Solvent controlled 2D structures of bottom-up fabricated nanoparticle superlattices

We demonstrate that oleyl phosphate ligand-stabilized iron oxide nanocubes as building blocks can be assembled into 2D supercrystalline mono- and multilayers on flat YSZ substrates within a few minutes using a simple spin-coating process. As a bottom-up process, the growth takes place in a layer-by-layer mode and therefore by tuning the spin-coating parameters, the exact number of deposited monolayers can be controlled. Furthermore, ex situ scanning electron and atomic force microscopy as well as X-ray reflectivity measurements give evidence that the choice of solvent allows the control of the lattice type of the final supercrystalline monolayers. This observation can be assigned to the different Hansen solubilities of the solvents used for the nanoparticle dispersion because it determines the size and morphology of the ligand shell surrounding the nanoparticle core. Here, by using toluene and chloroform as solvents, it can be controlled whether the resulting monolayers are ordered in a square or hexagonal supercrystalline lattice.

Strengthening Engineered Nanocrystal Three-Dimensional Superlattices via Ligand Conformation and Reactivity

Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.

Deformation Behavior of Cross-Linked Supercrystalline Nanocomposites: An in Situ SAXS/WAXS Study during Uniaxial Compression

With the ever-expanding functional applications of supercrystalline nanocomposites (a relatively new category of materials consisting of organically functionalized nanoparticles arranged into periodic structures), it becomes necessary to ensure their structural stability and understand their deformation and failure mechanisms. Inducing the cross-linking of the functionalizing organic ligands, for instance, leads to a remarkable enhancement of the nanocomposites' mechanical properties. It is however still unknown how the cross-linked organic phase redistributes applied loads, how the supercrystalline lattice accommodates the imposed deformations, and thus in general what phenomena govern the overall material's mechanical response. This work elucidates these aspects for cross-linked supercrystalline nanocomposites through an in situ small- and wide-angle X-ray scattering study combined with uniaxial pressing. Because of this loading condition, it emerges that the cross-linked ligands effectively carry and distribute loads homogeneously throughout the nanocomposites, while the superlattice deforms via rotation, slip, and local defects generation.

Defects and plasticity in ultrastrong supercrystalline nanocomposites

Supercrystalline nanocomposites are nanoarchitected materials with a growing range of applications but unexplored in their structural behavior. They typically consist of organically functionalized inorganic nanoparticles arranged into periodic structures analogous to crystalline lattices, including superlattice imperfections induced by processing or mechanical loading. Although featuring a variety of promising functional properties, their lack of mechanical robustness and unknown deformation mechanisms hamper their implementation into devices. We show that supercrystalline materials react to indentation with the same deformation patterns encountered in single crystals. Supercrystals accommodate plastic deformation in the form of pile-ups, dislocations, and slip bands. These phenomena occur, at least partially, also after cross-linking of the organic ligands, which leads to a multifold strengthening of the nanocomposites. The classic shear theories of crystalline materials are found to describe well the behavior of supercrystalline nanocomposites, which result to feature an elastoplastic behavior, accompanied by compaction.

Mapping the Mechanical Properties of Hierarchical Supercrystalline Ceramic-Organic Nanocomposites

Multiscale ceramic-organic supercrystalline nanocomposites with two levels of hierarchy have been developed via self-assembly with tailored content of the organic phase. These nanocomposites consist of organically functionalized ceramic nanoparticles forming supercrystalline micron-sized grains, which are in turn embedded in an organic-rich matrix. By applying an additional heat treatment step at mild temperatures (250-350 °C), the mechanical properties of the hierarchical nanocomposites are here enhanced. The heat treatment leads to partial removal and crosslinking of the organic phase, minimizing the volume occupied by the nanocomposites' soft phase and triggering the formation of covalent bonds through the organic ligands interfacing the ceramic nanoparticles. Elastic modulus and hardness up to 45 and 2.5 GPa are attained, while the hierarchical microstructure is preserved. The presence of an organic phase between the supercrystalline grains provides a toughening effect, by curbing indentation-induced cracks. A mapping of the nanocomposites' mechanical properties reveals the presence of multiple microstructural features and how they evolve with heat treatment temperature. A comparison with non-hierarchical, homogeneous supercrystalline nanocomposites with lower organic content confirms how the hierarchy-inducing organic excess results in toughening, while maintaining the beneficial effects of crosslinking on the materials' stiffness and hardness.

Modulating the Mechanical Properties of Supercrystalline Nanocomposite Materials via Solvent-Ligand Interactions

Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.

Iron oxide-based nanostructured ceramics with tailored magnetic and mechanical properties: development of mechanically robust, bulk superparamagnetic materials

Nanostructured iron-oxide based materials with tailored mechanical and magnetic behavior are produced in bulk form. By applying ultra-fast heating routines via spark plasma sintering (SPS) to supercrystalline pellets, materials with an enhanced combination of elastic modulus, hardness and saturation magnetization are achieved. Supercrystallinity - namely the arrangement of the constituent nanoparticles into periodic structures - is achieved through self-assembly of the organically-functionalized iron oxide nanoparticles. The optimization of the following SPS regime allows the control of organics' removal, necking, iron oxide phase transformations and nano-grain size retention, and thus the fine-tuning of both mechanical properties and magnetic response, up until the production of bulk mm-size superparamagnetic materials.

Hierarchical supercrystalline nanocomposites through the self-assembly of organically-modified ceramic nanoparticles

Biomaterials often display outstanding combinations of mechanical properties thanks to their hierarchical structuring, which occurs through a dynamically and biologically controlled growth and self-assembly of their main constituents, typically mineral and protein. However, it is still challenging to obtain this ordered multiscale structural organization in synthetic 3D-nanocomposite materials. Herein, we report a new bottom-up approach for the synthesis of macroscale hierarchical nanocomposite materials in a single step. By controlling the content of organic phase during the self-assembly of monodisperse organically-modified nanoparticles (iron oxide with oleyl phosphate), either purely supercrystalline or hierarchically structured supercrystalline nanocomposite materials are obtained. Beyond a critical concentration of organic phase, a hierarchical material is consistently formed. In such a hierarchical material, individual organically-modified ceramic nanoparticles (Level 0) self-assemble into supercrystals in face-centered cubic superlattices (Level 1), which in turn form granules of up to hundreds of micrometers (Level 2). These micrometric granules are the constituents of the final mm-sized material. This approach demonstrates that the local concentration of organic phase and nano-building blocks during self-assembly controls the final material's microstructure, and thus enables the fine-tuning of inorganic-organic nanocomposites' mechanical behavior, paving the way towards the design of novel high-performance structural materials.

Localized Overheating Phenomena and Optimization of Spark-Plasma Sintering Tooling Design

The present paper shows the application of a three-dimensional coupled electrical, thermal, mechanical finite element macro-scale modeling framework of Spark Plasma Sintering (SPS) to an actual problem of SPS tooling overheating, encountered during SPS experimentation. The overheating phenomenon is analyzed by varying the geometry of the tooling that exhibits the problem, namely by modeling various tooling configurations involving sequences of disk-shape spacers with step-wise increasing radii. The analysis is conducted by means of finite element simulations, intended to obtain temperature spatial distributions in the graphite press-forms, including punches, dies, and spacers; to identify the temperature peaks and their respective timing, and to propose a more suitable SPS tooling configuration with the avoidance of the overheating as a final aim. Electric currents-based Joule heating, heat transfer, mechanical conditions, and densification are imbedded in the model, utilizing the finite-element software COMSOL™, which possesses a distinguishing ability of coupling multiple physics. Thereby the implementation of a finite element method applicable to a broad range of SPS procedures is carried out, together with the more specific optimization of the SPS tooling design when dealing with excessive heating phenomena.

Long-term effects of intravenous high dose methylprednisolone pulses on bone mineral density in patients with multiple sclerosis

To determine the effects of high dose methylprednisolone (HDMP) pulses on bone mineral density (BMD) in patients with multiple sclerosis (MS), we studied 25 MS patients who received regular pulses of HDMP as well as pulses of HDMP for relapses, 18 MS patients who received HDMP at the same dose schedule only for relapses, and 61 healthy controls. We measured BMDs at lumbar spine and femoral neck and we assessed biochemical markers of bone metabolism and turnover. The average lifetime dosage of MP was 75.4 (SD 11.9) g in the pulsed HDMP group and 28.6 (SD 18.3) g in the HDMP for relapses group (P < 0.0001). Two MS patients (4.7%) and four controls (6.6%) had osteoporosis (P = NS), whereas 25 patients with MS (58.1%) and 21 controls (34.4%) had osteopenia (P = 0.016). BMDs measured at lumbar spine and femoral neck and biochemical indices of bone metabolism did not differ in MS patients and controls. BMD measures were not associated with lifetime methylprednisolone dosage. In partial correlation analysis, controlling for age, gender and menopausal status there was a significant inverse correlation between BMD at femoral neck and Expanded Disability Status Scale (EDSS) score (r = -0.31, P = 0.05). In conclusion, treatment with repeated HDMP pulses was not associated with osteoporosis in patients with MS who participated in a trial of methylprednisolone. However, osteopenia was observed more frequently in MS patients than healthy controls. Our data are reassuring, as them suggest that repeated pulses of methylprednisolone do not result in substantially increased risk of osteoporosis in MS patients. Moreover, osteopenia was found only in patients treated for relapses, who had a significantly higher EDSS score than patients in the HDMP group, suggesting that decreased mobility may contribute to bone loss more than corticosteroid use. BMD should be monitored in patients with MS, regardless of the use of methylprednisolone.

Efficacy and safety of alendronate for the treatment of osteoporosis in diffuse connective tissue diseases in children: a prospective multicenter study

OBJECTIVE

Osteopenia/osteoporosis is being increasingly reported as a complication of many chronic diseases, even in children. In this preliminary study, we evaluated the effect of an oral bisphosphonate (alendronate) on bone mass in children with diffuse connective tissue diseases.

METHODS

Thirty-eight children with low bone mass were treated with alendronate for 1 year; 38 children who had the same primary disorders as the study patients but in a less severe form served as untreated control patients. We were also able to evaluate changes in bone mass (before and after alendronate) in 16 of the treated patients whose bone mineral density (BMD) had been routinely measured before the present study was initiated.

RESULTS

BMD increased by a mean +/- SD of 14.9 +/- 9.8% (P < 0.002 versus baseline) in the treated patients (reaching the normal range in 13 patients), while the BMD was 2.6 +/- 5% (not significant versus baseline) in the control group (15 had a decrease). Most interestingly, there was a large increase in BMD (15.3 +/-9.9%) after alendronate therapy in the 16 children who had their BMD followed up in the year before the study, during which time they had shown little increase in BMD (1.03 +/- 6.3%), and often a decrease. Considering their condition, increases in the height of all patients was satisfactory. No new fractures were observed after alendronate therapy was initiated.

CONCLUSION

Bisphosphonates can be considered essential components of the treatment of secondary osteoporosis, not only in adults, but also in pediatric patients. Alendronate has a positive effect on secondary osteopenia/osteoporosis in children with connective tissue diseases.

Simple methods for nutritional assessment in hemodialyzed patients

Uremic patients undergoing hemodialysis are often catabolic and malnourished. To treat malnutrition effectively, a preliminary nutritional assessment is needed. Available techniques should enable the clinician to readily detect the presence of malnutrition and to follow the response to nutritional therapy. In a group of chronic uremic patients undergoing maintenance hemodialysis, the authors evaluated the nutritional status with the following indices: 1) assessment of the somatic fat and protein compartments by means of anthropometric measurements (weight/height ratio, triceps and subscapular skinfold thickness, and arm muscle circumference); 2) assessment of the visceral protein compartment (serum total protein, albumin, transferrin, pseudocholinesterase, C3, and immunoglobulin content); 3) assessment of cell-mediated immunity by means of skin tests ("skin window," PPD and phytohemagglutinin) and blood lymphocyte content; and 4) assessment of the dietary intake of nutrients with dietary diaries. Anthropometric indices, serum protein content (except immunoglobulins), and the immune response was generally lower than in normal subjects, suggesting a mixed marasmus-like and kwashiorkor-like pattern of protein-calorie malnutrition. The protein intake was normal, whereas the energy intake tended to be low. Protein intake was significantly correlated with the predialysis serum urea nitrogen. Due to the difficulties in improving oral energy intake and the negative nitrogen balance reported during the days of dialysis therapy, patients were given intravenous supplements of essential or essential and nonessential amino acids for 2 months. The effects of this short-term supplementation were limited.