Surface coatings are widely used for the protection of underlying materials from erosion or contamination by the external environment, with biomimetic organic coatings based on catecholamine chemistry gaining prominence in recent years. Such coatings have found use in the biomedical field, e.g., in diagnostics, implant manufacturing, and biosensing, with coatings based on polydopamine (PDA) being the most popular. This Review aims to summarize the chemistry of catechol(amine) coatings, in particular the adhesion and cohesion properties of catecholamine-based coatings. This will allow for the design and synthesis of new polymers and coating materials in a more rational manner, enabling the selection of parameters and conditions to precisely control the structure of the materials formed. Particular attention is paid to the formation mechanism, structure, and variables affecting the properties of PDA, which is the most widely reported catechol(amine) coating. The use of other catechol(amine) precursors to synthesize biomimetic coatings is also discussed. A summary of the different methods reported in the literature to effect specific chemical properties on catechol(amine) coatings will allow the reader to best choose the technique to tailor coating properties for specific applications.

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Mass spectroscopic studies using deuterium-labeling reveal a novel structure for polydopamine, deriving from dopaminochrome and uncyclized dopamine.

The definitive role of the intermediate 5,6dihydroxyindole (DHI) in the formation of polydopamine (PDA) coatings from aqueous dopamine (DA) has not been clearly elucidated and remains highly controversial. Our foray into this debate as reported in this study agrees with some reported assertions that DHI-based coatings are not synonymous with PDA coatings. Our conclusion arises from a systematic comparison of the components and properties of DHI-based coatings and PDA coatings. In addition, through careful copolymerization studies of DA and DHI, our studies reported herein unequivocally suggest that both DA and DHI are partial building blocks for PDA formation. Our results also provide additional evidence of the critical role of DHI in controlling the thickness of PDA coatings, through competitive events between PDA aggregation in solutions and deposition onto substrates. These findings highlight the complex interplay between both DHI and uncyclized DA moieties in the formation of adhesive catechol/amine materials.

The underwater in situ nano-deposition studies of 5,6-dihydroxyindole (DHI) have provided new insights into the controversial deposition mechanism(s) of DHI-based and polydopamine-based coatings.