Effect of the structure of imidazolium cations in [BF4](-)-type ionic liquids on direct electrochemistry and electrocatalysis of horseradish peroxidase in Nafion films.

The direct electrochemistry and bioelectrocatalysis of horseradish peroxidase (HRP) in Nafion films at glassy carbon electrode (GCE) was investigated in three [BF(4)](-)-type room-temperature ionic liquids (ILs) to understand the structural effect of imidazolium cations. The three ILs are 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF(4)]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim][BF(4)]). A small amount of water in the three ILs is indispensable for maintaining the electrochemical activity of HRP in Nafion films, and the optimum water contents decrease with the increase of alkyl chain length on imidazole ring. Analysis shows that the optimum water contents are primarily determined by the hydrophilicity of ILs used. In contrast to aqueous medium, ILs media facilitate the direct electron transfer of HRP, and the electrochemical parameters obtained in different ILs are obviously related to the nature of ILs. The direct electron transfer between HRP and GCE is a surface-confined quasi-reversible single electron transfer process. The apparent heterogeneous electron transfer rate constant decreases gradually with the increase of alkyl chain length on imidazole ring, but the changing extent is relatively small. The electrocatalytic reduction current of H(2)O(2) at the present electrode decreases obviously with the increase of alkyl chain length, and the mass transfer of H(2)O(2) via diffusion in ILs should be responsible for the change. In addition, the modified electrode has good stability and reproducibility; the ability to tolerate high levels of F(-) has been greatly enhanced due to the use of Nafion film. When an appropriate mediator is included in the sensing layer, a sensitive nonaqueous biosensor could be fabricated.