Rare-earth free sensitizer in NaLuCrF4:Er upconversion material

Upconversion phosphors are known as a material system that can convert near-infrared light into visible/ultraviolet emissions by sequentially absorbing multiple photons. The studies on upconversion materials often use two rare earth (RE) ions as a sensitizer-activator pair. We investigated the influences on luminescence intensity depending on Cr-doping content (x) of hexagonal NaLu_0.98–xCr_xF₄Er_0.02 (x = 0–0.9) upconversion material by substituting Lu³⁺ ions with Cr³⁺in the absence of Gd³⁺. The change in upconversion luminescence intensity appears with saddle-like shape. We suggest that Cr³⁺ ions play the dual role as a constituent in host lattice and a sensitizer in the upconversion process. Optimal conditions for gaining the strongest upconversion emission correspond to x = 0.3–0.5, where there are effective energy transfers between Cr³⁺ and Er³⁺ ions and CrEr dimers. Apart from these values, the emission intensity decreases rapidly which can be ascribed to the absence of multiple-photon absorption for the case of low Cr³⁺ contents, and to the coupling between Cr³⁺ and/or Er³⁺ ions for the case of high Cr³⁺ contents. Magnetization and electron-spin-resonant measurements were performed to understand the correlation between the optical and magnetic properties.     

Polyol-mediated synthesis of water-soluble LaF3:Yb,Er upconversion fluorescent nanocrystals

Well-crystallized LaF₃:Yb,Er nanoparticles were prepared by the polyol method and three kinds of polyols (glycol, diethylene glycol and glycerol) were chosen as the reaction medium respectively. All of the obtained LaF₃:Yb,Er nanoparticles have roughly spherical shapes, and the average sizes of these nanoparticles ranged from 5 to 7 nm. These nanoparticles could be well dispersed in water or ethanol to form colloidal solutions. When these nanoparticles were excited by the 980 nm laser, several upconversion emissions were observed.     

Synthesis of NaYF4:20% Yb3+,2% Er3+,2% Ce3+@NaYF4 nanorods and their size dependent uptake efficiency under flow condition

Lanthanide doped fluorescent nanoparticles have gained considerable attention in biomedical applications. However, the low uptake efficiency of nanoparticles by cells has limited their applications. In this work, we demonstrate how the uptake efficiency is affected by the size of nanoparticles under flow conditions. Using the same size NaYF₄:20% Yb³⁺,2% Er³⁺,2% Ce³⁺ (the contents of rare earths elements are in molar fraction) nanoparticles as core, NaYF₄:20% Yb³⁺,2% Er³⁺,2% Ce³⁺@NaYF₄ core–shell structured nanorods (NRs) with different sizes of 60–224 nm were synthesized by thermal decomposition and hot injection method. Under excitation at 980 nm, a strong upconversion green emission (541 nm, ²H_11/2 → ⁴I_15/2 of Er³⁺) is observed for all samples. The emission intensity for each size nanorod was calibrated and is found to depend on the width of NRs. Under flow conditions, the nanorods with 96 nm show a maximum uptake efficiency by endothelial cells. This work demonstrates the importance of optimizing the size for improving the uptake efficiency of lanthanide-doped nanoparticles.     

多孔硅红外光激发上转换兰光特性

采用电化学腐蚀法制备多孔硅（ＰＳ），测量了ＰＳ在近红外光（８００ｎｍ）激发下的光致发光（ＰＬ）谱和光致发光激发（ＰＬＥ）谱，结果表明ＰＳ具有良好的上转换荧光特性，并随着存放时间的延长，在一定期了内峰值强度有明显的增强，这种非线性光这响应的增强，被认为与空间量子限制效应作用下局域束缚激子被激发有关，硅片初始电阻率赵低发光强度越大。     

Phase transitions and upconversion luminescence in oxyfluoride glass ceramics containing Ba4Gd3F17 nanocrystals

Novel transparent Er³⁺ doped oxyfluoride glass-ceramics containing Ba₄Gd₃F₁₇ nanocrystals were prepared by melt quenching followed by heat treatment of as-prepared glasses. The phase composition and microstructure were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Intense upconversion luminescence (UCL) was detected. Longer characteristic decay times and splitting of the luminescence bands compared to the precursor glass indicated the incorporation of erbium ions in the crystalline phase. The spectroscopic properties of glass ceramics were compared with single phase cubic and rhombohedral Ba₄Gd₃F₁₇ ceramics. The unit cell parameters and atomic positions in the rhombohedral phase were calculated using Rietveld refinement. The local environment of Er³⁺ and the phonon energy of both polymorphs were analyzed using luminescence and Raman spectroscopy. In the glass ceramics, a phase transition from distorted metastable fluorite to ordered rhombohedral Ba₄Gd₃F₁₇ was observed and resulted in the enhancement of the efficiency of UCL.     

Rational design and synthesis of upconversion luminescence-based optomagnetic multifunctional nanorattles for drug delivery

Optomagnetic multifunctional composite based on upconversion luminescence nanomaterial is regarded as a promising strategy for bioimaging,disease diagnosis and targeted delivery of drugs.To explore a mesoporous nanostructure with excellent water dispersibility and high drug-loading capacity,a novel nanorattle-structured Fe3O4@SiO2@NaYF4∶Yb,Er magnetic upconversion nanorattle (MUCNR) was suc-cessfully designed by using Fe3O4 as core and NaYF4∶Yb,Er nanocrystals as shell.The microstructures and crystal phase of the as-prepared MUCNRs were evaluated by transmission electron microscopy,X-ray powder diffraction and N2 adsorption/desorption isotherms.The Kirkendall effect was adapted to explain the formation mechanism of the MUCNRs.The loading content and encapsulation efficiency of doxorubicin hydrochloride (DOX) could reach as high as 18.2％ and 60.7％,respectively.Moreover,the DOX loading MUCNR (DOX-MUCNR) system showed excellent sustained drug release and strong pH-dependent performance,which was conducive to drug release at the slightly acidic microenvironment of tumor.Microcalorimetry was used to quantify the interactions between the carrier structure and drug release rate directly.The heat release rates in the heat-flow diagrams are basically consistent with the DOX release rate,thereby showing that microcalorimetry assay not only provides a unique thermody-namic explanation for the structure-activity relationship of Fe3O4@SiO2@NaYF4∶Yb,Er MUCNRs but also provides powerful guidance to avoid the blind selection or design of drug carriers.Therefore,our work firmly provided a comprehensive perspective for using Fe3O4@SiO2@NaYF4∶Yb,Er MUCNRs as a remark-able magnetic targeted drug carrier.     

Enhancing upconversion of Nd3+ through Yb3+-mediated energy cycling towards temperature sensing

Photon upconversion of lanthanides has been a powerful means to convert low-energy photons into high-energy ones. However, in contrast to the mostly investigated lanthanide ions, it has remained a challenge for the efficient upconversion of Nd³⁺ due to the deleterious concentration quenching effect. Here we report an efficient strategy to enhance the upconversion of Nd³⁺ through the Yb³⁺-mediated energy cycling in a core-shell-shell nanostructure. Both Nd³⁺ and Yb³⁺ are confined in the interlayer, and the presence of Yb³⁺ in the Nd-sublattice provides a more matched energy for the upconversion transitions occurring at the intermediate state of Nd³⁺ towards much better population at its emissive levels. Moreover, this design also minimizes the possible cross-relaxation processes at both intermediate level and the emissive levels of Nd³⁺ which are the primary factors limiting the upconversion performance for the Nd³⁺-doped materials. Such energy cycling-enhanced upconversion shows promise in temperature sensing.     

Luminescence properties of zirconia nanocrystals prepared by solar physical vapor deposition

Zirconia nanocrystals have attracted considerable interest as biolabels, which can be used as probes for medical imaging and biosensor applications. However, zirconia particle agglomeration forms a major limitation to its use for biolabeling. In this backdrop, for the first time, well-separated zirconia nanocrystals were obtained in a Heliotron reactor (PROMES CNRS, France) via the solar physical vapor deposition (SPVD) method. As the raw material target for solar evaporation, zirconia nanopowders obtained via the sol–gel process were used. The luminescence and upconversion luminescence properties of the Sol Gel nanopowders were compared with those of the SPVD nanocrystals. Erbium was chosen as the luminescence center with ytterbium as the sensitizer, and along with these two dopants, niobium was also used. Niobium acts as a charge compensator to compensate for depletion in the charge due to the introduction of trivalent erbium and ytterbium at tetravalent zirconium sites. Consequently, the oxygen-vacancy concentration is reduced, and this results in a significant increase in the upconversion luminescence.The SPVD-prepared samples showed less agglomeration and a fine crystal structure as well as high luminescence, and thus, such samples can be of great interest for biolabeling applications.     

Upconversion under polychromatic excitation: Y2SiO5:Pr3+, Li+ converts violet,cyan, green,and yellow light into UVC

Due to their unique ability to amplify photon energies, upconversion (UC) materials are being studied for numerous potential applications to light-utilizing technologies. Many such uses of UC demand adequate conversion efficiency under low-power, broad spectrum irradiation; however, spectroscopic studies of UC phosphors have been most commonly conducted using monochromatic laser excitation that is resonant with the desired electronic transitions. In this study, we sought to elucidate the behavior of UC materials under polychromatic excitation by initiating visible-to-UV UC in a Pr³⁺-doped phosphor under multiple combined two-color excitation beams. Several wavelengths of light, which were not capable of producing UC emission when used alone, were found to contribute to UC when combined with other colors, indicating that polychromatic UC mechanisms warrant consideration in applications involving broad band excitation.     

Tunable structure and intensive upconversion photoluminescence for Ho3+-Yb3+ codoped bismuth titanate composite synthesized by sol-gel-combustion (SGC) method

Ho³⁺ and Yb³⁺ codoped bismuth titanate (BTO) composite powders with infrared to visible upconversion luminescence (UCL) function were prepared by SGC method. The effects of Ho³⁺ and Yb³⁺ doping content on the structure and property were investigated for BTO: xHo, 0.2 Yb (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) and BTO: 0.02Ho, yYb (y = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) samples. All the samples include three bismuth titanate phases (Bi₄Ti₃O₁₂, Bi₂Ti₂O₇, and Bi₂₀TiO₃₂), and the phase proportion can be tuned by changing Ho³⁺ and Yb³⁺ doping content. These powders are well crystalized with honeycomb-like microscopic structure, and with good absorption for 233 nm, 310 nm and 975 nm wavelength. The band gap can be tuned from 3.53 eV to 4.03 eV when increasing Yb³⁺ content from y = 0 to y = 0.9. A strong 530–580 nm green emission band and a relative weak 630–690 nm red one corresponding to Ho³⁺: ⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions appear in the UCL spectra for all the BTO: Ho, Yb samples when pumped at 980 nm. The emission intensities can well be tuned with various Ho³⁺ and Yb³⁺ content. The optimal UCL was obtained in BTO: 0.02Ho, 0.5 Yb for all the prepared samples. The energy transfer mechanism is analyzed by building a two-photon energy transfer model, which is proved by the relationship between emission intensities and pumping power measurement. The concentration quenching of Ho³⁺ is caused by cross relaxation of CR₁ and CR₂ (Ho: ⁵F₄, ⁵S₂ + ⁵I₈ → ⁵I₄ + ⁵I₇) and by CR₃ (Ho: ⁵F₄, ⁵S₂ + Yb: ²F_7/2 → Ho: ⁵I₆ + Yb: ²F_5/2) for Yb³⁺ quenching. The mean luminescence lifetime (τ_m) from Ho: ⁵S₂ decreases monotonously with the increase of Ho³⁺ and Yb³⁺ content.