Continuously Responsive Surface-Enhanced Raman Scattering Sensors for Long-Term On-Site Free Chlorine Monitoring in Aquatic Environments

Rapid industrialization and urbanization have significantly increased wastewater discharge into natural water bodies, thereby emphasizing the critical importance of precise long-term pollutant monitoring at urban sewage outfalls to protect aquatic ecosystems. Free chlorine (FC), widely used in disinfection, poses significant ecological risks when overdischarged. However, achieving reliable long-term FC monitoring at discharge points remains a substantial challenge. In this study, we developed a continuously responsive surface-enhanced Raman scattering (SERS) sensor for long-term on-site FC monitoring. The sensor integrated a flexible SERS probe functionalized with 4-aminothiophenol (4-ATP) for selective FC detection and a compact floating roller device for continuous signal acquisition. The sensor exhibited a low detection limit of 4.16 ppb and accurately quantified FC concentrations in real-world water samples, showcasing high sensitivity and reliability. Long-term deployment at sewage treatment plant (STP) outfalls further validated its robust monitoring capabilities under actual environmental conditions. This innovative SERS sensor offers a practical and accurate solution for long-term on-site FC monitoring, highlighting its potential as a scalable approach for environmental monitoring applications.