Electrochemical redox active electrodes

Although at present their use has been restricted to redox-active sensors in solution, it should be possible to immobilise these receptors at an electrode, and we may then have a simple redox-active electrode whose behaviour in solution is modified by the presence of ions. The high sensitivity of electrochemical techniques would then give us a sensitive and selective method of anion detection. 

Second, in designing new molecule-based electronic devices, one of the major goals is the precise control of the current flowing between the terminals. Electrochemical molecular junctions allow for control of the potentials of the electrodes with respect to the redox potential of incorporated redox-active molecules with well-defined, accessible, tunable energy states. These junctions represent unique systems able to predict precisely at which applied potential the current flow will take off. Even though the presence of a liquid electrolyte represents a detriment towards possible applications, they provide the concepts for designing molecular devices that mimic electronic functions and control electrical responses. 

Electrochemical communication between electrode-bound enzyme and an electrode was confirmed by such electrochemical characterizations as differential pulse voltammetxy. As shown in Fig. 11, reversible electron transfer of molecularly interfaced FDH was confirmed by differential pulse voltammetry. The electrochemical characteristics of the polypyrrole interfaced FDH electrode were compared with those of the FDH electrode. The important difference between the electrochemical activities of these two electrodes is as follows by the employment of a conductive PP interface, the redox potential of FDH shifted slightly as compared to the redox potential of PQQ, which prosthetic group of FDH and the electrode shuttling between the prosthetic group of FDH and the electrode through the PP interface. In addition, the anodic and cathodic peak shapes and peak currents of PP/FDH/Pt electrode were identical, which suggests reversibility of the electron transport process. 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

As can be seen, the increase in separation between the electrode surface and the redox-active site operated by the variable-in-length carbon chain from (CH2)3 to (CH2)n makes the process change from electrochemically reversible to irreversible. In fact, in agreement with the (simplified) Marcus equation, ket = 1013/e, the rate of the electron transfer is considerably reduced with the distance d. 

More recent approaches to the effects of the ligands on the redox activity of metal complexes are based upon the assumption that the electrode potential of a redox change involving a metal complex is determined by the additivity of the electronic contribution of all the ligands linked to the metal centre, or to the overall balance between the c-donor and the 7r-acceptor capability of each ligand.3 In particular two ligand electrochemical parameters have gained popularity ... 

In this chapter we describe the use of polyelectrolytes carrying redox-active centers on electrode surfaces with particular emphasis on organized layer-by-layer redox polyelectrolyte multilayers (RPEM). In redox-active polyelectrolyte multilayers the polyion-polyion intrinsic charge compensation can be broken by ion exchange driven by the electrochemical oxidation and reduction forming extrinsic polyion-counterion pairing. In this chapter we describe the structure, dynamics and applications of these systems. 

As a compromise between the above two approaches, the third approach adopts nonactive (inert) materials as working electrodes with neat electrolyte solutions and is the most widely used voltammetry technique for the characterization of electrolytes for batteries, capacitors, and fuel cells. Its advantage is the absence of the reversible redox processes and passivations that occur with active electrode materials, and therefore, a well-defined onset or threshold current can usually be determined. However, there is still a certain arbitrariness involved in this approach in the definition of onset of decomposition, and disparities often occur for a given electrolyte system when reported by different authors Therefore, caution should be taken when electrochemical stability data from different sources are compared. 

This section will discuss the electrochemical stabilities of different solvents and salts used in state-of-the-art electrolytes that were determined with nonactive electrodes (i.e., in the first and the third approaches). When active rather than inert electrodes are used as working surfaces, many complicated processes, including the reversible electrochemical redox chemistries as well as surface passivation, occur simultaneously. These related materials will be dealt with in a dedicated section (section 6). 

In MET, a low-molecular-weight, redox-active species, referred to as a mediator, is introduced to shuttle electrons between the enzyme active site and the electrode.In this case, the enzyme catalyzes the oxidation or reduction of the redox mediator. The reverse transformation (regeneration) of the mediator occurs on the electrode surface. The major characteristics of mediator-assisted electron transfer are that (i) the mediator acts as a cosubstrate for the enzymatic reaction and (ii) the electrochemical transformation of the mediator on the electrode has to be reversible. In these systems, the catalytic process involves enzymatic transformations of both the first substrate (fuel or oxidant) and the second substrate (mediator). The mediator is regenerated at the electrode surface, preferably at low overvoltage. The enzymatic reaction and the electrode reaction can be considered as separate yet coupled. 

Research on the use of CNT-MPc based electrode in electroanalytical chemistry is still in its infancy. Without doubt, there is an enormous potential for using CNT-MPc-based electrodes for applications in areas such as environmental, industrial, food, pharmaceutical, clinical, and biomedical fields. Few studies have only been attempted with MPc complexes with Co, Fe and Ni as the central metals, meaning that there are many open doors for research on these and many other MPc complexes as redox mediators for the development of electrochemical sensors. Given the many advantages of electrochemical techniques (especially sensitivity to redox-active analytes, and amenability to automation,... 

The peculiar electrochemical behaviour of electrode surfaces coated with a layer of phosphatidylcholine (PC) adds a new dimension to the practical applications of this type of sensor as it enables development of electrochemical processes within a lipid layer. Redox-active amphiphiles readily... 

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