Engineering Ligand-Interactive Hairpin-Containing I-Motif DNA as a Generic Light-Up System for Label-Free Cell-Membrane-Anchored Sulfur Dioxide Biosensors

Sulfur dioxide is one of the gaseous signaling molecules involved in many physiological and pathological processes of living cells, and label-free monitoring of the release of SO₂ on the cell surface provides a facile way to investigate the related cellular processes. Toward this goal, here we engineer a hairpin-containing i-motif (iM-Hp) DNA that effectively binds a benzothiazole-based SO₂-responsive fluorescent probe (PSMB) and remarkably promotes its fluorescence emission. When this light-up system is incubated with SO₂ in aqueous solution, the addition of HSO₃^- to the C═C bond of PSMB destroys the fluorophore structure, accompanied by a sharp decrease in the fluorescence intensity. In this way, SO₂ can be sensitively detected with a high specificity. Based on it, we further construct a cell membrane-anchored SO₂ fluorescent sensor for label-free monitoring the release of SO₂ from HeLa cells stimulated by lipopolysaccharide (LPS), a well-known inducer for cell inflammation. It shows that the level of SO₂ exocytosis increases gradually within 4 h after incubation with LPS, whereas the cell viability almost remains unchanged during this period. This finding discloses a postdrug progressive process of inflammatory cells, highlighting the significant potential of our designed light-up system in monitoring drug-induced early cell lesion.