Researchers have stumbled upon a cheap and simple method to produce graphene particles that shine like quantum dots (Nano Lett., DOI: 10.1021/nl2038979). These small flakes, which are often only two to three layers of carbon atoms thick, could be a less toxic alternative to the metal-based quantum dots that researchers use to image cells, the researchers say.
To label proteins and other cellular structures, biologists often use quantum dots made from cadmium telluride, lead sulfide, or cadmium arsenide. But synthesizing those materials creates toxic metallic waste, and researchers worry that the quantum dots could release toxic ions into the cells they want to study. Carbon-based quantum dots don’t contain these toxic metals, but have proven difficult to make.
Wei Gao, a graduate student at Rice University, Pulickel Ajayan, her adviser, and their colleagues accidentally synthesized graphene quantum dots from carbon fibers while trying to make materials that store energy. After treating the fibers with acid, one of the scientists found a small batch of carbon quantum dots.
The outer walls of the carbon fibers they started with contained islands of graphene, one-atom-thick sheets of carbons arrayed like honeycombs. By treating the fibers with hot nitric and sulfuric acid, the scientists broke apart the fibers and released carbon particles that were 1 to 5 nm in diameter. The acid also oxidized the graphene particles along their edges into graphene oxide. Unlike bulk graphene, at small particle sizes, graphene oxide can emit light.
The researchers found that the wavelength of light emitted by the quantum dots depends on their size and the structure of their edges. By tuning the temperature of the reaction of carbon fibers with acid, the researchers can change these physical properties and in turn adjust the wavelength of light the quantum dots emit, Gao explains.
To test the toxicity of the materials, the scientists exposed breast cancer cells to two sizes of graphene dots, which emit blue or green light. They found that the cells’ growth was similar to that of untreated cells.
Vikas Berry, a nanomaterials expert at Kansas State University, calls the method “a great step” toward a simple, high-yielding way to produce graphene quantum dots. He points out that other groups have previously synthesized graphene quantum dots from fibers, but he says those methods are more complicated than the new one.