A revised manuscript submitted to sensors and actuators B: Chemical illumination modification from an LED to a laser to improve the spatial resolution of IGZO thin film light-addressable potentiometric sensors in pH detections

Indium Gallium Zinc Oxide (IGZO) on Indium Tin Oxide (ITO) glass has been applied as the semiconductor substrate of a light-addressable potentiometric sensor (LAPS) with the advantages of immunity to room light inference and high photocurrent. For the first time, investigations into this IGZO LAPS including two-dimensional (2D) chemical imaging and reduction of the potential biorelative damage induced by the requirement of an ultraviolet (UV) light source are presented in this study. For the spatial resolution, a smaller aperture for the light spot provided by a light emitting diode (LED) makes the photocurrent decrease. To overcome this natural limitation, the type of AC modulation of light source is changed from a sine wave to a square wave with different duty cycles. The photocurrent and power consumption of a 365 nm LED with square wave and a duty cycle of 40 % are 42.2 % higher and 16.2 % lower than those of the conventional setup of a sine wave, respectively. The pH sensitivity, linearity, hysteresis and drift are comparable. The illumination wavelengths of 365, 405 and 435 nm are studied for photoresponse. 405 nm illumination with acceptable pH sensing performance can be suggested for IGZO LAPS for less bio damage concern. Additionally, a laser with a wavelength of 405 nm with spot size of 25 μm and scanning step of 2 μm is proven to have a clear pattern recognition down to 50 μm in 2D image. Further optimization of the spot size of the laser, thickness and composition of IGZO are suggested for better spatial resolution.