A fluorescent and colorimetric pH-indicator 1,5-bis(1-hydroxy-4-methoxphenyl)-1,4-pentadiene-3-one was synthesized based on the ketene structure as the fluorogen and phenolichydroxyl as the deprotonation group.The pH titration indicated that the probe displayed strong spectral sensitivity to pH in absorption spectra and fluorescence spectra.The absorption spectra were red-shifted and the solution undergoed distinct color changes from yellow to red with pH turning from acidity to basicity.The fluorescence spectra changed little under acidic solution,while there was more than 20-fold increase in the fluorescence emission intensity under basic solution.The fluorescence dissociation constant(pKa)was calculated to be 8.13 by using the formula of Henderson-Hasselbach,and the fluorescence quantum yield(in ethanol solution) was found to be 0.056 by comparing with the fluorescence quantum yield of quinine sulfate in 0.5 mol/L H2SO4.Furthermore,the probe also showed strong light resistant and anti-metal interference capability.