Fig. 4. (A) Cyclic voltammograms of 4.0 μM Cu²⁺ in 0.1 M PBS (pH 6.5) at different scan rates on PG/GCE. (B) Cyclic voltammograms of 4.0 μM Cd²⁺ in 0.1 M PBS (pH 6.5) at different scan rates on PG/GCE. (C) Cyclic voltammograms of 2.0 μM Hg²⁺ in 0.1 M PBS (pH 6.5) at different scan rates on PG/GCE. Inset: plot of peak currents (I_pa and I_pc) against the scan rate (ν).