Copper Fluor-4 (CF4) is a Cu⁺-specific fluorescent probe based on a rhodol dye scaffold. Copper Fluor-4 has high copper selectivity, particularly over zinc and iron, as well as abundant cellular alkali and alkaline earth metals. Copper Fluor-4 is stable in a physiologically relevant pH regime between 6 and 8 (Ex: 488 nm)^[1].