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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, R(max) and Δ(max) from SEED-R and SEED-I, respectively, are linearly proportional to amperometric peak current, I(max). The DNA binding is measured by a redox active dye, methylene blue, that intercalates in dsDNA, leading to an R(max). Concomitantly, the absence of Δ(max) 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 = f(T) = 1.25 × 10(−3), was observed. Below f(T), dsDNA molecules behaved as individual single-molecule nanoelectrodes. Above the crossover transition, R(max), 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 > f(T) regime.

Living systems must continuously receive substrates from subenvironment, and the population metabolic rate model is affected on this flow of substrates to be metabolized, its relevant variables, and the rate at which operates. This study focuses on the influences of resistances and bulk phase factors with in the subenvironment and by thermodynamic coupling on reaction-transport processes representing a simple enzymatic conversion of a substrate to a product. Thermodynamic coupling refers to mass flow, or a reaction velocity that occurs without or opposite to the direction imposed by its primary thermodynamic driving force. We considered the effects of (i) subenvironment resistances for the heat and mass flows of reacting substrate in the form of the ratios of Sherwood to Nusselt numbers, (ii) the subenvironment bulk phase temperatures and concentration of substrate, and (iii) the cross-coefficients responsible for the induced effects due to the thermodynamic coupling. In order to study these effects, the ...