MOF-808: A Metal–Organic Framework with Intrinsic Peroxidase-Like

Article Cite This: Inorg. Chem. 2018, 57, 9096−9104

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MOF-808: A Metal−Organic Framework with Intrinsic PeroxidaseLike Catalytic Activity at Neutral pH for Colorimetric Biosensing He-Qi Zheng,†,‡ Chun-Yan Liu,† Xue-Yu Zeng,§ Jin Chen,† Jian Lü,§ Rong-Guang Lin,† Rong Cao,*,‡ Zu-Jin Lin,*,†,‡ and Jin-Wei Su†

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†

Department of Applied Chemistry, College of Life Science, Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, People’s Republic of China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China § Fujian Provincial Key Laboratory of Soil Environmental Health and Regulation, College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, People’s Republic of China S Supporting Information *

ABSTRACT: Natural enzyme mimetics with high catalytic activity at nearly neutral pH values are highly desired for their applications in biological systems. Herein for the first time a stable MOF, namely MOF-808, has been shown to possess high intrinsic peroxidase-like catalytic activity under acidic, neutral, and alkaline conditions. As a novel peroxidase mimetic, MOF-808 can effectively catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine when H2O2 serves as oxidant, accompanied by a significant color variation in the solution. The catalytic activity and the color variation were greatly dependent on H2O2 concentration, and thus MOF-808 can be applied to the colorimetric sensing of H2O2. The H2O2 detection limit is 4.5 μM, and the linear range is 10 μM to 15 mM. In view of the significant inhibition effect produced by ascorbic acid, a facile and sensitive approach for colorimetric sensing of ascorbic acid was successfully established. The AA detection limit is 15 μM, and the linear range is 30−1030 μM. Further investigation found that the catalytic activity of MOF-808 could be mainly ascribed to the Zr−OH(OH2) groups. Such active Zr−OH(OH2) groups can be effectively shielded by gluconic acid, and subsequently the catalytic activity of MOF-808 was significantly suppressed. With these findings, a facile and selective colorimetric assay for glucose sensing has been successfully explored via combination of the glucose oxidation with the TMB oxidation. The glucose detection limit is 5.7 μM, and the linear range is 5.7−1700 μM. MOF-808 is one of the best colorimetric biosensors among the peroxidase mimics reported for H2O2, AA, and glucose detection.

1. INTRODUCTION

have drawn the most attention for their great potential application in bioanalysis. Up to now, both small molecules such as porphyrins6,7 and a variety of nanomaterials such as grapheme oxide (GO),8,9 carbon nanodots,10 metallic oxide nanoparticles,11 noble nanoparticles,12 etc. have been observed to have unique peroxidase-like catalytic activity. Some of them have been successfully employed as peroxidase mimics for the use of diagnostic detection.13 Given their excellent stability and biocompatibility, these peroxidase mimics are generally competent for bioanalysis. However, most of these peroxidase mimics suffer from a common drawback that their optimum reactions generally occur under acidic solutions with pH values of about 3−4 and their catalytic peroxidase-like activity is negligible under neutral conditions. This shortcoming seriously restricts their further applications in biological systems where near-neutral pH values are required (pH 5.0−7.4). Although

Natural enzymes are unique and illocal in nature, which can efficiently catalyze a series of vital reactions under mild reaction conditions. As catalysts, natural enzymes uaually are highly efficient, strongly substrate specific, and excellently stereoselective. Due to these merits, they have been attracting great interest in medicine, biotechnology, food processing, and environmental analysis. However, they are not only expensive and difficult to reuse but also harsh to prepare and store since they are susceptive to heat and easy to denature and deactivate. As a result, their wide applications have been severely impeded by the these intrinsic fatal shortcomings.1 Therefore, it is imperative to develop potential alternatives such as natural enzyme mimetics to circumvent the above problems. During the last few decades, a great number of efforts have been made to develop enzyme mimetics. So far, many enzyme mimetics including phosphotriesterase mimetics,2,3 cytochrome P450 mimetics,4 serine protease mimetics,5 etc. have been exploited. Among them, peroxidase mimetics in particular © 2018 American Chemical Society

Received: April 20, 2018 Published: July 11, 2018 9096

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Inorganic Chemistry

2. EXPERIMENTAL SECTION

several enzyme mimics showing good catalytic activity at nearly neutral pH values have been reported recently,14,15 it is still highly desirable to develop peroxidase mimics showing excellent catalytic activity at a near-neutral pH value for their applications in biological systems. Metal−organic frameworks, known as MOFs, are built from organic ligands and metal ions. MOFs are becoming an indispensable type of nonoporous material.16 The availability of various organic linkers endows it with the possibility of constructing MOFs with diverse structures and porosity.17−19 In view of their permanent porosity, large surface areas, uniform structure cavities, tunable pore sizes, and customizable pore functionalities, MOFs have been widely applied in storage and separation,20−24 catalysis,25−31 sensing,32−37 etc.28 In principle, MOFs are ideal candidates for enzyme mimetics since their uniform cavities endow them with the ability to decorate a high density of biomimetic active centers. Very recently, many functionalized MOFs were proved to be biocompatible38,39 and several MOFs have also been exploited as peroxidase mimics for colorimetric biosensing.40−43 For example, with inorganic Fe3O4 and CeO2 nanoparticles which possess intrinsic peroxidase-like activity as inspiration, several Fe(III)-based MOFs such as MIL-53,44 Fe-MIL-88 derivatives,45,46 MIL-68 and MIL-100,47 PCN-222,48 and two Cebased MOFs49,50 were recently reported to show a high peroxidase-like catalytic activity. Due to these MOFs possessing high catalytic activity, they were successfully employed as peroxidase mimics for colorimetric biosensing. Although excellent performance was shown under acidic conditions (pH 3−4), the deactivation that was found in HRP or other peroxidase mimics at near-neutral pH values was also observed for the previously reported MOF-based peroxidase mimics. Therefore, it is still a daunting challenge to exploit MOF-based peroxidase mimics with excellent catalytic activity at near-neutral pH values. Herein, a Zr-based MOF, [Zr6O4(μ3OH)4(OH)6(H2O)6(BTC)2]·nH2O (namely MOF-808), was found to exhibit high intrinsic peroxidase-like activity over a pH range of 3−10 and is capable of catalyzing oxidation of the peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) to the oxidized TMB (oxTMB) with a green color by H2O2 in a mixed water/ethanol solution. Remarkably, MOF-808 shows excellent catalytic activity under both acidic and neutral pH reaction conditions, which makes it greatly promising for applications in biological systems. As far as we know, MOF808 represents one of the few natural peroxidase mimetics and as well the first MOF-based peroxidase mimic possessing excellent catalytic properties at nearly neutral pH values. On the basis of the above findings, MOF-808 was employed as a novel peroxidase mimic to offer a colorimetric sensing of H2O2 at neutral pH. In addition, a significant inhibition effect was observed by ascorbic acid (AA) on the oxidation of TMB, providing a facile colorimetric method for detection of AA. Further studies demonstrated that the catalytic activity of MOF-808 can be mainly ascribed to the Zr−OH(OH2) active sites in Zr6O4 clusters. Such active Zr−OH(OH2) sites can strongly interact with gluconic acid (GA), which leads to a significant suppression effect in the oxidation of TMB. Given that gluconic acid can be readily generated in glucose oxidation catalyzed by glucose oxidase, a facile and selective colorimetric assay was further explored for glucose sensing by coupling the glucose oxidation with TMB oxidation.

2.1. Synthesis of MOF-808FA and MOF-808. MOF-808FA was synthesized on the basis of recently published articles with slight modifications.51 Briefly, ZrCl4 (233 mg, 1.0 mmol), 1,3,5-benzenetricarboxylic acid (H3BTC, 70.6 mg, 0.236 mmol), formic acid (5.6 mL, 98 mmol), and N,N′-dimethylformamide (DMF, 10 mL) were placed in a screw-capped vial and ultrasonically dissolved. The mixture was then placed in an oven at 135 °C for 24 h. After the mixture was cooled naturally to 30 °C in the oven, the white precipitate was filtered and then extracted with methanol overnight via a Soxhlet extractor. Subsequently, the obtained precipitate was dried in a drying oven at 80 °C overnight to obtain MOF-808FA. MOF-808FA (500 mg) was placed in a screw-capped vial. Then, 110 mL of DMF and 10 mL of concentrated HCl were subsequently added. The suspension was then incubated at 80 °C for 24 h. After it was naturally cooled to 30 °C, the solid was collected by suction filtration and then extracted with methanol by a Soxhlet extractor overnight. The resulting white precipitate was dried at 80 °C in a vacuum drying oven to obtain MOF-808. 2.2. Materials and Methods. ZrCl4, 1,3,5-benzenetricarboxylic acid, H2O2 (30%), and L-ascorbic acid (AA) were purchased from Energy Chemical. 3,3′,5,5′-Tetramethylbenzidine (TMB) and horseradish peroxidase (HRP) with biological reagents were purchased from Sangon Biotech (Shanghai) Co., Ltd. Lactose, sucrose, rhamnose, fructose, glucose, and gluconic acid (GA) were supplied by Sinopharm Chemical Reagent Co., Ltd. Glucose oxidase (GOx) with biological reagents was purchased from Aladdin. All other starting materials, solvents, and reagents of analytical grade were bought from Energy Chemical and directly used without further treatment. Powder X-ray diffraction (PXRD) data were acquired at room temperature on a Rigaku MiniFlex2 diffractometer with Cu Kα radiation, and the recording speed was 1° min−1. Scanning electron microscopy (SEM) images were obtained on a JSM6700-F scanning electron microscope. 1H NMR spectra were acquired on a Bruker Avance-400 MHz NMR spectrometer. The BET surface area and pore volume were obtained from the N2 sorption isotherms, which were tested in a Micrometrics ASAP 2460 surface area and pore size analyzer. Liquid nitrogen was used for the tests at 77 K. A density functional theory (DFT) model was adopted to calculate the pore size distribution. Prior to the tests, the samples were degassed under high vacuum at 120 °C for 8 h. UV−vis spectra were obtained with a UV2600 (Shimadzu) ultraviolet spectrophotometer. 2.3. Bioassay. 2.3.1. General Considerations. A 5 mM TMB ethanol solution was prepared by dissolving TMB in ethanol. Aqueous solutions of H2O2 of various concentrations were prepared by successive dilutions of purchased H2O2 (30%) in distilled water. Similarly, AA, GA, and glucose solutions of various concentrations were separately prepared by dilutions of the corresponding AA (50 mM), GA (50 mM), and glucose (50 mM) stock solutions, respectively. MOF-808 and MOF-808FA suspensions with a concentration of 1 mg mL−1 were separately prepared by dispersion of MOF-808FA or MOF-808 in deionized water and then ultrasonication for 5 h. The MOF suspensions were strongly stirred and kept at a preset temperature before use. 2.3.2. Typical TMB Oxidation Procedure Catalyzed by MOF-808. In a typical assay, 200 μL of 5 mM TMB ethanol solution, 1.5 mL of ethanol, 1.5 mL of 1 mg mL−1 MOF-808 suspension, 200 − x μL of H2O, and x μL (x = 10, 30, 50, 70) of H2O2 with various concentrations were sequentially placed into a 5 mL vial containing a stirring bar. The mixture (with a total volume of 3.4 mL) was vigorously stirred at 40 °C for 0.5 h and then immediately filtered with a syringe filter (PTFE, hydrophobic, 0.24 μm). The UV absorbances of the filtrates were tested to evaluate the content of oxTMB. 2.3.3. Kinetic Measurements. A 2.0 mL TMB (5 mM) ethanol solution, 15 mL of ethanol, 15 mL of 1 mg mL−1 MOF-808 suspension, 1.0 mL of H2O, and 1.0 mL of 500 mM H2O2 were sequentially placed in a 50 mL vial. Then the mixture was strongly 9097

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Inorganic Chemistry stirred at 40 °C for 0.5 h. During this period, 1.5 mL of the suspension was taken out at a preset time and immediately filtered with a syringe filter. The content of oxTMB in the filtrate was assessed through the UV absorbance. The Michaelis−Menten constant was calculated using a Lineweaver−Burk plot as follows:

Km 1 1 = + v0 vmax[S] Vmax where Km is the Michaelis constant, v0 and vmax represent the initial and the maximum reaction velocities, respectively, and [S] is the concentration of the substrate.11,52 2.4. AA and Glucose Detection Procedures. 2.4.1. Detection of Ascorbic Acid. Typically, 200 μL of 5 mM TMB ethanol solution, 1.5 mL of ethanol, 1.5 mL of 1 mg mL−1 MOF-808 suspension, 190 − x μL of H2O, x μL of aqueous solution containing ascorbic acid with various concentrations, and 10 μL of 5 M H2O2 were sequentially placed in a 5 mL vial containing a stirring bar. The mixture was vigorously stirred at 40 °C for 0.5 h and then immediately filtered with a syringe filter. The UV−vis spectrum of the filtrate was tested to assess the content of oxTMB. The GA detection procedure is similar to that of AA except that AA solutions were replaced by GA solutions. 2.4.2. Detection of Glucose. In a typical experiment, 500 μL of aqueous solutions of glucose with various concentrations was placed in a 1 mL reaction vial, and then 10 μL of 40 mg mL−1 GOx was added. This mixture was heated at 40 °C for 1.5 h. In another vial (5 mL), 200 μL of 5 mM TMB ethanol solution, 1.5 mL of ethanol, 1.5 mL of 1 mg mL−1 MOF-808 suspension, 200 μL of the above reaction solution, and 10 μL of 5 M H2O2 were sequentially added. The mixture was then vigorously stirred at 40 °C for 0.5 h and immediately filtered with a syringe filter. The UV−vis spectrum of the filtrate was tested to assess the content of oxTMB. The experimental procedure of selectivity analysis for glucose detection was the same as that of glucose detection except that glucose (1700 μM) was separately replaced by equimolar amounts of lactose, sucrose, rhamnose, and fructose.

Figure 1. Structure of MOF-808FA. The tetrahedral and adamantaneshaped cages are shown by blue and pink spheres, respectively; the coordinated formate ligands are pointing into the hexagonal windows of large adamantane-shaped cages (the carbon atoms in formate ligands are shown as gray balls for clarity).

similar to each other and coincide with the simulated pattern (Figure S2), indicating the purity and crystallinity of the two MOFs. The framework retained high crystallinity after treatment under acidic conditions, demonstrating the extremely high chemical stability of MOF-808. SEM images showed that MOF-808FA displays particles with sizes small than 500 nm (Figure S3a). In comparison with MOF-808FA, no significant morphology changes were observed for MOF808 and the particles of these two MOFs have the same extent of aggregation (Figure S3b). To assess the specific surface areas and the pore size distributions of these two MOFs, the N2 isotherms were obtained at 77 K. As shown in Figures S4 and S5, the Brunauer−Emmett−Teller (BET) surface area of MOF-808 reaches 1140 m2 g−1, which is slightly smaller than that of MOF-808FA (1390 m2 g−1). Pore distribution data calculated by density functional theory (DFT) showed that two types of micropores at around 10 and 18 Å were observed in MOF-808. The result was almost identical with that obtained from crystallographic data, where a diameter of approximately 9.7 Å was found for the large hexagonal windows and 18.4 Å for the adamantane-shaped cages, respectively. 3.2. Intrinsic Peroxidase-Like Catalytic Activity of MOF-808. MOF-808 shows intrinsic peroxidase-like catalytic activity in the TMB oxidation under neutral pH conditions (Figure 2). On addition of MOF-808 to a solution of TMB and H2O2, a green color was gradually observed (Figure 2A). UV− vis absorption tests showed that the green solution exhibits two remarkable absorption peaks at ca. 369 and ca. 661 nm. This result is in good agreement with that observed for the horseradish peroxidase enzyme, where the absorption bands originate from oxTMB.48 However, only colorless and faint

3. RESULTS AND DISCUSSION 3.1. Structures and Characterizations of MOF-808FA and MOF-808. MOF-808FA, formulated as [Zr6O4(μ3OH)4(FA)6(BTC)2]·nH2O where FA denotes formate ion, was synthesized according to an article previously reported.51 MOF-808FA is comprised of [Zr6O4(μ3-OH)4(FA)6] clusters and BTC3− linkers, in which each zirconium cluster is linked to six BTC3− ligands and each BTC3− ligand is linked to three zirconium clusters to give a 3D framework containing two types of cages (Figure 1). One is a tetrahedral cage with a diameter of 4.8 Å, and the other is an adamantane-shaped cage with a diameter of 18 Å. Similarly to other Zr-based MOFs such as NU-1000,53,54 monocarboxylic groups (i.e., formate in MOF-808FA) which coordinate to zirconium clusters in MOF808FA can be readily removed through treatment with 1 M hydrochloric acid,54 which was confirmed by the NMR spectrum (Figure S1). For convenience, the resulting acidtreated MOF sample without formate groups coordinated in zirconium clusters is denoted as MOF-808. In this case, the structure of MOF-808 is identical with that of MOF-808FA except that the coordinated formate ligands in MOF-808FA are replaced by coordinatde -OH and -OH2 groups. As shown in Figure 1, all coordinated formate ligands in MOF-808FA point toward the large hexagonal windows of adamantaneshaped cages, and thus all formate ligands are accessible via hexagonal windows with a diameter of approximately 10 Å. As a consequence, all of the Zr−OH(OH2) sites in MOF-808 should also be available through the large hexagonal windows. The PXRD patterns of MOF-808FA and MOF-808 are almost 9098

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Inorganic Chemistry

very low peroxidase-like catalytic ability in comparison with MOF-808 (Figure 2d). Since the structures of these two MOFs are identical except that the coordinated formate groups in MOF-808FA are substituted by coordinated −OH groups to form MOF-808, it is safe to conclude that the major catalytically active sites in MOF-808 were the coordinated −OH groups. Remarkably, after the removal of MOF-808 catalyst, the reaction almost completely stopped, giving almost the same conversion after reaction for another 10 min (Figure 2). The result demostrated that no catalytically active species were leached into the supernatant during the reaction and MOF-808 was truly a heterogeneous catalyst. Similar to the case for other peroxidase mimetics, the mechanism of the intrinsic peroxidase-like activity of MOF-808 may be ascribed to the ability to decompose H2O2 into •OH radicals through electron transfer.47,55 3.3. Optimal Reaction Conditions of MOF-808. The catalytic activity of most reported peroxidase mimics is dependent on pH values, temperature, MOF-808 dosage, and H2O2 dosage. Therefore, the effects of these factors on the catalytic activity of MOF-808 were also assessed. It is wellknown that TMB can be consumed under acidic conditions. To exclude the effects of background, we first studied the background over a pH range of 1−10. As shown in Figure S6, TMB can be readily consumed in the absence of a catalyst at pH values lower than 2. Without a catalyst, the TMB consumption velocity sharply decreased in association with the increasing pH values and no obvious reactions were observed when the pH was greater than 4. Considering the facts that MOF-808 is stable in the pH range 1−10 and that strong backgrounds were observed when the pH value waslower than 3, the catalytic activity of MOF-808 was only investigated in a pH range of 3−10 (Figure 3a). Remarkably, MOF-808 showed a high catalytic activity over the broad pH range of 3−10. With an increase in pH, the catalytic activity

Figure 2. Time-dependent absorbance of control experiments at 661 nm at 40 °C: (a) MOF-808, H2O2, and TMB; (b) H2O2 and TMB; (c) MOF-808 and TMB; (d) MOF-808FA, H2O2, and TMB. The red curve meansthat the MOF-808 catalyst was filtered after reaction for 20 min and then the clear filtrate was stirred for another 10 min. Typical reaction conditions: 200 μL of 5 mM TMB ethanol solution, 1.5 mL of ethanol, 1.5 mL of 1.0 mg mL−1 catalyst aqueous suspension, 100 μL of H2O, and 100 μL of 50 mM H2O2. The inset shows the corresponding photographs.

yellow solutions were observed in the absence of MOF-808 (Figure 2B) or H2O2 (Figure 2C), respectively. In addition, the solutions showed negligible absorption in the range of 350− 750 nm, demostrating that almost no oxidation reaction happened when either MOF-808 or H2O2 is absent. All of these observations demonstrated that MOF-808 has high peroxidase-like catalytic ability, which could effectively catalyze the TMB oxidation when H2O2 serves as oxidant. It is worth noting that the crystallinity of MOF-808 was well retained after the catalytic reaction (Figure S2). To reveal the catalytically active sites of MOF-808, control experiments were performed by the replacement of MOF-808 with MOF-808FA. Surprisingly, MOF-808FA only showed

Figure 3. Peroxidase-like catalytic activity of MOF-808 against (a) pH value, (b) temperature, (c) catalyst dosage, and (d) H2O2 concentration. The maximum activity in each graph was set as 100%. 9099

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Table 1. Apparent Michaelis−Menten Constant (Km) and Maximum Reaction Rate (Vmax) of MOF-808 and Other Materials catalyst MOF-808 HRP N-GODs GO-COOH H@M

substrate fixed

substrate varied

Km (mM)

Vmax (10−8 M s−1)

ref

TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2

H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB

1.06 0.0796 0.434 3.7 0.10 11.9 0.024 3.9 0.068 10.9

1.39 3.12 10.00 8.71 0.14 0.38 3.45 3.85 6.07 8.98

this work 11 10 52 56

Figure 4. (a) UV−vis absorbance spectra of TMB oxidation in neutral pH solution at varying H2O2 concentrations (10, 15, 30, 45, 75, 105, 150, 300, 450, 750, 1000, 1500, 4500, 7500, 10000, and 15000 μM) and (b) corresponding linear calibration plots for H2O2 detection. (c) UV−vis spectra of TMB oxidation in solutions with various AA concentrations (0, 30, 45, 75, 105, 150, 225, 300, 450, 750, and 1030 μM) as an inhibitor in a neutral pH solution and (d) corresponding linear calibration plot for AA detection. ΔA = A0 − Ai (A0 is the initial absorbance intensity at 661 nm without AA and Ai is the absorbance intensity at 661 nm with an AA concentration of i).

an increase in MOF-808 dosage and the optimal MOF-808 dosage was 500 μg mL−1. In addition, the catalytic activity first rose with an increase in H2O2 concentration and then gradually fell (Figure 3d). The optimum catalytic activity was observed at a H2O2 concentration of 44 mM. Such a H2O2 concentration is much higher than that of HRP to reach the maximum peroxidase activity,10 indicating that MOF-808 possesses more stable catalytic activity in comparison to HRP at a higher H2O2 concentration. 3.4. Steady-State Kinetic Assays of MOF-808. The kinetic parameters of MOF-808 were further investigated by the use of steady-state kinetics, which were performed using H2O2 and TMB as substrates at neutral pH and 40 °C. The kinetic experiments were carried out by fixing one substrate concentration while the other was changed. The initial reaction velocity was applied to the double reciprocal of the Michaelis− Menten equation.11 As shown in Figure S7, the reaction

gradually decreased but remained excellent even when the pH was 10. So far, the catalytic activity of most reported peroxidase mimics has been pH-dependent with an optimal pH value of ca. 3−4. As far as we know, MOF-808 represents one of the very few peroxidase mimics and as well the first MOF-based peroxidase mimic that exhibits high peroxidaselike catalytic activity at near-neutral pH values.15 Since nearly neutral pH conditions are generally required in many biological systems, MOF-808 with excellent catalytic activity at nearly neutral pH values may potentially serve as a peroxidase mimic for applications in biological systems. A temperature screen showed that MOF-808 exhibits the highest catalytic activity at 50 °C (Figure 3b). Considering that MOF808 also shows excellent catalytic activity at 40 °C, 40 °C was selected as the standard temperature for subsequent activity analysis, since it is closer to physiological temperature. As shown in Figure 3c, the catalytic activity rapidly increased with 9100

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Inorganic Chemistry

Table 2. Comparison of the Sensing Parameters among MOF-Based Peroxidase Mimics for Their Colorimetric Sensing of H2O2, AA, and Glucose peroxidase mimic

analyte

linear range (μM)

detection limit (μM)

ref

MOF-808 H@M MIL-53(Fe) Cu6(Trz)10(H2O)4[H2SiW12O40] MOF-808 MIL-53(Fe) MIL-68, MIL-100(Fe) MOF-808 H@M Fe-MIL-88NH2

H2O2 H2O2 H2O2 H2O2 AA AA AA glucose glucose Glucose

10−15000 2.0−500 0.95−19 10−50 30−1030 28.6−190.5 30−485 5.7−1700 10−300 2−300

4.5 NMa 0.13 1.37 15 15 6 5.7 NM 0.48

this work 56 44 58 this work 44 47 this work 56 59

a

NM = not mentioned.

decreased with an increase in AA concentration. As shown in Figure 4d, ΔA at 661 nm was proportional to the AA concentration on a logarithmic scale in the range of 30−1030 μM. The linear regression equation could be expressed as ΔA = −0.804 + 0.544 log CAA with a correlation coefficient of 0.9945. The detection limit was estimated to be 15 μM, which is compared with those of other MOF-based peroxidase mimics in Table 2. Again, MOF-808 shows the widest linear range in comparison with the other MOF-based colorimetric sensors for AA detection. In addition, the color variation for AA response in this assay was also obvious and can be readily distinguished by the naked eye (Figure 4d inset). 3.6. Detection of Glucose. In view of the Zr−OH(OH2) groups serving as the active catalytic sites in the catalytic TMB oxidation, the catalytic ability of MOF-808 could be readily tuned if the density of active Zr−OH(OH2) groups can be adjusted and controlled. Recent reports showed that monocarboxylic acid can effectively graft to Zr−OH(OH2) groups in Zr-based MOFs via either coordinate bonds or hydrogen bonds.33 Thus, a potential method to tune the density of Zr−OH(OH2) groups is by the introduction of monocarboxylic acid into the Zr-based MOFs. Gluconic acid, a major product of glucose oxidation catalyzed by glucose oxidase, was evaluated as to whether it can modify the peroxidase-like catalytic activity of MOF-808. UV−vis absorption spectra showed that the absorbance intensity at 661 nm was significantly lowered with an increase in gluconic acid concentration (Figure S8), indicating that the catalytic activity of MOF-808 was effectively suppressed by gluconic acid. Since gluconic acid can be quantitatively obtained in the GOx-catalyzed reaction, MOF-808, serving as a novel peroxidase mimic instead of HRP, was applied in the colorimetric sensing of glucose. In the assay, GOx catalyzed the glucose oxidation to produce glucose acid and H2O2. Then, one of the products, glucose acid, serving as an inhibitor, suppressed the oxidation of TMB; the other product, H2O2, can be neglected since a large quantity of H2O2 with a concentration of 15 mM was present in the reaction solution. Figure 5a shows the relationship between the glucose concentration and the UV−vis absorption spectra of the corresponding reaction solutions. The absorbance intensity at 661 nm gradually decreased as the glucose concentration increased under neutral pH conditions. In the range of 5−1700 μM, a linear correlation existed between ΔA and the glucose concentration on the logarithmic scale (Figure 5b). The linear regression equation can be expressed as ΔA = −0.027 + 0.234 log Cglucose. The correlation coefficient was 0.9801, and the

process followed the conventional enzymatic dynamic regulation of the Michaelis−Menten equation. The apparent steady-state kinetic parameters, such as the Michaelis−Menten constant (Km) and maximum initial velocity (Vmax), were calculated from a Lineweaver−Burk plot52 and are shown in Table 1. The Km value of MOF-808 with H2O2 as the substrate was much lower than those of HRP or other reported peroxidase mimics, indicating that the affinity between H2O2 and MOF-808 is higher than those of H2O2 and HRP (or other mimics). 3.5. Detection of H2O2 and AA. Given that both the catalytic activity of MOF-808 and the color variation of the resulting reaction solutions greatly relied on the concentration of H2O2, a colorimetric sensing of H2O2 has been established on the basis of the relationship between the concentration of H2O2 and the intensity of UV−vis absorbance at 661 nm. Figure 4a displays the UV−vis absorption spectra tested at varying concentrations of H2O2 ranging from 10 μM to 15 mM under optimal conditions (i.e., pH 7, 40 °C). The absorbance intensity at 661 nm increased as the H2O2 concentration increased. As shown in Figure 4b, the H2O2 concentration can be divided into two parts, which separately showed good linear correlation with the absorption intensity at 661 nm. One is H2O2 concentration ranging from 0.01 to 1 mM, where the linear regression equation could be expressed as A = 0.003 + 0.450CH2O2. In this part, the correlation coefficient is 0.9974 and the H2O2 detection limit is about 4.5 μM. The other is H2O2 concentration ranging from 1.0 to 15 mM, where the linear regression equation was A = 0.420+ 0.031CH2O2. In this part, the correlation coefficient is 0.9903. The performances of other MOF-based peroxidase mimics as colorimetric detections for H2O2 are summarized in Table 2. In comparison to other MOF-based colorimetric sensors, MOF-808 exhibits the widest linear range. As far as we know, MOF-808 represents one of the peroxidase mimics possessing the broadest linear range for the colorimetric sensing of H2O2.57 Simultaneously, the color variation for the resulting reaction solutions could be easily and obviously observed (Figure 4b inset), which offers a facile and simple approach to detect H2O2 by the naked eye even when the H2O2 concentration is very low. Remarkably, a trace amount of ascorbic acid (AA) can considerably suppress the TMB oxidation process to yield a nearly clear solution. In view of these observations, a colorimetric biosensing for AA on the basis of MOF-808 was readily developed. Figure 4c shows the UV−vis absorption spectra of the reaction solutions at various AA concentrations. The figure shows that the absorption intensity at 661 nm 9101

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Inorganic Chemistry

Figure 6. Selectivity of glucose sensing on the basis of a combination of GOx- and MOF-808-catalyzed oxidation.

the presence of TMB and H2O2, MOF-808 can produce a green color reaction, thus leading to a colorimetric sensing for H2O2. In view of the fact that AA has a significant inhibition effect on the catalytic activity of MOF-808, a facile colorimetric biosensing for AA was also successfully explored. Moreover, the peroxidase-like activity of MOF-808 was obviously suppressed by gluconic acid. Since gluconic acid is the major product of glucose oxidation catalyzed by glucose oxidase, a facile and selective colorimetric glucose sensing has been successfully developed through a combination of the glucose oxidation with the TMB oxidation. Remarkably, MOF-808 represents the first MOF-based peroxidase mimic that has excellent catalytic activity in the broad pH range 3−10. The high catalytic activity of MOF-808 in near-neutral pH solutions paves the way for its further applications in biological systems. Our work showed that MOF-808 is a unique material that possesses highly intrinsic peroxidase-like catalytic activity over a broad pH range and can be employed for biosensing of H2O2, AA, and glucose.

Figure 5. (a) UV−vis absorbance spectra and (b) corresponding linear calibration plot for glucose sensing by the combination of GOxcatalyzed glucose oxidation and MOF-808-catalyzed TMB oxidation. The glucose concentrations are 5.7, 17, 40, 57, 170, 400, 570, and 1700 μM. The inset shows photographs for colorimetric detection of glucose.

glucose detection limit was found to be 5.7 μM. As shown in Table 2, MOF-808 possesses the widest linear range among MOF-based colorimetric sensors for glucose detection and represents one of the peroxidase mimics having the broadest linear range.60 The color variation can be obviously observed by the naked eye for the glucose response (Figure 5b, inset). Since the glucose concentrations of healthy persons and diabetics are usually in the ranges in 3−8 and 9−40 mM, respectively, this proposed approach is a promising candidate for glucose sensing in serum samples. To evaluate the specificity of this proposed approach, control experiments were performed separately using lactose, sucrose, rhamnose, and fructose instead of glucose. The result showed that sucrose and lactose did not cause significant changes in the absorbance intensity at 661 nm, and rhamnose and fructose only showed a small decrease of the values (Figure 6). In contrast, the absorbance intensity at 661 nm sharply decreased in the case of glucose. This is because GOx has a high specificity to glucose in the catalytic oxidation. The result demostrated that the newly developed approch has an excellent selectivity toward glucose.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01097. NMR spectrum, N2 adsorption isotherms, PXRD patterns, SEM images, and UV−vis spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for R.C.: [email protected]. *E-mail for Z.-J.L.: [email protected]. ORCID

Jian Lü: 0000-0002-0015-8380 Rong Cao: 0000-0003-2384-791X Zu-Jin Lin: 0000-0003-2515-3356

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CONCLUSION In a nutshell, we have demonstrated that the water-stable MOF-808 possesses intrinsic peroxidase-like catalytic activity in the pH range of 3−10, efficiently catalyzing the TMB oxidation when H2O2 serves as oxidant. The catalytic activity of MOF-808 relies on temperature, catalyst dosage, and H2O2 concentration. As a novel enzyme mimetic, MOF-808 possesses high catalytic activity at near-neutral pH values and is easy to prepare and store, highly stable, and inexpensive, features which are almost impossible for natural enzymes. In

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (21520102001, 21521061, 91622114, and 21331006), Fujian Agriculture and Forestry University (118360020), Outstanding Youth Research Training Program of Fujian Agriculture and Forestry University (XJQ201616), and the State Key Laboratory of 9102

DOI: 10.1021/acs.inorgchem.8b01097 Inorg. Chem. 2018, 57, 9096−9104

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Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (20170028).

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