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|Green-fluorescing graphene quantum dots created at Rice University surround a blue-stained nucleus in a human breast cancer cell. Cells were placed in a solution with the quantum dots for four hours. The dots, each smaller than 5 nanometers, easily passed through the cell membranes, showing their potential value for bioimaging. (Credit: Ajayan Lab/Rice University)|
A Rice University laboratory has found a way to turn common carbon fiber into graphene quantum dots, tiny specks of matter with properties expected to prove useful in electronic, optical and biomedical applications.
The Rice lab of materials scientist Pulickel Ajayan, in collaboration with colleagues in China, India, Japan and the Texas Medical Center, discovered a one-step chemical process that is markedly simpler than established techniques for making graphene quantum dots. The results were published online this month in the American Chemical Society's journal Nano Letters.
"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," said Ajayan, Rice's Benjamin M. and Mary Greenwood Anderson Professor of Mechanical Engineering and Materials Science and of Chemistry. "We thought that as these nanodomains of graphitized carbons already exist in carbon fibers, which are cheap and plenty, why not use them as the precursor?"
Quantum dots, discovered in the 1980s, are semiconductors that contain a size- and shape-dependent band gap. These have been promising structures for applications that range from computers, LEDs, solar cells and lasers to medical imaging devices. The sub-5 nanometer 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 Rice researchers were attempting another experiment when they came across the technique. "We tried to selectively oxidize carbon fiber, and we found that was really hard," said Wei Gao, a Rice graduate student who worked on the project with lead author Juan Peng, a visiting student from Nanjing University who studied in Ajayan's lab last year. "We ended up with a solution and decided to look at a few drops with a transmission electron microscope."
The specks they saw were bits of graphene or, more precisely, oxidized nanodomains of graphene extracted via chemical treatment of carbon fiber. "That was a complete surprise," Gao said. "We call them quantum dots, but they're two-dimensional, so what we really have here are graphene quantum discs." Gao said other techniques are expensive and take weeks to make small batches of graphene quantum dots. "Our starting material is cheap, commercially available carbon fiber. In a one-step treatment, we get a large amount of quantum dots. I think that's the biggest advantage of our work," she said.
Further experimentation revealed interesting bits of information: The size of the dots, and thus their photoluminescent properties, could be controlled through processing at relatively low temperatures, from 80 to 120 degrees Celsius. "At 120, 100 and 80 degrees, we got blue, green and yellow luminescing dots," she said.
They also found the dots' edges tended to prefer the form known as zigzag. The edge of a sheet of graphene -- the single-atom-thick form of carbon -- determines its electrical characteristics, and zigzags are semiconducting.
Their luminescent properties give graphene quantum dots potential for imaging, protein analysis, cell tracking and other biomedical applications, Gao said. 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," Romero Alburto said. "In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too."
Co-authors include Angel Marti, assistant professor of chemistry and bioengineering, postdoctoral research associates Zheng Liu and Liehui Ge, senior research scientist Lawrence Alemany and graduate student Xiaobo Zhan, all of Rice; Rice alumnus Li Song of Shinshu University, Japan; Bipin Kumar Gupta of the National Physical Laboratory, New Delhi, India, who worked at the Ajayan Lab on an Indo-US Science and Technology Forum fellowship; Guanhui Gao of the Ocean University of China; research technician Sajna Antony Vithayathil of Baylor College of Medicine; Benny Abraham Kaipparettu, a postdoctoral researcher at Baylor College of Medicine; Takuya Hayashi, an associate professor of engineering at Shinshu University, Japan; and Jun-Jie Zhu, a professor of chemistry at Nanjing University.
The research was supported by Nanoholdings, the Office of Naval Research MURI program on graphene, the Natural Science Foundation of China, the National Basic Research Program of China, the Indo-US Science and Technology Forum and the Welch Foundation.
About Rice University
Located on a 300-acre forested campus in Houston, Rice University is consistently ranked among the nation's top 20 universities by U.S. News & World Report. Rice has highly respected schools of Architecture, Business, Continuing Studies, Engineering, Humanities, Music, Natural Sciences and Social Sciences and is known for its "unconventional wisdom." With 3,708 undergraduates and 2,374 graduate students, Rice's undergraduate student-to-faculty ratio is less than 6-to-1. Its residential college system builds close-knit communities and lifelong friendships, just one reason why Rice has been ranked No. 1 for best quality of life multiple times by the Princeton Review and No. 4 for "best value" among private universities by Kiplinger's Personal Finance. To read "What they're saying about Rice," go to www.rice.edu/nationalmedia/Rice.pdf.
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