Fig. 1. Cyclic voltammogram for the reductive desorption of -cysteine from a gold disk electrode in 0.5 M KOH solution (deaerated). Initial potential, −0.2 V; sweep rate, 20 mV s⁻¹. Modification of the gold electrode was performed by immersion of the electrodes into 1 mM -cysteine in phosphate buffer (pH 5.5) for 30 min.

Fig. 2. Filled circles: isotherm for adsorption of -cysteine on a gold electrode from reductive desorption measurements. Open circles: isotherm for Cu(II) complexation on an -cysteine coated gold electrode from chronoamperometry measurements. Error bars are ±1 standard deviation determined from three repeat measurements.

Fig. 3. Cyclic voltammograms of a gold electrode modified by immersion in 10 mM -cysteine for 30 min in phosphate buffer (pH 5.5). Dotted line: -cysteine modified electrode in phosphate buffer. Solid line: After subsequent immersion in 10 μM Cu²⁺ for 5 min and return to the phosphate buffer. Sweep rate: 100 mV s⁻¹.

Fig. 4. Plots of peak current versus scan rate for the copper--cysteine complex on gold. For conditions see Fig. 3.

Fig. 5. Cyclic voltammogram of a gold electrode modified with -cysteine that has been immersed in 0.1 M Cu²⁺ for 5 min. Electrolyte is phosphate buffer pH 5.5. Sweep rate: 100 mV s⁻¹.

Fig. 6. (A) Cu 2p_3/2 X-ray photoelectron spectra (a) -cysteine modified gold after immersion in 10.0 μM Cu²⁺ for 10 min. (b) -Cysteine modified gold after immersion in 0.1 M Cu²⁺ for 10 min. (B, C, D) X-ray photoelectron spectra (a) -cysteine adsorbed on gold from 10 mM phosphate buffer (pH 5.5); (b) -cysteine modified gold after immersion in 0.01 mM Cu²⁺ for 10 min. B: S 2p, C: N 1s, D: O 1s. a.u.=arbitrary units.

Fig. 7. Proposed structure of the Cu(-Cysteine)₂ complex at a gold surface.

Fig. 8. Calibration plot of OSWV peak height against concentration of Cu²⁺. Error bars are ±95% confidence interval determined from three replicate measurements.

Table 1. Determinations of Cu(II) in two samples of tap water