Abstract

Sol–gel films were electrodeposited on aluminum electrodes following the methodology we have developed and is based on applying negative potentials. This increases the pH at the surface, causing acceleration of the polymerization. Our process follows the “two step method”, in which the monomer is first hydrolyzed in acidic solution (pH ∼ 4) and only then the negative potential is applied, which consumes protons and releases hydroxyl ions, thus enhancing the condensation.

Films made of different monomers, i.e., tetraethoxysilane (TEOS), methyl trimethoxysilane and phenyl trimethoxysilane (PTMOS), were prepared, characterized and examined for their corrosion inhibition properties. Potentiodynamic polarization, electrochemical impedance spectroscopy, optical and scanning electron microscopy as well as atomic force microscopy have been used as a means of film characterization. Hydrophobic and steric silanes, such as PTMOS showed a considerable corrosion inhibition capacity as compared to the capacity exhibited by less hydrophobic and steric derivatives such as TEOS. The difference between the conventional dip-coating method and the electrodeposition approach for depositing sol–gel films was also examined, indicating a clear advantage of the latter.

Abstract

We report here on the successful incorporation of organic anions into a sol–gel film on Al as a means of enhancing the protection against corrosion. Following our previous study where we showed that hydrophobic sol–gel films provided pronounced corrosion inhibition, we studied the corrosion inhibition that phenylphosphonic acid (PPA) has when embedded inside a thin sol–gel coating on Al. The anion of this organic anion tends to stay inside a phenyltrimethoxysilane (PTMOS) based sol–gel film due to π-interactions. Our findings, which are derived primarily from potentiodynamic polarization measurements, electrochemical noise, scanning electron microscopy measurements and Auger electron spectroscopy (AES), clearly show that the organic phosphonate adds to the protection efficiency of the sol–gel film.

Abstract

This work points out that electrogeneration of silica gel (SG) films on glassy carbon electrodes (GCEs) can be applied to immobilize biomolecules – hemoglobin (Hb) or glucose oxidase (GOD) or both of them in mixture – without preventing their activity. These proteins were physically entrapped in the sol–gel material in the course of the electro-assisted deposition process applied to form the thin films onto the electrode surface. SG films were prepared from a precursor solution by applying a suitable cathodic potential likely to induce a local pH increase at the electrode/solution interface, accelerating thereby polycondensation of the silica precursors with concomitant film formation. Successful immobilization of proteins was checked by various physico-chemical techniques. Both Hb and GOD were found to undergo direct electron transfer, as demonstrated by cyclic voltammetry. GCE–SG–Hb gave rise to well-defined peaks at potentials E_c = −0.29 V and E_a = −0.17 V in acetate buffer, corresponding to the Fe^III/Fe^II redox system of heme group of the protein, while GCE–SG–GOD was characterized by the typical signals of FAD group at E_c = −0.41 V and E_a = −0.33 V in phosphate buffer. These two redox processes were also evidenced on a single voltammogram when both Hb and GOD were present together in the same SG film. Hb entrapped in the silica thin film displayed an electrocatalytic behavior towards O₂ and H₂O₂ in solution, respectively in the mM and μM concentration ranges. Immobilized GOD kept its biocatalytic properties towards glucose. Combined use of these two proteins in mixture has proven to be promising for detection of glucose in solution via the electrochemical monitoring of oxygen consumption (decrease of the oxygen electrocatalytic signal).

Abstract

TeO_x–SiO₂ composite films having third-order nonlinearities were prepared by electrochemically induced sol–gel deposition method on ITO substrate. The third-order optical nonlinearities of the films were measured by Z-scan technique. The third-order nonlinear susceptibilities (χ⁽³⁾) of the as-prepared films are 5.9 × 10⁻⁷ to 4.29 × 10⁻⁶ esu. The surface morphology and composition of the films were characterized by SEM/EDX, which identified that Te metallic particles well dispersed in TeO_x–SiO₂ gel films.