A method and device for the rapid detection of biomolecules (320) diffusing in a nanometer-confined slit (204) is claimed. In particular, the present invention relates to a novel concept of fluidic side apertures (205) that facilitates the filling of the device, the surface coating with biomolecules and that measures the affinity between fluorescently labeled biomolecules in aqueous solution with other biomolecules immobilized on surfaces.