A turn-on fluorescence sensor, Cu2+-c-mpg-C3N4, was developed for detection of CN- in aqueous solution by simply mixing cubic mesoporous graphitic carbon nitride (c-mpg-C3N4) and aqueous solution of Cu(NO3)(2). The highly sensitive detection of CN- with a detection limit of 80 nM is not only possible in aqueous solution but also in human blood serum.