UC Irvine

Advancements in bioimaging modalities have changed the face of modern medicine. However, current techniques such as MRI and CT utilize hazardous radiation, contrast agents with toxic effects, and possess with limited resolution for deep tissue imaging on length scales relevant for cellular processes. In vivo fluorescence imaging in the near-infrared (NIR) window (750-1700 nm) is an emerging deep tissue imaging technique that offers the possibility of achieving micron-scale spatial resolutions for clearer images of anatomical structures in real time. This thesis develops NIR-emissive DNA-stabilized silver nanoclusters (AgN-DNAs) as fluorophores for non-hazardous fluorescence imaging. We focused on studying the chemical and photophysical properties of AgN-DNAs emitting in NIR I (650-950 nm) wavelengths. First, we worked towards tuning the synthesis conditions to optimize the chemical yields of these nanoclusters, purifying monodisperse and single-emissive species in solution, and determining their chemical compositions. Additionally, we tested the stability of NIR-emissive AgN-DNAs in physiological conditions and aimed to protect these nanoclusters against degradation to evaluate their applicability for fluorescence imaging. We found that the post-reduction temperature used for the nanoclusters after synthesis can significantly increase the brightness of known NIR-emissive AgN-DNAs and decrease the time it takes to achieve maximum emission intensity. Also, new AgN-DNAs were found to evolve from red emissive AgN-DNAs upon changing post-reduction conditions. Mass-spectra of HPLC-purified NIR-emissive AgN-DNAs indicate that these nanoclusters are stabilized by 2 DNA strands, containing six neutral silver atoms and, in some cases, chloride ligands. Attempts to use polyamines to stabilize the AgN-DNAs in saline conditions did not aid in protecting them from emission quenching, but instead, some nanoclusters were found to be stable on their own without modification. Lastly, preliminary in vivo experiments utilized a promising NIR-emissive AgN-DNA in a mouse model, which displayed bright luminescence for several hours while indicating no overt signs of toxicity to the mouse’s liver or spleen. This work laid the groundwork for improved fundamental understanding of methods to discover new NIR-emissive AgN-DNAs, optimize their chemical yields, and establish them as suitable and safe fluorophores for bioimaging techniques.