Research description: As we all know, Selenium (Se) is an essential micronutrient element，insufficient or excessive intake of Se has been associated with a number of diseases. Selenoproteins are the key form of selenium that exist and play biological functions in vivo. In the past, the main study is to test the total selenoproteins or selenium in vivo, but ours is aim at detecting a single definite selenoprotein - glutathione peroxidase 4 (GPx4), which has a wide range of biological functions mainly the antioxidant function. Under the biorthogonal chemistry, we designed and synthesized the target functional ligand-three peptide mutations via quenched ligand -directed tosylate (Q-LDT) reagents. By tracking the fluorescence we can detect the active GPx4. By the way, one of the peptides can serve as an inhibitor of GPx4 and therefore can provide a theoretical basis and research for the subsequent development of anticancer drugs.