Coordination-driven multistep assembly of metal-polyphenol films and capsules

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.