Fiber-optic pH sensor based on poly(vinyl alcohol) (PVA)/poly(acrylic acid) (PAA) hydrogel coating.

Abstract

The growing needs for optical pH sensors in many applications in recent years; especially the biomedicine industry promotes the design of a bio-compatible optical pH sensor for regulation of the body functions and other biological processes.

In this paper, a new optical pH sensor utilizing Michelson interferometer in a standard single mode fiber (SMF) with one waist-enlarged fusion bitaper has been proposed and experimentally evaluated. The optical pH sensor presented is based on the swelling kinetics of nanostructured film such as hydrogel which was dip-coated onto the proposed fiber probe. The pH-sensitive hydrogel was prepared through the thermal crosslinking of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA). Following, the coated sensor was exposed to solutions of various pH which the shift in the interference fringes of the optical signals was determined. The performance of the sensor was then evaluated.

It was found that the sensor demonstrated a high sensitivity of 1.58nm/pH and exhibited high repeatability with minimal hysteresis. The sensor also displayed a low ionic strength sensitivity of 0.06nm/M and temperature sensitivity 0.014nm/◦C. In addition, experiments conducted also showed a long-term time stability of sensor with minute fluctuations of ±0.06 nm. Lastly, the proposed sensor achieved a fast response time in the second range with a rise and fall time of 1.40s and 1.20s respectively.