When dealing with nanoparticles, size usually indicates a number average. This is obtained directly by measuring the diameter of a sufficiently large number of unique particles, using electron or probe microscopy techniques, or indirectly by analyzing data collected by X-ray diffraction or light scattering.

In both cases, the result is a hystogram representing the nanoparticle distribution, or dispersity, across various intervals of size. A perfectly monodispersed distribution means that all the nanoparticles have the very same size; for most applications, this is the ideal condition, as a collection of identical particles is expected to be absolutely homogeneous in terms of typically size-dependent physico-chemical properties.

Due to the complexity in the synthesis of nanoparticles, achieving narrow distributions of size is extremely challenging and must not be given for granted: for this reason, the indication of size should be always integrated with some indication of the degree of monodispersity, often provided as a ± variation.

A broad size distribution implies that for a given average size, there exist a significant population of nanoparticles with much larger and much smaller sizes. Each of these distribution tails will affect the final properties: larger nanoparticles will settle faster, block larger pores and channels, scatter more light, show a smaller specific surface; smaller nanoparticles will be more difficult to precipitate, show enhanced quantum behaviors and a higher specific surface, determining for instance larger corrosion effects. This dishomogeneity transfers directly to processing and performance issues, thereby affecting negatively the final product performance.


When purchasing nanoparticles, make sure the size distribution on the box matches the size in the box or be ready to be disappointed.

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