279420 Preparation and Quantitative Analysis of PAMAM-Stabilized Metal Ions in Aqueous Solutions: Effect of pH and Dialysis
Preparation and quantitative analysis of PAMAM-stabilized Metal ions in aqueous solutions: Effect of pH and Dialysis
Eleni A. Kyriakidou1, Konstantin V. Khivantsev1, Christina Papadimitriou1, Thomas M. Gostanian2, Paul T. Fanson3, Oleg S. Alexeev1 and Michael D. Amiridis1*
1Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208 (USA)
2Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003 (USA)
3Toyota Technical Center USA, Inc., Ann Arbor, MI 48105 (USA)
A dendrimer-based preparation approach for heterogeneous catalysts is focused on the rational design of metal nanoparticles in solution and their subsequent delivery onto the surface of solid supports [1,2]. Poly(amidoamine) (PAMAM) dendrimers offer an opportunity to control the architecture and the size of metal nanoparticles in solution and maximize the uniformity of active metal sites in supported catalytic materials. While the synthesis of metal-dendrimer nanocomposites has been reported in the literature, little is known about the complexation chemistry of metal cations with PAMAM dendrimers in aqueous solution, the strength of metal-dendrimer interactions, and how these interactions can be affected by the preparation conditions [3,4]. Our specific objective was to explore the solution chemistry in the presence of both metal cations (e.g., Fe3+, Cu2+, Ni2+, Co2+, Au3+ and Ag+) and G4OH/G4NH2 dendrimers and to determine how the fraction of the different metal cations strongly bound to the dendrimers depends on the complexation time, the theoretical Metal/Dendrimer ratio, the solution pH, and the dialysis conditions. UV-vis, Atomic Absorption (AAS) and STEM were used to monitor changes in concentration, structure and size of the monometallic nanoparticles present during each step of the synthesis.
Generation four (G4OH/G4NH2) PAMAM dendrimers were mixed with salt precursor solutions at different molar ratios to initiate the complexation process. The complexation of Fe3+ and Cu2+ cations with the G4OH dendrimer in aqueous solution was monitored by tracing ligand-to-metal charge transfer UV-vis bands (LMCT) at 300, 350, 470 and 605 nm, respectively. The data collected for the Fe3+/G4OH system show that the intensity of the absorption bands remains unchanged with time, suggesting that complexation of Fe3+ with G4OH takes place rapidly. Data collected for different Fe3+/G4OH ratios indicate that approximately 90% of Fe3+ cations are strongly bound to the dendrimer, even under strongly acidic conditions (pH ≈ 2.3). This result suggests that the amide groups of the G4OH dendrimer may also participate in complexation process, given that tertiary amines are fully protonated under strongly acidic conditions. When similar experiments were performed with the Cu2+/G4OH system (pH ≤ 6.5), the complexation of Cu2+ cations with the dendrimer was also rapid. In this case, however, Cu2+ cations were not strongly bound to G4OH. When the complexation of Cu2+ was performed under pH controlled conditions (i.e., pH ~ 6.5), the number of metal cations strongly bound to the dendrimer was increased, suggesting that the solution pH plays a crucial role in the complexation process. UV-vis data collected for Ni2+/G4OH and Co2+/G4OH systems show the absence of LMCT band around 300 nm, indicating that these cations do not complex with the G4OH dendrimer molecules. This conclusion is consistent with AAS experiments that also show that both Ni2+ and Co2+ cations can be completely removed from Ni2+/G4OH and Co2+/G4OH solutions after approximately 10 h of dialysis. Autoreduction of gold and silver occurred in all G4OH solutions after 10 min and 4 days of complexation, respectively, as evidenced by the presence of a Surface Plasmon Resonance (SPR) absorption peak in the 450-600 nm region and the subsequent growth of this peak over an one week period. When amine-terminated dendrimers (G4NH2) were used instead, a slower SPR peak growth was observed during the first day and no evidence of reduction of the G4NH2–(Au3+)5 complexes could be obtained.
The results of this work show that complexation of metal ions with PAMAM dendrimers rely heavily on the metal nature, timing, the solution pH, and the metal-to-dendrimer molar ratios used. The composition of Metal-Dendrimer nanocomposites can be controlled to some extent by the solution acidity. These features offer an opportunity to control the composition of nanoparticles in solution and potentially prepare a variety of supported metal catalysts with uniform distributions of active sites.
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