Development of an ‘OFF-ON-OFF’ fluorescent pH sensor suitable for the study of intracellular pH

Abstract

Changes of pH in living organisms are closely related to various physiological functions, so fluorescent sensors of pH would be useful as experimental tools and possibly also for medical diagnosis. We previously reported a pH sensor with ‘OFF-ON-OFF’ -type fluorescence change, which would be potentially more useful than conventional sensors with ‘OFF-ON’ type change. It can be utilized as a sensor for a specific range of pH, however, the range with strong fluorescence is relatively wide, around pH 7–10, which renders it impractical for use in biological studies. So in the present work, we adjusted the pH detection range by the introduction of chloro groups at the ortho-positions to the phenolic hydroxyl groups, whose deprotonation could control fluorescence change, and obtained a sensor specifically responsive to pH around 6. This sensor was confirmed to be suitable for fluorescence imaging of changes of intracellular pH in living cells.