Enhancement of heterogeneous immunoassays using AC electroosmosis

Enhancing the transport of analyte to a transducer surface is of critical importance to many biosensors, especially for those designed to capture and detect large biomolecules (nucleic acids, proteins, etc.) which diffuse slowly and therefore cause inordinately long detection times. For microfluidic-based sensors, which are characterized by low Reynolds number flow, the mass transport is purely diffusive in nature, which eliminates the possibility of turbulent mixing. In this article, enhancement of heterogeneous immunoassays using AC electroosmosis was studied through the use of finite element modeling and fluorescent immunoassays. Simulations of interdigitated planar electrodes indicate that optimum electrode geometry (electrode width and spacing) scales with the chamber height and that binding times can be reduced by up to a factor of 6. Optimum frequency of electrical excitation of the electrodes was between 100 and 200 Hz, which was confirmed by fluorescent immunoassays. Immunoassays also show that binding of secondary antibody on a functionalized surface can be significantly enhanced, especially at the ends of each electrode without noticeable nonspecific binding. Unexpectedly, the antibody binding was not uniformly distributed across the surface but patterned preferentially on the edges of the electrode, which indicates the presence of dielectrophoresis. Analysis of fluorescent intensity shows an enhancement of about 1.9 times at the center of the device and an enhancement of about 6.7 times at the edges of the best device type compared to a non-mixed counterpart. This work contributes to the growing field of electrokinetic enhancement of biosensors through the use of microelectrodes built directly onto, or near the binding region of a transducer.