Chelation therapy-inspired design of a water-stable fluorescent probe for the effectual monitoring of copper(ii) ions in real water

Abstract

This work introduces a thought-provoking design to develop a water-soluble chemical probe, sodium 4-hydroxy-3-((E)-((E)-((2-hydroxynaphthalen-1yl)methylene)hydrazono)methyl) benzenesulfonate (SW2) and its analytical characterization for the efficient detection and monitoring of Cu²⁺ ions in a matrix of s-, d-, and f-metal ions in pure water. The water-stable molecular probe, SW2, in the presence of Cu²⁺ salts in pure water exhibits a fluorescence turn-off characteristic with a high detection limit, 3.8 μM, and irresistibly holds 4-cycle reversibility in the presence of sulphide ions without any significant loss of its chemosensing efficiency. Spectroscopic and computational studies ensure 1 : 1 complexation between SW2 and Cu²⁺ ions, leading to the formation of SW2–Cu²⁺ chelate, thus inducing dynamic quenching of SW2 emission, which subsequently reverts on the addition of S²⁻ ions in water. Additionally, the SW2–Cu²⁺ chelate was isolated in microcrystalline powder and the complexation was studied with mass spectrometry and EPR analysis. Computational analysis reveals the remarkable reduction in the S₀–S₁ energy level of the SW2–Cu²⁺ complex, which is attributed to the drastic quenching of the fluorescence intensity. Furthermore, SW2 was successfully applied to the detection of Cu²⁺ ions in tap and pond water. Interestingly, the probe is also effective for the determination of Cu²⁺ ions in the aqueous solution of a Cu-based fungicide (copper oxychloride), commercially available as Blitox in India, thus evaluating the effectiveness of SW2 in real sample analysis.