Electrochemical analysis of Fenuron herbicide by a carbon paste electrode 1 modified by a functionalized NiAl-Layered Double Hydroxide 2

12 Environmental pollution by usage of pesticides as fenuron increases health risk, due 13 to carcinogenic and teratogenic properties of these compounds. There are needs to develop a 14 rapid and cheaper detection methods for quantification of fenuron. In this work, an inorganic- 15 organic composite material was obtained by intercalation of sodium dioctylsulfosuccinate 16 (DSS) within the interlayer space of a Nickel-Aluminum Layered Double Hydroxide (NiAl- 17 LDH). The structure of the pristine LDH and the intercalated-LDH was confirmed using 18 Fourier transform infrared spectroscopy, X-ray diffraction and thermal analysis. The 19 modified LDH was used to elaborate an amperometric sensor for fenuron herbicide by 20 differential pulse voltammetry (DPV) via a carbon paste electrode (CPE). The 21 electrochemical procedure for fenuron analysis was based on the immersion of the working 22 electrode in the electrolytic solution containing appropriate amount of herbicide, followed by 23 voltammetry detection without any preconcentration step. The peak current obtained on the 24 CPE modified by the organo-LDH was 2-fold higher in comparison with the pristine 25 LDH/CPE. The observed increase in the signal of fenuron was attributed to the high 26 organophilic character of this composite material induced by the modification using DSS. 27 The effects of some experimental parameters (pH of medium and percentage of the modifier 28 in the paste) on the stripping response were investigated in order to optimize the sensitivity of 29 the organo-LDH modified electrode. Linear calibration curves were obtained in the fenuron 30 concentration ranging from 0.5 to 1 μmol.L -1 and 1 to 5 μmol.L -1 . The limit of detection 31 (LOD) calculated on the basis of a signal-to-noice ratio of 3 was 1.8×10 -9 mol.L -1 (low 32 concentration range) and the limit of quantification (LOQ) was 6×10 -9 mol.L -1 . The 33 interference effect of various inorganic ions likely to influence the stripping determination of 34 the fenuron was also examined, and the applicability of the method was verified by the 35 determination of fenuron in a river sample collected down-town Yaoundé.

long lifetime in the environment, its residues have been detected in ground and surface water 48 [3,4]. These residues constitute a very serious environmental issue for human health and 49 ecosystems, since FEN has been recognized to be toxic with mutagenicity and 50 carcinogenicity issues [5,6]. Its accurate detection in the environment (soils, water and crops) 51 is of critical importance in order to obtain the extent of contamination in a selected area and 52 recommendations for necessary removal procedures. Several analytical techniques have been 53 used for the analysis of phenylurea herbicide, mostly based on gas or liquid chromatography 54 [7][8][9]. Although, these methods have been successfully employed, they require long analysis 55 time and several pre-treatment steps. Also, some of them exhibit low sensitivity and 56 selectivity. Thus, the developments of simple, low cost and sensitive alternatives for FEN 57 detection are still needed. 58 Electroanalytical methods are very effective in the analysis of hazardous compounds 59 such as pesticides (use for disease and control pests), particulary linked to environmental 60 issues like water pollution. From literature works, different types of material have been 61 successfully used as modifier of electrodes for the detection of pesticides in water, including 62 titanium-dioxide nanoparticule and cetyltrimethylammonium bromide based carbon paste 63 electrode [10] and tungsten oxide hydrates nanorod modified carbon paste electrode for 64 detection of amitrole [11] and carbendazim [12]. Other well-known materials such as 65 Layered Double Hydroxide (LDH) have been used as electrode modifier [13][14][15] for the their biocompatibility and swelling properties [16,17]. Despite these attractive properties, 70 LDHs exhibit poor selectivity and restricted adsorption capacities for hydrophobic organic 71 compounds because of their hydrophilic surfaces [16][17][18]. In order to circumvent these 72 weaknesses, functionalization of LDH with various organic and inorganic components had 73 been recommended. Many works have been reported on the functionalization of LDH via the 74 intercalation of organophilic anionic surfactants in the interlayer space [14,19,20] Aldrich and a stock solution (0.01 mol.L -1 ) was prepared in deionized water. CaCl 2 , MgSO 4 , 109 NaCl, PbCl 2 and ZnCl 2 (from sigma Aldrich) were used to prepare solution for interferences 110 studies. The pH was adjusted by aliquots of NaOH and HCl (37%) purchased respectively 111 from BDH and Prolabo. All the aqueous solutions were prepared using deionized water.   The synthesis of organo-LDH (named NiAl-DSS) was performed using the 123 homogeneous co-precipitation method adapted from method reported in the literature for the 124 synthesis of modified LDH [28]. Practically, three solutions were prepared using freshly

145
FTIR spectra were recorded using Alpha spectrometer from Bruker Optics using the 146 KBr method in the spectral range from 4000 to 500 cm -1 . The spectra resolution was 4 cm -1 .

171
The FT-IR spectra of NiAl and intercalated NiAl samples are displayed in Figure 1A.

172
The broad band centered around 3415 cm -1 in the spectrum of NiAl was attributed to the 173 stretching vibration mode of hydroxyl group of LDH and water molecules (intercalated and 174 physically adsorbed) [29]. The bending vibration of these water molecules is also reflected in  Table SI 1 188 for band assignments.

189
The XRD pattern of the precursor material (NiAl) is shown in Figure 1B  results of the XRD studies, it could be inferred that the DSS anions were successfully 203 intercalated into the LDH galleries.

204
The results of thermogravimetric analysis of NiAl and NiAl-DSS are presented in Figure 1C 205 (a and b). The TGA curve of NiAl ( Figure 1C (a) showed two decomposition steps. The first 206 step was observed between 25° and 170°C with a weight loss of 10% which corresponds to 207 the water molecules adsorbed on the external surface of NiAl or in the interlayer surface. The 208 second weigh loss observed between 250° and 500°C, is due to the dehydroxylation of the 209 metal hydroxide layer [40,41]. The thermal behaviour of NiAl-DSS ( Figure 1C    on the CPE/NiAl-DSS is mainly a diffusion controlled process [13]. We Also noticed that, 282 the E p of the oxidation peak was also dependent on the scan rate. The peak potential shifted to

286
Where α is the electron transfer coefficient, Ks is the standard rate constant of the surface 287 reaction, v is the scan rate, n is the electron transfer numbers and E 0 is the formal potential.
Since for a totally irreversible electron transfer α was assumed as 0.5, the n was calculated to 294 be 1.63.

Effect of pH on FEN signal 313
The acidity of medium is a key parameter that can affect the mass transport on the 314 electrode surface, especially when the redox process involves proton transfer as in the present 315 case. Figure 5A represents the DPV curves of FEN 5×10 -5 M in acetate buffer 0.1 M on 316 CPE/NiAl-DSS when the pH was varied from 3 to 6. It was found that, the peak current and 317 potential varied with the pH of the electrolytic solution. The results in Figure 5B showed that 318 the sensitivity of the electrode increased in the pH range 4 < pH < 4.7 and then decreases 319 with a further increasing solution pH. The optimum value is reached at pH 4.7. In fact, a low 320 sensitivity obtained in the pH range 3 < pH < 4 can be attributed to the hydrolysis of the 321 metal ions, and instability of LDH [47]. The loss of sensitivity observed for the pH value up 322 to 4.7 could be explained by the fact that FEN was partially degradated in the pH range 5-6 323 [24]. Also shown in Figure 5A Table 1, which indicates that the 362 proposed sensor exhibited detection limits lower than those reported by certain authors for 363 trace analysis of some phenylurea pesticides [47,50,51,52].  Methylparathion) could somewhat reduce the selectivity of the method, and let us to propose 376 their elimiation from matrices before the quantification of FEN. In fact, according to 377 literature, inorganic ion like Pb 2+ can be remove in a medium using a typical chelating agent 378 like ethylene diamine tetraacetic acid (EDTA) [57][58][59]. Ghyphosate and methylparathion can 379 be remove using smectite modified by organosilane [60,61].

380
The analytical applicability of the modified electrode was applied to the determination  The quantitative recoveries of FEN in the real sample are summarized in Table 2. The 387 obtained values are in good agreement with the spiked value, indicating that the proposed 388 method is a good alternative for the analytical determination of this pesticide in the sample.

392
In this work, a NiAl-LDH intercalated by anionic surfactant was shown to be an