Surface-rare-earth-rich upconversion nanoparticles induced by heterovalent cation exchange with superior loading capacity

Surface modification of different functional molecules onto NaREF_(4)(RE=rare earth)upconversion nanoparticles(UCNPs)impart their multiple functionalities.Functional molecules can be loaded onto NaREF_(4) UCNPs through the formation of coordination bonds between the surface-exposed RE^(3+) ions and the appropriate chemical groups of functional molecules.The density of surface RE^(3+) ions directly determines the loading efficiency of Na REF4 UCNPs.However,NaREF_(4) is a binary cation system,rendering the surface-distributed Na;and RE^(3+) ions remains a mystery.Here,we develop an effective strategy to significantly enhance the density of surface RE^(3+) ions,thus maximizing the loading capacity of NaREF_(4) UCNPs.This strategy is based on a heterovalent cation exchange(HCE)reaction in the surface region in which Na^(+)ions are replaced by RE^(3+) ions.The density of surface ligands enhances from 3.6 to 8.8 molecules/nm^(2) after reaction,suggesting that the loading efficiency increases by approximately 150%.Benefiting from the improved loading capacity,we demonstrate such surface-RE-rich nanoparticles have the ability to offer higher colloidal stability and more desirable photodynamic therapy(PDT)efficacy.This work not only advances our understanding of cation exchange reactions in RE-based nanoparticles,but also provides significant value for considerable applications such as sensing,bioimaging,and therapy.