Basic Fluorescent Protonation-Type pH Probe Sensitive to Small ΔpK_a of Methanol and Ethanol

An optical pH probe is a simple and effective tool for determining an accurate pH value in its localized area. However, basic pH probes with pK_BH+ values above 8 have rarely been reported, although many components with high pK_a such as arginine play important roles in vivo. Herein, we introduce novel colorimetric and fluorescent basic probes 1–5, which are designed using push–pull-type aminoquinoline and aminobenzoquinoline fluorophores, with pK_BH+ values ranging from 8.4 to 9.9. After the basicity of the remarkably sensitive basic probe 4 was tuned, it was able to successfully distinguish between the pK_a values of MeOH (15.5) and EtOH (15.9), thus displaying selective protonation and fluorescence enhancement in MeOH over EtOH. Our pH probes can be used to detect MeOH poisoning in commercial EtOH products such as hand sanitizers, providing an effective solution to this problem observed during the COVID-19 pandemic.