Single Fiber-Optic pH Sensor Based on Changes in Reflection Accompanying Polymer Swelling

We have prepared fiber-optic pH sensors consisting of a small drop of aminated polystyrene on the tip of a single optical fiber with a core diameter of 100 microns. The sensor is prepared by dipcoating a partially polymerized solution and then completing the polymerization on the fiber. This is followed by amination with diethanolamine. The polymer formulation includes xylene/dodecane to introduce porosity and Kraton G1652, a styrene-ethylene, butylene-styrene, triblock copolymer as a toughening agent. The polymer swells as the amine groups are protonated. This is accompanied by an increase in clarity of the polymer and a decrease in the intensity of light reflected back into the optical fiber. Intensity decreases by over a factor of 2 as the pH is decreased from 8.0 to 6.5. The resulting sensor is small and mechanically stable with a response time of several minutes.