On-chip pH measurement using functionalized gel-microbeads positioned by optical tweezers

This paper demonstrates local pH measurement in a microchip using a pH-sensing gel-microbead. To achieve this, the gel-microbead made of a hydrophilic photo-crosslinkable resin was functionalized with the pH indicator bromothymol blue (BTB). The primary constituent of this photo-crosslinkable resin is poly(ethylene glycol). Gel-microbeads impregnated with BTB were obtained by stirring the mixture solution, which was composed of the resin, BTB, and an electrolyte solution. The gel-microbead is polymerized by UV illumination. The polymerized gel-microbead can be manipulated by optical tweezers and made to adhere to a glass surface. The local pH was measured from the color of the gel-microbead impregnated with BTB by calibrated color information in the YCrCb color space. We succeeded in measuring the local pH value using the pH-sensing gel-microbead by manipulating and positioning it at the desired point in the microchip.