Iron Gall Ink Revisited: In Situ Oxidation of Fe(II)–Tannin Complex for Fluidic‐Interface Engineering

Abstract

The ancient wisdom found in iron gall ink guides this work to a simple but advanced solution to the molecular engineering of fluidic interfaces. The Fe(II)–tannin coordination complex, a precursor of the iron gall ink, transforms into interface‐active Fe(III)–tannin species, by oxygen molecules, which form a self‐assembled layer at the fluidic interface spontaneously but still controllably. Kinetic studies show that the oxidation rate is directed by the counteranion of Fe(II) precursor salts, and FeCl2 is found to be more effective than FeSO4—an ingredient of iron gall ink—in the interfacial‐film fabrication. The optimized protocol leads to the formation of micrometer‐thick, free‐standing films at the air–water interface by continuously generating Fe(III)–tannic acid complexes in situ. The durable films formed are transferable, self‐healable, pliable, and postfunctionalizable, and are hardened further by transfer to the basic buffer. This O2‐instructed film formation can be applied to other fluidic interfaces that have high O2 level, demonstrated by emulsion stabilization and concurrent capsule formation at the oil–water interface with no aid of surfactants. The system, inspired by the iron gall ink, provides new vistas on interface engineering and related materials science.

Oxidation‐Mediated Kinetic Strategies for Engineering Metal–Phenolic Networks

“…Second, the gradual conversion of TA‐Fe II into TA‐Fe III and the low concentration of free TA‐Fe III complexes could provide additional opportunities for conformational rearrangement and self‐correction. Third, owing to the higher solubility of Fe II compared with that of Fe III (Fe III tends to form μ‐oxo‐bridged species in aqueous media; see the Supporting Information, Figure S1), Fe II can result in a higher degree of cross‐linking in the networks . Overall, both the coordination process and the subsequent oxidation of TA‐Fe II to TA‐Fe III are dependent on time and concentration, ultimately affording temporal and spatial control over the thickness and microstructure of the MPN films.…”

Iron Gall Ink Revisited: Natural Formulation for Black Hair-Dyeing

“…The blonde hair turned to dark brownish to the naked eyes, and the L* value was measured to be 26.2 ± 0.5 after dyeing with FeSO 4 . We thought that the observed difference in the dyeing capability was caused by differences in the anion-dependent oxidation kinetics that had previously been observed [10], but detailed mechanisms remain to be seen at this stage. Nonetheless, this finding is meaningful in the cosmetic formulation, because customers tend to be reluctant to use sulfate-containing cosmetic products.…”

“…Among a multitude of tannins, gallic acid was selected as a main dye ingredient in this study, considering that the blackish color in the iron gall ink originates mainly from gallic acid [7]. Tannic acid was additionally added to enhance the adhesion property of Fe(III)-tannin nanocomplex [10]. In our formulation with Fe(II), the steady O 2 oxidation of Fe(II) to Fe(III) in the Fe(II)-tannin complex, upon application to the hair, led to the continuous coating of Fe(III)-tannin nanocomplex and hair darkening, which could not be achieved by direct use of Fe(III) due to the rapid precipitation of Fe(III)-tannin complex.…”

“…Therefore, ink blackening, rather pale initially upon mixing, is achieved by the oxidation of Fe(II) to Fe(III) ions and semi-infinite self-assembly of Fe(III)-tannin complexes [8,9]. We have recently reported the use of the Fe(II)-tannin complex, not the final Fe(III)-tannin complex in the iron gall ink, as a reaction precursor for fluidic-interface engineering [10] and coacervate formation [11]. In addition to its water solubility, other advantages of Fe(II)-tannin complex use include reaction controllability, because the oxidation-initiation time and reaction rates could be controlled tightly in the formation of Fe(III)-tannin nanostructures.…”

Medical Applications Based on Supramolecular Self-Assembled Materials From Tannic Acid

“…The coordination assembly of TA occurs through the MPN structure to form metal–organic supramolecular assembly materials (Richardson et al, 2016 ). At present, more research is focused on the assembly of TA with iron or ferrous ions, which has many applications in the field of biomedicine such as antibacterial coating (Ko and Huang, 2020 ), clinical diagnosis, nanoprobe treatment (Zou et al, 2018 ; Yan et al, 2019 ), tooth desensitizers (Zhou et al, 2013 ), separation and protection of living cells (Kim et al, 2017 ), self-healing and functional films (Lee et al, 2018 ), and antioxidant coatings (Maerten et al, 2017 ). Ejima et al ( 2013 ) first proposed the synthesis of coordination complexes using natural polyphenol TA as an organic ligand and Fe(III) as an inorganic crosslinking agent, which can be used to prepare various films and particles on a series of substrates by one-step assembly.…”