Natural dye modified hemp fibrous foam for colorimetric NH3 sensing

Abstract

Environmental pH monitoring through color change has attracted much attention, especially for the application as food freshness indicator. However, how to unlock its safety by using natural resources has not been taken into account. Herein, a colorimetric pH foam indicator was developed by loading the natural dye anthocyanin onto hemp fiber (Anth-foam). Anth-foam made from biomass presented advantageous properties such as nontoxicity, low density, and high porosity. The colorimetric response of as-prepared Anth-foam to acidity and alkalinity in both aqueous and vaporous conditions were investigated using UV-Vis spectroscopy and CIELab analyses. All the color changes in pH range from 2 to 10 were perceivable by naked eyes. Compared with film-based indicator, the colorimetric responsive time was significantly shortened by 60% due to the porous structure of Anth-foam. The Anth-foam exhibited a greatly increased color change as NH 3 concentration increased to 250 ppm and an excellent cycling performance for retaining 89.5% of the original colorimetric response over several cycles. These properties made the Anth-foam a promising material for practical applications as a colorimetric pH indicator. • A porous colorimetric indicator was prepared from all natural resources. • Foam indicator responds faster than film indicator under the same conditions. • Sensitivity to different ammonia concentration was quantitatively investigated. • Color change of the foam indicator is reversible, leading to excellent reusability.