Production of Encapsulated Metal Nanoparticles

In order to produce metal nanoparticles encapsulated with organic ligands the precipitation method or brust method have to be used. This method is used to produce gold nanoparticles in organic liquids such as toluene that are generally not mix with water. The strategy exploited by this method is chemical bonding of a thiol ligand monolayer on the surface of the gold nanoparticles during formation.

In this method the chemical bonding of a thiol ligand monolayer on the surface of the gold nanoparticles during formation. This is an attachment of self-assembly thiolated monolayer and it occurs spontaneously with no external assistance, as the bond between the thiolated molecules and the gold surface is an extremely strong covalent bond.

This method involves the reaction of a chloroauric acid (HAuCl4) solution with a tetraoctyl-ammonium bromid (shortly: TOAB) solution in toluene, which serves as the gold phase transfer agent from water to the organic solvent and sodium borohydride which serves as a reducing agent. When adding TOAB dissolved in an organic phase to gold ions in water, the interaction with the gold ions transfer the gold to the organic solvent. In the organic solvent, the thiol molecule self-assembly can form on the nanoparticle surface, and the sodium borohydride serves as a source for the electrons necessary for the reaction.

TOAB does not bind strongly to the gold nanoparticles, so the solution will aggregate gradually over the course of approximately two weeks. In order to prevent this, one can add stronger binding agent such as thiol, which will covalently bind to gold, producing a permanent suspended solution. After the synthesis, some of the phase transfer agent may remain bound to nanoparticles, and this may affect physical properties such as solubility. In order to remove as much of this agent as possible, the nanoparticles must be further purified by extraction methods. Alkanethiol-protected gold nanoparticles can easily precipitated and then re-dissolved, facilitating washing or transferring to fresh or alternative solvents.