Laser Trapping Chemistry: From Polymer Assembly to Amino Acid Crystallization

Abstract

Laser trapping has served as a useful tool in physics and biology, but, before our work, chemists had not paid much attention to this technique because molecules are too small to be trapped in solution at room temperature. In late 1980s, we demonstrated laser trapping of micrometer-sized particles, developed various methodologies for their manipulation, ablation, and patterning in solution, and elucidated their dynamics and mechanism. In the 1990s, we started laser trapping studies on polymers, micelles, dendrimers, and gold, as well as polymer nanoparticles. Many groups also reported laser trapping studies of nanoclusters, DNA, colloidal suspensions, etc. Following these research streams, we have explored new molecular phenomena induced by laser trapping. Gradient force leading to trapping, mass transfer by local heating, and molecular reorientation following laser polarization are intimately coupled with molecular cluster and aggregate formation due to their intermolecular interactions, which depend on whether the trapping position is at the interface/surface or in solution. In this Account, we summarize our systematic studies on laser trapping chemistry and present some new advances and our future perspectives. We describe the laser trapping of nanoparticles, polymers, and amino acid clusters in solution by focusing a continuous wave 1064 nm laser beam on the molecules of interest and consider their dynamics and mechanism. In dilute solution, nanoparticles with weak mutual interactions are individually trapped at the focal point, while laser trapping of nanoparticles in concentrated solution assembles and confines numerous particles at the focal spot. The assembly of polymers during their laser trapping extends out from the focal point because of the interpolymer interactions, heat transfer, and solvent flow. When the trapping laser is focused at an interface between a thin heavy water solution film of glycine and a glass substrate, the assembled molecules nucleate and evolve to a liquid-liquid phase separation, or they will crystallize if the trapping laser is focused on the solution surface. Laser trapping can induce spatiotemporally the liquid and solid nucleation of glycine, and the dense liquid droplet or crystal formed can grow to a bulk scale. We can control the polymorph of the formed glycine crystal selectively by tuning trapping laser polarization and power. These results provide a new approach to elucidate dynamics and mechanism of crystallization and are the fundamental basis for studying not only enantioselective crystallization but also confined polymerization, trapping dynamics by ultrashort laser pulses, and resonance effect in laser trapping.

Hierarchical, Ultrathin Single-Crystal Nanowires of CdS Conveniently Produced in Laser-Induced Thermal Field

“…28 High-temperature accelerates reactions and thus leads to supersaturated regions formed in the beam center. 31 Violent convection induced by the gradient field transports the supersaturated solution into lower-temperature areas, leading to a burst of nucleation and the formation of nanostructures, in accordance with the Wilson−Frenkel growth theory. 28 In view of the above, we adopted a near-infrared CW laser to irradiate a precursor solution (with a specially added dye) for the synthesis of ultrathin CdS HNWs.…”

Optical trapping assembling of clusters and nanoparticles in solution by CW and femtosecond lasers

“…All these phenomena are schematically shown in Fig. 1 [14]. In parallel with the studies on crystallization and crystal growth, we are exploring new trapping dynamics of nanoparticles by femtosecond laser pulses and have found a unique behavior for polystyrene nanoparticles by the pulse [15].…”

A method for an approximate determination of a polymer-rich-domain concentration in phase-separated poly(N-isopropylacrylamide) aqueous solution by means of confocal Raman microspectroscopy combined with optical tweezers

“…Optical tweezers, which uses a tightly-focused laser beam to exert radiation force at a focal point [17], have been used for trapping and manipulating of microparticles such as living cells and polymer microspheres in bioscience [18]. This technique can also trap smaller nanoparticles such as polymer chains in aqueous solution [19]. Ishikawa et al first reported optical trapping of PNIPAM in an aqueous solution [20].…”