Electrochemical Determination of Diclofenac by Using ZIF-67/g-C3N4 Modified Electrode

A facial differential pulse voltammetric procedure using a glassy carbon electrode modified with zeolite imidazolate framework-67/graphitic carbon nitride (ZIF-67/g-C3N4) for the diclofenac (DCF) determination is demonstrated. ZIF-67/g-C3N4 with different mass ratios of the components was synthesized in a self-assembly process. The obtained materials were characterized by using X-ray diffraction, scanning electron microscopy (SEM), EDX-mapping, and nitrogen adsorption/desorption isotherms. The peak current varies linearly with the DCF concentration in the range of 0.2–2.2 μmol·L−1 and has a detection limit of 0.071 μmol·L−1. The modified electrode exhibits acceptable repeatability, reproducibility, and selectivity towards DCF. The proposed electrode allows determining DCF in human urine without pretreatment, and the results are comparable with those determined with HPLC.


Introduction
Diclofenac, 2-(2 ′ ,6 ′ -dicholoroanilino) phenylacetic acid (denoted as DCF), is used for the treatment of several diseases, such as ankylosing spondylitis, rheumatoid arthritis, and osteoarthritis [1]. However, diclofenac causes acute hepatotoxicity, and this chemical-driven liver damage leads to the change in the kidney function and gills in rainbow trout (O. mykiss). Diclofenac also presents acute toxicity to phytoplankton and zooplankton. Moreover, the possible synergetic effects with other pharmaceuticals or chemicals in the aquatic environment increase the environmental risk [2]. Various techniques have been developed to determine diclofenac because of the prominence of DCF in the environment and pharmaceutical and clinical applications. The techniques include capillary zone electrophoresis with electrochemical detection [3], high-performance liquid chromatography (HPLC) [4], HPLC combined with solid-phase extraction [5], and spectrofluorimetry [6]. Besides these techniques, electrochemical methods possess several advantages, such as high sensitivity, selectivity, and quick, cheap analysis for actual samples. These techniques use various types of porous materials to modify the working, such as ionic liquid/carbon nanotube-paste electrode [7], carboxyl/multiwalled carbon nanotubes/screen-printed carbon electrode [8], and gold/multiwalled carbon nanotube/glassy carbon electrode [9].
Zeolite imidazolate framework-67 (ZIF-67) is a subclass of metal-organic frameworks (MOFs). It is an organicinorganic hybrid solid with infinite and uniform crystalline coordination networks consisting of cobalt ions (II) and imidazolate ligands [10]. ZIF-67 exhibits various unique properties, such as thermal and chemical stability, high surface area, large pores, accessible coordinative unsaturated sites, and excellent chemical and solvent stability [10]. At present, ZIF-67 attracts increasing attention from researchers in various fields, such as adsorption and separation [11], catalysis [12], and gas separation [13]. However, its poor electroconductivity leads to limited applications in electrochemistry [14]. Graphitic carbon nitride (g-C 3 N 4 ) with graphitic-like structure possesses a unique ability to be simply prepared by thermally condensation of cheap nitrogen-rich compounds such as urea, melamine, and cyanimide [15,16], and this is unlike graphene, reduced graphene oxide, and graphitic oxide that require the more complex synthesis processes. It has gradually attracted attention in multidisciplinary areas due to its characteristic physicochemical properties, such as ability to resist attacks from strong acid/alkaline solution [17], moderate bandgap (~2.7 eV), and superior electronic properties [18]. Therefore, the combination of highly electroconductive g-C 3 N 4 with large surface area ZIF-67 might lead to versatile materials with properties of both components.
To the best of our knowledge, only a few papers have been reported on the voltammetric or amperometric detection of DCF on a ZIF-67/g-C 3 N 4 modified electrode. Therefore, to fabricate new electrodes, we expand our research on modifying glassy carbon electrodes (GCE). In the present work, we describe the determination of DCF by using a GCE electrode modified with ZIF-67/g-C 3 N 4 . This modified electrode exhibits sound electrocatalytic and accumulative effects on DCF and enables us to determine the DCF content in human urine with satisfactory results.  [10]. Briefly, 1.16 g of Co(NO 3 ) 2 ·6H 2 O and 1.31 g of 2-methylimidazole (Hmim) were dissolved in 100 mL of methanol separately. These two solutions were mixed so that the resulting mixture has the following molar composition: Co 2+ /Hmim/CH 3 OH = 1 : 4 : 1:2 and stirred for 24 h at ambient temperature. Then, the purple solid (ZIF-67) was collected by centrifuging, washed with ethanol five times, and dried at 80°C for 24 h.

Experimental
The g-C 3 N 4 was synthesized according to Yan et al. [19]. In brief, 10 g of melamine powder was put into an alumina crucible with a cover and heated at 500°C in a muffle furnace for 2 h.

2.
3. Instruments. The material was characterized by using Xray diffraction (XRD) with a D8-Advance device (Bruker, USA; Cu Kα radiation, λ = 1:5406 Å), scanning electron microscopy (SEM) with JMS-5300LV (USA), Raman spectroscopy with Xplora Plus (Horiba with 785 nm laser excitation), transmission electron microscopy (TEM) with JEM-2100, and nitrogen adsorption/desorption isotherms with a Micromeritics 2020 volumetric adsorption analyzer system (the sample was degassed at 150°C for 12 h). The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) model. The meso/microsurface area was calculated with the t-plot method. Electrochemical measurements were conducted with a CPA-HH5 computerized polarography analyzer (Vietnam). A threeelectrode system consisting of a working electrode (GCE, 0.28 cm 2 ), a reference electrode (Ag/AgCl in saturated KCl), and a counter electrode (platinum wire) was used for the measurements. The high-performance liquid chromatography (HPLC) was performed to measure DLF for the sake of comparison. The HPLC measurements were conducted on an HPLC Shimadzu with LC 20 AD pump, PDA SPD-M20A detector (Japan), C18 column (250 × 4:6 mm; particle size 5 μm). The mobile phase is a mixture of methanol/5 mM NaH 2 PO 4 (pH 2.3) (66 : 34 v/v). The flow rate was 1 mL·min -1 with an injection volume of 20 μL. For HPLC analysis, DCF detection was performed at a wavelength of 274 nm.

Preparation of Electrode and Actual Sample
2.4.1. Electrode Preparation. Prior to modification, the GCE was first polished with alumina slurry (0.05 μm) on a polishing pad, followed by successive ultrasonication in ethanol and distilled water. Then, the electrode was washed with double distilled water and dried at ambient temperature. The suspension of ZIF-67/g-C 3 N 4 was prepared by mixing 10 mg of ZIF-67/g-C 3 N 4 with 10 mL of ethanol under ultrasonication for 24 h. Five microliters of suspension was cast dropwise on the GCE surface and dried in an oven for a 2 Adsorption Science & Technology few minutes. The as-prepared electrode was denoted as ZIF-67/g-C 3 N 4 -GCE.

Real Samples.
Human urine samples were received from five healthy persons aged 25-30 and stored in a refrigerator at 5°C. About 5 mL of the sample was filtered through a 0.2 μm membrane. One milliliter of the supernatant was collected and diluted with 2 mL of pH 6.5 BRS to avoid complex interference. The resulting solution was transferred to a 100 mL electrochemical cell for analysis.

Results and Discussion
3.1. Characterization of Materials. The XRD patterns of g-C 3 N 4 , ZIF-67, and ZIF-67/g-C 3 N 4 are shown in Figure 1. Two peaks at around 13.7 and 27.1°are found in g-C 3 N 4 ((0/10)ZIF-67/g-C 3 N 4 )). It is well known that g-C 3 N 4 consists of tri-s-triazine building blocks [20]. The higher peak at 27.1°is assigned to the characteristic interlayer of aromatic systems, indexed for graphitic materials as (002), while the peak at 13.7°, indexed as (100), is attributed to the periodic arrangement of the condensed tri-s-triazine units in the sheets [21] (the inset of Figure 1). For the XRD pattern of (10/0)ZIF-67/g-C 3 (440) according to CCDC 671073, are clearly observed indicating that the (10/0)ZIF-67/g-C 3 N 4 is ZIF-67. The intensity of these peaks in ZIF-67/g-C 3 N 4 increases with an increase in the ZIF-67/g-C 3 N 4 fraction. This increase manifests the coexistence of g-C 3 N 4 and ZIF-67 phases in the composite. Clear characteristic peaks of ZIF-67 witness that the hybrid of ZIF-67 with g-C 3 N 4 has no significant effects on the crystal structure of ZIF-67. Nitrogen adsorption/desorption isotherms are employed to evaluate the samples' specific surface area and porosity ( Figure 2). The isotherm curves of ZIF-67 have a type I isotherm, indicating the presence of a microporous material. The g-C 3 N 4 has a type IV isotherm with an H3 hysteresis loop at high relative pressure between 0.46 and 1.0, confirming the existence of a mesoporous structure. The BET surface area (S BET ) and total pore volume (V pore ) are listed in Table 1.
Compared with pure ZIF-67, the ZIF-67/g-C 3 N 4 has the S BET declining from 1217.1 to 335.6 m 2 ·g -1 as the amount of g-C 3 N 4 increases. This decline might result from the low surface area of g-C 3 N 4 and blockage of micropores of the increasing g-C 3 N 4 amount in the composite. The primary test reveals that the (5/5)ZIF-67/g-C 3 N 4 sample has the highest electrocatalytic activity for DCF oxidation. Therefore, (5/5)ZIF-67/g-C 3 N 4 is selected for further experiments.
As shown in Figure 3(a), smooth rhombic dodecahedral-shaped crystals with 1.5 μm in width are observed for ZIF-67. The surface of g-C 3 N 4 exhibits a layered and platelet-like morphology (Figure 3(b)). Although several inclusions of g-C 3 N 4 appear on the ZIF-67 surface, the rhombic dodecahedral structure of ZIF-67 remains (Figure 3(c)). The TEM image illustrates a sheet-like structure of g-C 3 N 4 embroiled with dark-colored ZIF-67 crystals   The ZIF-67/g-C 3 N 4 structure is characterized by using Raman spectra (Figure 4). Typical characteristic peaks of g-C 3 N 4 at 470, 543, 702, 746, 978, and 1147 cm -1 are observed, and they are consistent with those in a previously published report [22]. The peaks at 702 and 978 cm -1 indicate the existence of the heptazine ring structure [23]. The peak at 702 cm -1 is ascribed to the inplane bending vibrations of the heptazine linkages, whereas the 978 cm -1 peak is assigned to the symmetric N-breathing mode of the heptazine units [24]. For ZIF-67, characteristic peaks at 686, 831, 951, 1044, 1147, 1178, 1307, 1385, 1459, and 1506 cm -1 are attributed to the cobalt ion [25], imidazolium ring puckering, δ H (out of plane), bending C-H (out of plane) (C4-C5), bending C-H (out of plane) (C2-H), stretching C-N, N-H wag, bending of CH 3 , stretching of C2-N1, and stretching of C-H (methyl), respectively [26]. The characteristic vibrations of ZIF-67 and g-C 3 N 4 in the Raman spectrum of ZIF-67/g-C 3 N 4 confirm the coexistence of ZIF-67 and g-C 3 N 4 again.
Although Figure 6 shows that ZIF-67 could attach to the surface of g-C 3 N 4 , such self-assembly can be further confirmed by studying the surface charges of ZIF-67, g-C 3 N 4 , and their composite ( Figure 6). These values are determined with the drift pH method [27]. As seen from the figure, the material's surface is positively charged when pH < 9:2 for ZIF-67 and pH < 3:8 for g-C 3 N 4 . Therefore, the electrostatic attraction between ZIF-67 and g-C 3 N 4 could form ZIF-67/g-C 3 N 4 with a positively charged surface at pH below 8.4.
The stability of (5/5)ZIF-67/g-C 3 N 4 in the solutions with different pHs is essential for the electrochemical application. Figure 7 presents the XRD patterns of (5/5)ZIF-67/g-C 3 N 4 immersed in water with different pHs for 5 hours. At pH less than 4, the characteristic diffractions of ZIF-67 are absent, revealing that the material is destroyed at these pHs. The peaks of ZIF-67 remain in the solutions with pH 5-10, indicating that ZIF-67 is stable in this pH range.

Electrochemical Determination of Diclofenac by
Using (5/5)ZIF-67/g-C 3 N 4 3.2.1. The Cyclic Voltammetry of DCF on (5/5)ZIF-67/g-C 3 N 4 Modified Electrode (ZC-GCE). The cyclic voltammograms (CVs) (Figure 8) are obtained in the presence of 5 mM DCF on GCE, ZIF-67-GCE, g-C 3 N 4 -GCE, and ZIF-67/g-C 3 N 4 -GCE in 0.2 M BRS pH 6. The bare GCE does not exhibit any electrochemical signals, indicating that this electrode cannot detect DCF. The modified electrodes (with ZIF-67, g-C 3 N 4 , or ZIF-67/g-C 3 N 4 ) exhibit an oxidation peak of DCF at around 0.68 V. In particular, the ZIF-67/g-C 3 N 4 modified electrode significantly enhances the electrochemical response, and the intensity of peak current depends on the ZIF-67 and g-C 3 N 4 mass ratio. The (5/5)ZIF-67/g-C 3 N 4 -GCE (henceforth denoted as ZC-GCE) provides the highest intensity with a peak potential of 0.671 V. The peak current is 2.1-fold and 2.12-fold higher than that of g-C 3 N 4 /GCE and ZIF-67-GCE, respectively. The substantial enhancement in the anodic peak current clearly shows the catalytic effect of ZIF-67/g-C 3 N 4 . No reduction peaks are observed on the reverse scan, indicating that the electron transfer on the ZC modified electrode is irreversible. In addition, the ZC-GCE does not exhibit an oxidation/reduction peak on the voltammogram in the solution without DCF, indicating the ZC-GCE is inactive in the studied potential range.
(1) Effect of pH. The influence of pH on the oxidation peak current of the 5:0 × 10 -3 M DCF solution is studied by using CV in the BR buffer. At pH 3-5, DCF oxidation at the elec-trode is not observed. It is possibly due to the unstable structure of ZIF-67 in ZIF-67/g-C 3 N 4 in this pH range (Figure 7). The peak current increases with pH and has the highest value at pH 6.5. Higher pHs witness a decrease of the peak current up to pH 8, and then the current increases again (Figure 9(c)). Therefore, pH 6.5 is selected for further experiments. The point of zero charge of ZIF-67/g-C 3 N 4 is 8.4, and the pK a of DCF is around 4 [28].  7 Adsorption Science & Technology of peak currents of DCF oxidation could not be explained by the electrostatic interaction between charged species on the opposite sites. This process may follow a mechanism different from electrostatic interaction.

Adsorption Science & Technology
When pH is greater than 5, the peak potential decreases, suggesting the involvement of protons in the oxidation reaction (Figure 9(d)). The peak potential in the DCF oxidation on the ZC-GCE decreases linearly with the pH of the buffer solution according to the following equation: E pa ðVÞ = -0:0514 × pH + 0:9409 ðr 2 = 0:9988Þ. The slope of the equation is close to the Nernstian value (−0.0592 mV) for the electrochemical processes involving an equal number of protons and electrons.
(2) Effect of Scan Rate. The scan rate of CVs is chosen from 0.1 to 0.5 mV·s -1 . In this range, the anodic peak current increases (Figure 10(a)). The highly linear relationship between I p and the scan rate manifests that the electrode process is predominantly controlled by adsorption [29] (Figure 10(b)). The scan rate effect is also examined from the function I p = f ðv 1/2 Þ plot. A straight line passing the origin suggests an adsorptioncontrolled electrode process [30]. This relationship in our study is I p = ð0:003 ± 0:011Þ + ð0:028 ± 0:006Þ × v 1/2 ; r = 0:927. With the 95% confident interval, the intercept passes 0 (between -0.008 and 0.014) and confirms an adsorptioncontrolled process again.
The relationship between the peak potential and the natural logarithm of the scan rate can provide the number of electrons transferred (n) on the electrode surface. For an irreversible system, this relationship is described by Laviron's equation [31]: where α is the electron-transfer coefficient, K s is the apparent charge-transfer rate constant, n is the number of electrons transferred, v is the scan rate (V·s -1 ), R = 8:314 J · mol -1 · K -1 , and F = 96500 C · mol -1 at 298 K. The linear regression of E p versus ln(v) is as follows: The value of ð1 -αÞ × n calculated from the slope of this straight line is 0.95. For an irreversible system, α is considered as 0.5 [32]; therefore, the value of n is equal to 1.9 (≈2). Considering the pH effect on the anodic peak current, we can conclude that the redox reaction on the modified electrode involves the transfer of two electrons and two protons.
These results are consistent with those reported by Madsen et al. [33] and Goyal et al. [34], in which DCF is oxidized to 5-hydrodiclofenac via losing two electrons and two protons, as shown in Scheme 1.
The enhancement of electrochemical signals could result from the synergic effect of ZIF-67 and g-C 3 N 4 . The open Co(II) sites, as a Lewis acid [35], could attract the carboxyl group in DCF, and as a Lewis base via acid-base interaction, the graphitic rings in g-C 3 N 4 could attract the benzene rings in DCF via π-π interaction. These arguments are schematically illustrated in Scheme 2.  (1) Linear Range and Detection Limit. The relationship between the peak current and DCF concentration is studied by using differential pulse voltammetry (DPV). In this case, the peak current increases linearly with the concentration of DCF, from 0.2 to 2.2 μM (Figure 11). The regression equation is I p = ð0:941 ± 0:02Þ + ð3:889 ± 0:03Þ × C ; r = 0:999. The LOD (3S/n) is 0.071 μM. The detection limit, the linear range, and the sensitivity of the proposed DPV method are compared with previously reported values for the determination of DCF (Table 2). Although the proposed electrode has a relatively narrow linear range, it has high sensitivity and a lower detection limit. This method can be used to determine DCF in pharmaceuticals and water samples in the micromole range.
(2) Reproducibility and Repeatability. The reproducibility is studied from four replicates measurements of DCF determination. To investigate the repeatability of this modified electrode, we conduct the measurements ten times with the 11 Adsorption Science & Technology sample electrode. Figure 12(a) shows very similar DPV curves of four scans on the same working electrode. The small RSD of 1.32% indicates good reproducibility of the proposed method. Figure 12(b) presents the values of peak currents measured on 10 distinct working electrodes under the same modified electrode procedure. The RSD varies from 0.151% to 5.315% in run 1 to run 10, manifesting excellent repeatability.
The biological sample is a very complex mixture consisting of various ions and molecules. Possible interfering electroactive species on the biological samples include inorganic salts (KHCO 3 , Na 2 SO 4 , CaCl 2 , and NH 4 NO 3 ) and organic compounds (uric acid, caffeine, paracetamol, and ascorbic acid). Table 3 reveals that inorganic salts do not significantly interfere with the measurement even at extremely high content (100-320 fold). Ascorbic acid does not seem to affect the peak current at 480-fold concentra-tion. Paracetamol, uric acid, and caffeine exert a medium effect on the peak current at a concentration of 40 to 80fold higher than that of DCF. These results confirm the relevant selectivity of the proposed method for DCF detection.   Figure 12: The DPV of four replicates measurement (a) and the values of peak current of DCF on ZC-GCE modified using (5 μL ZC (1 mg/mL) for ten times on the same electrode (b).  Table 4). The content of DCF in the samples is also determined with the HPLC method for comparison. When α = 0:05, the paired samples t-test proves that there is no statistically significant difference between the two methods (p two−tailed = 0:388 ð>0:05Þ; tð4Þ = 0:968). These results demonstrate a good performance of the presented method for the determination of DCF in urine samples.

Conclusion
In this research, we successfully synthesized ZIF-67/g-C 3 N 4 with the self-assembly method and used the material to modify a glassy carbon electrode for the electrocatalytic determination of diclofenac. The g-C 3 N 4 is highly dispersed in ZIF-67 through an ultrasonic assisted stir to form a composite of g-C 3 N 4 and ZIF-67. The enhancement of electrochemical signals is due to the synergic effect of ZIF-67 and g-C 3 N 4 . The oxidation of diclofenac on the modified electrode takes place with a two-electron-proton mechanism. The electrode process is controlled by adsorption. The proposed DPV method exhibits high sensitivity with a low detection limit and can be used to determine diclofenac at trace levels.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.