Zinc oxide (ZnOx) nanoparticles electrodeposited on the surface of glassy carbon (GC) electrode by applying potential step (−0.8V for 120 s) from a zinc acetate solution with 50 ◦C. Then, potential cycling was used for oxidation of guanine and producing an electroactive redox couple which strongly and irreversibly adsorbed on the ZnOx nanoparticles/modified electrode. Modified electrode shows a pair of well-defined nearly reversible and surface-controlled redox couple (Eo = 0.28V in pH 7 buffer solution), at pH range 2–12. The surface coverage ( ) and heterogeneous electron transfer rate constant (ks) of adsorbed redox couple were about 9.5×10−9 molcm−2 and 3.18(±0.20) s−1, respectively, indicating the high loading ability of ZnOx nanoparticles toward guanine oxidation product and great facilitation of the electron transfer between redox couple and ZnOx nanoparticles. The modified electrode exhibited excellent electrocatalytic activity toward l-cysteine oxidation. The kcat for l-cysteine oxidation was found to be 4.20(±0.20)×103M−1 s−1. The catalytic oxidation current allows the amperometric detection of lcysteine at potential of 0.5V with detection limit of 50 nM, linear response up to 20Mand sensitivity of 215.4 nAA−1 cm−2.
