Developing a Unique Hydrogen-Bond Network in a Uranyl Coordination Framework for Fuel Cell Applications.

Crystalline materials with persistent high anhydrous proton conductivity that can be directly used as a practical electrolyte of the intermediate-temperature proton exchange membrane fuel cells for durable power generation remain a substantial challenge. The present work proposes a unique way of the axial uranyl oxo atoms as hydrogen-bond acceptors to form a dense hydrogen-bonded network within a stable uranyl-based coordination polymer, UO2(H2PO3)2(C3N2H4)2 (HUP-3). It exhibits stable and efficient anhydrous proton conductivity over a super-wide temperature range (-40-170 °C). It was also assembled into a H2/O2 fuel cell as the electrolyte and shows a high electrical power density of 11.8 mW·cm-2 at 170 °C, which is among one of the highest values reported from crystalline solid electrolytes. The cell was tested for over 12 h without notable power loss.

Hinokitiol, an Advanced Bidentate Ligand for Uranyl Decorporation.

Despite the critical role actinide decorporation agents play in the emergency treatment of people in nuclear accidents and other scenarios that may cause internal contamination of actinides, new ligands have seldom been reported in recent decades because the current inventory has been limited to only a handful of functional groups. Therefore, new functional groups are always being urgently sought for the introduction of advanced actinide decorporation agents. Herein, a tropolone derivative, 2-hydroxy-6-(propan-2-yl)cyclohepta-2,4,6-trien-1-one (Hinokitiol or Hino), is proposed to be a promising candidate for this purpose by virtue of its well-demonstrated high membrane permeability and high affinity for metal ions. The coordination stoichiometry of Hino with uranyl is demonstrated to be 3:1 both in an aqueous solution (pH 7.4) and in the solid state. The results of a liquid-liquid extraction experiment further show that Hino exhibits strong chelating ability and selectivity toward uranyl over biological essential metal ions (i.e., Mn2+, Zn2+, Co2+, and Ni2+) with an extraction efficiency of >90.0%. The in vivo uranyl removal efficacies of Hino in kidneys and bone of mice are demonstrated to be 67.0% and 32.3%, respectively. On the basis of the observations described above, it is highly possible that further modification of Hino will lead to a large family of multidentate agents with enhanced uranyl decorporation ability.

Turn-up Luminescent Sensing of Ultraviolet Radiation by Lanthanide Metal-Organic Frameworks.

Here, we report a series of two-dimensional lanthanide metal-organic frameworks Ln-DBTPA (where DBTPA = 2,5-dibromoterephthalic acid and Ln = Tb (1), Eu (2), or Gd (3)) showing a unique turn-up responsiveness toward ultraviolet (UV) radiation. The luminescence enhancement was derived from the accumulated radicals that can promote the intersystem crossing process. The compound 1 shows an ultralow detection limit of 9.1 × 10-9 J toward UV radiation, representing a new type of luminescent UV detectors.

3-Hydroxy-2-Pyrrolidinone as a Potential Bidentate Ligand for in Vivo Chelation of Uranyl with Low Cytotoxicity and Moderate Decorporation Efficacy: A Solution Thermodynamics, Structural Chemistry, and in Vivo Uranyl Removal Survey.

Uranium poses a threat for severe renal and bone damage in vivo. With the rapid development of nuclear industry, it is more urgent than ever to search for potential in vivo uranium chelators. In this work, 3-hydroxy-2-pyrrolidinone (HPD) is investigated as a new potential uranium decorporation ligand. The potentiometric titration measurements were carried out, and the stability constants were determined to be log ß110 = 10.5(7), log ß120 = 20.7(9), and log ß130 = 28.2(4). The species distribution diagram shows that nearly all uranyl is complexed by HPD at pH 7.4 under the defined condition. A single crystal of uranyl and HPD complexes, [(UO2)3O(H2O)3(C4H6NO2)3]·NO3·12H2O (uranyl-HPD), was obtained via an evaporation method. The overall structure of uranyl-HPD is a trimer that consists of three uranyl units and three HPD ligands. The uranyl unit is equatorially coordinated by three oxygen atoms from two HPD agents, one coordinated water molecule, and one µ3-O atom that is shared by three uranyl units. The results of the cytotoxicity assay indicate that the ligand is less toxic than the chelators used clinically (i.e., DTPA-ZnNa3 and 3-hydroxy-1,2-dimethyl-4(1 H)-pyridone (DFP)). The results of the uranium removal assay using the NRK-52E cell show that it could reduce as much as 58% of the uranium content at the cellular level. Furthermore, the in vivo uranium decorporation assays demonstrate that HPD can remove 52% of uranium deposited in the kidney but shows poor uranium removal efficacy in the bone.

Three Mechanisms in One Material: Uranium Capture by a Polyoxometalate-Organic Framework through Combined Complexation, Chemical Reduction, and Photocatalytic Reduction.

The design and synthesis of uranium sorbent materials with high uptake efficiency, capacity and selectivity, as well as excellent hydrolytic stability and radiation resistance remains a challenge. Herein, a polyoxometalate (POM)-organic framework material (SCU-19) with a rare inclined polycatenation structure was designed, synthesized through a solvothermal method, and tested for uranium separation. Under dark conditions, SCU-19 can efficiently capture uranium through ligand complexation using its exposed oxo atoms and partial chemical reduction from UVI to UIV by the low-valent Mo atoms in the POM. An additional UVI photocatalytic reduction mechanism can occur under visible light irradiation, leading to a higher uranium removal without saturation and faster sorption kinetics. SCU-19 is the only uranium sorbent material with three distinct sorption mechanisms, as further demonstrated by X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) analysis.

Unique Proton Transportation Pathway in a Robust Inorganic Coordination Polymer Leading to Intrinsically High and Sustainable Anhydrous Proton Conductivity.

Although comprehensive progress has been made in the area of coordination polymer (CP)/metal-organic framework (MOF)-based proton-conducting materials over the past decade, searching for a CP/MOF with stable, intrinsic, high anhydrous proton conductivity that can be directly used as a practical electrolyte in an intermediate-temperature proton-exchange membrane fuel cell assembly for durable power generation remains a substantial challenge. Here, we introduce a new proton-conducting CP, (NH4)3[Zr(H2/3PO4)3] (ZrP), which consists of one-dimensional zirconium phosphate anionic chains and fully ordered charge-balancing NH4+ cations. X-ray crystallography, neutron powder diffraction, and variable-temperature solid-state NMR spectroscopy suggest that protons are disordered within an inherent hydrogen-bonded infinite chain of acid-base pairs (N-H···O-P), leading to a stable anhydrous proton conductivity of 1.45 × 10-3 S·cm-1 at 180 °C, one of the highest values among reported intermediate-temperature proton-conducting materials. First-principles and quantum molecular dynamics simulations were used to directly visualize the unique proton transport pathway involving very efficient proton exchange between NH4+ and phosphate pairs, which is distinct from the common guest encapsulation/dehydration/superprotonic transition mechanisms. ZrP as the electrolyte was further assembled into a H2/O2 fuel cell, which showed a record-high electrical power density of 12 mW·cm-2 at 180 °C among reported cells assembled from crystalline solid electrolytes, as well as a direct methanol fuel cell for the first time to demonstrate real applications. These cells were tested for over 15 h without notable power loss.

Monitoring Ultraviolet Radiation Dosage Based on a Luminescent Lanthanide Metal-Organic Framework.

A luminescent lanthanide metal-organic framework [Tb7(OH)8(H2O)6(IDA)3(COO)3]·4Cl·2H2O (Tb-IDA, IDA = iminodiacetic acid) was hydrothermally synthesized and structurally characterized. Monitoring ultraviolet radiation was achieved by correlating the dosage with the luminescence color change in doped Gd99Tb0.1Eu0.9-IDA compound. A linear relationship is developed across a broad range from blue to yellow within a CIE chromaticity diagram.

An Ultrastable Heterobimetallic Uranium(IV)/Vanadium(III) Solid Compound Protected by a Redox-Active Phosphite Ligand: Crystal Structure, Oxidative Dissolution, and First-Principles Simulation.

The first heterobimetallic uranium(IV)/vanadium(III) phosphite compound, Na2UV2(HPO3)6 (denoted as UVP), was synthesized via an in situ redox-active hydrothermal reaction. It exhibits superior hydrolytic and antioxidant stability compared to the majority of structures containing low-valent uranium or vanadium, further elucidated by first-principles simulations, and therefore shows potential applications in nuclear waste management.

Tunable 4f/5f Bimodal Emission in Europium-Incorporated Uranyl Coordination Polymers.

There have been numerous studies on emission-color regulation by the adjustment of molar amounts of multiple trivalent lanthanide cations, such as Eu3+, Tb3+, Dy3+, and others, in many types of solid host materials. Although uranyl emission originating from charge-transfer transitions has been well-recognized and investigated for many decades, as of now there is no report on tunable 4f/5f bimodal emission based on heterobimetallic lanthanide(III) and uranyl(VI) compounds. In most cases, complete energy transfer between uranyl(VI) and lanthanide(III) centers was observed. In this work, a series of isotypic-europium-incorporated uranyl coordination polymers, Eu@UO2L(DMF) (L2- = 3,5-pyridinedicarboxylate, denoted as 1-10, which represent the different Eu contents in UO2L(DMF); DMF = N,N-dimethylformamide), has been synthesized by solvothermal reactions. Crystallographic evidence of this series unveiled one-dimensional chains of UO22+ as pentagonal-bipyramidal units bridged by pyridinedicarboxylate with no defined, crystallographically unique site containing Eu, even for the products with high concentrations of Eu in this series. However, emission bands characteristic of Eu3+ were clearly observed in every product along with the characteristic uranyl-emission feature when observed with UV-vis fluorescence spectroscopy. Laser-ablation inductively coupled plasma mass spectrometry indicated that europium was concomitant with uranium, corroborating the incorporation of europium into crystals of UO2L(DMF). Systematic control of the solvent ratio (VH2O/VDMF) in each reaction gives rise to an enrichment of Eu3+ in the interior of UO2L(DMF). In addition, the color of emission of these compounds changed significantly from bright red to bright green with decreasing Eu content. This phenomenon occurs from the highly efficient energy transfer between the UO22+ and Eu3+ centers within each sample, providing the first case of a tunable 4f/5f bimodal emission in a mixed 4f/5f-elements-bearing metal-organic-hybrid material.

Facile and Efficient Decontamination of Thorium from Rare Earths Based on Selective Selenite Crystallization.

The coexistence of radioactive contaminants (e.g., thorium, uranium, and their daughters) in rare earth minerals introduces significant environmental, economic, and technological hurdles in modern rare earth production. Efficient, low cost, and green decontamination strategies are therefore desired to ameliorate this problem. We report here a single-step and quantitative decontamination strategy of thorium from rare earths based on a unique periodic trend in the formation of crystalline selenite compounds across the lanthanide series, where Ce(III) is fully oxidized in situ to Ce(IV). This gives rise to a crystallization system that is highly selective to trap tetravalent f-blocks while all other trivalent lanthanides completely remain in solution when coexist. These results are bolstered by first-principles calculations of lattice energies and an examination of bonding in these compounds. This system is contrasted with typical natural and synthetic systems, where trivalent and tetravalent f-block elements often cocrystallize. The separation factors after one round of crystallization were determined from binary systems of Th(IV)/La(III), Th(IV)/Eu(III), and Th(IV)/Yb(III) to reach 2.1 × 105, 1.2 × 105, and 9 × 104, respectively. Selective crystallization of thorium from a simulated monazite composite yields a separation factor of 1.9 × 103 with nearly quantitative removal of thorium.

Emergence of Uranium as a Distinct Metal Center for Building Intrinsic X-ray Scintillators.

The combination of high atomic number and high oxidation state in UVI materials gives rise to both high X-ray attenuation efficiency and intense green luminescence originating from ligand-to-metal charge transfer. These two features suggest that UVI materials might act as superior X-ray scintillators, but this postulate has remained substantially untested. Now the first observation of intense X-ray scintillation in a uranyl-organic framework (SCU-9) that is observable by the naked eye is reported. Combining the advantage in minimizing the non-radiative relaxation during the X-ray excitation process over those of inorganic salts of uranium, SCU-9 exhibits a very efficient X-ray to green light luminescence conversion. The luminescence intensity shows an essentially linear correlation with the received X-ray intensity, and is comparable with that of commercially available CsI:Tl. SCU-9 possesses an improved X-ray attenuation efficiency (E>20 keV) as well as enhanced radiation resistance and decreased hygroscopy compared to CsI:Tl.

Identifying the Recognition Site for Selective Trapping of 99TcO4- in a Hydrolytically Stable and Radiation Resistant Cationic Metal-Organic Framework.

Effective and selective removal of 99TcO4- from aqueous solution is highly desirable for both waste partitioning and contamination remediation purposes in the modern nuclear fuel cycle, but is of significant challenge. We report here a hydrolytically stable and radiation-resistant cationic metal-organic framework (MOF), SCU-101, exhibiting extremely fast removal kinetics, exceptional distribution coefficient, and high sorption capacity toward TcO4-. More importantly, this material can selectively remove TcO4- in the presence of large excesses of NO3- and SO42-, as even 6000 times of SO42- in excess does not significantly affect the sorption of TcO4-. These superior features endow that SCU-101 is capable of effectively separating TcO4- from Hanford low-level waste melter off-gas scrubber simulant stream. The sorption mechanism is directly unraveled by the single crystal structure of TcO4--incorporated SCU-101, as the first reported crystal structure to display TcO4- trapped in a sorbent material. A recognition site for the accommodation of TcO4- is visualized and is consistent with the DFT analysis results, while no such site can be resolved for other anions.

Systematic Investigation of the in Situ Reduction Process from U(VI) to U(IV) in a Phosphonate System under Mild Solvothermal Conditions.

The oxidation state greatly affects the chemical behavior of uranium in the nuclear fuel cycle and in the environment. Phosphonate ligands, on the other hand, show strong complexation toward uranium at different oxidation states and are widely used in nuclear fuel reprocessing. Therefore, in this work, the reduction behavior of U(VI) with the presence of a phosphonate ligand is investigated under mild solvothermal conditions. By adjusting the reaction time, temperature, and counterion species, a series of uranium diphosphonates including two U(VI), seven U(IV), two mixed-valent U(IV/VI), and one distinct U(IV/V/VI) compounds were obtained. All these compounds were characterized by single crystal X-ray diffraction and UV-vis-NIR absorption and fluorescence spectroscopy. The structural diversity among those compounds not only illustrates the intrinsic structural complexity in this system, but also illuminates the in situ reduction pathways that are affected by the variation of reaction conditions. The UV-vis-NIR absorption spectra of the tetravalent uranium compounds show that the absorption features are closely related to the local coordination environments of the uranium centers as well as the bonding modes of the phosphonate ligand. The fluorescence spectra of mixed-valent uranium compounds show unique emission features of U(VI) luminescence that are partially quenched by the multiple electronic transitions of U(IV) centers in the visible and NIR regions.

Substitutional Disorder of SeO32-/IO3- in the Crystalline Solid Matrix: Insights into the Fate of Radionuclides 79Se and 129I in the Environment.

As the crucial soluble species of long-lived radionuclides 129I and 79Se, iodate and selenite anions commonly share similar geometry of the trigonal pyramid XO3 (X = I, Se) but in different valence states. Although large amounts of investigations have been performed aiming at understanding the environmental behavior of these two anions individually, studies on cases when they coexist are extremely scarce. Structurally well-characterized natural/synthetic crystalline solids simultaneously incorporating these two anions as potential solubility-limiting products at the nuclear waste geological depository remain elusive. We report here a crystalline solid Th(IO3)2(SeO3) representing the first example of aliovalent substitution between IO3- and SeO32- sharing the same structural site, as demonstrated by single crystal X-ray diffraction, laser-ablation inductively coupled plasma mass spectrometry analysis, and spectroscopic techniques including infrared, Raman, and X-ray absorption spectroscopies. Sequentially, in the Eu(IO3)3 solid matrix, we demonstrated that the IO3- site can be sufficiently substituted by SeO32- in the presence of Th4+ via simultaneous incorporation of Th4+ and SeO32- in a charge-balancing mechanism. The obtained results provide insights into the environmental behavior of fission products 79Se and 129I: they may cocrystallize in one solid matrix and may be efficiently immobilized by incorporation into each other's solid phase through solid solution.

Mild Periodic Acid Flux and Hydrothermal Methods for the Synthesis of Crystalline f-Element-Bearing Iodate Compounds.

f-element-bearing iodate compounds are a large family mostly synthesized by hydrothermal reactions starting with actinide/lanthanide ions and iodic acid or iodate salt. In this work, we introduce melting periodic acid flux as a new reaction medium and provide a safe way for single-crystal growth of a series of new f-element iodate compounds including UO2(IO3)2·H2O (1), UO2(IO3)2(H2O)·HIO3 (2), α-Th(IO3)2(NO3)(OH) (3), ß-Th(IO3)2(NO3)(OH) (4), and (H3O)9Nd9(IO3)36·3HIO3 (5). The structures of these compounds deviate from those afforded from hydrothermal reactions. Specifically, compounds 1 and 2 exhibit pillared structures consisting of uranyl pentagonal bipyramids and iodate trigonal pyramids. Compounds 3 and 4 represent two new thorium iodate compounds that are constructed from subunits of thorium dimers. Compound 5 exhibits a flower-shaped trivalent lanthanide iodate structure with HIO3 molecules and H3O+ cations filled in the channels. The aliovalent replacement of f elements in 5 is available from a hydrothermal process, further generating compounds of Th2(IO3)8(H2O) (6) and Ce2(IO3)8(H2O) (7). The distinct absorption features are observed in isotypic compounds 5-7, where 7 shows typical semiconductor behavior with a band gap of 2.43 eV. Remarkably, noncentrosymmetric 1, 6, and 7 exhibit strong second-harmonic-generation efficiencies of 1.3, 3.2, and 9.2 times, respectively, that of the commercial material KH2PO4. Additionally, the temperature-dependent emission spectra of 1 and 2 were also collected showing typical emission features of uranyl units and a negative correlation between the intensities of the emissions with temperature. Clearly, the presented low-temperature melting inorganic acid flux synthesis would provide a facile and effective strategy to produce a large new family of structurally versatile and multifunctional f-element inorganic compounds.

Selenium Sequestration in a Cationic Layered Rare Earth Hydroxide: A Combined Batch Experiments and EXAFS Investigation.

Selenium is of great concern owing to its acutely toxic characteristic at elevated dosage and the long-term radiotoxicity of 79Se. The contents of selenium in industrial wastewater, agricultural runoff, and drinking water have to be constrained to a value of 50 µg/L as the maximum concentration limit. We reported here the selenium uptake using a structurally well-defined cationic layered rare earth hydroxide, Y2(OH)5Cl·1.5H2O. The sorption kinetics, isotherms, selectivity, and desorption of selenite and selenate on Y2(OH)5Cl·1.5H2O at pH 7 and 8.5 were systematically investigated using a batch method. The maximum sorption capacities of selenite and selenate are 207 and 124 mg/g, respectively, both representing the new records among those of inorganic sorbents. In the low concentration region, Y2(OH)5Cl·1.5H2O is able to almost completely remove selenium from aqueous solution even in the presence of competitive anions such as NO3-, Cl-, CO32-, SO42-, and HPO42-. The resulting concentration of selenium is below 10 µg/L, well meeting the strictest criterion for the drinking water. The selenate on loaded samples could be desorbed by rinsing with concentrated noncomplexing NaCl solutions whereas complexing ligands have to be employed to elute selenite for the material regeneration. After desorption, Y2(OH)5Cl·1.5H2O could be reused to remove selenate and selenite. In addition, the sorption mechanism was unraveled by the combination of EDS, FT-IR, Raman, PXRD, and EXAFS techniques. Specifically, the selenate ions were exchanged with chloride ions in the interlayer space, forming outer-sphere complexes. In comparison, besides anion exchange mechanism, the selenite ions were directly bound to the Y3+ center in the positively charged layer of [Y2(OH)5(H2O)]+ through strong bidentate binuclear inner-sphere complexation, consistent with the observation of the higher uptake of selenite over selenate. The results presented in this work confirm that the cationic layered rare earth hydroxide is an emerging and promising material for efficient removal of selenite and selenate as well as other anionic environmental pollutants.

Efficient and Selective Uptake of TcO4- by a Cationic Metal-Organic Framework Material with Open Ag+ Sites.

99Tc is one of the most problematic radioisotopes in used nuclear fuel owing to its combined features of high fission yield, long half-life, and high environmental mobility. There are only a handful of functional materials that can remove TcO4- anion from aqueous solution and identifying for new, stable materials with high anion-exchange capacities, fast kinetics, and good selectivity remains a challenge. We report here an 8-fold interpenetrated three-dimensional cationic metal-organic framework material, SCU-100, which is assembled from a tetradentate neutral nitrogen-donor ligand and two-coordinate Ag+ cations as potential open metal sites. The structure also contains a series of 1D channels filled with unbound nitrate anions. SCU-100 maintains its crystallinity in aqueous solution over a wide pH range from 1 to 13 and exhibits excellent ß and γ radiation-resistance. Initial anion exchange studies show that SCU-100 is able to both quantitatively and rapidly remove TcO4- from water within 30 min. The exchange capacity for the surrogate ReO4- reaches up to 541 mg/g and the distribution coefficient Kd is up to 1.9 × 105 mL/g, which are significantly higher than all previously tested inorganic anion sorbent materials. More importantly, SCU-100 can selectively capture TcO4- in the presence of large excess of competitive anions (NO3-, SO42-, CO32-, and PO43-) and remove as much as 87% of TcO4- from the Hanford low-level waste melter off-gas scrubber simulant stream within 2 h. The sorption mechanism is well elucidated by single crystal X-ray diffraction, showing that the sorbed ReO4- anion is able to selectively coordinate to the open Ag+ sites forming Ag-O-Re bonds and a series of hydrogen bonds. This further leads to a single-crystal-to-single-crystal transformation from an 8-fold interpenetrated framework with disordered nitrate anions to a 4-fold interpenetrated framework with fully ordered ReO4- anions. This work represents a practical case of TcO4- removal by a MOF material and demonstrates the promise of using this type of material as a scavenger for treating anionic radioactive contaminants during the nuclear waste partitioning and remediation processes.

Highly Sensitive and Selective Uranium Detection in Natural Water Systems Using a Luminescent Mesoporous Metal-Organic Framework Equipped with Abundant Lewis Basic Sites: A Combined Batch, X-ray Absorption Spectroscopy, and First Principles Simulation Investigation.

Uranium is not only a strategic resource for the nuclear industry but also a global contaminant with high toxicity. Although several strategies have been established for detecting uranyl ions in water, searching for new uranium sensor material with great sensitivity, selectivity, and stability remains a challenge. We introduce here a hydrolytically stable mesoporous terbium(III)-based MOF material compound 1, whose channels are as large as 27 Å × 23 Å and are equipped with abundant exposed Lewis basic sites, the luminescence intensity of which can be efficiently and selectively quenched by uranyl ions. The detection limit in deionized water reaches 0.9 µg/L, far below the maximum contamination standard of 30 µg/L in drinking water defined by the United States Environmental Protection Agency, making compound 1 currently the only MOF material that can achieve this goal. More importantly, this material exhibits great capability in detecting uranyl ions in natural water systems such as lake water and seawater with pH being adjusted to 4, where huge excesses of competing ions are present. The uranyl detection limits in Dushu Lake water and in seawater were calculated to be 14.0 and 3.5 µg/L, respectively. This great detection capability originates from the selective binding of uranyl ions onto the Lewis basic sites of the MOF material, as demonstrated by synchrotron radiation extended X-ray adsorption fine structure, X-ray adsorption near edge structure, and first principles calculations, further leading to an effective energy transfer between the uranyl ions and the MOF skeleton.

Highly Sensitive Detection of Ionizing Radiations by a Photoluminescent Uranyl Organic Framework.

Precise detection of low-dose X- and γ-radiations remains a challenge and is particularly important for studying biological effects under low-dose ionizing radiation, safety control in medical radiation treatment, survey of environmental radiation background, and monitoring cosmic radiations. We report here a photoluminescent uranium organic framework, whose photoluminescence intensity can be accurately correlated with the exposure dose of X- or γ-radiations. This allows for precise and instant detection of ionizing radiations down to the level of 10-4  Gy, representing a significant improvement on the detection limit of approximately two orders of magnitude, compared to other chemical dosimeters reported up to now. The electron paramagnetic resonance analysis suggests that with the exposure to radiations, the carbonyl double bonds break affording oxo-radicals that can be stabilized within the conjugated uranium oxalate-carboxylate sheet. This gives rise to a substantially enhanced equatorial bonding of the uranyl(VI) ions as elucidated by the single-crystal structure of the γ-ray irradiated material, and subsequently leads to a very effective photoluminescence quenching through phonon-assisted relaxation. The quenched sample can be easily recovered by heating, enabling recycled detection for multiple runs.

A Mixed-Valent Uranium Phosphonate Framework Containing U(IV) , U(V) , and U(VI).

It is shown that U(V) O2 (+) ions can reside at U(VI) O2 (2+) lattice sites during mild reduction and crystallization process under solvothermal conditions, yielding a complicated and rare mixed-valent uranium phosphonate compound that simultaneously contains U(IV) , U(V) , and U(VI) . The presence of uranium with three oxidation states was confirmed by various characterization techniques, including X-ray crystallography, X-ray photoelectron, electron paramagnetic resonance, FTIR, UV/Vis-NIR absorption, and synchrotron radiation X-ray absorption spectroscopy, and magnetism measurements.