During crystal growth, attachment/detachment processes of solute molecules on a crystal surface are key elementary processes that are still unclear. To reveal how the dynamics of attachment/detachment processes affect the crystallization process, one should know the behavior of individual molecules. Ensemble averaging approaches, such as spectroscopy and quartz crystal microbalance, usually need relatively large amount of molecules, and cannot obtain information on the locations and attachment/detachment events of individual molecules. Hence, ensemble averaging approaches are less promising for investigating the attachment/detachment processes during crystal growth. Single-molecule technique is a powerful way to study the behavior of individual molecules, avoiding ensemble averaging.
Associate professor Dai Guoliang (Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Sciences) collaborated with Professor Gen Sazaki (The Institute of Low Temperature Science, Hokkaido University, Japan; 2014 Chinese Academy of Sciences Visiting Professorships for Senior International Scientists) for several years and studied adsorption/desorption of molecules in solution crystal growth process. Total internal reflection fluorescence (TIRF) microscopy was applied by the researchers, by which they can track individual fluorescent molecules at a submicron size scale.
The adsorption/desorption behavior of mobile solute molecules at a solution-crystal interface has been explored using crystals of model used a monoclinic protein hen egg-white lysozyme (HEWL) and fluorescent-labeled HEWL (F-HEWL) molecules. The researchers visualized the attachment/detachment processes of individual F-HEWL molecules by a fluorescent single-molecule visualization technique. They first confirmed that under an equilibrium condition, there was an “induction period” (~120 min) of the attachment/detachment processes, during which period the number density remained constant. After the induction period, the number density increased linearly with the adsorption time. In addition, they performed similar measurements under a supersaturated condition and found that supersaturation significantly enhanced the attachment process after the induction time. The attachment/detachment processes finally reached a steady state, in which the attachment rate was higher than the detachment one. The above results was published in Crystal Growth & Design (G. Dai, et al, The attachment and detachment processes of individual lysozyme molecules on a surface of a monoclinic lysozyme crystal studied by fluorescent single-molecule visualization, Crystal Growth & Design, 2014, 14, 5303−5309.).
The researches were supported by Chinese Academy of Sciences, Ministry of Science and Technology, Hokkaido University.
A sectional view of an observation chamber. The black bold arrow shows the observed interface between a monoclinic HEWL crystal and a bulk HEWL solution.
A typical time-course of F-HEWL molecules appearing on the crystal surface taken 61 min after the molecules were introduced into an observation chamber
Changes in the net adsorption rates of F-HEWL molecules after the induction period (~120 min), as a function of the residence time . Dashed curves were fitting curves by exponential decay. Open circles: under an equilibrium condition, closed circles: under a supersaturated condition (C=1.55 mg/mL and Ce=1.1 mg/mL, here C and Ce denote HEWL concentration and solubility, respectively).