Fig. 7. (A) Two colour staining. E. coli cells stained with the mixture of CdSe QDs and AO. Red fluorescence and green fluorescence signals corresponds to CdSe QDs and AO, respectively. (B) Mercury sensing in E. coli cells. The cells were stained with CdSe QDs/AO and then, treated with Hg²⁺. AO green fluorescence indicates cells are live and the fluorescence quenching of CdSe QDs is attributed to the selective sensing of accumulated Hg²⁺ inside the cells (magnification – 100×).