Effects of pH and Oxidants on the First Steps of Polydopamine Formation: A Thermodynamic Approach

Abstract

We present a general thermodynamic top-down analysis of the effects of oxidants and pH on dopamine oxidation and cyclization, supplemented with UV–vis and electrochemical studies. The model is applicable to other catecholamines and various experimental conditions. The results show that the decisive physicochemical parameters in autoxidation are the pK values of the semiquinone and the amino group in the oxidized quinone. Addition of Ce(IV) or Fe(III) enhances dopamine oxidation in acidic media in aerobic and anaerobic conditions by the direct oxidation of dopamine and, in the presence of oxygen, also by the autoxidation of the formed semiquinone. At pH 4.5, the enhancement of the one-electron oxidation of dopamine explains the overall reaction enhancement, but at a lower pH, cyclization becomes rate-determining. Oxidation by Cu(II) at reasonable rates requires the presence of oxygen or chloride ions.

Oxidative Polymerization of Dopamine: A High-Definition Multifunctional Coatings for Electrospun Nanofibers - An Overview

“…Bernsmann et al have shown that the presence of stoichiometric excess of Cu 2+ ions in Tris buffer at pH 4.5 resulted in an increase in coating thickness of >70 nm which was difficult to achieve by conventional Tris-HCl route wherein the thickness did not increase beyond 45 ± 5 nm [17]. When compared to copper ions, the presence of other transition metal ions such as Fe 3+ and Ce 4+ has also been shown to accelerate the pDA coating under weakly acidic conditions [18]. Park et al reported the pDA coating under neutral pH by adding fourfold stoichiometric excess of vanadyl (VO 2+ ) ions to the dopamine solution.…”

Transferrin‐Coated Nanodiamond–Drug Conjugates for Milliwatt Photothermal Applications

“…Sodium periodate initiated polymerization of dopamine, and l ‐DOPA in pure MilliQ water was used to avoid buffer salts, which are known to destabilize colloidal NDs . In addition, this method provides faster deposition times than polymerization in alkaline buffers further reducing ND aggregation. In contrast to the well‐established dopamine surface polymerization, l ‐DOPA provides an additional carboxylic group and poly( l ‐DOPA) coatings exhibit higher hydrophilicity than polydopamine .…”

Tuning the antimicrobial activity of low molecular weight hydrogels using dopamine autoxidation

“…Several studies on the kinetic behaviour of dopamine autoxidation have been carried out. [30][31][32] The summary of these kinetic studies suggest that the autoxidation of dopamine is strongly pH-dependent with the rate of autoxidation faster at higher pH. 32 The increase in rate is due to the abundance of hydroxide ions which are required in the intramolecular Michael addition step.…”

“…Babbit, Lloyd and more recently Salomäki et al report the rate constant for the formation of aminochrome is first order. 30,31,33 At low pH, the Michael addition is the rate limiting step. 30 If we are able to control the rate of dopamine oxidation, we should be able to control the gelation kinetics of self-assembly.…”

“…We describe this linear relationship as an effect of the first order rate kinetics of dopamine oxidation. 30 As the pH decreases, the rate of dopamine oxidation is slower. The resulting gels are stiffer as the oxidation rate is slower (Fig.…”