A Novel Pentachlorophenol Electrochemical Sensor Based on Nickel-Cobalt Layered Double Hydroxide Doped with Reduced Graphene Oxide Composite

A highly sensitive non-enzymatic electrochemical pentachlorophenol (5-CP) sensor was successfully fabricated employing a multi-component sensing platform made of nickel-cobalt layered double hydroxide (NiCo-LDH) supported on green organic-inorganic nanohybrid (rGO-CuO) drop-casted on a gold electrode (AuE). The chemical and morphological properties of the as-synthesized nanostructures were investigated and confirmed by infrared spectroscopy (ATR) and scanning electron microscopy (SEM). The electrochemical measurements demonstrated that both the good conductivity of and the large active surface area of the hierarchical structure of NiCo-LDH/rGO-CuO favor the electrochemical redox reaction of 5-CP. In the optimized procedure, we have evaluated the analytical performance of the NiCo-LDH/rGO-CuO/AuE using cyclic voltammetry based on the current intensities of the redox peaks. Our findings indicate that the developed NiCo-LDH/rGO-CuO/AuE sensor exhibits a wide linear range from 1 to 50 μM while the limit of detection was estimated to be 12.64 nM for 5-CP. Moreover, the suggested 5-CP sensor displayed an excellent stability that might provide a robust sensing platform for the practical and reliable detection of 5-CP in various real samples.

Chlorophenols are commonly known as high toxicity organic pollutants produced after the combination of chlorine with phenols. 1 The United States Environmental Protection Agency (USEPA) and the World Health Organization (WHO) have consistently stated that pentachlorophenol (5-CP) in water is a serious problem. 2,3 Due to its limited degradability and thermal stability, 5-CP is also neurotoxic, mutagenic, teratogenic, and immunosuppressive. Additionally, 5-CP has the potential to damage DNA at high concentrations and can block a number of crucial enzymes. However, pentachlorophenol is largely manufactured as a pesticide, fungicide, and materials preservation. 4,5 Due to its harmful impact on human health and its ability to accumulate in the living with high toxicity, long persistence, and its ability to degrade cells causing carcinogenic and other adverse reactions, it is urgent to develop new methods to detect pentachlorophenol in a sensitive and selective manner in real samples.
Many attempts have been made to identify 5-CP in real sample, for instancegas chromatography, 6 thin-layer chromatography (TLC), 7 gas chromatography-mass spectrometry (GC-MS), 6 photoelectrochemical 8 and electrochemical sensors. [9][10][11] However, electrochemical techniques have attracted a lot of researchers' attention cause of their strong features such as fast response, easy operation, lower limits of detection and in addition, no need for sophisticated equipment. However, these reported electrochemical sensors are facing some drawbacks. Firstly, most of these sensors depend on expensive noble metals for the electrocatalytic reaction of 5-CP. 12 Secondly, some metal precursors used to prepare sensing platforms contains heavy metals and are unfriendly to the environment and its resources. Thirdly, these sensors may suffer from a narrowlinear range with a lower sensitivity. [13][14][15][16] As a result, sensitive, simple, and quick manufacture of new electrochemical sensor platforms is highly needed.
Electrochemical sensors are always in need of novel materials with new characteristics to overcome the limitations mentioned above. In this view, metal hydroxides with layered double hydroxides (LDH) have attracted considerable attention as catalysts for electrochemical sensors. [17][18][19] Many studies confirmed that have unique materials because of their two-dimensional structures, highly tunable interior architecture, excellent ion exchange capabilities, reasonable interlayer spaces, and high porosities. 20,21 For instance, NiFe-LDH was reported for nitrobenzene detection, 22 CuAl-LDH nanocomposite for Glyphosate detection in water, 23 magneto LDH/Fe 3 O 4 for tramadol sensing, 24 and also ZnAl-LDH was described for the simultaneous detection of vitamin C and aspirin. 25 The LDH morphology involves negatively charged interlayer anions and positively charged host layers. Incorporating anions and water between metal cations and the layers cause a larger interlayer space, leading to a magnificent redox ability. 26 Nickelcobalt layered double hydroxides (NiCo-LDH) is a typical twodimensional lamellar metal hydroxide and a strong candidate as a substrate for the non-enzymatic electrochemical sensors because of their 2D nanosheets active area, versatility in both structural morphology and chemical composition which can effectively boost the electrochemical performance and thus improve anion exchange capability. [27][28][29] Still, NiCo-LDH may suffer from redox kinetics, which is limited by the mass diffusion rate resulting in poor conductivity due to the high mass loading capacity. 30 Therefore, the integration of carbonbased nanomaterials such as carbon nanotubes (CNTs), 31 graphene oxide (GO), 32 and reduced graphene oxide (rGO) [33][34][35] can be an appropriate solution to this problem. Hence, rGO offers superb features such as high conductivity, strong stability, and the presence of defective sites which are chemically active, making it the perfect candidate for improving the NiCo-LDH loading capacity and further boosting the electrochemical performance by accelerating the electron transfer rate. 36,37 To the best of our knowledge, there is no earlier study reported the successful fabrication of a 5-CP sensing platform based on NiCo-LDH supported on green organic-inorganic nanohybrid namely rGO-CuO, which is expected to positively impact the sensor sensitivity, improving the electroanalytical performance toward the direct detection of pentachlorophenol.
In this paper, and for the first time though, NiCo-LDH/rGO-CuO based catalyst was reported for the electrochemical trace level detection of pentachlorophenol as a high toxicity organic pollutant. The NiCo-LDH/rGO-CuO structures were synthesized through a simple two-step method. The chemical and morphological characteristics were investigated by infrared spectroscopy (ATR) and z E-mail: hicham.meskher@.g.enp.edu.dz scanning electron microscopy (SEM) while the electrochemical behavior of 5-CP on the NiCo-LDH/rGO-CuO composite was further studied using cyclic voltammetry techniques. Based on this, a simple, eco-frienly, cost-effective, and fast response sensor was successfully fabricated for the sensitive detection of 5-CP. Experimental results demonstrated that the developed sensor could detect low levels of 5-CP with negligible interference, enabling it to be applied in real samples.
Sensor preparation.-Preparation of the modified electrodes.-To begin, a gold electrode (AuE) was cleaned by polishing it in a circular motion using an alumina slurry (0.3 μm). For comparision purposes, different electrodes were prepared by drop-casting 3 mg ml −1 of the synthesized materials on the active surface area of the godl electrodes. The prepared electrodes are named: NiCo-LDH/ AuE, rGO-CuO/AuE, and NiCo-LDH/rGO-CuO/AuE/AuE.

Green synthesis of the organic-inorganic hybrid (rGO-CuO).-
The organic-inorganic hybrid namely rGO-CuO was produced utilizing an aqueous extract of Artemisia. The plant was collected from an Algerian agricultural area in the north of Ouargla city. Just after, fresh Artemisia leaves were washed multiple times with tap water, then distilled water, to eliminate any dust particles. After drying, the plant leaves were crushed into a fine powder. Leaf extract was made by combining 30 g of fine plant powder with 100 ml of purified water and heating it at 60°C for around 10 min. After that, it was left at room temperature for 24 h. The leaf extract was filtered and kept at 4°C for further research.
To make rGO-CuO, 30 mg of GO was mixed with 50 ml of CuSO 4 (10 mM) 160 mg. The browny blue solution was mixed with 20 ml of aqueous extract of Artemisia leaves for 35 min at 80°C. The solution's color changed to dark, confirming the bio-reduction of GO and CuO formation. The mass obtained was washed three times with distilled water to eliminate any trace of impurties until the pH value reached 7, and then dried at room temperature for 72 h.
Preparation of NiCo-LDH.-The synthesis of NiCo-LDH is carried out as follows: two separate solutions of Co(NO 3 ) and Ni(NO 3 ) are prepared by dissolving 0.145 g of Co(NO 3 ).6H 2 O with 0.118 g Ni (NO 3 ), respectively. Each solution's pH was adjusted to 10 by adding NaOH (0.1 M) wisely. Finally, the two solutions of Ni(NO 3 ) and Co(NO 3 ), were gently put into a 100 ml beaker and stirred for 16 h at 90°C. The mass obtained was washed several times to eliminate salts and any possible impurities and to maintain a pH of 7. Finally, the material produced was dried at 80°C for 24 h.
Sensor charcterisation.-All electrochemical measurements were carried out using an electrochemical instrument Voltalab PGZ 301 connected to a computer and controlled by Voltamaster 4 software. An electrochemical cell containing three electrodes system was used where a saturated calomel electrode (SCE) was used as reference electrode, a platinum wire as a counter and a gold electrode (AuE)was used as the working electrode.

Results and Discussion
Characterization of the NiCo-LDH and rGO-CuO.-Fourier transform infrared spectra of the synthesized GO, rGO-CuO, and NiCo-LDH were examined to determine the different functional groups, and they are displayed in Fig. 1. Figure 1a depicts a comparison between the spectrumsof GO and rGO-CuO. GO exhibits absorption peaks at 3300, 1695, and 1110 cm −1 in these spectra due to hydroxyl, carboxyl, and epoxy functionalities presented in the composite. 38 The intensity of OH stretching in the rGO-CuO spectra was dramatically lowered, indicating that GO was reduced during the doping process, 39,40 while the CuO stretching vibrations cause the absorption peak at 515 cm −1 in rGO-CuO spectra. The present findings match those reported values in literature, 41 which confirms the successful synthesis of the rGO-CuO hybrid. The NiCo-LDH spectra (Fig. 1b) 44 The bands in the 500-800 cm −1 range were attributed to metal-oxygen bond stretching, specifically Ni-O and Co-O. Figure 2 shows the results of scanning electron microscope (SEM) investigation of the synthesized materials. Figures 2a and 2b depict the surface morphology of the prepared NiCo-LDH. The surface morphology of the produced NiCo-LDH revealed rough surfaces that make it more porous and appear to be agglomerated in sphere-like shapes. As a result, the produced LDH has a porous surface that allows it to be combined with other materials while also providing a good indicator of its affinity for the 5-CP molecule. Similarly, Figs. 2c and 2d describe the surface morphologies of rGO-CuO. Apart from the wrinkled layers, distribution of CuO particles on the basal planes of graphene is also evident in the SEM micrograph of rGO-CuO displaying the magnetic properties of rGO-CuO along with the dispersion and agglomeration processes of rGO.
Electrochemical characterization of the sensing system.-Different electrodes of bare AuE, NiCo-LDH/AuE, rGO-CuO/AuE and NiCo-LDH/rGO-CuO/AuE were characterized using CV measurements in 5 mM of potassium ferri-ferrocyanide solution containing 0.1 M KCl. As shown in Fig. 3, it can be clearly observed that both the NiCo-LDH/AuE and rGO-CuO/AuE show an increasing current intensity compared with the bare AuE. However, compared to NiCo-LDH/AuE, rGO-CuO/AuE boosted the signal more due the excellent conductivity of rGO, the good transfer rate of copper and to the synergic effect between CuO and rGO. On the other hand, to improve the sensitivity of the NiCo-LDH/AuE sensing system, NiCo-LDH were combined with the organic-inorganic hybrid namely rGO-CuO in 1/1 ratio (3 mg ml −1 ). As can be seen in Fig. 3, NiCo-LDH/rGO-CuO/AuE sensing based system showed good electrochemical performance compared to the bare AuE, NiCo-LDH/AuE and even to rGO-CuO/AuE. This good electrochemical performance of NiCo-LDH/rGO-CuO/AuE can be attributed to the excellent electronic conductivity of the synthesized rGO-CuO and the affinity reaction on the engineered-interface-based carboxyl groups in NiCo-LDH and the dual task combination between NiCo-LDH and rGO-CuO on AuE surface which islikely to have high interaction with 5-CP. 45,46 Effect of scan rate.-As shown in Fig. 4a, the influence of varied scan rates ranging from 20-80 mV s −1 on the electrochemical performance of the NiCo-LDH/rGO-CuO/AuE was also investigated. The magnitudes of the current intensity responses illustrated in Fig. 4a demonstrate a progressive rise as the scan rate increases. As a result, it was discovered that the size of the current response is linearly dependent on the scan rate, as shown in Fig. 4b    It can be seen from Fig. 5a, the magnitude of the electrochemical response current of the NiCo-LDH/rGO-CuO/AuE was observed to increase with an increase in the 5-CP concentrations. This was attributed to the successful interaction between the present catalyst and 5-CP molecule due to the synergic effect between the organicinorganic hybrid (rGO-CuO) and NiCo-LDH. 48,49 A linear plot was drawn between changes in current and the concentrations of 5-CP and it is almost linear (Fig. 5b), which is expressed in the following equation I (μA) = 0.0125 C(μM)+0.7796 (R 2 = 0.98876). The cyclic voltammetry results show that the prepared sensor showed a wide linear response to 5-CP concentrations ranging from 1 to 50 μM with a correlation coefficient of 0.98876 while the detection limit (LOD) has been estimated as 12.64 nM and the quantification limit (LOQ) was estimated to be 38.32 nM. In terms of limit of detection, we are on par with most of the recent reports (Table I) but as per our knowledge, this is not only the first report using such combination of an LDH with an organic-inorganic composite based on green synthesis to detect 5-CP, but also the present study reports the first use of NiCo-LDH based electrochemical sensor matrix to detect 5-CP.
Stability of the electrochemical sensor.-Stability is a critical stage in any sensor system and must be correctly calibrated. The electrochemical scanning stability of NiCo-LDH/rGO-CuO/AuE was investigated by measuring the current response of the sensing system for 20 cycles in 50 μM of 5-CP using CV, as shown in Fig. 6. The relative standard deviation (RSD) values were estimated from the current responses of NiCo-LDH/rGO-CuO/AuE to be equal 3.6%, indicating that the electrode was stable and no surface denaturation has occured. Thus, even after 20 cycles of applied  voltage, the sensor has no bias stability concerns and continues to function effectively in 5-CP detection.

Conclusions
For the first time, we proposed a green synthesis of an novel organic-inorganic hybrid rGO-CuO decorated on NiCo-LDH, and its use in the construction of a new sensitive electrochemical sensor for 5-CP detection was also invesitgated. The developed sensing system based on NiCo-LDH and rGO-CuO composite might specifically improve AuE sensitivity, resulting in an increase in ferricyanide peak current intensity employing the modified electrode. The fabricated sensor performance was evaluated and employed for the electrochemical measurement of 5-CP with a limit of detection around 12.64 nM while the linear range has been determined to be from 1 to 50 μM. The sensing system demonstrated ease of setup, acceptable measurement reusability, and high stability towards pentachlorophenol.