Effect of compartmentalization of donor and acceptor on the ultrafast resonance energy transfer from DAPI to silver nanoclusters

The mechanism and dynamics of excitation energy transfer (EET) from photo-excited 4',6-diamidino-2-phenylindole (DAPI) to silver nanoclusters (Ag NCs) and its subsequent modulation in the presence of cationic polymer poly(diallyldimethylammonium chloride) (PDADMAC) and Calf Thymus DNA (CT-DNA) have been demonstrated using steady-state fluorescence and femtosecond fluorescence upconversion techniques. The synthesized Ag NCs were characterized using FTIR, mass spectrometry, XPS, HRTEM, DLS, UV-Vis and PL spectroscopy. Mass spectrometric analysis reveals the formation of ultrasmall Ag4 NCs with a small amount of Ag-5 NCs. UV-Vis and PL spectra reveal distinct molecular-like optoelectronic behaviour of these ultrasmall Ag NCs. The dihydrolipoic acid-capped Ag NCs strongly quench the fluorescence of DAPI with concomitant increase in its photoluminescence (PL) intensity at 675 nm. This steady-state fluorescence quenching proceeds with a significant shortening of the fluorescence lifetime of DAPI in the presence of Ag NCs, signifying the nonradiative Forster resonance energy transfer (FRET) from DAPI to Ag NCs. Various energy transfer parameters have been estimated from FRET theory. The present FRET pair shows a characteristic Forster distance of 2.45 nm and can be utilized as a reporter of short-range distances in various FRET based applications. Moreover, this nonradiative FRET is completely suppressed in the presence of both 0.2 wt% PDADMAC and CT-DNA. Our results reveal selective compartmentalization of Ag NCs and DAPI in the presence of 0.2 wt% PDADMAC and CT-DNA, respectively. This selective compartmentalization of donor and acceptor and the subsequent modification of the FRET process may find application in various sensing, photovoltaic, and light harvesting applications.