In an article on the detection of Zn(II) that is released from neuronal tissue (brain, nerves) following trauma, E. Tomat and S.J. Lippard describe the development of special ligands that are highly selective for Zn and allow it to be imaged by a technique called confocal microscopy (Curr. Opin. Chem. Biol., 2010, 14, 225). Using your knowledge of the coordination chemistry of Zn, explain the principles underlying this research.