Shape Observation by Local illumination Using Reflected Light from Magnetically Controlled Guanine Crystal Platelet

Biogenic guanine crystal platelets act as micromirror with the high reflectivity because their refractive index is ~1.83. The diamagnetic anisotory enables the guanine crystals suspended in water to control the reflected light by a magnetic field of several hundred millitesla. In this paper, we demonstrate the shape observation by reflecting the light which enters from the side of the sample on the broadest surface of guanine crystal just below the sample. Reflected light illuminates the sample locally from an oblique direction by adjusting the permanent magnet position. In this case, higher contrast image like a dark-field illumination is observed compared to bright-field illumination. For reducing the magnetic field amplitude required for guanine crystal orientation to improve the controllability, various magnetic nanoparticles are examined to bind the guanine crystal platelets. As a result, the orientation of the crystal direction by low magnetic field and the high reflection property of light are obtained for Fe
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O
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nanoparticles with average particle size of 15 nm. The guanine crystal platelet can be oriented by a three axis Helmholtz coil. A high-contrast image comparable to that for guanine crystal without nanoparticles case is successfully obtained.