Single step creates quantum dots in bulk for biomedical imaging

Ashley N. Paddock, ashley.paddock@photonics.com

A simple one-step chemical process for turning carbon fiber into semiconducting nanocrystals could prove useful in biomedical, optical 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 Rice University materials scientist who discovered the process in collaboration with colleagues at the Texas Medical Center and in China, India and Japan. "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?"

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

Quantum dots — semiconductors that contain a size- and shape-dependent bandgap — were discovered in the 1980s and are seen as promising for applications ranging from medical imaging devices to computers, LEDs, solar cells and lasers. The sub-5-nm carbon-based quantum dot analogues are produced in bulk through a wet chemical process discovered at Rice. The particles 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.

When attempting to selectively oxidize carbon fiber, the team ended up with asolution that, when analyzed under a transmission electron microscope, was seen to contain oxidized nanodomains of graphene extracted via chemical treatment of carbon fiber.

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 quantum dots, each smaller than 5 nm, easily passed through the cell membranes, showing their potential value for bioimaging.

"That was a complete surprise," said graduate student Wei Gao. "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 require 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.

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 quantum dot. Graphene quantum dots may find use in electronics, optical and biomedical applications.

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 that the quantum 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 studies, but perhaps not optimal for deep tissues in humans."

The quantum dots could help bioimaging, she said. "In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications too."

The results were published online Jan. 4 in Nano Letters (doi: 10.1021/nl2038979).