Upconversion luminescence in sub-10 nm β-NaGdF4:Yb3+,Er3+ nanoparticles: an improved synthesis in anhydrous ionic liquids

Sub-10 nm β-NaGdF₄:18% Yb³⁺,2% Er³⁺ nanoparticles were synthesized in ethylene glycol and various ionic liquids under microwave heating. The products were characterized by powder X-ray diffraction, electron microscopy, and upconversion (UC) luminescence spectroscopy. After Yb³⁺ excitation at 970 nm, Er³⁺ ions are excited by energy transfer upconversion and show the typical green and red emission bands. The UC luminescence intensity was optimized with respect to reactant concentrations, solvents, and reaction temperature and time. The strongest UC emission was achieved for sub-20 nm core–shell nanoparticles which were obtained in the ionic liquid diallyldimethylammonium bis(trifluoromethanesulfonyl)amide from a two-step synthesis without intermediate separation. Strictly anhydrous reaction conditions, a high fluoride/rare earth ion ratio, and a core–shell structure are important parameters to obtain highly luminescent nanoparticles. These conditions reduce non-radiative losses due to defects and high energy acceptor modes of surface ligands. A low power excitation of the core–shell particles by 70 mW at 970 nm results in an impressive UC emission intensity of 0.12% compared to the bulk sample.

The microwave-assisted synthesis of β-NaGdF₄:Er³⁺,Yb³⁺ in anhydrous ionic liquids yields efficient upconversion luminescence nanoparticles. A core–shell structure raises the nanoparticle emission intensity to 0.12% of the bulk material.     

Epitaxial growth of ultrathin layers on the surface of sub-10 nm nanoparticles: the case of β-NaGdF4:Yb/Er@NaDyF4 nanoparticles

Upconversion core–shell nanoparticles have attracted a large amount of attention due to their multifunctionality and specific applications. In this work, based on a NaGdF₄ sub-10 nm ultrasmall nanocore, a series of core–shell upconversion nanoparticles with uniform size doped with Yb³⁺, Er³⁺ and NaDyF₄ shells with different thicknesses were synthesized by a facile sequential growth process. NaDyF₄ coated upconversion luminescent nanoparticles showed an obvious fluorescence quenching under excitation at 980 nm as a result of energy resonance transfer between Yb³⁺, Er³⁺ and Dy³⁺. NaGdF₄:Yb,Er@NaDyF₄ core–shell nanoparticles with ultrathin layer shells exhibited a better T₁-weighted MR contrast.

In this work, a series of core–shell upconversion nanoparticles with uniform size doped with Yb³⁺, Er³⁺ and NaDyF₄ shells with different thicknesses were synthesized by a facile sequential growth process.     

SiO2@MnOx@Na2WO4@SiO2 core–shell-derived catalyst for oxidative coupling of methane

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared and their fabrication was confirmed using transmission electron microscopy. The formation of Mn-based nanosheets on the silica spheres is important for the deposition of nanoscopic Na₂WO₄. The SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were used for the oxidative coupling of methane at a temperature of 700–800 °C at which the nanostructures were completely destroyed. Although the core–shell structures did not survive the high-temperature oxidative coupling of methane, the selective production of olefins and paraffins can be attributed to highly dispersed Na₂WO₄ derived from confined core–shell structures.

SiO₂@MnO_x@Na₂WO₄@SiO₂ core–shell catalysts were prepared for the oxidative coupling of methane.     

Aerosol synthesis of TiO2:Er3+/Yb3+ submicron-sized spherical particles and upconversion optimization for application as anti-counterfeiting materials

Er³⁺/Yb³⁺-doped TiO₂ up-conversion (UC) phosphors were prepared by spray pyrolysis, and the UC luminescence properties were optimized by changing the calcination temperature and the concentration of Er³⁺ and Yb³⁺ dopants. TiO₂:Er³⁺/Yb³⁺ showed green and red emissions due to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 transition and the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions, respectively. The R/G ratio between red (R) and green (G) emissions does not change significantly with Er concentration but increases linearly with increasing Yb³⁺ concentration. The dependence of UC luminescence intensity on 980 nm IR pumping power showed that both the red and green UC luminescence of TiO₂:Er³⁺/Yb³⁺ occurred through a typical two-photon process. In terms of achieving the highest red UC emission intensity, the optimal Er³⁺ and Yb³⁺ contents are 0.3% and 7.0%, respectively. The UC intensity of TiO₂:Er³⁺/Yb³⁺ particles increases until they are calcined at temperatures up to 600 °C and then decreases rapidly above 800 °C. This is because when the calcination temperature is 800 °C and higher, not only does the phase transition of TiO₂:Er³⁺/Yb³⁺ occur from anatase to rutile, but also the Yb₂Ti₂O₇ impurity phase is formed. According to SEM and TEM/EDX analysis, the prepared TiO₂:Er³⁺/Yb³⁺ UC powders have an average particle size of 680 nm, a spherical shape with a dense structure, and Er and Yb are uniformly dispersed throughout the particles without local separation. A mark prepared using TiO₂:Er³⁺/Yb³⁺ powder was found to have a UC emission high enough to be visually observed when irradiated with a portable 980 nm IR lamp.

TiO₂:Er/Yb spherical particles were synthesized by spray pyrolysis and their luminescence was optimized for application as anti-counterfeiting materials.     

Highly sensitive optical temperature sensing based on pump-power-dependent upconversion luminescence in LiZnPO4:Yb3+–Er3+/Ho3+ phosphors

In this work, various LiZnPO₄:0.5 mol% Ln³⁺ (Ln = Ho, Er) phosphors with different Yb³⁺ ion doping concentrations were synthesized by a sol–gel/Pechini method. X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques were used to evaluate the phase and morphology of the samples. The UC process was mentioned as the typical emission peaks of Er³⁺ and Ho³⁺. For Er³⁺ and Ho³⁺, different optical temperature sensing methods are included. The Boltzmann distribution was accompanied by the fluorescence intensity ratio (FIR) for the two green Er³⁺ emissions originating from thermally-coupled levels. The effect of pump power on sensor sensitivities was extensively studied. The temperature uncertainty is also evaluated. The red and green emissions generated from non-thermally-coupled levels were used for temperature sensing in the Ho³⁺-activated LiZnPO₄. High sensitivities were obtained in the phosphors, and the LiZnPO₄:Yb³⁺/Ho³⁺ showed the largest absolute sensitivities. LiZnPO₄:Yb³⁺–Er³⁺/Ho³⁺ phosphors may be useful in the development of new luminescent materials for optical temperature sensing.

Novel orthophosphate LiZnPO4:Yb³⁺–Er³⁺/Ho³⁺ with tunable luminescence have been synthesized via sol–gel/Pechini method for optical thermometry.     

Investigation on the upconversion luminescence and ratiometric thermal sensing of SrWO4:Yb3+/RE3+ (RE = Ho/Er) phosphors

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method. The upconversion (UC) luminescence properties of all the samples have been investigated under 980 nm excitation. Strong green emissions are obtained in the SrWO₄:Yb³⁺/Ho³⁺ and SrWO₄:Yb³⁺/Er³⁺ samples with the naked eyes. In a temperature range going from 303 K to 573 K, the UC emission spectra of the phosphors have been measured. Then the temperature sensing properties also have been discussed via fluorescence intensity ratio (FIR) technology. For the SrWO₄:Yb³⁺/Ho³⁺ phosphor, the FIR technologies based on thermal coupling levels (TCLs)(⁵F₄,⁵F₅) and non-thermal coupling levels (non-TCLs)(⁵S₂, ⁵F₄/⁵F₅) are used for investigating the sensitivity. The results show that the maximum absolute sensitivity reaches 0.0158 K⁻¹ with non-TCLs. As for Yb³⁺/Er³⁺ codoped SrWO₄ phosphor, the maximum absolute sensitivity reaches 0.013 K⁻¹ with TCLs (²H_11/2,⁴S_5/2) at a temperature of 513 K. These significant results demonstrate that the SrWO₄:Ho³⁺(Er³⁺)/Yb³⁺ phosphors are robust for optical temperature sensors.

SrWO₄ phosphors doped with Ho³⁺(Er³⁺)/Yb³⁺ are successfully prepared by a high temperature solid-state reaction method.     

Er3+/Yb3+ co-doped nanocrystals modified with 6-aminocaproic acid for temperature sensing in biomedicine

We report β-PbF₂:Er³⁺/Yb³⁺ nanocrystals (NCs) modified with 6-aminocaproic acid (6AA) via wet chemical etching of glass ceramics (GCs). NCs body-doped with trivalent rare-earth (RE³⁺) ions were released from the GCs by etching of the glass matrix and modified with bifunctional 6AA ligands to enhance their water solubility. They have good stability in water with an average diameter of 56 nm and display efficient green (521, 550 nm) and red (660 nm) emission under the excitation of a 940 nm laser. High absolute sensitivity (S_A) and relative sensitivity (S_R) (0.0027 K⁻¹ and 1.18% K⁻¹, respectively) and high resolution (0.25 K) were achieved for temperature sensing in the biological temperature range, using the fluorescence intensity ratio (FIR) technique. All of the experimental results indicate that the Er³⁺/Yb³⁺ co-doped NCs modified with 6AA may potentially be useful as fluorescent biological temperature sensors.

We report β-PbF₂:Er³⁺/Yb³⁺ nanocrystals (NCs) modified with 6-aminocaproic acid (6AA) via wet chemical etching of glass ceramics (GCs).     

Concentration and pump power-mediated color tunability,optical heating and temperature sensing via TCLs of red emission in an Er3+/Yb3+/Li+ co-doped ZnGa2O4 phosphor

Intense red upconversion luminescence was observed in the Er³⁺/Yb³⁺/Li⁺ co-doped ZnGa₂O₄ phosphor synthesized through the solid state reaction method for the first time. The structural characterization showed a large crystalline nature and an increase in the particle size via Li⁺ doping. The absorption spectra showed a large number of peaks in the UV-vis-NIR regions due to the Er³⁺ and Yb³⁺ ions. The Er³⁺/Yb³⁺ co-doped ZnGa₂O₄ phosphor exhibited green, red and NIR upconversion emissions on excitation with 980 nm radiation. The intensity of the red emission was relatively larger than that of the other emissions. The luminescence intensity versus pump power measurements revealed the number of required photons for these emissions. The phosphor showed very interesting color tunability as a function of Er³⁺ ion concentration and incident pump power. The luminescence intensity of the Er³⁺/Yb³⁺ co-doped phosphor was enhanced more than two times via Li⁺ doping. The enhancement in the luminescence intensity was proposed to be due to the increase in the crystallinity and particle size of the phosphor. The lifetimes of the ⁴S_3/2 and ⁴F_9/2 levels also increased in the presence of Li⁺ ions. The variation in the fluorescence intensity ratio (FIR) of the thermally coupled levels (TCLs) of the red emission with incident pump power offered effective optical heating in the phosphor. The temperature-induced FIR using TCLs of red emission exhibited a larger value of temperature sensing sensitivity in the presence of Li⁺ ions, which was up to 14 × 10⁻⁴ K⁻¹. Thus, the Er³⁺/Yb³⁺/Li⁺ co-doped ZnGa₂O₄ phosphor may be used in photonic, optical heating, and temperature sensing devices.

The Er³⁺/Yb³⁺/Li⁺ co-doped ZnGa₂O₄ phosphor gives intense red upconversion photoluminescence, color tunability with Er³⁺ ion concentration and incident pump power, R/G ratio, induced optical heating and temperature sensing characteristics.     

Effect of Bi3+ ion on upconversion-based induced optical heating and temperature sensing characteristics in the Er3+/Yb3+ co-doped La2O3 nano-phosphor

The upconversion-based optical heating and temperature sensing characteristics are investigated in the Er³⁺/Yb³⁺/Bi³⁺ tri-doped La₂O₃ nano-phosphor synthesized through a solution combustion method. The structural measurements reveal an increase in lattice parameters and particles size of the phosphor on increasing the concentrations of Bi³⁺ ions. The energy dispersive spectroscopic (EDS) measurements confirm the presence of La, Er, Yb, Bi and O elements in the tri-doped phosphor. The absorption spectra show the large number of bands due to Er³⁺, Yb³⁺ and Bi³⁺ ions. The Er³⁺/Yb³⁺ co-doped phosphor gives strong green emission bands at 523 and 548 nm upon 976 nm excitation due to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ion, respectively. The emission intensity of these bands is enhanced upto 15 times in the presence of Bi³⁺ ions. The emission intensities of the 523 and 548 nm bands vary non-linearly with the pump power. The fluorescence intensity ratio (FIR) of the thermally coupled 523 and 548 nm emission bands shows efficient optical heating in the tri-doped phosphor. The FIR of the 523 and 548 nm emission bands further varies with the increase in temperature of the phosphor. The relative temperature sensing sensitivity has been calculated to be 71 × 10⁻⁴ K⁻¹ at 450 K for the tri-doped phosphor. Thus, the Er³⁺/Yb³⁺/Bi³⁺ tri-doped La₂O₃ nano-phosphor may provide a platform to use it in the photonic devices, as an optical heater and temperature sensor.

The Er³⁺/Yb³⁺/Bi³⁺ tri-doped La₂O₃ nano-phosphor gives efficient induced optical heating and temperature sensing. This study is useful to understand these characteristics in different materials with various pump powers and temperatures.     

NaBiF4 upconversion nanoparticle-based electrochemiluminescent biosensor for E. coli O157 : H7 detection

Foodborne or water-borne pathogens pose great threats to human beings and animals. There is an urgent need to detect pathogens with cheap, rapid and sensitive point-of-care diagnostic assays. Herein, we report the electrochemiluminescent (ECL) behaviors of NaBiF₄ : Yb³⁺/Er³⁺ upconversion nanoparticles (UCNPs) which were synthesized via a fast and environment-friendly method at room temperature for the first time. The UCNPs together with K₂S₂O₈ exhibit high ECL intensity and stable cathodic signals. Further, the Au nanoparticles (Au NPs) and Anti-E. coli O157 : H7 antibody were assembled on the surface of UCNPs successively to construct a novel ECL immunosensor for the detection of deadly E. coli O157 : H7. The as-prepared ECL immunosensor reveals high sensitivity to E. coli O157 : H7 in a linear range of 200–100 000 CFU mL⁻¹, and the minimum detection limit could reach up to 138 CFU mL⁻¹. The designed UCNP-based biosensor demonstrates high specificity, good stability and remarkable repeatability, and the strategy will provide a sensitive and selective method for rapid detection of E. coli O157 : H7 in food safety and preclinical diagnosis.

The ECL behaviors of NaBiF₄ : Yb³⁺/Er³⁺ UCNPs synthesized via a fast and environment-friendly method are reported for the first time. UCNPs-based ECL biosensor shows a wide detection range with low detection limit of 138 CFU mL⁻¹ for E. coli O157 : H7.     

Interaction of LiYF4:Yb3+/Er3+/Ho3+/Tm3+@LiYF4:Yb3+ upconversion nanoparticles,molecularly imprinted polymers,and templates

In this work, LiYF₄:Yb_0.25³⁺/Er_0.01³⁺/Tm_0.01³⁺/Ho_0.01³⁺@LiYF₄:Yb_0.2³⁺ upconverting nanoparticles (UCNP) were used as luminescent materials for the preparation of molecular imprinting polymer nanocomposites. Three luminescent molecularly imprinted polymer (MIP) nanocomposites were prepared by in situ polymerization. The relationship between the functional monomers, templates, and upconversion nanoparticles was investigated. Two hydrophilic monomers (acrylic acid (AA) and acrylamide (AAm)) and one hydrophobic monomer (N-tert-butylacrylamide (TBAm)) were employed as functional monomers, while one amino acid (cysteine) and two proteins (albumin and hemoglobin) were employed as the templates to investigate the effect of their interaction with LiYF₄:Yb³⁺/Er³⁺/Ho³⁺/Tm³⁺@LiYF₄:Yb³⁺ core/shell UCNPs on the polymerization process, luminescence properties, and adsorption capacity. The results showed that the UCNPs were embedded in the polymeric matrix to form an irregular quasimicrospherical UCNPs@MIP with diameters ranging from several hundred nanometers to several micrometers depending on the functional monomer. The quenching effect was more pronounced for the adsorption of hemoglobin with UCNPs@MIP compared to cysteine and albumin. In addition, the adsorption capacities of the AA- and AAm-made UCNPs@MIP were greater than those of TBAm-made UCNPs@MIP. The rebinding of the templates onto UCNPs@MIP was very fast and approached equilibrium within 30 min, indicating that the synthesized UCNPs@MIP can be employed as fluorescent probes to offer rapid detection of molecules.

In this work, LiYF₄:Yb_0.25³⁺/Er_0.01³⁺/Tm_0.01³⁺/Ho_0.01³⁺@LiYF₄:Yb_0.2³⁺ upconverting nanoparticles (UCNP) were used as luminescent materials for the preparation of molecular imprinting polymer nanocomposites.     

An upconversion luminescence and temperature sensor based on Yb3+/Er3+ co-doped GdSr2AlO5

GdSr₂AlO₅:Yb³⁺/Er³⁺ micro-particles were synthesized by a simple solid state method. The structure, morphology, size and upconversion luminescence features have been characterized. These results indicated that GdSr₂AlO₅ has a contracted tetragonal cell and has irregular block shaped particles with sizes of about 5 μm. During upconversion, green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) (527 nm, 549 nm) and red (⁴F_9/2 → ⁴I_15/2) (665 nm) emissions had been observed, both of which occurred via a two-photon population process. In addition, green UC emission characteristics were studied, and it was found that its temperature ranged from 293 K to 473 K and the sensitivity was 0.0054 K⁻¹ at 473 K. This indicated that GdSr₂AlO₅:Yb³⁺/Er³⁺ micro-particles may have potential application in high temperature environments for safety signs.

GdSr₂AlO₅:Yb³⁺/Er³⁺ micro-particles were synthesized by a simple solid state method.     

Structure,luminescence and temperature sensing in rare earth doped glass ceramics containing NaY(WO4)2 nanocrystals

Novel rare earth doped glass ceramics containing NaY(WO₄)₂ nanocrystals were fabricated for the first time. The appearance of sharp diffraction peaks and well-resolved lattice fringes certifies the precipitation of NaY(WO₄)₂ nanocrystals with high crystallinity. After the crystallization process, significant changes in the photoluminescence emission spectra and fluorescence lifetime of Sm³⁺ ions are observed, which are ascribable to the enrichment of Sm³⁺ ions in the highly disordered NaY(WO₄)₂ nanocrystals. Under 980 nm excitation, characteristic green and red upconversion emission signals were detected and the enhanced upconversion luminescence of Er³⁺ ions in the glass ceramics was attributable to the incorporation into the low energy phonon NaY(WO₄)₂ nanocrystals. Based on the dependence of upconversion intensity on the excitation power, the upconversion mechanism of Er³⁺–Yb³⁺ ions was proposed. The temperature-dependent fluorescence intensity ratio (FIR) of the thermally-coupled ²H_11/2 and ⁴S_3/2 energy levels was determined at a low power density of 0.4125 W cm⁻². The maximum temperature sensitivity is 146 × 10⁻⁴ K⁻¹ at 523 K, which is mainly attributed to the highly disordered structure of NaY(WO₄)₂ nanocrystals and exhibits promising potential for optical temperature sensors.

A novel rare earth doped glass ceramic containing NaY(WO₄)₂ nanocrystals was fabricated and its temperature sensing properties were investigated.     

Core/shell upconversion nanoparticles with intense fluorescence for detecting doxorubicin in vivo

Doxorubicin (Dox) is a chemotherapy medication used to treat cancer. Herein, we report a rapid and efficient method for detecting Dox in vivo based on a NaGdF₄:Yb³⁺,Er³⁺@NaYF₄ core/shell upconversion nanoparticles (UCNPs) probe. We found that the intensity ratio of green to red emission (IGVRE) bands of the core/shell NaGdF₄:Yb³⁺,Er³⁺@NaYF₄ nanoparticles was sensitive to Dox in blood samples, and drops as the concentration of Dox increases. In addition, the proposed UCNPs probe possessed the advantage that no nanoparticles leaked into the living body, thus overcoming the intrinsic defect (difficulty in removing UCNPs from blood vessels) of the fluorescence resonance energy transfer (FRET) approach. This proposed UCNP probe design and results may provide some guidance for the real-time and efficient detection of Dox, and can be helpful in biomedical applications.

Doxorubicin (Dox) is a chemotherapy medication used to treat cancer.     

Enhanced photoluminescence of LEuH nanosheets: 2D photonic crystals self-assembled by core–shell SiO2@LEuH spheres

The enhancement in photoluminescence (PL) is a challenge for layered rare-earth hydroxides, which usually have weak PL due to the quenching effect of hydroxyls. In this work, we provide a strategy to enhance the PL behavior by constructing two-dimensional (2D) photonic crystals. The core–shell structured SiO₂@LEuH spheres were prepared by attaching the positively charged layered europium hydroxide (LEuH) nanosheets onto the negatively charged surfaces of the SiO₂ spheres; the core–shell spheres further formed a monolayered 2D colloidal crystal with the hexagonal lattice on a quartz substrate through an evaporation-induced assembly process. The 2D colloidal crystals exhibited a significantly enhanced photoluminescence at 611 nm related to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ compared with the SiO₂@LEuH spheres and the LEuH nanosheets dispersed in deionized water. The emission band of Eu³⁺ hardly changed in the three samples; therefore, the PL enhancement can be attributed to the emission band located at the short edge of the photonic band-gap of the 2D crystals.

A two-dimensional (2D) colloidal crystal of core–shell-structured SiO₂@LEuH spheres was prepared and showed enhanced photoluminescence at 611 nm (⁵D₀ → ⁷F₂).     

Controllable crystal form transformation and luminescence properties of up-conversion luminescent material K3Sc0.5Lu0.5F6: Er3+, Yb3+ with cryolite structure

In this paper, a novel cryolite-type up-conversion luminescent material K₃Sc_0.5Lu_0.5F₆: Er³⁺, Yb³⁺ with controllable crystal form was synthesized by a high temperature solid state method. K₃Sc_0.5Lu_0.5F₆: Er³⁺, Yb³⁺ can crystallize in monoclinic or cubic form at different temperatures. The composition, structure and up-conversion luminescence (UCL) properties of K₃Sc_0.5Lu_0.5F₆: Er³⁺, Yb³⁺ samples with different crystal form were investigated in detail. It is impressive that both monoclinic and cubic forms of K₃Sc_0.5Lu_0.5F₆: Er³⁺, Yb³⁺ show green emission (²H_11/2/⁴S_3/2→⁴I_15/2). The luminescence intensity of cubic K₃Sc_0.5Lu_0.5F₆ is much higher than that of the monoclinic form, and the reasons are also discussed in detail. The results show that the luminescence intensity of up-conversion materials can be effectively tuned by controlling the crystal form. According to the power dependent UCL intensity, the UCL mechanism and electronic transition process were discussed. In addition, the fluorescence decay curves were characterized and the thermal coupling levels (TCLs) of Er³⁺ (²H_11/2/⁴S_3/2 → ⁴I_15/2) in the range of 304–574 k were used to study the optical temperature sensing characteristics. All the results show that K₃Sc_0.5Lu_0.5F₆: Er³⁺, Yb³⁺ can be used in electronic components and have potential application value in temperature sensing fields.

Controllable crystal form transformation can effectively influence the up-conversion luminous intensity of cryolite materials.     

Effects of axial pressure on the evolution of core–shell heterogeneous structures and magnetic properties of Fe–Si soft magnetic powder cores during hot-press sintering

Silicon dioxide (SiO₂) has attracted much attention as an ideal coating material for iron (Fe)-based soft magnetic powder cores (SMPCs). However, maintaining the integrity and uniformity of Fe-based/SiO₂ core–shell heterostructures is still a challenge. The evolution mechanism of core–shell heterostructures determines the performance of Fe-based SMPCs. Herein, the evolution of the core–shell structures and heterogeneous interfaces of Fe–Si@SiO₂ SMPCs with axial pressure and the influence of the evolution on the SMPCs performance were investigated. The results show that in the axial pressure range of 10–15 kN, the core–shell heterostructures were gradually integrated, whereas the SiO₂ insulation coatings underwent an amorphous-to-crystalline transformation. At axial pressure above 16 kN, the Fe–Si powder melted partially, and the core–shell heterostructure collapsed due to overheating, caused by the gradient temperature field during the hot-press sintering. When the core–shell heterostructure was intact, the Fe–Si@SiO₂ SMPCs showed a permeability of over 38 with a wide and stable frequency range of 100–300 kHz, a saturation magnetisation of 231.7 emu g⁻¹, resistivity of 0.8 mΩ cm and total loss of 704.7 kW m⁻³ at 10 mT and 100 kHz. When the core–shell heterostructure was destroyed, the resistivity dropped dramatically and the loss increased to 765.0 and 897.4 kW m⁻³. These results show the relationship between the core–shell heterostructure of Fe–Si@SiO₂ SMPCs, axial pressure and magnetic properties, which would be vital in achieving high power density, high efficiency and miniaturisation in SMPCs.

Silicon dioxide (SiO₂) has attracted much attention as an ideal coating material for iron (Fe)-based soft magnetic powder cores (SMPCs).     

Enhanced upconversion luminescence of GdVO4:Er3+/Yb3+ prepared by spray pyrolysis using organic additives

Spray pyrolysis was applied to prepare Er³⁺/Yb³⁺-doped GdVO₄ particles, and their emission properties were investigated by varying the Er³⁺/Yb³⁺ content and the calcination temperature from 900 to 1400 °C. Ethylene glycol (EG), citric acid (CA) and N,N-dimethylformamide (DMF) were used as organic additives in order to improve the upconversion of GdVO₄:Er³⁺/Yb³⁺. The resulting GdVO₄:Er³⁺/Yb³⁺ particles show strong green emission due to ²H_11/2/⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ and weak red peak due to the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺. From the result observed by changing the pumping power of the near-infrared (NIR, 980 nm) laser, the observed green emission is caused by a typical two-photon process. In terms of achieving the highest upconversion luminescence, the optimal Er³⁺ and Yb³⁺ contents are 1.5% and 20% with respect to Gd, respectively. The luminescence intensity steadily increased as the calcination temperature was elevated up to 1200 °C due to the increment of crystallinity. The upconversion intensity showed a linear relationship with the crystallite size in all the calcination temperature range. Using the EG/CA/DMF mixture as organic additives improves the upconversion emission about 4.3 times higher than when no organic additives are used, due to the enhancement of crystallinity as well as the enlargement of primary particle size.

GdVO₄:Er/Yb fine particles with good upconversion luminescence was prepared by spray pyrolysis using organic additives.     

A hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core–shell structure for supercapacitor applications

Transition metal sulphides are becoming one of the promising materials for energy storage applications. Particularly, an advanced electrode material architecture, which gives favourable electronic and ionic conductivity, is highly in demand. Herein, a hierarchical NiCo₂S₄ honeycomb/NiCo₂S₄ nanosheet core–shell structure is reported for supercapacitor applications. The core–shell structure was in situ grown on a nickel foam via two consecutive hydrothermal processes, followed by an electrochemical deposition process. Moreover, we tuned the deposition cycle to get abundant active sites with gaps of suitable sizes between the walls of the honeycomb structure for efficient electrolyte diffusion routes. The 3D honeycomb core structure was used as superhighway for electron transport to the current collector, while the ultrathin shell structure offered a large surface area with short electron and ion diffusion paths, thus leading to the faster kinetics and higher utilization of active materials. Thus, using the synergistic advantages of the core material and the shell material, the as-synthesized optimized electrode material came up with an excellent specific capacitance of 17.56 F cm⁻² at a current density of 5 mA cm⁻² and the highest cycling stability of 88.2% after 5000 cycles of charge–discharge process. Such advanced electrode architectures are highly promising for the future electrode materials.

3D core–shell structure with excellent electronic and ionic transport, leading to faster kinetics and higher utilization of active material obtained.     

Dual-modal imaging and excellent anticancer efficiency of cisplatin and doxorubicin loaded NaGdF4:Yb3+/Er3+ nanoparticles

NaGdF₄:Yb³⁺/Er³⁺ nanoparticles were synthesized via a modified hydrothermal route. The dependence of structure and morphology on the dosage of sodium polyacrylate was studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The as-prepared nanoparticles could be used for T₂ weighted magnetic resonance imaging due to the paramagnetism of Gd³⁺. cis-dichlorodiamineplatinum (CDDP) could be loaded onto NaGdF₄:Yb³⁺/Er³⁺ nanoparticles through binding carboxyl in the form of Pt–O bonds, and doxorubicin (DOX) could be loaded via hydrogen bonding. DOX could also be loaded onto the NaGdF₄–CDDP composite in the same manner, and the loading efficiency of both drugs remained unchanged. Three as-prepared drug delivery systems were used for tumor inhibition both in vitro and in vivo, and the results indicated that NaGdF₄–CDDP–DOX displayed the greatest inhibitory capacity.

The drug delivery system NaGdF₄–CDDP–DOX showed best tumor inhibition capacity both in vitro and in vivo.