In order to overcome the serious deficiencies of the traditional aqueous centrifugation method in on-demand purification of metal nanoparticles, we have theoretically and experimentally developed a simple purification method based on the nanoparticles' surface area discrepancy, which can separate particles with the same mass but different shapes. As an example, we apply this method to obtain on demand homogeneous Au triangular nanoplates, and tune the plasmon modes of Au nanoplates into resonance with the emission of quantum dots to achieve fluorescence resonance energy transfer (FRET) between them. Moreover, due to their high homogeneity, the purified Au triangular nanoplates exhibit an excellent sensitivity to refractive index, as high as 963 nm RIU(-1) (approaching the theoretical value of 982 nm RIU(-1)), which leads to high gauging accuracy, up to 80 ppb and 1.0 U, for sensing bovine serum albumin and DNA polymerase in solution, respectively. Our work introduces a facile, effective strategy for the separation of nanoparticles, which could obtain building blocks with scalable uniform nanosize, providing a path to precise control of metal nanoparticle's plasmon modes and efficient fabrication of nanodevices.