Magnetic beads retention device for on-chip sandwich immuno-assay

We propose a new and original concept for the precise positioning and the geometrical retention of self-assembled magnetic microbeads in a structured microchannel. When brought in a homogeneous magnetic field, a solution of 500 nm diameter magnetic microbeads self-assembles in magnetic chains in the large sections of a periodically enlarge microchannel, thereby decreasing their magnetic dipolar energy. The self-assembled magnetic chains are retained over the cross-section of the microchannel in a flow velocity up to ~1 mm.s^-1, and strongly interact with the analytes of the solution. Our device is able to perform full on-chip direct and sandwich immuno-assays, in a total assay time of less than 30 min. It consumes nanoliters of sample and reagent solutions, with a fluorescent detection limit of ~1 ng.mL^-1 for biotinylated rabbit immunoglobulin G (IgG) and mouse IgG in the direct and two-site immuno-assay test, respectively.