Highly Sensitive Detection of Trace Tetracycline in Water Using a Metal-Organic Framework-Enabled Sensor

Due to the abuse application of antibiotics in the recent decades, a high level of antibiotics has been let out and remains in our environment. Electrochemical sensing is a useful method to sensitively detect antibiotics, and the key factor for a successful electrochemical sensor is the active electrode materials. In this study, a sensitive electrochemical sensing platform based on a metal-organic framework (MOF) of MIL-53 (Fe) was facilely fabricated. It shows highly selective and sensitive detection performance for trace tetracycline. Differential pulse voltammetry (DPV) was applied to analyze the detection of tetracycline. The linear range of tetracycline detection was 0.0643 μmol/L-1.53 μmol/L, and the limit of detection (LOD) is 0.0260 μmol/L. Furthermore, the MOF-enabled sensor can be effectively used in actual water bodies. The results indicate that the electrochemical sensor is a high potential sensing platform for tetracycline.


Introduction
Tetracycline is a typical antibiotic which has been widely used to treat infections caused by Gram-positive and Gramnegative bacterium in both human and animals. Furthermore, it has been also used as growth promoting agents in livestock breeding industry [1]. Because of its low cost and wide-spectrum antibacterial activity, abuse application of tetracycline was common in the past decades. As a result, high tetracycline residue was detected in lots of environmental areas nowadays, including soil, water, and animal productions [2]. Long-term exposure to antibiotics can cause directed toxicity or result in allergies, also boosting the prevalence of antibiotic resistance [3][4][5][6]. Tetracycline, as one of the most applied antibiotics, is considered to be carcinogenic, and depressive to bone growth besides the common adverse effects of antibiotics [7]. Tetracycline is a typical emerging contaminant (ECs), which usually refers to those highly concerned environmental substances because of their detecting frequency and potential dangers, but the environmental management details such as monitor regulations and emission control limits are still in blank [8]. Thereafter, highly sensitive and fast responsive techniques for tetracycline detection are urgently needed, which are important for the governors to formulate the suitable management policies.
Numerous methods have been developed for tetracycline detection. Chromatographic methods are the sensitive methods to detect tetracycline, including high performance liquid chromatography (HPLC) [9], liquid chromatography coupled to mass spectrometry (LCMS) [10], and thin-layer chromatography (TLC) [11]. Nevertheless, some inherent drawbacks, such as time-costing, needing highly professional operators, and luxury devices, significantly limit their wide application. Besides chromatographic methods, some other methods such as capillary electrophoresis (CE) [12] and enzyme-linked immunosorbent assay (ELISA) [13] are also applied to detect tetracycline with good performance. But inherent disadvantages including complicated pretreatments and skilled operation requirement still exist. Therefore, more convenient, rapid, sensitive, and selective techniques are   Adsorption Science & Technology highly required in tetracycline detection. Recently, tetracycline detection based on electrochemical sensors rapidly develops owing to their easy operation, high sensitivity, high selectivity, and real-time detection. Metal-organic frameworks (MOFs), a polymer built by assembling metal ions and lots of organic linkers, has been widely used in catalysis [14], adsorption [15], energy storage [16], sensors [17] and so on, due to their unique advantages of plentiful micropores, large specific surface area, abundant active sites, and varied conformations [18]. Therefore, facile whereas highly performed MOF-enabled electrochemical sensors are highly demanded.
In this study, a simple electrochemical sensing platform based on a Fe containing MOF, MIL-53 (Fe), modified glass carbon electrode (denoted as MIL-53 (Fe)/GCE) was facilely fabricated. The present work demonstrates that MIL-53 (Fe)/GCE shows highly sensitive detection performance for trace tetracycline, indicating its high potential in effective electrochemical sensor application due to the easy fabrication, high sensitivity, and low cost.   3 Adsorption Science & Technology 2.3. Preparation of MIL-53 (Fe)/GCE. 5 mg MIL-53 (Fe) was dispersed by ultrasonication in 5.0 mL deionized water to form a suspension (1 mg/mL). The GCE (diameter 3 mm) was polished with alumina powder and then ultrasonically washed with dilute nitric acid (V concentrated nitric acid : V water = 1 : 1), ethanol, and DI water sequentially. At last, it was dried at room temperature (RT). 8 μL suspension was dropped on the surface of the polished GCE. After it was dried, 8 μL 0.5% Nafion was applied in drops to the surface to obtain the MIL-53 (Fe) modified GCE, denoted as MIL-53 (Fe)/GCE, and then, it was wired for electrical measurements.

Electrochemical Experiments.
All the experiments were conducted in a standard three-electrode system by an  Adsorption Science & Technology electrochemical workstation (CHI660E). Platinum plate electrode and Ag/AgCl (saturated KCl) were employed as a counter electrode and a reference electrode, respectively, and GCE supported by catalyst was used as a working electrode. All subsequent experiments were carried out under the condition of constant temperature at 25°C. Before the experiment, nitrogen was used to purge the electrolyte for degassing. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were tested by adding certain amount of tetracycline in 1 M H 2 SO 4 . The scanning rate of CV is 100 mV/s, and the voltage range is -0.7~1.2 V. DPV was applied with voltage range of -0.8~1.5 V, pulse width of 50 ms, and pulse amplitude of 50 mV. MIL-53 (Fe) and the ligand p-phthalic acid (H 2 BDC) were investigated by FT-IR spectroscopy, and the results are indicated in Figure 2(a). One can see that MIL-53 (Fe) clearly shows distinct peaks at 1587, 1396, 745, and 550 cm -1 . The strong adsorption band appearing at 1396 cm -1 is ascribed to the symmetric vibrations of the -COOH group, confirming the presence of carboxylate linkages in the framework of MIL-53 (Fe) [22]. The peak at 745 cm -1 is due to the bending vibration of the C-H bond from the benzene ring [19,23]. Furthermore, the band at 550 cm -1 is related to the tensile trembling of the Fe-O bond [24], which indicates the formation of Fe-oxo cluster linking the metal and the carboxyl group. H 2 BDC shows a clear characteristic band of carboxyl group asymmetric vibrations at 1682 cm -1 . It is noted that this band in MIL-53 (Fe) shifts to 1587 cm -1 , indicating the carboxyl group of H 2 BDC coordinated with the metal center and formed the complex of MIL-53 (Fe) [25]. The FT-IR analysis indicate that MIL-53 (Fe) is successfully synthesized.

Results and Discussion
The structure of MIL-53 (Fe) was studied by XRD spectra. As shown in Figure 2(b), the spectra exhibits strong adsorption bands at 9.17°, 12.7°, 17.6°, and 25.5°, which are consistent with the characteristic XRD spectra of MIL-53 (Fe) [21]. This result confirms the successful preparation of MIL-53(Fe).
In order to study the surface area and pore diameter of the as-synthetic MIL-53(Fe), the material were subjected to BET analysis. As shown in Figure 2(c), the N 2 adsorptiondesorption isotherms shows a typical H3 type hysteresis ring, which indicates the existence of mesopores formed by accumulation. The BET special surface area of MIL-53 (Fe) is calculated to be 213.93 m 2 /g. The high BET special area and mesopores can increase the molecular interaction between the analyte and the active electrode material, promoting the sensing performance [26][27][28][29].

Electrochemical Characterization of MIL-53 (Fe).
The electrochemical behaviors of tetracycline over bare GCE and MIL-53 (Fe)/GCE were investigated in 1 M H 2 SO 4 via CV. It can be seen in Figure 3 that the reduction peak in the range of -0.35 to -0.55 V is sensitive, so it is measured as a response signal to compare the difference between the samples. It can be found that the bare GCE shows a little electrochemical response to tetracycline. As GCE was modified with MIL-53 (Fe), its response to tetracycline was significantly improved, which indicates the high electrochemical activity toward tetracycline of MIL-53 (Fe).
3.3. The Effect of pH. The electrochemical responses of the sensor toward tetracycline in different pH conditions were investigated by CV, and the pH was adjusted by H 2 SO 4 solution. The experiment was carried out under the following conditions: constant temperature (25°C), tetracycline concentration of 0.641 μmol/L, pH range of 0 to 2.6, voltage range of -0.7 V~1.2 V, and scanning rate of 100 mV/s. As indicated in Figure 4(a), the reduction peak potential (E p ) in the range of -0.35 to -0.55 V is sensitive; therefore, it is measured as a response signal. The reduction peak potential (E p ) and the related peak current (I p ) were plotted with pH values of the supporting electrolyte solution. As one can see in Figure 4(b), when the pH value rises from 0 to 2.6, the peak potential of the cathode changes negatively and is inversely proportional to pH, indicating that protons are directly related to electrochemical reactions [30]. The linear relationship between peak potential (E) and pH over MIL-53 (Fe)/GCE towards tetracycline can be expressed as follows: E = −0:05013pH − 0:3837 ðR 2 = 0:94942Þ. It can be found that the slope of the linear relationship is close to the  [31], indicating that the detection of tetracycline by MIL-53 (Fe) is an electrochemical reaction that the same amount of proton and electron is involved [30]. As one can see in Figure 4(c), when the pH value is 0, the I p value is the largest. When the pH rises from 0 to about 0.25, the value of I p significantly decreases. When the pH changes from 0.25 to 2, the I p changes a little but relatively stable as compared with the behaviors in other pH ranges, indicating pH does not have much effect on I p in this range. When the pH is higher than 2.0, the value of I p dramatically decreases as the pH rises. This result indicates that pH = 0 can be determined as the optimum pH condition, because the highest I p value is obtained.

Detecting Range and
Limit of MIL-53 (Fe)/GCE. The detection of tetracycline over MIL-53 (Fe)/GCE was measured by differential pulse voltammetry (DPV) (Figure 5(a)). A certain amount of tetracycline was added in 1 M H 2 SO 4 solution, and the DPV was carried out. The concentration of tetracycline and the related peak current (I p ) in DPV at~0.25 V (I p ) are linearly related over two different continuous concentration ranges ( Figure 5 The limit of detection (LOD) is calculated according to the IUPAC criterion (based on 3 times the signal-to-noise ratio, Equation (3)) [32].

Adsorption Science & Technology
where σ is the standard deviation of a blank (n = 20) and k is the slope of the calibration curve. Therefore, the limit of detection is estimated to be 0.0260 μmol/L.
Because tetracycline is one of the ECs, there is no official permitted limitation of tetracycline in water at present. It is no other than comparison of detecting performance of MIL-53 (Fe)/GCE and other tetracycline sensors can be used to evaluate the detecting efficiency of MIL-53 (Fe)/GCE. For this purpose, the parameters of detection techniques, linear range, and limit of detection of MIL-53 (Fe)/GCE were compared to other reported tetracycline sensors. As one can see in Table 1, the performance of MIL-53 (Fe)/GCE is comparable with those of others, even higher in some aspects (especially the detection limit). The comparison indicate that MIL-53 (Fe)/GCE is an effective sensing platform, which can sufficiently meet the requirement of tetracycline detection application.

Selective Study.
To evaluate the selectivity of the electrochemical sensor for tetracycline, the interfering substances (with significantly high concentrations than tetracycline) that may occur during the detection of tetracycline were studied. As shown in Figure 6, typical organic interferences such as phenol, hydroquinone, and catechol have a little effect on the detection of tetracycline. When inorganic ions such as Ce 3+ , NO 3-, K + , Cl -, Mn 2+ , Na + , and Fwas added, there is no significant change in the amperometric response of tetracycline. When other antibiotics (such as amoxicillin, ciprofloxacin, and norfloxacin) are present in the solution of tetracycline, there is almost no effect on the detection of tetracycline. The above results indicate that MIL-53 (Fe)/GCE has a high selectivity to detect tetracycline.
3.6. Reproducibility and Stability of Modified Electrodes. For the purpose of characterizing the repeatability of the sensor for tetracycline detection, seven different glassy carbon electrodes were applied to prepare seven different MIL-53 (Fe)/GCEs, and their tetracycline detection performance were measured. After 7 parallel measurements, the relative standard deviation (RSD) was obtained as 7.3% (Figure 7(a)), showing that the sensor has a high repeatability to detect tetracycline. For the purpose of investigating the stability of the sensor, 100 cycles of voltammetric scanning were performed. As indicated in Figure 7(b), after 100 CV cycles, there is no significant change in the reduction peak current, showing that the sensor has a high stability. The periodic stability of the sensor was evaluated, too. 3 weeks later after MIL-53 (Fe)/GCE was placed in air at RT, the sensor was employed to measure tetracycline via DPV again (detection conditions are the same as above). The results showed that an initial I p value of more than 96% was retained (Figure 7(c)), showing the high period stability of the sensor.
3.7. Application in Actual Water. Natural water samples including tap water and river water were used to evaluate the practical application of MIL-53(Fe)/GCE. Tetracycline (with three concentrations of 0.321 μM, 0.641 μM, and 1.278 μM) were spiked into the above actual water samples due to no tetracycline being detected in the original samples. The recovery results are shown in Table 2 2+ , Na + , and F -). It was pair system that one interference coexistent with tetracycline in one test, and the DPV responses of the sensor to tetracycline were recorded. Other conditions were the same as described elsewhere. The DPV response of the sensor to bare tetracycline was set as the value of 100%. 8 Adsorption Science & Technology see that the sensor has relative high recovery rates in actual water.

Conclusion
An electrochemical polymer sensor of tetracycline based on MIL-53 (Fe) was successfully developed in this work. It shows high tetracycline sensing performance in concentration range of 0.0643 μmol/L-0.769 μmol/L and 0.769-1.53 μmol/L, and the LOD can reach 0.0260 μmol/L. The sensor exhibits high reproducibility, anti-interference ability, and stability. Moreover, it can be efficiently used in actual water bodies. The polymer sensor can be facilely prepared and applied with high sensitivity. It has a high potential in practical application of trace tetracycline detection.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare no conflicts of interest.