Several electrochemical DNA biosensors for label-free detection of nucleic acid sequences, based on the molecular beacon principle, are discussed. These systems exploit 1). conformational movements of probe DNA, labelled with a redox active tag, upon hybridization (e.g. shown in the works of A. Heeger, briefly reviewed) and 2). conformational transitions in branched DNA molecules operating as molecular switches, changing between ‘open’ and ‘closed’ conformations in response to the local ionic conditions at the branch point (Figure). In the first case, electrochemical read-out of the changing signal from the redox tag is used for detection of the hybridization event. In the second case, the switching state of hybridized DNA is read-out optically using the fluorescence resonance energy transfer (FRET) occurring between donor and acceptor dyes located at the termini of the DNA junctions (Figure). In this case, the modulation of the conformational state is performed electrochemically, through electrochemical injection/removal of cations, thus producing change in the conformational state of the DNA. Two electrochemical cation-injection redox systems are shown: Zn/Zn²⁺ and polypyrrol-organic counteranion-Mg²⁺/ oxidised polypyrrol-organic counteranion (Figure). Chemistries of the electrode modification by DNA and perspectives and drawbacks for application of both methodologies for analysis of multiplexing gene systems “on-chip” are discussed. Current research on electrochemical detection of analytes through their binding to redox-labeled DNA, thus changing DNA conformational states, is also reported.

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