It has been expected that the gradient force in the electric field induced by the excitation of the localized surface plasmon resonance (LSPR) could retard the molecular Brownian motion, if molecules has enough polarizability to generate optical force beyond thermal fluctuation. In this study, we have attempted to observe the optical molecular manipulation at the gap of plasmonic bow-tie nanostructures in the bi-analyte solution of molecules. Through the evaluation of the surface diffusion process by electrochemical surface-enhanced Raman scattering (SERS) measurements, it has been found that the control of the electrochemical potential of the metal nanostructures realized the molecular selective manipulation. In addition, we have successfully observed the formation of unique phase of molecule condensation at electrified interface.
Optical manipulation has been used for the trapping of micrometer-scaled objects, but it is still difficult to control the motion of small molecules on the nanometer scale at room temperature. Plasmonic metal nanostructures are expected to be useful for the optical manipulation of nanoscale molecules using a highly localized electric field. We use the plasmonic Ag nanostructure for a demonstration of optical trapping through the observation using surface-enhanced Raman scattering (SERS) imaging. The optical measurements were conducted under electrochemical potential control to stabilize the nanostructure with target molecules, 4,4′-bipyridyl (44 bpy). Upon increasing the concentration of 44 bpy molecules in an electrolyte solution at room temperature, the blinking frequency of the SERS signal was different in both the spectra and imaging. The dwell time of the SERS signals was increased from several tens of milliseconds to a few seconds, which suggested the successful observation of plasmonic trapping of small molecules through the surface diffusion control. The observed results prove the importance for the control of the surface coverage of the molecules and its influence on surface diffusion under plasmonic molecular trapping.
It is predicted by various theoretical studies that nanometer size molecules could be trapped in the strong electromagnetic field due to its steep spatial gradient of the filed intensity. In this study, we have attempted to observe the plasmonic molecular trapping behavior in the mixed solution of 4,4’-bipyridine and 2,2’-bipyridine by surface enhanced Raman scattering measurements. In order to control the molecular optical trapping selectivity, we have introduced the electrochemical potential control into the system. The experimental results would indicate the achievement of the selective control of molecular optical trapping at room temperature in solution.