Metal-organic framework derived porous hollow ZnCo2S4 improving electrocatalytic oxidation of glucose

In this paper, ZnCo2S4 nanomaterial with a unique structure were synthesized by static and hydrothermal method and applied to the field of electrochemical sensing. Based on the advantages of metal-organic frameworks (MOF) materials with large specific surface areas, multiple active centers, and synergistic effect of multi-metals, we choose Zn and Co bimetallic MOF as the precursor material and synthesized ZnCo2S4 nanomaterials with large active area and multiple active centers after vulcanization. In addition, we verified the excellent electrochemical sensing performance of the prepared ZnCo2S4 nanomaterial through the catalytic oxidation experiment of glucose. The results showed that ZnCo2S4 nanomaterials with bimetallic MOF as a precursor had higher electrocatalytic activity for glucose oxidation than ZnCo2S4 nanomaterials without precursor and single metal sulfide. According to the analysis of the electrochemical performance of the material, the detection limit is extremely low (0.007 μM)and two linear range (3 ∼9 μM) and (10 ∼100 μM)for glucose. Therefore, we believe that the method of preparing polymetallic sulfides with MOF as a precursor is of great significance in electrochemical sensing.


Introduction
Non-enzyme-based electrochemical sensors combine sensing and detection of specific substances. They have the advantages of simple equipment, rapid detection, highly sensitive to the detected object in complex substances [1][2][3][4]. Therefore, they are widely used in agriculture, food analysis, environmental monitoring, biomedical detection, and other fields [5][6][7][8][9]. Among many non-enzymatic electrochemical sensing materials, transition metal sulfide materials (TMS) are widely used because of their low cost, low electronegativity, stable structure, and not easy to collapse [8][9][10][11][12][13][14][15]. Among them, polymetallic sulfide has been proven to be a promising electrode material because of its rich redox reactions and better electrochemical activity than single metal sulfide [15]. For example, Liu et al and Villian et al prepared Ni and Co bimetallic sulfides and applied them to electrochemical sensing. Due to the bimetallic replacement of Ni and Co, the surface-active sites of the material increase, which makes the electrode show a lower detection limit and a more comprehensive detection range [8,11].
However, polymetallic sulfide still has some limitations in electrochemical sensing because of its small specific surface area and few active sites, which cause its detection limit is high and its sensing performance is poor. In addition, due to the simple preparation method of one-step synthetic polymetallic sulfide, the distribution of elements cannot be regulated, resulting in the uneven distribution of elements, resulting in the uneven distribution of active sites and poor electrochemical performance [16][17][18][19][20]. Therefore, designing a polymetallic sulfide material with a high specific surface area and multiple active sites for non-enzymatic electrochemical sensing is very important. Using metal-organic frameworks (MOFs) as a precursor to synthesize polymetallic sulfide is a good idea, which has been widely concerned due to its large active area and uniform distribution of metal central sites. In synthesizing polymetallic MOF materials, the unique framework structure of MOF materials ensures the advantages of taking metal elements as active catalytic centers and evenly distributing them [20,21]. Therefore, multi-metal sulfides with MOF as a precursor have been widely used in electrochemistry [21][22][23][24][25]. Such as, Duan's group synthesized ZnS/CoS material with ZnCo-MOF as a precursor and used it as a test for chlorine. Using the uniform distribution of Zn and Co in MOF materials and the unique morphology of MOF materials greatly improves the electrochemical sensing performance of polymetallic sulfide and makes the detection limit lower and the sensitivity higher [26]. Li et al used NiZn-MOF as a precursor and vulcanized and used for high-performance supercapacitors. During the synthesis process, MOF's unique hollow structure increases the material's specific surface area and active sites. It solves the disadvantage of low capacitance and finally shows good performance [25]. Therefore, to solve the inherent shortcomings of polymetallic sulfide materials, we refer to the unique advantages of MOF materials in the above cases and choose MOF as the precursor of vulcanization treatment, so that the materials have better electrochemical performance [24,27].
In the present work, we report a sensitive and error-free non-enzyme sensing materials. The sensor comprises ZnCo 2 S 4 nanomaterial with ZnCo-MOF synthesized at room temperature as a precursor and hydrothermal vulcanization. Next, the performance of the prepared samples was tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Because of MOFs material as a precursor and bimetallic elements, ZnCo 2 S 4 with ZnCo-MOF as a precursor shows good electrocatalytic sensing performance in glucose compared with single metal sulfide material and ZnCo 2 S 4 without precursor. Amperometric analysis shows that the ZnCo 2 S 4 composite has low LOD and high sensitivity (Scheme 1). Therefore, we believe that it has essential research significance in the field of electrochemical sensing.   ZnCo-ZIF is formed after centrifugation and vacuum drying for 12 h. ZIF-8 and ZIF-67 are also made in the same way [33].

Synthesis of ZnCo 2 S 4
ZnCo 2 S 4 was synthesized by vulcanizing ZnCo-MOF. First, add TAA (0.2M) into 35 ml ethanol and 20 ml water. Then added ZnCo-ZIF into the above solution and mixed and stirred thoroughly for 2 min. Pour the obtained mixed solution into a 100 ml reaction kettle and vulcanized at 180°C for 3h. Centrifuge the sulfated sample with ethanol and deionized water, then the cleaned sample is placed in an oven at 60°C for 12 h. ZnS and CoS were fabricated with the same method.

Electrochemical tests
PGSTAT302N electrochemical workstation (Metrohm, CH) as an instrument for electrochemical testing, which includes cyclic voltammetry (CV) and electrochemical Impedance Spectroscopy (EIS). In this experiment, a three-electrode system was used, in which Ag/AgCl (saturated KCl) was chosen as the reference electrode and a platinum sheet was chosen as the counter electrode. The working electrode is made of NF; apply ZnCo 2 S 4 , conductive carbon, and PVDF to NF in a ratio of 8:2:2, and press it into a 1 * 1 cm 2 sheet. The glucose solution was tested in 0.1M KOH and kept at 25°C throug hout. Before amperometric detection, a suitable potential will be applied to the working electrode (versus Ag/AgCl) and the current is stabilized by cyclic CV testing before adding glucose. EIS testing at 10mV AC probe amplitude over a frequency range of 0.1Hz to 100kHz in an open circuit condition.

Morphology and structural analysis
To solve the defects of a small specific surface area, few active sites, and uneven distribution of elements of polymetallic sulfide materials, we synthesized ZnCo-MOF at room temperature and used it as a precursor material. Then it was vulcanized into ZnCo 2 S 4 material by hydrothermal method. To prove whether ZnCo-MOF and ZnCo 2 S 4 were synthesized successfully, we analyzed the crystal structures of ZnCo-MOF (figure 1(a)) and ZnCo 2 S 4 (figure 1(b)) after vulcanization by XRD. As shown in figure 1(a), the diffraction peaks of the synthesized ZIF-8 and ZIF-67 nanomaterials are the same, indicating that both are cubic structures. Moreover, the synthesized ZnCo-MOF not only has the same diffraction peaks as ZIF-8 and ZIF-67 but also has good crystallinity [29,30]. As shown in figure 1(b), the diffraction peaks at 28.6°, 33.1°, 47.5°, 56.4°, and 59.1°belong to the lattice planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) of ZnCo 2 S 4 (PDF#47-1656).
To determine the basic morphology and size of the material, ZnCo-MOF and ZnCo 2 S 4 were characterized by SEM. Figure 2(a) shows the SEM of ZnCo-MOF; as shown in the figure, ZnCo-MOF shows a regular dodecahedral morphology and is about 200 nm in size. After Vulcanized, the material produces a hollow structure and keeps the morphology of the dodecahedron unchanged ( figure 2(b)). To observe the distribution of various elements in ZnCo 2 S 4 , we characterized ZnCo 2 S 4 by energy dispersive spectroscopy (EDS). As shown in the figure, EDS image shows that Zn, Co and S are evenly distributed in ZnCo 2 S 4 (figure 2(c).). We proved the synthesis of ZnCo 2 S 4 with ZnCo-MOF as a precursor is an effective method to solve the uneven distribution of polymetallic sulfide elements. Therefore, compared with the polymetallic sulfide synthesized in one step, ZnCo 2 S 4 synthesized with ZnCo-MOF as a precursor has a more uniform distribution of active sites, larger catalytic area, and better catalytic performance. To prove that MOF material has a super large specific surface area, we evaluated the texture properties of ZnCo 2 S 4 material with ZnCo-MOF as a precursor and ZnCo 2 S 4 material synthesized in one step by N 2 adsorption-desorption isotherm. We performed bet tests on the two materials, respectively. Figure 2(d) shows an isotherm comparison diagram between the ZnCo 2 S 4 semiconductor and MOF precursor material. We can see from the figure that MOF precursor material had a far greater BET surface area (1312.8m 2 g) −1 than ZnCo 2 S 4 semiconductor material (12.3m 2 g) −1 . It is because MOF material has a unique porous and hollow structure, and its structural characteristics are maintained during sulfuring. Therefore, ZnCo 2 S 4 with ZnCo-MOF as a precursor has a higher specific surface area. The ultra-high specific surface area of ZnCo-MOF leads to many active sites in ZnCo 2 S 4 material, which makes ZnCo 2 S 4 have ultra-high electrochemical properties.
To prove the effectiveness of different valence states of various elements in ZnCo 2 S 4 on the catalytic oxidation of glucose, we have measured the different valence states of each element in ZnCo 2 S 4 by XPS tests. As shown in figure 3(a), the XPS survey indicates the synthesis of ZnCo 2 S 4 from Zn, Co and S. Of these, the peak value of Zn 2p3/2 track is 1021.8ev, and the peak value of Zn 2p1/2 track is 1044.9 eV (figure 3(b)) mapping, which confirms the presence of Zn in the ZnCo 2 S 4 in the Zn 2+ state., Co 2p shows a peak of Co 2p1/2 at 797.0eV and a peak of Co 2p3/2 at 781.3eV (figure 3(c)), and the analysis combined with the two satellite peaks (Sat.) concludes that Co is present in ZnCo 2 S 4 in both Co 3+ and Co 2+ valence states. The S 2p map (figure 3(d)) shows peaks at 161.6 eV and 162.6 eV corresponding to the S 2p3/2 and S 2p1/2 orbitals, with a characteristic peak of 163.7 for the metal-sulfur bond (S 2− ) and 168.5 eV for the SO 4 2− /HSO 4− characteristic peak. Based on the above analysis of the XPS pattern, it can be concluded that Zn 2+ , Co 2+ , Co 3+ , S 2− and SO 4 2− together form the ZnCo 2 S 4 material, which also proves the successful preparation of ZnCo 2 S 4 [15]. Besides, the concurrent presence of Co 2+ and Co 3+ also greatly increases the efficiency of ZnCo 2 S 4 in the catalytic oxidation of glucose. It is because the presence of Zn makes the Co in ZnCo 2 S 4 present Co 2+ and Co 3+ . Compared with only Co 2+ in CoS, the presence of Co 3+ can directly catalyze glucose oxidation, which greatly improves the efficiency of ZnCo 2 S 4 in the oxidation of glucose [31].

Electrochemical sensing performance of the ZnCo 2 S 4 composite
In the process of electrode surface modification, EIS is usually used to track the resistance of interface dynamics. EIS tested the electron transfer ability of various ZnCo-MOF and ZnCo 2 S 4 modified electrodes in 0.1M KOH. The EIS of ZnCo-MOF is curved, proving its electron transfer ability is poor ( figure 4(a)). The EIS of ZnCo 2 S 4 has a semicircle at high frequency and a slash at low frequency. It is because the vulcanized composite has stronger adhesion, which promotes charge transfer and improves conductivity.
To evaluate the catalytic performance of non-enzyme sensing ZnCo 2 S 4 , we first subjected the electrodes modified with different materials to a redox test. As shown in figure 4(b), Because the bare electrode has no active site, no redox reaction occurs, so no redox peak is generated. Behind Zn-MOF modification, Zn 2+ participates in oxidation-reduction, but the Zn element's single valence state of Zn element and limited oxidation-reduction shows a weak oxidation-reduction peak. With the addition of Co, the redox effect of the material is enhanced and accompanied by the shift of the peak position. Because after the addition of heteroatoms, the replacement reaction of the two elements exposes more metal ions to the surface and intensifies the redox [32]. After vulcanization, the redox peak increases significantly, indicating that vulcanization accelerates the electron transfer rate [34].
Subsequently, we compared the sensitivity of different catalytic materials to glucose solution (figure 4(c)); it can be seen from the figure that when 1 mM glucose is added to the electrolyte, the redox peak of ZnS does not change significantly, indicating that it has no apparent catalytic effect on glucose solution. This is because Zn itself has no significant catalytic impact on glucose and cannot catalyze the oxidation of glucose to gluconolactone through its valence change. Unlike Zn, the redox peak of CoS increased slightly after adding glucose solution to the electrolyte, and its reaction mechanism with glucose is as follows [ However, after doping Co and Zn, the redox peak of ZnCo 2 S 4 was significantly enhanced. This indicates that there are a large number of glucose catalytic oxidation reaction sites on the surface of porous ZnCo 2 S 4 material because of the replacement effect of CO on Zn, which leads to more active site exposure on the surface of ZnCo 2 S 4 . In an alkaline solution, due to the replacement of Zn, Co 2+ with high catalytic activity is easily converted to Co (OH) 2 and then oxidized to CoOOH intermediate to catalyze glucose oxidation to gluconolactone [28]. It reflects the synergy between the two elements (figure 4(c)).
As a comparison, we tested the CV curves of ZnCo 2 S 4 semiconductor material synthesized in one step in different concentrations of electrolyte. As shown in figure 4(d), ZnCo 2 S 4 synthesized in one step has no apparent linear relationship with glucose solutions of different concentrations, and the redox peak does not change significantly. It proves that there is no active site of glucose oxidation on this material's surface, so it cannot catalyze the oxidation of glucose. We believe this is due to the inherent shortcomings of polymetallic sulfides, such as small specific surface area, fewer active sites, uneven distribution of elements. It also proves that MOFs has a large active area, many active sites, and uniform element distribution. At the same time, it also shows that using MOF material as a precursor is a favorable factor in increasing the electrocatalytic activity of the samples.
To explore the influence of the reaction kinetics of the electrode on the electrocatalytic of the working electrode, we performed CV tests on ZnCo 2 S 4 /GCE in 1M KOH containing 0.1 mM glucose solution with different scan rates from 10 to 50 mV s −1 . The redox of glucose is gradually enhanced with increasing scan rate ( figure 5(a)), indicating that glucose can be rapidly oxidized on ZnCo 2 S 4 within a certain range. Implying that the ZnCo 2 S 4 electrode material has an excellent performance for glucose.
The calibration between the redox peak and the different scan rates for the ZnCo 2 S 4 material is shown in figure 5(b). The redox peak of ZnCo 2 S 4 /GCE to glucose is proportional to its scanning rate, and we believe may be caused by the diffusion of glucose on the electrode surface [8]. To confirm this conjecture, we analyzed the relationship between oxidation peak and scanning rate ( figure 5(c)). The equation for the relationship between the two is as follows [35,36]  b = 0.63, which is close to b = 0.5, thus suggesting that the transfer of glucose to the bimetallic organic framework sulfide electrode is a typical diffusion-controlled process [6].
The repeatability and stability of sensing materials are the key factors to evaluate the electrocatalytic effect of materials. We measured the relative current density of ZnCo 2 S 4 material in 0.1M KOH with 4 mM glucose solution for seven consecutive days ( figure 5(d)). The current response of the material reached more than 90% of the initial current response for seven consecutive days, which indicates that the material has good repeatability and stability for the electrocatalytic oxidation of glucose.
Then we added 0-4 mm glucose to 0.1M KOH electrolyte respectively, and we recorded its cyclic voltammetry curve ( figure 6(a)). When the glucose concentration increased, a rapid current density response was found, this confirms that the porous ZnCo 2 S 4 electrode has a rich glucose oxidation active sites. Thus, glucose is oxidized by electrocatalysis to gluconolactone [34].
To further prove the sensitivity of ZnCo 2 S 4 nanomaterials to glucose, we carried out chronoamperometric tests on the materials at 0.54V voltage. Under constant stirring, we add glucose solution of a certain concentration to 0.1M KOH electrolyte every 25s. The test results are shown in figure 6(b), each time glucose is added to the electrolyte, the constant signal will increase and show a relatively stable rising gradient. We believe that the main reason for this phenomenon is the synergistic effect of bimetals. The valence change between zinc and cobalt accelerates the electron transfer rate, and also accelerates the reaction rate of ZnCo 2 S 4 to glucose, thus showing a regular chronoamperometric response [9]. Figure 6(c) shows a gradual increase in the redox peak of the ZnCo 2 S 4 material as the concentration of the glucose solution in the electrolyte increases, and also demonstrates a good linear relationship between the two. In the calibrated linear graph, ZnCo 2 S 4 /GCE showed a good linear response at glucose solution levels of 3 μM-9 μM and 10 μM to 0.1 mM respectively. The linear regression equations for these two segments are y 1 = 6589.9 + 26.5x (R 2 = 0.99989) and y 2 = 6869 + 2.498x (R 2 = 0.99359), where x (mM) is the glucose concentration and the y (mA) is current values for different glucose concentrations. The limit of detection (LOD) was 0.007 μM reference formula LOD =3σ/b. However, the electrocatalytic oxidation of glucose by ZnCo 2 S 4 gradually saturates after the concentration of the glucose solution reaches 0.1 mM since the reaction's intermediates can decrease the modified electrode's active site and electrical conductivity. As a result, ZnCo 2 S 4 electrode materials exhibit high sensitivity, wide detection lines, and low detection limits, which also make them excellent sensing materials for the electrocatalytic oxidation of glucose. In addition, we compared ZnCo 2 S 4 with other glucose electrocatalytic oxidation materials, showing the excellent electrochemical properties of the ZnCo 2 S 4 material (table 1) [8,[37][38][39][40][41][42][43] To study the specificity of the sensor for glucose, we selected some common organic and inorganic substances that may cause interference, including NaCl, urea (UA), ascorbic acid (AA), hydrogen peroxide (H 2 O 2 ), and SO 4 2− , the concentration of each interfering sample is 1M, and the time interval is the 50s. The results are shown in figure 6(d), even if the concentration of the interfering sample is dozens of times higher than that of glucose, ZnCo 2 S 4 nanomaterials still have no obvious response to these interfering substances. The above results prove that ZnCo 2 S 4 nanomaterials only have an obvious response to glucose solution under the  interference of multiple interfering substances, which indicates that they have unique selectivity to glucose solution [7].

Mechanism discussion
Given the excellent catalytic oxidation phenomenon of ZnCo 2 S 4 for glucose, we discuss the mechanism. First, due to the synergistic effect between multiple metals and the low electronegativity of sulfide materials, we chose polymetallic sulfide as the electrocatalyst material. We chose Zn and Co because they have similar crystal structures and atomic radii. In ZnCo 2 S 4 , zinc exists in the form of Zn 2+ , but its effect on glucose oxidation is not apparent [44]. Therefore, we chose to dope Co to increase the metal sites between Zn and Co, thereby improving the electrocatalytic activity. The addition of Co greatly improved the electrocatalytic performance of ZnS. Co can replace Zn without changing its original structure. In ZnCo 2 S 4 , Co exists in the form of Co 2+ / Co 3+ . The cobalt hydroxide (Co-O-OH) produced by the Peroxidation of Co 2+ to Co 3+ can lead to faster glucose oxidation. Compared with CoS, the unique Co 3+ of ZnCo 2 S 4 can directly catalyze the oxidation of glucose, which greatly accelerates the reaction of the material to glucose [10,28]. That is to say, the addition of Co increases the charge transfer rate of ZnCo 2 S 4 to accelerate its electrocatalytic oxidation of glucose. At the same time, the presence of Zn greatly increases the Co 3+ /Co 2+ , and the cooperation of Zn and Co maximizes the efficiency of ZnCo 2 S 4 for the catalytic oxidation of glucose [31].
On this basis, we choose MOF structure as the precursor because its unique porous framework structure can increase the specific surface area and active site of the material while make the distribution of Zn and Co bimetals more uniform in the process of material synthesis so that the active site can be evenly distributed, which significantly improves the electrocatalytic performance of the material. Therefore, using MOF as the precursor material of ZnCo 2 S 4 is the best choice to maximize the electrocatalytic performance of ZnCo 2 S 4 .

Conclusions
This paper synthesized ZnCo 2 S 4 nanomaterials with bimetallic MOF as a precursor by a simple hydrothermal method. It was applied to the electrocatalytic sensing of glucose. Compared with CoS and ZnS-modified electrodes, ZnCo 2 S 4 modified electrode has high sensitivity and wide detection range for glucose electrocatalytic oxidation, due to its large electroactive surface area and bimetallic synergy. The oxidation-reduction reaction is accelerated by bimetals and their variable valence. At the same time, with the increase of glucose concentration in electrolytes, the electrocatalytic oxidation process of glucose is also greatly accelerated. We use its linear response to glucose to prove its good electrocatalytic performance. Therefore, the proposed ZnCo 2 S 4 composite prepared by ZnCo-MOF as precursor has been proved to be a promising electrode material for electrochemical sensing.