Activators Confined Upconversion Nanoprobe with Near-Unity Förster Resonance Energy Transfer Efficiency for Ultrasensitive Detection

Lanthanide-doped upconversion nanoparticles (UCNPs) as energy donors for Förster resonance energy transfer (FRET) are promising in biosensing, bioimaging, and therapeutic applications. However, traditional FRET-based UC nanoprobes show low efficiency and poor sensitivity because only partial activators in UCNPs possessing suitable distance with energy acceptors (<10 nm) can activate the FRET process. Herein, a novel excited-state energy distribution-modulated upconversion nanostructure is explored for highly efficient FRET. Integration of the optimal 4% Er3+ doped shell and 100% Yb3+ core achieves ∼4.5-fold UC enhancement compared with commonly used NaYF4:20%Yb3+,2%Er3+ nanoparticles, enabling maximum donation of excitation energy to an acceptor. The spatial confinement strategy shortens significantly the energy-transfer distance (∼4.5 nm) and thus demonstrates experimentally a 91.9% FRET efficiency inside the neutral red (NR)-conjugated NaYbF4@NaYF4:20%Yb3+,4%Er3+ nanoprobe, which greatly outperforms the NaYbF4@NaYF4:20%Yb3+,4%Er3+@SiO2@NR nanoprobe (27.7% efficiency). Theoretical FRET efficiency calculation and in situ single-nanoparticle FRET measurement further confirm the excellent energy-transfer behavior. The well-designed nanoprobe shows a much lower detection limit of 0.6 ng/mL and higher sensitivity and is superior to the reported NO2- probes. Our work provides a feasible strategy to exploit highly efficient FRET-based luminescence nanoprobes for ultrasensitive detection of analytes.