A fluorescence resonance energy transfer based pH probe for visualizing acidification in fungal cells

Fungi belong to a unique kingdom of organisms closely related to human beings. Monitoring the vital species in fungal cells, H⁺ as an example, reveals tremendous physiological and pathological processes that are crucial for everyone. Herein, we introduced fluorescence imaging with designed fluorophores into the pH imaging in fungal cells. A rhodamine-coumarin based fluorescence resonance energy transfer (FRET) ratiometric probe was successfully applied in visualizing pH and acidification in pathogenic Candida albicans. The probe was designed for pH through the H⁺-triggered reversible closed-open process of spiroboronate with high sensitivity and selectivity. It turns out that the coumarin group is more than a FRET donor, it significantly pushed the closed-open equilibrium constant of the probe from >8.0 into 6.0, the region that suits pH imaging of fungal cells. With excellent cellular permeability and ignorable biotoxicity in fungi, the probe manifested precise pH change from 7.5 to 6.5 in amiodarone treated fungal cells. We hope this work can shed some light on the fluorescence imaging of fungal cells with versatile molecular probes and invoke more interests in fungal cells.