Melanin-Inspired Composite Materials: From Nanoarchitectonics to Applications

Abstract

Synthetic melanin is a mimic of natural melanin analogue with intriguing properties such as metal−ion chelation, redox activity, adhesion, and broadband absorption. Melanin-inspired composite materials are formulated by assembly of melanin with other types of inorganic and organic components to target, combine, and build up the functionality, far beyond their natural capabilities. Developing efficient and universal methodologies to prepare melaninbased composite materials with unique functionality is vital for their further applications. In this review, we summarize three types of synthetic approaches, predoping, surface engineering, and physical blending, to access various melanin-inspired composite materials with distinctive structure and properties. The applications of melanininspired composite materials in free radical scavenging, bioimaging, antifouling, and catalytic applications are also reviewed. This review also concludes current challenges that must be addressed and research opportunities in future studies.

Precision Engineering of Polydopamine: Advances in Spatial Fabrication Techniques

“…This process enables the secondary immobilization of desired molecules onto pDA. In addition, the catalytic properties, redox reactivities, and photothermal conversion performance of pDA have emerged, which can be further tuned by incorporating various organic/inorganic additives. − As many of the unexplored secrets in the chemical characteristics of pDA become understood, a broader range of applications will likely emerge.…”

Melanin Upcycling: Creation of Polymeric Materials from Melanin Decomposition Products

“…Recent studies have also reported melanin production by bioprocesses using melanin-producing microorganisms such as fungi and bacteria − and efficient methods for extracting melanin from animal hair. − Thus, renewable melanin obtained from diverse biological resources can be regarded as a sustainable biomass resource. Traditionally, cuttlefish and squid inks have been chiefly discarded except for use as food products and ink for packaging containers. , In recent years, however, inspired by the excellent photofunctional properties of melanin, there has been a growing trend to create functional materials such as photothermal therapy, UV shielding, dye-sensitized solar cells, energy storage, optical, and organic bioelectronic materials by using natural melanin as is or in combination with other materials. ,− On the other hand, to our knowledge, an attempt has yet to be made to decompose melanin into a useful resource. This is probably due to the complex structure of melanin, a three-dimensional cross-linked polymer composed of indole compounds such as 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) (Figure a). , While lignin, a cross-linked polyphenol, also has a heterogeneous and complex structure, advances in its decomposition methods have led to the isolation and utilization of various aromatic compounds from lignin. − Preparation of biomass decomposition products with well-defined structures is an option as long as the functional groups in the compound are known.…”

Metal-Loaded Synthetic Melanin via Oxidative Polymerization of Neurotransmitter Norepinephrine Exhibiting High Photothermal Conversion

“…Melanin-like materials show high affinity for metal ions owing to the presence of abundant catechol groups, providing coordination sites for metal ions. − The introduction of metal ions serves to promote the formation of highly reactive supramolecular monomers, thereby accelerating the polymerization of catechol monomers and stabilize the molecular structure. , The unpaired electrons of metal ions (Mn 3+ , Fe 3+ , Co 2+ ) within the melanin structure play a pivotal role in biomedical fields such as magnetic resonance imaging. − Moreover, the unoccupied orbitals of metal ions can induce d–d transitions and ligand-to-metal charge transfer (LMCT) between catechol and metal ions to absorb photon energy, which enhances photothermal conversion. − For example, Yin et al prepared Fe 3+ -loaded PDA NPs through a predoping method with higher NIR photothermal conversion by LMCT than PDA for photothermal/immunotherapy . While metal ion chelation is commonly employed in diverse catechol materials, the preparation of metal-ion-loaded melanin-like poly­(norepinephrine) nanomaterials has not been reported yet.…”