High-resolution pH imaging sensor for microscopic observation of microorganisms

We have constructed a high-resolution pH imaging sensor, which enables one to observe the metabolic activity of microorganisms microscopically. A spatial resolution better than 10 mu m was achieved by thinning the Si substrate to 20 mu m, and by employing an infra-red laser as an illumination source. Using a 100 kHz digital sampling technique for signal detection, a pH image of 128 x 128 pixels was measured within 15 min. The pH imaging sensor was integrated into a system for microscopic observation of incubated biological specimens such as microorganisms. Using the pH imaging sensor, Escherichia coli (E. coli) colonies incubated on agar medium for 8 h (visual size similar to 0.2 mm diameter, number of cells similar to 800) were detected. The observed pH distribution was in agreement with the result of simulation based on the production and diffusion of acidic ions.