Cationic Donor–Two-Acceptor Dye–Graphene Quantum Dot Nanoconjugate for the Ratiometric Detection of Bisulfite Ions and Monitoring of SO₂ Levels in Heat-Stressed Cells

When the body temperature rises above 40.6 °C, heat stress in the cells causes mitochondrial damage. This damage can lead to apoptosis, multiple organ failure, and even death. The dysregulation of SO 2 levels in the mitochondria is linked to this heat stress, and its detection may act as an early indicator of heat stroke. Graphene quantum dots (GQDs), a common class of zero-dimensional carbon-based fluorescent nanomaterials, have shown immense potential as sensory probes, and it is a prospective candidate for monitoring SO 2 levels in living cells. Herein, we report a combination of a donor–two acceptor (D2A) red-emissive di-picolinium salt (PPy-Br) and hydroxy-functionalized graphene quantum dots (GQDs-OH) as the fluorescence resonance energy transfer (FRET)-based ratiometric sensor for bisulfite ions detection in aqueous media. The dye-GQD nanoconjugate displays excellent photostability and good aqueous dispersibility allowing the monitoring of SO 2 levels in living cells during heat stress. The PPy-Br:GQD nanoconjugate is indifferent to a large number of cations, anions, or biologically relevant species and displays a detection limit of 36 nM for the solution phase detection of bisulfite. PPy-Br is biocompatible with human cell lines and endocytoses into the cells to ensure monitoring of SO 2 levels in the mitochondrial milieu. PPy-Br-treated human breast cancer cells displayed a gradual decrease in fluorescence at temperatures above 40 °C, indicating an increase in SO 2 levels in heat-stressed cells. The PPy-Br:GQDs sensing conjugate is efficient in the real-time monitoring of intracellular SO 2 levels and demonstrates enough prospects for further explorations as a diagnostic kit for heat stroke monitoring.