Determination of pH with surface-enhanced Raman fiber optic probes

Surface-enhanced resonance Raman scattering, cystamine-coupled pH indicators, roughened optical fibers and silver substrate combine toform a sensitive pH probe. Methyl red (MR) and cresol red (CR) react with cystamine to form disulfides which, together with 4-pyridinethiol (4PT), are each adsorbed on the silver substrate of fiber optic probes. MR serves as a monoprotic near resonance indicator, CR as a diprotic resonance indicator and 4PT as a normal Raman indicator. A spectral test of the indicators in aqueous solution shows that the relative intensity of their peaks compared to those of their conjugate acid correlate well with their pH.