A novel BODIPY-based fluorescent probe for naked-eye detection of the highly alkaline pH

A novel alkaline pH-responsive probe based on an asymmetric aza-BODIPY was synthesized in a one-pot Schiff base formation reaction. This pH-sensitive probe comprises an asymmetric aza-BODIPY as the luminescent core, with a benzothiazole moiety connected via an imine bond serving as the recognition site. The probe exhibits a turn-off fluorescence response upon exposure to alkaline pH (9.6-12.4), while a bathochromic band in the absorption emerges due to its extended π-conjugation system, accompanied by a visible colorimetric change from yellow to orange to red. Furthermore, the probe responds linearly in the highly alkaline region, with a pK_a of 11.65. The recognition mechanism of the probe towards alkaline pH relies on the deprotonation of the imine group on the aza-BODIPY core, leading to an enhanced degree of π-electron conjugation. The quenched fluorescence intensity is attributed to the increased non-radiative decay of the deprotonated form of the probe. The probe demonstrates high reliability for practical applications due to its photostability and reversibility. This study provides new insights into the design of probes for detecting high alkaline pH levels.