The Inside Dope; Opposites Interfere; When Off-Target is Right On; Live Broadcasts

The Inside Dope

A New Technique May Speed the Development of Molecular Electronics

Often, things can be improved by a little “contamination.” Steel, for example, is iron with a bit of carbon mixed in. To produce materials for modern electronics, small amounts of impurities are introduced into silicon—a process called doping. It is these impurities that enable electricity to flow through the semiconductor and allow designers to control the electronic properties of the material.

Scientists at the Weizmann Institute of Science, together with colleagues from the U.S., recently succeeded in being the first to implement doping in the field of molecular electronics—the development of electronic components made of single layers of organic (carbon-based) molecules. Such components might be inexpensive, biodegradable, versatile, and easy to manipulate. The main problem with molecular electronics, however, is that the organic materials must first be made sufficiently pure, and then ways must be found to successfully dope these somewhat delicate systems.

This is what Prof. David Cahen and postdoctoral fellow Dr. Oliver Seitz of the Weizmann Institute’s Material and Interfaces Department, together with Drs. Ayelet Vilan and Hagai Cohen from the Chemical Research Support Unit and Prof. Antoine Kahn from Princeton University, did. They showed that such contamination is indeed possible, after they succeeded in purifying the molecular layer to such an extent that the remaining impurities did not affect the system’s electrical behavior. The scientists doped the “clean” monolayers by irradiating the surface with ultraviolet (UV) light or weak electron beams, changing chemical bonds between the carbon atoms that make up the molecular layer. These bonds ultimately influenced electronic transport through the molecules.

This achievement was recently described in the Journal of the American Chemical Society. The researchers foresee that this method may enable scientists and electronics engineers to substantially broaden the use of these organic monolayers in the field of nanoelectronics. Dr. Seitz: “If I am permitted to dream a little, it could be that this method will allow us to create types of electronics that are different, and maybe even more environmentally friendly, than the standard ones that are available today.”

Prof. David Cahen’s research is supported by the Nancy and Stephen Grand Research Center for Sensors and Security; the Philip M. Klutznick Fund for Research; Mr. Yehuda Bronicki, Israel; Mr. and Mrs. Yossie Hollander, Israel; and the Wolfson Family Charitable Trust. Prof. Cahen is the incumbent of the Rowland Schaefer Professorial Chair in Energy Research.

Opposites Interfere

In a classic physics experiment, photons (light particles), electrons, or any other quantum particles are fired, one at a time, at a sheet with two slits cut in it that sits in front of a recording plate. For photons, a photographic plate reveals an oscillating pattern (bands of light and dark)—a sign that each particle, behaving like a wave, has somehow passed through both slits simultaneously and interfered, canceling the light in some places and enhancing it in others.

If single quantum particles can exist in two places at once, and interfere with themselves in predictable patterns, what happens when there are two quantum particles? Can they interfere with each other? Prof. Mordehai Heiblum of the Weizmann Institute’s Condensed Matter Physics Department and his research team have been experimenting with electrons fired across special semiconductor devices. Quantum mechanics predicts that two electrons can indeed cause the same sort of interference as that of a single electron on one condition: that the two are identical to the point of being indistinguishable. Heiblum and his team showed that, because of such interference, these two particles are entangled—the actions of one are inextricably tied to the actions of the other—even though they come from completely different sources and never interact with each other. The team’s findings recently appeared in the journal Nature.

Dr. Izhar Neder and Nissim Ofek, together with Drs. Yunchul Chung, Diana Mahalu, and Vladimir Umansky, fired such identical electron pairs from opposite sides of their device, toward detectors that were placed two to a side of the device. In other words, each pair of detectors could detect the two particles arriving in one of two ways: particle 1 in detector 1 and particle 2 in detector 2, or, alternatively, particle 2 in detector 1 and particle 1 in detector 2. Since these two “choices” are indistinguishable, the choices interfere with each other in the same way as the two possible paths of a single quantum particle interfere. The scientists then investigated how the choice of one particle affected the pathway taken by the other, and found strong correlations between them. These correlations could be affected by changing, for example, the length of the path taken by one particle. This is the first time an oscillating interference pattern between two identical particles has been observed, proving, once again, the success of quantum theory.

Prof. Mordehai Heiblum’s research is supported by the Joseph H. and Belle R. Braun Center for Submicron Research; the Wolfson Family Charitable Trust; Hermann Mayer and Dan Mayer; and Mr. Roberto Kaminitz, Sao Paulo, Brazil. Prof. Heiblum is the incumbent of the Alex and Ida Sussman Professorial Chair in Submicron Electronics.

When Off-Target is Right On

All organisms perform intricate molecular computations to survive. Unlike manmade computer components that are meticulously ordered on a chip, the molecules that make up biological “computers” are diffuse within the cell. Yet these must pinpoint and then bind to specific counterparts while swimming in the cell’s thick, erratic molecular stew—something like finding a friend in a Tokyo subway station during rush hour.

In the classical view of molecular recognition, the binding molecules fit each other like a lock and key. Half a century of research has shown, however, that in numerous cases, the molecules need to deform in order to bind, as the key is not an exact fit for the molecular lock. Why would evolution choose such an inexact system?

The work of Dr. Tsvi Tlusty and research student Yonatan Savir of the Weizmann Institute’s Physics of Complex Systems Department, suggests a possible answer. A simple biophysical model they developed indicates that, in picking out the target molecule from a crowd of look-alikes, the recognizer has an advantage if it’s slightly off-target. This may appear to be counterintuitive: Why search for a key that does not match its lock exactly, and then require that the imperfect key warp its shape to fit the lock? The researchers’ model shows that the key’s deformation actually helps in discerning the right target. Although the energy required to deform the molecular key slightly lowers the probability of its binding to the right target, it also reduces the probability that it will bind to a wrong one by quite a bit. Thus, the quality of recognition—i.e., the ratio of the right to wrong binding probabilities increases. The research was published recently in the journal PLoS ONE.

This simple mechanism is coined “conformational proofreading” and may explain the observed deformations in many biological recognition systems. Furthermore, conformational proofreading may turn out be a crucial factor affecting the evolution of biological systems, and it may also be useful in the design of artificial molecular recognition systems.

Dr. Tsvi Tlusty’s research is supported by the Clore Center for Biological Physics; the Asher and Jeannette Alhadeff Research Award; and the Philip M. Klutznick Fund for Research.

Live Broadcasts

To help molecular biologists in the difficult task of keeping abreast of current events in the world of cells and organisms, they employ reporter genes to “broadcast” specific happenings. For example, if a scientist is interested in the whereabouts and activities of a certain gene, the reporter “follows” it, and when this gene is activated in any way, the reporter gene produces an easily detectable protein, such as green fluorescent protein (GFP). The scientists are then able to “read” this “report” and learn about the specific events that are occurring and in what regions.

The light given off by these proteins is scattered in the tissue, however, reducing the resolution of many images. An alternative to fluorescent proteins is reporters that would be detectable via magnetic resonance imaging (MRI). But for most of the candidate reporters proposed so far, a second material needs to be administered in addition to the reporter gene to allow the MRI to detect its signals. Unfortunately, processes such as fetal development and those that take place within the central nervous system present barriers to these additional substances

Prof. Michal Neeman and Dr. Batya Cohen of the Weizmann Institute’s Biological Regulation Department, along with Ph.D. students Keren Ziv and Vicki Plaks and colleagues, have now developed genetically modified mice that carry a promising candidate reporter named ferritin, which could circumvent these problems. Ferritin works by sequestering iron from cells. When it is overexpressed, iron uptake increases, causing signal changes in the surrounding environment that can be detected by MRI, without the need to administer an additional substance.

As recently described in the journal Nature Medicine, ferritin has so far successfully broadcast live reports via MRI detection from the liver, endothelial cells, and even during fetal development in pregnant mice, without the need for additional substances.

Prof. Michal Neeman’s research is supported by the Clore Center for Biological Physics. Prof. Neeman is the incumbent of the Helen and Morris Mauerberger Chair in Biological Sciences.