A single fluorescent probe for simultaneous detection of Zn2+, Cd2+, and Pb2+ in industrial wastewater with application in bio-imaging and molecular logic gates

Abstract

Detecting multiple metal ions, especially those from the same periodic group, is quite challenging with a single fluorophore-based sensing method because of their similar chemical properties. We introduce a unique fluorescent probe that can identify Zn²⁺, Cd²⁺, and Pb²⁺ simultaneously from their mixture, even in the presence of other metal ions, with the first two being in the same group of the periodic table. The design of the probe (L₁) is based on the selective binding of Zn²⁺ or Cd²⁺, which triggers the fluorescence response of the phenolic aldehyde chromophore moiety, changing it from the off-mode to the on-mode due to its linkage with 2-(((pyridin-2-yl)methylamino)methyl)phenol, which restricts the photo-induced electron transfer (PET) process upon metal ion binding. Pb²⁺ acts as a unique metal ion, replacing Cd(II) but not Zn(II) from their respective complexes with L₁. Using the preferential interaction of Pb²⁺ with the Cd(II)-complex, we were able to quantify not only Zn²⁺ and Cd²⁺, but also Pb²⁺ in their mixture with a detection limit below 0.01 μM. This can be achieved by formulating suitable analytical equations to determine the intensity value related to Cd(II)/L₁ from the total intensity affected by all three metal ions, as well as the intensity reduction caused by Pb²⁺ for Cd(II)/L₁. We measured Zn²⁺, Cd²⁺, and Pb²⁺ contents in nearby industrial wastewater. Fluorescence bio-imaging studies indicate that the probe is very effective in identifying elevated levels of Zn²⁺ and Cd²⁺ in cancer cells. Furthermore, we demonstrated that the reversible fluorescence responses of L₁ in the presence of Zn²⁺, Cd²⁺, and Pb²⁺ are suitable for application in molecular logic gates.