Evaluation of the intrinsic pH sensing performance of surface-enhanced Raman scattering pH probes

Surface-enhanced Raman spectroscopy (SERS) has shown a high promise in microenvironmental pH sensing in recent years, which is significant for understanding various biological processes and multiphase chemical reactions. Generally, the pH sensing capability of the SERS substrate is realized by the surface functionalization of the substrate with a pH sensitive molecule (pH probe). However, to the best of our knowledge, there is no systematic evaluation of mainstream SERS pH probes reported in literatures on the pH response range, resistance to co-present ions, accuracy and stability. Here a systematic analysis of common pH probes, including 4-mercaptobenzoic acid (4-MBA), 4-mercaptopyridine (4-Mpy), 2-aminobenzenethiol (2-ABT), and 4-aminothiophenol (4-ATP) has been carried out with a series of performance assessments. Density functional theory (DFT) calculation was carried out to investigate the vibration mode in SERS spectra. Moreover, the practical application comparison was conducted with urine samples. Overall, 4-MBA showed an excellent sensing performance in the neutral and alkaline area, and meanwhile, the pH response of 4-Mpy and 4-ATP was inclined to the acidic and neutral range. The pH probe 2-ABT is not recommend because of the poor performance.