Branched poly(ethylenimine) (PEI)-capped Au nanoparticles are prepared at room temperature using PEI as the reductant of hydrogen tetrachloroaurate (HAuCl4). The size of Au nanoparticles, ranging from 10 to 70 nm, is readily controlled by varying the relative amount of PEI used initially versus HAuCl4. The PEI-capped Au nanoparticles are further demonstrated to be assembled into a large area of 2-D aggregates at a toluene-water interface either by heating the mixture or by adding benzenethiol to the toluene phase at room temperature. Both films are quite homogeneous, but Au nanoparticles appear to be more closely packed in the film assembled via the mediation of benzenethiol. The optical property of the PEI-capped Au films is controlled by the amount of benzenethiol added to the toluene phase. The obtained large area of PEI-capped Au film exhibits strong SERS activity of benzenethiol and also exhibits a very intense SERS spectrum of 4-nitrobenzenethiol via a place-exchange reaction that takes place between benzenethiol and 4-nitrobenzenethiol. Because the proposed method is cost-effective and is suitable for the mass production of diverse Au films irrespective of the shapes of the underlying substrates, it is expected to play a significant role in the development of optical nanotechnology especially for surface plasmon-based analytical devices.