 
     
     
       44. The electrochemical sensor of  claim 39 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1V. 
     
     
       45. The electrochemical sensor of  claim 39 , wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       46. The electrochemical sensor of  claim 45 , wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       47. The electrochemical sensor of  claim 39 , configured for measuring a concentration of glucose in a fluid. 
     
     
       48. The electrochemical sensor of  claim 39 , configured for insertion into a subcutaneous tissue of a host. 
     
     
       49. The electrochemical sensor of  claim 39 , configured for implantation into a subcutaneous tissue of a host. 
     
     
       50. The electrochemical sensor of  claim 39 , configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       51. An electrochemical sensor for measuring a concentration of an analyte in a biological fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with the analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material that is permeable or impermeable to glucose but is permeable to oxygen, and wherein the sensor is configured such that the auxiliary electrode is located between the electroactive surface of the working electrode and the biological fluid being measured.   
     
     
       52. The electrochemical sensor of claim 51, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       53. The electrochemical sensor of claim 51, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       54. The electrochemical sensor of claim 51, wherein the polymer comprises a material having a molecular weight that allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       55. The electrochemical sensor of claim 51, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       56. The electrochemical sensor of claim 51, wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       57. The electrochemical sensor of claim 56, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       58. The electrochemical sensor of claim 51, configured for measuring a concentration of glucose in a fluid. 
     
     
       59. The electrochemical sensor of claim 51, configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       60. An electrochemical sensor for measuring a concentration of an analyte in a biological fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with the analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated at a location directly between the electroactive surface and the biological fluid being measured such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is directly situated on a surface of the auxiliary electrode, and wherein the polymer comprises a material that is permeable or impermeable to glucose but is permeable to oxygen and permeable to one or more interfering species.   
     
     
       61. The electrochemical sensor of claim 60, wherein the polymer comprises a material having a molecular weight that blocks glucose and allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       62. The electrochemical sensor of claim 60, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       63. The electrochemical sensor of claim 60, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       64. The electrochemical sensor of claim 60, wherein the polymer comprises a material having a molecular weight that allows transport therethrough of urate, ascorbate, and acetaminophen. 
     
     
       65. The electrochemical sensor of claim 60, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       66. The electrochemical sensor of claim 60, wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       67. The electrochemical sensor of claim 66, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       68. The electrochemical sensor of claim 60, configured for measuring a concentration of glucose in a fluid. 
     
     
       69. The electrochemical sensor of claim 60, configured for insertion into a subcutaneous tissue of a host. 
     
     
       70. The electrochemical sensor of claim 60, configured for implantation into a subcutaneous tissue of a host. 
     
     
       71. The electrochemical sensor of claim 60, configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       72. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with the analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to modify an electrochemical interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode such that at least a portion of the polymer is located more distal to the electroactive surface than the auxiliary electrode, and wherein the polymer comprises a material that is permeable or impermeable to glucose but is permeable to oxygen.   
     
     
       73. The electrochemical sensor of claim 72, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       74. The electrochemical sensor of claim 72, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       75. The electrochemical sensor of claim 72, wherein the polymer comprises a material that is permeable to an electrochemical interferent. 
     
     
       76. The electrochemical sensor of claim 72, wherein the polymer comprises a material that is impermeable to glucose but is permeable to oxygen and interferents. 
     
     
       77. The electrochemical sensor of claim 72, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       78. The electrochemical sensor of claim 72, wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       79. The electrochemical sensor of claim 78, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       80. The electrochemical sensor of claim 72, configured for measuring a concentration of glucose in a fluid. 
     
     
       81. The electrochemical sensor of claim 72, configured for insertion into a subcutaneous tissue of a host. 
     
     
       82. The electrochemical sensor of claim 72, configured for implantation into a subcutaneous tissue of a host. 
     
     
       83. The electrochemical sensor of claim 72, configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       84. The electrochemical sensor of claim 72, wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       85. The electrochemical sensor of claim 84, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       86. An electrochemical sensor for determining a presence or a concentration of an analyte in a fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with the analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to modify an electrochemical interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode, wherein the auxiliary electrode located within or adjacent to a membrane system such that at least a portion of the membrane system is located more distal to the electroactive surface than the auxiliary electrode, and wherein the membrane system comprises a polymer comprising a material having a molecular weight that allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen.   
     
     
       87. The electrochemical sensor of claim 86, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       88. The electrochemical sensor of claim 86, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       89. The electrochemical sensor of claim 86, wherein the polymer comprises a material that is permeable to an electrochemical interferent. 
     
     
       90. The electrochemical sensor of claim 86, wherein the polymer comprises a material that is permeable to interferents. 
     
     
       91. The electrochemical sensor of claim 86, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       92. The electrochemical sensor of claim 86, wherein the auxiliary electrode is configured to generate oxygen. 
     
     
       93. The electrochemical sensor of claim 92, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       94. The electrochemical sensor of claim 86, configured for measuring a concentration of glucose in a fluid. 
     
     
       95. The electrochemical sensor of claim 86, configured for insertion into a subcutaneous tissue of a host. 
     
     
       96. The electrochemical sensor of claim 86, configured for implantation into a subcutaneous tissue of a host. 
     
     
       97. The electrochemical sensor of claim 86, configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       98. An electrochemical sensor for measuring a concentration of an analyte in a biological fluid in a host, the sensor comprising:
 a membrane system comprising: a cell impermeable domain configured to contact a biological fluid in a host and an enzyme domain comprising enzyme, wherein the enzyme reacts with the analyte;   a working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode located within or adjacent to the membrane system and comprising a conductive material and configured to modify an electrochemical interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode.   
     
     
       99. The sensor of claim 98, configured for measuring a concentration of glucose in a host. 
     
     
       100. The sensor of claim 98, configured for insertion into contact with a subcutaneous tissue of a host. 
     
     
       101. The sensor of claim 98, configured for communication with the intravascular system of a host. 
     
     
       102. The sensor of claim 98, configured for continuous measurement of the analyte in a host. 
     
     
       103. The sensor of claim 98, further comprising a potentiostat operably connected to the working electrode. 
     
     
       104. The sensor of claim 103, wherein the potentiostat enables continuous measurement of the analyte in a host. 
     
     
       105. The sensor of claim 98, further comprising sensor electronics operably connected to the working electrode, wherein the sensor electronics are configured to transmit data to a receiver. 
     
     
       106. The sensor of claim 105, wherein the sensor electronics comprise an RF transceiver configured to wirelessly transmit the data to a receiver. 
     
     
       107. The sensor of claim 98, wherein the membrane system is configured to limit diffusion of the analyte there through. 
     
     
       108. The sensor of claim 107, wherein the membrane system further comprises a resistance domain configured to limit diffusion of the analyte there through. 
     
     
       109. The sensor of claim 98, wherein the membrane system is configured to limit or block one or more interfering species there through. 
     
     
       110. The sensor of claim 109, wherein the membrane system further comprises an interference domain configured to limit or block the one or more interfering species there through. 
     
     
       111. The sensor of claim 109, wherein the membrane system further comprises an electrolyte domain configured to provide the hydrophilicity at the working electrode. 
     
     
       112. The sensor of claim 98, wherein the membrane system is configured to provide a hydrophilicity at the working electrode. 
     
     
       113. The sensor of claim 98, wherein the cell impermeable domain is located more distal from the working electrode than any other domain of the membrane system such that the cell impermeable domain directly contacts the host when placed into contact with the host's dermis, subcutaneous tissue and/or intravascular system. 
     
     
       114. The sensor of claim 98, wherein the auxiliary electrode is located within the cell impermeable domain. 
     
     
       115. The sensor of claim 98, wherein the auxiliary electrode is located between the cell impermeable domain and the enzyme domain. 
     
     
       116. An electrochemical sensor for continuous measurement of a concentration of an analyte in an in vivo biological environment, the sensor comprising:
 a membrane comprising an outermost layer configured for protection of the sensor from the biological environment, wherein the membrane further comprises an enzyme configured to react with the analyte;   a working electrode comprising a conductive material and configured to measure a product of the reaction of the enzyme with the analyte; and   an auxiliary electrode located within or adjacent to the membrane and comprising a conductive material, wherein the auxiliary electrode is configured to modify an electrochemical interferent such that the electrochemical interferent is rendered substantially electrochemically non-reactive at the working electrode.   
     
     
       117. The sensor of claim 116, configured for measuring a concentration of glucose in a host. 
     
     
       118. The sensor of claim 116, configured for insertion into contact with a subcutaneous tissue of a host. 
     
     
       119. The sensor of claim 116, configured for communication with the intravascular system of a host. 
     
     
       120. The sensor of claim 116, further comprising a potentiostat operably connected to the working electrode. 
     
     
       121. The sensor of claim 120, wherein the potentiostat enables continuous measurement of the analyte in a host. 
     
     
       122. The sensor of claim 116, further comprising sensor electronics operably connected to the working electrode, wherein the sensor electronics are configured to transmit data to a receiver. 
     
     
       123. The sensor of claim 122, wherein the sensor electronics comprise an RF transceiver configured to wirelessly transmit the data to a receiver. 
     
     
       124. The sensor of claim 116, wherein the membrane is configured to limit diffusion of the analyte there through. 
     
     
       125. The sensor of claim 124, wherein the membrane further comprises a resistance domain configured to limit diffusion of the analyte there through. 
     
     
       126. The sensor of claim 116, wherein the membrane is configured to limit or block one or more interfering species there through. 
     
     
       127. The sensor of claim 126, wherein the membrane further comprises an interference domain configured to limit or block the one or more interfering species there through. 
     
     
       128. The sensor of claim 116, wherein the membrane is configured to provide a hydrophilicity at the working electrode. 
     
     
       129. The sensor of claim 128, wherein the membrane further comprises an electrolyte domain configured to provide the hydrophilicity at the working electrode. 
     
     
       130. The sensor of claim 116, wherein the cell impermeable domain is located more distal from the working electrode than any other domain of the membrane such that the cell impermeable domain directly contacts the host when placed into contact with the host's dermis, subcutaneous tissue and/or intravascular system. 
     
     
       131. The sensor of claim 116, wherein the auxiliary electrode is located within the outermost layer. 
     
     
       132. The sensor of claim 116, wherein the auxiliary electrode is located between the outermost layer and the enzyme. 
     
     
       133. An electrochemical sensor for measuring a concentration of an analyte in a biological fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with an analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of a reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, wherein the polymer comprises a material that is permeable or impermeable to glucose but is permeable to oxygen, and wherein the sensor is configured such that the auxiliary electrode is located between the electroactive surface of the working electrode and a biological fluid in which a concentration of the analyte is being measured;   wherein the sensor is configured for insertion or implantation into a subcutaneous tissue of a host.   
     
     
       134. The electrochemical sensor of claim 133, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       135. The electrochemical sensor of claim 133, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       136. The electrochemical sensor of claim 133, wherein the polymer comprises a material that is permeable to interfering species. 
     
     
       137. The electrochemical sensor of claim 136, wherein the polymer comprises a material having a molecular weight that allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       138. The electrochemical sensor of claim 133, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       139. The electrochemical sensor of claim 133, wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       140. The electrochemical sensor of claim 139, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       141. The electrochemical sensor of claim 133, configured for measuring a concentration of glucose in a fluid. 
     
     
       142. The electrochemical sensor of claim 133, configured for measuring a concentration of glucose substantially without an oxygen deficit. 
     
     
       143. An electrochemical sensor for measuring a concentration of an analyte in a biological fluid, the sensor comprising:
 a membrane system comprising an enzyme configured to react with an analyte;   an electroactive surface comprising a working electrode, the working electrode comprising a conductive material and configured to measure a product of a reaction of the enzyme with the analyte; and   an auxiliary electrode comprising a conductive material and configured to generate oxygen, wherein the auxiliary electrode is situated such that the oxygen generated diffuses to the enzyme or to the electroactive surface, wherein the auxiliary electrode comprises a polymer, wherein the polymer is situated on a surface of the auxiliary electrode, wherein the polymer comprises a material that is permeable or impermeable to glucose but is permeable to oxygen, wherein the polymer comprises a material that is permeable to interfering species, and wherein the sensor is configured such that the auxiliary electrode is located between the electroactive surface of the working electrode and a biological fluid in which a concentration of the analyte is being measured.   
     
     
       144. The electrochemical sensor of claim 143, wherein the auxiliary electrode comprises a conductive material selected from the group consisting of a conductive metal, a conductive polymer, and a blend of a conductive metal and a conductive polymer. 
     
     
       145. The electrochemical sensor of claim 143, wherein the auxiliary electrode comprises a form selected from the group consisting of a mesh, a grid, and a plurality of spaced wires. 
     
     
       146. The electrochemical sensor of claim 143, wherein the polymer comprises a material that is permeable to interfering species. 
     
     
       147. The electrochemical sensor of claim 146, wherein the polymer comprises a material having a molecular weight that allows transport therethrough of oxygen, urate, ascorbate, and acetaminophen. 
     
     
       148. The electrochemical sensor of claim 143, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.6 V. 
     
     
       149. The electrochemical sensor of claim 143, wherein the auxiliary electrode is configured to electrochemically modify an electrochemical interferent to render the electrochemical interferent substantially electrochemically non-reactive at the working electrode. 
     
     
       150. The electrochemical sensor of claim 149, wherein the auxiliary electrode is configured to be set at a potential of at least about +0.1 V. 
     
     
       151. The electrochemical sensor of claim 143, configured for measuring a concentration of glucose in a fluid. 
     
     
       152. The electrochemical sensor of claim 143, configured for measuring a concentration of glucose substantially without an oxygen deficit.