Investigation of Properties of Polyamido Amine (PAMAM) Dendrimers by Flourescence Spectroscopy

Fluorescence spectroscopy was used to investigate the properties of PAMAM dendrimers capped with fatty acid chains through ester and amide linkages. Several organic molecules were used as extrinsic fluorescent probes to investigate the spectral properties of fatty acid capped PAMAM dendrimers. We used four kinds of probes: pyrene (highly hydrophobic), 1-pyrenecarboxaldehyde (has a polar group), 1-pyrenemethylamine hydrochloride (has a cationic group), 1-anilino-8-naphthalene sulfonic acid (has an anionic group in equilibrium with undissociated acid) and 1-anilino-8-naphthalene-sulfonate magnesium salt (anionic). Most of our experiments were done in organic solvents or on hydrophobic glass slides because of the solubility properties of capped PAMAM dendrimers. Neither amide-capped nor ester-capped fatty acid derivatized PAMAM dendrimers interact with these extrinsic probes in methanol. Ester-caped dendrimers were also studied in a less polar solvent, THF. Increasing the concentration of ester-capped dendrimers led to a red shift in emission of 1-anilino-8-naphthalene sulfonic acid (ANSA) consistent with an increase in medium polarity. A similar study of ester-capped PAMAM dendrimers in THF conducted with 1-pyrenemethylamine hydrochloride suggested that this probe experienced decreased medium polarity. This finding might be explained by assuming that prior to dissolution this cationic probe is aggregated and reports an elevated effective polarity. Amide-capped PAMAM dendrimers were not sufficiently soluble in THF to conduct a study of concentration dependence. It was also observed that amide-capped and ester-capped dendrimers probably have simliar interaction with 1-pyrenemethylamine hydrochloride and 1-anilino-8-naphthalene sulfonic acid on the surfaces of hydrophobic glass slides