Recent results about the design, the fabrication and the characterization of Dayem bridges based nano-SQUIDs having a loop area ranging from 4 to 0.04 μm2 are reported. The devices have been patterned by electron beam lithography in a 20–25 nm thick Nb layer. The nanosensors have shown at T = 4.2 K an intrinsic magnetic flux resolution less than 1 mΦ0. Preliminary measurements on Fe3O4 nanoparticles having a size between 4 and 8 nm are shown.