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Nitrogen-vacancy (NV) centers in diamond allow measurement of magnetic fields at nanoscale. The working principle is based on the measurement of a NV center resonance frequency shift, detected by monitoring the center's photoluminescence. Here we present a technique based on lock-in detection and simultaneous excitation of the three peaks corresponding to hyperfine interaction between the NV center and the 14N nucleus.

We present an innovative experimental set-up that uses Nitrogen-Vacancy centres in diamonds to measure magnetic fields with the sensitivity of $\eta =68\pm 3~\mathrm{nT}/\sqrt{\mathrm{Hz}}$ η = 68 ± 3 nT / Hz at demonstrated (sub)cellular scale. The presented method of magnetic sensing, utilizing a lock-in based ODMR technique for the optical detection of microwave-driven spin resonances induced in NV centers, is characterized by the excellent magnetic sensitivity at such small scale and the full biocompatibility. The cellular scale is obtained using a NV-rich sensing layer of 15 nm thickness along z axis and a focused laser spot of $(10 \times 10)~\mu\mathrm{m}^{2}$ ( 10 × 10 ) μ m 2 in x-y plane. The biocompatibility derives from an accurate choice of the applied optical power. For this regard, we also report how the magnetic sensitivity changes for different applied laser power and discuss the limits of the sensitivity sustainable with biosystem at such small volume scale. As such, this method offers a whole range of research possibilities for biosciences.