Graphene and DNA: 'Wonder material' may hold key to fast, inexpensive genetic sequencing

But when scientists found a way—using, essentially, a piece of ordinary sticky tape—to peel off a layer of graphite that was just a single atom thick, they called the two-dimensional material graphene and, in 2010, won the Nobel Prize in physics for the discovery.

Now, researchers at the University of Delaware have conducted high-performance computer modeling to investigate a new approach for ultrafast DNA sequencing based on tiny holes, called nanopores, drilled into a sheet of graphene.

"Graphene is a two-dimensional sheet of carbon atoms arranged in a honeycomb pattern" Branislav Nikolic, associate professor of physics and astronomy, said. "The mechanical stability of graphene makes it possible to use an electron beam to sculpt a nanopore in a suspended sheet of graphene, as demonstrated in 2008 by Marija Drndic at the University of Pennsylvania.”

Graphene has been among the fastest-growing areas of study in nanoscience and technology over the past five years, Nikolic said. He calls it a wonder material that has remarkable mechanical, electronic and optical properties and is being investigated for a variety of applications as diverse as plastic packaging and next-generation gigahertz transistors.

In the sequencing that he and other physicists have proposed, a tiny hole a few nanometers in diameter is drilled into a sheet of graphene and DNA is threaded through that nanopore. Then, a current of ions flowing vertically through the pore or an electronic current flowing transversely through the graphene is used to detect the presence of different DNA bases within the nanopore.

“Since graphene is only one atom thick, the nanopore through which DNA is threaded has contact with only a single DNA base,” Nikolic said.

In 2010, three experimental teams—led by Jene Golovchenko of Harvard, Cees Dekker of Delft and Drndic—demonstrated DNA detection using nanopores in large-area graphene. However, Nikolic said, the process moved too quickly for the existing electronics to detect single DNA bases.

The new device concept proposed by the UD researchers uses graphene nanoribbons—thin strips of graphene that are less than 10 nanometers wide—with a nanopore drilled in their interior. Chemists, engineers, materials scientists and physicists have devised various methods over the past three years to fabricate nanoribbons with a specific zigzag pattern of carbon atoms along their edges, Nikolic said. Nanoribbons could enable fast and low-cost (less than $1,000) DNA sequencing, he said, because of the quantum-mechanically generated electronic currents that flow along those edges.

Such quick and inexpensive DNA sequencing could usher in an era of personalized medicine, Nikolic said.

"We used the knowledge acquired from several years of theoretical and computational research on the electronic transport in graphene to increase the magnitude of the detection current in our biosensor by a thousand to million times when compared to other recently considered devices," Nikolic said. "Two years ago, scientists would have told me our device was impossible, but there are so many people working on graphene that nothing is impossible anymore.

"Every time physicists think something is impossible, materials scientists or chemists come to the rescue—and vice versa."

Nikolic said he and postdoctoral researcher Kamal Saha have employed their home-grown massively parallel computational codes to simulate the operation of the proposed nanoelectronic biosensor from first principles, using the supercomputer Chimera that UD acquired with support from a National Science Foundation grant.

"This project has to run on 500-1,000 processors for several months continuously," he said. "We couldn't have done it without UD Chimera becoming fully operational in early 2011."

Nikolic, Saha and Drndic have recently published the results of this research in an article in the prestigious Nano Letters, a journal with an impact factor of 12.219 published by the American Chemical Society.  Colleagues, led by Drndic at the University of Pennsylvania, will now seek to fabricate the biosensors in their lab, guided by the simulations presented in the article. Nikolic said that this research synergy will, in turn, allow for simulations of improved device designs. 

Provided by University of Delaware (news : web)

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Graphene: Potential for modelling cell membrane systems

Graphene could also play an important role in the modelling of cell membranes. For example, the lipid bilayer is the fundamental structure of cell membranes, and the structure and dynamic of bilayer membranes govern the transport of materials and information in and out of cells.

Ryugo Tero and his colleagues in the Graphene Research Group at Toyohashi University of Technology have established a new procedure to fabricate artificial planar lipid membranes on graphene oxide (GO) and reduced graphene oxide (r-GO) as a means of detecting biomolecules such as lipids and proteins on and inside lipid bilayers.

An aqueous solution of GO was prepared by chemical exfoliation and dropped onto a thermally oxidized and cleaned SiO2/Si substrate (Fig.1A). The resulting GO/SiO2/Si was incubated in a vesicle suspension of phospholipid (dioleoylphosphatidylcholine: DOPC). Subsequent observation with an atomic force fluorescence microscopy (Fig.1B) and revealed the presence of two planar DOPC bilayer membranes stacked on GO with the assistance of calcium ion (5 mM), and that the DOPC bilayers on GO were fluid and continuous with the surrounding DOPC bilayers on the bare SiO2 surfaces (Fig. 1C).

Lipid bilayer/monolayer stacking structures were obtained on hydrophobic r-GO, which was produced by reducing GO with hydrazine vapour. Artificial lipid bilayers on graphene and its derivatives could be a new cell membrane model system for the researche on fundamental processes in cell membrane reactions.

These results will be a part of the presentation in MRS (Material Research Society) Spring Meeting 2012 at San Francisco on April 12 (Symposium EE: New Functional Nanocarbon Devices).

More information: Y. Okamoto, et al. 'Fabrication of Supported Lipid Bilayer on Graphene Oxide," IOP Journal of Physics: Conference Series (in press).

K. Tsuzuki, et al. 'Reduced Graphene Oxide as the Support for Lipid Bilayer Membrane,' IOP Journal of Physics: Conference Series (in press).

Provided by Toyohashi University of Technology

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Graphene battery demonstrated to power an LED

Researchers led by Zihan Xu of the Department of Applied Physics and Materials Research Centre at the Hong Kong Polytechnic University, attached silver and gold electrodes to a graphene sheet, typically 7 mm x 7 mm in area, mounted on a silicon substrate. The assembly was then immersed in a saturated solution of copper chloride (CuCl2), and was found to produce an electrical voltage of 0.35 V. They also found that six assemblies arranged in series produced enough electricity to power a light-emitting diode (LED). The device continued to produce around the same voltage for 25 days, but after a month it dropped to about 40 mV.

Graphene is a material that consists of a layer of carbon only one atom thick, and it has been the subject of intense research in recent years because of its unusual properties. One of these properties is an exceptionally high electron mobility.

Xu and colleagues write in their paper that they think the voltage arises from the kinetic energy of the copper ions in the copper chloride solution, which they say is enough to knock electrons out of the graphene, and that these electrons then flow through the sheet. They noted that the voltage increases when the copper chloride solution is heated, and varies with its concentration.

Enlarge

Experimental setup of six graphene devices connected with a commercial LED before (a) and after (b) it was lighted up. Image: arXiv:1203.0161v2

The group also found the voltage increased when the assembly was exposed to pulses of ultrasound, and they say this lends weight to the idea that kinetic energy is the source of the voltage, since the ultrasound would increase the velocity of the copper ions. Small voltages were also produced with ionic solutions such as NaCl and CuSO4. The researchers also carried out control experiments to rule out the possibility that chemical reactions were responsible for the voltage generated.

Dr. Wanlin Guo, the graduate supervisor of one of Xu’s team (Guoan Tai), expressed skepticism at the proposed mechanism, and added that he had so far been unable to reproduce the findings in his own experiments, in which he used graphene sheets of varying sizes, mounted on varying substrates, and with different kinds of electrodes. He was unable to achieve voltages greater than around 0.1 mV.

In 2011, a research group led by Nikhil Koratkar of New York’s Rensselaer Polytechnic Institute also reported on experiments in which graphene was shown to generate a voltage when an ionic solution was made to flow over the sheets. Dr. Guo, of Nanjing University in China, also refuted these results and carried out experiments that showed the interaction of the ions in solution with the electrodes was responsible for the voltage, rather than any interaction with the graphene.

If Xu’s “graphene battery” is harnessing the thermal energy of motion of the ions to generate electricity, this source of energy is essentially unlimited. The researchers say their experimental results provide a “huge breakthrough” in the research into self-powered technology.

More information: Self-Charged Graphene Battery Harvests Electricity from Thermal Energy of the Environment, arXiv:1203.0161v2 [cond-mat.mes-hall] http://arxiv.org/abs/1203.0161

Abstract
The energy of ionic thermal motion presents universally, which is as high as 4 kJbullet kg-1bullet K-1 in aqueous solution, where thermal velocity of ions is in the order of hundreds of meters per second at room temperature1,2. Moreover, the thermal velocity of ions can be maintained by the external environment, which means it is unlimited. However, little study has been reported on converting the ionic thermal energy into electricity. Here we present a graphene device with asymmetric electrodes configuration to capture such ionic thermal energy and convert it into electricity. An output voltage around 0.35 V was generated when the device was dipped into saturated CuCl2 solution, in which this value lasted over twenty days. A positive correlation between the open-circuit voltage and the temperature, as well as the cation concentration, was observed. Furthermore, we demonstrated that this finding is of practical value by lighting a commercial light-emitting diode up with six of such graphene devices connected in series. This finding provides a new way to understand the behavior of graphene at molecular scale and represents a huge breakthrough for the research of self-powered technology. Moreover, the finding will benefit quite a few applications, such as artificial organs, clean renewable energy and portable electronics.

? 2011 PhysOrg.com

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