A solid-state NMR investigation of the structure of mesoporous silica nanoparticle supported rhodium catalysts

A spatiotemporal drug release scaffold with antibiosis and bone regeneration for osteomyelitis

INTRODUCTION

Scaffolds loaded with antibacterial agents and osteogenic drugs are considered essential tools for repairing bone defects caused by osteomyelitis. However, the simultaneous release of two drugs leads to premature osteogenesis and subsequent sequestrum formation in the pathological situation of unthorough antibiosis.

OBJECTIVES

In this study, a spatiotemporal drug-release polydopamine-functionalized mesoporous silicon nanoparticle (MSN) core/shell drug delivery system loaded with antibacterial silver (Ag) nanoparticles and osteogenic dexamethasone (Dex) was constructed and introduced into a poly-l-lactic acid (PLLA) scaffold for osteomyelitis therapy.

METHODS

MSNs formed the inner core and were loaded with Dex through electrostatic adsorption (MSNs@Dex), and then polydopamine was used to seal the core through the self-assembly of dopamine as the outer shell (pMSNs@Dex). Ag nanoparticles were embedded in the polydopamine shell via an in situ growth technique. Finally, the Ag-pMSNs@Dex nanoparticles were introduced into PLLA scaffolds (Ag-pMSNs@Dex/PLLA) constructed by selective laser sintering (SLS).

RESULTS

The Ag-pMSNs@Dex/PLLA scaffold released Ag+ at the 12th hour, followed by the release of Dex starting on the fifth day. The experiments verified that the scaffold had excellent antibacterial performance against Escherichia coli and Staphylococcus aureus. Moreover, the scaffold significantly enhanced the osteogenic differentiation of mouse bone marrow mesenchymal stem cells.

CONCLUSION

The findings suggested that this spatiotemporal drug release scaffold had promising potential for osteomyelitis therapy.

Solid-state nuclear magnetic resonance studies of nanoparticles

In this trends article, we review seminal and recent studies using static and magic-angle spinning solid-state NMR to study the structure of nanoparticles and ligands attached to nanoparticles. Solid-state NMR techniques including one-dimensional multinuclear NMR, cross-polarization, techniques for measuring dipolar coupling and internuclear distances, and multidimensional NMR have provided insight into the core-shell structure of nanoparticles as well as the structure of ligands on the nanoparticle surface. Hyperpolarization techniques, in particular solid-state dynamic nuclear polarization (DNP), have enabled detailed studies of nanoparticle core-shell structure and surface chemistry, by allowing unprecedented levels of sensitivity to be achieved. The high signal-to-noise afforded by DNP has allowed homonuclear and heteronuclear correlation experiments involving nuclei with low natural abundance to be performed in reasonable experimental times, which previously would not have been possible. The use of DNP to study nanoparticles and their applications will be a fruitful area of study in the coming years as well.

Surface Enhanced DNP Assisted Solid-State NMR of Functionalized SiO2 Coated Polycarbonate Membranes

Surface enhanced solid-state NMR by dynamic nuclear polarization (DNP SENS) enables the characterization of the inner-pore surface functionalization of porous etched ion-track membranes exhibiting low specific surface areas compared to typical SBA- or MCM-type mesoporous silica materials. The membranes were conformally coated with a 5 nm thin SiO2 layer by atomic layer deposition. This layer was subsequently modified by aminopropyl silane linkers that allow further functionalization via the terminal amine group. The results evidence that in principle DNP SENS is a capable tool to analyze more complex porous systems, e.g. bioinspired functional etched ion-track membranes down to the molecular level. These results are relevant also for single nanopore systems, for which a direct analysis of the channel surface functionalization is not feasible by classical characterization methods. The applicability of DNP SENS to complex porous systems requires the optimization of the sample preparation and measurement parameters.

Functional Group Coverage and Conversion Quantification in Nanostructured Silica by 1H NMR

Silica nanostructured materials are important in many fields, including catalysis, imaging, and drug delivery, mainly due to the versatility of surface functionalization that can bestow a huge variety of chemical and physical properties. With most applications requiring precise control over this surface modification, characterization of surface composition and reactivity have become of extreme importance. We present a novel approach to track silica surface modification and quantify functional group coverage using only solution NMR. We test the method using different types of silica nanoparticles and surface modifications, to show that after dissolving the silica matrix, the ¹H NMR spectra can be resolved for every single component of the mixture. By using an internal standard, we are able to quantify the density of ligands and follow their sequential modification. Our work presents a fast, accurate, and straightforward method for surface characterization of silica nanostructures, using widely available NMR spectroscopy and small amounts of sample.

31P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts

The synthesis of novel robust and stable iridium-based immobilized catalysts on silica-polymer hybrid materials (Si-PB-Ir) is described. These catalysts are characterized by a combination of 1D 31P CP-MAS and 2D 31P-1H HETCOR and J-resolved multinuclear solid state NMR experiments. Different binding situations such as singly and multiply coordinated phosphines are identified. Density functional theory (DFT) calculations are performed to corroborate the interpretation of the experimental NMR data, in order to propose a structural model of the heterogenized catalysts. Finally, the catalytic activity of the Si-PB-Ir catalysts is investigated for the hydrogenation of styrene employing para-enriched hydrogen gas.

Natural abundance 15N NMR by dynamic nuclear polarization: fast analysis of binding sites of a novel amine-carboxyl-linked immobilized dirhodium catalyst

A novel heterogeneous dirhodium catalyst has been synthesized. This stable catalyst is constructed from dirhodium acetate dimer (Rh2(OAc)4) units, which are covalently linked to amine- and carboxyl-bifunctionalized mesoporous silica (SBA-15-NH2-COOH). It shows good efficiency in catalyzing the cyclopropanation reaction of styrene and ethyl diazoacetate (EDA) forming cis- and trans-1-ethoxycarbonyl-2-phenylcyclopropane. To characterize the structure of this catalyst and to confirm the successful immobilization, heteronuclear solid-state NMR experiments have been performed. The high application potential of dynamic nuclear polarization (DNP) NMR for the analysis of binding sites in this novel catalyst is demonstrated. Signal-enhanced (13)C CP MAS and (15)N CP MAS techniques have been employed to detect different carboxyl and amine binding sites in natural abundance on a fast time scale. The interpretation of the experimental chemical shift values for different binding sites has been corroborated by quantum chemical calculations on dirhodium model complexes.

Analysis of trivalent cation complexation to functionalized mesoporous silica using solid-state NMR spectroscopy

The first comprehensive study of Al(iii) and Sc(iii) interactions with a novel hybrid material, N-[5-(trimethoxysilyl)-2-aza-1-oxopentyl]caprolactam functionalized mesoporous silica, was conducted using solid-state NMR spectroscopy.

Solid-state NMR concepts for the investigation of supported transition metal catalysts and nanoparticles

In recent years, solid-state NMR spectroscopy has evolved into an important characterization tool for the study of solid catalysts and chemical processes on their surface. This interest is mainly triggered by the need of environmentally benign organic transformations ("green chemistry"), which has resulted in a large number of new catalytically active hybrid materials, which are organized on the meso- and nanoscale. Typical examples of these catalysts are supported homogeneous transition metal catalysts or transition metal nanoparticles (MNPs). Solid-state NMR spectroscopy is able to characterize both the structures of these materials and the chemical processes on the catalytic surface. This article presents recent trends both on the characterization of immobilized homogeneous transition metal catalysts and on the characterization of surface species on transition metal surfaces.

Immobilization and Characterization of RuCl2(PPh3)3 Mesoporous Silica SBA-3

The five-fold coordinated ruthenium (II) catalyst RuCl2(PPh3)3 is heterogenized on the surface of amine functionalized mesoporous silica material SBA-3 and studied by BET, XRD and 31P CP-MAS solid-state NMR. The spectra indicate the replacement of one or two of the triphenyl-phosphine groups in the course of the grafting. To distinguish between both possibilities two-dimensional J-resolved 31P MAS solid-state NMR combined with quantum chemical calculations is employed. The changes of the scalar coupling pattern between the neat catalyst and the immobilized catalyst suggest the replacement of two of the PPh3-groups by coordinative bonds to the amine-functions of the linker. DFT calculations reveal the absence of two chemically and magnetically equivalent phosphines and confirm the replacement of two PPh3-groups. This finding is similar to the results observed for the binding of the rhodium in the Wilkinson's catalyst, despite the different coordination of the metals (four-fold versus five-fold), the presence of two chlorine ligands and the different transition metal.

A Solid-State NMR Investigation of MQ Silicone Copolymers

The structure of MQ copolymers of the general chemical formula [(CH₃)₃SiO_0.5]_m [SiO₂]_n was characterized by means of solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The MQ copolymers are highly branched polycyclic compounds (densely cross-linked nanosized networks). MQ copolymers were prepared by hydrolytic polycondensation in active medium. ²⁹Si NMR spectra were obtained by single pulse excitation (or direct polarization, DP) and cross-polarization (CP) ²⁹Si{¹H} techniques in concert with MAS. It was shown that material consist of monofunctional M (≡SiO Si (CH₃)₃) and two types of tetrafunctional Q units: Q⁴ ((≡SiO)₄Si) and Q³ ((≡SiO)₃SiOH). Spin-lattice relaxation times T₁ measurements of ²⁹Si nuclei and analysis of ²⁹Si{¹H} variable contact time signal intensities allowed us to obtain quantitative data on the relative content of different sites in copolymers. These investigations indicate that MQ copolymers represent dense structure with core and shell.

Applications of high-resolution 1H solid-state NMR

This article reviews the large increase in applications of high-resolution (1)H magic-angle spinning (MAS) solid-state NMR, in particular two-dimensional heteronuclear and homonuclear (double-quantum and spin-diffusion NOESY-like exchange) experiments, in the last five years. These applications benefit from faster MAS frequencies (up to 80 kHz), higher magnetic fields (up to 1 GHz) and pulse sequence developments (e.g., homonuclear decoupling sequences applicable under moderate and fast MAS). (1)H solid-state NMR techniques are shown to provide unique structural insight for a diverse range of systems including pharmaceuticals, self-assembled supramolecular structures and silica-based inorganic-organic materials, such as microporous and mesoporous materials and heterogeneous organometallic catalysts, for which single-crystal diffraction structures cannot be obtained. The power of NMR crystallography approaches that combine experiment with first-principles calculations of NMR parameters (notably using the GIPAW approach) are demonstrated, e.g., to yield quantitative insight into hydrogen-bonding and aromatic CH-π interactions, as well as to generate trial three-dimensional packing arrangements. It is shown how temperature-dependent changes in the (1)H chemical shift, linewidth and DQ-filtered signal intensity can be analysed to determine the thermodynamics and kinetics of molecular level processes, such as the making and breaking of hydrogen bonds, with particular application to proton-conducting materials. Other applications to polymers and biopolymers, inorganic compounds and bioinorganic systems, paramagnetic compounds and proteins are presented. The potential of new technological advances such as DNP methods and new microcoil designs is described.