Breakthrough process enables graphene quantum dots to be created in bulk

A team from Rice University in Houston, Texas have found a one step chemical process that it claims is markedly simpler than established methods for making graphene quantum dots. The results have been published in Nano Letters. Rice lab of materials scientist, Pulickel Ajayan, said: "There have been several attempts to make graphene based quantum dots with specific electronic and luminescent properties using chemical breakdown or e-beam lithography of graphene layers. We thought that as these nanodomains of graphitised carbons already exist in carbon fibres, which are cheap and plenty, why not use them as the precursor?" Quantum dots are semiconductors that contain a size and shape dependent band gap. These have been promising structures for applications such as computers, leds, solar cells and medical imaging devices. The sub 5nm carbon based quantum dots produced in bulk through the wet chemical process discovered at Rice are highly soluble and their size can be controlled via the temperature at which they're created. The technique was discovered while the researchers were attempting another experiment. Wei Gao, a Rice graduate student, said: "We tried to selectively oxidise carbon fibre and we found that was really hard. We ended up with a solution and decided to look at a few drops with a transmission electron microscope." The specks they saw were oxidised nanodomains of graphene extracted via chemical treatment of carbon fibre. "That was a complete surprise," said Gao. "We call them quantum dots, but they're two dimensional, so what we really have here are graphene quantum discs." While other techniques are expensive and take weeks to make small batches of graphene quantum dots, the researchers' starting material is cheap, commercially available carbon fibre. "In a one step treatment, we get a large amount of quantum dots," said Gao. "I think that's the biggest advantage of our work." Further experimentation revealed the size of the dots and the resultant photoluminescent properties could be controlled through processing at relatively low temperatures from 80 to 120°C. "At 120, 100 and 80°C, we got blue, green and yellow luminescing dots," she said. It was also found that the dots' edges tended to prefer zigzag forms. The edge of a sheet of graphene determines its electrical characteristics and zigzags are semiconducting. The luminescent properties give graphene quantum dots potential for imaging, protein analysis, cell tracking and other biomedical applications. Tests at Houston's MD Anderson Cancer Center and Baylor College of Medicine on two human breast cancer lines showed the dots easily found their way into the cells' cytoplasm and did not interfere with their proliferation. "The green quantum dots yielded a very good image," said co-author Rebeca Romero Aburto, a graduate student in the Ajayan Lab who also studies at MD Anderson. "The advantage of graphene dots over fluorophores is that their fluorescence is more stable and they don't photobleach. They don't lose their fluorescence as easily. They have a depth limit, so they may be good for in vitro and in vivo (small animal) studies, but perhaps not optimal for deep tissues in humans. But everything has to start in the lab, and these could be an interesting approach to further explore for bioimaging. In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too."