INVESTIGATING POLYDOPAMINE VIA SOLUTION CHEMISTRY AND SURFACE ADHESION
Dopamine (DA) has been identified as the fundamental building block for spontaneous formation of polydopamine (PDA) films, which possess excellent adhesiveness on a variety of substrates. Under favorable conditions, DA undergoes self-polymerization, forming a thin PDA film on a substrate of interest. However, the reason behind the universal coating ability of PDA has not been completely understood. To have a better understanding of the mechanism of PDA polymerization as well as the interactions between PDA films and underlying substrates, PDA solution chemistry, and surface adhesion were investigated. For PDA solution chemistry, the sizes and shapes of PDA particles in solution were captured as a function of time. Open access to atmospheric oxygen is not necessary for initiating the oxidation of DA monomers and hence the polymerization of PDA, but essential in generating more homogenous PDA particles in solution. For PDA surface adhesion, DA deposition was carried out on four substrates with different wettability and functional groups: silicon wafer (Si), polydimethylsiloxane (DMS), amine-functionalized PDMS (AMS), and thiol-functionalized PDMS (SMS). It was established that PDA films with controlled thickness, homogeneity, and smoothness were obtained without prior surface equilibrium with buffer solution and in an unstirred DA solution with open access to atmospheric oxygen. Surface energetics plays a critical role in shaping the thickness, surface morphology, wettability, and stability of PDA films as well as the interactions between PDA films and substrates. The deposition of PDA films is more favorable on the hydrophobic substrates compared to the hydrophilic substrate. Wettability of the substrates is an essential factor contributing to the thickness of PDA films but does not influence the wettability of PDA films. The initial contact between DA monomers and Si substrate is critical in the formation of PDA films on Si, as Si substrate fails to form hydrogen bondings with PDA aggregates. Moreover, while on Si, PDA films are likely to form by direct nucleation, on polymer substrates, PDA films seem to experience three-dimensional island growth. Finally, PDA films on Si are less stable than those on polymer substrates after acid and base treatment, likely due to the lack of functional groups as well as the instability of the Si-O bond of the Si substrate in base.