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Developing a water-soluble supramolecular system for the detection and removal of Hg²⁺ is extremely needed but remains challenging. Herein, we reported the facile construction of a fluorescent supramolecular system (<b>H</b><b>⊃</b><b>G</b>) in 100% water through the self-assembly of carboxylatopillar[5]arene sodium salts (<b>H</b>) and diketopyrrolopyrrole-bridged bis(quaternary ammonium) guest (<b>G</b>) by host–guest interaction. With the addition of Hg²⁺, the fluorescence of <b>H</b><b>⊃</b><b>G</b> could be efficiently quenched. Since Hg²⁺ showed synergistic interactions (coordination and Hg²⁺- cavity interactions with <b>G</b> and <b>H</b>, respectively), crosslinked networks of <b>H</b><b>⊃</b><b>G</b>@Hg²⁺ were formed. A sensitive response to Hg²⁺ with excellent selectivity and a low limit of detection (LOD) of 7.17 × 10⁻⁷ M was obtained. Significantly, the quenching fluorescence of <b>H</b><b>⊃</b><b>G</b>@Hg²⁺ can be recovered after a simple treatment with Na₂S. The reusability of <b>H</b><b>⊃</b><b>G</b> for the detection of Hg²⁺ ions was retained for four cycles, indicating the <b>H</b><b>⊃</b><b>G</b> could be efficiently used in a reversible manner. In addition, the <b>H</b><b>⊃</b><b>G</b> could efficiently detect Hg²⁺ concentration in real samples (tap water and lake water). The developed supramolecular system in 100% water provides great potential in the treatment of Hg²⁺ detection and removal for environmental sustainability.