Metal–organic framework MIL-53(Fe): facile microwave-assisted synthesis and use as a highly active peroxidase mimetic for glucose biosensing

Abstract

The octahedral structure of MIL-53(Fe) was facilely prepared by a microwave (MW)-assisted approach, and confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The MIL-53(Fe) MOFs were further characterized by thermo gravimetric (TG) analysis and fourier transform infrared (FTIR) spectroscopy. It is found that the as-prepared MIL-53(Fe) exhibits intrinsic peroxidase-like activity, and could catalytically oxidize 3,3′,5,5′-tetramethylbenzidine (TMB), ABTS and OPD by H₂O₂ to produce a typical coloured reaction. The Michaelis–Menten behavior of the as-prepared MIL-53(Fe) was studied. The K_m value of the as-prepared MIL-53(Fe) with H₂O₂ as the substrate was 0.03 mM, which was at least seven times lower than that of Fe-MIL–88NH₂ and hemin@MIL-53(Al)–NH₂. Interesting, the K_m values of the as-prepared MIL-53(Fe) with H₂O₂ and TMB as the substrates were both lower than those of the MIL-53(Fe) obtained by conventional electric (CE) heating-based solvothermal method. This is probably attributed to the purely octahedral structure and small sized crystals of the MIL-53(Fe) obtained by MW-based synthesis method, confirming that the MW-based synthesis method promised advantages of simplicity, fast crystallization and good phase selectivity. Results of electron spin resonance (ESR) experiments indicated that the as-prepared MIL-53(Fe) exhibited catalytic ability to decompose H₂O₂ into ˙OH radicals. On this basis, a simple, sensitive and selective method for glucose detection was developed by coupling the oxidation of glucose catalyzed by glucose oxidase (GOx). As low as 0.25 μM glucose could be detected with a linear range from 0.25–20 μM. The proposed method was successfully used to determine glucose in real human serum samples.

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1. Introduction

The nanozymes are a group of nanomaterial-based artificial enzymes and have received increasing interest owing to their stability under harsh conditions, low cost and resistance to denaturation compared with natural enzymes.¹ Ferromagnetic nanoparticles (NPs),² metal oxide NPs,³ metal sulfide,⁴ metal NPs,⁵ and carbon-based nanomaterials⁶ are considered within this group and have been successfully employed as enzyme mimics for the detection of biologically important substances such as hydrogen peroxide and glucose. This opens the door for the development of a nanoscaled platform for sensing applications in the bioanalytical field. The rapidly growing number of publications in this field proves that nanozymes are one of the potential next-generation artificial enzymes.¹

On the other hand, as an emerging class of ultraporous crystalline materials, metal–organic frameworks (MOFs) are well known for their various applications.⁷ The MOFs are constructed by polydentate bridging ligands and inorganic nodes (metals ions, metal clusters). The peculiar structure endows MOFs with intriguing properties, such as large specific surface area, tunable porosity, good thermal stability and uniform structured nanoscale cavities, making MOFs useful in small-molecule sensing, separation, catalysis, storage and release of molecules of interest and so on. In addition to their intriguing properties indicated above, MOFs have been recently reported to show peroxidase-like catalytic activity⁸ for colorimetric biosensing. However, research on MOFs as enzyme mimetic is rare. Up to now, only limited Fe(III)-based MOFs, namely Fe-MIL–88NH₂, PCN–222(Fe) with porphyrinic Fe(III) centers, MIL-53(Fe), MIL-68(Fe) and MIL-100(Fe) were reported to exhibit intrinsic peroxidase-like catalytic activity. For example, Liu et al. illustrated that Fe-MIL–88NH₂ showed typical Michaelis–Menten kinetics and good affinity to TMB (3,3′,5,5′-tetramethylbenzidine).^8a Ai et al. demonstrated that MIL-53, a typical iron-based MOF, showed intrinsic peroxidase-like activity and could catalyze the oxidation of TMB, o-phenylenediamine (OPD), and 1,2,3-trihydroxybenzene (THB) by H₂O₂.^8b Jiang's group reported the efficient peroxidase-like activity of two iron(III)-based MOFs, namely MIL-68 and MIL-100, which could catalyze the oxidation of different peroxidase substrates by H₂O₂.^8c However, these previous works are at the proof-of-concept stage. The analytical application of such MOFs with enzyme-like activity is far from fully developed and still in its infancy. Furthermore, in these studies, all the used MOFs were synthesized via a solvothermal method by conventional electric (CE) heating, usually requiring reaction time as long as several days and leading to the formation of hybrid crystal morphologies.⁹

In contrast, microwave (MW) irradiation is a promising alternative technique to allow the whole material to be heated rapidly and simultaneously, resulting in fast crystallization and higher yield. Due to a consequence of the intense localized heating, MW-based synthesis method can reduce reaction time from days and hours by classical heating to minutes and seconds.^10a Recently, microwave-assisted synthesis method has been proposed for preparation of MOFs.^10b,c

By virtue of microwave energy, here, we report on a microwave-assisted approach for the fast and facile preparation of MIL-53(Fe). The as-prepared MIL-53(Fe) was well characterized for their structure and morphology. For the first time, we found that, as compared with MIL-53(Fe) by CE-based solvothermal methods, the as-prepared MIL-53(Fe) by a microwave (MW)-based route showed much higher peroxidase-like catalytic activity towards the oxidation of TMB by H₂O₂. The results showed that MW-based synthesis method promised advantages including simplicity, fast crystallization, good phase selectivity and higher yield, which is of great significance to understanding the catalytic mechanism of MOFs and eventually utilizing the MOFs as artificial enzyme in practical applications.

2. Experimental section

2.1. Material and reagents

All chemicals used in this work were of analytical grade and used as received without further purification. Glucose, fructose, lactose, and maltose were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), p-phthalic acid (BDC), glucose oxidase (GOx, EC1.1.3.4.47, 200 U mg⁻¹), 3,3′,5,5′-tetramethylbenzidine (TMB), o-phenylenediamine (OPD), and 2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) diammonium salt (ABTS) were purchased from Sigma-Aldrich (St. Louis, MO) and stored in a refrigerator at 4 °C. H₂O₂, FeCl₃·6H₂O, acetic acid, sodium acetate, ethanol, N,N-dimethylformamide (DMF) were obtained from Chongqing Taixin Chemical Co. Ltd. (Chongqing, China). Serum samples were from the Ninth People's Hospital of Chongqing (Beibei, Chongqing). Ultra-filtration tubes with cutoff molecular weight of 30 kDa were purchased from Millipore Corporation (Billerica, MA 01821, USA). Ultra pure water was prepared in the lab.

2.2. Preparation of MIL-53(Fe)

The MIL-53(Fe) was prepared by the MW method^10c with a modification. Briefly, in a typical procedure, a mixed solution of 0.45 g (1.63 mmol) of FeCl₃·6H₂O, 0.137 g of BDC (0.827 mmol) and 10 mL of DMF was transferred to a Teflon tube. The resulting mixture solution was then transferred into a microwave reactor (MARS-5, USA) and heated to 150 °C in 5 min and held at 150 °C for 10 min. The MW power was controlled at 600 W over the whole process. After the reaction, the resulting orange suspension was centrifugated and the obtained product was purified by a double treatment in ethanol and DMF at 60 °C for 1 h. After drying in a vacuum at 50 °C, a orange power was obtained with a yield of about 30%.

2.3. Instrumentation

A MARS-5 microwave heating apparatus was used for the preparation of MIL-53(Fe). Absorption measurements were performed on a UV-2450 spectrophotometer (Shimadzu, Japan). The scanning electron microscope (SEM) image was taken on Hitachi model S-4800 field emission scanning electron microscope (Hitachi, Japan) with an accelerating voltage of 30 kV. The X-ray diffraction (XRD) pattern of the as-prepared MIL-53(Fe) was obtained by XD-3 X-ray diffractometer (PuXi, Beijing, China) under the conditions of nickel filtered Cu Kα radiation (λ = 0.15406 nm) at current of 20 mA and a voltage of 36 kV. The scanning rate was 4° min⁻¹ in the angular range of 5–50° (2θ). The thermogravimetric (TG) curve was obtained on a TA-STDQ600 (Texas Instruments Inc., New Castle, DE, USA) instrument. Fourier transform infrared (FTIR) spectra of the as-prepared MIL-53(Fe) were obtained using a Tenson 27 Fourier Transform Infrared spectrometer (Bruker, Germany).

2.4. Electron spin resonance (ESR)

The 120 μL samples were prepared by adding 10 μL of 30% H₂O₂, 50 μL of 150 mg L⁻¹ MIL-53(Fe), 20 μL of 0.2 M DMPO and proper amount of 0.2 M NaAc buffer (pH = 3.5) into a plastic tube in the presence and absence of 5 μL of 0.01 M TMB, respectively. The resulting sample solution was transferred to a quartz capillary tube and placed in the ESR cavity. DMPO was used to trap the ˙OH radicals to form the DMPO–˙OH spin adduct. The ESR spectra were obtained on a Bruker E 500 Electron Spin Resonance with microwave bridge X-band (receiver gain, 60 dB; modulation amplitude, 3 Gauss; microwave power, 10 mW; modulation frequency, 9.85 GHz; Sweep Time 40.96 s).

2.5. Robustness and stability of the as-prepared MIL-53(Fe) as enzyme mimic

The robustness of the as-prepared MIL-53(Fe) was evaluated in this study. To perform this experiment, the as-prepared MIL-53(Fe) was first incubated at a range of temperature from 4 to 80 °C (4, 8, 14, 20, 25, 30, 40, 50, 60, 70 and 80 °C) and a range of pH from 2.0 to 10.0 (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0) for 2 h, respectively. Then their activities were measured under the following conditions: 0.2 M NaAc buffer (pH 4.0), 40 °C, 0.1 mM H₂O₂, 0.05 mM TMB and 15 mg L⁻¹ MIL-53(Fe). The storage stability of the as-prepared MIL-53(Fe) was evaluated by measuring its catalytic activity in different storage time. Note that the relative catalytic activity was used as criterion to evaluate the effects of various factors on the peroxidase-like activity of the MIL-53(Fe). The maximum point in each curve was set as 100%. Hence, the relative activity was defined as the ratio of absorbance at target point to the maximum point.

2.6. General procedure for colorimetric detection, kinetic analysis and glucose analysis

In a typical process, the solution of hydrogen peroxide with a given concentration was added to TMB solution (final concentration of 0.05 mM) in the presence of MIL-53(Fe) solution (final concentration of 15 mg L⁻¹) and the 0.2 M NaAc buffer (pH 4.0). The mixture was incubated at 40 °C for 20 min. Then UV-vis spectra measurements and photographs were taken.

Kinetic measurements were carried out by monitoring the absorbance change at 652 nm in every minute.

In order to investigate the kinetic characteristics, the velocity changes of the used reaction with varying concentrations of TMB and a fixed concentration of H₂O₂ or vice versa were obtained. For this, the concentrations of H₂O₂ and TMB were fixed at 0.02 mM and 0.05 mM, respectively. The apparent kinetic parameters were calculated based on the eqn (1):

v = V_max[S]/(K_m + [S]) (1)

where v is the initial velocity, V_max is the maximal reaction velocity, [S] is the concentration of substrate (TMB or H₂O₂), K_m is the Michaelis constant and K_m approximates the affinity of the enzyme for the substrate. K_m and V_max could be obtained from hyperbola curve fitting of kinetic measurements.

For glucose detection, 0.1 mL of 1 mg mL⁻¹ GOx was added to the mixture of 0.1 mL of different concentrations of glucose and 0.5 mL of 0.2 M NaAc buffer (pH 7.0). After the mixture was incubated at 37 °C for 30 min, the obtained solution was mixed with 0.25 mL of 1 mM TMB, 0.5 mL of MIL-53(Fe) (150 mg L⁻¹), and 3.55 mL of 0.2 M NaAc buffer (pH 4.0) for another 20 min incubation at 40 °C. The resulting solution was used for standard curve measurement.

For glucose determination in serum, the samples were first treated by ultra filtration with 30 kDa Amicon cell at 3000 rpm for 30 min. The filtrate was diluted by 20 folds. And then 0.1 mL of the diluted solution was added into 0.5 mL of 0.2 M NaAc buffer (pH 7.0) and 0.1 mL of 1 mg mL⁻¹ GOx. The obtained mixture was treated by the same way as glucose standard. In the selectivity experiments, 0.1 mM maltose, 0.1 mM lactose, and 0.1 mM fructose were used instead of 0.02 mM glucose.

3. Results and discussion

3.1. Characterization of the MIL-53(Fe) prepared by microwave-assisted synthesis

The MIL-53(Fe) was prepared by a microwave-assisted approach. The morphology and structure of the synthesized MIL-53(Fe) were identified by SEM, XRD, TG analysis and FTIR. The SEM image showed that the synthesized MIL-53(Fe) had octahedron morphology with a dimension of about 250–850 nm (Fig. 1A). However, the SEM image of MIL-53(Fe) obtained by CE heating is not so homogeneous and large size material was obtained (Fig. S1A ESI†), which is probably due to concomitant nucleation and crystal growth under CE heating.^11a The powder XRD indicates the successful preparation of MIL-53(Fe) because the information it shows is similar to the simulated one (Fig. 1B). It is worth to noting that XRD spectra of as-prepared MIL-53(Fe) don't match exactly the peaks of the simulated pattern of MIL-53(Fe). This is probably caused by the breathing effect of the framework with different guest molecular inside. MIL-53(Fe) belongs to a flexible structure.^11b It has been reported that some solvents, including DMF and H₂O, can cause breathing effect and framework shrinkage probably through hydrogen bonding between the oxygen atoms in the MOFs and guest molecule,^11c,d leading to significant variations of the lattices with the appearance of new diffraction peak.^11c,e In our case, DMF and ethanol were used for the purification of MIL-53(Fe). Because the cages of MIL-53 show a better affinity for DMF molecules than that for ethanol,^11d it is hard for DMF to be removed completely in a short time treatment (immersion for 1 h). So, we speculate that DMF or polar H₂O molecule (evidenced by TG result in Fig. S1B ESI†) residue in the synthesized MIL-53(Fe) can possibly form OH⋯O hydrogen bonds with the oxygen atoms of the bridging carboxylate groups,^11c leading to the variations of the lattices with appearance of an extra peak in XRD patterns. The peak shifting may be related to the difference in the overall concentration of the reaction mixture, which is a decisive factor for MOF formation.^11f In fact, compared to the simulated XRD patterns of the MIL-53(Fe), there exist peak shifting in the previous reports due to the different reaction conditions.^10b,11e,g,h Fig. S1B (Fig. S1B ESI†) presents the TG curve of the as-prepared MIL-53(Fe). The weight-loss stage below 200 °C is due to a loss of water and DMF in the sample. Further weight loss above 300 °C is related to the breakdown of BDC,^12a confirming the formation of the MIL-53(Fe). The material keeps weight decreasing above 500 °C, which likely corresponds to the elimination of the BDC linkers from the framework.^12b FTIR spectroscopy (Fig. S1C ESI†) shows the characteristic vibrational bands of the framework –O–C–O– groups around 1510 and 1390 cm⁻¹, confirming the presence of the dicarboxylate within the obtained MIL-53(Fe).^8a,b

Fig. 1 (A) SEM image of MIL-53(Fe). (B) The XRD spectra of the as-prepared MIL-53(Fe). The simulated XRD pattern of MIL-53(Fe) was based on the corresponding CheckCIF file of MIL-53(Fe).

3.2. The peroxidase-like activity of the MIL-53(Fe)

The peroxidase-like activity of the MIL-53(Fe) was evaluated by the catalytical oxidation of typical peroxidase substrates (TMB, ABTS and OPD) by H₂O₂. When MIL-53(Fe) was added to three substrates in the presence of H₂O₂, a typical colour could be seen (Fig. 2). In contrast, there were minor color variances in the absence of MIL-53(Fe). Taking TMB as an example, a conspicuous ascension at 652 nm could be monitored. A_{652 nm} in MIL-53(Fe)–TMB–H₂O₂ system were at least 3-fold higher than that in the TMB–H₂O₂ and MIL-53(Fe)–TMB systems, suggesting that MIL-53(Fe) had significant peroxidase-like catalytic activity. In addition, the absorbance at 652 nm increased with increasing H₂O₂ concentration (Fig. S2 ESI†). Thus, the MIL-53(Fe) could be potentially used as an effective enzyme mimic catalyst in biochemical applications because H₂O₂ is the product of many enzymatic reaction of important biochemical substance, such as glucose.

Fig. 2 (A) Images of production of colour product upon successive addition of H₂O₂ and MIL-53(Fe) to TMB, ABTS and OPD solution at pH 4.0 (0.2 M acetate buffer). (B) The UV-visible absorption spectra of 0.05 mM TMB with and without 15 mg L⁻¹ of MIL-53(Fe) in the absence and presence of 0.1 mM H₂O₂ for 20 min reaction at pH 4.0 (0.2 M acetate buffer) and 40 °C.

Similar to horseradish peroxidase (HRP), the peroxidase-like catalytic activity of the MIL-53(Fe) was dependent on pH, temperature and concentration. First, the effect of solution pH from 2.0 to 12.0 was investigated. The result demonstrated that the catalytic behavior of the MIL-53(Fe) was dependent on the pH (Fig. S3 ESI†). The catalytic activity of MIL-53(Fe) increased with increase of pH of MIL-53(Fe) solution from 2.0 to 4.0. However, it decreased with increase of pH from 4.0 to 6.0. Thus, 4.0 was chosen as the optimum pH value of MIL-53(Fe) solution for subsequent study. Second, the temperature effect was studied from 26 °C to 60 °C. As can be seen, the maximum catalytic activity of MIL-53(Fe) was obtained at 40 °C (Fig. S4, ESI†). Last, the effect of the MIL-53(Fe) concentration was investigated over the range of 2–40 mg L⁻¹ (Fig. S5 ESI†). It was found that the catalytic activity of MIL-53(Fe) increased fastly with increasing MIL-53(Fe) concentration in the range of 2 to 6 mg L⁻¹. However, when the concentration of MIL-53(Fe) was in the range of 6 to 15 mg L⁻¹, less than 10% increase of catalytic activity was observed. When the concentration of MIL-53(Fe) was above 15 mg L⁻¹, the catalytic activity of MIL-53(Fe) remained stable. Finally, 15 mg L⁻¹ of the MIL-53(Fe) was chosen for subsequent experiments. In summary, the maximum catalytic activity of the MIL-53(Fe) was obtained under the following optimal conditions: pH 4.0, 40 °C and 15 mg L⁻¹ MIL-53(Fe).

The apparent steady-state kinetic parameters for MIL-53(Fe) were determined. In a certain range of H₂O₂ or TMB concentration, typical Michaelis–Menten curves were observed (Fig. 3). The K_m and V_max were calculated from Lineweaver–Burk plots and listed in Table 1. K_m is an indicator of the affinity of enzyme for a substrate. The smaller the value of K_m, the stronger is the affinity between the enzyme and the substrate. The K_m value of MIL-53(Fe) with H₂O₂ as the substrate was 0.03 mM, which was at least seven times lower than that of Fe-MIL–88NH₂ and hemin@MIL-53(Al)–NH₂. This implies that the as-prepared MIL-53(Fe) has higher affinity for H₂O₂ than Fe-MIL–88NH₂ and hemin@MIL-53(Al)–NH₂. Interesting, it is noted that the K_m value of the as-prepared MIL-53(Fe) with H₂O₂ as the substrate was lower than that of the MIL-53(Fe) obtained by CE-based solvothermal method. In particular, the K_m value of the as-prepared MIL-53(Fe) with TMB as the substrate was about four times lower than the K_m of the CE-based MIL-53(Fe). These findings suggested that the as-prepared MIL-53(Fe) had higher affinity for H₂O₂ and TMB than MIL-53(Fe) obtained by CE-based solvothermal method. Such a better catalytic performance is likely attributed to the purely octahedral structure of the as-prepared MIL-53(Fe) (Fig. 1A) as compared with MIL-53(Fe) by CE (Fig. S1A ESI†), showing that MW-based synthesis method promised advantage of fast crystallization and good phase selectivity. In addition, a better catalytic performance of the as-prepared MIL-53(Fe) is likely related to formation of small sized MIL-53(Fe) by MW irradiation because small crystals of porous MOF materials are effective in catalysis.^11a

Fig. 3 Steady-state kinetic assay of the as-prepared MIL-53(Fe). Reaction conditions: 15 mg L⁻¹ MIL-53(Fe), temperature: 40 °C, 0.2 M NaAc buffer (pH 4.0). (A) The concentration of TMB was 0.05 mM and the H₂O₂ concentration was varied. (B) The concentration of H₂O₂ was 0.02 mM and the TMB concentration was varied.

Table 1 Comparison of the apparent Michaelis–Menten constant (K_m) and maximum reaction rate (V_max) of MIL-53(Fe) and other MOFs

Catalyst	Km/mM		Vmax/10^−8 M s^−1		Ref.
	H2O2	TMB	H2O2	TMB
Fe-MIL–88NH2	0.206	0.284	7.04	10.47	8a
MIL-53(Fe) by CE	0.04	1.08	1.86	8.78	8b
Hemin@MIL-53(Al)–NH2	10.90	0.068	8.98	6.07	8e
MIL-53(Fe) by MW	0.03	0.28	0.96	4.48	This work

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The robustness of the as-prepared MIL-53(Fe) as enzyme mimic was evaluated at a range of temperatures (4–80 °C) and pH (2.0–10.0) for 2 h. As can be seen, the as-prepared MIL-53(Fe) was indeed found to remain stable activity over a wide range of pH and temperatures (Fig. S6 ESI†). This indicates that the as-prepared MIL-53(Fe) has high temperature resistance and wide pH adaptability. After one month storage, about 85% catalytic activity was held (Fig. S7 ESI†), showing good long-term storage stability. The long-term storage stability and robustness of the as-prepared MIL-53(Fe) MOFs makes them suitable for a broad range of applications in the area of the analytical biochemistry. The reproducibility of the catalytic performance among three MIL-53(Fe) MOFs prepared at different time is satisfactory (RSD among inter-batch is less than 4% (Table S1 ESI†)).

To explore the catalytic mechanism of MIL-53(Fe), the catalytic efficiency of MIL-53(Fe) was measured by varying concentrations of H₂O₂ and a fixed concentration of TMB or vice versa. Fig. 4 displays the double reciprocal plots of initial velocity against the concentration of one substrate in a certain range of concentrations of the second substrate. Clearly, the double reciprocal plots revealed the characteristic parallel lines of a ping–pong mechanism, implying that MIL-53(Fe) reacts with the first substrate and then releases the first product before reacting with the second substrate.

Fig. 4 Double reciprocal plots of activity of the as-prepared MIL-53(Fe) with the concentration of one substrate (H₂O₂ or TMB) fixed and the other varied. Other reaction conditions: 15 mg L⁻¹ MIL-53(Fe), temperature: 40 °C, 0.2 M NaAc buffer (pH 4.0).

Previous reports have indicated that ˙OH radicals were generated from the decomposition of H₂O₂ during the catalytic reaction of various nanozymes.^5h,l In order to confirm the generation of ˙OH radicals, an ESR experiment was conducted. As shown in Fig. 5, the ESR spectra in the MIL-53(Fe)–H₂O₂ system displayed a typical 4-fold characteristic peak of the DMPO–˙OH adduct with an intensity ratio of 1 : 2 : 2 : 1. However, the DMPO–˙OH adduct signal intensity of the MIL-53(Fe)–H₂O₂–TMB system is much lower than that of the MIL-53(Fe)–H₂O₂ system, indicating that the generated ˙OH radicals attack TMB molecules and react with TMB to produce color reaction (Fig. 5).

Fig. 5 ESR spin-trapping spectra of H₂O₂–DMPO–MIL-53(Fe) system in the absence and presence of TMB. Conditions: 33 mM DMPO, 833 mM H₂O₂, 0.42 mM TMB, 62.5 mg L⁻¹ MIL-53(Fe) and 0.2 M NaAc buffer.

3.3. Analytical characteristics and application of the proposed method for glucose detection

Under the optimized conditions, a linear relationship for hydrogen peroxide was observed over the range of 0.25 to 20 μM (r² = 0.9916, n = 7). H₂O₂ is the main product of glucose oxidation by glucose oxidase, indicating that glucose detection can be realized by the proposed colorimetric method. The linear range for glucose was from 0.25 to 20 μM. The regression equation was ΔA = 0.0075[glucose] (μM) + 0.006, r² = 0.9929 (n = 7). The limit of detection (LOD) of this assay for glucose was 0.25 μM. For comparison, the detection limits and linear ranges for glucose detection by different MOFs and other nanozymes were summarized in Table 2. Compared with other reported approaches, the present method promises high sensitivity, low detection limit and relatively wide detection range.

Table 2 Comparison of MOFs and other nanozymes for glucose analysis^a

Catalyst	Substrate	Linear range (μM)	LOD (μM)	Ref.
a N/A: data not available.
Fe3O4 MNPs	ABTS	50–1000	30	2b
ZnFe2O4 MNPs	TMB	1.25–18.75	0.3	2j
Co3O4/rGO nanocomposites	TMB	1–100	1.0	3j
MoS2 nanosheets	TMB	5−150	1.2	4b
WS2 nanosheets	TMB	5−300	2.9	4c
(+)-Au NPs	TMB	18–1100	4	5b
Copper NCs	TMB	100–2000	100	5i
FeTe NRs	ABTS	1–100	0.38	5j
NiTe nanowires	ABTS	1−50	0.42	5k
GO–COOH	TMB	1–20	1.0	6a
Carbon dots	TMB	1–500	0.4	6e
Carbon NPs	TMB	25–100	20	6g
Graphitic C3N4	TMB	0.5–10	0.5	6i
Grapheme dots	TMB	0.5–200	0.5	6j
Fe-MIL–88NH2	TMB	2–300	0.48	8a
MIL-53(Fe) by CE	TMB	N/A	N/A	8b
MIL-68/MIL-100	TMB	N/A	N/A	8c
Hemin@MIL-53(Al)–NH2	TMB	10–300	N/A	8e
MIL-53(Fe) by MW	TMB	0.25–20	0.25	This work

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It is essential to evaluate the selectivity of the proposed method for glucose detection in real samples. The selectivity experiments were carried out using buffer, 0.1 mM maltose, 0.1 mM lactose and 0.1 mM fructose in place of 0.02 mM glucose under the same conditions. As can be seen, the absorbance of the studied system in the presence of the selected three saccharides was almost same as that of buffer solution (Fig. S8 ESI†). Furthermore, it was obviously lower than that in the of glucose, indicating good selectivity of the proposed method for glucose detection. In order to demonstrate the practical application of our proposed method in real samples such as human serum, four serum samples from local hospital were used for glucose detection. The results obtained by the proposed method agree well with those obtained by glucose meter (Table 3), confirming the reliability and precision of the proposed method for glucose detection in complicated samples.

Table 3 Results of glucose detection in serum samples

Serum	Proposed method^a (mM, n = 2)	Glucose meter method^b (mM)
a The blood samples were diluted 1000-fold for glucose determination by the proposed method.b The glucose determination was performed directly without dilution in the laboratory for clinical analysis, The Ninth People's Hospital of Chongqing.
1	4.81 ± 0.12	4.6
2	3.99 ± 0.12	4.0
3	4.22 ± 0.12	4.0
4	5.92 ± 0.05	6.2

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4. Conclusion

In summary, current work demonstrated that a microwave-assisted approach for the fast and facile preparation of metal–organic framework MIL-53(Fe) promised a simplicity, fast crystallization, higher yield, and good phase selectivity. The as-prepared MIL-53(Fe) exhibited enhanced peroxidase-like activity, and could catalytically oxidize TMB, ABTS and OPD by H₂O₂ to produce a coloured reaction. The as-prepared MIL-53(Fe) shows much higher affinity to H₂O₂ over other MOFs-based peroxidase mimetics and MIL-53(Fe) peroxidase mimetics by CE-based solvothermal method, which is probably due to the purely octahedral structure and small sized crystals of the MIL-53(Fe) obtained by MW-based synthesis method. The double reciprocal plots revealed the characteristic parallel lines of a ping–pong mechanism and implied that MIL-53(Fe) reacts with the first substrate and then releases the first product before reacting with the second substrate. ESR spectrum confirmed that MIL-53(Fe) presented efficiently catalytic ability to decompose H₂O₂ into ˙OH radicals. On this basis, a simple, sensitive, and selective colorimetric assay for glucose detection in serum samples was developed. It is believed that the as-prepared MIL-53(Fe) may have a great potential for biosensing.

Acknowledgements

The financial support by the Natural Science Foundation of China (no. 21275021) is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Supporting tables and figures. See DOI: 10.1039/c4ra15840g

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