Figure 5.

(a) UV-Vis absorption spectra of the DDTC-Cu²⁺ complex in wastewater with bands at ~450 nm, depending on the concentrations of the Cu²⁺ ion. The inset shows a photo of the DDTC-Cu²⁺ complex with [Cu²⁺] from 0 to 50 ppm in wastewater. (b) Cu²⁺ concentration-dependent SERS spectra of DDTC on AuNPs in wastewater samples after alkaline chlorination treatment. (c) A magnified view of the region from 1400 to 1550 cm⁻¹. (d) Three independent measurements of Raman intensities at vibrational bands of ~1504 cm⁻¹ were performed to provide the standard deviations and a linear fit for the concentration range between 1 and 50 ppm. The samples for the calibration curve were obtained by dilution of the initial wastewater (77.06 ppm Cu²⁺) with the DDTC-Cu²⁺ complex and the AuNP solution. The samples with the other Cu²⁺ concentrations could also made by changing the volumes of wastewater and DDTC-Cu²⁺ complex, accordingly.