Dynamics of Excited States in DNA Strands and DNA-Silver Nanoclusters

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Silver ions (Ag+) can bind to the nitrogen and oxygen atoms in DNA giving rise to unusual and interesting molecular structures. DNA acts as a scaffold that precisely controls the positions of individual silver ions. When another chemical reagent is added to provide electrons to the Ag+ ions, the array of Ag+ ions in the DNA become small clusters containing no more than a few dozen metal atoms. These miniscule clusters have remarkable optical properties, including for example fluorescence. Fluorescence is a process by which a molecule absorbs light at one wavelength and shortly afterwards emits light of another color. This is a property that macroscopic silver objects do not possess. Their fluorescence properties makes these silver clusters attractive for sensing and imaging applications. However, despite widespread interest in these clusters, very little is known about what controls their light emission. In this project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, Professor Bern Kohler of The Ohio State University and students working under his direction are investigating DNA-metal assemblies that are excited by short laser pulses (a few hundred femtoseconds, where a femtosecond is one quadrillionth of one second). The Kohler team observes how the color of the light emitted from the silver clusters depends on the color and time interval of the impinging laser pulse. Under some conditions, no light is emitted at all. These studies are revealing the mysteries of how light energy is absorbed by small metal clusters, and either emitted again as light or dissipated into the clusters' surroundings. The undergraduate and graduate students involved in this project are gaining experience in state-of the-art laser techniques, and the kinds of analysis needed to solve problems at the interface of spectroscopy and nanoscience. This multidisciplinary training is important for upholding the nation's workforce of scientists. This project targets fundamental understanding of excited electronic states formed by UV and visible excitation of DNA-metal complexes and nanostructures. Silver ions coordinate to the nitrogen and oxygen atoms of the nucleobases, giving rise to metal-mediated base pairs and a rich variety of self-assembled structures. Reduction of the bound silver ions produces compact metal nanostructures containing small numbers of silver atoms with remarkable, but poorly understood photophysical properties. Using femtosecond spectroscopy, nonradiative and radiative decay pathways are being observed and characterized in mononucleotide-metal ion complexes and their self-assembled nanostructures, and in DNA-hosted silver nanoclusters. Time-domain measurements are employed to develop a comprehensive photophysical model of silver nanoclusters. Fundamental understanding of the electronic interactions between DNA and metals can provide new insights into hybrid organic-inorganic interfaces that are attractive for controlling the separation and recombination of photogenerated carriers and redox equivalents. The study of few-atom noble metal nanoclusters by ultrafast spectroscopy promises to accelerate societally beneficial advances in science and technology. The ability to control and manipulate the excited states of metal nanoparticles could advance the fields of chemical imaging and sustainable energy production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.