In this work, we report a liquid crystal (LC)-based sensor to determine pH level of aqueous solutions. This sensor was fabricated by filling the LCs doped with a pH-sensitive molecule into a cupper grid on a glass substrate. Theoretically, the dopants are neutral and disperse freely in the LCs. When the sensor was immersed in the aqueous solution, the increases of pH can lead to the dissociation of the dopants, allowing them to align at the LC/aqueous interface. This phenomenon causes the reorientation of the LC molecules and therefore a bright-to-dark transition of the LC image was observed simply through naked-eye. In our research, we found that the critical pH value for the optical transition of LC sensors can be adjusted from 6.8 to 8.2 through the selection of dopants, while it can be adjusted from 6.2 to 7.0 through the selection of dopant concentrations. By arranging four individual sensors in a device with inlet and out channels, we demonstrated that the pH level of an aqueous flow system can be determined by the number of bright LC sensors shown in the device. Based on this strategy, we developed a device to monitor the pH safety level for drinking water ranging from pH 6.5 to 8.5. This device demonstrated fast response (~1 s), good stability, reversibility and its capability to measure the pH change in tap water and pond water, which make it suitable for real-time and continuous monitoring the pH change in various flow aqueous systems.
