An explosive situation: By using gold nanoparticles and taking advantage of the donor–acceptor interaction between trinitrotoluene (TNT) and cysteamine, the visualization of TNT can be achieved at picomolar levels (see picture). The color change from red to blue can be seen with the naked eye, which allows sensitive on‐the‐spot detection.

A gold‐based assay: By taking advantage of the unique optical properties of gold nanoparticles (AuNPs) and the cascade reactions of glucose, a simple but effective method has been successfully developed for the direct colorimetric visualization of glucose in the rat brain. GOD=glucose oxidase.