Science model systems for "true" charge separation, Closs says. That capabil ity is important, he adds, since fast charge separation traditionally has been checked by optical spectroscopy, an "inherently poor" technique for true charge separation. In contrast, he says, "Our technique can't work at all without true charge separation." Other team members include Argonne chemist Arthur G. Kostka, University of Chicago graduate stu dent David E. Budil, and Colin A. Wraight of the University of Illinois, Urbana. Major funding came from the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy. Detailed ac counts of the work will appear in forthcoming issues οι Proceedings of the National Academy of Sciences. Ward Worthy, Chicago
Brandeis team devises chemical oscillators Like the beat of a heart or the pulse of a nerve cell, oscillating systems are a familiar and important part of the biological world. In chemistry they are less common and seem more bi zarre—but oscillating systems exist there, too. The known examples of chemical oscillators have puzzled and fasci nated researchers for half a century. Why they swing from one peak to another and back again, instead of moving uniformly toward a thermodynamically predetermined endpoint is not only intellectually intriguing, but may well provide some clues to the nature of stability and why nature hasn't u run down" over the mil lennia. What may turn out to be an im portant step to understanding chemical oscillators comes from re searchers at Brandeis University, Waltham, Mass. These chemists have successfully engineered a system de signed to oscillate. Previous chemical oscillators have been discovered by accident, borrowed from nature, or were only slight modifications of previously existing systems. The chemists report their work in the current issue of the Journal of the American Chemical Society [103, 2133 (1981)]. "The work is based on a partial list of criteria to describe an oscillating system," explains Irving R. Epstein, associate professor of chemistry at Brandeis. Some of these criteria are necessary for oscillation, some are only helpful, and the researchers 28
C&ENMay4, 1981
don't claim that their list is complete. But it is complete enough that when a system is constructed that meets these criteria, it will oscillate. So far, the researchers have used their criteria to generate one funda mentally different oscillating chemi cal system and about a dozen slight modifications of that basic system. "We think this was not a matter of pure luck, and the method should work again," Epstein says. "Of course, the approach will not be fully dem onstrated until we have used it to make more than one fundamentally different system." Others working on the project with Epstein are Kenneth Kustin of Brandeis, Patrick De Kepper of the Paul Pascal Center for Research, Domaine University in France, who is currently working at Brandeis, and Miklos Orban of the Institute of In organic and Analytical Chemistry of Eotvos University, Budapest, Hun gary. Oscillation in the chemical sense means that the concentration of at least one of the components in a sys tem rises and falls dramatically over a period of time. It sometimes fluc tuates spatially, too, forming con centric rings of high reagent concen tration in an unstirred reaction vessel. Such a system, the Brandeis chemists say, must first of all be far from equilibrium or the oscillations will not be sustained. It also is helpful for the system to have two relatively stable steady states so that an oscil lation can consist of swinging from a system dominated by one of these states to a system dominated by the other. Such systems are called bi stable. The third property that is needed is a feedback mechanism to swing the system from one bistable state to the other. Although more elaborate feedback mechanisms do exist, an autocatalytic reaction frequently serves as the feedback step in chemi cally oscillating systems. The scientists keep their system far from equilibrium by carrying out their experiments in a continuousflow stirred tank reactor. They find an autocatalytic reaction that will give rise to two stable steady states under the same set of external crite ria. Once such a system has been identified, the next step is to seek a reaction that perturbs the stability of the two stable branches on a suitable time scale so that oscillation takes place. Thus the problem has been broken down into several pieces that can be tackled one at a time. In the Brandeis system, the auto
catalytic step is the arsenite-iodate reaction: 3H 3 As0 3 + I 0 3 - — 3H 3 As0 4 + I " This reaction is autocatalyzed by io dide ion. Iodide ion also will react with chlorite ion in a reaction that is autocatalyzed by iodine: 4H+ + C10 2 - + 4 1 — 2H 2 0 + CI" + 2I 2 Adding chlorite ion to the arseniteiodate system generates a system that oscillates with periods between 15 seconds and four minutes under the conditions of the experiment. This system is the prototype of a family of chemical oscillators, all of which contain both chlorite and iodate ions, but which use a variety of agents in place of arsenite ion. Ascorbic or malonic acid, potassium ferrocyanide, sodium sulfite, and so dium thiosulfate can all be used in place of arsenite to produce these systems, they find. The common feature appears to be that these reactants can participate in the au tocatalyzed reduction of iodate ion to iodide ion. The chemists see two features of their approach that may be widely applicable. The first is a list of criteria for such a system that is sufficiently simple and general that it should apply to a wide range of reactions. And secondly, this method splits the task of finding new oscillators into two more tractable problems: the discovery of bistable systems, and the design of suitable feedback reactions. They are hopeful that by using this method they and others will be able to develop a large enough variety of chemically oscillating systems to really come to understand them. Rebecca Rawls, Washington
Heavy ions used in micrography technique A technique with the potential for producing high-resolution micro graphs of intact, living cells has been developed by researchers at Lawrence Berkeley Laboratory. Dubbed heavy-ion microscopy, the technique uses a monoenergetic beam of neon or argon ions to produce a three-di mensional replica of a sample in a sheet of mica. The magnification potential of heavy ions has been recognized for nearly two decades, according to LBL's Cornelius A. Tobias, who de veloped the technique in collabora tion with Gerhard Kraft of the GSI
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Laboratory in Darmstadt, West Germany, and LBL researchers Tracy Yang, Eugene Benton, and Thomas Hayes in work supported by the De partment of Energy. A microscope's resolution is proportional to the wavelength of the illuminating ra diation, Tobias explains. Particles with higher mass have shorter wave lengths, and hence theoretically can provide greater resolution. However, heavy ions are difficult to focus, which blocked their use in mi croscopy. The technique developed by Tobias and Kraft circumvents the focusing problem. The sample is mounted or, in the case of some cells, grown on a mica sheet. The 1-MeV beam of ions passes through the sample, where the main interaction consists of collisions between the ions and sample electrons. These collisions cause the ions to lose energy. Because of the differential energy loss, the ions penetrate the mica to varying depths. Treatment with concentrated hydrofluoric acid dis solves the mica that has been pene trated by the ions, leaving a con toured impression of the sample. This impression can be photographed through a light microscope or with a scanning electron microscope. Although the resolution of heavyion microscopy is no better than electron microscopy, Tobias says, the technique has advantages. Electrons are easily scattered and can be used to look only at the surface or a thin seg ment of a sample. Heavy ions pass through much thicker samples. But more important, electron mi croscope specimens are mounted in a vacuum, which means they must be dehydrated. A heavy-ion beam oper ates in air, so there is the potential to look at wet mounted, living cells. D
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