Origins of Structural Elasticity in Metal–Phenolic Networks Probed by Super-Resolution Microscopy and Multiscale Simulations

Metal-phenolic networks (MPNs) are amorphous materials that can be used to engineer functional films and particles. A fundamental understanding of the heat-driven structural reorganization of MPNs can offer opportunities to rationally tune their properties (e.g., size, permeability, wettability, hydrophobicity) for applications such as drug delivery, sensing, and tissue engineering. Herein, we use a combination of single-molecule localization microscopy, theoretical electronic structure calculations, and all-atom molecular dynamics simulations to demonstrate that MPN plasticity is governed by both the inherent flexibility of the metal (Fe^III)-phenolic coordination center and the conformational elasticity of the phenolic building blocks (tannic acid, TA) that make up the metal-organic coordination complex. Thermal treatment (heating to 150 °C) of the flexible TA/Fe^III networks induces a considerable increase in the number of aromatic π-π interactions formed among TA moieties and leads to the formation of hydrophobic domains. In the case of MPN capsules, 15 min of heating induces structural rearrangements that cause the capsules to shrink (from ∼4 to ∼3 μm), resulting in a thicker (3-fold), less porous, and higher protein (e.g., bovine serum albumin) affinity MPN shell. In contrast, when a simple polyphenol such as gallic acid is complexed with Fe^III to form MPNs, rigid materials that are insensitive to temperature changes are obtained, and negligible structural rearrangement is observed upon heating. These findings are expected to facilitate the rational engineering of versatile TA-based MPN materials with tunable physiochemical properties for diverse applications.