A Cell-Surface-Specific Ratiometric Fluorescent Probe for Extracellular pH Sensing with Solid-State Fluorophore.

Extracellular acidity is correlated with the development of various pathological states and bulk pH measurements could not report surface acidity. In this study, we have developed a ratiometric fluorescent probe that aggregates upon interaction with cells, allowing persistent labeling of cells and in situ measurement of cell surface pH. The ternary nanoplatform is constructed by a convenient noncovalent combination of bovine serum albumin protected gold nanoclusters (BSA-AuNCs), fluorescein isothiocyanate (FITC) labeled cationic peptides (CPs), and FITC-free CPs. The red fluorescent AuNCs serve as reference fluorophore, while FITC labeled peptides act as specific recognition element for H+ and FITC unlabeled peptides are used for delivery. The probe displays a sensitive fluorescence ratiometric response for pH in the range of 5.0-9.5 with calculated p Ka of 7.2. Further studies have demonstrated that this nanosensor also has properties of high selectivity, reversibility to pH fluctuations, as well as low cytotoxicity. The new surface pH-measurement tool was validated in mapping extracellular pH and monitoring acidification regarding cell metabolism, demonstrating its potential for bioimaging and biosensing.