Metallic-Nanostructure-Enhanced Optical Trapping of Flexible Polymer Chains in Aqueous Solution As Revealed by Confocal Fluorescence Microspectroscopy

Abstract

Optical trapping of flexible polymer chains to a metallic nanostructured surface was explored by microscopic imaging and confocal fluorescence spectroscopy. A fluorescence-labeled poly­(<i>N</i>-isopropylacrylamide) was targeted, being a representative thermo-responsive polymer. Upon resonant plasmonic excitation, it was clearly observed that polymers were assembled into the excitation area to form molecular assemblies. Simultaneously, fluorescence from the area was obviously intensified, indicating an increase in the number of polymer chains at the area. The excitation threshold of light intensity that was required for obvious trapping was 1 kW/cm², which was much lower by a factor of 10⁴ than that for conventional trapping using a focused laser beam. The morphology of the assemblies was sensitive to excitation intensity. We precisely evaluated temperature rise (Δ<i><i>T</i></i>) around the metallic nanostructure upon plasmonic excitation: Δ<i><i>T</i></i> ≈ 10 K at 1 kW/cm² excitation. This temperature rise was an origin of a repulsive force that blocked stable trapping. On the basis of experimental observations and theoretical calculations, we quantitatively evaluated the plasmon-enhanced trapping force and the thermal repulsive force (Soret effect). The overall mechanisms that were involved in such plasmon-enhanced optical trapping are discussed in detail. The smooth catch-and-release trapping (manipulation) of polymer chains was successfully demonstrated