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.
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