Electroanalytical Performance of Graphene Paste Electrode Modified Al(III)-TiO2 Nanocomposites in Fipronil Solution

The new composite material Al(III)-TiO2 has been synthesized and applied as a modifier of graphene paste electrode for the determination of fipronil pesticide by cyclic voltammetry. The methods were to synthesis of Aluminum-Titanium dioxide (AT), preparation of graphene paste electrode with mass varied Al(III)-TiO2 (GAT) (0.05 g, 0.1 g, 0.2 g), and fipronil electroanalytic respons. Addition of Al(III)-TiO2 to the graphene paste electrode shows redox properties which are well characterized by a fast electron transfer process. Based on the results of measurements in a solution containing fipronil, it is known that fipronil is oxidized at a potential value of 0.26 V. Furthermore, the fipronil oxidation process on the GAT surface is influenced by diffusion control, this is powered by R value 0.91 when plotted between peak oxidation currents (Ipa) vs. root scan rate. Other results show that measurement linearity is in the range 0.01 to 0.09 μg/L with a limit of detection (LOD) value of 0.0164 μg/L. Moreover, GAT shows good stability in the determination of fipronil with% RSD equal to 5%.


Introduction
Organic pesticides have been widely used in agriculture because they have high performance to eradicate pests, diseases, and weeds in plants. However, there are negative impacts caused to living things due to contamination of hazardous substances. One type of insecticide that is classified as Pollutant Organic Persistance (POPs) is Fipronil (Maulidiyah et al., 2019). Fipronil is a member of a group of phenyl pyrazole compounds (5-amino-1-[2,6-dichloro-4-(trifluoro-methyl)phenyl]-4 (trifluoromethyl) sulfinyl]-1H-pyra-zole-3-carbonitrile) (Wang et al., 2016). Fipronil is a insecticide that is designed for plants such as wheat, rice, cotton, sorghum, corn, and other grains. Excessive use of pesticides can cause residues from pesticides to be difficult to accumulate in the environment and body (Kim et al., 2019).
Electrochemical techniques have been reported as techniques with good sensitivity and selectivity for the determination of fipronil pesticide compounds (Nurdin et al., 2019a). The presence of fipronil in the environment is a serious concern due to its toxic nature (Tu et al., 2019), the slow degradation process (Stafford et al., 2018), and its impact on aquatic animals and marine invertebrates (Gunasekara et al., 2007;Guo et al., 2018). The results of a recent study reported that fipronil has a high affinity for human receptors which triggers nerve disorders (Gan et al., 2012;Kim et al., 2019;Ly et al., 2019). In addition, the degradation of fipronil will produce derivative compounds such as desulfinyl which are far more dangerous than fipronil molecules (Vasylieva et al., 2015).
In general, the detection of fipronil was carried out using a high-performance liquid chromatograph (HPLC), thin-layer chromatography (TLC) method, and gas chromatography-mass spectrometry (GC-MS) technique (Vıĺchez et al., 2001;Jimenez et al., 2008;Wang et al., 2014). Furthermore, some measurement techniques based electrochemical that are being developed includes amperometry (Pesavento et al., 2009), potentiometry (Prasad et al., 2007, voltammetry (Alizadeh 2009), andchronopotentiometric (Đurović et al., 2016). Simple tool preparation (Zhang et al., 2014), low cost of measurement (Ensafi et al., 2013), short measurement time (Janegitz et al., 2012), and good stability (Yang et al., 2012) are the reasons why this technique continues to be developed for the determination of pesticide compounds. The performance of the voltammetry method is strongly influenced by the working electrode as the site of the oxidation reaction and reduction of the analyte molecule.
One of the working electrodes that have been developed is the graphene paste electrode. Graphene paste electrode is a working electrode in voltammetry. Graphene has excellent physical and chemical properties that can make it an electrical device and sensor. Graphene as a material with thin sheets of carbon atoms that form a regular hexagonal lattice has excellent electrical conductivity (Agusu et al., 2018). The excellent electrical conductivity of graphene makes it easy for the electron exchange between analyte molecules to be very efficient to be used as a base material for the working voltage of a voltammetry cell.
Modification of working electrodes is an important part of developing the technique. Modified working electrode carbon-nanotube (MWCNT)-glassy carbon electrodes (GCEs) (Montes et al., 2016), Graphite-polyurethane (GPU) composite electrode (Okumura et al., 2016), and ZnO@g-C3N4 modified glassy carbon electrode (Yin et al., 2019), and FeO.TiO2-CPE (Nurdin et al., 2019a) have been reported to be able to detect fipronil compounds in different samples. Although the modification of the electrode shows high sensitivity with a low detection limit, electrode modification is still an interesting study in the past year. It aims to get accurate measurements on different matrices such as food, soil, and water.
In this work, we report the performance of Al(III)-TiO2 nanocomposites as a new composite material in determining fipronil by cyclic voltammetry using graphene paste electrodes. In general, Al(III)-TiO2 is reported to have high oxidation and reduction currents (Desireé et al., 2015;Murtada et al., 2018), but its application has not been reported as a composite material for the development of general voltammetry sensors and specifically pesticide detection sensors. Graphene paste electrodes were chosen in this work by considering several advantages such as easily updated and modified, cheap, and very low current interference. Previous research, we have specifically reported TiO2 as an electrode composite to detect cypermethrin (Nurdin et al., 2019b) and phenol pesticides (Nurdin et al., 2019c).

Preparation of GAT Electrode
Preparation of GAT electrodes begins by synthesizing composite Aluminum-Titanium dioxide (AT). The synthesis process was carried out using TTIP and ATIP as precursors. In summary, ATIP powder is dissolved in TTIP solution and added citric acid as a complexing agent. Then the mixture was heated for 3 hours at a temperature of 200 o C and continued with a temperature of 650 o C for 5 hours. The obtained composites were mixed with graphene powder and paraffin oil at a temperature of 80 o C with a composite mass varied between 0.05 g, 0.1 g, and 0.2 g. The formed paste has been homogenized.
Glass tubes with a diameter of 3 mm were used as a paste container and connect to copper wire to provide current during electrochemical measurements. For comparison, we used the Graphene-TiO2 (GT) composite electrode whose preparation refers to the procedure reported by (Nurdin et al., 2019c). Graphene powder, paraffin oil, and TiO2 degussa were simultaneously mixed with a ratio of 7: 3: 1, and homogenized at 80 o C.

Electroanalytical Response of Fipronil
The electroanalytic response of the fipronil compound was carried out by the voltammetry method using the DY2100B potentiostat with three electrode systems namely GAT as the working electrode, Ag/AgCl saturated in 3.0 M KCl as the reference electrode and platinum wire as auxiliary electrodes. For analytical purposes, fipronil compounds with concentrations of 0.1 μg/L were made by mixing them in a mixture of hydrochloric acid and sodium sulfate as electrolytes. The fipronil concentration was applied in 0.01, 0.03, 0.05, 0.07, and 0.09 µg/L to determine the linearity areas and LOD. We also determination the stability for 14 times. Scheme of the experimental shown in Figure 1. Figure 2 shows the performance of the GAT electrode in detecting fipronil pesticides (solid line) in a solution containing a mixture of electrolytes supporting HCl and Na2SO4 of 0.1 M, respectively. Based on the figure, it can be seen that fipronil oxidizes at anodic potential (Ea) of 0.26 V with a current anodic peak (Ipa) of 262 µA. Compared to the Graphene-TiO2 (GT) electrode (dot line), the performance of GAT in fipronil oxidation is much better, this is seen with the value of Ea fipronil in the GT of 0.57 V with a small value of the Ipa and a widening peak.

GAT Response in Fipronil Solution
The results of previous studies reported that during the oxidation process, fipronil released 2 protons and electrons (Maulidiyah et al., 2019). The difference in the potential value of the two electrodes explains that the transfer of electrons between fipronil molecules and GAT electrodes is faster than GT. The combination of aluminum composite and TiO2 (AT) has a significant influence on the performance of the electrode. Both cause the conductivity and surface area of the GT to increase (Munir et al., 2019;Xu et al., 2019). In addition, it also increases catalytic activity and the effectiveness of the electrode surface in fipronil oxidation. In general, the electroanalytic response of GAT in fipronil oxidation is much faster than MWCNT/GCEs  The effect of AT modifier composition is also studied in this work. Like other modifiers, the mass of AT can affect the transfer of electrons and the resulting peak current on graphene paste electrode. Figure 3 shows that the addition of AT as much as 0.2 g (Figure 3c) causes electron transfer faster than the addition of AT at 0.05 g (Figure 3a), and AT at 0.1 g (Figure 3b). Addition of AT as much as 0.2 g causes a decrease in Ipa fipronil. Measurement during this work, a GAT electrode is used with an addition of AT of 0.2 g. The basic consideration is that the reduction in Ipa does not significantly change.
The difference in scan rate potential (20 mV/s, 50 mV/s, 100 mV/s, and 200 mV/s) in fipronil oxidation shows a linear relationship as shown in Figure 4. The fipronil oxidation current increases linearly with an increasing potential scan rate ( Figure 4A). This linear relationship results from GAT interactions with redox species during the formation of multiple electrical layers. The potential difference in scan rate does not cause a significant change in the value of Ea fipronil.  A high scan rate will cause the resulting diffusion layer to be thin so that the transfer of electrons on the electrode surface becomes easier and the peak current produced is also greater. Conversely, a small scan rate results in the thickness of the resulting diffusion layer, thus inhibiting the electron transfer process at the electrode surface and the resulting peak current is small (Nurdin et al., 2019b).

Linearity, Detection Limit, and GAT Stability
Linearity is the ability of the analytical method to produce a response that is directly proportional to the concentration of the analyte (Wibowo et al., 2017). The equation of a linear line is determined by making a plot between the resulting peak current (µA) and the concentration of the fipronil solution (µg/L). The detection limit of the measurement of fipronil pesticides is studied to find out the smallest amount of analyte in a sample that can still be detected and still gives a significant response compared to the blank. The GAT response in the fipronil analysis was also carried out with different fipronil concentrations ranges of 0.01 to 0.09 µg/L ( Figure 5).  In addition, GAT also shows good stability ( Figure 5C) in detection of fipronil with% RSD of 5% (Yuan-Yuan et al., 2015). Several comparisons of electrochemical techniques in detecting fipronil with the use of electrodes and different modifiers are shown in Table 1.

Effect of Disturbing Ions
The electrode selectivity used needs to be known before the stage application on the environment. One way to determine the level of selectivity is to add disturbing ions to the fipronil solution to determine the effectiveness of the electrodes in detecting pesticide compounds in the presence of other compounds or ions in the same analyte.  Figure 6 shows the effect of confounding ions on the electroanalytical GAT response in the fipronil analysis. The interfering ions tested are heavy metal ions such as Cd 2+ and Pb 2+ with different ion concentrations. The result shows that the GAT response to fipronil anodic currents has decreased. However, based on the calculation of the standard deviation carried out, the decline can still be tolerated.

Conclusion
The electroanalytic response of the GAT electrode in a solution containing fipronil has been studied in this work. GAT synthesis was carried out using the sol-gel method with the presence of TTIP and ATIP as precursors.
During the testing process, we observed that fipronil was oxidized at anodic potential of 0.26 V. This potential was recorded on the use of fipronil solution with a concentration of 0.1 μg / L in a mixture of electrolytes supporting HCl and Na2SO4 with a concentration of 0.1 M respectively. The addition of Al (III) -TiO2 nanocomposite by 0.2 g causes faster electron transfer at the GAT surface. The resulting detection limit of 0.0164 μg/L shows the sensitivity of measurement with good stability. In addition, the presence of disturbing ions in the form of metal ions Cd 2+ and Pb 2+ does not have a significant effect on the anodic current and potential of fipronil.

Acknowledgement
We grateful for financial support from DRPM