Low temperature CO oxidation catalysed by flower-like Ni–Co–O: how physicochemical properties influence catalytic performance

In this work, mesoporous Ni–Co composite oxides were synthesized by a facile liquid-precipitation method without the addition of surfactant, and their ability to catalyse a low temperature CO oxidation reaction was investigated. To explore the effect of the synergetic interaction between Ni and Co on the physicochemical properties and catalytic performance of these catalysts, the as-prepared samples were characterized using XRF, XRD, LRS, N₂-physisorption (BET), SEM, TEM, XPS, H₂-TPR, O₂-TPD and in situ DRIFTS characterization techniques. The results are as follows: (1) the doping of cobalt can reduces the size of NiO, thus massive amorphous NiO have formed and highly dispersed on the catalyst surface, resulting in the formation of abundant surface Ni²⁺ ions; (2) Ni²⁺ ions partially substitute Co³⁺ ions to form a Ni–Co spinel solid solution, generating an abundance of surface oxygen vacancies, which are vital for CO oxidation; (3) the Ni_0.8Co_0.2 catalyst exhibits the highest catalytic activity and a satisfactory stability for CO oxidation, whereas a larger cobalt content results in a decrease in activity, suggesting that the amorphous NiO phase is the dominant active phase instead of Co₃O₄ for CO oxidation; (4) the introduction of Co can alter the morphology of catalyst from plate-like to flower-like and then to dense granules. This morphological variation is related to the textural properties and catalytic performance of the catalysts. Lastly, a possible mechanism for CO oxidation reaction is tentatively proposed.

The flower-like catalyst possesses highly dispersed amorphous NiO and a high concentration of surface oxygen vacancies which are the central points for CO oxidation.     

Scalable and cost-effective Ag nanowires flexible transparent electrodes

Flexible transparent electrodes (TEs) are important for new electronic devices. This paper reports a scalable, cost effective Ag nanowires (AgNWs) TE, which is made of a SnO₂·xH₂O and AgNWs composite layer and a flexible polyethylene terephthalate (PET) bottom layer by a solution method at room temperature. The AgNWs/SnO₂·xH₂O composite TEs reveal a significant reduction of four orders in magnitude of sheet resistance, from 90 kΩ sq⁻¹ to 12 Ω sq⁻¹, while retaining transmittance of about 92% at 550 nm. This could be owing to the significant reduction of contact resistance for the weld-like junction of bound AgNWs. Compared with others, this method is characterized by filling gaps of the silver nanowire network with SnO₂·xH₂O. In addition, the adhesive forces between the AgNWs and the substrate are improved. This could be attributed to strong adhesion of SnO₂·xH₂O with the substrate. Moreover, this foldable transparent electrode is applicable for any non-planar surfaces and ultimately for future wearable optoelectronic devices.

This paper reports one of a scalable, cost effective Ag nanowires (AgNWs) TE, which reveals a significant reduction of four orders in magnitude of sheet resistance, from 90 kΩ sq⁻1 to 12 Ω sq⁻1, while keep transmittance of about 92% at 550 nm.     

White light-induced cell apoptosis by a conjugated polyelectrolyte through singlet oxygen generation

A cationic conjugated polyelectrolyte (CPE) PPET3 with a poly(p-phenylene ethynylene terthiophene) backbone and quaternary ammonium side chains was designed and synthesized. It serves as an efficient photosensitizer for photodynamic therapy under white light irradiation and induces cell death through the mitochondrial apoptosis pathway.

A conjugated polyelectrolyte PPET3 functions as an efficient photosensitizer for photodynamic therapy and induces cell apoptosis under white light irradiation.

Reactive oxygen species (ROS) act as a double-edged sword in cells in which moderate levels of ROS function as important messengers in intracellular signalling pathways.^1–3 However, overproduction of ROS may disrupt cellular homeostasis, cause oxidative damage to cellular constituents, and result in cell growth arrest or cell death.^2,4 Thus, it is a promising strategy to utilize ROS as cytotoxic agents to induce cell apoptosis. Photodynamic therapy (PDT) is an attractive ROS-mediated therapeutic modality.⁵ It utilizes the combination of a photosensitizer (PS), light (usually in the visible spectrum), and oxygen molecules to produce excess intracellular ROS, predominantly highly reactive singlet oxygen (¹O₂) that can oxidize and damage biomolecules.^6,7 However, the cell death mechanism induced by PDT is complex and depends on multiple factors, such as intracellular localization of PS, PS concentration, and light dose.⁸ Therefore, the mechanism of cell death is an ongoing topic of investigation in PDT.Various chemical compounds have been investigated as PSs in PDT, including porphyrins, phenothiazines, cyanines, borondipyrromethene dyes, and transition metal complexes.^9–11 In recent years, conjugated polyelectrolytes (CPEs), characterized by a delocalized π-electronic backbone and ionic side chains, have aroused considerable attention as admirable PSs due to their interesting optical properties, such as strong light-harvesting capability, high fluorescence quantum yields, and good photostability.^12–14 Since a cationic poly(p-phenylene ethynylene) (PPE) was first reported to kill bacteria under visible light irradiation in 2005,¹⁵ a variety of CPEs with backbones of PPEs,^16,17 poly(phenylene vinylene)s (PPVs),¹⁸ poly(fluorine-co-phenylene)s (PFPs),¹⁹ and poly(thiophene)s (PTs)^20,21 have been developed for use in light-activated antimicroorganism therapy. Moreover, other researches that have gained much attention include anticancer systems based on CPEs and the combination of CPEs with other traditional PSs to enhance ¹O₂ generation,^22–25 for example, the combined use of CPEs with porphyrin to enhance the generation of ¹O₂. However, few studies exist on the cellular response to CPE-mediated PDT. Even though a cationic PT has been reported to induce cell apoptosis by increasing activation of caspase-3 under irradiation,²² the upstream and downstream signal events elicited by CPEs-mediated PDT are still not fully understood.In this work, we report a cationic poly(p-phenylene ethynylene terthiophene) (PPET3) as a sensitizer for effective PDT under white light irradiation. Compared with the reference polyelectrolyte PPE, PPET3 showed more efficient photo-induced ¹O₂ generation and higher photocytotoxicity under identical conditions. The mechanisms of cell apoptosis induced by PPET3 in PDT treatment were evaluated through flow cytometric analysis, mitochondrial membrane potential (MMP) measurement, western blot, and confocal imaging analysis. The results suggest that PPET3 can induce mitochondrial apoptosis upon white light irradiation (Scheme 1).Open in a separate windowScheme 1Illustration of PPET3 as a photosensitizer to induce cell apoptosis.The molecular structures of PPET3 and PPE are shown in the ESI (Scheme S1^†), and their photophysical properties were characterized in pure water. PPET3 exhibited a wide absorption band located at 350–600 nm with a peak at 450 nm (Fig. S1, ESI^†), covering the spectral regions ranging from violet, blue, green to yellow and suggesting that the polymer can be excited by broad spectrum white light. However, PPE showed a narrower absorption band in the wavelength range of 350–500 nm (absorption peak at 393 nm), which might limit its excitation efficiency under white light irradiation. In addition, both molar extinction coefficients of PPET3 and PPE were determined to be about 1.5 × 10⁴ M⁻¹ cm⁻¹, indicating that the two CPEs have strong light-harvesting capability.Since PDT efficacy mostly depends on the ¹O₂ generation of the photosensitizer, the ability of PPET3 and PPE to photosensitize ¹O₂ was evaluated using 9,10-anthracenediyl-bis(methylene) dimalonic acid (ABDA) as the ¹O₂ detection reagent. ABDA can be selectively oxidized by ¹O₂ to form its corresponding endoperoxide component (Fig. 1a), which exhibited photobleaching.²⁶ Therefore, the loss of absorbance of ABDA can quantify the amount of ¹O₂ generation in solution. Fig. 1b shows the absorption spectra of ABDA in aqueous solutions containing different concentrations of PPET3 (5, 10, 20, and 50 μM) as a function of exposure time of white light (400–800 nm, power: 1 W, fluence rate: 100 mW cm⁻²). Under irradiation, the absorption peaks of ABDA monotonically decreased in intensity with an increase in exposure time, indicating an increased yield of ¹O₂. Similar behaviour was observed in the case of ABDA in the presence of PPE under identical experimental conditions (Fig. S2a, ESI^†). The ratio of the characteristic absorption peak of ABDA at 378 nm before and after irradiation against exposure time is summarized (Fig. 1c and S2b, ESI^†). In the presence of 5, 10, 20, and 50 μM PPET3 and after 10 min of white light illumination, the relative absorbance of ABDA at 378 nm decreased to 74.6%, 61.4%, 36.0%, and 12.3%, respectively, and decreased to 84.7%, 75.9%, 65.1% and 30.0% for the respective concentrations of PPE. The decrease rate of absorbance intensity in the presence of PPET3 was faster than that in PPE with equal concentration. That is, PPET3 displayed better ¹O₂ generation ability, which was attributed to the intersystem crossing effect enhanced by terthiophene units.^27,28 There was no obvious decrease in absorbance for the solution containing ABDA without any CPEs after irradiation, confirming that the decrease in ABDA absorbance intensity was caused by ¹O₂ generated from photosensitive CPEs instead of white light illumination.Open in a separate windowFig. 1(a) Schematic diagram of ¹O₂ generation and chemical reaction of ABDA with ¹O₂. (b) Absorption spectral changes of ABDA in the presence of 5, 10, 20, and 50 μM PPET3 over different periods of exposure time. (c) Plot of normalized absorbance of ABDA at 378 nm against exposure time in the solutions containing various concentrations of PPET3. (d) Relative cell viability of HeLa cells incubated with PPET3 at a series of concentrations with or without white light irradiation.To verify the PDT efficacy of PPET3 and PPE, the dark-toxicity and phototoxicity against human cervical carcinoma (HeLa) cells were measured using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. We first explored the parameters of the PDT dose (PS concentration and irradiation time). HeLa cells were treated with graded concentrations of PPET3 and PPE (5, 10, 20, 50, and 100 μM) and different doses of white light (5 and 10 min). As illustrated in Fig. 1d, the cell viability decreased rapidly with increasing concentrations of PPET3 and increasing white light dose. In contrast, PPET3 showed no obvious cytotoxicity (cell viability > 99%) in the dark even up to a concentration of 100 μM, suggesting water-soluble PPET3 has excellent biocompatibility and low dark cytotoxicity. In addition, the cells maintained high viability when white light irradiation was performed alone. These results demonstrate that photocytotoxicity was induced by ¹O₂ generated from photosensitive PPET3 and not by the irradiating white light. The reference polyelectrolyte, PPE generated less ¹O₂ under white light irradiation than PPET3 in identical concentration and displayed much less photocytotoxicity against HeLa cells under similar experimental conditions (Fig. S3, ESI^†). Even at the highest tested concentration (100 μM), HeLa cells maintained 84% viability after exposure to white light for 10 min. We speculate that the PDT efficacy of a photosensitizer in HeLa cells depends both on its ¹O₂ generation efficiency and other parameters, such as cellular uptake and retention.Earlier studies found that the internalized PPET3 translocated from lysosomes to mitochondria upon white light irradiation.²⁹ However, the responses of HeLa cells to the excessive ¹O₂ have not yet been investigated. Since mitochondria plays a vital role in cell apoptotic pathway, we explored the capability of ¹O₂ to induce cell apoptosis. The effects of PPET3-mediated PDT on apoptotic death in Annexin V-mFluor Violet 450/PI staining HeLa cells were assessed by flow cytometric analysis. Fig. 2 shows the percentages of viable (Annexin V negative, PI negative), early apoptotic (Annexin V positive, PI negative), late apoptotic (Annexin V positive, PI positive), and necrotic (Annexin V negative, PI positive) cells after different treatments. The cells treated with PPET3 (5 and 10 μM) had minimal detrimental effects without white light irradiation. However, after PPET3-treated cells were exposed to white light, the cells exhibited an increased percentage of both early apoptotic cells and late apoptotic cells. For 5 μM PPET3-treated cells under white light, the early apoptotic cells were 4.56% after 5 min irradiation, and increased to 11.6% after 10 min irradiation. When the treated concentration of PPET3 was increased to 10 μM, the percentage of early apoptotic cells was slightly increased to 9.32% and 12.4% after 5 and 10 min of irradiation, respectively. These results serve as evidence that PPET3-mediated PDT is capable of inducing cell apoptosis.Open in a separate windowFig. 2Flow cytometry quantification of different treatments of HeLa cells labeled with Annexin V-mFluor Violet 450/PI. The cells were incubated with PPET3 (5 and 10 μM) and irradiated by white light for 5 or 10 min.Since early apoptotic cells undergo characteristic depolarization of mitochondria,³⁰ detection of the mitochondrial membrane potential (MMP, ΔΨ_m) using tetramethylrhodamine (TMRM) was carried out by flow cytometry. TMRM is a lipophilic cationic rhodamine derivative, which will accumulate within mitochondria in an inverse proportion to ΔΨ_m.³¹Fig. 3 displays the histogram plots of HeLa cells after the treatment of PPET3 alone or combined with PPET3 and white light irradiation. Compared to HeLa cells treated solely with PPET3, the cells treated by combined PPET3 and irradiation exhibited weaker TMRM fluorescence, indicating mitochondrial membrane disruption in HeLa cells after combined treatment. Moreover, the intensity of TMRM fluorescence weakened further when the irradiation time was prolonged from 5 to 10 min. The above studies indicate that the MMP decreased with an increase in PPET3 concentration or light dose, which agree with the results from Annexin V-mFluor staining in Fig. 2.Open in a separate windowFig. 3Flow cytometry quantification of different treatments of HeLa cells labeled with TMRM (200 nM). The cells were incubated with PPET3 (5 and 10 μM) and irradiated with white light for 5 or 10 min.The loss of MMP was reported to facilitate cytochrome C release and activate apoptotic cascade reaction.³² To further analyse the mitochondria-mediated apoptosis pathway activated by PPET3-mediated PDT, the expression of related proteins was tested using western blotting. HeLa cells treated with PPET3 alone and with white light irradiation alone had nearly no changes in the expressions of the detected proteins compared with control HeLa cells (Fig. 4a, left). However, when the cells were treated with the combination of PPET3 and white light illumination, the expression of cleaved caspase-9, -3, and -7 increased, accompanying the cleavage of poly(ADP-ribose) polymerase (PARP), one of the main targets of cleaved caspase-3. Caspase-9 plays a pivotal role in the intrinsic mitochondrial apoptosis pathway, and the cleavage of caspase-9 presumably triggers a cascade of caspase activation events,³³ including the cleavage of caspase-7 and -3. PARP activated by cleaved caspase-3 is essential for cell apoptosis.³⁴ Our results reveal that PPET3 can efficiently induce apoptosis of HeLa cells via the mitochondrial apoptotic pathway.Open in a separate windowFig. 4(a) Western blot analysis of the expression of apoptosis-related proteins in HeLa cells. Left: HeLa cells were incubated with PPET3 (5 and 10 μM) and irradiated by white light for 5 or 10 min. The expression of the proteins was evaluated at 8 h post-treatment. Right: HeLa cells were incubated with PPET3 (5 μM) and irradiated by white light for 10 min. The expression of the proteins was evaluated at 0.5, 1, 2, 4, 6, and 8 h post-treatment, respectively. Ctrl: untreated cells, P: cells incubated with 5 μM PPET3 alone. (b) Confocal fluorescence images of PPET3 (5 μM)-treated HeLa cells after white light irradiation for 10 min. Cells were stained with Hoechst 33342. Scale bar: 5 μm.Furthermore, time-dependent cell apoptosis was also investigated using western blotting. The cells were treated with a combination of 5 μM PPET3 and 10 min of white light illumination. The expression of the caspase family as well as PARP was evaluated at 0.5, 1, 2, 4, 6, and 8 h post-treatment (Fig. 4a, right). In cells fractionated within 0.5–2 h post-treatment, there was almost no considerable change in the expression of cleaved caspase-9, -3, and -7 as well as cleaved PARP. However, when time was extended to 4 and 6 h, a decreasing amount of the cleaved caspases and PARP was observed. In addition, levels of the proteins were found to increase slightly 8 h post-treatment.To explore the morphological changes of nucleus induced by PPET3-mediated PDT treatment, Hoechst 33342 which has a bright blue fluorescence when combined with DNA double strands, was employed to label the nuclei. As shown in Fig. 4b, the nucleus of control cells treated with 5 μM PPET3 alone were evenly stained by Hoechst 33342. In contrast, the cells treated with the combination of 5 μM PPET3 and 10 min of white light illumination displayed chromatin margination (the yellow arrow in Fig. 4b), the disassembly of chromosomal territory, and formation of spatially organized nuclear apoptotic bodies (the white arrow in Fig. 4b). These results further indicate that the cells underwent apoptotic cell death after PPET3-mediated PDT treatment.     

ADC值在原发性脑淋巴瘤放疗疗效中的预测价值

目的探讨扩散加权成像中表观扩散系数(apparent diffusion coefficient,ADC)值对原发性脑弥漫大B细胞淋巴瘤放疗疗效的预测价值。材料与方法收集宁夏医科大学总医院2009至2017年经临床和病理活检证实为弥漫大B细胞淋巴瘤的20例原发性脑淋巴瘤患者(无免疫缺陷病史)资料,通过放疗前后轴位T1WI增强病灶强化最大径变化计算出肿瘤消退率进而将病灶分为完全缓解(complete remission,CR)组和非完全缓解(非CR)组,测量病灶放疗前的最小ADC(ADC_(min))值、平均ADC(ADCmean)值、最大ADC(ADC_(max))值,采用独立样本t检验验证放疗前CR组与非CR间有无差异,采用Pearson相关性分析评价肿瘤ADC值与肿瘤消退率的相关性,进而利用受试者工作特性曲线求得ADC值的阈值。结果放疗前ADC_(min)值与ADCmean值在CR组与非CR组间差异有统计学意义(P0.05),而ADC_(max)在两组间差异无统计学意义。ADC_(min)值、ADCmean值与肿瘤消退率有相关性(r=0.630、0.460,P=0.000、0.005),且分别大于0.602×10~(-3) mm~2/s、0.800×10~(-3) mm~2/s时患者放疗疗效,能在放疗周期完成后达到CR的效果。结论 ADC_(min)值及ADCmean值可以作为预测原发性脑淋巴放疗疗效的指标。     

Reconstruction of structure and function in tissue engineering of solid organs: Toward simulation of natural development based on decellularization

Failure of solid organs, such as the heart, liver, and kidney, remains a major cause of the world's mortality due to critical shortage of donor organs. Tissue engineering, which uses elements including cells, scaffolds, and growth factors to fabricate functional organs in vitro, is a promising strategy to mitigate the scarcity of transplantable organs. Within recent years, different construction strategies that guide the combination of tissue engineering elements have been applied in solid organ tissue engineering and have achieved much progress. Most attractively, construction strategy based on whole‐organ decellularization has become a popular and promising approach, because the overall structure of extracellular matrix can be well preserved. However, despite the preservation of whole structure, the current constructs derived from decellularization‐based strategy still perform partial functions of solid organs, due to several challenges, including preservation of functional extracellular matrix structure, implementation of functional recellularization, formation of functional vascular network, and realization of long‐term functional integration. This review overviews the status quo of solid organ tissue engineering, including both advances and challenges. We have also put forward a few techniques with potential to solve the challenges, mainly focusing on decellularization‐based construction strategy. We propose that the primary concept for constructing tissue‐engineered solid organs is fabricating functional organs based on intact structure via simulating the natural development and regeneration processes.     

Covalent incorporation of tobacco mosaic virus increases the stiffness of poly(ethylene glycol) diacrylate hydrogels

Hydrogels are versatile materials, finding applications as adsorbers, supports for biosensors and biocatalysts or as scaffolds for tissue engineering. A frequently used building block for chemically cross-linked hydrogels is poly(ethylene glycol) diacrylate (PEG-DA). However, after curing, PEG-DA hydrogels cannot be functionalized easily. In this contribution, the stiff, rod-like tobacco mosaic virus (TMV) is investigated as a functional additive to PEG-DA hydrogels. TMV consists of more than 2000 identical coat proteins and can therefore present more than 2000 functional sites per TMV available for coupling, and thus has been used as a template or building block for nano-scaled hybrid materials for many years. Here, PEG-DA (M_n = 700 g mol⁻¹) hydrogels are combined with a thiol-group presenting TMV mutant (TMV_Cys). By covalent coupling of TMV_Cys into the hydrogel matrix via the thiol-Michael reaction, the storage modulus of the hydrogels is increased compared to pure PEG-DA hydrogels and to hydrogels containing wildtype TMV (wt-TMV) which is not coupled covalently into the hydrogel matrix. In contrast, the swelling behaviour of the hydrogels is not altered by TMV_Cys or wt-TMV. Transmission electron microscopy reveals that the TMV particles are well dispersed in the hydrogels without any large aggregates. These findings give rise to the conclusion that well-defined hydrogels were obtained which offer the possibility to use the incorporated TMV as multivalent carrier templates e.g. for enzymes in future studies.

Tuning hydrogel properties with viruses.     

Sandwich-like transparent ceramic demonstrates ultraviolet and visible broadband downconversion luminescence

We report a novel sandwich-like Ce, Yb:Y_1.76La_0.18Zr_0.06O_x transparent ceramic, which shows efficient UV/Vis to NIR downconversion luminescence with a broad conversion window (250 nm to 650 nm), owing to its graded defective structure. The solar spectrum conversion efficiency is improved by 3.6 fold, compared with the homogenous material.

A novel sandwich-like transparent ceramic with a graded defective structure shows efficient downconversion luminescence with a broad conversion window.

The major obstacle limiting the efficiency of crystalline silicon (c-Si) solar cells is the mismatch between the spectral response curve of c-Si and the solar spectrum distribution.¹ The bandgap energy (E_g) of c-Si is approximately 1.1 eV. Sunlight photons of energy greater than the E_g cannot be absorbed efficiently by c-Si, with extra energy being wasted as heat losses, therefore the external quantum efficiency cannot be higher than the Shockley–Queisser limit (∼30%).¹ One way in which to break this limit is by using downconversion materials to convert the short wavelength part of the solar spectrum into wavelengths closer to E_g before it reaches the solar cell.^2,3 Previous efforts have shown that Re–Yb (Re = Tb, Tm, Pr or Ce) doped downconversion materials are promising materials for application in spectrum modified solar cells, which can convert high-energy UV/Vis photons into more (∼1000 nm) photons through Re → Yb cooperative energy transfer.^4–10 However, the narrow, discrete, line-like conversion band of Tb³⁺/Tm³⁺/Pr³⁺ ions, due to their forbidden f–f transition, is inefficient for harvesting of the broadband solar spectrum.^11,12 On the other hand, Ce ions demonstrate a broader absorption window (400–600 nm) owing to the allowed Ce 4f → 5d transition,^13–17 but this absorption range is still not wide enough for application in sunlight harvesting.Yttria based transparent ceramic has been an attractive optical material in the past decade because it shows high transparency (98% of theoretical transmittance),¹⁸ a wide transmittance range (250–2500 nm) and good chemical/thermal stability.¹⁹ These advantages make it a promising material for use in spectrum modified solar cells, either as a luminescent down-shifting layer (LDS) on the top surface of solar cells,²⁰ or as a luminescent solar concentrator (LSC) with solar cells coupled on its edges.²¹ Its good transparency in a large transmittance range indicates that photons with a wavelength beyond the conversion window can transmit through the spectral converter without optical losses, therefore common absorption of photovoltic materials such as c-Si will not be harmed.¹¹ In addition, the emerging transparent ceramic technology also makes it easy to fabricate novel optical materials with complex structures, such as gradient-doping ceramic composites or cladding-core configuration fibres.²²Here, we designed a novel sandwich-like Ce, Yb co-doped Y_1.76La_0.18Zr_0.06O_x (YLZO) transparent ceramic composite with a graded-defective structure. The introduction of defects improves the light absorption effectively, and the broadband downconversion luminescence with a large conversion window covering the whole UV-Vis region (250–650 nm) is realized for the first time. The sandwich ceramic also shows good transparency, which is necessary for avoiding the common absorption decrease of photovoltic materials, while being used as a spectral converter for solar cells.The sandwich-like transparent ceramic was prepared by vacuum sintering and a tailored air-annealing process (experimental details are shown in the ESI^†). In a typical routine (Fig. 1), the cold-isostatic-pressed disk type green body of raw powders was vacuum sintered at 1800 °C for 20 h under 1.0 × 10⁻³ Pa, and a dark brown color Ce, Yb:YLZO ceramic was obtained. The dark color of the ceramic is due to the high concentration of oxygen defects that come from high temperature sintering under a vacuum atmosphere, which is very common in transparent ceramic sintering.^23–25 In transparent ceramic fabrication, this is usually followed with long-term annealing at a relative high-temperature under an air atmosphere to eliminate the oxygen defects.^22,26 During the air annealing, the following defect reaction will occur:1Open in a separate windowFig. 1Synthesis procedure of the sandwich-like Ce, Yb:YLZO transparent ceramic.In which, V_O indicates the oxygen vacancy, and O_O is the oxygen at the lattice site. The limiting step of reaction (1) is generally oxygen diffusion in the solid ceramics at annealing temperatures. In this work, we controlled the annealing temperature and the time taken to partially eliminate the oxygen defects and obtain a sandwich-like oxygen defect graded distribution transparent ceramic as shown in Fig. 1 and ​and2a.2a. From Fig. 2a it can be seen that when the vacuum-sintered ceramic disk is annealed at 1100 °C under an air atmosphere for 2 h, the color reduces due to the diffusion of oxygen from air into the highly oxygen defective ceramic bulk, and a graded yellow/red/yellow colored sandwich-like Ce, Yb:YLZO ceramic is obtained.Open in a separate windowFig. 2(a) Photos of the sandwich-like Ce, Yb:YLZO ceramic. (b) Photos of the fully-annealed (yellow) and as-sintered (dark brown) Ce, Yb:YLZO ceramic. (c) and (d) SEM images. (e) XRD plots. (f) UV-Vis absorption spectra. (g) and (h) XPS spectra of Y 3d and O 1s. Y1: Y–O bond (156.5 and 158.5 eV), Y2: Y–OH bond (157.0 and 159.0 eV), Y3: Y₂O_3−x (155.8 and 157.9 eV), O1: lattice oxygen (528.9 eV), O₂: absorbed oxygen (531 eV) and O3: oxygen defects (529.6 eV). Fig. 2a and b also show that the ceramic is fully densified and transparent. The top and bottom layers are light yellow, owing to the doping of Ce ions and a low oxygen defect concentration.¹⁴ The color of the mid-layer is red, probably due to the light absorption of color centers caused by the high concentration of the oxygen defect.^22,23 In Fig. 2b, the color of the as-sintered ceramic changed from dark brown to a homogeneous light yellow when it was annealed at 1450 °C for 10 h under an air atmosphere.The SEM photos (Fig. 2c and d) show that the sandwich-like ceramic is a fully densified polycrystalline with a grain size around 10 μm. The XRD pattern (Fig. 2e) agrees well with the cubic Y₂O₃ phase (JCPDS 41-1105), and no second phases are observed in the XRD results. Fig. 2f shows the UV-Vis absorption spectra measured for the as-sintered Ce, Yb:YLZO, fully-annealed Ce, Yb:YLZO and fully-annealed Yb:YLZO ceramics. UV absorption (250–400 nm) is clearly observed in the fully-annealed Ce, Yb:YLZO ceramic, which can be attributed to the combination effect of the host lattice absorption,²⁷ Yb–O charge transfer absorption²⁸ and Ce 4f → 5d absorption.²⁹ The absorption window of the as-sintered Ce, Yb:YLZO ceramic extends into the visible region (<650 nm), indicating that the Vis light absorption can be enhanced by introducing oxygen defects. XPS results show that the Y 3d peaks of the as-sintered Ce, Yb:YLZO splits into three doublets corresponding to the Y–O bond,³⁰ the Y–OH bond,³¹ and the Y₂O_3−x.^32,33 After fully annealing in air, the Y₂O_3−x peak vanishes, indicating that the oxygen defect concentration is lower than the XPS detection limit (Fig. 2g). For the O 1s de-convoluted spectrum, there is an obvious oxygen defect peak (529.6 eV) in the as-sintered Ce, Yb:YLZO,^34,35 which decreases greatly after air annealing owing to the defect elimination during oxygen diffusion into the highly defective Ce, Yb:YLZO (Fig. 2h).Photoluminescence (PL) and photoluminescence excitation (PLE) spectra measurements were conducted at room temperature to study the downconversion properties (Fig. 3). It can be seen that the as-sintered, fully-annealed, and sandwich-like Ce, Yb:YLZO ceramics are all efficient downconversion materials. The PL emission peaks in the NIR region (Fig. 3c) are located at 976, 1030 and 1074 nm, which can be attributed to the Yb: ²F_5/2 → ²F_7/2 transition.³⁶ The peak positions of different specimens are consistent, which indicates that the introduction of oxygen defects does not change the position of the Yb 4f energy levels. It can be seen that the 976 nm emission peak decreases greatly in the sandwich Ce, Yb:YLZO ceramic under visible wavelength excitation. This is due to the reabsorption effect of the Yb ions,^37,38 which is detrimental to the downconversion efficiency, and we will aim to eliminate this in future work.Open in a separate windowFig. 3(a) PLE spectra measured with the fully-annealed, as-sintered and sandwich-like Ce, Yb:YLZO transparent ceramics (monitoring emission peak 1030 nm). The PLE spectra of Ce, Yb:glass, Ce, Yb:YAG and Pr, Yb:SrF₂ are also listed for comparison. (b) and (c) PL spectra measured with fully-annealed, as-sintered and sandwich-like Ce, Yb:YLZO transparent ceramics. Fig. 3b shows the PL spectra of the as-sintered, fully-annealed, and sandwich-like Ce, Yb:YLZO ceramic in the visible region. It can be seen that the fully-annealed Ce, Yb:YLZO ceramic shows ∼350 nm emission under 280 nm excitation, which is similar to that in Ce:Lu₂O₃ (ref. 39) and can be attributed to the Ce 5d → 4f emission. In the PL spectra of the sandwich Ce, Yb:YLZO ceramic, there is another broadband emission at around 450 nm, which is also found in the as-sintered Ce, Yb:YLZO ceramic under 400 nm excitation. This emission is caused by the introduction of oxygen defects, which indicates that there is probably a defect-induced mid-gap state under the Ce 5d level. Fig. 3a shows that the effective excitation band of the fully-annealed Ce, Yb:YLZO ceramic is located at 250–450 nm, which can be attributed to the Ce 4f → 5d transition.¹⁴ The excitation band of the as-sintered Ce, Yb:YLZO ceramic is located at 400–650 nm owing to the defect-induced red-shift effect. A typical Ce³⁺ transition band is not seen in the PLE spectra of the as-sintered Ce, Yb:YLZO ceramic, because the oxygen-defect-induced color centers can cause luminescence quenching in the short wavelength region where the Ce³⁺ transition is located.^23,40,41 Downconversion caused by the Yb–O charge transfer band has been observed in many host materials.^42,43 However, it is not seen in this work, because the Yb–O charge transfer band decays quickly in the bulk Y₂O₃ host (lifetime ∼ 72 ns) and quenches easily with increasing temperature (quenching temperature ∼ 130 K).²⁸In the PLE spectra of the sandwich-like Ce, Yb:YLZO ceramic, excitation bands in both the UV and Vis range (250–650 nm) have been detected (Fig. 3a, red line). The UV excitation band of the sandwich-like Ce, Yb:YLZO ceramic is similar to that of the fully-annealed Ce, Yb:YLZO ceramic, which can be attributed to the Ce 4f → 5d transition. The excitation peak position changes from approximately 300 nm to approximately 350 nm, probably due to the change of crystal field around the Ce³⁺ ions in the gradient defective structure. On the other hand, the Vis excitation band of the sandwich-like ceramic is similar to that of the as-sintered Ce, Yb:YLZO ceramic, indicating that the Vis to NIR downconversion of the sandwich-like ceramic contributes to the highly oxygen defective layer. These results confirm that the graded sandwich-like Ce, Yb:YLZO ceramic is a broadband (UV and Vis) downconversion material combining the conversion ability of both the fully-annealed and as-sintered Ce, Yb:YLZO. The conversion range is broader than that of any known downconversion material, such as Ce–Yb doped glass, Ce–Yb doped crystals or Pr–Yb doped materials.^14,44 Which demonstrates that the sandwich-like Ce, Yb:YLZO ceramic has a remarkable broadband spectral conversion ability converting photons in the whole UV-Vis region into approximately 1000 nm photons.Previous efforts have confirmed that there are a large amount of oxygen vacancies in vacuum sintered transparent ceramics.^23,24 First-principle calculation suggests that the introduction of oxygen vacancies can induce a mid-gap state in yttria-base materials (Fig. S1 and S2^†), which is probably the reason why the conversion band of the highly defective Ce, Yb:YLZO red-shifts from the UV to the Vis region. As shown in Fig. 4a, the usual Ce → Yb downconversion process contains three steps.¹⁴ Firstly, Ce ions absorb UV photons and jump to the excitation state. Secondly, energy transfers from the Ce ions to the Yb ions, pumping Yb ions to the excitation state. Thirdly, Yb ions jump back to the ground state, emitting photons at approximately 1000 nm. In highly oxygen defective materials, the absorption red-shifts to the Vis region, pumping Ce ions into a defect-induced mid-gap state (Fig. 4b). The following two steps stay the same, and the UV → NIR downconversion process becomes a Vis → NIR downconversion process. PL and PLE results suggest that this mid-gap level locates at about 2.5–2.8 eV above the 4f level of Ce ions, which agrees with the result calculated by the first principle theory (2.7–2.9 eV).⁴⁵ These results suggest a new strategy to adjust the spectral conversion window of the Ce–Yb downconversion material through defect tuning, which may help in the future development of spectral conversion materials and spectral modified photovoltaic devices.Open in a separate windowFig. 4(a) and (b) Energy transfer diagram of Ce → Yb downconversion luminescence process. (c) PL spectra using a solar simulator as the excitation source (inset: integrated luminescent intensity).For practical applications, downconversion materials with a broad excitation bandwidth are of significant advantage because more photons in the solar spectrum can be converted effectively to the desired wavelength.¹¹ To investigate the solar-spectrum conversion ability, the PL spectra of the fully-annealed, as-sintered and sandwich-like Ce, Yb:YLZO ceramic was measured under the illumination of a solar simulator. As shown in Fig. 4c, the solar-excited PL intensity of the sandwich-like Ce, Yb:YLZO ceramic is significantly stronger than that of the fully-annealed ceramic or the as-sintered ceramic, owing to its broader conversion window. In general, the integrated PL intensity is proportional to the spectral conversion efficiency.¹³ According to the result shown in the inset of Fig. 4c, the spectral conversion efficiency of the sandwich-like ceramic is 3.6 times larger than that of its homogenous counterpart, which demonstrates that designing a graded defective structure is an effective way to improve the efficiency of spectral conversion materials.In conclusion, we have developed a new strategy to tune the excitation band of Ce–Yb downconversion materials with oxygen defects, and broadband downconversion has been realized by preparing a low defect/high defect/low defect sandwich-like transparent ceramic with a controlled air-annealing treatment after vacuum sintering. This unique structure effectively extends the spectral conversion window and improves the solar conversion efficiency by 3.6 times, which may help to enhance the performance of spectral modified photovoltaic devices.     

Hypoxic Lung-Cancer-Derived Extracellular Vesicle MicroRNA-103a Increases the Oncogenic Effects of Macrophages by Targeting PTEN

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Amelioration of Neurosensory Structure and Function in Animal and Cellular Models of a Congenital Blindness

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