2. How have nanomaterials been defined?
All definitions of nanomaterials offered so far are based on size. Upper size limits proposed have varied from 100 nm up to 1000nm for pharmaceuticals. Lower size limits have also varied, from 1nm, to 0.1 nm. A lower limit has to be considered carefully because some individual molecules fall in the nanometre range, but may not be classified as nanomaterials.
The small dimensions involved can be measured outside – for individual particles or other structures – or inside. Internal measurements apply to aggregates of nanoparticles. Applying them more widely leads to including materials with nano-sized pores in the general classification of nanomaterials.
These ambiguities have prompted suggestions for other physical or chemical characteristics to be used to define nanomaterials. These have been applied in different ways at different times, and need close examination.