We investigated the time course of interaction between three different concentrations of iodide and three different concentrations of silver TNPs. As it showed in Supporting Information Fig.S14, the reaction rate is dependent on iodide and silver TNPs concentration, higher concentration, rapider speed. For detectioniodide in the range from 1.0 to 10.0 × 10−7M, at least half an hour was required, and more time of waiting will not affect the discriminating of critical color.