Redox-switchable gelation of unmodified cellulose nanocrystals

Abstract

We demonstrate a redox-switchable gelation system for unmodified cellulose nanocrystals (CNCs) driven by the oxidation of ferrocene (Fc). Upon oxidation of Fc to ferrocenium (Fc⁺), achieved either chemically or electrochemically, the increased ionic strength screens electrostatic repulsion between negatively charged CNC rods, thereby inducing physical gelation. The gel strength can be reversibly modulated: oxidation produces a strong gel, and subsequent reduction weakens it. Multiple redox cycles enable repeated switching, although complete recovery of the initial fluidity is not obtained. Rheological, spectroscopic, and electrochemical analyses confirm that the gelation arises from Fc⁺-mediated electrostatic screening without any chemical modification of CNCs. This simple, additive-driven strategy provides a sustainable platform for electro-responsive soft materials from renewable nanocrystals, with potential applications in drug delivery, microfluidic valves, and electro-assisted 3D printing.