Surface-Enhanced Raman Spectroscopy Detection of Trace Ni²⁺ in Aquatic Environments Using a Zincon Probe

Heavy metal ions pose persistent threats to ecological stability and human health due to their high toxicity, poor degradability, and long-term accumulation in the environment. Therefore, the development of analytical techniques capable of rapid, sensitive, and selective detection of these ions in complex aqueous matrices is of significant importance. In this study, the nickel ion (Ni²⁺), a typical contaminant frequently found in industrial effluents, was selected as the target analyte. The aim was to establish a probe-assisted surface-enhanced Raman scattering (SERS) method that enables efficient trace-level detection of Ni²⁺ even in complex sample matrices. Zincon was employed as a coordination-sensitive molecular probe. However, due to its sulfonic acid functional groups, Zincon cannot effectively adsorb onto the negatively charged surface of the citrate-stabilized silver nanoparticles (NPs). To address this issue, poly(diallyldimethylammonium chloride) (PDDA) was introduced to modify the NP surface, resulting in positively charged and highly stable SERS-active substrates that facilitate effective probe loading. UV-Vis spectroscopy was applied to characterize the Ni²⁺-Zincon coordination system and to elucidate the associated spectral evolution, thereby confirming the presence of coordination interactions and potential charge-transfer processes. Key experimental parameters, including halide type and concentration, probe dosage, and modification time, were systematically examined. Optimal conditions were identified by evaluating both signal intensity and measurement repeatability. The results showed that upon coordination with Ni²⁺, the characteristic absorption peak of Zincon at 495 nm red-shifted to 510 nm, accompanied by the emergence of a new absorption band at 665 nm. These spectral variations provide strong evidence of electron redistribution induced by complex formation. Under optimized conditions at pH 9, with 200 μmol/L Zincon, 10-min bromide-assisted modification, and 532 nm excitation, the PDDA-Ag NPs substrate generated stable and reproducible SERS signals. The Raman band at 730 cm⁻¹ exhibited a strong linear correlation with Ni²⁺ concentrations ranging from 10 nmol/L to 1 μmol/L (R² = 0.9942). The detection limit was as low as 0.0187 μmol/L, with a quantification limit of 0.0625 μmol/L, demonstrating the method's capability for quantitative trace Ni²⁺ detection in electroplating wastewater. Selectivity evaluations confirmed that the proposed sensing platform exhibited excellent resistance to interference from other common metal ions. When applied to real electroplating wastewater samples, the results obtained using this SERS method showed high agreement with those obtained by atomic absorption spectroscopy, exhibiting a relative standard deviation in low-concentration samples (RSD = 4.31%), confirming the method's good reproducibility and practical applicability. Further analysis indicated that the effective SERS enhancement primarily originates from the charge-transfer energy level of the Ni²⁺-Zincon complex, which is well-matched with the excitation wavelength. This mechanism provides theoretical guidance for the design of SERS-based heavy metal detection systems suitable for complex environmental matrices. Overall, the established probe-assisted SERS strategy offers a promising analytical approach for environmental monitoring, pollution source identification, and early risk warning.