[A] Absorption spectra of Film 4 measured in 50 mM glycine-phosphate buffer, pH 3.00, and at varying fluoride ion concentrations: 0.2, 2, and 22 μM, and 0.22, 0.7, 2.1, and 5.5 mM. The peak absorbance at 529 nm decreased monotonically with increasing fluoride concentration. [B] Typical response time of Al[OEP]/ETH-7075 sensing Film-4, measured in 50 mM glycine-phosphate buffer, pH 3.00, at different fluoride concentrations. Dashed line represents the recovery time when changing the sample from 19.8 mM to fluoride-free buffer solution.