Alleviating Luminescence Concentration Quenching in Upconversion Nanoparticles through Organic Dye Sensitization

The phenomenon of luminescence concentration quenching exists widely in lanthanide-based luminescent materials, setting a limit on the content of lanthanide emitter that can be used to hold the brightness. Here, we introduce a concept involving energy harvesting by a strong absorber and subsequent energy transfer to a lanthanide that largely alleviates concentration quenching. We apply this concept to Nd³⁺ emitters, and we show both experimentally and theoretically that the optimal doping concentration of Nd³⁺ in colloidal NaYF₄:Nd upconverting nanoparticles is increased from 2 to 20 mol% when an energy harvestor organic dye (indocyanine green, ICG) is anchored onto the nanoparticle surface, resulting in ∼10 times upconversion brightness. Theoretical analysis indicated that a combination of efficient photon harvesting due to the large absorption cross section of ICG (∼30 000 times higher than that of Nd³⁺), non-radiative energy transfer (efficiency ∼57%) from ICG to the surface bound Nd³⁺ ions, and energy migration among the Nd³⁺ ions was able to activate Nd³⁺ ions inside the nanoparticle at a rate comparable with that of the pronounced short-range quenching interaction at elevated Nd³⁺ concentrations. This resulted in the optimal concentration increase to produce significantly enhanced brightness. Theoretical modeling shows a good agreement with the experimental observation. This strategy can be utilized for a wide range of other lanthanide-doped nanomaterials being utilized for bioimaging and solar cell applications.