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Optical Ammonia Sensors for Environmental Applications

Triphenyl-methane based indicators bromophenol blue and bromocresol green were immobilized in ormosil sol-gel films prepared from phenyl-triethoxysilane in order to prepare colorimetric ammonia sensors. The sensing layers contained different molar ratios of the indicators, in this way—depending on the pKa values of the indicators—sensors with tailored dynamic ranges could be fabricated. Adsorption based immobilization was also used for the fabrication of chemical layers immersing anodized aluminum films in aqueous solutions of the indicators. Simultaneous immobilization of the indicators was considerably effortful on the anodized aluminum film, compared with the sol-gel film; therefore, the significant temperature dependency of the sensor response was used to shift the dynamic range. The effect of an ionic liquid additive was also investigated to increase the lifetime of these sensors. All sensors were tested spectrophotometrically, although a simple reflection based read-out device was also constructed using two light emitting diodes as light sources and two photodiodes as detectors. The device did not produce a concentration dependent signal but gave a digital yes or no (1 or 0) indicating that ammonia concentrations had reached a preset level.