Graphene-Enabled High-Performance Electrokinetic Focusing and Sensing.

Transverse isoelectric focusing, i.e., isoelectric focusing that is normal to the fluid-flow direction, is an electrokinetic method ideal for micro total analysis. However, a major challenge remains: There is no electrode system integrable in a microfluidic device to allow reliable transverse isoelectric focusing and electrokinetic sensing. Here, we overcome this barrier by developing devices that incorporate microelectrodes made of monolayer graphene. We find that the electrolysis stability over time for graphene microelectrodes is >10× improved compared to typical microfabricated inert-metal microelectrodes. Through transverse isoelectric focusing between graphene microelectrodes, within minutes, specific proteins can be separated and concentrated to scales of ∼100 μm. Based on the concentrating effect and the high optical transparency of graphene, we develop a three-dimensional multistream microfluidic strategy for label-free detection of the proteins at same processing position with a sensitivity that is ∼10× higher than those of the state-of-the-art label-free sensors. These results demonstrate the advantage of monolayer-graphene microelectrodes for high-performance electrokinetic analysis to allow lab-on-a-chips of maximal time and size efficiencies.