L-cystine-linked BODIPY-adsorbed monolayer MoS quantum dots for ratiometric fluorescent sensing of biothiols based on the inner filter effect.

This study fabricated a dual-emission probe consisting of monolayer MoS quantum dots (M - MoS QDs) and L-cystine-linked boron-dipyrromethene (L-Cys-BODIPY) molecules for ratiometric sensing of biothiols, thiol product-related enzyme reactions, and ratiometric imaging of glutathione (GSH)-related reactions in HeLa cells. The formation of L-Cys-BODIPY-adsorbed M - MoS QDs (named as BODIPY-M-MoS QDs) was demonstrated by comparing them with M - MoS QDs using transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The BODIPY-M-MoS QDs exhibited dual-emission bands, excellent biocompatibility, and good resistance to photobleaching. It was found that the adsorbed L-Cys-BODIPY molecules rarely quenched the fluorescence of M - MoS QDs, and meanwhile, they were self-quenched by π-π stacking between each BODIPY backbones. The presence of biothiols induced the reduction of weakly fluorescent L-Cys-BODIPY to strongly fluorescent of L-cysteine-conjugated BODIPY. Since having a much higher molar absorption coefficient than L-Cys-BODIPY, the liberated L-cysteine-conjugated BODIPY behaved as an effective inner filter to absorb the excitation light and subsequently quenched the fluorescence of M - MoS QDs. The appearance of L-cysteine-conjugated BODIPY could barely affected to the fluorescence lifetime of M - MoS QDs, confirming the inner filter effect of L-cysteine-conjugated BODIPY onto the fluorescence of M - MoS QDs. The present probe not only provided a linear ratiometric response to 1-10 mM GSH, 1-10 μM cysteine, and 1-10 μM of homocysteine but also remarkably showed the ratiometric detection of thiol products from the reactions of 1-900 units L S-adenosylhomocysteine (SAH) hydrolase and SAH as well as 1-850 units L GSH reductase and disulfide GSH. Additionally, the present probe was well-suited for ratiometric imaging of intracellular GSH levels in non-treated and drug-treated HeLa cells.