English

• Diabetes is one of the most common and complex diseases in the world. According to the data of the World Health Organization, there are more than 420 million people with diabetes worldwide [1]. Severe diabetes can lead to complications, such as diabetic retinopathy, cardiovascular disease, kidney disease and emotional disorders. α-Glucosidase (α-Glu) is a membrane binding enzyme located in the epithelium of the small intestine that can hydrolyze oligosaccharides/disaccharides into glucose and is considered to be one of the most critical targets for treating diabetes. The inhibitors of α-Glu can inhibit its enzyme activity, slow down the hydrolysis process of carbohydrates, and thus avoid an increase in postprandial blood sugar levels, so α-Glu inhibitors could be used to control blood sugar levels [2]. Many efforts have been made to search for inhibitors of α-Glu, including synthetic drugs (voglibose and miglitol) [3,4] and natural products (acarbose, luteolin, rutin, etc.) extracted from microorganisms and plants [5,6]. Natural inhibitors have better hypoglycemic effects, lower costs and fewer side effects than synthetic drugs [7]. Accordingly, the establishment of an effective strategy to detect α-Glu activity as well as screen natural α-Glu inhibitors is of great significance for the clinical treatment of diabetes.

  Various nanomaterials have been used as nanozymes for the detection of biological enzymes (α-Glu [8], acid phosphatase [9,10], alkaline phosphatase [11–13], acetylcholinesterase [14], etc.). Metal oxide nanozymes have received special attention because of their acid-base and redox properties. Cerium(Ⅳ) oxide (CeO₂) nanozyme is one of the most widely used metal oxide catalysts. Similar to the catalytic cycle of natural oxidoreductases, CeO₂ nanozyme owns a mixed oxidation state of Ce⁴⁺ and Ce³⁺, and displays high redox behavior by means of switching the two states [15]. However, the catalytic activity of CeO₂ still cannot meet the requirements of high affinity and detection sensitivity. Therefore, improving the catalytic activity of CeO₂ is important for developing high-performance nanozymes. According to the literature reports [16–19], doping metal ions into CeO₂ nanozymes is an effective way to regulate the surface, structure, and catalytic activity of pure CeO₂. The Cr³⁺-doped CeO₂ (Cr-CeO₂) nanozymes [16] can increase the content of Ce³⁺ and make the activity of enzyme 3–5 times higher than that of pure CeO₂. Jampaiah et al. [17] found that Fe³⁺-CeO₂ nanorods exhibited enhanced peroxidase-like activity. The doping of Pr increases the oxygen vacancy content in CeO₂ nanocubes, and greatly improves the affinity of the material to 3,3′, 5,5′-tetramethylbenzidine (TMB) [18]. However, there is limited research on Ni doped CeO₂ (Ni-CeO₂) nanozymes [19]. As a new type of ionic liquid, deep eutectic solvents (DESs), can be used as reactants, solvents and templates [20], playing an enormous advantage in the morphology and crystallography of nanomaterials [21]. In our previous work [22], we demonstrated that nanomaterials prepared in DESs have better nanozyme properties than those prepared in water. Therefore, the study of metal-doped CeO₂ in DESs may provide a new way to prepare highly active nanozymes.

  In this work, we developed Ni-doped CeO₂ nanorods with oxidase-like activity in DESs composed of L-Pro and Ce(NO₃)₃·6H₂O. Based on the inhibitory effect of ascorbic acid (AA) on the color reaction of TMB catalyzed by Ni-CeO₂, 2-O-α-D-glucopyranosyl-L-ascorbic acid (AAG) was acted as substrate to construct a new approach for the colorimetric determination of α-Glu activity. When the activity of α-Glu was inhibited, the oxidation reaction of TMB resumed. Herein, acarbose was used as a control to screen three terpenoids (ursolic acid, uleanolic acid, glycyrrhetinic acid) isolated from natural products (Scheme 1). The results showed that ursolic acid and oleanolic acid had good inhibitory effects. The constructed colorimetric sensor platform is simple and cost-effective and is expected to become a simple method for screening inhibitors in natural products.

  Scheme 1

  

  Scheme 1.  Schematic diagram of the procedure for determining α-Glu activity and screening natural inhibitors.

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  X-ray diffraction (XRD) was used to study the crystalline phase structure of Ni-CeO₂. The XRD pattern of Ni-CeO₂ is similar to the standard XRD pattern of CeO₂ (PDF No. 4-593) [23]. At the same time, we prepared pure CeO₂ under the same conditions, and its XRD pattern is consistent with the standard pattern (Fig. 1A). The diffraction peaks of (111), (200), (220) and (311) crystal planes can be found in Ni-CeO₂ and CeO₂, indicating that the crystallinity of the product is good. No peaks were found from other compounds such as Ce(OH)CO₃, NiO and Ni(OH)₂, indicating the feasibility of the method for preparing Ni-CeO₂. Compared with pure CeO₂, the XRD peaks of Ni-CeO₂ in Fig. 1A are slightly shifted to a lower angle [24], indicating that the successful incorporation of Ni²⁺ with smaller atomic radius into CeO₂ causes lattice distortion.

  Figure 1

  

  Figure 1.  (A) XRD of Ni-CeO₂ and pure CeO₂. SEM images of Ni-CeO₂ (B) and CeO₂ (C). (D) TEM, (E) HRTEM, (F) SAED, (G) EDX spectrum, (H) STEM-EDS mapping images of Ni-CeO₂.

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  The morphologies of Ni-CeO₂ and CeO₂ are displayed in Figs. 1B and C. Scanning electron microscopy (SEM) images show that Ni-CeO₂ is a nanorod, while CeO₂ is an aggregated particle. This indicates that Ni(NO₃)₂·6H₂O plays a certain role in the ordered growth of Ni-CeO₂, possibly because the presence of metal salts affects the hydrogen bonding in DESs. The structure and morphology of Ni-CeO₂ were further studied by TEM (Fig. 1D). The transmission electron microscopy (TEM) results are in good agreement with the SEM image. As shown in Fig. 1E, the (111) and (220) crystal surfaces of Ni-CeO₂ were observed by high resolution TEM (HRTEM). The crystal face spacing were 0.31 nm and 0.52 nm, respectively. The SAED image (Fig. 1F) exhibits a clear diffraction ring corresponding to the (311), (220), (200) and (111) crystal planes of CeO₂.

  Energy dispersive X-ray (EDX) diagram (Fig. 1G) demonstrates the presence of the elements Ni and Ce. In addition, the element mapping (Fig. 1H) shows that Ni is doped into CeO₂, and that Ni and Ce are uniformly distributed. The Raman spectrogram in Fig. S1A (Supporting information) shows the characteristic peak of CeO₂ at 455 cm⁻¹. The TGA diagram shows that Ni-CeO₂ has good thermal stability up to 200 ℃ (Fig. S1B in Supporting information). When the temperature is less than 100 ℃, the mass loss is the evaporation of water adsorbed on the Ni-CeO₂ surface. The mass loss curve decreases significantly in the temperature range of 200–244 ℃, and the inflection point is about 214 ℃. The total mass loss is approximately 22%. The results show that Ni²⁺ is inserted into the lattice of CeO₂, where DESs are the reactant, solvent and template for the formation of Ni-CeO₂ nanorods.

  In order to further elucidate the composition of Ni-CeO₂ and the effect of doped Ni on the product, XPS testing was conducted on Ni-CeO₂ and CeO₂. In the full spectrum of Ni-CeO₂ (Fig. 2A), peaks of O 1s, Ce 3d, and Ni 2p could be observed, suggesting the presence of Ce, O, and Ni in our prepared material. The relative content of Ni and Ce in the product was determined by ICP-MS to be approximately 23:1. The two peaks located at 856 eV as well as 873 eV in Fig. 2B correspond to the binding energies of Ni 2p_3/2 and Ni 2p_1/2, respectively [24], proving the existence of Ni²⁺. As depicted in Fig. 2C, in the Ce 3d spectrum of Ni-CeO₂, 916.8, 899.37, 904.2 and 881.2 eV correspond to Ce⁴⁺(Ce 3d_3/2, Ce 3d_5/2) and Ce³⁺(Ce 3d_3/2, Ce 3d_5/2) respectively [25], and the content of Ce³⁺ is 3.2 times that of Ce⁴⁺. The content of Ce³⁺ in Ni-CeO₂ is significantly increased compared to that in CeO₂ (Ce³⁺/Ce⁴⁺= 0.27) (Fig. 2D), indicating that the addition of Ni promotes an increase in the ratio of Ce³⁺ to Ce⁴⁺. As displayed in Figs. 2E and F, the oxygen vacancy to adsorbed oxygen ratios of Ni-CeO₂ and CeO₂ are 6 and 0.5, respectively. Compared with CeO₂ (Fig. 2F), Ni-CeO₂ (Fig. 2E) has more abundant oxygen vacancies, which is consistent with the conclusion of a high content of Ce³⁺ in Ni-CeO₂.

  Figure 2

  

  Figure 2.  (A) XPS spectrum of Ni-CeO₂. (B) Ni 2p XPS of Ni-CeO₂. Ce 3d XPS spectra of (C) Ni-CeO₂ and (D) CeO₂. O 1s XPS of (E) Ni-CeO₂ and (F) CeO₂.

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  Of course, the factors affecting Ni-CeO₂ morphology, such as temperature, the amount of metal salt and reaction time were optimized in the experiment, and it was found that the material had the best morphology when the temperature was 80 ℃, the amount of metal salt was 10 mmol/L, and the reaction time was 3 h (Figs. S2–S4 in Supporting information). Next, TMB was acted as the chromogenic substrate to investigate the oxidase-like activity of Ni-CeO₂. As illustrated in Fig. 3A, colorless TMB can be oxidized to oxidized TMB by Ni-CeO₂, the solution color turns blue, and the absorbance value at 652 nm increases significantly. As displayed in Fig. 3B, under the same conditions, the rate of TMB oxidation catalyzed by Ni-CeO₂ was approximately twice that of CeO₂, indicating that Ni doping plays an important role in enhancing the oxidase-like activity of CeO₂. Ni-CeO₂ exhibits better oxidase-like activity, which is attributed to the presence of Ni promoting the production of Ce³⁺, resulting in a higher Ce³⁺/Ce⁴⁺ ratio in the product [17].

  Figure 3

  

  Figure 3.  (A) Absorption spectra of Ni-CeO₂+TMB, Ni-CeO₂, and TMB solutions. (B) The reaction rate of Ni-CeO₂ and CeO₂ over time. (C) Absorption spectra with and without AA in the Ni-CeO₂/TMB system. (D) The absorbance measured in the presence of other ions (200 mmol/L) and small molecules (20 mmol/L) with and without AA (20 mmol/L).

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  In order to acquire the optimal oxidase-like activity of Ni-CeO₂, the influence of the concentration of Ni-CeO₂, the concentration of TMB, the pH value and the reaction temperature on the oxidase-like activity of Ni-CeO₂ was explored by means of the control variable method (Fig. S5 in Supporting information). The optimal Ni-CeO₂ concentration, TMB concentration, pH value and reaction temperature were 2 mg/mL, 10 mmol/L, 4.0, and 25 ℃, respectively.

  In addition, to further understand the oxidase-like properties of Ni-CeO₂, the steady-state kinetic parameters of TMB catalyzed by Ni-CeO₂ were also investigated. As seen in Fig. S6 in Supporting information, the K_m of Ni-CeO₂ obtained from the Lineweaver-Burk curve is 1.912 mmol/L, which is equivalent to or even smaller than the oxidase-like materials reported in the literature [26,27], indicating that Ni-CeO₂ has good binding ability to TMB.

  AA, as an antioxidant and free radical scavenger, can effectively inhibit the TMB oxidation catalyzed by Ni-CeO₂. As can be seen from Fig. 3C, after injecting AA into the Ni-CeO₂/TMB system, the absorbance of the solution decreases sharply at 652 nm, indicating that this strategy is feasible for the detection of AA. Considering that the determination of AA may be interfered by other ions and small molecules, the effects of Na⁺, K⁺, Ca²⁺, D-Glu, Gal, Fru, Gly-and Arg-on the detection system were studied. As presented in Fig. 3D, other metal ions and small molecules do not cause significant changes in absorbance values and do not interfere with the detection of AA. Thus, the analytical performance of Ni-CeO₂ for the determination of AA was investigated. It was clearly observed that the absorbance at 652 nm decreased with the increasing AA concentration (Fig. 4A), and a good linear relationship was obtained in the range of 2.0 µmol/L to 100 µmol/L between the change in absorbance and AA concentration (Fig. 4B). The LOD was computed to be 1.18 µmol/L. The analytical performance of the established methods is comparable to or superior to other reports (Table S1 in Supporting information) [9,28–35].

  Figure 4

  

  Figure 4.  (A) Absorption spectra of the Ni-CeO₂/TMB system with different concentrations of AA. (B) The linear relationship between AA concentration and ∆A at 652 nm (∆A = A₀ - A, where A₀ and A stand for the absorbance values of the solution at 652 nm in the absence and presence of AA, respectively). (C) Absorption spectra of the system with various α-Glu concentrations. (D) The linear relationship between ∆A and the log value of α-Glu activity. (∆A = A₀ - A, where A₀ and A represent the absorbance values of the solution at 652 nm in the absence and presence of α-Glu, respectively).

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  To assess the practicality and accuracy of our established method for AA in real samples, the detection of AA in human serum, urine and various fruit samples was further investigated. First, both human blood, urine and various juice samples from fresh fruits were centrifuged at 4500 rpm for 10 min and then the supernatants were collected, separately. The obtained human serum was diluted 5-fold, urine sample was diluted 50-fold, and various juice samples were diluted 100-fold. Subsequently, the standard addition methods were carried out. As listed in Table S2 (Supporting information), the initial content of AA in human serum is approximately 33.51 µmol/L, and the detected AA in fresh lemons, navel oranges and grapes were 72.85, 32.36, and 17.23 µmol/L, respectively. The spiked experimental results show that the recoveries are in the range of 95.98%–105.3%, with relative standard deviations (RSDs) less than 4.30%, which proved that the potential applicability and accuracy of our established strategy for AA assay in real samples.

  AA can be produced via α-Glu and AAG. Therefore, the proposed strategy, which sensitively responds to AA, can also be applied to measure the activity of α-Glu. As shown in Fig. S7 (Supporting information), AAG and α-Glu did not interfere with the detection system of Ni-CeO₂/TMB. When AAG and α-Glu exist simultaneously in the above system, AAG will be hydrolyzed to release AA, resulting in the reduction of oxidized TMB (ox-TMB) to TMB, with weakened absorbance at 652 nm [12]. Therefore, by means of measuring the absorbance reduction of the solution at 652 nm, the quantitative detection of α-Glu activity can be achieved. As illustrated in Fig. 4C, as the activity of α-Glu increased from 0.1 mU/mL to 500 mU/mL, the AA produced by α-Glu hydrolysis of AAG gradually increased, resulting in a gradual decrease in the maximum absorption value of the solution at 652 nm. Fig. 4D reveals that there is a good linear relationship between the change value of absorbance and the logarithm of the value of α-Glu activity with a detection limit of 0.050 mU/mL. The analytical performance of determining α-Glu activity in this work was better than that of many previously reported methods (Table S3 in Supporting information) [36–43].

  Because α-Glu inhibitors are related to the treatment of type Ⅱ diabetes, an increasing number of researchers have focused on the study of α-Glu inhibitors, especially to screen low-toxicity α-Glu inhibitors from natural products. Acarbose, a natural inhibitor provided by actinomycetes and a commercial antidiabetic drug [40], was used as a model analyte to investigate the approach's feasibility. As shown in Fig. S8A (Supporting information), acarbose does not affect the oxidase-like activity of Ni-CeO₂. When acarbose co-exists with AAG and α-Glu, it can reduce ox-TMB reduction caused by the hydrolysis product AA of AAG, indicating that this strategy can be applied for screening α-Glu inhibitors. In order to further demonstrate the applicability of our proposed approach, the effects of three terpenoids (ursolic acid, oleanolic acid and glycyrrhetinic acid) in ligustrum, glycyrrhetinic acid and glycyrrhetinic acid) on the activity of Ni-CeO₂ oxidase were investigated. As shown in Figs. S8B–D (Supporting information), ursolic acid, oleanolic acid and glycyrrhetinic acid did not interfere with the Ni-CeO₂/TMB detection system, proving that this colorimetric strategy can be used to screen α-Glu inhibitors from natural products.

  Using 1 mmol/L acarbose as a control, the inhibitory effects of 1 mmol/L three terpenoids (ursolic acid, oleanolic acid, and glycyrrhetinic acid) on 100 µL of 100 mU/mL α-Glu were measured. The calculated inhibition rates are shown in Table S4 (Supporting information). Ursolic acid and oleanolic acid have better inhibitory effects, while glycyrrhetinic acid has weaker inhibitory effects. And the IC₅₀ value of acarbose was 268 µmol/L (Fig. S9 in Supporting information), which was similar to the data reported in the literature [42], indicating the reliability of this strategy in screening inhibitors. In addition, the IC₅₀ values of ursolic acid and oleanolic acid were 620 µmol/L and 702 µmol/L, respectively (Fig. 5).

  Figure 5

  

  Figure 5.  Relationship between inhibition rate and concentration of (A) ursolic acid and (B) oleanolic acid.

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  In conclusion, we constructed a facile approach for the fabrication of Ni-CeO₂ nanorods in DESs composed of L-Pro and Ce(NO₃)₃·6H₂O. It has been proven that Ni plays a crucial role in the morphology and catalytic properties of materials. The incorporation of Ni promotes the production of reduced Ce³⁺ and increases the proportion of Ce³⁺ and Ce⁴⁺ in the Ni-CeO₂ product. In addition, Ni-CeO₂ nanorods exhibit enhanced oxidase-like activity, which can achieve the assay of α-Glu activity and its natural inhibitor screening through colorimetric sensing. The results indicate that the synthesis strategy is simple and efficient, and can be further extended to the preparation of other transition metal-doped CeO₂. In addition, this constructed strategy also provides a convenient method to screen potential anti-diabetes drugs from natural products.

  Declaration of competing interest

  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.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22374159), Regional Collaborative Innovation Project of Xinjiang (No. 2020E01051), CAS-JSPS Cooperation and Exchange Program (No. E20489GHZ1) and the Youth Innovation Promotion Association CAS (No. 2021420).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109463.

  
  
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  Scheme 1  Schematic diagram of the procedure for determining α-Glu activity and screening natural inhibitors.

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  Figure 1  (A) XRD of Ni-CeO₂ and pure CeO₂. SEM images of Ni-CeO₂ (B) and CeO₂ (C). (D) TEM, (E) HRTEM, (F) SAED, (G) EDX spectrum, (H) STEM-EDS mapping images of Ni-CeO₂.

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  Figure 2  (A) XPS spectrum of Ni-CeO₂. (B) Ni 2p XPS of Ni-CeO₂. Ce 3d XPS spectra of (C) Ni-CeO₂ and (D) CeO₂. O 1s XPS of (E) Ni-CeO₂ and (F) CeO₂.

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  Figure 3  (A) Absorption spectra of Ni-CeO₂+TMB, Ni-CeO₂, and TMB solutions. (B) The reaction rate of Ni-CeO₂ and CeO₂ over time. (C) Absorption spectra with and without AA in the Ni-CeO₂/TMB system. (D) The absorbance measured in the presence of other ions (200 mmol/L) and small molecules (20 mmol/L) with and without AA (20 mmol/L).

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  Figure 4  (A) Absorption spectra of the Ni-CeO₂/TMB system with different concentrations of AA. (B) The linear relationship between AA concentration and ∆A at 652 nm (∆A = A₀ - A, where A₀ and A stand for the absorbance values of the solution at 652 nm in the absence and presence of AA, respectively). (C) Absorption spectra of the system with various α-Glu concentrations. (D) The linear relationship between ∆A and the log value of α-Glu activity. (∆A = A₀ - A, where A₀ and A represent the absorbance values of the solution at 652 nm in the absence and presence of α-Glu, respectively).

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  Figure 5  Relationship between inhibition rate and concentration of (A) ursolic acid and (B) oleanolic acid.

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•