Electrochemical Generation of Steady-State Linear Concentration Gradients within Microfluidic Channels Perpendicular to the Flow Field.

A new design of microelectrode was introduced to generate electrochemically steady-state linear concentration gradients perpendicular to the flow direction throughout the cross section of microchannels. The shape and geometry of the electrode were established based on operating regimes at microchannel electrodes. Before implementation, optimal conditions were preliminary delineated by numerical simulations according to the flow velocity and microchannel dimensions. To assess experimentally these predictions, a specific microfluidic platform was developed with optimized geometry to simultaneously allow the generation of linear concentration gradients and the mapping of concentration profiles by confocal fluorescence microscopy. As a model, the electrochemical reduction of a quinone in the presence of fluorescein was selected to both generate and monitor a linear proton gradient. A good agreement was observed between theoretical and experimental data, establishing the proof of concept. These results should broaden the performance and applications of electrochemical platforms, particularly in the field of active control of microenvironments in biology, biochemistry, and analytical chemistry.