Probing electric fields inside microfluidic channels during electroosmotic flow with fast-scan cyclic voltammetry

Fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used in microfluidic channels. This method offers the advantage that it can resolve electroactive species not separated in the channel. In addition, this method provides a route to investigate the distribution of applied electrophoretic fields in microfluidic channels. To probe this, microelectrodes were inserted at various distances into channels and cyclic voltammograms recorded at 300 V/s were repeated at 0.1-s intervals. The use of a battery-powered laptop computer and potentiostat provided galvanic isolation between the applied electrophoretic field and the electrochemical measurements. In the absence of an external field, the peak potential for oxidation of the test solute, Ru(bpy)₃²⁺, was virtually unaltered by insertion of the microelectrode tip into the channel. When an electrophoretic field was applied, the peak potential for Ru(bpy)₃²⁺ oxidation shifted to more positive potentials in a manner that was directly proportional to the field in the channel. The shifts in peak potential observed with FSCV enabled direct compensation of the applied electrochemical potential. This approach was used to explore the electrophoretic field at the channel terminus. It was found to persist for more than 50 μm from the channel terminus. In addition, the degree of analyte dispersion was found to depend critically on the electrode position outside the channel.