Polystyrene particles in various sizes (81, 202, 350, 485, and 752 nm in diameter) at an approximately 2.6% solid () in deionized (DI) water concentration (Polyscience, Inc., Warrington, Pennsylvania) were used without further modification. Gold nanoparticles (Au NPs) in different sizes (18, 33, 52, and 126 nm in diameter) were synthesized by following the published protocol with slight modifications.15,27 The mean diameter of Au NPs was characterized by using a DLS particle analyzer (Zetasizer Nano series- ZEN 3600, Malvern Instruments, Worcestershire, UK). The mean size of each particle sample was obtained from at least 10 repeated measurements and further compared with the observed morphology under a transmission electron microscopy. Concentrations of Au NPs in different sizes were calculated using the UV/vis spectroscopic method as reported previously.28 The concentrated Au NPs in sizes of 18, 33, 52, and 126 nm in diameter, respectively, by 30 min centrifugation at 13,200, 9,000, 8,000, and 4,000 rpm in room temperature were resuspended in DI water at a concentration of . Before the OCT imaging, all particles were sonicated for at least 20 s to be homogeneous. Then, one hundred microliters of colloidal solution of Au NPs and polystyrene particles were dropped in a 96 well plate and incubated at room temperature for 3 h to equilibrate the temperature to minimize the fluctuation of Brownian motion by temperature. Spectrometer-based Fourier domain OCT was used for DV-OCT imaging. Raw data was processed and quantified within a 200 by 50 pixels area under the vertex of the colloidal particle surface for 5 consecutive images and averaged with nonzero data containing pixels.