Hierarchical carbon nanodot-nanosheet assemblies as active colloids exhibiting collective motion and environment-responsive emission.

Hierarchical organization of colloidal building blocks can give rise to emergent collective behavior inaccessible to individual constituents. This study shows that polycrystalline giant carbon nanodots and carbonaceous nanosheets, produced from a single reaction, can serve as building blocks for hierarchically organized active matter. In organic media, the giant CNDs spontaneously assemble into nanosheet architectures that form fluorescent microswimmers, exhibiting sustained propulsion, corkscrew-like helical motion, and nonreciprocal body deformation required for locomotion at low Reynolds numbers. Time-resolved trajectory analysis reveals interaction-mediated synchronization between neighbouring swimmers, including hydrodynamically coupled double-helical motion and long-range dynamic influence between distant assemblies. At higher concentrations, continuous optical excitation drives collective motion, producing macroscopic vortex formation observable directly by the naked eye. The assemblies further undergo fractal growth into leaf- and root-like crystalline architectures in the condensed state spanning centimeter length scales, highlighting the emergence of hierarchical organization across multiple dimensions. Interestingly, the assembled state induces an aggregation-driven fluorescence red-shift of more than 100 nm, generating yellow-orange emissive microswimmers from otherwise blue-emitting carbon nanodots. Exposure to trace water destabilizes the assemblies, releasing blue-emitting nanodots and disrupting the active state, thereby enabling rapid visual detection of small amounts of water in several organic solvents through distinct emission-colour transitions, including white-light generation at intermediate compositions. These findings establish structurally evolved carbon nanostructures as a platform for interaction-driven active matter and broaden the conceptual scope of carbon nanodot research from isolated fluorescent nanoparticles toward hierarchically organized, dynamically adaptive functional systems.