Magnetic relaxometry of methemoglobin by widefield nitrogen-vacancy microscopy

Hemoglobin (Hb) is a multifaceted protein, classified as a metalloprotein, chromoprotein, and globulin. It incorporates iron, which plays a crucial role in transporting oxygen within red blood cells. Hb functions by carrying oxygen from the respiratory organs to diverse tissues in the body, where it releases oxygen to fuel aerobic respiration, thus supporting the organism's metabolic processes. Hb can exist in several forms, primarily distinguished by the oxidation state of the iron in the heme group, including methemoglobin (MetHb). Measuring the concentration of MetHb is crucial because it cannot transport oxygen; hence, higher concentrations of MetHb in the blood causes methemoglobinemia. Here, we use optically detected magnetic relaxometry of paramagnetic iron spins in MetHb drop-cast onto a nanostructured diamond doped with shallow high-density nitrogen-vacancy (NV) spin qubits. We vary the concentration of MetHb in the range of 6 × 106–1.8 × 107 adsorbed Fe+3 spins per micrometer squared and observe an increase in the NV relaxation rate Γ1 (=1/T1, where T1 is the NV spin lattice relaxation time) up to 2 × 103 s−1. NV magnetic relaxometry of MetHb in phosphate-buffered saline solution shows a similar effect with an increase in Γ1 to 6.7 × 103 s−1 upon increasing the MetHb concentration to 100 μM. The increase in NV Γ1 is explained by the increased spin noise coming from the Fe+3 spins present in MetHb proteins. This study presents an additional usage of NV quantum sensors to detect paramagnetic centers of biomolecules at volumes below 100 picoliter.