Facile synthesis of water soluble fluorescent metal (Pt, Au, Ag and Cu) quantum clusters for the selective detection of Fe3+ ions as both fluorescent and colorimetric probes

A facile and green synthetic route is developed to prepare water soluble, green-fluorescent Pt, Au, Ag and Cu quantum clusters (QCs), employing innoxious L-histidine as a capping agent. These metal QCs give out emissions at 455-495 nm, with maximum excitation wavelengths ranging between 350 and 401 nm. Interestingly, these histidine-stabilized metal quantum clusters also possess non-linear optical properties like two-photon luminescence, which is detected when they are exposed to a femtosecond laser at 812 nm. Luminescence quantum yields of Pt (2.93%), Au (13.10%), Ag (6.97%) and Cu (8.29%) clusters are found to be distinctly different, and lifetime measurement results suggest that the emissions of metal clusters (4.59-7.39 ns) stem from singlet transitions among different electron energy levels rather than metal-ligand electron transfer behaviors. The Pt, Au, Ag and Cu quantum clusters have average sizes of 1.3-1.9 nm, and are mainly composed of 11, 9, 4 and 7 metal atoms, respectively, according to the electrospray ionization mass spectrometry results. Moreover, the as-prepared metal clusters are sensitive to aqueous ferric ions, both as highly selective fluorescent and colorimetric probes in a low and wide concentration range, and are promising in many biological applications.