Optical Sensors for High Acidities Based on Fluorescent Polymers

Abstract

An indirect method for monitoring acid concentrations from 0.1 to 10 M was developed on the basis of the fluorescence of linear-chain, rigid-rod polymers. These linear polymers have high strength and acid-resistant properties that are ideal for support materials, as well as having chemical functionalities that can be protonated under highly acidic conditions to provide a sensing mechanism. By immobilization of the polymers on the endface of an optical fiber, an acidity sensor for monitoring nitric and hydrochloric acids was developed with the use of the change in fluorescent emission as a function of acid concentration. Polymers tested for sensor applications include poly(phenylquinoline), poly(biphenylquinoline), and poly(phenylquinoxaline). The quinoxaline responded from 0.1 to 2 M nitric acid, and the quinolines responded from 2 to 10 M nitric acid. The performance of sensors developed with these materials resulted in a relative standard deviation of approximately 10%.