Carbon paste electroactive electrodes

In recent years, the electrodes of similar structure have also found application in the electroanalytical chemistry, for purposes of researching the electrochemical behavior of solid substances, etc. These electrodes are known as the modified carbon paste electrodes or carbon paste electroactive electrodes [2]. 

Fig. 7.10 (a) Schematic representation of the detection principle for non-electroactive species by means of indirect amperometry at the Cu-doped zeolite-modified electrode (ZME). (b) Ampero-metric responses in flow injection analysis of a Cu -doped zeolite Y carbon paste-modified electrode to successive injections of K samples (a) 5.0 x 10 M, (b) 1.0 x 10 M, (c) 2.5 X 10 M, and (d) 5.0 x 10 M. Flow rate 5 ml/min applied potential —0.25 V. The dimension of the tetrabutylammonium cation (TB A" ) carrier does not allow it to enter the zeolite and, hence, to interfere with K determination (Reproduced fiorn Ref. [147] with the permission of Elsevier)... 

CNTs have been one of the most actively studied electrode materials in the past few years due to their unique electronic and mechanical properties. From a chemistry point of view, CNTs are expected to exhibit inherent electrochemical properties similar to other carbon electrodes widely used in various electrochemical applications. Unlike other carbon-based nanomaterials such as C60 and C70 [31], CNTs show very different electrochemical properties. The subtle electronic properties suggest that carbon nanotubes will have the ability to mediate electron transfer reactions with electroactive species in solution when used as the electrode material. Up to now, carbon nanotube-based electrodes have been widely used in electrochemical sensing [32-35], CNT-modified electrodes show many advantages which are described in the following paragraphs. 

Paste electrodes - electrodes prepared by making a paste from an electron-nconducting material and a binder. The binder may be an electrolyte solution or inert oils like - Nujol. Paste electrodes can be part of batteries [i], where the electroactive material is used to prepare the paste, often with the addition of an inert electron conductor. In - electroanalytical chemistry, unmodified and modified - carbon paste electrodes are used. 

Researchers turned their attention to applications of silica gel as a new electrode material. Silica gel, which has a three-dimensional structure with high specific surface area and is electroinactive in an aqueous medimn can be used as a support for electroactive species during their formation and/or enzymes by adsorption or entrapment [92,93]. Patel et al. recently reported application of poljwinyl ferrocene immobilized on silica gel particles to construct glucose sensors. Efficiency of carbon paste electrodes prepared with these polymeric electron mediators and GOx was comparable to electrodes constructed with other ferrocene based polymeric electron transfer systems. The fact that 70% of initial anodic current was retained after a month when electrodes were kept in the buffer at room temperature shows that polymerization of monomer vinylferrocene in the pores of silica gel and entrapping GOx in the matrix of poljwinyl ferrocene appears to have added stability to the sensors [94]. 

A carbon paste electrode modified with Cu -doped clinoptilolite powder has been evaluated as an amperometric sensor for non-electroactive in flow injection analyses [48]. The conductivity of heulandite single crystals parallel to [100] has been studied under isothermal conditions as a function of the H2O content, small polar organic molecule concentration, and charge compensating cations. Results indicate that heulandite electrodes will be applicable for analytical purposes in aqueous solution [49]. 

In recent work, this same research group has reported on the use of modified carbon paste electrodes for the discrimination of vegetable oils [51]. The oils were used as an electroactive binder material of carbon paste electrodes and the responses of these electrodes immersed in various solutions were used to discriminate the oils. The polyphenol content of olive oil allows it to be differentiated from sunflower or corn oils. In addition, the voltammograms are influenced by the pH and the nature of the ions present, resulting in characteristic signals for PCA even olive oils of different quality (extra virgin, virgin, lampante, and refined) could be discriminated. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

Such a brief summary can be started with a brand new review [90] that pursues the recent achievements with new carbon pastes made of alternate carbons and binders for example, graphite powder-based solgel electrodes. Another review article then reflects the still stronger position of solid-state analysis herein, in a new renaissance of solid-phase electrochemistry with electroactive carbon paste electrodes, CPEEs [91]. 

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