Unraveling Eumelanin Radical Formation by Nanodiamond Optical Relaxometry in a Living Cell

Defect centers in nanodiamond allow the detection of tiny magnetic fields in their direct surrounding, which makes them an emerging tool for nanoscale sensing applications. The abundant pigment eumelanin plays a major role in biology and material science. We present the first time tracking of the comproportionation reaction in eumelanin by detecting and quantifying semiquinone radicals via the nitrogen-vacancy colour centre as single atom detector. A thin layer of eumelanin is polymerized on the surface of NDs and depending on the environmental conditions, such as pH, near infrared and ultraviolet light irradiation, the radicals undergo maintenance, formation, or disappearance in-situ. By combining experiments and theoretical simulations, we were able to quantify the local number and kinetics of free radicals in the eumelanin layer. Next, the nanodiamond sensor was transported into cells in endosomal vesicles and the number of radicals within the eumelanin layer was also determined. The new quantum sensor could provide valuable insights into the chemistry of eumelanin, which could shine light on eumelanin and melanin-related diseases.