Strategies for active colloidal manipulation with plasmonic tweezers (Conference Presentation)

Abstract

Controlled manipulation of nanoscale objects in fluidic media is one of the defining goals of modern nanotechnology. In this respect, optical traps based on highly localized electromagnetic fields around plasmonic nanostructures offer a promising solution in generating strong trapping forces at low levels of optical illumination. However, conventional plasmonic trapping occurs at predefined spots on the surface of a nanopatterned substrate where trapping is limited by the diffusion of colloidal objects into a small trapping volume which renders the process inherently slow. As we discuss here, this limitation can be overcome by integrating plasmonic nanostructures with magnetically driven helical nanoswimmers and maneuvering these mobile nanotweezers under optical illumination. In an alternate strategy, a similar functionality has been obtained in a unique nanophotonic device, where sub-micron colloids could be manipulated using optical forces alone. The strategy with magnetic nanoswimmers provide a working range that matches with state-of-the-art plasmonic tweezers and in-addition allows selective pickup, transport, release, and positioning of submicrometer objects over large areas in standard microfluidic environments with great speed and control. The MNTs can be used to manipulate one or many nano-objects in three dimensions and are applicable to a variety of materials beyond model colloids (e.g. silica, polystyrene) including living bacteria and fluorescent nanodiamonds. A crucial component of these tweezers is the generation of thermofluidic forces which provide an additional handle to trap and sort objects. The alternate strategy with optical forces, as we will explain in detail, works in a regime where optical absorption and therefore generated heat is minimized.