Defect-Activated and Surface-Modified Hexagonal Boron Nitride Nanoparticles toward Intracellular Quantum Sensing

van der Waals materials hosting spin defects have emerged as a new platform for solid-state quantum sensing. We report hexagonal boron nitride (hBN) nanoparticles containing negatively charged boron vacancy (<i>V</i>_<i>B</i>^-) centers that function as nanoscale quantum sensors. High-energy electron irradiation efficiently generated <i>V</i>_<i>B</i>^- centers, enabling clear optically detected magnetic resonance (ODMR) detection from nanoparticle ensembles. To improve dispersion and suppress nonspecific protein adsorption, we developed a two-step surface modification process using silica encapsulation followed by hyperbranched polyglycerol (HPG) grafting. The HPG coating enhanced colloidal stability and reduced nonspecific protein adsorption. ODMR thermometry revealed temperature-dependent frequency shifts with a thermal coefficient of approximately - 300 kHz/°C. Finally, we demonstrated proof-of-concept detection of ODMR signal in HeLa cells after uptake of the nanoparticles. These results establish <i>V</i>_<i>B</i>^--containing hBN nanoparticles as promising nanoscale quantum sensors for future intracellular applications.