3. What are the key criteria for defining nanomaterials?
The importance of size is implied by the actual term nanomaterial, using the "nano” prefix. In the standard system of international units, the prefix denotes a measure of 1 thousand millionth (10-9) of some unit. In metric sizes, anything between 1nm and 999 nm is on the nanoscale.
Upper size limit
Although 999 nm is still formally on the nanoscale, a very commonly used upper limit for nanomaterial size is 100 nm. This covers most nanomaterials, but there are exceptions. Nanomaterials clumped together can have outside dimensions larger than 100 nm, as can those which have been modified by adding a coating or an unusually large chemical group such as a long-chain organic molecule.
These kinds of materials will fall in the definition if it covers internal structures, not simply external dimensions, of the specified size. However, nanoporous materials are not usually considered to be true nanomaterials, but would also be included in the definition if internal structure was the only criterion. Additional criteria are needed to rule in the right things, while ruling them out.
Lower size limit
The lower size limit faces the opposite problem – not excluding nanomaterials which need assessment but including some preparations which are not truly nanomaterials. A single nanometre is on the scale of atoms and molecules. Some large proteins, for instance, are around 5 nm across, and can link into much bigger structures. So at the nanometre scale the distinction between molecules, nanoclusters and nanoparticles is not always clear. Other difficult cases include tubes and fibres which can be less than 1 nm across but more than 100 nm long, thus falling outside the normal definition at both ends of the scale.
Measurement methods matter
Any size used in a definition has to be measured on the material. This is not straightforward. Different methods can yield different results – registering the presence of a coating or measuring the size of the underlying particles, for example. The right technique to use also varies depending on whether nanoparticles are in a fine powder, dispersed in a liquid or embedded in some other solid material. So size measurements need to be stated along with the method used to produce the figures.
If there are agreed upper and lower limits, there must also be an idea how much of some sample falls inside the limits to be classed as a nanomaterial. Few nanomaterials are of uniform size, and size measurements need to take account of the range of sizes present. Reports of size measurements need to estimate the size distribution of nanoparticles, and this has to be taken into account in any agreed definition. Some defined percentage of the material in the nano-range will lead to the material being classified as a nanomaterial.
For example, a batch of spherical particles might be classified as nanomaterial if more than 0.15 per cent of them have a diameter below 100 nm. Estimating whether this threshold is reached depends on having reliable information on the statistical distribution of particles of different sizes in the material.
3.2 Surface area
Surface area is useful to know because many of the properties which make nanomaterials interesting depend on their large surface area compared to larger-scale preparations. It may also be a more meaningful measure than size when nanomaterials consist of agglomerates of small particles, or have small pores. A commonly used measure is the volume specific surface area, or VSSA. This puts a value on all the external or internal surfaces of a material, per unit of volume, and is usually given as so many square metres per cubic centimetre (m2/cm3).
It then remains to decide what values of the VSSA can be considered to identify a nanomaterial. One limit would be derived from the VSSA of 100nm sized spherical particles at unit density. The threshold for a nanomaterial would then be a VSSA greater than or equal to 60 m2/cm3.
Methods for assessing VSSA in powders and dry solids are well established. Measurement of powders and particles dispersed in liquids is more difficult.