Optical Trapping-Induced New Polymorphism of β-Cyclodextrin in Unsaturated Solution

Abstract

A new polymorph of β-cyclodextrin (β-CD) was produced by optical trapping with a 1064 nm continuous wave laser beam in D2O solution. Upon optical trapping of β-CD in unsaturated solutions, a single prism crystal or multiple plate crystals were formed at and near the laser focus. The resulting crystals differed in morphology and in the Raman spectrum. Furthermore, the solubility of the plate crystal in D2O was much lower than that of the prism crystal, implying that the former crystal is thermodynamically more stable than the latter. The single-crystal X-ray crystallographic analysis showed that the prism crystal has practically the same crystallographic parameters as the crystal reported for β-CD dodecahydrate and is thought to be the thermodynamically most stable phase. These results reveal that the plate crystal is a new polymorph characteristic of optical trapping. Crucially, this is the first success to produce a new polymorph via the laser-induced crystallization method. More interestingly, the generation probability of these two polymorphs turned out to be a critical function of the initial β-CD concentration, laser power, and laser polarization mode. In particular, the polymorphism could be entirely controlled by adjusting the initial concentration, which allowed us to obtain pure prism and plate crystals at high and low CD concentrations, respectively. The dynamics and mechanism of this unique optical trapping-induced polymorphism are discussed in terms of the pre-nucleation cluster size determined by the initial concentration, the trapping efficiency as a function of the cluster size and the laser power, and the molecular alignment in the pre-nucleation clusters steered by the laser polarization mode.