Modified electrodes characterization

Calfuman K, Aguirre M, Villagra D, Yanez C, Arevalo C, Matsuhiro B, Mendoza L, Isaacs M (2010) Nafion/tetraruthenated porphyrin glassy carbon-modified electrode characterization and voltammetric smdies of sulfite oxidation in water-ethanol solutions. J Solid State Electrochem 14(6) 1065-1072... 

For the in situ characterization of modified electrodes, the method of choice is electrochemical analysis by cyclic voltammetry, ac voltammetry, chronoamperometry or chronocoulometry, or rotating disk voltametry. Cyclic voltammograms are easy to interpret from a qualitative point of view (Fig, 1). The other methods are less direct but they can yield quantitative data more readily. 

SETM) give hope that the full structural characterization of modified electrodes will be achieved in the near future. 

Fig. 2a-c. Kinetic zone diagram for the catalysis at redox modified electrodes a. The kinetic zones are characterized by capital letters R control by rate of mediation reaction, S control by rate of subtrate diffusion, E control by electron diffusion rate, combinations are mixed and borderline cases b. The kinetic parameters on the axes are given in the form of characteristic currents i, current due to exchange reaction, ig current due to electron diffusion, iji current due to substrate diffusion c. The signpost on the left indicates how a position in the diagram will move on changing experimental parameters c% bulk concentration of substrate c, Cq catalyst concentration in the film Dj, Dg diffusion coefficients of substrate and electrons k, rate constant of exchange reaction k distribution coefficient of substrate between film and solution d> film thickness (from ref. 

The initial hurdle to overcome in the biosensor application of a nucleic acid is that involving its stable attachment on a transducing element which commonly includes a metallic electrode. In the first part of this chapter, we wish to introduce our approach for DNA immobilization (Scheme 1). A detailed characterization of the immobilization chemistry is also presented. In the second part, we follow the development of work from our laboratory on chemical sensor applications of the DNA-modified electrode involving a biosensor for DNA-binding molecules and an electrochemical gene sensor. 

Vertically aligned CNT-modified electrodes are based on a more elaborated technique than other methods, and microscopic images are used to characterize the integrity of this type of electrode. The technique has been applied for the immobilization of enzymes and DNA, and the sensors based on this technique have shown a lower detection limit than those based on other methods. More research activities using this technique, particularly with low density CNT arrays, are expected in the near future because of its sensitivity and versatility. 

Repeated scanning results in a continuous increase in the size of the wave at 0.6 V vs. Ag+/Ag, which is believed to be the oxidation wave of the polythiophene backbone of poly(I). The amount of polymer produced per unit of charge in the deposition has not been measured, but is qualitatively similar to polythiophene itself. A film of poly(I) on the Pt electrode can be seen with the naked eye. After deposition of poly(I), the polymer-modified electrode is transferred, after thorough rinsing, to clean electrolyte for further characterization. 

One other design developed by Wang s group uses the same base sensor (GCE), which is coated with a layer of poly(4-vinylpyridine) (PVP). This cationic polyelectrolyte was one of the first polymers used to modify electrode surfaces [27]. Much research effort in this context has been directed to the characterization of the transport and electrostatic binding of multi-charged anions at PVP-coated electrodes. The ability of this polymer... 

For further characterization of the protein multilayer, two electrochemical techniques were employed independently. The first technique involves amperometric measurements of the GOx multilayer-modified electrode in the presence of glucose. Figure 15 plots the output current (A/) of the electrode to 1 mM glucose as a function of the deposition number. The response current depends linearly on the number... 

The electrochemical behavior of a modified electrode ultimately depends on structural details at the molecular level. For example, the molecular-level interaction between the redox site in the film and the solvent from the contacting solution phase might play an important role in the electrochemical response. Molecular-level details are often difficult to infer from electrochemical methods alone, but do lend themselves to spectroscopic analyses. In recent years there has been an explosion of new spectroscopic techniques for characterizing modified electrodes and the electrode-solution interface in general [44,45]. In this section, we review some of these spectroelectrochemical methods. 

To characterize the properties of molecules and polymer films attached to an electrode surface, a wide variety of methods have been used to measure the electroactivity, chemical reactivity, and surface structure of the electrode-immobilized materials [9]. These methods have been primarily electrochemical and spectral as indicated in Table I. Suffice it to say that a multidisciplinary approach is needed to adequately characterize chemically modified electrodes combining electrochemical methods with surface analysis techniques and a variety of other chemical and physical approaches. 

In the last 20 years, a big effort has been made to characterize different reaction schemes taking place at modified electrodes, with special focus on the case of biomolecules in what has been called Protein Film Voltammetry [79-83]. Among the different situations analyzed with multipulse techniques (including Cyclic Voltammetry), it can be cited the surface ECE process [89, 90] and surface reactions preceded by homogeneous chemical reactions [91]. For a more detailed revision of the different mechanisms analyzed in the case of SWV, see [19]. 

The detection of many analytes on unmodified electrodes occurs at high potentials, hence the need for modified electrodes. Even though different types of SAMs have been reported, only a few of them have been employed as catalysts for electrochemical reactions. Most work has concentrated on the characterization of SAMs, with limited studies devoted to detection. Table 3 summarizes the analytes which have been detected by employing NIR-absorbing MPcs (such as MnPc and TiPc) containing amino or alkylthio substituents. In Table 3, the electrodes are modified by SAMs, polymers, electrodeposion, and adsorption. 

Trollund E, Ardiles P, Aguirre M J, Biaggio SR, Rocha-Filho RC (2000) Spectroelectrochemical and electrical characterization of poly(cobalt-tetra aminophthalocyanine)-modified electrodes electrocatalytic oxidation of hydrazine. Polyhedron 19(22-23) 2303-2312... 

SERS has also been found to be valuable in characterizing modified electrodes and electrochemical processes (61). SERS was used to observe the sequential electrochemical formation of adsorbed O2-, OH- and H2O on a silver electrode in 0.1-0.001 M alkali chloride solution containing... 

It has been shown that the thickness of the polypyrrole (PPy) film has a significant effect on the electrode performance.17 Figure 6 shows the dependence of the response of PPy/PQQ modified electrode to 10 mM DMAET (A) and 10 mM DEAET (B) as a function of PPy film thickness. As film thickness increases the oxidation current increases for both DMAET and DEAET, presumably due to increases in the amount of PQQ loaded in the PPy film. The maximum current for the oxidation of PQQH2 is observed when 200 nm films are used. When the PPy film thickness was larger then 200 nm a decrease in the sensor response was observed, which could be due to increased resistance (R ) of the thicker film. The optimum 200 nm PPy film thickness was used to characterize the performance of the electrode for amperometric detection of thiols. 

Characterization of modified electrodes can be carried out by electrochemical, spectroscopic, and microscopic methods. Of the electrochemical methods we stress cyclic voltammetry, chronocoulometry, and impedance, which combined together measure the number of redox centres, film conductivity, kinetics of the electrode processes, etc. Almost all the non-electrochemical techniques described in Chapter 12 have been employed for the characterization of modified electrodes. 

Since the appearance of the redox [ii, iii] and conducting [iv] polymer-modified electrodes much effort has been made concerning the development and characterization of electrodes modified with electroactive polymeric materials, as well as their application in various fields such as -> sensors, actuators, ion exchangers, -> batteries, -> supercapacitors, -> photovoltaic devices, -> corrosion protection, -> electrocatalysis, -> elec-trochromic devices, electroluminescent devices (- electroluminescence) [i, v-viii]. See also -> electrochemically stimulated conformational relaxation (ESCR) model, and -> surface-modified electrodes. 

We briefly mention here the use of the ferrocene/ferrocenium redox couple to mediate electron transfer on the oxidation (anodic) side, especially in derivatized electrode. This broad area has been reviewed [349]. For instance, polymers and dendrimers containing ferrocene units have been used to derivatize electrodes and mediate electron transfer between a substrate and the anode. Recently, ferrocene dendrimers up to a theoretical number of 243 ferrocene units were synthesized, reversibly oxidized, and shown to make stable derivatized electrodes. Thus, these polyferrocene dendrimers behave as molecular batteries (Scheme 42). These modified electrodes are characterized by the identical potential for the anodic and cathodic peak in cyclic voltammetry and by a linear relationship between the sweep rate and the intensity [134, 135]. Electrodes modified with ferrocene dendrimers were shown to be efficient mediators [357-359]. For the sake of convenience, the redox process of a smaller ferrocene dendrimer is represented below. 

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