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Research Alert: Nov. 19, 2013

Graphene nanoribbons for ‘reading’ DNA

If we wanted to count the number of people in a crowd, we could make on the fly estimates, very likely to be imprecise, or we could ask each person to pass through a turnstile. The latter resembles the model that EPFL researchers have used for creating a “DNA reader” that is able to detect the passage of individual DNA molecules through a tiny hole: a nanopore with integrated graphene transistor.

Direct electrical readout from the graphene transistors is used to detect DNA translocation events. Nanopore, DNA and the graphene nanoribbon are shown in this schematic (which is not to scale).

The DNA molecules are diluted in a solution containing ions and are driven by an electric field through a membrane with a nanopore. When the molecule goes through the orifice, it provokes a slight perturbation to the field, detectable not only by the modulations in ionic current but also by concomitant modulation in the graphene transistor current. Based on this information, it is possible to determine whether a DNA molecule has passed through the membrane or not.

This system is based on a method that has been known for over a dozen years. The original technique was not as reliable since it presented a number of shortcomings such as clogging pores and lack of precision, among others. “We thought that we would be able to solve these problems by creating a membrane as thin as possible while maintaining the orifice’s strength,” said Aleksandra Radenovic from the Laboratory of Nanoscale Biology at EPFL. Together with Floriano Traversi, postdoctoral student, and colleagues from the Laboratory of Nanoscale Electronics and Structures, she came across the material that turned out to be both the strongest and most resilient: graphene, which consists of a single layer of carbon molecules. The strips of graphene or nanoribbons used in the experiment were produced at EPFL, thanks to the work carried out at the Center for Micro Nanotechnology (CMI) and the Center for Electron Microscopy (CIME).

“Through an amazing coincidence, continued the researcher, the graphene layer’s thickness measures 0.335 nm, which exactly fits the gap existing between two DNA bases, whereas in the materials used so far there was a 15 nm thickness.” As a result, while previously it was not possible to individually analyze the passage of DNA bases through these “long” tunnels – at a molecular scale –, the new method is likely to provide a much higher precision. Eventually, it could be used for DNA sequencing.

However they are not there yet. In only five milliseconds, up to 50,000 DNA bases can pass through the pores. The electric output signal is not clear enough for “reading” the live sequence of the DNA strand passage. “However, the possibility of detecting the passage of DNA with graphene nanoribbons is a breakthrough as well as a significant opportunity”, said Aleksandra Radenovic. She noted that, for example, the device is also able to detect the passage of other kinds of proteins and provide information on their size and/or shape.

Taking a new look at carbon nanotubes

Despite their almost incomprehensibly small size – a diameter about one ten-thousandth the thickness of a human hair – single-walled carbon nanotubes come in a plethora of different “species,” each with its own structure and unique combination of electronic and optical properties. Characterizing the structure and properties of an individual carbon nanotube has involved a lot of guesswork – until now.

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have developed a technique that can be used to identify the structure of an individual carbon nanotube and characterize its electronic and optical properties in a functional device.

New way to dissolve semiconductors holds promise for future of electronics

Semiconductors could become even more versatile as researchers make headway on a novel, inexpensive way to turn them into thin films. Their report on a new liquid that can quickly dissolve nine types of key semiconductors appears in the Journal of the American Chemical Society.

Richard L. Brutchey and David H. Webber note that making low-cost, semiconducting thin films on a large scale holds promise for improving a number of electronic applications, including solar cells. The problem has been finding a liquid that can dissolve semiconductors so that they can be subsequently solution-processed using inexpensive methods. Hydrazine can do the trick for many of these materials, but as a compound that is sometimes used in rocket fuel, it is explosive and highly toxic. It’s also a poor option for making semiconducting thin films en masse. Brutchey and his team decided to search for a safer solution.

They found an answer in a mixture of two compounds that could dissolve a set of important semiconducting materials called chalcogenides at room temperature and normal air pressure. The researchers state, “We believe these initial results indicate that the chemistry can be further extended to other families of chalcogenide materials and may hold promise for applications that would benefit from solution deposition of semiconductor thin films.”

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