Porous Photo-Fenton Catalysts Rapidly Triggered by Levodopa-Based Mussel-Inspired Coatings for Enhanced Dye Degradation and Sterilization.

The advanced oxidation process of the photo-Fenton reaction can produce hydroxyl radicals with extremely strong oxidizing properties for the efficient and green degradation of various chemical and microbial pollutants. Herein, we report an approach to fabricating heterogeneous Fenton catalysts of ß-FeOOH nanorods on porous substrates triggered by mussel-inspired coatings of levodopa (3,4-dihydroxy-phenyl-l-alanine, l-DOPA) and polyethylenimine (PEI) for efficient photocatalytic dyes' degradation and sterilization. The l-DOPA-based coatings not only promote the formation and immobilization of ß-FeOOH nanorods on the porous substrates by strong coordination between catechol/carboxyl groups and Fe3+ but also improve the energy band structure of the Fenton catalysts through a valence band blue shift and band gap narrowing. The photo-Fenton catalysts prepared by the l-DOPA-based coatings exhibit high electron transport efficiency and improved utilization of sunlight. Only 2 h of mineralization is needed to fabricate these catalysts with excellent photocatalytic efficiency, in which the degradation efficiency of methylene blue can reach 99% within 30 min, whereas the sterilization efficiency of E. coli/S. aureus can reach 93%/94% within 20 min of the photo-Fenton reaction. Additionally, the prepared catalysts reveal a high photodegradation performance for various dyes including methylene blue, methyl blue, methyl orange, direct yellow, and rhodamine B. Furthermore, the catalysts retain high dye degradation efficiencies of above 90% after five photodegradation cycles, indicating cycling performance and good stability.

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 5,6-dihydroxyindole, 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.

Co-deposition Kinetics of Polydopamine/Polyethyleneimine Coatings: Effects of Solution Composition and Substrate Surface.

Polydopamine-based chemistry has been employed for various surface modifications attributed to the advantages of universality, versatility, and simplicity. Co-deposition of polydopamine (PDA) with polyethyleneimine (PEI) has then been proposed to realize one-step fabrication of functional coatings with improved morphology uniformity, surface hydrophilicity, and chemical stability. Herein, we report the co-deposition kinetics related to the solution composition with different dopamine/PEI ratios, PEI molecular weights, dopamine/PEI concentrations, and the substrate surface with varying chemistry and wettability. The addition of PEI to dopamine solution suppresses the precipitation of PDA aggregates, resulting in an expanded time window of steady co-deposition compared with that of PDA deposition. Low-molecular-weight PEI at low concentration accelerates the co-deposition process, while high-molecular-weight PEI and high concentration of either PEI or dopamine/PEI are detrimental to the co-deposition efficiency. Meanwhile, the surface morphology and chemical composition of the co-deposition coatings can be regulated by the solution conditions during co-deposition. Moreover, obvious deviations in the co-deposition rate and the amount of substrates bearing various functional groups, such as alkyl, phenyl, hydroxyl, and carboxyl, are revealed, which are quite different from PDA deposition. The initial adsorption rates further reflect the change in interactions between the aggregates and these substrates caused by PEI, which follows the sequence of carboxyl > hydroxyl > alkyl > phenyl. These results provide deep insights into the PDA/PEI co-deposition process on various substrates.