A General Technique to Investigate the Aggregation of Nanoparticles by Transmission Electron Microscopy

The aggregation state of nanoparticles (NPs) must be precisely known in order to study the structure-property relationship and to evaluate the exploitability of NPs dispersion for a given application. Here we report a general technique for sample preparation to investigate with transmission electron microscopy (TEM) the state of aggregation of NPs dispersed in liquid solution. Following a simple procedure which requires few minutes, the aggregates of NPs are "frozen" in a polymeric matrix simultaneously to their deposition on a TEM grid. Our technique is of general applicability and it avoids the use of cryo-TEM, which is more expensive, more time consuming and less common than ordinary TEM. Compared to the investigation of NPs aggregation in the liquid phase with dynamic light scattering, our approach avoids the problem of shielding by large aggregates and it allows the full exploitation of TEM advantages, primarily the reliable determination of shape and size of each aggregate and the precise evaluation of the number of single NPs forming each cluster. As an example, we demonstrate the use of our technique to study two frequent topics related to aggregation: the plasmon properties of gold NPs aggregates and the stability of iron oxide NPs in physiological environment. The methodology described here will be useful to advance the knowledge about how aggregation influences the physical-chemical properties of NPs.