Ultraefficient Cascade Energy Transfer in Dye-Sensitized Core/Shell Fluoride Nanoparticles

The multistep sequential dye → Nd3+ → Yb3+ energy transfer leading to significantly enhanced emission at 1 μm has been investigated in core-shell CaF2 nanoparticles. We demonstrate that, by controlling the relative positions and the distance between energy donor (Nd3+) and acceptor (Yb3+) units through the confinement of the donor into a thin shell well below the Förster's radius, virtually fully efficient Yb3+ sensitization can be achieved. Optimized and facile synthetic protocols by employing a hot injection approach allowed the controlled deposition of a ∼0.4 nm thick Nd3+-doped outer shell on Yb3+-doped core nanoparticles of less than 4 nm diameter. The fluorescein isothiocyanate (FITC) dye on the surface of the nanoparticles acts as efficient visible-light harvester, enabling the sensitization via nonradiative energy transfer of emitting lanthanide ions (Ln3+). The short distance between Nd3+ and Yb3+ allows for ultraefficient (∼90%) interlanthanide energy transfer resulting in an Yb3+ sensitization efficiency of over 90% thanks to the "bridging effect" of Nd3+ energy donors. As a result, the overall near-infrared quantum yield increases by ∼40% with respect to dye-only sensitized Yb3+ and a total enhancement of about 2100× the 1 μm luminescence intensity with respect to directly excited Yb-only nanoparticles, which is the highest figure of merit reported in literature so far for NIR-emitting analogous systems, is observed. The achievement of sensitization efficiencies so far only obtained in tightly bonded lanthanide molecular complexes, through a design strategy of general validity, opens new perspectives in regard to the potential application of this type of nanoparticle for optical amplification at 1 μm.