Cobalt-Doped ZIF-8 Nanoparticle-Decorated Fiber Optic Sensor for Copper Ion Detection

Copper is a toxic water contaminant; its quantification below the maximum contaminant level (MCL) of 1.3 ppm by a sensitive, field-deployable sensor becomes critical. Here, the cobalt-doped zeolitic-imidazolate framework (Co/ZIF-8) nanomaterial was developed as a novel, highly selective, sensitive, and stable chemoreceptor as an alternative to ZIF-8 for Cu(II) ion detection. A dip-type optical absorption Cu(II) ion sensor was realized by an in situ deposition of Co/ZIF-8 nanoparticles over a highly sensitive U-bent fiber optic sensor (FOS) probe with a remarkable high evanescent wave-based absorption (EWA) sensitivity. The Co/ZIF-8 coated FOS works on the principle of attenuation of the light passing through the FOS by the Co/ZIF-8 nanoparticles due to EWA at 400 nm wavelength upon interaction with Cu(II) ions and increases proportional to the Cu(II) ions concentrations. The interaction of Cu(II) ions with Co/ZIF-8 nanoparticles was extensively characterized with the help of UV-visible absorption, scanning electron microscopy and EDAX analysis, X-ray diffraction, and FTIR absorption spectral analysis. Cu(II) ions were found to replace Zn(II) in the Co/ZIF-8, forming Cu-imidazole complexes over and off the nanoparticles, leaving the surface rich with Co- and Cu-imidazole complexes and thereby altering the optical and morphological properties of the Co/ZIF-8. The sensor demonstrated an excellent dynamic range of 0.05-100 ppm with an experimental LoD of 50 ppb using a simple LED-photodetector-based setup, selectivity for Cu(II) against 12 other potential interfering metal ions, and a shelf life of at least 3 months. The facile and scalable Co/ZIF-8 deposition process, simpler instrumentation, excellent sensor parameters, and good recovery rates with field water samples make this strategy highly promising for on-site environmental monitoring.