Caprolactone-Based Biodegradable Polymer for Selective, Sensitive Detection and Removal of Cu²⁺ Ions from Water

Even though heavy and transition metals originated in the earth's crust, the significant human exposure and environmental pollution consequences of anthropogenic activities include industrial production and waste, mining and smelting operations, and agricultural and domestic usage of metals. Because of their nonbiodegradable nature, heavy metal ions such as Cu²⁺ accumulate very quickly in plants and edible animals, ultimately ending up in the human food cycle. Therefore, to nullify the detrimental effects of Cu²⁺ ions for the sake of the environment and living organisms, we are motivated to design a sensor molecule that can not only detect Cu²⁺ ions but also remove them selectively from the water medium. To detect the Cu²⁺ ions, we synthesized a monomer (NCu) and its biodegradable caprolactone-based polymer (PNCu). It was observed that both NCu and PNCu showed higher selectivity toward Cu²⁺ ions by changing the color from colorless to yellow, with a limit of detection value of 29 nM and 0.3 μM. Furthermore, removing the Cu²⁺ ions from the water solution was also accomplished by introducing the hydrophobicity of the polymer (PNCu) through the ring-opening polymerization process. Due to increased hydrophobicity, the polymer produced a yellow color precipitate upon adding Cu²⁺ ions to the solution; thus, removal of the metal ion is possible using our designed polymer and its detection ability. We checked the removal efficiency of our polymer by using UV-vis spectroscopy and EDX analysis, which indicated that almost all of the copper is removed by our polymer. Therefore, to our knowledge, this is the first biodegradable caprolactone-based polymer for colorimetric turn-on detection and separation of the Cu²⁺ ions from the water.