Electrodes modification using

The M-N4 monomers are widely employed for electrocatalysis . The monomers readily adsorb onto electrodes to form electroactive surfaces. The methods employed for the adsorption are often fast. Carbon electrodes are popular for the fabrication of CME using M-N4 complexes. The following are some of the methods which have been employed in electrode modification using M-N4 monomers ... 

Metallophthalocyanines (MPcs) are well known as electrode modifiers for elec-trocatalytic applications. MPcs whose metal J-orbitals lie between the highest occupied molecular orbital (HOMO) and lowest imoccupied molecular orbital (LUMO) levels of the Pc ligand, such as Mn, Fe, and Co, are known to be good electrocatalysts [1]. A good electrocatalyst must lower the redox potential, increase sensitivity and selectivity, and should show stability. In our previous book chapter [1], electrode modification using mononuclear phthalocyanines (Pcs) was extensively reviewed. The current chapter will review electrocatalytic behavior of mononuclear MPc complexes when combined with various nanomaterials. In addition, the chapter presents electrode modification using the least studied, binu-clear phthalocyanines alone or in the presence of nanomaterials. 

There are several methods for modifying electrodes using MPc complexes when alone or in the presence of nanomaterials. Only self-assembled monolayer, click chemistry, adsorption, and grafdng will be discussed since they are the ones mainly reported for electrode modification using Pcs in the presence of nanomaterials. 

Electrode Modifications Using QDs and Their Conjugates with Pcs... 

Table 3 Electrode modification using Pcs and metal nanoparticles... 

The chapter summarizes the dilFerent types of electrode modifications using ph-thalocyanines alone or in the presence of nanomaterials. Simple adsorption is a prominent method employed in the presence of nanomaterials. The review includes electrode modification using binuclear phthalocyanines which are less studied compared to mononuclear phthalocyanines. The nanomaterials most studies in the presence of bis phthalocyanines and mononuclear phthalocyanines are carbon nanombes. Some studies on quantum dots and metal nanoparticles are also presented. Electrode characterizalion methods using different techniques are included. X-ray photoelectron spectroscopy, in particular, has been shown to be very effective on the characterization of modified electrodes. 

Coates M, Nyokong T (2012) Electrode modification using iron metallophthalocyanine through click chemistry and axial ligation with pyridine. J Electroanal Chem 687 111-116... 

Fig. 2.20 Most common modes of the electrode modifications using ILs for different sensing applications...

Among the variety of materials used for electrode modification the electroactive organic and inorganic polymers seem to be the most prominant ones. In this chapter the electroactive polycrystals of transition metals, hexacyanoferrates, will be discussed for the development of chemical and biological sensors. 

The first reports on direct electrochemistry of a redox active protein were published in 1977 by Hill [49] and Kuwana [50], They independently reported that cytochrome c (cyt c) exhibited virtually reversible electrochemistry on gold and tin doped indium oxide (ITO) electrodes as revealed by cyclic voltammetry, respectively. Unlike using specific promoters to realize direct electrochemistry of protein in the earlier studies, recently a novel approach that only employed specific modifications of the electrode surface without promoters was developed. From then on, achieving reversible, direct electron transfer between redox proteins and electrodes without using any mediators and promoters had made great accomplishments. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

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. 

However, there are still some drawbacks in the use of CNT that need to be solved for practical applications. Not only large reversible capacities but also large irreversible capacities have been reported on CNTs [179, 183]. Such irreversible capacity together with the lack of voltage plateau during lithium extraction (hysteresis) limits the use of nanotubes as electrode material in LI Bs. However, an active study (based on CNT treatments, surface modification, use of CNT nanocomposite matrices, etc.) is being carried out in order to overcome such difficulties [184]. 

However, the oxidation of NAD(P)H to NAD(P)+ at practical rates on most electrode materials proceeds only at high overpotentials [182] and often fouling of the electrode surface has been observed. This situation induces the search for suitable mediators or electrode modification processes to accelerate the highly irreversible oxidation of NAD(P)H. The stability of the reaction products in the presence of each other is also a necessary condition for usefulness. 

The size, shape, and material of the electrode can be tailored to the application. Problems of response time, for example, often can be solved by using a smaller electrode. The use of new electrode materials, including possibilities for modification of the electrode surfaces, could lead to new measurement caDa ilities. Thi. mol i sizes tl itid r ce f the de el d-... 

Another method of stabilizing the surface of semiconductor electrodes relates to an electrolyte modification, using solution mediators that efficiently accept the electron from the semiconductor to subsequently reduce C02. In aqueous solution, Taniguchi et al. used a p-GaP photoelectrode in the presence of 15-crown-5 ether at a potential of-0.95 V (versus SCE) [119]. In this case, current efficiencies of 44%, 15%, and 4% were observed for methanol, formic acid, and formaldehyde, respectively. 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Some reports have dealt with the immobilization of intact PSI onto electrodes, intended for photoelectrochemical devices. Cliffel and co-workers reported the adsorption of intact PSI onto an Au electrode surface using SAM modification of the electrode preliminary to PSI adsorption.54-57 In these reports, PSI and SAM were connected by hydrogen bonding, and the orientation of PSI on the electrode varied. The orientation of PSI on the electrode is important, as the junction between PSI and electrodes favorably locates near the electron transfer chain. Therefore, direct modification of the electron transfer chain in PSI, based on molecular-level assembly, is a powerful tool for PSI use as a photonic device however, few investigations have employed PSI as a photonic device based on molecular-level assembly. 

The most important selectivity parameter of electrodes for voltammetric sensors is the applied potential. Ideally, the electrode potentials of the redox couples would be sufficiently far apart for there to be no interference between different species. Unfortunately this is not the case, and it is necessary to look for greater selectivity. We can discriminate better between the different species present in solution through a correct choice of conditions for the study of the electrode reaction electrode material (Chapter 7) in some cases through surface modification use of hydrodynamic electrodes (Chapter 8) application of potential sweep... 

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