Fast and Reliable Synthesis of Melanin Nanoparticles with Fine-Tuned Metal Adsorption Capacities for Studying Heavy Metal Ions Uptake

PURPOSE: Adsorption and uptake of heavy metals by polymeric nanoparticles is driven by a variety of physicochemical processes. In this work, we examined heavy metal uptake by synthetic melanin nanoparticles and analyzed physicochemical properties that affect the extent of metal uptake by the nanoparticles. METHODS: Eumelanin nanoparticles were synthesized in a one-pot fast process from a 5,6-diacetoxy indole precursor that is hydrolyzed in situ into dihydroxy indole (DHI). The method allows the possibility of changing the level of sodium ions that ends up in the nanoparticles. Two variants of synthetic DHI–melanin (low-sodium and high sodium variants) were evaluated and demonstrated different relative adsorption efficiencies for heavy metal cations. RESULTS AND DISCUSSION: For the low-sodium DHI–melanin and in terms of percentages of metal ion removal, the relative order of extraction from 50 ppm solutions was Zn(2+) > Cd(2+) > Ni(2+) > Co(2+) > Cu(2+) > Pb(2+), with the extraction percentages ranging from 90% down to 76%, for a 30-minute adsorption time before equilibrium. The lower-sodium DHI–melanin consistently removed more Zn(2+) than the higher-sodium variant. Electron microscopy (SEM) showed an increase in melanin particle size after metal ions uptake. In addition, X-ray photoelectron spectroscopy (XPS) of DHI–melanin particles with depth profiling after Zn ions uptake supported particle swelling and ion transport within the particles. CONCLUSION: These initial studies showed the potential of this straightforward synthesis to obtain synthetic DHI–melanin nanoparticles similar to those from biological sources with the possibility to fine-tune their metal adsorption capacity. These synthetic nanoparticles can be used either for the removal of a variety of metal ions or to mimic and study mechanisms of metal uptake by melanin deriving from biological sources, with the potential to understand, for instance, differential heavy metal uptake by various melanic pigments.