Room-temperature detection of single 20 nm super-paramagnetic   nanoparticles with an imaging magnetometer
  We demonstrate room temperature detection of single 20 nm super-paramagnetic nanoparticles (SPNs) with a wide-field optical microscope platform suitable for biological integration. The particles are made of magnetite (Fe3O4) and are thus non-toxic and biocompatible. Detection is accomplished via optically detected magnetic resonance imaging using nitrogen-vacancy defect centers in diamond, resulting in a DC magnetic field detection limit of 2.3 {\mu}T. This marks a large step forward in the detection of SPNs, and we expect that it will allow for the development of magnetic-field-based biosensors capable of detecting a single molecular binding event. 
Michael Gould, Russell Barbour, Nicole Thomas, Hamed Arami, Kannan M.   Krishnan and Kai-Mei Fu
cond-mat.mes-hall physics.optics quant-ph