A fluorescent probe with serum albumin as a signal amplifier for real-time sensing of HSO3− in solution, mitochondria of animal cells and rice roots†

Endogenous release of HSO₃⁻ during the enzymatic oxidation of sulfur containing amino acids in mitochondria or insufficiency of sulfite oxidase results in the accumulation of sulfite and thiosulfate in biological fluids affecting mitochondrial homeostasis of brain mitochondria associated with serious clinical symptoms related to neurological disorders. The red fluorescent probe MGQ undergoes self-assembly in water and reveals aggregation induced quenching of fluorescence. MGQ reveals 143-fold and 179-fold increases in fluorescence intensity at 645 nm, respectively, in the presence of HSA and BSA and does not significantly differentiate between two albumins. The detailed studies of MGQ have been performed in the presence of BSA. The presence of other enzymes/proteins and amino acids, viz. pepsin, trypsin, lysozyme, Bromelain, lysine, histidine, hemoglobin, etc., does not affect the fluorescence of MGQ or MGQ–BSA solutions and points to high selectivity towards BSA. The limit of detection for BSA is 10 nM. In PBS buffer, MGQ in the absence of BSA does not react with HSO₃⁻ and sluggishly in a 1 : 1 ethanol–water mixture. However, in the confined space of BSA/HSA, MGQ displays a signal amplification, undergoes instantaneous Michael type addition of HSO₃⁻ and results in a ratiometric change in fluorescence intensity in ≤1.5 min with the decrease of red fluorescence at 645 nm and emergence of green fluorescence at 515 nm. The LOD for the detection of HSO₃⁻ is 4 nM.