Computational and Experimental Studies on Mixed-Phase Copper Species Deposited on Nitrogen-Doped Carbon Nanofibers for Electrochemical Sensing of Norfloxacin

This work reports the synthesis of mixed-phase copper species deposited on nitrogen-doped carbon nanofibers (Cu@N-CNF) involving thermal annealing of the CNF followed by solvothermal treatment for the deposition of copper species. The developed sensor was applied for the electrochemical sensing of antibiotic norfloxacin (NOR). The Cu@N-CNF was characterized using X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), high-resolution transmission electron microscopy, and Raman spectroscopy. Hybrid morphology of Cu@N-CNF was revealed showing oval patches deposited over fibrous structure with intermittent dark spots. XRD revealed that copper is present in its various forms such as CuO, Cu(OH)₂, along with the g-C₃N₄ sheets. Furthermore, XPS analysis provided the atomic percentages of C 1s, N 1s, Cu 2p, and O 1s, while FTIR analysis identified the presence of various functional groups. The antibiotic NOR was detected with cyclic voltammetry and differential normal pulse voltammetry techniques. A linear relationship was obtained between current (μA) response and concentration of NOR varying from 0.6 to 11.7 µM with a limit of detection (LOD) of 400 nM. The limit of quantification obtained is 1.4 μM with improved sensitivity of 4.53 µA/µM·cm². The developed sensor was also tested on human urine samples with a 1.97 μM LOD. It was further observed that the oxidation of NOR followed a diffusion-controlled mechanism. The physicochemical properties and interaction dynamics between the analyte and material were thoroughly investigated using a combination of electrochemical analysis and density-functional theory calculations. The deposition of mixed-phase Cu species on N-CNF decreases the highest occupied molecular orbital-lowest unoccupied molecular orbital gap in Cu@N-CNF that indicates the stability of the material.