Metal-Organic Framework MIL-53(Fe): Synthesis, Electrochemical Characterization, and Application in Development of a Novel and Sensitive Electrochemical Sensor for Detection of Cadmium Ions in Aqueous Solutions

A metal-organic frameworkMIL-53(Fe) was successfully synthesized by a simple hydrothermal method. A synthesizedMIL-53(Fe) sample was characterized, and results indicated that the formed MIL-53(Fe) was a single phase with small particle size of 0.8 μm and homogeneous particle size distribution was obtained. The synthesized MIL-53(Fe) has been used to modify a glassy carbon electrode (GCE) by a drop-casting technique. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements of the MIL-53(Fe)-modified GCE showed that the MIL-53(Fe) was successfully immobilized onto the GCE electrode surface and the electrochemical behavior of the GCE/MIL-53(Fe) electrode was stable. In addition, several electrochemical parameters of MIL-53(Fe)-modified GCE (GCE/MIL-53(Fe)) including the heterogeneous standard rate constant (k0) and the electrochemically effective surface area (A) were calculated. Obtained results demonstrated that the synthesized MIL-53(Fe) with the small particle size, highly homogeneous particle size, and high electrochemically effective surface area was able to significantly enhance the electrochemical response signal of the working electrode. Therefore, the GCE/MIL-53(Fe) electrode has been used as a highly sensitive electrochemical sensor for cadmium ion (Cd(II)) monitoring in aqueous solution using differential pulse voltammetry (DPV) technique. The response signal of the electrochemical sensor increased linearly in the Cd(II) ion concentration range from 150 nM to 450 nM with the limit of detection (LOD) of 16 nM.


Introduction
The pollution of water sources caused by heavy metal ions, inorganic substances, organic matters, and bacteria from different agricultural, industrial, and medical waste sources has seriously affected people's lives and the ecosystems and organisms in the aquatic environment [1][2][3]. Determining the concentration of pollutants is an important step in environmental monitoring and supervision and is a basis for offering solutions to protect the environment. Chromatography and its associated techniques such as atomic emission spectroscopy (AES), atomic fluorescence spectroscopy (AFS), atomic absorption spectroscopy, and inductively coupled plasma mass spectrometry are common and effective methods for identifying environmental contaminants [3,4]. However, the above techniques showed many disadvantages such as long testing times, expensive equipment, and complex measurement processes [3,5]. Therefore, development of new techniques, which possess the advantages of high sensitivity, quick detection, and ease of use, is needed for the onsite, real-time, and continuous monitoring of environmental pollutants [6][7][8][9][10][11].
In which, electrochemical sensors can offer advantages of low detection limits, wide linear response ranges, and good stability and reproducibility. Moreover, electrochemical sensors can be easily prepared from many advanced materials to improve sensitivity and selectivity. In general, an electrochemical sensor is configured from two main parts, which are named a sensing element and a transducer. When the sensing element interacts with an analyte, the transducer can convert the sensed information into an electrical signal [10,[12][13][14]. The important characteristics of the sensor include sensitivity, stability, selectivity, response time, cost, and reusability [9]. To improve the sensitivity of electrochemical sensors, advanced materials have been applied to modified electrodes, including nanostructured conducting polymers [12,15], metal and metal oxide nanoparticles [16][17][18], nanostructured carbon materials (such as porous carbon, carbon nanotube, and graphene/graphene oxide) [17][18][19], and metal-organic frameworks (MOFs) [6,9,[20][21][22][23][24]. Among them, MOF materials which are constructed by metal ions or clusters and organic ligands via coordination bonds have unique properties including their structural diversity, flexible framework functionality, large metal cluster density, high stability, abundant adsorption sites, and a large surface area exceeding that of activated carbons and zeolites which are traditional porous materials [9,21,[23][24][25][26]. Moreover, the physical and chemical characteristics of MOFs can be customized and precisely designed, depending on how they are synthesized [27]. The development of MOF material-based electrochemical sensors for determining pollutants in water has practical significance and high potential in the field of the environment [9,28]. Therefore, a number of MOFs have been used in electrochemical sensors to detect organic substances, inorganic ions, antibiotics, and heavy metals in aqueous solutions [9, 20-24, 26, 29, 30].
MIL-53(Fe) (MIL: Materials of Institute Lavoisier), a class of MOFs generated by a combination between iron(III) cations and 1,4-dicarboxylic acid, consists of threedimensional networks or secondary building units which contain FeO 6 hexagonal chains and dicarboxylate anions [31]. The emergent features of MIL-53(Fe) compared with other MOFs are chemically stable and have lower toxic metal centers and green, sustainable pathways for fabrication [31]. However, only few works have been dedicated to the development of an electrochemical sensor based on MIL-53 for detection of heavy metal ions in aqueous solutions (MIL-53(Al) [10]). Therefore, in this work, an effective hydrothermal method was carried out to synthesize a metal-organic framework MIL-53(Fe), which was then used to modify the glassy carbon electrode (GCE) for sensitive electrochemical determination of Cd(II) ion concentration in aqueous solutions. In addition, the specific characteristics of the synthesized MIL-53(Fe) were also carefully analyzed by various chemical-physical techniques. ) n OH, PEG 2000) was purchased from Alfa Aesar. Chitosan (CS, MW = 400,000 g mol −1 , degree of acetylation ðDAÞ = 70%) was prepared by deacetylation of chitin in our laboratory [32,33]. Acetate buffer solution 0.1 M (ABS) with pH 5.0 was prepared by using the appropriate amounts of sodium acetate and acetic acid.

Instrumentations.
A PGSTAT302N AutoLab electrochemical workstation (Netherlands) was used to perform electrochemical measurements. X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Advance diffractometer equipped with Cu-Kα radiation (λ = 1:54056 Å). Scanning electron microscope (SEM) images of MIL-53(Fe) were investigated using a Hitachi S4800 scanning electron microscope. The structure of MIL-53(Fe) was examined using Fourier transform infrared spectroscopy (FT-IR) spectra measured with a Nicolet 6700 FT-IR spectrometer. Transmission electron microscope (TEM) images were taken using a TECNAI F20-G2 high-resolution transmission electron microscope.
2.2. Synthesis of MIL-53(Fe). 0.623 g of FeCl 3 ·6H 2 O and 0.383 g of TPA were mixed in 50 mL of DMF solvent using an IKA magnetic stirrer with a stirring rate of 200 rpm for 10 minutes to get a solution. Then, the solution was added in an autoclave, and the hydrothermal synthesis was performed at 150°C for 12 hours. The product obtained after the hydrothermal process was filtrated, washed with 150 mL of MeOH and distilled water, and then dispersed in distilled water and stirred on the IKA magnetic stirrer with a stirring rate of 200 rpm for 15 hours. After that, the product was vacuum-dried at 170°C for 12 hours. Finally, the synthesized product was finely ground and stored in a desiccator.  [31,34]. These peaks shift slightly compared to the previous reports due to the different reaction conditions [35,36]. Comparing XRD patterns of sample B and sample C (samples were synthesized with the hydrothermal time of 12 and 15 hours, respectively), the hydrothermal time for the formation of MIL-53(Fe) was finally chosen to be 12 hours because of the sharper and higher characteristic peaks, which implied that the crystals were formed completely. The FT-IR spectrum of the MIL-53(Fe) (Figure 1(b)) exhibits the typical vibrational bands of the car-boxylic acid function in the region of 1400-1700 cm −1 . The adsorption band of the carboxyl group of the ligand coordinated to the Fe(III) ion is visible at 1585 cm −1 . The peak at 749 cm −1 corresponds to the C-H bonding vibration of the benzene rings [37,38]. Moreover, the presence of the Fe-O vibration at 545 cm −1 indicates the formation of a metaloxo bond between the carboxylic group of terephthalic acid and the Fe(III) [39]. The surface morphology of the MIL-53(Fe) was investigated by using SEM that is shown in Figure 2(a), which can be observed that the MIL-53(Fe)'s particle size is quite small and homogeneous with the distribution of the particle size around 0.8 μm (Figure 2(b)). The TEM images (Figures 2(c) and 2(d)) indicate that there are small pseudospherical particles in the range of 5-8 nm which were attached to the surface of the MIL-53(Fe) crystals [26]. The small particle size and the homogeneous particle size distribution of the obtained MIL-53(Fe) have an important role in improving the repeatability and reproducibility of electrochemical sensors. Figure 3 shows the CV (Figure 3(a)) and EIS spectra (Figure 3(b)) of the GCE/bare (curve A) and GCE/MIL-53(Fe) (curve B) electrodes performed in K 3 Fe(CN) 6 /K 4-Fe(CN) 6 (0.005 M) and 0.1 M KCl solution. It can be seen that there are two peaks on the two CV curves corresponding to the oxidation of Fe(CN) 6 4− and the reduction of Fe(CN) 6 3 − (Figure 3(a)). In the case of the GCE/MIL-53(Fe) electrode (curve B), the peak current is higher than that of the bare GCE electrode (curve A). This result can be attributed to the increase in the electroactive area when the MIL-53(Fe) was loaded on the GCE, leading to the accumulation of the electroactive species on the electrode surface and the increase in the current density. The EIS spectra (Figure 3(b)) of the two different electrodes consist of two parts: a semicircle which characterizes the charge transfer process and a linear region which characterizes the diffusion process. These EIS spectra can be simulated by an equivalent circuit based on the Randles model (Figure 3(b), inset). This equivalent circuit

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consists of a solution resistance (R s ), a charge transfer resistance (R ct ), a constant phase element (Q CPE ), and a Warburg diffusion coefficient (W). Fitted R ct results of the GCE/MIL-53(Fe) and bare GCE were 425 Ω and 828 Ω, respectively, which implied that the MIL-53(Fe) layer-modified GCE with the small particle size, the homogeneous particle size distri-bution, and the high electrochemically effective surface area was able to significantly enhance the electrochemical response of the working electrode. These EIS results are suitable for the CV results obtained in Figure 3(a).
CVs of the GCE/MIL-53(Fe) electrode in 0.1 M KCl solution containing 5 mM Fe(CN) 6 3-/4at different scan rates   Figure 4(a). From these obtained data, the relationships between the currents of anodic and cathodic peaks and the square root of the potential scan rate are plotted in Figure 4(b), i.e., with the relationship between I pa vs. ν 1/2 (marked by the red line with the triangle symbols) y = 0:0014 × ν 1/2 -1:2563 × 10 -4 R 2 = 0:9857 À Á ð1Þ and I pc vs. ν 1/2 (the blue line with the circle symbols) y = -0:0014 × ν 1/2 + 1:0267 × 10 -4 R 2 = 0:9903 In the cases of both anodic and cathodic peaks, the peak currents expressed linear dependence on the square root of the potential scan rate, indicating that the electrochemical processes occurred in a diffusion-controlled regime on the MIL-53(Fe)-modified GCE electrode [40]. Besides, the relationships between the potentials of anodic and cathodic peaks and ln ν were investigated, and their corresponding linear fit lines for each set of experimental data have been shown in Figure 4(c), i.e., E pa vs. ln ν (the green line with the square symbols) y = 0:0178 × ln ν + 0:3099 R 2 = 0:9940 and E pc vs. ln ν (the orange line with the star symbols) y = -0:0292 × ln ν + 0:0294 R 2 = 0:9969 The gradually increasing trend of the potential peak difference (ΔE = E pa -E pc ) with the potential scan rate ν  6 3-/4at different scan rates of 25,30,35,40,45,50,55,60,65,70, and 75 mV s -1 ; (b) the relationships between the currents of anodic and cathodic peaks and the square root of the scan rate (ν 1/2 ) and their corresponding linear fit lines: I pa , the red line with the triangle symbols, and I pc , the blue line with the circle symbols; (c) the relationships between the potentials of anodic and cathodic peaks and ln ν and their corresponding linear fit lines: E pa , the green line with the square symbols, and E pc , the orange line with the star symbols. 5 Advances in Polymer Technology demonstrates that the electrochemical processes which happened on the GCE/MIL-53(Fe) electrode surface are quasireversible [41]. Moreover, as can be seen in Figure 4(c), both the potentials of anodic and cathodic peaks depend linearly on the natural logarithm of the scan rate, and these relationships can be expressed by the following equations [41,42]: where ν (V s -1 ) is the scan rate, E 0 (V) is the formal standard potential, α is the charge transfer coefficient, n is the number of transported electrons, k 0 (cm s -1 ) is the electron transfer rate constant, D (cm 2 s -1 ) is the diffusion coefficient, R is gas constant (R = 8:314 J mol -1 K -1 ), T is working temperature (T = 298 K), and F is Faraday's constant (F = 96480 C mol -1 ). The values of α and n can be calculated from the slopes of E pa vs. ln ν and E pc vs. ln ν, and the results are that the value of α is 0.379 and the value of n is 1.161. The value of n verified that the oxidation of Fe(CN) 6 4and the reduction of Fe(CN) 6 3which occurred on the GCE/MIL-53(Fe) electrode surface are through monoelectronic steps. The diffusion coefficients (D O and D R ) of ferri-and ferrocyanide ions in 0.1 M KCl medium at 25°C are 7:20 × 10 -6 cm 2 s -1 and 6:66 × 10 -6 cm 2 s -1 , respectively [43]. From the intercepts of E pa vs. ln ν and E pc vs. ln ν and using E 0 = ðE pa + E pc Þ/2 = 0:19 V [41], the values of k 0 were calculated as 1:069 × 10 -3 cm s -1 and 1:489 × 10 -3 cm s -1 for the anodic and the cathodic branches, respectively, and the average value was 1:279 × 10 -3 cm s -1 . These results further confirmed that, in the considered scan rate range, the oxidation of Fe(CN) 6 4and the reduction of Fe(CN) 6 3which occurred on the GCE/MIL-53(Fe) electrode surface are quasireversible.
The electrochemically effective surface area (A) of modified electrodes is an important factor increasing electrochemical active sites, enhancing electrochemical response, and improving sensitivity of electrochemical sensors [44]. In this work, the electrochemically effective surface area of the GCE/MIL-53(Fe) electrode was determined by CV data analysis by using the Randles-Sevcik equation for quasireversible reactions as follows [45]: where ν (V s -1 ), α, n, and D (cm 2 s -1 ) were mentioned in Equations (5)- (8). A (cm 2 ) is the electrochemically effective surface area of the electrode, C 0 (mol cm -3 ) is the concentration of redox species, and I p (A) is the redox peak current. From Equation (9) and using the slope of the linear dependence of the anodic peak current on the square root of the potential scan rate (Figure 4(b)), the value of A of the GCE/-MIL-53(Fe) electrode was calculated to be 0.453 cm 2 . Thus, the electrochemically effective surface area of the modified GCE electrodes using the synthesized MIL-53(Fe) is significantly larger than that of the GCE electrodes (A GCE = 0:071 cm 2 ). The above EIS and CV results verified that the synthesized MIL-53(Fe) material was successfully immobilized on the GCE electrode surface, the electrochemical activity of the GCE/MIL-53(Fe) electrode was stable, and the MIL-53(Fe) with a high electrochemically effective surface area was able to significantly enhance the electrochemical response of the WE. These data also further confirmed that in this study, the drop-casting method used to immobilize the MIL-53(Fe) on the WE surface was simple, convenient, and effective.   Advances in Polymer Technology (20 μM). This result implies that the electrochemical sensor based on the MIL-53(Fe)-modified GCE can be used for determination of Cd(II) ions in aqueous solutions. These obtained results can be attributed to the MIL-53(Fe) material with abundant adsorption sites and a large surface area, which can adsorb Cd(II) ions from the aqueous solution, leading the Cd(II) ion enrichment process onto the GCE/MIL-53(Fe) surface. Therefore, the electrochemical measurements were performed to detect the presence of Cd(II) ions on the electrode surface at low concentration. In addition, the enhanced current at the GCE/MIL-53(Fe) electrode compared to the bare GCE also indicates that the obtained MIL-53(Fe) with the high electrochemically effective surface area exhibits electrocatalytic activity to the reduction of the target. These above results suggest that using MIL-53(Fe) material in development of the electrochemical sensor for detection of Cd(II) ions is totally suitable. Figure 6(a) shows the DPV response results of the GCE/-MIL-53(Fe) with different Cd(II) concentrations. As can be seen, the increase in the concentration of Cd(II) target caused an increase in the peak current (I peak ) of the responses. The change in the electrochemical signal was obtained even at low concentration of Cd(II) ions (150 nM). And this phenomenon becomes more obvious when the target concentra-tion continues to rise (up to 450 nM). The above results are explained that when Cd(II) ions in the aqueous solutions increase, the amount of Cd(II) ions accumulated on the surface of the MIL-53(Fe) layer increases, which leads to increasing in response current of the DPV results. The sensitivity of the electrochemical sensors was investigated by plotting I peak versus different Cd 2+ concentrations (Figure 6(b)). It shows that a linear relationship between I peak vs. [Cd 2+ ] in the range from 150 nM to 450 nM is I peak ðμAÞ = 0:2946 + 0:0017 × ½Cd 2+ (nM) with a correlation coefficient of R 2 = 0:9809. Based on a signal-to-noise ratio characteristic of 3 (S/N = 3) [32,46], the detection limit of Cd 2+ ions on GCE/MIL-53(Fe) was estimated to be 16 nM. The fabricated electrochemical sensor was compared to someone else's which has been reported in the previous studies (Table 1). Results in Table 1 indicate that the GCE/MIL-53(Fe) in this work has a great applied potential for Cd 2+ determination on the terms including simple fabrication, direct detection, rapid analysis time, and high sensitivity.

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

Conflicts of Interest
The authors declare that they have no conflicts of interest regarding the publication of this paper.