Fluorescein (F) species were encapsulated inside nanosized zeolites
L and Y by their synthesis from adsorbed precursors (phthalic anhydride (Ph) and resorcinol (R)). The applied nanozeolites were modified either with H+
cations and they played roles of the catalysts for synthesis of F and of the matrices for resulting dye molecules. The F loading, its tautomeric forms and optical properties of obtained composites depended on zeolite structure, morphology and on introduced cations. The composites usually showed higher fluorescence intensity than the pristine dye. The highest fluorescence intensity was noticed for the sample with nano-ZnL matrix. This intensity was much higher than that observed for composite with conventional micro-sized Zn L zeolite. Regarding an efficient fluorescence and nanosize of the obtained composites they seem promising fluorescent tracer for medical (theranostic) applications.