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.
Keywords: cyanine dyes; fluorescence imaging; pH probe; pyrrole; ratiometric; turn-on.