The effect of temperature and solvent on the morphology of microcapsules doped with a europium β-diketonate complex

Fluorescent microcapsules doped with a europium β-diketonate complex were fabricated for the first time by stepwise adsorption of polyelectrolytes and europium complex using the layer-by-layer technique. The influence of temperature and solvent treatment on the morphology of the microcapsules was investigated. Intense red light emission of the microcapsules could be clearly observed by fluorescence microscopy before and after treatment. Remarkable shrinking, decrease of the inner volume and increase of the wall thickness were observed using atomic force microscopy (AFM) and transmission electron microscopy (TEM) after thermal treatment. The shrinkage induced by annealing could be recovered by dissolving in ethanol solution, which was confirmed by AFM and TEM. Morphology variation of the luminescent microcapsules induced by annealing or solvent are both attributed to the molecular rearrangement of polyelectrolytes. While the shrinkage by annealing is an entropy driven process with formation of more coiled conformations of polyelectrolytes the morphology variation by ethanol might be due to the effective screening of electrostatic interaction within the polyelectrolyte multilayers and the changed interaction between hydrophobic fragments present in the polyelectrolytes.