Biosensors are all about the limit of detection¹ imposed by the molecular level changes in expression of biomolecules to hope to perform reliable diagnostics before symptoms of a disease appears. A special interest was driven to the DNA target, mimicking the liver-specific micro-Ribonucleic Acid 122² (miRNA122). DNA hybridization is the most prized method compared to direct sequencing all the more now that long range electron transfer through the DNA duplex π-staking has been demonstrated³. The electronic coupling within its inner core of stacked array of heterocyclic aromatic base pairs is very sensitive to local disruptions such as mismatches making a DNA biosensor particularly sequence specific. In a DNA biosensor, the target sequence is recognized by a complementary DNA probe and hybridized. The basis pairing according to Watson and Crick’s rules⁴ is converted into an electrical signal. A 23-base DNA probe was self-assembled on the gold microelectrode via thiol adsorption. Long-range electron transfer was chosen for the monitoring of the hybridization step. Indeed, this direct transduction was significantly enhanced due to DNA-duplex π-stacking and the use of the redox methylene blue as DNA intercalator⁵. The electrochemical properties of the sensor were screened with the [Fe(III)(CN)₆]^3-/[Fe(II)(CN)₆]^4- redox couple in droplets by using cyclic voltammetry (CV) in a two-electrode configuration which is more adapted in the case of microliter biological samples. Voltammograms centered at potential zero are observed as expected for a working UME and a counter electrode made from the same metal and immersed in the same electrolyte. The 2 mm-gold counter electrode (very high area compared to the 25 μm gold working electrode) can be considered as a pseudo-reference electrode allowing a feeble potential drift during measurements. When working with thiolated SAM absorbed on gold substrates, the matter of desorption is not trivial all the more in chloride electrolytes. The reliability of 10^-14 M limit of detection determined for DNA target quantification was deduced from differential current density measured compared with the blank measurements. The data analysis highlights the possible role of the desorption phenomenon of the thiolated DNA probes from the sensor surface that could explained the residual current densities measured for concentrations lower than 10^-14M.

1           M. Lazerges and F. Bedioui, Anal. Bioanal. Chem., 2013, 405(11), 3705-3714.

2           J. a. Wilson and S. M. Sagan, Curr. Opin. Virol., 2014, 7, 11–18.

3           S. O. Kelley, E. M. Boon, J. K. Barton, N. M. Jackson and M. G. Hill, 1999, 27, 4830–4837.

4           J. D. Watson and F. H. C. Crick, Nature, 1953, 171, 737–738.

5           M. Lazerges, V. T. Tal, P. Bigey, D. Scherman and F. Bedioui, Sensors Actuators B Chem., 2013, 182, 510–513.