Recent progress in the green synthesis of rare-earth doped upconversion nanophosphors for optical bioimaging from cells to animals

Rare-earth doped upconversion nanophosphors(UCNPs), which convert low energy near-infrared(NIR) photons into high energy photons such as ultraviolet, visible light and NIR light, have found various applications in optical bioimaging. In this review article, we summarize recent advances in the synthesis and applications of UCNPs achieved by us and other groups in the past few years. The approaches for the synthesis of UCNPs are presented,with an emphasis on the role of green chemistry in the advancement of this field, followed by a focused overview on their latest applications in optical bioimaging from subcellular structures through cells to living animals. Challenges and opportunities for the use of UCNPs in biomedical diagnosis and therapy are discussed.

Upconversion photoluminescence of Er-doped Bi_(4)Ti_(3)O_(12) ceramics enhanced by vacancy clusters revealed by positron annihilation spectroscopy

Doping of rare earth elements into Bi_(4)Ti_(3)O_(12)can significantly enhance the upconversion photoluminescence(UCPL)properties,but its structure-property relationship is still unclear.In this work,Er-doped bismuth titanate Bi_(4-x)Er_(x)Ti_(3)O_(12)(x=0,0.1,0.2,0.3,0.4,0.5)ceramics were synthesized via solid-state reaction method.The x-ray diffraction analysis confirmed the orthorhombic crystalline structure of the Bi4-xErxTi3012ceramics without any secondary phases.Experiments and calculations of positron annihilation spectroscopy were carried out to characterize their defect structure.The comparison between the experimental and calculated lifetime revealed that vacancy clusters were the main defects in the ceramics.The increase of the intensity of the second positron lifetime component(I_(2))of Bi_(3.5)Er_(0.5)Ti_(3)O_(12)ceramics indicated the presence of a high concentration of vacancy clusters.The UCPL spectra showed the sudden enhanced UCPL performance in Bi3.7Er0.3Ti3O12and Bi_(3.5)Er_(0.5)Ti_(3)O_(12)ceramics,which were consistent with the variation of the second positron lifetime component(I2).These results indicate that the enhanced UCPL properties are influenced not only by the concentrations of rare earth ions but also by the concentration of vacancy clusters present within the ceramics.

Controlled Assembly of Gold and Rare-Earth Upconversion Nanoparticles for Ratiometric Sensing Applications

In this paper, the Au nanoparticles and rare-earth （RE） upconversion nanoparticles （NPs） were respectively synthesized by using polyelectrolyte as the capping agents. Since the synthesized Au NPs and RE NPs had the similar size and surface conditions, Their mixture were employed in a pH sensing application. Benefited from the good spectral overlap between the RE upconversion emission bands and pH-tunable surface plasmon bands of the Au NPs, the pH-induced manipulation of green-to-red emission intensity ratio of the upconversion fluorescence was achieved in the Au-RE mixture. The results demonstrate a rapid ratiometric approach for pH sensing, which is more efficient than traditional sensing methods that depend on single intensity-based responses to analytes.

Highly sensitive mid-infrared upconversion detection based on external-cavity pump enhancement

Sensitive mid-infrared(MIR)detection is in high demand in various applications,ranging from remote sensing,infrared surveillance,and environmental monitoring to industrial inspection.Among others,upconversion infrared detectors have recently attracted increasing attention due to their advantageous features of high sensitivity,fast response,and room-temperature operation.However,it remains challenging to realize high-performance passive MIR sensing due to the stringent requirement of high-power continuouswave pumping.Here,we propose and implement a high-efficiency and low-noise MIR upconversion detection system based on pumping enhancement via a low-loss optical cavity.Specifically,a singlelongitudinal-mode pump at 1064 nm is significantly enhanced by a factor of 36,thus allowing for a peak conversion efficiency of up to 22%at an intracavity average power of 55 W.The corresponding noise equivalent power is achieved as low as 0.3 fW∕Hz^(1∕2),which indicates at least a 10-fold improvement over previous results.Notably,the involved single-frequency pumping would facilitate high-fidelity spectral mapping,which is particularly attractive for high-precision MIR upconversion spectroscopy in photonstarved scenarios.

Study on the hydrogen absorption properties of a YGdTbDyHo rare-earth high-entropy alloy

This study investigated the microstructure and hydrogen absorption properties of a rare-earth high-entropy alloy(HEA),YGdTbDyHo.Results indicated that the YGdTbDyHo alloy had a microstructure of equiaxed grains,with the alloy elements distributed homogeneously.Upon hydrogen absorption,the phase structure of the HEA changed from a solid solution with an hexagonal-close-packed(HCP)structure to a high-entropy hydride with an faced-centered-cubic(FCC)structure without any secondary phase precipitated.The alloy demonstrated a maximum hydrogen storage capacity of 2.33 H/M(hydrogen atom/metal atom)at 723 K,with an enthalpy change(ΔH)of-141.09 kJ·mol^(-1)and an entropy change(ΔS)of-119.14 J·mol^(-1)·K^(-1).The kinetic mechanism of hydrogen absorption was hydride nucleation and growth,with an apparent activation energy(E_(a))of 20.90 kJ·mol^(-1).Without any activation,the YGdTbDyHo alloy could absorb hydrogen quickly(180 s at 923 K)with nearly no incubation period observed.The reason for the obtained value of 2.33 H/M was that the hydrogen atoms occupied both tetrahedral and octahedral interstices.These results demonstrate the potential application of HEAs as a high-capacity hydrogen storage material with a large H/M ratio,which can be used in the deuterium storage field.

Review on the application of upconversion nanomaterials in heavy metal detection

As a widespread element,heavy metals have a significant impact on human health and threaten human health.It is of great significance to develop analytical technologies that can detect heavy metal ions quickly and accurately.In comparison to conventional fluorescent materials such as organic dyes,quantum dot(QD)labels,and carbon quantum dots(CD),fluorescence detection technology utilizing lanthanide(Ln)ion-doped upconversion nanoparticles(UCNPs)stands out due to its distinctive attributes.These include a notably reduced autofluorescence background,enhanced tissue penetration capabilities,biocompatibility with cellular tissues,and minimal photodamage inflicted on biological samples.The utilization of this technology has garnered considerable attention across multiple fields.In the domain of heavy metal detection,traditional laboratory methods necessitate costly instrumentation and a fully equipped laboratory,involving intricate sample processing procedures and protracted detection periods,as well as a demand for skilled personnel.In contrast,the implementation of this material offers rapid and cost-effective detection,significantly mitigating the technical barriers for operators.Consequently,this represents an exceptional avenue to curtail expenses and broaden the scope of detection within the analytical process.This paper reviews the research progress of UCNPs in the detection of heavy metal ions,encompassing a brief elucidation of the luminescence principle of upconversion nanomaterials and commonly used detection principles.Additionally,it provides a detailed overview of the research status of several common non-metal ions and essential heavy metals.Furthermore,it summarizes the current focal points in UCNP detection and discusses the challenges and prospects associated with it.

Fundamental Study of Rare-earth-containing Catalytic Reduction Systems for End-group Functionalization of Telechelic Low-molecular-weight Fluoropolymers

Herein the use of rare-earth compounds in catalytic reduction systems for the end-group functionalization of carboxyl-terminated low-molecularweight fluoropolymers was explored.Leveraging the high catalytic activity and selectivity of rare-earth compounds along with no residual impact on polymer product's performance,highly efficient catalytic reduction systems containing sodium borohydride(NaBH_(4))and rare-earth chloride(RECl_(3))were specifically designed for a telechelic carboxyl-terminated liquid fluoroeslastomer,aiming to facilitate the conversion of chainend carboxyl groups into hydroxyl groups and improvement in end-group reactivity.To achieve this,lanthanum chloride(LaCl_(3)),cerium chloride(CeCl_(3)),and neodymium chloride(NdCl_(3))were used separately to form catalytic reduction systems with NaBH_(4).The effects of solvent dosage,reaction temperature,reaction time length,and reductant dosage on carboxylic conversion were investigated,and the molecular chain structure,molecular weight,and functional group content of the raw materials and the products were analyzed and characterized by means of infrared spectroscopy(FTIR),proton nuclear magnetic resonance(^(1)H-NMR),fluorine-19 nuclear magnetic resonance(^(19)F-NMR),gel permeation chromatography(GPC),and chemical titration.Moreover,the catalytic activity and selectivity of the rare-earth chlorides,as well as the corresponding underlying interactions were discussed.Results indicated that the rare-earth-containing catalytic reduction systems studied in this work could efficiently convert the chain-end carboxyl groups into highly active hydroxyl groups,with a highest conversion up to 87.0%and differing catalytic reduction activities ranked as NaBH_(4)/CeCl_(3)>NaBH_(4)/LaCl_(3)>NaBH_(4)/NdCl_(3).Compared with the conventional lithium aluminum hydride(LiAIH_(4))reduction system,the NaBH_(4)/RECl_(3)systems provide multiple advantages such as mild reaction conditions,high conversion ratio with good selectivity,and environmental innocuity,and are potentially applicable as new reduction-catalysis combinations for the synthesis and functionalization of polymer materials.

Synthesis and upconversion luminescence of Lu_2O_3:Yb^(3+),Tm^(3+) nanocrystals

Lutetium oxide nanocrystals codoped with Tm3＋ and Yb3＋ were synthesized by the reverse-like co-precipitation method, using ammonium hydrogen carbonate as precipitant. Effects of the Tm3＋, Yb3＋ molar fractions and calcination temperature on the structural and upconversion luminescent properties of the Lu2O3 nanocrystals were investigated. The XRD results show that all the prepared nanocrystals can be readily indexed to pure cubic phase of Lu2O3 and indicate good crystallinity. The experimental results show that concentration quenching occurs when the mole fraction of Tm3＋ is above 0.2%. The optimal Tm3＋ and Yb3＋ doped molar fractions are 0.2% and 2%, respectively. The strong blue （490 nm） and the weak red （653 nm） emissions from the prepared nanocrystals were observed under 980 nm laser excitation, and attributed to the 1G4→3H6 and IG4→3F4 transitions of Tm3＋, respectively. Power-dependent study reveals that the 1G4 levels of Tm3＋ can be populated by three-step energy transfer process. The upconversion emission intensities of 490 nm and 653 nm increase gradually with the increase of calcination temperature. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing number of OH- groups and the enlarged nanoerystal size.

Effect of Er substituting sites on upconversion luminescence of Er^(3+)-doped BaTiO_3 films

Erbium-doped BaTiO3 films on LaNiO3/Si substrates were fabricated by sol-gel method. The crystalline structure, morphologies and upconversion （UC） luminescence properties of films were respectively investigated by X-ray diffraction （XRD）, atomic force microcopy （AFM） and photoluminescence （PL）. The results indicate that both of the microstructure and luminescence are found to be dependent on Er^3＋ substituting sites. The samples with A-site substitution have smaller lattice constants, larger grains and smoother surface than those with B-site substitution. The photoluminescence spectra show that both of the samples have two stronger green emission bands centered at 528 and 548 nm and a weak red emission band centered at 673 nm, which correspond to the relaxation of Er^3＋ from ^2H11/2, ^4S3/2, and ^4F9/2 levels to the ground level ^4I15/2, respectively. Compared with B-site doped films, A-site doped films have a stronger integrated intensity of green emissions and a weaker relative intensity of red emissions. The differences could be explained by the crystalline quality and cross relaxation （CR） process.

Degradation of dyestuff wastewater using visible light in the presence of a novel nano TiO_2 catalyst doped with upconversion luminescence agent

A new upconversion luminescence agent, 40CdF2·60BaF2·0.8ErO3, was synthesized and its fluorescent spectra were determined. This upconversion luminescence agent can emit five upconversion fluorescent peaks shown in the fluorescent spectra whose wavelengths are all below 387 nm under the excitation of 488 nm visible light. This upconversion luminescence agent was mixed into nano rutile TiO2 powder by ultrasonic and boiling dispersion and the novel doped nano TiO2 photocatalyst utilizing visible light was firstly prepared. The doped TiO2 powder was charactered by XRD and TEM and its photocatalytic activity was tested through the photocatalytic degradation of methyl orange as a model compound under the visible light irradiation emitted by six three basic color lamps. In order to compare the photocatalytic activities, the same experiment was carried out for undoped TiO2 powder. The degradation ratio of methyl orange in the presence of doped nano TiO2 powder reached 32.5% under visible light irradiation at 20 h which was obviously higher than the corresponding 1.64% in the presence of undoped nano TiO2 powder, which indicate the upconversion luminescence agent prepared as dopant can effectively turn visible lights to ultraviolet lights that are absorbed by nano TiO2 particles to produce the electron-cavity pairs. All the results show that the nano rutile TiO2 powder doped with upconversion luminescence agent is a promising photocatalyst using sunlight for treating the industry dye wastewater in great force.

Upconversion properties of Y_2O_3:Er films prepared by sol-gel method

Y2O3：Er^3＋ films were prepared by a simple sol-gel process. The structural properties of Y2O3：Er^3＋ films were characterized with X-ray diffraction, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The results indicated that the Y2O3：Er^3＋ films might have high upconversion efficiency because of their low vibrational energy. Under 785 and 980 nm laser excitation, the samples showed green （^2H11/2→^4I15/2, ^4S3/2→^4I15/2） and red （^4F9/2→^4I15/2） upconversion emissions. The upconversion mechanisms were studied in detail through laser power dependence. Excited state absorption and energy transfer process were discussed as possible upconversion mechanisms. The cross relaxation process in Er^3＋ was also investigated.

Upconversion Luminescence Properties of Ho^(3+),Tm^(3+),Yb^(3+) Co-Doped Nanocrystal NaYF_4 Synthesized by Hydrothermal Method

Nanocrystal of upconversion （UC） phosphor Ho^3＋, Tm^3＋ , and Yb^3＋ co-doped NaYF4 was prepared by the hydrothermal method in the presence of the complexing agent EDTA. Under 980 nm diode laser excitation, the impact of different concentrations of Ho^3＋ ion on the UC luminescence intensity was discussed. The law of luminescence intensity versus pump power shows that the 474 nm blue emission, 538 nm green emission, and 642 nm red emission are all due to the two-photon process, while the 450 nm blue emission is a three-photon process. The UC mechanism and processes were also analyzed. The sample was characterized by transmission electron microscopy （TEM） and X-ray diffraction （XRD）. The result shows that Ho^3＋ ,Tm^3＋ , and Yb^3＋ co-doped NaYF4 prepared by the hydrothermal method exhibits a hexagonal nanocrystal.

Synthesis and upconversion luminescence properties of NaYF_4:Yb^(3+)/Er^(3+) microspheres

Cubic NaYF4：Yb^3＋（20%）/Er^3＋（1%） microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm excitation, ultraviolet （^4G11/2→^4I15/2）, violet （^2H9/2→^4I15/2）, green （^4F7/2→^4I15/2, 2H11/2→^4I15/2, and ^4S3/2→^4I15/2）, and red （^4F9/2→^4I15/2） upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process, n, was determined to be 3.89, 1.61, 2.55, and 1.09 for the ultraviolet, violet, green, and red emissions, respectively. Obviously, n=3.89 indicated that a four-photon process was involved in populating the ^4G11/2 state, and n=2.55 indicated that a three-photon process was involved in populating the ^4F7/2/^2H11/2/^4S3/2 levels. For the violet and red emissions, the population of the states ^2H9/2 and ^4F9/2 separately came from three-photon and two-photon processes. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the depletion of the intermediate excited states.

10BaF_2:NaF,Na_3AlF_6/TiO_2 composite as a novel visible-light-driven photocatalyst based on upconversion emission

A rare-earth free upconversion luminescent material, 10BaF2：NaF, Na3AIF6, is synthesized by a hydrothermal method. The study of fluorescent spectrum indicates that it can convert visible light （550 nm-610 nm） into ultraviolet light （290 nm 350 nm）, and two emission peaks at 304 nm and 324 nm are observed under the excitation of 583 nm at room temperature. Subsequently, 10BaF2：NaF, Na3AIF6/TiO2 composite photocatalyst is prepared and its catalytic activity is evaluated by the photocatalytic reduction of CO2 under visible light irradiation （λ〉 515 nm）. The results show that 10BaF2：NaF, Na3AIF6/TiO2 is a more effective photocatalyst for CO2 reduction than pure TiO2, their corresponding methanol yields are 179 and 0 μmol/g-cat under the same conditions. Additionally, the mechanism of photocatalytic reduction of CO2 on 10BaF2：NaF, Na3AIF6/TiO2 is proposed.

Upconversion luminescence of Y_2O_3:Er^(3+), Yb^(3+) nanoparticles prepared by a homogeneous precipitation method

Y2O3： Er^3＋, Yb^3＋ nanoparticles were synthesized by a homogeneous precipitation method without and with different concentrations of EDTA 2Na. Upconversion luminescence spectra of the samples were studied under 980 nm laser excitation. The results of XRD showed that the obtained Y2O3：Er^3＋,Yb^3＋ nanoparticles were of a cubic structure. The average crystallite sizes calculated were in the range of 28-40 nm. Green and red upconversion emission were observed, and attributed to ^2H11/2,^4S3/2→^4I15/2 and ^4F9/2→^4I15/2 transitions of the ion, respectively. The ratio of the intensity of green emission to that of red emission drastically changed with a change in the EDTA 2Na concentration. In the sample synthesized without EDTA, the relative intensity of the green emission was weaker than that of the red emission. The relative intensities of green emission increased with the increased amount of EDTA 2Na used. The possible upconversion luminescence mechanisms were discussed.

Effect of Yb^(3+) concentration on the structures and upconversion luminescence properties of Y_2O_3:Er^(3+) ultrafine phosphors

Y2O3：Er^3＋ ultrafine phosphors with a varying Yb^3＋ ion concentration were prepared by a urea homogeneous precipitation method. The results of XRD show that all the samples are of a pure cubic structure and the average crystallite sizes can be calculated as 45, 34, and 28 nm for Y2O3：Er^3＋ ultrafine phosphors with Yb^3＋ ion concentrations of 0, 10%, and 20%, respectively. The lattice constant and cell volume of the ultrafine phosphors decrease with enhancing Yb^3＋ ion concentration. The upconversion luminescence spectra of all the samples were studied under 980 nm laser excitation. The strong green and red upconversion emission were observed, and attributed to the ^2H11/2→^4I15/2, ^4S3/2 → ^4I15/2 and ^4F9/2 →^4I15/2 transitions of Er^3＋, respectively. The intensity of red emission increases with increasing Yb^3＋ ion concentration. The effect of Yb^3＋ ion concentration on the structures and upconversion luminescence mechanism were discussed.

Study on the green upconversion saturation in Er^(3+) doped oxyfluoride tellurite glass

The upconversion emission spectra of Er^3＋ doped oxyfluoride tellurite glasses excited by 808 nm laser diode （LD） were measured. The dependence of 550 nm upconversion emission on the excitation intensity was analyzed. Quadratic intensity dependence was only observed at weak excitation intensity. With increasing the excitation intensity, saturation was turned out. The experimental results were fitted to a model based on the rate equations.

Upconversion Nanoparticles-Encoded Hydrogel Microbeads-Based Multiplexed Protein Detection

Fluorescently encoded microbeads are in demand for multiplexed applications in different fields.Compared to organic dye-based commercially available Luminex's x MAP technology, upconversion nanoparticles(UCNPs) are better alternatives due to their large antiStokes shift, photostability, nil background, and single wavelength excitation. Here, we developed a new multiplexed detection system using UCNPs for encoding poly(ethylene glycol) diacrylate(PEGDA) microbeads as well as for labeling reporter antibody. However, to prepare UCNPs-encoded microbeads, currently used swellingbased encapsulation leads to non-uniformity, which is undesirable for fluorescence-based multiplexing. Hence,we utilized droplet microfluidics to obtain encoded microbeads of uniform size, shape, and UCNPs distribution inside. Additionally, PEGDA microbeads lack functionality for probe antibodies conjugation on their surface.Methods to functionalize the surface of PEGDA microbeads(acrylic acid incorporation, polydopamine coating)reported thus far quench the fluorescence of UCNPs. Here,PEGDA microbeads surface was coated with silica followed by carboxyl modification without compromising the fluorescence intensity of UCNPs. In this study, droplet microfluidics-assisted UCNPs-encoded microbeads of uniform shape, size, and fluorescence were prepared.Multiple color codes were generated by mixing UCNPs emitting red and green colors at different ratios prior to encapsulation. UCNPs emitting blue color were used to label the reporter antibody. Probe antibodies were covalently immobilized on red UCNPs-encoded microbeads for specific capture of human serum albumin(HSA) as a model protein. The system was also demonstrated for multiplexed detection of both human C-reactive protein(hCRP) and HSA protein by immobilizing anti-h CRP antibodies on green UCNPs.

Preparation and upconversion luminescence of YAG(Y_3Al_5O_(12)):Yb^(3+),Ho^(3+) nanocrystals

Cubic YAG：Yb3＋, Ho3＋ pure phase nanocrystals were synthesized by using coprecipition nitrate and ammonium hydrogen carbonate as raw materials.After calcining the precipitates at 800 °C, the resultant YAG：Yb3＋, Ho3＋ nanocrystals were nearly spheric and the particle size was about 40 nm.Intense upconversion spectra were observed on the powder compact pumped by a 980 nm continuous wave diode laser, and green emission centered at 549 nm, red emission centered at 667 nm, and NIR centered at 760 nm were all due to two photons process, which originated from 5S2（5F4）→5I8, 5F5→5I8, and 5S2（5F4）→5I7 transitions, respectively.

Upconversion Luminescence of Ce^(3+) Doped BK7 Glass by Femtosecond Laser Irradiation

Near-infrared to visible upconversion luminescence was observed in a multicomponent silicate （BK7） glass containing Ce^3 ＋ ions under focused infrared femtosecond laser irradiation. The emission spectra show that the upconversion luminescence comes from the 4f-5d transition of the Ce^3 ＋ ions. The relationship between the intensity of the Ce^3 ＋ emission and the pump power reveals that a three-photon absorption predominates in the conversion process from the near-infrared into the blue luminescence. The analysis of the upconversion mechanism suggests that the upconversion luminescence may come from a three-photon simultaneous absorption that leads to a population of the 5d level in which the characteristic luminescence occurs.