Supramolecular design and applications of polyphenol-based architecture: A review

Advances in Colloid and Interface Science 272 (2019) 102019

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Historical perspective

Supramolecular design and applications of polyphenol-based architecture: A review Hongshan Liang a,c, Bin Zhou b, Di Wu a,c, Jing Li a,c, Bin Li a,c,d,⁎ a

College of Food Science and Technology, Huazhong Agricultural University, Wuhan 430070, China School of Food and Biological Engineering, Hubei University of Technology, Wuhan 430068, China Key Laboratory of Environment Correlative Dietology, Huazhong Agricultural University, Ministry of Education, China d Functional Food Engineering & Technology Research Center of Hubei Province, China b c

a r t i c l e

i n f o

Article history: 5 July 2019 Available online 13 August 2019 Keywords: Coordination bonding Hydrogen bonded Electrostatic interaction Self-polymerizing

a b s t r a c t Polyphenol-based materials are of wide-spread interest because of the unique properties of the polyphenol itself. Tannic acid, contains high level of galloyl groups, could be coordinated to a range of metal ions to generate robust mental ion-TA films on substrate or even forming hollow capsules. These films or capsules can be used in the field of sensing, separation and catalysis, most importantly in drug/nutraceutical delivery, allowing for the high loading efficiency, high mechanical and thermal stability, pH-responsive disassembly and fluorescence behavior. Additionally, such coating could also provide protection of the sensitive molecules and cells. With the numerous carbonyl and phenolic functional groups, TA has also been demonstrated to form strong hydrogen bonded multilayers with various non-ionic polymers. The properties of the hydrogen-bonded system were highly influenced by the chemical structure of the polymers, which will change the behavior of pH-, temperature- or ionic strength-responsive release of the loading molecules. Additionally, the ionization of galloyl phenol group was attributed to the interaction between TA and other ionic polymers by electrostatic interaction. The electrostatic interaction/hydrogen bonding derived TA/polyme$$%r complexes could deposit on glass slides, microcores or even forming hollow capsules, promising in their applicability to nutraceutical encapsulation, delivery and depot. Notably, polyphenols self-polymerizing could also deposit coatings on different substrates without any exogenous additives, while the comprehensive undertanding about the selfpolymerizing mechenism remains unclear. This review provides a promising prospect for utilizing polyphenol-based materials to design versatile architecture in different system, used in the field of chemistry and materials science. © 2019 Published by Elsevier B.V.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coordination-mediated metal-TA network . . . . . . . . . . . . . . . . . . . . 2.1. Feasibility of coordination bonding architecture . . . . . . . . . . . . . . . 2.2. Deposition on different substrates . . . . . . . . . . . . . . . . . . . . . 2.3. Applications of metal-TA network . . . . . . . . . . . . . . . . . . . . . 2.3.1. Application in drug delivery . . . . . . . . . . . . . . . . . . . 2.3.2. Application in biomedical imaging. . . . . . . . . . . . . . . . . 2.3.3. Application in catalysis . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Other applications . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen bonding-based polymer-TA assembly . . . . . . . . . . . . . . . . . . 3.1. Effect of chemical structure on hydrogen bonding-based polymer-TA assembly 3.2. Applications of hydrogen bonding-based polymer-TA assembly. . . . . . . . 3.3. Electrostatic-based assembly of TA with ionic polymers . . . . . . . . . . . 3.4. Self-polymerizing of polyphenols . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding author at: College of Food Science and Technology, Huazhong Agricultural University, Wuhan 430070, China. E-mail address: [email protected] (B. Li).

https://doi.org/10.1016/j.cis.2019.102019 0001-8686/© 2019 Published by Elsevier B.V.

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4. Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction Plant polyphenols are widely distributed in plant tissues and plays a critical role in a range of biological functions such as photosynthesis, structural support, oxygen transportation and adhesion [1,2]. With the high content of dihydroxyphenyl (catechol) and trihydroxyphenyl (gallic acid), plant polyphenols display versatile physical and chemical properties, including absorption of UV radiation, radical scavenging, and metal ion complexation [2]. Significant interest has been given to building blocks based on these properties of polyphenols in the field of chemistry and materials science. For example, the polyphenols could be used in the context of surface modification, as catechols are known to deposit thin films on the surface of substrate through covalent and noncovalent interactions [3]. Specifically, polyphenols can assemble into functional materials with enhanced mechanical/thermal stability and stimuli-responsiveness [4]. Members of this large family of compounds, tannic acid (TA), contains five digalloyl ester groups covalently attached to a central glucose core, is supported as a unique plant polyphenol [5]. This hydrolyzable tannin provide diverse functional properties, used as an antioxidant, anticarcinogenic, antimicrobial agent [6,7]. Except for the bioactive properties, TA is commonly used in materials engineering due to special structural properties that facilitate interactions with a variety of materials via multiple reaction pathways, including coordination bonding, electrostatic, hydrogen bonding, and hydrophobic interactions [8–12]. With the development of versatile materials engineering strategies, supramolecular thin films, stimuli-responsive nano- or microparticles, multifunctional hydrogel and the like, exhibit a promising prospect in advanced materials design and applications. Polymers pair with complementary functional groups have been widely used to fabricate thin films or decorate the formed substrates to impact additional properties, demonstrating potential for sensing, separation and catalysis [13–15]. Polymers themselves could also assemble facile and versatile microcapsules for biomedical and environmental applications. Additionally, the assembly of polymers into hollow capsules has recently attracted interest because of their desirable properties, including high loading efficiency, high mechanical and thermal stability, and pHresponsive disassembly [16]. Also the hollow capsules can be used for protection of sensitive bioactives, cells, and bacteria [16,17]. Although polydopamine (pDA) has been widely studies for its preferred surface deposition property, the high costs of pDA and the specific dark colour of pDA coatings may limit its applications in many fields [18]. Inspired by the finding that thin polyphenol coatings form spontaneously on surfaces exposed to these polyphenol-rich beverages, plant polyphenols could deposit on different substrates as monolayer adsorbates or as ingredients in multicomponent coatings [2,19–21]. As a kind of low-cost plant polyphenols, TA has a high surface bonding affinity which can be easily deposited on the surface of the substrates, used as a polyphenolic drug or an agent to further interact with other components. It has reported TA could deposit multifunctional coatings onto the surface of many organic and inorganic substrates to improve their surface properties [22,23]. Specially, TA could form assembly with other polymers, giving rise to broad applications in materials design. Herein, giving a summary of TA-based materials will provide a comprehensive reference for designing versatile architecture in different system. In this highlight, we focus on the interactions between TA and other molecular, emphasizing on tailoring the properties of multilayer capsules or films on different substrates based on coordination bonding, hydrogen bonding, electrostatic and covalent interactions. Knowledge

of the mechanism of such materials that dominates the efficacious engineering strategies will help us to understand experimental observations and, most importantly, to design safe, multi-functional systems intended for various applications. 2. Coordination-mediated metal-TA network 2.1. Feasibility of coordination bonding architecture Advances in design of supramolecular architectures highly promote the smart engineering of versatile self-assembly processes in the last two decades. The exploitation of organic and inorganic building blocks to prepare hybrid materials has attracted widespread interest due to the coupled benefits of inorganic and organic components [24,25]. Among the organic-inorganic hybrid materials, metal-organic coordination materials (MOCMs) are an important class, in which metal ions are linked together by compatible organic bridging ligands [26]. Considerable effort has been put into the design and synthesis of such MOCMs, because of their stimuli responsiveness to the environment, combined beneficial physicochemical properties of both metals and organic materials, diverse functionality achieved by changing the supramolecular architectures. So the MOCMs provide new opportunities for engineering multifunctional systems for applications in separation, heterogeneous catalysis, analysis, drug delivery, etc. [25]. Different polymers were chose as the ligand to coordinate with metal ions to form coordination complexes, such as aminofunctionalized mesoporous silica [27], chitosan [28], catechol [29], anticancer drugs (daunorubicin hydrochloride [30], mitoxantrone [31], etc) and others [32]. Such non-covalent coordination systems have attracted widespread interest due to their desirable properties [33,34], including selective permeability [34], high mechanical [35,36] and thermal stability [37]. The formation and cleavage of metal ion-ligand coordination bonds are sensitive to external pH variations which also impact the pH-responsive disassembly of the formulations [38–40]. The main mechanism involved is that both metal ions and protons are Lewis acids and compete to combine with the ligand, which is a Lewis base [27,31,41]. Che et al. developed novel coordination bonding systems in functionalized mesoporous materials for the pH-responsive drug delivery [27,28,42–44]. In these systems, the coordination bonding was based on “host-metal-drug” architecture. Here host represented functional groups of the carriers. The coordination anticancer drugs can be released form the system by the cleavage of either the “host-metal” or the “metal-drug” coordinate bonding which response to pH variations (Fig. 1). Additionally, the drug release rate was significantly affect by the functional group of mesoporous materials, metal sources and the structure of loading drug, due to the different coordination bonding strengths of the architectures [45]. Plant polyphenols is an important class to act as ligands for coordination architecture. Generally, the catechol or galloyl groups in phenolic compounds provide binding sites for metal ions to chelate [46,47]. For TA, the metal chelation is a salient feature, upon which it can act as a polydentate ligand for metal ion coordination. Therefore, TA, which contains high level of galloyl groups, can form highly stable complexes with different mental ions. As illustrated in Fig. 2a, TA could be coordinated to 18 different metal ions to generate robust mental ion-TA films on substrate [1]. Mental ion-TA films formed just upon mixing TA and metal ions in water at ambient temperature. It was reported that after remove the template metal ion-TA covered, metal ion-TA network films could form hollow capsules. The thickness and stability of these mental

H. Liang et al. / Advances in Colloid and Interface Science 272 (2019) 102019

Fig. 1. Schematic mechanism of the release of guest molecules by cleavage of coordination bonds in pH-responsive systems depending on the relative strength of the “host–metal” and “metal–guest” interactions. Reproduced with permission from Ref. [45]. Copyright 2013, Coordination Chemical Reviews.

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ion-TA capsules are dependent on the spices of metal ion used as well as the metal ion feed concentrations [36]. When FeIII was chose as the linkage center, at pH N7, three galloyl groups from TA can react with each FeIII ion to form a stable octahedral complex. When decrease the pH value to the range of 3–6, two galloyl groups were coordinated to each FeIII ion. At pH b 2, only one galloyl group interacted with FeIII ion (Fig. 2b), so the coordination bonding between metal ions and TA is pHdependent. Moreover, the molar ratios of TA to FeIII ion will highly affect the film thickness and morphology of the capsules. At molar ratios of 1:1 to 3:1 between FeIII and TA, it was possible to form stable capsules. As the feeding concentration of FeIII ion increased, the thickness of the capsules strengthened. Above this ratio range, more capsules will aggregate, leading to the grainy surface. Below the ratio of 1:1, only few capsules could be formed. Different metal ion chose resulted in different pH-disassembly kinetics of the metal-TA capsules. When AlIII was chose as the center, as the pH decreased from 7.4 to 5.0, AlIII-TA capsules disassembled gradually. Below the pH 5, the speed of disassembly increased dramatically. This disassembly kinetics for the AlIII-TA capsules has a promising application in cancer drug delivery, as the capsules are relatively stable at the pH of the blood-stream and gradually disassembled at lower pH value, adapted their properties as a function of pH environments [1]. In Fig. 2c, it illustrated the pH-disassembly kinetics of metal-TA capsules prepared from three kinds of model metals.

Fig. 2. (a) Assembly of metal-TA network (MPN) capsules from 18 kinds of metals; Scale bars are 5 mm. Inset images are photographs of MPN capsule suspensions. (b) pH-dependent transition of dominant FeIII-TA complexation state; R represents the remainder of the TA molecule. (c) pH-dependent disassembly kinetics of CuII-TA, AlIII-TA and ZrIV-TA capsules at different pH values, as assessed by flow cytometry. The results were displayed as the mean ± standard deviation (n = 3). Reproduced with permission from Ref. [1]. Copyright 2014, Angewandte Chemie International Edition.

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Fig. 3. Assembly of TA and metal ions to form a MPN films on a particulate template, followed by the subsequent formation of a metal-TA network capsule after removal of the template. Reproduced with permission from Ref. [1]. Copyright 2014, Angewandte Chemie International Edition.

2.2. Deposition on different substrates Catechol-functionalized molecules and their derivatives perform well as binding agents for coating on a wide variety of inorganic and organic substrates with different surface properties [13,14,48]. Simple immersion of substrates in an aqueous solution of TA resulted in adsorption of TA onto the template surface [23,49], followed by spontaneous deposition of a thin adherent polymer film after cross-linked by metal ion complexation (Fig. 3) [50,51]. Ejima reported that TA and FeIII could assemble coordination complexes on a range of substrates regardless of the materials, shape or surface charge of the substrates [36]. The Energy-dispersive X-ray mapping images illustrated that the distribution patterns of metal, C and O maps matched well with high-angle annular dark-field images, which indicated that metal and TA was well distributed on the surface of the substrates. The material-independent surface coatings have been used to deposit on many inorganic materials such as silicon substrate [52–54], calcium carbonate [55], single cell [4] and Au nanoparticles (NPs) [56]. Moreover, it has proved this metal-TA coating could deposit on organic surfaces, including the polymer fiber and protein or polysaccharide NPs [11,57–59].

2.3. Applications of metal-TA network Except for the pH-responsive disassembly, the metal-polyphenolic coatings show great promise for a wide range of applications in the field of drug delivery, protection of the loadings, catalysis, etc. [60],

motivated by the many diverse functions of plant polyphenols and metal ions in nature.

2.3.1. Application in drug delivery Hollow polymeric capsules, a kind of delivery system, have been extensively studied due to the high loading efficiency, controllable and multi-functional properties of build-up materials [60–63]. One prominent example for hollow capsules preparation is described by the assembly of metal-polyphenolic on different substrates, followed by the removal of the templates [55]. As reported, AlIII-TA coordination films were rapidly coated on the calcium carbonate (CaCO3) templates in pH 8.0 buffer solution. Before this coating step, CaCO3 firstly deposited large amounts of doxorubicin hydrochloride (DOX) into the porous interior of templates. The hollow capsules can be obtained after the removal of CaCO3 templates. Duo to the pH-dependent disassembly of complexes between metal ions and TA, DOX-loaded AlIII-TA capsules exhibit pH-disassembly kinetics as the pH decreased from 7.4 to 5.0 [55]. As known, the pH environment between extracellular fluids and tumor tissues or inside endosomal and lysosomal ompartments is pH ≈ 7.4 and pH 6.8–4.5 [64–66]. So this kind of hollow capsules illustrated enhanced intracellular release of cargo while staying stable under physiological conditions, which corresponded to a desirable profile for drug delivery. Metal-polyphenol coordination complexes can form controllable films on the surface of inorganic templates or form hollow capsules to impact a pH-responsive release of the cargo. Recently, we reported a new pH-responsive release system based on metal-TA architecture on

Fig. 4. (a), Illustration of the synthesis and structures of DOX-loaded zein NPs coated by metal-TA films and the proposed model for pH-dependent drug release in intracellular delivery processing. (b) and (c), TEM image and size distribution of zein NPs and zein-TA/CuII NPs respectively. In vitro release profiles of DOX-loaded zein NPs (d) and zein-TA/CuII NPs (e) in buffer solution under different pH conditions. Data displayed as mean ± SD (n = 3). Reproduced with permission from Ref. [11]. Copyright 2015, Colloids and Surface B: Biointerfaces.

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organic NPs [11]. This delivery system was fabricated by first adsorption of the polyphenol on zein NPs, followed by the addition of metal ion solution and directed by the formed pH-dependent, multivalent coordination bonding (Fig. 4a). The introduction of these metal-polyphenol films impacted controllable pH-dependent release of the loaded guest molecules and the release behavior was tailored by the feeding metal ion species (Fig. 4b–e). Additionally, the prepared metal-TA coated zein NPs demonstrated good stability to maintain particle size in cell culture medium compared with zein NPs. Due to the reducing property, TA on the surface of zein NPs could reduce Au3+ ions to AuNPs which may provide greatly potential in the photothermal therapy of cancers and other diseases. The nano- or micro-carriers are highly affected by the surface properties when entering the body environment, such as the blood, protein-rich environments [67–69]. As reported, serum proteins could interact with the carrier and form a protein corona, which affect the cellular uptake of the carrier or even mask the targeted ability [70]. The metal-TA coatings also could exhibit strong nonspecific interactions with biological systems [71–73], limited the application in long blood circulation system. The incorporation of polyethylene glycol (PEG) into the delivery system could suppress protein adsorption [74–77]. So the researchers functionalized TA with PEG to form PEG-polyphenol conjugate and then assembled by adding metal ion into the PEG-polyphenol solution [78]. The introduction of PEG did not affect the pHdegradability of capsules. Moreover, PEG-functionalized capsules exhibited higher resistance to nonspecific protein adsorption and cellular association. Caruso et.al reported a kind of hyaluronic acid (HA)-based metal-phenolic capsules for improving the

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targeting specificity of the system [79]. The conjugate of high molecular HA into TA was negligibly affected by the presence of protein coronas leading to significant targeting specificity. Based on these studies, their group further prepared phenolicfunctionalized HA and PEG coatings cross-linked by metal ions on CaCO3 templates shown in Fig. 5a [80]. The introduction of HA enhanced the binding and association of the functionalized capsules towards its primary receptor in MAD-MB-231 cells and the incorporation of PEG dramatically minimized nonspecific interactions with BT-474 cells (Fig. 5b, c). 2.3.2. Application in biomedical imaging In order to impart imaging properties to metal-TA capsules, lanthanide ions such as EuIII and TbIII were introduced into this system [81–83]. 2-thenoyltrifluoroacetone (TTA) and acetylacetone (AA) always used as a coligand, serve to facilitate intramolecular energy transfer from coligands to lanthanide ions and excite state stimulating metal centered luminescence [84,85]. The fluorescence photographs of EuIIITTA-TA and TbIII-AA-TA capsules suspension can be seen in Fig. 6a. In our group, we also implied EuIII-TTA-TA complexes into our prepared NPs (Fig. 6b). After incubation with the coated NPs, the red fluorescence appearing in the cytoplasm and nuclei of HepG2 cells in Fig. 6c. The good fluorescence property of the coated NPs, make it highly promising for use in biomedical imaging [86]. Caruso group also incorporated radioactive 64CuII into metal-TA capsules for positron emission tomography (PET) and magnetic resonance imaging [87] [1]. The results illustrated that 64CuII-TA hollow capsules or 64 CuII-TA film coated NPs are efficient PET-active vehicles, and used as probe for tracking the biodistribution of the carrier.

Fig. 5. (a) Schematic illustration of assembled capsules based on HA-PEG conjugated MPN with different HA/PEG ratios to a suspension of CaCO3 particles, followed by dissolving the templates. (b) Deconvolution microscopy images of MPNHA-PEG-III capsules incubated with MDA-MB-231 cells and BT-474 cells in complete media with 10% FBS at 37 °C for 1, 4, 8, and 12 h. Cell nuclei and membrane were stained with Hoechst 33342 (blue) and WGA-594 (red), respectively. MPNHA-PEG-III capsules were fluorescently labeled with AF48. Scale bars are 10 μm. (c) Percentage of cells associated with MPNHA-PEG-III capsules after different incubation times in complete media (mean ± SD, n = 6). The capsule-to-cell ratio was set to 100:1 for all cell experiments. Reproduced with permission from Ref. [80]. Copyright 2016, Biomacromolecules.

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Fig. 6. (a) Normalized fluorescence spectra of capsule suspensions of EuIII-TTA-TA and TbIII-AA-TA excited at 360 nm. Insets are photographs of the corresponding capsule suspensions, excited at 254 and 365 nm. Scale bars are 1 cm. (b) Fluorescence spectra and (c) photographs of the corresponding suspensions of EuIII-TTA-TA films coated zein/HTCC NPs excited at 360 nm with different EuIII ions concentration. (d) CLSM images of intracellular uptake of EuIII 3 -TTA-TA films coated zein/HTCC NPs by HepG2 cells. Cells were counter-stained with DAPI for nuclei. The scale bars represent 20 μm. Reproduced with permission from Ref. [1]. Copyright 2014, Angewandte Chemie International Edition; Ref. [87]. Copyright 2016, ACS Biomaterials Science & Engineering.

2.3.3. Application in catalysis Noble metal NPs have been widely used in the field of heterogeneous catalysis duo to these large specific surface area and more importantly increased exposed metal atoms [88,89]. However, the tendency to aggregate may lead to the loss of their initial activities [90–93]. The introduction of shell onto the noble metal core surface could hinder the aggregation of encapsulated cores [94,95]. Additionally, the unique property of the shell may increase the catalytic activity [20,96,97]. Cai et.al introduced a new controllable supramolecular shell based on metal-TA architecture onto gold NPs [56]. Duo to the high surface binding affinity, TA can easily absorb onto Au NPs surface. The catalytic activity of Au@TA-Fe composites was significantly enhanced attributed to the interaction of the reaction substrate and TA. The pH-dependent coordination bonding between TA and metal ions endowed different catalytic activity of Au@TA-Fe composites in different pH conditions. Compared with bare Au NPs, the Au@TA-Fe composites performed good reusability. This innovative approach based on metal-phenolic films with unique characteristics can be used in the synthesis of other core-shell nanosystems for various applications.

2.3.4. Other applications As known, metal-TA complexes could form shell around the prepared NPs to impact functional properties of the system. It has reported that the incorporation of these non-covalent coordination complexes could also be used for cell-encapsulation (Fig. 7a) [4]. This tough and compatible shell provided a physical barrier to the biological actions of the encapsulated cells, increasing the in vitro stability of cells (Fig. 7b and c). Additionally, these metal-organic films on individual yeast could response to external stimuli and be degraded under cytocompatible conditions. Jiang et al. rendered a simple and convenient approach for the fabrication of coordination-enabled synthesis of TA-TiIV ultrathin

microcapsules with weak pH-response. The prepared TA-TiIV microcapsules exhibited high stability in a broad range of pH values, preserved their intact wall structure under extreme acidic/alkaline conditions. In particular, these microcapsules utilized for enzyme immobilization displayed the high catalytic performance, especially the superior pH and thermal stabilities [98].

3. Hydrogen bonding-based polymer-TA assembly Most interests are focus on utilizing the unique properties of TA to build up architectures. Just as aforementioned, these systems are based on the preferred bonding affinity and metal ion chelation ability. However, materials based on hydrogen bonding between TA and polymers presents new opportunities in biomedical and bioengineering fields since it has been considered to be response to small external stimuli, namely temperature, pH and ionic strength [99]. TA with the numerous carbonyl and phenolic functional groups has been demonstrated to be a key component for binding with various biomolecules based on hydrogen bonding [19,100]. It can act as excellent H donors to form strong hydrogen bonded stable assemblies with compounds containing carbonyl groups without the need for chemical crosslinking. The high pKa value (−8.5) of TA makes the hydrogenbonded system persist at acidic and neutral pH values. In addition, the properties of TA/polymer based hydrogen-bonded system, e.g., disintegration pH range, can be tailored by the nature of the components and by employing various processing methods [101]. Previously, most of hydrogen-bonded system suffered from the problems of unstability and easy dissolution under physiological conditions, resulting from the responsiveness in biologically and physiologically relevant pH range in different environmental conditions [102,103]. Non-ionic polymers such as poly(ethylene oxide) (PEO), poly(N-vinylpyrrolidone) (PVPON), poly(N-vinylamide) derivatives, PEG and the like are crucial for use in building up hydrogen-bonded

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Fig. 7. (a) Schematic representation for controlled formation and degradation of the TA-FeIII shell on individual S. cerevisiae. (b) Enhanced tolerance of yeast@TA-FeIII against UV-C tolerance. (c) Cytoprotection ability of the TA-FeIII nanoshell against silver NPs with different sizes in diameter. Yeast@[TA-FeIII]4 was treated with FDA and propidium iodide (PI) for live/dead staining (green: live, red: dead). All scale bars are 20 mm. Reproduced with permission from Ref. [4]. Copyright 2014, Angewandte Chemie International Edition.

system, due to their uncharged biocompatible functional properties [104–108]. 3.1. Effect of chemical structure on hydrogen bonding-based polymer-TA assembly As known, the strength of hydrogen bonds determined by the chemical nature of the hydrogen accepting and donating polymers and in addition the processing conditions, controlled the pH- or ionic-stability [109,110]. Association of TA with polymers with different molecular arrangement in the repeating unit highly affects the construction and pHstability of hydrogen-bonded multilayers. Kharlampieva et al. reported that when composed of weakly associating polymer pairs including PEO, poly(vinyl methyl ether) (PVME) or poly(methacrylic acid) (PMAA), hydrogen-bonded films dissolved at lower pH values compared with the strongly associating polymer pairs, such as PVPON, poly(N-vinylcaprolactam) (PVCL) or PMAA [102,111]. Significance of the contribution of molecular weight of polymers was also demonstrated. Vladimir V and co-workers constructed hollow microcapsules with responsive properties on the base of TA combined with a range of neutral polymers, PVPON, poly(N-vinylcaprolactam) (PVCL) or poly (N-isopropylacrylamide) (PNIPAM). The results suggested that the strength of association between TA and a polymer and molecular weight of polymers highly affected the shell thickness of the produced capsules. At the tested molecular weight of polymers, there witnessed an increase in micro-roughness compared with common values for ionic-based films [112]. In addition, the deposition on different substrates may also affect the bilayer thickness. As reported earlier, hydrogen-bonded films adsorbed on planar substrates always resulted in slightly thicker films compared with particulate ones [113,114]. It is worth noting that these fabricated hydrogen-bonded TA/polymer shells exhibited a high pH-stability in the pH range from 2 to 10. The effect of different arrangement of atoms in a polymer repeating unit on the hydrogen-bonded association of polymers and TA was investigated by

Erel and co-workers. They reported different conformational behavior of poly(N-isopropylacrylamide) (PNIPAM) and poly(2-isopropyl-2oxazoline) (PIPOX) in water determined the thickness and stability of the formed multilayers in different pH conditions. The introducing amino groups at the PIPOX chain also shifted the critical dissolution pH to higher values and resulted in gradual dissolution of the films in a wide pH range of 9–12 [115]. Similarly, Takemoto et.al compared the hydrogen-bonding interactions of interpolymer complexes composed of nonionic and low toxicity alkylated poly(N-vinylamide)s and TA. Duo to the lower steric hindrance of formamide groups in poly(Nethyl-N-vinylformamide) (PNEF) compared with the acetamide groups in poly(N-methyl-N-vinylacetamide) (PNMA) (Fig. 8a and b), the stability of prepared multilayered films composed of PNEF and TA were stable in water, while those of PNMA and TA were unstable seen from Fig. 8c and d [108]. 3.2. Applications of hydrogen bonding-based polymer-TA assembly Multiple biolayers could form coatings on various cores or even form hollow microcapsules, capable of absorbing functional molecules such as dyes or drugs, leading to the pH-, temperature-, or ionic strength-responsive release of the loading molecules. TA/PVPON hydrogen-bonded multilayers or hollow capsules have been used for drug encapsulation and long-term storage reported by Kozlovskaya et al. [116]. In this work, the chemical and physical properties of TA hydrogen-bonded multilayer films were comprehensively investigated by varying fabrication conditions including changes in pH values and salt concentration. DOX, a module drug, can be successfully encapsulated within TA-PVPON capsules with a high loading capacity prepared at pH 7.4 (0.01 M). Additionally, the release of DOX showed a pHdependent manner (Fig. 9). Kharlampieva and co-workers reported a novel type of cytoprotective coatings designed from TA and PVPON [117]. Such hydrogen-bonded TA/PVPON hollow multilayer shells allowed conformal coatings on individual, living pancreatic islets,

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Fig. 8. Chemical structures of PNEF (a) and PNMA (b). QCM frequency shift during LbL assembly (n = 3). The PNEF (c) and PNMA (d) in acetonitrile and TA in acetonitrile were alternately assembled on a QCM substrate for a 12-step assembly at a concentration of 10 unit mM at 25 °C. The odd-numbered step is a PNEF or PNMA step (opened circle), and the even-numbered step is a TA step (closed circle). The QCM substrate was rinsed with water for 5 min (13th step), 5 min (14th step), 10 min (15th step), and 20 min (16th step) after 12-step assembly (gray circle). Reproduced with permission from Ref. [108]. Copyright 2015, Langmuir.

providing physical islet protection and preventing efficient autoreactive T cell responses. The proposed materials, further coupled with various active reagents (e.g. catalytic antioxidants, insulinotropic and immunoisolation moieties), offers new opportunities in the area of

advanced multifunctional materials. Other researcher also used neutral TA/PVPON multilayered films for living-cell coating to maintain high viability, because of its low toxicity and appropriate permeability of nutrients or inducer molecules [17,118].

Fig. 9. (a) Confocal microscopy images of (TA/PVPON)8 shells, (b) with encapsulated DOX and (c) their superimposed image. (d) pH-dependent release of DOX from PEI(TA/PVPON)8 capsules. Reproduced with permission from Ref. [117]. Copyright 2014, Soft Matter.

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In order to impact temperature-triggered “on-demand” release of drugs, the temperature-responsive polymer or block copolymer was introduced by da Fonseca Antunes, et al. [119] and by Zhu et al. [120]. Poly (2-(n-propyl)-2- oxazoline) (PnPropOx) and poly(N-vinylpyrrolidone)b-poly(N-isopropylacrylamide) (PVPON-b-PNIPAM) was used as the temperature-responsive part of the hydrogen bonded system, tailor the temperature-controlled on-off drug release. Zhang et.al used the layer-by-layer (LbL) assembled TA/PVPON films as a module delivery system for polyphenolic drug release [6]. The growth behavior of TA/PVPON films is temperature-dependent which showed relatively strong hydrogen bonding between PVPON and TA at a low temperature while partial breakage of the hydrogen bonds upon heating. Because of reversible/dynamic nature of hydrogen bonding, the release process of TA can be finely controlled by external conditions, such as temperature, pH values and ionic strength. The released TA retains its ability to scavenge harmful radicals. Subsequently, PEG, as hydrogen acceptor, was also bonded with polyphenolic drugs, as hydrogen donor, to form hydrogen-bonded films for sustained drug delivery [9]. The films fabricated from polyphenol and PEG demonstrated zero-order release of polyphenolic drugs, including TA, epigallocatechingallate, proantho cyanidinss, and theaflavin-3′-gallate. Hydrogen-bonded precipitated complexes of TA and PEG were also used to fabricate water-enabled self-healing coatings on various substrates explored by Du and coworkers. Such robust coatings were obtained after drying, yet swollen and softened under water or in moist environment (shown in Fig. 10), resulting in repeatedly healing capacity via rebuilding hydrogen bonds between TA and PEG. TA, capturing reactive oxygen species, also endowed the coatings with high antioxidant capacities [121]. In our group, we first prepared zein/TA colloidal NPs and TA on the surface of the colloidal particles were hydrogen-bonded with PEG to form hydrogen-bonded coatings [7]. Here TA was used as the hydrogen donor, which could interact with the ether group (–O–) of PEG. The fabricated dynamic hydrogen bonding between TA and PEG tailored the stimuli-responsive release of TA. Because of the hydrophobic property of zein, hydrophobic, unstable compound could encapsulate into zein NPs first, and then coated with TA/PEG coatings, achieving the controlled release of the loading compound, as well as providing efficient protection against UV light. So the TA/polymer films could not only be used as a reservoir for the active therapeutic cargo but also a coating to modulate surface, response to small external stimuli. Other polymers such as methylcellulose

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[122,123], BSA [124] could also form hollow capsules with TA based on hydrogen bonding for sustained drug release and encapsulation and release of fragrance respectively. 3.3. Electrostatic-based assembly of TA with ionic polymers The phenolic groups of TA were response for its acidic properties, which, as a weak polyacid, could form strong hydrogen bonds with compounds. In addition, the ionization of galloyl phenol group was attributed to the interaction between TA and other ionic polymers by electrostatic interaction [125]. Taking into account the properties of TA, many efforts have focused on using the negatively charged structural blocks for LBL electrostatic assembly with positive polyions [126–129]. Shutava chose two different polycations, strong poly(dimethyldiallylamide) (PDDA) and weak poly (allylamine) (PAH) to assemble with TA for alternative deposition on glass slides, MnCO3 microcores or even forming empty capsules. PAH with primary amine groups can form hydrogen bonds with phenolic rings of TA while formation of such bonds was not possible in the case of PDDA, leading to the slightly higher amount of polyphenol deposited on PAH layers [130]. The formed capsules or films based on TA/ polycation complexes showed pH-dependent permeability, extending possible applications of multilayered polyelectrolyte membranes in controlled drug release systems. Additionally, this work provided insight into designation of TA-based assemblies coupled with positively charged natural polyelectrolytes with primary amine groups. In the following, their group used TA assembled alternatively with chitosan deposited on flat supports [131]. For hollow TA/chitosan capsules, the pH-permeability threshold lowered down to pH 5, as compared with polyallylamine/polystyrene sulfonate microcapsules. So the prepared biocompatible TA/chitosan films or microcapsules have advantages in drug encapsulation, delivery and depot systems. After that, their group further studied the reaction of radical cations of 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+·) with PAH/TA multilayers [132]. ABTS+· interacted with TA started from the surface of the multilayer PAH/TA films or capsules and gradually propagated deep into the film. So these TA containing films or capsules could protect the loading compound from the action of free radicals. TA/Polyethylene imine (PEI) polyplex particles was also synthesized by the electrostatic interaction between the phenolic groups of TA and positively charged ammonium groups of PEI reported by Sahiner et al. [133]. The prepared polyplex particles were more prone to decompose

Fig. 10. Scheme for the formation of TA-PEG hydrogen-bonded complexes and the fabrication of the coatings. Reproduced with permission from Ref. [121]. Copyright 2016, Advanced Materials Interfaces.

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at acidic and basic pH than in neutral pH conditions, resulting from the strong interaction of positive and negative groups on the polymeric structure in acidic and basic conditions. The high antioxidant and antimicrobial properties, coupled with low cytotoxicity make these particles as good materials for versatile applications. Sukhishvili and the co-workers systematically studied the association of TA with neutral or charged polymers in solution and at surfaces assembled by hydrogen bonding and electrostatic interactions, respectively [99]. Hydrogen-bonded films based on TA and neutral polymers could be constructed at low pH condition (pH 2), while it was not possible for electrostatically stabilized films of TA with 90% quaternized poly(4-vinylpyridine) (Q90). Duo to the combined electrostatic and hydrogen-bonding interactions in the multilayers of TA with PVPON containing 20 mol% of primary amino groups (PVPON-NH2–20), this system demonstrated highly stablility in a wide pH range from 1.3 to 11.7. By varying the nature of the polymers, they fabricated TA/polymer films with controlled dissolution pH, making such coatings attractive for future biomedical applications. In order to suppress the nonspecific absorption in biomedical applications, a highly stable, protein-resistant films via the LBL assembly of poly(sulfobetaine methacrylate) (PSBMA)and TA is generally conducted by Ren and the co-workers [134]. The driving force for this assembly is attributed to electrostatic interactions between deprotonated phenol groups of TA and the quaternary ammoniums of PSBMA. The dendritic structure and pKa value of phenol groups of TA all play important roles for the stability of the TA/PSBMA films against pH and ionic strength. The prepared films illustrated excellent protein resistance behavior, which may be of great promise in antifouling biomaterials and separation membranes. 3.4. Self-polymerizing of polyphenols As mentioned above, most of approaches to polyphenol derived coatings involve the use of polymers, enzymes, multivalent metal ions, or multistep deposition processes [135–137]. We favour an approach to nanocoating formation that is analogous to polydopamine coatings, for example, requiring only aqueous buffer and polyphenol precursor [138]. As we all known, plant polyphenols are products of the secondary metabolism of plants exhibiting a large number of catechol and gallol groups which are similar with dopamine [139]. In resent years, the

strong interfacial properties of some polyphenols were recently exploited in forming functionally versatile nanocoatings. Firstly, Sileika et al. surprisingly found that polyphenol-rich beverages, e.g. wine or tea infusion, left undisturbed in cup for a period of time will spontaneously form a thin polyphenol coatings on surfaces of the cup with different materials. Subsequently, they investigated a group of polyphenol molecules with trihydroxyphenyl functional group, exhibiting similar self-polymerizing and adhesive properties on a variety of different materials based on catechol chemistry with out any exogenous additives [2]. These nanocoatings also display excellent ability to scavenge radical and non-radical reactive oxygen species. Furthermore, about 20 natural and synthetic precursors containing gallol, catechol, or phenol structural units (listed in Fig. 11) were selected to investigated their coating potential [138]. One important outcome of this research is the expansion of known molecular precursors, including TA, epigallocatechin gallate, pyrogallol, catechin, epigallocatechin, morin, catechol, and hydroxyhydroquinone [138]. And among these polyphenol or phenol structural units, morin was a notable exception, emerging as the only one known example of a flavonoids coating precursor that does not contain a catechol or gallol group [138]. Besides, not all of the polyphenols with catechol or gallol group could form a coating, such as ellagic acid, rutin and quercetin. However, everything is not absolute, Oliver et al. successfully prepared water soluble quercetin oligomers of 3100 g/mol by a facile procedure [140]. This result indicated that quercetin has a potential to form nanocoating under suitable condition. Presently, most of the literature reported that the formation of coating from polyphenol were under alkaline condition in the presence of oxygen [141,142]. Although, the detailed polymerization mechanism of polyphenols is unclear, plenty of researchs declared that the –OH belonged to catechol or gallol group were oxidized into quinone state by different ways was a critical step [137,141]. Interestingly, we found that the materials formed through polypehonol polymerization revealed different stimuli-responsive. Liu et al. fabricated a biodegradable polyphenol nanocoating on mesoporous silica through an in situ selfpolymerization method under in a weakly alkaline buffer without the aid of any other organic reagents or crosslinkers. This polyphenol coating were stable under physiological conditions and could be biodegraded by acidic pH and intracellular glutathione [141]. Howerver, chen et al. successfully prepared a green luminescent, monodisperse, smooth, porous and hollow spheres. This strategy simply involved

Fig. 11. Chemical structures of tannin-inspired compounds studied as coating precursors; 1. Gallic acid, 2. Ellagic acid, 3. Tannic acid, 4. catechin, 5. epicatechin gallate, 6. Epigallocatechin gallate, 7. Quercetin, 8. Morin, 9. Naringenin, 10. Rutin, 11. Naringin, 12. Phloroglucinol, 13. Pyrogallol, 14. catechol, 15. Hydroquinone, 16. Resorcinol, 17. Hydroxyhydroquinone, 18. Phenol, 19. Resveratrol. Reproduced with permission from Ref. [138]. Copyright 2014, Royal Society of Chemistry.

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Cu2+ and temperature mediated oxidative coupling assembly of green tea polyphenols in water. Unexpectedly, this polymeric polyphenol spheres are GSH responsive, acid resistant but alkali-responsive [143]. Results from the both research inspire us that maybe we could design some materials with different functions via different polymerized strategy to broaden the potential application of polyphenol-materials. 4. Conclusion and outlook Plant polyphenol-inspired materials exhibited a promising prospect in advanced materials design and applications. TA, a kind of low-cost plant polyphenols, has a high surface bonding affinity which can be easily deposited on the surface of the substrates, used as a polyphenolic drug or an agent to further interact with other components. Such TAbased coatings can deposit on different substrates, including organic, inorganic templates, cell surface or even tissues. The unique properties of TA also impacted versatile functions in different system, demonstrating potential for sensing, separation and catalysis. The high metal ion chelation ability makes it a good candidate to act as a polydentate ligand for metal ion coordination. Through a simple and rapid conformal coating method, TA could be coordinated to 18 different metal ions to generate robust mental ion-TA films on a range of substrates to prepare various films and particles. Additionally, after removal of the templates, it could form hollow metal-TA capsules. These films or hollow capsules demonstrated pH-disassembly kinetics and this behavior were dependent on the spices of metal ion used as well as the metal ion feed concentrations. Hollow metal-TA capsules have been extensively studied in delivery system due to the high loading efficiency, controllable and multi-functional properties of build-up materials. The introduction of lanthanide ions such as EuIII and TbIII, also impart imaging properties of metal-TA capsules. Specifically, such capsules could provide a protection of the encapsulated cells or even improve the catalytic activity of noble metal NPs. Notably, TA with the numerous carbonyl and phenolic functional groups has been demonstrated to be excellent H donors to form strong hydrogen bonded stable multilayers with various non-ionic polymers. The hydrogen-bonded multilayers could form coatings on various cores or even form hollow microcapsules. In addition, the chemical structure of the polymers significantly affected the properties of the assemblies, changing the behavior of pH-, temperature-, or ionic strength-responsive release of the loading molecules. The ionization of galloyl phenol group was also attributed to the interaction between TA and other ionic polymers by electrostatic interaction. Through LBL methods, the electrostatic interaction derived TA/polycation complexes could deposit on glass slides, microcores or even forming empty capsules, using in the field of drug encapsulation, delivery and depot. Self-polymerizing of various polyphenols including TA could specially deposited on different substrates without any exogenous additives. Coating deposition was most effective from weakly alkaline buffer solution as compared to pure water. Additionally, the existence of Cu2+ or relative higher temperature could accelerate the forming of oxidative coupling assembly of green tea polyphenols. Although this system suggests a bright future for drug delivery, cell protection, catalysis et.al, considerable challenges remain. First, the detailed investigation of the formation based on metal-TA architecture on organic particles including protein or polysaccharide particles, must be conducted to further understand the mechanism. In our previous work, the incorporation of TA-FeIII films on zein NPs dramatically affected the pH-disassembly kinetics of TA-FeIII films as compared with the rare TA-FeIII capsules. Second, the interaction of zwitterionic polymers with polyphenols should be further studied, to comprehensively understand the experimental observations as well as design multifunctional systems intended for various applications. Third, the mechanism of polyphenols self-polymerizing remains unclear. So it is important to continue experimental observations to determine a general rule for the self-polymerizing.

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Acknowledgments This work was financially supported by National Natural Science Foundation of China (Grant No. 31801586), Hubei Provincial Natural Science Foundation of China (Grant No. 2018CFB233) and the Fundamental Research Funds for the Central Universities (Grant No. 2662018QD016). The authors greatly thank colleagues of Key Laboratory of Environment Correlative Dietology of Huazhong Agricultural University for offering many conveniences. References [1] Guo J, Ping Y, Ejima H, Alt K, Meissner M, Richardson JJ, et al. Engineering multifunctional capsules through the assembly of metal-phenolic networks. Angew Chem Int Ed 2014;53:5546–51. [2] Sileika TS, Barrett DG, Zhang RKHA, Lau PB. Messersmith colorless multifunctional coatings inspired by polyphenols found in tea, chocolate, and wine. Angew Chem Int Ed 2013;52:10766–70. [3] Lee HNF, Scherer PB. Messersmith single-molecule mechanics of mussel adhesion. Proc Natl Acad Sci 2006;103:12999–3003. 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