Professor Paul W. Bohn of the University of Notre Dame is supported by the Chemical Measurement and Imaging Program in the Division of Chemistry to develop multifunctional nanoscale fluidic devices. The project investigates how devices can be constructed so that a single structure can simultaneously switch fluid flow, sense the presence of specific molecules and subsequently subject these molecules to treatment - perhaps remediation of an environmental pollutant or destruction of a harmful biomolecule circulating in the body. Electrical potentials applied to different regions of the device cause small volumes of fluid to move through specific channels for sensing and treatment. The projects seek to combine this electrically-activated fluid flow with electrochemical reactions and with sensitive methods of chemical sensing - based either on the luminescence of the molecules after electrochemical reaction or the interaction of high frequency excitations in metals with the surrounding fluid. One critical challenge in carrying out the electrochemical reactions is that they must occur far from ground potential, where such reactions routinely are carried out. Thus, the project aims to develop new ways to perform electrochemical transformations at arbitrarily large voltages. The second challenge is that because the structures are so small, only a tiny number of molecules may react at any given time. Thus, the project seeks to develop new highly sensitive methods of following electron transfer reactions that are applicable to ultrasmall samples, even to single molecules.
The broader impact of this program on society is felt in three ways. First, the scientific work addresses the control and manipulation of fluid flow in miniature analytical devices, a problem of growing interest to the international scientific community. Second, the scientific work contributes to scientific human resource development through the involvement of a diverse range of students with participation from under-represented groups and women. The third area lies in helping students learn about the international dimension of science through a student exchange with one of the world's leading bioelectrochemistry laboratories at Cambridge University, Cambridge, UK.
The principal focus of this project was to develop and study new kinds of ultrasmall electrical and electrochemical structures capable of manipulating very small amounts of matter – typically from 1 to 1,000,000 molecules. The structures used to handle these samples were of nanometer dimensions in either 2 (1-D nanostructures) or all 3 (0-D nanostructures) dimensions, and electrodes capable of carrying out electron transfer reactions were built into them. The simplest structures were constructed as two-electrode 0-D reactors for highly sensitive chemical analysis. By incorporating two electrodes into a nanopore with a volume of a billionth of a billionth of a liter, molecules could be passed back and forth between the electrodes very rapidly, allowing the same molecule to be measured many times before it finally escaped the pore by diffusing out the top, thus greatly enhancing the sensitivity. This approach to nanometer scale electrochemistry was shown to enhance the sensitivity of electrochemical measurements and in some cases to help with selectivity, i.e. discriminating the desired signal against a background of unwanted interfering molecules. Another set of electrochemical reactors were fashioned from 1-D nanostructures which allowed a very useful kind of coupling between fluid flow and surface electrochemical reactions. By constructing the electrode in the form of an annular ring inside of the 1-dimensional nanofluidic structure, very large reaction velocities were achieved – in fact, under optimum conditions, it was possible to convert every molecule brought to the electrode surface - limited only by the rate at which they could be delivered. The key scientific problem that was solved to enable this high efficiency processing involved establishing very small, but precise, electrochemical potential differences, e.g. 0.5 volt, at arbitrarily high potentials – hundreds to thousands of volts across the pore. A different kind of nanostructure, 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. However, the presence of molecules at or near the surface of the ASJ can affect the transmission of the electron through the junction. We studied the fluctuations – essentially the chemical noise – associated with the adsorption and desorption of molecules on the ASJs as a way of understanding the chemical dynamics of small populations of molecules. Furthermore, we were able to do these experiments with electrochemical potential control and show that making the junction more oxidizing increased the fluctuations, which points the way to making and breaking these small nanowires controllably. 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. 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, environmental detection, and new types of electronic and photonic devices. In order to broaden its impact on the larger scientific and professional community, this research project was linked to the creation of a new kind of program in Analytical Sciences & Engineering in which chemical measurement science is developed as a multi-department, multi-college activity. By bringing together students and scholars with diverse backgrounds and preparation to focus on advanced analytical measurements, we aspire to achieve better and more diverse training and ultimately to improve the quality of the science itself.
