Decoding eumelanin's spin label signature: a comprehensive EPR analysis

Eumelanin is a black-brown natural pigment found throughout nature. As a material class, it possesses unique physical and chemical properties that entice much attention from chemists, physicists, and material scientists due to its potential use in medical and sustainable organic (bio)electronics settings. To harness its potential, accurate spectroscopical characterization is a primary requirement for the efficient engineering of eumelanin-based electronic devices. A key spectroscopic technique is electron paramagnetic resonance (EPR) since the persistent paramagnetism of eumelanin is a spin-label signature of its molecular state. This review provides a comprehensive discussion on the origin of eumelanin paramagnetism through a multifrequency EPR approach and highlights the current multi-species point-of-view. Through electron paramagnetic resonance spectroscopy, one secret of eumelanin was unlocked revealing its intricate paramagnetic signature.