Microbiosensor for acetylcholine and choline based on electropolymerization/sol–gel derived composite membrane

Amperometric enzyme biosensors for the determination of acetylcholine (ACh) and choline (Ch) have been described. For the fabrication of the biosensors, N-acetylaniline (nAN) was first electropolymerized on a Pt electrode surface to be served as a permselective layer to reject interferences. Bovine serum albumin (BSA) and choline oxidase (CHOD) were co-immobilized in a zinc oxide (ZnO) sol–gel membrane on the above modified Pt electrode for a Ch sensor, or CHOD, acetylcholinesterase (AChE) and BSA immobilized together for an ACh/Ch sensor. The poly (N-acetylaniline) (pnAN) film was the first time used for an ACh/Ch sensor and found to have excellent anti-interference ability, and the BSA in the sol–gel can improve the stability and activity of the enzymes. Amperometric detection of ACh and Ch were realized at an applied potential of +0.6 V versus SCE. The resulting sensors were characterized by fast response, expanded linear range and low interference from endogenous electroactive species. Temperature and pH dependence and stability of the sensor were investigated. The optimal ACh/Ch sensor gave a linear response range of 1.0 × 10⁻⁶ to 1.5 × 10⁻³ M to ACh with a detection limit (S/N = 3) of 6.0 × 10⁻⁷ M and a linear response range up to 1.6 × 10⁻³ M to Ch with a detection limit of 5.0 × 10⁻⁷ M. The biosensor demonstrated a 95% response within less than 10 s.     

Low-potential stable detection of β-NADH at sol–gel derived carbon composite electrodes

Sol–gel derived carbon composite electrodes, prepared from different non-silicate metal alkoxide precursors, offer a substantial decrease in the overvoltage of the NADH oxidation reaction (compared to ordinary carbon electrodes). Such promotion is attributed to acceleration of the proton-transfer step by the metal-oxide component of the composite. Passivation problems, accrued by accumulation of reaction products, are also greatly minimized. Both titania–, zirconia–sol–gel carbon composite electrodes thus offer a highly sensitive and stable anodic detection of NADH at +0.2 V. Greatly improved retention of the redox mediator Meldola Blue within the sol–gel network permits convenient measurements at NADH at −0.1 V. These improvements indicate great promise for the design of dehydrogenase-based amperometic biosensors. An intrinsic activation action by the metal-oxide component is also reported towards the oxidation of hydrazine, hydrogen peroxide, ascorbic acid and catechol. Low-potential detection of NADH is also illustrated at microfabricated titania/carbon screen-printed electrodes.     

Cr bound catalytically active sol–gel siloxane polymers

Several types of Cr bound siloxane polymers were prepared by various modes of polymerization. The co-polymerization of (EtO)₃SiPhCr(CO)₃ and Si(OMe)₄ by the sol–gel process, and its subsequent curing, led to a hydrogenation reactive polymer catalyst. Its catalytic reactivity was retained throughout several cycles, contrary to siloxane polymers prepared by different methods. The hydrogenation reaction was studied with methyl sorbate, 3-nonen-2-one, and 1-octyne. Regio- and stereoselectivities were studied. Cyclohexane as solvent was found to be superior to THF in retaining the catalytic activity upon recycling of the polymeric catalyst in the hydrogenation reactions.     

Renewable silica sol–gel derived carbon composite based glucose biosensor

An amperometric mediated glucose biosensor has been developed based on a sol–gel derived carbon composite material. Glucose oxidase and the mediator vinylferrocene have been immobilised within the porous, rigid and organically modified silicate network in the composite material. The organic group in the silicate network controls the hydrophobicity of the electrode surface and thus limits the wettability of the electrode surface. Various important fabrication factors controlling the biosensor performance have been investigated systematically. The glucose biosensor can be renewed easily in a reproducible manner by a simple polishing step and it has a long operational lifetime. Applicability of the biosensor has been demonstrated in real samples and the results obtained by this biosensor corroborate well with a classical UV spectrophotometric technique.     

TPR/TPO characterization of cobalt–silicon mixed oxide nanocomposites prepared by sol–gel

Cobalt–silicon mixed oxides with Co/Si ratio of 10/90 (10Co), 20/80 (20Co) and 30/70 (30Co) were prepared by a modified sol–gel method. The materials treated in air at 400 and 600 °C were characterized by SEM and TPR/TPO techniques. TPR measurements showed that in all samples only a fraction of Co was present as Co₃O₄ and as amorphous silicate and was reducible by H₂ within 800 °C, while a part was not reducible under TPR conditions. The fraction of Co not reducible decreased with increasing Co content. A TPO/TPR cycle gave rise to an increase of the fraction of not reducible Co.     

An anhydrous sol–gel system derived from methyldichlorosilane

Pyridine‐catalyzed reactions of methyldichlorosilane with bis(trimethylsilyl)carbodiimide afford a dichlorosilane‐derived anhydrous sol–­gel material. Both trimethylchlorosilane formation and Si–H disproportionation act as crosslinking mechanisms. The dried gels form amorphous or crystalline materials, depending on the sample history. The xerogels gave Si/C/N ceramics when pyrolyzed at 1200 °C. Copyright © 1999 John Wiley & Sons, Ltd.     

Photoluminescence and photoactivity of titania particles prepared by the sol–gel technique: effect of calcination temperature

The photoluminescence (PL) characteristics of anatase titania particles prepared by the sol–gel method were investigated and correlated to their photocatalytic behavior with respect to the change of calcination temperature. It was found that the photoluminescence intensity measured at 77 K was gradually increased by increasing the calcination temperature due to the reduction of the internal defects which are responsible for the radiationless recombination of photoexcited electron/hole pairs. Also, the calcination temperature was found to influence the maximum peak position (λ) of the photoluminescence spectra of titania. That is, a blue shift of the photoluminescence spectrum occurs as a consequence of the enlargement of the energy-gap between the lowest excited state and the ground state of titania as increasing the calcination temperature. The quenching behavior of the photoluminescence at 77 K was monitored by in situ supplying oxygen at 77 K in order to investigate what happened to the surface of titania by the calcination. The quenching intensity was monotonically increased with increasing the calcinations temperature. Based on the above results, we concluded that the calcination of titania at higher temperature produces more surface-active sites easily reacting with oxygen molecules as well as improving the crystallinity of anatase phase. Consequently, higher temperature heat treatment of anatase titania particles makes it possible to get higher photoactivity as long as no significant rutile phase is formed.     

Pd-based sulfated zirconia prepared by a single step sol–gel procedure for lean NOx reduction

Pd-based sulfated zirconia catalysts have been prepared through a single step (one-pot) sol–gel preparation technique, in which both sulfate and Pd precursors were dissolved in an organic solution before the gelation step. Observation of the calcination procedure through TGA/DSC and mass spectrometry revealed that the addition of increasing amounts of Pd resulted in the evolution of organic precursor species at lower temperatures. In situ XRD experiments showed that tetragonal zirconia is formed at lower temperatures and larger zirconia crystallites are formed when Pd is added to the gel. Although tetragonal zirconia was the only phase observed through XRD, Raman spectra of samples calcined at 700 °C showed the presence of both the tetragonal and the monoclinic phase, indicating a surface phase transition. DRIFTS experiments showed NO species adsorbed on Pd²⁺ cations. Pd/SZ catalysts prepared through this single step method were active for the reduction of NO₂ with CH₄ under lean conditions. Calcination temperature had a significant effect on this activity, with samples calcined at 700 °C being much more active than those calcined at 600 °C, despite the observed transition to the monoclinic phase. This activity may be linked to observed changes in the surface sulfate species at higher calcination temperatures.     

Novel fiber-optic biosensor based on immobilized glutathione S-transferase and sol–gel entrapped bromcresol green for the determination of atrazine

The aim of this work was to investigate, for the first time, the potential of the enzyme glutathione S-transferase I (isoenzyme GST-I) for uses in analytical chemistry. A novel fiber-optic biosensor for the detection and determination of the triazine herbicide atrazine was developed based on maize GST-I expressed in E. coli. The sensing bioactive material was a three-layer mini-sandwich. The enzyme was immobilized on the outer layer that consisted of a hydrophilic polyvinylidenefluoride membrane. This membrane was supported on an inner glass disk by means of an intermediate binder sol–gel layer that incorporated bromcresol green (BCG). The biosensor operated in a static mode at 25 °C and the rate of the enzymatic reaction, using atrazine as a substrate, served as an analytical signal. A calibration curve was obtained for atrazine, with analytically useful concentration range 2.52–125 μM. The sensor detection limit was 0.84 μM. The reproducibility of atrazine sensing was in the order of ±3–5%. The method was successfully applied to the determination of this herbicide in real water samples, without sample preparation steps. Atrazine recovery ranged between 85 and 110%. No interference from other pesticides, such as alachlor and carbaryl was observed in the absence of atrazine. The immobilized enzyme retained about 75% of its original activity after 1 month use. Simply unscrewing the terminal holding ring of the probe and placing a new bioactive sandwich could easily replace a deteriorated mini-sandwich.     

Polymer–filler interactions in sol–gel derived polymer/silica hybrid nanocomposites

The effect of polymer–filler interaction on solvent swelling and dynamic mechanical properties of the sol–gel derived acrylic rubber (ACM)/silica, epoxidized natural rubber (ENR)/silica, and poly (vinyl alcohol) (PVA)/silica hybrid nanocomposites has been described for the first time. Tetraethoxysilane (TEOS) at three different concentrations (10, 30, and 50 wt %) was used as the precursor for in situ silica generation. Equilibrium swelling of the hybrid nanocomposites in respective solvents at ambient condition showed highest volume fraction of the polymer in the swollen gel in PVA/silica system and least in ACM/silica, with ENR/silica recording an intermediate value. The Kraus constant (C) also followed a similar trend. In dynamic mechanical analysis, the storage modulus dropped at higher strain (>1%), which indicated disengagement of polymer segments from the filler surfaces. This drop was maximum in ACM/silica, intermediate in ENR/silica, and minimum in PVA/silica, both at 50 and 70 °C. The drop in modulus with theoretical volume fraction of silica (ϕ) was interpreted with the help of a Power law model ΔE′ = a₁ϕ, where a₁ was a constant and b₁ was primarily a filler attachment parameter. Strain dependence of loss modulus was observed in ACM/silica hybrid nanocomposites, while ENR/silica and PVA/silica nanocomposites showed almost strain‐independent behavior. The storage modulus showed sharp increase with increasing frequency in ACM/silica system, while that was lower in both ENR/silica (at higher frequency) and PVA/silica systems (in the entire frequency spectrum). The increase in modulus with ϕ also followed similar model ΔE′ = a₂ϕ proposed in the strain sweep mode. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2399–2412, 2005     

An optical pH sensor with extended detection range based on fluoresceinamine covalently bound to sol–gel support

An optical pH sensor was developed based on the fluorophor, fluoresceinamine isomer II (FA), covalently immobilized in a sol–gel matrix. This sol–gel matrix was created by the copolymerization of two precursors, methyltriethoxysilane (MTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS), in an ethanolic solution. Fluoresceinamine was covalently bound to the glycidoxypropyl chain group of GPTMS, thereby preventing it from leaching. Moreover, the immobilization of the fluoresceinamine also extended its linear detection range. The sensor showed good repeatability, a short response time of less than 8 s, high long-term stability and no temperature effect in the biologically relevant range. In the pH range of 4–10, the sensor was very sensitive and its linear range was found to be between pH 6 and 9 (R² = 0.995).     

Density fractionated hollow silica microspheres with high-yield by non-polymeric sol–gel/emulsion route

Hollow silica microspheres were synthesized by non-polymeric sol–gel/emulsion technique using tetra ethyl orthosilicate (TEOS) as a source of silica. A sol mixture of TEOS, water, ethanol and acid was emulsified in a solution of light paraffin oil and surfactant (Span-80). Calcined spheres were density fractionated between density ranges: <1.0, 1.0–1.594, 1.594–1.74 and >1.74 g cm⁻³. The samples were characterized by optical and scanning electron microscopy with energy dispersive X-ray analysis, Fourier transform infrared spectroscopy and laser diffraction size analyzer. Spheres of densities lower than 1.74 g cm⁻³ were found to be hollow as observed from scanning electron microscopy (SEM) images and their yield was maximized to 100% by using a specific TEOS volume ratio with respect to volumes of surfactant and oil. Decreasing the calcination temperature from 700 to 500 °C enhances the yield of hollow spheres emphasizing importance of slower diffusion kinetics at lower calcination temperature. Outer diameters of spheres were between 5 and 60 μm with mean diameter expectedly increasing with increase in TEOS sol volume and with decrease in sphere density. It is proposed that silica shells form via hydrolysis and polycondensation at oil–water/ethanol interface in the water-in-oil emulsion, which subsequently form hollow spheres on removal of water–ethanol during calcination.     

High surface area nickel aluminate spinels prepared by a sol–gel method

The oxide spinel NiAl₂O₄ and spinel-type solid solutions Al₂O₃–NiAl₂O₄ (at Ni/Al=1:4, and Ni/Al=1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase cubic materials having the spinel-type structure. The cubic lattice parameter, a_o, was found to decrease gradually with increasing aluminium content of the mixed metal oxides. The specific surface area (determined by nitrogen adsorption at 77 K) was found to be in the range of 200–300 m² g⁻¹. The materials were found to be basically mesoporous, the most frequent pore radius being in the range 3.2–6.4 nm. IR spectroscopy of CO adsorbed at liquid nitrogen temperature gave a main band at 2186–2195 cm⁻¹, which was assigned to the C---O stretching vibration of surface Al³⁺CO adducts where coordinatively unsaturated Al³⁺ ions act as Lewis acid centres.     

Amperometric detection of cytochrome c by capillary electrophoresis at a sol–gel carbon composite electrode

Capillary electrophoresis (CE) was employed for the determination of cytochrome c using a wall-jet amperometric detector consisting a copper(I) oxide-modified sol–gel carbon composite electrode (CCE), which exhibits a sensitive electrocatalytic response for the oxidation of cytochrome c. The optimum conditions of separation and detection are 0.08 M NaOH for the separation solution, 12 kV for separation voltage and +0.60 V versus saturated calomel electrode (SCE) for the detection potential. Calibration was linear over the concentration range 1–600 μM with the limit of detection of 3.4 μM, based on a signal-to-noise ratio (S/N) of 3.     

Ionic transport phenomenon across sol–gel derived organic–inorganic composite mono-valent cation selective membranes

Organic–inorganic composite mono-valent cation selective membranes (MCSMs) were prepared by sol–gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic segment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical, thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for mono-valent cations, ionic diffusion and water transport number. These properties suggested the suitability of MCSMs, especially Si-65%, for electro-separation of Na⁺ from Ca²⁺, Mg²⁺, and Fe³⁺. The effect of electrolyte solution on the characteristics of the current–voltage (i–v) curve in MCSM was studied based on the concentration polarization. Electro-transport of different ions in terms of plateau length and concentration profiles for different ions in the solution phase, diffusion boundary layer and membrane phase were presented. Information obtained from i–v curve analysis were validated by electrodialysis (ED) experiments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor of different ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability of MCSMs for separating cations.     

Optochemical sensing by immobilizing fluorophore-encapsulating liposomes in sol–gel thin films

The chemical stability of optochemical sensors depends largely on the physiochemical properties of the supportive matrix of the sensor and on the method used to immobilize sensing reagents to the supportive matrix of the sensor. Leaking of physically immobilized sensing reagents from the matrix support decreases the stability of the sensor and its overall usefulness. Covalent immobilization eliminates leakage of the sensing reagent from the support but may lead to alteration of spectral properties and loss of analyte response. This paper presents a new method for physical immobilization of polar fluorescence dyes in a sensing support. The method is based on the immobilization of fluorescent dye encapsulating liposomes in a sol–gel film of micrometer thickness. The encapsulation of the dye molecules in the liposomes effectively increases the molecular dimensions of the sensing reagent, thus preventing its leakage from the matrix support. This paper describes the analytical properties of a pH sensor fabricated by immobilizing carboxyfluorescein-encapsulating liposomes in a sol–gel thin film. The sensor shows excellent stability with respect to dye leaking which in turn leads to high reproducibility and sensitivity of about 0.01 pH units. The linear dynamic range of the sensor is between pH 6 and 7.5 and its response time is at the sub-seconds time scale.     

Synthesis of silica–silver wires by a sol–gel technique

SiO₂–Ag wires were synthesized by a sol–gel technique. A two step approach was followed, focusing mainly on the effect of acid concentration on the first stage and processing temperature on the second. This acid-catalyzed reaction on the first stage yielded SiO₂–AgCl wires with diameters as low as 800 nm average, and lengths ranging up to 100 μm, as determined by LV-SEM and TEM. A thermal treatment at different temperatures on the second step, under H₂ atmosphere, yields silica–silver unidirectional structures. The chemical composition of these structures was determined by EDS, indicating the presence of Si, O and Ag. The transformation of the wires as a function of temperature under reducing atmosphere was followed by electron microscopy analysis. At 400 °C and above the silica starts to cover the reduced silver while maintaining the unidirectional conformation, suggesting a tendency to form silver wires covered by a silica layer.     

Cobalt–silicon mixed oxide nanocomposites by modified sol–gel method

Cobalt–silicon mixed oxide materials (Co/Si=0.111, 0.250 and 0.428) were synthesised starting from Co(NO₃)₂·6H₂O and Si(OC₂H₅)₄ using a modified sol–gel method. Structural, textural and surface chemical properties were investigated by thermogravimetric/differential thermal analyses (TG/DTA), XRD, UV–vis, FT-IR spectroscopy and N₂ adsorption at −196 °C. The nature of cobalt species and their interactions with the siloxane matrix were strongly depending on both the cobalt loading and the heat treatment. All dried gels were amorphous and contained Co²⁺ ions forming both tetrahedral and octahedral complexes with the siloxane matrix. After treatment at 400 °C, the sample with lowest Co content appeared amorphous and contained only Co²⁺ tetrahedral complexes, while at higher cobalt loading Co₃O₄ was present as the only crystalline phase, besides Co²⁺ ions strongly interacting with siloxane matrix. At 850 °C, in all samples crystalline Co₂SiO₄ was formed and was the only crystallising phase for the nanocomposite with the lowest cobalt content. All materials retained high surface areas also after treatments at 600 °C and exhibited surface Lewis acidity, due to cationic sites. The presence of cobalt affected the textural properties of the siloxane matrix decreasing microporosity and increasing mesoporosity.     

Sol–gel synthesis and crystallisation of 3CaO·2SiO2 glassy powders

A gel of composition 3CaO·2SiO₂ was synthesised by means of sol–gel route involving hydrolysis of tetramethyl orthosilicate (TMOS) with calcium nitrate in alcoholic medium, and polycondensation reaction. The gel was submitted to DTA and TGA analysis and the resulted material was examined by FTIR spectroscopy and X-ray powder diffractometry. The thermal treatments required for the gel into glass conversion led to a partially devitrified product. The XRD analysis showed that the structure obtained is highly depolymerised.     

Preparation and characterization of a supported Si3N4 membrane via a non-aqueous sol–gel process

We report a novel fabrication process of a mesoporous Si₃N₄ membrane on an Al₂O₃ support via a non-aqueous sol–gel technique. The membrane was prepared by dipping an -Al₂O₃ support disk into a silicon diimide sol that was synthesized by catalytic ammonolysis of tris(dimethylamino)silylamine, H₂NSi(NMe₂)₃. The SEM image and nitrogen adsorption analysis indicate that amorphous Si₃N₄ layers with nano-sized pores have formed on the surface and also inside the pores of the Al₂O₃ support disk. The new membrane demonstrates high selective absorption of NO₂, suggesting a potential application as a selective filter for gas sensors.