Relaxometry imaging of superparamagnetic magnetite nanoparticles at ambient conditions

We present a novel technique to image superparamagnetic iron oxide nanoparticles via their fluctuating magnetic fields. The detection is based on the nitrogen-vacancy (NV) color center in diamond, which allows optically detected magnetic resonance (ODMR) measurements on its electron spin structure. In combination with an atomic-force-microscope, this atomic-sized color center maps ambient magnetic fields in a wide frequency range from DC up to several GHz [1], while retaining a high spatial resolution in the sub-nanometer range [2]. We demonstrate imaging of single 10 nm sized magnetite nanoparticles using this spin noise detection technique. By fitting simulations (Ornstein-Uhlenbeck process) to the data, we are able to infer additional information on such a particle and its dynamics, like the attempt frequency and the anisotropy constant [3]. This is of high interest to the proposed application of magnetite nanoparticles as an alternative MRI contrast agent or to the field of particle-aided tumor hyperthermia. [1] E. Sch\"{a}fer-Nolte et al., Phys. Rev. Lett. \textbf{113}, 217204 (2014) [2] P. Maletinsky et al., Nat. Nanotech. \textbf{7}, 320 (2012) [3] D. Schmid-Lorch et al., Nano Lett. \textbf{15}, 4942 (2015)