Carbon dots – fluorescent carbon nanoparticles – can be used for in vivo cell imaging with both one- and two-photon excitation, researchers have discovered. The team, with members from Clemson University, from Peking University in Beijing and from Shanghai University in China, created and identified carbon dots back in 2006, and it has been researching the various applications of the particles ever since. The particles had previously been proved useful as in vitro imaging agents.To test their efficacy as imaging agents, the investigators subcutaneously injected a solution of carbon dots or of zinc sulfide-doped carbon dots into the backs of female DBA/1 mice. The mice were then imaged using an in vivo imaging system with 470-nm excitation and 525-nm emission filters. Both versions of the nanoparticles could easily be seen, though the doped version glowed more brightly, and both types also proved to have a relatively slow diffusion rate and a long fluorescence lifetime of approximately 24 hours. When the excitation wavelength at each injection site was lengthened to 545 nm with a 620-nm emission filter, red fluorescence emissions were produced.An in vivo image of the fluorescence produced by intravenously injected carbon dots has been color-coded to show the intensity, ranging from blue at the low end to red at the high end. Image courtesy of Sheng-Tao Yang, Clemson University.When the scientists injected the mice with the carbon dot solution intravenously, they found that the particles were visible mainly in the animal’s bladder and urine. Upon examining the organs ex vivo, they detected a small amount of fluorescence in the liver. The minimal uptake of the nanoparticles into the test subjects’ organs could mean that these carbon dots would offer an alternative to other nanoparticles and nanotubes, which have demonstrated a much larger hepatic uptake.“There is growing evidence suggesting that carbon dots represent a new class of quantum-dots-like optical nanomaterials for a variety of applications, while remaining non- or minimally toxic,” said Ya-Ping Sun, one of the researchers. The team plans to continue researching the particles, with the goal of ultimately developing imaging agents for clinical use.