Single DNA-Shelled Silver Nanoclusters Probed by Tip Enhanced Fluorescence Spectroscopy

Abstract

Significant advances in nanoimaging have been made by development of aperture-less tip enhanced fluorescent microscopy, in which so called near field effect enhances the absorption and emission rates and allows overcoming the diffraction limit. Tip Enhanced Fluorescent Spectroscopy (TEFS) involves a combination of classical scanning tunneling microscope (STM) with optical confocal microscope where incident light is focused on the tip of metal probe enhancing electromagnetic field nearby.We applied TEFS technique for investigation of DNA-stabilized fluorescent Ag nanoclusters. DNA-stabilized Ag nanoclusters make up a class of water-soluble fluorophores consisted of 1-10 silver atoms. They exhibit high chemical stability, good biocompatibility and high absorbance and quantum yield. An assembled structure of compact globule with silver clusters inside was obtained using calf thymus DNA condensed by synthetic polycation (poly)allylamine. Compact fluorescent globules 100 nm in size containing 1-2 silver clusters were visualized by TEFS technique. Fourfold enhanced fluorescent signal was obtained and fluorescent spectra of single clusters were registered. Such Ag-DNA-polymer assembled structure seems to be promising in creation highly stable and bright nanoparticles containing multiple emitters therein. TEFS technique thus appears to be a powerful approach for single molecule spectroscopy and superresolution bioimaging.