Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

Molecular mechanism of Thioflavin-T binding to amyloid fibrils.

Because of the high catalytic activities and substrate specificity, natural enzymes have been widely used in industrial, medical, and biological fields, etc. Although promising, they often suffer from intrinsic shortcomings such as high cost, low operational stability, and difficulties of recycling. To overcome these shortcomings, researchers have been devoted to the exploration of artificial enzyme mimics for a long time. Since the discovery of ferromagnetic nanoparticles with intrinsic horseradish peroxidase-like activity in 2007, a large amount of studies on nanozymes have been constantly emerging in the next decade. Nanozymes are one kind of nanomaterials with enzymatic catalytic properties. Compared with natural enzymes, nanozymes have the advantages such as low cost, high stability and durability, which have been widely used in industrial, medical, and biological fields. A thorough understanding of the possible catalytic mechanisms will contribute to the development of novel and high-efficient nanozymes, and the rational regulations of the activities of nanozymes are of great significance. In this review, we systematically introduce the classification, catalytic mechanism, activity regulation as well as recent research progress of nanozymes in the field of biosensing, environmental protection, and disease treatments, etc. in the past years. We also propose the current challenges of nanozymes as well as their future research focus. We anticipate this review may be of significance for the field to understand the properties of nanozymes and the development of novel nanomaterials with enzyme mimicking activities.

Nanozymes: Classification, Catalytic Mechanisms, Activity Regulation, and Applications

Bone tissue is a complex biocomposite material with a variety of organic (e.g., proteins, cells) and inorganic (e.g., hydroxyapatite crystals) components hierarchically organized with nano/microscale precision. Based on the understanding of such hierarchical organization of bone tissue and its unique mechanical properties, efforts are being made to mimic these organic–inorganic hybrid biocomposites. A key factor for the successful designing of complex, hybrid biomaterials is the facilitation and control of adhesion at the interfaces, as many current synthetic biomaterials are inert, lacking interfacial bioactivity. In this regard, researchers have focused on controlling the interface by surface modifications, but the development of a simple, unified way to biofunctionalize diverse organic and inorganic materials remains a critical challenge. Here, a universal biomineralization route, called polydopamine-assisted hydroxyapatite formation (pHAF), that can be applied to virtually any type and morphology of scaffold materials is demonstrated. Inspired by the adhesion mechanism of mussels, the pHAF method can readily integrate hydroxyapatites on ceramics, noble metals, semiconductors, and synthetic polymers, irrespective of their size and morphology (e.g., porosity and shape). Surface-anchored catecholamine moieties in polydopamine enriches the interface with calcium ions, facilitating the formation of hydroxyapatite crystals that are aligned to the c-axes, parallel to the polydopamine layer as observed in natural hydroxyapatites in mineralized tissues. This universal surface biomineralization can be an innovative foundation for future tissue engineering.

Mussel-Inspired Polydopamine Coating as a Universal Route to Hydroxyapatite Crystallization

General functionalization route for cell adhesion on non-wetting surfaces.

The abnormal deposition and aggregation of beta-amyloid (Abeta) on brain tissues are considered to be one of the characteristic neuropathological features of Alzheimer's disease (AD). Environmental conditions such as metal ions, pH, and cell membranes are associated with Abeta deposition and plaque formation. According to the amyloid cascade hypothesis of AD, the deposition of Abeta42 oligomers as diffuse plaques in vivo is an important earliest event, leading to the formation of fibrillar amyloid plaques by the further accumulation of soluble Abeta under certain environmental conditions. In order to characterize the effect of metal ions on amyloid deposition and plaque growth on a solid surface, we prepared a synthetic template by immobilizing Abeta oligomers onto a N-hydroxysuccinimide ester-activated solid surface. According to our study using ex situ atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FT-IR), and thioflavin T (ThT) fluorescence spectroscopy, Cu2+ and Zn2+ ions accelerated both Abeta40 and Abeta42 deposition but resulted only in the formation of "amorphous" aggregates. In contrast, Fe3+ induced the deposition of "fibrillar" amyloid plaques at neutral pH. Under mildly acidic environments, the formation of fibrillar amyloid plaques was not induced by any metal ion tested in this work. Using secondary ion mass spectroscopy (SIMS) analysis, we found that binding Cu ions to Abeta deposits on a solid template occurred by the possible reduction of Cu ions during the interaction of Abeta with Cu2+. Our results may provide insights into the role of metal ions on the formation of fibrillar or amorphous amyloid plaques in AD.

Metal ions differentially influence the aggregation and deposition of Alzheimer's beta-amyloid on a solid template.

The self-assembly of peptide-based building blocks into nanostructures is an attractive route for fabricating novel bio-inspired materials because of their capacity for molecular recognition and functional flexibility as well as the mild conditions required in the fabrication process. Among various peptide-based building blocks forming nanostructures, the simplest building blocks are aromatic dipeptides like diphenylalanine, which can readily self-assemble into nanotubes in aqueous solutions at ambient conditions. According to literature, the peptide nanotubes could be used in versatile applications for casting conducting metal nanowires, enhancing the sensitivity of electrochemical detection of biomolecules, and fabricating nano-fluidic channels or peptide liquid crystals. Although the self-assembly of peptides into nanostructured materials had been extensively studied, little progress had been made in the alignment and positioning of peptide nanostructures on a solid surface. Major obstacles include the complexity of current ‘solution-based’ approaches to peptide nanofabrication, causing dispersion and agglomeration problems, which also require the chemical modification of surface and peptide nanostructures. In the present study, we report a novel solid-phase growth of crystalline peptide nanowires at high temperatures driven by aniline vapor under anhydrous conditions. The formation of vertically well-aligned peptide nanowires on a solid surface were investigated through multiples tools, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), matrix-assisted laser desorption/ ionization time-of-flight (MALDI-TOF) mass spectrometry, and thermal analytical tools like the differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). We prepared an amorphous peptide thin film by drying a drop of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) solution containing diphenylalanine on a Si substrate. We conducted the experiment under strictly anhydrous conditions in a

High-Temperature Self-Assembly of Peptides into Vertically Well-Aligned Nanowires by Aniline Vapor**

We report the synthesis of novel diphenylalanine/cobalt(II,III) oxide (Co3O4) composite nanowires by peptide self-assembly. Peptide nanowires were prepared by treating amorphous diphenylalanine film with aniline vapor at an elevated temperature. They were hybridized with Co3O4 nanocrystals through the reduction of cobalt ions in an aqueous solution using sodium borohydride (NaBH4) without any complex processes such as heat treatment. The formation of peptide/Co3O4 composite nanowires was characterized using multiple tools, such as electron microscopies and elemental analysis, and their potential application as a negative electrode for Li-ion batteries was explored by constructing Swagelok-type cells with hybrid nanowires as a working electrode and examining their charge/discharge behavior. The present study provides a useful approach for the synthesis of functional metal oxide nanomaterials by demonstrating the feasibility of peptide/Co3O4 hybrid nanowires as an energy storage material.

Jungki Ryu

Papers

Mussel-Inspired Polydopamine Coating as a Universal Route to Hydroxyapatite Crystallization

General functionalization route for cell adhesion on non-wetting surfaces.

Metal ions differentially influence the aggregation and deposition of Alzheimer's beta-amyloid on a solid template.

High-Temperature Self-Assembly of Peptides into Vertically Well-Aligned Nanowires by Aniline Vapor**

Synthesis of Diphenylalanine/Cobalt Oxide Hybrid Nanowires and Their Application to Energy Storage