An Application of a Schiff-Base Type Reaction in the Synthesis of a New Rhodamine-Based Hg(II)-Sensing Agent

A facile synthesis procedure, whereby 9-Anthraldehyde (AA) is coupled to aminated rhodamine (AR) via a Schiff base-type reaction, is reported. The applicability and performance of the obtained material (AA-AR) as a sensing agent was studied towards 16 metal cations (i.e. Li⁺, Na⁺, Ag⁺, Ca²⁺, Ba²⁺, Co²⁺, Cs⁺, Cu²⁺, Mg²⁺, Hg²⁺, Mn²⁺, Pb²⁺, Ni²⁺, Sr²⁺, Zn²⁺, Al³⁺). Among the studied metals, an extraordinary selectivity was observed for Hg²⁺, and the observed selectivity was found not to be influenced by the presence of other cations and some common anions (i.e. Br^-, Cl^-, I^-, HPO₄^2-, H₂PO₄^-, NO₃^-, NO₂^-, ClO₄^-, AcO^-, HSO₄^-, SO₄^2-, Cr₂O₇^-, CO₃^2-, OH^- and HCO₃^-). The material, AA-AR, exhibited such a high selectivity and sensitivity towards Hg²⁺ that it could be detected even by naked eyes. The Hg²⁺-sensing property of AA-AR was found not to be limited to colorimetric detections so that a high fluorescent nature of the compound was also observed upon binding Hg²⁺ ion. The detection limit, which is correspondent to fluorescence emission intensity, was found as 0.87 μM. The underlying mechanism of sensing property was studied by using some spectroscopic techniques such as FT-IR, ¹H-NMR, ¹³C-NMR, and UV-Vis. (Job-plot). In the final course of the experiments, the performance of AA-AR in cell-imaging was also studied, and even trace amounts of Hg²⁺ in living cells could be detected by the studied probe. Thus, the applicability of a new synthesis approach in producing a highly efficient new fluorescence sensor for the detection of Hg²⁺ ions is discussed in detail.