Ctrl-CF4-S2 is a control probe for the chemically modified Copper Fluor-4 (CF4), in which two of the four thioether ligands of the CF4 probe are replaced with methylene groups. Unlike CF4, which is a fluorescent probe used to detect the presence and distribution of copper ions, Ctrl-CF4-S2 does not respond to copper ions. This lack of response helps eliminate background signals from copper ions, thereby confirming whether the signals from CF4 accurately reflect the dynamic changes of copper ions within biological systems.