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Novel and Highly Sensitive Electrochemical Sensor for the Determination of Oxytetracycline Based on Fluorine-Doped Activated Carbon and Hydrophobic Deep Eutectic Solvents

Residues of oxytetracycline (OTC), a veterinary antibiotic and growth promoter, can be present in animal-derived foods; their consumption is harmful to human health and their presence must therefore be detected and regulated. However, the maximum residue limit is low, and consequently highly sensitive and accurate detectors are required to detect the residues. In this study, a novel highly sensitive electrochemical sensor for the detection of OTC was developed using a screen-printed electrode modified with fluorine-doped activated carbon (F-AC/SPE) combined with a novel deep eutectic solvent (DES). The modification of activated carbon by doping with fluorine atoms (F-AC) enhanced the adsorption and electrical activity of the activated carbon. The novel hydrophobic DES was prepared from tetrabutylammonium bromide (TBABr) and a fatty acid (malonic acid) using a green synthesis method. The addition of the DES increased the electrochemical response of F-AC for OTC detection; furthermore, it induced preconcentration of OTC, which increased its detectability. The electrostatic interactions between DES and OTC as well as the adsorption of OTC on the surface of the modified electrode through H-bonding and π–π interactions helped in OTC detection, which was quantified based on the decrease in the anodic peak potential (Epa = 0.3 V) of AC. The electrochemical behavior of the modified electrode was investigated by cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy. Under optimum conditions, the calibration plot of OTC exhibited a linear response in the range 5–1500 μg L–1, with a detection limit of 1.74 μg L–1. The fabricated electrochemical sensor was successfully applied to determine the OTC in shrimp pond and shrimp samples with recoveries of 83.8–100.5% and 93.3–104.5%, respectively. In addition to the high sensitivity of OTC detection, the proposed electrochemical sensor is simple, cost-effective, and environmentally friendly.


INTRODUCTION
Oxytetracycline (OTC) is an antibiotic in the tetracycline group that has been widely used in fields such as veterinary antibiotics and as animal growth promoters for the prevention and treatment of livestock diseases. 1,2 Residues of OTC can therefore be found in animal-derived foods; however, consumption of excessive residual antibiotics can seriously threaten human health. 3 Therefore, regulatory organizations such as the Food and Agriculture Organization (FAO), Food and Drug Authority (FDA), and Environmental Protection Agency (EPA) have set a maximum residue limit (MRL) of 0.2−1.2 mg L −1 for OTC in animal-derived foods. 4 As the MRL of OTC is low, sensitive detection techniques are required.
Several analytical techniques have been applied to detect OTC; accurate qualitative and quantitative results can be achieved by chromatographic methods. 5,6 Recently, electrochemical sensors have gained considerable attention owing to their high sensitivity and selectivity, fast analysis, rapid process, and low cost. 7 In recent years, there has been great development of new materials for electrochemical applications such as carbon nanomaterials 8−10 and metal nanoparticles. 11−13 The attractiveness of nanomaterials including a unique characteristic, high chemical reactivity, and high surface area 14,15 significantly increases the sensitivity of electrochemical measures. 16 The attractive characteristic of the material can allow its use in the development of modified electrodes for the determination of target analytes with environmental concerns.
The modification of electrodes with nanomaterials has attracted attention, especially, carbon nanomaterials. Activated carbon (AC) is produced from environmental wastes with a high carbon content. AC is a low-cost material with distinguishable properties like a high specific surface area, high porosity, and desired surface functionalization available for adsorption or chemical reactions. 17 In our previous study, 18 we synthesized AC from waste coffee grounds and modified AC with CuO-NPs for the detection of methyl parathion in soil samples. However, modification of AC is required to enhance its conductivity: doping of carbon nanomaterials can induce changes in their structure and properties that are favorable for catalytic applications. Thus, doping of AC is one approach to improve its electrical conductivity for enhanced detection of OTC.
Heteroatom-doped carbon materials have attracted particular attention because of their low cost and superior electrochemical properties, conductivity, and chemical stability. 19,20 Typically, heteroatom-doped carbon catalysts have been fabricated using nitrogen, 21,22 boron, 22−24 and phosphorus 22,25,26 as the doping elements and through post-treatment methods at high temperatures. A new class of halogen-doped carbon materials (i.e., fluorine, 27 chlorine, 28 bromine, 29 and iodine 30 ) has, however, emerged as a promising choice for the efficient modification of carbon materials. Moreover, the surface area and electrical double-layer capacitance of carbon materials can be improved for use in supercapacitors by doping them with halogen atoms via a simple and effective method. 31,32 Among halogen-doped carbon materials, fluorine-doped carbon materials exhibit a highly efficient approach to increase the electrical conductivity: fluorine atoms enhance polarization owing to the presence of highly electronegative fluorine functional groups and refinement of pore structures/ surfaces. 33 Therefore, fabricating fluorine-doped carbon with a high fluorine content, intact carbon structure, and superior electrochemical performance using a facile synthesis approach is critical.
Deep eutectic solvents (DESs) are eutectic mixtures comprising hydrogen bond donors and acceptors. 34,35 The interactions between these components lower the melting point of the mixture to below those of the individual components, leading to a liquid phase at room temperature. DESs are easy to prepare and offer several advantages 34,35 such as solubility of various organic and inorganic species, high biodegradability, low toxicity, and low cost. Among the different types of DESs, hydrophobic DESs have received considerable attention in recent years. However, to date, only a few hydrophobic DESs have been reported. Hydrophobic DESs are based on poorly water-soluble components such as tetraalkylammonium salts, 36 long-chain carboxylic acids, 37 menthol, 38 and lidocaine. 39 Ruggeria et al. studied the chemical and electrochemical properties of a hydrophobic DES that was prepared by mixing tetrabutylammonium chloride and decanoic acid. 40 Its hydrophobic nature was mostly extracted with aqueous media, such as volatile fatty acids 36 and pesticides. 41 The addition of a small amount of water to the hydrophobic DES not only can dramatically improve its electrical conductivity, but it can also improve the viscosity to a value suitable for electrochemical analysis. DESs are particularly appealing green systems for application in electrochemistry, such as in sensing and electrodeposition. 40 For example, hydrophobic DESs can induce preconcentration of compounds to be detected during electrochemical sensing, which is highly advantageous when the amount of compounds to be detected is very low. Hence, it is challenging to synthesize a novel DES to increase the performance of electrochemical sensors.
In this study, a novel and highly sensitive electrochemical sensor using a fluorine-doped activated carbon (F-AC)modified screen-printed electrode (SPE) was fabricated for OTC detection. DES was used to induce the preconcentration of the target analyte to improve OTC detection. This is the first report on the use of a hydrophobic DES composed of tetrabutylammonium salt (tetrabutylammonium bromide (TBABr)) and fatty acids (malonic acid) to increase the sensitivity of OTC detection; the DES was prepared using a simple and green synthesis method. In this study, OTC was indirectly detected by decreasing the anodic peak current of F-AC in the presence of OTC. Modification of the SPE with F-AC combined with DES enhanced electrochemical sensitivity and selectivity for OTC detection. Various experimental parameters, including the modification of the electrode surfaces and the electrochemical behavior of OTC, were thoroughly investigated. Lastly, the proposed electrochemical sensor was successfully applied to determine OTC in shrimp pond samples.

Apparatus.
All electrochemical measurements were carried out using an electrochemical workstation (AutoLab, PGSTAT302N, Switzerland) containing 5 mL of the solution in a conventional three-electrode system at room temperature, using an SPE modified by fluorinated AC as the working electrode (F-AC/SPE), Ag/AgCl (3 mol L −1 NaCl) as the reference electrode (Hebei, China), and a platinum wire as the counter electrode (Hebei, China). The surface morphology and composition of the modified electrodes were examined using field-emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDS) (Helios NanoLab G3 CX, FEI, USA). X-ray photoelectron spectroscopy (XPS; AXIS Ultra DLD, UK) was employed to analyze the elemental compositions of F and AC.

Fabrication of the Modified Electrodes.
AC from waste coffee grounds was synthesized according to a previous study 18 and used as a precursor to synthesize F-AC. F-AC was carried out in accordance with a previous study 42 by mixing 1 mg of AC, 50 mL of ammonium hydroxide, and 200 mL of distilled water under vigorous stirring for 30 min at room temperature. Hydrofluoric acid (5 mL) was added to the solution, which was then stirred for 24 h at room temperature and filtered to separate the AC. The filtered AC was washed thrice with distilled water until a neutral solution (pH 7.0) was obtained. The prepared sample was dried for 24 h at 80°C to obtain the final F-AC powder. F-AC (3 mg) was dissolved in DMF (1 mL) and sonicated for 30 min to obtain a homogenous suspension. The F-AC suspension (5 μL) was dropped onto the SPE surface, which was dried at room temperature to obtain F-AC/SPE. Before each measurement, a potential ranging from −1.0 to +1.0 V with a scan rate of 100 mV s −1 was applied to the modified electrode in a phosphate buffer (pH 7.0) and repeated for 20 cycles to clean the electrode surface. The phosphate buffer solutions were prepared in a pH range of 5.0−10.5 by mixing solutions of 0.1 mol L −1 NaH 2 PO 4 ·2H 2 O and 0.1 mol L −1 Na 2 HPO 4 · 2H 2 O. The pH of the acidic solution was adjusted by adding 0.1 mol L −1 H 3 PO 4 , while the addition of 0.1 mol L −1 NaOH was used to adjust the pH of the basic solution. All phosphate buffer solutions were stored at room temperature until the time of use in the electrochemical analysis.

Sample Preparation.
Shrimp pond samples (WS−1− 4) and shrimp samples (SH−1−4) were collected from a local market in Khon Kaen, Thailand. The shrimp pond samples were filtered through a 0.45 μm membrane before analysis. Meanwhile, shrimp samples were mixed until a homogeneous texture was obtained, accurately weighed (1.0000 g), and then placed in a 15 mL centrifuge tube to which McIlvain buffer (10 mL, pH 4.0) was added to extract the analyte. The samples were mixed by vortexing for 30 s and then centrifuged at 5000 rpm for 20 min. The resulting supernatant was filtered through a 0.45 μm membrane before analysis using the proposed electrochemical sensor.
The McIlvain buffer solution (pH 4.0) was prepared by dissolving 7.5 g of disodium hydrogen phosphate dehydrate, 6.5 g of citric acid monohydrate, and 1.86 g of EDTA in water and diluting to 500 mL. The stock buffer solution was adjusted to pH 4.0 using 1 mol L −1 HCl. 6 2.5. Electrochemical Analysis of Oxytetracycline. A 5 mL standard OTC or sample solution in phosphate buffer (pH 9.5) was detected by differential pulse voltammetry (DPV). The prepared DES (75 μL) was then added to each solution and stirred at 650 rpm for 20 s prior to electrochemical analysis using DES-F-AC/GCE. DPV measurements were performed from −1.0 to +0.6 V, and all experiments were performed in triplicate.

Characterization of the Modified Electrode.
The surface morphology and composition of AC/SPE and F-AC/ SPE were characterized using SEM and EDS. Figure 1A shows the surface morphology of AC, which exhibits irregular shapes. 43 F-AC exhibits a surface morphology similar to that of AC and has a smooth surface, as shown in Figure 1C. 44,45 Moreover, EDS of the synthesized AC (inset; Figure 1A,B) displayed a composition of approximately 88% carbon atoms and 12% oxygen atoms, while that of F-AC/SPE displayed 49.5% carbon atoms, 41.6% oxygen atoms, and 0.6% fluorine atoms (inset; Figure 1C,D). Therefore, the successful fabrication of the proposed electrode was confirmed. The synthesized AC and F-AC were characterized using XPS to study the chemical composition ( Figure 2). The XPS spectrum of the synthesized AC ( Figure 2A) displayed peaks at 285.0 and 532.0 eV, which indicated the presence of carbon and oxygen atoms, respectively. The high-resolution C 1s spectra of AC ( Figure 2B Figure 2D illustrates the high-resolution C 1s spectra of F-AC, wherein an additional peak corresponding to the C−F bond (289.8 eV) was observed, which was not detected for AC. 46 The relatively high C−F binding energy was due to the large electronegativity difference between the components; the electronegativity difference enhanced the conductivity of the activated carbon by facilitating charge transfer between fluorine and carbon. 47,48 Thus, the XPS results revealed that the doping of fluorine atoms on AC was successful in fabricating F-AC.

Electrochemical Behavior of Modified Electrodes.
The electrochemical efficiencies of the modified electrodes were characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in 5 mmol L −1 [Fe(CN) 6 ] 3−/4− containing 0.1 mol L −1 KCl. As shown in Figure 3A, the response of [Fe(CN) 6 ] 3−/4− at bare SPE was the lowest value including the peak separation (ΔE) of 0.20 V at bare SPE obtained from the anodic peak at 0.00 V and the cathodic peak at −0.20 V and current of [Fe(CN) 6 ] 3−/4− due to a small surface area (0.069 cm 2 ) and weak conductivity. After the modification with either AC or F-AC (AC/SPE and F-AC/SPE), the peak currents were clearly increased and the peak potentials were shifted to a negative value providing ΔE of 0.18 and 0.15 V for AC/SPE and F-AC/SPE, respectively. Furthermore, when F-AC combined with DESs (DES-F-AC/ SPE), the response of [Fe(CN) 6 ] 3−/4− showed the highest peak current resulting from the synergistic effect of F-AC and DES. The ΔE of 0.15 V was obtained from DES-F-AC/SPE and another one peak at 0.65 V (cathodic peak) was the peak potential of the DES, indicating that electron transfer was facilitated.
According to the Randles−Sevcik equation, 49 the electroactive surface area can also be calculated as respectively. The DES-F-AC/SPE showed the highest electrochemical response because (i) F-AC has a high surface area and sp 2 hybridized carbon atoms constitute the main structure, leading to a much lower electron transfer resistance and high conductivity, and (ii) the presence of DES induced more porosity in F-AC, thus increasing the surface area for target analyte adsorption. Furthermore, the high conductivity of the DES accelerated the electron transfer between the analyte and electrode surface. 50 EIS was used to study the interfacial properties of the surface-modified electrodes. A modified Randles circuit was chosen to fit the impedance data. The circuit parameters corresponding to the electron transfer resistance (R ct ) and Warburg impedance are both parallel to the double-layer capacitance. Figure 3B shows the impedance spectra of (a) bare SPE, (b) AC/SPE, (c) F-AC/SPE, and (d) DES-F-AC/ SPE. The R ct at the SPE was evaluated to be 54.6 Ω, which dropped to 7.97 Ω at the AC/SPE and 4.94 Ω at the F-AC/ SPE. DES-F-AC/SPE exhibited the lowest R ct of 3.14 Ω. Therefore, the decrease in R ct proves the significant acceleration of electron transfer by DESs and F-AC. The impedance results confirmed the successful fabrication of DES-F-AC on the bare SPE surface.
The electrochemical properties of OTC on F-AC/SPE and DES-F-AC/SPE were studied by CV. A comparison of the electrochemical behavior of OTC on F-AC/SPE and DES-F-AC/SPE is shown in Figure 4. The F-AC/SPE (inset A) and DES-F-AC/SPE (inset B) cyclic voltammograms demonstrated the oxidation reaction of F-AC at an anodic peak potential (E pa ) of ∼0.30 V. 51 The decrease in the current (ΔI) of AC in the presence of OTC was compared to that without OTC. The ΔI of AC on DES-F-AC/SPE was higher than that on F-AC/ SPE owing to the adsorption of OTC on the surface of F-AC through electrostatic interactions, hydrogen bonding, and π−π interactions, as shown in Figure 5. 52 The adsorption mechanism of OTC on the electrode surface was investigated based on the effect of the scan rate (ν) on the peak current and potential at the F-AC/SPE. The CV curves of F-AC/SPE were obtained at different scan rates over the range 10−100 mV s −1 in the presence of 200 μg L −1 OTC ( Figure  S1A). The peak current of the AC increased, and the potential shifted with increasing scan rate, confirming the irreversibility of the electrode process. 53 The relationship between the change in current (ΔI) and the scan rate (ν) was linear, with a regression equation of ΔI = 0.0041ν + 0.0518 (R 2 = 0.9973), as shown in Figure S1B. The relationship between the peak potential and scan rate is based on the where E p is the peak potential, E 0 is the formal potential (V), T is the temperature (298.15 K), α is the electron transfer coefficient, n is the electron transfer number, k 0 is the rate constant, F is the Faraday constant (F = 96,485 C mol −1 , R = 8.314 J K −1 mol −1 ), and ν is the scan rate (V s −1 ). The relationship between the oxidation peak potential (E pa ) and the natural logarithm of the scan rate (lnν) is shown in Figure  S1C. The oxidation peaks moved directly toward a positive potential for various scanning rates. E pa was linearly proportional to ln ν and could be described by the linear equation E p = 0.085 lnν + 0.0514 (R 2 = 0.9902). The slope of E p against lnν was calculated according to eq 2; a calculated value of 0.30 was achieved for αn, with α being assumed as 0.5 for the irreversible progress. 55 Therefore, the number of electrons in the OTC oxidation reaction was evaluated to be 1, which is consistent with previous reports. 56 The reaction between OTC and F-AC/SPE is shown in Figure 5.

Effect of Concentration and Volume of F-AC.
In this study, the electrochemical behavior of OTC adsorbed on modified electrodes was studied using CV and DPV. The peak current of F-AC decreased significantly owing to the adsorption of OTC on the surface of the electrode through electrostatic interactions, hydrogen bonding, and π−π electron donor−acceptor interactions. 52 Therefore, the results were investigated based on the change in the current (ΔI) of F-AC with and without OTC. Although OTC affected the oxidation peaks of F-AC, only the oxidation peak (anodic current) of F-AC was considered.
The effect of the concentration and volume of the F-AC suspension on the modified electrode (F-AC/SPE) was studied over the ranges 1−10 mg/mL and 2−10 μL, respectively; the effect of F-AC concentration is presented in Figure S2A and the effect of the F-AC volume in Figure S2B. It was found that the ΔI of F-AC/SPE increased slightly with an increase in the concentration of the F-AC suspension from 1 to 3 mg mL −1 ; above this concentration (>3 mg mL −1 ), the ΔI of F-AC decreased slightly. Meanwhile, the ΔI of F-AC increased as the volume of the F-AC suspension increased from 2 to 5 μL; further increase in the volume (>5 μL) caused a decrease in the ΔI. The ΔI of F-AC decreased when the concentration and volume of F-AC increased beyond 3 mg mL −1 and 5 μL, respectively, because the thick layer of F-AC blocked the electron transfer from the F-AC to the electrode surface. Therefore, the optimum concentration and amount of F-AC were selected as 3 mg mL −1 and 5 μL, respectively.

Effect of DESs.
The effects of DESs including type, mole ratio, and volume of DESs were also investigated by DPV. The effect of various type of DES-based TBABr (hydrogen bond acceptor) with different fatty acids (hydrogen bond donor) including MA, TA, OA, PheBA, HepA, OctA, or NanoA is shown in Figure S3A, TBABr:MA gave the highest ΔI of F-AC in the presence of 200 μg L −1 OTC in 0.1 mol L −1 phosphate buffer (pH 9.5), whereas the results of TBABr:TA and TBABr:OA were not shown because they were not be synthesized. Thus, TBABr:MA was chosen for further experiment. The TBABr:MA mole ratio was also studied, and the result is shown in Figure S3B. For TBABr:MA, 2:1 is the optimum ratio to form DESs, which gave highest ΔI of F-AC. It is because the high ratio of TBABr causes a high positive charge to interact with anionic charge of OTC. The DES volume was studied in range of 50−125 μL, as shown in Figure  S3C. The ΔI increased until the DES volume up to 75 μL. After this, the ΔI was decreased because DES covered the F-AC surface. The DES hindered the electrochemical behavior between OTC and the surface of electrodes. Therefore, the optimum DES volume is 75 μL.

Effect of pH.
The influence of pH on the DES system was studied over a pH range of 8.5−10.5 at DES-F-AC/SPE using 0.1 mol L −1 phosphate buffer. The results ( Figure S4A) showed that ΔI increased until pH 9.5 and then sharply decreased. DES is of the ionic hydrophobic type that acts as a highly efficient extractant for a wide range of compounds. 57 At pH < pK a3 , OTC has one positive charge, one zwitterion, and one negative charge, which interact poorly with the cationic ACS Omega http://pubs.acs.org/journal/acsodf Article DES. However, at pH > pK a3 , OTC produced two negative charges and was fully deprotonated; thus, the sensitivity of OTC increased owing to the electrostatic interactions between the anionic OTC and the cationic DES. Hence, a phosphate buffer with a pH of 9.5 was chosen for the DES-F-AC/SPE system. Moreover, the number of protons and electrons transferred in the reaction of OTC were investigated using F-AC/SPE in the pH range of 5.0−7.5. At low pH values, OTC was positively charged (pH < pK a1 (3.6)). One negative charge was obtained for OTC at pH values higher than that of pK a2 (>7.3). At high pH values, OTC had two negative charges (pH > pK a3 (8.9)). At pH values ranging between pK a1 and pK a2 , OTC molecules existed in neutral or zwitterionic forms. 55 Fig-Figure S4B,C shows the influence of pH on the peak current and ΔI of F-AC, respectively, in the presence of OTC. It was found that the peak intensity and ΔI increased with increasing pH until pH 6.0, and thereafter significantly decreased. Moreover, the voltammograms showed that when the pH decreased, the anodic and cathodic peak potentials of F-AC were more positive, suggesting the involvement of protons in the oxidation of OTC. At pH 6.0, the zwitterion OTC was easily oxidized, and the maximum peak current of AC was achieved. According to the Nernst equation, 49 the relationship between E p and pH is described as: where E p is the peak potential (V), m and n are the numbers of protons and electrons in the electrochemical reaction, respectively, and b is the intercept of the equation. Plots of the peak potential (E p ) and pH ( Figure S4D) revealed that the linear relationship can be expressed as follows: E p = −0.0690pH + 0.9099 (R 2 = 0.9988). It is clearly seen that the linear slope, 69.0 mV pH −1 , is close to the theoretical value, 59.2 mV pH −1 , 58 implying that the numbers of protons and electrons transferred in the reaction of OTC were equal.

Selectivity of OTC Detection.
To evaluate the selectivity of the fabricated sensor, the effect of potential interference from species such as antibiotics (amoxicillin, tetracycline, doxycycline, chlortetracycline, ciprofloxacin, and norfloxacin) and cations (Cu 2+ , Fe 3+ , Na + , and Zn 2+ ) were studied by DPV using the DES-F-AC/SPE containing 200 μg L −1 of OTC and 200 μg L −1 of antibiotics and cations. The results clearly showed that all studied substances did not significantly interfered MP detection ( Figure 6A), while CTC, DOX, and TET had an effect on the DPV signal, which may be because these compounds are in the same chemical group as OTC. The selectivity of the modified electrodes for OTC detection using another antibiotic and metal ions containing OTC at the same concentration is shown in Figure 6B. The results showed that the change in the F-AC signal was still ≥95%. Thus, OTC exhibited the highest sensitivity under the studied conditions for the fabricated electrochemical sensor.

Quantitative Analysis and Method Validation.
The analytical performance of the fabricated sensor, DES-F-AC/SPE, was evaluated using DPV under optimal conditions. The analytical features studied were the linearity, detection limit (LOD), and quantification limit (LOQ). Figure 7 displays the voltammograms of F-AC recorded in a potential range of −1.0 to +1.0 V in different OTC concentrations using the proposed electrochemical sensors. The current corresponding to F-AC decreased with increasing OTC concentration from 0 to 1500 μg L −1 . The results obtained from the DES-F-AC/SPE ranged from 5 to 1500 μg L −1 and displayed linearity, with a  determination coefficient (R 2 ) of 0.9981 (inset; Figure 7). The LOD and LOQ were calculated according to the following equations: LOD = 3σ/S and LOQ = 10σ/S, where σ is the standard deviation of the blank (n = 7) and S is the slope of the calibration plot. The LOD and LOQ obtained from the DES-F-AC/SPE were 1.74 and 5.80 μg L −1 , respectively. The results indicated that the DES-F-AC/SPE exhibited a high sensitivity for OTC detection. In addition, the performance of the DES-F-AC/SPE sensor was compared with that of previously reported sensors, as summarized in Table 1. While the linear range of the fabricated sensor was comparable to that of other sensors, this sensor was more sensitive than most reported sensors, which provided a low LOD. In addition, the proposed sensor DES-F-AC/SPE is simple to fabricate and cost-effective. a PoAP/MWCNTs/Pt: Poly-ortho-aminophenol and multiwalled carbon nanotubes deposited on the platinum electrode. b Zn-Mt/GCE: Zinc cation-exchanged montmorillonitemodified glassy carbon electrode. c Cu 2 O microsphere/GCE: copper(I) oxide microspheremodified glassy carbon electrode. d Ta 2 O 5 -ErGO/GCE: Tantalum pentoxide nanoparticleelectrochemically reduced graphene oxide nanocompositemodified glassy carbon electrode.

Detection of OTC in Real Samples.
The performance of the DES-F-AC/SPE sensor in detecting OTC in shrimp pond and shrimp samples was tested. In this work, to ensure that the sensor exhibited a high performance for the detection of OTC, AC was modified by doping with fluorine atoms and the electrical conductivity was increased using DESs. The detection of OTC occurred through the electrostatic interactions between DES and OTC under alkaline conditions and the adsorption of OTC on the F-AC surface through H-bonding and π−π interactions. The OTC in shrimp samples was extracted by adding the McIlvaine buffer (pH 4.0). Shrimp pond and shrimp samples were spiked with OTC at three concentration levels (6, 500, and 1000 μg L −1 ), and the detection was performed in triplicate. The recoveries of OTC were achieved in the range of 83.8−100.5% and 93.3− 104.5% for the water samples from the shrimp pond and shrimp samples, respectively. The RSDs of OTC detection in all samples was less than 6.70%, as shown in Tables 2 and 3. The results from the fabricated sensor at 500 and 1000 ng L −1 OTC were compared with those from HPLC and were in good agreement. Thus, the fabricated sensor (DES-F-AC/SPE) effectively and accurately detected OTC.

CONCLUSIONS
A novel and sensitive electrochemical sensor using the SPE modified with F-AC in the presence of DES (DES-F-AC/SPE) was fabricated to detect OTC. The facile doping of AC with fluorine increased its surface area and conductivity. Moreover, the novel DES made from TBABr and malic acid increased the sensitivity of F-AC for the detection of OTC, while also inducing preconcentration of OTC to increase its detectability. OTC determination was achieved through electrostatic interactions between DES and OTC, which facilitated adsorption of OTC on the F-AC surface through H-bonding and π−π interactions. The electrochemical sensor exhibited an