Poly(Amidoamine) Dendrimer-Templated Nanocomposites. 1. Synthesis of Zerovalent Copper Nanoclusters

Poly(Amidoamine) Dendrimer-Templated Nanocomposites. 1. Synthesis of Zerovalent Copper Nanocluster

Non-Traditional Intrinsic Luminescence (NTIL): Dynamic Quenching Demonstrates the Presence of Two Distinct Fluorophore Types Associated with NTIL Behavior in Pyrrolidone-Terminated PAMAM Dendrimers

Historically, poly­(amidoamine) (PAMAM) dendrimers were the first macromolecular structures reported to exhibit “non-traditional intrinsic luminescence” (NTIL) properties. Initially, this unique intrinsic luminescent property suggested the possibility of dendrimer-based biological imaging without the need for conjugating external labels. Unfortunately, low NTIL intensity levels exhibited by most simple surface-modified PAMAMs presented a serious barrier to progress in that area. Unexpectedly, a simple surface modification of amine-terminated PAMAM dendrimers with N-(4-carbomethoxy)­pyrrolidone groups (4-CMP) was found to dramatically increase NTIL fluorescence intensity (i.e., >50-fold) while substantially enhancing biocompatibility and reducing cytotoxicity/complement activation properties. This current study focuses on the use of conventional and time-resolved spectroscopic measurements to characterize the NTIL behavior of 4-CMP PAMAM dendrimers over four generation levels (i.e., G2–G5). We describe and discuss the impact of polymer size and composition on NTIL intensity levels and quantum yields. Fluorescence lifetime measurements revealed two discrete major lifetime components, which were similar for all dendrimer generations and remained unaffected by changes in pH. Time-resolved fluorescence quenching studies involving a collisional quencher (methyl red) and a dynamic proximity quencher (nitrobenzoxadiazole dipeptide derivative) provided evidence for two spatially separated NTIL-type emission sites within this 4-CMP PAMAM dendrimer series. In summary, these results provide important insights into the molecular-level NTIL mechanism and demonstrate the critical role of pyrrolidone surface modification as well as separate contributions made by interior dendrimer components to the observed enhancement of NTIL fluorescence intensity

Characterization of Starburst Dendrimers by EPR. 3. Aggregational Processes of a Positively Charged Nitroxide Surfactant

The aggregation characteristics of aqueous solutions of a positively charged nitroxide surfactant (CAT16) in the presence and absence of half-generation polyamidoamine starburst dendrimers (n.5-SBDs) have been investigated by electron paramagnetic resonance (EPR). Computer simulation of the EPR spectra allowed the convenient extraction of several parameters that were related to the supramolecular structure of the aggregates formed by CAT16 and SBDs. From examination of the EPR spectra as a function of variation of the concentration of CAT16, the concentration of SBDs, and the ionic strength and application of the EPR parameters available from simulation of the spectra, a paradigm for the structure and dynamics of the aggregates formed by CAT16 in the presence and absence of SBDs under various conditions is deduced. A study of the fluorescence quenching of pyrene in the presence of CAT16 is compared to a previous investigation of the interaction of SBDs with cationic surfactants. It is concluded that at low SBD concentration, for earlier generation SBDs (G < 3.5), whose size is smaller than or comparable to the size of the CAT16 micellar aggregates, the SBDs act as “guests” that bind to the micelles that serve as “hosts”. In contrast, at low SBD concentration of the later generation SBD (G > 3.5), the size of the SBD is now larger than that of the micelles so that the latter can serve as “guests” for the former. A bilayer aggregate of the surfactant on the SBD is proposed. Finally, at high concentration of the later generation SBD, it is proposed that because of the large number of sites compared to the number of surfactants, an aggregate in which two or more SBDs are bridged by bilayers is formed by the surfactant and coexists with CAT16 micelles

Effect of Protonation and PAMAM Dendrimer Size on the Complexation and Dynamic Mobility of 2-Naphthol

The supramolecular dynamics of triplet excited states of the 2-naphthol@dendrimer (2-NpOH@n-SBD) noncovalently bound complex have been studied by fluorescence spectroscopy and laser flash photolysis. By varying the acidity and dendrimer size, polyamidoamine dendrimers are shown to possess both nonspecific and specific binding properties. For example, the studies show that 2-naphthol binds preferentially to the tertiary amine groups within the dendrimer interior and can be released by lowering the pH of the solution. On a per binding site basis, studies of the dynamics of 32-NpOH*@n-SBD reveal that the overall dynamics (entry and exit rate constants) are enhanced by increasing the acidity of the solution. Furthermore, the amount of protonation of tertiary amine groups within 2-SBD, 4-SBD, and 6-SBD was determined to be ∼12%, ∼10%, and <1%, respectively, at pH 6. The results presented here demonstrate that, in addition to being able to complex transition metals, organic molecules that are capable of hydrogen bonding to the internal amine moieties can also be bound to dendrimers

Preparation of Fullerene-Shell Dendrimer-Core Nanoconjugates

Generation 4 amine-terminated polyamidoamine dendrimer (PAMAM G4) was allowed to react with an excess of buckminsterfullerene (C60) to form a nanoconjugate containing a PAMAM core and C60 shell. The PAMAM−C60 conjugate was characterized by MALDI-TOF, TGA, UV−vis, and IR spectroscopy. Approximately thirty shell fullerenes surround each dendrimer core. The conjugates catalyze photooxidation of thioanisole by generation of singlet oxygen (1O2). The oxidation reactions occur in both organic and aqueous solvents, but the reactivity is enhanced in aqueous solution, possibly due to a nanoreactor effect resulting from diffusion of hydrophobic reactant molecules into dendrimer cavities

Intrinsic Fluorescence of Triazine Dendrimers Provides a New Approach to Study Dendrimer Structure and Conformational Dynamics

We present basic spectroscopic studies of 5 triazine dendrimers ranging from generations one through nine, G1, G3, G5, G7, and G9, based on the intrinsic fluorescence of these molecules. The extinction spectra of each generation can be separated into two components; the absorption spectra from triazine chromophores and Rayleigh scattering by dendrimer particles. Rayleigh scattering into the UV spectral range is significant and may contribute more than 50% to the measured light attenuation (extinction) for larger dendrimer generations. Deviations from the Rayleigh model at long wavelengths (where the triazine chromophore does not absorb) are clear indications of dendrimer aggregation. These larger particles can be eliminated by dilution and sonication. Importantly, this model system represents a comprehensive case study where the intrinsic fluorescence of the dendrimer when combined with insights from molecular dynamics (MD) simulations can be utilized to probe molecular conformations and dynamics. Experimental results from fluorescence lifetimes, time-resolved anisotropies, and diffusional quenching indicate an increasingly compact core as size increases from G1 to G5. This trend is reversed for G7 and G9 generations, which present more extended, and porous structures, less dense cores, and a denser peripheries. Simulations corroborate this picture and better anchor intuition of the behavior of these molecules