Studying Gold Nanoparticle Dimer Dynamics by Using Optical Tweezing and Darkfield Microscopy

Metallic nanoparticles, particularly gold nanospheres (AuNS), are known to be plasmonic in the visible region of the electromagnetic spectrum, a property exploited in various optical applications. Particularly, when two AuNS are brought close together, such that their surfaces are separated by a small fraction of their overall diameter, under illumination, the plasmonic field in the nano-gap is sufficiently strong, creating an "optical hotspot," which allows for single molecule sensitivity to surface-enhanced Raman spectroscopy (SERS) and other chemical signatures. This localized surface plasmon resonance (LSPR) of the dimer can be tuned by changing the inter-particle gap but approaches for achieving this resonance control in solution are lacking. In this study, we utilize a custom-built Optical Tweezer (OT) instrument integrated with a darkfield microscope to manipulate the inter-particle distances of AuNS dimers in aqueous solutions, where their high polarizability and large dielectric constant make them ideal for optical trapping. First, we investigate how these gaps can be tuned via ligands that both promote AuNS dimerization but have distinct lengths. Next, we show how increasing the trapping laser power raises the localized temperature of the dimer, promoting the break-down of the intermolecular interactions between the ligands on the surface of the particle and enabling its controlled separation. Overall, this research makes progress towards achieving sub-nanometer control over the inter-particle distance in AuNP dimers, enhancing the precision of optical hotspots for single-molecule sensing.