Direct Evidence for the Polymeric Nature of Polydopamine

Abstract

Inspired by the adhesive proteins of mussels, polydopamine (pDA) has emerged as one of the most widely employed materials for surface functionalization. Despite numerous attempts at characterization, little consensus has emerged regarding whether pDA is a covalent polymer or a noncovalent aggregate of low molecular weight species. Here, we employed single-molecule force spectroscopy (SMFS) to characterize pDA films. Retraction of a pDA-coated cantilever from an oxide surface shows the characteristic features of a polymer with contour lengths of up to 200 nm. pDA polymers are generally weakly bound to the surface through much of their contour length, with occasional "sticky" points. Our findings represent the first direct evidence for the polymeric nature of pDA and provide a foundation upon which to better understand and tailor its physicochemical properties.

Keywords: coatings; polydopamine; polymers; single-molecule force spectroscopy; surface modification.

Figure 1.. Peeling of polymer chains in SMFS.

a, Schematic of the SMFS experiments showing a pDA coated cantilever approaching onto a bare substrate and deflecting when in contact with the surface. b, SEM image of a pDA coated cantilever. c, F-D traces showing plateaus of constant force. d, Schematic showing peeling off polymer chains from the surface upon retraction of the cantilever. e, Distribution of peeling force obtained by measuring the height of plateaus. f, Distribution of plateau length and the corresponding apparent molecular weight values.

Figure 2.. Stretching of polymer chains along sticky locations.

a, Representative F-D traces showing stretching events in the middle of plateaus. b, Histogram showing the unbinding force of stretching events (N = 306). Inset shows schematic visualization of a pDA polymer chain that is adsorbed to a surface via mostly weak interactions along its contour length with occasional strong interactions with the surface (indicated in blue). Given the heterogeneous nature of pDA, the origins of the strong interactions at sticky points are unknown, but could represent catechol, quinone or other functional groups known to be present in pDA.

Figure 3.. Intermolecular interactions between pDA molecules.

a, Schematic of experiments showing a pDA coated cantilever approaching a pDA coated substrate. b, The F-D traces shown represent eight successive traces obtained during approach of a pDA coated cantilever onto a pDA coated substrate.

Figure 4.. in-situ time dependent SMFS during pDA formation.

a, Schematic of the in-situ experiments whereby the AFM liquid cell was filled with freshly prepared monomer solution (2 mg/ml dopamine.HCl in 100 mM bicine buffer at pH 8.5) and force spectroscopy was performed during the first two hours of pDA formation b, Representative F-D traces collected during in-situ polymerization of pDA; after two hours, particles formed in the solution and possibly also pDA deposition on the cantilever mirror surfaces interfered with the reflection of the laser light, making further measurements impossible.