Compared with classical fluorescent dyes such as fluorescein, quantum dots emit brighter fluorescence that lasts longer with repeated excitation. However, because they historically were used in physics and engineering applications before their entrance into the biology world, most existing quantum dots are made of materials that are OK for machines but that are toxic when inside living cells or organisms.In the past, biologists have attempted to eliminate the toxicity by coating the quantum dots with silica or a similarly nontoxic material. However, this coat cannot guarantee complete protection from the highly toxic core materials.Applied Physics Letters, April 28, 2008, Vol. 92, 173902. Quantum dots of silicon carbide do not need a protective layer because silicon carbide is nontoxic. Without the silicon-carbide quantum dots, nothing is visible in fluorescence mode. The quantum dots also improve the contrast of the cells in phase-contrast mode. Images reprinted with permission of Applied Physics Letters. In the Jan. 21, 2005, issue of Physical Review Letters, researchers from Nanjing University in China and from City University of Hong Kong described quantum dots consisting of silicon carbide crystals with cubic structure. Silicon carbide is nontoxic, so the quantum dots do not require a protective coat. Additionally, they are chemically inert and stable. The same researchers put the quantum dots and water in test tubes and left them out in the laboratory air for seven months, and the quantum dots still fluoresced brightly and remained uniformly dispersed in the water, as reported in the November 2006 issue of Progress in Materials Science.Now the silicon-carbide quantum dots have been used for live-cell imaging by Jacques Botsoa and colleagues from Université de Lyon 1 and from Institut National des Sciences Appliquées de Lyon, both in France. The researchers placed the silicon-carbide quantum dots with some cells on a microscope slide. They viewed the cells with UV/violet excitation from 2.92 to 3.5 eV with an observation spectral range They found that the silicon-carbide quantum dots easily entered the cells and brightly labeled them. Most were localized in the nucleus.The quantum dots primarily label the nucleus. Botsoa said that the nuclear localization of these quantum dots needs further study but that the quantum dots may be useful for studies of nuclear trafficking. A possible limitation of the accumulation of these quantum dots in the nucleus is that it could alter the function of the cells. However, the researchers plan to functionalize the quantum dots, with antibodies for example, which could change how the nanoparticles are distributed. They performed fluorescence experiments up to one week after incubating the quantum dots with the cells, and the cells were still alive and well. Botsoa said that they plan to test the quantum dots in living organisms.