Recently a definition of inorganic nanoparticles from an environmental, health and safety perspective was proposed by Auffan and coworkers (read the article from Nature Nanotechnology).

The idea whas than quoted by Hayley Birch on an article published on Chemistry World, the RSC magazine) with the appealing title "All that is small is not nano". Birch's review reads: "According to the researchers, nanomaterials should be categorised based on novel properties that are related to their small size - not, crucially, their size alone". (read the article from RSC website).

A commentary from Bice Fubini, Head of The Centre Scansetti,  was published as a letter on Chemistry World (Dec. 2009 issue).

I find excellent the idea of defining as "nanoparticles" only those particles which differ in some physico-chemical features from the bulk or bigger particles, thus constituting something new on a material science basis. This definition really helps in clarifying a very messy use of the word.
However, when it comes to regulations concerning their potential health effects and environmental damage, aspects other than the "new properties" acquired at the nanolevel have to be taken into account, merely related to their nano-size:

• If the damaging effect is related to the particle surface, "nanoparticles", will show effects orders of magnitude larger, when compared to the same mass dose of bigger particles (as usual for regulatory agencies). By the same token, even larger effects will be detected if the effect is related to the number of particles
• Even without gaining new properties, finely divided materials exhibit enhanced kinetics as compared to bigger particles because of a larger density of reactive defective sites, which are usually the catalytic centres for the release of highly damaging free radicals
• Nanoparticles have sizes comparable to proteins and may gain access to several biological compartments including the cardiovascular system and the brain, usually protected from bigger particles. Thus new potentially damaging scenarios may be attained, unknown to traditional particle toxicology

Taking into account these facts, the proposal by Auffan et al. should be welcomed and accepted by the scientific community.

Dr. Bice Fubini MRSC
Head of the Interdepartmental Center "G.Scansetti" for the Study of Asbestos and other Toxic Particulates of the University of Torino (I)