来源：ACS Publications
To enhance the upconversion luminescence (UCL) of small-sized nanoparticles, core–shell nanostructures are employed to effectively isolate emitting ions from surface quenchers. However, the synthesis of such structures is frequently compromised by cation intermixing, a result of core dissolution during shell growth. This unintended ion diffusion undermines the passivation function of the inert shell, as emitting ions relocated near surface defects contribute to nonradiative energy loss, thus diminishing luminescence enhancement. This detrimental effect is especially severe for sub-10 nm UCNPs. To address this challenge, we developed a “dissolution-suppressed shell growth” (DSSG) strategy that enables the fabrication of well-defined core–shell nanostructures. By systematically investigating the nanoparticle dissolution process, we established that a Y3+-enriched ionic environment effectively stabilizes the core during shell deposition. Coupled with the use of small-sized α-NaYF4 nanoparticles as shell precursors, the DSSG approach promotes the epitaxial growth of high-quality inert shells with minimal cation intermixing. The resulting core–shell UCNPs exhibit remarkable optical enhancement over those prepared by the conventional hot-injection method, showing over a 100-fold increase in emission intensity and an approximately 6-fold extension in luminescence lifetime. Notably, even for sub-10 nm nanostructures, a 4.4-fold lifetime enhancement is achieved with an ultrathin shell of only 1.6 nm. Consequently, the performance-optimized sub-10 nm core–shell UCNPs are expected to facilitate their widespread adoption, particularly in bioimaging.