In addition to alkanethiols and phosphine derivatives, alkylamines have been investigated as capping agents in the synthesis of organically dispersible gold nanoparticles. However, reports pertaining to gold nanoparticle derivatization with alkylamines are relatively scarce and their interaction with the underlying gold support is poorly understood. In this paper, we attempt a more detailed examination of this problem and present results on the Fourier transform infrared spectroscopy, thermogravimetry, nuclear magnetic resonance, and X-ray photoemission (XPS) characterization of gold nanoparticles capped with the alkylamines laurylamine (LAM) and octadecylamine (ODA). The capping of the gold nanoparticles with the alkylamines was accomplished during phase transfer of aqueous gold nanoparticles to chloroform containing fatty amine molecules. Thermogravimetry and XPS analysis of purified powders of the amine-capped gold nanoparticles indicated the presence of two different modes of binding of the alkylamines with the gold surface. The weakly bound component is attributed to the formation of an electrostatic complex between protonated amine molecules and surface-bound AuCl/AuCl ions, while the more strongly bound species is tentatively assigned to a complex of the form [AuCl(NHR)]. The alkylamine monolayer on the gold nanoparticle surface may be place exchanged with other amine derivatives present in solution.

Pepsin−colloidal gold conjugates were prepared by a simple protein-friendly process and the enzymatic activity of the bioconjugates is reported. The pepsin−gold conjugates are obtained by mixing colloidal gold and protein solutions at pH = 3 and, thereafter, centrifugation, washing, and redispersion of the pepsin−gold conjugate material in water. The bioconjugates in solution were characterized by UV−vis spectroscopy, fluorescence spectroscopy, and biocatalytic activity measurements while films of the bioconjugate material obtained by solvent evaporation on suitable substrates were further analyzed by scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX), transmission electron spectroscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). While TEM and SEM measurements showed aggregates of the enzyme/colloidal gold conjugates, the intactness of secondary and tertiary structures of the enzyme, as determined by FTIR and fluorescence spectroscopies and confirmed by biocatalytic activity measurements, clearly indicates that the enzyme is stable in its natural state and is possibly stabilized by the colloidal gold particles. The enzyme in the pepsin−Au bioconjugate retained substantial biocatalytic activity and was more stable than the free enzyme in solution.

The organization of nanoparticles into superstructures of predefined geometry is an important challenge in the area of nanoscale architecture. Attractive Coulombic interaction between positively charged amine groups on gold particle surfaces and negatively charged phosphate backbones of DNA molecules (see Figure) drives the self‐assembly of gold nanoparticles into linear supercluster structures.

The organization of gold nanoparticles at the liquid-liquid interface between the gold hydrosol and benzene as well as anthracene in chloroform is described. Vigorous stirring of the biphasic mixture results in almost complete transfer of the gold nanoparticles from the aqueous to the benzene phase and the subsequent assembly of the gold nanoparticles at the liquid-liquid interface. In the case of anthracene in chloroform, the gold nanoparticles assembled directly at the interface forming an extremely flexible membrane. The gold nanoparticle films formed at the interface in both cases could be transferred onto different solid supports and were analyzed by a host of techniques. The films show reasonable two-dimensional ordering of the gold nanoparticles over large length scales. It was observed that the benzene and anthracene molecules are strongly bound to the gold particle surface, presumably through cation-π interactions between the aromatic molecules and nanoparticle surface-bound Au + ions, thus opening up a hitherto unexplored avenue for the assembly of gold nanoparticles.

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