Combining Optical Tweezers and Laser Induced Nucleation to Get Liquid Phase Separation and Polymorphically Selective Crystals

A laser-induced phase-separated (LIPS) solution droplet formed by tightly focusing a continuous-wave, near-infrared laser beam at the glass/solution interface of a mm-thick layer of glycine in D₂O was irradiated with a single unfocused ns near-IR laser pulse in order to study the effect of nonphotochemical laser-induced nucleation (NPLIN) on the droplet, as well as to help characterize the behavior of the LIPS droplet. This revealed that NPLIN could nucleate crystals within a LIPS droplet and that the LIPS droplet was observed to be more labile to spontaneous nucleation than the control for the first 40 min of relaxation. The resulting crystals were analyzed using powder X-ray diffraction, and 100% of crystals formed within the LIPS droplet induced by NPLIN and by spontaneous nucleation were α-glycine. The results indicate that the LIPS droplet and the surrounding solution are not equilibrium phases of aqueous glycine, but phases in which gradient optical forces have induced a partitioning of large and small solute clusters.