Coumarin-4-Doped Sol-Gel Coating Film as Optical pH Sensor

Coumarin 4, a pH indicator, was entrapped in a silica matrix coated as a thin film onto a silica substrate by the sol-gel technique. The absorbance and fluorescence changes of the immobilized indicator were examined in the pH sensing range. The increase in absorption and fluorescence intensities as the acid form of the immobilized coumarin 4 is converted to its basic form, is dependent on pH. The dynamic range and the response time of this probe were pH 3-9 and less than 10 s, respectively.