¹²⁵Te NMR Probes of Tellurium Oxide Crystals: Shielding-Structure Correlations

The local environments around tellurium atoms in a series of tellurium oxide crystals were probed by ¹²⁵Te solid-state NMR spectroscopy. Crystals with distinct TeO_<i>n</i> units (<i>n</i> from 3 to 6), including Na₂TeO₃, α-TeO₂ and γ-TeO₂, Te₂O­(PO₄)₂, K₃LaTe₂O₉, BaZnTe₂O₇, and CsYTe₃O₈ were studied. The latter four were synthesized through a solid-state process. X-ray diffraction was used to confirm the successful syntheses. The ¹²⁵Te chemical shift was found to exhibit a strong linear correlation with the Te coordination number. The ¹²⁵Te chemical-shift components (δ₁₁, δ₂₂, and δ₃₃) of the TeO₄ units were further correlated to the O–Te–O-bond angles. With the aid of ¹²⁵Te NMR, it is likely that these relations can be used to estimate the coordination states of Te atoms in unknown Te crystals and glasses

Relating ¹³⁹La Quadrupolar Coupling Constants to Polyhedral Distortion in Crystalline Structures

A broad series of crystalline lanthanum oxide-based materials has been investigated through high-field ¹³⁹La solid state nuclear magnetic resonance (ssNMR) spectroscopy and ab initio density functional theory (DFT) calculations. The ¹³⁹La NMR spectra of LaBGeO₅, LaBSiO₅, LaBO₃, LaPO₄·1.8H₂O, La₂(SO₄)₃·9H₂O, and La₂(CO₃)₃·8H₂O are reported for the first time. Both newly reported and literature values of ¹³⁹La quadrupolar coupling constants (<i>C</i>_Q) are related to various quantitative expressions of polyhedral distortion, including sphericity (Σ) and ellipsoid span (ϵ). The compounds were separated into two groups based upon their polyhedral distortion behavior: compounds with the general formula LaMO₃, where M is a trivalent cation; compounds with different general formulas. The ¹³⁹La <i>C</i>_Q of the LaMO₃ family was found to correlate best with ϵ. The ¹³⁹La <i>C</i>_Q of non-LaMO₃ compounds correlates adequately to ϵ but is better described by Σ. The ¹³⁹La isotropic chemical shift (δ_iso^CS) of the non-LaMO₃ compounds is negatively correlated with the lanthanum coordination number; there is insufficient data from the LaMO₃ compounds to draw conclusions relating to chemical shift. DFT calculations of NMR parameters prove to be a sensitive probe of the quality of input geometry, with predicted parameters agreeing with experiment except in cases where the crystal structure is suspect

Control of Ultrasmall Sub-10 nm Ligand-Functionalized Fluorescent Core–Shell Silica Nanoparticle Growth in Water

Ultrasmall fluorescent silica nanoparticles (SNPs) and core–shell SNPs surface functionalized with polyethylene glycol (PEG), specific surface ligands, and overall SNP size in the regime below 10 nm are of rapidly increasing interest for clinical applications, because of their favorable biodistribution and safety profiles. Here, we present an aqueous synthesis methodology for the preparation of narrowly size-dispersed SNPs and core–shell SNPs with size control below 1 nm, i.e., at the level of a single atomic layer. Different types of fluorophores, including near-infrared (NIR) emitters, can be covalently encapsulated. Brightness can be enhanced via addition of extra silica shells. This methodology further enables synthesis of <10 nm sized fluorescent core and core–shell SNPs with previously unknown compositions. In particular, the addition of an aluminum sol gel precursor leads to fluorescent aluminosilicate nanoparticles (ASNPs) and core–shell ASNPs. Encapsulation efficiency and brightness of highly negatively charged NIR fluorophores is enhanced, relative to the corresponding SNPs without aluminum. Resulting particles show quantum yields of ∼0.8, i.e., starting to approach the theoretical brightness limit. All particles can be PEGylated providing steric stability. Finally, heterobifunctional PEGs can be employed to introduce ligands onto the PEGylated particle surface of fluorescent SNPs, core–shell SNPS, and their aluminum-containing analogues, producing ligand-functionalized <10 nm NIR fluorescent nanoprobes. In order to distinguish these water-based-synthesis-derived materials from the earlier alcohol-based modified Stöber process derived fluorescent core–shell SNPs referred to as Cornell dots or C dots, the SNPs and ASNPs described here and synthesized in water will be referred to as Cornell prime dots or C′ dots and AlC′ dots. These organic–inorganic hybrid nanomaterials may find applications in nanomedicine, including cancer diagnostics and therapy (theranostics)

Self-Assembled Gyroidal Mesoporous Polymer-Derived High Temperature Ceramic Monoliths

Polymer-derived ceramics (PDCs) have enabled the development of nonoxide ceramic coatings and fibers with exceptional thermo-mechanical stability. Here, we report the self-assembly based synthesis of gyroidal (space group Q²³⁰, <i>Ia</i>3̅<i>d</i>) mesoporous silicon oxynitride ceramic monoliths by pyrolysis of blends of commercially available preceramic polysilazane with a structure-directing triblock terpolymer up to temperatures of 1000 °C. Monoliths had pore diameters of 9.4 ± 1.1 nm and surface area of 160 m²/g. The three-dimensionally (3D) ordered periodic pore structure of the as-made hybrids acts to relieve stresses by allowing the escape of gases formed during ceramization. This process in turn enables the retention of smooth monoliths during ceramization under ammonia, a process that both adds nitrogen to the material and removes carbon pyrolysis products. The monoliths are appealing for high-temperature applications such as catalyst supports and microelectromechanical system (MEMS) devices including gas and pressure sensors, as well as strong, stiff, and creep-resistant scaffolds for ordered interpenetrating phase composites

Stimuli-Responsive Shapeshifting Mesoporous Silica Nanoparticles

Stimuli-responsive materials have attracted great interest in catalysis, sensing, and drug delivery applications and are typically constituted by soft components. We present a one-pot synthetic method for a type of inorganic silica-based shape change material that is responsive to water vapor exposure. After the wetting treatment, the cross-sectional shape of aminated mesoporous silica nanoparticles (MSNs) with hexagonal pore lattice changed from hexagonal to six-angle-star, accompanied by the loss of periodic mesostructural order. Nitrogen sorption measurements suggested that the wetting treatment induced a shrinkage of mesopores resulting in a broad size distribution and decreased mesopore volume. Solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy of samples after wetting treatment displayed a higher degree of silica condensation, indicating that the shape change was associated with the formation of more siloxane bonds within the silica matrix. On the basis of material characterization results, a mechanism for the observed anisotropic shrinkage is suggested based on a buckling deformation induced by capillary forces in the presence of a threshold amount of water vapor available beyond a humidity of about 50%. The work presented here may open a path toward novel stimuli-responsive materials based on inorganic components

Zero Thermal Expansion in ZrMgMo₃O₁₂: NMR Crystallography Reveals Origins of Thermoelastic Properties

The coefficient of thermal expansion of ZrMgMo₃O₁₂ has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (αl = (1.6 ± 0.2) × 10^–7 K^–1 from 25 to 450 °C by X-ray diffraction (XRD)). ZrMgMo₃O₁₂ belongs to the family of AMgM₃O₁₂ materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo₃O₁₂ by nuclear magnetic resonance (NMR) crystallography, using ⁹¹Zr, ²⁵Mg, ⁹⁵Mo, and ¹⁷O magic angle spinning (MAS) and ¹⁷O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo₃O₁₂ shows significant ionic conductivity, a property that is also related to its structure