Polydopamine and eumelanin: from structure-property relationships to a unified tailoring strategy

Polydopamine Induced in-Situ Formation of Metallic Nanoparticles in Confined Microchannels of Porous Membrane as Flexible Catalytic Reactor

Oxidant-regulated polymerization of dopamine was exploited, for the first time, for effective surface engineering of the well-defined cylindrical pores of nuclear track-etched membranes (NTEMs) to develop novel catalytic membrane reactor. First, in the presence of a strong oxidant, controlled synthesis of polydopamine (PDA) with tunable particle size was achieved, allowing a homogeneous deposition to the confined pore channels of NTEMs. The PDA interfaces rich in catechol and amine groups provided enhanced hydrophilicity to promote mass transport across the membrane and abundant nucleation sites for formation and stabilization of metallic nanoparticles (NPs). In-situ reductive growth of multiple metallic NPs, including Pd, Ag, and Au, was then achieved inside the cylindrical pores of NTEMs. Using the functionalized membrane as a catalytic reactor, efficient reduction of 4-nitrophenol (4-NP) was demonstrated in a flow-through mode. Moreover, after dissolution removal of the NTEMs, self-sustained one-dimensional (1D) PDA/M (M = Pd, Ag, or Au) hybrid nanotubes (NTs), with determined aspect ratio and a length reaching up to 10 μm, were obtained for catalysis of 4-NP in a batch reaction mode. This study established a facile and versatile method, by rational tuning of the polymerization behavior of dopamine, for effective modification of confined microscale/nanoscale cavities with different surface characteristics. The integration of PDA chemistry with NTEMs would provide more opportunities for development of novel catalytic membrane reactors as well as for the tailored synthesis of functional 1D nanotubes for broadened applications.

Cytotoxicity of paramagnetic cations-Loaded polydopamine nanoparticles

Polydopamine (PD) is a synthetic melanin pigment of great importance in biomedicine, where its affinity for metallic cations, especially paramagnetic ions, has sparked interest in its use in the development of magnetic resonance imaging (MRI) contrast agents. In this work, we report the cytotoxicity of metal-enriched PD nanoparticles on NIH3T3, a healthy cell line and BT474, a breast cancer cell line. Remarkably, it was found that the metal- enriched PD particles (Mⁿ⁺ = Fe³⁺, Fe²⁺ and Cu²⁺) were highly cytotoxic to the breast cancer cells, even after 24 h of treatment. Although, this effect was not selective systems, since an acute cytotoxic effect was also observed on the healthy cell line, this system can be considered as starting point for designing advanced antineoplastic agents.

Structural Requirements of Alkylglyceryl-l-Ascorbic Acid Derivatives for Melanogenesis Inhibitory Activity

l-Ascorbic acid has multifunctional benefits on skin aesthetics, including inhibition of melanin production, and is widely used in cosmetics. It, however, has low stability and poor skin penetration. We hypothesize that alkylglyceryl-l-ascorbic acid derivatives, highly stable vitamin C–alkylglycerol conjugates, would have similar anti-melanogenic activity with better stability and penetration. We test 28 alkylglyceryl-l-ascorbic acid derivatives (1–28) on theophylline-stimulated B16 melanoma 4A5 cells to determine if they inhibit melanogenesis and establish any structure–function relationships. Although not the most potent inhibitors, 3-O-(2,3-dihydroxypropyl)-2-O-hexyl-l-ascorbic acid (6, IC₅₀ = 81.4 µM) and 2-O-(2,3-dihydroxypropyl)-3-O-hexyl-l-ascorbic acid (20, IC₅₀ = 117 µM) are deemed the best candidate derivatives based on their inhibitory activities and low toxicities. These derivatives are also found to be more stable than l-ascorbic acid and to have favorable characteristics for skin penetration. The following structural requirements for inhibitory activity of alkylglyceryl-l-ascorbic acid derivatives are also determined: (i) alkylation of glyceryl-l-ascorbic acid is essential for inhibitory activity; (ii) the 3-O-alkyl-derivatives (2–14) exhibit stronger inhibitory activity than the corresponding 2-O-alkyl-derivatives (16–28); and (iii) derivatives with longer alkyl chains have stronger inhibitory activities. Mechanistically, our studies suggest that l-ascorbic acid derivatives exert their effects by suppressing the mRNA expression of tyrosinase and tyrosine-related protein-1.

Recent progress in the biomedical applications of polydopamine nanostructures

Polydopamine is a dark brown-black insoluble biopolymer produced by autoxidation of dopamine. Although its structure and polymerization mechanism have not been fully understood, there has been a rapid growth in the synthesis and applications of polydopamine nanostructures in biomedical fields such as drug delivery, photothermal therapy, bone and tissue engineering, and cell adhesion and patterning, as well as antimicrobial applications. This article is dedicated to reviewing some of the recent polydopamine developments in these biomedical fields. Firstly, the polymerization mechanism is introduced with a discussion of the factors that influence the polymerization process. The discussion is followed by the introduction of various forms of polydopamine nanostructures and their recent applications in biomedical fields, especially in drug delivery. Finally, the review is summarized followed by brief comments on the future prospects of polydopamine.

Cyclodextrin-Based Magnetic Nanoparticles for Cancer Therapy

Polydopamine (PDA)-coated magnetic nanoparticles functionalized with mono-6-thio-β-cyclodextrin (SH-βCD) were obtained and characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Nuclear and Magnetic Resonance Imaging (NMR and MRI), and doxorubicin (DOXO)-loading experiments. The liver cancer cellular internalization of DOXO-loaded nanoparticles was investigated by confocal imaging microscopy. Synthesized nanomaterials bearing a chemotherapeutic drug and a layer of polydopamine capable of absorbing near-infrared light show high performance in the combined chemo- and photothermal therapy (CT-PTT) of liver cancer due to the synergistic effect of both modalities as demonstrated in vitro. Moreover, our material exhibits improved T₂ contrast properties, which have been verified using Carr-Purcell-Meiboom-Gill pulse sequence and MRI Spin-Echo imaging of the nanoparticles dispersed in the agarose gel phantoms. Therefore, the presented results cast new light on the preparation of polydopamine-based magnetic theranostic nanomaterials, as well as on the proper methodology for investigation of magnetic nanoparticles in high field MRI experiments. The prepared material is a robust theranostic nanoasystem with great potential in nanomedicine.

Polydopamine Surface Chemistry: A Decade of Discovery

Polydopamine is one of the simplest and most versatile approaches to functionalizing material surfaces, having been inspired by the adhesive nature of catechols and amines in mussel adhesive proteins. Since its first report in 2007, a decade of studies on polydopamine molecular structure, deposition conditions, and physicochemical properties have ensued. During this time, potential uses of polydopamine coatings have expanded in many unforeseen directions, seemingly only limited by the creativity of researchers seeking simple solutions to manipulating surface chemistry. In this review, we describe the current state of the art in polydopamine coating methods, describe efforts underway to uncover and tailor the complex structure and chemical properties of polydopamine, and identify emerging trends and needs in polydopamine research, including the use of dopamine analogs, nitrogen-free polyphenolic precursors, and improvement of coating mechanical properties.

The photoreactive free radical in eumelanin

Melanin is the primary photoprotecting pigment in humans as well as being implicated in the development of deadly melanoma. The material also conducts electricity and has thus become a bioelectronic model for proton-to-electron transduction. Central to these phenomena are its spin properties-notably two linked species derived from carbon-centered and semiquinone radicals. Using a novel in situ photoinduced electron paramagnetic resonance technique with simultaneous electrical measurements, we have elucidated for the first time the distinct photoreactivity of the two different radical species. We find that the production of the semiquinone is light- and water-driven, explaining the electrical properties and revealing biologically relevant photoreactivity.

Well-defined functional mesoporous silica/polymer hybrids prepared by an ICAR ATRP technique integrated with bio-inspired polydopamine chemistry for lithium isotope separation

Mesoporous silica/polymer hybrids with well-preserved mesoporosity were prepared by integrating the initiators for continuous activator regeneration (ICAR) atom transfer radical polymerization (ATRP) technique with the bio-inspired polydopamine (PDA) chemistry. By manipulating the auto-oxidative polymerization of dopamine, uniform PDA layers were deposited on the surfaces and pore walls of ordered mesoporous silicas (OMSs), thereby promoting the immobilization of ATRP initiators. Poly(glycidyl methacrylate) (PGMA) brushes were then grown from the OMSs by using the ICAR ATRP technique. The evolution of the mesoporous silica/polymer hybrids during synthesis, in terms of morphology, structure, surface and porous properties, was detailed. And, parameters influencing the controlled growth of polymer chains in the ICAR ATRP system were studied. Taking advantage of the abundant epoxy groups in the PGMA platform, post-functionalization of the mesoporous silica/polymer hybrids by the covalent attachment of macrocyclic ligands for the adsorptive separation of lithium isotopes was realized. Adsorption behavior of the functionalized hybrids toward lithium ions was fully investigated, highlighting the good selectivity, and effects of temperature, solvent and counter ions. The ability for lithium isotope separation was evaluated. A higher separation factor could be obtained in systems with softer counter anions and lower polarity solvents. More importantly, due to the versatility of the ICAR ATRP technique, combined with the non-surface specific PDA chemistry, the methodology established in this work would provide new opportunities for the preparation of advanced organic-inorganic porous hybrids for broadened applications.

Kinetics of Melanin Polymerization during Enzymatic and Nonenzymatic Oxidation

Melanin is an abundant biopigment in the animal kingdom, but its structure remains poorly understood. This is a substantial impediment to understanding the mechanistic origin of its observed functions. Proposed models of melanin structure include aggregates of both linear and macrocyclic units and noncovalently held monomers. Both models are broadly in agreement with current experimental data. To constrain the structural and kinetic models of melanin, experimental data of high resolution with chemical specificity accompanied by atomistic modeling are required. We have addressed this by obtaining electronic absorption, infrared, and ultraviolet resonance Raman (RR) spectra of melanin at several wavelengths of excitation that are sensitive to small changes in structure. From these experiments, we observed kinetics of the formation of different species en route to melanin polymerization. Exclusive chemical signatures of monomer 3,4-dihydroxyphenylalanine (dopa), intermediate dopachrome (DC), and early-time polymer are established through their vibrational bands at 1292, 1670, and 1616 cm^-1 respectively. Direct evidence of reduced heterogeneity of melanin oligomers in tyrosinase-induced formation is provided from experimental measurements of vibrational bandwidths. Models made with density functional theory show that the linear homopolymeric structures of 5,6-dihydroxyindole can account for experimentally observed wavenumbers and broad bandwidth in Raman spectra of dopa-melanin. We capture resonance Raman (RR) signature of DC, the intermediate stabilized by the enzyme tyrosinase, for the first time in an enzyme-assisted melanization reaction using 488 nm excitation wavelength and propose that this wavelength can be used to probe reaction intermediates of melanin formation in solution.

Polymeric peptide pigments with sequence-encoded properties

Melanins are a family of heterogeneous polymeric pigments that provide ultraviolet (UV) light protection, structural support, coloration, and free radical scavenging. Formed by oxidative oligomerization of catecholic small molecules, the physical properties of melanins are influenced by covalent and noncovalent disorder. We report the use of tyrosine-containing tripeptides as tunable precursors for polymeric pigments. In these structures, phenols are presented in a (supra-)molecular context dictated by the positions of the amino acids in the peptide sequence. Oxidative polymerization can be tuned in a sequence-dependent manner, resulting in peptide sequence-encoded properties such as UV absorbance, morphology, coloration, and electrochemical properties over a considerable range. Short peptides have low barriers to application and can be easily scaled, suggesting near-term applications in cosmetics and biomedicine.

Polydopamine and Polydopamine-Silane Hybrid Surface Treatments in Structural Adhesive Applications

Numerous studies have focused on the remarkable adhesive properties of polydopamine, which can bind to substrates with a wide range of surface energies, even under aqueous conditions. This behavior suggests that polydopamine may be an attractive option as a surface treatment in structural bonding applications, where good bond durability is required. Here, we assessed polydopamine as a surface treatment for bonding aluminum plates with an epoxy resin. A model epoxy adhesive consisting of diglycidyl ether of bisphenol A (DGEBA) and Jeffamine D230 polyetheramine was employed, and lap shear measurements (ASTM D1002 10) were made (i) under dry conditions to examine initial bond strength and (ii) after exposure to hot/wet (63 °C in water for 14 days) conditions to assess bond durability. Surprisingly, our results showed that polydopamine alone as a surface treatment provided no benefit beyond that obtained by exposing the substrates to an alkaline solution of tris buffer used for the deposition of polydopamine. This implies that polydopamine has a potential Achilles' heel, namely, the formation of a weak boundary layer that was identified using X-ray photoelectron spectroscopy (XPS) of the fractured surfaces. In fact, for longer deposition times (2.5 and 18 h), the tris buffer-treated surface outperformed the polydopamine surface treatments, suggesting that tris buffer plays a unique role in improving adhesive performance even in the absence of polydopamine. We further showed that the use of polydopamine-3-aminopropyltriethoxysilane (APTES) hybrid surface treatments provided significant improvements in bond durability at extended deposition times relative to both polydopamine and an untreated control.

Melanin-Based Functional Materials

Melanin biopolymers are currently the focus of growing interest for a broad range of applications at the cutting edge of biomedical research and technology. This Special Issue presents a collection of papers dealing with melanin-type materials, e.g., polydopamine, for classic and innovative applications, offering a stimulating perspective of current trends in the field. Besides basic scientists, the Special Issue is directed to researchers from industries and companies that are willing to invest in melanin research for innovative and inspiring solutions.

Closer to the polydopamine structure: new insights from a combined 13C/1H/2H solid-state NMR study on deuterated samples

¹³C/¹H/²H ss-NMR on deuterated samples provide strong experimental evidence for the most probable monomer connectivity, π–π stacking, and the water dynamics in polydopamine.

Expedient synthesis of eumelanin-inspired 5,6-dihydroxyindole-2-carboxylate ethyl ester derivatives

Dihydroxyindoles such as 5,6-dihydroxyindole-2-carboxylic acid (DHICA) are the main monomer units of eumelanin, the black to brown pigment in humans, and have emerging biological roles beyond melanin. Elaboration of commercially available 5,6-dimethoxy-2-carboxylate ethyl ester provides ready access to DHICA-inspired small molecules, including 3-(hetero)aryl-indoles and 4,7-di-(hetero)aryl-indoles.

Two concise syntheses of novel aryl- and heteroaryl-substituted 5,6-dimethoxyindole-2-carboxylate ethyl esters utilizing regioselective halogenation/dehalogenation and Suzuki coupling are presented.

In situ insights into the nanoscale deposition of 5,6-dihydroxyindole-based coatings and the implications on the underwater adhesion mechanism of polydopamine coatings

The biomimetic coating polydopamine (PDA) has emerged as a promising coating material for various applications. However, the mechanism of PDA deposition onto surfaces is not fully understood, and the coating components of PDA and its relation to the putative intermediate 5,6-dihydroxyindole (DHI) are still controversial. This investigation discloses the deposition mechanisms of dopamine (DA)-based coatings and DHI-based coatings onto silicon surfaces by monitoring the nanoscale deposition of both coatings in situ using high-precision ellipsometry. We posit that the rapid and instantaneous nano-deposition of PDA coatings onto silicon surface in the initial stages critically involves the oxidation of DHI and/or its related oligomers. Our studies also show that the slow conversion of DA to DHI in PDA solution and the coupling between DA and DHI-derived precursors could be crucial for subsequent PDA coating growth. These findings elucidate the critical role of DHI, acting as an ‘initiator’ and a ‘cross linker’, in the PDA coating formation. Overall, our study provides important information on the early stage nano-deposition behavior in the construction of PDA coatings and DHI-based coatings.

The underwater in situ nano-deposition studies of 5,6-dihydroxyindole (DHI) have provided new insights into the controversial deposition mechanism(s) of DHI-based and polydopamine-based coatings.

Polydopamine and eumelanin models in various oxidation states

The most stable molecular structures of PDA and eumelanin are proposed.

Polydopamine nanoparticles kill cancer cells

Polydopamine (PD) is a synthetic melanin analogue of growing importance in the field of biomedicine, especially with respect to cancer research, due, in part, to its biocompatibility.

Polydopamine films change their physicochemical and antimicrobial properties with a change in reaction conditions

The present finding provides insight into the different chemistry, morphologies and properties of the designed polydopamine films with controlled antibacterial/antifouling properties.

Biomimetic Oxygen-Evolving Photobacteria Based on Amino Acid and Porphyrin Hierarchical Self-Organization

Biomimetic organization provides a promising strategy to develop functional materials and understand biological processes. However, how to mimic complex biological systems using simple biomolecular units remains a great challenge. Herein, we design and fabricate a biomimetic cyanobacteria model based on self-integration of small bioinspired molecules, including amphiphilic amino acid, 3,4-dihydroxyphenylalanine (DOPA), and metalloporphyrin and cobalt oxide nanoparticles (Co₃O₄ NPs), with the assistance of chemical conjugation and molecular self-assembly. The assembled amino acid fiber can be modified by DOPA to form covalently bound DOPA melanin containing hydroxyl and quinone species via Schiff base reaction. The adhering template can further tune the self-assembly of metalloporphyrin and Co₃O₄ NPs into J-aggregation and dispersive distribution, respectively, mainly via coordination binding. Metalloporphyrin molecules in the resulting hybrid fibers capture light; quinone species accept the excited electrons, and Co₃O₄ NPs catalyze water oxidation. Thus, the essential components of the photosystem-II protein complex in cyanobacteria are simplified and engineered into a simple framework, still retaining a similar photosynthetic mechanism. In addition, this architecture leads to efficient coupling of antenna, quinone-type reaction center, and photocatalyst, which increases the flux of light energy from antenna to reaction center for charge separation, resulting in enhanced oxygen evolution rate with excellent sustainability.

Colour-Value Based Method for Polydopamine Coating-Stability Characterization on Polyethersulfone Membranes

Porous polyethersulfone membranes as used in oenology were investigated in order to evaluate temperature-dependent permeances in a temperature range from 10 to 35 °C. A temperature correction factor was determined for this type of membrane to get accurate and comparable results for further developments. Moreover, the membranes were modified with a bio-inspired polydopamine coating in order to reduce fouling. The performance of the membranes could be increased with respect to permeance and flux recovery under cross-flow conditions. In order to test the applicability and stability of the coating layer, they were treated with basic and acidic cleaning agents as used in industry for fouled membranes. The chemical stability of the coating layer was studied under basic and acidic conditions, by systematic observation of the colour change of the coated membranes over treatment time.

Polydopamine Based Colloidal Materials: Synthesis and Applications

Polydopamine is a synthetic analogue of natural melanin (eumelanin) produced from oxidative polymerization of dopamine. Owing to its strong adhesion ability, versatile chemical reactivity, biocompatibility and biodegradation, polydopamine is commonly applied as a versatile linker to synthesize colloidal materials with diverse structures, unique physicochemical properties and tunable functions, which allow for a broad scope of applications including biomedicine, sensing, catalysis, environment and energy. In this personal account, we discuss first about the different synthetic approaches of polydopamine, as well as its polymerization mechanism, and then with a comprehensive overview of recent progress in the synthesis and applications of polydopamine-based colloidal materials. Finally, we summarize this personal account with future perspectives.

Mussel-inspired polydopamine-polyethylenimine conjugated nanoparticles as efficient gene delivery vectors for mammalian cells

Efficient delivery of DNA to cells is the primary concern to address the objective of gene therapy. Many attempts have been made to develop polymeric carriers for gene delivery. To have an efficient carrier, it is vital to understand the properties of the vector for better stability, transfection efficiency and minimal toxicity. Branched polyethylenimine (bPEI) has been considered as the 'gold standard' for gene delivery but suffers a major drawback of exhibiting high cytotoxicity. Here, we have attempted to develop a mussel-derived polymer, polydopamine (PDA), conjugated polyethylenimine nanoparticles in such a way that the toxic nature of bPEI is suppressed by the conversion of free primary amine groups to secondary and tertiary amines. Keeping the amount of PDA fixed, varying amounts of bPEIs of different molecular weights (25, 10 and 1.8kDa) were conjugated via Michael addition and/or Schiff base. A trend in hydrodynamic size of the conjugated nanoparticles was observed in the range from 160 to 300nm and zeta potential from +12-30mV in the projected three series, viz., (i) PDA₁-²⁵bPEI_0.5, PDA₁-²⁵bPEI₁, PDA₁-²⁵bPEI₂; (ii) PDA₁-¹⁰bPEI_0.5, PDA₁-¹⁰bPEI₁, PDA₁-¹⁰bPEI₂; and (iii) PDA₁-^1.8bPEI_0.5, PDA₁-^1.8bPEI₁, PDA₁-^1.8bPEI₂. A visible trend in the DNA condensation ability and buffering capacity was also noticed. Further, cell cytotoxicity assays revealed that pDNA complexes of PDA-bPEI nanoparticles were non-toxic to mammalian cells and these complexes exhibited several folds higher transfection efficiency than the complexes of native bPEIs as demonstrated by fluorescence measurements and flow cytometry. Altogether, the results advocate the promising potential of these conjugated nanoparticles for future in vivo applications.

Replacing Nitrogen by Sulfur: From Structurally Disordered Eumelanins to Regioregular Thiomelanin Polymers

The oxidative polymerization of 5,6-dihydroxybenzothiophene (DHBT), the sulfur analog of the key eumelanin building block 5,6-dihydroxyindole (DHI), was investigated to probe the role of nitrogen in eumelanin build-up and properties. Unlike DHI, which gives a typical black insoluble eumelanin polymer on oxidation, DHBT is converted to a grayish amorphous solid (referred to as thiomelanin) with visible absorption and electron paramagnetic resonance properties different from those of DHI melanin. Mass spectrometry experiments revealed gradational mixtures of oligomers up to the decamer level. Quite unexpectedly, nuclear magnetic resonance (NMR) analysis of the early oligomer fractions indicated linear, 4-, and 7-linked structures in marked contrast with DHI, which gives highly complex mixtures of partially degraded oligomers. Density functional theory (DFT) calculations supported the tendency of DHBT to couple via the 4- and 7-positions. These results uncover the role of nitrogen as a major determinant of the structural diversity generated by the polymerization of DHI, and point to replacement by sulfur as a viable entry to regioregular eumelanin-type materials for potential applications for surface functionalization by dip coating.

Ultrathin thermoresponsive self-folding 3D graphene

Graphene and other two-dimensional materials have unique physical and chemical properties of broad relevance. It has been suggested that the transformation of these atomically planar materials to three-dimensional (3D) geometries by bending, wrinkling, or folding could significantly alter their properties and lead to novel structures and devices with compact form factors, but strategies to enable this shape change remain limited. We report a benign thermally responsive method to fold and unfold monolayer graphene into predesigned, ordered 3D structures. The methodology involves the surface functionalization of monolayer graphene using ultrathin noncovalently bonded mussel-inspired polydopamine and thermoresponsive poly(N-isopropylacrylamide) brushes. The functionalized graphene is micropatterned and self-folds into ordered 3D structures with reversible deformation under a full control by temperature. The structures are characterized using spectroscopy and microscopy, and self-folding is rationalized using a multiscale molecular dynamics model. Our work demonstrates the potential to design and fabricate ordered 3D graphene structures with predictable shape and dynamics. We highlight applicability by encapsulating live cells and creating nonlinear resistor and creased transistor devices.

Mussel-Inspired Polyglycerol Coatings with Controlled Wettability: From Superhydrophilic to Superhydrophobic Surface Coatings

Facile approaches to substrate-independent surface coatings with special wettability properties, such as superhydrophobicity, superhydrophilicity, and superamphiphobicity, have been limited. To address this problem, we combined two separate biomimetic concepts of mussel-inspired adhesion and highly hierarchical lotuslike surface structures to develop a universal fabrication method for various superwetting systems on any kind of material. In this feature article, we summarize our work on mussel-inspired polyglycerol (MI-dPG) and its application in the area of superwetting interfacial materials. MI-dPG mimics not only the functional groups of mfp-5 but also their molecular weight and molecular structure, which results in strong and rapid adhesion to the substrate. Furthermore, the MI-dPG coating process provides precise roughness control. The construction of highly hierarchical and superhydrophilic structures was achieved either directly by pH-controlled aggregation or in combination with nanoparticles. Subsequent postmodification of these highly hierarchical structures with different fluorinated or nonfluorinated hydrophobic molecules yielded a surface with superhydrophobic and even superamphiphobic properties.

Mussel-Inspired Polydopamine Coating on Tobacco Mosaic Virus: One-Dimensional Hybrid Nanofibers for Gold Nanoparticle Growth

One-dimensional (1D) hybrid nanofibers with surface-deposited gold nanoparticles (AuNPs) have been fabricated by self-assembly of rod-like tobacco mosaic virus (TMV) with mussel-inspired polymerization of dopamine and in situ reduction of gold ion, providing a method for sensing the endocytic pathway of nanomaterial.

Biopolymer-Drug Conjugate Nanotheranostics for Multimodal Imaging-Guided Synergistic Cancer Photothermal-Chemotherapy

Some of the biomedical polymer-drug conjugates are being translated into clinical trials; however, they intrinsically lack photothermal and multi-imaging capabilities, hindering them from imaging-guided precision cancer therapy and complete tumor regression. We introduce a new concept of all-in-one biopolymer-drug conjugate nanotheranostics and prepare a kind of intracellular pH-sensitive polydopamine-doxorubicin (DOX) conjugate nanoparticles (PDCNs) under mild conditions. Significantly, this strategy integrates polymeric prodrug-induced chemotherapy (CT), near-infrared (NIR) light-mediated photothermal therapy (PT), and triple modalities including DOX self-fluorescence, photothermal, and photoacoustic (PA) imaging into one conjugate nanoparticle. The PDCNs present excellent photothermal property, dual stimuli-triggered drug release behavior, and about 12.4-fold blood circulation time compared to free DOX. Small animal fluorescent imaging technique confirms that PDCNs have preferential tumor accumulation effect in vivo, giving a 12.8-fold DOX higher than the control at 12 h postinjection. Upon NIR laser irradiation (5 min, 808 nm, and 2 W·cm^-2), the PDCN-mediated photothermal effect can quickly elevate the tumor over 50 °C, exhibiting good photothermal and PA imaging functions, of which the PA amplitude is 3.6-fold greater than the control. In vitro and in vivo assays persuasively verify that intravenous photothermal-CT of PDCNs produces synergistic antitumor activity compared to single PT or CT, achieving complete tumor ablation during the evaluation period.

The Supramolecular Buildup of Eumelanin: Structures, Mechanisms, Controllability

Research on the supramolecular buildup of eumelanin has gained high momentum in the last years. Several new aspects regarding the involved structures and mechanisms have been established, which has led to a better understanding of the entire process. This review intends to provide a clearly laid-out summary of previous and new findings regarding structures, mechanisms, and controllability. With respect to materials applications, the aspect of controllability is of supreme importance. A focus of this review is therefore set on a novel method with high potential for specific synthesis of various, isolated particle morphologies. Finally, open questions and possibilities for their elucidation are discussed.

Anti-Melanogenic Effects of Flavonoid Glycosides from Limonium tetragonum (Thunb.) Bullock via Inhibition of Tyrosinase and Tyrosinase-Related Proteins

Overproduction and stimulation of tyrosinase result in increased melanogenesis of which several skin disorders such as freckles, spots, and hyperpigmentation appear as complications. Limonium tetragonum is a halophyte well-known for its antioxidative properties. This study investigated the anti-melanogenic effects of solvent-partitioned L. tetragonum extracts (LTEs) and its bioactive constituents, two isolated flavonoid glycosides. Current study followed a set of experiments on B16-F10 mouse melanoma cell model with a focus on tyrosinase activity and production. The anti-melanogenic capacity of LTEs was confirmed by their tyrosinase inhibitory effects, prevention of DOPA oxidation, and suppression of melanin production. The inhibition of tyrosinase and DOPA oxidation by LTEs was suggested to be related with the downregulation of microphthalmia-associated transcription factor, tyrosinase, tyrosinase-related protein-1, and tyrosinase-related protein-2, verified with mRNA and protein expression levels. Among all tested LTEs, 85% aq. MeOH and n-BuOH were found to be the most active fractions which later yielded the two known compounds, myricetin 3-galactoside and quercetin 3-O-β-galactopyronaside. The anti-melanogenic potential of the compounds were confirmed by their tyrosinase inhibitory effects. These results suggested that L. tetragonum may serve as a potential source of bioactive substances with effective anti-melanogenesis properties.

A one step method for the functional and property modification of DOPA based nanocoatings

Biomimetic poly(catecholamine) coatings have gained much attention in recent years due to their versatility as functional materials. Despite this, only limited methods are available to modify the function and property of poly(catecholamine) coatings, primarily through post-modification methods. Our approach reported herein provides a simple approach to the fabrication of novel functionalized poly(catecholamine) coatings. The strategy employs the copolymerization of N-Ac-3,4-dihydroxyphenylalanine methyl ester (NADOPAMe) with nucleophilic additives, giving rise to nano-coatings on various surfaces including plastic, metal, glass and polymers. With the appropriate choice of nucleophilic additives, coatings with desired properties can be achieved. This is demonstrated through the fabrication of a redox responsive coating based on NADOPAMe with cysteamine as additive, which shows a concentration-dependent glutathione (GSH) responsive behavior. The ability to utilize this as a controlled release system is also demonstrated.

Melanin-Based Contrast Agents for Biomedical Optoacoustic Imaging and Theranostic Applications

Optoacoustic imaging emerged in early 1990s as a new biomedical imaging technology that generates images by illuminating tissues with short laser pulses and detecting resulting ultrasound waves. This technique takes advantage of the spectroscopic approach to molecular imaging, and delivers high-resolution images in the depth of tissue. Resolution of the optoacoustic imaging is scalable, so that biomedical systems from cellular organelles to large organs can be visualized and, more importantly, characterized based on their optical absorption coefficient, which is proportional to the concentration of absorbing chromophores. Optoacoustic imaging was shown to be useful in both preclinical research using small animal models and in clinical applications. Applications in the field of molecular imaging offer abundant opportunities for the development of highly specific and effective contrast agents for quantitative optoacoustic imaging. Recent efforts are being made in the direction of nontoxic biodegradable contrast agents (such as nanoparticles made of melanin) that are potentially applicable in clinical optoacoustic imaging. In order to increase the efficiency and specificity of contrast agents and probes, they need to be made smart and capable of controlled accumulation in the target cells. This review was written in recognition of the potential breakthroughs in medical optoacoustic imaging that can be enabled by efficient and nontoxic melanin-based optoacoustic contrast agents.

Interactions of iron, dopamine and neuromelanin pathways in brain aging and Parkinson's disease

There are several interrelated mechanisms involving iron, dopamine, and neuromelanin in neurons. Neuromelanin accumulates during aging and is the catecholamine-derived pigment of the dopamine neurons of the substantia nigra and norepinephrine neurons of the locus coeruleus, the two neuronal populations most targeted in Parkinson's disease. Many cellular redox reactions rely on iron, however an altered distribution of reactive iron is cytotoxic. In fact, increased levels of iron in the brain of Parkinson's disease patients are present. Dopamine accumulation can induce neuronal death; however, excess dopamine can be removed by converting it into a stable compound like neuromelanin, and this process rescues the cell. Interestingly, the main iron compound in dopamine and norepinephrine neurons is the neuromelanin-iron complex, since neuromelanin is an effective metal chelator. Neuromelanin serves to trap iron and provide neuronal protection from oxidative stress. This equilibrium between iron, dopamine, and neuromelanin is crucial for cell homeostasis and in some cellular circumstances can be disrupted. Indeed, when neuromelanin-containing organelles accumulate high load of toxins and iron during aging a neurodegenerative process can be triggered. In addition, neuromelanin released by degenerating neurons activates microglia and the latter cause neurons death with further release of neuromelanin, then starting a self-propelling mechanism of neuroinflammation and neurodegeneration. Considering the above issues, age-related accumulation of neuromelanin in dopamine neurons shows an interesting link between aging and neurodegeneration.

Polydopamine Coating To Stabilize a Free-Standing Lipid Bilayer for Channel Sensing

An appropriate method to study the function of membrane channels is to insert them into free-standing lipid bilayers and to record the ion conductance across the membrane. The insulating property of a free-standing lipid bilayer versus the single-channel conductivity provides sufficient sensitivity to detect minor changes in the pathway of ions along the channel. A potential application is to use membrane channels as label-free sensors for molecules, with DNA sequencing as its most prominent application. However, the inherent instability of free-standing bilayers limits broader use as a biosensor. Here we report on a possible stabilization of free-standing lipid bilayers using polydopamine deposition from dopamine-containing solutions in the presence of an oxidant. This stabilization treatment can be initiated after protein reconstitution and is compatible with most reconstitution protocols.

Deciphering Molecular Mechanisms of Interface Buildup and Stability in Porous Si/Eumelanin Hybrids

Porous Si/eumelanin hybrids are a novel class of organic-inorganic hybrid materials that hold considerable promise for photovoltaic applications. Current progress toward device setup is, however, hindered by photocurrent stability issues, which require a detailed understanding of the mechanisms underlying the buildup and consolidation of the eumelanin-silicon interface. Herein we report an integrated experimental and computational study aimed at probing interface stability via surface modification and eumelanin manipulation, and at modeling the organic-inorganic interface via formation of a 5,6-dihydroxyindole (DHI) tetramer and its adhesion to silicon. The results indicated that mild silicon oxidation increases photocurrent stability via enhancement of the DHI-surface interaction, and that higher oxidation states in DHI oligomers create more favorable conditions for the efficient adhesion of growing eumelanin.

Melanin and Melanin-Related Polymers as Materials with Biomedical and Biotechnological Applications-Cuttlefish Ink and Mussel Foot Proteins as Inspired Biomolecules

The huge development of bioengineering during the last years has boosted the search for new bioinspired materials, with tunable chemical, mechanical, and optoelectronic properties for the design of semiconductors, batteries, biosensors, imaging and therapy probes, adhesive hydrogels, tissue restoration, photoprotectors, etc. These new materials should complement or replace metallic or organic polymers that cause cytotoxicity and some adverse health effects. One of the most interesting biomaterials is melanin and synthetic melanin-related molecules. Melanin has a controversial molecular structure, dependent on the conditions of polymerization, and therefore tunable. It is found in animal hair and skin, although one of the common sources is cuttlefish (Sepia officinalis) ink. On the other hand, mussels synthesize adhesive proteins to anchor these marine animals to wet surfaces. Both melanin and mussel foot proteins contain a high number of catecholic residues, and their properties are related to these groups. Dopamine (DA) can easily polymerize to get polydopamine melanin (PDAM), that somehow shares properties with melanin and mussel proteins. Furthermore, PDAM can easily be conjugated with other components. This review accounts for the main aspects of melanin, as well as DA-based melanin-like materials, related to their biomedical and biotechnological applications.

Composite Materials and Films Based on Melanins, Polydopamine, and Other Catecholamine-Based Materials

Polydopamine (PDA) is related to eumelanins in its composition and structure. These pigments allow the design, inspired by natural materials, of composite nanoparticles and films for applications in the field of energy conversion and the design of biomaterials. This short review summarizes the main advances in the design of PDA-based composites with inorganic and organic materials.

Rapid Deposition of Uniform Polydopamine Coatings on Nanoparticle Surfaces with Controllable Thickness

Polydopamine is a bioinspired, versatile material that can adhere to bulk and nanoscale surfaces made of disparate materials to improve their physical and chemical properties in many applications. The typical methods to coat polydopamine on the nanoparticle substrates usually take several hours to a day. This work successfully applies a dispersion method to form a controllable, uniform coating on a nanoparticle surface within minutes. Using plasmonic Ag nanoparticles as a substrate, the coating thickness can be monitored using a spectroscopic method based on the extinction peak shifts of the Ag nanoparticles. The deposition rate increases with dopamine concentration; however, too much excess dopamine leads to the formation of free dopamine particles. The optimized concentration of dopamine (i.e., ∼6 mM) can be applied to other nanoparticles by normalizing the number of particles to maintain a constant concentration of dopamine per unit surface area (i.e., 1.70 × 10⁴ dopamine/nm²). The molecular dynamics simulation reveals that the amount of hydrogen bonding increases with water content, suggesting that sufficient mixing using the dispersion tool facilitates the formation of hydrogen bonding, thus rapidly depositing PDA on the nanoparticle surface. The physical and chemical properties (e.g., pH response and thermal stability) can be tailored by varying the coating thickness due to the changes in the number of hydrogen bonds and the conformation of π-π interactions. This dispersion method provides a facile means to control the PDA coating thickness on nanoparticle surfaces and thus the surface properties of nanoparticles toward various applications.

Polydopamine Generates Hydroxyl Free Radicals under Ultraviolet-Light Illumination

Polydopamine (PDA) generally demonstrates as an efficient free-radical scavenger. However, its free-radical chemistry under illumination is unclear, which becomes important in view of growing studies of polydopamine applications in photoprotector and photothermal therapy. In this study, for the first time, we reported an experimental investigation of the generation of hydroxyl free radicals from ultraviolet (UV)-illuminated polydopamine in an aqueous environment. By using terephthalic acid as fluorescent probe, we measured hydroxyl radicals generated from UV-illuminated polydopamine with different shapes and sizes. The morphology of PDA shows significant influence on its productions of hydroxyl free radicals. Through characterizations of UV-vis absorption spectroscopy, fluorescence spectroscopy, X-ray photoelectron spectrometry, mass spectrometry, and thermogravimetric analysis, we demonstrated the change of PDA nanomaterials brought by UV-light illumination in composition and thermal stability. We proposed a tentative mechanism for interpreting the relationship between morphology and photostability of PDA nanomaterials. These results reveal underlying complexity of polydopamine chemistry under light illumination that will deepen our understanding and benefit its further application.

Synthetic Melanin E-Ink

Extensive efforts have been devoted to the development of surfactant-free electronic ink (E-ink) with excellent display resolution for high-definition resolution display. Herein, we report the use of polydopamine-based synthetic melanin, a class of functional nanoparticles with similar chemical compositions and physical properties to those of naturally occurring melanin, as a new E-ink material. It was found that such E-ink displays could achieve ultrahigh resolution (>10 000 ppi) and low power consumption (operation voltage of only 1 V) in aqueous solutions. Interestingly, simple oxidation of synthetic melanin nanoparticles enables the generation of intrinsic fluorescence, allowing further development of fluorescent E-ink displays with nanoscale resolution. We describe these bioinspired materials in an initial proof-of-concept study and propose that synthetic melanin nanoparticles will be suitable for electronic nanoinks with a potential wide range of applications in molecular patterning and fluorescence bioimaging.