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Preparation of polydopamine and polydopamine/cation coatings. DA (2 mg/ml) was dissolved in 4 ml of DDW and mixed with different amounts of 0.2 N NaOH solution (20, 40, 60, 120, 240, 300, 360, or 420 μl). Each DA/NaOH solution was measured to determine its pH value, with the values being as follows: pH 7.4 (20 μl of NaOH), pH 7.7 (40 μl of NaOH), pH 8.2 (60 μl of NaOH), pH 8.8 (120 μl of NaOH), pH 9.8 (240 μl of NaOH), pH 10.2 (300 μl of NaOH), pH 10.3 (360 μl of NaOH), and pH 10.8 (420 μl of NaOH). Various substrates with a 1 × 1–cm2 size were placed in six-well plates with 3 ml of DA/NaOH solution. After 24 hours of shaking at 70 rpm, the polydopamine-coated surfaces were washed with DDW and then dried by blowing nitrogen gas. Polydopamine/cation coating was conducted under St-Bc conditions (pH 9.8). Potassium chloride (KCl) and tetramethylammonium chloride (NMe4+Cl) were dissolved in DDW. DA (2 mg/ml) was dissolved in 4 ml of different concentrations of KCl solution (20, 50, or 100 mM) or NMe4+Cl solution (0.1, 0.5, or 1 M) and then added to 240 μl of 0.2 N NaOH (pH 9.8). Various substrates with a 1 × 1–cm2 size were placed in six-well plates with 3 ml of DA/cation solution and then shaken at 70 rpm for 1, 3, 5, 8, 12, or 24 hours. After coating, the polydopamine-coated substrates were washed with DDW and dried by blowing nitrogen gas.

Characterization of polydopamine-coated surfaces. The thickness of the polydopamine coatings was measured by a spectroscopic ellipsometer (M2000D, JA Woollam Co. Inc.) with a He-Ne laser (632.8 nm) at a 70° incidence angle. The atomic composition of the surfaces was analyzed using XPS (K-Alpha, Thermo VG Scientific). The surface morphologies were observed with a scanning electron microscope (Inspect F50, FEI). The static water contact angles of the surfaces were compared before and after coating with a Phoenix 300 goniometer (Surface Electro Optics Co. Ltd.) by dropping 5 μl of DDW on the surfaces.

Delamination of polydopamine coating via base treatment. TPU films and TiO2 wafers were coated with polydopamine under the Md-Bc condition (pH 8.8) for 24 hours. The polydopamine-coated TPU films with a 3 × 3–cm2 size were dipped into 5 ml of 0.1 N NaOH (pH 12), DDW, or 0.1 N KOH (pH 12) solution for 1 hour, which was recorded as a video. The polydopamine coating in NaOH was delaminated from the TPU film, whereas those in KOH and DDW remained. Each TiO2 wafer with a 1 × 1–cm2 size was treated with 3 ml of either 0.1 N NaOH or 0.1 N KOH (pH 11) for 1 hour. After base treatment, TiO2 wafers were washed with DDW and dried by blowing N2 gas. The changes in the atomic compositions of the polydopamine-coated TiO2 surfaces were analyzed by XPS after base treatment.

UV-vis absorption analyses for DA and natural melanin polymerization. In total, DA (0.2 mg/ml) was dissolved in 10 mM tris buffer (pH 8.5). The absorbance of both DA solutions was monitored for 12 hours using a UV-vis spectrophotometer (HP8453, Hewlett Packard). Next, melanin (2 mg/ml) from S. officinalis was dissolved in 10 mM tris buffer and centrifuged. The UV-vis absorbance of the supernatant solution was measured.

HPLC-MS analyses with the polydopamine solution. DA (2 mg/ml) was oxidized for 24 hours under Md-Bc (pH 8.8) and St-Bc (pH 9.8) conditions by adjusting the pH values with 0.2 N NaOH. Both polydopamine solutions were filtered (0.45-μm pore size; Minisart, Sartorious Stedim Biotech), and then, 100 μl of each solution was injected into the HPLC equipment (Agilent 1260 Series). The solution passed through a C18 column (3-μm particle size, Supelco Analytical) at a flow rate of 0.5 ml/min, and the eluents were detected using UV-vis spectroscopy (280, 320, and 420 nm). The gradient conditions were as follows: 5 to 90% acetonitrile in 0.1% trifluoroacetic acid over periods ranging from 5 to 35 min, 95% acetonitrile at 40 min, and 5% acetonitrile at 43 min.

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