Optical Sensor Monitors pH

A team at Kyushu University in Fukuoka, Japan, says its laser-based pH sensor will be at least as sensitive as a fluorometric device. The sensor, reported in Analytical Chemistry, comprises a gel grating of the pH-sensitive thymolphthalein, which is transparent at pH <9.3 and blue at pH >10.5, and a 2-mW, 633-nm HeNe laser light source. A photodiode and digital oscilloscope measure the diffracted beam's intensity, and a digital camera records the diffraction pattern, which changes in response to alkalinity.