Fiber-optic sensor for salt concentration based on polymer swelling coupled to optical displacement

Abstract

A sensor that responds reversibly to salt concentration was prepared in which the sensing element is an ionic polymer, either sulfonated polystyrene or sulfonated dextran, that shrinks as the ionic strength is increased. The polymer is coupled to a reflector such that changes in polymer size cause a change in reflector position, which in turn causes a change in the intensity of light reflected back into an optical fiber. The range of salt concentrations that cause a change in polymer size depends on the charge density on the polymer. Larger intensity changes are observed as the degree of polymer cross-linking decreases. Intensity changes are larger for smaller beads. Plots of intensity vs. concentration vary from salt to salt but follow concentration more closely than ionic strength. The response is more sensitive with a two-fiber optical arrangement than with a single fiber.