Redox Labels and Indicators Based on Transition Metals and Organic Electroactive Moieties for Electrochemical Nucleic Acids Sensing
Natural nucleobases are electrochemically active, producing analytically useful oxidation and reduction signals (usually measured at carbon or mercury-based electrodes, respectively). Electroactive indicators and labels are used in electrochemical nucleic acids sensing to improve selectivity and sensitivity of the biosensors and bioassays and to expand the palette of applicable electrode materials. Electrochemical activity of these species is conferred by either redox-active transition metals, or organic electroactive moieties. The non-covalently interacting indicators interact with DNA in a structure-selective manner, usually preferring double-stranded DNA, via groove binding or intercalation between base pairs within the DNA double helix. These indicators are used mostly in DNA hybridization sensors due to their ability to discriminate between single-stranded probes and probe target hybrid duplexes. A variety of electroactive moieties has been attached to nucleic acids covalently to create labeled DNA targets or signaling probes. Covalent DNA labeling offers a clear discrimination between complementary strands forming the DNA double helix, which further improves selectivity of the hybridization assays. Moreover, redox coding of particular nucleobases can be utilized in electrochemical DNA sequencing and single nucleotide polymorphism typing. Besides classical synthesis of modified nucleic acids via solid phase phosphoramidite methodology, more facile and versatile approaches have been introduced, based on either chemical modification of natural DNA components (such as thymine bases with osmium tetroxide complexes), or enzymatic incorporation of modified nucleotides using deoxynucleotide triphosphate conjugates bearing electroactive labels. Examples of electroanalytical applications of labeled nucleic acids are discussed.