Novel strategy for energy transfer via Ho3+ as a bridge in upconversion nanoparticles

Lanthanide-doped upconversion nanoparticles (UCNPs) have been extensively investigated owing to their unique advantages. So far, the most favored modality of UCNPs is still the classical Yb–A (where A refers to Er 3+ , Tm 3+ , and Ho 3+ )-coupled nanosystems under 980-nm excitation. However, the frequently used Yb 3+ ion with the single-excited 2 F 5/2 state can barely transfer energy to other acceptors owing to its lack of matched excited levels. Herein, a novel strategy is proposed for energy transfer via Ho 3+ as a bridge ion coupling donor-acceptor pairs (e.g., Yb 3+ –Nd 3+ ), where little direct energy transfer occurs, and the involved energy transport mechanisms are investigated. The findings reveal that long-distance energy transportation can occur in the Ho 3+ sublattice owing to the energy migration capability of Ho 3+ and that Ho 3+ as a bridge ion can realize interparticle energy transfer for upconversion luminescence. Moreover, well-designed NaYbF 4 :40%Ho@NaYF 4 :10%Nd core-shell nanoparticles with multimode responsiveness can emit different colored lights under varying excitation wavelengths, which are demonstrated in high-level anticounterfeiting and information identification. Therefore, this work paves a new way to understand the functionality of the Ho 3+ bridge in energy transfer upconversion and offers strategies for broadening energy transfer pathways and finely manipulating photon upconversion.