Abstract
The facile preparation of conformal polydopamine (PDA) films on broad classes of materials has prompted extensive research into a wide variety of potential applications for PDA. The constituent molecular species in PDA exhibit diverse chemical moieties, and therefore highly variable properties of PDA-based devices may evolve with post-processing conditions. Here we report the use of redox-inactive cations for oxidative post-processing of deposited PDA films. PDA films incubated in alkaline CaCl2 solutions exhibit accelerated oxidative evolution in a dose-dependent manner. PDA films incubated in CaCl2 solutions exhibit 53% of the oxidative charge transfer compared to pristine PDA films. Carboxylic acid groups generated from the oxidation process lower the isoelectric point of PDA films from pH = 4.0 ± 0.2 to pH = 3.1 ± 0.3. PDA films exposed to CaCl2 solutions during post-processing also enhance Fe2+/Fe3+ chelation compared to pristine PDA films. These data illustrate that the molecular heterogeneity and non-equilibrium character of as-deposited PDA films afford control over the final composition by choosing post-processing conditions, but also demands forethought into how the performance of PDA-incorporated devices may change over time in salt solutions.
Highlights
The autoxidation of dopamine in alkaline aqueous solutions produces a dark insoluble material known as polydopamine (PDA)
Oxidative chemical synthesis of PDA films from dopamine is a complex multi-step process (Scheme 1) that includes redox processes, cyclization, polymerization, and various cleavage reactions that may serve as control points for altering the composition of PDA [15,22,28]
Reactive monomers produced along the dopamine oxidation pathway oligomerize (observed up to the octamer level in PDA [22,28] or 30-mer for solely 5,6-dihydroxyindole (DHI) polymerization [31]) and aggregate into disordered graphite-like nanostructures [32] akin to naturally occurring melanins [33,34]
Summary
The autoxidation of dopamine in alkaline aqueous solutions produces a dark insoluble material known as polydopamine (PDA). Catechol-bearing PDA exhibits robust surface adhesion [2,3], metal chelation [4], and electrooxidative control over redox states [5] These unique properties have prompted PDA’s exploration for numerous applications [6,7] in antifouling surfaces [8,9], biointerfaces [10,11], and high surface-area aqueous metal sorption devices [12,13,14]. The intrinsic oxidation mechanism of dopamine is sensitive to dissolved O2, dopamine, and hydronium concentration [15], buffer composition and substrate chemistry influence PDA film growth [16,17]. Post-synthesis treatments such as thermal annealing [11,24] or
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
