Sub-20 nm LiErF₄-Based Upconversion Nanophosphors for Simultaneous Imaging and Photothermal Therapeutics

Abstract

This study reports red-emitting LiErF4:Tm-based upconversion nanophosphors (UCNPs) with small size and high brightness for simultaneous upconversion luminescence (UCL) imaging and photothermal therapeutics. Strong red UCL is realized from LiErF4-based UCNPs by doping Tm3+ into LiErF4 and forming multishells on the LiErF4:Tm core. The ultrasmall LiErF4:Tm UCNPs are facilely synthesized by using rare-earth-oleate precursors and a mixed solvent of oleic acid and 1-octadecene. The size of the ultrasmall LiErF4:Tm­(0.3%) UCNPs is finely tuned from 2.8 to 10.4 nm, and LiErF4:Tm­(0.3%)/LiGdF4 core/shell (C/S) UCNPs show small sizes ranging from 8.6 to 13.2 nm. Notably, 11.4 nm-sized C/S UCNPs show bright red light. Furthermore, LiErF4:Tm-based core/triple-shell (C/T-S) UCNPs are synthesized, and they show dramatic enhancement of red UCL (∼4242-fold enhancement compared with the core under 980 nm excitation and ∼89-fold and ∼14-fold enhancement compared with the C/S UCNPs under 800 and 1532 nm excitation, respectively). Besides UCL enhancement, Nd3+ ions doped in the LiErF4:Tm-based C/T-S UCNPs generate heat under 800 nm irradiation, allowing the C/T-S UCNPs to be applied as photothermal therapeutic agents. The ultrasmall (<20 nm) and bright red-emitting LiErF4:Tm-based C/T-S UCNPs have the potential for simultaneous imaging and photothermal therapeutic applications.

Orthogonal R/G/B Upconversion Luminescence-based Full-Color Tunable Upconversion Nanophosphors for Transparent Displays

“…Although LiErF 4 :Tm shell can show red UCL under 800 nm excitation, this red UCL under 800 nm is negligible since the absorbance of Er 3+ at 800 nm NIR light is low and efficiency of the LiErF 4 :Tm under 800 nm is low. 24 In this case, the emission from the Er 3+ in the S 2 is dominant and a strong green emission peak is observed under 800 nm excitation. However, for 980 nm excitation, emission intensity at the red spectral region was stronger than that at the green spectral region, and the red peak at ∼670 nm is attributed to the emission from the Er 3+ in the S 5 .…”

“…21 In addition, the red emission peak was observed via 4 F 9/2 → 4 I 15/2 transition of Er 3+ under 1532 nm NIR light. 20,24 It is worth noting that the excitation of orthogonal pure R/G/ B UCL from the UCNPs is attributed to our C/6S nanostructure, i.e., the configuration of the blue-emitting core, green-emitting inner shell, and red-emitting outer shell in the LiREF 4 -based composition. Although photon blocking layers were not introduced to our C/6S UCNPs, the excitation wavelength-dependent R/G/B UCL was apparently displayed in the C/6S UCNPs (Figure 3a).…”

“…In this study, we synthesized LiREF 4 -based C/M-S UCNPs since the LiREF 4 is known as the host for efficient UCL, 21,22 and small-sized LiREF 4 -based UCNPs showing efficient R/G/B UCL can be synthesized. 6,23,24 As shown in Figure 1a, six kinds of core/shell/shellstructured UCNPs can be considered and R/G/B UCL can be generated from each core, the first shell, and the second shell, separately. Among these structures, we chose the B(core)/G(shell)/R(shell) structure for bright UCL.…”

“…However, for 980 nm excitation, emission intensity at the red spectral region was stronger than that at the green spectral region, and the red peak at ∼670 nm is attributed to the emission from the Er 3+ in the S 5 . Since the LiErF 4 :Tm exhibits much stronger red UCL under 980 nm excitation than under 800 nm excitation, 24 the Er 3+ emission from the LiErF 4 :Tm layer was dominant under 980 nm excitation, while that from the LiGdF 4 :Yb,Er layer was dominant under 800 nm excitation. As a result, the different ratios of red to green peaks were observed for 980 and 800 nm excitations.…”

“…These studies inspire us to create an orthogonal R/G/B UCL-based full-color-emitting single UCNP which can be excited with 800, 980, and 1532 nm NIR light, respectively. In this study, we synthesized LiREF 4 -based C/M-S UCNPs since the LiREF 4 is known as the host for efficient UCL, , and small-sized LiREF 4 -based UCNPs showing efficient R/G/B UCL can be synthesized. ,, …”