Structural Properties of Epoxy-Silica Barrier Coatings for Corrosion Protection of Reinforcing Steel.

Reinforcement steel extensively applied in civil construction is susceptible to corrosion due to the carbonation process in reinforced concrete and chloride ions diffusion. Epoxy-silica-based coatings are a promising option to guarantee the long-term stability of reinforced concrete structures. In this study, the influence of the proportion between the poly (bisphenol-A-co-epichlorhydrin) resin (DGEBA) and the curing agent diethylenetriamine (DETA) on the structural, morphological, and barrier properties of epoxy-silica nanocomposites were evaluated. To simulate different stages of concrete aging, electrochemical impedance spectroscopy (EIS) assays were performed for coated samples in a 3.5 wt.% NaCl solution (pH 7) and in simulated concrete pore solutions (SCPS), which represent the hydration environment in fresh concrete (SCPS2, pH 14) and after carbonation (SCPS1, pH 8). The results showed that coatings with an intermediate DETA to DGEBA ratio of 0.4, presented the best long-term corrosion protection with a low-frequency impedance modulus of up to 3.8 GΩ cm2 in NaCl and SCPS1 solutions. Small-angle X-ray scattering and atomic force microscopy analysis revealed that the best performance observed for the intermediate DETA proportion is associated with the presence of larger silica nanodomains, which act as a filler in the cross-linked epoxy matrix, thus favoring the formation of an efficient diffusion barrier.

Zeta Potential and Colloidal Stability Predictions for Inorganic Nanoparticle Dispersions: Effects of Experimental Conditions and Electrokinetic Models on the Interpretation of Results.

In this work, a set of experimental electrophoretic mobility (µe) data was used to show how inappropriate selection of the electrokinetic model used to calculate the zeta potential (ζ-potential) can compromise the interpretation of the results for nanoparticles (NPs). The main consequences of using ζ-potential values as criteria to indicate the colloidal stability of NP dispersions are discussed based on DLVO interaction energy predictions. For this, magnetite (Fe3O4) NPs were synthesized and characterized as a model system for performing electrokinetic experiments. The results showed that the Fe3O4 NPs formed mass fractal aggregates in solution, so the ζ-potential could not be determined under ideal conditions when µe depends on the NP radius. In addition, the Dukhin number (Du) estimated from potentiometric titration results indicated that stagnant layer conduction (SLC) could not be neglected for this system. The electrokinetic models that do not consider SLC grossly underestimated the ζ-potential values for the Fe3O4 NPs. The DLVO interaction energy predictions for the colloidal stability of the Fe3O4 NP dispersions also depended on the electrokinetic model used to calculate the ζ-potential. The results obtained for the Fe3O4 NP dispersions also suggested that, contrary to many reports in the literature, high ζ-potential values do not necessarily reflect high colloidal stability for charge-stabilized NP dispersions.

Fenton-like degradation of sulfathiazole using copper-modified MgFe-CO3 layered double hydroxide.

The catalytic activity of layered double hydroxides, with and without insertion of copper, was evaluated in a heterogeneous Fenton process for degradation of the antibiotic sulfathiazole (STZ). The characterizations with different techniques revealed lamellar structures formed by stacking of layers containing magnesium, iron, and copper cations. The insertion of copper in the lamellar structure increased the specific area of the material and the degradation kinetics, achieving complete STZ removal after 90 min. X-ray photoelectron spectroscopy analysis showed the presence of Cu(II) and Cu(I) surface sites, which contributed to the generation of hydroxyl and hydroperoxyl/superoxide radicals. It also indicated an increase of Cu(I) content after use. For both materials, but specially for LDH without copper, addition of tert-butyl alcohol and p-benzoquinone hindered STZ degradation, indicating the importance of hydroxyl and hydroperoxyl/superoxide radicals in the degradation process, respectively. These results demonstrated the potential of copper-modified MgFe-CO3 as a catalyst for the degradation of emerging contaminants, offering the benefits of easy preparation and high efficiency in the Fenton process.

PMMA-silica nanocomposite coating: Effective corrosion protection and biocompatibility for a Ti6Al4V alloy.

Ti6Al4V is the mostly applied metallic alloy for orthopedic and dental implants, however, its lack of osseointegration and poor long-term corrosion resistance often leads to a secondary surgical intervention, recovery delay and toxicity to the surrounding tissue. As a potential solution of these issues poly(methyl methacrylate)-silicon dioxide (PMMA-silica) coatings have been applied on a Ti6Al4V alloy to act simultaneously as an anticorrosive barrier and bioactive film. The nanocomposite, composed of PMMA covalently bonded to the silica phase through 3-(trimethoxysilyl)propyl methacrylate (MPTS), has been synthesized combining the sol-gel process with radical polymerization of methyl methacrylate. The 5 µm thick coatings deposited on Ti6Al4V have a smooth surface, are homogeneous, transparent, free of pores and cracks, and show a strong adhesion to the metallic substrate (11.6 MPa). Electrochemical impedance spectroscopy results proved an excellent anticorrosive performance of the coating, with an impedance modulus of 26 GΩ cm2 and long-term durability in simulated body fluid (SBF) solution. Moreover, after 21 days of immersion in SBF, the PMMA-silica coating presented apatite crystal deposits, which suggests in vivo bone bioactivity. This was confirmed by biological characterization showing enhanced osteoblast proliferation, explained by the increased surface free energy and protein adsorption. The obtained results suggest that PMMA-silica hybrids can act in a dual role as efficient anticorrosive and bioactive coating for Ti6Al4V alloys.

Quick-EXAFS and Raman monitoring of activation, reaction and deactivation of NiCu catalysts obtained from hydrotalcite-like precursors.

The understanding of phase transformation upon activation, reaction and deactivation of catalysts is of prime importance for tailoring catalysts with better performances. Herein we combined Quick-EXAFS and Raman spectroscopies in operando conditions through the monitoring of reaction products by mass spectrometry in order to study in depth active species and deactivating ones for Ethanol Steam Reforming reaction. Quick-EXAFS data analyzed by multivariate analysis allows one to determine the nickel and copper species involved during the activation of a Ni-Cu hydrotalcite-like precursors. Upon reaction and regeneration monitoring, Raman spectroscopy combined with mass spectrometry highlights the side products formed upon ESR leading to the formation of amorphous coke species encapsulating active metallic species and inducing catalyst deactivation. The coke encapsulation of active species was demonstrated by the simultaneous observation of oxidation of nickel and copper as soon as the amorphous coke was burnt by the oxidative regeneration treatment. Formation of filamentous coke species is also confirmed as causing little impact in catalyst deactivation.

In situ three-dimensional imaging of strain in gold nanocrystals during catalytic oxidation.

The chemical properties of materials are dependent on dynamic changes in their three-dimensional (3D) structure as well as on the reactive environment. We report an in situ 3D imaging study of defect dynamics of a single gold nanocrystal. Our findings offer an insight into its dynamic nanostructure and unravel the formation of a nanotwin network under CO oxidation conditions. In situ/operando defect dynamics imaging paves the way to elucidate chemical processes at the single nano-object level towards defect-engineered nanomaterials.

The Critical Role of Thioacetamide Concentration in the Formation of ZnO/ZnS Heterostructures by Sol-Gel Process.

ZnO/ZnS heterostructures have emerged as an attractive approach for tailoring the properties of particles comprising these semiconductors. They can be synthesized using low temperature sol-gel routes. The present work yields insight into the mechanisms involved in the formation of ZnO/ZnS nanostructures. ZnO colloidal suspensions, prepared by hydrolysis and condensation of a Zn acetate precursor solution, were allowed to react with an ethanolic thioacetamide solution (TAA) as sulfur source. The reactions were monitored in situ by Small Angle X-ray Scattering (SAXS) and UV-vis spectroscopy, and the final colloidal suspensions were characterized by High Resolution Transmission Electron Microscopy (HRTEM). The powders extracted at the end of the reactions were analyzed by X-ray Absorption spectroscopy (XAS) and X-ray diffraction (XRD). Depending on TAA concentration, different nanostructures were revealed. ZnO and ZnS phases were mainly obtained at low and high TAA concentrations, respectively. At intermediate TAA concentrations, we evidenced the formation of ZnO/ZnS heterostructures. ZnS formation could take place via direct crystal growth involving Zn ions remaining in solution and S ions provided by TAA and/or chemical conversion of ZnO to ZnS. The combination of all the characterization techniques was crucial to elucidate the reaction steps and the nature of the final products.

Loaded Ce-Ag organic-inorganic hybrids and their antibacterial activity.

There are requirements for surfaces with antibacterial properties in various technological fields. U-PEO hybrids with antibacterial properties were synthesized by the sol-gel process, incorporating combinations of cerium and silver salts at different silver molar fractions (0, 0.02, 0.05, 0.10, and 1) relative to the total amount of doped cations. The loaded hybrids were characterized by TGA, XRD, and Raman spectroscopy. Release tests were performed using UV-vis spectroscopy, and the antibacterial properties of the hybrids were studied in agar tests and turbidimetry assays. The nanostructural evolution of the hybrids during the release of the antibacterial agents was investigated by in situ SAXS. XRD results showed the presence of the AgCl crystalline phase in the loaded hybrids from a silver molar fraction of 0.05. Raman spectroscopy evidenced the interaction of silver cations with the polymeric part of the hybrid. SAXS results confirmed these interactions and showed that cerium species interacted with both organic and inorganic parts of the hybrids. The loaded U-PEO hybrids were found to release all the incorporated cerium in 1h, while the hybrid containing 100% of silver released only 78% of the incorporated silver. All the loaded hybrids displayed antibacterial activity against the Pseudomonas aeruginosa bacterium. The antibacterial activity was found to increase with silver molar fraction. Due to its high antibacterial activity and low silver molar fraction, the loaded hybrid with silver molar fraction of 0.10 seemed to be a good compromise between efficiency, esthetic transparency, and photostability.

Liquid Foam Templates Associated with the Sol-Gel Process for Production of Zirconia Ceramic Foams.

The unique properties of ceramic foams enable their use in a variety of applications. This work investigated the effects of different parameters on the production of zirconia ceramic foam using the sol-gel process associated with liquid foam templates. Evaluation was made of the influence of the thermal treatment temperature on the porous and crystalline characteristics of foams manufactured using different amounts of sodium dodecylsulfate (SDS) surfactant. A maximum pore volume, with high porosity (94%) and a bimodal pore size distribution, was observed for the ceramic foam produced with 10% SDS. Macropores, with an average size of around 30 µm, were obtained irrespective of the SDS amount, while the average size of the supermesopores increased systematically as the SDS amount was increased up to 10%, after which it decreased. X-ray diffraction analyses showed that the sample treated at 500 °C was amorphous, while crystallization into a tetragonal metastable phase occurred at 600 °C due to the presence of sulfate groups in the zirconia structure. At 800 and 1000 °C the monoclinic phase was observed, which is thermodynamically stable at these temperatures.

Immobilization of streptavidin in sol-gel films: Application on the diagnosis of hepatitis C virus.

Recent advances have accelerated the development of biosensors for the analysis of specific gene sequences. In this kind of biosensor, a DNA probe is immobilized on a transducer and the hybridization with the target DNA is monitored by suitable methodology. In the present work, the streptavidin (STA) was encapsulated in thin films siloxane-poly(propylene oxide) hybrids prepared by sol-gel method and deposited on the graphite electrode surface by dip-coating process. Biotinylated 18-mer probes were immobilized through STA and a novel amperometric DNA biosensor for the detection and genotyping of the hepatitis C virus (genotypes 1, 2A/C, 2B and 3) is described. The HCV RNA from serum was submitted to reverse transcriptase-linked polymerase chain reaction (RT-PCR) and biotin-labeled cDNA was obtained. Thus, the cDNA was hybridized to the target-specific oligonucleotide probe immobilized on the graphite electrode surface and following the avidin-peroxidase conjugate was added. The enzymatic response was investigated by constant potential amperometry at -0.45V versus Ag/AgCl using H(2)O(2) and KI solutions. HCV RNA negative and positive controls and positive samples of sera patients were analyzed and the results were compared to commercial kit. The proposed methodology appeared to be suitable and convenient tool for streptavidin immobilization and diagnose of HCV disease.