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

Surface modification of different functional molecules onto NaREF₄ (RE = rare earth) upconversion nanoparticles (UCNPs) impart their multiple functionalities. Functional molecules can be loaded onto NaREF₄ UCNPs through the formation of coordination bonds between the surface-exposed RE³⁺ ions and the appropriate chemical groups of functional molecules. The density of surface RE³⁺ ions directly determines the loading efficiency of NaREF₄ UCNPs. However, NaREF₄ is a binary cation system, rendering the surface-distributed Na⁺ and RE³⁺ ions remains a mystery. Here, we develop an effective strategy to significantly enhance the density of surface RE³⁺ ions, thus maximizing the loading capacity of NaREF₄ UCNPs. This strategy is based on a heterovalent cation exchange (HCE) reaction in the surface region in which Na⁺ ions are replaced by RE³⁺ ions. The density of surface ligands enhances from 3.6 to 8.8 molecules/nm² 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.