A novel detection method for neohesperidin dihydrochalcone (NHDC) based on ZIF-8/ErGO composite modified electrode in milk

doi:10.21203/rs.3.rs-2594408/v1

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A novel detection method for neohesperidin dihydrochalcone (NHDC) based on ZIF-8/ErGO composite modified electrode in milk

https://doi.org/10.21203/rs.3.rs-2594408/v1

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Neohesperidin dihydrochalcone (NHDC), as a high-intensity sweetener, has been widely used to be a new food additive in recent years. In this study, zeolitic imidazolate framework (ZIF)-8/electrochemical reduced graphene oxide (ErGO) composite materials were favorably constructed onto the surface of glassy carbon electrode (GCE) by two simple steps without any dispersant, and a ZIF-8/ErGO modified GCE for an electrochemical sensing of NHDC was developed. The morphology and its structure of the composite materials were characterized by scanning electron microscopy and Raman spectroscopy. The electrochemical behavior of NHDC at different modified electrodes and its redox processes was investigated by cyclic voltammetry (CV). Under the optimized conditions, the linear sweep voltammetry (LSV) method was used to detect NHDC. The linear relationship between oxidation peak current of NHDC and its concentration is ranged from 80 nM to 80 μM and the detection limit is calculated to be 31.5 nM (based on S/N=3). The selectivity and reproducibility of the modified electrode were evaluated. Finally, the modified electrode was used to detect NHDC in milk samples with a recovery rate of 98.3%~106.3%, which implied its feasibility for the determination of NHDC in actual samples.

ZIF-8

ErGO

Neohesperidin Dihydrochalcone

Modified electrode

Voltammetry

As a new food additive, neohesperidin dihydrochalcone (NHDC) has been extensively used as sweeteners, flavoring agents, corrigents, flavors and fragrances, and feed additives, etc. due to its high sweetness, low calories, good stability and non-toxicity. In medicine aspect, it has been shown that NHDC possesses the efficacy of protecting the liver [1], lowering blood sugar and blood lipids [2, 3], improving the intestinal probiotic flora [4], anti-inflammatory [5], and antioxidant [6]. Just because of its wide-ranging application in the field of food additive and pharmaceutical adjuvant, the control of its content is the key link during the whole process of production. Therefore, it is necessary to develop a rapid and sensitive analysis and detection method for NHDC in food and pharmaceuticals. The traditional analysis methods include high performance liquid chromatography [7], high performance liquid chromatography-mass spectrometry [8, 9, 10], capillary electrophoresis [11], etc. Electrochemical analysis is also an important method owing to the advantages of fast response, simple operation, time saving, low instrument cost, high sensitivity, good selectivity and so on.

Metal organic framework (MOF) is a crystalline porous material formed by self-assembly with coordination of metal ions (metal clusters) and organic ligands [12]. Zeolite imidazole framework materials (ZIFs) are a branch of metal-organic framework materials, which have large specific surface area, high porosity, topological diversity and good chemical and thermal stability. According to the report, ZIFs have been applied in drug delivery [13], gas adsorption [14], gas storage [15], heterogeneous catalysis [16], electrochemical sensors [17, 18], and energy storage and collection [19, 20]. Among them, ZIF-8, as one of the most common MOFs, is easily prepared with Zn as a metal source and 2-methylimidazole as a ligand at room temperature. Moreover, ZIF-8 has the characteristics of high specific surface [21] and porous structure [22], which makes it become a popular material in the field of electrochemical analysis. However, its electrical conductivity is not good reportedly [23]. Therefore, constructing composite materials with ZIF-8 by the introduction of high conductivity materials becomes the preferred strategy in order to improve the response of modified electrodes to the trace target analyte. For example, Zheng et al [24] prepared graphene and ZIF-8 (G-ZIF-8 for short) composite modified electrode and applied it to electrochemical sensing of dopamine. The results manifested that G-ZIF-8 composite shows good electrochemical response to dopamine due to the synergistic effect between highly conductive graphene and porous ZIF-8. Dong et al [25] also synthesized reduced graphene oxide and ZIF-8 (rGO@ZIF-8) composite material to construct a sensitive electrochemical sensor for dopamine. The sensor showed sufficient repeatability and durability, which was also ascribed to the synergistic amplification effect of rGO and ZIF-8.

In comparison with small molecules, the electroactive group is difficult to directly contact the electrode surface for NHDC with high molecular weight. Therefore, it is a decisive factor to shorten the electron transfer distance between the active group and the electrode surface by seeking appreciate modified electrode materials. It was guessed that the above description is the main reason and thus there are very few reports for the electrochemical detect of NHDC in the literature. In the past recent years, only Yang et al published their studies about voltammetry determination of NHDC based on single-walled nanocarbon tubes (SWNTs) modified glassy carbon electrode (GCE) in 2014. It was even found that there were almost no reports on the detection of NHDC by using MOF material modified electrode. In the present work, we first synthesized ZIF-8 and graphene oxide, respectively, and then fixed the composite of ZIF-8 and graphene oxide onto the surface by electrochemical method without adding any dispersant. ZIF-8 and the electro-reduced graphene oxide (ZIF-8/ErGO) modified GCE obtained were used to detect NHDC in solution and real samples.

2.1 Materials

Zn(NO₃)₂·6H₂O and 2-methylimidazole were provided by Sinopharm Chemical Reagent Co., LTD and Bidde Pharmaceutical, respectively. Mcllvaine buffer solution (MBS) with different pH values was prepared by mixing 0.1 M citric acid (Sinopham Chemical Reagent Co., LTD.) and 0.2 M Na₂HPO₄ (Sinopham Chemical Reagent Co., LTD.) in a certain proportion. NHDC (C₂₈H₃₆O₁₅, 95%) was purchased from Aladdin Shanghai Reagent Co., LTD and its standard solution was dissovled in ethanol. All other solutions were prepared with double-distilled water.

2.2 Apparatus

The electrochemical test was carried out by electrochemical workstation (CHI660E, Shanghai, China) in a three-electrode system. The glassy carbon electrode (GCE, diameter 3 mm) or modified glassy carbon electrode was selected as the working electrode, saturated calomel electrode as the reference electrode, and platinum wire electrode as the counter electrode. The surface morphology of the modified electrode was given by ultra-high resolution cold field scanning electron microscope (SEM, SU8000, Hitachi, Japan), and Raman spectra of modified electrode material containing graphene was obtained by confocal microscopic Raman spectroscopy (Renishaw New Mills, UK).

2.3 ZIF-8 nanocrystals synthesized

According to literature reports [26], ZIF-8 material was synthesized at room temperature. First, 0.734 g of Zn(NO₃)₂.6H₂O and 0.406 g of 2-methylimidazole were dissolved in 20 mL and 80 mL methanol, respectively. Then the zinc nitrate solution was slowly added to the 2-methylimidazole solution with the assistance of ultrasonic, and stood for 30 min. Finally, white ZIF-8 nanocrystals were collected by centrifugation, washed with methanol for 3 times, and immediately dried in an oven at 60 ℃ for 12 h.

2.4 Preparation of ZIF-8/ErGO modified electrodes

The graphene oxide material was prepared according to the method reported by our research group [27]. The ZIF-8/GO dispersion was obtained by adding 1 mg ZIF-8 and 1.5 mg GO into 1 mL double distilled water for 30 min under ultrasonic. Then, 8 µL of the dispersion were dropped onto the surface of GCE polished in advance and dried naturally. Right after, the ZIF-8/GO modified GCE was inserted into pH = 7 phosphate buffer solution (PBS) and treated with N₂ for 30 min. Finally, ZIF-8/electrochemical reduction graphene oxide (ErGO) composite modified GCE (ZIF-8/ErGO GCE for short) was obtained by consecutive cyclic voltammetry (CV) in the potential window of 0.6 ~ -1.5 V at a scan rate of 0.1 V/s for 25 cycles (Fig. S1). ErGO GCE and ZIF-8 GCE were prepared according to the above similar process. Figure 1 shows the schematic diagram of preparation process of ZIF-8/ErGO GCE and detect of NHDC.

2.5 Electrochemical measurements

The modified electrode was placed into a beaker containing different concentrations of NHDC in 10 mL MBS with pH = 5.0 in a three-electrode system and enriched by open circuit under magnetic stirring. The study of electrochemical behavior of NHDC was carried out by CV in the potential range of 0 ~ 0.8 V at 0.05 V/s. The detection of NHDC was executed by linear sweep voltammetry (LSV) in the potential range of 0.4 ~ 0.7 V at 0.05 V/s. After each test, the modified electrode was restituted by CV in the potential range of 0 ~ 0.8 V at 0.1 V/s for 20 cycles in neutral PBS.

2.6 Pretreatment of milk sample

First, 10 mL of a native brand milk was removed into a 50 mL beaker, and then 20 mL of absolute ethanol was added into the above beaker to precipitate protein and lipid, sealed with plastic wrapped stood for 24 h. At last, the clear liquid at middle layer was taken out for later use.

3.1 SEM characterization of ZIF-8, GO, ErGO, and ZIF-8/ErGO

ZIF-8, GO, ErGO, and ZIF-8/ErGO were modified at the surface of glassy carbon (GC) sheet according to the method of Experiment 2.4. The SEM images of them are exhibited in Fig. 2. It can be seen from Fig. 2a and 2b that there are obvious wrinkles on the surface of GO and ErGO modified GC sheet, which is a distinctive feature of graphene. Figure 2c indicates that ZIF-8 particles with uniform size are distributed on the surface of GC sheet. While the composite film of ZIF-8 and ErGO is fixed onto the GC sheet, it clearly shows that ZIF-8 material looks like being cladded by ErGO (shown in Fig. 2d). That pleated protuberance of graphene and porous structure of ZIF-8 work in conjunction to increase the surface area and conductivity of GCE. Thus, it is conjectured that the ZIF-8/ErGO GCE can improve the electrochemical response of NHDC by their synergistic effect.

3.2 Raman Characterization

As shown in Fig. 3, Raman spectroscopy was used to test and analyze the characteristic bands of carbon material at the surface of modified electrode, and the results showed that D and G peaks of the carbon atom crystal appeared at 1353.71 cm^− 1 and 1592.79 cm^− 1both for ZIF-8/GO GCE and ZIF-8/ErGO GCE [28], while there was almost no peak observed for ZIF-8 GCE. In general, the D peak is the disordered vibration of the carbon atom crystal, which represents the defect of the carbon atom lattice. The G peak originates from the plane stretching vibration of sp² hybrid carbon atom. The peak intensity ratio of I_D/I_G for ZIF-8/GO GCE is calculated to be 1.08. After electrochemical reduction, an increase intensity ratio for I_D/I_G with a value of 1.78 is obtained for ZIF-8/ErGO GCE, indicating that some sp³ hybrid carbon group such as carbon hydroxyl, carboxyl, other functional groups in GO is reduced and there are many defects in the ErGO, which demonstrates the successful reduction of GO in situ at ZIF-8/GO GCE by cyclic voltammetric method.

3.3 Electrochemical impedance spectroscopy characterization

Figure 4 shows the electrochemical impedance spectroscopy (EIS) of different electrodes in 0.1 M KCl containing 5 mM Fe[(CN)₆]^3−/4−. The typical impedance spectra are observed at all electrodes in Nyquist plots of Fig. 4. That is, the characteristic impedance semicircle appears in the high frequency region, indicating a charge transfer-controlled process for the redox of Fe[(CN)₆]^3−/4− at the electrode surface; while the straight line appeared in the low frequency region reveals a diffusion-controlled process. Ordinarily, the diameter of semicircle mirrors the charge transfer resistance (R_ct). After fitting by equivalent circuit diagram, the R_ct values of different electrodes are 137.31 Ω, 182.52 Ω, 11.12 Ω, and 23.85 Ω for bare GCE, ZIF-8 GCE, ErGO GCE and ZIF-8/ErGO GCE, respectively, manifesting that the larger impedance for ZIF-8 shows poor conductivity while ErGO displays excellent conductivity. Concurrently, the smaller value of 23.85 Ω for ZIF-8/ErGO GCE also exhibits favorable conductivity by compositing ZIF-8 and ErGO, which becomes an ideal modified electrode material in this study.

3.4 Electrochemical response of NHDC at different electrodes

The CV responses of NHDC at bare GCE, ZIF-8 GCE, ErGO GCE and ZIF-8/ErGO composite film electrode are exhibited in Fig. 5. From yellow curve of inset, no peaks appear, indicating that there are no response for NHDC at bare GCE. After GCE is modified with ZIF-8, ErGO and ZIF-8/ErGO composite film, respectively, the electrochemical response of NHDC at the electrodes improves notably in comparison with bare GCE. Especially for the ZIF-8/ErGO composite film modified electrode (red curve), three obvious redox peaks are observed during two cycles of CV measurements. For the first cycle, a strong oxidation peak (P_a1) at 0.506 V and a reduction peak (P_c) at 0.198 V are obtained. Moreover, the peak current of P_a1 at ZIF-8/ErGO composite film electrode is about 3.8 times as much as that of ErGO GCE and 42 times of ZIF-8 GCE. The probably reason is stemed from the synergistic effect between good adsorption of ZIF-8 and high conductivity of ErGO. Thus, the sensitivity for NHDC is improved drastically at ZIF-8/ErGO GCE. For the second cycle, another new oxidation peak (P_a2) appears at 0.227 V, which is one of a couple of redox peaks with P_c, caused by reversible redox process. It can also be concluded that P_a1 is produced by an irreversible oxidation process. Strangely, the peak current of P_a1 remarkably diminishes, which is due to the blocked pore channel caused by the oxidation product of NHDC in the first cycle. In the subsequent procedure, P_a1 in the first cycle is used for the determination of NHDC.

3.5 Calculation of reaction kinetic constant

In order to study the overall redox process of NHDC at the ZIF-8/ErGO modified electrode surface and calculate the related kinetic parameters, the CV tests of NHDC was performed by two successive cycles at different scan rates, as shown in Fig. 6a and 6b, where 6a is the data for the first cycle and 6b for the second cycle. It can be seen that with the increase of the scan speed, three redox peaks current of NHDC also increases, two oxidation peaks potential shifts positively and one reduction peak potential slightly shift negatively. Figure 6c ~ 6f show the relationships between peak current and scan rate, or between peak potential and scan rate. Firstly, Fig. 6c gives good linear relationship between peak current of P_a1 (i_Pa1) and square root of scan rate (υ^1/2), which reveals that the first irreversible oxidation of NHDC is a diffusion-controlled process. In the same way, the linear fit between peaks current of P_c (i_Pc) and P_a2 (i_Pa2) and its scan rate (υ) is manifested in Fig. 6d. But unlike Fig. 6c, the linear relationship demonstrates that the reversible redox of the first oxidation product of NHDC is an adsorption-controlled process in Fig. 6d. Secondly, for the irreversible redox process, some kinetic reaction parameters can be calculated according to the Laviron equation[29]:

${ E}_{P}={E}^{\theta }+\frac{RT}{\alpha nF}ln(\frac{RT{k}_{s}}{\alpha nF}$ ) + $\frac{RT}{\alpha nF}ln\upsilon$ (1)

where α is the electron transfer coefficient, k_s is the apparent rate constant, n is the electron transfer number, υ is the scan rate, E^θ is the standard potential, and other constants have their common sense. Figure 6e and 6f display the relationships between peaks potential (E_Pa1, E_Pc, E_Pa2) and narural logarithm of scan rate (lnυ), respectively. The linear regression equation of E_Pa1 and lnυ is E_Pa1=0.03037lnυ + 0.6149 (R² = 0.9908). In terms of Laviron Eq. (1), the value of αn is calculated to be 0.85. For the irreversible diffusion-controlled process, usually α = 0.5, that is, n = 1.70 ≈ 2, indicating that the first irreversible oxidation of NHDC is a two-electron process. Finally, Eq. (1) is transformed into the following Equations according to Laviron theory, that is:

E _pc =E ^θ $- \frac{RT}{\alpha nF}$ lnυ (1–1)

E _pa =E ^θ $- \frac{RT}{(1-\alpha )nF}$ lnυ (1–2)

lgk _s = $\alpha {lg}\left(1-\alpha \right)+\left(1-\alpha \right)lg\alpha -lg\frac{RT}{nF\upsilon }$ - $\alpha ($ 1- $\alpha )\frac{nF\varDelta Ep}{2.3RT}$ (1–3)

where all symbols have the same sense as Eq. (1). By linear fit for Fig. 6f, the corresponding linear regression equations of E_Pa2 (or E_Pc) ~ lnυ are as follows: E_Pa2=0.0195lnυ + 0.3088 (R² = 0.9957); E_Pc=0.0294lnυ + 0.1086 (R² = 0.9901). The results of calculation express α with a value of 0.40 (α = 0.40) and n with a value of 2.19 (n ≈ 2) according to Eq. (1–1) and (1–2), indicating that the reversible redox of NHDC’s oxidation products is a two-electron process. According to Eq. (1–3), it can also be calculated that the apparent rate constant k_s of NHDC’s oxidation products at a scan rate of 0.05 V/s is 0.88 s^− 1at ZIF-8/ErGO modified electrode surface.

3.6 Optimization of experimental conditions

3.6.1 Effects of supporting electrolytes

0.1 M phosphate buffer solution (PBS), MBS, NH₄Cl-NH₃·H₂O, HAc-NaAc and HCl-C₆H₅O₇Na₃ were prepared, respectively. In this study, all the above buffer solutions with an approximate pH value was obtained by mixing a certain solution with volume ratio, and then 80 µM NHDC solution was added for the first cycle of CV test. The results imply that the largest oxidation peak current (i_pa1) is achieved while the supporting electrolyte is MBS, so the buffer solution is chosen as MBS.

3.6.2 Effects of ratio and volume of ZIF-8/ErGO

For the initial dispersion of ZIF-8 and GO, its ratio and volume determine the ratio of ZIF-8 to ErGO and the composite film thickness at the electrode, and ultimately influence the sensitivity of modified electrode. Thus, it is necessary to explore the effects of ratio and volume of ZIF-8/ErGO on determination of NHDC. The different mass ratios of ZIF-8 to GO with 5:1, 2:1, 1:1, 2:3 and 1:2 were mixed by ultrasonic dispersion, and then 8 µL of each dispersion was dropped onto the surface of electrode, the CV response of each modified electrode containing 80 µM NHDC was recorded (shown in Fig. S2a). The experimental results show that when the mass ratio of ZIF-8 to GO is 2:3, the i_pa1 value in the oxidation process of NHDC was the largest among all ratios, so the mass ratio of ZIF-8 to GO (same as ZIF-8 to ErGO) of 2:3 was selected as the optimal ratio.

Soon afterwards, each different amount of ZIF-8/ GO dispersion with a fixed mass ratio of 2:3 were carefully transferred onto the surface of the glassy carbon electrode, including 2 µL, 4 µL, 6 µL, 8 µL and 10 µL. According to the same method as introduced in Experiment 2.4, the as-prepared ZIF-8/ErGO modified electrode with different thickness was applied to separately detect NHDC, as shown in Fig. S2b. The experimental results show that i_pa1 of NHDC increases continuously with the increase of the modification volume, until the peak current reaches the maximum while the modification volume is 8 µL, and later the peak current decreases as the modification amount continuing rising. The probable reason is that quantities of ZIF-8 with poor conductivity will hinder the electron transport of NHDC on the electrode surface as the ZIF-8/ErGO film thickens with increasing the volume of dispersion. Therefore, 8 µL is selected as the best modification volume during the experiment.

3.6.3 Effect of pH

pH of solution also influences the peak current and peak potential of NHDC redox. The effect of pH on the redox of NHDC was tested in MBS in the range of pH 3 ~ 7, as exhibited in Fig. 7a (the first cycle) and 7b (the second cycle). As pH increases, peak current of P_a1 increases firstly and then decreases (seen in star symbols of Fig. 7c). Therefore, pH 5.0 MBS is chosen for further study in the experiment. Figure 7c also separately depicts the dependence of each redox peak potential of NHDC on pH. It is vividly found that each peak potential (E_p) of P_a1, P_c and P_a2 shifts to the negative direction with the increase of pH, indicating that H⁺ was involved in the redox process of NHDC. In addition, each peak potential of redox of NHDC shows a good linear relationship with pH. The linear regression equations are expressed as follows: E_pa1=0.0343pH + 0.7139, R² = 0.9964; E_pc=0.0531pH + 0.4469, R² = 0.9933; E_pa2= 0.0604pH + 0.5482, R² = 0.9922. According to the Nernst equation:

$$\frac{d{E}_{p}}{d\text{p}\text{H}}=\frac{2.303mRT}{nF}$$

Where m and n are the transfer number of proton and electron, respectively, and other symbols have the conventional meanings. Thus, for P_a1, it can be calculated that m/n = 0.57 ≈ 0.5, and m = 1 according to the result of n = 2 provided in Step 3.5,suggesting that the first oxidation of NHDC on the surface of the modified electrode is a one-proton and two-electron process. For P_c and P_a1, it can also be obtained that the transfer number of proton and electron is equal in the subsequent redox process in the light of the similar computing method, that is, m = n = 2, so the redox process of the second step for NHDC is a two-proton and two-electron transfer process. Figure 7d reveals the whole redox mechanism of NHDC at the ZIF-8/ErGO GCE, which is consistent with the result reported in the literature [30].

3.6.4 Effect of enrichment time

As described in the above discussion, the first oxidation of NHDC is an adsorption-controlled process, thus enrichment time will affect the current response of NHDC. In this experiment, 80 µM NHDC was added into MBS with pH = 5 and the CV responses at the ZIF-8/ErGO GCE were recorded by enabling preconditioning of time with open-circuit under magnetic stirring. The enrichment time was started from 30 s and raised every 30 s. The experimental results show that, with the increase of enrichment time, i_pa1 gradually increases until it reaches the maximum at 240 s, and finally i_pa1 tends to be stable (seen in Fig. S3), so the enrichment time is set as 240 s in this experiment.

3.7 Standard curve for NHDC detect

Under the optimized experimental conditions, LSV method was used to determine NHDC in the concentration range of 0.08 ~ 80 µM, as shown in Fig. 8a. The oxidation peak current of NHDC climbs with its concentration. Figure 8b unveils the linear relationship between the oxidation peak current (i_pa1) of NHDC and its concentrations (c_NHDC) from 80 nM to 80 µM, and the fitting linear equation is stated as: i_pa1(µA) = 0.4892 + 0.7674 c_NHDC(µM) (R² = 0.9922). The detection limit was 31.5 nM (S/N = 3), according to Eq. (3):

LOD= $\frac{3\sigma }{R}$ (3)

Where σ represents the standard deviation of i_pa1 at the lowest concentration under the operating curve (σ = 0.008057 µA after repeated measurement for 11 times); R represents the slope of the fit line.

3.8 Anti-interference and reproducibility of ZIF-8/ErGO GCE

In order to test the anti-interference ability of ZIF-8/ErGO GCE, Fig. 9 records the oxidation peak current response of NHDC by LSV method under optimized conditions while some inorganic ions or organic substances are individually added to MBS containing 10 µM NHDC. .Considering the possible interference in food such as milk or drink, the NHDC solutions coexisting with 400-fold concentration of NaCl, MgCl₂, FeCl₃, KNO₃ and CuSO₄, respectively, were detected. The experimental results show that the relative deviation of i_pa1 was less than ±5%, indicating that the modified electrode has preeminent ability of anti-interference for NHDC detection in the presence of the above ions. In the same way, 50-fold concentration of sucrose and glucose, and 10-fold concentration of dopamine, ascorbic acid and p-nitrophenol were also added, respectively, there is still no interference for the detection of NHDC. Therefore, ZIF-8/ErGO GCE has strong anti-interference property.

To characterize the reproducibility of ZIF-8/ErGO GCE, the same modified electrode was parallel tested for 7 times by LSV method under optimized conditions. Due to strong adsorption of NHDC and its redox product on the electrode, the modified electrode should be first placed in blank neutral PBS buffer solution for 10 cycles of CV after each test, and then the next test was conducted. The relative standard deviation (RSD) of i_pa1 was 4.67% for 10 µM NHDC, indicating that the modified electrode has good reproducibility. Furthermore, the 7 batches of ZIF-8/ErGO GCE were attained according the same procedure for fabricating electrode, and the RSD of i_pa1 was only 3.56%, also demonstrating good reproducibility.

3.9 Detection of NHDC in milk samples

The milk samples of 100 µL, 500 µL and 1000 µL, pretreated as stated in Experiment 2.6, were diluted to constant volume of 10 mL with pH = 5 MBS, respectively. Under the optimized experimental conditions, the LSV method was used for several tests, and the results show that no NHDC was detected in the milk samples or the content of NHDC in the samples was quite low.

After that, the standard addition method was chosen to test the practical application of the modified electrode by adding a certain amount of NHDC solution to the above diluted milk samples. While NHDC of 1.6 µM, 2.4 µM and 4.8 µM was added for parallel measurements of three times, respectively, its recoveries are determined in the range of 98.3% and 106.3% (shown in Table 1). The results were reliable. Therefore, this method can be used for the detection of NHDC content in milk.

Table 1 Detection of NHDC in milk samples (n=3)

Samples of milk

Found/μM

Added/μM

Detected/μM

Recovery/%

1.60

2.40

4.80

1.70

2.36

4.76

106.3

98.3

99.2

A sensitive electrochemical modified electrode for NHDC determination was constructed by fixing ZIF-8 and ErGO composite onto the surface of glassy carbon electrode. The redox mechanism of NHDC at the modified electrode is deduced by jointly studying variation of scan rate and pH of solution, that is, one-proton and two-electron oxidation process is first occured and then two-proton and two-electron redox reactions are performed. The experimental conditions such as supporting electrolyte, pH, ratio of ZIF-8 to ErGO and etc. are optimized. Finally, the LSV method was used to quantitatively detect NHDC in solution and milk samples, the results are satisfactory due to its good sensitivity, low detect limit, good anti-interference and repeatability performance. Although no NHDC was detected in the milk samples for testing, the favorable recoveries of 98.3%-106.3% also validate its application perspective.

Acknowledgements

This work was sponsored by Project of Wuhan Education Bureau (No. CXY2020001), Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (No. JDGD-202214), and Graduate Student Research and Innovation Fund of Jianghan University (2022).

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Funding

This work was supported by Project of Wuhan Education Bureau (No. CXY2020001), Opening Project of Key Laboratory of Optoelectronic Chemical Materials and Devices of Ministry of Education, Jianghan University (No. JDGD-202214), and Graduate Student Research and Innovation Fund of Jianghan University (2022).

Author Contribution

Liang Zhang finished all the experiments and wrote the original manuscript. Junhui Xu designed the research plan and prepared figure 1. Jing Qian prepared figures 2-3. Yazhen Wang guided all the research plan and modified the whole manuscript. All authors reviewed the manuscript.

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A novel detection method for neohesperidin dihydrochalcone (NHDC) based on ZIF-8/ErGO composite modified electrode in milk

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Abstract

Figures

1. Introduction

2. Experiment

2.2 Apparatus

2.3 ZIF-8 nanocrystals synthesized

2.4 Preparation of ZIF-8/ErGO modified electrodes

2.5 Electrochemical measurements

2.6 Pretreatment of milk sample

3. Results And Discussion

3.1 SEM characterization of ZIF-8, GO, ErGO, and ZIF-8/ErGO

3.2 Raman Characterization

3.3 Electrochemical impedance spectroscopy characterization

3.4 Electrochemical response of NHDC at different electrodes

3.5 Calculation of reaction kinetic constant

3.6 Optimization of experimental conditions

3.6.1 Effects of supporting electrolytes

3.6.2 Effects of ratio and volume of ZIF-8/ErGO

3.6.3 Effect of pH

3.6.4 Effect of enrichment time

3.7 Standard curve for NHDC detect

3.8 Anti-interference and reproducibility of ZIF-8/ErGO GCE

3.9 Detection of NHDC in milk samples

4. Conclusions

Declarations

References

Additional Declarations

Supplementary Files

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