Iron Gall Ink Revisited: Visible Light-Induced, Eosin-Mediated Acceleration of Fe2+ Oxidation in Pattern Nanoarchitectonics of Fe3+-Tannic Acid Films.

Abstract

Inspired by iron gall ink (IGI), the Fe2+ ion is utilized for the continuous formation of Fe3+-tannic acid (TA) films through its in situ oxidation to the Fe3+ ion. Although several IGI-based strategies have been developed to meet different application requirements, there remains a need for the precise kinetic control for Fe2+ oxidation, along with one-step spatial controllability. This work demonstrates a visible light-induced method for oxidizing Fe2+ to Fe3+ ions, achieving the kinetic control over Fe3+-TA film formation. Photoexcitation of eosin Y leads to significantly accelerated film formation in a continuous manner, as shown by an 11-fold increase in film thickness compared with the air oxidation-based IGI method. Mechanistic studies reveal that the process is O2-dependent, and the kinetics of the film-forming process could be easily tuned by changing the light intensity or eosin Y concentration. Furthermore, the spatiotemporal controllability of the photoreaction allows for the pattern generation of Fe3+-TA complexes, proteins, and cells.