Intrinsic Fluorescence of Carboxylate-Terminated Polyamido Amine Dendrimers

Abstract

The “intrinsic” fluorescence of carboxylate-terminated polyamido amine (PAMAM-CT) dendrimers is studied by two fluorescence techniques—excitation-emission matrices (EEMs) and lifetimes. The EEMs show similar spectral profiles for all dendrimer generations (a broad peak with an excitation and emission maximum of 380 and 440 nm, respectively) and an overall increase in relative fluorescence emission with increasing generation. Three distinct, fairly discrete lifetimes are also recovered. The shortest lifetime (0.2–0.4 ns) is a background signal from the solvent. The two longer lifetimes (1.3–2.5 and 4.2–7.1 ns) are attributed to the dendrimer. There is a general shift to longer lifetimes in τ₂ and τ₃, with increasing generation (Gn). Both lifetimes nearly double in magnitude, going from G2.5 to G7.5, which is indicative of a specific fluorescent component being in a more protected or constrained microenvironment. These results imply that the dendrimer becomes densely packed with increasing generation. The weak, but detectable, fluorescence is most likely due to an n → π transition from the amido groups throughout the dendritic structure. Even though the exact nature of PAMAM-CT fluorescence is not fully understood, it is clear that this property shows the unique aspects of the architecture of these dendrimers and can be utilized in their characterization.