Label-Free Optical pH Measurement Based on Chitosan-TEOS-PDMS Hydrogel Layer for Microfluidic Applications

The optical pH sensor based on smart hydrogel material was designed for use in microfluidic applications. The chitosan-tetraethyl orthosilicate (chitosan-TEOS) interpenetrating network was synthesized as a pH-sensitive part of the sensor. This chitosan-TEOS-PDMS layer was bonded by oxygen plasma treatment to a thin layer of polydimethylsiloxane (PDMS), which enabled easily integrating of the sensor layer into the microfluidic chips. Also, the thickness of this PDMS layer worked as an adjustable parameter to control the sensitivity of the sensor. In addition, the optical astigmatism method (the method used in CD/DVD optical pickup unit) was utilized to measure the pH-induced variations of the sensor layer by monitoring the laser beam profile on a charge-coupled device (CCD). The experimental results showed that the fabricated sensor responded to pH changes and became stable in less than 90 s. It showed a sensitivity of 54 and 94 pixel/pH using the spin-coated PDMS thickness of 75 and 28 $\mu \text{m}$ , respectively.