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.