Codeposition of Levodopa and Polyethyleneimine: Reaction Mechanism and Coating Construction

Mussel-inspired poly(catecholamine) coatings from polydopamine (PDA) have been widely studied to design functional coatings for various materials. The chemical precursor of dopamine (DA), levodopa (L-DOPA, 3,4-dihydroxyphenyl-L-alanine), is known as the main element of mussel adhesive foot protein, but it is relatively hard to be constructed into a desirable coating on a given material surface under the same conditions as those for DA. Herein, we report a codeposition strategy to achieve the rapid fabrication of mussel-inspired coatings by L-DOPAwith polyethyleneimine (PEI) and to deeply understand the formation mechanism of those aggregates and coatings from L-DOPA/PEI. DFT calculations, fluorescence spectra, nuclear magnetic resonance analysis, and liquid chromatography-tandem mass spectrometry identification demonstrate that the formation of L-DOPA/PEI aggregates is effectively accelerated by PEI crosslinking with those intermediates of oxidized L-DOPA, including L-DOPAquinone and 5,6-dihydroxyindole-2-carboxylic acid as well as 5,6dihydroxyindole, through Michael-addition and Schiff-base reactions. Therefore, we can facilely control the growth rate and the particle size of the L-DOPA/PEI aggregates in the deposition solution by adjusting the concentration of PEI. The coating formation rate of L-DOPA/PEI is four times faster than that of PDA and DA/PEI within 12 h. These L-DOPA/PEI coatings are demonstrated to display potential as structure colors, superhydrophilic surfaces, and antibacterial materials.