Investigate how small silver clusters interact with DNA. The overall goal is to control the fluorescence spectra of these strongly emitting metal species by altering the DNA sequence. These chromophores are innovative in several respects. First, chromophores with different emission spectra are straightforward and inexpensive to prepare because the nucleobases control cluster formation. Second, the DNA-clusters complexes are biocompatible due to their encapsulation by DNA and due to their small size. Third, the clusters have favorable spectroscopic properties for imaging in biological environments. The expertise of Professor Dickson in the areas of high-sensitivity spectroscopy and DNA arrays will be critical for identifying the optimal sequences via high-throughput parallel screening of a comprehensive range of sequences. Professor Petty's background in DNA thermodynamics and structure will be the basis of the further scrutiny of the effect of reaction conditions on the formation and spectra of the clusters. The fluorescence photophysical parameters of the silver-DNA conjugates will show how both the base sequence and the solution environment influence the DNA-silver cluster adducts. This project will develop undergraduate and graduate students by conducting novel research related to biophotonics. The established and productive collaboration between the undergraduate program at Furman and the graduate program at Georgia Tech will provide multiple opportunities for learning and teaching. The students will learn to organize and disseminate the outcomes of their research activities through presentations at professional meetings and through publication in the primary literature. A wider impact will be in the classroom, as the new instruments and experiments will be integrated into the physical chemistry and nanoscience laboratories. A postdoctoral associate will be mentored for a future faculty career at an undergraduate-focused institution.
Few atom clusters of silver possess distinct electronic transitions spanning the visible and near-infrared spectral region, and we have pioneered the development of a new class of biocompatible chromophores based on silver clusters (Fig. 1). An outcome of our work has been the development of a new class of nanomaterial-based biological sensors (Fig. 2). Our studies are focusing on synthesis and characterization. A key feature of these molecular silver clusters is that their growth is stringently attenuated via nucleobase coordination. A major accomplishment from our studies is development of synthetic techniques to create specific silver clusters. Characterization of particular clusters has been accomplished by measuring their spectra and size. Through our spectral studies, we have characterized transiently populated dark electronic states that can be optically modulated to identify clusters in high background samples. Through chromatographic purification and elemental analysis, we have characterized a range of clusters with sizes from 5 -22 atoms. With these synthetic tools and the new perspectives on cluster size, we are now aggressively pursuing a unique feature of DNA-silver cluster conjugates because they both report and change DNA structure. Our studies developed a sensor strand with two general components: a cluster template favors a specific silver cluster and a recognition site hybridizes with a target oligonucleotide. The composite strand selectively harbors a ~11 atom silver cluster that is emissively silent and absorbs at 400 nm and it folds the host strand. With association of the target and recognition site, stark distinctions in both the spectrum and the structure ensue. With one DNA template, hybridization opens the conjugate to expose the template binding site for a ~11 atom cluster with near-infrared absorption at 720 nm and strong emission. Through variations in the recognition site appended to the cluster template, this spectral and structural transformation provides a platform for a general optically-based sensor of oligonucleotides. The interplay between base-pairing and cluster driven changes in the DNA secondary structure suggest that fine control of the cluster environment can selectively direct the synthesis of particular silver clusters. We are particularly interested in developing sensors that function in native biological environments. The near infrared emission from the cluster will facilitate detection in these chemically and spectrally challenging environments. With another DNA template, hybridization develops a cluster with blue-green emission. We are working to develop this cluster for biological sensing by utilizing its temperature sensitive spectra.
