were performed with a CHI660B electrochemical
station (CHEN HUA Instruments Co., Shanghai, China). A
conventional three-electrode system consisted of a glassy carbon
working electrode with a diameter of 3.0 mm, a counter electrode
made of platinum wire and an Ag/AgCl reference electrode. All
electrochemical measurements were carried out in a 10 mL
cell. All potentials were given with respect to the Ag/AgCl
electrode.