Electrochemical Characterization of Poly 3,4-ethylenedioxythiophene (PEDOT)-Modified Electrodes in Acetonitrile/Water/Trifluoroacetic Acid Solutions and Application in Magnetohydrodynamics Microfluidics

Abstract

Electrochemical characterization of electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT)-modified gold band electrodes on a chip was performed in water/acetonitrile mixtures (0–90% (v/v) CHs3CN) containing 0.1% (v/v) trifluoroacetic acid (TFA) as the electrolyte. The performance of applied-current redox magnetohydrodynamic (R-MHD) microfluidics at a coplanar arrangement of parallel PEDOT-modified pumping electrodes in a confined chamber on a 0.37 T NdFeB magnet was also established under these conditions, at applied currents of 25–100 μA, extending R-MHD operation to mixed solvent environments beyond purely aqueous ones. Chronopotentiometry (CP), chronoamperometry (CA), and cyclic voltammetry (CV) were performed across three electrode configurations, with responses described by a serial RC circuit model. PEDOT capacitance is relatively invariant across the full composition range: 32 ± 2, 33.0 ± 0.4, and 28 ± 2 mF cm–2 for the 3-electrode semi-infinite, 2-electrode semi-infinite, and 2-electrode confined configurations, respectively. Electrochemically-measured PEDOT resistance scales approximately proportionally with solution resistivity (inverse conductivity) as %CH3CN increases, attributed to declining ionic mobility rather than polymer-solvent interactions, and for fixed cutoff potentials, restricts the maximum achievable current and R-MHD pumping duration. Velocity profile shapes in the R-MHD chamber were consistent across solution compositions, with velocity magnitudes scaling linearly with 1/(dynamic viscosity, μ) and applied current (24–39 μm s–1 at 25 μA and 95–170 μm s–1 at 100 μA for 0–80% CH3CN). A wide rectangular channel model incorporating a hydraulic resistance scaling factor fg accurately predicts velocities from independently measurable quantities. Fluid displacement tracks 1/μ across most of the composition range, as decreasing viscosity offsets increasing PEDOT resistance with increasing %CH3CN. Above 70% CH3CN, displacement continues to increase at 25 μA but decreases at higher currents where iR drop dominates. These studies address previously unexplored solution compositions directly relevant to mobile phase selection in reversed-phase chromatographic applications.