The present invention provides a method for quantifying moving heterologous nanoparticles in a suspension by imaging. The quantifying method comprises acquiring at least one z-stack of images within the suspension; tracking the nanoparticles in the images to identify unique nanoparticles; and enumerating the unique nanoparticles. Also provided is a method for characterizing size distribution of moving heterologous nanoparticles in a suspension by imaging. The characterization method comprises acquiring time lapse images; tracking the nanoparticles in the images to identify unique nanoparticles; determining the locations of each of the unique particles; determining the size of each of the unique nanoparticles; and aggregating the sizes of the unique nanoparticles.