DETERMINATION OF PH WITH SURFACE-ENHANCED RAMAN FIBER OPTIC PROBES

Methyl red, cresol red, and 4-pyridinethiol were examined for their ability to determine the pH at a surface. Methyl red and cresol red were coupled to cystamine. This produced a disulfide derivatized indicator which formed robust monolayers on sliver substrates. The spectroscopic technique used to examine these compounds was surface-enhanced Raman spectroscopy. We determined that all of these compounds possessed surface-enhanced Raman bands which were characteristic of the molecular structure associated with the indicator and its conjugate acid. Methyl red represents a near resonance indicator. It showed a linear log (I 1400/I 1615) vs pH relationship from pH 2 to 4.5. Some interference due to buff er anions was observed. Cresol red was examined as a resonance Raman indicator. Its linear range was pH 2-8 for the ratio of I 1600/I 1390. An inflection In the calibration curve was observed near pH 5. Cresol red was tested on an optical fiber for Its usefulness as a remote pH probe. It functioned very well and was used to follow a flowing, pH gradient produced by adjusting a reservoir pH with acetic acid. 4-Pyridinethiol is a normal Raman indicator. It possessed a linear log (I 1005/I 1100) vs pH relationship between pH 5 and pH 9. The calibration curves, including the inflection in the case of cresol red, are explained with Gouy-Chapman-Stern theory.