Investigating the effect of Yb3+ and Er3+ concentration on red/green luminescent ratio in β-NaYF4: Er, Yb nanocrystals using spectroscopic techniques

Enhancement of the Photoluminescent Intensity of YVO4: Er, Yb UCNPs Using a Simple Coprecipitation Synthesis Method

Yttrium vanadate (YVO4) is a non-toxic ceramic matrix that, when doped with lanthanides, can be used as a photoluminescent biosensor. In this study, we meticulously synthesized upconversion nanoparticles (UCNPs) of YVO4 via chemical coprecipitation, using Er3+ and Yb3+ ions for codoping. The light emission achieved through upconversion mechanisms enables the excitation of nanoparticles with infrared light rather than ultraviolet light, enhancing the potential of current bioimaging techniques. The light emission intensity of our YVO4: Er, Yb UCNPs, a key factor in their effectiveness, depended on various easily adjustable factors during the synthesis, such as the dopant concentration, the heat treatment, and the cleaning process. The UCNPs were characterized using a range of advanced techniques, including X-ray diffraction (XRD) and Rietveld refinements, as well as Raman, photoluminescence (PL), and ultraviolet-visible (UV-vis) spectroscopies, and high-resolution transmission electron microscopy (HRTEM). We found the most convenient stoichiometry to obtain the YVO4: Er, Yb UCNPs and showed that a rigorous thermal treatment was necessary to achieve light emission through upconversion mechanisms. We also discovered that some porosity characteristics can be promoted in the YVO4: Er, Yb UCNPs during the cleaning process, depending on the solvent employed. The porosity and morphology of the nanoparticles could be predicted using the microstrain values obtained from the refinement of the crystalline structures. All these meticulous steps in our research have enabled us to develop an efficient synthesis pathway to produce YVO4: Er, Yb UCNPs with high photoluminescent intensity.

Upconversion Emission and Dual-Mode Sensing Characteristics of NaYF4:Yb3+/Er3+ Microcrystals at High and Ultralow Temperatures

In this study, we investigate micrometer-sized NaYF4 crystals double-doped with Yb3+/Er3+ lanthanide ions, designed for temperature-sensing applications. In contrast to previous studies, which focused predominantly on the high-temperature regime, our investigation spans a comprehensive range of both high and ultralow temperatures. We explore the relationship between temperature and the upconversion luminescence (UCL) spectra in both frequency and time domains. Our findings highlight the strong dependence of these spectral characteristics of lanthanide-doped NaYF4 crystals on temperature. Furthermore, we introduce a dual-mode luminescence temperature measurement technique, leveraging the upconversion emission intensity ratio for both green and red emissions. This study also examines the correlation between temperature sensing, energy level disparities, and thermal coupling in Er3+ ions across various temperature scales. Our research contributes to advancing the understanding and application of lanthanide-doped materials, setting a foundation for future innovations in temperature sensing across diverse fields.

Controllable structural and optical properties of NaYF4:Tm, Yb microparticles by Yb3+ doping for anti-counterfeiting

Hexagonal NaYF4:Tm, Yb upconversion (UC) phosphors with excellent UC luminescence quantum efficiency and chemical stability meet demands for applications in bioimaging and anti-counterfeiting printing. In this work, a series of NaYF4:Tm, Yb upconversion microparticles (UCMPs) with different concentrations of Yb were synthesized by a hydrothermal method. Then, the UCMPs become hydrophilic through surface oxidation of the oleic acid (C-18) ligand to azelaic acid (C-9) using the Lemieux-von Rodloff reagent. The structure and morphology of UCMPs were investigated by X-ray diffraction and scanning electron microscopy. The optical properties were studied using diffusion reflectance spectroscopy and photoluminescent spectroscopy under 980 nm laser irradiation. The emission peaks of the Tm³⁺ ions are 450, 474, 650, 690, and 800 nm, attributed to the transitions from the excited state to ground state ³H₆. These emissions are the results of two or three photon absorption through multi-step resonance energy transfer from excited Yb³⁺, confirmed via a power-dependent luminescence study. The results show that the crystal phases and luminescence properties of the NaYF4:Tm, Yb UCMPs are controlled by changing the Yb doping concentration. The printed patterns are readable under the excitation of a 980 nm LED. Moreover, the zeta potential analysis shows that the UCMPs after surface oxidation are water dispersible. In particular, the naked eye can observe the enormous upconversion emissions in UCMPs. These findings indicated that this fluorescent material is an ideal candidate for anti-counterfeiting and biological applications.

Enhancement of single upconversion nanoparticle imaging by topologically segregated core-shell structure with inward energy migration

Manipulating topological arrangement is a powerful tool for tuning energy migration in natural photosynthetic proteins and artificial polymers. Here, we report an inorganic optical nanosystem composed of NaErF₄ and NaYbF₄, in which topological arrangement enhanced upconversion luminescence. Three architectures are designed for considerations pertaining to energy migration and energy transfer within nanoparticles: outside-in, inside-out, and local energy transfer. The outside-in architecture produces the maximum upconversion luminescence, around 6-times brighter than that of the inside-out at the single-particle level. Monte Carlo simulation suggests a topology-dependent energy migration favoring the upconversion luminescence of outside-in structure. The optimized outside-in structure shows more than an order of magnitude enhancement of upconversion brightness compared to the conventional core-shell structure at the single-particle level and is used for long-term single-particle tracking in living cells. Our findings enable rational nanoprobe engineering for single-molecule imaging and also reveal counter-intuitive relationships between upconversion nanoparticle structure and optical properties.

Recent Progresses in NIR-II Luminescent Bio/Chemo Sensors Based on Lanthanide Nanocrystals

Fluorescent bio/chemosensors are widely used in the field of biological research and medical diagnosis, with the advantages of non-invasiveness, high sensitivity, and good selectivity. In particular, luminescent bio/chemosensors, based on lanthanide nanocrystals (LnNCs) with a second near-infrared (NIR-II) emission, have attracted much attention, owing to greater penetration depth, aside from the merits of narrow emission band, abundant emission lines, and long lifetimes. In this review, NIR-II LnNCs-based bio/chemo sensors are summarized from the perspectives of the mechanisms of NIR-II luminescence, synthesis method of LnNCs, strategy of luminescence enhancement, sensing mechanism, and targeted bio/chemo category. Finally, the problems that exist in present LnNCs-based bio/chemosensors are discussed, and the future development trend is prospected.

Luminescent Ink Based on Upconversion of NaYF4:Er,Yb@MA Nanoparticles: Environmental Friendly Synthesis and Structural and Spectroscopic Assessment

NaYF4:Er,Yb upconversion luminescent nanoparticles (UCNPs) were prepared by hydrothermal methods at 180 °C for 24 h. The X-ray diffraction (XRD) and TEM (transmission electron microscopy) images show that the resulting 60 nm UCNPs possess a hexagonal structure. In this work, maleic anhydride (MA) was grafted on the surface of UCNPs to induce hydrophilic properties. The photoluminescence spectra (PL) show upconversion emissions centered around 545 nm and 660 nm under excitation at 980 nm. The luminescent inks, including UCNPs@MA, polyvinyl alcohol (PVA), deionized water (DI), and ethylene glycol (EG), exhibit suitable properties for screen printing, such as high stability, emission intensity, and tunable dynamic viscosity. The printed patterns with a height of 5 mm and a width of 1.5 mm were clearly observed under the irradiation of a 980 nm laser. Our strategy provides a new route for the controlled synthesis of hydrophilic UCNPs, and shows that the UCNPs@MAs have great potential in applications of anti-counterfeiting packing.