Abstract
SummaryMaterial‐independent adhesive action derived from polycatechol structures has been intensively studied due to its high applicability in surface engineering. Here, we for the first time demonstrate that a dihydroxynaphthalene‐based fungal melanin mimetic, which exhibit a catechol‐free structure, can act as a coating agent for material‐independent surface modifications on the nanoscale. This mimetic was made by using laccase to catalyse the oxidative polymerization of specifically 2,7‐dihydroxynaphthalene. Analyses of the product of this reaction, using Fourier transform infrared‐attenuated total reflectance and X‐ray photoelectron spectroscopy, bactericidal action, charge‐dependent sorption behaviour, phenol content, Zeta potential measurements and free radical scavenging activity, yielded results consistent with it containing hydroxyphenyl groups. Moreover, nuclear magnetic resonance analyses of the product revealed that C‐O coupling and C‐C coupling were the main mechanisms for its synthesis, thus clearly excluding a catechol structure in the polymerization. This product, termed poly(2,7‐DHN), was successfully deposited onto a wide variety of solid surfaces, including metals, polymeric materials, ceramics, biosurfaces and mineral complexes. The melanin‐like polymerization could be used to co‐immobilize other organic molecules, forming functional surfaces. In addition, the hydroxyphenyl group contained in the coated poly(2,7‐DHN) induced secondary metal chelation/reduction and adhesion with proteins, suggesting the potential of this poly(2,7‐DHN) layer to serve as a platform material for a variety of surface engineering applications. Moreover, the novel physicochemical properties of the poly(2,7‐DHN) illuminate its potential applications as bactericidal, radical‐scavenging and pollutant‐sorbing agents.
Highlights
For a material to achieve effective adhesion and coating on a solid surface, it requires functional moieties that can strongly bind to the substrate
The anabolic action of laccase on 1,8-DHN in vivo is known to lead to fungal melanogenesis, as shown in Scheme 1
Reacting laccase with 2,7-DHN led to efficient oxidative polymerization and gave rise to polyaromatics containing multiple hydroxyphenyl groups
Summary
For a material to achieve effective adhesion and coating on a solid surface, it requires functional moieties that can strongly bind to the substrate These groups should be capable of forming diverse bonds with several different kinds of surfaces and, once the initial coating has been assembled, the binding should be durable against external stresses such as exposure to water and contamination with various substances. Chang diverse materials (McDonald et al, 1996; Jeon et al, 2009, 2010, 2013a; Sileika et al, 2013) This finding strongly supports the idea that the previously reported catechol-based adhesives, including those whose adhesive action derives from polydopamine coating and catechol conjugation, could be reproduced by materials displaying polyphenolic moieties. We carried out immobilization experiments with a coated layer of poly(2,7-DHN) and demonstrated that the polymeric layer could act as a platform for a variety of surface engineering purposes
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