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

Abstract

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 SO2 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 zerodimensional carbon-based fluorescent nanomaterials, have shown immense potential as sensory probes, and it is a prospective candidate for monitoring SO2 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 SO2 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 SO2 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 SO2 levels in heat-stressed cells. The PPy-Br:GQDs sensing conjugate is efficient in the real-time monitoring of intracellular SO2 levels and demonstrates enough prospects for further explorations as a diagnostic kit for heat stroke monitoring.