3-methyl-4-nitrophenol (MNP) is the main by-product of the organophosphate insecticide fenitrothion (FT), used in locust control. MNP is highly toxic because it is an endocrine disruptor and then may cause adverse in the biological systems. Then, it is necessary to develop analytical methods for determination of this pollutant in the environment. In this sense, we reported herein the development of an electrochemical sensor for the detection of 3-methyl-4-nitrophenol (MNP), one of the metabolites of fenitrothion (FT), by using naked and modified carbon fiber microelectrode (CFME) by nickel tetrasulfonated phthalocyanine polymer (CFME/p-NiTSPc). The voltammogram showed that MNP presents one irreversible anodic peak corresponding to the oxidation of the phenol group at 0.9 V vs Ag/AgCl. The effect of pH of the buffer on the peak current and SWV parameters such as frequency, scan increment and pulse amplitude were investigated in order to optimize the electrochemical response of the sensor. The obtained results lead to the following optimum value: pH = 6; frequency = 25 Hz, pulse amplitude = 50 mV, scan increment = 10 mV. With these optimum values, the calibration curves show that the peak current varied linearly upon MNP concentration leading to a limit of detection (LoD) for naked CFME close to 3 μg/L whereas for CFME modified by p-NiTSPc, it reaches 0.75 μg/L. This results prove that the presence of p-NiTSPc increasing the sensitivity of the sensor could be used to monitor 3-methyl-4-nitrophenol residue in real matrix.
The relentless pursue of improving productivity and agricultural products quality has led man to a massive use of pesticides. Thus, according to the United Nations Organization statistics Food and Agriculture Organization (FAO), in Burkina Faso over 500 tons of pesticides are used each year since 2010 [
All electrochemical measurements were performed using a portable potentiostat (Palmsens) controlled by software and obtained through International Foundation for Science (IFS) individual grant.
3-methyl-4-nitrophenol (MNP), as a powder with analytical grade, was purchased from Sigma-Aldrich, and a stock solution (5 mg/L) was prepared in distilled water. Phosphate buffer solution (PBS) 0.2 M was used as supporting electrolyte and prepared by mixing appropriate amounts of potassium phosphate monobasic (KH2PO4) and potassium phosphate dibasic (K2HPO4) in MilliQ water. NiTSPc monomers were purchased by Sigma and used as received.
Platinum electrode (250 µm) and Ag/AgCl/KCl 1 M were used as counter and reference electrodes respectively. The working electrode was a naked carbon fiber microelectrode (CFME) (diameter Φ = 12 µm) or modified by p-NiTSPc film (p-NiTSPc/CFME) and all chemical are purchasing from Sigma Aldrich and are used without further purification.
Electrochemical behavior of MNP at naked and modified carbon fiber microelectrode was investigated using cyclic voltammetry. Square wave voltammetry, which is well-known to be more sensitive has been used to analyze the amount of MNP in aqueous samples by following the irreversible oxidation peak. Phosphate buffer at pH 6 containing 2 mM of Fe(CN)64− was scanned in cyclic voltammetry (CV) for electrochemical characterization of the performance of naked and modified working electrodes.
Prior to modification the working electrode was cleaned electrochemically in 0.5 M H2SO4 with ethanol (1:1, v/v) by performing 10 cyclic voltammograms at the scan rate 50 mV/s. After cleaning, the working electrode was modified following the method already reported [
SEM experiments were performed on CFME and CFME modified with p-NiTSPc (CFME/p-NiTSPc) using a JEOL type JSM-6301F (SCIAM, Angers university). Images obtained were from secondary electrons under 3 - 5 keV with magnifications situated between 3000 and 5000.
Cyclic voltammogram of 30 mg/L of MNP in PBS 0.2 M pH 6 at the scan rate of 50 mV/s is presented in
As it is illustrated on
ireversible oxidation peak at around 0.90 V versus Ag/AgCl/KCl 1 M in comparison with the blank. This peak could be assigned to the oxidation of the phenol group and the irreversibility is due to the existence of chemical reaction following the electron transfer. This result is in agreement with the literature data [
When cyclic voltammetry is realized on FT during scanning potential in cathodic domain (
During the reverse scan, one may note the presence of reversible peak at −0.05 V vs Ag/AgCl/KCl 1 M corresponding to the oxidation/reduction of hydroxylamine electrogenerated to nitroso group and this agrees with the literature [
Based on these results, on may conclude that FT and its metabolite MNP could be differentiate in anodic domain by the oxidation of phenol group. To improve the detection of the residues of MNP in environment, we carried out by square wave voltammetry (SWV), the effects some parameters such as the pH value and the electrochemical parameters of the method on the electrochemical oxidation of MNP using naked and modified CFME.
To optimize the electrochemical response of MNP by square wave voltammetry (SWV), the effect of pH of the buffer and the electrochemical parameters method (SWV) such as frequency, pulse increments and pulsation were investigated.
As illustrated on
peak potential upon pH value shows a linear decrease of the oxidation peak potential with the increase of the pH value from 2 to 7 (
in electrons than phenol and therefore more easily oxidable. This result agrees with the literature [
The optimum parameters for determination of the electrochemical response of MNP by poising the optimum pH value on CFME have been determined by using SWV. The optimum values obtained are a frequency of 25 Hz, a pulse amplitude of 50 mV and a scan increment of 10 mV. Therefore, these optimum values are being used in the rest of the work.
r2 = 0.995. Electrochemical method sensitivity was evaluated by calculating the LoD and LoQ and the values obtained were respectively 3 µg/L and 10 µg/L.
In order to improve the sensitivity CFME was modified with p-NiTSPc as previously described.
As with the CFME, effect of pH on peak current intensity and peak potential and SWV parameters were investigated with CFME/p-NiTSPc and the results are similar to that obtained previously. Current peak is maximum at pH 7 and the optimum parameters obtained for determination of MNP on CFME/p-NiTSPc are frequency = 25 Hz, pulse amplitude = 50 mV, scan increment = 10 mV (data not showed). In such optimal conditions, SW voltammograms of MNP for different concentration using CFME/p-NiTSPc are presented in
In addition, we studied the reproducibility and stability of the sensor and the obtained results (Data not shown) shown that these sensors remained reproducible and stable. Indeed after 4 weeks of storage in laboratory temperature (25˚C), it lost only 5% of its response.
MNP determination was performed using SWV and following the oxidative peak current of phenol group at naked CFME and modified CFME with p-NiTSPc. The effects of pH and SWV parameters (frequency, scan increment and pulse amplitude) were investigated and the pKa was estimated. Peak current varied linearly with concentration in two regions for both sensors. The LoD calculated for MNP on CFME is 3 µg/L. This value is reduced to 0.75 µg/L with the CFME/p-NiTSPc. This results show that modification with p-NiTSPc has increased sensor sensitivity and can be used for determination of MNP in natural waters.
Authors would like to thank International Science Program (ISP) for supporting financially this work through African Network of Electroanalytical Chemists (ANEC).
Bako, Y.F.R., Kabore, B. and Tapsoba I. (2017) Electrochemical Sensors Based on Modification of Carbon Fiber Microelectrode by Nickel Phthalocyanine Polymer for 3-Methyl-4-Nitrophenol Analysis in Water. Materials Sciences and Applications, 8, 798-810. https://doi.org/10.4236/msa.2017.811058