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ABSTRACT A novel colorimetric and fluorescent pH sensor derived from iminocoumarin and thiophene-carboxaldehyde was designed and synthesized. The structures of the dye and related compounds were characterized by IR, 1H NMR, 13C NMR, and mass spectra. Color change from green to yellow of the new sensor solution in ethanol-water with the decrease in the pH value from 7 to 2 was observed by the naked eye. Under acidic conditions, the intensity of the maximum fluorescence emission peak of the sensor increased gradually with the decrease in the acidity of the solution (the increase in the pH value from 2 to 7) and attained to the maximum value at about pH 6. Under basic conditions, the fluorescence intensity of the emission peak of the sensor did not exhibit a distinct change at pH ≤ 11.85, and the fluorescence was quenched at pH 13.36, concomitant with the green color of the solution turning pale. The sensor can be used as a fluorescent pH probe in the presence of common metal cations and anions without interference. [ABSTRACT FROM AUTHOR]