Kinetics of polydopamine film deposition as a function of pH and dopamine concentration: insights in the polydopamine deposition mechanism

The formation of "polydopamine" thin films becomes a popular method to confer multifunctionality to solid-liquid interfaces through the available catechol groups of such films. The mechanism of film formation is, however, not well elucidated, and most investigators use the protocol developed by Messersmith et al. (H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426.) using a dopamine solution at a constant concentration of 2 g L(-1) in the presence of Tris(hydroxymethyl aminomethane) at pH 8.5. A particular finding of this initial study was that the film thickness reaches a constant value (almost substrate independent) of about 40 nm. Herein, we investigate the change in the polydopamine film thickness, morphology, surface energy and electrochemical properties as a function of the concentration of the dopamine solution put in the presence of silicon substrates. As a surprising finding, we observe a constant increase in the maximal film thickness with an increase in the dopamine solution between 0.1 and 5 g L(-1). The surface morphology is also markedly affected by the concentration of the dopamine solution, whereas the different components of the surface energy stay unaffected by the dopamine solution concentration. In addition, electrochemical impedance spectroscopy shows that the higher the initial dopamine concentration, the more rapidly compact and impermeable films are formed. Finally, we propose a model for the deposition of polydopamine films taking all our findings into account. This model relies on a rate equation taking into account both attractive and repulsive interactions between small polydopamine aggregates on the surface and in solution.