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We have developed a surface stress sensor based on a microelectromechanical Fabry–Perot interferometer with high wavelength selectivity by using Au half-mirrors, for highly sensitive label-free biosensing. When the target molecule is adsorbed by the antigen–antibody reaction onto a movable membrane with a thin Au film, which acts as an upper mirror of the optical interferometer, the amount of deflection of the movable membrane deflected by the change in surface stress can be detected with high sensitivity. To improve the signal at the small membrane deflection region of this biosensor resulting in detection of low concentration molecules, by integrating 50 nm-thick Au half-mirrors, the wavelength selectivity of the optical interferometer has been successfully improved 6.6 times. Furthermore, the peak shift in the reflection spectrum due to the adsorption of bovine serum albumin (BSA) antigen with a concentration of 10 ng ml−l by the antigen–antibody reaction was spectroscopically measured on the fabricated optical interferometer, and the deflection amount of the movable membrane after 10 min treatment was 2.4 times larger than that of nonspecific adsorption with the avidin molecules. This result indicated that the proposed sensor can be used for selective detection of low-concentration target antigen molecules.

A charge-transfer-type pH sensor operated by charge accumulation was demonstrated, and it was confirmed that its pH sensitivity was 1130 mV/pH with five signal integration cycles. The most familiar pH sensor using a semiconductor is the ion-sensitive field effect transistor (ISFET). The sensitivity of our pH sensor was 20 times higher than that of the ISFET. The sensitivity of the ISFET, which is similar in structure to a metal–insulator–semiconductor (MIS) structure, is restricted by the Nernst equation (59 mV/pH at room temperature). The output signal from an ISFET disappears into its noise as pH variation becomes small. The charge-transfer-type pH sensor increases the pH output signal to integrate signals in the time domain that are noise-free. It is expected that with this sensor it is possible to measure small pH signal variations.

32 × 32 pH image sensors were successfully fabricated by using the CCD/CMOS image sensor technique, and real time imaging of a chemical reaction and pH distribution was carried out for the first time. The pH variations by a chemical reaction are observed by 200 ms step (i.e. 5 flames per sec). The pH image sensor was able to take a pH image of mouse stomach successfully. It means that the novel image sensor can be applied to a biomedical and biochemical field.