Unveiling the Critical Influence of pH on Arsenic Detection by Surface-Enhanced Raman Spectroscopy

Exacerbated by ongoing anthropogenic activities, inorganic arsenic (As) contamination of drinking water poses a significant health threat worldwide. Current quantification and speciation strategies rely on resource-extensive laboratory analyses. Providing rapid, low-cost, and reproducible measurements, surface-enhanced Raman spectroscopy (SERS) has emerged as a promising As detection tool in both laboratory and field settings. This study enhances understanding of SERS for As speciation using unmodified citrate-capped gold nanoparticles (cit-AuNPs). Herein, changes to the Raman fingerprints of As(III) and As(V) oxyanions are characterized across a wide pH range and under high/low dissolved oxygen concentrations. The quantification efficacy of the cit-AuNPs for As(III) and As(V) is also identified as being pH-dependent with quantifications achieved as low as 10 ppb and 1 ppm, respectively. These values are acquired near pH 8, where the highest SERS intensities are observed. Low oxygen conditions have minimal impact on the spectra, suggesting a limited role for As oxidation in its detection and speciation under ambient conditions. These findings offer novel insights into the interactions between As and cit-AuNPs across varying pH levels, and establish a foundation for deploying Au-based SERS to quantify and speciate inorganic As in environmental settings.