4. What other measurable properties of nanomaterials might be relevant?
- 4.1 Surface modification
- 4.2 Other physical or chemical characteristics
- 4.3 Other distinctions between classes of nanomaterials
There is a wide range of other properties of nanomaterials which are potentially measurable and may be of interest for risk assessment. Often, they relate to the reason the material was developed. The question then arises whether any can be used to define nanomaterials.
4.1 Surface modification
Many nanomaterials acquire modified surfaces as part of their preparation. This happens through applying chemical coatings, or through physical processing. Some particles end up with multiple coatings or shells. Such coatings affect the way the particles behave and interact, and especially their possible effects in living things. The same applies to coating acquired after preparation through contact with materials in the environment.
In general, each core and coating has to be considered as a specific preparation and there are many and varied combinations. This wide variety makes the properties of coating unsuitable for use in any definition of nanomaterials.
4.2 Other physical or chemical characteristics
Many other characteristics of individual nanomaterials can be defined and measured. They include:
Changes in crystal structures when particle sizes are small.
Whether materials are oxidising or reducing agents, chemically speaking – otherwise expressed as their tendency to gain or lose electrons, measured by their so-called redox potential.
(Photo)catalysis and radical formation – Some materials generate highly reactive chemicals known as free radicals when exposed to light, and the effect varies with size.
Surface charge (zeta potential) – some materials carry a surface charge when they are dispersed in a solution. This results in a cloud of opposite charges from the fluid around each particle, and the electrical field this creates can be measured.
Water solubility –knowing whether a material is soluble, or not, in water, is important for risk assessment. It has to be carefully defined in itself, and is not the same as dissolution, sometimes used to mean individual nanomaterial units separating in a fluid. As before, coatings and other modifications can affect both solubility and dispersion of nanomaterials.
All of these properties, as well as others such as electrical conductivity, light scattering, and magnetism, may be relevant for risk assessment. But none vary entirely predictably with size, and none apply to all nanomaterials. For these reasons, while useful to measure, none can contribute to any straightforward definition which is intended to embrace all nanomaterials.
4.3 Other distinctions between classes of nanomaterials
There are other terms which have cropped up in discussion of nanomaterials, but their definitions are generally too loose to be of help in the general discussion about defining nanomaterials more broadly.
For example, "soft” nanomaterials has usually referred to materials that are biodegradable, in contrast to "hard” nanomaterials, which are not. The terms are sometimes used along with organic and inorganic, sometimes not. There is also a distinction made between manufactured and natural nanomaterials. These two need to be supplemented by those which are not actually manufactured but nevertheless result from human action, like ultra fine emissions from combustion plants.
There are also some difficulties with categorising composite nanomaterials and with the various uses of the term nanocomposite. This can mean materials such as liposomes, which carry nanoparticle sized cargoes. It can also mean bulk materials impregnated with nanoscale particles, such as the carbon black that may be found in car tyres. These would not be considered as nanomaterials, although such composites might give rise to nanomaterials if they decompose during disposal.
Any definition needs to cover all these possible subtypes of nanomaterials, with appropriate inclusions and exclusions.