Specific Reactivity Arising at the Nanolevel

In the past few decades, the standard elementary school definition of the difference between atoms and molecules was: the molecule is the tiniest part of a substance, which keeps its properties unchanged. This, as is now clear to any chemist, is incorrect. How many molecules (or atoms or ions) do we need to obtain a piece of material, which bears the same properties of the bulk of it? Or conversely, until what point will a reduction in size not yield a marked variation in some physicochemical properties? Such variation may also reflect different biological activity of the NP with respect to the bulk. TiO2 NPs and quantum dots (e.g., CdSe) belong to this category.

Titania has been employed in a large spectrum of products, including pharmaceutics and cosmetics. TiO2 NPs are produced and largely employed in sunscreens, as they are powerful absorbent of UV radiations. Titania is a very white powder, but when the particle size becomes of the order of

target molecules

corner

Figure 2 Enhanced surface reactivity: defective position of atoms or ions, with unsatisfied valencies or vacancies in the coordination sphere, at the surface.

target molecules

free radica!

corner active surface site

Figure 2 Enhanced surface reactivity: defective position of atoms or ions, with unsatisfied valencies or vacancies in the coordination sphere, at the surface.

magnitude of the visible wavelength it becomes transparent, thus much appropriate for its cosmetic usage. Unfortunately, in these circumstances the particles also exhibit a marked photocatalytic activity, which yields ROS release and which may damage the skin. In order to prepare safe powders to be used in sunscreen products, researchers have to look for conditions in which the transparency is maintained but the photocatalytic activity is inhibited. A major role is played by the crystal structure, rutile being much less photoactive than anatase (2), and by appropriate coating procedures.

CdSe nanocrystals are cytotoxic. When coated with a ZnS (CdSe/ZnS) the critical concentration, up to which no toxic effect is observed, is increased by a factor of 10 (15). Cytotoxicity also decreases when a ligand shell of mercaptopropionic acid coats the particles, even if such shell is not stable enough to prevent the release of Cd2+ ions from the particle surface, suggesting a role of surface phenomena. Moreover cytotoxicity correlated with the internalization of the particle and not only with the release of toxic Cd2+ ions. Thus, if such material is to be employed, research on the most appropriate coatings will be urgently required.

Was this article helpful?

0 0

Post a comment