Synthesis of fluorescein by a ship-in-a-bottle method in different zeolites

Composites consisting of fluorescein (F) entrapped inside various zeolite structures (i.e. FAU, LTL, MFI, and LTA) were prepared by catalytic synthesis of the dye from its precursors (phthalic anhydride and resorcinol) adsorbed in the zeolites. The zeolites (mostly H forms) played the role of the catalyst and of the matrix encapsulating the resulting dye molecules. It was revealed that the F loading and optical properties of the composites depended on the zeolite structure. The attained F loading corresponded to the zeolite pore volume and aperture size (HY > HL > HZSM-5 ≫ NaA). The fluorescence efficiency (i.e. intensity, stability and bleaching resistance) of the composites was much higher than that of pristine F and depended on the dye dispersion in the matrix (HL > HZSM-5 > HY ≫ NaA). Various zeolitic matrices could be used for the controlled preparation of F composites suitable for different applications (optical, microelectronic, and medical). The attempts to encapsulate F during crystallization of the zeolites (LTA, FAU, and LTL) appeared not to be successful.