Distinctive Fluorescence Response of Eosins Confined in PAMAM Dendrimers. A Spectroscopic Study

Binding of Eos-B to PAMAM dendrimers was evaluated in buffered aqueous solutions, by means of absorption and fluorescence spectroscopies. Progressive changes in the spectra of the dye were recorded with increasing amounts of PAMAM. The corresponding binding constants (K_bind) were estimated by using a 1:1 equilibrium model and different fitting approximations were discussed. After analyzing six generations of PAMAM (G0-G5), a clear correlation of the K_bind values with the size of the dendrimer was observed. In addition, the effect of pH was also tested, recording a significant increase in the complex stability when the pH was changed from 10 to 7. This behavior was ascribed to the electrostatic interaction between protonated amino moieties in the PAMAM structure and anionic dye. Furthermore, a fluorescence enhancement of Eos-B was observed in the presence of PAMAM, in contrast to the fluorescence quenching recorded for the structurally analogous Eos-Y. This was interpreted in terms of the ability of Eos-B to form H-bonds with the solvent conferred by the nitro group which Eos-Y lacks. Since dendrimers are protonated at physiological pH and can act as a membrane model, the present results suggest a potential use of Eos-B as probe for protic environments in biological tests.