No abstract

Polydopamine (PDA) formed by the oxidation of dopamine is an important polymer, in particular, for coating various surfaces. It is composed of dihydroxyindole, indoledione, and dopamine units, which are assumed to be covalently linked. Although PDA has been applied in a manifold way, its structure is still under discussion. Similarities have been observed in melanins/eumelanins as naturally occurring, deeply colored polymer pigments derived from L-DOPA. Recently, an alternative structure was proposed for PDA wherein dihydroxyindoline, indolinedione, and eventually dopamine units are not covalently linked to each other but are held together by hydrogen bonding between oxygen atoms or π stacking. In this study, we show that this structural proposal is very unlikely to occur taking into account unambiguous results obtained by different analytical methods, among them (13)C CPPI MAS NMR (cross-polarization polarization-inversion magic angle spinning NMR), (1)H MAS NMR (magic angle spinning NMR), and ES-HRMS (electrospray ionization high-resolution mass spectrometry) for the first time in addition to XPS (X-ray photoelectron spectroscopy) and FTIR spectroscopy. The results give rise to a verified structural assignment of PDA wherein dihydroxyindole and indoledione units with different degrees of (un)saturation are covalently linked by C-C bonds between their benzene rings. Furthermore, proof of open-chain (dopamine) monomer units in PDA is provided. Advanced DFT calculations imply the arrangements of several PDA chains preferably by quinone-hydroquinone-type interactions in a parallel or antiparallel manner. From all of these results, a number of hypotheses published before could be experimentally supported or were found to be contradictory, thus leading to a better understanding of the PDA structure.

Polydopamine (PDA) is a polymer easily obtained by oxidation of dopamine. It is composed of indole and dopamine units in various oxidation states and to a lesser extent of pyrroles. It adheres to all type of surfaces even under water due to its abundant catechol moieties assisted by amino groups. This property together with a widespread reactivity to nucleophiles and electrophiles allowing linkage of a variety of entities renders PDA extremely interesting for various applications in biology, biomedicine, membranes, catalysis, materials and water purification. The field of PDA is violently developing. The present review gives an overview about the chemistry and properties of PDA and its analogues with the focus on recent publications. Their widespread applications are occasionally touched. Analogues are obtained by two strategies: post‐modification of PDA and oxidative polymerization of dopamine analogues. Scope and limitations of these strategies are worked out giving impulses for future research in the field.

Polydopamine analogues (PDANAs) extend the prospering field of polydopamine (PDA) considerably. They include additional functional groups providing altered properties interesting for various applications. PDANAs can be obtained by post functionalization of PDA, by oxidative polymerization of dopamine derivatives or by copolymerization of dopamine with dopamine derivatives, other monomers or reactive polymers. This review covers the state of the art and gives insight into the huge potential of the field to be explored in the future. © 2016 Society of Chemical Industry