UV–vis absorption spectra data were performed on a UV-2550 Spectrophotometer (Shimadzu, Japan). Fluorescence spectra data were collected with a RF-5301PC fluorescence spectrophotometer (Shimadzu, Japan). TEM images were conducted on a Hitachi H7000 electron microscope. Fluorescence intensity decay curves were measured on a FELIX32 system (Photo Technology International). The sensor was put in a ZF-20D black-box type UV analyzer (Shanghai electro-optical instrument factory), and the fluorescent photos were taken by a digital camera. The fluorescence emission spectrum of the solution was then measured 15 min later. All optical measurements were performed at room temperature under ambient conditions, and the excitation wavelength (λex) was 388 nm with the slit of 5 nm and 10 nm, respectively.