How thick, uniform and smooth are the polydopamine coating layers obtained under different oxidation conditions? An in-depth AFM study

To clarify inconsistencies that still exist in the literature data, a systematic AFM study of polydopamine coating layers is performed for three different oxidants: air-O2, NaIO4, and KMnO4. The focus was on assessing reproducibility and uniformity in the measured morphological parameters by analyzing a statistically relevant set of AFM data. Mechanical stability was investigated by comparing only washed with washed, followed by 30 min sonicated samples. The PDA coating always contains two components: a quasi-flat film, with intrinsic roughness in the order of 1 nm, and many “sedimented” PDA aggregates lying on the top of this film. The larger aggregates were found loosely bound to the film, they could be easily detached by sonication, leaving surfaces with more uniform coverage and reduced roughness, up to one order of magnitude less in some cases. The largest thickness was found under NaIO4 oxidation, followed by KMnO4 and O2, which contradicts recent reports of thicknesses approaching 1 µm for KMnO4 oxidation, the value determined here being almost 30 times less. The causes for this large difference and other inconsistencies were identified and discussed. Overall, the present study demonstrates the importance of establishing a standardized protocol for morphological characterization of PDA coatings.