Structure and dynamics of optically directed self-assembly of nanoparticles

Abstract

Self-assembly of nanoparticles leading to the formation of colloidal clusters often serves as the representative analogue for understanding molecular assembly. Unravelling the in situ structure and dynamics of such clusters in liquid suspensions is highly challenging. Presently colloidal clusters are first isolated from their generating environment and then their structures are probed by light scattering methods. In order to measure the in situ structure and dynamics of colloidal clusters, we have generated them using the high-repetition-rate femtosecond laser pulse optical tweezer. Since the constituent of our dimer, trimer or tetramer clusters are 250 nm radius two-photon resonant fluorophore coated nanospheres under the optical trap, they inherently produce Two-Photon Fluorescence, which undergo intra-nanosphere Fluorescence Energy Transfer. This unique energy transfer signature, in turn, enables us to visualize structures and orientations of these colloidal clusters during the process of their formation and subsequent dynamics in a liquid suspension. We also show that due to shape-birefringence, orientation and structural control of these colloidal clusters are possible as the polarization of the trapping laser is changed from linear to circular. We thus report important progress in sampling the smallest possible aggregates of nanoparticles, dimers, trimers or tetramers, formed early in the self-assembly process.

Optical Force-Induced Dynamics of Assembling, Rearrangement, and Three-Dimensional Pistol-like Ejection of Microparticles at the Solution Surface

“…In single NPs level, Goswami et al have reported that femtosecond laser enables manipulation and construction of 250 nm PS NPs with a specific 3D geometry. Not only optical trapping potential but also assembly of NPs can be controlled by femtosecond laser polarization 53,54 . Two-photon fluorescence is used for tracking the movement of individual NPs and for analyzing their geometry 55 .…”

“…These findings are explained in terms of single transient assembly formation in the optical trap. − In the single NP level, Goswami et al have reported that the femtosecond laser enables manipulation and construction of 250 nm PS NPs with a specific 3D geometry. Not only optical trapping potential but also assembly of NPs can be controlled by femtosecond laser polarization. , Two-photon fluorescence is used for tracking the movement of individual NPs and for analyzing their geometry . Even the hierarchical self-assembly was demonstrated and discussed, which will be a useful reference for fabricating the assembly as the bottom-up approach.…”

Anomalously Large Assembly Formation of Polystyrene Nanoparticles by Optical Trapping at the Solution Surface

“…The other is the structuring of NPs will be more controllable by laser power and polarization. For example, Goswami et al demonstrated for different configurations of a few 250 nm PS NPs by utilizing femtosecond laser in solution. − This interesting trial enables individual arrangement in NP assembly at the solution surface and interface too.…”

Nanoparticle Assembling Dynamics Induced by Pulsed Optical Force

“…The assembly formation of NPs by femtosecond laser in single NPs level is studied by Goswami et al. They manipulated dye‐doped 250 nm PS NPs and constructed a specific 3D geometry, [78,79] where two‐photon fluorescence is used for tracking the movement of individual NPs and for analyzing their geometry [80] . Even hierarchical self‐assembly was demonstrated and discussed, [81] which will be a useful reference for fabricating the assembly as the bottom‐up approach.…”