Phenolic materials have long been known for their use in inks, wood coatings, and leather tanning. However, there has recently been a renewed interest in engineering advanced materials from phenolic ...building blocks. The intrinsic properties of phenolic compounds, such as metal chelation, hydrogen bonding, pH responsiveness, redox potentials, radical scavenging, polymerization, and light absorbance, have made them a distinct class of structural motifs for the synthesis of functional materials. Materials prepared from phenolic compounds often retain many of these useful properties with synergistic effects in applications ranging from catalysis to biomedicine. This Review provides an overview of the diverse functional materials that can be prepared from natural and synthetic phenolic building blocks, as well as their applications.
Phenolics on the rise: Recent advances in the preparation of functional materials, namely thin films, particles, and bulk materials, based on phenolic building blocks and their diverse applications, ranging from biomedicine to catalysis, are highlighted. This Review will help integrate various disciplines that use phenols and serve as a guide for the development of future materials.
Conspectus Polyphenols are naturally occurring compounds that are ubiquitous in plants and display a spectrum of physical, chemical, and biological properties. For example, they are antioxidants, ...have therapeutic properties, absorb UV radiation, and complex with metal ions. Additionally, polyphenols display high adherence, which has been exploited for assembling nanostructured materials. We previously reviewed the assembly of different phenolic materials and their applications (Angew. Chem. Int. Ed. 2019, 58, 1904–1927); however, there is a need for a summary of the fundamental interactions that govern the assembly, stability, and function of polyphenol-based materials. A detailed understanding of interactions between polyphenols and various other building blocks will facilitate the rational design and assembly of advanced polyphenol particles for specific applications. This Account discusses how different interactions and bonding (i.e., hydrogen, π, hydrophobic, metal coordination, covalent, and electrostatic) can be leveraged to assemble and stabilize polyphenol-based particles for diverse applications. In polyphenol-mediated assembly strategies, the polyphenols typically exert more than one type of stabilizing attractive force. However, one interaction often dominates the assembly process and dictates the physicochemical behavior of the particles, which in turn influences potential applications. This Account is thus divided into sections that each focus on a key interaction with relevant examples of applications to highlight structure–function relationships. For example, metal coordination generally becomes weaker at lower pH, which makes it possible to engineer metal–phenolic materials with a pH-responsive disassembly profile suitable for drug delivery. Engineered particles, such as hollow capsules, mesoporous and core–shell particles, and self-assembled nanoparticles are some of the systems that are covered to highlight how polyphenols interact with other building blocks and therefore make up the major focus of this Account. Some of the applications of these materials exemplified in this Account include drug delivery, catalysis, environmental remediation, and forensics. Finally, a perspective is provided on the current challenges and trends in polyphenol-mediated particle assembly, and viable near-term strategies for further elucidating the interplay of various competing interactions in particle formation are discussed. This Account is also expected to serve as a reference to guide fundamental research and facilitate the rational design of polyphenol-based materials for diverse emerging applications.
Materials assembled by coordination interactions between naturally abundant polyphenols and metals are of interest for a wide range of applications, including crystallization, catalysis, and drug ...delivery. Such an interest has led to the development of thin films with tunable, dynamic properties, however, creating bulk materials remains a challenge. Reported here is a class of metallogels formed by direct gelation between inexpensive, naturally abundant tannic acid and group(IV) metal ions. The metallogels exhibit diverse properties, including self‐healing and transparency, and can be doped with various materials by in situ co‐gelation. The robustness and flexibility, combined with the ease, low cost, and scalability of the coordination‐driven assembly process make these metallogels potential candidates for chemical, biomedical, and environmental applications.
Ti‐tannic gels: The direct gelation of the natural polyphenol tannic acid, by coordination‐driven supramolecular assembly with titanium(IV), is reported. These metallogels exhibit a range of properties, including transparency, injectability, moldability, self‐healing, shape persistence, adhesiveness, and tunable mechanics.
We report the assembly of metal-polyphenol complex (MPC) films and capsules through the sequential deposition of iron(III) ions (Fe(III)) and a natural polyphenol, tannic acid (TA), driven by ...metal–ligand coordination. Stable Fe(III)/TA films and capsules were formed, indicating lateral and longitudinal cross-linking of TA by Fe(III) in the film structure. Quartz crystal microbalance, ultraviolet–visible (UV-vis) spectrophotometry, and X-ray photoelectron spectroscopy were carried out to quantitatively analyze the film growth. A comparison of the MPC capsules prepared through multistep assembly with those obtained through one-step deposition, as reported previously Ejima et al., Science 2013, 341, 154–156, reveals substantial differences in the nature of complexation and in their physicochemical properties, including permeability, stiffness, and degradability. This study highlights the importance of engineering MPC films with different properties through implementing different assembly methods.
It is well-understood that during the liquid-to-solid phase transition of alloys, elements segregate in the bulk phase with the formation of microstructures. In contrast, we show here that in a Bi-Ga ...alloy system, highly ordered nanopatterns emerge preferentially at the alloy surfaces during solidification. We observed a variety of transition, hybrid and crystal-defect-like patterns, in addition to lamellar and rod-like structures. Combining experiments and molecular dynamics simulations, we investigated the influence of the superficial Bi and Ga
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layers during surface solidification and elucidated the pattern-formation mechanisms, which involve surface-catalysed heterogeneous nucleation. We further demonstrated the dynamic nature and robustness of the phenomenon under different solidification conditions and for various alloy systems. The surface patterns we observed enable high-spatial-resolution nanoscale-infrared and surface-enhanced Raman mapping, which reveal promising potential for surface- and nanoscale-based applications.
Surface patterning of liquid metals (LMs) is a key processing step for LM‐based functional systems. Current patterning methods are substrate specific and largely suffer from undesired ...imperfections—restricting their widespread applications. Inspired by the universal catechol adhesion chemistry observed in nature, LM inks stabilized by the assembly of a naturally abundant polyphenol, tannic acid, has been developed. The intrinsic adhesive properties of tannic acid containing multiple catechol/gallol groups, allow the inks to be applied to a variety of substrates ranging from flexible to rigid, metallic to plastics and flat to curved, even using a ballpoint pen. This method can be further extended from hand‐written texts to complex conductive patterns using an automated setup. In addition, capacitive touch and hazardous heavy metal ion sensors have been patterned, leveraging from the synergistic combination of polyphenols and LMs. Overall, this strategy provides a unique platform to manipulate LMs from hand‐written pattern to complex designs onto the substrate of choice, that has remained challenging to achieve otherwise.
Assembly of a ubiquitous natural polyphenol, tannic acid, allows the preparation of a new class of adhesive liquid metal inks and their utilization in the direct pen writing and patterning of liquid metals in a substrate‐independent manner—an important feature that has remained challenging using the current methods for liquid metal based conductive patterning.
Gut microbiota dynamically participate in diverse physiological activities with direct impact on the host’s health. A range of factors associated with the highly complex intestinal flora ecosystem ...poses challenges in regulating the homeostasis of microbiota. The consumption of live probiotic bacteria, in principle, can address these challenges and confer health benefits. In this context, one of the major problems is ensuring the survival of probiotic cells when faced with physical and chemical assaults during their intake and subsequent gastrointestinal passage to the gut. Advances in the field have focused on improving conventional encapsulation techniques in the microscale to achieve high cell viability, gastric and temperature resistance, and longer shelf lives. However, these microencapsulation approaches are known to have limitations with possible difficulties in clinical translation. In this Perspective, we present a brief overview of the current progress of different probiotic encapsulation methods and highlight the contemporary and emerging single-cell encapsulation strategies using nanocoatings for individual probiotic cells. Finally, we discuss the relative advantages of various nanoencapsulation approaches and the future trend toward developing coated probiotics with advanced features and health benefits.
The use of supramolecular gel media for the crystallization of active pharmaceutical ingredients (APIs) is of interest for controlling crystal size, morphology, and polymorphism, as these features ...determine the performance of pharmaceutical formulations. In contrast to supramolecular systems prepared from synthetic gelators, herein, supramolecular metallogels based on a natural polyphenol (tannic acid) are used for the crystallization of APIs. The gel‐grown API crystals exhibit considerable differences in size, morphology, and polymorphism when compared with those formed in solutions. These physical features can also be tailored by varying the gel composition and additives, suggesting an influence of the gel medium on the crystallization outcomes. Furthermore, these gel–API crystal composites can be used for sustained drug release, indicating their potential as drug delivery systems. The facile preparation of these supramolecular gels and the use of naturally abundant components in their synthesis provide a generic platform for studying gel‐mediated crystallization of diverse APIs.
Simple yet elegant, the controlled crystallization of active pharmaceutical ingredients (APIs) in supramolecular metallogel media using a natural polyphenol, tannic acid, is reported. The presented system possesses inherent advantages, including ease of preparation, and the use of inexpensive components and the ability to incorporate various additives, thus providing a generic platform to study the crystallization of diverse APIs.
Hybrid conformal coatings, such as metal–phenolic networks (MPNs) that are constructed from the coordination-driven assembly of natural phenolic ligands, are of interest in areas including ...biomedicine, separations, and energy. To date, most MPN coatings have been prepared by immersing substrates in solutions containing the phenolic ligands and metal ions, which is a suitable method for coating small or flexible objects. In contrast, more industrially relevant methods for coating and patterning large substrates, such as spray assembly, have been explored to a lesser extent toward the fabrication of MPNs, particularly regarding the effect of process variables on MPN growth. Herein, a spray assembly method was used to fabricate MPN coatings with various phenolic building blocks and metal ions and their formation and patterning were explored for different applications. Different process parameters including solvent, pH, and metal–ligand pair allowed for control over the film properties such as thickness and roughness. On the basis of these investigations, a potential route for the formation of spray-assembled MPN films was proposed. Conditions favoring the formation of bis complexes could produce thicker coatings than those favoring the formation of mono or tris complexes. Finally, the spray-assembled MPNs were used to generate superhydrophilic membranes for oil–water separation and colorless films for UV shielding. The present study provides insights into the chemistry of MPN assembly and holds promise for advancing the fabrication of multifunctional hybrid materials.
The use of natural compounds for preparing hybrid molecular films—such as surface coatings made from metal–phenolic networks (MPNs)—is of interest in areas ranging from catalysis and separations to ...biomedicine. However, to date, the film growth of MPNs has been observed to proceed in discrete steps (≈10 nm per step) where the coordination‐driven interfacial assembly ceases beyond a finite time (≈1 min). Here, it is demonstrated that the assembly process for MPNs can be modulated from discrete to continuous by utilizing solid‐state reactants (i.e., rusted iron objects). Gallic acid etches iron from rust and produces chelate complexes in solution that continuously assemble at the interface of solid substrates dispersed in the system. The result is stable, continuous growth of MPN films. The presented double dynamic process—that is, etching and self‐assembly—provides new insights into the chemistry of MPN assembly while enabling control over the MPN film thickness by simply varying the reaction time.
Don't throw away your rusted objects! Rust can do wonders. A continuous assembly approach for the fabrication of metal–phenolic networks (MPNs) with tailored thicknesses using rusted objects as solid‐state iron sources is reported. Using this strategy, it is demonstrated that the assembly process for MPNs can be modulated from discrete to continuous.
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