Utilizing Hydrogel-Isolated Dual-Carboxyl Surface-Enhanced Raman Scattering Probe for Accurate Online pH Monitoring

Surface-enhanced Raman scattering (SERS)-based pH sensors have been widely applied; However, the used 4-mercaptobenzoic acid (4-MBA) probes exhibit small pH-sensitive peak changes (carboxyl group) and strong susceptibility to interference, leading to inaccurate measurements. To address these limitations, we developed a SERS pH sensor using 2,5-dimercaptoterephthalic acid (2,5-DMTA) as the probe, which contains dual carboxyl groups. These carboxyl groups undergo reversible protonation–deprotonation, producing pronounced and reproducible spectral responses that enhance detection accuracy. The developed sensor enabled reliable detection across the acidic pH range of 0–7, showing good linearity ( R 2 = 0.9786) and compensating for the weak acidic response of 4-MBA. Importantly, the 2,5-DMTA-based pH sensor demonstrated much better detection accuracy (detection relative standard deviation RSD less than 5%) than the 4-MBA-based SERS pH sensor (detection RSD ≈ 20%). To further improve measurement accuracy in complex matrices, the sensor was embedded in hot agarose to form an AuNP@hydrogel substrate, effectively suppressing interference from small molecules. Moreover, the developed sensor also shows satisfactory online pH monitoring features, including good reversibility (≥6 cycles), high stability of continuous measurements (30 min), and long-term storage stability (30 days). Integrated with a microfluidic 3D-printed flow cell, the system enabled rapid response (∼120 s) and online pH monitoring, and was successfully applied to continuous testing in lake water. Overall, this SERS platform provides a robust and accurate solution for SERS-based pH detection under acidic and complex environmental conditions.