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There is a need to accurately detect, characterize, and quantify nanoparticles in suspensions. This study helps to understand the complex interactions between similar types of nanoparticles. Before initiating a study of metal nanoparticles, five submicron PS beads with sizes between 200 nm and 1 mu m were used to derive a reference scale that was useful in evaluating the flow cytometer for functionality, sensitivity, resolution, and reproducibility. Side scatter intensity (SSC) from metal nanoparticles was obtained simultaneously from 405 nm and 488 nm lasers. The 405 nm laser generally yielded histogram distributions with smaller CVs, less side scatter intensity, better separation indices between beads and decreased scatter differences between different sized particles compared with the 488 nm laser. Submicron particles must be diluted to 10(6) and 10(7) particles/mL before flow cytometer analysis to avoid coincidence counting artifacts. When particles were too concentrated the following occurred: swarm, electronic overload, coincidence counting, activation of doublet discrimination and rejection circuitry, increase of mean SSC histogram distributions, alterations of SSC and pulse width histogram shape, decrease and fluctuations in counting rate and decrease or elimination of particulate water noise and 1 mu m reference bead. To insure that the concentrations were in the proper counting range, the nanoparticle samples were mixed with a known concentration of 1 mu m counting beads. Sequential dilutions of metal nanoparticles in a 1 mu m counting bead suspension helped determine the diluted concentration needed for flow cytometer analysis. It was found that the original concentrated nanoparticle samples had to be diluted, between 1: 10,000 and 1: 100,000, before characterization by flow cytometry. The concentration of silver or gold nanoparticles in the undiluted sample were determined by comparing them with a known concentration (1.9 x 10(6) beads/mL) of 1 mu m polystyrene reference beads. Published 2015Wiley Periodicals Inc., on behalf of ISAC