Size is certainly the most defining characteristics of our nanoparticles. While the nano- prefix refers to the unit of length of the nanometer (10-9 m, or 1 billionth of meter), nanomaterials are in general required to have at least one dimension smaller than 100 nm.

The deep reason behind the 100 nm limit is that once that size is crossed, several physico-chemical properties of matter deviate progressively from the bulk phase behavior, and several quantum effects start kicking in.

Color changes; electric and magnetic properties are enhanced; hardness increases; inert materials become efficient chemical catalysts; light scattering becomes negligible.

And beyond the quirkiness of quantum mechanics, a small size presents some evident geometrical implications.

The specific surface, i.e. the surface area of all the nano-objects that make up 1 gram of material, is usually in the 100s of m2 per gram. This means that 1 gram of nanoparticle can interact with 100 m2 of a polymer matrix, of polluted water, of reactive gas.

A nano-object is comparable in size with several biological organelles, micropores, microtubules, getting access to usually precluded biochemical environments. On surfaces, it can infiltrate deeper crevices, fill smaller voids, run through tighter channels. More individual objects can be arranged per unit volume or unit surface, thereby exponentially increasing the density of stored information, as witnessed by the corresponding growth of the electronics industry.

Below a certain size range, particles become less prone to settling as gravitational forces are counterbalanced by diffusion to regions of lower particle concentration, which is inversely proportional to particle size. Therefore if the particle size is properly engineered the dispersion will be indefinitely stable.

Size can be measured by a variety of direct and indirect techniques, including Electron Microscopies, X-ray diffraction, Dynamic Light Scattering. Optical techniques are unfortunately of little use, as nanoparticles are one order of magnitude smaller than the wavelentgh of visible light.

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

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