Dual-target sensors: the effect of the encapsulation route on pH measurements and ammonia monitoring

Abstract

Six different sensors were prepared using hydrolytic and non-hydrolytic sol–gel routes and three different pH indicators as receptor elements. These materials were employed as dual-target sensors for ammonia gas sensing and pH measurements in real samples. Ultraviolet–visible diffuse reflectance spectroscopy and a colorimetric analysis were used to evaluate the pH transitions and the color change that resulted from the ammonia that was employed as a probe for gas sensing. The colorimetric analysis was performed using the CIELAB color space model with the D65 standard illuminant. The resulting $\Updelta {\text{E}}_{\text{ab}}^{*}$ values correlated with the response time for the observation of the first visible color change detected by the human eye (N = 50), and the average time values ranged from 48 to 117 s. In addition, the response times were evaluated as a function of sample characteristics, namely, the gender, age and eye color. Only groups of these characteristics exhibited relevance to the response time and could be associated with a specific range of time. The sensors were also employed for pH measurements of an aqueous solution, and a double-color scale could be established both with the sensors synthesized by the acid route and with the pH indicators alizarin red and brilliant yellow. From this color scale, it was possible to identify a shift in the turning point of the pH indicator from the free molecule to the encapsulated molecule. The double-color scale was applied to evaluate real samples from a galvanic industry using the synthesized encapsulated sensors.