Methods of synthesizing a DOPA-melanin (DM) polymer are disclosed, as well as compositions comprising the DM polymer and uses thereof. The method comprises contacting a reactant such as 3,4-dihydroxyphenylalanine (DOPA) with a high concentration aqueous salt solution under oxidative conditions. The resulting DM polymer may form as insoluble particles or as a coating on a substrate surface. Exemplary uses for the DM polymers include removing heavy metal ions from contaminated solutions or capturing and delivering cationic drugs such as gentamicin.