Special Issue "Sol-Gel Technique"

Guest Editor
Prof. Dr. Jay B. Benziger
Department of Chemical Engineering, Princeton University, Engineering Quad., Room A-407, Princeton, NJ 08544-5263, USA
Website: http://pemfc.princeton.edu/benziger.html
E-Mail:

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.

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 Materials

Submission

All papers should be submitted to ijms@mdpi.org. To be published continuously until the deadline and papers will be listed together at the special issue website.

Submitted papers should not have been published previously, nor be under consideration for publication elsewhere. All papers are refereed through a peer-review process. A guide for authors is available on the Instructions for Authors page. The International Journal of Molecular Sciences is an international peer-reviewed monthly journal published by Molecular Diversity Preservation International.

Open Access publication fees are 800 CHF per paper. English correction fees and/or formatting fees (250 CHF) will be added in certain cases (1050 CHF per paper for those papers that require extensive additional formatting and/or English corrections).

silica gels; aerogels; xerogels; optical fiber cladding; sol gel derived waveguides; abrasive resistant coatings or hybrid glass coatings; antireflective coatings; titania sol gel photocatalysts; aerogel windows; sol gel catalysts; adsorbents; ceramic foams; sol gel membranes; sol gel NOx sensors; colloidal gels; sol gel transition; self cleaning coatings; ceramic spin coating; ceramic dip coating; sol gel composite

Type of Paper: Review
Title: Photocatalytic Prepared by Sol-Gel Method for Vocs Removal-A Review
Authors: Ting-Ke Tseng and Hsin Chu *
Affiliations: Dept. of Environmental Engineering, National Cheng Kung University, Tainan 70101, Taiwan; * Author to whom correspondence should be addressed: E-mail: chuhsin@mail.ncku.edu.tw
Abstract: The sol-gel process is a wet-chemical technique (chemical solution deposition) widely used recently in the fields of materials science, ceramic engineering and especially for preparing the photocatalysts. Volatile organic compounds (VOCs) are prevalent components of indoor air pollution. Among the approaches to remove VOCs from indoor air, photocatalytic oxidation (PCO) is regarded as a promising method. This paper is a review of the status of research on Sol-gel method preparation of the photocatalysts and the PCO purification of VOCs in indoor air. The review and discussion will focus on the preparation and coating of various photocatalysts, reaction pathways, intermediates generated by PCO and an overview of various PCO reactors and their models described in the literatures.

Type of Paper: Article
Title: Whole Cell Imprinting in Sol-Gel Materials for Bacterial Recognition in Liquids (Macromolecular Fingerprinting)
Authors: Robert Armon *, Jeanna Starosvetsky and Tally Cohen
Affiliations: Faculty of Civil & Environmental Engineering, Division of Environmental, Water & Agricultural Eng‏., Technion-Israel Institute of Technology, Haifa 32000, Israel; * Author to whom correspondence should be addressed; E-mail: cvrrobi@tx.technion.ac.il
Abstract: Molecular imprinting in different polymers has been one of the main focuses in advanced materials sciences. The molecular imprinting nano-technology allows detection from molecules to macro scale organisms (yeast cells, viruses and bacteria) by simple use of a tailored material applied on a suitable transducer. The polymer layer is left with exact fingerprints of the molecule or organism in a honeycomb shape following removal of the “stamp”. Such a versatile polymer layer can be achieved through sol-gel technology of metal alkoxydes (siloxanes). This nano-technique can be developed further to differentiate among various bacterial species in spite of their close morphology (i.e. rod shape bacteria such as E. coli, Salmonella, Pseudomonas, etc.) by further improvement of the above technique involving bio-molecular tools. Our main working hypothesis was that imprinting of a specific bacterium previously treated with specific antibodies or lectins on their outer membrane, in order that when already imprinted, the antibodies or lectins will be left as specific anchors on the imprinted hallows. Preliminary results point toward a simpler solution in imprinting process in which outer membrane fractions are left in sol-gel cavity following bacterial cells release from sol-gel template. This feature will allow detecting on line specifically various bacterial pathogens with similar morphology but with different antigenic surface properties.

Type of Paper: Article
Title: Study on Laser Processing of Nanostructured Vanadium Oxides
Authors: S. Beke¹, L. Kőrösi², K. Sugioka¹ and K. Midorikawa¹
Affiliations: ¹ Laser Technology Laboratory, RIKEN–Advanced Science Institute, Hirosawa 2-1, Wako, Saitama 351-0198, Japan; E-Mails: beke@scientist.com (S.B.); ksugioka@riken.jp (K.S.); kmidori@riken.jp (K.M.)
2 Supramolecular and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, University of Szeged, Dóm tér 8, H-6720 Szeged, Hungary; E-Mail: L.Korosi@chem.u-szeged.hu (L.K.)
Abstract: V₂O₅ gels have been known and investigated since the end of the XIX. century. Interest in them started to grow in the 1980’s by the discovery of their semiconducting properties and their use in antistatic coatings in the photographic industry. The fast growing development of the sol-gel process brought new interest in V₂O₅ gels. Following a short summary of the vanadium oxides and the V₂O₅ gels, we report on laser processing of V₂O₅•nH₂O thin films prepared by sol-gel method. The nanocrystalline films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transmission electron microscopy (TEM) and Scanning Electron Microscopy (SEM). These characterization techniques revealed that the applied laser intensity is a key factor in the final composition of the vanadium oxide films. The original xerogel structure disappears and the films become completely polycrystalline in an orthorhombic structure. At the same time, the originally fibrillar-like particles disappear and irregular shaped, layer structured V₂O₅ particles are created. We made also a comparison between this technique and the pulsed laser ablation regarding the particle sizes and their influences on the quantum confinement effect. XPS spectra show that due to the laser radiation of the sol-gel films the O/V ratio increases when using higher intensities.
Keywords: vanadium oxide thin films; sol-gel process; pulsed laser ablation; laser radiation; X-ray diffraction; X-ray photoelectron spectroscopy; TEM; SEM; Bohr radius

Type of Paper: Article
Title: An in-Vitro Controlled Release Study of Valproic Acid Encapsulated within a Titania Ceramic Matrix
Authors: M. J. Uddin ^1,2,3,*, D. Mondal ⁴, C. A. Morris ³, U. Diebold ^2,* and R. D. Gonzalez ¹
Affiliations: ¹ Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA 70118, USA; E-Mail: juddin@tulane.edu (M.J.U.); ² Department of Physics and Engineering Physics, Tulane University, New Orleans, LA 70118, USA; E-Mail: diebold@tulane.edu (U.D.); ³ Department of Microbiology and Immunology, Tulane University, New Orleans, LA 70112, USA; ⁴ Department of Pharmacology, Tulane University, New Orleans, LA 70112
Abstract: This paper addresses quantitative kinetic parameters, which ensures a long term controlled release of an antiepileptic drug, such as valproic acid, directly release to damaged tissue in the temporal lobe of the brain. The purpose of this approach is to eliminate drug diffusion through the blood brain barrier (BBB) in order to decrease the drug dose required using systematic administration. The results of this in-vitro study are compared to previous in-vivo studies performed on animal models. Two types of studies were performed involving different approach, in which the concentration gradient between the implanted drug and the solution was varied. In the first study, a static approach was used. In this study, the encapsulated drug was directly released to a methanol solvent and the concentration of VPA in solution was maintained using massspectrometer-gas chromatography. These studies showed that adsorption-desorption equilibria were established following a period of approximately 400 hrs. In the second study, a dynamic approach was used in which the valproic acid was removed by changing the solution prior to each concentration measurement. This approach results in a maximum concentration gradient. Using this approach, maximum drug release was obtained following a period of 500 hrs. The exact in-vivo rate of release must follow a rate somewhere in between these two extremes. The synthesis of the titania ceramic implanted devices was performed using sol-gel methods in which varying amounts of valproic acid were added during the hydrolysis step of the synthesis. The resulting samples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and BET surface area analysis. The materials had high surface area (550-680 m²g^-1), and pore size distributions between 4-5 nm. Dynamic VPA release showed pseudo first order kinetics and was independent of drug loading.