Three-dimensional microfluidic confinement for efficient sample delivery to biosensor surfaces. application to immunoassays on planar optical waveguides.
A microchip-based flow confinement method for rapid delivery of small sample volumes to sensor surfaces is described. For flow confinement, a sample flow is joined with a perpendicular makeup flow of water or sample medium. Under laminar flow conditions, the makeup flow confines the sample into a thin layer above the sensing area and increases its velocity. This can benefit mass transport limited processes such as DNA hybridization or heterogeneous immunoassays. For proof of concept, this method was applied to a high-affinity immunoassay with excess capture antibody. Rabbit IgG was immobilized onto a silicon nitride waveguide. Cy5-labeled anti-rabbit IgG was hydrodynamically pumped over the immobilized zone through an attached 3D-PDMS flow cell with 20-microm-deep microchannels. The degree of confinement was adjusted through the volume flow rate of the confining flow. Evanescent field-based fluorescence detection enabled monitoring of the binding event. Assays were allowed to reach equilibrium to enable sensorgram normalization for inter-run comparison. The corresponding assay completion times could be reduced from 55 min for static drop conditions to 13 min for 25:1 flow confinement (ratio of confining to sample flow). For typical analytical applications, where equilibrium formation is not required, the faster response should translate to very short analysis times. Concurrently with the faster binding, sample consumption was reduced by 96% compared to conventional whole-channel sample delivery. Diffusional loss of analyte into the confining layer was identified as the main limitation of flow confinement, particularly for long sensing pads.