Sol-gel Structure Research Articles

Spherical LDH-Ag°-montmorillonite heterocoagulated system with a pH-dependent sol-gel structure for controlled accessibility of AgNPs immobilized on the clay lamellae.

Aqueous suspensions of spherical ZnMgAl-layered double hydroxides [LDH(sph)] and antibacterial silver nanoparticles (AgNPs) deposited on the lamellae of montmorillonite were used for the synthesis of composites, which behave like coherent gels at low pH (≲4.5) and incoherent sols at higher pH (≳4.5). The composition of the composite was chosen as LDH(sph)/Ag°-montm. = 25:75 wt % in order to ensure a sol-gel transition that can also be characterized by viscometry. This pH-sensitive heterocoagulated system consisting of oppositely charged colloid particles was suitable for the release of antimicrobial AgNPs immobilized on the clay lamellae via a pH-controlled gel-sol transition. The heterocoagulation process was also characterized by surface charge titration measurements. Spherical LDH/Ag°-montmorillonite composite samples were identified by X-ray diffraction (XRD) measurements. The morphological properties of the composites were studied, and the presence of the heterocoagulated structure was confirmed by scanning electron microscopy (SEM). The nanoscale structure of the LDH(sph)-Ag°-montmorillonite composite obtained was also verified by small-angle X-ray scattering (SAXS), and the rheological characteristics were studied at various pH values. The viscosity and yield value of the composite decreased by an order of magnitude upon increasing the pH from 3.0 to 5.5. The sol-gel transition of the composite suspension was reversible in the previously mentioned pH range.

Synthesis of organosilicon sol-gel matrices and preparation of heterogeneous biocatalysts based on them

Hybrid organosilicon structures prepared by the sol-gel method from tetraethoxysilane, methyltriethoxysilane, and polyethylene glycol 3000 under the conditions of base catalysis were used as a matrix for immobilization of whole cells of methylotrophic yeast for developing heterogeneous biocatalysts. The influence of the precursor ratio on the time of formation of the organosilicon sol-gel structure under the conditions of base catalysis was examined. According to the IR data, the structure obtained with 50 vol % methyltriethoxysilane has the highest content of Si-O-Si bonds. Examination by scanning electron microscopy showed that microorganism cells under these conditions are centers of formation of the sol-gel structure.

Facile Electrospinning Preparation of Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> Nanobelts

PVP/[Y(NO3)3+Al (NO3)3] composite nanobelts were fabricated via electrospinning combined with sol-gel process and novel structure of Y3Al5O12(denoted as YAG for short) nanobelts have been obtained after calcination of the relevant composite nanobelts. The structural properties were characterized by X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). XRD analysis indicated that the composite nanobelts were amorphous, and YAG nanobelts were cubic in structure with space group Ia3d. FTIR analysis manifested that pure YAG nanobelts were formed at 900oC. SEM analysis and histograms revealed that the width of the composite nanobelts and YAG nanobelts were 3.5 μm and 2.4 μm, and the thickness were 240 nm and 112 nm, respectively, under the 95% confidence level. The formation mechanism of YAG nanobelts was discussed in detail.

RhEPO/EPO discrimination with ultrasensitive electrochemical biosensor based on sandwich-type nano-Au/ZnO sol–gel/nano-Au signal amplification

This research established a non-labeled electrochemical biosensor for discrimination of recombinant human erythropoietin (rhEPO) and endogenous erythropoietin (EPO). We prepared a glassy carbon electrode (GCE) modified by a unique sandwich-like nano-Au/ZnO sol–gel/nano-Au compound membrane for signal amplification. The porous sol–gel structure facilitates protein activity maintenance and thermostability. Nano-Au is characterized by a large specific surface area, high surface activity, high absorbability, and good electro-conductivity and biocompatibility. By combining the advantages of both ZnO sol–gel and nano-Au, the amount of erythropoietin receptor (EPOR) increased substantially, and electron transfer of EPOR protein and electrode surface increased accordingly. In the present study, the effects of experimental conditions such as nano-Au electrodeposition time and nano-Au concentration were investigated by cyclic voltammetry, and the process of GCE modification was characterized electrochemically. We successfully developed a new method for electrochemical detection of trace rhEPO/EPO. More importantly, the response current change (ΔI) of the nano-Au/ZnO sol–gel/nano-Au modified GCE increases 3-fold when compared with that of the unmodified electrode and the sensor detection sensitivity increases significantly. In conclusion, this electrochemical biosensor is simple to prepare and allows fast, accurate, and specific detection of trace rhEPO in clinical monitoring and stimulant discrimination.

Nanostructural control of the release of macromolecules from silica sol–gels

The therapeutic use of biological molecules such as growth factors and monoclonal antibodies is challenging in view of their limited half-life in vivo. This has elicited the interest in delivery materials that can protect these molecules until released over extended periods of time. Although previous studies have shown controlled release of biologically functional BMP-2 and TGF-β from silica sol–gels, more versatile release conditions are desirable. This study focuses on the relationship between room temperature processed silica sol–gel synthesis conditions and the nanopore size and size distribution of the sol–gels. Furthermore, the effect on release of large molecules with a size up to 70kDa is determined. Dextran, a hydrophilic polysaccharide, was selected as a large model molecule at molecular sizes of 10, 40 and 70kDa, as it enabled us to determine a size effect uniquely without possible confounding chemical effects arising from the various molecules used. Previously, acid catalysis was performed at a pH value of 1.8 below the isoelectric point of silica. Herein the silica synthesis was pursued using acid catalysis at either pH 1.8 or 3.05 first, followed by catalysis at higher values by adding base. This results in a mesoporous structure with an abundance of pores around 3.5nm. The data show that all molecular sizes can be released in a controlled manner. The data also reveal a unique in vivo approach to enable release of large biological molecules: the use more labile sol–gel structures by acid catalyzing above the pH value of the isoelectric point of silica; upon immersion in a physiological fluid the pores expand to reach an average size of 3.5nm, thereby facilitating molecular out-diffusion.

A novel tantalum-based sol–gel packed microextraction syringe for highly specific enrichment of phosphopeptides in MALDI-MS applications

A new tantalum-based sol-gel material was synthesized using a unique sol-gel synthesis pathway by PEG incorporation into the sol-gel structure without performing a calcination step. This improved its chemical and physical properties for the high capacity and selective enrichment of phosphopeptides from protein digests in complex biological media. The specificity of the tantalum-based sol-gel material for phosphopeptides was evaluated and compared with tantalum(V) oxide (Ta2O5) in different phosphopeptide enrichment applications. The tantalum-based sol-gel and tantalum(V) oxide were characterized in detail using FT-IR spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM), and also using a surface area and pore size analyzer. In the characterization studies, the surface morphology, pore volume, crystallinity of the materials and PEG incorporation into the sol-gel structure to produce a more hydrophilic material were successfully demonstrated. The X-ray diffractograms of the two different materials were compared and it was noted that the broad signals of the tantalum-based sol-gel clearly represented the amorphous structure of the sol-gel material, which was more likely to create enough surface area and to provide more accessible tantalum atoms for phosphopeptides to be easily adsorbed when compared with the neat and more crystalline structure of Ta2O5. Therefore, the phosphopeptide enrichment performance of the tantalum-based sol-gels was found to be remarkably higher than the more crystalline Ta2O5 in our studies. Phosphopeptides at femtomole levels could be selectively enriched using the tantalum-based sol-gel and detected with a higher signal-to-noise ratio by matrix-assisted laser desorption/ionization-mass spectrometer (MALDI-MS). Moreover, phosphopeptides in a tryptic digest of non-fat bovine milk as a complex real-world biological sample were retained with higher yield using a tantalum-based sol-gel. Additionally, the sol-gel material was packed into a standard syringe (0.5 mL) to enhance the ease of use of the sol-gel material and for the elimination of additional mixing and separation procedures during the adsorption, washing and elution steps of the enrichment procedure. It was found that up to 28 phosphopeptides in milk digest were easily detectable by MALDI-MS at femtomole levels (around 20 fmol) using the microextraction syringe within less than one minute.

Concentration Dependence on the Shape and Size of Sol–Gel-Derived Yttria-Stabilized Zirconia Ceramic Features by Soft Lithographic Patterning

Typical surface areas of 5 × 5 mm(2) were patterned with high-aspect-ratio micrometer- and submicrometer-sized structures of yttria-stabilized zirconia using a combination of micromolding in capillaries and sol-gel chemistry. The influence of precursor solution concentration and mold geometry on the final shape and dimensions of the patterned structures was investigated. At a precursor concentration of [Zr] = 0.724 mol/dm(3), isolated objects-due to the controlled cracking of patterned films-such as crosses (height 1.4 μm, width 6.0 μm) and "dog bones" (height 800-900 nm, width 900 nm) or patterned films (height 450 nm) were obtained, depending on the mold geometry. Lower precursor concentrations led to differently sized and shaped structures, with changes in dimensions of more than an order of magnitude. Employing a precursor concentration of [Zr] = 0.036 mol/dm(3) yielded isolated rings (height 100-150 nm, line width 20 nm) and squares (height 40 nm, line width 40 nm). A better understanding of the relationship between the precursor concentration, mold geometry, and observed coherent crack patterns in as-dried sol-gel structures may lead to new techniques in patterning isolated features.

Covalent immobilization of a fluorescent pH-sensitive naphthalimide dye in sol–gel films

A pH indicator dye was covalently linked to inorganic–organic hybrid sol–gel layers via its carboxyl function by the formation of an amide bond. For this, the dye was activated by 2-succinimido-1,1,3,3-tetramethyluronium tetrafluoroborate and linked to N-(3-(trimethoxysilyl)propyl)-ethylendiamine. Different ratios of tetraethoxy-silane, diisobutyldimethoxysilane and 3-glycidoxypropyltrimethoxysilane were evaluated to tailor the performance of the sensing material. Fluorescence spectroscopy of the optimised sensor layers with a molar ratio of organically modified siloxane to tetraethoxysilane of 25.9 and of dye/amino groups of 1.16, showed a reversible fluorescence signal increase of 117 % upon protonation, and a pk a 6.5. The signal changes were caused by photoinduced electron transfer between the methylpiperazine moiety and the naphthalimide fluorophore, its efficiency being modulated by protonation of the methylpiperazine nitrogen. The influence of parameters such as synthesis, dip-coating process and heat-treatment on the performance of the sensor layers was investigated. Optimum signal changes were obtained when heating the sol–gel structure up to 170 °C.

Sol–gel simulation—II: Mechanical response

A novel computational procedure is proposed to predict the outstanding mechanical properties of sol–gel structures. An aggregation algorithm incorporating Brownian motion and chemical reactions is used to recreate the sol–gel structures at molecular scale. Just like in the physical colloidal aggregation process, the computational aggregation process produces structures with fractal features. Such fractal character leads to a recursion algorithm for calculating mechanical properties at any scale using a recursive multiscale approach. The mechanical properties are then predicted at each scale by calculating the effective properties using the Finite Element Method. It is shown that Young's modulus naturally follows a power law relationship with density, whereas Poisson's ratio displays more complicated behavior. Also, it is shown that Young's modulus and Poisson's ratio depend on a) the mass distribution of the structure, which is influenced by the Brownian motion and chemical reactivity during the aggregation process, and b) the connectivity, which is also influenced by additional processes as sintering and/or aging. Finally, it is shown that the Young's modulus and Poisson's ratio can be correlated to scattering intensity of sintered and/or aged structures.

Sol–gel simulation—I: Scattering response

Scattering of sol–gel structures is investigated computationally. Sol-gels are recreated through an aggregation algorithm incorporating Brownian motion and chemical reactions. Using the fractal character of sol–gels, the concept of recursion is introduced as a tool to perform multi scale computation of the response of sol–gels through the different scales from the molecular level to the macro scale. The concept is illustrated with the prediction of scattering intensity. The relationship between scattering intensity and functionality is investigated, noting that the latter is a function of the Brownian motion and chemical reactivity. Computational simulation tools are developed to predict scattering intensity as a function of density and reactivity, the former represented by the number of particles, or clusters, in the simulation box. Then, the results are correlated to an analytical model that reveals the critical wave number, or critical scale, at which percolation occurs.

Photovoltaic responses of ZnO/Si heterojunctions synthesized by sol-gel method

ZnO/Si heterojunctions were prepared by the sol-gel method and hexagonal polycrystalline wurtzite structures with pores were observed by a field emission scanning electron microscope. The steady photovoltage properties of ZnO/Si heterojunctions were obtained under the illumination of a 532 nm continuum solid state laser with a 50 Hz chopper. In addition ns photoresponse signals were found when the samples were excited by a ps pulsed laser at room temperature. A possible mechanism was proposed to describe the photovoltaic process in the ZnO/Si heterojunction.

Electrochemically patterning sol–gel structures on conducting and insulating surfaces

A new approach for the local deposition of sol-gel films on conducting and insulating surfaces using scanning electrochemical microscopy (SECM) via the feedback and direct modes is presented. Patterning is based on enhancing sol-gel condensation by altering the local pH due to water electrolysis as a result of applying negative potentials.

Sodium silicate based sol–gel structures for generating pressure-driven flow in microfluidic channels

In this article, we report the design of a microchip based hydraulic pump that employs a sodium silicate derived sol–gel structure for generating pressure-driven flow within a microfluidic network. The reported sol–gel structure was fabricated in a chosen location of our device by selectively retaining sodium silicate solution within a sub-micrometer deep segment via capillary forces, and then providing the precursor material appropriate thermal treatment. It was shown that while the molecular weight cut-off for these membranes is at least an order of magnitude smaller than their photo-polymerized counterparts, their electrical conductance is significant. Moreover, unlike their polymeric counterparts these structures were found to be capable of blocking electroosmotic flow, thereby generating a pressure-gradient around their interface with an open microchannel upon application of an electric field across the microchannel–membrane junction. In this work, a fraction of the resulting hydrodynamic flow was successfully guided to an electric field-free analysis channel to implement a pressure-driven assay. Our experiments show that the pressure-driven velocity produced in the analysis channel of our device varied linearly with the voltage applied across the sol–gel membrane and was nearly independent of the cross-sectional dimensions of the membrane and the microfluidic channels. With our current design pressure-driven velocities up to 1.7 mm/s were generated for an applied voltage of 2 kV, which easily covers the range of flow speeds that can minimize the plate height in most microfluidic separations. Finally, the functionality of our device was demonstrated by implementing a reverse phase chromatographic separation in the analysis channel of our device using the pressure-driven flow generated on-chip.

Doubly patternable epoxy based sol–gel structures by UV and soft lithography

The sol–gel synthesis of hybrid materials offers special opportunities to combine polymer and glass properties and to create promising candidates for photonic applications. We report on the optical and morphological characterization of a new photosensitive epoxy based sol–gel system. Germanium ethoxide and 3-Glycidoxypropyltrimethoxysilane were used as precursors for hybrid sol-gel planar coatings. A photoacid generator was added to solution in order to allow the epoxy photopolymerization when the films are selectively exposed to UV light. The refractive index increase (Δ n = 0.015) induced by UV light allows the direct patterning of waveguiding structures having good morphological quality. Stripes and beam splitters were defined by direct UV exposition on silicon substrates. Moreover, we present structures made by thermal imprinting method. Silicon masters have been used to transfer relief gratings on the photopolymerizable epoxy sol-gel materials. Combining thermal imprinting technique and UV light exposition through a photomask, complex structures can be realized such as light couplers, sensors and wavelength filters.

Nanochannel arrays as supports for proton exchange membranes in microfluidic fuel cells

In this article, we report the use of nanochannel arrays as supports for proton exchange membranes in microfluidic fuel cells. The proposed design has been demonstrated by fabricating a sodium silicate based sol–gel structure within such an array bridging two microchannels containing the fuel (HCOOH) and the oxidant (KMnO 4) streams. A voltage was generated in this system by bringing two platinum electrodes in contact with these solutions and then connecting them through an external circuitry. With this current design, we have been able to generate an open circuit potential of 1.31 V and a maximum current of 31.2 μA at 25 °C.

Synthesis and Selected Properties of Silicate Hybrids Containing Sepharose

The main purpose of the present work is the sol-gel synthesis, structure and application of hybrid inorganic–organic hybrids based on different silica precursors and addition of the organic compound sepharose (SP). The structural evolution of the hybrid materials containing different amounts of SP is examined. Formation of silica nanocomposites by self-assembling processes was studied by AFM and roughness analysis. The average size of nanoparticles on the sample surface is about 7 to 14 nm and the formation of their self-organized structures is observed. The hybrids are used for immobilization of bacterial cells, producers of thermostable nitrilase. The biocatalysts show good operational stability for a long period-18 days, as well as high thermostability. The degradation capability is greater for the encapsulated cells in the hybrid matrix with 5% SP.

Patterning High-Aspect-Ratio Sol–Gel Structures by Microtransfer Molding

Organic–inorganic silicon-based polymers can be tailor-made to obtain materials with mechanical, chemical, optical, or electric properties that suit a great variety of applications and are compatible with microfabrication technologies. Herein, this work reports on the one-step processing of microstructures of a sol–gel polymeric material on flat substrates by microtransfer molding. A careful selection of silanes and the experimental conditions for the generation of the three-dimensional polymeric network was of key importance to obtain structures of up to 70 µm high having an aspect ratio of up to 10:1 (height/width), with excellent periodicity and reproducibility. Besides, a very low degree of shrinkage of the sol–gel polymer was shown upon aging and drying steps, thereby enabling the perfect control over the fabrication of the final structure. Pillars, wells, and lines of different dimensions were fabricated and characterized by optical, scanning electron, and atomic force microscopy techniques. The controlled development of sol–gel microstructures using a simple fabrication technology may widen the range of promising applications of these materials in areas such as optoelectronics or chemical sensing.

IMPROVED FERROELECTRIC PROPERTIES OF SrBi4−0.1Pr0.1Ti4O15 THIN FILMS

ABSTRACT Thin films with the composition of SrBi4Ti4O15(SBTi) and SrBi4−0.1Pr0.1Ti4O15 (SBPTi) have been prepared on Pt/TiO2/SiO2/Si substrates by sol-gel method structure characterization of the SBT and SBPT thin films was revealed by x-ray diffraction (XRD). Using Pt/SBPT/Pt/TiO2/SiO2/Si configuration, ferroelectric properties of the film were well established with the hysteresis loop. A notable enlargement of remnant polarization (2Pr) and a decrease of the coercive field (Ec) are observed. The 2Pr of SBPTi reaches a value as large as34.76 μC/cm2, nearly twice greater than that of SrBi4Ti4O15 without Pr-doping. The SBPTi film shows little change of Pns and −Pns up to 1010 switching cycles, suggesting an excellent fatigue-endurance characteristic.

Thermal spray coatings of highly reinforced aluminium matrix composites with sol–gel silica coated SiC particles

Low porosity coatings made of aluminium matrix composites reinforced with SiC particles were fabricated by a thermal spray method. The initial spray material used was a mixture of aluminium powder and SiC particles which were directly fed in the thermal spray gun flame without any previous sintering. Sol–gel silica coatings with different structural characteristics were deposited on the SiC particles before mixing with aluminium powder to increase the wettability of reinforcement by molten aluminium during the spray while avoiding the appearance of any degrading reaction between aluminium and SiC particles. The thermal spray coatings were manufactured with different reinforcement degrees from 4 vol.% to 47 vol.% SiC using mixtures of sol–gel coated SiC particles in the 20% to 70% range. Porosity was in most cases below 1%. The relevance of using sol–gel coatings on reinforcement particles and of the sol–gel structure is analysed.

Effect of Sol?Gel Encapsulation on Lipase Structure and Function: A Small Angle Neutron Scattering Study

The application of small angle neutron scattering (SANS) to the characterisation of sol–gel hosts containing biomolecules offers the opportunity to explore the relationship between gel structure and catalyst. A model system involving the immobilisation of Candida antarctica lipase B (CALB) was investigated. Gels were produced by fluoride-catalysed hydrolysis of fixed ratios of tetramethylorthosilicate (TMOS) and methyltrimethoxysilane (MTMS). Phase separation between the enzyme and the evolving sol–gel matrix was minimised by incorporating glycerol into the sol–gel precursor solution. The potential stabilising effect of the NaF catalyst upon the enzyme was also investigated. Scattering studies were conducted on both immobilised lipase, and lipase in free solution. Scattering studies on free enzyme provided evidence of multiple populations of enzyme aggregates and showed that choice of solvent affected the degree of aggregation. Both NaF and glycerol affected neutron scattering, indicating changes in lipase conformation. Increasing glycerol concentration increased the degree of aggregation and produced differences in solvent packing on the surface of protein molecules. Initial evidence from SANS data indicated that the presence of the enzyme during gel formation conferred structural changes on the gel matrix. Modelling the effect of sol–gel encapsulation on lipase requires comparison of data from free enzyme to the immobilised form. Removal of the enzyme from the sol–gel structure, post gelation, is necessary to better characterise the modified matrix. This methodological problem will be the subject of future investigations.