A robust and effective composite film based on geuam gum (GG) and hydrophilic ionic liquid (ILs) 1-heptyl-3-methyl-imidazolium chloride ([HMIM]Cl) was prepared. The composite film can be readily used as an immobilization matrix to entrap cytochrome c (Cyt c). UV-Vis spectroscopy showed that Cyt c retained its native structure in the composite film. A pair of well-defined and quasi-reversible redox peaks of Cyt c was obtained in 0.10 mol•L^－1 pH 7.0 phosphate buffer solution (PBS) at the GG-Cyt c-ILs composite film modified glassy carbon electrode (GG-Cyt c-ILs/GCE), indicating the direct electron transfer between Cyt c and the underlying electrode. The electron transfer rate constant (k_s) of Cyt c on the electrode surface was 3.87 s^－1. The GG-Cyt c-ILs/GCE showed an excellent electrocatalytic activity toward the reduction of H₂O₂. The catalytic peak current was linear to H₂O₂ concentration in the range of 1.0×10^－6～2.7×10^－4 mol•L^－1, with a detection limit of 5.0×10^－7 mol•L^－1. The apparent Michaelis-Menten constant (K_m) was estimated to be 0.56 mmol•L^－1. Based on the GG-ILs composite film, a third-generation reagentless biosensor could be constructed for the determination of H₂O₂.

Key words: cytochrome c, ionic liquid, geuam gum, electrochemistry, electrocatalysis