recent advances in rare earth co-doped luminescent

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Cite this: Inorg. Chem. Front., 2021,8, 4158

Received 29th May 2021,Accepted 2nd July 2021

DOI: 10.1039/d1qi00681a

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Recent advances in rare earth co-dopedluminescent tungsten oxygen complexes

Bing Yan, Hechen Wu, Pengtao Ma, * Jingyang Niu * and Jingping Wang *

Rare earth (RE) materials have been employed in almost every corner of modern industry, such as lighting

applications, defense technology, and industrial catalysis. Particularly, RE-based tungsten oxygen com-

plexes have been intensively studied across academia and industry for their easy manufacturing, high

stability and low cost, and more interestingly, novel optical properties. To date, some novel multi-RE tung-

sten oxygen complexes in various aggregation states and morphologies have been synthesized and inves-

tigated in several fields. In this review, we will talk about the synthesis and characterization of RE co-

doped tungsten oxygen complex materials, related luminescent mechanism studies, optical properties

and applications of these complexes, with the aim of the further development of new materials for optical

and biological applications.

Introduction

Rare earths (RE), composed of the lanthanide series fromcerium to lutetium plus yttrium and lanthanum, are called“the vitamins of modern industry”.1–6 With abundant 4fenergy levels arising from the 4fn inner shell configurations,the RE exhibit fluorescent emission via intra-4f or 4f–5d tran-sitions, which can lead to high emission color purity and largeStokes shifts, avoiding interference from the excitation wave-

length in the emission spectra.7–10 Consequently, RE basedluminescent materials have attracted increasing research inter-est, comparable to the rapid development of transition metalbased polymers in recent years.11,12 In fact, the direct exci-tation of RE3+ ions turns out to be a relatively inefficientprocess because of the forbidden character of 4f transitions,which gives rise to extremely low molar absorptioncoefficients.13–15 Furthermore, the luminescence of the REconstituents could be quenched from the vibrational overtonesof –OH, –NH or –CH bonds of adjacent ligand and solventmolecules.16–18 Based on the above drawbacks, doping techno-logy is widely used to synthesize RE based luminescentmaterials by incorporating atoms or ions into host lattices toyield hybrid materials with desirable properties andfunctions.19–21 Energy transfer from the excited doped atoms

Bing Yan

Bing Yan was born in Henan,China, in 1996. In 2017, shebegan to conduct scientificresearch in the group of Prof.Jingyang Niu at ChemicalEngineering of Henan University,and received her B.S. degree in2019. Currently, she is pursuingher M.S. under the guidance ofDr Pengtao Ma. Her researchinterest is focused on the prepa-ration and luminescence per-formance of polyoxometalates. Hechen Wu

Hechen Wu was born in 1993.He received his BS degree andMS degree from the College ofChemistry and ChemicalEngineering of Henan Universityin 2016 and 2019, respectively.He is currently pursuing a PhDdegree at the Department ofChemistry of Fudan University.His research interests focus onthe luminescence performance ofmetal–oxide clusters and kineticsof molecular reactions.

Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and

Chemical Engineering, Henan University, Kaifeng, Henan 475004, P. R. China.

E-mail: [email protected], [email protected], [email protected];

Fax: (+86)-371-23886876

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or ions (hosts) to the RE luminescence centers (guests) canovercome limitations through the optical antenna effect, wherethe excited doped constituents act as a rigid matrix to protectthe RE centers from coupling to high frequency vibrations.22

Among various hosts, derivatives of tungsten oxygen com-plexes, WxOy

z−, can be seen as potent candidates for their mildsynthesis conditions, unique structures, and excellent chemi-cal and thermal stability.23–27 Meanwhile, tungsten oxygencomplexes present strong absorption in the ultraviolet (UV)region owing to the W–O charge transfer transitions, whichplay an important role in enhancing the luminescence of REcenters. Additionally, the tungsten oxygen complexes are ableto coordinate to RE3+ ions as feasible O-donor ligands.28 As oftoday, there are many successful reports on single RE dopedtungsten oxygen complexes and their luminescence propertieshave been well investigated.29,30 Additionally, the scheelite-likedouble tungstate phosphors with a formula of MRE(WO4)2(M = alkali metal) and multi-RE co-doped polyoxotungstates(POTs) are also gaining increasing interest as promising hoststhat embed RE centers. Particularly, the double tungstatephosphors, MRE(WO4)2, have promising luminescence pro-perties, excellent physical and chemical stability, low sinteringtemperatures, and environmentally friendly characteristics.POTs are advantageous for their structural diversity, and novelphysical and chemical properties; as a result, they yield manyfunctional materials with great diversity through the doping ofRE centers. Noteworthily, RE embedded POTs have demon-strated unique luminescent properties because of the sensitiz-ation effect between RE centers and POTs, empowered by theO → W ligand-to-metal charge-transfer (LMCT) photoexcitationprocess.31–34

RE-based luminescent materials are usually good candi-dates, achieving white light emission, though they are limitedby some bottlenecks, including but not limited to inadequateemission components and low color rendering index.35–37 To

date, the most popular solution is to co-dope a sensitizer andan activator concurrently into the same host matrix and lever-age the energy transfer processes between the sensitizer andactivator components. There have been several successful tung-sten oxygen complex hosts reported using this co-dopingmethod. In 2014, Liu and co-workers prepared the phosphorNaGd(WO4)2:Tm

3+,Dy3+,Eu3+ via a facile hydrothermal process,and analyzed the energy transfer process and efficiency fromTm3+ to Dy3+ and then to Eu3+ ions.38 In 2018, our group syn-thesized a series of Dy3+/Er3+ co-doped polyoxometalates(POMs) (DyxEr(1−x)-POM), and investigated the energy transferprocess from Er3+ to Dy3+ using time-resolved emission spec-troscopy (TRES).39 Compared with RE co-doped non-tungstenoxygen complexes, the RE co-doped tungsten oxygen com-plexes display evident advantages and features in many ways,such as high thermal stability, excellent luminescence per-formance assisted by energy transfer effects, and no environ-mental hazards.

In short, research into RE co-doped tungsten oxygen com-plexes is of huge significance in designing and developingluminescent materials. Additionally, RE co-doped tungstenoxygen complexes have also found immense interest in bio-technology, including but not limited to temperature sensing,tumor inhibition, and multi-imaging.40–45 More importantly,much progress has been made recently in expanding the appli-cations of RE co-doped POTs. Herein, we would like to sum-marize the state-of-the-art progress on RE co-doped tungstenoxygen complexes, from the perspective of synthesis strategies,luminescent mechanism, structural motifs and related appli-cations. Specifically, the first three sections briefly discuss thesynthesis strategies, related luminescent mechanisms, andstructural architectures respectively. We then focus on theirrecent applications in the fourth section. Additionally, con-clusions together with a brief outlook in this field are given atthe end.

Pengtao Ma

Pengtao Ma obtained his BA andMA degrees in 2003 and 2007from Henan University. Then, hejoined the faculty of HenanUniversity as an assistant pro-fessor in 2008. In 2016, hereceived his PhD degree underthe supervision of Prof. JingyangNiu at Henan University, and hewas promoted to an associateprofessor in 2017. His researchinterests focus on the synthesis,crystal structure, photochro-mism, photoluminescence andmagnetism properties of poly-oxometalate-based crystallinematerials.

Jingyang Niu

Jingyang Niu is the SpecialProfessor of Chemistry of HenanProvince at Henan University. Heobtained his BA, MA and PhDdegrees in 1986, 1989 and 1996from Henan University,Northeastern Normal Universityand Nanjing University, respect-ively. His research interests focuson the synthesis, crystal structureand properties of novel polyoxo-metalates. He has receivedseveral awards, including theDistinguished Young Scholars of

Henan Province, National Government Special Allowance recipi-ent, and Thousands of Intellectual Plans of Central Plain. He isleader of the Henan Province Key Laboratory of Polyoxometalateat Henan University.

Inorganic Chemistry Frontiers Review

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Synthesis strategies

It is well known that the aggregation states and morphologiesare primary structural types of RE co-doped tungsten oxygenmaterials, which are usually relayed by different synthesisstrategies. To date, there are two major synthesis strategiesdependent on controlling the reaction temperature: the con-ventional solution approach and the high temperature syn-thesis approach. The conventional solution approach is a feas-ible method to prepare crystalline materials with molecularaggregation states, whereas the high temperature synthesisapproach tends to generate materials in forms of extendedstructures, nanostructures or in the amorphous state.

Conventional solution synthesis strategy

The conventional solution synthesis strategy is widely used inpreparing RE co-doped POTs. Generally speaking, the conven-tional solution synthesis strategy further breaks down into twosynthesis methods, namely, the one-pot method and the step-by-step assembly method. They majorly differ in the form of thereaction’s raw materials: the one-pot method is usually performedusing simple sodium tungstate raw materials, whereas the step-by-step assembly method is carried out in an assembly systememploying pre-designed POT precursors. To date, the one-pot pro-cedure is widely applied for its relatively straightforwardimplementation, although the reaction process and reaction out-comes are hard to control. Several successful RE co-doped POTexamples have been reported so far using the conventionalsolution synthesis strategy. For example, Zhao’s group syn-thesized a double-oxalate-bridging tetra-Gd3+ containing POT,Na10[Gd2(C2O4)(H2O)4(OH)W4O16]2·30H2O, and its Eu3+/Tb3+ ionco-doped POTs via the one-pot reaction strategy, and explored theenergy transfer mechanism between Eu3+ and Tb3+ ions and theircolor-tunable photoluminescence property.46 Meanwhile, byapplying the step by step assembly method, our group preparedthe double-tartaric bridging Tm-substituted POT derivative

[N(CH3)4]6K3H7[Tm(C4H2O6)(α-PW11O39)]2·27H2O and Dy3+/Tm3+

ion co-doped POT derivatives [N(CH3)4]6K3H7[DyxTm1−x(C4H2O6)(α-PW11O39)]2·27H2O, and discovered the color-tunable photo-luminescence of Dy3+/Tm3+ co-doped POTs from blue to white toyellow with increasing doping proportions of Dy3+ ions.47 Yet, wehave to admit that the synthesis of RE co-doped POTs stillremains a challenging task, as the assembly process can beaffected by many factors, such as the species and stoichiometricratio of raw materials, pH environment, reaction temperature,and counter cationic components.

High temperature synthesis strategy

The high temperature synthesis strategy is another importantsynthesis method, which is capable of preparing RE co-dopedtungsten oxygen materials in extended structural or nano-structural forms. As of today, the high temperature synthesisstrategy mainly includes the solid state reaction method, thehydrothermal method, and the melt quenching method.

The solid state reaction method has advantages including asimple reaction setup, high reaction yield, and low-cost. Indetail, appropriate stoichiometric amounts of startingmaterials (usually the oxide of tungsten and RE together withsome salts) are ground thoroughly in an agate mortar withsolvent, followed by collection in a crucible and sintered at acertain high temperature. After cooling to room temperature,the obtained samples are ground into powder for further inves-tigation. Sufficient contact between reaction agents has provedto be an essential reaction condition. There are several suc-cessful reports using this method, for instance, Wu and co-workers synthesized Dy3+ and/or Sm3+ doped LiLa(WO4)2 phos-phors utilizing the solid state reaction method, and investi-gated the phase and luminescence properties.48 But, someinherent drawbacks also need to be overcome, such as energyconsumption, product separation and purification.

The hydrothermal method is a popular synthesis approachto RE co-doped POTs for its easy experimental process, whereraw materials are mixed in aqueous solution firstly, and thenplaced in an autoclave, followed by sealing and heating underhigh temperature and high pressure. Experiments are easy toperform, however, the reaction progress could be affected bymultiple factors, such as hydrothermal temperature, reactantconcentration, doped ion type and concentration, and pHvalue. Also, it is almost impossible to monitor the productgrowth progress clearly. In 2018, Zhai’s group reported afamily of color-tunable NaGd(WO4)2 (NGW):Tm3+,Dy3+ phos-phors via the hydrothermal method.49

The melt quenching method is a widely studied synthesisroute towards the preparation of glass materials. Firstly, theoxides of silica, calcium, phosphate, sodium, tungsten and REstarting materials are ground and mixed into powdered formby using a ball mill; then the ground mixtures are melted in aplatinum or alumina crucible under heating in muffle furnaces(≥1400 °C) and kept for a period of time for homogenizationpurposes; finally, the melted materials are quickly quenchedin a graphite mold or water. Possible advantages of thismethod over other synthesis methods include the easy experi-

Jingping Wang

Jingping Wang is the SpecialProfessor of Chemistry of HenanUniversity. She received her BAand MA from NortheasternNormal University in 1986 and1989, respectively. From 1989 to2002 she was an assistant pro-fessor, a lecturer and an associ-ate professor and then shebecame a professor at HenanUniversity. She has receivedseveral awards, including theMay 1st Labour Medal of HenanProvince and Excellent

University Key Teacher of Henan Province. Her research expertiseis in the synthesis, crystal structure and properties of novel polyox-ometalates and their potential application in different fields.

Review Inorganic Chemistry Frontiers


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mental operation, while impurities may be present in the finalproducts. Using this strategy, Tu et al. designed and preparedEr3+/Yb3+ co-doped TeO2-WO3-La2O3-Na2O (TWLN) glasses,which exhibit temperature sensing properties over a tempera-ture range of 293–569 K.41

The related luminescent mechanism

RE ions, the 4f electron configurations of which are shielded byelectron-filled 5s, 5p and 6s subshells, have displayed character-istic emission spectra with long lifetimes and high emissioncolor purity.50,51 The RE-based luminescent materials can bedivided into two categories from the perspective of the lumines-cent mechanism: upconversion (UC) and downconversiongroups. Most RE-based luminescent materials present weakluminescent properties because of the low molar absorptioncoefficients originating from the parity-forbidden effect of f–felectron transitions. A reliable strategy to overcome such a bot-tleneck is the introduction of sensitizers into the RE system.

The RE co-doped tungsten oxygen complexes have revealedfascinating photoluminescence properties, as the RE ions canbe sensitized by photoexcitation of the O → W LMCT band ofthe tungsten matrix through the intramolecular energy trans-fer mechanism. Apart from that, the tungsten oxygen com-plexes are a good choice to act as the matrix for their strongchemical and thermal stability. The doped RE ions play a keyrole in triggering energy transfer, as a result, compensating fortheir color imbalance and emission instability and presentingnovel luminescent properties. The acceptor RE3+ ion can besensitized to induce the characteristic emissions via the energytransfer process from certain RE3+ ions (donor or sensitizer) orthe tungsten oxygen clusters.

As for the downconversion materials, energy transferbetween two RE3+ ions has been intensively discussed in manystudies.39,46–49,52–54 Certainly, there are also reports that energytransfer occurs between three RE3+ ions.38,55 Our group hasinvestigated the possible energy transfer progress from POT toDy3+/Er3+ ions and between Er3+ and Dy3+ ions supported byTRES results.39 Similar to the energy transfer process happen-ing between two RE3+ ions, in our system, the POT serves as asensitizer and transfers energy to Dy3+ and Er3+ firstly, andthen energy transfer occurs between the Dy3+ and Er3+ ions(Fig. 1). Liu’s group also studied the energy transfer process inTm3+, Dy3+, and Eu3+ tri-doped NaGd(WO4)2 phosphors via cal-culating the rates and efficiencies of energy transfer betweenTm3+ and Dy3+ as well as Dy3+ and Eu3+.38

The UC process, as an anti-Stokes emission phenomenon,is achievable through sequential absorption of excitedphotons, which is an example of a non-linear optical process.The mechanisms of UC luminescence can be divided intothree cases: excited state absorption, energy transfer andphoton avalanche.56 The UC luminescence mechanisminvolved in the RE co-doped luminescent tungsten oxygenmaterials is mostly energy transfer. The excitation of theenergy transfer process in UC is realized through energy trans-

fer between two neighboring ions. The dopant concentrationplays a key role in the energy transfer efficiency. The Yb3+/Er3+

co-doped system is the one most intensively researchedbecause of efficient energy transfer between Yb3+ and Er3+

ions. The energy transfer process can be split into three steps:firstly, the conversion of ground state 2F7/2 to

2F5/2 of Yb3+, fol-

lowed by the transfer of the f–f state photon energy to theground state of Er3+, and lastly, the UC emission is inducedsuccessfully (Fig. 2).24 The Yb3+/Tm3+ and Yb3+/Ho3+ systemsbasically share similar energy transfer processes.

Representative structural types

RE substituted POTs, similar to other single RE doped tung-sten-oxo cluster compounds, are apt to crystallize into species

Fig. 1 A schematic energy level diagram demonstrating the energytransfer process from POT to Dy3+/Er3+ ions and characteristic emissionsof Dy3+ and Er3+ ions (dotted line denotes non-radiative transition).Reprinted with permission from ref. 39. Copyright 2018 AmericanChemical Society.

Fig. 2 A schematic energy level diagram demonstrating the energytransfer process from Yb3+ to Er3+ ions. Reproduced from ref. 24 withpermission from the Royal Society of Chemistry.



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with specific structures. Very similarly, RE co-doped crystallinePOTs can also crystallize into single crystals, and their struc-tures are usually characterized and confirmed by single crystalX-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD).As we know, SCXRD is a technique to determine the crystalstructure from the X-ray diffraction effect of a single crystal,which is powerful technology to determine the exact structureof single crystal materials. The PXRD technique is used forqualitative analysis, the determination of lattice constants andstress analysis of crystalline solid powder samples, and thus, itcan quantify the particle size, crystallinity and even provide aprecise X-ray structure. The RE3+ ion components can be ana-lysed by inductively coupled plasma (ICP). With thesemeasurements, the structures of RE co-doped crystalline POTscan be specified.

Quite differently, the RE co-doped tungstate materials withnanostructures are usually tested and characterized by PXRD,scanning electron microscopy (SEM), and transmission elec-tron microscopy (TEM). SEM and TEM are important charac-terization methods for high resolution microtopography ana-lysis; these provide insights to understand the morphologyand particle size of micro-/nanostructured materials. However,for nanostructured RE co-doped tungstate materials, it isdifficult to clearly characterize and identify their structures.According to the different dimensionality and aggregationstates, the RE co-doped tungsten oxygen compounds withdefined structures or morphology can be divided into threecategories: molecular, extended, and nanostructured. Only afew examples have been selected to illustrate these structuraltypes in this section, though more examples, including syn-thesis strategies, structural types, and corresponding appli-cations, are listed in Table 1.

Molecular

Molecular aggregates represent RE substituted POT com-pounds with zero-dimensional extensions. The RE substitutedPOT compounds can be divided into two parts: RE-POT andRE-POT-L. The RE-POT anions and their counterbalancingcations are discrete in the solid structure, and further stabil-ized by electrostatic interactions. The RE-POT-L compoundsare different from the RE-POT examples, as their anions aremodified by aromatic or aliphatic ligands, such as 1,10-phe-nantroline, 2-picolinicate or tartaric acid. In this sense, theorganic ligands work as “antennas” to harvest light. Then,energy is transferred from the excited state of the ligand to theRE ions to emit characteristic light. More importantly, thequantum yield of RE-POT-L is higher than that of RE-POT.57–59

Our group synthesized [N(CH3)4]6K3H7[DyxEr1−x(C4H2O6)(α-PW11O39)]2·27H2O (DyxEr(1−x)-POT) in 2018, the structures ofwhich are the same as the isomorph of Dy-POT, as confirmedby PXRD patterns and IR spectra (Fig. 3a).39 The dimeric polya-nion skeleton of Dy-POT can be seen as two Keggin type[α-PW11O39]

7− building blocks connected by the [RE(C4H2O6)]2

2− segment (Fig. 3b). Both RE centers display squareantiprism coordination geometry (Fig. 3c and d). Recently,Zhao’s group reported a series of Ho3+/Tm3+ co-doped POTs,

[H2N(CH3)2]10H3[SeO4RE5(H2O)7(Se2W14O52)2]·40H2O. TheseRE co-doped POTs contain a dimeric sandwich-type polyanion,[SeO4RE5(H2O)7(Se2W14O52)2]

13−, which is composed of twotetra-vacant Dawson-type [Se2W14O52]

12− units capturing a[SeO4Ho5(H2O)7]

11+ cluster through lacunary oxygen atomsfrom [Se2W14O52]

12− fragments.52 Also, most RE co-dopedPOTs are prepared through a conventional solution synthesisstrategy.

Extended

The RE co-doped tungsten oxygen compounds with extendedaggregate structures are formed by RE or counter cationswhere RE and/or counter cations serve as linkers to connectPOT or WO4

2− together. Zhao’s group reported a family of diva-cant Lindqvist RE doped dimeric POTs, Na10[RE2(C2O4)(H2O)4(OH)W4O16]2·30H2O (RE = Gd/Eu/Tb) (RE4W48).

46 The[RE2(C2O4)(H2O)4(OH)W4O16]2

10− polyanion is composed offour RE3+ ions, two oxalate ligands, and two [W4O16]

8− seg-ments that can be seen as the plenary Lindqvist [W6O19]

2−

segment upon loss of two {WO6} building blocks (Fig. 4a). Thelength of the polyanion is about 1.61 nm. The oxalate ligandsserve as organic linkers to connect two [RE2(H2O)4(OH)W4O16]

3− through four RE ions in the same mono-cappedsquare antiprismatic geometry. Then the dinuclear[Na2O3(H2O)8]

4− clusters connect to each other through aNa1A center via edge-sharing mode, leading to a 1D inorganicNa–O cluster chain (Fig. 4b). Most interestingly, the 1D chainconnects Re4W48 together, thus presenting an attractive 3Dporous framework (Fig. 4c).

Otherwise, an extended system based on LiGd(WO4)2(LGW):RE was reported in 2015, which showed potential applicationin a white light-emitting diode (WLED). The RE doped into theLGW does not cause changes to the crystal structure of LGWand qualifies as an isomorphic structure because the PXRDpatterns of these samples are consistent with standard data forLGW. Thus, these LiGd(WO4)2(LGW):RE samples have beencrystallized in a tetragonal system.60 Liu and co-workers madethe novel octahedral microcrystals by doping the RE into NaGd(WO4)2.

38 It is shown that the XRD patterns of these sampleare assigned to a pure tetragonal phase of NaGd(WO4)2, inaccordance with the standard values of the PDF card, indicat-ing that the doped RE has no influence over the NaGd(WO4)2host. Thus, the two cations Na+/RE3+ are in 8-fold coordinatedsites, as shown in Fig. 5a. Taking the NGW:Dy3+ phosphor asan example, the field emission scanning electron microscope(FESEM) and energy-dispersive X-ray spectrometer (EDS)results are shown in Fig. 5b–d. The NGW:Dy3+ phosphorclearly shows an octahedral microcrystal with an averagelength of 2 μm. The EDS pattern exhibits its chemical compo-sition, including Na, Gd, Dy, W, and O elements.

Nanostructured

Nanostructured materials have been getting much more atten-tion due to their strong potential for various applications. Fu’sgroup reported silica-coated Gd2(WO4)3:Yb

3+/Ho3+ nano-particles with a uniform distribution. The SEM and TEM



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Table 1 Typical structural types and the related applications of RE co-doped tungsten oxygen complexes

MaterialStructuraltype Synthesis strategy Application Ref.Host Dopant

[N(CH3)4]6K3H7[Dy(C4H2O6)(α-PW11O39)]2·27H2O Er Molecular Solution synthesis WLED/enhancing 39[N(CH3)4]6K3H7[Tm(C4H2O6)(α-PW11O39)]2·27H2O Dy Molecular Solution synthesis Multicolor/WLED 47[H2N(CH3)2]10H3[SeO4Ho5(H2O)7(Se2W14O52)2]·40H2O Tm Molecular Solution synthesis Enhancing 52[H2N(CH3)2]8Na4H2[Er2(OH) (B-α-TeW7O28)Sn2(CH3)4(W5O18)]2·18H2O

Yb Molecular Solution synthesis Enhancing 53

[N(CH3)4]3K2Eu(C7H5O2)(H2O)2(α-PW11O39)·11H2O Tb/Tm Molecular Solution synthesis Multicolor /WLED 55Na9EuW10O36·32H2O Sm/Tb Molecular Solution synthesis Multicolor 82Na17{(WO4)[Tb(H2O)(Ac) (B-α-SbW9O31(OH)2)]3}·50H2O

Eu/Gd/Dy Molecular Hydrothermal synthesis WLED 102

Na10[Gd2(C2O4)(H2O)4(OH)W4O16]2·30H2O Eu/Tb Extended Solution synthesis Multicolor/WLED 46Gd2(WO4)3 Er/Yb Extended Solid-state reaction Enhancing 24NaGd(WO4)2 Tm/Dy/Eu Extended Hydrothermal synthesis Multicolor 38Bi3Ti1.5W0.5O9 Yb/Er Extended Solid state reaction

methodTemperature sensor 45

LiLa(WO4)2 Dy/Sm Extended Solid state reaction. Enhancing/WLED 48NaBi(WO4)2 Dy/Tm Extended Solid state method WLED 54LiGd(WO4)2 Tm/Tb/Dy/

EuExtended Solid state reaction. WLED 60

CaGd2(WO4)4 Eu/Tb Extended Hydrothermal synthesis Multicolor 62CaGd2(WO4)4 Tb/Eu Extended Solid state reaction Enhancing 64LiGd(MoO4)2−x(WO4)x Sm/Dy Extended Solid state reaction Multicolor 66CaWO4 Tb/Eu Extended Solid state reaction Multicolor 72LiGd(WO4)2 Eu Extended Solid state reaction Enhancing 75Gd2W1−xMoxO6 Eu Extended Solid state reaction Enhancing 76Y6WxMo(1−x)O12 Eu Extended Solid state reaction Enhancing 77Sr9Eu2W4−xMoxO24 Extended Solid state reaction Enhancing 78NaY(WO4)2/Y6WO12 Eu Extended Hydrothermal synthesis Multicolor 86{[La2(DMF)8(H2O)6][ZnW12O40]}·4DMF Eu/Tb Extended Solution synthesis Multicolor 81{[Eu2(DMF)8(H2O)6][ZnW12O40]}·4DMF Tb Extended Solution synthesis Multicolor 81K2Gd(1−x)(PO4)(WO4) Dy Extended Solid state reaction WLED 101NaY(WO4)2 Tm/Dy/Eu Extended Solid state reaction WLED 103NaSrLa(MO4)3 [M = Mo and W] Eu Extended Solid state reaction WLED 105La2W2O9 Eu Extended Solid state reaction Red emission 107NaLaW2O7(OH)2(H2O) Eu/Tb/Sm/

DyExtended Hydrothermal synthesis Enhancing 108

LuWO6 Sm Extended Solid state reaction WLED 112LiLa(MoO4)x(WO4)2−x Tm/Dy Extended Solid state reaction WLED 113K2Ln(PO4)(WO4) (Ln = Y, Gd and Lu) Tb/Eu Extended Solid state reaction Multicolor 114NaGd(WO4)2 Eu Extended Hydrothermal synthesis Optical imaging 115Y2WO6 Ho Extended Solid state reaction Temperature sensor 119Ce10W22O81 Tb Nanostructure Hydrothermal synthesis Multicolor 25Gd2(WO4)3:@SiO2 Yb/Ho Nanostructure One-pot co-precipitation Multi-imaging/

temperature sensor/tumor inhibition

40

KBaGd(WO4)3 Dy/Eu Nanostructure Solid state reaction Multicolor 70Sr9La2W4O24 Sm/Eu Nanostructure Solid state reaction Multicolor 73Ca0.5Y1_x(WO4)2 Pr, Sm, Eu,

Tb, Dy, Yb/Er

Nanostructure Solid state reaction Enhancing 83

RE2(MO4)3 (RE = Y, La, Gd, Lu; M = W, Mo) Eu/Sm/Dy Nanostructure Hydrothermal synthesis Multicolor 87TeO2-Na2O-WO3 Er/Yb Nanostructure Melt quenching method Enhancing 93CaWO4 Tb/Eu/Dy Nanostructure Co-precipitation method WLED 111TeO2-WO3-La2O3-Na2O Er/Yb Amorphous Melt quenching method Temperature sensor 41TeO2-WO3 Er/Yb Amorphous Melt quenching method Temperature sensor 42TeO2-Li2CO3-WO3 Yb/Er Amorphous Melt quenching method Biotechnology 43NaGd(WO4)2 Eu Amorphous Melt quenching method Enhancing 44NaGd(WO4)2 Er /Yb Amorphous Melt quenching method Temperature sensor 44PbO-B2O3-3WO3-Al2O3 Dy/Tb Amorphous Melt quenching method Enhancing 84PbF2-25WO3-TeO2 Tm/Yb Amorphous Melt quenching method Enhancing 94TeO2-WO3 Ho/Yb Amorphous Melt quenching method Enhancing 95NaPO3-WO3 Nd/Tb Amorphous Melt quenching method Enhancing 110



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images of these nanoparticles reveal that the nanoparticles aremonodisperse with an average size of about 130 nm. Inaddition, Ho/Gd/Yb/W/Si was coated on the surface of thesenanoparticles, as proved by the elemental distributions.40

Laguna et al. synthesized Eu-doped NaGd(WO4)2 nanopho-sphors with a spherical shape, which showed the potential for

application in pH sensing. A TEM image of these nanospheresindicates a mean diameter of 88 nm.61 Interestingly,Kaczmarek et al. prepared Tb3+:Ce10W22O81 samples (I and II)built from nanosheets with different morphologies.25 Asshown in Fig. 6a and b, nanosheets I displayed an irregularmorphology when dioctyl sodium sulfosuccinate was absentfrom the reaction. While nanosheets II formed a flower-likestructure when dioctyl sodium sulfosuccinate was added to thereaction. The correlation between structures and optical pro-perties will be discussed in the luminescence performancesection in this review.

Fig. 3 (a) The presentation of RE-POT; (b) coordination mode of RE3+

ions and tartrate ligands; (c) coordination environment of the RE1 site;(d) coordination environment of the RE2 site. Reprinted with permissionfrom ref. 39. Copyright 2018 American Chemical Society.

Fig. 4 (a) The presentation of RE4W8; (b) the sinusoidal Na–O clusterchain; (c) the intriguing 3D extended porous framework. Reprinted withpermission from ref. 46. Copyright 2019 American Chemical Society.

Fig. 5 (a) Crystal structure of NGW; (b and c) FESEM images; (d) EDSpattern of NGW:Dy3+ phosphor. Reprinted with permission from ref. 38.Copyright 2014 American Chemical Society.

Fig. 6 SEM images at different magnifications of samples I (a and b)and II (c and d). Reproduced from ref. 25 with permission from the RoyalSociety of Chemistry.



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Related applicationsLuminescence performance

In recent years, the luminescence performance of RE co-dopedtungsten oxygen complexes has been studied deeply. Somemeasurements like the excitation spectra, emission spectra,lifetime of excited states, and TRES are widely used to evaluatethe luminescence properties. The excitation spectra reflectresponses to an external excitation wavelength, while the emis-sion spectra are generated by objects that emit light directly.When a substance is excited by a laser beam, its moleculesabsorb energy and are excited from the ground state to theexcited state, then return back to the ground state through aradiation transition process. When the excitation wavelength isremoved, the time required for the fluorescence intensity toreduce to 1/e of its maximum level is called the fluorescencelifetime. The curve of the decay lifespan can be fitted by anexponential function, thus calculating lifetime values. Inaddition, TRES reflects the variation trend of the luminescenceintensity in real time, providing insights into understandingthe luminescent kinetics. The luminescent studies of RE co-doped tungsten oxygen complexes mainly focus on multi-coloremission, enhancing luminescence and WLEDs.

The ability to output multicolor emission by RE co-dopedtungsten oxygen complexes is highly desirable for theirpotential applications in many fields, for instance, WLED,lighting, and displays. A number of strategies have beenimplemented to achieve multiple color emissions of RE co-doped tungsten oxygen complexes, including but not limitedtuning of the excitation wavelength, tuning the relative inten-sity of emission peaks, and controlling host sizes and/ortypes.25,38,46,47,53–55,62–89

In 2015, Kaczmarek’s group prepared Ce10W22O81 microma-terials via the hydrothermal synthesis method, and thendoped 5% Tb3+ ions into the cerium tungstate matrixes; thiswas inspired by the feasible energy transfer between Tb3+ andCe3+ ions.25 There are two broad bands reported in the exci-tation spectrum, one at ∼260 nm assigned to the W–O chargetransfer band and the other is located at ∼366 nm, which isattributed to Ce3+. The samples are observed to have thecharacteristic peaks of Tb3+ when excited at 260 nm, where the

CIE chromaticity coordinates are calculated as (0.23, 0.32) andthe emitting color is green. Nevertheless, upon excitation at366 nm, only one broad band at around 460 nm is observed inthe emission spectrum, which belongs to the transition bandof Ce3+.

Different from the excitation wavelength tuning approach,multi-color emission is also feasible via tuning the relativeintensity of multiple peaks, as a unique feature of RE basedluminescent materials. The reasoning is obvious because REspecies have unique sets of energy levels, in the form of exhi-biting different emission peaks with distinguishable spectro-scopic fingerprints. As a result, the doping combination caneasily tune the relative intensity of the emission peaks, asreported by many groups. For instance, our group reported aseries of Tm3+/Dy3+ co-doped POMs, [N(CH3)4]6K3H7[DyxTm1−x(C4H2O6)(α-PW11O39)]2·27H2O, whichdisplayed color-tunable properties.47 Different samples cangive a tuneable emitting color from blue to yellow throughadjusting the combinations of Tm3+/Dy3+. Zhao’s groupdemonstrated that multiple emission colors were achievableby doping Tb3+ ions into Na10[Eu2(C2O4)(H2O)4(OH)W4O16]2·30H2O. The samples emit green in the absence ofTb3+, while the color can change to red by adding Tb3+ indifferent ratios.46 Liu et al. co-doped Tm3+, Dy3+, and Eu3+ intoNaGd(WO4)2 (NGW) and synthesized novel octahedral micro-crystals, where the color could be tuned by energy transferbetween activator ions.38 Hence, the suite of NGW:Tm3+,Dy3+,Eu3+ phosphors can exhibit abundant color-tunable emissions.As shown in Fig. 7, the color tone can be tuned from coolwhite to red under 365 nm excitation upon increasing the con-centration of Eu3+ from 0 to 0.14 when Dy3+ = 0.03. Points 7and 8 in Fig. 7b lie in the bright blue area as the NGW:0.01Tm3+ sample gets excited under 359 or 355 nm, which showsthe potential for making blue LEDs. When Dy3+, showing ayellow color tone, was doped, the white was tuned (points9–12).

Other than the excitation wavelength and RE doping con-centration, the emission color can also be affected by the host.For example, Kaczmarek et al. reported several cases of Tb3+

doped cerium tungstate matrixes.25 It is interesting that thesamples presented different emission colors depending on

Fig. 7 (a–c) CIE chromaticity diagram with different RE3+ co-doped concentrations; (d) corresponding luminescence photographs of NGW:Tm3+,Dy3+,Eu3+ phosphors. Reprinted with permission from ref. 38. Copyright 2014 American Chemical Society.



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their hosts. For instance, the emission color of 5% Tb3+:Ce2(WO4)3 is green, while 5% Tb3+:Ce10W22O81 is blue. Moreinterestingly, the host morphology also turns out to be a factordetermining the emission color. The emission color isobserved as green–blue when Ce10W22O81 is assembled fromirregular micro-sized nanosheets.

In recent years, great efforts have been devoted to investi-gating RE co-doped materials for optical applications, and oneof the core evaluation metrics is luminescence stability. Takingthe work of Ning’s group as an example, they designed andsynthesized LiGd1−x(WO4)2:Eu

3+x (x = 0, 0.05, 0.2, 0.4, 0.6, 0.8,

0.9 and 1.0) through a facile solid-state reaction, and reportedits luminescence properties.75 Fig. 8a shows the emissionspectra of the LiGd1−x(WO4)2:Eu

3+x phosphors series;

obviously, the emission intensity increases overall with anincreasing amount of Eu3+ and reaches a maximum when x =0.8. Then the emission intensity begins to decrease owing tothe quenching effect caused by highly aggregated Eu3+ ions.The fluorescence lifetimes were measured, as shown inFig. 8b, and resulted in a good fitting using a first-order expo-nential function. The lifetime trend is similar to that of theemission intensity. The lifetime of Eu3+ steadily rises with anincreasing amount of Eu3+, and reaches a maximum when theEu3+ content equals 80%, and then decreases upon furtheradding Eu3+ ions. The CIE chromaticity coordinate of LiGd(WO4)2:80%Eu3+ is (0.656, 0.334), which is very close to that ofstandard red light (0.670, 0.330), implying the high colorpurity of LiGd(WO4)2:80%Eu3+ complexes. Thus, the LiGd(WO4)2:80%Eu3+ sample is considered to be a potential redemitting component, as supported by its high emission inten-sity and long fluorescence lifetime.

Recently, Zhao’ group prepared a family of Er/Ybco-doped POTs, [H2N(CH3)2]8Na4H2[Er(OH)(B-α-TeW7O28)RESn2(CH3)4(W5O8)]2·18H2O (RE = Er/Yb), and systematicallyinvestigated their luminescence properties in the visible andNIR regions, along with their UC emission properties.53 Thereare no obvious shifts of the characteristic emission bands ofEr3+ accompanied by the addition of the Yb3+ ion, but theemission intensity rises first and gradually decreases. Theemission intensity reaches a maximum with a mass ratio ofEr/Yb of 0.40 : 0.60. The reason for the change is the energytransfer process occurring from the excitation of Yb3+ ions toEr3+ ions on account of the fact that the 2F5/2 of Yb and 4I11/2of Er share similar energy values when increasing the concen-tration of Yb3+; besides, the emission intensity can be affectedby the decrease of the Er3+ concentration. However, such aphenomenon was not observed in the NIR region.Dramatically, the UC emission process occurred again and theUC emission reaches an apex when the radio of Er/Yb is0.06 : 0.94, mainly owing to strong cross-relaxation withinthe dopants. However, the emission intensity can bequenched inevitably if the concentration is too high. Veryrecently, Zhao et al. also probed the luminescence behavior ofHo3+/Tm3+ co-doped POT derivatives based on [H2N(CH3)2]10H3[SeO4Ho5(H2O)7(Se2W14O52)2]·40H2O in the visibleregion.52 In the photoluminescence emission spectra, the

characteristic emission bands of Ho3+ and Tm3+ coexist con-currently, while the characteristic emission bands of Tm3+

ions are slightly redshifted and the emission peaks of Ho3+

ions are slightly blueshifted when increasing the concentrationof Tm3+ ions (Fig. 9a). Moreover, the emission intensity of

Fig. 8 (a) Photoluminescence spectra of LiGd1−x(WO4)2:x%Eu3+ andCaWO4:y%Tb3+ samples; (b) Eu3+ decay curves of host emission inLiGd1−x(WO4)2 samples with different Eu3+ doping concentrations.Reprinted with permission from ref. 75. Copyright 2016 Royal Society ofChemistry and the Centre National de la Recherche Scientifique.



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Ho3+ keeps increasing until reaching a maximum when themolar ratio of Ho3+/Tm3+ is 0.50 : 0.50 and then starts todecrease, whereas the emission intensity of Tm3+ increasesconstantly. From Fig. 9b and c, the lifetime increases monoto-nically with increasing Tm3+ amounts, and reaches themaximum point with τ* = 5.18 μs where Ho3+ : Tm3+ is0.50 : 0.50. Our group has also made progress in this field. Wesynthesized a series of DyxEr(1−x)-POM (x = 0–1) derivativesbased on previously reported [N(CH3)4]6K3H7[Dy(C4H2O6)(α-PW11O39)]2·27H2O (Dy-POM) and explored their lumine-scence properties in depth.39 Three characteristic emissionbands of Dy3+ are observed, centered at 480, 573 and 663 nm,when excited at 367 nm (Fig. 10a). With increasing Er3+-doping

amounts, the emission intensity of Dy3+ gradually decreases;for instance, the emission intensity of these peaks drops to∼5% of the emission intensity of Dy-POT when the Er3+ ratio ishigher than 50%. Interestingly, no characteristic peaks of Er3+

ions were observed even when x = 0.1, probably because theemission intensity of Er3+ is much lower than that of Dy3+ ions.The CIE chromaticity coordinates were obtained from the photo-luminescence emission spectra (Fig. 10b). The consistent CIEchromaticity of these samples proves that doped Er3+ has aminor effect on the optical properties, except for the emissionintensity. The lifetime of these samples decreases as the dopedEr3+ amount rises (Fig. 10d). These samples were exposed to UVirradiation for 300 min to study emission intensity changes. Asshown in Fig. 11a–f, the emission intensity and lifetimedecreases upon 300 min irradiation (x = 0, 0.9). In contrast,when x = 0.8, the intensity stays the same during the first30 min irradiation, and the intensity drops to ∼85% of theinitial intensity after 300 min of irradiation (Fig. 11g and h).The τ1 value of Dy0.8Er0.2-POM is approximately constant duringthe irradiation process (Fig. 11i). In short, Dy0.8Er0.2-POM main-tains its emission intensity well and has a higher photostability.

Apart from changing the concentration of RE3+, the emis-sion intensity can also be improved through changing thehosts. Jeong et al. reported a series of Eu3+-activatedGd2W1−xMoxO6 phosphors.76 Through adjusting the pro-portions of W and Mo species, they observed that the emissionintensity was greatly enhanced when x = 0.95; this showed∼2.5 times improvement compared with x = 0.

Great progress has been made in UC materials to improvetheir stability for potential applications in biomedicalimaging, therapeutics, and energy conversion.43,90–95

Fig. 9 (a) Comparison of photoluminescence emission spectrabetween different molar ratios of Ho3+/Tm3+ co-doped samples underexcitation at 325 nm at room temperature; (b) the luminescence decaycurve of the Ho3+/Tm3+ co-doped state; (c) average decay time evol-ution of various Ho3+/Tm3+ co-doped samples monitored at 648 nm.Reprinted with permission from ref. 52. Copyright 2020 John Wiley &Sons Ltd.

Fig. 10 (a) Photoluminescence emission spectra of the DyxEr(1−x)-POMpowder sample (λex = 367 nm); (b) the CIE 1931 chromaticity coordinatescorresponding to the photoluminescence spectra of DyxEr(1−x)-POM; (c)corresponding line charts of the emission intensity at 480 nm, 573 nmand 663 nm of DyxEr(1−x)-POM; (d) photoluminescence decay time ofthe DyxEr(1−x)-POM powder sample. Reprinted with permission from ref.39. Copyright 2018 American Chemical Society.



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WLEDs, a hot topic in solid-state lighting areas, haveattracted considerable attention for their high energyefficiency, strong brightness, and low power consumption.RE3+ ions are widely used in making WLEDs materials, sincedifferent RE3+ ions can emit inherently characteristicCommission International de I’Eclairage (CIE) luminescentcolor coordinates. For example, Tm3+ is one of the mostcommon elements in making white light emitting materialsfor its unique blue characteristic emission. Besides, Eu3+ iswidely used for red color emission, Dy3+ or Sm3+ for yellowcolor, and Tb3+ for green color emission. Usually, co-doped REions in the host can achieve white light emission principallythrough two strategies: (i) co-doping blue-emitting and yellow-emitting ions into the host in a suitable ratio; and (ii) dynami-cally tuning the emission ratio of green, red, and blue emit-ters. With that, lots of multi-center RE3+ based luminescentmaterials containing two or more types of RE3+ ions have beenprepared, with the aim of achieving white lightemissions.96–114

In 2018, our group synthesized a series of double-tartaricbridging POT derivatives, [N(CH3)4]6K3H7[DyxTm1−x(C4H2O6)(α-PW11O39)]2·27H2O.

47 The CIE luminescent color coordinatesof 1 (x = 1) and 2 (x = 0) are yellow and blue, and the connec-tion line from the points of 1 to 8 on the chromaticity diagramappear as cross in the white area. Based on these, derivatives3–8 were prepared by adjusting the radio of Tm3+/Dy3+, andtheir emission spectra were investigated. The correspondingCIE coordinates obviously imply that 6 (x = 0.25) falls withinthe white-light area of the CIE chromaticity diagram (0.325,0.349), close to standard white light (0.333, 0.333) (Fig. 12a).In order to deepen research in such a field, just recently, weprobed the luminescence behavior of a series of dimeric POTs,[N(CH3)4]3K2EuxTbyTm1−x−y(C7H5O2)(H2O)2(α-PW11O39)·11H2O(5–11).55 As described earlier, white light emission could beachieved by reasonably tuning the combination of red, greenand blue colors. In theory, adjusting the relative molar ratio ofEu3+/Tb3+/Tm3+ components could result in white emission,since Eu3+ and Tb3+ components are able to provide the red

Fig. 11 Photoluminescence emission spectra (λex = 367 nm) of Dy-POM (a), Dy0.9Er0.1-POM (d) and Dy0.8Er0.2-POM (g); corresponding variationrules of emission intensity at 480, 573 and 663 nm of Dy-POM (b), Dy0.9Er0.1-POM (e) and Dy0.8Er0.2-POM (h); decay time diagrams of Dy-POM (c),Dy0.9Er0.1-POM (f) and Dy0.8Er0.2-POM (i) upon excitation at 367 nm and emission at 573 nm under continuous UV irradiation (300 W xenon lamp)for 300 min. Reprinted with permission from ref. 39. Copyright 2018 American Chemical Society.



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and green emitting color separately, and blue emitting color isavailable by overlapping Tm3+ and surrounding organic/in-organic ligands. As expected, white light emission was success-fully achieved with a controlled ratio of Eu3+ : Tb3+ : Tm3+ com-ponents of 0.06 : 0.10 : 0.84 (Fig. 12b). In the same year, Zhou’sgroup prepared a series of Na9EumTbnCe1−m−nW10O36 basedwhite-light emitting thin films through a spin-coatingmethod.74 When m = 0.0123 and n = 0.246, the thin film iscapable of generating white light emission. More interestingly,the emission intensity of the films could be regulated bytuning sample concentrations. Recently, studies on RE co-doped tungsten oxide materials have also been widelyreported. In 2015, Gong’s group synthesized a family of LiGd(WO4)2(LGW):RE3+ (RE: Tm, Tb, Dy, Eu) phosphors through asolid-state reaction method.60 In this work, white light emis-sion in a single-phase host was achieved through combiningTm3+, Tb3+, and Eu3+ together in a co-doped LGW host. Indetail, LGW:2% Tm3+,4% Tb3+,x% Eu3+ phosphors were syn-thesized and their digital luminescence photographs areshown in Fig. 12c, which clearly demonstrate white light emis-sion when x = 3. In 2017, Guo and co-workers selected K2Gd(PO4)(WO4) as the host to obtain novel phosphate-tungstatephosphors by doping with Dy3+.101 The results indicate thatthe KGPW:xDy3+ samples have warm white emission with CIEat (x = 0.33, y = 0.36), which is quite close to the NationalTelevision Standards Committee (NTSC) white point (x = 0.33,y = 0.33) when x = 0.05 (Fig. 12d).

Apart from the work mentioned above, there are manyother reports on emitting red, blue, yellow, and green colors

with the potential for obtaining white emission. For example,Reddy’s group studied a red emitting phosphor based on LiGd(WO4)2:xEu

3+ complexes.97 Kalimuthu et al. prepared a seriesof xSm3+: LiGd(WO4)2 phosphors with strong reddish-orangeemissions.98 Last but not least, Tb3+ doped NaGd(WO4)2 withdefined uniform square plate-like microcrystals was reportedto present intensive green emission.99

Biological applications

Recently, several novel RE co-doped UC materials have beenstudied as new optical markers exhibiting potential in the bio-medical imaging field. In 2018, Yang’s group reported theX-ray luminescence nanoprobe PEG-NaGd(WO4)2:Eu, whichwas used for optical bioimaging based on the energy transferUC mechanism.115 10% Eu3+ was doped into the complexesand the maximum emission intensity was achieved at λ =615 nm. Impressively, its emission intensity even outper-formed all other X-ray luminescence nanoprobes reported pre-viously, making it a promising bioimaging marker candidate.The feasibility of X-ray luminescence nanoprobes for biologicalapplications was also investigated through comparing themagainst InP/ZnS quantum dots (QDs). As shown in Fig. 13a, noobvious fluorescence of cell medium, cell lysis and mouse hairwas observed under UV or X-ray excitation with InP/ZnSquantum dots (QDs) or Eu3+-activated PEG-NaGd(WO4)2 nano-rods (PEG-NGW:Eu). Mice were imaged under UV or X-ray exci-tation after intramuscular injection or (30 min) intravenousinjection of the InP/ZnS QDs and PEG-NGW:Eu nanorods intomice (Fig. 13b). Obviously, there is a strong auto-fluorescencein mice using UV-excited QDs for fluorescence imaging, whilealmost no fluorescence background is observed with the X-ray-excited luminescence nanoprobe. Furthermore, it can be

Fig. 12 (a) CIE chromaticity diagram corresponding to emissions of3–8; (b) CIE chromaticity diagram corresponds to emissions of 5–11; (c)the variety of CIE chromaticity coordinates of the as-prepared phos-phors. (d) CIE chromaticity coordinates and the correlated color temp-eratures (Tcct). (a–d) Reprinted/reproduced with permission from ref. 47,55, 60 and 101. Copyright 2018 Royal Society of Chemistry. Copyright2019 Elsevier B.V. Copyright 2015 the Royal Society of Chemistry.Copyright 2017 the Royal Society of Chemistry.

Fig. 13 (a) Luminescence imaging of the cell medium, cell lysis, andmouse hair under UV or X-ray excitation; (b) luminescence imaging ofmice before and after intramuscular injection of InP/ZnS QDs (left) andPEG-NGW:Eu nanorods (right). The red and black dashed circles rep-resent target sites and background sites, respectively; (c) luminescenceimaging of mice before and after (30 min) intravenous injection of InP/ZnS QDs (left) and PEG-NGW:Eu nanorods (right); (d) comparison ofsignal-to-background ratio in mice. Reprinted with permission from ref.115. Copyright 2018 Royal Society of Chemistry.



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clearly observed that there is strong significant fluorescenceemitted from the PEG-NGW:Eu nanorods for mice treatedthrough intravenous injection (Fig. 13d). These aforemen-tioned results indicate that X-ray luminescence imaging canserve as a highly sensitive imaging technique for medical diag-nosis applications. Based on the signal-to-background ratiocalculated, it is revealed that the as-synthesized X-ray lumine-scence nanoprobes have a better signal-to-background ratiothan the 1/T1 versus Gd3+ ion concentration, which impliesthat PEG-NGW:Eu nanorods exhibit a stronger positive con-trast effect. PEG-NGW:Eu nanorods were also employed aspromising contrast materials for enhanced X-ray computedtomography imaging for excellent X-ray attenuation of W ions.

In 2019, Gd2(WO4)3:Yb3+/Ho3+@SiO2 (Gd2(WO4)3@SiO2)

nanoparticles, with a uniform distribution (∼130 nm), weresynthesized via a one-pot co-precipitation method.40 Themacroscopic yellow fluorescence was detected under NIR lightirradiation owing to the scheelite-like iso-structure ofGd2(WO4)3:Yb

3+/Ho3+@SiO2 nanoparticles (Gd2(WO4)3@SiO2-Pt-PEG), implying their potential for fluorescence imaging(Fig. 14a). After incubating nanoparticles in CT26 cells, the UCluminescence (UCL) signals were measured after differentincubation times, as shown in Fig. 14b. The blue fluorescencefrom DAPI in the nuclei is observable clearly at the very begin-ning. After incubation for 10 min, the fluorescence begins toappear in the cytoplasm and reaches maximum intensity after6-h incubation. The results indicate that the CT26 cells take upthe Gd2(WO4)3@SiO2-Pt-PEG UC nanoparticles (UCNPs) over afew hours. In addition, the magnetic resonance (MR) imagingproperty of the Gd2(WO4)3@SiO2-Pt-PEG UCNPs has also beeninvestigated for the advantageous contrast effect of Gd3+

(Fig. 14c). It is obvious that the amount of Gd3+ is positively

correlated to the MR signal. As shown in Fig. 14d, the relax-ation rate (1/T1) strongly correlates to the Gd3+ concentrationin a linear manner. Additionally, the test in vivo shows thatinjection is positive for MR imaging, which indicates that theGd2(WO4)3@SiO2-Pt-PEG UCNPs may serve for UCL and MRdual-mode imaging in the future.

Sensor

Additionally, RE co-doped tungstate materials have also dis-played great potential for sensing applications, for instance,pH sensing and temperature sensing. As we all know, pHsensing is a particular case for biosensors. For this appli-cation, it is independent of the local fluorophore concen-tration, which needs ratiometric sensors.116 In 2017, Ocañaand co-workers synthesized Eu-doped NaGd(WO4)2 nanopho-sphors with a spherical shape at low temperature.61 By coatingEu3+-based nanoparticles with fluorescein, a wide pH range(pH = 4–10) ratiometric sensor was developed. As shown inFig. 15a, the characteristic emission band of Eu3+ cations stays

Fig. 14 (a) The UC emission spectrum; (b) in vitro UCL imaging atdifferent times; (c) in vitro T1-weighted MR imaging at different concen-trations; (d) relaxation rate r1 versus different molar concentrations; (e)pre-injection; (f ) after injection in situ. Reprinted with permission fromref. 40. Copyright 2019 Royal Society of Chemistry.

Fig. 15 (a) Emission spectra of the as-synthesized Eu(6%):NaGd(WO4)2NPs coated with poly(fluorescein isothiocyanate allylamine hydro-chloride) at pH 4 (blue), 7 (green), and 12 (red). Emission of Eu3+ (rightpart) was collected with excitation at λex = 250 nm, while λex for fluor-escein was 490 nm (emission on the left part). All spectra are rep-resented with the same scale; (b) intensity ratio of the fluorescein(maximum λem = 512 nm, λex = 490 nm) and Eu3+ (λem = 611 nm, λex =250 nm) emissions at different pH values. Reproduced from ref. 61 withpermission from the Royal Society of Chemistry.



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constant with a peak at 616 nm, while the broad emissionband centered at 512 nm exhibits a clear dependence on pHchanges. Thereby, the sigmoidal responses obtained from thecalibration curve of the emission intensity difference betweenfluorescein and Eu3+ cations against pH values demonstratesits feasibility in pH sensing over the pH range of 4 to 10(Fig. 15b).

Temperature sensing is another promising application forRE co-doped tungstate materials.41–45,117–119 Tu et al. designeda portable all-fiber thermometer using Er3+/Yb3+ co-dopedTWLN glasses, leveraging the fluorescence intensity ratio (FIR)of Er3+ ions, in which Er3+ ions lie in asymmetric environ-ments in TWLN glass, according to Judd–Ofelt analysis.41 It isobvious that the UC emission intensity at 523 nm increasesfirstly and then decreases, while the UC emission intensity at540 nm decreases continuously over the temperature range of293–569 K (Fig. 16a), which is also intuitively shown inFig. 16b. Fitting of the dependence of FIR on temperature T isshown in Fig. 16c. The absolute and relative temperature sensi-tivity, Sa and Sr, are given in Fig. 16d, where Sa reaches itsmaximum value of 86.7 × 10−4 K−1 at 553 K, which is one ofthe highest values reported in the literature.

Conclusion

In this review, the synthesis strategies, luminescent mecha-nism, structural types, and related applications of RE co-dopedtungsten oxygen complexes are systematically introduced.Firstly, we reviewed the typical synthesis strategies for makingRE co-doped tungsten oxygen complexes, followed by elaborat-ing the energy transfer process using several certified methodsincluding TRES, lifetime measurements and related calcu-

lations to uncover the luminescent mechanism. Moreover,different structural types and characterization methods havealso been introduced for RE co-doped tungsten oxygen com-plexes: molecular and extended structures characterized bySCXRD and/or PXRD analysis; nanostructured morphologiesdemonstrated by SEM and/or TEM techniques. We thenfocused on the novel luminescence properties of RE co-dopedtungstate materials in the fields of multi-color emission,enhanced luminescence and WLEDs. Furthermore, biologicalapplications of RE co-doped tungstate materials, such as bio-imaging, pH sensing, and temperature sensing, are also dis-cussed. The main goal of this review is to present readers withthe recent advances of RE-based tungsten oxygen complexes,including synthesis strategies associated with the luminescentmechanism, structural features and related applications, withthe hope of inspiring further research work in this field andextending the applications of RE co-doped tungsten oxygencomplexes into a wider range of areas.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (22071043, 22071044, 21771053and 21771054), the Major Project of Science and Technology,Education Department of Henan Province (20A150010).

Notes and references

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recent advances in rare earth co-doped luminescent

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