Carbon Paste as the Electrode Material

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. 

The classical modification processes for solid homogeneous electrodes are (i) film and (ii) membrane covering, as well as (iii) adsorption, and (iv) covalent attachment immobilization of the modifier. These avenues are also open to modify carbon paste as the most popular representative of heterogeneous electrode materials. But, because of its composite character, it facilitates simpler ways of modification by direct addition of the modifier to the paste either during or after the preparation of the material. The term direct mixing was coined by Baldwin [106], though the history of modification of CPEs dates back to the mid-1960s, when Kuwana was the first who added electroactive components to the paste [107]. (In his case, however, there had been no intention to alter the CPE characteristics, and the purpose was to study the redox behavior of the adduct in a nonaqueous environment.)... 

The choice of electrode material is more critical in LCEC than in the usual electroanalytical experiment, primarily due to the mechanical ruggedness and long-term stability required. Carbon paste (an admixture of graphite powder and a dielectric material) remains a useful choice as an electrode material for LCEC. While carbon paste can be used in nonaqueous solvents if formulated... 

S-enalapril assay can be done using the potentiometric electrode based on impregnation of 2-hydroxy-3-trimethylammoniopropyl-/i-cyclodextrin (as chloride salt) solution in a carbon paste, in the 3.6 x 10 5-6.4 x 10-2 mol/L (pH between 3.0 and 6.0) concentration range with a detection limit of 1.0 x 10 5 mol/L [25]. The slope is near-Nernstian 55.00 mV/decade of concentration. The average recovery of S-enalapril raw material is 99.96% (RSD — 0.098%). The potentiometric selectivity coefficient over D-proline (6.5 x 10 4) proved the sensor s enantioselectivity. S-enalapril was determined from pharmaceutical tablets with an average recovery of 99.59% (RSD — 0.20%). 

Several useful schemes for attaching nucleic acid probes onto electrode surfaces have thus been developed [2-8]. The exact immobilization protocol often depends on the electrode material used for signal transduction. Common probe immobilization schemes include attachment of biotin-functionalized probes to avidin-coated surfaces [15], self-assembly of organized monolayers of thiol-functionalized probes onto gold transducers [16], carbodiimide covalent binding to an activated surface [17], as well as adsorptive accumulation onto carbon-paste or thick-film carbon electrodes [15-30]. 

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). 

In the previous text, attention was paid to a feature of carbon pastes as a unique electrode material of heterogeneous nature, reflecting both main carbon paste constituents in rather specific behavior that differs from mixture to mixture, depending not only on the type and quality of the respective component, but also on the way in which the respective electrode or sensor is constructed and employed in practical experiments. Typical properties and some specific features of traditional carbon paste mixtures are as follows ... 

Chemical and electrochemical inactivity Similar to solid carbons used in electrode configurations, carbon pastes keep the property of chemical and electrochemical inactivity and can be characterized as a fine material with high resistance against unwanted transformations during electrode processes [2, 5, 35, 50]. However, polarization at extreme potentials may cause substantial changes at the carbon electrode surface as shown later, in point (8-f). 

A milestone was the invention of carbon paste by Ralph Buzz Adams in 1958, which favoured, due to its heterogeneous paste-like composition, simple and multiple modification of the electrode material even with labile biological components. The first amperometric sensor, the famous Clark oxygen electrode was described in by Feland Clark 1954. Currently, new substances have a primary position in electrochemical literature, such as boron-doped diamond or nanostruc-tured materials. 

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