Rational Design of Ratiometric Near-Infrared Fluorescent pH Probes with Various pKa Values, Based on Aminocyanine

Abstract

Novel ratiometric, near-infrared fluorescent pH probes with various pK_a values have been designed and synthesized on the basis of aminocyanine bearing a diamine moiety, and their photochemical properties were evaluated. Under acidic conditions, these pH probes showed a 46- to 83-nm red shift of the absorption maximum. This change is sufficiently large to permit their use as ratiometric pH probes, and is reversible, whereas monoamine-substituted aminocyanines showed irreversible changes because of their instability under acidic conditions. Furthermore, the pK_a values of these probes can be predicted from the calculated pK_a values of the diamine moieties, obtained from the SciFinder database. This design strategy is very simple and flexible, and should be applicable to develop NIR pH probes for various applications.