Magnetic Relaxometry of Methemoglobin by Widefield Nitrogen-Vacancy Microscopy

Abstract: 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 concentration of MetHb in the blood causes methemoglobinemia. Here, we use optically detected magnetic relaxometry of paramagnetic iron spins in MetHb drop-casted onto nanostructured diamond doped with shallow high density nitrogen vacancy (NV) spin qubits. We modify the MetHb concentration in the range of 6 x 10^6 - 1.8 x 10^7 adsorbed Fe+3 spins per um^2 and observe an increase of the NV relaxation rate G1 (= 1/T1, T1 is NV spin lattice relaxation time) up to 2 x 10^3 s^-1. NV magnetic relaxometry of MetHb in phosphate-buffered saline solution shows a similar effect with an increase of G1 to 6.7 x 10e3 s^-1 upon increasing the MetHb concentration to 100 uM. The increase of NV G1 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.