Direct electrochemistry and nitric oxide interaction of heme proteins adsorbed on nanocrystalline tin oxide electrodes

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 TiO2 electrodes. In this paper, we extend these studies to nanocrystalline SnO2 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 SnO2 electrode is demonstrated to be preferable to the TiO2 electrode for electrochemical studies of these proteins, as the potential window over which the SnO2 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 SnO2 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/SnO2 and Hb/SnO2 electrodes, respectively. Finally, we demonstrate that the high protein loading and electrical conductivity of Hb/SnO2 films allow the electrochemical sensing of nitric oxide. Electrochemical sensing is demonstrated, with a limit of detection of 1 muM.