Quantification of indicator content in silica-based pH solid sensors by diffuse reflectance spectroscopy

Abstract

The amount of alizarin red, brilliant yellow or acridine encapsulated within silica matrixes as colorimetric pH sensors, produced by three sol–gel routes (acid catalyzed, base catalyzed and non-hydrolytic routes), was investigated using diffuse reflectance spectroscopy in the UV-Vis region. Univariate calibration curves using silica (commercial or sol–gel synthesized) as a blank were linear (R² > 0.99) but did not allow coherent quantification of the pH indicator, likely due to the matrix effect. The standard addition method using the silica blank (produced by the corresponding sol–gel route) to correct background afforded coherent results with standard deviation between 0.017 and 0.59, depending on the sol–gel route and indicator.