RICE (US) — Scientists have developed a one-step chemical process to turn carbon fibers into graphene quantum dots.
The Rice University researchers say the new method is markedly simpler than established techniques for making graphene quantum dots—tiny specks of matter expected to prove useful in electronic, optical, and biomedical applications.
“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,” says Pulickel Ajayan, 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 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,” says Wei Gao, a Rice graduate student. “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 says.
“We call them quantum dots, but they’re two-dimensional, so what we really have here are graphene quantum discs.” Gao says 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 says.
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 says.
As reported in the journal Nano Letters, 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 says. 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 cytoplasm and did not interfere with the cells’ proliferation.
“The green quantum dots yielded a very good image,” says co-author Rebeca Romero Aburto, a graduate student in the Ajayan lab.
“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 says. “In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too.”
Collaborators include additional researchers at Rice; Baylor College of Medicine; Shinshu University, Japan; the National Physical Laboratory, India; the Ocean University of China; and Nanjing University, China.
The research was supported by Nanoholdings, the Office of Naval Research, the Natural Science Foundation of China, the National Basic Research Program of China, the Indo-US Science and Technology Forum, and the Welch Foundation.
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