The Chemistry of Polydopamine Film Formation: The Amine-Quinone Interplay

Abstract

Despite extensive investigations over the past decade, the chemical basis of the extraordinary underwater adhesion properties of polydopamine (PDA) has remained not entirely understood. The bulk of evidence points to PDA wet adhesion as a complex process based on film deposition, and growth in which primary amine groups, besides catechol moieties, play a central role. However, the detailed interplay of chemical interactions underlying the dynamics of film formation has not yet been elucidated. Herein, we report the results of a series of experiments showing that coating formation from dopamine at pH 9.0 in carbonate buffer: (a) Requires high dopamine concentrations (>1 mM); (b) is due to species produced in the early stages of dopamine autoxidation; (c) is accelerated by equimolar amounts of periodate causing fast conversion to the o-quinone; and (d) is enhanced by the addition of hexamethylenediamine (HMDA) and other long chain aliphatic amines even at low dopamine concentrations (<1 mM). It is proposed that concentration-dependent PDA film formation reflects the competition between intermolecular amine-quinone condensation processes, leading to adhesive cross-linked oligomer structures, and the intramolecular cyclization route forming little adhesive 5,6-dihydroxyindole (DHI) units. Film growth would then be sustained by dopamine and other soluble species that can be adsorbed on the surface.

Keywords: amines; film; periodate; polydopamine; polymerization; quinone.

Figure 1

Ultraviolet–visible (UV–vis) absorption spectra of quartz substrates subjected to dip-coating with dopamine 1 mM (red trace) and 10 mM (blue trace).

Figure 2

Ultraviolet–visible (UV–vis) and film thickness analysis. (a) UV–vis spectra of quartzes dipped into 1 mM solution of dopamine in 0.05 M carbonate buffer (pH 9.0) in the presence of equimolar amounts of various amines over 24 h. (b) Average film thickness as determined by atomic force microscopy (AFM). Data are shown as mean ± standard deviation (SD) of three independent experiments.

Figure 3

Atomic force microscopy (AFM) analysis of the polydopamine (PDA) film obtained in the presence of hexamethylenediamine (HMDA). (a) Bright-field image of the investigated sample region collected by 20× microscope objective. (b) AFM image of the area indicated by the yellow square in the optical image. Film thickness: 40 ± 15 nm.

Figure 4

Kinetics of polydopamine (PDA) film formation. (a) Evolution of the ultraviolet–visible (UV–vis) spectra of PDA film formation with quartz substrates dipped into the mixture for 1 h at different reaction times. (b) Absorbance of the films at two selected wavelengths. Data are shown as mean ± standard deviation (SD) of three independent experiments.

Figure 5

Kinetics of polydopamine (PDA) film formation: autoxidation vs. periodate. (a) Evolution of the ultraviolet–visible (UV–vis) spectra of PDA film formed by dopamine autoxidation or periodate-induced oxidation. (b) Time course of 400 nm absorption development for PDA film formed by dopamine autoxidation (red line) vs. periodate induced oxidation (blue line). Data are shown as mean ± standard deviation (SD) of three independent experiments.

Figure 6

Atomic force microscopy (AFM) and micro-Raman analysis of polydopamine (PDA) films deposited at 24 h following autoxidation (1) and periodate oxidation (2). (a) Bright-field image of the investigated sample region collected by 20× microscope objective. (b) AFM image of the area indicated by the yellow square in the optical image. Average grain size: 200 nm (1), 150 nm (2). Film thickness: 100 ± 30 nm (1), 55 ± 15 nm (2). (c) Raman spectrum. (d) Micro-Raman image relative to the red sample region in the optical image.

Figure 7

¹³C solid-state nuclear magnetic resonance (NMR) spectra of the polydopamine (PDA) samples produced by periodate oxidation (green trace) and autoxidation (red trace). The spectrum of dopamine (black trace) is reported as a reference (marked signals are due to spinning side bands).

Figure 8

Ultraviolet–visible (UV–visible) spectra of polydopamine (PDA) coating (blue trace), and PDA coating immersed in a 0.1 mM dopamine solution at pH 9.0 for 3 (green trace) and 6 h (red trace).

Figure 9

Schematic illustration of a possible interplay of polydopamine (PDA) film deposition (yellow) and growth pathways (red).