Enhancing electrochemical DNA biosensing with active sensitisers

Biosensors for DNA (genosensors) are of significant interest because they address the need for rapid diagnosis of, e.g., genetic disorders and infections by pathogens. Electrochemical genosensors are of particular interest because they are miniaturised more readily than optical sensors, typically require small sample volumes, little sample pre-treatment, and are suitable for use with portable instrumentation by minimally trained clinical personnel.
We here present the combination of two strategies towards sequence-selective DNA detection, viz. intrinsically sequence-selective electrochemical detection of DNA hybridisation and detection of double-stranded DNA by electronically active molecules. A capture strand, either DNA or PNA (peptide nucleic acid) is immobilised on an electrode, using immobilisation procedures which have previously been optimised for DNA detection using electrochemically impedance spectroscopy. Following hybridisation, an electronically active molecule binds to the duplex DNA on the electrode, establishing a detectable electronic contact, thus acting as “sensitisers” for the electrochemical detection.
Electrochemical impedance spectroscopy and cyclic voltammetry measurements were carried out using two types of sensitisers: a Co aqphen complex binding to dsDNA through intercalation and a cationic quaterthiophene binding through the minor groove.
The intercalation of a cobalt(II) complex containing mixed-ligands of 12,17-dihydronaphtho-2,3-hdipyrido-3,2-a:2',3'-c-phenazine-12,17-dione (aqphen) and glycolic acid (GA), [Co(GA)2(aqphen)] with DNA was investigated by electronic absorption spectroscopy. The corresponding phen complex, with an oxidation potential of 0.12 V vs Ag/AgCl, had previously shown weak intercalation. Increasing the conjugation with an anthraquinone increased significantly the binding affinity of the complex.
The use of an optoelectronically active cationic quaterthiophene as a sensitiser was also explored. This water-soluble quaterthiophene interacts with dsDNA with an affinity of 10^5 M-1 and has an oxidation potential of 1.0 V vs Ag/AgCl.
Both sensitisers show promising enhancement of electrochemical DNA detection and highlight structural features favouring selectivity in sensing, paving the way towards a new generation of point-of-care electrochemical biosensors for genotyping.