Structure of polydopamine: a never-ending story?

Area of Science:

• Polymer Chemistry
• Materials Science
• Surface Science

Background:

• Polydopamine (PDA) is a versatile polymer formed by dopamine oxidation, widely used for surface coatings.
• Its precise molecular structure, particularly the linkage between units, remains debated.
• Previous hypotheses suggested non-covalent interactions, contrasting with assumed covalent bonding.

Purpose of the Study:

• To definitively elucidate the molecular structure of polydopamine (PDA).
• To experimentally verify or refute proposed non-covalent structural models of PDA.
• To provide a verified structural assignment for PDA to resolve existing scientific discrepancies.

Main Methods:

• Utilized advanced analytical techniques including 13C CPPI MAS NMR, 1H MAS NMR, and ES-HRMS.
• Incorporated complementary methods such as XPS and FTIR spectroscopy.
• Performed advanced Density Functional Theory (DFT) calculations to model chain arrangements.

Main Results:

• Provided unambiguous evidence against a structure based solely on hydrogen bonding.
• Verified a PDA structure with covalently linked dihydroxyindole and indoledione units via C-C bonds.
• Confirmed the presence of open-chain dopamine monomer units within the PDA structure.
• DFT calculations suggest PDA chains associate via quinone-hydroquinone interactions.

Conclusions:

• The study experimentally refutes the proposed hydrogen-bonded structure of PDA.
• A verified PDA structure consists of covalently linked aromatic units and open-chain monomers.
• This research clarifies PDA's molecular architecture, resolving prior hypotheses and enhancing its application potential.