A novel colorimetric and fluorometric chemosensor for H g2' detection based on poly (p-phenyleneethynylene) (PPE) containing benzo[2,1,3]thiadiazole (BDT) has been developed. A highly Hg 2+_selective fluorescence quenching property in conjunction with a visible colorimetric change from yellow to violet can be observed, which indicates that PPE-BDT can serve as a sensitive 'naked-eye' indicator for Hg'+.