The reduction of Ag+ in metallic silver on pseudomelanin films allows for antibacterial activity but does not imply unpaired electrons

Recent Advances in Dopamine-Based Membrane Surface Modification and Its Membrane Distillation Applications

Surface modification of membranes is essential for improving flux and resistance to contamination for membranes. This is of great significance for membrane distillation, which relies on the vapor pressure difference across the membrane as the driving force. In recent years, biomimetic mussel-inspired substances have become the research hotspots. Among them, dopamine serves as surface modifiers that would achieve highly desirable and effective membrane applications owing to their unique physicochemical properties, such as universal adhesion, enhanced hydrophilicity, tunable reducibility, and excellent thermal conductivity. The incorporation of a hydrophilic layer, along with the utilization of photothermal properties and post-functionalization capabilities in modified membranes, effectively addresses challenges such as low flux, contamination susceptibility, and temperature polarization during membrane distillation. However, to the best of our knowledge, there is still a lack of comprehensive and in-depth discussions. Therefore, this paper systematically compiles the modification method of dopamine on the membrane surface and summarizes its application and mechanism in membrane distillation for the first time. It is believed that this paper would provide a reference for dopamine-assisted membrane separation during production, and further promote its practical application.

Permanent Low-Toxicity Hair Dye Based on Pregrafting Melanin with Cystine

A pregrafting strategy is presented to construct covalent bridges between synthetic melanin, i.e., polydopamine (PDA), and hair for permanent hair dyeing. As a result, PDA is more uniformly distributed throughout the hair surface, and the dyed hair shows higher color intensity and better durability to washing than the control samples dyed with PDA directly at the same conditions.

Preparation of mussel-inspired silver/polydopamine antibacterial biofilms on Ti-6Al-4V for dental applications

The properties of osseointegration and antibacterial ability is vital import for dental materials. Herein, we designed the multilayer TC4-Ag-polydopamine coatings, to provide TC4 with slow-release antibacterial properties whilst maintaining cytocompatibility. In brief, thickness of Ag inner layer can be easily controlled by magnetron sputtering technology. The resulting top polydopamine layer protected the Ag well from corrosion and gave a sustained release of Ag⁺ up to one month. In addition, the prepared TC4-Ag-polydopamine samples with Ag thickness of 20 and 30 nm, showed high hydrophilic performance with the contact-angle less than 20°, low cytotoxicity and good cytocompatibility. Expectedly, it could become a prospective candidate for future slow-release antibacterial dental materials.

Copper Electroless Metallization of Cellulose Paper via Polydopamine Coating and Silver Catalyst

The paper presents the results of copper electroless metallization of cellulose paper with the use of a polydopamine coating and silver catalyst. The polydopamine coating was deposited via a simple dip method using a dopamine hydrochloride solution in 10 mM TRIS-HCl buffer with a pH of 8.5. The research showed that as a result of this process, cellulose fibers were covered with a homogeneous layer of polydopamine. The unique properties of the polydopamine coating allowed the reduction of silver ions from silver nitrate solution and the deposition of silver atoms on the paper surface. Deposited silver served as a catalyst in the autocatalytic electroless copper-plating process. The copper layer covered the entire surface of the paper sheet after 5 min of metallization, favorably affecting the electrical properties of this material by lowering the surface resistivity. The deposited copper layer was further characterized by good adhesive strength and high susceptibility to deformation.

Polydopamine and dopamine interfere with tetrazolium-based cytotoxicity assays and produce exaggerated cytocompatibility inferences

Tetrazolium-based assays such as the MTT assay have been commonly employed in evaluating biocompatibility. Here, we show that PDA (or its precursor dopamine (DA)) spontaneously reduces MTT and produces exaggerated cytocompatibility inferences. The extent of interference depends on the method of DA polymerization. We observed that the trypan blue exclusion assay allowed more accurate determination of cell viability in the presence of DA- and PDA-based nanomaterials.

Copper–CNT interfacing with Cu-doped polydopamine in CNT carpet: copper nucleation and resistance decrease upon soft annealing

The annealing of carpets of CNT coated with Cu-doped polydopamine lowers the carpets sheet resistance and induces the reduction of Cu ions into metallic Cu particles nucleating at the CNT surface.

Applications of Polydopamine-Modified Scaffolds in the Peripheral Nerve Tissue Engineering

Peripheral nerve injury is a common and complicated traumatic disease in clinical neurosurgery. With the rapid advancement and development of medical technologies, novel tissue engineering provides alternative therapies such as nerve conduit transplantation. It has achieved significant outcomes. The scaffold surface modification is vital to the reconstruction of a pro-healing interface. Polydopamine has high chemical activity, adhesion, hydrophilicity, hygroscopicity, stability, biocompatibility, and other properties. It is often used in the surface modification of biomaterials, especially in the peripheral nerve regeneration. The present review discusses that polydopamine can promote the adhesion, proliferation, and differentiation of neural stem cells and the growth of neuronal processes. Polydopamine is widely used in the surface modification of nerve conduits and has a potential application prospect of repairing peripheral nerve injury. Polydopamine-modified scaffolds are promising in the peripheral nerve tissue engineering.

Biomimetic catechol-based adhesive polymers for dispersion of polytetrafluoroethylene (PTFE) nanoparticles in an aqueous medium

Biomimetic synthetic functional materials are valuable for a large number of practical applications with improved or tunable performance. In this paper, we present a series of mussel-inspired biomimetic catechol-containing copolymers synthesized from dopamine methacrylamide (DMA) and 2-(2-ethoxyethoxy)ethyl acrylate (EEA) and abbreviated as poly(PDMA-PEEA). The successfully synthesized adhesive polymers allow adhering polytetrafluoroethylene (PTFE) and were used for coating PTFE particles in organic solvent and re-dispersion in an aqueous medium. Adhesive polymer coated PTFE particles were efficiently used as a nanoreactor for generating silver (Ag) metal nanoparticles (NPs).

Coating of polydopamine on polyurethane open cell foams to design soft structured supports for molecular catalysts

Polydopamine-coated polyurethane open cell foams are used as structured supports for molecular catalysts through the covalent anchoring of alkoxysilyl arms by the catechol groups of the mussel-inspired layer. This strong bonding prevents their leaching. No alteration of the mechanical properties of the flexible support is observed after repeated uses of the catalytic materials.

Boric Acid as an Efficient Agent for the Control of Polydopamine Self-Assembly and Surface Properties

The deposition of polydopamine (PDA) films on surfaces, a versatile deposition method with respect to the nature of the used substrate, is unfortunately accompanied by deposition of insoluble precipitates in solution after a prolonged oxidation time of dopamine solutions. Therefore, there is evident interest to find methods able to stop the deposition of PDA on surfaces and to simultaneously control the self-assembly of PDA in solution to get stable colloidal aggregates. In addition to proposed methods relying on the use of polymers like poly(vinyl alcohol) and proteins like human serum albumin, we show herein that boric acid is an efficient adjuvant that is simultaneously able to stop the self-assembly of PDA in solution as well as on surfaces and to change the adhesive properties of the resulting PDA coatings.

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.

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.

Mussel-inspired deposition of copper on titanium for bacterial inhibition and enhanced osseointegration in a periprosthetic infection model

Periprosthetic infection represents one of the most devastating complications in orthopedic surgeries.

Antibacterial activity and osseointegration of silver-coated poly(ether ether ketone) prepared using the polydopamine-assisted deposition technique

PEEK-PDA-Ag substrates may be a promising orthopaedic implant material due to the outstanding biocompatibility and antibacterial properties.

Catecholamine-functionalized graphene as a biomimetic redox shuttle for solar water oxidation

In natural photosynthesis, solar energy is converted to chemical energy through a cascaded, photoinduced charge transfer chain that consists of primary and secondary acceptor quinones (i.e., Q_A and Q_B). This leads to an exceptionally high near-unity quantum yield. Inspired by the unique multistep architecture of charge transfer in nature, we have synthesized a catecholamine-functionalized, reduced graphene oxide (RGO) film as a redox mediator that can mimic quinone acceptors in photosystem II. We used polynorepinephrine (PNE) as a redox-shuttling chemical. We also used it to coat graphene oxide (GO) and to reduce GO to RGO. The quinone ligands in PNE, which are characterized by a charge transfer involving two electrons and two protons, acted as electron acceptors that facilitated charge transfer in photocatalytic water oxidation. Furthermore, PNE-coated RGO film promoted fast charge separation in [Ru(bpy)₃]²⁺ and increased the activity of cobalt phosphate on photocatalytic water oxidation more than two-fold. The results suggest that our bio-inspired strategy for the construction of a forward charge transfer pathway can provide more opportunities to realize efficient artificial photosynthesis.

Mussel-inspired Functionalization of Cotton for Nano-catalyst Support and Its Application in a Fixed-bed System with High Performance

Inspired by the composition of adhesive and reductive proteins secreted by marine mussels, polydopamine (PDA) was used to coat cotton microfiber (CMF), and then acted as reducing agent for the growth of Pd nanoparticles on PDA coated CMF (PDA@CMF) composites. The resultant CMF@PDA/Pd composites were then packed in a column for the further use in fixed-bed system. For the catalysis of the reduction of 4-nitrophenol, the flow rate of the 4-aminophenol solution (0.5 mM) was as high as 60 mL/min. The obtained fixed-bed system even exhibited superior performance to conventional batch reaction process because it greatly facilitated the efficiency of the catalytic fibers. Consequently, its turnover frequency (TOF) was up to 1.587 min⁻¹, while the TOF in the conventional batch reaction was 0.643 min⁻¹. The catalytic fibers also showed good recyclability, which can be recycled for nine successive cycles without a loss of activity. Furthermore, the catalytic system based on CMF@PDA/Pd can also be applied for Suzuki coupling reaction with the iodobenzene conversion up to 96.7%. The strategy to prepare CMF@PDA/Pd catalytic fixed bed was simple, economical and scalable, which can also be applied for coating different microfibers and loading other noble metal nanoparticles, was amenable for automated industrial processes.

Synthesis of core–shell structured alumina/Cu microspheres using activation by silver nanoparticles deposited on polydopamine-coated surfaces

A facile and efficient approach to preparing alumina/Cu composite microspheres with silver-decorated polydopamine coating is proposed.

Physicochemical perspective on "polydopamine" and "poly(catecholamine)" films for their applications in biomaterial coatings

Bioinspired poly(catecholamine) based coatings, mostly "polydopamine," were conceived based on the chemistry used by mussels to adhere strongly to the surface of stones and wood in water and to remain attached to their substrates even under conditions of strong shear stresses. These kinds of films can in turn be easily modified with a plethora of molecules and inorganic (nano)materials. This review shows that poly(catecholamine) based coatings are an ideal film forming method for applications in the field of biomaterials. It is written from a physicochemical and a materials science perspective and discusses optical, chemical, electrochemical, and mechanical properties of polydopamine films. It further demonstrates that a better understanding of the polydopamine film deposition mechanism is warranted to improve the properties of these coatings even further.

Synthesis of poly(dihydroxystyrene-block-styrene) (PDHSt-b-PSt) by the RAFT process and preparation of organic-solvent-dispersive Ag NPs by automatic reduction of metal ions in the presence of PDHSt-b-PSt

We proposed a block copolymer, poly(dihydroxystyrene-block-styrene) (PDHSt-b-PSt), that contains catechol groups in the side chains of PDHSt moieties. Since catechol groups automatically reduce silver (Ag) ions to their metallic state, the block copolymer was used as a reductant to synthesize organic-solvent-dispersive Ag NPs (NPs) stabilized with the block copolymer at room temperature. Ag NP sizes were controlled by changing molecular weights of PDHSt of the block copolymer.

Ultrafine Pd nanoparticles encapsulated in microporous Co3O4 hollow nanospheres for in situ molecular detection of living cells

Recent progress in the in situ molecular detection of living cells has attracted tremendous research interests due to its great significance in biochemical, physiological, and pathological investigation. Especially for the electrochemical detection of hydrogen peroxide (H2O2) released by living cells, the highly efficient and cost-effective electrocatalysts are highly desirable. In this work, we develop a novel type of microporous Co3O4 hollow nanospheres containing encapsulated Pd nanoparticles (Pd@Co3O4). Owing to the synergy effect between the permeable microporous Co3O4 shell and the ultrafine Pd nanoparticles that encapsulated in it, the resultant Pd@Co3O4 based electrode exhibits excellent electrochemical sensor performance toward H2O2, even when the content of Pd in Pd@Co3O4 hollow nanospheres is as low as 1.14 wt %, which enable it be used for real-time tracking of the secretion of H2O2 in different types of living human cells.

In situ polydopamine-assisted deposition of silver nanoparticles on a two dimensional support as an inexpensive and highly efficient SERS substrate

Versatile substrates were modified with polydopamine followed by in situ AgNP deposition to fabricate a cheap, flexible and disposable SERS substrate.

Substrate independent coating formation and anti-biofouling performance improvement of mussel inspired polydopamine

Anti-biofouling performance of mussel inspired polydopamine coating can be improved significantly by simple coordination, oxidation, heating or grafting treatment.

Polydopamine Films from the Forgotten Air/Water Interface

The formation of polydopamine under mild oxidation conditions from dopamine solutions with mechanical agitation leads to the formation of films that can functionalize all kinds of materials. In the absence of stirring of the solution, we report the formation of polydopamine films at the air/water interface (PDA A/W) and suggest that it arises from an homogeneous nucleation process. These films grow two times faster than in solution and can be deposited on hydrophilic or hydrophobic substrates by the Langmuir-Schaeffer technique. Thanks to this new method, porous and hydrophobic materials like polytetrafluoroethylene (PTFE) membranes can be completely covered with a 35 nm thick PDA A/W film after only 3h of reaction. Finally the oxidation of a monomer followed by a polymerization in water is not exclusive to polydopamine since we also transferred polyaniline functional films from the air/water interface to solid substrates. These findings suggest that self-assembly from a solution containing hydrophilic monomers undergoing a chemical transformation (here oxidation and oligomerization) could be a general method to produce films at the liquid/air interface.

Synthesis of polydopamine at the femtoliter scale and confined fabrication of Ag nanoparticles on surfaces

Polydopamine is synthesized in confined femtolitre sized droplets and used as green nanoreactors to fabricate Ag nanoparticles on surfaces.

Metal deposition by electroless plating on polydopamine functionalized micro- and nanoparticles

A novel approach for the fabrication of metal coated micro- and nanoparticles by functionalization with a thin polydopamine layer followed by electroless plating is reported. The particles are initially coated with polydopamine via self-polymerization. The resulting polydopamine coated particles have a surface rich in catechols and amino groups, resulting in a high affinity toward metal ions. Thus, they provide an effective platform for selective electroless metal deposition without further activation and sensitization steps. The combination of a polydopamine-based functionalization with electroless plating ensures a simple, scalable, and cost-effective metal coating strategy. Silver-plated tungsten carbide microparticles, copper-plated tungsten carbide microparticles, and copper-plated alumina nanoparticles were successfully fabricated, showing also the high versatility of the method, since the polymerization of dopamine leads to the formation of an adherent polydopamine layer on the surface of particles of any material and size. The metal coated particles produced with this process are particularly well suited for the production of metal matrix composites, since the metal coating increases the wettability of the particles by the metal, promoting their integration within the matrix. Such composite materials are used in a variety of applications including electrical contacts, components for the automotive industries, magnets, and electromagnetic interference shielding.

Kinetics of polydopamine film deposition as a function of pH and dopamine concentration: insights in the polydopamine deposition mechanism

The formation of "polydopamine" thin films becomes a popular method to confer multifunctionality to solid-liquid interfaces through the available catechol groups of such films. The mechanism of film formation is, however, not well elucidated, and most investigators use the protocol developed by Messersmith et al. (H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Science 318 (2007) 426.) using a dopamine solution at a constant concentration of 2 g L(-1) in the presence of Tris(hydroxymethyl aminomethane) at pH 8.5. A particular finding of this initial study was that the film thickness reaches a constant value (almost substrate independent) of about 40 nm. Herein, we investigate the change in the polydopamine film thickness, morphology, surface energy and electrochemical properties as a function of the concentration of the dopamine solution put in the presence of silicon substrates. As a surprising finding, we observe a constant increase in the maximal film thickness with an increase in the dopamine solution between 0.1 and 5 g L(-1). The surface morphology is also markedly affected by the concentration of the dopamine solution, whereas the different components of the surface energy stay unaffected by the dopamine solution concentration. In addition, electrochemical impedance spectroscopy shows that the higher the initial dopamine concentration, the more rapidly compact and impermeable films are formed. Finally, we propose a model for the deposition of polydopamine films taking all our findings into account. This model relies on a rate equation taking into account both attractive and repulsive interactions between small polydopamine aggregates on the surface and in solution.