Wet Gel Research Articles

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.

Production of more ideal uranium trioxide microspheres for the sol-gel microsphere pelletization process without the use of carbon

Renewed interest by the U.S. Department of Energy in the sol-gel microsphere pelletization process led to an evaluation of dried uranium trioxide (UO3) microspheres produced without the use of carbon by the internal gelation process at Oak Ridge National Laboratory. An earlier study has shown that less dense UO3 microspheres led to uranium dioxide pellets with higher densities. This effort identified feed formulations that produced dried UO3 microspheres with the lowest tap densities for gelation temperatures of 333, 343, 353, and 363 K. The tap densities of these dried UO3 microspheres ranged from 680 to 820 kg/m3, which were significantly lower than the previously reported low UO3 density of 900 kg/m3. This study confirmed an earlier observation that the effects of gelation conditions on the properties of the gel sphere are complex since only two of the four best formulations were similar in composition. The need to age the wet gel spheres in the gelation medium for 20 min at the end of the experiment was also confirmed.

Adhesive, self-healing and antibacterial properties of Cu-coordinated soft gel based on histamine-conjugated polyaspartamide

Metal-ligand coordination bonding is a non-covalent interaction that has been extensively studied as an effective way to improve the mechanical properties of hydrogels or to generate novel supramolecular self-healing gels. In this study, biodegradable polyaspartamide derivatives conjugated with histamine were synthesized and used to prepare the metal-coordinated supramolecular gel with several different metal-ions such as Cu(II), Ni(II), and Zn(II) in an aqueous solution. The resulting gels showed high adhesive properties on glass and plastics substrates, and the adhesive strength could be modulated by using different metal-ion species as well as the concentration and medium pH. In particular, the Cu(II)-coordinated gel exhibited a reversible self-healing behavior and good antibacterial activity. These wholly bio-based supramolecular polymer gels have potential for various biomedical applications with their multifunctional properties comprising adhesive, self-healing and antimicrobial properties in wet gel state.

Convenient preparation of monolithic aerogel-like organosilica material through solid decomposition

Unlike aerogel preparation processes usually adopted, where a wet gel is first prepared and then solvent eliminated by supercritical extraction or ambient pressure evaporation, which often involves repeated time-consuming solvent exchange steps, the present process gives aerogel-like organosilica material through simple solid sublimation. In this process, a readily decomposable solid powder, ammonium carbonate (AC), is wetted with a resinous polymer solution of polymethylsilsesquioxane (PMSQ) and moulded into a desired shape. After PMSQ solution transforms to gel under catalysis by dibutyltin dilaurate, the sample is elevated to a higher temperature for the solvent to evaporate and AC to decompose. The obtained porous product has porosity up to 98.2% and bulk density of 0.035 g cm−3 and nitrogen adsorption-desorption measurement gives BET surface area of 103 m2 g−1, when 20 wt% PMSQ is used. FTIR spectra confirm the crosslinking of PMSQ after heating. Scanning electron microscopy (SEM) image shows that the obtained product is highly porous and aerogel-like. The porous product shows excellent oil collecting capability of 11 g g−1 porous solid in an oil-collecting test made by vacuum oil on water. This method is straightforward and convenient to use for highly porous aerogel-like material preparation.

Facile preparation of ultralight polymer‐derived SiOCN ceramic aerogels with hierarchical pore structure

AbstractSiOCN ceramic aerogels with lightweight, high surface areas, and macro‐meso pores have been synthesized by a facile method combining freeze‐drying technique and polymer‐derived ceramic route. The wet gels are synthesized via the hydrosilylation reaction between polysilazane and divinylbenzene with cyclohexane as solvent. The solvent is then removed by a freeze‐drying process to form pre‐ceramic aerogels. The SiOCN ceramic aerogels are finally obtained by pyrolyzing the pre‐ceramic aerogels at 1000°C in ultrahigh purity N2. The thermogravimetric and mass spectrometry system (TG/DSC‐MS) is used to investigate the polymer‐to‐ceramic conversion process during pyrolysis. The phase composition, structure, and morphology of the SiOCN ceramic aerogels are investigated by XRD, FT‐IR, XPS, and SEM. The results show that SiOCN ceramic aerogels are composed of amorphous matrix phase and “free carbon” phase. The SiOCN aerogels possess three‐dimensional (3D) network porous structure with low density (0.19 g/cm3), high specific surface area (134 m2/g), large pore volume (0.49 cm3/g), and hierarchical pore structures of both macro and meso pores. The formation mechanism and evolution process of SiOCN ceramic aerogels are discussed.

Studies of the characteristic and reaction mechanism of silica xerogel by sol–gel method catalyzed by alkylbiguanide as a strong organic base

Treatment of tetraethyl orthosilicate with 1,2-diisopropyl-4,4,5,5-tetra-methyl biguanide (A) as a highly strong base immediately gave silica gel by means of hydrolysis and condensation reaction at room temperature. The resulting wet gel was transparent and showed high density after dryness. From the results of gas adsorption and BET analysis, silica gel obtained by the treatment of strong base A had larger specific surface area and pore volume than silica gel that was prepared by a regular or less strong base such as tetramethylammonium hydroxide (TMAH). FTIR analysis revealed that the peak strength of Si-OH bond at 960 cm−1 of silica gel prepared by highly strong base A was smaller than that of TMAH. To understand the mechanism behind such difference, a mixture of diphenylsilandiol and dimethoxydiphenylsilane were reacted with highly strong base A, and the resulting products comprised linear-chain siloxane oligomer and octaphenylcyclotetrasiloxane. Our results indicate that silanol generated by hydrolysis of TEOS is activated by A and the activated silanol undergoes subsequent direct reaction with unhydrolyzed alkoxy silane to give condensation products in ethanol. Such a direct polycondensation between silanol and alkoxy silane brought by highly strong base A led to three-dimensional crosslinking having a higher bulk density of silica gel.

Effect of B-site doping on electrical conductivity of YAlO3 based electrolytes for solid oxide fuel cells

Solid oxide fuel cells (SOFCs) have emerged as high temperature fuel cell technology operating at temperatures around 1000 °C. Lowering the operating temperature enables the use of cheaper materials while maintaining high power outputs. Electrolytes with ABO3-type perovskite structure are good ionic conductors and are promising materials for SOFCs. In this study, a systematic investigation on the synthesis and characterization of Mg- substituted YAlO3 system has been performed. The samples have been synthesized by wet chemical citrate gel route and the electrical conductivity measurements have been conducted in air between 300 and 800 °C. Effect of composition of the phases on total conductivity has been analyzed employing X-ray diffraction. The influence of microstructure on total conductivity has been studied using scanning electron microscopy and orientation imaging microscopy.

A wearable button-like system for long-term multiple biopotential monitoring using non-contact electrodes

Abstract High medical expenditure on chronic diseases in recent years promotes proactive healthcare and health management. Wearable devices provide a good solution for long-term monitoring for both in-hospital and out-of-hospital healthcare; however, long-term contact may cause comfort issues. This paper introduces the design, manufacturing, and experimental validation of a button-like wearable system for long-term multiple biopotential monitoring using non-contact electrodes, namely NCMB-Button. The button-like system is capable of acquiring ECG signals through multiple layers of cloth without directly contacting human skin and other multiple biopotential signals, including EMG and EEG without using traditional wet gel. The detecting performance is achieved and enhanced by introducing a feedback loop into the skin to reduce motion artifacts. The system is packaged into a 39 mm × 32 mm × 17 mm 3D printed case, featuring low-weight and low-cost. The total weight of the system is 24.0 g. The detection performance is validated by testing in multiple motion scenarios through different types of cloth. The results show that ECG signals were able to be detected through the thickness of 2.60 mm of fabric cloth with fast recovery from motion artifacts. Two types of EMG signals were also detected during the validation experiments. The EEG signals were detected during sleepiness condition in which the alpha wave was visible as the indicator. The NCMB-Button demonstrates the feasibility of long-term biopotential monitoring for various applications without interrupting daily activities.

Chlorine-free, monolithic lanthanide series rare earth oxide aerogels via epoxide-assisted sol-gel method

Synthesis of chlorine-free, rare earth oxide aerogels from the lanthanide series was achieved using a modified epoxide-assisted sol-gel method. An ethanolic solution of the hydrated metal nitrate, propylene oxide, and ammonium carbonate was found to gel upon heating to 333 K. Critical point drying of the wet gel in CO2 yielded monolithic aerogels. Most of the aerogels were amorphous as-prepared, but became nano-crystalline after calcination at 923 K in air. The aerogels had high surface areas (up to 150 m2/g), low densities (40–225 mg/cm3), and were photoluminescent.

From random glass networks to random silica gel networks and their use as host for biocatalytic applications

Silica aerogels have in common with silica glass to present a random 3-dimensional network of Si(-O-)4 molecular tetrahedrons. The main difference stands on the presence of a high volume ratio of macropores and mesopores, which may exceed 90% by volume in aerogels. By comparison, silica glass is a dense material. Consequently, an aerogel can entrap relatively big macromolecules in its pores. In the present article, the work done to observe the random network structure of oxide aerogels during the last thirty years, by the present author and his co-workers, is reviewed. As an application, the entrapment of two types of enzymes in silica aerogels is summarized. The first one comprises two types of lipases, used as biocatalysts of esterification reactions in organic solvents. The second one is carbonic anhydrase, applicable in the capture of CO2 in aqueous un-dried wet gels. The influence of the gel network on the enzyme activity and of the enzymes on the aerogels network structure, are both discussed. Overall, these investigations were independent from to the work by Phalippou and co-workers, but they largely beneficiated from cross synergy with them.

Synthesis of colloidal nanosilica from waste glass powder as a low cost precursor

Waste glass powder was used as a low cost precursor for production of colloidal nanosilica for the first time. The process includes production of wet silica gel and thermal peptization of the wet gel. Purification of the glass powder and wet gel production were initiated by acid washing. The obtained powder was reacted with sodium hydroxide to produce wet silica gel. Type of the applied acid was examined in one factor at a time route. Temperature of the alkaline step and concentrations of the applied acid and base were investigated using Taguchi design of experiments. After finding the best combination of the investigated factor levels in production of the wet gel, time of the stabilization in thermal peptization was studied. Characterizations of the wet gel and colloidal silica were performed by XRF, DLS, FESEM, TEM, FTIR and N2 sorption evaluation. Accordingly pure and stable colloidal nanosilica (98.50%) with average particle size of 21.9 nm was produced from the glass powder successfully. Specific surface area of the dried porous optimum sample was 83.63 m2/g.

Synthesis of amorphous MoSx and MoSx/carbon nanotubes composite aerogels as effective hydrogen evolution reaction catalysts

This work reported the preparation of amorphous MoSx aerogel and MoSx/carbon nanotubes composite aerogels by a modified epoxide addition sol–gel method. In this process, propylene epoxide scavenged the protons from a S–H-contained organic acid (dl-Mercaptosuccinic acid) and promoted the interactions between (NH4)6Mo7O24•4H2O and the functional groups in the organic acid to form a gel. The sulfur and molybdenum contained a wet gel turned into an amorphous MoSx aerogel after supercritical drying in ethanol. Carbon nanotubes can be further incorporated into the aerogel backbone, which can extend the specific surface area and alter the pore structures in the composite aerogels. Such composite aerogels showed good catalytic performance in electrochemical hydrogen evolution reactions.

Fabrication and Testing of Catalytic Aerogels Prepared Via Rapid Supercritical Extraction.

Protocols for preparing and testing catalytic aerogels by incorporating metal species into silica and alumina aerogel platforms are presented. Three preparation methods are described: (a) the incorporation of metal salts into silica or alumina wet gels using an impregnation method; (b) the incorporation of metal salts into alumina wet gels using a co-precursor method; and (c) the addition of metal nanoparticles directly into a silica aerogel precursor mixture. The methods utilize a hydraulic hot press, which allows for rapid (<6 h) supercritical extraction and results in aerogels of low density (0.10 g/mL) and high surface area (200-800 m2/g). While the work presented here focuses on the use of copper salts and copper nanoparticles, the approach can be implemented using other metal salts and nanoparticles. A protocol for testing the three-way catalytic ability of these aerogels for automotive pollution mitigation is also presented. This technique uses custom-built equipment, the Union Catalytic Testbed (UCAT), in which a simulated exhaust mixture is passed over an aerogel sample at a controlled temperature and flow rate. The system is capable of measuring the ability of the catalytic aerogels, under both oxidizing and reducing conditions, to convert CO, NO and unburned hydrocarbons (HCs) to less harmful species (CO2, H2O and N2). Example catalytic results are presented for the aerogels described.

Millimeter-Size Spherical Polyurea Aerogel Beads with Narrow Size Distribution.

We report the room temperature synthesis of spherical millimeter-size polyurea (PUA) aerogel beads. Wet-gels of said beads were obtained by dripping a propylene carbonate solution of an aliphatic triisocyanate based on isocyanurate nodes into a mixture of ethylenediamine and heavy mineral oil. Drying the resulting wet spherical gels with supercritical fluid (SCF) CO2 afforded spherical aerogel beads with a mean diameter of 2.7 mm, and a narrow size distribution (full width at half maximum: 0.4 mm). Spherical PUA aerogel beads had low density (0.166 ± 0.001 g cm–3), high porosity (87% v/v) and high surface area (197 m2 g–1). IR, 1H magic angle spinning (MAS) and 13C cross-polarization magic angle spinning (CPMAS) NMR showed the characteristic peaks of urea and the isocyanurate ring. Scanning electron microscopy (SEM) showed the presence of a thin, yet porous skin on the surface of the beads with a different (denser) morphology than their interior. The synthetic method shown here is simple, cost-efficient and suitable for large-scale production of PUA aerogel beads.

Chemical tailoring of porous aluminum oxide xerogels

In the present work a new low-cost sol-gel method has been developed to prepare highly porous alumina systems. The prime consideration of this research work was to avoid the complicated and laborious preparation technique and the expensive drying processes. Aluminum oxide hydroxide gel systems were synthesized by a new sol-gel route. The sol-gel process starts from only two chemical compounds; from an aluminum salt and a solvent, and it does not adopt any basic agent or complex formation. All gel systems were dried at atmospheric pressure to obtain xerogels. Different chemical agents (such as gelation agent propylene oxide, surfactant characteristic citric acid and ethyl acetate or basic agent carbamide and urotropine) were applied. Their application was varied in the steps of the sol-gel procedure. Some wet gels were subjected to different treatments (microwave radiation and freezing in vacuum) before the drying process. The cryogels evolved by lyophilization was provided for the comparison. The route of propylene oxide’ addition to system reacted for 24 h produces a bead-like structure with the highest porosity and smallest pores in the present work.

Improvement of Thermal Stability of ZrO&lt;sub&gt;2&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt; Aerogel Modified by Ca(II) Cations

Ca (II) modified ZrO2-SiO2 aerogel (CaZSA) with excellent thermal stability at 1000 °C was prepared by aging the ZrO2-SiO2 wet gel in calcium nitrate (Ca (NO3)2) ethanol solution followed with alcohol supercritical fluid drying method. The reaction between surface hydroxyl groups on the aerogel and Ca (II) ions played an important role in reducing the high temperature activity and inhibiting the particle growth caused by the condensation of hydroxyl groups of aerogel. Moreover, tiny secondary-phase particles, Ca (II) ions, introduced along grain boundaries serve as the pinning particles to inhibit the crystallization of ZrO2-SiO2 aerogel (ZSA). Thus the high-temperature stability of ZSA was significantly improved by surface modification with Ca (II). The specific surface area of CaZSA still maintained 223 m2·g-1 after 1000 °C calcination, which was 52.7% higher than that of ZSA at the same treatment condition.

Low shrinkage, mechanically strong polyimide hybrid aerogels containing hollow mesoporous silica nanospheres

Low shrinkage, mechanically-strong polyimide hybrid aerogels were synthesized using a supercritical carbon dioxide (scCO2) drying process. Polyimide hybrid aerogels of biphenyl tetracarboxylic dianhydride (BPDA) and 4,4′-oxydianiline (ODA) containing surface-modified hollow mesoporous silica (AHMS) nanoparticles were prepared by chemical imidization and scCO2 drying. To avoid the high volume shrinkage of wet gels that occurs mainly during the drying process, AHMS nanoparticles were introduced to the network structure of the PI aerogels as a crosslinker. All aerogels exhibited a highly-interconnected, network-like structure of polyimide nanofibers with an average fiber diameter of approximately 27.6 nm and 92.8% porosity. These hybrid aerogels exhibited an excellent compression modulus, thermal stability, and high surface area, as well as low density, low shrinkage, and low thermal conductivity.

Catalytic aldol condensation of formaldehyde with acetic acid on titanium phosphates modified by different techniques

X-ray amorphous titanium phosphate (TiP) prepared via precipitation from titanylsulfate showed low catalytic performances in the process of aldol condensation of acetic acid with formaldehyde for production of acrylic acid (AA). As-precipitated TiP was modified in the form of wet gel and dried xerogel using hydrothermal (HTT), microwave (MWT) and mechanochemical (MChT) treatments. The physical–chemical characteristics of initial and modified TiP were studied using XRD, FTIR, adsorption–desorption of nitrogen, ammonia thermo-programmed desorption. All types of treatment lead to significant but irregular changes in crystal and porous structure as well as surface acidity of TiP catalysts. The crystalline titanium hydro- and pyrophosphates are formed under hydrothermal conditions (HTT and MWT). These phases exhibit increased activity and selectivity with respect to acrylic acid. The formation of meso-macroporous structure also contributes to this. The most efficient catalyst contains crystalline titanium hydro- and pyrophosphates and has high total acidity with maximum content of strong acid sites, multimodal meso-macroporous structure and the highest share of macropores: AA yield on this catalyst is 61% at 80% acrylic acid selectivity. It can be prepared via HTT of xerogel at 300 °C.

Ultra-low shrinkage chitosan aerogels trussed with polyvinyl alcohol

Naturally-derived materials have attracted numerous attention owing to a unique native set of properties, but the significant shrinkage of biomass aerogel, from its final wet gel to aerogel, remains a formidable challenge. Here we explore a strategy based on supramolecular interaction and covalent crosslinking to overcome the inherent large shrinkage of chitosan aerogel, by reinforcing its molecular-level structure. The introduction of linear polyvinyl alcohol (PVA) chains can adequately interpenetrate with crosslinked chitosan chains to form strong intermolecular network architecture. Chitosan aerogel trussed with PVA enables to be readily fine-tuned, simultaneously giving rise to nano-sized cellular voids inside the aerogel. Chitosan aerogel obtained shows good heat insulation, applicable compressive property and exceptional processing feature of special-shaped components. We also find an emergent phenomenon that the addition of PVA may steer desirable orientation shrinkage, and linear elasticity at low strain in terms of chitosan aerogel.

Design of High-Density Electrodes For EEG Acquisition.

In a 256-channel electrode cap for electroencephalogram (EEG) acquisition, the inter-space between adjacent electrodes is around 20mm. Theoretical and experimental evidence predict that improving the density of electrode can get more information from the added electrodes. 10mm or less center distance, corresponding to 1000 electrodes on a full head EEG cap, might be a more proper density to current EEG analysis methods. To develop high-density electrode array with center distance equal or less than 10mm, one must make sure that the adjacent electrodes are electrical isolated. It is difficult to avoid short circuit when common wet electrodes are used to build high-density electrode array. The contact area (about 28mm2 with diameter of 6mm) and gelling method make short circuit easily happen. To provide more isolation space between adjacent electrodes, the contact area of the proposed electrode should be less than 8mm2. To restrict the diffusion of the electrolyte, a customized hydrogel is used to replace the conventional gel. Compared with common wet electrode and gel, preliminary tests indicate that the high-density hydrogel-Ag/AgCl electrodes perform well at the impedance, isolation, as well as data quality in EEG acquisition.