The size uniformity and dispersity of inorganic nanoparticles are important when they are dispersed in organic phase and serve as the precursor for particle-deposited nanosensors. However, most of the nanoparticles are dispersed into coating solutions right after surface modification, where the natural tendency of nanoparticles to agglomerate reduces the monodispersity and size uniformity during the coating process. By initiation of microdroplet coalescence, a method for in situ dispersion of surface-modified Au nanoparticles is presented in this study. Under optimal conditions, about 88% of the modified Au nanoparticles with an average size of 19 nm could be in situ dispersed. On the basis of dip-coating of the monodispersed Au nanocolloids, a SERS (surface-enhanced Raman scattering) film was fabricated by uniformly depositing particles onto the surface and tremendous enhancement of Raman scattering was achieved. The SERS film could be used to detect a representative contaminant, trans-1,2-bis(4-pyridyl)ethylene (BPE), with a detection limit of down to 10 ppb.