Carbonaceous Moiety

Characterization of Two Main Carbon Paste Components 11.2.2.1 Carbonaceous Moiety Carbon Material in Traditional Mixtures 

Any carbon powder chosen for the preparation of carbon paste or its possible alternative with the primary function to act as the sensor proper should obey three principal criteria (i) particles in the micrometric scale and with uniform distribution, (ii) high chemical purity, and (iii) controlled adsorption capabilities. 

Together with other typical features such as (iv) high chemical inertness and (v) excellent conductivity, this is fidfilled by the majority of so-far used synthetic graphite available under various trademarks Acheson, UCP, GP, SMMS, RW-B, etc., see [2, 7, 9] and references therein) or supplied as graphite powder for spectroscopy by renowned international chemical companies, such as Merck, Sigma-Aldrich, Fluka, or Riedel-de-Haen. 

Highly suitable is also the specially refined natural product from South Bohemian graphite mines in Middle Europe marketed as a gear lubricant CR [35]. For common carbon paste mixtures, both synthetic and natural graphite are dominant in the final formulas, whereas other related materials listed in Table 11.1 are used only scarcely. 

As almost everywhere in science, grandiose commencement and a remarkable progress in the use of newly synthesized allotropic forms of carbon on the verge of the new millennium has principally changed electrochemistry with carbon pastes and, in fact, kicked off a completely new era in the field [5,11,12]. 

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Half a decade ago, carbon paste had celebrated a notable jubilee 50 years of existence and extraordinarily wide applicability across the areas of electrochemistry and electroanalysis [1-3]. Herein, it can be remembered that carbon paste - that is, a thick mixture of a carbonaceous moiety with a suitable (usually liquid) binder - had originally been discovered as a certain by-product, coming from unsuccessful experimentation with carbon powder-based suspension intended to accomplish an electrode with renewable surface for anodic oxidations [2, 3]. 

On the other hand, these newly acquired properties - primarily, distinct catalytic capabilities of most of these materials [5, 20, 35] - evoke an undeniable dilemma. Are such binary composites still unmodified carbon pastes if the respective carbon moiety behaves like typical modifier One possible answer came with the quickly advancing research, when many propagators of these new configurations had started to prefer ternary mixtures with the new carbon added as the third constituent [5, 11]. Then, the resultant formulas contain two carbonaceous materials (i) the ordinary graphite, representing the electrode proper and (ii) the new one acting as a modifier. And if one adopts this distribution of both possible functions, the two-component mixtures containing new carbon can be classified as a novel type of (binary) carbon paste with carbonaceous moiety in the dual role (it means, without further specification by the term unmodified). 

The carbonaceous moiety and a binder are mutually mixed in a ratio typically based on empirical experience [5,7,9,16,38-41,50-52,58] the optimal carbon-to-pasting liquid ratio for traditional carbon pastes usually being 1.0 g 0.5 ml. The two main components are usually hand-mixed in a mortar using a pestle with sufficient pressure, and advisably in two or three consecutive steps, when the formed mass is ripped off from the vessel wall to the bottom and homogenized... 

Presently, there appears to be two well-developed areas in the analysis of CME surface the evaluation of surface topography by SEM and the determination of elemental composition by XPS. Methodology development now needs to be directed toward those techniques that will provide molecular description of surface, especially in situ measurements during electrolyses. Of the available spectroscopic methods such as IR and Raman, the sensitivity in a reflectance mode seems to be the major problem. There is a continuing need for better approaches to quantitation of the surface concentration, spatial distribution and orientation of immobilized molecular moieties. Carbonaceous... 

Adsorption is an easy way to attach nucleic acids to solid surfaces, since no reagents or modified-DNA are required, as shown in Fig. 3.3. These features have promoted extensive use of adsorption as immobilization methodology in genetic analysis. The mainly claimed disadvantages of adsorption with respect to covalent immobilization are (i) nucleic acids may be readily desorbed from the substrate and (ii) base moieties may be unavailable for hybridization if they are bonded to the substrate in multiple sites [76]. However, the electrochemical detection strategy based on the intrinsic oxidation of DNA requires the DNA to be adsorbed in close contact with the electrochemical substrate by multisite attachment, as schematically shown in Fig. 3.4. This multisite attachment of DNA can be thus detrimental for its hybridization but is crucial for the detection based on its oxidation signals. The common method for the multisite physical adsorption of DNA on carbonaceous-based materials can be classified into dry or wet adsorptions. 

Sulfur cathodes that undergo reversible stepwise reduction in the presence of Li ions from S to Li2S via a series of Li2S (Li sulfides) moieties The main substrates for sulfur cathodes are carbonaceous materials (usually on Al foil current collectors) [7]. 

The resultant behavior of each carbon paste and the corresponding CPE(s) always reflects the type and quahty of carbonaceous and binding moieties used. With certain simplification, it can be stated that mechanical, physicochemical, and electrochemical properties of CPEs are close to those exhibited by related solid electrodes from graphite and similar materials nevertheless, because of the presence of the pasting liquid, there are also some specific characteristics or even completely unique features [4, 5, 7, 9,16,40,43, 51—60]. 

The catalytic hydrogenation probably occurs via the incorporation of hydrogen atoms into a weakly adsorbed ethene, maybe even on a carbonaceous layer [56]. This ethene then forms ethyl moieties and eventually forms ethane, released as product form the surface [36, 55, 56]. Flowever, all these processes are competing with the formation of ethylidyne. As a consequence of the competitive formation the efficiency of the reaction under vacuum is low and the formation of ethane only accounts for a small percentage of the initial amount of ethene. For example, measurements found that only 10 % of a saturated ethene layer were converted via self-hydrogenation to ethane at 283 K [54]. 

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