We chemically synthesized charged gold nanoparticles coated with hydrophobic organic molecules, which are stable in hexane but unstable in toluene. If a hexane droplet with charged gold nanoparticles is mixed with a larger toluene droplet, they immediately float to the airtoluene interface forming a monolayer of close-packed gold nanoparticles. The monolayer can be deposited to any substrate simply by the solvent molecules' evaporation with no limit in size and without using any sophisticated instrument. As a demonstration, we fabricated a very large monolayer of close-packed gold nanoparticles covering the whole surface of a 3 inch silicon wafer. We further showed that excess organic ligand molecules do not affect the charged gold nanoparticles' property of floating to and forming a monolayer at the air-toluene interface. However, these very slowly evaporating molecules remain on a substrate, affect the two-dimensional ordering, and significantly reduce the contrast in scanning electron microscopy.