pH optical sensor based on thin films of sol–gel with bromocresol purple

Abstract This paper presents an optical sensor for pH measurement based on thin film applied onto a silica substrate. The thin films were prepared through the sol–gel process, by using bromocresol purple (BCP) as photoresist indicator by means of hydrolysis and acid catalyzed reaction of tetraethylorthosilicate (TEOS) with the surfactant sodiumdodecylsulfate (SDS). As for the excitation light source, we used the wavelength of 470 nm, and we measured the fluorescence response by using three optical filters: 600.0, 632.8 and 650.0 nm. The highest fluorescence intensities were obtained for lower pH values, since the absorption of the film at the wavelength of 470 nm is higher for lower pH values. The sensor presented a greater variation in tension on the photodiode (11.65 mV) when we used the 632.8 nm filter, considering the pH patterns of 4 and 10. The pH sensor showed great performance for measuring pH in the range between 4 and 10.