Development of near-field optic/atomic force microscope for biological materials in aqueous solutions

This paper reports improvements of optical fiber cantilevers and the scanning near-field optical microscopy imaging of biological materials in liquid. In our scanning near-field optical/atomic-force microscope (SNOAM), the scanning of an optical fiber cantilever over the specimen was controlled by dynamic mode AFM to reduce damage to the probe and soft specimens. The typical resonant frequency of the optical fiber cantilever was 19.5 kHz, while it was 23.0 kHz in air. The Q-factor of the cantilever depended on the vibration amplitude and was typically 260-600 in air and 40-240 in water. The relationship between the vibration amplitude and the average sample-probe separation indicated that the cantilever worked in the non-contact mode in water, while it worked in the cyclic-contact mode in air. Cultured cells in aqueous solutions were visualized by the SNOAM, indicating that the SNOAM is suitable to observe soft specimens.