We propose a robust method to distinguish isolated single gold nanoparticles (AuNP monomers) and their dimers under Brownian\nmotion, a key for ultrasensitive homogeneous bioassays, including AuNP sandwich assays. To detect dimers and distinguish them\nfrom a larger number of monomers in aqueous solution, single-particle polarization microscopy was performed. For the accurate\ndetection of individual particles, the optical anisotropy and rotational diffusion time are measured because a dimer is much more\nanisotropic than the nearly spherical monomer and the rotational diffusion time of a dimer is four times that of a monomer.\nBy employing an autocorrelation analysis, we defined a measure of distinguishing that simultaneously enables high detection\nprobability and low error probability.The detection platform offers homogeneous DNA hybridization assays and immunoassays at\nthe subpicomolar level.
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