Spin Dynamics and Magnetization in Sepia Melanin by Electron Paramagnetic Resonance and Vibrating Sample Magnetometry

Abstract

Sepia melanin, a eumelanin member of the melanin biopigment family, is part of the dark Sepia ink liberated by cuttlefish for defence and camouflage. It takes its name from Sepia Officinalis, a common species of cuttlefish from where Sepia melanin is extracted. Sepia melanin contains stable radicals i.e. carbon centered radicals (CCR) and semiquinone radicals (SQR), making it a promising candidate for organic spintronic and bioelectronic applications. The behavior of the radicals is strongly influenced by hydration, as water molecules interact with redox-active sites and modulate charge and spin configuration. In this study, we investigate the magnetic behavior of Sepia melanin in both dry and hydrated states using Electron Paramagnetic Resonance (EPR) spectroscopy in ambient conditions and Vibrating Sample Magnetometry (VSM) across a temperature range of 150-360 K. EPR analysis revealed a decrease in signal intensity, a slight increase in g-value and an increase in transverse relaxation time upon hydration, in agreement with literature. VSM measurements suggested weak ferromagnetism in both states, with saturation magnetization (Ms) and remanent magnetization (Mr) decreasing upon hydration. As expected, this ferromagnetic behavior weakens with increasing temperature. Despite this decline, ferromagnetic behavior persists above 360 K. To discard the presence of metallic chemical elements possibly contributing to the magnetic behavior, we conducted an Instrumental Neutron Activation Analysis (INAA) study. In addition, we probed the surface elemental composition by X-ray Photoelectron Spectroscopy (XPS). While INAA revealed trace concentrations of Fe, Cu, Ni, and Co in the bulk, the presence of such elements was below the detection limit of XPS. These findings suggest that the observed magnetism could arise predominantly from the organic radical matrix, particularly CCR and SQR, rather than contributions from metals.