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Synthesis, Modification and Characterisation of NanoCarbon Electrodes for Determination of Nucleic Acids

Electroanalytical techniques have been widely used for the determination of nucleic acids. These analyses are possible to divide according many parameters, including type of material of a working electrode (mercury, carbon, gold) or applied electrochemical method and/or structure of detected system (presence or absence of biocompound(s)). Electrochemical determination of nucleic acid(s) on carbon electrodes is possible to divide into the two basic groups as it follows: (i) hybridisation based techniques and (ii) oxidation of DNA bases. Application of hybridisation reaction is very widespread due to the specificity of determination and low limit of detection. The other way, oxidation of DNA bases is presented too. Improvement of physic-chemical properties of carbon electrodes is possible via a modification of its surface. The modification of surface is possible, in general, by chemical entities or biocompounds according to the suggested detection system. The following text is focused on the electroanalytical determination of nucleic acids by direct detection (mainly by oxidation of DNA bases) using chemically modified carbon electrodes. The construction of biosensors was not included.

Carbon materials for electroanalysis
Carbon nanostructures have been in the centre of research activities since the discovery of fullerene in 1985. Interest in carbon and its importance can be demonstrated by Nobel Prize obtained by Andre Geim and Kostya Novoselov in 2010 for their research on unexpected properties of one atom thick layer of graphene. Various carbon nanostructures (Fig. 1) like carbon nanotubes (CNT), graphene, graphite oxide and fullerene are the most studied ones as promising materials with applications in different technologies and in biology and medicine (Roy et al. 2012). Carbon nanotubes can be described as rolled up graphene sheets with no overlapping edges. Their diameters typically vary from 1 to 100 nm and their lengths can be several orders of magnitude larger, up to millimetres, even centimetres long. A carbon nanotube is a graphene sheet rolled into a cylinder typically several nanometres in diameter and the ends are capped with half fullerene balls. This is, in the fact, the minimum energy conformation of a graphite layer of finite size. The properties of the nanotubes depend on the arrangement of the graphene sheets, the diameter and length of the tubes and the nanostructure. The multi-walled nanotubes (MWNTs) consist of a coaxial assembly of several single-walled nanotubes (SWNTs), separated from one another by ~0.34 nm, which is slightly more than the interlayer distance in single-crystal graphite. Double-walled carbon nanotubes (DWNTs), which consist of two graphene layers only, represent another form of nanotubes. The graphene honeycomb lattice is composed of two equivalent sub-lattices of carbon atoms bonded together with ? bonds. Each carbon atom in the lattice has a ? orbital that contributes to a delocalized network of electrons. Graphene possess 1 D structure and monolayer or few layer graphene is known.

Graphite oxide is also planar, but there are also oxygen atoms involved in the structure. Fullerenes are composed of carbon, such as C60 and C70, which may carry additional functional groups. Fullerenes are spherical molecules containing aromatic moieties, but despite their extensive conjugation they behave chemically and physically as electron-poor alkenes rather than electron-rich aromatic systems. Fullerene is a rigid substance with 12 pentagons and 20 hexagons constituting a single C60 molecule with a hybridization of sp2 for all the carbon atoms and a length of C-C bond of 1.46 A. It follows the Euler theorem of spherical network closure where the pentagons are responsible for the formation of curvature in the fullerene structure. The most stable is C60 followed by C70, C76, C78, C80, C82, and C84. In terms of addition reactions, it shows some similarity to olefins. Especially, the purely carbon-based fullerenes exhibit very low solubility in water. However, fullerenes have the ability to form stable aggregate clusters with nanoscale dimensions upon contact with water (Roy et al. 2012).

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