Abstract: The invention relates to a method for detecting an acid or base in an environment, measuring the concentration of an acid or base in an environment, or measuring the pH of an environment, by contacting the environment containing the acid or the base with a sensor, wherein the sensor is composed of an index of refraction transducer having an outer surface; at least one compound having at least one functional group of a Lewis acid, a Lewis base, a Bronsted acid, a Bronsted base, or a combination thereof, wherein the compound is on or near the outer surface of the transducer, wherein the functional group can interact with the acid or base in the environment to induce a change in index of refraction on or near the outer surface of the transducer, with the provisos that (i) the compound does not undergo a color change when contacted with the acid or base, and (ii) when there are naturally occurring functional groups, no transducer attached compounds, and exactly one overlayer, then the overlayer is not poly(vinyl