Nano Today
Volume 12, February 2017, Pages 136-148
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Review
Metal-phenolic networks as a versatile platform to engineer nanomaterials and biointerfaces

https://doi.org/10.1016/j.nantod.2016.12.012Get rights and content

Highlights

  • We review metal-phenolic networks (MPNs), a versatile class of surface coatings and nanostructured functional materials.

  • Diverse substrates can be coated owing to the universal adherent properties of phenolic moieties.

  • Coordinated metal ions can be used for cross-linking the films and for engineering additional functionality.

  • Applications including sensing, imaging, drug delivery, live-cell protection, and catalysis are highlighted.

Abstract

Surface modification is crucial for conferring novel functionalities to objects and interfaces. However, simple yet versatile strategies for the surface modification of multiple classes of nanomaterials, including biointerfaces, are rare, as the chemical interactions between the surface modifiers and the substrates need to be tailored on a case-by-case basis. Recently, metal-phenolic networks (MPNs) have emerged as a versatile surface modifier based on the universal adherent properties of phenolic molecules, namely the constituent gallol and catechol groups. Additionally, the dynamic interactions between metal ions and phenolic molecules confer additional functionalities to the MPNs, such as stimuli-responsiveness. Given the interest in MPNs for nanomaterial and biointerface engineering, this review aims to provide an overview of the assembly process, physicochemical properties and applications of MPN coatings.

Introduction

Coatings are useful for engineering specific and desired properties into materials. Moreover, the conformal coating of nanoscale objects is more important than ever, as nanotechnology is being adopted by disparate scientific fields. For example, thiol chemistry [1] is commonly used to engineer the surface properties of gold nanoparticles (AuNPs), thereby controlling pharmacokinetic and organ/tissue distribution during targeted drug delivery and photothermal cancer therapy [2]. Specifically, thiolated biocompatible polymers such as poly(ethylene glycol) (PEG) can be conjugated to AuNPs to reduce cytotoxicity, improve their in vivo stability, and prolong their systemic circulation lifetime [3]. For hydroxy-terminated substrates, including silicon and glass, silane coupling agents (e.g., R–Si(OH)3) are widely used for surface modification [4]. Phosphonic acids (e.g., R–PO3H2) also readily react with a range of metal oxides to form a dense coating on the surface, and phosphonic acids are sometimes superior to silane coupling agents because of the higher robustness of metal–OP over metal–OSi bonds [5]. Phosphonic acid-mediated coating prevents oxygen diffusion toward the metal surfaces, thereby protecting the coated materials from corrosion [6]. However, these technologies are seldom applicable to a wide variety of substrates, as they require specific interactions with the substrate [7].

Facile coating techniques should be substrate-independent, and therefore applicable to a substrate regardless of its chemical composition, size, shape, or stiffness. To develop such coatings, solution-based noncovalent interactions (e.g., electrostatic interaction, hydrogen bonding, hydrophobic attraction, and van der Waals interaction) are exploited, as these interactions occur on nearly all types of surfaces. Additionally, as most individual noncovalent interactions are not strong enough to stably tether the coating to a surface, the adhesive units are generally incorporated into polymer backbones to generate multivalency [7]. For example, layer-by-layer (LbL) assembly can be utilized to fabricate polymeric thin films on a wide range of surfaces through the sequential adsorption of polyanions and polycations [8], [9]. Unlike spin coating and chemical vapor deposition, LbL assembly can be used to coat both planar and particulate substrates. This topology-independent conformal nature allows for the deposition of homogeneous coatings on colloidal materials of different composition (e.g., polymer, inorganic, liquid droplets, gas bubbles) and geometry (e.g., spheres, rods, fibers) [10]. Despite the significant progress being made, the requisite multistep layering process of LbL assembly is often labor intensive and time consuming. To reduce the manual involvement and speed up the assembly time, a number of studies have focused on accelerating deposition kinetics and automating labor-intensive steps with robotic immersion machines [9].

Instead of utilizing multiple steps, the self-polymerization of dopamine is a one-step method to produce an adherent coating on a wide variety of surfaces [11]. Dopamine is oxidized into 5,6-dihydroxyindole in alkaline solutions and further polymerized to form three dimensionally cross-linked networks, although a substantial amount of unpolymerized dopamine and 5,6-dihydroxyindole also remain inside the network [12]. Dopamine has catechol and amine groups that are analogous to adhesive proteins secreted by marine mussels, such as Mytilus edulis, which secretes highly basic adhesive proteins containing large amounts of lysine and 3,4-dihydroxyphenylalanine residues [13]. Amines contribute to the electrostatic interaction of the proteins and also help remove the hydrated salt layer on surfaces to allow for catechol binding [14]. Catechols can mediate either coordination interactions or bidentate hydrogen bonding to metal oxide surfaces [15]. Although, the deposition of polydopamine coatings is a one-step process, the rate of film deposition decreases as the precursor dopamine concentration increases, and it can take 10 h to reach a thickness of 20 nm [16].

Recently, we reported a rapid and low cost method for the conformal coating of different substrates, including bulk materials, nanomaterials, and biointerfaces [17]. In this method, the naturally occurring polyphenol, tannic acid (TA), and metal ions (e.g., FeIII ions) are simply mixed in one-pot in the presence of the substrates. Film deposition occurs due to the adsorption of the polyphenols and simultaneous cross-linking of TA by FeIII. Adjacent hydroxyl groups of TA provide chelating sites for FeIII ions, and the large number of gallol groups on TA facilitates efficient coordination-driven cross-linking, thereby resulting in three dimensionally stabilized metal-phenolic networks (MPNs). Moreover, film deposition is completed within several minutes, and TA and FeIII are readily available and inexpensive.

Since they were first reported in 2013, MPN coatings have become convenient functional nanocoatings on a diverse array of substrates (Fig. 1 and Table 1). The choices of phenolic ligands and metal ions used for film assembly have expanded, allowing for the facile generation of diverse films for a variety of applications. In this review, we summarize recent research progress in the assembly of MPNs on nanomaterials and biointerfaces. First, the assembly process and mechanisms of MPN formation are described. We then introduce specific examples of MPN-coatings on nanomaterials and biointerfaces. We conclude with a future outlook on MPN coatings.

Section snippets

What are MPNs?

MPNs are supramolecular network structures consisting of metal ions coordinated to phenolic ligands. These are ideal candidates for accessing new functionalities in stimuli-responsive coatings, because they combine specific functions imparted by the metal ions with the high affinity of phenolics to a wide range of surfaces. Crystalline porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) composed of phenolic ligands with metal ions [56], [57] also fall into the class of MPNs,

Conclusions and outlook

This review highlights some of the recent advances in the MPN coating of nanomaterials and biointerfaces. While this approach is only in its infancy, a diverse array of MPN-coated objects (including graphene, nanodiamonds, AuNPs, yeast, mammalian cells, teeth) have been coated. However, there is still significant scope for improving and expanding upon the choice of phenolic ligands used for MPN construction. Commercially available TA is in fact a mixture of gallotannins and other galloylated

Acknowledgment

This research was supported by the Australian Research Council (ARC) Centre of Excellence in Convergent Bio-Nano Science and Technology (project number CE140100036). This work was also supported by the ARC under the Australian Laureate Fellowship (F.C., FL120100030) scheme.

Hirotaka Ejima was born in Kanagawa Prefecture, Japan in 1983. He completed his PhD under Professor T. Serizawa in 2011 at The University of Tokyo. He then joined the research group of Professor F. Caruso at The University of Melbourne as a JSPS postdoctoral fellow. After spending two and a half years in Australia, he moved back to Japan, and is now an associate professor at the University of Tokyo. His current research interests are in developing functional nanomaterials based on renewable

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  • Cited by (0)

    Hirotaka Ejima was born in Kanagawa Prefecture, Japan in 1983. He completed his PhD under Professor T. Serizawa in 2011 at The University of Tokyo. He then joined the research group of Professor F. Caruso at The University of Melbourne as a JSPS postdoctoral fellow. After spending two and a half years in Australia, he moved back to Japan, and is now an associate professor at the University of Tokyo. His current research interests are in developing functional nanomaterials based on renewable bioresources.

    Joseph J. Richardson was born in the ‘Sunshine State’ Florida, USA in 1988. He received his Bachelor’s degree in Philosophy and his Master’s in Industrial and Systems Engineering from the University of Florida. J.J. completed his PhD in early 2015 researching thin film deposition strategies under the supervision of Professor F. Caruso at The University of Melbourne. Currently, he is a Postdoctoral Fellow at CSIRO studying metal-organic hybrid systems for biomedicine.

    Frank Caruso is a professor and ARC Australian Laureate Fellow at the University of Melbourne. He received his PhD degree in 1994 from the University of Melbourne, and then moved to the CSIRO Division of Chemicals and Polymers in Melbourne. He was an Alexander von Humboldt Research Fellow and then group leader at the Max Planck Institute of Colloids and Interfaces from 1997 to 2002. His research interests focus on developing advanced nano- and biomaterials for biotechnology and medicine.

    1

    Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

    2

    CSIRO Manufacturing, CSIRO Private Bag 10, Clayton South, Victoria 3169, Australia.

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