(320e) Micro- pH Tuner and Its Applications Based On Field-Enhanced Water Dissociation In Bipolar Nanoporous Membranes

Control of pH in microscale environments offers many intriguing possibilities for several biological applications such as on-chip microbioreaction (proteolysis, protein crystallization etc) and cell or biomolecular separation.Conventional methods for changing local pH include diffusion of CO₂, addition of acid and base solutions and electrolysis. These methods, however, are either difficult to miniaturize or require microvalves to control the flow of different pH solutions. Electrolysis produces hydrogen and oxygen bubbles that impede microfluidic flow and other reaction products that contaminate the sample. To overcome the aforementioned obstacles, we present a novel strategy to regulate pH in in microscale environments by injecting controllable amounts of protons and hydroxide ions via field-enhanced dissociation of water molecules. The dissociation occurs at the ion-depleted junction of reverse-biased bipolar membranes, fabricated by photo-patterning ion-exchange polymers into microfluidic chips. The bipolar membrane generates and separates H⁺ and OH^- ions without gas production or contaminant generation of electron-transfer reactions. Furthermore, it effects pH changes with no need of additional acidic/basic solutions that dilute analyte concentrations. Combining with properly designed microchannel configurations, the approach allows us to sustain robust local on-chip pH and pH gradients to improve several microfluidic operations, such as micro-reactors and molecular separation in microfluidic isoelectric focusing chips.