Gold polydispersity

Furthermore, the reaction scheme implies that the molecular weight distribution is Poisson-like — i.e. very narrow — as it had been shown earlier on theoretical basis by Flory 8), Gold 9), and Szwarc l0>. Even though two (or more) types of active species add monomer at very different rates, the polydispersity remains narrow, provided solvation/desolvation and ionic dissociation/association processes are fast U). 

Considerable research effort was focused on systems of colloidal gold of which a broad variety of synthetic procedures were reported [140 b, fj. While native colloidal gold solutions are only stable for a restricted time, Brust et al. [141] were able to overcome this problem by developing a simple method for the in situ preparation of alkyl thiol-stabihzed gold nanoparticles. This synthetic route yields air-stable and easy to handle passivated nanoparticles of moderate polydispersity, and is now commonly employed for the preparation of inorganic-organic core-shell composites. Such composites are used as catalytic systems with principally two different functions of the protective 3D-SAM layer. Either the metal nanoparticle core can be used as the catalytically active center and the thiol layer is only used to stabihze the system [142], or the 3D-SAM is used as a Hnker system to chemically attach further catalytic functions [143]. 

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]. 

In the remainder of this section we see how the theoretical calculations of Mie account for the observed spectrum of colloidal gold. In the next section we consider the inverse problem for a simpler system how to interpret the experimental spectrum of sulfur sols in terms of the size and concentration of the particles. Both of these example systems consist of relatively monodisperse particles. Polydispersity complicates the spectrum of a colloid since the same x value will occur at different X values for spheres of different radii according to Equations (99M101). 

Eq. (17) predicts that, when Pn is 100, the polydispersity is equal to 1.01, so that the polymer is virtually monodisperse. However, such ideal monodisperse polymers have scarcely been synthesized. The lowest values of polydispersity (Mw/Mn = 1.05-1.10) have been attained in homogeneous anionic polymerization 43). Gold 41) calculated the polydispersity of a polymer in the living polymerization with a slow initiation reaction and showed that the value of Pw/Pn increases slightly to a maximum (1.33) with an increase in polymerization time, followed by a decrease toward 1.00. Other factors affecting the molecular weight distribution of living polymer have been discussed in several papers 5S 60). 

SFFF can measure gold particles down to 15 nm. Particle size distribution was determined from peak broadening caused by polydispersity of the sample. Peak broadening due to instrument imperfections was also detected. The results were compared with data from SEM and PCS. SEM gave a mean diameter of 20 nm sizing by PC was not possible due to aggregation [86]. 

In contrast, TEM and PXRD data of Au MOE-5 show polydispersed Au particles in a size range of 5-20 nm (see Fig. 17), with a metal loading in Au MOF-5 determined to be 48 wt%. The gold particles appear to interact more weakly with the host matrix than the Pd, Ru and Cu particles and thus larger agglomerates are formed possibly by diffusion of the particles to the outer surface. 

Tuorila, P, The rapid and slow coagulation of polydispersed systems gold and alumina dispersions, Kolloidchem. Beihefte, 22, 191, 1926. 

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