We developed a new chemical synthesis method for monodisperse gold nanoparticles coated with 1-dodecanethiol (DDT) molecules in non-polar organic solvents. We made a big modification to the Brust-Schiffrin method, which is to omit the phase-transfer agent, tetraoctylammonium bromide. First, gold nanoparticles smaller than 3 nm in diameter are made in water by reducing gold-chloride complex anions with borohydride anions and hydroxide anions. Then they are phase-transferred to hexane by being coated with DDT molecules during the simple shaking of the glass vial vigorously by hand. These small gold nanoparticles are grown larger to the diameter of 7 nm by heating in DDT after removing hexane via evaporation. Nearly monodisperse gold nanoparticles are also obtained by making 7 nm gold nanoparticles directly in water and transferring them to hexane with the help of acetone.

Surprisingly these gold nanoparticles coated with DDT are very stable in hexane as expected, but are slightly unstable in toluene. As a result they float at the air-toluene interface of a toluene droplet. This special property is used to fabricate monolayer films of gold nanoparticles on any substrate including the whole surface of a 3 inch silicon wafer. This toluene droplet deposition method for gold nanoparticles is similar to a Langmuir monolayer of amphiphilic molecules at the air-water interface, but is much simpler without need of any equipment like a compressor and a substrate dipper-lifter. The deposition of a gold nanoparticles monolayer is done simply by removing toluene molecules by evaporation.

Our new synthesis method has many practical merits in addition to the high quality in size monodispersity and the special property of floating at the air-toluene interface. It is extremely simple, highly reproducible, and fast, taking less than 10 minutes or 1 hour depending on the growth method. Also there is no need for cleaning the reaction byproducts, since they stay in the water phase.

A few exemplary results will be presented including a two-dimensional superlattice domain larger than 20 μm of 7 nm gold nanoparticles with hexagonal close-packing order and fabrication of substrates for surface-enhanced Raman scattering.