A nano-powered green and chemically synthesized Au/MWCNT modified electrochemical sensor for methylene blue detection in river water

This study explores the development of novel gold nanoparticle (AuNP) and multi-walled carbon nanotube (MWCNT) nanocomposites for methylene blue (MB) dye detection, leveraging both green (Au_grn) and chemical (Au_chm) synthesis methods. A thorough analysis of the nanomaterials synthesized using green and chemical routes was performed utilizing Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), revealing particle sizes of 13.66 nm and 14.86 nm for Au_chm and Au_grn, respectively. UV-visible spectroscopy (UV) and X-ray diffraction (XRD) reveal crystallite sizes ranging from 5.36 nm to 21.26 nm. Electrochemical analysis <i>via</i> cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and square-wave voltammetry (SWV) revealed distinct current responses among the synthesized materials. EASA, which is the electrochemical active surface area of the electrodes was calculated, and the values were 0.053 cm² (Au_chm/MWCNTs), 0.031 cm² (Au_grn/MWCNTs), 0.024 cm² (MWCNTs), 0.006 cm² (Au_grn), 0.005 cm² (Au_chm), and 0.002 cm² (bare). EIS showed <i>R</i> _ct values in the following order: 32.20 Ω < 34.02 Ω < 36.61 Ω < 3.4 × 10⁵ Ω < 3.7 × 10⁵ Ω < 5.6 × 10⁵ Ω for Au_grn/MWCNTs, MWCNTs, Au_chm/MWCNTs, Au_chm, Au_grn, and bare electrode, respectively, which correlated with CV oxidation peaks in FeCN, except for the bare electrode due to the <i>n</i>-value of 0.87. The oxidation current response in MB decreased in the order of 124.29 μA for MWCNTs, 114.77 μA for Au_grn/MWCNTs, 60.85 μA for Au_chm/MWCNTs, 18.96 μA for Au_chm, 2.81 μA for bare, and 2.08 μA for Au_grn. The limits of detection (LOD) and quantification (LOQ) were determined to be 20.62 nM and 62.51 nM for Au_chm/MWCNTs and 20.23 nM and 61.30 nM for Au_grn/MWCNTs, respectively, indicating slightly superior sensitivity for Au_grn/MWCNTs. Analysis of real-life environmental samples demonstrated the practical applicability of the synthesized electrodes, with recovery percentages ranging from 90% to 107% (<i>n</i> = 3). These findings underscore the dependability and sensitivity of the developed nanocomposites for MB detection.