Synthesis of NaLuF₄-based nanocrystals and large enhancement of upconversion luminescence of NaLuF₄:Gd, Yb, Er by coating an active shell for bioimaging

Abstract

A series of NaLuF4-based hexagonal phase upconversion nanocrystals (UCNs) were synthesized by a facile solvothermal method and the properties of the UCNs were investigated. The results show that the as-prepared nanocrystals exhibit pure hexagonal lattice structures, uniform morphologies, high monodispersities and excellent upconversion luminescence. The upconversion luminescence (UCL) intensities of the UCNs can be enhanced by coating with a shell of NaLuF4. More interestingly, the UCL intensities of active-shell coated nanocrystals (NaLuF4:Gd, Yb, Er@NaLuF4:Yb, Ho and NaLuF4:Gd, Yb, Er@NaLuF4:Yb) are remarkably higher than that of inert-shell coated nanocrystals (NaLuF4:Gd, Yb, Er@NaLuF4), and NaLuF4:Gd, Yb, Er@NaLuF4:Yb, Ho is higher than NaLuF4:Gd, Yb, Er@NaLuF4:Yb. The mechanisms of upconversion luminescence enhancement are discussed in detail. The bioimaging application of the nanocrystals showed that bright upconversion luminescence was observed when UCNs-labeled HeLa cells were excited with 980 nm light. This study presents a facile method for the synthesis of NaLuF4-based upconversion nanocrystals with intense luminescence that can be used as potential fluorescent probes for sensitive bioimaging, and the suggested mechanism could provide new insights into fabrication of upconversion materials with high upconversion fluorescence.

Synthesis of Multicolor Core/Shell NaLuF4:Yb3+/Ln3+@CaF2 Upconversion Nanocrystals

“…These superb attributes promise their applications in biological imaging [5,6,7,8], bio-detection [9,10], and three-dimensional display [11,12,13]. This motivation fuels a range of works to synthesize UCNCs with controlled size and morphology, as well as to prepare epitaxial core/shell UCNCs with enhanced efficiency and multifunction for theranostic applications, such as, NaYF 4 :Yb/Er@NaYF 4 [14], (NaLuF 4 :Gd 3+ /Yb 3+ /Er 3+ )@NaLuF 4 :Yb 3+ [15], and NaYbF 4 :Er@NaGdF 4 core/shell nanostructures [16]. …”

Structural and optical properties in Tm3+/Tm3+–Yb3+ doped NaLuF4 glass‐ceramics

“…NaLuF 4 exhibits two possible crystalline phases, cubic ( α ) and hexagonal ( β ), and it is an exceptional converter host for RE 3+ ions 18,19 . Nevertheless, the number of publications regarding NaLuF 4 GCs obtained by melting‐quenching is relatively scarce mainly due to the difficulty to obtain the desired crystalline phase with the proper degree of crystallization 20,21 .…”

A Polyol-Mediated Fluoride Ions Slow-Releasing Strategy for the Phase-Controlled Synthesis of Photofunctional Mesocrystals

“…The former is a high-temperature metastable phase, while the latter remains thermodynamically stable [7,8]. The past decades have witnessed much exploration of its controlled synthesis and up-/down-conversion luminescent properties [1,4,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Compared with considerable work on α→β phase transformation [1,4,9,10,11,12,13,14,15,16,17,18,19], the fabrication of α-NaYF 4 as well as the investigation involving the β→α transformation process have been neglected [26,30,33,34].…”

“…Compared with considerable work on α→β phase transformation [1,4,9,10,11,12,13,14,15,16,17,18,19], the fabrication of α-NaYF 4 as well as the investigation involving the β→α transformation process have been neglected [26,30,33,34]. So far, some strategies have been developed to fabricate α-phase NaYF 4 nano-/mocro-crystal, such as a liquid–solid–solution (LSS) procedure [21], polyol method [22], two-phase interfacial route [23], microwave-assisted ionic liquid (IL)-based technique [24], modified solvothermal approach [25], and self-sacrificing template multiple-step route [26,27]. Furthermore, introducing Mn 2+ ( r = 81 pm) with a smaller size than Y 3+ ( r = 89 pm) into an NaYF 4 host can dominate, forming pure α-phase NaYF 4 nanoparticles [28].…”