Optical sensing of biological processes with nitrogen-vacancy centers in nanodiamonds

Nitrogen-vacancy (NV) centers in nanodiamonds are very attractive probes for sensing chemical reactions and biological processes. Their relaxation times are especially sensitive to the surrounding environment and can be probed with an all-optical protocol. To induce strong variations in the relaxation, the nanodiamonds can be brought in proximity to Gadolinium (Gd) ions which are strongly paramagnetic and can effectively quench the NV relaxation time. By optically monitoring the relaxation, it is possible to observe chemical and biological processes that affect the distance between Gd and nanodiamond. Here we first directly cover the nanodiamonds with chelated Gd and characterize the quenching effect for different Gd concentrations on an ensemble of nanodiamonds with an average diameter of around 20 nm. We will further show how the nanodiamonds can be connected with Gd via a cleavable peptide sequence. Specific enzymes, released during apoptosis, can cut the connection thus separating NVs from Gd and decreasing the relaxation rate. The nanodiamond sensors have the potential to be used simultaneously as drug delivery probes in addition to optical sensing, hence they might allow for monitoring chemotherapy.