Flat gold nanostructures by the reduction of chloroaurate ions constrained to a monolayer at the air-water interface

Abstract

Formation of flat gold nanostructures occurs by the reduction of a Langmuir monolayer of hydrophobized chloroaurate ions using anthranilic acid as a reducing agent present in the subphase. Vigorous shaking of the aqueous chloroauric acid solution with a solution of surfactant, such as octadecylamine (ODA) or benzyldimethylstearylammonium chloride (BDSAC) in chloroform results in rapid transfer of chloroaurate ions (AuCl₄⁻) from the aqueous phase to the chloroform phase. Strong electrostatic interactions between negatively charged chloroaurate ions and cationic head groups of ODA and BDSAC molecules, making the gold ions sufficiently hydrophobic, are believed to be responsible for the transfer of AuCl₄⁻ ions from the aqueous to the organic phase. Surface pressure-area (Π-A) isotherm measurements reveal that these hydrophobized chloroaurate ions behave as amphiphilic molecules and form stable Langmuir monolayers on the acidic aqueous subphase. Spreading of hydrophobized chloroaurate ions on the surface of aqueous anthranilic acid solution results in the immobilization of AuCl₄⁻ ions strictly at the two dimensional surface. Hence, further reduction of these AuCl₄⁻ ions by anthranilic acid molecules from the subphase leads to the formation of highly anisotropic, flat gold nanostructures at the air-water interface. The capping of gold nanoparticles formed at the air-water interface by ODA and BDSAC enables their facile transfer as multilayers onto suitable solid substrates by the Langmuir-Blodgett (LB) technique. Multilayer Langmuir-Blodgett films were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), electron diffraction and X-ray photoelectron spectroscopy.