Affiliation: Furman University
Title: Revealing the Colors of Silver using DNA
Hosted by: Dr. Kohler
This talk will focus on molecular silver clusters with ~10 atoms that are encapsulated by DNA strands. Three issues will be discussed.
I. How are silver clusters different from bulk silver and silver nanoparticles? I will discuss their energy levels. The delocalized and densely-packed bands of molecular orbitals in bulk metals reorganize as the physical dimensions of a metal shrink. For example, a silver cluster with N~100 atoms and a diameter ~1.5 nm has an average energy spacing ~1.2 eV, or 50X the thermal energy (kBT) at room temperature. Thus, discrete and sparsely-spaced energy levels constrain the valence electrons. As a result, the smallest metal particles shed the familiar signatures of bulk metals and behave more like molecules. For example, distinct colors develop because the HOMO-LUMO gaps are large and vary with the cluster size, and luminescence emerges because the dispersed electronic states are weakly coupled. II. How are specific clusters synthesized? Silver clusters develop in aqueous solution when they are protected by ligands. We are using DNA strands, and these serve two functions. First, they bind and locally concentrate Ag+ and thus promote the coalescence of chemically-reduced silvers. Second, the primary sequence and secondary structure define specific cluster environments. III. How can DNA fine-tune cluster environments? I will discuss hybridization-induced fluorescence. A dim cluster with violet absorption switches to a fluorescent cluster via DNA hybridization. A common type of DNA template has two components: the 3' sequence hybridizes with target oligonucleotides and the 5' sequence forms specific silver clusters. These composite strands exclusively harbor an ∼11 silver atom cluster that absorbs at 400 nm with limited emission. When a target hybridizes with these single-stranded conjugates, cluster absorption shifts and strong emission develops. I will discuss methods that address this spectral transformation.
