The biosensor was developed on a gold disk electrode (1.6 mm diameter, MF-2014, Bioanalytical Systems, Inc., West Lafayette, IN). Prior to use, the gold disk electrode was polished using 0.3 mm alumina slurry, sonicated for at least 10 min in distilled water and then incubated in Piranha solution for 15 min and washed with DI water. Before modification, the gold disk electrode was sonicated in acetone and ethanol for 15 min consecutively, and dried under nitrogen gas flow.