Nanodiamond Emulsions for Enhanced Quantum Sensing and Click-Chemistry Conjugation

Nanodiamonds containing nitrogen-vacancy (NV) centers can serve as colloidal quantum sensors of local fields in biological and chemical environments. However, nanodiamond surfaces are challenging to modify without degrading their colloidal stability or the NV center's optical and spin properties. We present a simple and general method to coat nanodiamonds with a thin emulsion layer that preserves their quantum features, maintains their colloidal stability, and provides functional groups for subsequent cross-linking and click-chemistry conjugation reactions. To demonstrate this technique, we decorated nanodiamonds with combinations of carboxyl- and azide-terminated amphiphiles that enable conjugation using two different strategies. A theoretical model is developed to understand the effect of the emulsion layer on the NV center's spin lifetime, and T-1 relaxometry is employed to quantify the nanodiamonds' chemical sensitivity to paramagnetic ions. This general approach to nanodiamond surface functionalization will enable advances in quantum nanomedicine and biological sensing.