Research ArticleSURFACE CHEMISTRY

Progressive fuzzy cation-π assembly of biological catecholamines

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Science Advances  07 Sep 2018:
Vol. 4, no. 9, eaat7457
DOI: 10.1126/sciadv.aat7457
  • Fig. 1 A newly proposed progressive assembly for polydopamine coating.

    Dopamine is sequentially oxidized to various heterogeneous derivatives (dopamine-quinone, leukochrome, dopachrome, DHI, and dicatechol) via covalent bonds (pink arrow). The covalently bonded oligomers are physically assembled via hydrogen bonds, π-π stacking, van der Waals interactions (gray box), and cation-π interactions (blue box), forming the polydopamine coating.

  • Fig. 2 Delamination of polydopamine (dopamine-melanin) coating under St-Bc solutions.

    (A) Photos show the color changes before and after polydopamine coating at different pH values [Md-Bc, pH 8.8 (left); St-Bc, pH 9.8 (middle); and unmodified (right)]. PDMS, poly(dimethyl siloxane). (B) The thickness of the polydopamine coating on TiO2 substrates at different pH values and the corresponding contact angle images of the polydopamine-coated substrate immersed in Md-Bc solutions (7.4 < pH < 8.8; left) or St-Bc solutions (9.8 < pH < 10.8; middle) and the bare TiO2 substrate (right). (C) Delamination scheme for the polydopamine coating by potential deprotonation in strong alkaline solutions. (D) The polydopamine coating on TPU film was peeled off in 0.1 N NaOH (pH 12) solution.

  • 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 (N1s, –NH3+ in blue, –NH2 in red, and –NH– in green), sodium (Na1s), and potassium (K2p) 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).

  • Fig. 4 An application of cation-π–mediated progressive assembly on material-independent superhydrophilic coatings.

    (A) Plausible substructures of polydopamine with ammonium cations (that is, tetramethylammonium, NMe4+) via cation-π interactions between the polydopamine progressive building block indole ring and NMe4+ under St-Bc conditions, resulting in a superhydrophilic coating. (B) The static water contact angle for a variety of substrates without any treatment (top row) and one with a polydopamine/NMe4+ coating on the surface as a superhydrophilic modifier (bottom row). PEEK, polyether ether ketone; PS, polystyrene. (C) The chemical status of nitrogen elements in the polydopamine coating at St-Bc pH in the presence of NMe4+ (concentrations of 0.1, 0.5, or 1.0 M). The amount of the intercalated ammonium (indicated as a signature peak at 402.3 eV) increased with the presence of coatings with higher concentrations of NMe4+. (D) Changes in the water contact angle according to the NMe4+ Cl concentrations and coating times (red line for 1 M NMe4+, blue line for 0.5 M NMe4+, and black line for 0.1 M NMe4+). A water contact angle below 5° is defined as superhydrophilic.

  • Fig. 5 “Function gaining” by progressive cation-π assembly.

    (A) UV-visible (vis) absorbance of eumelanin from Sepia officinalis dissolved in tris buffer (pH 8.5). A.U., arbitrary units. (B) Continuous monitoring of polydopamine formation in tris buffer (pH 8.5) up to 12 hours via the UV-vis absorption spectra. The reaction solution became dark brown to black after 12 hours (the arrow). (C) Covalent and noncovalent progressive assemblies of eumelanin-mimetic polydopamine to gain light protection functions over time.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/9/eaat7457/DC1

    Fig. S1. Chemical structure of oligomers formed during polydopamine coating under St-Bc pH conditions analyzed by HPLC-MS with a C18 column.

    Fig. S2. XPS analyses of polydopamine-coated TiO2 surfaces after 1 hour of strong base treatment.

    Fig. S3. Maintenance of polydopamine coating on a TPU film in 0.1 N KOH (pH 12) solution for 1 hour.

    Fig. S4. Intercalation of NMe4+ on polydopamine coating with pH variations.

    Fig. S5. Wettability of polydopamine-coated substrates in Md-Bc and St-Bc conditions in the presence and absence of 1 M choline, a precursor to neurotransmitter ACh.

    Fig. S6. Surface morphologies of the NMe4+-intercalated polydopamine functionalized surfaces at St-Bc with 0.1 M NMe4+ (top) and 1 M NMe4+ (bottom) by SEM and atomic force microscopy analyses.

    Movie S1. Stability of polydopamine coating in NaOH, DDW, and KOH solution.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Chemical structure of oligomers formed during polydopamine coating under St-Bc pH conditions analyzed by HPLC-MS with a C18 column.
    • Fig. S2. XPS analyses of polydopamine-coated TiO2 surfaces after 1 hour of strong base treatment.
    • Fig. S3. Maintenance of polydopamine coating on a TPU film in 0.1 N KOH (pH 12) solution for 1 hour.
    • Fig. S4. Intercalation of NMe4+ on polydopamine coating with pH variations.
    • Fig. S5. Wettability of polydopamine-coated substrates in Md-Bc and St-Bc conditions in the presence and absence of 1 M choline, a precursor to neurotransmitter ACh.
    • Fig. S6. Surface morphologies of the NMe4+-intercalated polydopamine functionalized surfaces at St-Bc with 0.1 M NMe4+ (top) and 1 M NMe4+ (bottom) by SEM and atomic force microscopy analyses.

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Stability of polydopamine coating in NaOH, DDW, and KOH solution.

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