(105f) 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)

* amiridis@engr.sc.edu

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.

References

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