Effect of halide anions on the morphology of anisotropic silver nanoparticles printed on paper and applications in selective sensing.

Selective and sensitive detection of halides in water is critically important in the diagnosis of diseases and assessment of aquatic ecosystems. The recognition is usually carried out by numerous analytical techniques such as ion chromatography, fluorescence, near-infrared spectroscopy, ion-selective electrode and light scattering. Although these methods offer high sensitivity, they are expensive and time-consuming. Therefore, great efforts have been focused on the sensing of halide anions in water through a simple, fast, low-cost and portable paper-based colorimetric platform. In particular, the sensing process with silver nanoparticles interacting with analytes is of great chemical and biological interest. In this work, anisotropic silver nanoparticles in size and shape embedded in epoxy resin were prepared and subsequently printed on filter papers of cellulose. The influence of water and halide anions (Cl−, Br− and I−) on the morphology of these nanoparticles are investigated by scanning electron microscopy, Raman shift and diffuse reflectance spectroscopy. The mechanism involves aggregation of faceted silver nanoparticles induced by interactions between carbonyl groups of the resin and water. Subsequently, the interaction between halide anions and the aggregated silver nanoparticles causes oxidative etching on their surface, shape conversion (faceted to quasi-spherical shapes), decreasing in the particle size and formation of chain–like or compact aggregates resulting in strong interparticle plasmonic coupling. This selective mechanism was tested for the naked-eye colorimetric recognition of chloride anions in a complex simulated physiological system, whose results show promising perspectives to develop diagnostic-sensing devices. [ABSTRACT FROM AUTHOR]