Electrodeposition of guanine oxidation product onto zinc oxide nanoparticles: Application to nanomolar detection of l-cysteine

Zinc oxide (ZnOx) nanoparticles electrodeposited on the surface of glassy carbon (GC) electrode by applying potential step (−0.8 V 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′=0.28 VEo′=0.28 V in pH 7 buffer solution), at pH range 2–12. The surface coverage (Γ) and heterogeneous electron transfer rate constant (ks) of adsorbed redox couple were about 9.5 × 10−9 mol cm−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) × 103 M−1 s−1. The catalytic oxidation current allows the amperometric detection of l-cysteine at potential of 0.5 V with detection limit of 50 nM, linear response up to 20 μM and sensitivity of 215.4 nA μ A−1 cm−2.