NaPiDro - Nanoelectrodes in Picoleter Droplets: Towards Single Molecule Chemistry

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In this proposal the applicants aim for understanding the redox behavior of small ensembles of molecules -- eventually down to single molecules. We will use electrochemical techniques as they allow, in contrast to optical techniques, the observation of important reactions without introducing large quantities of thermal energy into the studied system. A particular interesting reaction is the oxygen reduction reaction (ORR) which is important in biology as well as in a number of technical systems. Two aspects of this reaction will be addressed: i) We would like to observe and describe the formation and follow-up reaction of reactive oxygen species (ROS). The formation of such compounds, such as superoxide, is well documented for organic electrolytes. There are more and more indications that those species also form in aqueous environment but decompose very quickly or enter into follow-up reactions. The nanoscopic dimension of the new type of electrochemical cell proposed in this project will enable the detection of such molecules before they enter into follow-up reactions or diffuse to the solution bulk. ii) We aim for quantifying the catalytic activity of a small number of enzyme molecules (laccase and bilirubin oxidase) for oxygen reduction and follow this activity over time in order to understand their deactivation (gradual vs. stepwise deactivation of single enzyme molecules). The required sensitivity and limits of detections will be reached by combining two achievements of the applicant groups, namely the use and structural characterization of nanoelectrodes of variable materials and arrangements with a new technique of dispensing attoliter to nanoliters droplets with immediate positional manipulation of the formed droplets. This ability will be used to direct such small volumes of electrolytes onto electrochemical cells formed by micrometer- and nanometer-sized working and counter electrodes. This will form a hitherto unknown combination of a nanometer-sized electrode (or electrochemical cell) with an adjustable liquid volume of 1E-18 to 1E-9 L. The droplet provides an environment within which extremely fast mass transport is possible so that either redox cycling can be used to reach the sensitivity required for activity analysis of single enzyme molecules over time or to achieve a total electrochemical conversion within short time. With respect to short-lived species the microscopic liquid phase of the droplet prevents their diffusional escape or spreading and allows the build-up of measurable concentrations with a low amount of converted substances.