A ratiometric fluorescent pH probe based on keto–enol tautomerization for imaging of living cells in extreme acidity

Abstract

6-(Diethylamino)-2,3-dihydro-1H-xanthene-4-carbaldehyde (DDXC), a reported synthetic intermediate for near-infrared fluorescent dyes, was developed into a fluorescent pH probe for extreme acidity. The unique sensing mechanism of DDXC for pH is based on the reversible protonation of the carbonyl oxygen followed by keto–enol tautomerization. The probe displays a linear ratiometric fluorescence response (I₅₁₂/I₅₈₀) to H⁺ over the extremely acidic range of pH 2.0–4.0 with a pK_a of 3.11, and features high fluorescence quantum yield (Φ = 0.60) and excellent selectivity. More importantly, the probe can be applied to ratiometric fluorescence imaging of pH changes in living cells, making it a potential molecular tool for pH-related cell biology study.