Electrochemical detection of thiamethoxam in food samples based on Co3O4 [email protected] carbon nitride composite

Ecotoxicology and Environmental Safety 189 (2020) 110035

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Electrochemical detection of thiamethoxam in food samples based on Co3O4 Nanoparticle@Graphitic carbon nitride composite

T

Jaysiva Ganesamurthi, Murugan Keerthi, Shen-Ming Chen∗, Ragurethinam Shanmugam Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, No.1, Section 3, Chung-Hsiao East Road, Taipei, 106, Taiwan

A R T I C LE I N FO

A B S T R A C T

Keywords: Insecticide Neonicotinoids Electrochemical sensor Differential pulse voltammetry Screen-printed carbon electrode

Thiamethoxam is a class of neonicotinoid insecticide widely used in agriculture. Due to their high water solubility, thiamethoxam can be transported to surface waters and have the potential to be toxic to human life. Herein, a simple and robust method is presented for the detection of thiamethoxam based on hydrothermally synthesized nanoparticles of cobalt oxide into the graphitic carbon nitride composite (Co3O4@g-C3N4 NC). The materials were well characterized by XRD, FT-IR, XPS, FESEM, HRTEM, EDX, and UV–vis which provide crystalline nature, structure, and composition. The impedance measurement shows an intimate electrode/electrolyte interface by casting Co3O4@g-C3N4 onto a screen-printed carbon electrode (SPCE), delivering an interfacial resistance as low as 12.5 Ωcm2. The cyclic voltammetry and differential pulse voltammetry measurements exhibit the nanocomposite as a superior electrocatalyst for the electrochemical detection of thiamethoxam and achieved a low detection limit of 4.9 nM with a wide linear range of 0.01–420 μM. The present work also demonstrates a promising strategy for electrochemical detection of thiamethoxam in real samples such as potato and brown rice.

1. Introduction Thiamethoxam (TMX) is a synthetic neonicotinoid insecticide, which is derived from nicotine (Ajermoun et al., 2019; Jeschke et al., 2010) and commonly used in agriculture fields to control of sucking insect pests (such as leafhoppers, whiteflies, and aphids), weeds, and pathogens (Hou et al., 2013; Jeschke and Nauen, 2008). As a promising insecticide, TMX can be used as a spray or seed treatment for the plants to kill insect pests (Ruddle et al., 2018). Thus, it effectively interacts with nicotinic acetylcholine receptors of the insect central nervous system and causing damage in their nervous, as a result of death or paralysis (Kurwadkar et al., 2016; Raina-Fulton, 2015; Robinson, 2019). Moreover, the utilization of thiamethoxam has been increased worldwide and suppressed the commercial use of organophosphates, carbamates, and pyrethroids. It is mainly due to their low mammalian toxicity, synthetic actions, lower affinity to vertebrates, and their efficacy (Pickford et al., 2018). However, TMX is inevitably released into the environment due to its extensive utility, and its residues form can be detected in agricultural lands and environmental waters, which causes adversely toxic effects to the nontarget organisms (Bartell et al., 2018; Raby et al., 2018). Regarding this European country, food safety authority has limited the maximum residue level range of TMX in agricultural products should be 0.02 mg/kg and 5.0 mg/kg (Xie et al., ∗

2017). Therefore, the determination of TMX residues in agricultural, environmental waters and food products is important necessary to ensure health risks to animals and humans. Several analytical techniques have been developed for the determination of TMX such as high-performance liquid chromatography (HPLC) (Seccia et al., 2008; Watanabe et al., 2007), mass spectrometry (MS) (Di Muccio et al., 2006; Fidente et al., 2005; Seccia et al., 2005), thermal lens spectrometry (TLS) (Guzsvány et al., 2007), enzyme-linked immunosorbent assay (ELISA) (Kim et al., 2003), and electrochemical sensor (Ajermoun et al., 2019). Among these techniques, electrochemical sensor attention as attracted more due to their advantages including low cost, ease of operation, rapid response, high sensitivity, and selectivity. For example, A. Kumaravel and M. Chandrasekaran (Kumaravel and Chandrasekaran, 2012) demonstrated the electrodeposition synthesis of nanosilver/SDS modified glassy carbon electrode for electrochemical detection of TMX. The result showed a good linear range of 0.1–9 μM with the detection limit of 0.1 μM. However, it is still necessary to improve the low level detection of TMX for environmental safety. Thus, designing excellent electrocatalytic materials for the electrode is a challenging approach. Recently, cobalt oxide (Co3O4) has been paid intensive attention for its wide range of applications such as pseudocapacitors (Wu et al., 2011; Xia et al., 2011), photocatalysis (Ji et al., 2019; Xiao et al., 2008), overall evolution reactions (Wang et al., 2018; Wei et al., 2019), and

Corresponding author. E-mail addresses: [email protected], [email protected] (S.-M. Chen).

https://doi.org/10.1016/j.ecoenv.2019.110035 Received 2 September 2019; Received in revised form 26 November 2019; Accepted 29 November 2019 0147-6513/ © 2019 Elsevier Inc. All rights reserved.

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Scheme 1. The schematic diagram for the synthesis of Co3O4@g-C3N4 and its electrochemical sensing application towards TMX.

2. Experimental section

electrocatalyst (Xu et al., 2012; Zhong et al., 2014), which is because of their unique physicochemical properties such as conductivity, thermal stabilities and great reversibility. However, the obtained results of most reported Co3O4-based electrodes are still far lower in electrochemical performance (Haldorai et al., 2016; Zhang et al., 2013). To improve the electrochemical performance of Co3O4 electrodes have a general strategy like fabrication of Co3O4 nanoparticles with desirable architecture, large surface areas, and high conductivity material such as activated carbon, porous carbon and graphene derivatives. Regarding this, graphitic carbon nitride (g-C3N4) is a metal-free semiconductor, which termed as an N-substituted graphitic π – π conjugated 2D layered structure consisting of periodic aromatic tris-triazine repeating units connected by planar amino groups. Owing to its excellent chemical and thermal stability, unique electronic structure, cost-effectiveness, and easy preparation; g-C3N4 has triggered intense research investigation in the applications of solar water splitting (Li et al., 2015; Ye et al., 2015), visible-light photocatalytic activity (Cui et al., 2017; Dong et al., 2013), CO2-reduction (Di et al., 2017; Ye et al., 2015), electrochemical sensing (Zhuang et al., 2015; Zou et al., 2018). In addition, g-C3N4 can be easily coordinated with other materials through its intrinsic structure, which possesses abundant pyridine nitrogen groups having lone pair of electrons capable of providing more metal coordination sites as catalytically active sites during the electrocatalytic process. Thus, it would be a great advance if Co3O4 can be developed as an excellent electrocatalyst for electrochemical performance by combining the abovementioned strategies. Herein, we successfully fabricated Co3O4@g-C3N4 nanocomposite via a facile hydrothermal method and acts as a highly efficient electrocatalyst toward the electrochemical determination of TMX. The unique nanocomposite provides a larger electrochemically active surface area, more reaction sites, and fast electron transportation. As a result, the as-synthesized Co3O4@g-C3N4 exhibits superior electrocatalytic performance towards TMX detection. Furthermore, Co3O4@g-C3N4 shows appreciable recovery results in the practical analysis of potato and brown rice samples.

2.1. Chemicals and reagents Melamine (purity, 99%), cobalt (II) chloride (CoCl2.6H2O, ≥98.0%), thiamethoxam (C8H10ClN5O3S), sodium phosphate dibasic (Na2HPO4), sodium phosphate monobasic (NaH2PO4), sodium hydroxide (NaOH, ≥98.0%), hydrochloric acid (HCl, 36.5–38.0%) were all brought from Sigma-Aldrich and used without further purification. All other chemicals used in this work were of standard analytical grade. The 0.1 M phosphate buffer (PB) was prepared from the mixing of Na2HPO4 and NaH2PO4; NaOH and H2SO4 were used for pH optimization. Ethanol and double distilled water were used for solution preparation and washing purposes in this experiment. For real sample analysis, potato (family: bintje) and brown rice (local market product) were purchased from the local market. 2.2. Instrumentations The as-synthesized samples structure, morphology, and composition were all studied by using various instrumentations such as field emission scanning electron microscopes (FESEM, Hitachi S-3000 H), scanning electron microscopes (SEM, JSM-6510) with bonded of energydispersive X-ray (EDX) and high-resolution transmission electron microscopy (HRTEM, H-7600, Hitachi, Japan) with the attachment of energy-dispersive X-ray (EDX, HORIBA EMAX X-ACT, Sensor +24 V = 16 W, resolution at 5.9 keV). The crystallinity of all samples were investigated by using powder x-ray diffraction (XRD, XPERT-PRO, PAN analytical B.V., The Netherlands) and the diffractometer with Cu Kα radiation (k = 1.54 Å). The presence of functional groups in the samples were observed by employing fourier-transform infrared spectroscopy (FT-IR, PerkinElmer IR spectrometer). The chemical and valence state of each individual element was investigated by using X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The UV–vis analysis was carried out in Jasco V-770 instrument. The electrochemical performance was examined with cyclic voltammetry (CV, CHI 1205C) and differential pulse voltammetry (DPV, CHI 900) 2

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electrochemical workstations (CH Instruments Company, made in the U.S.A). The three-electrode system was used in this experiment through a screen-printed carbon electrode (SPCE) as the working electrode (working area = 0.071 cm2), a silver wire as the counter electrode, and Ag/AgCl (saturated KCl) as the reference electrode. 2.3. Synthesis of g-C3N4 and Co3O4@g-C3N4 As mentioned in the Scheme 1, the bulk g-C3N4 was synthesized by direct heating of melamine powder following to previously reported literature (Liu et al., 2019). Specifically, 3 g of melamine was taken in a covered alumina ceramic boat. Then, it was heated in a muffle furnace for 4 h at a temperature of 550 °C with a heating rate of 5 °C/min. As a result, a pale yellow color g-C3N4 was obtained at the end of the heating process. Further, it was collected and ground by using mortal pestle before final use in the nanocomposite. For the synthesis of Co3O4@gC3N4, 1 g of g-C3N4 was dispersed into 10 ml of distilled water by a magnetic stirrer. After stirring for 15 min, 30 ml of CoCl2.6H2O (0.25 M) and 15 ml of NaOH (1 M) aqueous solution were added under stirrer to form a homogeneous solution. The resulting solution was transferred into a 50 ml Teflon-lined autoclave and maintained at 120 °C for 5 h. The autoclave was allowed to cool down naturally at room temperature and the as-obtained Co3O4@g-C3N4 was dried in an oven for 12 h at 60 °C. Pristine Co(OH)2 was prepared following similar procedures, without the addition of g-C3N4 to the hydrothermal reaction. The resulting pristine Co(OH)2 was annealed at 350 °C for 1 h in a muffle furnace to obtain pristine Co3O4 and used for control experiments. 2.4. Modification of screen-printed carbon electrode The as-synthesized Co3O4@g-C3N4 was dispersed in distilled water (1:2) and allowed to ultrasonication treatment for 30 min. Then, the dispersed solution was drop cast on the carbon surface of SPCE (precleaned) and dried at room temperature (25–27 °C). Finally, the electrode was used for electrochemical experiments. A similar procedure was followed to modify the control electrodes. In order to verify the chemical and physical stability of Co3O4@g-C3N4 modified SPCE, CV analysis was performed under the potential range from −0.2 V to +0.6 V (vs. Ag/AgCl) in N2-saturated 0.1 PB (pH 7) at a scan rate of 50 mV s−1. The results are depicted in the supplementary information (Fig. S1); the electrode showed 98% of its first cycle current after hundred consecutive cycles, indicating the Co3O4@g-C3N4 is a suitable electrode for the electrocatalytic applications.

Fig. 1. (a) XRD and (b) FT-IR of g-C3N4, Co3O4, and Co3O4@g-C3N4.

the N–H stretching vibration of –NH2 or = NH groups of uncondensed melamine units (Cui et al., 2012). The bands at about 1200–1650 cm−1 are assigned to the typical stretching vibration modes of C–N heterocyclic tri-s-triazine ring. The band appeared at 808 cm−1 is due to tri-striazine bending vibration modes in the g-C3N4 structure. For Co3O4, two bands appeared at 675 and 490 cm−1 are corresponding to the typical stretching vibration modes of Co(II) and Co(III) (Lin et al., 2003). These characteristic FT-IR observations also appear in the Co3O4@g-C3N4. This is a strong indication that Co3O4 was incorporated in the g-C3N4 homogenously. The XPS analysis was performed in order to intensely characterize the surface elemental proportion, valence state and chemical environment of the individual element in the Co3O4@g-C3N4 nanocomposite. The elements of C, N, Co, and O were detected in the overall survey spectrum (Fig. 2a). The high-resolution XPS spectrum of C1s exhibits three deconvolution peaks and shown in Fig. 2b. The sharp peak located at the binding energies of 288.7 eV corresponds to the C–N]C in heterocycle rings. The second peak at the binding energies of 285.4 eV is ascribed to the C–O bond, and the third peak at 282.0 eV is assignable to C–C in adventitious carbon (Lin et al., 2018). Fig. 2c displays the high-resolution spectra of N 1s, a high-intensity peak at 397.8 eV was observed and it corresponds to the sp2-hybridized nitrogen (C–N]C)

3. Results and discussion 3.1. Structural characterization of Co3O4@g-C3N4 The phase and crystalline nature of the g-C3N4, Co3O4, and Co3O4@ g-C3N4 were investigated by using XRD and given in Fig. 1a. The XRD pattern of as-prepared g-C3N4 shows two diffraction peaks, the primary intense peak (002) at 27.34° is a characteristic reflection of graphitic carbon formed by inter-layer stacking of structural tri-s-triazine rings. The second weak peak at 16.5° is ascribed to the reflection at (100), corresponding to the in-plane structural repeating units of tri-s-triazine [JCPDS 87–1526] (Ge, 2011). In XRD pattern of Co3O4, major diffraction peaks at 2θ = 18.9°, 30.1°, 37.99°, 52.84°, and 60.72° are observed and corresponding to their (111), (220), (311), (422), and (511) planes, respectively. It is well consistent with the crystallographic planes of Co3O4 [JCPDS: 43–1003] (Zhu et al., 2017). Finally, the nanocomposite exhibits all characteristic peaks of g-C3N4 and Co3O4, indicating the successful formation of the nanocomposite. The functional properties of g-C3N4, Co3O4, and Co3O4@g-C3N4 were analyzed using FT-IR spectroscopy (Fig. 1b). FT-IR spectra of gC3N4 exhibited a board peak at about 3250 cm−1, which is attributed to 3

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Fig. 2. The XPS analysis of Co3O4@g-C3N4; (a) overall survey, (b) C 1s, (c) N 1s, (d) Co 2p, and (e) O 1s.

arrangement of nano hexagonal-shaped Co3O4 in exfoliated g-C3N4 sheets. Furthermore, the EDX mapping given in Figs. S3e and f indicate that the compositional distributions of Co, O, C and N elements in Co3O4@g-C3N4 were uniform. In order to investigate the interaction between the Co3O4 and gC3N4, the UV–vis analysis was performed. The obtained UV–vis spectrum given in Fig. S4 The UV–vis spectrum of pure g-C3N4 shows the absorbance peak at 200–450 nm, which is due to charge transfer from the valence band of N atom (2p orbitals) to the conduction band of C atom (2p orbital) in the carbon nitride. An intense peak at 384 nm due to n–π* transition caused by the electron transfer from a nitrogen nonbonding to aromatic antibonding. The decreasing absorption peak intensity and slight redshift to higher wavelength in the Co3O4@g-C3N4 were observed as compared with pure g-C3N4. This result is due to the formation of coordinate covalent bond interaction between lone pair of electrons in the nitrogen from g-C3N4 and Co from Co3O4 (Kumar et al., 2016).

(Gu et al., 2014). The high-resolution spectra of Co 2p (Fig. 2d) show two main characteristic peaks at 781.7 and 797.8 eV that can be related to Co 2p3/2 and Co 2p1/2 spin-orbit lines, respectively (He and Tao, 2018). In the high-resolution XPS spectrum of O1s (Fig. 2e), there are three peaks are found according to the peak fitting result. The peak at 532.0 eV corresponds to N–C–O groups, the peak at 530 eV is due to an oxygen atom in the cobalt particles, while other peaks related to different oxygen species (Wu et al., 2016). 3.2. Morphological characterization of Co3O4@g-C3N4 The morphology and structure of the as-prepared g-C3N4, Co3O4, and Co3O4@g-C3N4 were investigated by using FESEM and HRTEM. A graphitic stacking structure could be observed in the FESEM image of gC3N4, as shown in Fig. 3a. The FESEM image of Co3O4 (Fig. 3b) showed nano hexagonal shapes type of morphologies and the average length of Co3O4 nano hexagonal was estimated to be 80–150 nm. In FESEM images of Co3O4@g-C3N4, the surface of g-C3N4 with the loading of nano hexagonal-shaped Co3O4 displayed some vertically and densely packed Co3O4 aggregates in g-C3N4 layer (Fig. 3c and d). The SEM-EDX mapping of Co3O4@g-C3N4 results are shown in Fig. S2, indicates that the mixed presence of Co, O, C and N elements in the nanocomposite. Furthermore, the SEM-EDX quantitative elemental analysis (Fig. 3e) reveals the presence of cobalt, oxygen, carbon and nitrogen atoms with a weight percentage of 48.43, 27.52, 16.73, and 7.32, respectively. The HRTEM image of g-C3N4 shows thick sheets like morphology which is resulting from their exfoliation of bulk g-C3N4 and displayed in Fig. S3a. A nano hexagonal shape like morphology was observed from the HRTEM images of Co3O4 (see Fig. S3b) and its well consistent with FESEM results. Whereas, the HRTEM image of Co3O4@g-C3N4 reveals thick exfoliated sheet-like g-C3N4 with the vertical presence of nano hexagonal-shaped Co3O4 and shown in Fig. S3c. The higher magnification of Co3O4@g-C3N4 (Fig. S3d) clearly revealed the vertical

3.3. Impedance studies The electron transferability of the bare SPCE, g-C3N4, Co3O4, and Co3O4@g-C3N4 modified SPCEs were examined by using electrochemical impedance spectroscopy (EIS) in presence of 5 mM [Fe (CN)6]−3/−4 with 0.1 M KCl solution. The Nyquist plots in the frequency range of 100 kHz to 10 mHz in Fig. 4a is fitted by the Randles circuit model as shown in inset; Fig. 4a. The Rct, Rs, Zw, and Cdl are such as charge-transfer resistance, electrolyte solution resistance, Warburg impedance and double layer capacitance, respectively. The Nyquist plot of Co3O4@g-C3N4/SPCE showed smaller Rct value of 12.5 Ω cm2 (see inset Fig. 4a) compared to the Rct value of bare SPCE (471 Ω cm2), gC3N4/SPCE (355 Ω cm2), Co3O4/SPCE (225 Ω cm2). The smaller Rct value is attributed to the fast electron transfer process between the electrode and the electrolyte, thus suggesting higher electrical 4

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Fig. 3. The FESEM images of (a) g-C3N4, (b) Co3O4 and (c–d) Co3O4@g-C3N4; (e) EDX quantitative elemental analysis.

curves of Co3O4@g-C3N4/SPCE at various concentrations of TMX (25–200 μM) in N2-saturated 0.1 M PB (pH 7) at a scan rate of 50 mV s−1. The peak current response gradually increased with augmenting the concentration of TMX from 25 to 200 μM. Besides, the results in Fig. 4e demonstrated good linearity between the current response and the TMX concentration with linear regression and coefficient (R2) of y = −0.0955x - 2.3643 and 0.9989, respectively. The scan rate performance of the Co3O4@g-C3N4/SPCE has been studied at different scan rates from 10 to 100 mV s−1 in 50 μM of TMX with N2-saturated 0.1 M PB (pH 7). Fig. 4f shows the peak current response of TMX increased with an increase in scan rates from 10 to 100 mV s−1. On the other hand, the peak current potential shifted towards the negative potential while increase the scan rates (see inset Fig. 4f), indicated that the Co3O4@g-C3N4 has good electron transport and reaction kinetics. In addition, Fig. 4g shows an excellent linear plot between the peak current response and the scan rate in mV s−1 with regression and coefficient of y = −0.0205x - 4.2473 and R2 = 0.9964, respectively. These results presented that the reaction kinetics of Co3O4@g-C3N4/SPCE were thoroughly occurred by the diffusion-controlled process.

conductivity of Co3O4@g-C3N4. Furthermore, the CV method was carried out for Co3O4@g-C3N4, Co3O4, g-C3N4 and bare SPCE under 0.5 M KCl containing 0.1 M of [Fe(CN)6]−3/−4. The results are shown in Supplementary Information Fig. S5; The higher peak current response and smaller peak to peak separation was observed at Co3O4@g-C3N4 modified electrode than other modified electrode. This result indicates the higher electron conductivity and faster redox reaction in the system due to the synergistic effect between Co3O4 and g-C3N4. 3.4. Electrochemical detection of TMX The cyclic voltammetry (CV) measurements were carried out to evaluate the electrocatalytic properties of bare SPCE, g-C3N4, Co3O4, and Co3O4@g-C3N4 modified SPCEs in presence of 100 μM of TMX containing N2-saturated 0.1 M PB (pH 7) at a scan rate of 50 mV s−1 and shown in Fig. 4b. The bare SPCE exhibited very poor electrochemical activity with 100 μM of TMX due to its poor conductivity and the small surface area of activity. The CV response of Co3O4 and g-C3N4 modified SPCEs revealed better electrochemical activity with 100 μM of TMX because of low electrolyte resistance of the Co3O4 and higher catalytic activity of g-C3N4. However, the Co3O4@g-C3N4 is much more active than either pure Co3O4 or g-C3N4 (Fig. 4c). The increased activity of the Co3O4@g-C3N4 may be attributed to synergistic effects between the Co3O4 and g-C3N4 with the enhanced electrochemically active surface area and excellent conductivity, which can improve the conductivity and charge transfer across the electrode-electrolyte interface. Therefore, the Co3O4 mixing of g-C3N4 plays a key role in improving the electrocatalytic activity of SPCE for electrochemical detection of TMX. It is noted that the CV curves of all observed samples revealed no reversible peak. Therefore, it is assumed that the electro-reduction of TMX involves 4e− and 4H+ transfer irreversible reduction processes, which can be ascribed to the –NO2 irreversibly into –NHOH group. The schematic illustration of the irreversible electro-reduction mechanism of TMX was shown in Fig. 4h and thus, also similar to the earlier reported literature of TMX electro-reduction (Kumaravel and Chandrasekaran, 2012). To further understand the electrochemical performance of Co3O4@ g-C3N4 was evaluated by using the CV method. Fig. 4d shows the CV

4. DPV analysis of TMX In order to attain the low detection limit of TMX at the Co3O4@gC3N4/SPCE, DPV analysis was carried out. Fig. 5a shows the DPV response of Co3O4@g-C3N4 at a total concentration range of TMX (0.01–420 μM) in N2-saturated 0.1 M PB (pH 7). The electrode revealed a rapid increase in peak current response with various consecutive addition of TMX concentration from 0.01 to 420 μM. Fig. 5b shows the linear dependence between the peak current response and the concentration of TMX with regression of y = −0.2077x + 6.4751, where y represents the current response value (μA) and x denotes to the concentration of TMX (μM) and the coefficient (R2) of 0.9906. From these results, the limit of detection (LOD) was calculated to be 4.9 nm using the standard equation of LOD = 3 S/q (Boopathy et al., 2018). where ‘q’ is the slope value (0.2077 μA μM−1) from the calibration plot, and ‘S’ is the standard deviation obtained from the five measurements of the blank signal (0.00034 μA). The sensitivity was found to be 5

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Fig. 4. (a) EIS measurements of bare SPCE, g-C3N4, Co3O4, and Co3O4@g-C3N4 modified SPCEs in 0.5 M KCl containing 0.1 M of [Fe(CN)6]−3/−4. (b) CVs of bare SPCE, g-C3N4, Co3O4, and Co3O4@g-C3N4 modified SPCEs under 0.1 M PB (pH 7) containing 100 μM of TMX at scan rate of 50 mV s−1, (c) histogram of peak current versus electrodes, (d) CVs of various concentration of TMX (25–200 μM) at the Co3O4@g-C3N4/SPCE, (e) the plot of current response versus [TMX]/μM, (f) CVs of 50 μM of TMX at different scan rates from 10 to 100 mV s−1 over the Co3O4@g-C3N4/SPCE, (g) the plot of current response versus scan rates in mV s−1. (h) Schematic illustration of electrochemical reduction of TMX.

12.2136 μA μM−1 cm−2. The results presented in Table 1 show the significance of our work in terms of improved analytical values for electrochemical detection of TMX to previous work recently reported in the literature.

electrode retained 98.6% from its initial day, demonstrated the good stability of our proposed method. The reproducibility of the electrode was investigated on determining the current response of five independently modified electrodes to the 100 μM of TMX. From the result shown in Supplementary Information Fig. S6a; the relative standard deviation (RSD) was calculated to be 4.28%, indicating the excellent reproducibility of the method. Furthermore, the repeatability of this sensor was evaluated through seven consecutive repeatability measurements of a single electrode for the current response of TMX 100 μM. The result depicted in Fig. S6b shows the current response of electrode for repeatability measurement of TMX and the RSD is calculated to be 5.75%, evident the excellent repeatability of this sensor.

4.1. Effect of interferences The selectivity of Co3O4@g-C3N4/SPCE toward TMX was evaluated under the presence of interfering species such as metal ions; Na+, K+, Fe2+, Mg2+, NH4+, and nitro compounds; methyl parathion (MP), ethyl parathion (EP), fenitrothion (FT), imidacloprid (IMI). Fig. 5c revealed the obtained recovery current of TMX (100 μM) at the Co3O4@gC3N4/SPCE in presence of 10-fold higher concentration of metal ions and nitro compounds. It is clear that the current response of TMX (100 μM) in the presence of 10-fold of metal ions and nitro compounds suffered slightly. The variations range of recovery current was from −0.04% to −5.8%, which indicated the Co3O4@g-C3N4 is an excellent suitable electrode for selectivity of TMX.

5. Real-time analysis of TMX For the preparation of potato stock solution, the purchased potato was directly peeled and was soaked into 50 ml of 0.1 M PB (pH 7) for 3–4 h. After that, the solution was centrifuged at 6000 rpm to settle down the heavy particles (sand) and impurities. Then, the transparent solution was removed to the analysis. The solution was tested in DPV measurement and found to be an absence of TMX. However, the stock solution was prepared by adding a known amount of TMX into the solution. Further, the stock solution was measured under the DPV method with N2-saturated 0.1 M PB (pH 7) at different consecutive addition of stock solution (0–15 μM) and shown in Fig. 6a.

4.2. Stability, reproducibility, and repeatability The stability of the electrode was evaluated by using a single asproposed modified electrode to the response of TMX (100 μM) for 30 days. The experiments were performed every day and the electrode stored in a refrigerator at 5 °C. The result as shown in Fig. 5d; the 6

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Fig. 5. (a) DPV responses of Co3O4@gC3N4/SPCE at within the total concentration of TMX (0.01–420 μM), (b) plot of peak current response against the concentration of TMX (μM), (c) the bar diagram of Co3O4@g-C3N4/SPCE at presence of interfering compounds such as Na+, K+, Fe2+, Mg2+, NH4+, methyl parathion, ethyl parathion, fenitrothion, and imidacloprid, (d) the histogram of Co3O4@g-C3N4/SPCE stability at TMX detection for 30 days.

Table 1 Comparison of Co3O4@g-C3N4/SPCE with the previous earlier reported electrodes for electrochemical detection of TMX. Modified electrode a

b

Nanosilver/SDS /GCE Bismuth film electrode/GCE Mercury film electrode/GCE b-CD-rGO/GCE MIPc-GN/GCE Co3O4@g-C3N4/SPCEd a b c d e

Methods

Linear range (μM)

LOD (μM)

Reference

DPV DPV DPV LSVe LSV DPV

10–90 4.3–154.3 2.64–154.3 0.5–16 0.5–20 0.1–420

0.1 1.3 0.89 0.27 0.04 0.0049

Kumaravel and Chandrasekaran (2012) Guzsvány et al. (2006) Guzsvány et al. (2006) Zhai et al. (2017) Xie et al. (2017) This work

SDS-sodium dodecyl sulfate. GCE-glassy carbon electrode. MIP-GN-molecularly imprinted polymer-based graphene. SPCE-screen-printed carbon electrode. LSV-Linear sweep voltammetry.

Fig. 6. The real-time detection of TMX at Co3O4@g-C3N4/SPCE in (a) Potato and (b) brown rice samples under N2-saturated 0.1 M PB (pH 7). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 7

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For the preparation of brown rice stock solution, the obtained brown rice was directly soaked into 50 ml of 0.1 M PB (pH 7) for 1–2 h. Then, the solution was centrifuged at 6000 rpm to remove impurities and the upper layer solution was taken to analysis. It is found free of TMX in the solution. However, the stock solution was prepared by adding a known amount of TMX into the solution. DPV analysis was carried out to measure the stock solution under N2-saturated 0.1 M PB (pH 7) at different consecutive addition of stock solution (0–15 μM) and depicted in Fig. 6b. The results of potato and brown rice stock solutions were calculated in terms of added, found and recovery of TMX. Furthermore, the HPLC method was carried out to compare our proposed electrochemical method (see Table S1 in supplementary information). The found and recovery results revealed that the Co3O4@g-C3N4/SPCE is a good suitable electrode material for TMX detection in all food samples.

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6. Conclusion We synthesized Co3O4@g-C3N4 nanocomposite by a simple hydrothermal method and demonstrated it as a superior heterogeneous catalyst for electrochemical detection of TMX. The crystal structure and morphology were revealed by the XRD, FT-IR, XPS, FESEM, and HRTEM techniques. The charge transfer resistance is evaluated, which is favorable for mass electron transfer. Furthermore, the cyclic voltammetry and differential pulse voltammetry method demonstrated a superior response of Co3O4@g-C3N4 towards TMX detection than those of Co3O4 and g-C3N4. More significantly, the detection limit of the Co3O4@g-C3N4 nanocomposite towards TMX detection was found to be 4.9 nm with a wide linear range and high sensitivity. The results obtained from the recovery and competitive experiments ensure the applicability of the developed method for the selective detection of TMX under the coexistence of metal cations and nitro compounds. Thus, the intense advantages including high sensitivity, excellent selectivity, and stability of the developed method provide significant activities towards the real-time sensing of TMX in all food samples. Author contribution statement Mr. Mr. Jaysiva Ganesamurthi and Miss. Murugan Keerthi designed the work. Both the authors contributed for preparing main manuscript, characterizing the materials, and carrying out the electrochemical studies. Ragurethinam Shanmugam helped to prepare all the figures. Prof. Shen-Ming Chen has given the financial support. Declaration of competing interest “The authors have no conflicts to declare”. Acknowledgements This work was supported by the Ministry of Science and Technology, Taiwan (MOST 107-2113-M-027-005-MY3). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ecoenv.2019.110035. References Ajermoun, N., et al., 2019. Electrocatalytic activity of the metallic silver electrode for thiamethoxam reduction: application for the detection of a neonicotinoid in tomato and orange samples. J. Sci. Food Agric. 99, 4407–4413. Bartell, S.M., et al., 2018. Modeling the effects of thiamethoxam on Midwestern farm ponds and emergent wetlands. Environ. Toxicol. Chem. 37, 738–754. Boopathy, G., et al., 2018. Graphene oxide/α-MnO2 binary nanosheets based non-enzymatic biosensor for pico-molar level electrochemical detection of biomarker

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