“Nowadays, much effort is invested in the creation of new materials on the nanometre scale which are hoped will find application in clinical diagnosis and therapy in the future,” Wolfgang Parak of Ludwig Maximilians University told nanotechweb.org. “However, before introducing such particles into humans we have to be aware of the potential risks. This is an obvious question to ask because several of the proposed nanomaterials contain toxic elements such as cadmium.”

Parak and colleagues tested the toxicity of fluorescent CdSe and CdSe/ZnS nanoparticles. The particles were either coated with mercaptopropionic acid, embedded in a silica shell or surrounded by an amphiphilic polymer shell. The scientists added the nanomaterials to a number of different cell cultures.

The team examined the uptake by the cells of the fluorescent nanocrystals using differential interference contrast and fluorescence microscopy.

“Nanoparticles are ingested in a nonspecific way by cells,” said Parak. “Since many nanoparticles are built from materials containing toxic elements, toxic ions can be released from the particles upon corrosion.”

According to Parak, there are two strategies to reduce such effects. “By embedding the particles in inert shells, corrosion can be slowed down,” he explained. “By coating the particles with molecules that reduce the uptake of the particles by cells, toxic ions are released outside but not inside the cells. In addition we could show that nanoparticles built from inert materials - such as gold - can also poison cells by sticking to their surfaces in large numbers.”

The presence of a ZnS shell in the CdSe/ZnS nanoparticles appeared to reduce nanoparticle toxicity. In some cases, a silica coating bearing polyethylene groups (PEG-silane) decreased uptake of the nanoparticles by cells.

The scientists also measured the adherence of the cells to check the effect of ingested particles on their behaviour. “Cell attachment to surfaces is an active process and will certainly be impaired when cells are poisoned,” said Parak. “The adhesion assay described in our study can be automated and so yields enough data for reliable statistics.”

Parak says that scientists should perform such quantitative toxicity tests whenever they are thinking of using nanoparticles for medical applications.

The researchers reported their work in Nano Letters.

• Parak and colleagues at Ludwig Maximilians University and the Technical University of Munich have also published a topical review on “Labelling of cells with quantum dots” in Nanotechnology.