Control of catalytically generated electroosmotic fluid flow through surface zeta potential engineering

The catalytic decomposition of hydrogen peroxide at bimetallic junctions has been shown to produce a local electric field capable of driving both electroosmotic fluid flow and electrophoretic forces on charged particles. The direction of the electroosmotic fluid flow depends on the metal surface zeta potential. We demonstrate that the direction of fluid flow can be controlled by the use of self-assembled monolayers to engineer the surface charge. In particular, silver disks were patterned on a gold-coated silicon surface. In the presence of hydrogen peroxide, gold behaves as an anode generating protons, and silver behaves as a cathode consuming protons, thus forming a local electric field extending from the gold surface to the silver surface. This electric field distribution results in an outward (away from the silver disk) electrophoretic force on a negative tracer and an inward force on a positive tracer. By selectively functionalizing the gold surface with either negatively charged carboxylic-acid-functionalized monolayers or positively charged amine-functionalized monolayers, we demonstrate the ability to align the direction of the electroosmotic fluid flow with the direction of the electrophoretic force on a given charged particle. A method for photolithographically patterning the monolayers on the gold surface while protecting the silver surface with an inert polymer layer has also been developed. This ability to engineer the direction of local catalytically produced forces provides the possibility of creating numerous fluidic and sorting devices.