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Progress Report 2010-2011

The primary goal of this research is to synthesize monodisperse nanoparticle-cored dendrimers (NCDs) using our synthetic strategy in which dendrons are linked to functionalized nanoparticles by single coupling reaction. Currently known NCDs are quite polydisperse and unlike dendrimers with uniform molecular weight and density. The availability of highly monodisperse NCDs will allow us to further elucidate the relationships between primary structural elements in these nanostructures and their optical and/or electronic properties.

During the second year of this grant, the progress was made on the attempted syntheses of NCDs by coupling reactions using COOH-functionalized Au₂₅ nanoparticles and dendrons shown in Figure 1. The functionalized nanoparticles were prepared by incorporation of p-mercaptobenzoic acid (MBA) to phenylethanethiolate-protected Au₂₅ nanoparticles in acetone. The formation of MBA-Au₂₅ nanoparticles was confirmed by FT-IR analyses showing C=O and O-H bands and UV-vis spectra displaying the distinct peaks at 670, 460, and 390 nm, which are indicatives of molecule-like electronic levels of Au₂₅ nanoparticles. Transmission electron microscopy results also confirmed the monodispersity of MBA-Au₂₅ nanoparticles. Among these dendrons we used for NCD syntheses, the polyarylether dendron (G2-PAE) and the polypropyleneimine dendron (G2-PPI) were synthesized in the first year. The third dendron (G1-ATE) was purchased from the Frontier Scientific.

Figure 1. Dendrons used for the attachment to MBA-Au₂₅ nanoparticles

First, we attempted to synthesize monodisperse NCDs by reacting G2-PAE with MBA-Au₂₅ nanoparticles in the presence of ester coupling reagents, N,N'-dicyclohexylcarbodiimide (DCC) and 2,6-dimethylaminopryidine (DMAP). Our previous results showed that the same reaction condition works well for the attachment of PAE dendrons to larger (polydisperse) Au₃₁₄ nanoparticles [Langmuir 2008, 24, 6924]. However, the reaction between G2-PAE and MBA-Au₂₅ nanoparticles produced insoluble aggregated particles. This was likely due to the lower stability of Au₂₅ nanoparticles especially in the presence of DMAP. The synthesis of DMAP-stabilized Au nanoparticles has previously been reported [Burgess et. al. J. Phys. Chem. C 2008, 112, 2872]. This suggested that the association of DMAP to the surface ligands of MBA-Au₂₅ nanoparticles might have caused a partial removal of protecting monolayers and subsequent aggregation of nanoparticles.

In our second attempt, we used 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) instead of DCC to increase reactivity in polar solvents and avoided the use of DMAP (or any other bases) by focusing on amide coupling of NH₂-fucntionalized dendrons (G2-PPI and G1-ATE) to MBA-Au₂₅ nanoparticles. THF and acetone were used as solvents because all dendrons and MBA-Au₂₅ nanoparticles are soluble in these solvents. Although EDC was initially soluble in these solvents, we found that the EDC-MBA-Au₂₅ nanoparticle adducts were insoluble and precipitated out during the reaction. Addition of G2-PPI or G1-ATE dendrons in the solution containing EDC-MBA-Au₂₅ nanoparticle adducts resulted in the slow dissolution (days) of these adducts back into solution. IR spectra of obtained products showed a peak at 1646 cm^-1 suggesting the presence of amide bonds. The spectra also showed a large O-H stretching absorption bands within the region between 3500-2500 cm^-1 and a C=O absorption peak at 1699 cm^-1 suggesting the presence of unreacted carboxylic acid groups in the interior of products (NCDs). Thermogravimetric analysis (TGA) confirmed an increase in ~5% of organic fractions when compared to TGA results of MBA-Au₂₅ nanoparticles. TGA data were correlated to about 2-3 dendron attachments to each Au nanoparticle. However, UV-vis spectra of the produced NCDs showed only an exponential decay without any distinct peaks corresponding to molecule-like electronic levels of Au₂₅ nanoparticles. Since the unique spectroscopic UV-visible absorption spectra of the Au₂₅ core allows us to determine any changes to the size of the gold core as we proceed through the synthesis, the result clearly suggested a possibility of core size evolution during the NCD synthesis. TEM results of NCDs confirmed this core size evolution (from ~ 1 nm to ~ 2 nm) during the reaction. We suspect that the precipitation of EDC-MBA-Au₂₅ nanoparticle adducts during the reaction and the subsequent slow coupling reactions are the reasons for the core size evolution of Au₂₅ nanoparticles.

We are trying to investigate if the use of water-soluble nanoparticles and water-soluble dendrons can solve the problems of precipitation and slow coupling reactions during the NCD synthesis, because EDC adducts are known to be soluble in water. We are currently working on the synthesis of water-soluble glutathione-capped Au₂₅ nanoparticles. We also obtained water soluble dendron (PFd-G2-ArNH2-acetonide) from Polymer Factory for the attachment. When successful, a combination of MALDI-TOF/TOF-MS spectrometry, TEM, and X-ray powder diffraction (XRD) will be used for the detailed characterization of monodisperse NCDs.

As an alternate project, we developed a synthetic method for alkanethiolate-capped Pd nanoparticles that can act as efficient catalysts for various organic reactions. Pd nanoparticles were produced by the borohydride reduction of K₂PdCl₄ in toluene/H₂O using sodium S-dodecylthiosulfate as a source for the stabilizing ligands. The analyses using ¹H NMR, FT-IR, TEM, UV-vis spectroscopy, and TGA suggested that the monolayer capped Pd nanoparticles from sodium S-dodecylthiosulfate are quite comparable in composition and core size from those previously prepared from dodecanethiol. However, the catalytic activity of Pd nanoparticles generated from S-dodecylthiosulfate was found to be much greater than that of Pd nanoparticles prepared from dodecanethiol. The increased catalytic activity of Pd nanoparticles was due to the lower ligand density (organic weight fraction) of Pd nanoparticles generated from S-dodecylthiosulfate. These Pd nanoparticles could catalyze the isomerization of allyl alcohols to the corresponding carbonyl compounds in a relatively high efficiency and with a high selectivity. Both kinetic and thermodynamic effects controlled the catalytic reactions of various substituted allyl alcohols. In general, less substituted allyl alcohols including prop-2-en-1-ol and pent-1-en-3-ol were isomerized to the corresponding aldehyde or ketone more efficiently. More substituted allyl alcohols such as but-2-en-1-ol and 3-methylbut-2-en-1-ol did not undergo isomerization under the same condition. However, the presence of reactive, less substituted allyl alcohols was found to promote the isomerization of poorly reactive, more substituted allyl alcohols. This research resulted in two published articles in Journal of Materials Chemistry and Applied Catalysis A: General.