The conformation of calmodulin (CaM) changes from closed confi guration to open one, converting to a claviform dumbbell-shaped biomolecule upon Ca 2+ -binding. A hybrid probe of graphene oxide (GO) cationic conjugated polymer for detection of the conformation transition of CaM by using FRET technique is demonstrated. The stronger hydrophobic interaction and weaker electrostatic repulsion leads to more CaM adsorption to the surface of GO upon binding with Ca 2+ than that of CaM in the absence of Ca 2+ (apoCaM),resulting in much farther proximity between poly[(9,9-bis(6′- N , N , N - rimethylammonium) hexyl)-fl uorenylene phenylene dibromide] (PFP) and green fl uorescent protein labeled at the N-terminus of CaM and therefore much weaker FRET effi ciency for PFP/Ca 2+ /CaM in comparison with that of PFP/apoCaM in the presence of GO. Notably, the assembly of CaM with GO is quantitatively and reversibly controlled by Ca 2+ ions.