Nanometer-sized colloidal Au nanopaprticles (AuNPs) has received a great deal of attention in recent years for their diverse chemical and physical properties. Gold nanoparticles present fascinating aspects such as their assembly into multiple types for applications in materials science, showing properties potentially relevant to nano-scale electronic and optic devices, magnetic metirals, multifunctional catalysts, chemical recognition, and biosensors. Several approaches have been developed to prepare monolayer protected clusters which are stabilized by a covalently attached shell of ligand molecules. While alkanethiolate-proteted nanoparticles are soluble in nonpolar organic solvents, the nanoparticles readily soluble in aqueous media that do not aggregate non-specifically due to electrostatic interactions are required for most biological or biomedical applications. In order to prepare stable, water-soluble gold nanoparticles, a number of different stabilizers such as tiopronin, glutathione, mercaptosuccinic acid, sulfonic acid, and ammonium ions have been used to result in the nanoparticles that are readily dispersible in water. However, gold nanoparticles protected with arylthiolates bearing OH, NH2, COOH functional groups were found to be soluble only in polar organic solvents but not in water. A general synthetic method producing new water-soluble gold nanoparticles has been developed using a thioethyl-pendant cyclic tetraammine, 1- thioethyl-4, 8, 11-trimethylcyclam as a capping molecule. A single-phase synthetic route of Au nanoparticles can be performed in a high concentration and partly control their quantum size.
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