Core-Shell Architecture for Engineered Biocompatible Quantum Nanoprobes*

Nitrogen-vacancy (NV) centers in nanodiamonds (NDs) have emerged as versatile qubit sensors capable of probing biophysical parameters not accessible by conventional techniques. Their applications range from probing fundamental cellular processes to novel diagnostic devices. Although powerful, applications have been limited by sensitivity and difficulties in targeting desired biological compartments. In this work, we present a core-shell architecture designed to address these challenges. We show that an engineered protective shell simultaneously increases the NV coherence properties while serving as a scaffold for biochemical modifications. Our approach is based on an initial encapsulation of the NDs with a high-quality SiO2 shell followed by silanization for subsequent surface functionalization. Single and double quantum relaxation measurements reveal prolonged longitudinal spin relaxation times, likely caused by a reduction in defect density associated with the diamond surface. Furthermore, in cell labeling assays, our engineered NDs allow us to efficiently target specific subcellular sites. 

The demonstrated techniques enable the next generation of diamond-based quantum sensing experiments in the life sciences where sensitivity and cellular targeting are vital elements.