A thin-film pH sensor based on vibrational spectroscopy has been developed. The sensor was constructed by the immobilization of Congo Red at a base-hydrolyzed cellulose acetate film that had been coated onto a ZnSe internal reflection element. The protonation of the azo groups of Congo Red was monitored as a function of pH with infrared internal reflection spectroscopy. The response characteristics of this sensor demonstrate the potential utility of applying infrared spectroscopy to detect the response of thin film sensors. Opportunities to design sensors based on the molecular specificity of infrared spectroscopy are briefly discussed.

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