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Surface modification of natural ink particles for hair coloring

We report a method to modify the surface of size-controlled ink particles (SIPs), consisting of natural eumelanin isolated from ink sacs of Sepia officinalis, through simple chemical reactions. The carboxyl group on the SIP was replaced with an amino group using cationization reagents. The original SIPs have a highly negative potential of −50.8 ± 0.6 mV. This value can be modified by cationization reagents. Increases in the surface potentials of the SIPs decreased the effect of blocking the aggregation through electrostatic repulsion. The original SIPs were repelled on negatively charged surfaces but adsorbed on positively charged surfaces. The opposite result was obtained in SIPs modified with cationization reagents. The specific interaction between the carboxyl group and the amino group might play a more important role than the overall electrostatic interactions at neutral pH. The results of this study should serve in the development of hair colorants utilizing SIPs.