Sol‐Gel/Polyelectrolyte Active Corrosion Protection System

This work presents a new type of feed‐back active coating with inhibitor‐containing reservoirs for corrosion protection of metallic substrates. The reservoirs are composed of stratified layers of oppositely charged polyelectrolytes deposited on AA2024 aluminum alloy coated with hybrid sol‐gel film. The layer‐by‐layer assembled polyelectrolyte film with the entrapped corrosion inhibitor is constructed by sequential spray‐coating deposition of water solutions of poly(ethyleneimine), poly(sodium styrenesulfonate) and 8‐hydroxyquiniline on the top of the sol‐gel coating. The active corrosion protection of AA2024 alloy coated with SiO₂/ZrO₂ sol‐gel film and modified by polyelectrolytes is demonstrated by electrochemical impedance spectroscopy and scanning vibrating electrode technique. The results obtained here show that polyelectrolyte films deposited atop of the hybrid sol‐gel coating on AA2024 alloy remarkably improve the long‐term protection performance providing additional “intelligent” anticorrosion effect that results from delivery of inhibiting species “on demand”. This becomes possible since the configuration of the polyelectrolyte molecules depends on the presence of H⁺ ions making the polyelectrolyte film sensitive to the pH of the surrounding solution. The source of local pH changes is the corrosion process starting in the micro‐ and nano‐defects leading to increased permeability of the polyelectrolyte reservoir and, consequently, to controllable release of entrapped inhibitor.     

Application of Inhibitor‐Loaded Halloysite Nanotubes in Active Anti‐Corrosive Coatings

Halloysite particles are aluminum‐silicate hollow cylinders with a length of 0.5–1 µm, an outer diameter of ca. 50 nm and a lumen of 15 nm. These nanotubes are used for loading and sustained release of corrosion inhibitors. The inhibitor is kept inside the particles infinitely long under dry conditions. Here, halloysite nanotubes filled with anticorrosive agents are embedded into a SiO_x–ZrO_x hybrid film. An aluminum plate is dip‐coated and immersed into 0.1 M sodium chloride aqueous solution for corrosion tests. A defect in the sol–gel coating induces pitting corrosion on the metal accompanied by a strong anodic activity. The inhibitor is released within one hour from halloysite nanotubes at corrosion spots and suppresses the corrosion process. The anodic activity is successfully restrained and the protection remains for a long time period of immersion in NaCl water solution. The self‐healing effect of the sol–gel coating doped with inhibitor‐loaded halloysite nanotubes is demonstrated in situ via scanning vibrating electrode technique measurements.     

Nanocontainer‐Based Anticorrosive Coatings: Effect of the Container Size on the Self‐Healing Performance

Organic coatings based on inhibitor loaded inorganic containers for smart corrosion inhibition are presented. The overall coating performance is strongly influenced by the containers as well as their inhibitor capacity, compatibility with the coating matrix, and size. The important effect of container size is described for the first time in this work by investigating two types of mesoporous silica containers of different diameters: 80 and 700 nm. The coating physical properties (thickness and adhesion) are comparable for both container types. In contrast, the coating barrier properties are strongly influenced by the container size as assessed with electrochemical impedance spectroscopy (EIS). The incorporation of bigger containers reduces the coating resistance by a factor of two. Surprisingly, despite the similar amounts (20 wt%) of loaded inhibitor (2‐mercaptobenzothiazole), different active inhibition ability is detected with the scanning vibrating electrode technique (SVET). Therefore, it is found that coatings with smaller containers exhibit better self‐healing performance.     

Self‐Healing and Antifouling Multifunctional Coatings Based on pH and Sulfide Ion Sensitive Nanocontainers

Application of mesoporous silica nanoparticles (MSNs) as delivery tools for self‐healing coatings is limited by spontaneous leakage and specifically responsive release of small molecular inhibitors. In this work, a pH/sulfide ion responsive release system based on MSNs using a Cu‐BTA complex forms at the openings of the mesopores into which BTA (corrosion inhibitor) and benzalkonium chloride (biocide) are loaded. The spontaneous leakage of active species is completely avoided and premature release of the loaded composition was lowered to 0.02. The responsive release begins when the pH is lower than 5 or [S^2?] is higher than 0.02 mM (about 0.6 ppm). The hybrid coating containing the responsive release system exhibits feedback self‐healing property sensitive to lowering of pH and sulfide ion concentration and, at the same time, provides a high barrier level for a long time. Due to incorporation of biocide in the release system, the coating is also provided with antifouling properties.     

Hollow Two‐Layered Chiral Nanoparticles Consisting of Optically Active Helical Polymer/Silica: Preparation and Application for Enantioselective Crystallization

This article reports for the first time a novel category of hollow organic@inorganic hybrid two‐layered nanoparticles (NPs), in which the inner layer is formed by optically active helical polyacetylene, and the outer layer by silica. Such NPs show remarkable optical activity and are successfully used for enantioselective crystallization. To prepare such NPs, n‐butyl acrylate undergoes radical polymerization to first form poly(n‐butyl acrylate) (PBA) cores two shells by catalytic polymerization of substituted acetylene and sol–gel approach of TEOS (tetraethyl orthosilicate), respectively. Removal of the PBA cores provides the expected hollow core/shell NPs. The intense dircular dichroism (CD) effects demonstrate that the hollow chiral NPs possess considerable optical activity, arising from the helical substituted polyacetylenes forming the inner layer. The hollow NPs are further used as chiral templates to induce enantioselective crystallization of racemic alanines, demonstrating the significant potential applications of the hollow chiral NPs in chiral technologies. Also of particular significance is the detailed process of the induced crystallization observed by TEM. The strategy for preparing the hollow hybrid chiral NPs should be highlighted since it combines free radical polymerization and catalytic polymerization with sol–gel process in a single system, by which numerous advanced materials will be accessible.     

Biological Functionalization of a Sol–Gel Coating for the Mitigation of Microbial‐induced Corrosion

The addition of biocides to water systems has been the most direct attempt to limit or mitigate the formation of surface biofilms that lead to the development of microbial‐induced corrosion (MIC), a process that costs industry millions of pounds annually. The efficacy of this approach is dependent upon a) the biocide being delivered to all water–metal substrate interaction sites and b) the biocide remaining active, therein preventing biofilm development/growth. Anti‐fouling biocide additions to water supplies, or anti‐fungal additives in paints, have an adverse impact upon the environment, for example, tri‐butyl tin is toxic to aquatic life. An alternative non‐toxic approach currently being pursued by the authors involves localizing a biologically active reagent close to the metal substrate. Localization in this case is achieved by synthesizing a hybrid functional sol–gel coating and encapsulating a suitable non‐pathogenic biological component within the sol–gel matrix. Distribution within the sol–gel coating of vegetative bacteria or spores has been confirmed using various microscopy techniques. Electrochemical data in the form of polarization resistance, impedance spectroscopy, and electrochemical noise all show that the presence of either P. fragi or P. polymyxa within a sol–gel coating leads to a corrosion resistance improvement at least 10 times that of an equivalent abiotic coating. Confirmation of this improvement is derived from six months of field trial testing conducted in a tidal estuarine environment. Comparative tests that use a control sol–gel coating immersed within nutrient poor artificial seawater that contained freely suspended P. polymyxa reveal no marked improvement over a control sample in the absence of the endospores.     

Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly

Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Engineering of Mesoporous Silica Coated Carbon‐Based Materials Optimized for an Ultrahigh Doxorubicin Payload and a Drug Release Activated by pH,T, and NIR‐light

Among the challenges in nanomedicine, engineering nanomaterials able to combine imaging and multitherapies is hugely needed to address issues of a personalized treatment. In that context, a novel class of drug releasing and remotely activated nanocomposites based on carbon‐based materials coated with mesoporous silica (MS) and loaded with an outstanding level of the antitumoral drug doxorubicin (DOX) is designed. First, carbon nanotubes (CNTs) and graphene sheets (called “few‐layer graphene” FLG) are processed to afford a distribution size that is more suitable for nanomedicine applications. Then, the controlled coating of MS shells having a thickness tailored with the sol–gel parameters (amount of precursor, sol–gel time) around the sliced CNTs and exfoliated FLGs is reported. Furthermore, the drug loading in such mesoporous nanocomposites is investigated and the surface modification with an aminopropyltriethoxysilane (APTS) coating leading to a controlled polysiloxane layer provides an ultrahigh payload of DOX (up to several folds the mass of the initial composites). Such new CNT‐based nanocomposites are demonstrated to release DOX at low acidic pH, high temperature (T), and remotely when they are excited by near infrared (NIR) light. Such nanoconstructs may find applications as components of innovative biomedical scaffolds for phototherapy combined with drug delivery.     

Transparent Mesoporous Nanocomposite Films for Self‐Cleaning Applications

A versatile approach is studied for the elaboration of TiO₂ based photocatalytic coatings for self‐cleaning applications on transparent substrates. The basic principle of the synthesis relies on the use of preformed TiO₂ colloidal particles that are further dispersed within a transparent silica binder with a mesoporous structure. Film porosity in the nanometer range is controlled by achieving the sol–gel silica condensation around self‐organized micellar assemblies of a templating copolymer surfactant. The latter also acts as a stabilizer for the TiO₂ particles, thus preserving their high dispersion within the film so that excellent optical properties are maintained even for high TiO₂ loading (up to 50 %). Studies of photodegradation kinetics show that such mesoporous films are at least 15 times more active than films synthesized with a usual microporous silica binder. Moreover, the measured quantum‐yield efficiency (1.1 %) is found to be among the highest reported up to now. Improved photoactivity of the films is discussed as resulting from the closer proximity between the organic molecules and the surface of the TiO₂ crystallites as well as the improved diffusion rate of water and oxygen through the interconnected pore network.     

Surface‐Engineered Nanocontainers for Entrapment of Corrosion Inhibitors

The release properties and reloading ability of polyelectrolyte‐modified halloysite nanotubes, polyelectrolyte‐modified SiO₂ nanoparticles, and polyelectrolyte capsules are studied. Three containers are distinguished by keeping the low‐molecular‐weight corrosion inhibitor benzotriazole in a hollow lumen inside or within the polyelectrolyte matrix and allowing release in either one direction or into all space dimensions. Polyelectrolyte shells, which modify the outer surface of the nanocontainers, are fabricated by using layer‐by‐layer assembly of poly(diallyldimethylammonium chloride)/poly(styrene sulfonate), poly(allylamine hydrochloride)/poly(styrene sulfonate), and poly(allylamine hydrochloride)/poly(methacrylic acid) polyelectrolyte bilayers. All nanocontainers reveal an increase of the benzotriazole release in aqueous solution at alkaline or acidic pH. The highest reloading efficiency (up to 80 %) is observed for halloysite‐based nanocontainers; however, after five reloading cycles the efficiency decreases to 20 %. The application of appropriate nanocontainers depends on the demands required from feedback‐active anticorrosion coatings. For coatings where the immediate release of the inhibitor is necessary, SiO₂‐based or halloysite‐based nanocontainers with a shell consisting of weak polyelectrolytes are preferable. When continuous, gradual release is required, halloysite‐based nanocontainers with a shell consisting of one weak and one or two strong polyelectrolytes are preferable.     

New hybrid organic–inorganic sol–gel positive resist

The direct nanopatterning of a novel hybrid organic–inorganic sol–gel film based on bridged polysilsesquioxanes (BPS) using X-ray synchrotron radiation is reported. The main advantages of a direct fabrication technique with respect to conventional photolithography are represented by the possibility to bypass some typical post-exposure lithographic steps and to avoid the use of a sacrificial layer. The distinctive features rendering hybrid BPS-based material innovative for photolithographic applications are: the patternability as resist, the positive tone behaviour exhibited under X-ray irradiation, the porous structure demonstrated at low temperature, and the possibility to widely tailor material electro-optical and structural properties to experimental needs. A systematic investigation of the interactions between sol–gel BPS films based on the bis(triethoxysilyl)benzene precursor and soft X-rays is conducted. Under X-ray exposure, BPS-based films suffer structural changes attributed to the organic bridge breaking, and become soluble in suitable acidic aqueous solutions, producing final lithographies of sub-micron resolution, high contrast and good edge definition.     

Highly Efficient Self‐Healable and Dual Responsive Cellulose‐Based Hydrogels for Controlled Release and 3D Cell Culture

To face the increasing demand of self‐healing hydrogels with biocompatibility and high performances, a new class of cellulose‐based self‐healing hydrogels are constructed through dynamic covalent acylhydrazone linkages. The carboxyethyl cellulose‐graft‐dithiodipropionate dihydrazide and dibenzaldehyde‐terminated poly(ethylene glycol) are synthesized, and then the hydrogels are formed from their mixed solutions under 4‐amino‐DL‐phenylalanine (4a‐Phe) catalysis. The chemical structure, as well as microscopic morphologies, gelation times, mechanical and self‐healing performances of the hydrogels are investigated with ¹H NMR, Fourier transform infrared spectroscopy, atomic force microscopy, rheological and compression measurements. Their gelation times can be controlled by varying the total polymer concentration or 4a‐Phe content. The resulted hydrogels exhibit excellent self‐healing ability with a high healing efficiency (≈96%) and good mechanical properties. Moreover, the hydrogels display pH/redox dual responsive sol‐gel transition behaviors, and are applied successfully to the controlled release of doxorubicin. Importantly, benefitting from the excellent biocompatibility and the reversibly cross‐linked networks, the hydrogels can function as suitable 3D culture scaffolds for L929 cells, leading to the encapsulated cells maintaining a high viability and proliferative capacity. Therefore, the cellulose‐based self‐healing hydrogels show potential applications in drug delivery and 3D cell culture for tissue engineering.     

Amyloid Fibrils form Hybrid Colloidal Gels and Aerogels with Dispersed CaCO3 Nanoparticles

New hybrid colloidal gels are reported formed by amyloid fibrils and CaCO₃ nanoparticles (CaNPs), with Ca²⁺ as charge screening ions and CaNPs as physical crosslinking agents to establish and stabilize the network. The gel is characterized by rheological measurements and diffusing wave spectroscopy, complemented by microscopic observations using transmission and scanning electron microscopy. The hybrid colloidal gels show a two orders of magnitude improved gel strength at significantly shorter gelation times compared to previous calcium ion‐induced amyloid fibril gels. Supercritical CO₂‐dried colloidal aerogels allow demonstrating that amyloid fibrils, combined with smaller (higher specific surface area) CaNPs, constitute a denser fibrils network, resulting in stronger gels. By varying the amyloid fibril concentration and the CaNPs size and concentration, the complete phase diagram is mapped out. This enables identifying the sol–gel phase transition and a window for gel formation, which widens with increasing CaNPs size. Finally pH responsiveness and self‐healing properties of this hybrid colloidal gel are also demonstrated, making these systems a suitable candidate for biological applications.     

Inside Front Cover: Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly (Adv. Funct. Mater. 7/2006)

The synthesis of three‐dimensionally ordered, transparent gold‐nanocrystal (NC)/silica superlattice thin films using the self‐assembly (by spin‐coating) of water‐soluble gold nanocrystal micelles and soluble silica is reported by Fan and co‐workers on p. 891. The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. Nanocrystals and their ordered arrays hold many important applications in fields such as catalysis, surface‐enhanced Raman spectroscopy based sensors, memory storage, and electronic and optical nanodevices. Herein, a simple and general method to synthesize ordered, three‐dimensional, transparent gold nanocrystal/silica superlattice thin films by self‐assembly of gold nanocrystal micelles with silica or organosilsesquioxane by spin‐coating is reported. The self‐assembly process is conducted under acidic sol–gel conditions (ca. pH 2), ensuring spin‐solution homogeneity and stability and facilitating the formation of ordered and transparent gold nanocrystal/silica films. The monodisperse nanocrystals are organized within inorganic host matrices as a face‐centered cubic mesostructure, and characterized by transmission electron spectroscopy and X‐ray diffraction.     

Template‐Free Sol‐Gel Preparation of Superhydrophobic ORMOSIL Films for Double‐Wavelength Broadband Antireflective Coatings

A double‐layer double‐wavelength antireflective (AR) coating with 100% transmittance at both 1064 nm and 532 nm, which is very important in high power laser systems, is designed using thin film design software (TFCalc). The refractive indices for the bottom and top layers of the designed AR coating are about 1.30 and 1.14. A simple, template‐free sol‐gel route is proposed to prepare the superhydrophobic ORMOSIL (organically modified silicate) thin film, which has an ultralow refractive index, by silica particle surface modification using hexamethylisilazane (HMDS); this treatment decreases the refractive index of the silica thin film from 1.23 to 1.13. The formation mechanism of the ultralow refractive index thin film is proposed. The particle surface modification with HMDS significantly improves the hydrophobicity of the coated film; the water contact angle of the film increases from 23.4° to 160°. The bottom layer, which has a refractive index of 1.30, is prepared from acid‐catalyzed and base‐catalyzed mixed silica sol. A double‐layer silica AR coating is obtained with transmittances of 99.6% and 99.8% at 532 nm and 1064 nm, respectively.     

Polyaniline–Silica Composite Conductive Capsules and Hollow Spheres

In this paper, we report on the preparation of monodisperse polyaniline (PANi)–silica composite capsules and hollow spheres on monodisperse core–gel‐shell template particles. An extension of the previously reported inward growth method was used. The samples were self‐stabilized without external additives. The core–gel‐shell particles were prepared by the inward sulfonation of monodisperse polystyrene particles. The introduced sulfonic acid and sulfone groups are responsible for the gel properties. The gel‐shell thickness and core size were synchronously controlled over the whole particle radius range. After aniline (ANi) monomer was preferentially absorbed in the sulfonated polystyrene shell, PANi was formed by polymerization. PANi was doped in situ with a sulfonic acid group to give the capsules a high conductivity. PANi hollow spheres were derived after the polystyrene cores were dissolved: their cavity size and shell thickness were synchronously controlled by using different core–gel‐shell particles. The PANi–silica composite capsules and hollow spheres were therefore prepared by a sol–gel process using tetraethylorthosilicate in the conducting shell. The PANi shell became more robust while maintaining the same conductivity level. Morphological results indicate that the PANi and silica formed a bicontinuous network. Fourier‐transform infrared (FTIR) spectra revealed that the hydrogen bonding in the PANi–gel shell was enhanced after the silica phase was incorporated, which could explain the high conductivity level after the silica phase was added. In a converse procedure, silica capsules and hollow spheres were prepared by a sol–gel process that incorporated tetraethylorthosilicate into the core–gel‐shell templates, which was followed by the absorption and polymerization of aniline in the silica shell thus forming PANi–silica composite capsules and hollow spheres. The silica capsules and hollow spheres thereby became conductive.     

Dynamic Catalysis in Aerobic Oxidation by Sol–Gel Living Materials

The catalytic activity of hybrid organic–inorganic silica glasses doped with the ruthenium species tetra‐n‐propylammonium perruthenate (TPAP) in the aerial oxidation of alcohols to carbonyl compounds, either in toluene or in dense‐phase CO₂, substantially increases with time several months after the xerogels' preparation, yielding the most active ruthenium‐based aerobic‐oxidation catalysts reported thus far. The doped sol–gels are living materials, and an explanation of the observed reactivity enhancement is given, which is thought to have general validity for future applications to a wide variety of relevant heterogeneous processes.     

A Low Temperature Route toward Hierarchically Structured Titania Films for Thin Hybrid Solar Cells

The requirement of high‐temperature calcination for titanium dioxide in (solid‐state) dye‐sensitized solar cells (DSSCs) implies challenges with respect to reduced energy consumption and the potential for flexible photovoltaic devices. Moreover, the use of dye molecules increases production costs and leads to problems related with dye bleaching. Therefore, fabrication of dye‐free hybrid solar cells at low temperature is a promising alternative for current DSSC technology. In this work the authors fabricate hierarchically structured titania thin films by combining a polystyrene‐block‐polyethylene oxide template assisted sol–gel synthesis with nano‐imprint lithography at low temperatures. The achieved films are filled with poly(3‐hexylthiophene) to form the active layer of hybrid solar cells. The surface morphology is probed via scanning electron microscopy and atomic force microscopy, and the bulk film morphology is examined with grazing incidence X‐ray scattering. Good light absorption by the active layer is proven by UV–vis spectroscopy. An enhancement in light absorption is observed and ascribed to light scattering in mesoporous titania films with imprinted superstructures. Accordingly a better photovoltaic performance is found for nano‐imprinted solar cells at various angles of light incidence.     

Fully Printed Flexible Smart Hybrid Hydrogels

A printable hybrid hydrogel is fabricated by embedding poly(N‐isopropylacrylamide) (PNIPAm) microparticles within a water‐rich silica‐alumina(Si/Al)‐based gel matrix. The hybrid gel holds water content of up to 70 wt%, due to its unique Si/Al matrix. The hybrid hydrogel can respond to both heat and electrical stimuli, and can be directly printed layer‐by‐layer using a commercial 3‐dimensional printer, without requiring any curing. The hybrid ink is printed onto a transparent, flexible conductive electrode composed of silver nanoparticles and sustains bending angles of up to 180°, which enables patterning of various flexible devices such as smart windows and a 3D optical waveguide valve.     

Silica Films with Bimodal Pore Structure Prepared by Using Membranes as Templates and Amphiphiles as Porogens

Extended porous silica films with thicknesses in the range of 60 to 130 μm and pores on both the meso‐ and macroscale have been prepared by simultaneously using porous membrane templates and amphiphilic supramolecular aggregates as porogens. The macropore size is determined by the cellulose acetate or polyamide membrane structure and the mesopores by the chosen ethylene‐oxide‐based molecular self‐assembly (block copolymer or non‐ionic surfactants). Both the template and the porogen are removed during an annealing step leaving the amorphous silica material with a porous structure that results from sol–gel chemistry occurring in the aqueous domains of the amphiphilic liquid‐crystalline phases and casting of the initial template membrane. The surface area and total pore volume of the inorganic films vary from 473 to 856 m² g^–1, and 0.50 to 0.73 cm³ g^–1, respectively, depending on the choice of template and porogen. The combined benefits of both macro‐ and mesopores can potentially be obtained in one film. Such materials are envisaged to have applications in areas of large molecule (biomolecule) separation and catalysis. Enhanced gas and liquid flow rates through such membranes, due to the presence of the larger pores, also makes them attractive as supports for other catalytic materials.