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  • One-step process turns carbon fibers into graphene QDs

Photonics Spectra
Mar 2012
Ashley N. Paddock, ashley.paddock@photonics.com

HOUSTON – Common carbon fiber can be turned into graphene quantum dots (QDs) in a one-step chemical process that is much simpler than established techniques. This discovery could prove useful for optical, biomedical and electronic 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,” said Pulickel Ajayan, a materials scientist at Rice University. “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?” Ajayan’s lab collaborated with colleagues in China, India, Japan and the Texas Medical Center to discover the process.

Quantum dots, discovered in the 1980s, are semiconductors that contain a size- and shape-dependent bandgap. These have been promising structures for applications that range from computers, LEDs, solar cells and lasers to medical imaging devices. The sub-5-nm carbon-based QDs produced in bulk through the new wet chemical process are highly soluble, and their size can be controlled via the temperature at which they’re created.


This transmission electron microscope image shows a graphene quantum dot with zigzag edges. The QDs can be created in bulk from carbon fiber through a chemical process discovered at Rice University. Courtesy of Ajayan Lab/Rice University.


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,” said Wei Gao, a 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 oxidized nano-domains 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 that other techniques are expensive, and they 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 that the size of the QDs, and their photoluminescent properties, could be controlled through processing at relatively low temperatures, from 80 to 120 °C. “At 120, 100 and 80 degrees, we got blue, green and yellow luminescing [quantum] dots,” she said.


Dark spots on a transmission electron microscope grid are graphene quantum dots made through a wet chemical process at Rice University. The inset is a close-up of one QD. Graphene QDs may find use in electronics, optical and biomedical applications.


They also found that the QDs’ edges tended to prefer the form known as zig-zag. The edge of a sheet of graphene – the single-atom-thick form of carbon – determines its electrical characteristics, and zigzags are semiconducting.

Luminescent properties give graphene quantum dots the 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 that the QDs 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 Rebeca Romero Aburto, a graduate student in the Ajayan Lab who also studies at MD Anderson. “The advantage of graphene [quantum] 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 studies, but perhaps not optimal for deep tissues in humans.”


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 QDs for four hours. The QDs, each smaller than 5 nm, easily passed through the cell membranes, showing their potential value for bioimaging.


The quantum dots could be an interesting approach for further exploration of bioimaging, she said. “In the future, these graphene QDs could have high impact because they can be conjugated with other entities for sensing applications too.”

The results were published online in Nano Letters (doi: 10.1021/nl2038979). 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.


GLOSSARY
fluorescence
The emission of light or other electromagnetic radiation of longer wavelengths by a substance as a result of the absorption of some other radiation of shorter wavelengths, provided the emission continues only as long as the stimulus producing it is maintained. In other words, fluorescence is the luminescence that persists for less than about 10-8 s after excitation.
optical
Pertaining to optics and the phenomena of light.
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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