Noble metal clusters of just a few atoms have unique and fascinating optical properties. In particular, they are fluorescent: they re-emit absorbed photons with a characteristic down-shift in energy. But such tiny bare clusters are quite chemically reactive, particularly with each other. If allowed to collide, these clusters will agglomerate into larger structures that are “dark” (that is, non-fluorescent). Studying bare metal clusters thus requires elaborate experimental approaches, such as creating cluster beams in vacuum or realizing cryogenic deposition schemes to embed clusters in non-reactive, noble gas matrices. In addition to being incompatible with practical applications, these experimental geometries are also incompatible with measurements of the properties of individual clusters. Instead, bare cluster studies query large ensembles in which variations amongst individual clusters will “wash out” important fundamental features, such as intrinsic linewidths.
With the use of ligands noble metal clusters can be made stable in solution. Metal clusters stabilized by biopolymers, such as DNA and RNA, are particularly interesting due to their promise for applications. We study few-atom silver clusters stabilized by DNA and RNA, which we call Ag-DNAs and Ag-RNAs. Encapsulation of Ag clusters within DNA strands makes the clusters chemically stable enough for purification, allowing homogenous cluster populations to be studied in solution and investigated at the single molecule level.
We study the underlying origins of the striking properties of Ag-DNAs, such as the enormous color space their fluorescence spans (from the blue through the near-IR), the their widely varying quantum yields and photostabilities, and the mechanisms for the Stokes shift and the single emitter linewidths. Understanding these basics will help us develop better synthetic strategies, which are much needed for navigating the space of 10^12 distinct nucleic acid strands available at the 20 base length used in many emerging applications. We are also continuing development of purification techniques for this novel class of hybrid metal-nucleic acid material and are exploring the use of large data set analysis to better understand how DNA sequence selects cluster color. We aim to develop applications that take advantage of the special properties of Ag-DNAs, which have unique potential for advanced fluorescence sensing and logic devices that hold silver clusters within precisely ordered DNA and RNA nanostructures created by modern self-assembly protocols.
For prospective graduate and undergraduate researchers
The work we do is a combination of chemistry, physics, and biophysics. Regardless of your department affiliation, if you are interested in joining a project or suggesting a new one, please get in touch.