Efficient excitation-energy transfer in ion-based organic nanoparticles with versatile tunability of the fluorescence...

Organic nanoparticles consisting of 3,3'-diethylthiacyanine (TC) and ethidium (ETD) dyes are synthesized by ion-association between the cationic dye mixture (10 % ETD doping) and the tetrakis(4-fluorophenyl)borate (TFPB) anion, in the presence of a neutral stabilizing polymer, in aqueous solution. Doping with ETD makes the particle size smaller than without doping. Size tuning can also be conducted by varying the molar ratio (ρ) of the loaded anion to the cationic dyes. The fluorescence spectrum of TC shows good overlap with the absorption of ETD in the 450-600 nm wavelength region, so efficient excitation-energy transfer from TC (donor) to ETD (acceptor) is observed, yielding organic nanoparticles whose fluorescence colours are tunable. Upon ETD doping, the emission colour changes significantly from greenish-blue to reddish or whitish. This change is mainly dependent on ρ. For the doped nanoparticle sample with ρ=1, the intensity of fluorescence ascribed to ETD is ∼150-fold higher than that from pure ETD nanoparticles (efficient antenna effect). Non-radiative Förster resonance-energy transfer (FRET) is the dominant mechanism for the ETD fluorescence enhancement. The organic nanoparticles of a binary dye system fabricated by the ion-association method act as efficient light-harvesting antennae, which are capable of transferring light energy to the dopant acceptors in very close proximity to the donors, and can have multi-wavelength emission colours with high fluorescence quantum yields.