Electrochemical characteristics of immobilized double-stranded DNA (dsDNA) on a Au electrode were 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, 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 alkanethiol 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 × 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 toward a planar-like monolayer. The SEED-R peak at f ∼ 3.3 × 10-4 (∼270 dsDNA molecules) was (statistically) robust, corresponding to a responsivity of ∼0.45 zeptomoles of dsDNA/spot. Differential pulse voltammetry in the single-molecule regime estimated that the current per dsDNA molecule was ∼4.1 fA. Compared with published amperometric results, the reported semilogarithmic dependence on target concentration is in the f > fT regime.
ASJC Scopus subject areas
- Analytical Chemistry