Solid-State Fluorescent Organic Polymers for Visual Detection and Elimination of Heavy Metals in Water

Abstract

Selective sensing and removal of toxic heavy metals from water are highly essential since their presence poses significant health and environmental hazards. Herein, we designed and synthesized a novel fluorescent nonconjugated organic polymer by strategically incorporating two key functional groups, namely, a dansyl fluorophore and dithiocarbamate (DTC). Different characterization techniques, including ¹H nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDAX), Fourier transform infrared (FTIR), and fluorescence spectroscopy, were performed to understand its structure and material properties. The quantum yield of 4.72% and its solid-state fluorescence indicate that it has potential for various applications in several technological and scientific domains. In this study, we investigated a specific application involving the detection and elimination of heavy metals from water. Interestingly, the presence of dansyl and DTC moieties demonstrated remarkable selectivity toward Cu²⁺, Co²⁺, Ni²⁺, Fe³⁺, and Fe²⁺ sensing, displaying distinct color changes specific to each metal. Cu²⁺ resulted in a yellow color, Co²⁺ showed a green color, Ni²⁺ displayed a pale yellowish-green color, and Fe²⁺/Fe³⁺ exhibited a brown color. The LOD (limit of detection) for each metal was obtained in the nanomolar range by using a fluorescence spectrometer and the micromolar range from UV–visible spectra: 13.27 nM and 0.518 μM for Cu²⁺, 8.27 nM and 0.581 μM for Co²⁺, 14.36 nM and 0.140 μM for Ni²⁺, 14.95 nM and 0.174 μM for Fe²⁺, and 15.54 nM and 0.33 μM for Fe³⁺. Moreover, the DTC functionality on its backbone facilitates effective interaction with the aforementioned heavy metals, subsequently removing them from water (except Fe²⁺ and Fe³⁺), validating its dual functionality as both an indicator and a purifier for heavy metals in water. The polymer exhibited a maximum adsorption capacity of 192.30 mg/g for Cu²⁺, 159.74 mg/g for Co²⁺, and 181.81 mg/g for Ni²⁺. Furthermore, this approach exhibits versatility in crafting fluorescent polymers with adjustable attributes that are suitable for a wide range of applications.