In this study, we developed electrochemical sensors based on the composite of hydroxylated multiwalled carbon nanotubes (MWCNT-OH) and graphene for paraoxon-ethyl detection as pesticide residues in agricultural products. Chemical treatment was employed to produce MWCNT-OH from pristine MWCNT and its composite with graphene was subsequently characterized using FTIR, Raman spectroscopy, FESEM-EDX, TEM, and XPS techniques. The MWCNT-OH/graphene composite was employed as an electrode modifier on the glassy carbon electrode (GCE) surface, and its electroanalytical performances were studied using differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) techniques. It was revealed the optimum composition ratio between MWCNT-OH and graphene was 2:8, for paraoxon-ethyl detection at pH 7. This could be attributed to the enhanced electrocatalytic activity in the MWCNT-OH/graphene composite which displayed a linear range of paraoxon-ethyl concentration as 0.1–100 μM with a lower detection limit of 10 nM and a good sensitivity of 1.60 μA μM cm− 2. In addition, the proposed sensor shows good reproducibility, stability, and selectivity in the presence of 10 different interfering compounds including other pesticides. Ultimately, this proposed sensor was tested to determine the paraoxon-ethyl concentrations in green apples and cabbage as samples of agricultural products. The obtained concentrations of paraoxon-ethyl from this proposed sensor show no significant difference with standard spectrophotometric techniques suggesting this sensing platform might be further developed as a rapid detection of pesticide residue in agricultural products.

MWCNT-OHgraphene composite sensor for nonenzymatic detection of paraoxon-ethyl in agricultural samples
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Reagents and apparatus
        2.2 Instrumentations
        2.3 Synthesis of hydroxylated multiwalled carbon nanotubes (MWCNT-OH)
        2.4 Preparation of a glassy carbon electrode modified with the composite of MWCNT-OH and graphene (MWCNT-OHgrapheneGCE)
        2.5 Investigation of the electrochemical behavior of MWCNT-OHgrapheneGCE
        2.6 The investigation of the analytical performance of MWCNT-OHgrapheneGCE
        2.7 Determination of paraoxon-ethyl concentration in agricultural products using MWCNT-OHgrapheneGCE
    3 Results and discussion
        3.1 The characterization of electrode modifier materials using FTIR, Raman spectroscopy, FESEM-EDS, and TEM
        3.2 Material characterization using X-ray photoelectron spectroscopy (XPS) technique
        3.3 The investigation of the electrochemical behavior of the modified GCEs using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) techniques
        3.4 The electrochemical response of paraoxon-ethyl reaction in different electrodes
        3.5 The investigation of the electroanalytical performance of MWCNT-OHgrapheneGCE to detect paraoxon-ethyl via its reduction current response
        3.6 Real sample analysis using MWCNT-OHgrapheneGCE
    4 Conclusion
    Acknowledgements 
    References

• Budi Riza Putra(Research Center for Metallurgy, National Research and Innovation Agency (BRIN), PUSPIPTEK Gd. 470, South Tangerang, Banten 15315, Indonesia)
• Elda Nurwidayanti(Analytical Chemistry Division, Department of Chemistry, Faculty of Mathematics and Natural Sciences, Bogor Agricultural University, Bogor 16680, Indonesia)
• Wulan Tri Wahyuni(Analytical Chemistry Division, Department of Chemistry, Faculty of Mathematics and Natural Sciences, Bogor Agricultural University, Bogor 16680, Indonesia, Tropical Biopharmaca Research Center, Bogor Agricultural University, Bogor 16680, Indonesia) Corresponding author
• Siti Fadilah(Directorate of Laboratory Management, Research Facilities, and Science and Technology Park, National Research and Innovation Agency (BRIN), Jakarta Pusat, Indonesia)
• Munawar Khalil(Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok 16424, Indonesia, Low Dimension Materials Lab., Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok 16424, Indonesia, Institute for Advanced Sustainable Materials Resarch and Technology (INA‑SMART), Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok 16424, Indonesia)
• Erus Rustami(Department of Physics, Faculty of Mathematics and Natural Sciences, Bogor Agricultural University, Bogor 16680, Indonesia)