In the present article, the synthesis of zeolite imidazole framework-11 (ZIF-11) by ultrasonic-assisted hydrothermal process and its application as an electrode modifier for electrochemical determination of uric acid in urine are demonstrated. The obtained materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nitrogen adsorption/desorption isotherms. It was found that the ZIF-11 with rhombic dodecahedron topology and high surface area (1066 m2.g-1) was synthesized in a certain temperature and found in around 25–40°C, and other crystalline phases of zinc benzimidazolate deferring from ZIF-11 phase were found in less 25°C or higher than 40°C. The ZIF-11 is stable in the pH range 6-10. The modification of glassy carbon electrode was performed with ZIF-67 using the drop-casting procedure. The present ZIF-11 modified electrode was employed to study the electrochemical behavior of uric acid (UA). UA oxidation is catalyzed by this electrode in aqueous buffer solution (pH 7) with a decrease of 70 mV in overpotential compared to glassy carbon electrode. With the differential pulse–anodic stripping voltammetry (DP-ASV) method, the oxidation current of UA versus its concentration shows good linearity in the range 20–540μM () with a detection limit of 0.48 μM (). The obtained ZIF-11 modified electrode was applied in the detection of UA content in urine samples, and satisfied results were obtained.

1. Introduction

Uric acid (UA, C5H4N4O3) is a natural waste product from the digestion of foods that contain purines. This acid is the main final product of purine metabolism. Abnormal UA levels are usually related to various disorders such as gout/arthritis, hyperuricemia, kidney disease, and Lesch–Nyhan syndrome. Hence, the detection of the UA with a facile method is clinically important because it serves as a marker for the determination of the mentioned diseases. For this purpose, various techniques have been used, such as enzymatic system [1], calorimetry [2], fluorescence [3], high-performance liquid chromatography [4], gas chromatography-mass spectrometry [5], and voltammetry [6]. However, these techniques require expensive equipment, sample pretreatment, skill training, harmful solvents, and considerable time, which limit their application. Meanwhile, electrochemical analysis has several advantages over conventional analytical methods in terms of sensitivity, accuracy, easy operation, and low cost. However, a weak point of electrochemical methods is poor selectivity due to the interferences of other electroactive species and high overpotentials for oxidation [7]. To overcome this obstacle, much attention has been paid for electrode surface modification with porous materials that have hierarchical structures, such as ordered mesoporous carbon functionalized with ferrocene carboxylic acid [8], uricase/carbon nanotube/carboxymethyl cellulose [9], poly (3,4-ethylene dioxythiophene) doped with graphene oxide [10], mesoporous Ag/TiO2-SnO2 on g-C3N4 nanosheets and decorated with a hierarchical core−shell MOF [11], and others.

Zeolite imidazole frameworks are a subclass of metal-organic frameworks (MOFs) with tunable pore size and chemical functionality, associated with extraordinarily thermal and chemical resistance [12]. Zeolite imidazole framework-11 (ZIF-11) comprises 48 Zn (II) ions within a unit cell, and each Zn (II) ion is tetrahedral-coordinated to the four nitrogen atoms of 4-benzimidazole (bIm). ZIF-11 with rhombic dodecahedron (RHO) topology exhibits larger cages connected through pore apertures and possesses unusual thermal and chemical stability in water and organic solvents [13]. Thanks to these exceptional properties, ZIF-11 is one of the most promising ZIFs [13]. It has several potential applications, such as membrane [14], gas separation [15], catalysis [16], and others. However, the application of ZIF-11 in electrochemistry is limited. To further enhance the electrochemical application, the surface area, pore properties, morphology, and particle size should be improved and controlled by the improvement of the synthesis method. According to our best knowledge, few papers have reported using ZIF-11 as an electrode modifier for electrochemical determination of uric acid in urine.

In the present work, the ZIF-11 was synthesized by ultrasonic-assisted hydrothermal process. The temperature influence on the ZIF-11 phase formation and its stability in aqueous solutions with different pHs are studied. Besides, the electrochemical determination of uric acid on a glassy carbon electrode modified with ZIF-11 is demonstrated.

2. Experimental

2.1. Materials

Zinc acetate dihydrate (Zn (CH3COO)2·2H2O, >98%), benzimidazole (C7H6N2, 98%), uric acid (>99%), boric acid (H3BO3, >99%), acetic acid, (CH3COOH, 99.7%), and phosphoric acid (H3PO4, 85% in water) are obtained from Merck. Britton–Robinson buffer (BRB) was prepared by mixing 0.5 M boric acid, 0.5 M acetic acid (99%), and 0.5 M phosphoric acid (85%).

2.2. Synthesis of ZIF-11

The synthesis of ZIF-11 was conducted according to reference [17]. Briefly, 3.3 g of zinc acetate was added to a homogeneous solution composed of 3.6 g benzimidazole, 144 g methanol, 138 g toluene, and 1.7 g ammonium hydroxide under ultrasonic stirring at a certain temperature (5, 15, 25, 40, 60, 80, and 100°C) for 3 hours. The resultant solids were rinsed with methanol several times and dried overnight at 80°C to obtain zinc benzimidazolate.

2.3. Instrument

The crystalline information and morphology of ZIF-11 were obtained from X-ray diffraction (XRD, JED-2300 JEOL) and scanning electron microscopy (SEM, JEOL-JSM 5410 LV) data. Its textural properties were investigated via nitrogen adsorption–desorption isotherms data (Micromeritics Tristar 3000 at 77 K).

A conventional three-electrode system (CPA-HH5 computerized polarography analyzer (Vietnam)) consists of a bare glassy carbon electrode (GCE, ) or a ZIF-11-modified GCE as a working electrode, an Ag/AgCl electrode as a reference electrode, and a platinum wire electrode as an auxiliary electrode. High-performance liquid chromatography (HPLC) for uric acid analysis was conducted on a Shimadzu 2030 HPLC system with a UV–Vis detector .

2.4. Modification of ZIF-11 and Preparation of Real Samples

The glassy carbon electrode was polished sequentially with 0.05 μm α-Al2O3 until a mirror-smooth surface appeared, washed with distilled water, and sonicated in ethanol for 10 min. The electrode was modified with ZIF-11 by using the drop-casting method. Five microlitres of a mixture containing 1 mg ZIF-11 in 1 mL methanol were dropped from a microsyringe onto the electrode and dried naturally prior to use. Three human urine samples were obtained from healthy volunteers. Before analysis, the samples were filtered through a 0.2 μm filter membrane and diluted five times with 0.1 M BRB solution (pH 7) to reduce the matrix interferents. Take 10 mL of the diluted sample for UA analysis using the DP-AVS and HPLC method. For recovery calculation, the filtered urine was spiked with the stock solution of UA, followed by diluted five times with 0.1 BRB solution pH 7, and both DP-AVS and HPLC measurements were performed.

2.5. Electrochemical Measurements

The cyclic voltammetry (CV) and DP-DPV measurements were performed in a BRB solution at ambient temperature. In a general procedure, 1 mL of BRB containing a UA standard or sample solution was put into a clean electrochemical cell. Before determination, the preconcentration was performed under an open circuit and agitation for 60 s. The CV measurements were performed from −0.2 V to 0.8 V with a scan rate of 0.125 V·s–1. The DPV measurements were carried out with a scanning voltage from 0.2 to 0.8 V with a scan rate of 0.125 mV·s–1, pulse amplitude, 70 mV, and quiet time, 5 s. After each measurement, the modified electrodes were washed with distilled water, and they were then put into a blank BRB solution for CV scanning five cycles with the potentials from 0 to 0.8 V to give a fresh electrode surface.

3. Result and Discussion

3.1. Synthesis of ZIF-11

Figure 1 presents the XRD patterns of zinc and the benzimidazolate synthesized at 5, 15, 25, 40, 60, and 100°C. It can be seen that reaction temperature substantially affects the crystalline phase of zinc benzimidazolate. The XRD characteristic peaks of ZIF-11 appear in the range of 25–40°C and belong to space group Fm-3 m, according to the CCDC No. 602545. Outside this range, the crystalline phase of zinc benzimidazolate is different from that of ZIF-11. Hence, the ZIF-11 phase is formed in the temperature range of about 25–40°C. Since MOFs are considered as “soft porous structure” [18], the phase transition at different synthetic conditions has often been observed [19]. The determination of a specific phase of zinc benzimidazolate outside this temperature is beyond the scope of this work.

The SEM images of ZIFs synthesized at different temperatures are shown in Figure 2. At low temperatures (5–15°C), cuboids of around 4–6 μm are in length form. At a higher temperature (25°C), ZIF-11 exhibits the RHO topology with regular rhombic dodecahedron particles with 2–4 μm. The particle size of rhombic dodecahedrons increases to 3–7 μm at 40°C, and the edges of the rhombic dodecahedrons become less sharp. The particles develop into rectangular cylindrical blocks composed of several layers at 60°C. At 80°C and higher temperatures, ZIF-11 exhibits irregular shapes with stacking layers and cylindrical blocks.

The surface area of zinc benzimidazolate is studied from its nitrogen adsorption/desorption isotherms (Table 1 and Figure 3). The material synthesized at 25 and 40°C exhibits the isothermal curves belonging to type I according to IUPAC classification and has a BET-specific surface area of 1066 and 347 m2·g–1, respectively. The material synthesized at high temperature (60 and 100°C) or low temperature (5 and 15°C) possesses a very low BET-specific surface area (2.2-8 m2·g–1). Since the gaps between the ZIF-11 particles synthesized at these temperatures are smaller than the kinetic diameter of nitrogen (3.0 against 3.6 Å) [13, 20], the specific surface area of ZIF-11 calculated from nitrogen adsorption measurements is smaller than that of other members of ZIF family, such as ZIF-8 or ZIF-12 [21]. The ultrasonic-assisted hydrothermal process enhanced significantly the surface area of ZIF-11. It could explain by the fact that the acoustic cavitation force inducing a series of compressions and rarefactions in the molecules of the present solvent made highly dispersive and uniform ZIF-11 particles. The specific surface area of the obtained ZIF-11 (1066 m2·g–1) is much higher than those synthesized with other procedures, for example, 745 m2·g–1 (microwave heating) [17], 97 m2·g–1 (ethanol solvent, room temperature) [21], and 350.29 m2·g–1 (methanol solvent, room temperature) [15]. In addition, it is worth noting that the desorption line of ZIF-11 and composites does not form a loop at a low relative pressure (). Such shapes of isotherm curves have been observed for ZIF-11 in several previous publications [16]. This phenomenon may be due to the swelling effect, commonly observed in complex microporous materials with throats and cavities [22, 23].

The stability of ZIF-11 in the aqueous solution with various pHs is critical for the material’s application. New peaks on the XRD patterns of ZIF-11 in the pH range of 1–3 indicate a change in the material structure, while the XRD patterns remain unchanged at higher pHs (Figure 4(b)). This fact implies that ZIF-11 is stable in the pH range of 4–10.

3.2. Voltammetric Determination of Uric Acid on ZIF-11-Modified GCE
3.2.1. Cyclic Voltammetric Behavior of ZIF-11/GCE

A sharp peak on the CV curve obtained on ZIF-11/GCE is observed at a lower (0.28 V) compared with an obtuse peak on bare GCE () (Figure 5). In the buffer solution without UA, no redox peaks are observed on the ZIF-11/GCE, indicating that ZIF-11 is electrochemically inactive towards UA oxidation. The oxidation of UA on ZIF-11/GCE exhibits a higher current and a lower potential. The current value of the oxidation peak of UA on ZIF-11/GCE is approximately 2.2 times higher than that on GCE. This fact indicates that ZIF-11, as an electrode modifier, increases the number of adsorption sites and the active surface and thus enhancing the catalytic activity.

The electrochemically active surface of bare GCE and ZIF-11/GCE can be obtained from the Randles-Sevcik equation (1) [24]. where (μA) is the anodic peak current; is the number of electrons transferred in the oxidation and reduction process of the redox probe ; is the concentration of ; D is the diffusion coefficient

The effective surface area could be obtained from the slope of vs. linear plot. The linear equation is presented in equations (2) and (3) (Figure 6).

The calculated effective surface area of ZIF-67/GCE (0.082 cm2) is higher than that of bare GCE (0.068 cm2), which is about 1.2 times.

(1) Effect of pH. The UA current peak shifts to a less positive potential as pH increases from 2 to 9 (Figure 7(a)), indicating that the reactions at the electrode are related to protons. The linear plot of the peak potential vs. pH is shown in Figure 7(b) (equation (4)).

The slope of this line (0.056) is close to the theoretical slope (0.059 μA/pH) of the Nernst equation. This fact indicates that the number of electrons and protons involved in the reaction is equal. Figure 7(c) depicts that pH less than 7 seems to have little influence on the anodic peak current, but at higher pH, the anodic peak current decreases. As we know that uric acid is a diprotic acid with pKa1 being 5.4 and pKa2 being 10.3 [25], at , it exists as an anion (urate), and at , it exists in the neutral form. The point of zero charge (Pzc) for ZIF-11 estimated with the pH drift method is 5.8 (Figure 7(d)). When the pH of the buffer solution is less than 5.8, the proton concentration has little influence on the anodic peak current. However, at pH greater than 5.8, the repulsion interaction between the urate and the negatively charged surface of ZIF-11 results in a decrease in the anodic peak current. The highest peak current is obtained at pH 7 (Figure 7(d)), and therefore, this pH is selected for further experiments.

(2) Effect of Scan Rate. The kinetics of electrode reaction is investigated by using the peak current values of UA oxidation with scan rates from 0.1 to 0.5 V·s–1 (Figure 8(a)). The anodic peak current, , varies linearly with according to equation (5)

The intercept varies from –10 to –4, and it means that the line in Figure 8(b) does not pass the origin, indicating that the reactions on ZIF-11/GCE are controlled by diffusion [26, 27].

For an irreversible system, the relationship between and lnv is described by the Laviron equation [28]. where ; .

The linear plot of vs. ln (v) provides the value of . The influence of ln (v) on is expressed in equation (7) (Figure 8(c))

For an irreversible system, the value of is accepted as 0.5 [29], and when the value of is 0.89, is found to be 1.77. Since an equal number of protons and electrons is involved in the reactions on the electrode, in this case, equal to 2 is acceptable. These results manifest that two electrons and two protons take part in the redox process of UA. A proposed oxidation mechanism of UA at ZIF-11/GCE is provided in Scheme 1.

3.2.2. Operational Parameters

Figure 9(a) indicates that the peak current of DP-ASV curves increases with the accumulation time up to 15 seconds, reaching a maximum, and then remains practically unchanged. This fact indicates that the accumulation of UA at the electrode surface reaches saturation. Accumulation potential has little influence on the peak current, probably because UA is almost neutral at pH 7. Hence, accumulation on an open circuit is adopted. The accumulation potential with a low deviation (–0.2 V) (Figure 9(b)) and accumulation time (15 seconds) is selected for further experiments.

3.2.3. Repeatability, Reproducibility, Selectivity, and Limit of Detection

In the repeatability tests, DP-ASV curves are measured with three solutions containing 10, 50, and 100μM UA in four successive cycles (Figure 10). The relative standard deviation is acceptable as it is less than ( is the relative standard deviation calculated for the Horwitz equation, , where is the fraction concentration). The relative standard deviations (RSDs) of the current response are calculated to be 2.31% for 10 μM UA, 2.24% for 50 μM UA, and 1.00% for 100μM UA, which are less than (7.40, 5.81, and 5.23, respectively). These results validate that the fabricated ZIF-11/GCE has acceptable repeatability.

The RSD (4.21%) of 10 replicates is less than (7.40%), indicating that the proposed method possesses good reproducibility.

The selectivity of the ZIF-11/GCE is estimated by spiking possible interfering electroactive species coexisting in the real samples. In the present work, the tolerance limit is considered as the concentration of the interferent that induces a relative error of 5% in the determination of 0.5 μM UA. When the relative error of the detection peak current raised by an interferent is lower than 5%, it is considered that this substance has no interference [30, 31]. According to this criterion, no interference is observed for the binary mixture composed of 0.5 μM UA and glucose, ascorbic acid, sodium benzoate, saccharine (less 15 μM or 30-fold concentration), xanthine, hypoxanthine (less 10 μM or 20-fold concentration), KCl, Na2CO3, Na2SO4, and CaCO3 (25 0.5 μM or less 50-fold concentration) (Table 2). These results show that the present modified electrode has good anti-interference ability, which allows its further application in real biological samples.

Under optimal conditions, the DP-ASV technique is used to determine UA (Figure 11(a)). The increases linearly with the UA concentration in the range of 20–540μM (equation (8)).

The sensitivity and LOD obtained as are 0.174 μμM–1·cm–2 and 0.48 μM, respectively (Figure 11(b)).

Table 3 compares the analytical performance of the proposed ZIF-11/GCE and some previously reported electrodes. The present electrode is comparable to or even better than the other electrodes. The obvious advantage is that the ZIF-11 material is synthesized simpler than the composites with multiple components reported. ZIF-11/GCE exhibited better than some of the electrodes modified using pyrolytic graphite, uricase/carbon nanotube/carboxymethylcellulose, or ordered mesoporous carbon (OMC) functionalized with ferrocene carboxylic acid but failed to methylcellulose/graphene oxide/iron oxide nanohydrogel, zinc tetraaminophthalocyanine functionalized graphene nanosheets.

3.2.4. Real Sample Analysis

Three urine samples are analyzed by using the fabricated electrode, and the HPLC method is also used for the sake of comparison. The acceptable recovery of UA is 94.52 to 104.48%. Besides, the paired samples -test was performed to compare the analytical results between the proposed method and HPLC. There was no significant difference in uric acid content for the results of DP-ASV analysis (; ) (: mean; SD: Standard deviation) and for HPLC analysis (; ), , . These results indicate that ZIF-11/GCE can be effectively employed for the UA determination in real biological samples with excellent concurrent results (Table 4).

4. Conclusions

The reaction temperature effects remarkably on the formation of the ZIF-11 phase. ZIF-11 materials could be obtained in a temperature range of about 25–40°C. The zinc benzimimidazole synthesized out of this temperature range possesses the crystalline structure differing from the ZIF-11 phase and low specific surface area. A novel ZIF-11 modified GCE has been developed to determine UA. The UA oxidation is remarkably catalyzed with an increased peak current and a decreased overpotential. The determination was successively achieved at ZIF-11/GCE. The proposed method with good stability and reproducibility has been employed to the determination of UA in a real urine sample with acceptable results.

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.