In Situ Quantum Detection of Ferritin Using Spin Defects in Hexagonal Boron Nitride

In situ detection of paramagnetic ions, including ions and biological macromolecules, is critical for fundamental research and applications in biology, chemistry, and medicine. The negatively charged boron vacancy (V_B^-) defects in hexagonal boron nitride (hBN) have emerged as versatile, highly sensitive quantum sensors for detecting various physical quantities. To further extend the scope and make practical applications of the quantum sensing, in this work, we demonstrate in situ quantum detection of the paramagnetic Mn²⁺ ions and ferritin in both solution and dry states using V_B^- defects in hBN. The experiments show that the optically detected magnetic resonance (ODMR) contrasts of V_B^- defects decrease with increasing concentrations of Mn²⁺ and ferritin in solution states; however, the ODMR contrasts remain unchanged in the case of dry states. This phenomenon is attributed to magnetic-noise-induced depopulation and modulated solution conductivity by solution paramagnetic ions. At the same time, the spin longitudinal relaxation rates monotonically increase with concentration in both solution and dried states as a function of ions concentration due to the magnetic noise from paramagnetic ions. Moreover, V_B^- defects can also distinguish different ionic species based on their distinct relaxation rates, requiring no prior knowledge. Finally, the all-optical relaxation method is also adopted to efficiently detect ferritin and paramagnetic ions. Collectively, our work establishes V_B^- defects in hBN-based sensors as a versatile quantum sensing platform for different species of paramagnetic ions in biology and chemistry, offering dual-modal detection methods including ODMR and T₁ relaxation with operational flexibility and high sensitivity.