Characterisation of poly(neutral red) modified carbon film electrodes; application as a redox mediator for biosensors

The polymer redox mediator, poly(neutral red) (PNR), has been synthesised and characterised electrochemically to investigate the best electropolymerisation and mediation conditions for application in enzyme biosensors and to clarify the mechanism of action. Neutral red was electropolymerised by potential cycling on carbon film electrode substrates by allowing the monomer to be oxidised during the full 20 cycles of polymerisation or reducing the positive limit of the potential window after the first 2 cycles to impede monomer oxidation with a view to obtaining longer polymer chains and a lesser degree of branching. Comparison was made with glassy carbon substrates. The PNR films on carbon film electrodes were characterised using cyclic voltammetry and electrochemical impedance spectroscopy, as well as in glucose biosensors prepared with PNR. Glucose oxidase enzyme was immobilised by encapsulation in silica sol-gel and compared with that obtained by cross-linking with glutaraldehyde. The biosensors were evaluated by chronoamperometry in 0.1 M phosphate buffer saline solution, pH 7.0, and showed evidence of electron transfer between the enzyme cofactor flavin adenine dinucleotide and PNR dissolved in the enzyme layer competing with PNR-mediated electrochemical degradation of H₂O₂ formed during the enzymatic process. This paper is dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

Photolithographic Patterning and Doping of Silica Xerogel Films

This study shows that conventional photolithography can be applied for patterning native or organic dye-doped silica films (0.5 m thick) obtained via a base-catalyzed sol-gel process. Photoresist was spin-coated onto high optical quality xerogel films, soft-baked, exposed to UV irradiation through a photomask, and developed with a commercial photoresist developing solution. Etching away of the photoresist-unprotected areas of the silica films was carried out with a dilute HF solution, while the remaining unexposed photoresist was removed with acetone. Interdigitated array patterns with features as small as 0.5 mm show a smooth surface and extremely sharp interfaces. Densification of the films at 550°C for 2 h decreases the film thickness by 11%, increases the refractive index from 1.420 to 1.456, and allows for well-defined patterning down to length scales of 10 m. Since the densification conditions are incompatible with organic dopants, it is demonstrated that sol-gel films can be doped after pattering (post-doping) by adsorption of cationic dyes from solution. Scanning electron microscopy reveals that the microstructure of patterned sol-gel films is similar to that of bulk monoliths, indicating that the photolithographic procedure is not harmful to the film quality. All patterned films demonstrate highly regular light diffraction patterns.     

Effects of Poly(ethylene glycol) Doping on the Behavior of Pyrene, Rhodamine 6G, and Acrylodan-Labeled Bovine Serum Albumin Sequestered within Tetramethylorthosilane-Derived Sol-Gel-Processed Composites

We investigate the effects of controlled poly(ethylene glycol) (PEG) doping on the behavior of pyrene, rhodamine 6G (R6G), and acrylodan-labeled bovine serum albumin (BSA-Ac) sequestered within tetramethylorthosilicate (TMOS)-derived sol-gel-processed materials. To probe the dipolarity of the local environment within the composite we performed static fluorescence measurements on pyrene as the composites aged. We found that small levels of PEG loading effected significant enhancements in the local dipolarity surrounding the average pyrene molecule. Time-resolved fluorescence anisotropy measurements were used to follow the rotational reorientation dynamics of R6G as the composites aged. As the PEG loading increased, the R6G reorientational mobility increased. Nitrogen adsorption techniques were used to quantify the effects of PEG doping level on the surface area and final xerogel pore features. A large reduction in surface area was observed with PEG doping, but no detectable change in pore size was noted. The effects of PEG doping on a biomolecule were probed by following the time-resolved fluorescence anisotropy decay of BSA-Ac. These results showed that PEG doping resulted in increased biomolecule dynamics relative to that found for a neat, undoped TMOS-derived composites. Together these results show that PEG doping can be used to tune the sol-gel-processed composite dipolarity, alter the mobility of dopants sequestered within the composite, control analyte acessibility to the sensing chemistry, and modulate the internal dynamics within a biodopant.     

Preparation of Mullite Fibers by the Sol-Gel Method

Mullite fibers were synthesized from an aqueous solution of aluminum isopropoxide, aluminum nitrate and tetraethylorthosilicate by the Sol-Gel method. The starting solution was prepared by adding aluminum isopropoxide and tetraethylorthosilicate simultaneously to an aluminum nitrate aqueous solution. The spinnability of the starting solution was investigated by varying the molar ratio of aluminum isopropoxide/aluminum nitrate in the solution. Gel fibers were obtained from solutions in a composition range of aluminum isopropoxide/aluminum nitrate of 3 to 5. Gels obtained from spinnable solutions appeared to have high homogeneity in the mixing of the Al/Si components, leading to a sharp 980°C exothermic peak in the DTA curve and a mullitization temperature of 1000°C. However, in the case of gels obtained from unspinnable solutions, a small 980°C exothermic peak was detected, and the mullitization temperature was as high as 1200°C, probably due to greater heterogeneity in the mixing of the Al/Si components.     

Pyrolysis study of fluorinated sol-gel silica

Fluorinated silica gels at various fluorine content were prepared via sol-gel by hydrolysis of 3,3,3-trifluoropropyltrimethoxysilane and tetraethoxysilane mixtures. The gels, of nominal stoichiometry Si(CH₂CH₂CF₃)_XO_(2-X/2)(X=0.1-1), were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS) and N₂ adsorption analysis. The thermal stability of the fluorinated samples was investigated by coupling thermogravimetric measurements with mass spectrometric and gas chromatographic analyses of the evolved gaseous species. The chemical reactions occurring in the gel matrices during heating were siloxane chain rearrangements involving condensation between residual hydroxyl and ethoxyl groups in the 100-350°C temperature range, whereas the thermal decomposition of the fluoroalkyl groups were observed at higher temperatures (450-600°C). The release of the fluoroalkyl moieties also involved C-F/Si-O bond exchanges inside the siloxane chains, with gas-phase evolution of different fluorinated silicon units. This revised version was published online in July 2006 with corrections to the Cover Date.     

New Photoluminescence Phenomena of Ge／SiO2 Glass Synthesized by Sol-gel Method

New Ge/SiO2 glasses have been synthesized by heating the GeO2/SiO2 dry gels under H2 gas at 700℃. The resulting fluorescence spectra show that this kind of Ge/SiO2 glasses emit strong photoluminescence at 392 nm (3.12 eV), medium strong photoluminescence at 600 nm (2.05 eV) and weak photoluminescence at 770 nm (1.60 eV) respectively. Possible photoluminescence mechanisms are also discussed based on the results of X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS).     

Sol-Gel Fabrication and Properties of Silica Cores of Optical Fibers Doped with Yb3+, Er3+, Al2 O3 or TiO2

Optical cores of preforms for drawing optical fibers doped with Er3+ and Yb3+ were fabricated by the sol-gel method with the aim of increasing the thickness of glass layers coated in a single coating cycle and to determine the relation between the preparation conditions and optical properties of the fibers. Al2O3-P2O5-SiO2 and TiO2-P2O5-SiO2 glasses have been studied as matrices for entrapping the rare-earth elements. Input sols have been prepared from silicon and titanium alkoxides, AlCl3, ErCl3, YbCl3, POCl3, water and a modifier under acidic catalysis of HCl. The sols were coated on the inner wall of a silica substrate tube and the gel layers were sintered at high temperatures up to 2000°C after which the tube was collapsed into the preform. Continuous and homogenous glass films with the maximum thickness of about 8 m were fabricated. The influence of high-temperature heat treatment of the layers on their composition and optical attenuation was observed. The amplified stimulated emission of Er3+ around 1.55 m was measured under the excitation of the fibers by an Nd : YAG laser at 1.064 m.     

Zn2SiO4基体掺杂稀土离子纳米粉体的制备和光谱特性

采用sol-gel法在Zn2SiO4基体中掺杂稀土离子Eu^3 和Y，制备了Zn2SiO4:Eu及Zn2SiO4:Eu,Y纳米粉体，研究不同稀土离子浓度对荧光强度的影响，并采用热重－差热分析，X射线粉末衍射，荧光光谱分析等技术手段进行表征，目标产物的平均粒径分别为31nm和27nm。     

A Systematic Study of the Use of DL-Tartaric Acid in the Synthesis of Silica Materials Obtained by the Sol-Gel Method

DL-tartaric acid was used as a template for the formation of silica nanotubes and spheres by the sol-gel method from tetraethylorthosilicate (TEOS) as silica source. The reactions were carried out in ethanol/water mixtures in the presence of aqueous ammonia, between 0°C and 75°C, using both stirred and non-stirred conditions. TEM and SEM images show that the yield and microstructure of the silica is influenced by the synthetic conditions (temperature, ammonia (aq) concentration, gelation time, solvent mixture). It was observed that the chiral form of the tartaric acid used and the diffusion of TEOS to the template determines the eventual silica structure.     

Sulfide Gels and Films: Products of Non-Oxide Gelation

The preparation of concentrated sols and transparent stiff gels of II-VI semiconductors nanocrystals is reported. A two-step process for the production of cadmium sulfide is reported. Sol stabilization and gelation control are achieved through successive passivation and depassivation of the surface of the nanocrystals which are complexed with thiols. The mechanisms driving the aggregation and the gelation are explained on the basis of NMR and SAXS experiments. Thin films as well as monoliths can be produced. The general principles of the method presented are not restricted to chalcogenide systems and thus enlarge the domain of application of the inorganic sol-gel process.