The immobilization methodology of enzyme and redox complex on electrode surfaces can have an impact on the magnitude and stability of amperometric current response, with implications for application as biosensor and fuel cell enzyme electrodes. Here we report on an investigation of carboxymethyl dextran (CMD) and polyacrylic acid polymers, bearing carboxylic functional groups, as chemical supports for immobilization of amine-containing osmium redox complexes and enzymes at electrode surfaces. Crosscoupling using carbodiimide reagent of the CMD polymer support, [Os (2,2 -bipyridine) 2 (4-aminomethyl pyridine)Cl].PF 6 redox complex, glucose oxidase (GOx) and multiwall carbon nanotubes (MWCNT), provides a 3-dimensional matrix for catalytic electrooxidation of glucose yielding current density of 1.0 ± 0.2 mA cm −2 and 4.5 ± 1.0 mA cm −2 at 0.45 V vs. Ag/AgCl, in 50 mM phosphate buffer saline (pH 7.4, 37 • C) containing 5 mM and saturated glucose amounts, respectively. Similar enzyme electrodes, but instead using [Os (4,4 -dimethoxy-2,2 -bipyridine) 2 (4-aminomethyl pyridine)Cl].PF 6 of lower redox potential, produce current densities of 0.83 ± 0.21 mA cm −2 in 5 mM glucose and 3.4 ± 0.7 mA cm −2 in saturated glucose solution at 0.2 V vs. Ag/AgCl thus showing promise for application as low potential glucose oxidizing biosensors and as anodes for in-vivo enzymatic fuel cells for power generation. Enzyme electrodes for glucose oxidation are of increasing interest due to their potential applications as biosensors and as anodes in membrane-less fuel cells operating on sugar as a fuel. Such enzymatic fuel cells (EFC) can use enzymes as specific catalysts to oxidize glucose at the anode and reduce oxygen at the cathode, that when combined as a fuel cell convert chemical energy into electrical power.1-3 The advantages to the use of enzyme-based catalysts are substrate specificity, which can eliminate the need for casings and ion exchange membranes in assembled fuel cells, and of being capable of operating under moderate ambient conditions, compared to metal catalysts. 1,2A substantial body of research exists on approaches for maximising catalytic current capture as a result of enzyme redox reactions in enzyme electrodes through the co-immobilization of enzymes and electron-shuttling mediators within redox-conducting hydrogels on solid electrodes. For example, over the past two decades, Heller and co-workers 3 have pioneered use of epoxy cross-linking of electrostatic adducts of redox enzyme and osmium redox polymers, using polyvinylimidazole (PVI) as the polymer backbone, at electrode surfaces to provide "wired" enzyme electrodes capable of producing glucose oxidation current. Mediated electron transfer by electron-hopping selfexchange within these hydrogels allows connection between redox active sites of enzymes and electrode surfaces thus generating bioelectrocatalytic current.4-6 As mediators, osmium based polypyridyl complexes are broadly explored, 6-8 as these complexes possess advantages over iron and ruthenium based ...

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Graphite electrodes are modified with a redox polymer, [Os(4,4′‐dimethoxy‐2,2′‐bipyridine)2(polyvinylimidazole)10Cl]+ (E°′=−0.02 V vs Ag/AgCl (3 M KCl), crosslinked with a flavin adenine dinucleotide glucose dehydrogenase and multi‐walled carbon nanotubes for electrocatalytic oxidation of glucose. The enzyme electrodes provide 52 % higher current density, 1.22±0.10 mA cm−2 in 50 mM phosphate‐buffered saline at 37 °C containing 5 mM glucose, when component amounts are optimised using a design of experiments approach compared to one‐factor‐at‐a‐time. Current densities of 0.84±0.15 mA cm−2 were achieved in the presence of oxygen for these enzyme electrodes. Further analysis of the model allowed for altering of the electrode components while maintaining similar current densities, 0.78±0.11 mA cm−2 with 34 % less enzyme. Application of the cost‐effective anodes in membrane‐less enzymatic fuel cells is demonstrated by connection to cathodes prepared by co‐immobilisation of [Os(2,2′‐bipyridine)2(polyvinylimidazole)10Cl+] redox polymer, Myrothecium verrucaria bilirubin oxidase and multi‐walled carbon nanotubes on graphite electrodes. Power densities of up to 285 μW cm−2, 146 μW cm−2 and 60 μW cm−2 are achieved in pseudo‐physiological buffer, artificial plasma and human plasma respectively, showing promise for in vivo or ex vivo power generation under these conditions.

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