Our laboratory has a long standing expertise in the characterization of nanomaterials and colloids in biological and environmental media. It is one of the few in the world that has multiple, state-of-the art, particle characterization techniques in a single facility.
Currently, dynamic light scattering (DLS) and microscopic techniques (mainly transmission electron microscopy, TEM) are most often employed to obtain size distributions of nanoparticles. We have significant experience with both techniques. In DLS, particle sizes (z average radii) are evaluated by modeling their scattered light using Mie theory. Because scattering varies strongly with particle radius (6th power dependency), contaminating particles (e.g. dust) and particle aggregates can easily mask the signal of the smaller nanoparticles. Furthermore, the techniques are ill suited for small (< 10 nm) or weakly scattering particles (e.g. C or Si based particles). While algorithms have been developed to correct for polydispersity, they invariably make numerous (often unwarranted) assumptions and are rarely appropriate for more than 2 distinct size classes.
In contrast, due to their ability to characterize single particles, the microscopic techniques can provide reasonably accurate number average dimensions that have little experimental bias. They are nonetheless also limited at the lower size scales, especially for the electron poor elements. Furthermore, the microscopic techniques have very high capital and running costs, are extremely labor intensive (typically image analysis required on >100 particles on triplicate grids) and cannot accurately measure aggregation or aggregation kinetics.
We have acquired a number of complementary, state-of-the-art techniques that circumvent many of the problems associated with DLS and TEM.