Effects of sol-gel processing parameters on the phases and microstructures of HA films

Bioactive hydroxyapatite (HA) films were fabricated by a sol–gel method and triethylphosphate and calcium nitrate were used as the phosphorus and calcium precursors, respectively. The effects of the heat treatment temperature, pH level and substrate materials on the phases and microstructures of HA films were studied by X-ray diffraction (XRD), scanning electronic microscopy (SEM) and electronic probe microanalysis (EPMA) and so on. The results show that all the sol–gel films are composed of the phases of HA, CaO, TiO₂ and CaTiO₃. With increasing the calcining temperature, the crystallinity of the films increases, the structure becomes more compact and changes from granular and lamellar to cellular structure, and the Ca/P ratio increases slightly because of the loss of P in the films. The addition of ammonia (adjusting the pH level to be about 7.5) can increase the HA content in the films, and the difference of substrate materials only has a little influence on the microstructure of the sol–gel films.     

Electrochemically induced sol-gel deposition of zirconia thin films

A novel electrochemical method for deposition of ZrO(2) thin films is described. The films, 50-600 nm thick, were obtained by applying moderate positive or negative potentials (+2.5 V to -1.5 V versus SHE) on conducting surfaces immersed in a 2-propanol solution of zirconium tetra-n-propoxide [Zr(OPr)(4)] in the presence of minute quantities of water (water/monomer molar ratios in the range of 10(-5) to 10(-1)), which was the limiting reagent. Oxidative electrochemical formation of solvated H(+) and reductive formation of OH(-) catalyze the hydrolysis and condensation of the metal alkoxide precursor. The magnitude of the applied potential and its duration provide a convenient way of controlling the film thickness. The films consist of an amorphous phase, as revealed by XRD measurements. The effects of different parameters, such as the applied potential and its duration, the amount of added water and the current-time characteristics, were studied. A mechanism for the electrodeposition of the zirconia films which is in accordance with our findings is proposed.     

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.     

STEM and EDXS characterisation of physico-chemical reactions at the periphery of sol-gel derived Zn-substituted hydroxyapatites during interactions with biological fluids

With its good properties of biocompatibility and bioactivity hydroxyapatite (HA) is highly used as bone substitutes and as coatings on metallic prostheses. In order to improve the bioactive properties of HA, we have elaborated Zn²⁺ doped hydroxyapatite. Zn²⁺ ions substitute for Ca²⁺ cations in the HA structure and four Zn concentrations (Zn/Zn + Ca) were prepared at 0.5, 1, 2 and 5 at.%. To study physico-chemical reactions at the materials periphery, we immersed the bioceramics into biological fluids for intervals from 1 day to 20 days. The surface changes were studied at the nanometer scale by scanning transmission electron microscopy associated with energy dispersive X-ray spectroscopy. After 20 days of immersion, we observed the formation of a calcium–phosphate layer at the periphery of the HA doped with 5% zinc. This layer contains magnesium and its thickness was around 200 nm. Formation of this Ca–P–Mg layer represents the bioactive properties of 5% Zn-substituted hydroxyapatite. This biologically active layer improves the properties of HA and will permit a chemical bond between the ceramic and bone.     

Combinatorial synthesis of thin mixed oxide films and automated study of their piezoelectric properties

The development of an automated production of thin films and the characterization of their piezoelectric properties in high-throughput are described. A library of 50 undoped as well as doped lead zirconate titanate Pb(Zr,Ti)O₃ (PZT) coatings was produced by sol deposition. Afterwards, the piezoelectric properties of the library films were analyzed by automated atomic force microscopy employing the ultrasonic piezo-mode.     

Oxidation at a sol-gel composite electrode doped with a dirhodium-substituted polyoxometalate for amperometric detection of peptides separated by HPLC

Oxidation of a variety of compounds, including methionine (Met), using a complex formed between dirhodium(II) acetate and the lacunary form of phosphotungstic acid as the catalyst is effective over a wide range of conditions, including pH 2–10. Thus, amperometric detection at a composite in which this complex is immobilized in a sol–gel material does not place restrictions on selection of conditions for separations by reverse-phase HPLC. A demonstration of this point is shown by a study of Met, Met–Phe, Phe–Met, Met–Met, and Gly–Met–Gly (Phe, phenylalanine; Gly, glycine). Using a 0.05 M phosphate buffer at pH 6.7, a C18 column, and a flow rate of 1 ml min⁻¹, capacity factors for Met, Gly–Met–Gly, Met–Met, and Phe–Met were 1.4, 2.1, 5.6, and 34, respectively. Phe–Met and Met–Phe co-eluted.     

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.     

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.     

Preparation of Al2O3/TiO2 composite sol-gel fiber for headspace solid-phase microextraction of chlorinated organic solvents from urine

A new solid-phase microextraction fiber based on alumina/titania sol-gel-coated on copper wire for headspace sampling of chlorinated organic solvents (chloroform, carbon tetrachloride, trichloroethene, and tetrachloroethene) from urine samples is introduced. The influences of fiber coating composition and microextraction conditions (extraction temperature, extraction time, and ionic strength of the sample matrix) on the fiber performance were investigated. Also, the influence of temperature and time on desorption of analytes from fiber was studied. The proposed fiber has high capacity and demonstrates fast sampling of chlorinated organic solvents from urine samples with high sensitivity. The relative standard deviation (RSD, n=5) for all analytes was below 6.5%.     

Application of amperometric sol-gel biosensor to flow injection determination of glucose

A glucose biosensor with enzyme immobilised by sol–gel technology was constructed and evaluated. The glucose biosensor reported is based on encapsulated GOX within a sol–gel glass, prepared with 3-aminopropyltriethoxy silane, 2-(3,4-epoxycyclohexyl)-ethyltrimetoxy silane and HCl. A flow system incorporating the amperometric biosensor constructed was developed for the determination of glucose in the 1×10⁻⁴–5×10⁻³ mol l⁻¹ range with a precision of 1.5%. The results obtained for the analysis of electrolytic solution for iv administration and human serum samples showed good agreement between the proposed method and the reference procedure, with relative error <5%.     

Molecularly imprinted polymer grafted on polysaccharide microsphere surface by the sol-gel process for protein recognition

An interfacial organic–inorganic hybridization concept was applied to the preparation of a new spherical imprinted material for protein recognition. The functional biopolymer chitosan (CS), shaped as microsphere and high-density cross-linked, constituted of the polysaccharide core for surface imprinting. After the model template protein, bovine serum albumin, was covalently immobilized by forming imine bonds with the functional amine groups of CS, two kinds of organic siloxane (3-aminopropyltrimethoxysiloxane: APTMS, and tetraethoxysiloxane: TEOS) assembled and polymerized on the polysaccharide–protein surface via sol–gel process in aqueous solution at room temperature. After template removal, the protein-imprinted sol–gel surface exhibited a prevalent preference for the template protein in adsorption experiments, as compared with four contrastive proteins. Bioinformatics methods were also employed to investigate the imprinting process and the recognition effect. The influence of siloxane type, pH, siloxane/water ratio on template removal and recognition selectivity was assessed. Under optimized imprinting conditions, a large quantity of well-distributed pores was observed on the immobilized-template imprinted surface. The surface-imprinted adsorbent offered a fast kinetics for template re-adsorption and could be reused. Compared with the imprinted material prepared with free-template, material prepared with immobilized-template possessed higher adsorption capacity towards template protein. Easy preparation of the described imprinted material, high affinity and good reusability make this approach attractive and broadly applicable in biotechnology for down-stream processing and biosensor.     

The role of thermal analysis in optimization of the electrochromic effect of nickel oxide thin films, prepared by the sol–gel method: Part II

Thin films and the corresponding xerogels were prepared from nickel acetate precursor using the sol–gel dip-coating technique. The differences in thermal stability of the two forms of samples were studied by dynamic and isothermal thermogravimetry. For thin films, the onset decomposition temperature of acetate groups was 230 °C and for the xerogel 250 °C. During thermal decomposition, the formation of nanosized nickel oxide took place. Carbonate ions, which were formed during thermal decompostion of acetate groups, remained either free or bidentately coordinated to nickel. In situ monochromatic optical transmittance changes showed that an optical stability up to the 100th cycle was already achieved for films heated for 15 min at the isothermal temperature (thermal decompositon 25%). Comparison of the results obtained for nickel sulfate (Part I) and nickel acetate precursors shows that at least two parameters, the precursor used and the degree of thermal treatment, have considerable influence on the thermal stability of the thin film and also on its electrochromic response during the cycling process.     

Sol–gel immobilized room-temperature phosphorescent metal-chelate as luminescent oxygen sensing material

The chelate formed by 8-hydroxy-7-iodo-5-quinolinesulfonic acid (ferron) with aluminium exhibits strong room-temperature phosphorescence (RTP) when retained on a solid support. In a previous paper we have found that sol–gel technology is a very useful approach for developing RTP optical sensors as a new way to immobilize lumiphors. Sol–gel active phases proved to exhibit a high physical rigidity that enhanced relative RTP intensities and triplet lifetimes of the immobilized probe. In this paper we present an optical sensing phase prepared using the Al–ferron chelate which displays RTP entrapped in a sol–gel glass matrix for the determination of very low levels of oxygen both dissolved in water and organic solvents and in gaseous media. The sol–gel sensing material has proved to be chemically stable for at least 6 months under ambient storage conditions. Besides a high reproducibility in the formation of the sensing materials and no leaching or bleaching of the trapped reagent (neither in the gas phase nor in water or organic solvents) was observed. Oxygen was determined by continuous flow and flow injection methods using both intensity and triplet lifetime measurements. Both methods provided a fast response, good reproducibility and detection limits of 0.0005% (v/v) in the gas phase and <0.01 mg l⁻¹ for dissolved oxygen. An exhaustive study of the effect of some possible interferents present in the gas phase or in solution demonstrated the high specificity of this phosphorescent probe. This highly sensitive oxygen probe has been successfully applied to dissolved oxygen determinations in river and tap waters and its coupling to fiber optics for RTP in-situ monitoring or remote sensing of oxygen has been evaluated.     

An improved bonded-polydimethylsiloxane solid-phase microextraction fiber obtained by a sol-gel/silica particle blend

A novel procedure for solid-phase microextraction fiber preparation is presented, which combines the use of a rigid titanium alloy wire as a substrate with a blend of PDMS sol–gel mixture/silica particles, as a way of increasing both the mechanical robustness and the extracting capability of the sol–gel fibers. The 30 μm average thick fibers displayed an improvement in the extraction capacity as compared to the previous sol–gel PDMS fibers, due to a greater load of stable covalently bonded sol–gel PDMS. The observed extraction capacity was comparable to that of 100 μm non-bonded PDMS fiber, having in this case the advantages of the superior robustness and stability conferred, respectively, by the unbreakable substrate and the sol–gel intrinsic characteristics. Repeatability (n = 3) ranged 1–8% while fiber production reproducibility (n = 3) ranged 15–25%. The presence of the silica particles was found to have no direct influence on the kinetics and mechanism of the extraction process, thus being possible to consider the new procedure as a refinement of the previous ones. The applicability potential of the devised fiber was illustrated with the analysis of gasoline under the context of arson samples.     

FT-IR and Raman spectroscopic study of di-urea cross-linked poly(oxyethylene)/siloxane ormolytes doped with Zn ions

The interactions occurring in di-urea (NHC(O)NH) cross-linked poly(oxyethylene) (POE)/siloxane hybrids (di-ureasils) doped with zinc triflate (Zn(CF₃SO₃)₂) were investigated by Fourier Transform infrared (FT-IR) and Raman (FT-Raman) spectroscopies. Bonding of the Zn²⁺ ions to the urea carbonyl oxygen atoms occurs in the entire range of compositions studied (∞ > n ≥ 1, where n, salt content, is the molar ratio of oxyethylene moieties per Zn²⁺ ion). At n > 20 the incorporation of the guest cations progressively reduces the number of free CO groups. At n = 20 the saturation of the urea cross-links is attained and the Zn²⁺ ions start to coordinate to the POE chains giving rise to the formation of a crystalline POE/Zn(CF₃SO₃)₂ complex. The latter process occurs at the expense of the destruction of the hydrogen-bonded POE/urea structures of the host di-ureasil structure. New hydrogen-bonded associations, more ordered than the urea–urea aggregates present in the non-doped matrix and including Zn²⁺OC coordination, emerge in parallel. “Free” and weakly coordinated CF₃SO₃⁻ ions, present in all the xerogels studied, appear to be the main charge carriers of the conductivity maximum of this family of ormolytes located at n = 60 at 30 °C. In materials with n ≤ 20 contact ion pairs, “cross-link separated” ions pairs and higher ionic aggregates appear. The data reported demonstrate that the behaviour of the di-ureasils doped with triflate salts depends on the type of cation.     

Cooled internal reflection element for infrared chemical sensing of volatile to semi-volatile organic compounds in the headspace of aqueous solutions

In this study, the cooling effect was applied to an evanescent wave type infrared (IR) chemical sensing method to effectively trap volatile organic compounds (VOCs), which have been absorbed in the hydrophobic film coated around the internal reflection element (IRE). The detection of VOCs in aqueous solutions was taken in the headspace of the aqueous solution. This method eliminates the long-term instability of hydrophobic film soaked in an aqueous solution and the potential spectral interference caused by the matrix of the aqueous solution. Thermal energy has been applied to the aqueous solution to assist in the evaporation of VOCs out of the aqueous matrix. By applying a cooling system to the IRE, the excess thermal energy can be removed leading to more stable IR signals. After examination of organic compounds with vapour pressure (P_v) ranging from 0.017 to 150 Torr, significant differences were found between IR signals from cooled and un-cooled systems. Because the thermal conductivity of the IRE used in IR detection is typically low; the efficiency in removing the thermal energy is limited. By heating the aqueous solutions to different temperatures, the IR signals showed that the sample temperature was limited to around 80 °C. The IR signal determination results for five different volatility organic compounds indicated that the optimal heating temperature was not necessary to match with the volatilities of organic compounds in cooling system. The linear regression coefficient (R²) of the standard curve for sample concentrations in the range 5-200 μg ml⁻¹ was generally higher than 0.991 and the detection limit was around a few hundred ng ml⁻¹, which was two to three times lower than that of un-cooled system.     

Application of poly(dimethylsiloxane) fiber sol-gel coated onto NiTi alloy electrodeposited with zirconium oxide for the determination of organochlorine pesticides in herbal infusions

A PDMS fiber sol-gel coated onto an NiTi alloy previously electrodeposited with zirconium oxide (named NiTi-ZrO(2)-PDMS) was applied to the determination of organochlorine pesticides (OCPs) in infusions of peppermint (Mentha piperita L.), lemon grass (Cymbopogon citratus Stapf), chamomile (Matricaria recutita L.), lemon balm (Melissa officinalis L.), and anise seeds (Pimpinella anisum L.). Salting-out effect, extraction time, and extraction temperature were optimized firstly by means of a full-factorial design and then using a Doehlert matrix. No salt addition and 50 min of extraction at 70 degrees C were the optimum conditions. Satisfactory LODs in the range of 2-17 ng/L, as well as good correlation coefficients (at least 0.9981) in the linear range studied, were obtained. Calibration was successfully applied using an infusion of M. recutita L. and recovery tests were performed to ensure the accuracy of the method, with values in the range of 77-120%. Comparison of the NiTi-ZrO(2)-PDMS with commercially available PDMS fibers showed that the proposed fiber has an extraction efficiency comparable to that of PDMS 30 microm for the compounds evaluated, demonstrating its potential applicability.     

Coupled enzyme reaction microarrays based on pin-printing of sol–gel derived biomaterials

We report on the development of a new class of protein microarrays based on the co-immobilization of multiple components within a single pin-printed sol–gel array element. In the first case, the microarraying of a coupled two enzyme reaction involving glucose oxidase and horseradish peroxidase along with the fluorogenic reagent Amplex Red is demonstrated to allow “reagentless” fluorimetric detection of glucose. The second system involved the detection of urea using co-immobilized urease and fluorescein dextran, which works on the basis of a pH induced change in fluorescein emission intensity upon production of ammonium carbonate owing to hydrolysis of urea. In both the cases, it is demonstrated that the changes in intensity from the array are time-dependent, consistent with the enzyme catalyzed reaction, showing that such arrays can be used for kinetic studies. The rate of intensity change was also found to be dependent on the concentration of analyte added to the array, showing that such arrays could be useful for quantitative multianalyte biosensing. Inhibition of urease by the competitive inhibitor thiourea is also demonstrated on a microarray, demonstrating that sol–gel-based microarrays may find use in high-throughput drug-screening applications.     

Determination of the diffusion coefficient of water into atelocollagen type I thin films by attenuated total reflection Fourier transform infrared spectroscopy

 The diffusion coefficient of water into thin polymer layers of glutar aldehyde cross-linked atelocollagen type I matrix (sample 1, the typical layer thickness was about 0.065 × 10⁻³ m) at 23 °C was 1.142 × 10⁻¹⁰ m² s⁻¹. At twice the concentration of the cross-linking agent (sample 2) the diffusion coefficient was 2.795 × 10⁻¹⁰ m² s⁻¹. This increase was attributed to the more ordered morphology and the creation of ordered microvoids in the film. A larger surface area is then available for the transport of diffusing molecules, allowing a higher penetration rate of the solvent. Received: 11 January 2000/Accepted: 6 May 2000     

Novel unbreakable solid-phase microextraction fiber by electrodeposition of silica sol-gel on gold

A new technique for preparation of an unbreakable solid-phase microextraction (SPME) fiber, using sol-gel technology is developed. Primarily, an ultrathin two-dimensional intermediate film was prepared by hydrolysis of 3-(trimethoxysilyl)-1-propanthiol self-assembled monolayer grafted onto gold, then a stationary phase by electrodeposition of 3-(trimethoxysilyl)propylmethacrylate as a precursor, tetramethyl orthosilicate and polyethylene glycol as a coating polymer was produced. The scanning electron microscopy images revealed that the new fiber exhibits a rather porous and homogenous surface. The thermal stability of the fabricated fiber was investigated by thermogravimetric analysis. The applicability of the prepared fiber coating in conjunction with gas chromatography-mass spectrometry was examined by SPME of polycyclic aromatic hydrocarbons, as model analytes, from aquatic media. An extraction time of 20 min at 50 °C gave maximum peak areas when NaCl, 15% was added to the aqueous samples. Limits of detection were in the range of 0.01-0.02 ng/mL and relative standard deviation values were in the range of 4-16% at 1 ng/mL. The developed method was successfully applied for the analysis of real water samples while the relative recovery percentage was in the range of 102-118%.