In Vitro: After incubation with PMQA (20 μM) for 1 h, weak green fluorescence appeared, presumably caused by the probe capturing intracellular zinc from its native ligands since PMQA has high affinity for zinc. The response of PMQA to Zn2+ can be reversed by N,N,N’,N’-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or EDTA. When the stock solution of PMQA (0.1 mM in Tris Buffer) is kept at room temperature for a week, there is no significant loss of PMQA determined by HPLC analysis.