Paper-based colorimetric sensor for detection of chloride anions in water using an epoxy-silver nanocomposite

An epoxy-silver nanocomposite printed on paper was prepared for colorimetric detection of chloride anions (Cl$^-$) in aqueous solution. This paper-based sensor provides a promising platform with attractive advantages such as simple fabrication, intense colors, fast naked-eye response, and high specificity toward Cl$^-$ detection. The sensor undergoes a color change from yellow-orange to chestnut-brown in the presence of water and turns to green-brown in the presence of Cl$^-$. A good linear relationship ($R^2$=0.9754) between logarithmic Cl$^-$ concentration and the extinction intensity difference at 515 nm was observed at concentrations in the 20-400 mM range with a limit of detection (LOD) of 14 mM, far beyond the usual concentrations at which most of the Cl$^-$ colorimetric sensors are limited. It was also proposed a sensing mechanism based on the oxidative etching of anisotropic silver nanoparticles (from 20 to 250 nm) by Cl$^-$ and subsequent formation of chain-like aggregates, resulting in strong interparticle plasmonic coupling. This paper-based sensor can distinguish Cl$^-$ from other ions such as F$^{-}$, OH$^{-}$, NO$_{3}^{-}$, SO$_{4}^{2-}$, HPO$_{4}^{2-}$, H$_{2}$PO$_{4}^{-}$, H$^{+}$, K$^{+}$, Na$^{+}$, NH$_{4}^{+}$, Zn$^{2+}$, and Co$^{2+}$ and from mixtures of these ions. The nanosensor was also tested to recognize Cl$^{-}$ in seawater and a commercial electrolyte solution, even using volumes as small as 4 $\mu$L, suggesting its easy inclusion in portable devices. This novel colorimetric platform is undoubtedly useful for recognizing Cl$^-$ in environmental and physiological systems.