High-efficiency and water-quenching-resistant Tb3+-based nanoparticles for single-particle imaging

Zhiwang Cai,Xin Guo,Meifeng Wang,Daoyu Xia,Wei Wei,Yiru Qin,Yongjun Hu,Huan Tang,Xiaoyu Zhao,Qiuqiang Zhan

doi:10.1515/nanoph-2021-0010

Abstract

Abstract The structure of the host lattice has a substantial influence on the optical properties of lanthanide-doped luminescent materials. Hexagonal-phase (β-phase) NaREF4 (RE = rare earth) is the most commonly used crystal structure for lanthanide-doped upconversion nanoparticles (UCNPs) owing to its high upconversion (UC) efficiency. In this work, we report, for the first time, that more efficient cooperative sensitization upconversion (CSU) can be achieved in cubic-phase (α-phase) NaREF4 UCNPs instead of their β-phase counterparts. With the passivation of an inert shell, the UC emission intensity of α-NaYbF4:Tb40%@CaF2 is 10.5 times higher than that of β-NaYbF4:Tb40%@NaYF4. We propose that the high-symmetry crystal structure of the α phase facilitates the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters, which are particularly beneficial for CSU. Moreover, we prove that such Tb3+-based UCNPs are almost impervious to water quenching because of the large energy gap (∼15,000 cm−1) that existed in Tb3+ between its lowest emit-ting level (5D4) and next low-lying level (7F0). Finally, their potential application for single-nanoparticle imaging has also been demonstrated. As expected, the α-core-shell UCNPs measured at the single-nanoparticle level are estimated to be 9-fold brighter than their β-core-shell counterparts. Importantly, the α-NaYbF4:Tb40%@CaF2 UCNPs offer exciting opportunities for realizing single-nanoparticle imaging at ultralow irradiance (30 W/cm2).

Highlights

We propose that the highsymmetry crystal structure of the α phase facilitates the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters, which are beneficial for cooperative sensitization upconversion (CSU)
The host material is of great importance for lanthanidedoped upconversion nanoparticles (UCNPs), which play a crucial role in determining their upconversion (UC) efficiency, emission colors, and potential applications [1,2,3]
Given that the as-prepared β-NaYbF4:Tb40% UCNPs have an average size of 13.2 ± 0.6 nm (Figures 1b and S2b), the α-NaYbF4:Tb40% UCNPs with an average size of 13.0 ± 1.8 nm (Figures 1a and S2a) were synthesized through the aforementioned homostructured epitaxial growth strategy

Summary

Introduction

The host material is of great importance for lanthanidedoped upconversion nanoparticles (UCNPs), which play a crucial role in determining their upconversion (UC) efficiency, emission colors, and potential applications [1,2,3]. We demonstrate that high-efficiency CSU can be achieved in α-phase instead of β-phase NaREF4 Both α- and β-NaYbF4:Tb40% core-only UCNPs, and their derivative core–shell UCNPs with highly comparable particle sizes, were synthesized and systematically investigated. From the view of crystal structure, α-phase NaREF4 is featured with higher-symmetry cation sites, benefiting the formations of [Yb–Yb] dimers and [Yb–Yb–Tb] clusters in the lattice. The existence of such dimers and clusters is able to dramatically boost the efficiency of CSU. As lanthanide-doped UCNPs have been recently considered as ideal tools for single-nanoparticle imaging, we further investigated the as-prepared α- and β-phase core–shell UCNPs for such application. The α-core-shell UCNPs are bright enough for single UCNP observation, and their brightness is approximately an order of magnitude higher than that of β-core-shell counterparts

Results and discussion

Experimental section

Conclusions