Nanoparticles are tiny materials (under 1000 nm in size) that have specific physicochemical properties different to bulk materials of the same composition [...] nanoparticles can act on living cells at the nanolevel resulting not only in biologically desirable, but also in undesirable effects. (Source: Medina C, Santos-Martinez MJ, Radomski A, Corrigan OI, Radomski MW. Nanoparticles: pharmacological and toxicological significance. Br J Pharmacol. 2007 Mar;150(5):552-8)