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Electrochemical Characteristics of DNA Modified Electrode as a Function of Percent Binding.

Electrochemical characteristics of immobilized dsDNA on Au electrode was studied as a function of coverage using a home-built optoelectrochemical method. The method allows probing of local redox processes on a 6-µm spot by measuring both differential reflectivity (SEED-R) and interferometry (SEED-I). The former is sensitive to redox ions that tend to adsorb to the electrode while SEED-I is sensitive to nonadsorbing ions. The redox reaction maxima, Rmax and Δmax from SEED-R and SEED-I, respectively, are linearly proportional to amperometric peak current, Imax. The DNA binding is measured by a redox active dye, methylene blue (MB), that intercalates in dsDNA leading to an Rmax. Concomitantly, the absence of Δmax for [Fe(CN)6]4-/3- by SEED-I ensures that there is no leakage-current from voids/defects in the alkane thiol passivation layer, at the same spot of measurement. The binding was regulated electrochemically to obtain the binding fraction, f, ranging about three orders of magnitude. A remarkably sharp transition, f = fT = 1.25 x 10-3 was observed. Below fT, dsDNA molecules behaved as individual single-molecule nanoelectrodes. Above the crossover transition, Rmax, per dsDNA molecule dropped rapidly as f-1/2 towards a planarlike monolayer. The SEED-R peak at f ~3.3 x 10-4 (~270 dsDNA molecules), was (statistically) robust corresponding to a responsivity of ~0.45 zeptomoles of dsDNA/spot. Differential pulse voltammetry (DPV) in the single-molecule regime estimated that the current per dsDNA molecule was ~4.1 fA. Comparing with published amperometric results, the reported semi-logarithmic dependence on target concentration is in f > fT regime.