Nano Lett. 14, 6443–6448 (2014)

Developing a nanoscale magnetic sensing technique applicable to individual molecules could enable revolutionary advances in the physical, biological and medical sciences. However, such magnetic sensing devices ideally require the ability to perform sensitive detection of very weak magnetic fields with nanometre-scale resolution in ambient conditions at room temperature. Now, Alex Sushkov and co-workers from the USA, China, Germany and Russia have developed an all-optical method to detect the electron spin of a single molecule using a shallow nitrogen–vacancy (NV) centre in bulk diamond. The proof-of-principle experiment was implemented by attaching paramagnetic molecules containing Gd3+ to the surface of a bulk diamond crystal with widely separated NV centres located at a nominal depth of 6 nm below the diamond surface. With the aid of gold nanoparticles as markers, the positions of the NV centres and the molecules were determined by fluorescence imaging and atomic force microscopy. The international team of researchers measured the spin-state-dependent fluorescence following a 532 nm laser pulse with variable delay time. When the NV centre was in the proximity of a Gd3+ molecule, the measured NV spin-state population relaxation rate was significantly increased. Once the molecule was removed, the relaxation returned to the intrinsic rate. The sensing radius of an NV centre was determined to be approximately 12 nm.