A method of chemically modifying the poly (methyl methacrylate) (hereinafter "PMMA") clad surface of an optical fiber to introduce amino groups. N-butyl lithium in a suitable organic solvent with ethylene diamine is applied to the clad surface of the optical fiber in a substantially oxygen-free atmosphere, such as nitrogen, at approximately 10.degree. C. to 40.degree. C. for about one to two hours. A pH-sensitive dye with isothiocyanate functionality can be bounded to the modified clad surface resulting in a pH sensor based on fiber optics.