A technique is proposed to enhance microfluidic immuno-sensors, for example, immunoassays, in which a ligand immobilized on a microchannel wall specifically binds analyte flowing through the channel. These sensors can be limited in both response time and sensitivity by the diffusion of analyte to the sensing surface. In certain applications, the sensitivity and response of these heterogeneous immunoassays may be improved by using AC electrokinetically-driven microscale fluid motion to enhance antigen motion towards immobilized ligands. Specifically, the electrothermal effect is used to micro-stir analyte near the binding surface. Numerical simulations of antigen in a microchannel flow subjected to the electrothermal effect show that 6 V(rms) applied to electrodes near a binding region can increase binding in the first few minutes by a factor of seven. The effectiveness of electrothermal stirring is a strong function of the Damköhler number. The greatest binding enhancement is possible for high Damköhler numbers, where the reaction is limited by diffusion. Based on these results, the utility of this technique for diffusion-limited microfluidic sensor applications is demonstrated.

AC-driven electrothermal flow is used to enhance the temporal performance of heterogeneous immuno-sensors in microfluidic systems by nearly an order of magnitude. AC electrokinetic forces are used to generate electrothermal flow, which in turn produces a circular stirring fluid motion that enhances the transport of diffusion-limited proteins. This provides more binding opportunities between suspended antigens and wall-immobilized antibodies. We investigate experimentally the effectiveness of electrothermal stirring, using a biotin-streptavidin heterogeneous assay, in which biotin is immobilized, and fluorescently-labeled streptavidin is suspended in a high conductivity buffer (sigma = 1.0 S m(-1)). Microfabricated electrodes were integrated within a microwell and driven at a frequency of f= 200 kHz and 10 V(rms). Fluorescent intensity measurements show that for a five minute assay, electrothermal stirring increases the binding rate by a factor of almost nine. Similar binding improvement was measured for longer assays, up to fifteen minutes. The electrothermal enhancement of this assay was modeled numerically and agrees with experimental binding rates. The measured fluid velocity of 22 +/- 2 microm s(-1) was significantly lower than that predicted by the numerical model, 1.1 mm s(-1), but nevertheless shows the same fourth power dependence on applied potential. The results demonstrate the ability for electrothermal stirring to reliably improve the response time and sensitivity within a given time limit for microfluidic diffusion-limited sensors.

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