A method and a device are provided for distinguishing a specific binding from a less specific binding between at least one magnetic nanoparticle and a surface of another entity by applying a magnetic field and detecting a physical parameter relating to magnetic nanoparticle rotational or motional freedom while the magnetic nanoparticle is attached to the surface. The method and device may be applied to in-vivo and in-vitro biomolecular diagnostics. The sensor combines in one sensor the detection of magnetic particles or labels and determination of the binding quality and the properties of magnetic particles or labels which are bound to the surface of another entity.