Influence of Ionic Strength on the Deposition of Metal–Phenolic Networks

Abstract

Metal-phenolic networks (MPNs) are a versatile class of self-assembled materials that are able to form functional thin films on various substrates with potential applications in areas including drug delivery and catalysis. Different metal ions (e.g., Fe, Cu) and phenols (e.g., tannic acid, gallic acid) have been investigated for MPN film assembly; however, a mechanistic understanding of the thermodynamics governing MPN formation remains largely unexplored. To date, MPNs have been deposited at low ionic strengths (<5 mM), resulting in films with typical thicknesses of ∼10 nm, and it is still unclear how a bulk complexation reaction results in homogeneous thin films when a substrate is present. Herein we explore the influence of ionic strength (0-2 M NaCl) on the conformation of MPN precursors in solution and how this determines the final thickness and morphology of MPN films. Specifically, the film thickness increases from 10 nm in 0 M NaCl to 12 nm in 0.5 M NaCl and 15 nm in 1 M NaCl, after which the films grow rougher rather than thicker. For example, the root-mean-square roughness values of the films are constant below 1 M NaCl at 1.5 nm; in contrast, the roughness is 3 nm at 1 M NaCl and increases to 5 nm at 2 M NaCl. Small-angle X-ray scattering and molecular dynamics simulations allow for comparisons to be made with chelated metals and polyelectrolyte thin films. For example, at a higher ionic strength (2 M NaCl), sodium ions shield the galloyl groups of tannic acid, allowing them to extend away from the Fe center and interact with other MPN complexes in solution to form thicker and rougher films. As the properties of films determine their final performance and application, the ability to tune both thickness and roughness using salts may allow for new applications of MPNs.

Metal-dependent inhibition of amyloid fibril formation: synergistic effects of cobalt–tannic acid networks

“…Similar results were observed for the Co-TA2 dimer systems, with an average of 6 ± 2 water oxygens positioned within a 4 Å radius of cobalt. The Co-TA2 dimers had a larger contact area (20 ± 1%) when compared to the Fe-TA3 dimers (13 ± 3%, as determined by our previous study; 46 Table S4). Additionally, the Co-TA2 dimers had relatively more aromatic contacts and fewer intermolecular hydrogen bond occupancies when compared to the Fe-TA3 dimers (13 ± 5 and 3 ± 1% compared to 9 ± 3 and 9 ± 2%, respectively; Table S4).…”

Spray Assembly of Metal–Phenolic Networks: Formation, Growth, and Applications

“…The effect of ionic strength on the spray formation of TA-Fe III films was also examined, where an increase in ionic strength led to thicker and rougher MPN coatings ( Figure S16), which is consistent with our previous study on the formation of TA-Fe III films by discrete deposition. 40 On the basis of the above observations, it can be concluded that the physicochemical properties (thickness and complex coordination state) of the MPN coatings prepared via spray assembly can be tuned by adjusting the pH, solvent, ionic strength, and/or precursor molar ratio and concentration. 41 spray coating allows for the rapid patterning of large areas.…”

Nanobiohybrids: Materials approaches for bioaugmentation

“…Moreover, the MPN coating is substrate independent, i.e., it can be formed on almost any type of surfaces due to the nonspecific binding of polyphenols with various surfaces and the simultaneous cross-linking of polyphenols by metal ions. In addition, the MPN-coating morphology, roughness, thickness, degradation profiles, and surface functionalities can be engineered through choice of the building blocks (51,60,61). Owing to their excellent biocompatibility, MPN coatings have also been exploited at the interface of bioentities such as proteins, alkaloids, polysaccharides, and even living cells and tissues under physiological conditions (62).…”