AIP’s Physics News HighlightsMay 2, 2012

1. Electronic Nose Out In Front

Chemical sensors are exceedingly good at detecting a single substance or a class of chemicals, even at highly rarified concentrations. Biological noses, however, are vastly more versatile and capable of discriminating subtle cues that would confound their engineered counterparts. Unfortunately, even highly trained noses do leave a certain ambiguity when relaying a signal and are not particularly suited for work in specialized situations like operating rooms. A new DNA-based chemical sensor appears to be both extremely sensitive and discerning, making it an important stride on the path to an all-electronic nose.

A team of researchers report in a paper published in the American Institute of Physics’ journal AIP Advances that specially tailored strands of DNA attached to carbon nanotubes can tell the difference between very similar molecules, even those that have an identical chemical makeup. “We’re trying to develop this into an electronic nose system,” says A.T. Charlie Johnson, a physicist at the University of Pennsylvania and study co-author. “We used this system to distinguish between optical isomers, molecules that are nearly identical except that one is structurally reversed – a mirror image.”

The system works by affixing DNA strands to carbon nanotubes, which are excellent electrical conductors. The DNA strands have been fine-tuned to respond to particular chemicals, so when strands come in contact with a target chemical – even at very low concentrations – it produces a measurable electrical signal along the nanotube. The sensors were able to check for molecules that differ by as little as one carbon atom. Though the researchers are not the first to observe this effect, they have achieved an unprecedented level of differentiation for an all-electronic chemical detector. “What I’m focusing on is the size of the difference in the signal,” says Johnson.

The researchers are next interested in creating something akin to an actual electronic nose consisting of many individual DNA-based sensors performing the same role as an olfactory receptor. The goal is to have a system that is highly versatile and sensitive with wide-scale applications. For example, the chemical dimethylsulfone is associated with skin cancer. The human nose cannot detect this volatile but it could be detected with the new sensor at concentrations as low as 25 parts per billion.

Acknowledgements: This work was supported by the Army Research Office and by the Nano/Bio Interface Center through the National Science Foundation.

Article: “DNA-decorated carbon nanotube-based FETs as ultrasensitive chemical sensors: Discrimination of homologues, structural isomers, and optical isomers” is published in AIP Advances.

Authors: S.M. Khamis (1), R.A. Jones (1), A.T. Charlie Johnson (1), G. Preti (2,3), J. Kwak (2), and A. Gelperin (2,4).(1) Department of Physics and Astronomy, University of Pennsylvania, Philadelphia (2) Monell Chemical Senses Center, Philadelphia(3) Department of Dermatology, University of Pennsylvania(4) Princeton Neuroscience Institute, Princeton University, New Jersey

2. Tiny Channel Cleanses Blood

Margination, the natural phenomenon where bacteria and leukocytes (white blood cells) move toward the sides of blood vessels, is the inspiration for a novel method for treating sepsis, a systemic and often dangerous inflammatory response to microbial infection in the blood. A team of researchers at the Massachusetts Institute of Technology and the National University of Singapore has designed a branchlike system of microfluidic channels, 20 micrometers (20 millionths of a meter, or about one-fifth the size of a grain of sand) high by 20 micrometers wide, that mimic the marginizing action of vessels on bacteria and inflammatory cellular components (leukocytes and platelets) to separate them from red blood cells. The microchannel network is etched onto a polymer chip by the same techniques used for manufacturing integrated circuits. As infected whole blood flows through the first part of the microchannel, red cells migrate toward the center while the unwanted cell types flow toward the side walls. Like a biological railway junction, the second part of the microchannel is divided into three branches with red cells taking the middle path and the marginated microbes, leukocytes, and platelets moving into the two outer ones. A second three-branch junction further purifies the red cell fraction, which could then be returned to a patient in a real-life situation. In their experiment with the prototype device, the researchers demonstrated highly efficient removal of the bacteria Escherichia coli (80 percent) and the yeast Saccharomyces cerevisiae (90 percent) as well as a greater than 80 percent depletion of inflammatory cellular components. The researchers also designed and tested a larger blood cleansing system consisting of six microfluidic channel networks in parallel. Currently, they are conducting a small-scale animal test to validate the efficacy of the technique in vivo.

Acknowledgements: This work was supported financially by the Singapore-MIT Alliance for Research and Technology (SMART) Centre (BioSyM IRG), the DARPA DLT (Dialysis- Like Therapeutics) program, and the Agency for Science, Technology and Research (A*STAR) Singapore.

Article: “A microfluidics approach towards high-throughput pathogen removal from blood using margination” is accepted for publication in Biomicrofluidics.

Authors: Han Wei Hou (1,2), Hiong Yap Gan (1,3), Ali Asgar S. Bhagat (2,#), Leon D. Li (1,4), Chwee Teck Lim (2,5,6), and Jongyoon Han (1,2,7).(1) Department of Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts(2) BioSystems and Micromechanics (BioSyM) IRG, Singapore-MIT Alliance for Research and Technology (SMART) Centre, Singapore(3) A*STAR Singapore Institute of Manufacturing Technology (SIMTech), Singapore(4) Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts(5) Department of Bioengineering, National University of Singapore, Singapore(6) Mechanobiology Institute, National University of Singapore, Singapore(7) Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts # Current Address: Clearbridge BioMedics Pte Ltd, Singapore

3. Mining for Heat

Underground mining is a sweaty job, and not just because of the hard work it takes to haul ore: Mining tunnels fill with heat naturally emitted from the surrounding rock. A group of researchers from McGill University in Canada has taken a systematic look at how such heat might be put to use once mines are closed. They calculate that each kilometer of a typical deep underground mine could produce 150 kW of heat, enough to warm 5 to 10 Canadian households during off-peak times.

A number of communities in Canada and Europe already use geothermal energy from abandoned mines. Noting these successful, site-specific applications, the McGill research team strove to develop a general model that could be used by engineers to predict the geothermal energy potential of other underground mines. In a paper accepted for publication in the American Institute of Physics’ Journal of Renewable and Sustainable Energy, the researchers analyze the heat flow through mine tunnels flooded with water. In such situations, hot water from within the mine can be pumped to the surface, the heat extracted, and the cool water returned to the ground. For the system to be sustainable, heat must not be removed more quickly than it can be replenished by the surrounding rock. The team’s model can be used to analyze the thermal behavior of a mine under different heat extraction scenarios.

“Abandoned mines demand costly perpetual monitoring and remediating. Geothermal use of the mine will offset these costs and help the mining industry to become more sustainable,” says Seyed Ali Ghoreishi Madiseh, lead author on the paper. The team estimates that up to one million Canadians could benefit from mine geothermal energy, with an even greater potential benefit for more densely populated countries such as Great Britain.

Acknowledgements: The authors acknowledge support from Vale Company and the Mitacs Accelerate program.

Article: “Sustainable heat extraction from abandoned mine tunnels: a numerical model” is accepted for publication in Journal of Renewable and Sustainable Energy.

Authors: S.A. Ghoreishi Madiseh (1), Mory M. Ghomshei (2), F.P. Hassani (2), and F. Abbasy (2).(1) Department of Mechanical Engineering, McGill University, Montreal, Canada.(2) Department of Mining and Materials Engineering, McGill University, Montreal, Canada.

4. Wired for Avalanches – and Learning

The brain’s neurons are coupled together into vast and complex networks called circuits. Yet despite their complexity, these circuits are capable of displaying striking examples of collective behavior such as the phenomenon known as "neuronal avalanches," brief bursts of activity in a group of interconnected neurons that set off a cascade of increasing excitation. In a paper published in the American Institute of Physics' journal Chaos, an international team of researchers from China, Hong Kong, and Australia explores connections between neuronal avalanches and a model of learning – a rule for how neurons “choose” to connect among themselves in response to stimuli. The learning model, called spike time-dependent plasticity, is based on observations of real behavior in the brain.

The researchers’ simulations reveal that the complex neuronal circuit obtained from the learning model would also be good at generating neuronal avalanches. This agreement between the model and a real, proven behavior of neurons suggests that the learning model is an accurate way to describe how the brain processes information. The authors say their work could aid an understanding of how learning could lead to the formation of cortical structures in the brain, as well as why the resulting structures are so efficient at processing large amounts of information. “While [the finding] is entirely consistent with existing neurophysiology, our work is the first to provide this concrete link” between this particular learning rule and neuronal avalanches, says co-author Michael Small of the University of Western Australia. “It provides a simple, and therefore perhaps surprising, explanation for how a system as complex as the cortex can generate such striking collective behavior.”Acknowledgements: This research was supported by a Hong Kong University Grants Council, Competitive Earmarked Research Grant.

Article: “Neuronal avalanches of a self-organized neural network with active-neuron-dominant structure” is published in Chaos.

Authors: Xiumin Li (1,2) and Michael Small (3,2).(1) College of Automation, Chongqing University, China(2) Department of Electronic and Information Engineering, Hong Kong Polytechnic University(3) School of Mathematics and Statistics, University of Western Australia

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Upcoming Conferences of Interest- The Optical Society’s Conference on Lasers and Electro-Optics (CLEO) will be held May 6 – 11, 2012, in San Jose, Calif.http://www.cleoconference.org/- The Acoustical Society of America’s 163rd meeting will be held May 13 – 18, 2012, in Hong Kong.http://acousticalsociety.org/- The American Astronomical Society’s 220th meeting will be held June 10 – 14, 2012, in Anchorage, Alaska.http://aas.org/meetings/aas220

Physics Today: May Articleshttp://www.physicstoday.org

1. Molding the flow of light: Photonics in astronomy: Light gathered and focused by a telescope must often be refocused onto spectrographs and other complex instruments. To such ends, astronomers are coming to realize the benefits of photonics.2. Insights from the classical atom: Decades after the 1920s rise of quantum mechanics, the classical mechanical framework remained a useful lens through which to examine ionization, scattering, and other atomic processes.3. A tale of openness and secrecy: The Philadelphia Story: A now little-known manuscript prepared by nine young physicists as a statement about the futility of scientific secrecy quickly became a test of the limits of free discourse in the nuclear age.

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