Polydopamine (PDA) is a major type of artificial melanin material with many interesting properties such as antioxidant activity, free-radical scavenging, high photothermal conversion efficiency, and strong metal-ion chelation. The high affinity of PDA to a wide range of metals/metal ions has offered a new class of functional metal-containing polydopamine (MPDA) nanomaterials with promising functions and extensive applications. Understanding and controlling the metal coordination environment is vital to achieve desirable functions for which such materials can be exploited. MPDA nanomaterials with metal/metal ions as the active functions are reviewed, including their synthesis and metal coordination environment and their applications in catalysis, batteries, solar cells, capacitors, medical imaging, cancer therapy, antifouling, and antibacterial coating. The current trends, limitations, and future directions of this area are also explored.

Metal-Containing Polydopamine Nanomaterials: Catalysis, Energy, and Theranostics

Stimuli responsiveness has long been a fascinating feature of smart material design. Polydopamine (PDA), a nature inspired polymeric pigment, exhibits excellent photo-responsive properties and has active surface functionality for loading various responsive motifs, making it a promising candidate for the construction of stimuli-responsive smart functional materials. PDA has long been considered as a robust coating material, but its responsive feature has rarely been emphasized in the past reviews. Herein, we present the first effort to summarize recent advances in the design strategies, responsive mechanisms, and diverse applications of stimuli-responsive PDA-based smart materials; the stimuli include light, pH, chemicals, temperature, humidity, electric fields, mechanical force, magnetic fields, and ultrasound. Moreover, the current trends, challenges, and future directions of stimuli-responsive PDA-based materials are also elaborated.

Stimuli-responsive polydopamine-based smart materials

Recently, the development of polydopamine (PDA) has demonstrated numerous excellent performances in free radical scavenging, UV shielding, photothermal conversion, and biocompatibility. These unique properties enable PDA to be widely used as efficient antibacterial materials for various applications. Accordingly, PDA antibacterial materials mainly include free-standing PDA materials and PDA-based composite materials. In this review, an overview of PDA antibacterial materials is provided to summarize these two types of antibacterial materials in detail, including the fabrication strategies and antibacterial mechanisms. The future development and challenges of PDA in this field are also presented. It is hoped that this review will provide an insight into the future development of antibacterial functional materials based on PDA.

Polydopamine antibacterial materials

Polyphenols are a class of ubiquitous compounds distributed in nature, with fascinating inherent biocompatible, bioadhesive, antioxidant, and antibacterial properties. The unique polyphenolic structures based on catechol or pyrogallol moieties allow for strong non-covalent interactions (e.g., multiple hydrogen bonding, electrostatic, and cation–π interactions) as well as covalent interactions (e.g., Michael addition/Schiff-base reaction, radical coupling reaction, and dynamic coordination interactions with boronate or metal ions). This review article provides an overview of the polyphenol-based scaffolds including the hydrogels, films, and nanofibers that have emerged from chemical and functional signatures during the past years. A full description of the structure–function relationships in terms of their utilization in wound healing, bone regeneration, and electroactive tissue engineering is also carefully discussed, which may pave the path towards the rational design and facile preparation of next-generation polyphenol scaffolds for tissue engineering applications.

Polyphenol scaffolds in tissue engineering

Polydopamine (PDA) fluorescent materials have recently gained much attention due to their unique physicochemical and biological properties. Since the first report on fluorescent PDA nanoparticles, many efforts have been made to develop new fabrication strategies to achieve their desirable luminescence properties. This feature article summarizes the recent advances and current highlights in fluorescent PDA materials including fabrication methods, nanomaterial morphologies, luminescence mechanisms and applications for imaging and sensing in the last few years. We believe that this review will provide new insights into the mechanistic aspect of PDA fluorescence for guiding preparation approaches and new applications.

Recent developments in polydopamine fluorescent nanomaterials

Flexible Polydopamine Bioelectronics

The therapeutic potential of nanomaterials toward oxidative damage relevant diseases has attracted great attentions by offering promising advantages compared with conventional antioxidants. Although different kinds of nanoantioxidants have been well developed, the facile fabrication of robust and efficient nanoscavengers is still met with challenges like the use of toxic and high-cost subunits, the involvement of multistep synthetic process, and redundant purification work. Herein, a direct fabrication strategy toward polyphenol nanoparticles with tunable size, excellent biocompatibility, and reactive oxygen species (ROS) scavenging capacities from grape seed via an enzymatic polymerization method is reported. The resulting nanoparticles can efficiently prevent cell damage from ROS and exert promising in vivo antioxidant therapeutic effects on several oxidative stress-related diseases, including accelerating wound healing, inhibiting ulcerative colitis, and regulating the oxidative stress in dry eye disease. This study can stimulate the development of more kinds of low-cost, safe, and efficient biomass-based antioxidative nanomaterials via similar fabrication methodologies.

Therapeutic Nanoparticles from Grape Seed for Modulating Oxidative Stress

Bioinspired fluorescent dihydroxyindoles oligomers

Synthetic melanin facilitates MnO supercapacitors with high specific capacitance and wide operation potential window

The usage of exogenous antioxidant materials to relieve oxidative stress offers an important strategy for the therapy of oxidative stress-induced injuries. However, the fabrication processes toward the antioxidant materials usually require the involvement of extra metal ions and organic agents, as well as sophisticated purification steps, which might cause tremendous environmental stress and induce unpredictable side effects in vivo. To address these issues, herein, we proposed a novel strategy to fabricate green nanoparticles for efficiently modulating oxidative stress, which was facilely prepared from tea polyphenol extracts (originated from green tea) via a green enzymatic polymerization-based chemistry method. The resulting nanoparticles possessed a uniform spherical morphology and good stability in water and biomedium and demonstrated excellent radical scavenging properties. These nanoparticle scavengers could effectively prevent intracellular oxidative damage, accelerate wound recovery, and protect the kidneys from reactive oxygen species damaging in the acute kidney injury model. We hope this work will inspire the further development of more types of green nanoparticles for antioxidant therapies via similar synthetic strategies using green biomass materials.

Tianyou Wang

Papers

Flexible Polydopamine Bioelectronics

Therapeutic Nanoparticles from Grape Seed for Modulating Oxidative Stress

Bioinspired fluorescent dihydroxyindoles oligomers

Synthetic melanin facilitates MnO supercapacitors with high specific capacitance and wide operation potential window

Green Nanoparticle Scavengers against Oxidative Stress.