Solution NMR methods for structural and thermodynamic investigation of nanoparticle adsorption equilibria

Characterization of dynamic processes occurring at the nanoparticle (NP) surface is crucial for developing new and more efficient NP catalysts and materials. Thus, a vast amount of research has been dedicated to developing techniques to characterize sorption equilibria. Over recent years, solution NMR spectroscopy has emerged as a preferred tool for investigating ligand-NP interactions. Indeed, due to its ability to probe exchange dynamics over a wide range of timescales with atomic resolution, solution NMR can provide structural, kinetic, and thermodynamic information on sorption equilibria involving multiple adsorbed species and intermediate states. In this contribution, we review solution NMR methods for characterizing ligand-NP interactions, and provide examples of practical applications using these methods as standalone techniques. In addition, we illustrate how the integrated analysis of several NMR datasets was employed to elucidate the role played by support-substrate interactions in mediating the phenol hydrogenation reaction catalyzed by ceria-supported Pd nanoparticles.

An X-state solid-liquid mixture with unusual mechanical properties formed by water and coordination polymer nanosheet nanoarchitectonics

Removal of water from a mixture of water and inorganic solids usually leads to a drastic change in the mechanical properties from liquid-like to brittle solid type. Here, we demonstrate that there is an unusual state, the X state, formed by naturally drying up a dough which is composed of Ni(H₂O)₂[(Ni(CN)₄]·H₂O nanosheets and water. This X-state mixture shows mechanical characteristics different from both pure liquids and brittle solids. The deformation curve of the X-state mixture contains three linear parts. However, the deformation is recoverable because the deformation curve can be repeated as long as fracture does not occur. The ice-like water formed among the nanosheets is believed to be an important reason for generating this X-state mixture. The unique properties of the X-state mixture may find applications, such as impact absorbents, which require materials with considerable strength, modulus and toughness.

Metal organic framework/polyelectrolyte composites for water vapor sorption applications

Metal-organic framework (MOF) core particles of MIL-101(Cr), aluminum fumarate (Basolite® A520), MIL-53-TDC, zirconium fumarate, and UiO-66 were modified by adsorption of thin polyelectrolyte (PE)-based shells without deterioration of their crystal structure. By applying different PEs and depositing a single layer (MOF/PE) or one to three layer-by-layer assembled bilayers (MOF/LbL), the mass percent of shell material in the composite was varied from 0.6-2.5% to 50%. Under a constant relative pressure of water vapor, the moisture uptake by a MOF/PE and a MOF/LbL is rather comparable with its S-shaped curvature to that of pristine MOFs. The relevant differences, such as a shift of the ascending adsorption part to lower/higher relative pressure or an increase/decrease in water uptake in selected regions, are associated with the core-shell structure and related to the morphological changes of the MOF powders. The hydrophilic surface promotes the formation of liquid menisci at the points of contact between particles and accelerates the moisture uptake and loss. A decrease in water sorption under an atmosphere with high humidity by some composites can be associated with the inhibition of liquid water condensation by the more hydrophobic shells.

Interaction of Reactive Gases with Platinum Aerosol Particles at Room Temperature: Effects on Morphology and Surface Properties

Nanoparticles produced in technical aerosol processes exhibit often dendritic structures, composed of primary particles. Surprisingly, a small but consistent discrepancy was observed between the results of common aggregation models and in situ measurements of structural parameters, such as fractal dimension or mass-mobility exponent. A phenomenon which has received little attention so far is the interaction of agglomerates with admixed gases, which might be responsible for this discrepancy. In this work, we present an analytical series, which underlines the agglomerate morphology depending on the reducing or oxidizing nature of a carrier gas for platinum particles. When hydrogen is added to openly structured particles, as investigated by tandem differential mobility analysis (DMA) and transmission electron microscopy (TEM) analysis, Pt particles compact already at room temperature, resulting in an increased fractal dimension. Aerosol Photoemission Spectroscopy (APES) was also able to demonstrate the interaction of a gas with a nanoscaled platinum surface, resulting in a changed sintering behavior for reducing and oxidizing atmospheres in comparison to nitrogen. The main message of this work is about the structural change of particles exposed to a new environment after complete particle formation. We suspect significant implications for the interpretation of agglomerate formation, as many aerosol processes involve reactive gases or slightly contaminated gases in terms of trace amounts of unintended species.

Carbon fiber reinforced polymers for implantable medical devices

Carbon fibers reinforced polymers (CFRPs) are prolifically finding applications in the medical field, moving beyond the aerospace and automotive industries. Owing to its high strength-to-weight ratio, lightness and radiolucency, CFRP-based materials are emerging to replace traditional metal-based medical implants. Numerous types of polymers matrices can be incorporated with carbon fiber using various manufacturing methods, creating composites with distinct properties. Thus, prior to biomedical application, comprehensive evaluation of material properties, biocompatibility and safety are of paramount importance. In this study, we systematically evaluated a series of novel CFRPs, aiming at analyzing biocompatibility for future development into medical implants or implantable drug delivery systems. These CFRPs were produced either via Carbon Fiber-Sheet Molding Compound or Fused Deposition Modelling-based additive manufacturing. Unlike conventional methods, both fabrication processes afford high production rates in a time-and cost-effective manner. Importantly, they offer rapid prototyping and customization in view of personalized medical devices. Here, we investigate the physicochemical and surface properties, material mutagenicity or cytotoxicity of 20 CFRPs, inclusive of 2 surface finishes, as well as acute and sub-chronic toxicity in mice and rabbits, respectively. We demonstrate that despite moderate in vitro physicochemical and surface changes over time, most of the CFRPs were non-mutagenic and non-cytotoxic, as well as biocompatible in small animal models. Future work will entail extensive material assessment in the context of orthopedic applications such as evaluating potential for osseointegration, and a chronic toxicity study in a larger animal model, pigs.

RH Sensing by Means of TiO2 Nanoparticles: A Comparison among Different Sensing Techniques Based on Modeling and Chemical/Physical Interpretation

TiO2 nanoparticles coating has been proven to be an extremely performing sensing material for relative humidity (RH) measurements. The chemical activity of TiO2 toward water vapor adsorption and the very large surface to volume ratio typical of nanostructures are ideal characteristics for the development of RH fast and sensitive sensors. Different sensor technologies can be used in conjunction with this material to realize devices with satisfactory performance. In this paper, the authors aim to describe and discuss the main different possible choices and highlight the advantages and disadvantages, and linking them both to the underlying mechanism of water adsorption on the TiO2 sensing layer and to the modification of the electrical behavior due to the water adsorption. In particular, the authors start from results obtained by depositing TiO2 nanoparticles on a novel MEMS microbalance operating at low frequency, which allows to sense only the adsorbed water mass, and they exploit the sensor output to obtain a dynamic model of the water adsorption. They also link these results to those obtained with a Quartz Crystal Microbalance (QCM) functionalized with the same material operating at 10 MHz as a part of an oscillator. Finally, they establish a link with the results obtained by an RH impedance sensor, which exploits the same active material and the same deposition technique. With this sensor technology, the conductive and electrical behavior of the sensing and adsorbed films play a role. The whole work tries to unravel the different phenomena that contribute to the response of RH sensors not only based on TiO2 nanoparticles but also, more generally, based on nanostructured metal oxide materials.

CP2K: An electronic structure and molecular dynamics software package - Quickstep: Efficient and accurate electronic structure calculations

CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post-Hartree-Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

Water Adsorption Isotherms on Fly Ash from Several Sources

In this study, horizontal attenuated total reflection (HATR) Fourier-transform infrared (FT-IR) spectroscopy was combined with quartz crystal microbalance (QCM) gravimetry to investigate the adsorption isotherms of water on fly ash, a byproduct of coal combustion in power plants. Because of composition variability with the source region, water uptake was studied at room temperature as a function of relative humidity (RH) on fly ash from several regions: United States, India, The Netherlands, and Germany. The FT-IR spectra show water features growth as a function of RH, with water absorbing on the particle surface in both an ordered (ice-like) and a disordered (liquid-like) structure. The QCM data was modeled using the Brunauer, Emmett, and Teller (BET) adsorption isotherm model. The BET model was found to describe the data well over the entire range of RH, showing that water uptake on fly ash takes place mostly on the surface of the particle, even for poorly combusted samples. In addition, the source region and power-plant efficiency play important roles in the water uptake and ice nucleation (IN) ability of fly ash. The difference in the observed water uptake and IN behavior between the four samples and mullite (3Al₂O₃·2SiO₂), the aluminosilicate main component of fly ash, is attributed to differences in composition and the density of OH binding sites on the surface of each sample. A discussion is presented on the RH required to reach monolayer coverage on each sample as well as a comparison between surface sites of fly ash samples and enthalpies of adsorption of water between the samples and mullite.

Following the Assembly of Iron Oxide Nanocubes by Video Microscopy and Quartz Crystal Microbalance with Dissipation Monitoring

We have studied the growth of ordered arrays by evaporation-induced self-assembly of iron oxide nanocubes with edge lengths of 6.8 and 10.1 nm using video microscopy (VM) and quartz crystal microbalance with dissipation monitoring (QCM-D). Ex situ electron diffraction of the ordered arrays demonstrates that the crystal axes of the nanocubes are coaligned and confirms that the ordered arrays are mesocrystals. Time-resolved video microscopy shows that growth of the highly ordered arrays at slow solvent evaporation is controlled by particle diffusion and can be described by a simple growth model. The growth of each mesocrystal depends only on the number of nanoparticles within the accessible region irrespective of the relative time of formation. The mass of the dried mesocrystals estimated from the analysis of the bandwidth-shift-to-frequency-shift ratio correlates well with the total mass of the oleate-coated nanoparticles in the deposited dispersion drop.

Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) yields better Hydrolytical Stability of Biocompatible SiOx Thin Films on Implant Alumina Ceramics compared to Rapid Thermal Evaporation Physical Vapor Deposition (PVD)

Densely sintered aluminum oxide (α-Al2O3) is chemically and biologically inert. To improve the interaction with biomolecules and cells, its surface has to be modified prior to use in biomedical applications. In this study, we compared two deposition techniques for adhesion promoting SiOx films to facilitate the coupling of stable organosilane monolayers on monolithic α-alumina; physical vapor deposition (PVD) by thermal evaporation and plasma enhanced chemical vapor deposition (PE-CVD). We also investigated the influence of etching on the formation of silanol surface groups using hydrogen peroxide and sulfuric acid solutions. The film characteristics, that is, surface morphology and surface chemistry, as well as the film stability and its adhesion properties under accelerated aging conditions were characterized by means of X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), inductively coupled plasma-optical emission spectroscopy (ICP-OES), and tensile strength tests. Differences in surface functionalization were investigated via two model organosilanes as well as the cell-cytotoxicity and viability on murine fibroblasts and human mesenchymal stromal cells (hMSC). We found that both SiOx interfaces did not affect the cell viability of both cell types. No significant differences between both films with regard to their interfacial tensile strength were detected, although failure mode analyses revealed a higher interfacial stability of the PE-CVD films compared to the PVD films. Twenty-eight day exposure to simulated body fluid (SBF) at 37 °C revealed a partial delamination of the thermally deposited PVD films whereas the PE-CVD films stayed largely intact. SiOx layers deposited by both PVD and PE-CVD may thus serve as viable adhesion-promoters for subsequent organosilane coupling agent binding to α-alumina. However, PE-CVD appears to be favorable for long-term direct film exposure to aqueous solutions.

Influence of surface functionalization on the hydrophilic character of mesoporous silica nanoparticles

We report the synthesis and surface functionalization of MCM-41-like mesoporous silica nanoparticles (MSNs) with spheroidal shape and particle size of 141 ± 41 nm. The success of surface functionalization with aminopropyl and sodium ethylcarboxylate groups (giving amino-MSN and carboxy-MSN, respectively) was ascertained by infrared spectroscopy and ζ potential measurements. The former showed the decrease of surface silanol groups and the corresponding appearance of signals related to NH2 bending mode (δNH2) at 1595 cm(-1) and COO(-) stretching (νas and νsym) at 1562 and 1418 cm(-1). The latter showed a change in surface charge, in that the isoelectric point (IEP) changed from pH 3-4.5 to 8.5 when the MSN was functionalized with the amino groups, while carboxy-MSN showed a more negative charge in the whole pH range with respect to MSN. The hydrophilic character of the prepared materials was ascertained by quantitative microgravimetric measurements, allowing the calculation of the average isosteric adsorption heat (q[combining macron]st). This was found to be 51 ± 3 kJ mol(-1), 61 ± 4, and 65 ± 3 kJ mol(-1) for MSN, amino-MSN, and carboxy-MSN samples, respectively. The increase in q[combining macron]st after functionalization can be ascribed to the specific interaction of water molecules with the functionalizing agents, in agreement with a higher basicity with respect to silanol groups. Moreover, the possibility of multiple H-bonding interactions of water molecules with the carboxylate anion is put forward to account for the higher water uptake with respect to parent MSN.

Viscoelastic and durability analysis of nanostructured composite layers of polyelectrolyte and nanoparticles

We have evaluated the abrasion and bending durabilities of stacked polymer/nanoparticle layer-by-layer films.