  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. Deviations in Hb concentration in the blood have been linked to
various medical conditions, including anemia and other blood disorders. Here,
we use optical detected magnetic relaxometry of paramagnetic iron spins in Hb
drop-casted onto nanostructured diamond doped with shallow (~ 5.5 nm) high
density nitrogen vacancy (NV) spin qubits. We modify the Hb concentration in
the range of 6 x 10^6 to 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 Hb in
phosphate-buffered saline solution show a similar effect with an increase of G1
to 6.7 x 10^3 s^-1 upon increasing the Hb concentration to 100 uM. The increase
of NV G1 is explained by the increased spin noise coming from the Fe+3 spins
present in Hb proteins. This study presents an additional usage of NV quantum
sensors to detect paramagnetic centers in biomolecules.
