Phase-mode operation of FDM-LAPS

The light-addressable Potentiometrie sensor (LAPS) is a chemical sensor based on the field effect in semiconductor. In a LAPS, a modulated light illuminates the sensor plate and the photocur-rent is measured as the sensor signal, which varies with the ion concentration at the illuminated spot. A plurality of spots on the sensing surface can be simultaneously measured by frequency division multiplex (FDM) using light beams modulated at different frequencies. In this study, we combined the phase measurement of the photocurrent with FDM-LAPS. While the amplitude of the photocurrent is affected by fluctuation of light intensity and recombination of photocarriers at defects in the semiconductor, the phase of the photocurrent is expected to be robust against such influences. As an example, a simultaneous multi-point measurement of pH values in a flow channel was demonstrated.