Chemical imaging sensor for observation of microscopic pH-distribution

A novel chemical imaging sensor has been developed using a Si semiconductor. The sensor (Si₃N₄/SiO₂/Si) is in contact with the electrolyte solution that serves as an objective sample. The backside of the Si substrate is illuminated by a modulated laser beam. The AC photocurrent flows though the electrolyte-insulator-semiconductor (EIS). The intensity of the photocurrent depends on the electrolyte solution pH value on only the illuminated region. A two-dimensional pH distribution can be obtained by scanning the focused laser beam. The sensor visualizes the pH distribution in the gel solution induced by ion exchange resin and microorganisms. The spatial resolution of this sensor is restricted by the Si substrate thickness as well as the laser beam spot size. A practical sensor with a spatial resolution better than 5 μm was fabricated using a SOI wafer instead of Si wafer, and anisotropic chemical etching instead of mechanical polishing. Such a high spatial resolution makes it possible to detect the microscopic pH distribution in solution. A reduction of spatial resolution due to the lateral spread of the depletion layer was estimated at approximately 80 nm in the present sensor system. The pH resolution of this sensor was also estimated to be 0.01 pH.