In this research supported by the Analytical and Surface Chemistry Program, the goals are to understand the role played by fluctuating chemical equilibrium in developing signals at nanostructured chemical sensors; and to use this understanding to develop new electrochemical routes to chemical sensing and to fabricating stimulus-responsive structures. A key characteristic of nanostructures is the role played by fluctuations in populations of reactants and products at equilibrium - fluctuations that are significant only when the number of molecules participating in the reaction is small. This project addresses these fundamental issues of chemical reactivity by first developing robust fabrication methods to produce atom-scale junctions (ASJs) - metallic wires a few atoms wide. Then, the ASJs are used to study fluctuating adsorption and desorption of electron donor molecules and electrochemical processes that can be used either for chemical sensing or to template nanoscale patterns into supermolecular architectures.
The broader impact of the project will be felt principally through human resource development. Ongoing activities such as the Beginning Faculty Workshop at PittCon and the Summer Program at Clark Atlanta University will be continued. In addition, a substantial international experience for graduate students will be developed as part of the project. The students will collaborate with members of the Life Sciences Interface group at the Tyndall National Institute (TNI) in Cork, Ireland and will spend up to 30 days each year at the TNI. The success of the proposed international component will be judged by the number of jointly authored conference presentations and peer-reviewed publications, how effectively the seed funding is leveraged to obtain funding for follow-on joint efforts between the two labs and by exit interviews.
The principal focus of this project was to develop new kinds of ultrasmall electrical and electrochemical structures capable of manipulating molecules. Being so small they can handle tiny amounts of chemicals, even down to a single molecule – or an amount equivalent to a billionth of a billionth of single human hair. One kind of structure, built by a variant of electroplating, is a very small wire which, at its narrowest point, may consist of a chain of single gold atoms, called an atom-scale junction, or ASJ. The process by which current is conducted in these ASJs is very different than that in larger structures. In fact, electrons can move from one side of the wire to the other at a rate limited only by the intrinsic quantum properties of the electrons themselves. In order to make the ASJs, a new kind of electroplating had to be invented in which individual gold atoms could be added to the growing wire one-at-a-time. Interest in making these structures was spurred by the knowledge that they can be exceptionally sensitive chemical sensors, responding, for example, to the presence of toxins or pollutants. Another ultrasmall structure built in the project is a new type of electrochemical reaction vessel - capable of producing minute, but controllable, amounts of chemical products on demand. These products are available for a subsequent reaction, but because they are produced in an ultrasmall volume the distance they must travel to reach the reaction site is tiny, and the subsequent reaction can be enhanced relative to the same system in a bench-scale vessel. Chemists are interested in how reactions occur and how they can be made more useful. One key descriptor, the reaction rate, controls how much useful product can be made in a given time interval. At its most basic the rate is determined by two elementary steps: the transport of reactants to the site of reaction and their subsequent combination to produce a product. This laboratory is interested in how artificial nanostructures, with volumes as small as a millionth of a millionth of a drop, can be used to enhance reaction rates both by limiting the distance that reactants have to travel to find a suitable reaction partner and by enhancing the behavior of catalysts in ultrasmall containers. Ultimately, there is interest in using one set of electrodes to produce a reactant which can then be consumed with high efficiency at another electrode placed adjacent to it. There are many situations where having enhanced reaction velocities is important. One specific example being pursued uses the ultrasmall electrochemical reaction vessel to produce molecular hydrogen which can then be combined with environmental pollutants, such as perchlorate or nitrate, at a downstream catalyst. Because the reactant does not have to travel far, it can be used very efficiently. This project focused on the development of functional nanostructures and methods of measurement, which are critical for generating portable, economical and disposable devices for chemical and biological sensing and environmental detection. In order to broaden its impact on the larger scientific and professional community, this research project supported active exchanges with international research groups - including hosting long-term visits from researchers at the Tyndall National Institute (TNI) in Cork, Ireland, the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan and the School of Materials Science and Engineering, Harbin Institute of Technology, China. The collaboration combines complementary skills and capabilities, and the exposure to international scientists gained by the US students will be invaluable in preparing them for their future careers.
