Colorimetric and fluorescence sensing of pH with a Schiff-base molecule

2-Hydroxy-5-methyl-3-((quinolin-6-ylimino)methyl)benzaldehyde (HL) has been found to be a ratiometric fluorescent chemosensor for pH. It has been synthesized by Schiff-base condensation reaction between 4-methyl-2,6-diformylphenol and 6-aminoquinoline in 1:1 ratio in methanol and characterized by various standard methods. Fluorescence peak of it appears at 464 nm (excitation 360 nm) with strong intensity at low pH in Britton Robinson buffer. With the gradual increase in pH value of the medium, intensity at 464 nm decreases and a new peak emerges at 529 nm with gradual increase in its intensity. Fluorescence intensity of HL remains almost unaffected in the presence of different cations and anions both in acidic and basic media. Fluorescence intensity can be successively altered to its original value with cyclic change of pH between 4.0 and 10.0 indicating stability of the probe in both the media. pK_a of HL has been determined to be 6.24. Colorless probe in strong acidic medium turns green in pH of ~ 6 or more under visible light. Under same condition, color changes from blue to green with the illumination of UV irradiation. These changes are handy to detect different pH medium and are in accordance with UV–vis spectral results. Phenolic proton of HL deprotonates in higher pH as is also evident from the theoretical calculations. This probe has been applied to detect pH regions using lemon juice and river (Ganga) water.