(5d) Investigation of Natural pH Gradient Formation within Microchannels

When an electric field is applied between an electrode pair in an aqueous medium, H+ and OH- ions are generated at anode and cathode surfaces, respectively. In micro- and nano-scale systems, the mass transport of these ions due to diffusion and electrokinetic transport can be significant enough that they develop concentration gradients across the system. The resulting ?natural pH gradients? can be used for some techniques such as isoelectric focusing. On the other hand, natural pH gradients can complicate other systems by causing spatial changes such as nonuniform wall surface charges. The roles of diffusion, electrophoretic transport and transport due to electroosmotic flow were compared. COMSOL simulations were used to predict the length scales for each transport mechanism as a function of electric field strength, concentration in the electrode fluid wells, reaction rate at the electrode surface, as well as time scales needed for a pH gradient to develop down microchannels traversing the electrodes. The decay of established gradients after stoppage of the electric field was also simulated and experimentally observed in order to determine system relaxation times. To experimentally validate the simulation results, pH changes in a microchannel were measured by fluorescence microscopy. The pH-sensitive fluorescent dyes SNARF-1 dextran conjugate (SNARF-1) and FITC Isomer I (FITC) were used to measure pH at different locations and points in time during and after the application of an electric field, and the measured changes in the pH gradient were compared with the simulation results. This work has the potential to elucidate subtle ion movements in common electrokinetic microdevices such that processes can be engineered to avoid or take advantage of these natural ion gradients.