"Carbon is hardly considered to be a semiconductor, so luminescent carbon nanoparticles are very interesting both fundamentally and practically," said Ya-Ping Sun of Clemson University. "It represents a new platform for the development of luminescent nanomaterials for a wide range of applications."

Sun and colleagues prepared the carbon nanoparticles by laser ablation from a carbon target in the presence of water vapour. The resulting particles were around 5 nm in diameter. The team treated the particles with nitric acid solution before passivating their surfaces by adding a polymer coating.

The polymer consisted of diamine-terminated oligomeric poly(ethylene glycol) PEG1500N or poly(propionylethyleneimine-co-ethyleneimine (PPEI-EI). When illuminated, the coated carbon nanoparticles photoluminesced, emitting light with visible and near-infrared wavelengths.

Carbon nanoparticles without polymer coatings did not luminesce, however. The team believes that holes on the carbon surface act as energy traps, becoming emissive when stabilized by the polymer coating. The polymer also enables the scientists to attach antibodies or other labelling materials to the nanoparticle.

The carbon dots could have applications in medical imaging, biological sensing and light-emitting diodes. The researchers say they labelled anthrax-like spores with luminescent carbon dots in a laboratory study.

The team reported its work in the Journal of the American Chemical Society.