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[MIS10-03] “Manipulated” Organic Crystallization and Two-step Dissolution Process Attained by Plasmonic Optical Tweezers
Keywords:Plasmonic Optical Tweezres, Crystallization of Organic Molecule, Two-step Dissolution Process
Figure 1 shows schematic illustration of experimental setup. Periodic Au nanolattice the unit structure of which is gammadion with the size of 500 nm was fabricated on a cover glass by electron beam lithography. An aqueous solution of acetaminophen saturated at room temperature (24oC) was dropped on the Au nanolattice, and then solution thin layer supported by the nanolattice was formed by capillarity. Near-infrared continuous-wave left-handed circularly polarized laser (1064 nm, 6.4×109 W/m2) was focused to the nanolattice supporting the solution thin layer by passing through an objective lens equipped on an inverted polarized-light optical microscope. In-situ microscopic observation for the vicinity of the focal spot was also performed simultaneous with the laser irradiation.
Figure 2 shows time-lapse micrographs of the in-situ observation in the vicinity of the focal spot. After the laser irradiation, crystals with the size of 1000 ~ 2000 nm were precipitated 19 mm away from the focal spot in annular pattern. The position of the annular pattern moved while following the change of in-plane position of the focal spot via dissolution/precipitation process, indicating that position of the crystal is manipulatable. When the plasmon excitation by laser irradiation was stopped, the crystals were disappeared via two-step dissolution process; the crystals first transformed to dense liquid droplets, and then the dense droplet disappeared through molecular diffusion.
Generally, large temperature gradient generates in the vicinity of the focal spot when localized surface plasmon resonance was excited, because of the heat generation through electron-phonon scattering. Thus, the large temperature gradient exerts thermophoretic force outward from the focal spot on acetaminophen molecules. Since the electrical field gradient force attracts the molecules towards the focal spot, the annular-patterned precipitation can be interpreted as the consequence of the balancing the electrical field gradient force as attractive force and the thermophoretic force as repulsive force. Therefore, our observation strongly suggested that plasmonic optical tweezers can precisely control crystallization.
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