A Novel Synthesis Strategy for Nanodiamond–Lanthanide Core–Shell Hybrids Toward Advanced Quantum Applications

Fluorescent nanodiamonds (FNDs) with negatively charged nitrogen vacancy (NV − ) and silicon vacancy (SiV − ) centers are powerful platforms for quantum sensing and bioimaging, yet their functionality can be greatly enhanced through integration with lanthanide, such as those exhibiting fluorescent upconversion or laser cooling. Despite extensive efforts ranging from surface functionalization and direct mixing to chemical etching, no established method has successfully produced uniform shells around FNDs. Here, we present a scalable hydrothermal strategy enabled by electrolyte‐mediated activation of NDs with Li + or Na + ions followed by controlled growth of a crystalline and uniform Y(Li/Na)F 4 :Yb,Er shell in a hydrothermal environment. This process yields integrated core–shell nanohybrids that preserve NV spin activity and enable engineered optical and magnetic functionality. The lanthanide shell is addressable under NIR excitation, while NV spin readout and magnetic‐field‐dependent ODMR are obtained under visible excitation. Together, these complementary modalities enable magnetic‐field modulation and provide a platform to explore laser refrigeration, supporting next‐generation quantum technologies and high‐resolution bioimaging.