A nano-sized Cu-MOF with high peroxidase-like activity and its potential application in colorimetric detection of H₂O₂ and glucose

Abstract

Peroxidase widely exists in nature and can be applied for the diagnosis and detection of H₂O₂, glucose, ascorbic acid and other aspects. However, the natural peroxidase has low stability and its catalytic efficiency is easily affected by external conditions. In this work, a copper-based metal–organic framework (Cu-MOF) was prepared by hydrothermal method, and characterized by means of XRD, SEM, FT-IR and EDS. The synthesized Cu-MOF material showed high peroxidase-like activity and could be utilized to catalyze the oxidation of o-phenylenediamine (OPDA) and 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The steady-state kinetics experiments of the oxidation of OPDA and TMB catalyzed by Cu-MOF were performed, and the kinetic parameters were obtained by linear least-squares fitting to Lineweaver–Burk plot. The results indicated that the affinity of Cu-MOF towards TMB and OPDA was close to that of the natural horseradish peroxidase (HRP). The as-prepared Cu-MOF can be applied for colorimetric detection of H₂O₂ and glucose with wide linear ranges of 5 to 300 μM and 50 to 500 μM for H₂O₂ and glucose, respectively. Furthermore, the specificity of detection of glucose was compared with other sugar species interference such as sucrose, lactose and maltose. In addition, the detection of ascorbic acid and sodium thiosulfate was also performed upon the inhibition of TMB oxidation. Based on the high catalytic activity, affinity and wide linear range, the as-prepared Cu-MOF may be used for artificial enzyme mimics in the fields of catalysis, biosensors, medicines and food industry.

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1. Introduction

Metal–organic frameworks (MOFs) are crystalline porous framework materials that are formed with organic ligands and metal ions through coordination bonds with a periodic network structure.¹ MOFs have been widely used in catalysis,² sensing,³ gas adsorption and separation,⁴ luminescence⁵ and other fields due to their large specific surface area, unsaturated sites, and structural and functional diversity.^6–8 MOFs materials have been shown to have great application prospects with more and more kinds of MOFs and their composite materials having been discovered. In the field of catalysis, MOFs with high catalytic efficiency have been reported. Qin et al. found that hollow mesoporous MOF exhibited superior catalytic performance when loading Pd nanoparticles toward benzyl alcohol oxidation.⁹ Furthermore, Muhammad Fiaz et al. synthesized high efficient oxygen evolution reaction (OER) catalyst NiS@MOF-5, which can be coated on Ni-foam to form NiS@MOF-5/NF and showed OER catalytic activity and excellent stability.¹⁰ Besides, Nguyenet et al. found that Ni-MOF-74 possessed ultrahigh catalytic activity for the arylation of azoles.¹¹

Enzyme, also called biocatalyst, has the characteristics of high efficiency, specificity and mild reaction conditions.^12,13 Peroxidase is a kind of enzyme, which is widely existed in nature and can participate in the metabolism of organisms,¹⁴ it can also be used for the diagnosis and detection of H₂O₂, glucose, ascorbic acid.^15,16 However, natural enzymes usually have drawbacks of the high cost of preparation and purification, special storage conditions and low stability.^15,17–20 The enzyme activity can only be best performed under suitable conditions, such as optimal temperature and acid-alkali conditions.^13,21,22 Therefore, people begin to pay attention to the research of artificial mimic peroxidase.^23–27

It has been reported that Fe₃O₄ could catalyze the oxidation of TMB and OPDA to generate blue and orange color reactions, respectively.²⁸ Since then, many researchers have carried out research on the application of Fe₃O₄ mimic peroxidase in the field of glucose detection.^29,30 Later, it has been discovered that Co₃O₄ nanoparticles,³¹ V₂O₅ nanowires,³² CuO nanoparticles,³³ MnO₂ microspheres³⁴ and Ag₂O,²¹ etc. also have intrinsic catalytic activity on classic peroxidase substrates in the presence of H₂O₂. At present, colorimetry is the most widely used method for glucose detection. The principle is that glucose oxidase (GOX) catalyzes the oxidation of glucose and the reduction of O₂ to H₂O₂, then the simulated peroxidase catalyzes H₂O₂ to produce hydroxyl free groups to oxidize substrates (TMB, DAB, OPDA, etc.) to produce color reaction.³⁵

As a peroxidase mimetic enzyme, MOFs have substantive applications in colorimetric detection of some substances such as H₂O₂ and glucose. It has been reported that some iron-containing MOFs, such as MIL-53(Fe),³⁶ MIL-88(Fe)³⁷ and MIL-68(Fe)³⁸ possess the properties of peroxidase mimics. In the presence of H₂O₂, the hydrothermally synthesized MIL-53(Fe) can catalyze the oxidation of TMB and OPDA, which has been applied to the detection of actual samples such as glucose and serum with a good linear range and selectivity. Later, it was found that precious metals such as Au and other metals,³⁹ and composite metals (such as bimetallic) also have the catalytic activity of mimetic enzymes,⁴⁰ which makes MOFs as a popular new material in the simulation of peroxidase.

Several Cu-MOFs were reported previously that different organic compounds were used as coordination ligands, such as uric acid,⁴¹ 2-aminoterephthalic acid⁴² and 1,10-phenanthroline-2,9-dicarboxylic acid.⁴³ In this work, we prepared a Cu-MOF by using a simple hydrothermal method with 1,3,5-benzenetricarboxylic acid (H₃BTC) as ligand. The peroxidase-like activity of the as-prepared Cu-MOF was investigated by the oxidation reaction of TMB and OPDA with H₂O₂. The Cu-MOF showed high peroxidase-like activity, which can be used for catalyzing OPDA and TMB to generate colored products in the presence of H₂O₂. To investigate the possible oxidation mechanism of the Cu-MOF, typical Michaelis–Menten curves were obtained through steady-state kinetic experiments. Furthermore, the color reaction of TMB with H₂O₂ can be inhibited by some reductive substances. Based on these findings, we obtained satisfied results with the Cu-MOF for colorimetric detection of H₂O₂, ascorbic acid, sodium thiosulfate and glucose.

2. Experimental

2.1. Materials

1,3,5-Benzenetricarboxylic acid (H₃BTC) and TMB were obtained from Shanghai Titan Scientific Co, Ltd (Shanghai, China). OPDA, hydrogen peroxide (H₂O₂, 30%), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), sodium acetate (NaAc), acetic acid (HAc), ethanol absolute, phosphate buffered saline (PBS), sodium hydroxide (NaOH), hydrochloric acid (HCl), GOX, glucose, maltose, lactose, sucrose were obtained from Chendu Kelong Chemical Reagent Company. All the reagents were of analytical reagent grade and all the aqueous solutions were prepared with deionized water.

2.2. Characterizations

X-ray powder diffraction (XRD) spectroscopy was performed on Rigaku Dmax/Ultima LV X-ray powder diffractometer. Fourier transform infrared (FT-IR) spectrometer (Nicolet-6700) was applied to record FTIR spectroscopy. Scanning electron microscopy (SEM) image and energy dispersive X-ray spectroscopy (EDS) image were taken by a Hitachi S4800 scanning electron microscope.

2.3. Cu-MOF preparation

The Cu-MOF was prepared by using hydrothermal method. Briefly, 0.45 g (2.14 mmol) H₃BTC was dissolved in 48 mL absolute ethanol and stirred for 10 min. Next, addition of 0.75 g (3.1 mmol) of Cu(NO₃)₂·3H₂O to the above solution and continued stirring for 10 min. Then, transferred the mixed solution into a Teflon-lined stainless autoclave. After heated at 120 °C for 12 h and cooled down to the room temperature, collected the Cu-MOF by centrifugation and washed 3 times with absolute ethanol, then dried in a vacuum drying-oven for 24 h at 60 °C to obtain the target product.⁴⁴

2.4. Peroxidase-like activity

The catalytic oxidation of TMB by H₂O₂ was performed in 100 mM acetate buffer (pH 4.0) in the presence of Cu-MOF catalyst. Briefly, 2.4 mL of 0.06 mg mL⁻¹ Cu-MOF (Cu-MOF solid was dispersed in acetate buffer by ultrasonication), 300 μL TMB (0.3 mM, dissolve the DMF) and 300 μL H₂O₂ (0.6 mM) were mixed and reacted for 20 min at 30 °C. The maximum absorption wavelength of TMB oxidized product (652 nm)⁴⁵ was measured with a UV-Vis spectrophotometer. For OPDA oxidation, 1 mL of 0.06 mg mL⁻¹ Cu-MOF solution (Cu-MOF solid was dispersed in deionized water), 1 mL OPDA (0.4 mM, dissolve the deionized water) and 1 mL H₂O₂ (0.6 mM) were reacted for 20 min at 30 °C. The maximum absorption wavelength of OPDA oxidized product (416 nm)¹⁵ was measured by a UV-Vis spectrophotometer.

2.5. Cu-MOF kinetics measurements

First, 6 mg Cu-MOF solid was dispersed in 100 mL acetate buffer (pH 4.0) to prepare 0.06 mg mL⁻¹ Cu-MOF solution. The concentration of the TMB was fixed at 0.2 mM and a series of H₂O₂ with different concentrations (10 mM–90 mM) were prepared. During the reaction, 2.6 mL Cu-MOF solution, 0.1 mL H₂O₂ solution and 0.3 mL TMB were mixed with a total volume of 3 mL at 30 °C. The reaction product was sampled at regular intervals and spectroscopically detected at 652 nm by using a UV-Vis spectrophotometer. Similar experiments were performed by varying the concentration of TMB (0.1 mM–0.9 mM) with fixed concentration of H₂O₂ (3.5 mM).

For OPDA oxidation, 6 mg Cu-MOF solid was dispersed in 100 mL 0.1 M PBS buffer (pH 7.4) to prepare 0.06 mg mL⁻¹ Cu-MOF solution. The concentration of OPDA was fixed at 0.4 mM with a series of concentration of H₂O₂ solutions (0.02 mM–0.6 mM). During the reaction, 1 mL Cu-MOF solution, 1 mL H₂O₂ and 1 mL OPDA were mixed with a total volume of 3 mL at 30 °C. The reaction product was sampled at regular intervals and spectroscopically detected at 416 nm by a UV-vis spectrophotometer. Similar measurements were carried out by varying the concentration of OPDA (0.05 mM–1 mM) with fixed concentration of H₂O₂ (0.6 mM). By monitoring the absorbance at 652 nm and 416 nm for TMB and OPDA oxidation, the steady-state reaction rates were obtained with the Lambert Beer's law: A = ε × b × C. A is absorbance which can be measured; ε is the molar extinction coefficient of OPDA oxidation product (ε = 16.7 mM⁻¹ cm⁻¹)⁴⁶ and TMB oxidation product (ε = 39 mM⁻¹ cm⁻¹),⁴⁷ b is the path length of light (b = 1 cm). The rate V can be calculated by the change of concentration with time: V = Δc/Δt. Michaelis–Menten equation, V = V_max[S]/(K_m + [S]), was used to obtain the apparent kinetic parameters by nonlinear least square fitting the absorbance data. The V_max is the reaction velocity of an enzyme with saturated substrate. K_m is so called Michaelis constant, which represents the substrate concentration when enzymatic reaction velocity V reaches the half of the V_max. Usually, the V_max and K_m values can be determined by Lineweaver–Burk double reciprocal model (1/V = (K_m/V_max)·(1/[S]) + 1/V_max). A straight line is formed by plotting 1/V as a function of 1/[S], and the intercept of this line on X axis represents −1/K_m, and Y axis represents 1/V_max.

The experimental procedures of the effects of pH, catalyst concentration and reaction temperature on the peroxidase-like activity of Cu-MOF, and the detection of H₂O₂, ascorbic acid, sodium thiosulfate and glucose were similar to the above-mentioned experimental process (see ESI† for details). The data presented in this work were the averages of at least two measurements with an error less than 5% unless otherwise stated.

3. Results and discussions

3.1. Characterizations

The XRD pattern of the Cu-MOF is shown in Fig. 1. The characteristic peaks at 2θ = 6.7°, 9.5°, 11.6°, 13.4°, 14.9°, 17.5°, 19.1°, 25.9° and 39.1° correspond to the (200), (220), (222), (400), (420), (511), (440), (731) and (882) crystal planes.^48–50 The narrow and sharp peaks and the main peaks are coincided well with the simulated peaks, which prove that the synthesized material is the target product. The SEM image shows that the Cu-MOF sample is mainly composed of polyhedron, and some irregular particles can also be seen on the surface of the sample (Fig. 2). The FTIR spectrum of Cu-MOF is shown in Fig. S1,† the peaks at 1370 cm⁻¹ can be attributed to the aromatic ring extension. 690–900 cm ⁻¹ were assigned to the out-of-plane bending vibration of aromatic hydrocarbons C–H.⁵¹ The absorption bands at 600–800 cm⁻¹ are due to lattice vibrations of Cu–O, Cu–O–Cu and O–Cu–O. The stretching vibration of benzene ring is at 1579 cm⁻¹, whereas the peak at 3404 cm⁻¹ can be ascribed to water molecules and hydroxyl groups on the surface of the sample.^51–55 The energy dispersive spectrum (EDS) and element mapping indicate that Cu, O and C elements are coexisted and distributed evenly in the sample (Fig. S2†).

Fig. 1 The XRD pattern of Cu-MOF.

Fig. 2 The SEM images of Cu-MOF with different magnifications.

3.2. Peroxidase activity

The peroxidase-like activity of the Cu-MOF was evaluated by the catalytic reaction of OPDA and TMB with H₂O₂. Fig. 3A shows that the Cu-MOF can catalyze H₂O₂ to oxidize OPDA at 30 °C, which produce a color product since the solution become yellow after the reaction (Fig. 3A, inset). The oxidation product of OPDA shows an absorption band at wavelength of 416 nm in the visible region (Fig. 3A). Besides, the Cu-MOF can also catalyze the oxidation of TMB by H₂O₂, and the oxidation product of TMB exhibited a broad absorption band at 652 nm in the visible region and a peak at 371 nm in the UV region (Fig. 3B). The color of the TMB solution changed to blue after oxidation by H₂O₂ (Fig. 3B, inset). The absorption peaks at 416 nm and 652 nm for OPDA and TMB solutions, respectively, were not observed in the UV-Vis spectra without addition of Cu-MOF (Fig. 3A and B), and the corresponding solutions were still colorless (the little glass bottles on the right of the insets in Fig. 3A and B, respectively). These results proved that the colored products generated by the reaction of OPDA and TMB with H₂O₂ were indeed catalyzed by the Cu-MOF material.

Fig. 3 The UV-Vis spectra of OPDA (A) and TMB (B) (insets: photographs of OPDA and TMB oxidized with H₂O₂) with and without Cu-MOF material. (C and D) Are the corresponding reaction equations for Cu-MOF catalyzed OPDA and TMB, respectively.

Fig. 3C and D show the corresponding reaction equations for Cu-MOF catalyzed OPDA and TMB oxidation, respectively. In the presence of Cu-MOF as catalysts, OPDA and TMB are oxidized by H₂O₂, producing a yellow-colored dimer product (2,3-diaminophenazine) (Fig. 3C) and a blue oxidized product complex (Fig. 3D), and H₂O₂ is reduced to H₂O simultaneously. The 371 and 652 nm bands in the absorption spectrum of TMB oxidation product are a charge transfer complex consisting of the diamine (TMB) as a donor and the diimine dication (TMB²⁺) as an acceptor.^56,57

3.3. Experimental condition optimization

Natural enzymes are easily inactivated under extreme conditions such as strong acid, strong base and high temperature. Herein, the effect of pH, temperature, and catalyst concentration were investigated for the peroxidase activity. First, we explored the effect of pH. For the OPDA, the reaction activity increased from pH 3.0 to pH 4.5, while decreased with further pH increase up to 9.0 (Fig. 4A), which similar to that of the peroxidase activity of HRP effected by pH. While for the TMB, the reaction activity increased from pH 3.0 to pH 4.0, while decreased with further increasing of the pH (Fig. 4B). We further investigated the effect of temperature on the peroxidase-like activity of Cu-MOF. It can be seen that the catalytic activity of Cu-MOF increased from 25 °C to 70 °C when oxidation of OPDA with H₂O₂ (Fig. 4C) and the activity of the enzyme was not lose even up to 70 °C. While the highest activity was at 30 °C for TMB oxidation, and the color of the oxidation product faded marvelously at 45 °C (Fig. 4D), which indicated that the enzyme had been inactivated. Finally, we studied the catalyst concentration effect and the results are shown in the Fig. 4E and F. The activity of OPDA oxidation increased linearly with the increase of catalyst between 0.03 and 0.06 mg mL⁻¹, which reached maximum at 0.06 mg mL⁻¹ and then tends to be flat. However, the concentration of catalyst had little effect on the activity for the oxidation of TMB with the activity higher than 70% between 0.03 and 0.09 mg mL⁻¹.

Fig. 4 The effect of pH for oxidation of OPDA (A) and TMB (B), temperature effect for oxidation of OPDA (C) and TMB (D), and the effect of catalyst concentration for oxidation of OPDA (E) and TMB (F).

3.4. Steady-state kinetics study

Steady-state kinetics assays were carried out to explore the mechanism of the peroxidase-like activity of the synthesized Cu-MOF. The kinetic data were measured by fixing concentration of H₂O₂ and varying the OPDA (or TMB) concentration or vice versa with Cu-MOF. The formation rates of the OPDA and TMB oxidized products were monitored from the increasing of the absorbance at 416 nm and 652 nm, respectively. The Fig. 5A showed the reaction rate of Cu-MOF peroxidase gradually increased with the increase of H₂O₂ when fixing OPDA. While when the H₂O₂ increases to a certain extent, the increasing rate of the reaction rate becomes slower and gradually flattens out. Secondly, when the concentration of H₂O₂ is fixed, the reaction rate of Cu-MOF peroxidase gradually increases with the increase of OPDA (Fig. 5B). However, when the concentration of OPDA increases to a certain extent, the reaction rate also gradually slows down. The fitting parameters (K_m and V_max) could be obtained through hyperbola curve fitting, which were shown in the Fig. 5C and D.

Fig. 5 Steady-state kinetics for the reaction of OPDA with H₂O₂ in the present of Cu-MOF. (A and C) Fixed OPDA concentration (0.4 mM) and varied H₂O₂ concentration (0.02 mM–0.6 mM); (B and D) fixed H₂O₂ concentration (0.6 mM) and varied OPDA concentration (0.05 mM–1 mM). The curves in panels A and B were obtained by nonlinear least square fitting to Michaelis–Menten equation, V = V_max·[S]/(K_m + [S]), and the lines in panels C and D were obtained by linear least square fitting to Lineweaver–Burk double reciprocal model, 1/V = (K_m/V_max)·(1/[S]) + 1/V_max.

The steady-state kinetics of TMB oxidation by H₂O₂ catalyzed with Cu-MOF is shown in Fig. 6A. The reaction rate of TMB oxidation gradually increased with the increasing of H₂O₂ when fixing TMB. While when the H₂O₂ increases to a certain extent, the increasing of the reaction rate becomes slower and gradually flattens out. Secondly, when the concentration of H₂O₂ is fixed, the reaction rate of Cu-MOF peroxidase gradually increases with the increase of TMB (Fig. 6B). However, when the concentration of TMB increases to a certain extent, the reaction rate also gradually slows down. By using the Lineweave–Burk double-reciprocal model to fit the data in the Michaelis curve, the straight lines can be obtained as shown in the Fig. 6C and D.

Fig. 6 Steady-state kinetics for the reaction of TMB with H₂O₂ in the present of Cu-MOF. (A and C) Fixed TMB concentration (0.2 mM) and varied H₂O₂ concentration (10 mM–90 mM); (B and D) fixed H₂O₂ concentration (3.5 mM) and varied TMB concentration (0.1 mM–0.9 mM). The curves in panels A and B and the lines in panels C and D were obtained as stated in Fig. 5.

The kinetic parameters K_m and V_max for the catalyst of Cu-MOF were calculated through the intercept and slope in the straight line. Tables S1 and S2† list the K_m and V_max values of several catalysts for substrates OPDA, TMB and H₂O₂. K_m value is one of the characteristic constant of enzymes, which indicates the affinity between enzyme and substrate. The higher the K_m value, the smaller the affinity between enzyme and substrate. These results showed that the K_m values of the Cu-MOF for OPDA and TMB were 0.54 mM and 0.456 mM, respectively. And the K_m values of natural HRP for OPDA and TMB were 0.59 mM and 0.434 mM, respectively.^28,58 Therefore, the affinity of Cu-MOF was similar to that of natural HRP, indicating it can be used as an artificial peroxidase. Through further comparison, we found that the affinity of Cu-MOF for OPDA and TMB is better than some of the published peroxidase mimetic materials, such as the affinity of Fe₃O₄@Cu@Cu₂O to OPDA and the affinity of Cu NCs or MoO₂ to TMB (Tables S1 and S2†).

3.5. Colorimetric detection of H₂O₂, ascorbic acid and sodium thiosulfate

The oxidation of TMB by H₂O₂ via the catalysis of Cu-MOF to generate a blue product, and the reaction rate was proportional to the concentration of H₂O₂. Based on this principle, a colorimetric method to detect H₂O₂ was established. As shown in Fig. 7A, the absorbance at 652 nm increased with the concentration increasing of H₂O₂ from 5 μM to 400 μM. A well linear relationship can be achieved between the absorbance intensity and H₂O₂ concentration from 5 μM to 300 μM (Fig. 7B). The detection limit was 4.6 μM and the correlation coefficient was 0.997. The detection limit was determined by LOD = KS₀/S. K is the numerical factor chosen on the basis of the confidence level desired. S₀ is the standard deviation⁵⁹ of the blank measurements (n = 11, K = 3), while S is the slope of the calibration curve. Table S3† list several peroxidase mimics for colorimetric detection of H₂O₂. According to the Table S3,† the linear range of the as-prepared Cu-MOF is wider than some of the published peroxidase mimics such as CuS-GNS,⁶⁰ and the detect limitation is also lower than some reported peroxidase mimics such as MnO₂,⁵⁸ indicated that Cu-MOF is suitable for the colorimetric detection of H₂O₂.

Fig. 7 Colorimetric detection of H₂O₂. Dose–response curve (A) and linear calibration plot (B) for the detection of H₂O₂ with Cu-MOF.

Moreover, it was found that the color reaction of TMB with H₂O₂ could be inhibited by ascorbic acid (AA) and sodium thiosulfate (Na₂S₂O₃). Accordingly, a colorimetric detection method of AA and Na₂S₂O₃ was also developed based on the color reaction inhibition (ESI, Fig. S3 and S4†). It should be pointed out that the detection of AA and Na₂S₂O₃ is not specific, for other reductive substance such as Na₂SO₃ can also inhibit the color reaction of TMB with H₂O₂.^21,61

3.6. Glucose detection

GOX is a typical oxidoreductase that can be used to catalyze the oxidation of glucose to produce glucono-δ-lactone and H₂O₂.⁶² It can be used in clinical diagnosis, rapid and accurate determination of glucose content in body fluid, which provides reliable data for doctors to accurately judge the patient's condition.^37,63,64 As shown in the Fig. 8A and B, the absorbance at 652 nm for TMB oxidation increased with the glucose concentration from 5 μM to 500 μM. A well linear relationship can be obtained between the intensity of absorbance and the glucose concentration from 5 μM to 500 μM. The detection limit is 4.7 μM and the correlation coefficient is 0.993.

Fig. 8 The colorimetric detection of glucose. Dose–response curve (A) and linear calibration plot (B) for glucose detection with Cu-MOF. (C) Determination of the selectivity detection for glucose with 300 μM glucose, 2 mM sucrose, 2 mM lactose, and 2 mM maltose. (D) A proposed reaction mechanism for detection of glucose.

To explore the selectivity of glucose detection, we performed the colorimetric reaction by replacing glucose with other sugar species, such as sucrose, lactose or maltose. The results show that the absorbance increase at 652 nm is very low for the selected sugar species instead of glucose (Fig. 8C). Therefore, the developed colorimetric detection method has high selectivity for the detection of glucose.

The reaction mechanism for glucose detection is illustrated in Fig. 8D. First, GOX catalyzed the oxidation of glucose to gluconic acid and O₂ dissolved in the solution was reduced to H₂O₂. Then the produced H₂O₂ oxidized TMB to a blue-colored oxidation product (TMBox) via the catalysis of Cu-MOF. The production of TMBox is proportional to the amount of H₂O₂, and the yield of H₂O₂ is proportional to the amount of glucose. Therefore, the production of TMBox is proportional to the amount of glucose and glucose can be detected by measuring the amount of TMBox. It also can be seen from the reaction mechanism that the detection of glucose is achieved by measuring the content of H₂O₂, thus the selectivity of glucose detection depends on the selectivity of GOX to glucose, but not on the catalysis of Cu-MOF.

Table S4† listed several peroxidase mimic catalysts for the colorimetric detection of glucose. According to the Table S4,† the linear range of the as-prepared Cu-MOF is wider than that of NiFe₂O₄ MNPs and Ch-Ag NPs.^65,66 The detection limit is relatively large, which indicated that Cu-MOF has better linear range and detection limit to detect glucose. These results demonstrated that this colorimetric method may has potential application for determine glucose concentration.

4. Conclusion

In conclusion, we prepared Cu-MOF with high peroxidase-like activity by a simple hydrothermal method. In the presence of H₂O₂, Cu-MOF can catalyze the oxidation of OPDA and TMB. Kinetics studies suggested that the affinity of Cu-MOF to TMB and OPDA is close to that of HRP. Based on the color reaction of TMB with H₂O₂, colorimetric methods for the detection of H₂O₂ and glucose were established. Moreover, the detection of ascorbic acid and sodium thiosulfate was also performed upon the inhibition of TMB oxidation. These results indicate that Cu-MOF based peroxidase-like activity may be applied in the fields of catalysis, biosensor, food detection and environmental monitoring.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was funded by the Open Project of Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province (CSPC2016-3-2), and the Opening Project of Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education (LZJ2002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra04877e

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