Protected Clusters

Following this report, several modifications of the method with regard to the stabilizing ligands or reaction conditions have been described. Some interesting examples are described in the following paragraphs. 

The first attempted modifications focused on the use of highly polar thiols to obtain very small water soluble MPCs. Thus, for example, the synthesis of gluthatione 

Tiopronin water soluble Au clusters have been described and their characterization by HRTEM and TGA has shown clusters of about 1.8 nm and a composition of Au2oi(Tiopronin)85 [65]. Polymer protected water soluble gold MPCs have also been synthesized using thiolated polyethyleneglycol (SH-PEG). H RTEM analysis displays a polydisperse population of clusters of 2.8 1 nm size and TGA results yields an average Au807(SH-PEG)98 composition [66]. 

Other modifications to the reaction conditions of the Brust-Schiffrin method, such as a reduction temperature of — 78 °C and the use of a hyperexcess of hexanethiol, results in an Au38(thiolate)24, based on observations, LDI-TOF mass spectrometry, TGA analysis and elemental analysis [69]. The influence of preparation temperature on the size and monodispersity of dodecylthiol monolayer protected gold clusters has also been reported. Both and SAXS measurements show that higher temperatures increase polydispersity. This modification of poly-dispersity may be related to the existence of a dynamic exchange of thiols at the particle surface with thiols in the solvent [70]. 

Another Au38 cluster core has been reported using phenylethanethiolate as protecting ligand, giving rise to 1.1 nm core diameter nanoparticles, as revealed by 

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A group of water-soluble glutathione-protected clusters with defined chemical compositions have been synthesized by reducing Au3+ ions in the presence of glutathione. This produces a mixture of quantum clusters that are the kinetically... 

Templeton AC, Wuelfing MP, Murray RW. Monolayer protected cluster molecules. Acc Chem Res 2000 33 27-36. 

The chemical reactivity of nanoparticle surfaces, presents interesting additional opportunities for evaluating nanoparticle surface composition. Some noble metal particles (Pd and Au in particular) can be extracted from the PAMAM dendrimer interiors into organic solution with long-chain thiols [37]. The resulting nanoparticles, referred to as Monolayer Protected Clusters (MPCs), retain the size distributions and spectroscopic characteristics of the original DENs and allow for recycling the expensive dendrimer [16]. 

Fig. 33 Optical activity of a chiral ligand-protected gold nanocluster [Au25(SRcys)18]. (a) Optimized geometry, (b) Calculated CD spectrum of the cysteine-protected cluster black), and experimental spectrum for a glutathione-protected cluster (red). Reprinted with permission from [299]. Copyright 2010 American Chemical Society...

The advantage of a uniform size must be qualified for colloids, having a size distribution per se which, however, can be relatively narrow in some cases. In principle the solubility of ligand-protected clusters and colloids makes their use as homogeneous catalysts possible. But as it has turned out, cluster solutions tend to decompose during catalytic processes. The isolation of an unchanged cluster material after a homogeneously catalyzed reaction is indicated in only a very... 

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