Modified electrodes behavior

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

A. Eftekhari, Electrochemical behavior and electrocatalytic activity of a zinc hexacyanoferrate film directly modified electrode. J. Electroanal. Chem. 537, 59-66 (2002). 

Thin-film ideal or Nemstian behavior is the starting point to explain the voltammetric behavior of polyelectrolyte-modified electrodes. This condition is fulfilled when (i) the timescale of the experiment is slower than the characteristic timescale for charge transport (fjD pp, with Ithe film thickness) in the film, that is all redox within the film are in electrochemical equibbrium at any time, (ii) the activity of redox sites is equal to their concentration and (iii) all couples have the same redox potential. For these conditions, anodic and cathodic current-potential waves are mirror images (zero peak splitting) and current is proportional to the scan rate [121]. Under this regime, there exists an analytical expression for the current-potential curve ... 

It is important to clarify that there have been, in the literature, some examples of electrochemical processes on CNT-modified electrodes on which an apparent electrocatalytic process associated to the CNTs seems to take place (that is from the edge-plane-like sites) where in fact that was not the case. An example is the apparent electrocatalytic oxidation ofhydrazine at MWNTelectrodes [64,65]. Such electrochemical behavior has been demonstrated to be a consequence of iron impurities contained in the CNTs that were responsible for the observed electrocatalytic effects (Figure 3.7). Therefore, caution is needed when reporting catalytic effects of CNTs under a given redox system and a careful comparison vdth, for instance, edge HOPG is mandatory to make sure that the CNTs are the responsible for the electrochemical enhancement. 

CNT randomly dispersed composites Many soft and rigid composites of carbon nanotubes have been reported [17]. The first carbon-nanotube-modified electrode was made from a carbon-nanotube paste using bromoform as an organic binder (though other binders are currently used for the paste formation, i.e. mineral oil) [105]. In this first application, the electrochemistry of dopamine was proved and a reversible behavior was found to occur at low potentials with rates of electron transfer much faster than those observed for graphite electrodes. Carbon-nanotube paste electrodes share the advantages of the classical carbon paste electrode (CPE) such as the feasibility to incorporate different substances, low background current, chemical inertness and an easy renewal nature [106,107]. The added value with CNTs comes from the enhancement of the electron-transfer reactions due to the already discussed mechanisms. 

Chen and coworkers were the first to describe the electrochemical behavior of PB NPs [41]. They prepared solutions of PB NPs by reaction of Fe(III) with Fe(CN)6 in the presence of H2O2, giving 30-50-nm diameter NPs. These were then immobilized at cysteine-modified Au electrodes with pendant amine groups by prolonged (10 h) exposure of the modified electrode to a solution of the NPs. This produced a monolayer of immobilized NPs that was subsequently examined using cyclic voltammetry. Interestingly, they observed two redox processes near 0.25 V vs. 

Without doubt, the most noteworthy aspect of the redox behavior of the synthesized organometallic dendritic macromolecules 1-6, having a predetermined number of noninteracting ferrocenyl redox centers, is their ability to modify electrode surfaces. In this way, for the first time, electrode surfaces have been successfully modified with films of dendrimers containing reversible four- and eight-electron redox systems, resulting in detectable electroactive materials persistently attached to the electrode surfaces. ... 

CV investigations of 6-mercaptopurine and 8-mercaptoquinoline SAMs on pc-Au electrodes have been presented by Madueno et al. [186] and He etal. [187], respectively. Several model electrode reactions involving various redox probes were studied using such modified electrodes. Baunach and Kolb etal. [188] have deposited copper on disordered benzyl mercaptan film on Au(lll) surfaces. They have also studied the behavior of benzyl mercaptan SAM on Au(lll) in H2SO4 solution using CV and STM. Structural and electrical properties of SAMs based on tetrathiafulvalene derivatives on Au(lll) were investigated. These mono-layers were disordered, or at least loosely... 

Cooke and coworkers have reported preparation of flavin-modified electrodes using a similar electropolymerization procedure.34 They have also studied electrodes coated with self-assembled monolayers (SAMs) formed from both flavin39 and phenanthrenequinone disulfides.59 The monolayers are stable in CH2C12 solution, and, as with the electrodes formed from 30, show redox-dependent binding behavior similar to that seen in solution. Interestingly, the phenanthrenequinone SAM... 

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. 

In the last decade, there has been a large number of reports on synthetic macro-molecule-metal complexes concerning their complexation, catalytic activities, redox reactions, adsorptions of gaseous molecules and metal ions, photochemical behavior, biochemical effects, modified electrodes, semiconductive and conductive materials, and so on. 

Electroanalytical sensors based on amperometric measurements at chemically modified electrodes are in the early stages of development. The modes of modification can take many forms, but the most common approach at the present time is the immobilization of ions and molecules in polymer films which are applied to bare metal, semiconductor, and carbon electrodes. Such surface-modified electrodes exhibit unique electrochemical behavior which has been exploited for a variety of applications. 

In recent years, considerable effort has gone into the development of a new class of electrochemical devices called chemically modified electrodes. While conventional electrodes are typified by generally nonspecific electrochemical behavior, i.e., they serve primarily as sites for heterogeneous electron transfer, the redox (reduction-oxidation) characteristics of chemically modified electrodes may be tailored to enhance desired redox processes over others. Thus, the chemical modification of an electrode surface can lead to a wide variety of effects including the retardation or acceleration of electrochemical reaction rates, protection of electrodes, electro-optical phenomena, and enhancement of electroanalytical specificity and sensitivity. As a result of the importance of these effects, a relatively new field of research has developed in which the... 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

The general electrochemical behavior of surface-bound molecules is treated in Sect. 6.4. The response of a simple electron transfer reaction in Multipulse Chronoamperometry and Chronocoulometry, CSCV, CV, and Cyclic Staircase Voltcoulometry and Cyclic Voltcoulometry is also presented. Multielectronic processes and first- and second-order electrocatalytic reactions at modified electrodes are also discussed extensively. 

The electrochemical behavior of molecules attached to conducting surfaces (i.e., electrode surfaces) forming electro-active monolayers is discussed in the following sections. This situation has frequently been called modified electrodes in the literature [39]. The electro-active character of these monolayers arises from the presence of redox molecules on them which are susceptible to transfer to or receive charge from the supporting electrode as well as from species in solution (in this last situation, the attached molecules act as redox mediators between the electrode and the solution and are responsible for the appearance of electrocatalytic processes [39, 40]). Multipulse and Sweep Electrochemical techniques like SCV and CV have proven to be very necessary tools for understanding the behavior of these interfaces and the processes taking place at them. 

Electrochemical Behavior of PPy-GOD Film Electrode with Pharmaceutical Drugs. Although the chemically modified electrode has been developed for more than a decade, and many kinds of materials have been used for the modification of the electrode surface, the enzyme modified electrode has rarely been used for the study of electroactive species. This is probably due to the fact that the enzyme is not electronically conductive and also it is difficult to immobilize an enzyme on the electrode surface. So far, biological lipids have been used to modify the electrode, and the modified electrode shows a selectivity for hydrophobic molecules because the lipid molecule is hydrophobic (37-39). 

Polymer-modified electrodes have shown considerable utility as redox catalysts. In many cases, modified electrode surfaces show an improved electrochemical behavior towards redox species in solution, thus allowing them to be oxidized or reduced at less extreme potentials. In this manner, overpotentials can be eliminated and more selective determination of target molecules can be achieved. In this discussion, a mediated reduction process will be considered, although similar considerations can be used to discuss mediated oxidation processes. This mediation process between a surface-bound redox couple A/B and a solution-based species Y can be describes by the following ... 

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