Highly sensitive and selective turn-on fluorescent probes for Cu²⁺ based on rhodamine B

A new rhodamine B-based fluorescent probe RL for Cu²⁺ has been designed, synthesized, and characterized. It exhibits a very high selectivity and sensitivity for the detection of Cu²⁺ in CH₃CN : H₂O = 1 : 9 (v/v) buffered with Britton-Robinson (pH = 7.02), the detection limit can reach 0.739 nM, which is the lowest reported. Furthermore, the probe RL can be made into test papers to detect the cupric ions in drinking water by the naked eye. RL can also be used for imaging Cu²⁺ in living cells and is almost non-toxic.