Nanocrystalline, nanoporous metal oxide films are a novel substrate for protein immobilization. Such electrodes allow both electrochemical and spectroelectrochemical studies of the protein redox function and, moreover, provide an attractive approach for the development of both optical and electrochemical biosensors. Previous studies have largely focused on the use of nanocrystalline TiO 2 electrodes. In this paper, we extend these studies to nanocrystalline SnO 2 electrodes. Immobilization of two proteins, cytochrome c (Cyt- c ) and hemoglobin (Hb) is observed to proceed with similar protein loadings for both metal oxides. However, the SnO 2 electrode is demonstrated to be preferable to the TiO 2 electrode for electrochemical studies of these proteins, as the potential window over which the SnO 2 film is conducting is shifted to more positive potentials by 500 mV. Cyclic voltammetry and spectroelectrochemistry are employed to demonstrate that this shift allows the observation of reversible oxidation and reduction of both heme proteins on the SnO 2 electrodes without the use of any electron-transfer promoters or mediators. Electron-transfer rate constants of 1 ± 0.03 and 0.53 ± 0.03 s - 1 are determined for the Cyt- c /SnO 2 and Hb/SnO 2 electrodes, respectively. Finally, we demonstrate that the high protein loading and electrical conductivity of Hb/SnO 2 films allow the electrochemical sensing of nitric oxide. Electrochemical sensing is demonstrated, with a limit of detection of 1 μM.
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Topoglidis et al. (2003) studied this question.