Kit for capturing a molecule contained in a sample comprising: a. magnetic nanoparticles having as the largest dimension a dimension less than 1 μm, said nanoparticles each being coupled to at least one capture element, said at least one capture element specifically binding to said molecule, and b. a support for capturing said magnetic nanoparticles comprising or consisting essentially of at least one magnetic layer, said magnetic layer comprising a juxtaposition, possibly repeated, of at least one first and one second region, the first region comprising magnetic particles polarised in a first direction, and the second region comprising magnetic particles not polarised or polarised in a second direction different from the first polarisation direction of the magnetic particles of the first region, such that said at least one magnetic layer generates a magnetic field having at least one intensity variation of at least 0.1 mT at a distance of at least 1 μm from said at least one magnetic layer, said at least one intensity variation defining a maximum and a minimum of the standard of the intensity of said magnetic field, such as to define at said maximum of the standard of said magnetic field an area for capturing the magnetic nanoparticles on the capture support, said at least one magnetic layer being a flexible magnetic strip comprising magnetic composite materials randomly distributed, or oriented along a pre-orientation axis, in a polymer, the magnetic layer having a retentivity of 2,000 to 30,000 μm. 10 -4 T, the retentivity being equal to the magnetic moment of the magnetic layer divided by the surface of the magnetic layer. a. magnetic nanoparticles having as the largest dimension a dimension less than 1 μm, said nanoparticles each being coupled to at least one capture element, said at least one capture element specifically binding to said molecule, and b. a support for capturing said magnetic nanoparticles comprising or consisting essentially of at least one magnetic layer, said magnetic layer comprising a juxtaposition, possibly repeated, of at least one first and one second region, the first region comprising magnetic particles polarised in a first direction, and the second region comprising magnetic particles not polarised or polarised in a second direction different from the first polarisation direction of the magnetic particles of the first region, such that said at least one magnetic layer generates a magnetic field having at least one intensity variation of at least 0.1 mT at a distance of at least 1 μm from said at least one magnetic layer, said at least one intensity variation defining a maximum and a minimum of the standard of the intensity of said magnetic field, such as to define at said maximum of the standard of said magnetic field an area for capturing the magnetic nanoparticles on the capture support, said at least one magnetic layer being a flexible magnetic strip comprising magnetic composite materials randomly distributed, or oriented along a pre-orientation axis, in a polymer, the magnetic layer having a retentivity of 2,000 to 30,000 μm. 10 -4 T, the retentivity being equal to the magnetic moment of the magnetic layer divided by the surface of the magnetic layer.