Deposition of low-density thick silica films from burning sol-gel derived alcogels

In the current study we show that the combustion of sol-gel derived alcogels with specifically tailored composition leads to the release of silica nanoparticles from the burning alcogel in a controlled manner which enables direct deposition of the released nanoparticles into low-density silica thick films. The process has some similarities to flame spray pyrolysis but requires no aerosol generator or other sophisticated instrumental setup. By the proper choice of catalysts and mixture of silicon alkoxides for the synthesis of the alcogel, preferential hydrolysis and polycondensation of one of the alkoxides is achieved. This leads to the formation of an alcogel with volatile silica precursor trapped in the gel pores. Resulting alcogels were burned to deposit uniform porous silica films with density of ~0.1 g/cm³ and primary particle size of ~10 nm. Demonstrated method yields silanol-free silica directly, without additional treatment steps and enables straightforward control over the deposition rate and coarseness of the layer by simple adjustment of the composition of the silica alcogel. The maximum layer thickness is limited only by the deposition time (in the current work up to 134 μm). Such technique of porous oxide film preparation could potentially be extended to the preparation of porous films from other oxides by using respective metal alkoxides as precursors.

Simple and ultrasensitive electrochemical sensor for oxalic acid detection in real samples by one step co-electrodeposition strategy

Oxalic acid (OA), naturally available in vegetables and foodstuffs derived from them, easily combines with calcium and iron to form insoluble oxalates. Their chelation will result in various renal diseases; thus, the accurate determination of OA is quite significant in the evaluation of food quality and healthcare settings. Here, we developed an electrochemically induced alcohol-free sol-gel method to obtain platinum nanoparticles (PtNPs) adhered with porous silica on glassy carbon electrode (PSiO₂-PtNPs/GCE) by a one-step process, which can be potentially used as an excellent catalyst towards electrochemical oxidation of OA for the first time. Without any redox mediator, PSiO₂-PtNPs/GCE exhibited a low oxidation overpotential and a significantly high current signal, achieving a wide linear range of concentration from 0 to 45 μM and a detection limit as low as to 25 nM for OA detection. Moreover, this present alcohol-free sol-gel approach towards OA determination was verified in real samples, which is promising for foodstuff analysis and clinical diagnosis. Graphical abstract.

Sustainable hybrid silica extracted from rice husk with polyvinyl alcohol and nicotinic acid as multi adsorbent for textile wastewater treatment

A new eco-friendly compound was prepared for the treatment of textile wastewater containing mixed dyes with various ranges of toxicity. Porous silica was extracted from a black liquor by-product using a simple method and characterized by porous morphology (the pore size ranged between 12 and 41 nm). The silica is the main corrosive agent present in the black liquor; thus, the extraction of silica from the black liquor was considered detoxification process. The extracted porous silica was used as a precursor material to prepare the hybrid material based on polyvinyl alcohol (PVA) as a binder polymer and functionalized by nicotinic acid. The multifunction prepared hybrid was characterized by FT-IR, TGA, DTGA, SEM, and EDX. The porous size of the prepared hybrid varied from 96 nm to 620 nm and presents a high thermal stability in comparison with its parent materials. The adsorption of cationic and anionic dyes was carried out. The adsorption kinetics parameters were fitted with pseudo-first-order and pseudo-second-order kinetic models for methyl orange (MO) and methylene blue (MB), respectively. The adsorption parameters indicated that the Langmuir model is better to describe the adsorption of dyes on the hybrid material. The maximum adsorption capacity was 484 and 771 mg/g for MO and MB, respectively.

Spray-drying water-based assembly of hierarchical and ordered mesoporous silica microparticles with enhanced pore accessibility for efficient bio-adsorption

The fast and scalable spray-drying-assisted evaporation-induced self-assembly (EISA) synthesis of hierarchically porous SBA-15-type silica microparticles from a water-based system is demonstrated. The SBA-15-type silica microparticles has bowl-like shapes, uniform micro-sizes (∼90 µm), large ordered mesopores (∼9.5 nm), hierarchical meso-/macropores (20-100 nm) and open surfaces. In the synthesis, soft- and hard-templating approaches are combined in a single rapid drying process with a non-ionic tri-block copolymer (F127) and a water-insoluble polymer colloid (Eudragit RS, 120 nm) as the co-templates. The RS polymer colloid plays three important roles. First, the RS nanoparticles can be partially dissolved by in-situ generated ethanol to form RS polymer chains. The RS chains swell and modulate the hydrophilic-hydrophobic balance of F127 micelles to allow the formation of an ordered mesostructure with large mesopore sizes. Without RS, only worm-like mesostructure with much smaller mesopore sizes can be formed. Second, part of the RS nanoparticles plays a role in templating the hierarchical pores distributed throughout the microparticles. Third, part of the RS polymer forms surface "skins" and "bumps", which can be removed by calcination to enable a more open surface structure to overcome the low pore accessibility issue of spray-dried porous microparticles. The obtained materials have high surface areas (315-510 m² g^-1) and large pore volumes (0.64-1.0 cm³ g^-1), which are dependent on RS concentration, HCl concentration, silica precursor hydrolysis time and drying temperature. The representative materials are promising for the adsorption of lysozyme. The adsorption occurs at a >three-fold faster rate, in a five-fold larger capacity (an increase from 20 to 100 mg g^-1) and without pore blockage compared with the adsorption of lysozyme onto spray-dried microparticles of similar physicochemical properties obtained without the use of RS.

Amine-functionalized, porous silica-coated NaYF4:Yb/Er upconversion nanophosphors for efficient delivery of doxorubicin and curcumin

Upconversion nanoparticles (UCNP) with unique multi-photon excitation photo-luminescence properties have been extensively explored as novel contrast agents for low-background biomedical imaging. There is an increasing interest in employing UCNPs as carrier for drug delivery as these offers a unique opportunity to combine therapy and diagnostics in one platform (theranostics). In the present work, we report microwave-assisted synthesis of hexagonal NaYF₄:Yb/Er UCNPs coated with porous silica and functionalized with amine (UCNP@mSiO₂). The UCNP@mSiO₂ were investigated for controlled delivery of a chemotherapeutic agent, doxorubicin (DOX, hydrophilic), and a chemosensitizing agent, curcumin (CCM, hydrophobic). The drug loading was relatively higher for DOX (17.4%), in comparison to CCM (8.1%). The cumulative drug release from DOX-loaded UCNP@mSiO₂ were 30 and 41% at physiological (7.4) and tumoral (6.4) pH, following a pseudo Fickian release pattern, whereas the release from CCM-loaded UCNP@mSiO₂ were 27 and 50% at pH 7.4 and 6.4, following a non-Fickian and pseudo-Fickian release patterns, respectively. Both DOX and CCM-loaded UCNP@mSiO₂ exhibited pH-dependent controlled drug delivery but the effect was more pronounced for CCM, the hydrophobic chemosensitizer. Cell viability assay using HeLa cells showed that DOX-loaded UCNP@mSiO₂ inhibit cell growth in a dose-dependent manner, similar to free DOX, but the cell inhibition activity of free CCM was lower than CCM passively entrapped in UCNP@mSiO₂. Confocal microscopy studies revealed cell uptake of both the drug by HeLa cells. Thus, UCNP@mSiO₂ exhibited the unique capability to deliver hydrophilic and hydrophobic drugs, individually. UCNP@mSiO₂ carrier, equipped with theranostic capabilities, may potentially be used for pH-responsive release of chemotherapeutic agents in cancer environment.

Peroxidase-immobilized porous silica particles for in situ formation of peroxidase-free hydrogels with attenuated immune responses

Enzymatic crosslinking chemistry using horseradish peroxidase (HRP) has been widely utilized as an effective approach to fabricating injectable hydrogels with high efficiency under mild reaction conditions. However, their clinical applications are limited by the immunogenicity of the plant-derived enzyme. Herein we report the design, synthesis and characterization of HRP-immobilized porous silica particles (HRP-particles) and their use for in situ formation of HRP-free hydrogels. HRP was immobilized on aminopropyl-modified porous silica particles of 70-140 µm in diameter via poly(ethylene glycol) spacers of different molecular weights by reductive amination reaction. Two different HRP-free hydrogels based on dextran-tyramine and gelatin-hydroxyphenylpropionic acid (GHPA) conjugates were produced by passing a solution containing the conjugates and H₂O₂ through a syringe packed with HRP-particles. The storage modulus and gelation rate of both hydrogels were tunable by varying the contact time between the polymer solution and HRP-particles. Our in vitro study revealed that HRP-free GHPA hydrogel was less stimulatory to activated mouse macrophages than HRP-containing GHPA hydrogel with the same stiffness. Furthermore, HRP-free GHPA hydrogel exhibited remarkably lower levels of local and systemic inflammation than HRP-containing one upon subcutaneous injection in immunocompetent C57BL/6J mice. The attenuated immunogenicity of HRP-free GHPA hydrogels makes them an attractive platform for tissue engineering applications. STATEMENT OF SIGNIFICANCE: The immunogenicity of HRP is a significant issue for clinical application of HRP-catalyzed in situ forming hydrogels. HRP-particles are developed to overcome the safety concerns by fabricating HRP-free hydrogels. The porosity of silica particles and molecular weight of poly(ethylene glycol) spacers are discovered as important factors determining the catalytic ability of HRP-particles to induce the in situ crosslinking of polymer-phenol conjugates. Although several articles speculate the potential of HRP to trigger immune responses when administered as a part of hydrogel formulation, no literature has attempted to investigate the immunogenicity of HRP-containing hydrogels in comparison with HRP-free hydrogels. Our findings suggest that the immunogenicity issue should be carefully considered before clinical translation of HRP-containing hydrogels.

Facile synthesis of smartaminosilane modified- SnO2/porous silica nanocomposite for high efficiency removal of lead ions and bacterial inactivation

The aim of the present study is to synthesize a new and proficient nanoadsorbent for rapid removal of heavy metals and disinfection of microorganisms. The proposed nanoadsorbent was fabricated using SnO₂ nanoparticles as the core, coated with mesoporous silica and further modified with 3-Aminopropyl triethoxysilane to render SnO₂/PSi/NH₂ nanocomposite. The nanocomposite was characterized using Fourier Transform Infrared (FTIR), X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Nitrogen adsorption-desorption analysis. The potential of the resultant SnO₂/PSi/NH₂ nanocomposite for the convenient removal of Lead ions in a batch systems was investigated as a function of solution pH, contact time, adsorbent dosage, temperature and metal ion concentration. The adsorption behavior was in good agreement with Sips and Langmuir isotherm models. The maximum adsorption capacity of SnO₂/PSi/NH₂ was 653.62 mg g^-1. Furthermore, the desorption experiments demonstrated that the proposed nanocomposite could be used frequently for at least three consecutive cycles with minor losses in adsorption performance. The bacterial inactivation ability of SnO₂/PSi/NH₂ toward E-Coli and S. aureus bacteria was also evaluated using disk diffusion and linear cultivation tests, according to which the SnO₂/PSi/NH₂ nanocomposite possessed exceptional disinfection ability toward both bacteria, specifically S. aureus.

Biogenic porous silica and silicon sourced from Mexican Giant Horsetail (Equisetum myriochaetum) and their application as supports for enzyme immobilization

Porous silica-based materials are attractive for biomedical applications due to their biocompatibility and biodegradable character. In addition, inorganic supports such as porous silicon are being developed due to integrated circuit chip compatibility and tunable properties leading to a wide range of multidisciplinary applications. In this contribution, biosilica extracted from a rarely studied plant material (Equisetum Myriochaetum), its conversion to silicon and the potential for both materials to be used as supports for enzyme immobilization are investigated. E. myriochaetum was subject to conventional acid digestion to extract biogenic silica with a% yield remarkably higher (up to 3 times) than for other Equisetum sp. (i.e. E. Arvense). The surface area of the isolated silica was ∼400 m²/g, suitable for biotechnological applications. Biogenic silicon was obtained by magnesiothermic reduction. The materials were characterized by SEM-EDX, XRD, FT-IR, ICP-OES, TGA and BET analysis and did not contain significant levels of class 1 heavy elements (such as Pb, Cd, Hg and As). Two commercial peroxidases, horseradish peroxidase (HRP) and Coprinus cinereus peroxidase (CiP) were immobilized onto the biogenic materials using three different functionalization routes: (A) carbodiimide, (B) amine + glutaraldehyde and (C) amine + carbodiimide. Although both biogenic silica and porous silicon could be used as supports differences in behaviour were observed for the two enzymes. For HRP, loading onto biogenic silica via the glutaraldehyde immobilization technique (route B) was most effective. The loading of CiP showed a much higher peroxidase activity onto porous silicon than silica functionalized by the carbodiimide method (route A). From the properties of the extracted materials obtained from Equisetum Myriochaetum and the immobilization results observed, these materials appear to be promising for industrial and biomedical applications.

Stable solidification of silica-based ammonium molybdophosphate by allophane: Application to treatment of radioactive cesium in secondary solid wastes generated from fukushima

Silica-based ammonium molybdophosphate (AMP/SiO₂) is an absorbent material that can effectively remove Cs from radioactive-contaminated wastewater (RCW) generated by Fukushima nuclide accident. Pressing/sintering method was used for final disposal of secondary waste (spent absorbent) to achieve the volume reduction of AMP-Cs/SiO₂ (AMP/SiO₂ saturation adsorption of Cs) and stable solidification of Cs by adding natural allophane. The structure of AMP-Cs completely collapsed at approximately 700°C, and most Mo and P species in AMP sublimed. The optimal sintering temperature was estimated as 900°C. The stable crystalline phase of Cs₄Al₄Si₂₀O₄₈ was recrystallized by the reaction of Cs₂O, Al₂O₃, and SiO₂, and the immobilization ratio of Cs was approximately 100%. The leachability of Cs from the sintered product in distilled water was approximately 0.41%. The high immobilization and low leachability of Cs were attributed to the excellent solidification properties of the sintered products of AMP-Cs/SiO₂-allophane.

Supersaturated silica-lipid hybrids (super-SLH): An improved solid-state lipid-based oral drug delivery system with enhanced drug loading

The method of supersaturation for achieving high drug loads in lipid-based formulations is under exploited and relatively unexplored, especially in the case of solid-state lipid-based formulations. Silica-lipid hybrids are solid-state lipid-based formulations designed for improving the oral delivery of poorly water-soluble drugs. However, their application to compounds of low potency and requiring large doses is limited by their low drug loading capacity. Here, an innovative technique to fabricate supersaturated silica-lipid hybrid formulations (super-SLH) has been established and the relationship between drug load and performance investigated. Using the model poorly water-soluble drug, ibuprofen, super-SLH was fabricated possessing drug loads ranging from 8 to 44% w/w, i.e. greater than the previously developed standard ibuprofen silica-lipid hybrids (5.6% w/w). Drug crystallinity of the encapsulated ibuprofen ranged from non-crystalline to part-crystalline with an increase in drug load. Super-SLH achieved improved rates and extents of dissolution when compared to pure ibuprofen, regardless of the drug load. The percentage increase in dissolution extent at 60 min varied from 200 to 600%. The results of the current study indicate that supersaturation greatly improves drug loading and that 16-25% w/w is the optimum loading level which retains optimal dissolution behaviour for the oral delivery of ibuprofen, which has the potential to be translated to other poorly water-soluble drugs.

Ex vivo skin permeation and penetration of nonivamide from and in vivo skin tolerability of film-forming formulations containing porous silica

AIM

The purpose of this study was to evaluate skin permeation and penetration of nonivamide which has been formulated in novel film-forming formulations (FFFs). These formulations aim to prolong the availability of capsaicinoids which are used in long-term treatment of chronic pruritus.

METHODS

An oily solution of nonivamide was loaded into porous silica particles which then were suspended in an aqueous dispersion of a sustained release polymer. Permeation and penetration experiments were performed ex vivo with postauricular porcine skin using modified Franz diffusion cells. The penetrated drug amount was assessed ex vivo by skin surface biopsy followed by cryo-sectioning. Furthermore, in vivo skin irritation experiments were performed to compare the potential skin irritation caused by the FFFs to conventionally used semi-solid formulations.

RESULTS

Permeation rates of nonivamide from FFF through the skin are comparable to that from clinically used immediate release formulations. This elucidates the therapeutic safety profile of the novel FFF. Penetration studies confirmed the prolonged drug availability at the site of action. FFFs were found not to irritate the skin of healthy volunteers.

CONCLUSION

FFFs with sustained nonivamide penetration represent safe and easy-to-use formulations. They therefore may improve the treatment of chronic pruritus with capsaicinoids by enhancing patient compliance through a sustained release regime.

Enhancement of Peroxidase Stability Against Oxidative Self-Inactivation by Co-immobilization with a Redox-Active Protein in Mesoporous Silicon and Silica Microparticles

The study of the stability enhancement of a peroxidase immobilized onto mesoporous silicon/silica microparticles is presented. Peroxidases tend to get inactivated in the presence of hydrogen peroxide, their essential co-substrate, following an auto-inactivation mechanism. In order to minimize this inactivation, a second protein was co-immobilized to act as an electron acceptor and thus increase the stability against self-oxidation of peroxidase. Two heme proteins were immobilized into the microparticles: a fungal commercial peroxidase and cytochrome c from equine heart. Two types of biocatalysts were prepared: one with only covalently immobilized peroxidase (one-protein system) and another based on covalent co-immobilization of peroxidase and cytochrome c (two-protein system), both immobilized by using carbodiimide chemistry. The amount of immobilized protein was estimated spectrophotometrically, and the characterization of the biocatalyst support matrix was performed using Brunauer-Emmett-Teller (BET), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), and Fourier transform infrared (FTIR) analyses. Stability studies show that co-immobilization with the two-protein system enhances the oxidative stability of peroxidase almost four times with respect to the one-protein system. Thermal stability analysis shows that the immobilization of peroxidase in derivatized porous silicon microparticles does not protect the protein from thermal denaturation, whereas biogenic silica microparticles confer significant thermal stabilization.

C,N-bipyrazole receptor grafted onto a porous silica surface as a novel adsorbent based polymer hybrid

A simple heterogeneous synthesis of pure adsorbent based polymer hybrid made by condensing a functionalized C,N-bipyrazole with a 3-glycidoxypropyl-trimethoxysilane silylant agent, previously anchored on a silica surface was developed. The formed material (SG2P) was characterized through elemental analysis, FT-IR spectroscopy, (13)C NMR of solid state, scanning electron microscope (SEM), and was studied and evaluated by determination of the surface area using the BET equation, the adsorption and desorption capability using the isotherm of nitrogen and B.J.H. pore sizes. The new material exhibits good thermal stability determined by thermogravimetry curves and good chemical stability was examined in various acidic and buffer solutions (pH 1-7). The binding and adsorption abilities of SG2P were investigated for Hg(2+), Cd(2+), Pb(2+), Zn(2+), K(+), Na(+) and Li(+) cations and compared to the results of classical liquid-liquid extraction with the unbound C,N-bipyrazole compound. The grafting at the surface of silica does not affect complexing properties of the ligand and the SG2P exhibits a high selectivity toward Hg(2+) ion with no complexation being observed towards zinc and alkali metals. The extracted and the complexing cation percentages were determined by atomic absorption measurements.

Inclusion of fenofibrate in a series of mesoporous silicas using microwave irradiation

A selection of porous silicas were combined with a model drug using a recently developed, controlled microwave heating process to determine if the application of microwave irradiation could enhance subsequent drug release. Five mesoporous silica types were investigated (core shell, core shell rehydrox, SBA-15, silica gel, SYLOID®) and, for comparison, one non-porous silica (stober). These were formulated using a tailored microwave heating method at drug/excipient ratios of 1:1, 1:3 and 1:5. In addition, all experiments were performed both in the presence and absence of water, used as a fluidising media to aid interaction between drug and support, and compared with results obtained using more traditional heating methods. All formulations were then characterised using differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transformation infrared spectroscopy (FT-IR). Pharmaceutical performance was investigated using in vitro drug release studies. A significant enhancement in the release profile of fenofibrate was observed for formulations prepared using microwave heating in the absence of water for five of the six silica based formulations. Of all the formulations analysed, the greatest extent of drug release within the experimental 30 min was the 1:5 core shell rehydrox achieving a total of 86.6 ± 2.8%. The non-porous (stober) particles did not exhibit an increased release of the drug under any experimental conditions studied. This anomaly is thought to be a result of the comparatively small surface area of the silica particles, thus preventing the adsorption of drug molecules.