Chemically modified substrate surfaces

An exciting prospect in synthetic organic electrochemistry is the selective synthesis of specific optical isomers by taking advantage of the asymmetry afforded by certain types of surface sites achieved with chemically modified electrode surfaces on substrates such as carbon. 

In another study, the so-called sohd-liquid-solid (SLS) technique was used to prepare a silicon oxide nanofiber surface [64]. The amorphous sihcon oxide nanofibers shown in Fig. 10b were made by heating a silicon substrate coated with a thin gold layer at 1100°C for 3 h in a lutrogen atmosphere in contrast to the previous example, no additional source of silicon materials was used in this case. As before, the nanofiber growth starts at the interface of the Au/Si alloy droplet and the Si substrate and is maintained by the diffusion of the Si atoms from the substrate to the interface [65]. The surface was then UV/ozone-treated to generate surface hydroxyl groups that were subsequently reacted with perfluorodecyltrichlorosilane. The chemically modified nanofiber surface exhibited a WCA of 152°. 

After the cleaning process, other techniques are used to prepare the surface of the substrate for coating. Some techniques include drying, surface etching, and chemical surface preparation. Examples of chemical surface preparation include the formation of an oxide layer or the monolayer assembly of an adhesion promoter on the surface. These processes modify the surface of the substrates so as to facilitate the subsequent deposition process. In surface preparation, frequently, the hydrophilic/hydrophobic character of the surface is controlled to match the coating solution properties. For example, Van Driessche et al.19 reported on improving the wettability of Ni-4at%W tapes... 

The ink-jet process relies on using a piezoelectric printhead that can create deformation on a closed cavity through the application of an electric field. This causes the fluid in the cavity to be ejected through the nozzle whose volume is determined by the applied voltage, nozzle diameter, and ink viscosity. The final width of the drop of the substrate is a result of the volume of fluid expelled and the thickness of the droplet on the surface. In addition, the drop placement is critical to the ultimate resolution of the display. Typical volumes expelled from a printhead are 10 to 40 pi, resulting in a subpixel width between 65 and 100 pm. Drop accuracies of +15 pm have been reported such that resolutions better than 130 ppi are achievable however, because the solvent to polymer ratio is so high, the drops must be contained during the evaporation process to obtain the desired resolution and film thickness. This containment can be a patterned photoresist layer that has been chemically modified so that the EL polymer ink does not stick to it. 

In the category of silicone coatings used for surface modification of the specific substrates, functional silicone fluids are often used, which can selectively interact with the chemical groups of the substrate, thus modifying its surface properties. The use of functional silicones in the textile industry has been discussed in a number of recent publications.5 421 422 The use of different types of high-performace silicone-coated textiles, which include elastomers and resins, has recently been reviewed.423 The use of functional silicones in personal-care products, for example, in shampoos and hair conditioners, mentioned before,381 provides another well-known example. 

Hill etal. (2001) modified wood surfaces with methacrylic anhydride and grafted the activated surface with styrene in order to see if this would improve the UV stability of the modified substrate. There was no evidence to suggest that UV stability was improved either by chemical modification or by modification plus grafting. 

Moisture acts as a debonding agent through one of or a combination of the following mechanisms 1) attack of the metallic surface to form a weak, hydrated oxide interface, 2) moisture assisted chemical bond breakdown, or 3) attack of the adhesive. (2 ) A primary drawback to good durability of metal/adhesive bonds in wet environments is the ever present substrate surface oxide. Under normal circumstances, the oxide layer can be altered, but not entirely removed. Since both metal oxides and water are relatively polar, water will preferentially adsorb onto the oxide surface, and so create a weak boundary layer at the adhesive/metal interface. For the purposes of this work, the detrimental effects of moisture upon the adhesive itself will be neglected. The nitrile rubber modified adhesive used here contains few hydrolyzable ester linkages and therefore will be considered to remain essentially stable. 

The corrosion resistance and polymer-bonding compatibilities of the lonizable organophosphonates and the neutral organo-silanes are directly related to their inherent chemical properties. Specifically, NTMP inhibits the hydration of AI2O2 and maintains or Improves bond durability with a nitrile-modified epoxy adhesive which is cured at an elevated temperature. The mercaptopropyl silane, in addition to these properties, is compatible with a room temperature-cured epoxy-polyamide primer and also exhibits resistance to localized environmental corrosion. These results, in conjunction with the adsorbed Inhibitor films and the metal substrate surfaces, are subsequently discussed. 

An improved adsorption of DNA bases has been observed at a chemically modified electrode based on a Nafion/ruthenium oxide pyrochlore (Pb2Ru2-x FhxOj-y modified GC (CME). Nafion is a polyanionic perfiuorosulfonated ionomer with selective permeability due to accumulation of large hydrophobic cations rather than small hydrophilic ones. The Nafion coating was demonstrated to improve the accumulation of DNA bases, while the ruthenium oxide pyrochlore proved to have electrocatalytic effects towards the oxidation of G and A. The inherent catalytic activity of the CME results from the Nafion-bound oxide surface being hydrated. The catalytically active centers are the hydrated surface-boimd oxy-metal groups which act as binding centers for substrates [50]. 

The charges are usually mainly located at the particle or substrate surface. Their origin is generally mainly due to the chemical terminations of these surfaces. These terminations are in equilibrium with the solution and can therefore be modified by the pH or by some species, to a certain extent in the same way as ion exchange resins. 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

The second topic of this chapter is the role of coordination compounds in advancing electrochemical objectives, particularly in the sphere of chemically modified electrodes. This involves the modification of the surface of a metallic or semiconductor electrode, sometimes by chemical reaction with surface groups and sometimes by adsorption. The attached substrate may be able to ligate, or it may be able to accept by exchange some electroactive species. Possibly some poetic licence will be allowed in defining such species since many interesting data have been obtained with ferrocene derivatives thus these organometallic compounds will be considered coordination compounds for the purpose of this chapter. 

This review gives a brief summary of the "types of chemically modified electrodes, their fabrication, and some examples of their uses. One especially promising area of application is that of selective chemical analysis. In general, the approach used is to attach to the electrode surface electrochemically reactive molecules which have electrocatalytic activity toward specific substrates or analytes. In addition, the incorporation of biochemical systems should greatly extend the usefulness of these devices for analytical purposes. 

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