Novel self-assembled cobalt sulfide nanorods/reduced graphene oxide as an efficient nano-catalyst for dual sensing of 4-nitrophenol.

A sensitive and sustainable electrochemical sensor based on cobalt sulfide nanorods/reduced graphene oxide (CoS-NR/rGO) was fabricated for the detection of 4-nitrophenol (4-NP). rGO derived from recycled plastic waste, served as a conductive and eco-friendly matrix, while CoS-NR were synthesized via a simple chemical route using cobalt acetate and thiourea. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed uniform CoS-NR anchored on thin rGO sheets, forming a high surface area architecture conducive to rapid charge transfer. X-ray diffraction (XRD) confirmed the hexagonal CoS phase and the (002) peak of rGO, whereas Raman spectroscopy identified the D and G bands of rGO and characteristic E g, F 12g, F 22g, and A 1g vibrational modes for CoS. X-ray photoelectron spectroscopy (XPS) verified the presence of Co2+/Co3+, S2-, and sp2-hybridized carbon, indicating strong chemical bonding and homogeneous distribution. Electrochemical characterization using cyclic voltammetry (CV) demonstrated enhanced catalytic activity toward 4-NP reduction due to the synergistic effect between CoS and rGO. The sensor achieved high sensitivity (0.118 µA µM-1), a low detection limit (LOD = 2.4 µM), and excellent repeatability over ten cycles. Optimization via response surface methodology (RSM) revealed a maximum current response at pH 7 under increased analyte concentration and scan rate. These results establish CoS-NR/rGO as an efficient, reproducible, and environmentally friendly platform for detecting phenolic pollutants.