Ultrasmall, water dispersible, TWEEN80 modified Yb:Er:NaGd(WO4)2 nanoparticles with record upconversion ratiometric thermal sensitivity and their internalization by mesenchymal stem cells

Heterogeneous Oxysulfide@Fluoride Core/Shell Nanocrystals for Upconversion-Based Nanothermometry

Lanthanide (Ln³⁺)-doped upconversion nanoparticles (UCNPs) often suffer from weak luminescence, especially when their sizes are ultrasmall (less than 10 nm). Enhancing the upconversion luminescence (UCL) efficiency of ultrasmall UCNPs has remained a challenge that must be undertaken if any practical applications are to be envisaged. Herein, we present a Ln³⁺-doped oxysulfide@fluoride core/shell heterostructure which shows efficient UCL properties under 980 nm excitation and good stability in solution. Through epitaxial heterogeneous growth, a ∼4 nm optically inert β-NaYF₄ shell was coated onto ∼5 nm ultrasmall Gd₂O₂S:20%Yb,1%Tm. These Gd₂O₂S:20%Yb,1%Tm@NaYF₄ core/shell UCNPs exhibit a more than 800-fold increase in UCL intensity compared to the unprotected core, a 180-fold increase in luminescence decay time of the ³H₄ → ³H₆ Tm³⁺ transition from 5 to 900 μs, and an upconversion quantum yield (UCQY) of 0.76% at an excitation power density of 155 W/cm². Likewise, Gd₂O₂S:20%Yb,2%Er@NaYF₄ core/shell UCNPs show a nearly 5000-fold increase of their UCL intensity compared to the Gd₂O₂S:20%Yb,2%Er core and a maximum UCQY of 0.61%. In the Yb/Er core-shell UCNP system, the observed variation of luminescence intensity ratio seems to originate from a change in lattice strain as the temperature is elevated. For nanothermometry applications, the thermal sensitivities based on thermally coupled levels are estimated for both Yb/Tm and Yb/Er doped Gd₂O₂S@NaYF₄ core/shell UCNPs.

Effect of the Size and Shape of Ho, Tm:KLu(WO4)2 Nanoparticles on Their Self-Assessed Photothermal Properties

The incorporation of oleic acid and oleylamine, acting as organic surfactant coatings for a novel solvothermal synthesis procedure, resulted in the formation of monoclinic KLu(WO4)2 nanocrystals. The formation of this crystalline phase was confirmed structurally from X-ray powder diffraction patterns and Raman vibrational modes, and thermally by differential thermal analysis. The transmission electron microscopy images confirm the nanodimensional size (~12 nm and ~16 nm for microwave-assisted and conventional autoclave solvothermal synthesis) of the particles and no agglomeration, contrary to the traditional modified sol-gel Pechini methodology. Upon doping with holmium (III) and thulium (III) lanthanide ions, these nanocrystals can generate simultaneously photoluminescence and heat, acting as nanothermometers and as photothermal agents in the third biological window, i.e., self-assessed photothermal agents, upon excitation with 808 nm near infrared, lying in the first biological window. The emissions of these nanocrystals, regardless of the solvothermal synthetic methodology applied to synthesize them, are located at 1.45 μm, 1.8 μm and 1.96 μm, attributed to the 3H4 → 3F4 and 3F4 → 3H6 electronic transition of Tm3+ and 5I7 → 5I8 electronic transition of Ho3+, respectively. The self-assessing properties of these nanocrystals are studied as a function of their size and shape and compared to the ones prepared by the modified sol-gel Pechini methodology, revealing that the small nanocrystals obtained by the hydrothermal methods have the ability to generate heat more efficiently, but their capacity to sense temperature is not as good as that of the nanoparticles prepared by the modified sol-gel Pechnini method, revealing that the synthesis method influences the performance of these self-assessed photothermal agents. The self-assessing ability of these nanocrystals in the third biological window is proven via an ex-vivo experiment, achieving thermal knowledge and heat generation at a maximum penetration depth of 2 mm.

Upconversion luminescence and temperature sensing properties of NaGd(WO4)2:Yb3+/Er3+@SiO2 core-shell nanoparticles

Optical thermometry based on the fluorescence intensity ratio (FIR) of two thermally coupled levels in lanthanide ions has potential application in non-contact optical temperature sensing techniques. In this work, a shell of SiO2 with tunable thickness was uniformly coated on NaGd(WO4)2:Yb3+/Er3+ core upconversion nanoparticles (UCNPs). The effects of the silica shell on UC luminescence and thermal sensing properties of core-shell NaGd(WO4)2:Yb3+/Er3+@SiO2 UCNPs were investigated. Under 980 nm laser excitation, the temperature-dependent UC emission spectra of obtained samples were measured. The FIR was analyzed based on the thermally coupled 2H11/2 and 4S3/2 levels of Er3+ in the biological temperature range of 300-350 K, in which the Boltzmann distribution is applied. The emission from the upper 2H11/2 state within Er3+ was enhanced as temperature increased due to the thermal effect. Absolute sensitivities (S A) and relative sensitivities (S R) of the core and core-shell UCNPs were calculated. It was found that after SiO2 coating, the maximum S A was enhanced by ∼2-fold (1.03% K-1 at 350 K). Especially, S A was as high as 2.14% K-1 at 350 K by analyzing the FIR of the non-thermally coupled 2H11/2 and 4F9/2 levels.

Synthesis, functionalization and properties of uniform europium-doped sodium lanthanum tungstate and molybdate (NaLa(XO4)2, X = Mo,W) probes for luminescent and X-ray computed tomography bioimaging

A one-pot simple procedure for the synthesis of uniform, ellipsoidal Eu³⁺-doped sodium lanthanum tungstate and molybdate (NaLa(XO₄)₂, X  = W, Mo) nanophosphors, functionalized with carboxylate groups, is described. The method is based on a homogeneous precipitation process at 120 °C from appropriate Na⁺, Ln³⁺ and tungstate or molybdate precursors dissolved in ethylene glycol/water mixtures containing polyacrylic acid. A comparative study of the luminescent properties of both luminescent materials as a function of the Eu³⁺ doping level has been performed to find the optimum nanophosphor, whose efficiency as X-ray computed tomography contrast agent is also evaluated and compared with that of a commercial probe. Finally, the cell viability and colloidal stability in physiological pH medium of the optimum samples have also been studied to assess their suitability for biomedical applications.

Thermal enhancement and quenching of upconversion emission in nanocrystals

Photoluminescence is a powerful tool in temperature sensing. Recently, the application of upconversion (UC) nanocrystals (NCs) has shown great potential for nanothermometry due to high spatial resolution, superior accuracy, and its non-invasive nature. In addition to spectral changes upon heating, anomalous thermal enhancement of UC emission has been reported for UC NCs, but the underlying mechanism remains unclear. Here, we report on NaY(WO₄)₂ doped with the Er³⁺-Yb³⁺ UC couple in NCs and the bulk material, and investigate the temperature-dependent luminescence in both air and dry nitrogen. For UC NCs in air, strong thermal enhancement of UC emission is observed with good reversibility and accompanied by a lengthening of the decay time for the Er³⁺ UC emission and Yb³⁺ IR emission. In contrast, the measurements carried out on NCs in dry nitrogen demonstrate a transition from thermal enhancement in the first cycle to thermal quenching in the subsequent cycles. The thermal quenching is similar to that in bulk materials. Thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) measurements reveal the presence of water coupled on the NC surface that evaporates upon heating up to ∼470 K but is readsorbed upon cooling. Based on these observations, we explain the anomalous thermal enhancement of UC NCs in air by quenching of the Yb³⁺ and Er³⁺ emissions via surface adsorbed water molecules. The present study highlights the importance of careful characterization of surface adsorbed molecules which is crucial for understanding the luminescence properties of NCs, and enables the exploration of UC NCs with higher quantum efficiencies.

In situ reversible color variation of a ready-made upconversion material using the designed component of a three-state fluorescence switching system

In recent years, upconversion materials have attracted considerable attention because of their unique physicochemical features. Numerous studies have focused on the synthesis of upconversion materials with different colors. However, an easier way to vary the upconversion colors without changing the materials' components has not been extensively studied. In this study, we realized the in situ color variation of the designed upconversion material with the help of a three-state fluorescence switching hybrid device. The device was composed of Prussian blue and upconversion materials; the former element functioned as a fluorescence resonance energy transfer acceptor and the latter acted as a donor. Smartly applying the RGB color model guaranteed multicolor of the device. Moreover, the highest fluorescence contrast of the three-state fluorescence switching system was 86% (larger than the result of a previous study), and the three-state reversibility was remarkable; this was probably owing to the unique layer-by-layer dripping/electrodepositing assembly method. To the best of our knowledge, the in situ reversible color variation of the ready-made upconversion material has been demonstrated for the first time.

Double NIR laser stimulation and enhancing the thermal sensitivity of Er3+/Tm3+/Nd3+ doped multilayer core-shell nanoparticles

Non-contact thermal sensors are important devices to study cellular processes and monitor temperature in vivo. Herein, a novel highly sensitive nanothermometer based on NaYF₄:Yb,Er@ NaYF₄@NaYF₄:Yb,Tm@ NaYF₄:Nd (denoted as Er@Y@Tm@Nd) was prepared by a facile solvothermal method. When excited by the near-infrared (NIR) light of 808 and 980 nm, the as-prepared Er@Y@Tm@Nd nanoparticles could emit both blue and green light, respectively, since the lanthanide cations responsible for these emissions are gathered inside this nanostructure. The green and blue light intensity ratio exhibits obvious temperature dependence in the range of the physiological temperature. Additionally, the fluorescence intensity of Er³⁺ and Tm³⁺ are also greatly enhanced due to the multilayer structure that implies avoiding the Er³⁺ and Tm³⁺ energy cross-relaxation by introduction of a NaYF₄ wall between them. The as-prepared core-shell-shell-shell structure with Er³⁺ and Tm³⁺ in different layers improves dozens of times of the thermal sensitivity based on the non-thermal coupling levels of the probe: the maximum values for the sensitivity are 2.95% K^-1 (I _Er-521/I _Tm-450) and 6.30% K^-1 (I _Tm-474/I _Er-541) when excited by 980 and 808 nm laser sources, respectively. These values are well above those previously reported (<0.7% K^-1), indicating that the prepared nanostructures are temperature sensors with excellent thermal sensitivity and sensitive to NIR wavelength excitation that makes them highly preferred for thermal detection.

Synthesis and functionalization of NaGdF4:Yb,Er@NaGdF4 core-shell nanoparticles for possible application as multimodal contrast agents

Upconverting nanoparticles (UCNPs) are promising, new imaging probes capable of serving as multimodal contrast agents. In this study, monodisperse and ultrasmall core and core-shell UCNPs were synthesized via a thermal decomposition method. Furthermore, it was shown that the epitaxial growth of a NaGdF₄ optical inert layer covering the NaGdF₄:Yb,Er core effectively minimizes surface quenching due to the spatial isolation of the core from the surroundings. The mean diameter of the synthesized core and core-shell nanoparticles was ≈8 and ≈16 nm, respectively. Hydrophobic UCNPs were converted into hydrophilic ones using a nonionic surfactant Tween 80. The successful coating of the UCNPs by Tween 80 has been confirmed by Fourier transform infrared (FTIR) spectroscopy. Scanning electron microscopy (SEM), powder X-ray diffraction (XRD), photoluminescence (PL) spectra and magnetic resonance (MR) T1 relaxation measurements were used to characterize the size, crystal structure, optical and magnetic properties of the core and core-shell nanoparticles. Moreover, Tween 80-coated core-shell nanoparticles presented enhanced optical and MR signal intensity, good colloidal stability, low cytotoxicity and nonspecific internalization into two different breast cancer cell lines, which indicates that these nanoparticles could be applied as an efficient, dual-modal contrast probe for in vivo bioimaging.