http://www.mdpi.com/journal/materials/special_issues/sol-gel-technique-materials/ Dear Colleagues, Sol-gels are versatile materials made by condensing a solution (sol) of metal oxide precursors into three dimensional networks. The gels are bi-phasic systems in which a continuous fluid phase fills the space inside a polymerized network. The gels can be dried in controlled fashion to produce porous solids with unique thermal, mechanical, optical and chemical properties. Sol-gel materials have grown in importance over the past 30 years as chemists and engineers have learned how to vary the reactants and processing conditions to tailor material properties for specific applications. The early work with sol-gels focused on those made of silica, derived by condensation of silanols groups (Si-OH), as illustrated in by reaction (1). Reaction 1. Condensation of silanols into a gel. The silanols condense by forming water leading to a network of Si-O-Si bonds. The quaternary functionality of the Si results in a three dimensional network. The silanols groups may be on the surface of nanometer sized silica particles or could be formed by hydrolysis of silicone alkoxides as illustrated in reaction (2). Reaction 2. Hydrolysis of silicon alkoxides to produce silanols. The silanols subsequently undergo condensation reactions to produce silica gels. Silicon has four functional groups that can undergo condensation. By altering the reaction conditions (temperature, concentration, pH, solvent and reactant), the degree and nature of the condensation reactions can be controlled. At low pH conditions the rate of condensation slows down with degree of branching resulting in low cross-link density and very porous gels. In contrast high pH will accompany rapid condensation that can produce dense particles that precipitate from the solution. The gels are filled with liquid, generally a water alcohol solution. Controlled drying of the gel is employed to tailor the porosity and composition for specific applications. Highly porous materials can be produced that are exceptional thermal insulators. Alternatively dense gels may be employed as thin film protective coatings for lenses. Advances in chemistry and the chemical precursors available for sol-gel processing have made it an extremely flexible process for materials synthesis. We are no longer limited to silica gels or even to metals oxides. It is now possible to make sol gels materials from almost any transition metal, as well as make composite materials. The applications of sol-gel materials have grown as the synthesis and processing methods have opened new vistas of material properties. A list of some of major applications of sol-gels is given in Table 1. Table 1. Applications of Sol-gel materials.   Application Sol-Gel Material Optical fibers High purity doped silica gel films for optical fiber precursors Protective optical coatings Abrasion resistance silica gel coatings on plastic substrates Anti-reflective optical coatings Laser windows, smart windows Thermal insulation for windows Aerogel window spacers, solar collector coatings High Temperature Refractory Insulation Ceramic foams Chemical Sensors Thin film NOx sensors, sol-gel coated crystal oscillators Catalysts and Adsorbents Silica alumina solid acid catalysts, high surface area catalyst supports, Silica gel desiccant Ceramic membranes Sol-gel molecular sieves, antibacterial filters Abrasives Alumina abrasives Dental sealants and fillers Hydroxyapatite   Sol-gel technology offers many important advantages in materials processing. The nanometer structure of the gels permits low temperature processing of ceramic materials so that ceramics and plastics can be combined in hybrid materials. The introduction of metal alkoxides precursors for sol-gels made possible the production of high purity materials that dramatically improved the quality of optical fibers. The pore structure and large surface areas associated with sol-gel materials has been essential to the development of catalysts and adsorbents making possible improved production of gasoline and removing impurities for automobile exhausts and new photocatalysts for splitting water. We now have a tool box of chemical and processing methods to tailor sol-gels to tackle new materials technologies. In this special issue we present reviews of the synthesis and processing techniques to produce sol-gel materials. Jay B. Benziger, Ph.D.Guest Editor Related Special Issue Sol-Gel Technique in the International Journal of Molecular Sciences {snippet name="submission_info"} http://www.mdpi.com/1996-1944/4/3/469/ The paper describes development of a sol-gel based immunoaffinity method for the steroid hormone levonorgestrel (LNG) and the effects of changes in the sol-gel matrix format on the activity of the entrapped antibodies (Abs) and on matrix structure. The best sol-gel format for Ab entrapment was found to be a tetramethoxysilane (TMOS) based matrix at a TMOS:water ratio of 1:8, containing 10% polyethylene glycol (PEG) of MW 0.4 kDa. Addition of higher percentages of PEG or a higher MW PEG did not improve activity. No activity was obtained with a TMOS:water ratio of 1:12, most likely because of the very dense polymer that resulted from these polymerization conditions. Only minor differences in the non-specific binding were obtained with the various formats. TMOS was found to be more effective than tetrakis (2-hydroxyethyl)orthosilicate (THEOS) for entrapment of anti-levonorgestrel (LNG) Abs. However, aging the THEOS-based sol-gel for a few weeks at 4 °C stabilized the entrapped Abs and increased its binding capacity. Confocal fluorescent microscopy with fluorescein isothiocyanate (FITC) labeled immunoglobulines (IgGs) entrapped in the sol-gel matrix showed that the entrapped Abs were distributed homogenously within the gel. Scanning electron microscopy (SEM) images have shown the diverse structures of the various sol-gel formats and precursors. http://www.mdpi.com/1996-1944/4/3/469/ Fri, 25 Feb 2011 00:00:00 CET Materials 2011-02-25 4 3 Article 469 486 1996-1944 Sol-Gel Entrapped Levonorgestrel Antibodies: Activity and Structural Changes as a Function of Different Polymer Formats 2011-02-25 doi: 10.3390/ma4030469 Moran Shalev Altstein Miriam http://www.mdpi.com/1996-1944/4/2/448/ In this work we investigate the effect of iron oxide embedded in silica matrices as a function of Fe/Si molar ratio and sol pH. To achieve homogeneous dispersion of iron oxide particles, iron nitrate nonahydrate was dissolved in hydrogen peroxide and was mixed with tetraethyl orthosilicate and ethanol in a sol-gel synthesis method. Increasing the calcination temperature led to a reduction in surface area, although the average pore radius remained almost constant at about 10 Å, independent of the Fe/Si molar ratio or sol pH. Hence, the densification of the matrix was accompanied by similar reduction in pore volume. However, calcination at 700 °C resulted in samples with similar surface area though the iron oxide content increased from 5% to 50% Fe/Si molar ratio. As metal oxide particles have lower surface area than polymeric silica structures, these results strongly suggest that the iron oxides opposed the silica structure collapse. The effect of sol pH was found to be less significant than the Fe/Si molar ratio in the formation of molecular sieve structures derived from iron oxide silica. http://www.mdpi.com/1996-1944/4/2/448/ Thu, 17 Feb 2011 00:00:00 CET Materials 2011-02-17 4 2 Article 448 456 1996-1944 Iron Oxide Silica Derived from Sol-Gel Synthesis 2011-02-17 doi: 10.3390/ma4020448 Adi Darmawan Simon Smart Anne Julbe João Carlos Diniz da Costa http://www.mdpi.com/1996-1944/3/4/2815/ Monolithic materials have become very popular because of various applications, especially within chromatography and catalysis. Large surface areas and multimodal porosities are great advantages for these applications. New sol-gel preparation methods utilizing phase separation or nanocasting have opened the possibility for preparing materials of other oxides than silica. In this review, we present different synthesis methods for inorganic, non-silica monolithic materials. Some examples of application of the materials are also included. http://www.mdpi.com/1996-1944/3/4/2815/ Wed, 21 Apr 2010 00:00:00 CEST Materials 2010-04-21 3 4 Review 2815 2833 1996-1944 Sol-Gel Synthesis of Non-Silica Monolithic Materials 2010-04-21 doi: 10.3390/ma3042815 Gaweł Gaweł Øye http://www.mdpi.com/1996-1944/3/4/2567/ Low temperature methods have been applied to the synthesis of many advanced materials. Non-hydrolytic sol-gel (NHSG) processes offer an elegant route to stable and metastable phases at low temperatures. Excellent atomic level homogeneity gives access to polymorphs that are difficult or impossible to obtain by other methods. The NHSG approach is most commonly applied to the preparation of metal oxides, but can be easily extended to metal sulfides. Exploration of experimental variables allows control over product stoichiometry and crystal structure. This paper reviews the application of NHSG chemistry to the synthesis of negative thermal expansion oxides and selected metal sulfides. http://www.mdpi.com/1996-1944/3/4/2567/ Tue, 06 Apr 2010 00:00:00 CEST Materials 2010-04-06 3 4 Review 2567 2587 1996-1944 Novel Materials through Non-Hydrolytic Sol-Gel Processing: Negative Thermal Expansion Oxides and Beyond 2010-04-06 doi: 10.3390/ma3042567 Lind Gates Pedoussaut Baiz http://www.mdpi.com/1996-1944/3/4/2196/ The sol-gel technique is earning the worldwide attention of researchers in the field of material science, due to its versatility in synthesizing inorganic ceramic materials at mild conditions. High purity, homogeneity, controlled porosity, stable temperature and nanoscale structuring are the most remarkable features offered by this method for generating highly sensitive and selective matrices to incorporate analyte molecules. The crafting of sol-gel sensors through molecular imprinting has put great influence on the development of innovative chemical sensors, which can be seen from the growing number of publications in this field. The review provides a brief overview of sol-gel sensor applications, and discusses the contribution of molecular imprinting in exploring the new world of sensors. http://www.mdpi.com/1996-1944/3/4/2196/ Wed, 24 Mar 2010 00:00:00 CET Materials 2010-03-24 3 4 Review 2196 2217 1996-1944 Chemical Sensors Based on Molecularly Imprinted Sol-Gel Materials 2010-03-24 doi: 10.3390/ma3042196 Mujahid Lieberzeit Dickert