Nanomedicine is still in its early stages and many fluorescent nanomaterials, like cadmium selenide quantum dots needed to image tumours or kill cancer cells, are highly toxic to humans. Scientists are striving to develop non-toxic alternatives to these nanomaterials and it is important that such particles are safely eliminated from the body in a reasonable amount of time after they have performed their imaging or tumour-killing function.

Now, Michael Sailor of the University of California at San Diego and colleagues at MIT and the University of California at Santa Barbara have made the first non-toxic, brightly luminescent silicon-based nanoparticles able to carry drugs, travel to a tumour and be imaged. The particles then degrade into harmless by-products that can be eliminated safely from the body in around five days.

The researchers made the nanoparticles by exposing silicon wafers (the same as those used in the semiconductor industry) to an electric current, which produces nanosized holes in the wafers. The resulting porous film then breaks up into nanoparticles when ultrasound is applied. These nanoparticles are photoluminescent and glow red when exposed to UV light.

The team succeeded in imaging the particles for several hours in a mouse by detecting their luminescence with a fluorescence scanner. After this time, the particles degrade and are safely excreted by the kidneys. "This is the first luminescent particle that was purposely designed to minimize toxic side effects," said Sailor.

An added advantage of the particles is that the anti-cancer drug, doxrubucin, can be loaded into them at relatively high levels (1 mg of drug per kilogram of body mass). This is important as the drug usually needs to be administered at much higher loads to be effective, but it is toxic at these concentrations if not sequestered in the nanoparticle.

The work was published in Nature Materials.