Graphene-stabilized copper nanoparticles as an air-stable substitute for silver and gold in low-cost ink-jet printable electronics

Metallic copper nanoparticles were synthesized by a bottom-up approach, and in situ coated with protective shells of graphene in order to get a metal nanopowder of high air stability and chemical inertness. Using an amphiphilic surfactant, a water-based copper nanocolloid could be prepared and successfully printed onto a polymer substrate by conventional ink-jet printing using household printers. The dried printed patterns exhibited strong metallic gloss and an electrical conductivity of >1 S cm(-1) without the need for a sintering or densification step. This conductivity currently limits use in electronics to low current application or shielding and decorative effects. The high stability of graphene-coated copper nanoparticles makes them economically a most attractive alternative to silver or gold nanocolloids, and will strongly facilitate the industrial use of metal nanocolloids in consumer goods.

Advanced piezoresistance of extended metal-insulator core-shell nanoparticle assemblies

Assembled metal-insulator nanoparticles with a core-shell geometry provide access to materials containing a large number (>10(6)) of tunneling barriers. We demonstrate the production of ceramic coated metal nanoparticles exhibiting an exceptional pressure-sensitive conductivity. We further show that graphene bi- and trilayers on 20 nm copper nanoparticles are insulating in such a core-shell geometry and show a similar pressure-dependent conductivity. This demonstrates that core-shell metal-insulator assemblies offer a route to alternative sensing materials.

Removal of oxide nanoparticles in a model wastewater treatment plant: influence of agglomeration and surfactants on clearing efficiency

The rapidly increasing production of engineered nanoparticles has created a demand for particle removal from industrial and communal wastewater streams. Efficient removal is particularly important in view of increasing long-term persistence and evidence for considerable ecotoxicity of specific nanoparticles. The present work investigates the use of a model wastewater treatment plant for removal of oxide nanoparticles. While a majority of the nanoparticles could be captured through adhesion to clearing sludge, a significant fraction of the engineered nanoparticles escaped the wastewater plant's clearing system, and up to 6 wt % of the model compound cerium oxide was found in the exit stream of the model plant. Our study demonstrates a significant influence of surface charge and the addition of dispersion stabilizing surfactants as routinely used in the preparation of nanoparticle derived products. A detailed investigation on the agglomeration of oxide nanoparticles in wastewater streams revealed a high stabilization of the particles against clearance (adsorption on the bacteria from the sludge). This unexpected finding suggests a need to investigate nanoparticle clearance in more detail and demonstrates the complex interactions between dissolved species and the nanoparticles within the continuously changing environment of the clearing sludge.

Do bioactive glasses convey a disinfecting mechanism beyond a mere increase in pH?

AIM

To test whether bioactive glasses kill microbiota via mineralization or the release of ions other than sodium.

METHODOLOGY

Flame-spray synthesis was applied to produce nanometric glasses of different sodium content and constant Ca/P ratio: 28S5, 45S5 and 77S. Calcium hydroxide and nanometric tricalcium phosphate (TCP) were used as controls. Apatite induction was monitored by Raman spectroscopy. Bovine dentine disks with adherent Enterococcus faecalis cells were exposed to test and control suspensions or buffered solutions for 1 h, 1 day and 1 week. Colony-forming units were counted and disks were inspected using scanning electron microscopy. Suspension supernatants and solutions were analysed for their pH, osmolarity, calcium and silicon content.

RESULTS

Sodium containing glasses induced pH levels above 12, compared with less than pH 9 with sodium-free 77S. Calcium hydroxide, 45S5 and 28S5 killed all bacteria after 1 day and lysed them after 1 week. TCP caused the highest apatite induction and substantial calcification on bacteria adhering to dentine, but did not reduce viable counts. 77S achieved disinfection after 1 week without visible apatite formation, whilst the buffer solution at pH 9 caused only minimal reduction in counts.

CONCLUSION

Bioactive glasses have a directly and an indirectly pH-related antibacterial effect. The effect not directly linked to pH is because of ion release rather than mineralization.

Micro-organism-triggered release of silver nanoparticles from biodegradable oxide carriers allows preparation of self-sterilizing polymer surfaces

The antimicrobial activity of silver has attracted significant research interest and contributes to an exponentially growing use of this noble metal in commodity products. In this investigation, we describe a general approach to increase the antimicrobial activity of a silver-containing surface by two to three orders of magnitude. The use of 1-2-nm silver particles decorating the surface of 20-50-nm carrier particles consisting of a phosphate-based, biodegradable ceramic allows the triggered release of silver in the presence of a growing microorganism. This effect is based on the organism's requirements for mineral uptake during growth creating a flux of calcium, phosphate, and other ions to the organism. The growing micro-organism dissolves the carrier containing these nutrients and thereby releases the silver nanoparticles. Further, we demonstrate the rapid self-sterilization of polymer surfaces containing silver on calcium phosphate nanoparticles using a series of human pathogens. Colony-forming units (viable bacteria or fungi counts) have been routinely reduced below detection limit and suggest application of these self-sterilizing surfaces in hospital environments, food and pharmaceutical processing, and personal care.

Comparison of nanoscale and microscale bioactive glass on the properties of P(3HB)/Bioglass composites

This study compares the effects of introducing micro (m-BG) and nanoscale (n-BG) bioactive glass particles on the various properties (thermal, mechanical and microstructural) of poly(3hydroxybutyrate) (P(3HB))/bioactive glass composite systems. P(3HB)/bioactive glass composite films with three different concentrations of m-BG and n-BG (10, 20 and 30 wt%, respectively) were prepared by a solvent casting technique. The addition of n-BG particles had a significant stiffening effect on the composites, modulus when compared with m-BG. However, there were no significant differences in the thermal properties of the composites due to the addition of n-BG and m-BG particles. The systematic addition of n-BG particles induced a nanostructured topography on the surface of the composites, which was not visible by SEM in m-BG composites. This surface effect induced by n-BG particles considerably improved the total protein adsorption on the n-BG composites compared to the unfilled polymer and the m-BG composites. A short term in vitro degradation (30 days) study in simulated body fluid (SBF) showed a high level of bioactivity as well as higher water absorption for the P(3HB)/n-BG composites. Furthermore, a cell proliferation study using MG-63 cells demonstrated the good biocompatibility of both types of P(3HB)/bioactive glass composite systems. The results of this investigation confirm that the addition of nanosized bioactive glass particles had a more significant effect on the mechanical and structural properties of a composite system in comparison with microparticles, as well as enhancing protein adsorption, two desirable effects for the application of the composites in tissue engineering.

Cotton wool-like nanocomposite biomaterials prepared by electrospinning:In vitrobioactivity and osteogenic differentiation of human mesenchymal stem cells

The present study evaluates the in vitro biomedical performance of an electrospun, flexible, and cotton wool-like poly(lactide-co-glycolide) (PLGA)/amorphous tricalcium phosphate (ATCP) nanocomposite. Experiments on in vitro biomineralization, applicability in model defects and a cell culture study with human mesenchymal stem cells (hMSC) allowed assessing the application of the material for potential use as a bone graft. Scaffolds with different flame made ATCP nanoparticle loadings were prepared by electrospinning of a PLGA-based composite. Immersion in simulated body fluid showed significant deposition of a hydroxyapatite layer only on the surface of ATCP doped PLGA (up to 175% mass gain within 15 days for PLGA/ATCP 60:40). Proliferation and osteogenic differentiation of hMSC on different nanocomposites were assessed by incubating cells in osteogenic medium for 4 weeks. Proper adhesion and an unaffected morphology of the cells were observed by confocal laser scanning microscopy for all samples. Fluorometric quantification of dsDNA and analysis of ALP activity revealed no significant difference between the tested scaffolds and excluded any acute cytotoxic effects of the nanoparticles. The osteocalcin content for all scaffolds was 0.12-0.19 ng/ng DNA confirming osteogenic differentiation of human mesenchymal stem cells on these flexible bone implants.

Remineralization of human dentin using ultrafine bioactive glass particles

Bioactive glass nanoparticles synthesized by flame spray synthesis were tested for their remineralization capabilities in vitro. After artificial demineralization with EDTA, human dentin was treated with 20-50nm size bioactive glass nanoparticles or a micrometer-sized, commercial reference material (PerioGlas) for up to 30 days. The degree of remineralization was measured using quantitative gravimetric methods (thermogravimetry, elemental analysis) and element-sensitive scanning electron microscopy imaging to detect new mineral precipitated on or within the demineralized tooth matrix. After treatment with bioactive glass nanoparticles for 10 or 30 days a pronounced increase in mineral content of the dentin samples suggested a rapid remineralization. The mechanical properties of the remineralized dentin samples were well below the stability of natural dentin. It is suggested that this lack of mechanical reconstitution may be attributed to an imperfect arrangement of the newly deposited mineral within the demineralized tooth matrix. Nevertheless, the substantially higher remineralization rate induced by nanometer-sized vs. micrometric bioactive glass particles corroborated the importance of particle size in clinical bioglass applications.

Preparation of an ultra fast binding cement from calcium silicate-based mixed oxide nanoparticles

Building construction takes time, in part because the binding process of cement is based on the slow re-crystallization and precipitation of calcium silicate species. Since the material's reactivity is surface area limited, a reduction in particle size of Portland cements has been used to prepare faster binding formulations. The present work investigates a new and direct, one-step preparation of calcium silicate-based nanoparticles of a typical Portland cement composition by flame spray synthesis. Isothermal calorimetry revealed that the hardening of this new nano-cement corroborated a more than tenfold increase of initial reactivity with different reaction kinetics if compared to conventionally prepared cements. At present, the unfavourably high porosity of nano-cements, however, underlines the need for additional improvements of chemical composition and formulation to make these highly reactive materials applicable to modern construction work, where load-bearing strength is of importance.

Effect of sodium hypochlorite on human root dentine – mechanical, chemical and structural evaluation

AIM

To investigate the mechanical, chemical and structural alterations of human root dentine following exposure to ascending sodium hypochlorite concentrations.

METHODOLOGY

Three-point bending tests were carried out on standardized root dentine bars (n = 8 per group, sectioned from sound extracted human third molar teeth) to evaluate their flexural strength and modulus of elasticity after immersion in 5 mL of water (control), 1% NaOCl, 5% NaOCl or 9% NaOCl at 37 degrees C for 1 h. Additional dentine specimens were studied using microelemental analysis, light microscopy following bulk staining with basic fuchsin, and scanning electron microscopy (SEM). Numerical data were compared using one-way ANOVA. Bonferroni's correction was applied for multiple testing.

RESULTS

Immersion in 1% NaOCl did not cause a significant drop in elastic modulus or flexural strength values in comparison to water, whilst immersion in 5% and 9% hypochlorite reduced these values by half (P < 0.05). Both, carbon and nitrogen contents of the specimens were significantly (P < 0.05) reduced by 5% and 9% NaOCl, whilst 1% NaOCl had no such effect. Exposure to 5% NaOCl rendered the superficial 80-100 mum of the intertubular dentine permeable to basic fuchsin. Three-dimensional SEM reconstructions of partly demineralized specimens showed NaOCl concentration-dependent matrix deterioration. Backscattered electron micrographs revealed that hypochlorite at any of the tested concentrations left the inorganic dentine components intact.

CONCLUSIONS

The current data link the concentration-dependent hypochlorite effect on the mechanical dentine properties with the dissolution of organic dentine components.

Antimicrobial effect of nanometric bioactive glass 45S5

Most recent advances in nanomaterials fabrication have given access to complex materials such as SiO(2)-Na(2)O-CaO-P(2)O(5) bioactive glasses in the form of amorphous nanoparticles of 20- to 60-nm size. The clinically interesting antimicrobial properties of commercially available, micron-sized bioactive glass 45S5 have been attributed to the continuous liberation of alkaline species during application. Here, we tested the hypothesis that, based on its more than ten-fold higher specific surface area, nanometric bioactive glass releases more alkaline species, and consequently displays a stronger antimicrobial effect, than the currently applied micron-sized material. Ionic dissolution profiles were monitored in simulated body fluid. Antimicrobial efficacy was assessed against clinical isolates of enterococci from persisting root canal infections. The shift from micron- to nano-sized treatment materials afforded a ten-fold increase in silica release and solution pH elevation by more than three units. Furthermore, the killing efficacy was substantially higher with the new material against all tested strains.

Exposure of engineered nanoparticles to human lung epithelial cells: influence of chemical composition and catalytic activity on oxidative stress

The chemical and catalytic activity of nanoparticles has strongly contributed to the current tremendous interest in engineered nanomaterials and often serves as a guiding principle for the design of functional materials. Since it has most recently become evident that such active materials can enter into cells or organisms, the present study investigates the level of intracellular oxidations after exposure to iron-, cobalt-, manganese-, and titania-containing silica nanoparticles and the corresponding pure oxides in vitro. The resulting oxidative stress was quantitatively measured as the release of reactive oxygen species (ROS). The use of thoroughly characterized nanoparticles of the same morphology, comparable size, shape, and degree of agglomeration allowed separation of physical (rate of particle uptake, agglomeration, sedimentation) and chemical effects (oxidations). Three sets of control experiments elucidated the role of nanoparticles as carriers for heavy metal uptake and excluded a potential interference of the biological assay with the nanomaterial. The present results indicate that the particles could efficiently enter the cells by a Trojan-horse type mechanism which provoked an up to eight times higher oxidative stress in the case of cobalt or manganese if compared to reference cultures exposed to aqueous solutions of the same metals. A systematic investigation on iron-containing nanoparticles as used in industrial fine chemical synthesis demonstrated that the presence of catalytic activity could strongly alter the damaging action of a nanomaterial. This indicates that a proactive development of nanomaterials and their risk assessment should consider chemical and catalytic properties of nanomaterials beyond a mere focus on physical properties such as size, shape, and degree of agglomeration.

Highly sensitive optical detection of humidity on polymer/metal nanoparticle hybrid films

Porous metal films for optical humidity sensing were prepared from copper nanoparticles protected by a 2-3 nm carbon coating, a silicon tenside, and a polymeric wetting agent. Exposure to water or solvent vapor revealed an exceptional sensitivity with optical shifts in the visible light range of up to 50 nm for a change of 1% in relative humidity. These properties could be attributed to a combination of surface plasmon resonance effects at low humidity and thin film interference at higher water or solvent concentration in the surrounding air. The simple concept and use of ultra-low-cost materials suggests application of such porous metal-film-based optical humidity sensors in large-scale applications for food handling, storage, and transport.

Direct Coupling of Bromine-Mediated Methane Activation and Carbon-Deposit Gasification

The direct bromination of methane offers a quite selective (>98 %) route towards methane activation but shifts the problem of fuel production to converting and handling corrosive methyl bromide. The direct conversion of methyl bromide, at about 200 degrees C, into light hydrocarbons can be catalyzed under pressure by AlBr(3) resulting in the formation of propane-rich mixtures of light hydrocarbons, carbonaceous deposits, and HBr. After releasing the gaseous products, the addition of hydrogen at 260 degrees C allows a quantitative conversion of the carbonaceous deposits into the same range of light hydrocarbons. These second-stage products efficiently contribute to the overall process yield while enabling a full regeneration of the catalyst's activity. This oxygen-free process is compared to the conversion of methyl bromide on zeolites and the currently used methanol-to-gasoline (MTG) process in terms of product distributions and apparent energy of activation. A detailed chemical analysis of the intermediates revealed the presence of a carbon pool consisting of highly substituted benzene and cyclopentadiene derivatives, as observed on zeolites used in the MTG process. This similarity suggests that the currently used oxygen-based syngas/MTG process for methane conversion may be extended to a bromine-mediated process by using methyl bromide as an intermediate instead of methanol.

The degree and kind of agglomeration affect carbon nanotube cytotoxicity

The urgent need for toxicological studies on carbon nanotubes (CNTs) has arisen from the rapidly emerging applications of CNTs well beyond material science and engineering. In order to provide a basis for comparison to existing epidemiological data, we have investigated CNTs at various degrees of agglomeration using an in vitro cytotoxicity study with human MSTO-211H cells. Non-cytotoxic polyoxyethylene sorbitan monooleate was found to well-disperse CNT. In the present study, the cytotoxic effects of well-dispersed CNT were compared with that of conventionally purified rope-like agglomerated CNTs and asbestos as a reference. While suspended CNT-bundles were less cytotoxic than asbestos, rope-like agglomerates induced more pronounced cytotoxic effects than asbestos fibres at the same concentrations. The study underlines the need for thorough materials characterization prior to toxicological studies and corroborates the role of agglomeration in the cytotoxic effect of nanomaterials.

Grain growth resistance and increased hardness of bulk nanocrystalline fcc cobalt prepared by a bottom-up approach

Bulk nanocrystalline cobalt was prepared from reducing flame-spray-derived cobalt nanopowders exclusively in the face-centred cubic (fcc) modification. Compacts of approximately 60% density were obtained from uniaxial compression at room temperature and showed a strong resistance towards grain growth upon subsequent sintering to 90% relative density. The nanocrystalline structure remained stable well above 1000 degrees C and resulted in a pore-rich metal with about 10(15) nanovoids cm(-3). These sintered compacts displayed an up to three times higher bulk hardness if compared to conventional cobalt and local ductility as evidenced from scanning electron microscopy and nanoindentation. The strong grain-growth resistance and consequent increase in material hardness are discussed in respect of the presence of nanovoids, twin boundaries and material contamination.

Interaction between human cathepsins K, L, and S and elastins: mechanism of elastinolysis and inhibition by macromolecular inhibitors

Proteolytic degradation of elastic fibers is associated with a broad spectrum of pathological conditions such as atherosclerosis and pulmonary emphysema. We have studied the interaction between elastins and human cysteine cathepsins K, L, and S, which are known to participate in elastinolytic activity in vivo. The enzymes showed distinctive preferences in degrading elastins from bovine neck ligament, aorta, and lung. Different susceptibility of these elastins to proteolysis was attributed to morphological differences observed by scanning electron microscopy. Kinetics of cathepsin binding to the insoluble substrate showed that the process occurs in two steps. The enzyme is initially adsorbed on the elastin surface in a nonproductive manner and then rearranges to form a catalytically competent complex. In contrast, soluble elastin is bound directly in a catalytically productive manner. Studies of enzyme partitioning between the phases showed that cathepsin K favors adsorption on elastin; cathepsin L prefers the aqueous environment, and cathepsin S is equally distributed among both phases. Our results suggest that elastinolysis by cysteine cathepsins proceeds in cycles of enzyme adsorption, binding of a susceptible peptide moiety, hydrolysis, and desorption. Alternatively, the enzyme may also form a new catalytic complex without prior desorption and re-adsorption. In both cases the active center of the enzymes remains at least partly accessible to inhibitors. Elastinolytic activity was readily abolished by cystatins, indicating that, unlike enzymes such as leukocyte elastase, pathological elastinolytic cysteine cathepsins might represent less problematic drug targets. In contrast, thyropins were relatively inefficient in preventing elastinolysis by cysteine cathepsins.

Inorganic nanoparticles for transfection of mammalian cells and removal of viruses from aqueous solutions

Owing to their small size, synthetic nanoparticles show unprecedented biophysical and biochemical properties which may foster novel advances in life-science research. Using flame-spray synthesis technology we have produced non-coated aluminum-, calcium-, cerium-, and zirconium-derived inorganic metal oxide nanoparticles which not only exhibit high affinity for nucleic acids, but can sequester such compounds from aqueous solution. This non-covalent DNA-binding capacity was successfully used to transiently transfect a variety of mammalian cells including human, reaching transfection efficiencies which compared favorably with classic calcium phosphate precipitation (CaP) procedures and lipofection. In this straightforward protocol, transfection was enabled by simply mixing nanoparticles with DNA in solution prior to addition to the target cell population. Transiently transfected cells showed higher production levels of the human secreted glycoprotein SEAP compared to isogenic populations transfected with established technologies. Inorganic metal oxide nanoparticles also showed a high binding capacity to human-pathogenic viruses including adenovirus, adeno-associated virus and human immunodeficiency virus type 1 and were able to clear these pathogens from aqueous solutions. The DNA transfection and viral clearance capacities of inorganic metal oxide nanoparticles may provide cost-effective biopharmaceutical manufacturing and water treatment in developing countries.

Comparison of amorphous TCP nanoparticles to micron-sized α-TCP as starting materials for calcium phosphate cements

The development of degradable bone cements with a mineral composition similar to natural bone was investigated using highly reactive calcium phosphate phases as starting materials. Mixtures of XRD-amorphous, glassy tricalcium phosphate (amorphous-TCP) nanoparticles of 25-60 nm size and micron sized, milled alpha-TCP were set by hydration with sodium phosphate buffer and investigated for possible application as single component calcium phosphate cements (CPCs). Isothermal calorimetry allowed a precise tracking of the setting process. Amorphous-TCP nanoparticles converted into calcium deficient hydroxyapatite with cement setting times below 12 min. The total energy release by the material during hardening corroborated the importance of high specific surface area and phase composition, that is, amorphous state of the nanometric starting material as repeatedly suggested earlier. The phase composition of the resulting CPCs was characterized by X-ray diffraction before and after setting. The morphology was investigated by nitrogen adsorption, scanning, and transmission electron microscopy and revealed the formation of highly porous calcium deficient hydroxyapatite with specific surface areas of up to 160 m(2) g(-1) after setting. In contrast to the very fast reaction time and highest specific surface area, the mechanical stability of the resulting CPC is still insufficient and requires further improvement.