Harnessing Energy Enrichment Strategy for Designing Innovative Sandwich-Structured Upconversion Nanoprobes

Despite the various advantages of upconversion nanoparticles (UCNPs), the paradoxes of high luminescence resonance energy transfer (LRET) efficiency and low quantum yield remain a bottleneck for broader sensing applications. Herein, novel sandwich-structured UCNPs (SWUCNPs, NaYbF4:(30%Gd)@NaYbF4:Er(2%)@NaYF4) with a core-middle shell-outer shell structure were synthesized. The SWUCNPs maintained a high LRET efficiency by confining the luminescent center of Er3+ in the middle shell. Moreover, the NaYbF4:(30%Gd3+) inner core significantly increased the absorption of excitation light, while the host lattice of NaYbF4 in the middle shell facilitated the energy transfer from the core to the emitting ions as well as further increased the absorption for 980 nm excitation light. Both the strategy of energy enrichment and optimizing the energy migration were beneficial to boosting the strong UCL and high LRET efficiency. Compared with the traditional SWUCNPs (NaYF4@NaYF4:Yb/Er@NaYF4), the upconversion luminescence (UCL) intensity of the as-prepared SWUCNPs was greatly improved, with the UCL at 540 nm increased by 56 times and the UCL at 655 nm increased by 117 times. In proof-of-concept applications, this innovative structure has been utilized for high-sensitivity detection of ascorbic acid. The strategy reported here opens a novel pathway toward the preparation of highly sensitive upconversion nanoprobes.