Using ac-field-induced electro-osmosis to accelerate biomolecular binding in fiber-optic sensing chips with microstructures.

This article reports the use of ac-field-induced charges at the corners of microstructures on fiber-optic sensing chips to generate electro-osmotic vortex flows in flow cell channels that can accelerate solute binding on the fiber. The sensing chip made of a cyclic olefin copolymer COC substrate contained a flow cell channel of dimensions 15 mm x 1 mm x 1 mm. A partially unclad optical fiber was placed within the channel. Relief-like strip structures of 25-mum thickness fabricated on the channel bottom were produced with an injection-molding process. The external electric field lines penetrating through the corners of the plastic microstructures induce charges on the corner surfaces to build up electrical double layers. When a high-frequency ac field (approximately 100 kHz) is used to flip the field polarities quickly, neutralization of the induced charge cannot be accomplished. The electrical double layer is therefore sustained. When absorbed charges in the double layer are driven by the external field, electro-osmotic flows are generated. The unclad portion of the fiber was coated with biotin-functionalized gold nanoparticles. The streptavidin solution was filled in the channel from the feeding tube, and the ac field (approximately 50 V/cm) was subsequently turned on for 30 s. The ac-field-induced electro-osmotic flows can accelerate solute transport in the sensing channel to enhance the binding kinetics of streptavidin molecules with biotin probes implanted on the gold nanoparticle surface. As a result, the fiber-optic localized plasmon resonance (FO-LPR) sensing signal becomes steady as soon as the external field is turned off. In contrast, the signal cannot reach steady state until 200-300 s in a typical static sensing cell. A significant reduction in the sensing response time is demonstrated. The binding assay of streptavidin with immobilized biotin on gold nanoparticle-coated sensing fibers was validated using this mixing device. The detection limit for streptavidin of approximately 10(-11) M is close to the reported values obtained using static cells. Similarly, the sensing response time of an orchid Odontoglossum ringspot virus (ORSV) sample was reduced from 1000 to 330 s when an external field was applied to mix the fluid for 60 s, even though the detection limit was maintained.