Distance‐Confined Ultrathin Upconversion Nanoplatelets Enabling Highly Efficient Luminescence Resonance Energy Transfer

ABSTRACT Luminescence resonance energy transfer (LRET) is highly sensitive to the donor–acceptor distance, rendering the geometry of the donor critically important. Conventional upconversion nanoparticles (C‐UCNPs, based on β‐NaYF 4 as the host) exist as isotropic spheres, wherein only the near‐surface activators contribute to LRET. Although the core–shell design can reduce the donor–acceptor distance, it requires complex multistep synthesis with careful shell control. In this study, distance‐confined ultrathin upconversion nanoplatelets (DC‐UCNPLs, based on Cs 2 NaYF 6 as the host) with sub‐10 nm thickness are reported. In this system the platelet geometry positions the majority of activators within the Förster distance, inherently favoring distance‐confined LRET (DC‐LRET). As a result, the DC‐UCNPLs exhibited a 4.3‐fold higher acceptor‐to‐donor emission ratio than the C‐UCNPs; Monte Carlo simulations confirmed this intrinsic advantage. In the reverse configuration, where IR‐806 acted as a donor and the DC‐UCNPLs as acceptors, the process was defined as reverse distance‐confined LRET (RDC‐LRET), corresponding to dye‐sensitized upconversion. This yielded an extraordinary 1471‐fold increase in upconversion luminescence under 808 nm excitation. Overall, these DC‐UCNPLs establish a structurally uniform, scalable, and bidirectional donor–acceptor architecture, thereby offering a powerful strategy for next‐generation nanoprobes in bioimaging, sensing, and energy conversion.