Design of novel and modified dual optode membrane based on carbon dots for both ultratrace copper(ii) and cobalt(ii): derivative spectrophotometric and central composite design study

Abstract

A selective, accurate and highly sensitive optical chemical sensor (optode) for the simultaneous determination of copper(II) and cobalt(II) without any prior separation or purification ion sensing was developed. The optode was designed by incorporating carbon dots (CDs) and N′-(2-hydroxy-5-iodobenzylidene)isonicotinohydrazide (HIBIN) in plasticized polyvinyl chloride. The membrane composition, such as the ionophore, ionic additive and plasticizer/immobilizer mass ratio, was optimized by central composite design (CCD). Overlaps between the spectra of copper(II) and cobalt(II) complexes were eliminated via a second-derivative zero-crossing spectrophotometric method. The constructed membrane was characterized by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). The synthesized CDs were investigated via FE-SEM, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) analyses. Cross-sectional SEM images showed that the average thickness of the prepared thin film membrane was 306.6 nm. The proposed optical sensor displayed a calibration response for Cu(II) and Co(II) ions over concentration ranges of 9.23 × 10⁻⁸ to 1.08 × 10⁻⁵ M, with a limit of detection of 28.1 nM, and 2.36 × 10⁻⁷ to 1.03 × 10⁻⁵ M, with a limit of detection of 13 nM, respectively, whereas its response time for both ions was determined to be 4.5 min. The stability, reproducibility, regenerability and repeatability of the proposed optical sensor were studied, which revealed good results. The proposed optical sensor was employed successfully for the simultaneous determination of Cu(II) and Co(II) ions in various water samples.