Sensor and method for measuring the areal density of magnetic nanoparticles on a micro-array

The present invention relates to a method and a device for magnetic detection of binding of biological molecules on a biochip. A magnetoresistive sensor device for measuring an areal density of magnetic nanoparticles on a micro-array, the magnetic nanoparticles ( 15 ) being directly or indirectly coupled to the target sample ( 11 ), is described. The magnetoresistive sensor device comprises a substrate ( 3 ) having attached thereto binding sites ( 9 ) able to selectively bind target sample ( 11 ), and a magnetoresistive sensor for detecting the magnetic field of the nanoparticles ( 15 ) coupled to the target sample. The magnetoresistive sensor comprises a plurality of magnetoresistive sensing elements ( 17, 19 ), the width and length dimensions of which are at least a factor 10 or more, preferably a factor 100 or more larger than the diameter of the nanoparticles ( 15 ). A corresponding method is described as well.