Contrasting Trivalent Lanthanide and Actinide Complexation by Polyoxometalates via Solution-State NMR.

Deciphering the solution chemistry and speciation of actinides is inherently difficult due to radioactivity, rarity, and cost constraints, especially for transplutonium elements. In this context, the development of new chelating platforms for actinides and associated spectroscopic techniques is particularly important. In this study, we investigate a relatively overlooked class of chelators for actinide binding, namely, polyoxometalates (POMs). We provide the first NMR measurements on americium-POM and curium-POM complexes, using one-dimensional (1D) 31P NMR, variable-temperature NMR, and spin-lattice relaxation time (T1) experiments. The proposed POM-NMR approach allows for the study of trivalent f-elements even when only microgram amounts are available and in phosphate-containing solutions where f-elements are typically insoluble. The solution-state speciation of trivalent americium, curium, plus multiple lanthanide ions (La3+, Nd3+, Sm3+, Eu3+, Yb3+, and Lu3+), in the presence of the model POM ligand PW11O397- was elucidated and revealed the concurrent formation of two stable complexes, [MIII(PW11O39)(H2O)x]4- and [MIII(PW11O39)2]11-. Interconversion reaction constants, reaction enthalpies, and reaction entropies were derived from the NMR data. The NMR results also provide experimental evidence of the weakly paramagnetic nature of the Am3+ and Cm3+ ions in solution. Furthermore, the study reveals a previously unnoticed periodicity break along the f-element series with the reversal of T1 relaxation times of the 1:1 and 1:2 complexes and the preferential formation of the long T1 species for the early lanthanides versus the short T1 species for the late lanthanides, americium, and curium. Given the broad variety of POM ligands that exist, with many of them containing NMR-active nuclei, the combined POM-NMR approach reported here opens a new avenue to investigate difficult-to-study elements such as heavy actinides and other radionuclides.

Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale.

The synthesis and study of radioactive compounds are both inherently limited by their toxicity, cost and isotope scarcity. Traditional methods using small inorganic or organic complexes typically require milligrams of sample-per attempt-which for some isotopes is equivalent to the world's annual supply. Here we demonstrate that polyoxometalates (POMs) enable the facile formation, crystallization, handling and detailed characterization of metal-ligand complexes from microgram quantities owing to their high molecular weight and controllable solubility properties. Three curium-POM complexes were prepared, using just 1-10 µg per synthesis of the rare isotope 248Cm3+, and characterized by single-crystal X-ray diffraction, showing an eight-coordinated Cm3+ centre. Moreover, spectrophotometric, fluorescence, NMR and Raman analyses of several f-block element-POM complexes, including 243Am3+ and 248Cm3+, showed otherwise unnoticeable differences between their solution versus solid-state chemistry, and actinide versus lanthanide behaviour. This POM-driven strategy represents a viable path to isolate even rarer complexes, notably with actinium or transcalifornium elements.

Solid phase characterization and transformation of illite mineral with gas-phase ammonia treatment.

In situ remediation applications of ammonia (NH3) gas have potential for sequestration of subsurface contamination. Ammonia gas injections initially increase the pore water pH leading to mineral dissolution followed by formation of secondary precipitates as the pH is neutralized. However, there is a lack of understanding of fundamental alteration processes due to NH3 treatment. In these batch studies, phyllosilicate minerals (illite and montmorillonite) were exposed to NH3 gas with subsequent aeration to simulate in situ remediation. Following treatments, solids were characterized using a variety of techniques, including X-ray diffraction, N2 adsorption-desorption analysis for surface area, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), and microscopy methods to investigate physicochemical transformations. Results indicate that, at high pH, the clays are altered as observed by differences in morphology and particle size via microscopy. However, the two clays interact differently with NH3. While montmorillonite interlayers collapsed due to intercalation, illite layers were unaffected as confirmed by FTIR analysis. Further, structural changes in silicate ([SiO4]n-) and aluminol (Al-OH) groups were identified by NMR and FTIR. This research showed that mineral alteration processes occur during and after NH3 gas treatment which may be used to remove radionuclides from the aqueous phase through sorption, co-precipitation, and coating with secondary phyllosilicate alteration products.

Fast Permeation of Small Ions in Carbon Nanotubes.

Simulations and experiments have revealed enormous transport rates through carbon nanotube (CNT) channels when a pressure gradient drives fluid flow, but comparatively little attention has been given to concentration-driven transport despite its importance in many fields. Here, membranes are fabricated with a known number of single-walled CNTs as fluid transport pathways to precisely quantify the diffusive flow through CNTs. Contrary to early experimental studies that assumed bulk or hindered diffusion, measurements in this work indicate that the permeability of small ions through single-walled CNT channels is more than an order of magnitude higher than through the bulk. This flow enhancement scales with the ion free energy of transfer from bulk solutions to a nanoconfined, lower-dielectric environment. Reported results suggest that CNT membranes can unlock dialysis processes with unprecedented efficiency.

Additive Manufacturing of Optical Quality Germania-Silica Glasses.

Direct ink writing (DIW) three-dimensional (3D) printing provides a revolutionary approach to fabricating components with gradients in material properties. Herein, we report a method for generating colloidal germania feedstock and germania-silica inks for the production of optical quality germania-silica (GeO2-SiO2) glasses by DIW, making available a new material composition for the development of multimaterial and functionally graded optical quality glasses and ceramics by additive manufacturing. Colloidal germania and silica particles are prepared by a base-catalyzed sol-gel method and converted to printable shear-thinning suspensions with desired viscoelastic properties for DIW. The volatile solvents are then evaporated, and the green bodies are calcined and sintered to produce transparent, crack-free glasses. Chemical and structural evolution of GeO2-SiO2 glasses is confirmed by nuclear magnetic resonance, X-ray diffraction, and Raman spectroscopy. UV-vis transmission and optical homogeneity measurements reveal comparable performance of the 3D printed GeO2-SiO2 glasses to glasses produced using conventional approaches and improved performance over 3D printed TiO2-SiO2 inks. Moreover, because GeO2-SiO2 inks are compatible with DIW technology, they offer exciting options for forming new materials with patterned compositions such as gradients in the refractive index that cannot be achieved with conventional manufacturing approaches.

Microbe Encapsulation for Selective Rare-Earth Recovery from Electronic Waste Leachates.

Rare earth elements (REEs) are indispensable components of many green technologies and of increasing demand globally. However, refining REEs from raw materials using current technologies is energy intensive and enviromentally damaging. Here, we describe the development of a novel biosorption-based flow-through process for selective REE recovery from electronic wastes. An Escherichia coli strain previously engineered to display lanthanide-binding tags on the cell surface was encapsulated within a permeable polyethylene glycol diacrylate (PEGDA) hydrogel at high cell density using an emulsion process. This microbe bead adsorbent contained a homogenous distribution of cells whose surface functional groups remained accessible and effective for selective REE adsorption. The microbe beads were packed into fixed-bed columns, and breakthrough experiments demonstrated effective Nd extraction at a flow velocity of up to 3 m/h at pH 4-6. The microbe bead columns were stable for reuse, retaining 85% of the adsorption capacity after nine consecutive adsorption/desorption cycles. A bench-scale breakthrough curve with a NdFeB magnet leachate revealed a two-bed volume increase in breakthrough points for REEs compared to non-REE impurities and 97% REE purity of the adsorbed fraction upon breakthrough. These results demonstrate that the microbe beads are capable of repeatedly separating REEs from non-REE metals in a column system, paving the way for a biomass-based REE recovery system.

Chemically directing d-block heterometallics to nanocrystal surfaces as molecular beacons of surface structure.

Our understanding of structure and bonding in nanoscale materials is incomplete without knowledge of their surface structure. Needed are better surveying capabilities responsive not only to different atoms at the surface, but also their respective coordination environments. We report here that d-block organometallics, when placed at nanocrystal surfaces through heterometallic bonds, serve as molecular beacons broadcasting local surface structure in atomic detail. This unique ability stems from their elemental specificity and the sensitivity of their d-orbital level alignment to local coordination environment, which can be assessed spectroscopically. Re-surfacing cadmium and lead chalcogenide nanocrystals with iron- or ruthenium-based molecular beacons is readily accomplished with trimethylsilylated cyclopentadienyl metal carbonyls. For PbSe nanocrystals with iron-based beacons, we show how core-level X-ray spectroscopies and DFT calculations enrich our understanding of both charge and atomic reorganization at the surface when beacons are bound.

Engineering strategy to improve peptide analogs: from structure-based computational design to tumor homing.

We present a chemical strategy to engineer analogs of the tumor-homing peptide CREKA (Cys-Arg-Glu-Lys-Ala), which binds to fibrin and fibrin-associated clotted plasma proteins in tumor vessels (Simberg et al. in Proc Natl Acad Sci USA 104:932-936, 2007) with improved ability to inhibit tumor growth. Computer modeling using a combination of simulated annealing and molecular dynamics were carried out to design targeted replacements aimed at enhancing the stability of the bioactive conformation of CREKA. Because this conformation presents a pocket-like shape with the charged groups of Arg, Glu and Lys pointing outward, non-proteinogenic amino acids α-methyl and N-methyl derivatives of Arg, Glu and Lys were selected, rationally designed and incorporated into CREKA analogs. The stabilization of the bioactive conformation predicted by the modeling for the different CREKA analogs matched the tumor fluorescence results, with tumor accumulation increasing with stabilization. Here we report the modeling, synthetic procedures, and new biological assays used to test the efficacy and utility of the analogs. Combined, our results show how studies based on multi-disciplinary collaboration can converge and lead to useful biomedical advances.

Blood clot initiation by mesocellular foams: dependence on nanopore size and enzyme immobilization.

Porous silica materials are attractive for hemorrhage control because of their blood clot promoting surface chemistry, the wide variety of surface topologies and porous structures that can be created, and the potential ability to achieve high loading of therapeutic proteins within the silica support. We show that silica cell-window size variation in the nanometers to tens of nanometers range greatly affects the rate at which blood clots are formed in human plasma, indicating that window sizes in this size range directly impact the accessibility and diffusion of clotting-promoting proteins to and from the interior surfaces and pore volume of mesocellular foams (MCFs). These studies point toward a critical window size at which the clotting speed is minimized and serve as a model for the design of more effective wound-dressing materials. We demonstrate that the clotting times of plasma exposed to MCF materials are dramatically reduced by immobilizing thrombin in the pores of the MCF, validating the utility of enzyme-immobilized mesoporous silicas in biomedical applications.