Two-Dimensional Growth Rate Control of L-Phenylalanine Crystal by Laser Trapping in Unsaturated Aqueous Solution

Abstract

The growth rate control of single L-phenylalanine plate-like anhydrous crystal is successfully demonstrated by laser trapping at an air/solution interface of the unsaturated aqueous solution. Focusing a continuous-wave near-infrared laser beam into the interface generates single L-phenylalanine crystal at the focal spot even under unsaturated condition. Subsequently, the plane area of the generated crystal becomes larger linearly with time under continued laser irradiation into the crystal central part. Two-dimensional crystal growth rate defined as a slope of the temporal change in the crystal plane area strongly depends on initial solution concentration as well as irradiation time until single crystal formation is confirmed by eye under a microscope. When the laser power is decreased after the crystallization, the growth rate is slowed down accordingly. Thus, the two-dimensional growth rate is arbitrarily controlled by tuning the laser power. As the critical phenomenon underlying the crystal growth, we propose that a dense domain consisting of a large number of the liquid-like clusters is formed prior to the crystallization. The dynamics and mechanism of the two-dimensional crystal growth is discussed by considering the supply of the solutes to the crystal edge from the cluster domain dependent on the laser power.