Technological significance of sol-gel process and process-induced variations in sol-gel materials and coatings

A significant aspect of sol-gel technology is the capability it provides to affect the substructure of materials by controlling the nature and the kinetics of chemical reactions. This capability allows us to produce novel materials, design unique molecular and pore morphologies, circumvent high-temperature reactions, and modify material properties. The modifications include strongly thermodynamic-dependent high-temperature properties such as sintering, crystallization, and viscosity in glass and ceramic materials. A particularly exciting area for investigation is the optical-electronic field, where a significant dependence of electro-optical properties and photosensitivity on process-induced molecular-structural variations occurs. Understanding the basis for the creation of structural variations in sol-gel processes should have significant impact on the technologies and systems that use these materials. In this article, some fundamental aspects of alkoxide-based, sol-gel processes and thermochemical bases for process-induced structural variates are discussed.     

Encapsulation of the ferritin protein in sol-gel derived silica glasses

A significant recent development in sol-gel science has been the encapsulation of biomolecules such as proteins and enzymes in optically transparent silica glasses. This paper reports on the encapsulation of an iron (Fe) storage protein, ferritin, to develop a magnetic silica glass. Native ferritin, which has a nanometer-sized microcrystalline Fe oxide core, was encapsulated in optically transparent silica glasses using the sol-gel process. Fe could be released from ferritin but could not be reconstituted into apoferritin when the protein was trapped in the pores of the glass. Transmission electron microscopy of ferritin-doped aged silica gels indicated that crystallinity of the Fe oxide core was retained upon sol-gel encapsulation. Magnetic measurements on ferritin-doped silica gels indicated the material to be paramagnetic, but not superparamagnetic.     

Protein micropatterning on bifunctional organic-inorganic sol-gel hybrid materials

Active protein micropatterns and microarrays made by selective localization are popular candidates for medical diagnostics, such as biosensors, bioMEMS, and basic protein studies. In this paper, we present a simple fabrication process of thick (approximately 20 microm) protein micropatterning using capillary force lithography with bifunctional sol-gel hybrid materials. Because bifunctional sol-gel hybrid material can have both an amine function for linking with protein and a methacryl function for photocuring, proteins such as streptavidin can be immobilized directly on thick bifunctional sol-gel hybrid micropatterns. Another advantage of the bifunctional sol-gel hybrid materials is the high selective stability of the amine group on bifunctional sol-gel hybrid patterns. Because amine function is regularly contained in each siloxane oligomers, immobilizing sites for streptavidin are widely distributed on the surface of thick hybrid micropatterns. The micropatterning processes of active proteins using efficient bifunctional sol-gel hybrid materials will be useful for the development of future bioengineered systems because they can save several processing steps and reduce costs.     

Novel silica-biopolymer nanocomposites: the silica sol-gel process in biopolymer organogels

A novel sol-gel nanocomposite material is introduced which was produced by silica sol-gel processes in microemulsion organogels. Microemulsion organogels are formed by a percolating droplet network containing a biopolymer (e.g. gelatin, chitosan, etc.). Microemulsion organogels have water localized in the droplet network which permits the spatial confinement of hydrolysis and polycondensation to microheterogeneous reaction sites. In the case of microemulsion organogels, the oxid gel forms an interpenetrating network with the gelatin maze. While the formation of the microemulsion and the microemulsion organogel is controlled by interfacial processes, the aggregation process of the inorganic oxide particles is objected to gravitational forces. Therefore, we propose this system for sol-gel studies under low or microgravitational conditions. The structural features of the obtained silica-gelatin nanocomposites will be described and compared with silica-biopolymer composites of non-gelling biopolymers in microemulsions (e.g., chitosan). Comparison is made with the silica gels that form in the parent microemulsions.     

UV-nanoimprinting lithography of Bragg Gratings on hybrid sol-gel based channel waveguides

We report on the fabrication in a single step of a channel grating loaded waveguide on Titanium based hybrid sol-gel material.This result has been accomplished by the merging of several lithographic techniques, namely conventional, laser interference, and soft lithography.Conventional lithographic processes have been employed for fabricating channel waveguides on a previously holographically written planar photopolymerizable sol-gel film. Such structures have been used as a master to produce a negative replica in polydimethylsiloxane (PDMS) and subsequently exploited to reproduce the master patterns by UV-nanoimprinting on photopolymerizable hybrid sol-gel coatings (titanium and 3-(trimethoxysilyl)propyl methacrylate).Optical and morphological characterization of the various fabrication steps and of the final device have been reported and discussed.     

Applications of sol-gel technology in imaging

New materials need to be developed to fulfill the future requirements of the imaging business. This applies to the development of new products, but also to the improvement of current products of processes in different areas such as image capture, storage, manipulation, display or printing. Examples of such materials based on sol-gel technology are given, including conductive oxides for antistatic films, sensors or membranes for environmental applications and optical films. For each example specific key issues have to be addressed, so that the material can be used in real applications.     

Characterization of sol-gel immobilized lipases

The immobilization of lipases within a chemically inert hydrophobic sol-gel support, which is prepared by polycondensation of hydrolyzed tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) or iso-butyltrimethoxysilane (iso-BTMS), results in heterocatalysts. The heterocatalysts so prepared showed a dramatically enhanced catalytic activity and stability as measured by the hydrolysis and transesterification of soybean oil. The lipase/sol-gel materials were characterized by nitrogen adsorption to determine their specific surface area. Solid state NMR was used to reveal the degree of cross-linking of the sol-gel materials. Scanning electron microscopy and atomic force microscopy were used to observe the morphology of the biocatalysts. Transmission electron microscopy and confocal microscopy were used to investigate the enzyme distribution within the sol-gel materials. The characterization studies showed that the most active lipase-containing sol-gel was a non-porous amorphous material with enzyme randomly distributed throughout the sol-gel material. The activity of the immobilized enzyme did not correlate to the degree of cross-linking or the specific surface area of the sol-gel materials. The highly retained activity of the immobilized enzyme was more likely attributed to the conformational changes of the enzyme during the immobilization, which result in enzyme's fixation in a more favorable conformation and to the lipophilic environment of the hybrid matrix structure which facilitates the transport of the hydrophobic substrate to the active sites.     

The current state of sol-gel technology

The current state of sol-gel technology has been reviewed mainly from the standpoint of microstructures of materials which can be achieved by the sol-gel method. It has been shown that the sol-gel method makes it possible to produce a great variety of high technology materials by providing the existing substances with a significant microstructure and producing the material with a novel microstructure. There are microstructures characterized by micropores, preferential crystal orientation, inorganic-organic and inorganic-inorganic composites of hybrid nature and gradient composition. These characteristic microstructures are related to the properties and applications of the materials.     

Release of nitric oxide from a sol-gel hybrid material containing a photoactive manganese nitrosyl upon illumination with visible light

A polyurethane-coated sol-gel material containing the photoactive Mn nitrosyl [Mn(PaPy3)(NO)]ClO4 rapidly releases NO with high quantum efficiency when exposed to visible light of low intensity. This rigid and strongly colored hybrid material is a convenient point source of NO that can only be triggered with light. Successful delivery of NO to biological targets, such as proteins, by this material has also been demonstrated.     

Current sol-gel activities in Japan

In Japan, sol-gel research activities are very prosperous and there are many successful commercial products. It appears that this trend will continue into the future. In this article, basic sol-gel research carried out in Japanese universities and applications of the sol-gel method to industrial production will be described. It should be noted that many unique studies, including photocatalysts and products involving coatings applied to automobiles and display panels, are found in Japan.     

Polysaccharides as a template for silicate generated by sol-gel processes

The polysaccharides, as established previously (Yu.A. Shchipunov, J. Colloid Interface Sci. 268 (2003) 68; Yu.A. Shchipunov, T.Yu. Karpenko, Langmuir 20 (2004) 3882), can manipulate the formation of hybrid silica nanocomposites by sol-gel processes. Here atomic force microscopy was applied to show whether carbohydrate macromolecules serve as a template for silicate. Mica was used as a substrate to adsorb polysaccharide. It was found that its surface is not neutral to the sol-gel processes, providing the silica precipitation. To hinder it, the mica was protected by a monomolecular film of arachidic acid with the help of a Langmuir-Blodgett technique. Hydrophobically modified cationic hydrohyethylcellulose was adsorbed from a diluted aqueous solution. It was demonstrated that the carbohydrate macromolecules located on the hydrophobic surface did promote silica precipitation, serving as a template.     

Electrochemical redox reaction in a silica sol-gel glass with embedded organic electrolyte

The electrochemical redox reaction of ferrocene was studied in silica sol-gel glass with embedded organic electrolyte. This material has been prepared by mixing hydrolysed tetramethylorthosilicate sol with propylene carbonate LiClO₄ solution containing ferrocene and further gelation in a form of block or film. The electrochemical behaviour of encapsulated ferrocene was studied by cyclic voltammetry, chronoamperometry, differential pulse voltammetry and impedance spectroscopy on ultramicroelectrodes. The shape of the cyclic voltammograms corresponding to the electrooxidation of ferrocene in sol-gel block is similar to that obtained in liquid electrolyte and it does not depend on gel aging. The current substantially decreases during the first few days after gelation. Later it becomes weakly dependent on aging and the apparent diffusion coefficient of ferrocene in the gel block is about half of that in liquid electrolyte. The electrooxidation of ferrocene also occurs in film of the analogous sol-gel material cast on the surface of the electrode assembly.     

Low-temperature preparation of nanocrystalline anatase films through a sol-gel route

Nanocrystalline anatase (TiO₂) films were prepared at very low temperature through a sol-gel route using titanium isopropoxide and hydrogen peroxide in ethanol. Crystallization occurred after film deposition at 35°C in an atmosphere saturated with water vapor. Both thin and thick films of nanocrystalline anatase were prepared. Observed particle size in crystallized films is approximately 20–40 nm as measured with AFM. No residual organic material was apparent through FTIR after crystallization occurred. Dynamic light scattering studies performed on this system indicate that particle size measured in solution is strongly dependent on the amount of agitation samples received prior to measurement.     

Functional coatings: The sol-gel approach

CEA's sol-gel laboratory is specialized in the development of innovative sol-gel optical coatings and has extended its application field to membrane materials and coatings for energy conversion, to electric coatings for microelectronics devices and to thin films for gas sensing. This article describes, by way of examples, the laboratory's research on sol-gel functional coatings, including nanomaterial synthesis, organic-inorganic hybrid-based solution preparation as well as deposition process development and prototyping.     

Introducing hydrophilic carbon nanoparticles into hydrophilic sol-gel film electrodes

A hydrophilic carbon nanoparticle–sol-gel electrode with good electrical conductivity within the sol-gel matrix is prepared. Sulfonated carbon nanoparticles with high hydrophilicity and of 10–20 nm diameter (Emperor 2000) are co-deposited onto tin-doped indium oxide substrates employing a sol-gel technique. The resulting carbon nanoparticle-sol-gel composite electrodes are characterized as a function of composition and salt (KCl) additive. Scanning electron microscopy and voltammetry in the absence and in the presence of a solution redox system suggest that the composite electrode films can be made electrically conducting and highly porous to promote electron transport and transfer. The effect of the presence of hydrophilic carbon nanoparticles is explored for the following processes: (1) double layer charging, (2) diffusion and adsorption of the electrochemically reversible solution redox system 1,1′-ferrocenedimethanol, (3) electron transfer to the electrochemically irreversible redox system hydrogen peroxide, and (4) electron transfer to the redox liquid tert-butylferrocene deposited into the porous composite electrode film. The extended electrochemically active hydrophilic surface area is beneficial in particular for surface sensitive processes (1) and (3), and it provides an extended solid|organic liquid|aqueous solution boundary for reaction (4). The carbon nanoparticle–sol-gel composite electrodes are optimized to provide good electrical conductivity and to remain stable during electrochemical investigation.     

On the interaction of encapsulated pH indicator species within a silica matrix produced by three sol-gel routes

Solid acid-base sensors were prepared by encapsulating two pH indicators (brilliant yellow or acridine) within a silica matrix by the sol-gel method using three different routes: (1) non-hydrolytic, (2) acid catalyzed and (3) base catalyzed. The interactions of the silica-indicator with the resulting materials were then investigated by cyclic and differential pulse voltammetry. Complementary, ultraviolet-visible, photoacoustic spectroscopy was employed for the characterization of the interactions by monitoring the band shifts (bathochromic or hypsochromic, depending on the sol-gel route) between the neat pH indicators and those encapsulated within the silica network. Furthermore, X-ray photoelectron spectroscopy showed that the N 1s binding energy in brilliant yellow was shifted for the material resulting from the acid route. The electrochemical behavior and the pH indicator interactions with the silica network were dependent on the nature of the employed sol-gel route. For the sensors prepared with acridine, the interactions with the silica network took place through the nitrogen group from the pyridinic ring. For the brilliant yellow indicator, different behaviors were observed depending on the route, suggesting different processes during preparation or analysis. For the basic catalyzed and non-hydrolytic routes, it was not possible to assign a specific interaction. Nevertheless, it seemed that interactions might have taken place through the hydroxyl and/or sulphonic groups. Furthermore, for the brilliant yellow sensor prepared through the acid route, it was possible to show that the interaction probably or partially occurred through the azo groups.     

TG/DTG/DTA/DSC as a tool for studying deposition by the sol-gel process on materials obtained by rapid prototyping

In rapid prototyping (RP), building 3D physical prototypes involves the addition of material in layers. The sol-gel route is an alternative to produce multicomponent oxide materials with chemical, physical and thermal properties that cannot be obtained by other processes. The sol-gel method allows for the preparation of coatings on several kinds of materials, directly influencing the materials’ properties. In this work, metal oxides were prepared by the sol-gel process and deposited further by dip-coating technique on ABS and Nylon substrates obtained by RP. The resulting coating presented good adhesion to the substrates. The obtained materials were characterized by scanning electron microscopy (SEM) and thermal analysis (TA).     

Influence of pyrogenic particles on the micromechanical behavior of thin sol-gel layers

Coatings based on sol-gel technology with different types of nanoparticles embedded into the sol-gel matrix were fabricated, and the resulting properties were investigated. Pyrogenic silica nanoparticles were added to the sol before coating. The silica particles varied in primary particle size and agglomerate size, and in their surface modification. The particles were wetted in ethanol and dispersed to certain finenesses. The difference in agglomerate size was partly caused by varying particle types, but also by the dispersing processes that were applied to the particles. The resulting coatings were examined by visual appearance and SEM microscopy. Furthermore, their micromechanical properties were determined by nanoindentation. The results show an important influence from the added nanoparticles and their properties on the visual appearance as well as the micromechanical behavior of the sol-gel coatings. It is shown that, in fact, the particle size distribution can have a major impact on the coating properties as well as the surface modification.     

Fabrication of anisotropic porous silica monoliths by means of magnetically controlled phase separation in sol-gel processes

Sol-gel accompanied by phase separation is an established method for the preparation of porous silica monoliths with well-defined macroporosity, which find numerous applications. In this work, we demonstrate how the addition of (superpara)magnetic nanocolloids as templates to a system undergoing a sol-gel transition with phase separation leads to the creation of monoliths with a strongly anisotropic structure. It is known that magnetic nanocolloids respond to the application of an external magnetic field by self-assembling into columnar structures. The application of a magnetic field during the chemically driven spinodal decomposition induced by the sol-gel transition allows one to break the symmetry of the system and promote the growth of elongated needle-like silica domains incorporating the magnetic nanocolloids, aligned in the direction of the field. It is found that this microstructure imparts a strong mechanical anisotropy to the materials, with a ratio between the Young's modulus values measured in a direction parallel and perpendicular to the one of the field as high as 150, and an overall smaller average macropores size as compared to isotropic monoliths. The microstructure and properties of the porous monoliths can be controlled by changing both the system composition and the strength of the applied magnetic field. Our monoliths represent the first example of materials prepared by magnetically controlling a phase transition occurring via spinodal decomposition.