Considering the significance and urgency for the recognition and sensing of anions specifically, especially those of biological relevance, herein, a simple and reliable colorimetric iodide sensor that based on pH-dependent interaction of silver nanoparticles (AgNPs) and H2O2 was developed. In acidic medium, AgNPs reacted with H2O2 to produce Ag(+) and powerful oxidizing species. The powerful oxidizing species could etch AgNPs seriously. While, iodide acted as an antioxidant could protect AgNPs from oxidation-etching by the powerful oxidizing species. In neutral and alkaline medium, the reaction of AgNPs and H2O2 mainly produce Ag(+). The existence of iodide could complex with Ag(+), forming AgI, which paved the way for aggregation of AgNPs. Based on the different responses of iodide to these different products of the reaction between H2O2 and AgNPs in solutions with different pH, iodide with concentrations down to 1 nM in acidic medium, 6 nM in neutral medium, and 100 nM in alkaline medium could be detected by naked-eye. More importantly, urinary iodide had been detected successfully. This simple and speedy method, which also exhibited remarkable selectivity and outstanding sensitivity, not only innovated the field of iodide recognition but also opened up a novel insight of the application of AgNPs.
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