Spatial Heterogeneity of Sulfate in Tap Water across China: Insights from a Rapid and Reliable Surface-Enhanced Raman Spectroscopy Analysis

As a key anion in the biogeochemical cycle and life science, the analysis of trace-level sulfate anion (SO₄^2-) is a routine yet laborious laboratory task. Traditional lab-based methods are inadequate for real-time, high-spatiotemporal-resolution screening and tracking on national/global scales. To address this challenge, a surface-enhanced Raman spectroscopy (SERS)-based strategy was developed, achieving sensitive qualitative analysis (≥1 μg/L) within 1 min/sample and accurate quantitative analysis (relative standard deviation < 10%) within 3-5 min/sample for SO₄^2- in diverse water samples. The selective capture and detection of SO₄^2- are realized via intermolecular hydrogen bonding with the adsorbed thiourea, using positively charged Au NPs as the SERS substrate. The rapid screening of SO₄^2- levels in tap water from major cities across China displayed high spatial heterogeneity, consistent with recently reported theoretical prediction, influenced by an overall interplay of climatic, hydro-geochemical, and anthropogenic drivers. Critically, this SERS platform is expected to establish a high-throughput sensing framework for monitoring SO₄^2- dynamics on national/global scales, generating spatially resolved real-time data sets that empower data-driven sulfur cycle modeling. Moreover, it holds potential to revolutionize real-time environmental monitoring of other environmentally relevant anions, such as nitrate, nitrite, and perchlorate.