Development and Applications of Fluorescent Indicators for Mg2+ and Zn2+

In a study of the spectroscopic behavior of two Schiff base derivatives, salicylaldehyde salicylhydrazone (1) and salicylaldehyde benzoylhydrazone (2), Schiff base 1 has high selectivity for Zn2+ ion not only in abiotic systems but also in living cells. The ion selectivity of 1 for Zn2+ can be switched for Mg2+ by swapping the solvent from ethanol−water to DMF (N,N-dimethylformamide)−water mixtures. Imine 2 is a good fluorescent probe for Zn2+ in ethanol−water media. Many other ions tested, such as Li+, Na+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Ag+, Cd2+, Sn2+, Ba2+, Hg2+, and Pb2+, failed to induce any spectral change in various solvents. The selectivity mechanism of 1 and 2 for metal ions is based on a combinational effect of proton transfer (ESPT), C═N isomerization, and chelation-enhanced fluorescence (CHEF). The coordination modes of the complexes were investigated.