Fig. 3. Polydopamine coating through K⁺ assistance in delaminating conditions consisting of strong alkaline pH.

(A) A schematic description for resuming polydopamine coating in the presence of potassium ions after delamination in a strong alkaline condition. (B) The polydopamine coating thickness at pH 8.8 [a method described in (19)] (Md-Bc, blue bar), pH 9.8 (St-Bc, red bar), and pH 9.8 with 20, 50, or 100 mM KCl (St-Bc/K⁺, green bars). (C) Surface morphology by scanning electron microscopy (SEM) and the static water K⁺/St-Bc contact angles in increasing KCl concentration [20 mM (left), 50 mM (middle), and 100 mM (right)]. Scale bars, 2 μm. (D) XPS analysis of nitrogen (N_1s, –NH₃⁺ in blue, –NH₂ in red, and –NH– in green), sodium (Na_1s), and potassium (K_2p) elements showing cation-to-neutral (–NH₃⁺→–NH₂) charge conversion from Md-Bc to St-Bc transfer (that is, elimination of cation-π). The surface-bound Na⁺ and K⁺ ions were also detected. (E) Assembly force transitions from strong cation-π interactions between the protonated amines and π system (left, Md-Bc conditions) to weak cation-π interactions between Na⁺ and the π system due to pH-induced deprotonation (middle, St-Bc conditions). Finally, cation-π interactions are resumed by the addition of K⁺ under the deprotonation conditions (right, St-Bc/K⁺ conditions).