2.1. Preparation of Melanin Nanoparticles

We synthesized melanin-based nanomaterials using nanochemistry-based approaches that allow for controlling the size and size distributions of the nanoparticles. The melanin nanoparticles were prepared using the oxidative polymerization of the organic monomeric units, which is similar to the biological mechanism of melanin formation by melanocytes in live organisms [2,3]. The monomeric units of the melanin differ depending on their chemical compositions, in the case of eumelanin, the monomeric units were 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid. The synthetic melanin nanoparticles in this study are prepared from dopamine hydrochloride (Sigma-Aldrich, St. Luis, MO, USA) as the starting precursor. The natural pigments (sepia melanin) are extracted from biological sources [2]. By adjusting synthetic parameters such as pH, temperature, and reaction time, the chemical compositions and sizes of the nanoparticles were controlled. The nanoparticles were separated from the solution batch by centrifugation (Beckman Coulter, Indianapolis, IN, USA) at 18,000 RPM 3 to 4 times. Melanin nanoparticle ink was prepared using deionized (DI) water as a solvent. This procedure allows synthesizing different melanin types with changing the precursor as the starting material.