New approaches and standardized test procedures to study the impact of nanoparticles on living cells are urgently needed for the evaluation of potential hazards relating human exposure to nanoparticles. An important aspect of nanoparticle toxicity, in contrast to molecular toxicity, is the fact that the preparation and way of administration of the nanoparticles plays a crucial role. The importance of nanoparticle characterization before conducting experiments for in vitro toxicity assessments is well known.

The fascinating properties of nanoparticles in part triggered the rapid development of nanotechnology and its commercial application. Nevertheless, its widespread use in food products, sunscreens, toothpastes, skin care products, antibacterial silver coatings and paints continues to raise concerns about its potential toxicity and long term environmental effects. There are first studies that investigate the toxicity of prototype NPs such as TiO₃, C₆₀, quantum dots, carbon nanotubes and gold.

It is well known that toxic effects brought about by exposure to nanoparticles are related to the ability of these nanoparticles to catalyze the production of reactive oxygen species and to bind irreversibly to membranes or DNA. This causes interference at multiple levels of cellular metabolism, signalling and genetic alterations. Studies, so far, point towards a majority of intracellular rather than extracellular interferences, posing the question of how nanoparticles enter the cells of utmost importance.

Scientists at the Ludwig-Maximilians-Universität in Germany [Alberola and Rädler, doi:10.1016/j.biomaterials.2009.03.031] have successfully used semiconductor quantum dots (QDs) preparation on solid surfaces and the subsequent internalization by cells seeded on top of the nanoparticle layer. Controlled densities of nanoparticles were adsorbed to solid surfaces coated with extra-cellular macromolecules.

The scientists measured the fraction of QDs taken up by the cells by automated fluorescence microscopy z-scans. The results showed that nanoparticles aggregate during the uptake process, forming clusters inside cells that are able to enter the cell nucleus. Nanoparticle uptake is dependent on surface functionalization and can be hindered by increasing the strength of the adhesion force between nanoparticles and the surface. Studying time dependent uptake the scientists were able to show that particles are able to exit cells.

Results might also help in the development of drug and gene delivery systems, since understanding cellular uptake within and from the extra-cellular matrix is a key aspect in developing efficient vectors.