The localized enhanced near field on nanostructures has been attracting much attention for a template for size-selective optical trapping (tweezers) beyond the diffraction limit. The near-field optical trapping has mainly been studied using metallic substrates such as Au nanodot pairs, periodic Al nanoslits, nanoapertures on an Au film, etc. In this paper, we newly propose a Mie-scattered-near-field optical trapping scheme for size-selective photocatalytic application using pairs of poly-rutile TiO2 nanospheres. The optical intensity distribution in a 3D-nanogap space between the nanospheres was simulated by a 3D FDTD method. The simulation system consists of the two TiO2 nanospheres placed on a silica substrate in water. The 400-nm excitation laser is used for both the near-field trapping and the photocatalyst excitation. The optical trapping forces were calculated based on the near-field optical intensity distribution. The trapping stiffness for 20-nm polystyrene sphere at a gap distance of 20 nm was 6.4 pN/nm/W. The optical force vector shows that the object like virus can be trapped with sufficient forces into the nanogap space and then is driven into the direct surface of the TiO2 sphere. This result suggests that this system works as a photocatalytic trapping for killing virus, protein, etc.
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