Noting the toxicity and potential environmental hazard of silicon-based semiconductor quantum dots, researchers at Clemson University in South Carolina have created carbon nanodots with photoluminescence that is dependent upon particle size and surface passivation.

Silicon nanostructures are strongly luminescent when their surfaces are oxidized, and the researchers, led by chemistry professor Ya-Ping Sun, theorized that carbon would act similarly. They report in the May 23 ASAP edition of the Journal of the American Chemical Society that, to test this, they used a Q-switched Nd:YAG laser at 1064 nm to ablate nanoscale particles from a pressed, baked, cured and annealed mixture of graphite and cement.

The resulting particles were not photoluminescent but became so when passivated with such organic molecules as a diamine-terminated oligomeric form of polyethylene glycol or polypropionylethyleneimine-co-ethyleneimine. The dots emitted from the visible to the near-infrared, with larger dots less luminescent than smaller ones and different passivating molecules yielding different emission curves. Because the passivation process was organic, the dots could be conjugated with bioactive molecules for bioimaging. Multiplexing of the dots could create different labels simultaneously. The details of how this might be done still must be worked out.