We have prepared a turn-on fluorescent probe for biothiols based on bromoketo coumarin (KC-Br). The emission intensity of the coumarin chromophore is modulated by both the heavy atom effect and internal charge transfer (ICT) process. The probe KC-Br is intrinsically nonfluorescent; however, after being reacted with thiols, the bromide moiety is substituted by the -SH group, which elicits a significant fluorescence increase. We surmised the free -NH2 group would further react with carbonyl in the Cys/Hcy-substituted intermediate product yielding to Schiff base compound KC-Cys/KC-Hcy, but not in compound KC-GSH. The ICT effect has a stronger influence in compound KC-GSH than that in compound KC-Cys/KC-Hcy, resulting in compound KC-GSH having a stronger fluorescence. Thus, the probe has a good selectivity for GSH over other various biologically relevant species and even two other similar biothiols (Cys/Hcy) and could image glutathione (GSH) in living cells. We expect the design concept presented in this work would be widely used for the design of fluorescent probes for distinguishing among biothiols.
We introduce a new FRET strategy to construct a ratiometric fluorescent H2S sensor. The ratio emission signal of the coumarin-naphthalimide dyad is modulated by the FRET process, which works in coordination with the ICT mechanism. The FRET process on/off is controlled through tuning the overlap level of the donor emission spectrum with the acceptor absorption via modulation of the acceptor fluorophore absorption wavelength. was applied to visualize both the intracellular exogenous and endogenous H2S through blue and green emission channels.
The development of new functional fluorescent dyes has attracted great attention. Herein we have described a novel strategy to design a unique type of cyanine dyes by attaching two indolium moieties at the α-positions of the pyrrole core. The new type of cyanine dyes is named as PyCy fluorophores. Importantly, PyCy dyes can exhibit an exceptional feature, fluorescence turn-on response at pH varying from acidic to near-neutral conditions, and a ratiometric fluorescence response at pH varying from near-neutral to basic conditions. By taking advantage of the fluorescence turn-on response of PyCy2 at pH varying from acidic to near-neutral conditions and emission properties of PyCy2, we have demonstrated that a small-molecule fluorescent probe can image pH variations in living cells. Furthermore, we have demonstrated that PyCy2 can sense real-time pH changes under alkaline conditions induced by enzymes based on the ratiometric fluorescence response of PyCy2 at pH varying from near-neutral to basic conditions. We expect that the new design strategy for PyCy fluorophores may prompt the development of a wide variety of cyanine derivatives with desirable properties.
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