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.
The limited toolbox for conducting polymer (CP) microscale fabrication and characterization hampers the development of applications such as sensors and actuators. To address this issue, a robust and integrated methodology is presented, capable of electrochemical fabrication and characterization of CPs in a highly localized manner, allowing for CP patterning and spatial mapping of voltammetric response. This is enabled by scanning probe microscopy (SPM) tipped with a single‐barreled micropipette to electrochemically polymerize CP microspot arrays, demonstrated for 3,4‐ethylenedioxythiophene and aniline monomers. Stationary electropolymerization produces individual microspots; lateral movement produces long microribbons; retraction produces extruded microstructures. Subsequently the same SPM setup is tipped with a double‐barreled micropipette to carry out localized cyclic voltammetry. The micropipettes are filled with saline solutions in contact with Ag/AgCl electrodes, forming a thin meniscus of solution at the micropipette tip, which enable an automated approach in air and subsequent contact with the surface. The flexibility of this novel technique is demonstrated by application to 2D poly(3,4‐ethylenedioxythiophene) (PEDOT) microspots, microribbons and nanowires, plus polyaniline (PANI) microstructures and self‐assembled thin films. Finally, setting up a dynamic electrochemical cell allowed for voltammetric–amperometric imaging, simultaneously mapping the morphology and current response of CPs. Future refinements towards the nanoscale through smaller‐tipped pipettes should open up new opportunities for voltammetric response mapping of individual CP nanostructures.
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