Iron Gall Ink\nRevisited: Visible Light-Induced, Eosin-Mediated\nAcceleration of Fe<sup>2+</sup> Oxidation in Pattern Nanoarchitectonics\nof Fe<sup>3+</sup>-Tannic Acid Films

Inspired by iron gall ink (IGI),\nthe Fe<sup>2+</sup> ion is utilized\nfor the continuous formation of Fe<sup>3+</sup>-tannic acid (TA) films\nthrough its in situ oxidation to the Fe<sup>3+</sup> ion. Although\nseveral IGI-based strategies have been developed to meet different\napplication requirements, there remains a need for the precise kinetic\ncontrol for Fe<sup>2+</sup> oxidation, along with one-step spatial\ncontrollability. This work demonstrates a visible light-induced method\nfor oxidizing Fe<sup>2+</sup> to Fe<sup>3+</sup> ions, achieving the\nkinetic control over Fe<sup>3+</sup>-TA film formation. Photoexcitation\nof eosin Y leads to significantly accelerated film formation in a\ncontinuous manner, as shown by an 11-fold increase in film thickness\ncompared with the air oxidation-based IGI method. Mechanistic studies\nreveal that the process is O<sub>2</sub>-dependent, and the kinetics\nof the film-forming process could be easily tuned by changing the\nlight intensity or eosin Y concentration. Furthermore, the spatiotemporal\ncontrollability of the photoreaction allows for the pattern generation\nof Fe<sup>3+</sup>-TA complexes, proteins, and cells.