Label-free oligonucleotide sensors that use a change in the electrode kinetics of the redox reaction of the negatively charged Fe(CN)(6)(3-/4-) redox couple to signal the formation of a DNA duplex with a surface-conjugated probe nucleotide are investigated. Electrochemically active conducting polymers (ECPs) can advantageously be used both as the active electrode and as the means of surface conjugation of the probe nucleotide. Here, we demonstrate that the sensitivity of the detection of the surface-complementary oligonucleotide can significantly be improved, into the low nanomolar range, by forming the ECP as a highly porous, very rough layer by growing it using electrochemical polymerization on a microelectrode. In comparison, smoother surfaces formed on macroelectrodes had detection sensitivity in the low micromolar range. We propose Donnan exclusion of the redox couple from small pores as the reason for the enhanced sensitivity. We discuss the effects using a simple patch model for the electrochemical kinetics and use the model to derive the equilibrium binding constant and binding kinetic rate constants for the surface hybridization reaction. We use the electrochemically active copolymer of pyrrole (Py) and 3-pyrrolylacrylic acid (PAA) [poly(Py-co-PAA)] as the sensing electrode and binding surface and measure the surface hybridization-induced changes in electrode kinetics of Fe(CN)(6)(3-/4-) by electrochemical impedance spectroscopy.