Sulfur-Bridged Co(II)-Thiacalix[4]arene Metal–Organic Framework as an Electrochemical Sensor for the Determination of Toxic Heavy Metals

A novel sulfur-bridged metal-organic framework (MOF) [Co(TIC4R-I)_0.25Cl₂]·3CH₃OH (Co-TIC4R-I) based on thiacalix[4]arene derivatives was successfully obtained using a solvothermal method. Remarkably, adjacent TIC4R-I ligands were linked via Co(II) cations to form a three-dimensional (3D) microporous architecture. Subsequently, Co-TIC4R-I was modified on a glassy carbon electrode (Co-TIC4R-I/GCE) to produce an electrochemical sensor for the detection of heavy-metal ions (HMIs), namely, Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, in aqueous solutions. It was found that Co-TIC4R-I/GCE exhibited wide linear detection ranges of 0.10-17.00, 0.05-16.00, 0.05-10.00, and 0.80-15.00 μM for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, respectively, in addition to low limit of detection (LOD) values of 0.017, 0.008, 0.016, and 0.007 μM. Moreover, the fabricated sensor employed for the simultaneous detection of these metals has achieved LOD values of 0.0067, 0.0027, 0.0064, and 0.0037 μM for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, respectively. The sensor also exhibited satisfactory selectivity, reproducibility, and stability. Furthermore, the relative standard deviation (RSD) values of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ were 3.29, 3.73, 3.11, and 1.97%, respectively. Moreover, the fabricated sensor could sensitively detect HMIs in various environmental samples. The high performance of the sensor was attributed to its sulfur adsorption sites and abundant phenyl rings. Overall, the sensor described herein provides an efficient method for the determination of extremely low concentrations of HMIs in aqueous samples.