Experimental Findings about the Fluorescence Emission of Generation 4.0 and 4.5 Polyamidoamine Dendrimers

Abstract

Physicochemical characterization of polyamidoamine (PAMAM) dendrimers of generation 4.0 amine-terminated (DG4.0) and 4.5 carboxy-ended (DG4.5) was done. We have measured the pKa of the inner tertiary amine, the surface primary-amine, and carboxyl-terminal groups. We have conducted UV-Vis absorption and fluorescence emission experiments as a function of pH. We have made a 4th derivative analysis of the UV-Vis absorption experiments and compare the results with classical amide such as dimethylformamide. Our results have permitted us to calculate the pKa of the groups sensible to the pH in both dendrimers and compare the results with theoretical studies. On the one hand, the pKa values found for DG4.0 were 10.0 � 0.5 and 7.1 � 0.2, for primary and tertiary amines groups, respectively. On the other hand, the pKa values found for the DG4.5 were 3.5 � 0.7 and 6.8 � 0.4, for the carboxylic and tertiary amine groups. Our experimental results agree with theoretical ones. We offer probes that the dendrimers have two non-traditional fluorophores, where one of them is the inner amide bond. The dendrimers show fluorescence emission in all the tested pH.

pH- and Functionalization-Dependent Host–Guest Interactions Between Fluorescein and Various Poly(amidoamine) Dendrimers

“…Such exterior amines are absent for G6OH, which does not interact with fluorescein at high pH. Instead, G6OH exclusively possesses tertiary interior amines that have a p K a value between 6 and 7. − This p K a is consistent with the onset of fluorescein-G6OH interactions (Figure ). Therefore, the data in this work support previous conclusions that electrostatic forces between PAMAM’s amine groups and anions are the main driver for dendrimer-guest association in these systems. ,, This conclusion also explains why fluorescein’s dianionic form is stabilized in the presence of G6NH 2 and G6OH.…”

“…A second indicator for the role of amines is the pH interval in which fluorescein and the various dendrimers interact. Specifically, the dendrimer-fluorescein interactions in this work correlate strongly with dendrimer protonation: the exterior amine groups of G6NH 2 have a p K a value between 9 and 10. − In other words, the exterior amines start protonating at pH values that match the onset of fluorescein-G6NH 2 interactions (Figure ). Such exterior amines are absent for G6OH, which does not interact with fluorescein at high pH.…”

“…For each experiment, the results will be described in the context of incrementally decreasing the pH from 11 to 3. Even though the fluorescein-dendrimer interactions are independent of the direction of the pH change (increase or decrease), the present work only focused on an acidic titration experimental protocol, which is in alignment with previous dendrimer titration studies. , In addition, this work focuses on fluorescein-PAMAM interactions as a function of dendrimer generation and solution pH. In order to not further expand the experimental parameter space, we do not study the effect of the dendrimer-guest ratio and fix this ratio to 1:1 by having both species in solution at a concentration of 10 μM.…”

“…This dendrimer possesses tertiary amines in its core and can contain various terminating groups on its outer shell . Each of these groups has its own p K a value, so the interior and exterior protonation of a dendrimer depends on the pH of its vicinal environment. − Because protonated amines carry a positive charge, the degree of protonation causes nearby amines to repel each other . When this happens throughout a dendrimer, its conformation changes: increased protonation causes the dendrimer to swell and open up for potential guest molecules. , Additionally, protonated amines can interact electrostatically with dissolved anionic species.…”