A supramolecular gel platform for selective naked-eye recognition of thorium(IV) ion

The naked-eye detection of Th⁴⁺ poses a critical analytical challenge. While stimulus-responsive hydrogels offer a potential solution by converting recognition events into visible macroscopic phase transitions, this promising mechanism has not been specifically explored for Th⁴⁺. We demonstrate a supramolecular hydrogel platform using a 1,1'-binaphthyl derivative (BINAB). BINAB hydrogel forms through the self-assembly of molecules into a 3D fibrous network via hydrogen bonding and π-π stacking, with the resulting network confining the solvent to induce gelation. The gel exhibits thorium-induced collapse and displays high specificity toward Th⁴⁺. Mechanism studies indicate that the recognition involves the terminal carboxyl groups of BINAB forming a 1 : 1 complex with Th⁴⁺, whereby this high-affinity interaction triggers selective precipitation and disintegration of the gel network, enabling specific and naked-eye recognition of Th⁴⁺ among 24 metal ions.