Modification of screen printed electrode using reduced graphene oxide-gold nanoparticles for voltammetric detection of diuron and fenitrothion

In recent years, increasing attention has been given to the distortion of the aquatic ecosystem around the world. The risk of chemical pollution in water bodies is getting worst day by day. One of the possible sources of these pollutants is pesticides. Thus, the development of an on-line selective and sensitive monitoring system needs to be given priority. The use of electrochemical sensor is the most promising monitoring system due to its low instrumentation cost, simple operation, short response time, highly sensitivity and selectivity compared to other conventional monitoring systems. Recently, the use of nanomaterials such as reduced graphene oxide (rGO) and gold nanoparticles (AuNPs) have been given more attention among researchers around the world. They have been extensively used as sensing materials due to their outstanding properties in terms of conductivity, effective surface area, stability and catalytic effect. In this project, two different approaches have been used to fabricate the electrochemical sensor materials based on rGO/AuNPs nanocomposite for detection of diuron and fenitrothion as herbicide and insecticide respectively in natural waters. In the first work, rGO/AuNPs nanocomposite was synthesized based on electrochemical co-reduction of graphene oxide and chloroauric acid via cyclic voltammetry technique on the surface of screen printed electrode (SPE). The fabricated sensor (rGO-AuNPs/SPE) showed higher sensitivity towards diuron and its cathodic peak current was directly correlated to the diuron concentration. The field emission scanning electron microscopy (FESEM) image shows the uniform distribution of AuNPs on the surface of rGO nanosheets. The presence of rGO nanosheets was further proven by Raman Spectroscopy. Under optimized conditions, the cathodic peak current was proportional to the diuron concentration over a wide range between 0.5 to 30.0 μg mL-1 with the detection limit of 0.125 μg mL-1 (S/N=3). The proposed diuron electrochemical sensor also exhibited a relative standard deviation of 4.25% for six replicate analysis of 10.0 μg mL-1 diuron and the response of the electrode was declined up to 20% after keeping for 30 days in ambient temperature. In addition, the sensor was successfully employed for the determination of diuron in real natural water samples including lake and sea water. In spite of the fact that rGO-AuNPs/SPE sensor (via electrochemical reduction of GO) is environmentally green technique, nevertheless, the modified substrate is polarized in a short potential range due to incomplete reduction of GO functional groups. Therefore, in the second study, a new synthesis procedure was reported for the preparation of rGO/AuNPs nanocomposite using ethylenediamine (en) as a cross- linker for chemical reduction of GO functional groups in order to fabricate a polarized sensor. The constructed nanocomposite (AuNPs/rGO-en) was homogenized in dimethylformamide (DMF) and drop-casted on a SPE to fabricate an electrochemical sensor (AuNPs/rGO-en/SPE) which was sensitive to fenitrothion. The nanocomposite electrode was characterized with FESEM, X-Ray diffraction (XRD) spectroscopy, Fourier transform infrared spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS), Raman spectroscopy and cyclic voltammetry (CV). The anodic peak current at around 0.06 V was proportional to fenitrothion concentration over a wide range of 0.1 to 6.25 ng mL-1 with the limit of detection of 0.036 ng mL-1 (S/N=3). Moreover, in order to evaluate the repeatability of the AuNP/en-rGO/SPE, the peak currents of fenitrothion at two different concentrations (0.5 ng mL-1 and 4.0 ng mL-1) were determined successively under optimum conditions for six times with the same modified sensor, and the RSD values were found to be 4.1% and 4.3%, respectively, exhibiting good repeatability. The stability study showed that the oxidation peak current of fenitrothion at 4.0 ng mL-1 decreased 2.2% after recording 50 successivecyclic voltammograms. In addition, the proposed senor was successfully employed for the determination of fenitrothion residue in the natural water samples including tap and lake water. The validity of the response was checked with gas chromatography as a standard method and the result was in agreement with constructed sensor.

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