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

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.