(a) Binding isotherm of Cu2+ ions to synthetic-eumelanin-coated discs; titration of Cu(NO3)2 · 2.5H2O in 50 mM NaNO3 (aq.) at initial pH 4.75. (b) Binding isotherm of Cd2+ ions to synthetic-eumelanin-coated discs; titration of Cd(NO3)2 · 4H2O in 50 mM NaNO3 (aq.) at initial pH 4.75. (c) Binding isotherm of Zn2+ ions to synthetic-eumelanin-coated discs; titration of Zn(NO3)2 · 6H2O in 50 mM NaNO3 (aq.) at initial pH 4.75. (a–c) Data are average values from at least three independent titrations, each with different discs; error bars represent one standard deviation.
