Direct analysis of total tannins encapsulated in silica matrices by ultraviolet-visible diffuse reflectance spectroscopy

Tannins from Acacia mearnsii were encapsulated by four different sol-gel routes (acid-catalyzed hydrolytic, basic-catalyzed hydrolytic, non-hydrolytic and silica) and were quantified by diffuse reflectance spectroscopy in the ultraviolet-visible region (DRS UV-vis). Calibration curves were prepared by adding tannin solutions at different concentrations to silica (via standard addition) and by the encapsulation of tannins by four different sol-gel routes at the same concentrations. The standard addition method on a silica matrix provided poor linear calibration curves due to tannin oxidation. The present study demonstrated the feasibility of using DRS UV-vis as a tool for the quantification of tannins in a silica matrix and identified conditions necessary for obtaining reliable quantitative data. Confocal microscopy measurements indicated that the prepared standards consisted of uniformly distributed materials, but colorimetric measurements indicated that the color change, probably due to tannin oxidation, might affect the feasibility of the measurements by diffuse reflectance. This approach represents a potential alternative for the quantification of the total tannins encapsulated in solid matrices in applications such as natural additives for intelligent packaging, antimicrobiological patches and leather tanning.