Chirality, a neglected physico-chemical property of nanomaterials? A mini-review on the occurrence and importance of chirality on their toxicity.

Toxicity assessments are required for nanomaterials which are being used in many fields such as medicines, electronics, pesticides, clothes and construction materials. One factor that usually affects toxicity of chemicals is chirality. This paper reviews the existence of chirality in nanoparticles and critically analyses the implications of chirality on toxicological risk assessment of nanoparticles. Chirality and optical activity arise from lack of symmetry, where an object is non-superimposable on its mirror image. Optical isomers or enantiomers have similar physico-chemical properties but only differ in their optical activity and their interaction with biological systems. Chirality is not limited to organic molecules, but also exists in inorganic compounds and crystals. Studies have also shown that chirality can be bestowed onto nanoparticles by adsorption of chiral molecules, and by careful design of the crystal to expose chiral kinked and stepped surface structures. Just as chirality has been shown to affect the biological activities of conventional chemicals, nanoparticles functionalised with different enantiomers have been shown to exhibit enantioselectivity and different toxicities. However, at the moment more studies on the ligands and the conditions under which chirality occurs in NPs, as well as on the effects of NP chirality on protein adsorption kinetics and thermodynamics. Nevertheless, this cursory analysis has shown the importance of chirality on biological activity of nanoparticles. In cases where there is a potential for the existence of chirality in nanomaterials (either intrinsic or extrinsic), there may be need for adequate consideration of the effects of chirality.