Silica Sol-gel Materials Research Articles

Effect of Sol-Gel Silica Matrices on the Chemical Properties of Adsorbed/Entrapped Compounds.

The sol-gel process enables the preparation of silica-based matrices with tailored composition and properties that can be used in a variety of applications, including catalysis, controlled release, sensors, separation, etc. Commonly, it is assumed that silica matrices prepared via the sol-gel synthesis route are "inert" and, therefore, do not affect the properties of the substrate or the catalyst. This short review points out that porous silica affects the properties of adsorbed/entrapped species and, in some cases, takes an active part in the reactions. The charged matrix affects the diffusion of ions, thus affecting catalytic and adsorption processes. Furthermore, recent results point out that ≡Si-O. radicals are long-lived and participate in redox processes. Thus, clearly, porous silica is not an inert matrix as commonly considered.

Determination of hazardous vapors from the thermal decomposition of organochlorinated silica xerogels with adsorptive properties

The incorporation of organic groups into sol-gel silica materials is known to have a noticeable impact on the properties and structure of the resulting xerogels due to the combination of the properties inherent to the organic fragments (functionality and flexibility) with the mechanical and structural stability of the inorganic matrix. However, the reduction of the inorganic content in the materials could be detrimental to their thermal stability properties, limiting the range of their potential applications. Therefore, this work aims to evaluate the thermal stability of hybrid inorganic-organic silica xerogels prepared from mixtures of tetraethoxysilane and organochlorinated triethoxysilane precursors. To this end, a series of four materials with a molar percentage of organochlorinated precursor fixed at 10%, but differing in the type of organic group (chloroalkyls varying in the alkyl-chain length and chlorophenyl), has been selected as model case study. The gases and vapors released during the thermal decomposition of the samples under N2 atmosphere have been analyzed and their components determined and quantified using a thermogravimetric analyzer coupled to a Fourier-transform infrared spectrophotometer and to a gas chromatography-mass spectrometry unit. These analyses have allowed to identify up to three different thermal events for the pyrolysis of the organochlorinated xerogel materials and to elucidate the reaction pathways associated with such processes. These mechanisms have been found to be strongly dependent on the specific nature of the organic group.

Design and fabrication of a new sol-gel based sensor for the visual detection of oxalate and investigation of the effect of boiling on the oxalate content of some foods

Sol–gel chemistry is a unique method for constructing high-performance flexible sensors. Sol-gel silica matrices doped with a controlled amount of sensing agent (dopant) lead to the fabrication of suitable sensors. According to this knowledge, we decided to describe the development and performance characteristics of the sol-gel glasses with a nanoporous structure as the novel naked eye sensor for fast detection of oxalate based on the incorporation of indole inside pores of the silica matrix obtained by sol–gel technology. Several sols were made with different water: alkoxide ratios and pHs to design a highly sensitive and selective thin film without dopant leaching. The Field emission scanning electron microscopy (FE-SEM) technique was employed to evaluate the surface morphology and porosity growth. Also, the sol–gel films were characterized by Fourier-transform infrared (FTIR) spectroscopy. The optimum performance in terms of sensitivity, leaching, and response time was achieved using water: alkoxide ratio of 4:1, water acidity (HCl concentration) of 0.01 M, and ethanolic solution of Indole 0.01 mol L-1. The calibration plot was obtained within 0.01–6.00 μg mL−1 of oxalate with a detection limit value of 0.005 μg mL−1. Meanwhile, the prepared sensor exhibited excellent and acceptable RSDs for both inter-day and intra-day precision. The present sensor was used to measure oxalate in various samples and investigate the effects of the celeryʼs cooking time and the brewing duration of tea on oxalate content.

Revisiting carboxylic group functionalization of silica sol-gel materials.

The carboxylic chemical group is a ubiquitous moiety present in amino acids, a ligand for transition metals, a colloidal stabilizer, and a weak acidic ion-exchanger in polymeric resins and given this property, it is attractive for responsive materials or nanopore-based gating applications. As the number of uses increases, subtle requirements are imposed on this molecular group when anchored to various platforms for the functioning of an integrated chemical system. In this context, silica stands as an inert and multipurpose platform that enables the anchoring of multiple chemical entities combined through several orthogonal synthesis methods on the interface. Surface chemical modification relies on the use of organoalkoxysilanes that must meet the demand of tuned chemical properties; this, in turn, urges for innovative approaches for having an improved, but simple, organic toolbox. Starting from commonly available molecular precursors, several approaches have emerged: hydrosilylation, click thiol-ene additions, the use of carbodiimides or the reaction between cyclic anhydrides and anchored amines. In this review, we analyze the importance of the COOH groups in the area of materials science and the commercial availability of COOH-based silanes and present new approaches for obtaining COOH-based organoalkoxide precursors. Undoubtedly, this will attract widespread interest for the ultimate design of highly integrated chemical platforms.

Investigation of Hybrid Films Based on Fluorinated Silica Materials Prepared by Sol–Gel Processing

In this research, fluorinated silica materials were prepared through sol–gel processing with tetraethylorthosilicate (TEOS), triethoxymethylsilane (MTES), and trimethoxyhexadecylsilane (HDTMES), using a fluorinated solution (FS) under acidic medium. The fluorinated solution (FS) was obtained by diluting the perfluorooctanoic acid (PFOA) in 2-propanol. These fluorinated sol–gel silica materials were placed on the glass surfaces in order to achieve the antireflective and hydrophobic fluorinated hybrid films. The structure and surface properties of the final samples were investigated by Fourier transform infrared spectroscopy (FTIR), ultraviolet/visible spectroscopy, thermogravimetric analysis (TGA), atomic force microscopy (AFM), and contact angle (CA) determinations. FTIR spectra demonstrated the presence of a silica network modified with alkyl and fluoroalkyl groups. Thermal analysis showed that the fluorinated sol–gel silica materials prepared with HDTMES have a good thermostability in comparison with other samples. Ultraviolet/visible spectra indicated that the fluorinated hybrid films present a reflectance of ~9.5%, measured at 550 nm. The water contact angle analysis found that the wettability of fluorinated hybrid films was changed from hydrophilic (64°) to hydrophobic (~104°). These hybrid films based on fluorinated sol–gel silica materials can be useful in various electronics and optics fields.

High porosity silica aerogel thin film to monitoring vanadium: Synthesis, characterization and implementation

Sol–gel chemistry is a very unique method for constructing high-performance flexible sensors. Sol-gel silica matrices doped with a controlled amount of sensing agent (dopant) lead to the fabrication of suitable sensors. According to this knowledge, we decided to describe the development and performance characteristics of the sol-gel glasses with a nanoporous structure as novel naked eye sensor for fast detection of vanadium (V) based on physical entrapment of 4-(2-pyridylazo) resorcinol inside pores of the silica matrix obtained by sol–gel technology. To design a highly sensitive and selective thin film without dopant leaching, several sols were made with different water: alkoxide ratios and pHs. In summary, this study revealed that the characteristics of porous matrix as well as its average pore size could play significant roles in almost all critical parameters affecting the sensor performance, including sensitivity, selectivity, and response time. The FE-SEM technique equipped with EDAX was also employed to evaluate the surface morphology, porosity growth, and elemental identification of the thin films. The calibration plot was obtained within 0.004–3.000 μg mL−1 of vanadium (V) with a detection limit value of 0.0014 μg mL−1.

Tissue engineering using scaffolds for bone reconstruction: a review of sol-gel silica materials for bone morphogenetic proteins (BMP) encapsulation and release

Tissue engineering using scaffolds for bone reconstruction: a review of sol-gel silica materials for bone morphogenetic proteins (BMP) encapsulation and release

Effect of Modified Silica Materials on Polyvinyl Chloride (PVC) Substrates to Obtain Transparent and Hydrophobic Hybrid Coatings

In this research, we report a simple and inexpensive way to prepare transparent and hydrophobic hybrid coatings through deposition of different silica materials on polyvinyl chloride (PVC) substrates. The silica materials were prepared using an acid-catalyzed sol–gel method at room temperature (25 ± 2 °C), using alkoxysilanes: tetraethoxysilane (TEOS), as the silica source, and ethoxydimethylvinylsilane (DMVES), triethoxyoctylsilane (OTES), and trimethoxyhexadecylsilane (HDTMES), as modifier agents. The obtained materials were characterized (either as powders or as thin films) by Fourier-transform infrared spectroscopy (FTIR), UV/Vis spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and water contact-angle measurements. UV/Vis spectra showed that the PVC substrate coated with the silica material containing TEOS/DMVES/OTES had a transmittance of about 90% in the wavelength range of 650–780 nm. The water contact angles increased from 83° for uncoated PVC substrate to ~94° for PVC substrates coated with the sol–gel silica materials. These PVC films with hybrid silica coatings can be used as the materials for outdoor applications, such as energy-generating solar panel window blinds or PVC clear Windmaster outdoor blinds.

Strategy to enhance catalytic activity and stability of sol–gel oxidoreductases

Oxidoreductases are widely recognized for their capability to degrade phenolic pollutants and versatile. However, the lack of enzyme stability makes this technique unrealistic for industrial applications. In order to enhance their catalytic activity, stability and reusability, oxidoreductases namely laccases and peroxidases were entrapped in sol–gel silica and their catalytic activities were measured by an enzymatic assay using 2,6-dimethoxyphenol and guaiacol as substrates, respectively. The sol–gel silica matrices acted as a polymeric framework around the enzyme is a promising tool for improving enzyme stability. After entrapment, the catalytic activity and stability of sol–gel laccase and peroxidase toward pH, temperature and storage duration remarkably enhanced.

Ecological formulation for improving resveratrol stability and release in aqueous environment

Resveratrol (RES) is a naturally occurring product with numerous biological activities. Despite its potential benefits, its use is limited due to its low aqueous stability and solubility in its native form. The porous sol–gel silica materials which are able to entrap different organic molecules represent new studied release carriers. The aim of this work was to generate a solid matrix to encapsulate RES ensuring protection, increased solubility and release in solutions. A non-toxic ingredient, namely β-cyclodextrin (β-CD), able to form inclusion complexes (ICs) with RES has been used. Ecological formulations have been processed by entrapping the RES containing ICs in silica matrices obtained from a silica colloidal sol by the aqueous route of the sol–gel method. Characterization methods (DSC, FTIR, UV–Vis, fluorescence studies, SEM) have evidenced the presence of RES–β-CD inclusion complex in the silica powder, RES stability in the matrix and its release in aqueous and organic solutions, and the morphology of the carrier. An evaluation of the antioxidant activity of RES in the present formulation was performed by the chemiluminescence assay and RES release profile in aqueous solutions was obtained by HPLC–MS. The resulted materials can find applications in different domains.

Sol-gel coatings made using methyl-modified alkoxysilanes: The balance between protection and bioactivation

The reported osteogenic properties of the hybrid silica sol-gel materials make these compositions perfect candidates for bone tissue engineering applications. The aim of this study was the synthesis and characterisation of hybrid silica coatings, obtained using mixtures of tetraethyl orthosilicate (TEOS) and three different methyl-modified alkoxysilanes: trimethoxymethylsilane (MTMS), dimethyldiethoxysilane (DMDES) or polydimethylsiloxane (PDMS). A comparison of the properties of these materials can reveal the best candidate for the coatings on metallic prostheses. After optimising the synthesis parameters, the developed coatings were characterised using Fourier transform infrared spectrometry (FT-IR), 1H and 29Si solid-state nuclear magnetic resonance (1H-NMR and 29Si-MNR), cross-cut tests, scanning electron microscopy (SEM), contact angle measurements, optical profilometry, hydrolytic degradation tests and electrochemical corrosion analysis. Homogeneous and well-adhering coatings were obtained using the three methyl-modified reagents. However, different degrees of protection against corrosion, different hydrophilicity and varying degradation kinetics were observed for different precursors. The MTMS-based coating showed the highest hydrophilicity and degradation kinetics; these properties can be associated with increased bioactivity (Si release). In contrast, the PDMS and DMDES-based coatings showed augmented resistance to corrosion and lower permeability to water and, consequently, improved protection of metallic surfaces. From the physicochemical point of view, all these materials displayed interesting characteristics, relevant for coatings to be used in biomedical applications.

Self-encapsulation and controlled release of recombinant proteins using novel silica-forming peptides as fusion linkers

Recently, the potential use of biomimetic silica as smart matrices for the auto-encapsulation and controlled release of functional proteins has gained increased interest because of the mild synthesis conditions. Inspired by biological silicification, in this study, we studied novel silica-forming peptides (SFPs), Volp1 and Salp1, to mediate the generation of silica hybrids in vitro. The fusion of SFPs to model fluorescent proteins directed their auto-encapsulation in wet sol-gel silica materials. Furthermore, the SFPs served as affinity linkers for the immobilization of recombinant proteins in silica. Interestingly, the SFP fusion proteins modulated silicic acid polycondensation and allowed for the self-immobilization of SFP fusion proteins in two distinct silica formulations depending on the ionic strength—precipitated silica particles or wet silica gel. The controlled release of Salp1/Volp1 fusion proteins from silica matrices was significantly greater than that of the silaffin R5 fusion proteins. Subsequently, we showed that multiple SFP-tagged proteins homogenously entrapped within a silica matrix could be separately released following pre-incubation with different concentrations of l-arginine solution. These new findings provide a simple and reproducible route for silica hybrid formation for in situ stable auto-encapsulation and the sustained release of recombinant proteins with potential applications in biotechnology.

Effects of TiO2 crystal structure on the luminescence quenching of [Ru(bpy)2(dppz)]2 +-intercalated into DNA

The intercalation of [Ru(bpy)2(dppz)]2+ labeled as Ru(II) (bpy=2,2′-bipyridine and dppz=dipyrido[3,2,-a:2′,3′-c]phenazine) into herring sperm DNA leads to the formation of emissive Ru(II)-DNA dyads, which can be quenched by TiO2 nanoparticles (NPs) and sol-gel silica matrices at heterogeneous interfaces. The calcinations temperature exhibits a remarkable influence on the luminescence quenching of the Ru(II)-DNA dyads by TiO2 NPs. With increasing calcinations temperature in the range from 200 to 850°C, the anatase-to-rutile TiO2 crystal structure transformation increases the average particle size and hydrodynamic diameter of TiO2 and DNA@TiO2. The anatase TiO2 has the stronger ability to unbind the Ru(II)-DNA dyads than the rutile TiO2 at room temperature. The TiO2 NPs and sol-gel silica matrices can quench the luminescence of the Ru(II) complex intercalated into DNA by selectively capturing the negatively DNA and positively charged Ru(II) complex to unbind the dyads, respectively. This present results provide new insights into the luminescence quenching and competitive binding of dye-labeled DNA dyads by inorganic NPs.

Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

In present work, the optical and structural properties of silica sol-gel glasses and glass-ceramic materials singly- and doubly-doped with Eu3+ and Gd3+ ions were investigated. The optical properties of studied systems were determined based on absorption, excitation and emission spectra as well as luminescence decay analysis. Conducted studies clearly indicated a significant enhancement of visible emission originated from Eu3+ ions as a result of changing the excitation mechanism, via Gd3+→Eu3+ energy transfer. The luminescence intensity R-ratio was analyzed before and after heat-treatment process upon excitation at γex=393 nm and γex=273 nm. Moreover, the influence of excitation wavelength on luminescence decay time of the 5D0 excited state was also analyzed. The Gd3+→Eu3+ energy transfer efficiencies for precursor and annealed samples were calculated based on luminescence lifetime of the 6P7/2 level of Gd3+ ions. The X-ray diffraction measurements were conducted to verify the nature of obtained sol-gel materials. In result, the formation of orthorhombic GdF3 nanocrystal phase dispersed in amorphous silica glass host was identified after annealing. Obtained results clearly indicated an incorporation of Eu3+ activators into formed GdF3 nanocrystals. Thus, conducted heat-treatment process led to considerable changes in surrounding environment around Eu3+ ions. Actually, it was found that energy transfer phenomenon and heat-treatment process were responsible for significant improvement of Eu3+ luminescence in studied sol-gel samples.

Luminescent properties of composite scintillators based on PPO and o-POPOP doped SiO2 xerogel matrices

New composite scintillation detectors were obtained by incorporation of PPO and o-POPOP organic scintillators into porous sol–gel silica matrices. Composites possess high photoluminescence intensity and decay time in nanosecond range. The absolute light yield of composite scintillators at excitation by alpha-radiation is about 4000–5000 photons/MeV and the pulse–height resolution is about 30%. The investigations of time-resolved luminescence of composites performed under excitation by synchrotron radiation in the 3.7–25eV range have shown that the non-radiative energy transfer between host matrix and dopant molecules occurs via singlet states of SiO2 oxygen-deficient centers.

Structural and optical properties of Eu3+/Gd3+ ions in silica xerogels and powders obtained by sol–gel method

The xerogels and sol–gel powders with different quantitative composition have been studied based on excitation and emission measurements as well as luminescence decay analysis. The structure of prepared silica sol–gel materials was examined using FT-IR spectroscopy technique. The photoluminescence spectra of Eu3+ ions were registered upon two different excitation wavelengths related to direct excitation of Eu3+ ions (λexc = 393 nm) and indirect excitation through energy transfer process from Gd3+ to Eu3+ (λexc = 273 nm). Upon direct excitation of Eu3+ (7F0 → 5L6 transition) the characteristic emission bands assigned to the 5D0 → 7FJ(0–4) electronic transitions were observed. Also, it was found that enhanced luminescence of Eu3+ is a result of change the excitation parameter through energy transfer phenomenon (8S7/2 → 6IJ transition of Gd3+) and occurred in both xerogels and powder samples. In this way, a UV photon absorbed by Gd3+ is converted into visible light emitted by Eu3+ ions. Obtained results clearly indicated that high concentration of Gd3+ ions in powders led to particularly strong enhance the emission in red spectral range and allowed for more than 12-times prolongation of luminescence lifetime for the 5D0 excited state of Eu3+ compared to silica xerogel samples. The values of energy transfer efficiency were also estimated based on luminescence decay times of the 6P7/2 state of Gd3+ ions.

Physisorption data for methyl-hybrid silica gels

The porous structure of inorganic silica and methyl-ormosil gels were studied using nitrogen adsorption at 77 K. Organically, modified silica gels were produced from mixtures of tetraethoxysilane (TEOS) and triethoxy(methyl)silane (MTES), under basic and acid sol–gel catalysis. Starting from pure TEOS, mixtures of increasing MTES content have been prepared. Although not new, silica-based surface morphology is still a challenging subject, particularly when related to silica functionalise systems. N2 adsorption isotherms were used, and the validity of BET model has been discussed for ormosil systems. The pore size distribution of inorganic and hybrid sol–gel silica materials were calculated by using the adsorption branch of the N2 isotherm. Compared with inorganic silica gel, the microstructure of the resulting hybrid gel has been modified—sorption capacities and average pore radius have intensely increased. Acid and basic catalyst determine the type of gel microstructure. Further, the presence of the more flexible ≡Si–CH3 bonds is the responsible for the increasing mobility during gelation, and for the presence of not energetically equivalent terminal groups, which will affect the final pore structure by the suppleness capacity. The rigid and regular pore shape assumption is no more valid. For MTES content higher than 2 mol%, the N2 isotherms reached no more the equilibrium. N2 adsorption isotherms are though not very suitable for surface characterisation of hybrid silica gels, over a limit organic moiety concentration.

Molecularly imprinted silica-silver nanowires for tryptophan recognition

We report on silver nanowires (AgNWs) coated with molecularly imprinted silica (MIP SiO2) for recognition of tryptophan (Trp). The use of AgNWs as a template confers an imprinted material with adequate mechanical strength and with a capability of recognizing Trp due to its nanomorphology when compared to spherical microparticles with a similar surface-to-volume ratio. Studies on adsorption isotherms showed the MIP-SiO2-AgNWs to exhibit homogeneous affinity sites with narrow affinity distribution. This suggests that the synthesized material behaves as a 1D nanomaterial with a large area and small thickness with very similar affinity sites. Trp release from MIP-SiO2-AgNWs was demonstrated to be dominated by the diffusion rate of Trp as controlled by the specific interactions with the imprinted silica shell. Considering these results and the lack of toxicity of silica sol-gel materials, the material offers potential in the field of drug or pharmaceutical controlled delivery, but also in optoelectronic devices, electrodes and sensors.

Erratum to: Preserving the activity of enzymes under harsh oxidizing conditions: sol–gel entrapped alkaline phosphatase exposed to bromine

Chemically reactive sol–gel matrices hold the ability of protecting entrapped enzymes from destruction by external harsh chemicals. We show this concept by exposing alkaline phosphatase (AlP) to a strong oxidizing agent—bromine. In solution, AlP is immediately destroyed by this oxidant. When AlP was entrapped in hybrid silica sol–gel materials carrying double bonds, the reactivity of AlP was preserved after exposure to bromine under conditions which totally destroy it in solution. The matrices studied were vinylated and allylated silicas, and their protectability was compared to n-alkylated silicas and to silica itself. For instance, the reactivity of AlP entrapped in allylated silica after exposure to 25.6 mM bromine solution is 40 times higher than its reactivity when entrapped in pure silica; and in solution the enzyme is totally destroyed at this concentration. Molecular level mechanisms for these observations are proposed.

Combining mercury thermoporometry with integrated gas sorption and mercury porosimetry to improve accuracy of pore-size distributions for disordered solids

The typical approach to analysing raw data, from common pore characterization methods such as gas sorption and mercury porosimetry, to obtain pore size distributions for disordered porous solids generally makes several critical assumptions that impact the accuracy of the void space descriptors thereby obtained. These assumptions can lead to errors in pore size of as much as 500%. In this work, we eliminated these assumptions by employing novel experiments involving fully integrated gas sorption, mercury porosimetry and mercury thermoporometry techniques. The entrapment of mercury following porosimetry allowed the isolation (for study) of a particular subset of pores within a much larger interconnected network. Hence, a degree of specificity of findings to particular pores, more commonly associated with use of templated, model porous solids, can also be achieved for disordered materials. Gas sorption experiments were conducted in series, both before and after mercury porosimetry, on the same sample, and the mercury entrapped following porosimetry was used as the probe fluid for theromporometry. Hence, even if one technique, on its own, is indirect, requiring unsubstantiated assumptions, the fully integrated combination of techniques described here permits the validation of assumptions used in one technique by another. Using controlled-pore glasses as model materials, mercury porosimetry scanning curves were used to establish the correct correspondence between the appropriate Gibbs–Thomson parameter, and the nature of the meniscus geometry in melting, for thermoporometry measurements on entrapped mercury. Mercury thermoporometry has been used to validate the pore sizes, for a series of sol–gel silica materials, obtained from mercury porosimetry data using the independently-calibrated Kloubek correlations. The pore sizes obtained for sol–gel silicas from porosimetry and thermoporometry have been shown to differ substantially from those obtained via gas sorption and NLDFT analysis. DRIFTS data for the samples studied has suggested that the cause of this discrepancy may arise from significant differences in the surface chemistries between the samples studied here and that used to calibrate the NLDFT potentials.