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The paramagnetic properties of size-controlled ink particles isolated from the ink sacs of Sepia officinalis were studied by electron spin resonance (ESR) spectroscopy. Both the size-controlled ink particles and synthetic melanins seemingly yielded similar ESR spectra consisting of a singlet with a slightly asymmetrical signal. However, the progressive microwave power saturation revealed a clear difference between their paramagnetic behaviors. In comparison with synthetic melanins, the ESR spectra of the ink particles readily reached saturation, indicating a long spin–lattice relaxation time. On the other hand, the ESR linewidth depended on particle size. This implies that the particle size is related to the distance between paramagnetic species in the particles. Hence, it is reasonable that the large ink particle has the weakest spin–spin interaction among these samples. The employment of the size-controlled ink particles enabled us to determine precisely the paramagnetic parameters of Sepia inks.

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