Fluorescent detection of copper ions with acylhydrazine-based probes: Effects of substitute and its position

Four naphthalimide-based compounds (NHA, NHB, NHC and NHD) bearing carbazide or semi-carbazide at different positions were designed and synthesized for the detection of cooper ions. The presence of copper ions caused different spectral changes of the four compounds: the fluorescence of NHA and NHB were enhanced, while that of NHC was quenched, and little spectral changes of NHD were observed. The mechanism studies reveal that: copper ions promoted the hydrolysis of the probe containing semi-carbazide group (NHA), leading to evident fluorescence enhancement with clear red-shift; while it formed complex with the probes bearing carbazide (NHB and NHC), resulting in fluorescence “turn-on” or “turn-off”. The substituted position of carbazide group in naphthalimide affected the fluorescence significantly: strong photoinduced electron transfer occurred as it at 4-position (NHB), whereas the PET process was not easy to happen as the group at imine-N (NHC). Copper ions increased the fluorescence of NHB by blocking the PET process, while it could initiate a d-PET process and quenched the fluorescence of NHC. Fluorescence bio-imaging experiments demonstrate that both NHB and NHC exhibit good cell permeability and can monitor Cu2+ as well as thiols in living cells.