Sol–gel derived organic–inorganic hybrid materials: synthesis,characterizations and applications

Organic/inorganic hybrid materials prepared by the sol–gel approach have rapidly become a fascinating new field of research in materials science. The explosion of activity in this area in the past decade has made tremendous progress in both the fundamental understanding of the sol–gel process and the development and applications of new organic/inorganic hybrid materials. Polymer-inorganic nanocomposite present an interesting approach to improve the separation properties of polymer material because they possess properties of both organic and inorganic such as good permeability, selectivity, mechanical strength, and thermal and chemical stability. Composite material derived by combining the sol–gel approach and organic polymers synthesis of hybrid material were the focus area of review It has also been demonstrated in this review that a more complete understanding of their structure–property behavior can be gained by employing many of the standard tools that are utilized for developing similar structure–property relationships of organic polymers. This review article is introductory in nature and gives introduction to composite materials/nanocomposite, their applications and the methods commonly employed for their synthesis and characterization. A brief literature survey on the polysaccharide templated and polysaccharide/protein dual templated synthesis of silica composite materials is also presented in this review article.     

Process optimization of sol–gel derived colloidal photonic crystals

Three-dimensional photonic bandgap structures have been synthesized by a colloidal/sol–gel route, starting with the self-organization of polystyrene microspheres into opals by dip-coating, sedimentation or vertical convective self-assembly, followed by sol–gel infiltration of the interstices with silica, titania or a silica-titania mixture, by dip-coating and removal of the polymeric template. The structural and optical properties of the opals and inverse opals prepared by this method have been studied by scanning electron microscopy and visible infra-red spectroscopies to assess the relationship between their structure and the photonic properties obtained. The optical transmission and reflection spectra of the opal and inverse opal structures have also been simulated by the Translight Software code, using the Transfer Matrix method, for different numbers of stacked layers, showing reasonable agreement with the experimental results. By optimizing the fabrication parameters, colloidal photonic crystals of good quality have been obtained, with reduced defect concentrations and increased mechanical strength.     

Catalytic activity and stability of laccase entrapped in sol–gel silica with additives

This study investigated the effects of different additives and precursors on the catalytic activity of laccase entrapped in sol–gel silica. It was found that the laccase catalytic activity and stability of sol–gel laccase could be enhanced if the entrapment was performed in the presence of additives such as PVA, PEG and APTS. The use of TEOS as a precursor showed slightly higher laccase catalytic activity compared to TMOS. The PVA as an additive showed a better catalytic activity enhancement compared to the PEG and APTMS with the optimum PVA concentration of 0.03 mg/mL. The optimal temperatures of sol–gel laccase without and with additives were found to be at 40 and 27°C, respectively. After 70 days of storage at 27°C, the catalytic activity of the immobilized sol–gel laccase with additives maintained its catalytic activity compared to only 30% of its original catalytic activity for the sol–gel laccase without additives.     

Oxalic acid sensors based on sol–gel nanostructured TiO<Subscript>2</Subscript> films

Titanium (IV) oxide semiconducting layers were prepared by means of the templated sol–gel method and deposited on conductive ITO substrates. The films were described by a series of techniques involving X-ray diffraction (XRD), Raman spectroscopy, X-ray reflectivity (XRR), atomic force microscopy (AFM), scanning electron microscopy (SEM) and ultraviolet–visible spectroscopy (UV–Vis). The photo-excitation properties of the films were characterized by electrochemical tests and evaluated from the obtained polarization curves. The generated photocurrents were measured in the presence of the hole-scavengers—oxalic acid and formic acid in the electrolyte. It was shown that especially in the case of oxalic acid the developed system can be used as an efficient and simpler concentration sensor. The relationship between values of the generated photocurrent and the layers’ thicknesses was also investigated.     

Optimization in the immobilization of penicillin G acylase by entrapment in xerogel particles with magnetic properties

Biocatalysis presents a sound alternative to chemical synthesis in the field of drug production, given the highly selective nature of biological catalysts. Penicillin G Acylase (PGA) from E. coli is currently used to hydrolyze penicillin G (PG) and catalyzes the synthesis of β-lactam antibiotics. In this work, particular emphasis is given to recent developments in penicillin G acylase immobilization, by entrapment simultaneously with nano-magnetic particles in a silica matrix. The sol–gel biocatalytic particles were prepared either by a conventional method (crushed powder) or by a more recent approach, based in an emulsion system using 150 mM AOT/isooctane, which allowed for the formation of spherical micro- and nanobeads. The effects on PGA activity of different sol–gel precursors, additives, enzyme concentration, aging, drying conditions and mechanical stability were evaluated. After these optimization studies, a mechanically stable carrier based on porous xerogels silica matrixes, starting from tetramethoxysilane (TMOS) with 65–67% PGA activity yield in these carriers allowed an immobilization yield of 74 mg protein g_{dry sol–gel}⁻¹ and 930 Ug_{dry sol–gel}⁻¹ for specific activity were obtained.     

Probing the dynamics of domain III of human serum albumin entrapped in sol–gel derived silica using a Sudlow’s site II specific fluorescent ligand

Previous studies from our lab reported on the use of time-resolved fluorescence anisotropy (TRFA) to probe the dynamics of domains I and II within the model protein, human serum albumin (HSA), in solution and when entrapped into sol–gel derived silica. In order to further our understanding of the dynamics within this multi-domain protein, TRFA was used to measure the dynamics of domain III of the protein. For this purpose, the fluorescence ligand dansylsarcosine (DS), which has a 400-fold higher emission intensity in the bound state relative to the free state and an emission lifetime of >22 ns when bound to Sudlow’s site II (domain III) in HSA, was selected. This probe is able to accurately report on slow rotational motions (up to 300 ns correlation time) and the bound form of the probe can be selectively measured at 475 nm, ensuring that the dynamics reflect only the properly folded form of the protein. The mobility of HSA with bound dansylsarcosine (HSA–DS) was evaluated in solution and after entrapment in sol–gel derived silica prepared from sodium silicate under varying ionic strength and pH conditions. The results here show that (1) the 43 ns global rotational correlation time of HSA in buffered solution can be accurately measured via labeling with DS with no interference from faster local or segmental motions; (2) the global motion of HSA in silica is greatly hindered immediately after encapsulation, with no correlation time faster than 300 ns discernable, indicative of strong templating of the silica around domain III of the native protein; and (3) the addition of salt and variation of pH have essentially no effect on HSA mobility, ruling out electrostatics as the primary interaction restricting HSA motion. The results from this study are compared to past studies using intrinsic tryptophan fluorescence (domain II) or fluorescein-labeled HSA (domain I), and demonstrate that motion observed using such probes likely reflects differential mobility of the three domains, consistent with domain III of HSA adsorbing to or templating with silica upon entrapment while the other domains protrude into the pore. Restricted motion of domain III of HSA was also observed in silica materials derived from diglycerylsilane or tetraethylorthosilicate, showing that templating is not dependent on the silica precursor or processing conditions.     

Effect of sol–gel hydrophobicity on the distribution and structure of different proteins in organically modified sol–gel thin films

Although the use of silica sol–gels for protein entrapment has been studied extensively our understanding of the interactions between the immobilization matrix and the entrapped biomolecules is still relatively poor. Non-invasive in situ spectroscopic characterization is a promising approach to gain a better understanding of the fundamentals governing sol–gel immobilization of biomolecules. This work describes the application of Fourier transform infrared (FTIR) microscopy to determine the influence of modifying the sol–gel hydrophobicity, by varying the content of the organically modified precursor propyltrimethoxysilane (PTMS), on the distribution and structure of three model proteins (lysozyme [EC 3.2.1.17], lipase [EC 3.1.1.3] and bovine serum albumin (BSA)) in silica sol–gel thin films. FTIR analysis of the overall immobilized protein positional distribution showed a Gaussian type distribution. FTIR microscopic mapping however, revealed that the spatial distribution of proteins was heterogeneous in the sol–gel thin films. When this positional information provided by FTIR microscopy was taken into account, areas of high protein concentration (clusters) were found and were not found to be homogeneously distributed. The shape of these clusters was found to depend on the type of protein entrapped, and in some cases on the composition of the sol–gel. Positional analysis of the distribution of the organically modified precursor PTMS in relation to the protein distribution was also conducted. The localized concentration of PTMS was found to positively correlate with the protein concentration in the case of lipase and negatively correlate in the case of lysozyme and BSA. These results indicate that lysozyme and BSA concentration was higher in areas of low hydrophobicity, while lipase concentration was higher in areas of high hydrophobicity within the sol–gel. Additionally, as determined by peak shape analysis of the amide I peak a higher PTMS content appeared to conserve protein structure in high concentration clusters for lipase. In contrast, lysozyme and BSA, appeared to retain their structure in high concentration clusters better at lower PTMS contents. A hypothesis speculating on the nature of the hydrophobic/hydrophilic interactions between the proteins and the sol–gel domains as the reason for these differences is presented.     

FT-IR microscopy characterization of sol–gel layers prior and after glucose oxidase immobilization for biosensing applications

Micro-Attenuated Total Reflection (ATR) Fourier Transform Infrared spectroscopy was used to investigate sol–gel layers for biosensing applications prior and after glucose oxidase (GOD) immobilization. The changes occurring in sol–gel infrared spectrum after GOD immobilization were clearly evidenced confirming the retaining of the enzyme activity. Moreover, micro-ATR experimental technique allowed us to investigate the spatial distribution of enzyme concentration. The non-destructive nature of our approach also enabled to monitor the time stability of sol–gel layers and of embedded GOD. The temporal evolution of some peaks in infrared spectra of these sol–gel layers was compared with absorption and steady-state fluorescence measurements. The results reported here confirm that micro-ATR infrared spectroscopy can be usefully employed for a non- or minimally invasive detailed characterization of supports for enzyme immobilization.     

Fractionation of nano- and microparticles in a rotating conoidal coiled column

A method for the continuous-flow fractionation of particles in a transverse centrifugal field in a rotating conoidal coiled (RCC) column has been developed. A model of a planetary centrifuge with a conoidal drum of a special construction has been tested. The effects of the rotation and revolution speed of the conoidal RCC, as well as the direction and pumping rate of the mobile phase on the behavior of particles smaller than 1 μm have been studied. The conditions have been selected and optimized for the retention and elution of spherical particles of the ‘150 nm’, ‘400 nm’, and ‘900 nm’ standard samples of silica gel (Polyscience Inc.). The possibility of using RCC for the analysis and production of monodisperse standard particle samples has been demonstrated. In particular, the ‘900 nm’ particles have been separated from admixtures of small (100–200 nm) particles and nonspherical 1–2 μm particles present in the sample. The separated fractions have been characterized by electron microscopy.     

Materials doping through sol–gel chemistry: a little something can make a big difference

Several examples of sol–gel preparation of doped materials are taken to illustrate the various situations where the doping elements are responsible for the main function of the material or govern its structure. Other examples are used to illustrate that sometimes unexpected effects can be observed like structural modification and the appearance of new properties. Rare earth doped scintillators demonstrate higher homogeneity for materials prepared via sol–gel chemistry when compared with classical solid state reaction. The XRD study of rare earth doped orthoborates shows that doping can affect the vaterite to calcite phase transition observed in these compounds. A Raman spectroscopic study has been performed on doped silica xerogels and it has been shown that doping ions can modify greatly the densification process in these amorphous materials. Finally, it has been evidenced that sol–gel chemistry allows the preparation of bioactive ceramics with enhanced properties. In particular Zn-doped HAP with anti inflammatory properties has been prepared and Sr-doped bioactive glasses have demonstrated superior in-vitro bioactivity as evidenced by PIXE-RBS study.     

Preparing undoped and Mn-doped ZnO nanoparticles: a comparison between sol–gel and gel-combustion methods

Undoped and Mn doped ZnO nanoparticles were synthesized by two wet chemical techniques: sol–gel and gel-combustion. We were able to prepare Mn-doped ZnO nanoparticles free from the second phases at calcining temperatures 400 and 500 °C using sol–gel and gel combustion, respectively. Complete crystallization occurs in both methods, but it is found that the crystallization is better performed in the ZnO based solid solution prepared by the sol–gel method. TEM images show that the average size of the nanoparticles synthesized by gel-combustion is smaller than that of prepared by sol–gel method. Optical characterizations such as vibrational properties, lattice dynamical parameters, absorption edges and optical band gap energies were also carried out by FTIR and UV–Vis spectroscopies. The quantitative estimations led to the conclusion that the effects of the method on the particle size and the optical band gap energy of the prepared samples are very significant.     

Bio-hybrid materials for immunoassay-based sensing of cortisol

Sol–gel encapsulation has been used as the basis for detecting cortisol by an immunoassay approach. Previous research showed that antibodies immobilized in the pores of a sol–gel derived silica were able to bind cortisol and be used as an immunosensor. However, this approach was not effective when measuring cortisol levels in human serum because of interference from other fluorescence sources. The present paper describes a protocol which overcomes these limitations and enables sol–gel immunoassays to effectively measure cortisol in human serum over the physiological range of cortisol blood concentrations in an adult (2–28 μg/dL). The method involves a standard additions approach in which various amounts of cortisol are added to the serum. The cortisol concentration values obtained with our sol–gel immunoassay were typically within 10% of the values obtained by traditional analytical methods. The protocol presented here represents a significant contribution to sol–gel sensing and immunoassays in particular, because of the ability to detect an analyte in human serum. In addition, this work reports the first comparison between results from a sol–gel immunosensor and an alternative immuno-binding method for analyte detection.     

Sol–gel one-pot synthesis in soft conditions of mesoporous silica materials ready for drug delivery system

The present work reveals a new and simple strategy, a one-step sol–gel procedure, to encapsulate a low water-soluble drug in silica mesostructured microparticles and to improve its release in physiological media. The synthesis of these new materials is based on the efficient solubilisation of a poorly water-soluble drug in surfactant micelles (Tween 80, a pharmaceutical excipient) which act as template for the silica network. A strict control of the sol–gel process and the parameters procedure in soft conditions (concentration, pH, temperature) was applied to reach the solubilisation limit of the drug in the micellar solution so as to optimise its encapsulation. Even if this one-pot procedure could appear limited by the low drug loading, it could provide an interesting alternative for the formulation of many recent highly active but very poorly soluble drugs.     

PVA-Based Sol–Gel Synthesis and Characterization of CdO–ZnO Nanocomposite

CdO–ZnO nanocomposite was fabricated by a sol–gel pyrrolysis method based on the poly vinyl alcohol (PVA) polymeric network. The prepared nanocomposite was carefully characterized using scanning electron microscopy, X-Ray dispersive energy analysis, ICP-atomic emission spectroscopy, X-Ray diffraction, transmission electron microscopy and UV–visible spectroscopy. The structure, composition, and morphology of this composite depend on a number of aspects: the amounts of cadmium salt, zinc salt, and PVA in the initial solution, the solvent composition, and the pyrrolysis temperature. The obtained results showed that the nanocomposite had excellent linear nanoclusters created from nanograins. Each nanograin was made of a CdO core, completely covered by ZnO layers. Total diameter of each nanograin was 70–90 nm.     

Silica covered porphyrin microstructures obtained in sol–gel processes

In this article, we report on the formation of well-defined highly emissive silica-covered porphyrin microstructures in base-catalyzed sol–gel processes. The microstructures were obtained by self-assembly of 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin (mTHPP) at room temperature. Tetraethoxysilane (TEOS) was used as a silica precursor. The hybrid mTHPP- silica particles were characterized by means of reflectance UV–Vis and microscopy techniques including atomic force microscopy, scanning electron microscopy (SEM) and confocal fluorescence microscopy (CM). The SEM and TEM observations revealed that depending on the porphyrin concentration used in the synthesis, the shape of the hybrid mTHPP-silica particles has changed from ribbon-like (c _mTHPP = 2.09 mM) to rhombus-like structures (c _mTHPP = 4.35 mM). The ribbons were straight-edged, uniform in width (1.2–1.8 μm) and height (350–400 nm), and variable in length (40–100 μm). The rhombs were 1–3.7 μm in height, 7–25 μm in length, and 3.5–15 μm in width, and the ratio of length to width was uniform and equal to ca. 1.8–2. UV–Vis absorption spectra indicated that the J-aggregates and H-aggregates formed in the systems with lower and higher porphyrin content, respectively. Formation of different type of porphyrin aggregates in both systems resulted in different emission spectra, as it was shown with CM.     

Microencapsulation of an acrylate monomer in silica particles by sol–gel process

The sol–gel synthesis strategies combined with the templated growth of organic–inorganic hybrid networks provide access to an immense new area of innovative multi-functional advanced materials. One possible way to prepare such new advanced materials is to encapsulate liquid active agents (such as monomers, dyes, catalysts and hardeners) in microcapsules. Silica microcapsules of tetraethylortosilicate (TEOS) and 3-(trimethoxysilyl)propyl methacrylate (MPTS) were prepared in a precursor-monomer/NH₄OH water microemulsion system. Trimethylolpropane triacrylate (TMPTA)—a trifunctional monomer useful in manufacturing of coatings, inks and adhesives—and a corresponding photoinitiator (DAROCUR 1173) were entrapped inside the obtained microcapsules. MPTS was used to increase compatibility between TMPTA and the sol–gel precursors. As stability agent we added a “home made” product resulted from functionalization of poly (ethylene glycol) methyl ether (MPEG) with (3-isocyanatopropyl) triethoxysilane (NCOTEOS). Were obtained microcapsules containing incorporated monomer and having average particle size in range of 0.5–50 μm. Thermal analysis, morphology study and the increase of the silica microcapsules average diameter, measured by DLS technique confirm the monomer encapsulation.     

Detection of thrombin-specific oligonucleotides using quartz crystal microbalances coated with silica sol–gel glass thin films

A silica sol–gel glass containing thrombin was spin-coated on a quartz crystal and used to measure the binding of thrombin with thrombin-specific aptamers. The porous structures of the silica thin films were optimized to facilitate binding of the aptamers with thrombin by varying the solution pH and the concentration of polyethylene glycol. The hydrophilic silica sol–gel glass prevented aptamers from adsorbing nonspecifically and the absence of covalent bonding allowed the entrapped thrombin molecules to retain their reactivity. Further, the mass sensitivity of the quartz crystal microbalance was enhanced by encapsulating more thrombin molecules inside the porous sol–gel glasses.     

Influence of cerium concentration on the structure and properties of silica-methacrylate sol–gel coatings

The aim of this work was to study the effect of the incorporation of cerium nitrate in a silica-methacrylate sol–gel hybrid matrix reinforced with silica nanoparticles. Sols, coatings and powders have been studied, focusing specially in the determination of the redox ratio Ce³⁺/Ce⁴⁺ and films structure. Sols have been characterised using viscosity measurements and FT-IR spectroscopy, and powders and coatings obtained with different Ce contents through UV–Vis and FT-IR spectroscopy, TGA, TEM, AFM and FE-SEM. The goal was to reach the best compromise between maximum cerium concentration and coating stability to better understand the mechanisms acting in active anti-corrosive processes.     

Evaluation of Polycation-Stabilized Lactate Oxidase in a Silica Sol–Gel as a Biosensor Platform

 Whereas glucose oxidase and related proteins are encapsulated readily in silica sol–gels, α-hydroxy enzymes such as lactate oxidase (LOx), are reported to be damaged by electrostatic interaction with these matrices. Based on a previous report, poly(ethyleneimine), PEI, was evaluated as a protecting compound under conditions suited to analytical measurements. With LOx and PEI co-encapsulated in a silica sol–gel, the enzyme retained 62% of its initial activity after 20 days. In the absence of PEI, activity was lost during the processing. Batch analytical measurements with enzyme-doped sol–gel yielded a linear response over the range 0.5–2.0 mM lactate and a detection limit of 0.03 mM lactate. Both simple incorporation of LOx in a silica sol–gel and an alternative protection method, blocking the ion-exchange sites on silica with La(III), failed. These negative results supported the hypothesis that the efficacy of PEI was related to its formation of a protective sheath around the enzyme. Author for correspondence. E-mail: coxja@muohio.edu Received July 29, 2002; accepted December 15, 2002 Published online May 19, 2003     

Improved silica xerogel film processing for MALDI-TOF–MS quantitative analysis of peptides and small molecules

In this work, sol–gel derived silica films were prepared for direct desorption/ionization of organic compounds in MALDI-TOF–MS analysis with the aim of improving method precision and of reducing interfering signals at low m/z values. Two commonly used MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA), were incorporated into the sol–gel network in order to absorb laser energy and to induce analyte desorption/ionization with low or absent background signals in the mass spectra. To achieve a reproducible xerogel film formation, experimental parameters for its deposition were optimized. The gel matrices were characterized by Fourier Transform Infrared (FT-IR) spectroscopy, X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) analysis. The results proved the embedding of the matrix molecules in a disperse form into the homogeneous sol–gel material. The sol–gel matrix was then tested for the qualitative and quantitative analysis of two reference peptides, such as Bradykinin and P₁₄R. In addition, spectral quality and method performance were assessed for quantitation of melamine, a low-molecular weight compound of food safety concern. In all cases, high quality spectra and excellent mass accuracy (between 3.5 and 13 ppm) were observed. Furthermore, the experimental results evidenced a significant improvement of the measurement repeatability on spot and between spots (relative standard deviation <10%), with respect to the traditional dried-droplet sample deposition method. Good sensitivity and linearity in the concentration range explored were obtained for peptides and melamine, demonstrating the suitability of the sol–gel-based matrix to be used for quantitative analysis.