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.

Phenolic materials have long been known for their use in inks, wood coatings, and leather tanning. However, recently there has 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. The present review provides an overview of the diverse functional materials that can be prepared from phenolic building blocks, bridging the various fields currently studying and using phenolic compounds. Natural and synthetic phenolic compounds are first discussed, followed by the assembly of functional materials. The engineered phenolic materials are grouped under three broad categories: thin films (e.g., metal-phenolic networks and polydopamine); particles (e.g., metal-organic frameworks and superstructures); and bulk materials (e.g., gels). Applications of phenolic-based materials are then presented, focusing mainly on mechanical (e.g., adhesives), biological (e.g., drug delivery), and environmental and energy (e.g., separation and catalysis) applications. Several examples of their emerging applications are also included. Finally, potential routes for the continued integration of disparate fields using phenolic building blocks and fundamental questions still requiring investigation are highlighted, and an outlook of the field is provided.

Coordination chemistry of natural polyphenols and transition metals allows rapid self-assembly of conformal coatings on diverse substrates. Herein, we report that this coordination-driven self-assembly process applies to simple phenolic molecules with monotopic or ditopic chelating sites (as opposed to macromolecular, multitopic polyphenols), leading to surface-confined amorphous films upon metal coordination. Films fabricated from gallic acid, pyrogallol, and pyrocatechol, which are the major monomeric building blocks of polyphenols, have been studied in detail. Pyrocatechol, with one vicinal diol group (i.e., bidentate), has been observed to be the limiting case for such assembly. This study expands the toolbox of available phenolic ligands for the formation of surface-confined amorphous films, which may find application in catalysis, energy, optoelectronics, and the biomedical sciences. ■ INTRODUCTIONModular control over the rational design of supramolecular architectures has been achieved in the last two decades by smart engineering of coordination-driven self-assembly processes. 1 Early prediction of the inherent preferences for directionality and binding affinity within the complementary building blocks of coordination complexes has paved the way for fabricating structures with extended networks of metal clusters bridged by compatible organic ligands. 2,3 Porous coordination polymers or metal−organic frameworks (MOFs) with distinct spatial and geometrical arrangements of the interconnecting motifs are examples of such organic−inorganic hybrid materials. 4−7 These crystalline materials with structurally encoded nano-and microporosities have potential application for gas storage, separations, and sensing. 8−13 On the other hand, surface-bound or freestanding amorphous thin films/coatings are another class of network materials of importance in several branches of science, 14−16 where polymeric compounds are commonly used structural components. Research has also focused on exploring novel strategies to incorporate inorganic moieties in polymeric films to obtain functional hybrid materials that exploit the synergistic effects of the organic and inorganic constituents. 17,18 In this context, processes utilizing self-assembly of coordination complexes are a promising strategy toward facile engineering of thin films with defined properties.Recently, we reported a facile assembly approach that exploits metal−polyphenol interactions, specifically between tannic acid (TA) and iron(III) (Fe III ) ions, to form thin films. 19 Our interest in these metal−polyphenol systems arises from the facile and versatile nature of the assembly process, which produces tunable, dynamic materials. Using TA as a ligand, we demonstrated the formation of capsules with engineered pHresponsive degradation, luminescence, and positron emission, by judicious choice of the incorporated metal, 20 as well as pHresponsive drug delivery vectors 21 and cytoprotective coatings. 22 Furthermore, we reported the assembly of Fe IIIpolyphenol capsules fro...

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 polyphenolbased materials. A detailed understanding of interactions between polyphenols and various other Association for the Advancement of Science. (c) Reproduced with permission from ref. 16.

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.

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.

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The nanoengineering of particles is of interest for both fundamental and applied science. How particles are made substantially affects their properties and quality, and therefore usefulness. Disseminating current understanding of particle engineering can help facilitate the use of existing technologies, as well as guide future developments. Herein, we describe three methods used in our laboratory for the nanoengineering of particles, based on templated assembly, and discuss important considerations for each. Firstly, we describe the use of layer-by-layer assembly for depositing multilayered nanofilms on particle surfaces to generate core-shell particles and hollow capsules. Secondly, we detail the use of mesoporous silica templating for the engineering of porous polymer replica particles. Thirdly, we describe how the coordination of phenolic compounds and metal ions can be used to fabricate thin films via metal-phenolic network formation on particle templates. We provide stepwise, easy-to-follow guides for each method, and discuss commonly encountered challenges and obstacles, with considerations for how to alter these protocols to achieve desired particle properties. While we intend for these guides to be easily accessible to researchers new to particle engineering, we believe they can also provide useful insight to all researchers working in the field of engineering advanced particles.