Fabrication and characterization of a highly sensitive surface plasmon resonance based fiber optic pH sensor utilizing high index layer and smart hydrogel

We report the fabrication and characterization of a surface plasmon resonance based fiber optic sensor to measure the pH of an aqueous fluid. The sensing probe is prepared by coating an unclad core of the optical fiber with three consecutive layers of silver, silicon and pH-sensitive smart hydrogel. The change in pH of the fluid around the probe causes the swelling/shrinkage of the hydrogel layer resulting in the change in its refractive index. The sensor works on wavelength interrogation method. The change in refractive index causes a change in the resonance wavelength in the transmitted spectrum. Experimental results show a blue shift in the resonance wavelength on the increase in the pH of the aqueous solution. The sensor operates in the low and high pH range. The influence of environmental temperature (in 16-28 °C range) on its performance is found to be negligible. Further, the sensor possesses short response time, stability and remarkable sensitivity. The detection accuracy of the sensor increases as the pH of the fluid increases. The additional advantages of the present probe are small size probe, no effect of source fluctuation/intensity reduction on its performance because the sensor is based on wavelength interrogation and its low cost because the operational wavelength range is in visible region.