Although semiconductor quantum dots have shown promise as bright fluorescent labels, concerns about their toxicity remain, particularly for biological uses. Now a group of scientists at Clemson University in South Carolina has demonstrated that carbon nanoparticles subjected to an organic surface treatment also shine brightly, potentially making them suitable for applications in biosensing, medical imaging and the construction of novel LEDs.Carbon nanoparticles passivated with organic molecules may be suitable for use as fluorescent labels in biological imaging. The potential of the "carbon dots" for multiplexed biolabeling was demonstrated by photographing them through various bandpass filters under one excitation wavelength (a) and by directly photographing them under various excitation wavelengths (b). Courtesy of Ya-Ping Sun, Clemson University.The researchers began with the idea that carbon should be able to mimic silicon. Surface-oxidized silicon nanocrystals are strongly luminescent, which is thought to be a bandgap-related effect. Based on their results, said Clemson chemistry professor Ya-Ping Sun, they believe that carbon luminescence is fundamentally different and is not related to the material’s bandgap.In their work, the scientists used a Spectra-Physics Nd:YAG laser operating at 1064 nm to generate dots a few nanometers in diameter by laser ablation. When passivated with an organic molecule, the “carbon dots” responded to excitation by emitting from the visible to the near-infrared, which the investigators quantified from the blue to the near-IR in 20-nm steps using a Horiba Jobin Yvon spectrometer. At 400 nm, the quantum yield ranged from 4 percent to more than 10 percent; the researchers attributed the lower figure to incomplete surface passivation.Because they lack the toxicity of semiconductor quantum dots, the passivated carbon particles may be suitable for in vivo applications, including the detection of biological warfare agents. Here, the luminescent carbon dots tag spores of Bacillus subtilis — a simulant of B. anthracis, the bacterium that causes anthrax.The two organic molecules employed in the experiments have no visible or near-ultraviolet chromophores and so could not be the source of the response. They did produce different emission curves. Larger dots were less luminescent than smaller ones, which, along with other results, indicated that the photoluminescence is the result of quantum confinement of energy traps on the surface of the particles. Given the variation in luminescence resulting from particle size and surface passivation, it may be possible to use the carbon nanodots for multiplexed biolabeling.Sun noted that various biomedical applications, as well as LED uses, are being investigated along with fundamental improvements such as increasing the brightness and the quantum yields.Journal of the American Chemical Society, online May 23, 2006, doi:10.1021/ja062677d.