Excited-State Relaxation and Forster Resonance Energy Transfer in an Organic Fluorophore/Silver Nanocluster Dyad

A single-stranded DNA-based (ssDNA) dyad was constructed comprising 15 silver atoms stabilized by a ssDNA scaffold (DNA-AgNC) and an Alexa 546 fluorophore bound to the 5' end. The Alexa 546 was chosen to function as a Forster resonance energy transfer (FRET) donor for the AgNC. Time-correlated single photon counting (TCSPC) experiments allowed unraveling the excited-state relaxation processes of the purified DNA-AgNC-only system. The TCSPC results revealed slow relaxation dynamics and a red shift of the emission spectrum during the excited-state lifetime. The results from the model systems were needed to understand the more complicated decay pathways present in the collected high-performance liquid chromatography fraction, which contained the dyad (37% of the emissive population). In the dyad system, the FRET efficiency between donor and acceptor was determined to be 94% using TCSPC, yielding a center-to-center distance of 4.6 nm. To date, only limited structural information on DNA-AgNCs is available and the use of TCSPC and FRET can provide information on the center-to-center distance between chromophores and provide positional information in nanostructures composed of AgNCs.