Electrochemically deposited sol-gel films

Besides the apphcations described above, electrochemically deposited sol-gel films have also been used for fabricating solid-phase microextraction (SPME) fibers, nonlinear optical films, antireflection coatings, and electrocatalytic and... 

Deepa, P. N., M. Kanungo, G. Claycomb, P. M. A. Sherwood, and M. M. Collinson, 2003. Electrochemically deposited sol-gel-derived silicate films as a viable alternative in thin-film design. Anal Chem 75 5399-405. 

Carrington, N. A., Yong, L. and Xue, Z. L. (2006), Electrochemical deposition of sol-gel films for enhanced chromium(VI) determination in aqueous solutions. Anal. Chim. Acta, 572(1) 17-24. 

Most film and particle formation techniques can be divided into gas-phase and liquid-phase deposition processes, which are briefly discussed in this section. Deposition of metal and metal oxides from metal enolate sources results from application of CVD, ALD, spin-coating, electrochemical and sol-gel methods, which are discussed in detail... 

Sol-gel films can be used to immobilize biomolecules. For example, Dong etal. developed thin sol-gel films on electrodes in which enzymes horseradish peroxidase (HRP) and glucose oxidase (GOD) were used to determine enzyme substrates [12]. A vapor-deposition sol-gel process [13] featuring a thin film of aqueous HRP solution into which titanium isopropoxide vapor was diffused to form a Ti02 sol-gel film incorporating HRP prevented denaturation of the enzyme. Surfactants were used in preparation of sol-gel films incorporating redox proteins on electrodes to improve porosity and electrochemical and catalytic performance [14]. 

The most common sol—gel system that has been electrochemically deposited is based on silane, although electrochemical deposition of non-silicon sol-gel films from metal alkoxides has also been reported [3-6]. Early studies mainly dealt with the electrochemical deposition behavior of different silanes and their mixtures. The silanes include tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and organofunctional silanes such as 3-aminopropyltriethoxysilane (APTES) and 3-mercaptopropyltrimethoxysilane (MPTMS). The electrochemically deposited silane films find applications in electroanalysis, corrosion... 

The electrochemical deposition approach is versatile and applicable to various sol-gel systems, including silanes and metal alkoxides. Most of the research has focused on the electrodeposition of silane-based sol-gel films. The latter are typically electrodeposited from aqueous solutions of the monomers in the presence of ethanol or methanol as a cosolvent The deposition is usually carried out at cathodic potentials, although a few groups [11,14,15] also reported the anodic electrodeposition process. Electrodeposition of non-silicon sol-gel films from... 

The electrochemical deposition of sol-gel films provides an alternative for shifting the pH on the substrate. In aerated aqueous media, it is well known that by applying cathodic potential, the following reactions occur at the electrode surfece [16,18] ... 

Figure 12.2 Scheme of the cathodic electrochemical deposition of sol-gel films. 

A main advantage of electrochemical deposition is the ease of manipulation by deposition parameters, such as potential, time, concentration of monomers, and water content in the deposition solution. Many works have shown that the thickness and properties of sol-gel films are strongly dependent on the deposition conditions [2,18,21,36]. Specifically, nanostructured sol-gel films can be electro-deposited using a template as a building block. The templates used include an anodized aluminum oxide (AAO) membrane and a cetyltrimethylammonium bromide (CTAB) surfectant [28,37]. In addition, the electrodeposition of sol-gel films can also be spatially confined to conductive patterns on the electrode surface [38], or localized by a microelectrode using scanning electrochemical microscopy (SECM) [39,40]. 

It should be noted that although the effect of deposition parameters on the electrochemical deposition of sol-gel films has been thoroughly investigated by many different groups aiming at tuning the thickness and other properties (e.g., hydrophobicity, roughness, and corrosion resistance) of the films, the manipulation of the deposition process is still based on experience in different systems. 

The main driving force for the electrochemical deposition of sol—gel films is the electrolysis of H2O, which not only generates OH ions but also induces H2 evolution on the cathode. The latter causes the deposited films to be porous. This structure may deteriorate the corrosion resistance of the films as reported by Hu et al. [18,42], but on the other hand it is beneficial for the application of the films in electroanalysis, as it does not completely block electron transfer on the electrode [20,28]. The pores induced by H2 are large, and the obtained films are macroporous consisting of silica particles with diameter of a few himdred nanometers. In order to better tune the morphology of the films, templates are desired for electrodeposition. 

Surfactants have also been used as a soft template for the electrochemical deposition of mesoporous silica films. Walcarius et al [28] demonstrated the elec-trochemically assisted self-assembly of surfactant-templated silica thm films on various conductive substrates (Au, Pt, glassy carbon, ITO, and Cu). The films were highly ordered and homogeneous over wide areas. Figure 12.10 shows the TEM images of the electrodeposited mesoporous silica films. Hexagonal pores are seen to be well ahgned perpendicular to the surface. It is believed that the cathodic potential applied on the substrate not only drove the sol-gel film... 

Local Electrochemical Deposition of Sol-Gel Films by Scanning Electrochemical Microscopy... 

The electrochemical deposition of sol-gel films could also be localized using a microelectrode. The experiments were carried out on SECM, which is known as... 

In summary, the electrochemical deposition of sol-gel films can be well manipulated by deposition parameters such as potential, time, and concentration of the... 

The electrochemical deposition approach was initially used for depositing orga-nofunctional silane sol-gel films for promoting adhesion between aluminum alloy and epoxy resin [Ij. Later, it was used for preparing silane films for corrosion protection of metals, electrochemical stripping analysis of metal ions, and microextraction of organic compounds [18,20,22,42,45]. The recent development of the electrochemical deposition of sol-gel-based composite films opens various applications such as encapsulating proteins, enzymes, and bacteria. In this section, we will show three main applications of eiectrodeposited sol-gel films, that is, corrosion protection and adhesion promotion, electrochemical sensors, and finally biocomposite films. Other applications such as solid-phase microextraction (SPME), nonlinear optics, antireflection, electrocatalysis, and superhydrophobic films are also discussed. 

Sol-gel-derived silane-based materials have been proven as a suitable matrix for entrapment of bioactive species, due to biocompatibility of silica and the mild operating conditions. We recall that the solutions used for cathodic electrochemical deposition of silane-based sol-gel films are usually mild acidic (pH 3-6), and the deposition is achieved by electrochemically driving the inter- cial pH near the cathode to mild basic, about pH 8. This environment is favorable for maintaining the activity of biological species such as proteins, enzymes, and bacteria. Many researchers have reported the fabrication of biosensing films by co-electrodeposition of silane with different bioactive substances. The essence of the concept is to entrap bioactive substances within the sol-gel matrix during... 

The above studies suggest that electrochemical deposition provides a versatile, yet simple route for immobilizing proteins, enzymes, and bacteria in silane sol-gel films. The biological species remained active in the electrodeposited composite films, allowing their applications for biosensing. More sophisticated films with redox mediators, metal nanoparticles, and CNTs have also been constructed via the sol-gel co-electrodeposition approach. 

Although the electrodeposition of sol-gel films can be well manipulated based on the experimental data, the kinetics of the deposition process is still not quantified. This is desired to be solved in the near future as it is important for better understanding and controlling the process. Other future aspects may include the electrodeposition from nonaqueous solutions using other electrochemically induced catalytic reactions, and improving the spatial resolution of patterning. 

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