Electrochemical Behavior of Graphite Electrodes Modified by Spin-Coating with Sol−Gel-Entrapped Ionomers

Highly sensitive detection of hexavalent chromium utilizing a sol-gel/carbon nanotube modified electrode

A pyridine-functionalized thin film has been fabricated to selectively preconcentrate Cr(VI) anions for electrochemical detection in the 5-300 μg L-1 range. Glassy carbon electrodes were modified through physical deposition of single-walled carbon nanotubes (SWNTs) on the electrode surface, followed by electrochemical deposition of a sol-gel containing a 2-pyridine functional group. The use of SWNTs has increased sensitivity for Cr(VI) detection in aqueous solutions, providing a detection limit of 0.8 μg L-1.

A Novel Dye-Doped Sol–Gel Silica Sorbent for the Removal of Cobalt

A TAR [4-(2-thiazolylazo)resorcinol] doped sol–gel silica sorbent (red colour, porous, stable, hard, non-swelling) was prepared, characterized and investigated for the removal of Co(II) from aqueous solutions. The kinetics, adsorption isotherm, equilibration time and effect of pH on such removal were studied to optimize the conditions necessary for large-scale application. Rapid equilibration was observed with adsorption equilibria being attained within 10 min. The adsorption of cobalt ions onto the plain (undoped) sol–gel silica was negligible. The maximum adsorption of Co(II) ions onto the TAR-doped sol–gel silica from single solutions was 12.6 mmol/g. The TAR-doped sol–gel silica could be regenerated by washing with a 0.1 M HCl solution to give a maximum regeneration value as high as 98.5%. The TAR-doped sol–gel silica could be used for more than three consecutive adsorption/desorption cycles without experiencing any considerable loss of adsorption capacity. The adsorption process and the nature of the Co–TAR complex were discussed.

Diamond nanoparticles as a way to improve electron transfer in sol-gel L-lactate biosensing platforms

In the present work, we have included for the first time diamond nanoparticles (DNPs) in a sol-gel matrix derived from (3-mercaptopropyl)-trimethoxysilane (MPTS) in order to improve electron transfer in a lactate oxidase (LOx) based electrochemical biosensing platform. Firstly, an exhaustive AFM study, including topographical, surface potential (KFM) and capacitance gradient (CG) measurements, of each step involved in the biosensing platform development was performed. The platform is based on gold electrodes (Au) modified with the sol-gel matrix (Au/MPTS) in which diamond nanoparticles (Au/MPTS/DNPs) and lactate oxidase (Au/MPTS/DNPs/LOx) have been included. For the sake of comparison, we have also characterized a gold electrode directly modified with DNPs (Au/DNPs). Secondly, the electrochemical behavior of a redox mediator (hydroxymethyl-ferrocene, HMF) was evaluated at the platforms mentioned above. The response of Au/MPTS/DNPs/LOx towards lactate was obtained. A linear concentration range from 0.053 mM to 1.6 mM, a sensitivity of 2.6 μA mM(-1) and a detection limit of 16 μM were obtained. These analytical properties are comparable to other biosensors, presenting also as advantages that DNPs are inexpensive, environment-friendly and easy-handled nanomaterials. Finally, the developed biosensor was applied for lactate determination in wine samples.

Efficacy of a new tan doped sol-gel sorbent for uptake of zinc

A TAN (1-(2-Thiazolylazo)-2-naphthol)) doped sol gel silica sorbent (reddish colour, porous, stable, hard, non- swelling) were prepared and investigated for the removal of Zn (II) from aqueous solutions. The kinetics, adsorption isotherm, equilibration time and pH effect on the removal were studied from non competitive aqueous solution to optimize the conditions to be utilized on a large scale. Fast equilibration was observed and adsorption equilibria were reached within 30minutes. Adsorption of zinc ions on the blank (un-doped) sol gel was found negligible. The maximum adsorption of Zn (II) ions onto the TAN doped sol-gel from single solutions were 2.33mg/g (0.035mmol/g). TAN doped sol gel can be regenerated through washing with a solution of 0.1M HCl solution. The maximum regeneration value was as high as 99.0. The TAN-doped sol gel silica is suitable at least for four adsorption-desorption cycles without experiencing considerable loss of adsorption capacity. The uptake route and the nature of the Zn-TAN complex have been discussed.

What can be learned from single molecule spectroscopy? Applications to sol-gel-derived silica materials

Single molecule spectroscopic methods are now being widely employed to probe the nanometer scale properties of sol-gel-derived silica materials. This article reviews a subset of the recent literature in this area and provides salient examples of the new information that can be obtained. The materials covered include inorganic and organically-modified silica, along with surfactant-templated mesoporous materials. Studies of molecule-matrix interactions based on ionic, hydrogen bonding and hydrophobic interactions are reviewed, highlighting the impacts of these interactions on mass transport phenomena. Quantitative investigations of molecular diffusion by single molecule tracking and fluorescence correlation spectroscopy are also covered, focusing on the characterization of anisotropic and hindered diffusion in mesoporous systems. Single molecule polarity studies are described and the new information that can be obtained from the resulting inhomogeneous distributions is discussed. Likewise, single molecule studies of silica acidity properties are reviewed, including observation of nanoscale buffering phenomena due to the chemistry of surface silanols. Finally, related single nanoparticle studies of macroporous silicas are also discussed.

Factors affecting the preparation and properties of electrodeposited silica thin films functionalized with amine or thiol groups

Well-adherent sol-gel-derived silica films functionalized with amine or thiol groups have been electrogenerated on gold electrodes and both the deposition process and the film properties have been studied by various physicochemical techniques. Electrodeposition was achieved by combining the formation of a self-assembled "nanoglue" on the electrode surface, the sol-gel process, and the electrochemical manipulation of pH to catalyze polycondensation of the precursors. Gold electrodes pretreated with mercaptopropyltrimethoxysilane (MPTMS) were immersed in sol solutions containing the selected precursors (tetraethoxysilane, TEOS, in mixture with (3-aminopropyl)triethoxysilane, APTES, or MPTMS) where they underwent a cathodic electrolysis to generate the hydroxyl ions that are necessary to catalyze the formation of the organosilica films on the electrode surface. Special attention was given to analyze the effects of deposition time and applied potential and to compare APTES and MPTMS films. Characterization was made using quartz crystal microbalance, scanning electron microscopy, cyclic voltammetry, and atomic force microscopy (including in situ monitoring). The electrodeposition process was found to occur at two growing rates: a first slow stage giving rise to rather homogeneous, yet rough, films with thickness in the sub-mum range (increasing continuously when increasing the deposition time), which was followed by a faster gelification step resulting in much thicker (>1 microm) and rougher macroporous deposits. These two successive situations were observed independently on the applied potential except that more cathodic values led to narrower sub-microm ranges (as expected from the larger amounts of the electrogenerated hydroxyl catalyst). Thiol-functionalized silica films were deposited more rapidly than the amine ones and, for both of them, permeability to redox probe was found to decrease when increasing the film thickness because of higher resistance to mass transport.

Sol-gel based surface modification of electrodes for electro analysis

In chemical analysis and electrochemical catalysis modified electrodes have been used in a wide range. In addition to the several methods of electrode modification reported in literature, the sol-gel route offers a possibility of preparing ceramic like films under rather mild conditions. This enhances the possibility of incorporation of temperature sensitive particles such as enzymes, microorganisms, proteins and several other biomolecules into such composite layers, resulting in very inert and stable matrices useful for analytical applications. The present paper, in addition to providing an overview of the various analytical applications of sol-gel processes, presents the fabrication and evaluation, of a novel sol-gel based potentiometric sensor for pH measurement. The performance characteristics of the sol-gel based pH sensor is evaluated using buffers in the pH range 2-10 prepared using standard buffer compositions reported in literature. The results show a linear correlation between negative logarithm of hydrogen ion concentration and measured millivolt response.

Electrochemical deposition of sol-gel films for enhanced chromium(VI) determination in aqueous solutions

A pyridine-functionalized sol-gel film has been formed by electrodeposition at a glassy carbon electrode surface. When this protonated film is exposed to a Cr(VI) solution, the Cr(VI) anions are preconcentrated at the electrode surface. Using square wave voltammetry, the Cr(VI) species are reduced to Cr(III), and a peak current corresponding to this reduction is generated at 0.17V. The peak currents can be correlated with the Cr(VI) concentration. The functionalized sol-gel films demonstrated an enhanced sensitivity for Cr(VI) in aqueous solutions, providing a limit of detection at the low ppb level. Interference studies also displayed the high selectivity of the films for Cr(VI), and the system was able to tolerate a large excess of Cr(III) with no adverse affects. The reported electrodeposition method of film formation uses commercially available reagents and yields films quickly and reproducibly. The growth of these sol-gel films was monitored using an electrochemical quartz crystal microbalance (EQCM), and they were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The reported work shows the promise of such an electrode for use in Cr(VI) sensing applications.

Co-immobilization of glucose oxidase and hexokinase on silicate hybrid sol-gel membrane for glucose and ATP detections

The co-immobilization of glucose oxidase (GOD) and hexokinase/glucose-6-phosphate dehydrogenase (HEX) in the silica hybrid sol-gel film for development of amperometric biosensors was investigated. The silica hybrid film fabricated by hydrolysis of the mixture of tetraethyl orthosilicate and 3-(trimethoxysiyl)propyl methacrylate possessed a three-dimension vesicle structure and good uniformity and conformability, and was ready for enzyme immobilization. The electrochemical and spectroscopic measurements showed that the silica hybrid sol-gel provided excellent matrice for the enzyme immobilization and that the immobilized enzyme retained its bioactivity effectively. The immobilized GOD could catalyze the oxidation of glucose, which could be used to determine glucose at +1.0 V without help of any mediator. The competition between GOD and HEX for the substrate glucose involving ATP as a co-substrate led to a decrease of the glucose response, which allowed us to develop an ATP sensor with a good stability. The fabricated silica hybrid sol-gel matrice offered a stage for further study of immobilization and electrochemistry of proteins.

Electrogenerated chemiluminescence biosensor with alcohol dehydrogenase and tris(2,2′-bipyridyl)ruthenium (II) immobilized in sol–gel hybrid material

An ethanol biosensor based on electrogenerated chemiluminescence detection was developed. Electrogenerated chemiluminescence reagent tris(2,2'-bipyridyl)ruthenium (II) and alcohol dehydrogenase were immobilized in the same sol-gel hybrid film. The copolymer poly(vinyl alcohol) with 4-vinylpyridine and cation exchanger Nafion were incorporated into sol-gel film to provide the microenvironment for retaining the activity of enzyme and immobilize tris(2,2'-bipyridyl)ruthenium (II). The design was simpler than the previous two-layer format. The experimental conditions, such as scan rate, pH and concentration of the cofactor were investigated. The intensity of electrogenerated chemiluminescence increased linearly with ethanol concentration from 2.5x10(-5) to 5.0x10(-2) M and detection limit was 1.0x10(-5) M. The prepared biosensor exhibited high sensitivity, wide linear range and good stability.

Controlling diffusion in sol-gel derived monoliths

Redox probes were trapped within a silica monolith prepared in part with organoalkoxysilanes containing a quaternary ammonium functional group. The diffusion coefficients of the entrapped molecules were measured as the gels were slowly dried using chronoamperometry and cyclic voltammetry with ultramicroelectrodes. Gel-entrapped cobalt(II) tris(bipyridine) (Co(bpy)(3)(2+)) diffuses at rates similar to that measured in the sols by incorporating a small amount of the positively charged functional group in the matrix. In comparison, the diffusion coefficient of gel-entrapped ferricyanide (Fe(CN)(6)(3-)) drops an order of magnitude relative to its value in the sol soon after gelation. These results demonstrate the ease at which diffusion in hydrated gels can be easily controlled by simply changing the charge on the walls of the silica host.

Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device. 9. Incorporation of planar waveguide technology

Incorporation of planar waveguide technology into a spectroelectrochemical sensor is described. In this sensor design, a potassium ion-exchanged BK7 glass waveguide was over-coated with a thin film of indium tin oxide (ITO) that served as an optically transparent electrode. A chemically selective film was spin-coated on top of the ITO film. The sensor supported five optical modes at 442 nm and three at 633 nm. Investigations on the impact of the ITO film on the optical properties of the waveguide and on the spectroelectrochemical performance of the sensor are reported. Sensing was based on the change in attenuation of light propagated through the waveguide resulting from an optically absorbing analyte. By applying either a triangular or square wave excitation potential waveform, electromodulation of the optical signal has been demonstrated with Fe(CN)6(3-/4-) as a model electroactive couple that partitions into a PDMDAAC-SiO2 film [where PDMDAAC = poly(dimethyldiallylammonium chloride)] and absorbs at 442 nm.

Silica sol-gel composite film as an encapsulation matrix for the construction of an amperometric tyrosinase-based biosensor

An amperometric tyrosinase enzyme electrode for the determination of phenols was developed by a simple and effective immobilization method using sol-gel techniques. A grafting copolymer was introduced into sol-gel solution and the composition of the resultant organic-inorganic composite material was optimized, the tyrosinase retained its activity in the sol-gel thin film and its response to several phenol compounds was determined at 0 mV vs. Ag/AgCl (sat. KCl). The dependences of the current response on pH, oxygen level and temperature were studied, and the stability of the biosensor was also evaluated. The sensitivity of the biosensor for catechol, phenol and p-cresol was 59.6, 23.1 and 39.4 microA/mM, respectively. The enzyme electrode maintained 73% of its original activity after intermittent use for three weeks when storing in a dry state at 4 degrees C.