We demonstrate that the amino acid tyrosine is an excellent reducing agent under alkaline conditions and may be used to reduce Ag+ ions to synthesize stable silver nanoparticles in water. The tyrosine-reduced silver nanoparticles may be separated out as a powder that is readily redispersible in water. The silver ion reduction at high pH occurs due to ionization of the phenolic group in tyrosine that is then capable of reducing Ag+ ions and is in turn converted to a semi-quinone structure. These silver nanoparticles can easily be transferred to chloroform containing the cationic surfactant octadecylamine by an electrostatic complexation process. The now hydrophobic silver nanoparticles may be spread on the surface of water and assembled into highly ordered, linear superstructures that could be transferred as multilayers onto suitable supports by the versatile Langmuir-Blodgett technique. Further, tyrosine molecules bound to the surface of Au nanoparticles through amine groups in the amino acid may be used to selectively reduce silver ions at high pH on the surface of the Au nanoparticles, thus leading to a simple strategy for realizing phase-pure Au core-Ag shell nanostructures.

We demonstrate the phase transfer of dodecylamine (DDA)-capped colloidal gold particles dispersed in an organic solvent into water containing the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Vigorous shaking of the biphasic mixture results in the rapid phase transfer of DDA-capped gold nanoparticles from the organic to aqueous phase, the aqueous phase acquiring a pink, foamlike appearance. Drying of the colored aqueous phase results in the formation of a highly stable reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of a DDA primary monolayer directly coordinated to the gold particle surface and a secondary monolayer of CTAB, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle phase transfer and render them water-dispersible. The CTAB-DDA stabilized particles may be dispersed in water at very high nanoparticle concentrations with stability even in the presence of high amounts of electrolyte and over a wide range of solution pH.

The single-step synthesis of silver nanoparticles by the reduction of silver ions present in the subphase under alkaline conditions by 3-pentadecylphenol (3-PDP) Langmuir monolayer and their assembly into ordered two-dimensional structures is described. The reduction of the silver ions occurs by electron transfer from ionized phenol groups of 3-PDP, which then stabilize the particles against aggregation. Similar reduction of aqueous silver ions was carried out at the interface with toluene bearing 3-PDP and yielded monodisperse silver nanoparticles that may be separated as a dry powder and redispersed in a range of organic solvents. The mechanism of reduction of silver ions by 3-PDP under alkaline conditions was studied by UV−visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) spectroscopy, transmission electron microscopy (TEM), and chemical analysis by X-ray photoelectron spectroscopy (XPS), and is discussed in detail.

The one-step electrostatic complexation, reduction of aqueous chloroaurate ions and capping of the gold nanoparticles thus formed by hexadecylaniline Langmuir monolayers, is described. This process leads to the formation of highly oriented gold nanoribbons strongly bound to the Langmuir monolayer (images alongside) that may be transferred as multilayers by the Langmuir−Blodgett technique. The unusual morphology of the gold nanostructures is believed to be a consequence of highly localized reduction of the gold ions by the Langmuir monolayer.

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