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

Abstract

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 Er³⁺, Tm³⁺, and Ho³⁺)-coupled nanosystems under 980-nm excitation. However, the frequently used Yb³⁺ ion with the single-excited ²F_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 Ho³⁺ as a bridge ion coupling donor-acceptor pairs (e.g., Yb³⁺–Nd³⁺), 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 Ho³⁺ sublattice owing to the energy migration capability of Ho³⁺ and that Ho³⁺ as a bridge ion can realize interparticle energy transfer for upconversion luminescence. Moreover, well-designed NaYbF₄:40%Ho@NaYF₄:10%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 Ho³⁺ bridge in energy transfer upconversion and offers strategies for broadening energy transfer pathways and finely manipulating photon upconversion.

摘要

镧系掺杂上转换纳米粒子(UCNPs)由于其具有独特优点而被广 泛研究. 目前最受青睐的980 nm激发UCNPs仍是经典的Yb–A耦合纳米 体系(A = Er³⁺, Tm³⁺和Ho³⁺). 但是, 仅具有单个激发态(²F_5/2)的Yb³⁺因缺 乏匹配的激发态能级而难以向一些其他受体离子传递能量. 在本文中, 我们提出了一种能量传递新策略, 即以Ho³⁺为桥联剂来连接原本难以 进行直接能量传递的供体–受体对(如Yb³⁺–Nd³⁺), 并探究了供体、桥 联剂和受体间所涉及的能量传输机制. 此外, 我们发现Ho³⁺离子间能进 行能量迁移, 使得Ho³⁺次晶格内可发生长距离的能量输运, 同时证实 Ho³⁺作为桥联剂亦可实现粒子间的能量转移而获得上转换发光. 我们 设计的NaYbF₄:40%Ho@NaYF₄:10%Nd核壳纳米粒子表现出多模态响 应性, 即在不同激发波长下可发出不同颜色的光, 其被证明适用于高等 级防伪和信息识别. 总之, 这些研究结果有助于深入理解Ho³⁺桥联剂在 能量传递上转换中的功能机制, 并为拓宽能量传递途径及精细操控光 子上转换提供了方法策略.