Hydrochromic CsPbBr3 Nanocrystals for Anti-Counterfeiting.

Hydrochromic materials that can reversibly change color upon water treatment have attracted much attention due to their potential applications in diverse fields. Here, for the first time, we report that space-confined CsPbBr 3  nanocrystals (NCs) are hydrochromic. When CsPbBr 3  NCs are loaded into a porous matrix, reversible transition between luminescent CsPbBr 3  and non-luminescent CsPb 2 Br 5  can be achieved upon the exposure/removal of water. The potential applications of hydrochromic CsPbBr 3  NCs in anti-counterfeiting are demonstrated by using CsPbBr 3  NCs@mesoporous silica nanospheres (around 100 nm) as the starting material. Due to the small particle size and negatively charged surface, the as-prepared particles can be laser-jet printed with high precision and high speed. We demonstrate the excellent stability over repeated transformation cycles without color fade. This new discovery may not only deepen the understanding of CsPbX 3 , but also open a new way to design CsPbX 3  materials for new applications.