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 (R² = 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⁻, NO₃⁻, SO₄²⁻, HPO₄²⁻, H₂PO₄⁻, H⁺, K⁺, Na⁺, NH₄⁺, Zn²⁺, and Co²⁺ 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 µL, suggesting its easy inclusion in portable devices. This novel colorimetric platform is undoubtedly useful for recognizing Cl⁻ in environmental and physiological systems.