Corrole functionalized iron oxide nanocomposites as enhanced peroxidase mimic and their application in H₂O₂ and glucose colorimetric sensing

 Linna Gao,^1# Leyou Zhang,^1# Xintian Lyu,³ Guifen Lu^2* and Qingyun Liu^1*

¹ School of Chemistry and Chemical Engineering,Harbin Institute of Technology, 92 Xidazhi Street, Harbin 150001, China

² College of Chemical and Environmental Engineering,Shandong University of Science and Technology, Qingdao 266590, PR China

³ Integrated Composites Laboratory (ICL),Department of Chemical & Bimolecular Engineering,University of Tennessee, Knoxville, TN 37996 USA

* Corresponding Author(E-mail) : qyliu@sdust.edu.cn

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Abstract

For the first time, functional Corrole molecules modified iron oxide (Fe₃O₄) magnetic nanoparticles (MNPs) were prepared by a facile two-step method. The Corrole-Fe₃O₄ nanocomposites exhibited a higher peroxidase-like activity than that of pure Fe₃O₄ nanoparticles, and accelerated the oxidation of peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) with the help of hydrogen peroxide (H₂O₂) only in 5 min, attributing to hydroxyl radicals (·OH) generated in the process of oxidation of TMB. Kinetic analysis showed that the catalytic behaviors followed the typical Michaelis-Menten kinetics. Additionally, Corrole-Fe₃O₄ exhibits several advantages including low cost, easy separation, facile fabrication, and high catalytic efficiency. Based on the catalytic activity of Corrole-Fe₃O₄ nanocomposites, a simple, sensitive, and selective colorimetric biosensor for H₂O₂ and glucose determination was successfully designed. The linear relationships of absorbance of oxidized TMB at 652 nm with H₂O₂ or glucose concentration were obtained from 10 µM to 100 µM with a detection limit of 3.6 µM and 4 µM to 40 µM with the detection limit as low as 2.46 µM, respectively. The results demonstrate that the Corrole-Fe₃O₄ nanocomposites have potential applications in bioanalysis and biodetection.

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Table of Content

A colorimetric sensor using the glucose oxidase (GOx) and Corrole-Fe₃O₄ catalyzed reactions is reported for detecting glucose.

 

 

 

Keywords

 Corrole      Corrole-Fe₃O₄     nanocomposites      peroxidase      mimetic     colorimetric     glucose

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1. Introduction

Colorimetric biosensing associated with enzymes has gained considerable attention not only due to its simplicity and practicability but also without requirement for any expensive and sophisticated instruments.^1-9 It is known to us that the natural enzyme, HRP, has been playing key roles in colorimetric biosensing in the past few years. However, several limitations including expensive preparation and purification, low stability, and harsh storage, inhibited the application of natural enzyme,¹⁰ thus leading to a high desire to construct and fabricate enzyme mimics with similar or even superior properties.^11,12

Recently, with the development of study of nanomaterials,^13,14 more and more ferromagnetic nanoparticles have attracted considerable attention because of their large specific surface area and high surface reactivity^15-17 as well as their unique magnetic properties and potential applications in the fields of drug delivery,¹⁸ biological separation¹⁹ and biological catalysis.^20-23 For example, Fe₃O₄ magnetic nanoparticles (MNPs) generally considered to be biologically and chemically inert have been first reported to exhibit an intrinsic peroxidas-like catalytic activity,²⁴ which captures intense interesting in magnetic nanomaterial-related enzyme mimics. Subsequently, investigations about magnetic nanomaterial as alternatives to natural enzymes are further explored ranging from single magnetic nanoparticles to composites together with various potential applications in biocatalytic field. For example, Wang et al developed a coprecipitation method to prepare Fe₃O₄ MNPs and achieved the detection of H₂O₂ and glucose.^10-12 Chen et al established a colorimetric assay method for the detection of melamine using Fe₃O₄ MNPs.²⁵ Yan et al developed a novel Fe₃O₄ magnetic nanoparticle peroxidase mimetic based colorimetric method for the rapid detection of organophosphorus pesticides and nerve agents.²⁶ Furthermore, based on the Fe₃O₄ magnetic materials, various composited magnetic nanomaterials have been focused on.^27,28 Yuan et al investigated the enzyme-mimic activities of the Ag@Fe₃O₄ nanocomposite.²⁹ Kang et al reported a facile route to synthesize size tunable Fe₃O₄-carbon nitride nanotube (CNNT) hybrids with high peroxidase mimetic activity.³⁰ Gu et al found that Prussian blue modified iron oxide magnetic nanoparticles exhibited enhanced peroxidase-like catalytic activity.³¹ Liu and coworkers have studied functional organic molecules modified magnetic nanoparticles which demonstrated the enhanced peroxidase mimetic activity.^32-34 The previous studies prove that people have paid attention to the nanocomposites based on magnetic materials as peroxidase mimics.

Corroles, which have a somewhat more condensed N4 coordination core and a much more electron-rich π system³⁵ have attracted enormous interest in recent years.^36-42 This is not only because of improved synthetic methods that make them more readily available but also because of their potential applications in the fields of photodynamic therapy, solar cells, catalysis and sensors.^39,40,43-45 Nowadays, its functionalization and further applications as catalysts, dyes for solar energy conversion and in medicine are attracting increasing interest.^39,43,46,47 However, up to date, there are no reports on using Corroles to modify Fe₃O₄ magnetic nanoparticles, which can be employed as colorimetric sensing materials to detect H₂O₂ and glucose.

In this work, for the first time, we selected functional Corrole molecules to modify iron oxide (Fe₃O₄) magnetic nanoparticles, which demonstrated the enhanced peroxidase-like activity. The Corrole functionalized Fe₃O₄ nanoparticles (Corrole-Fe₃O₄ nanocomposites) could accelerate to oxidize the peroxidase substrate 3,3',5,5'-tetramethylbenzidine (TMB) in the presence of H₂O₂, suggesting a higher catalytic activity than that of pure Fe₃O₄ nanoparticles. On the basis of the experimental results, we have successfully developed a simple, selective, colorimetric and visual method for detecting glucose (Scheme 1).

Scheme 1. Schematic illustration of a colorimetric sensor for glucose detection using glucose oxidase (GOx) and Corrole-Fe₃O₄ catalyzed reactions.

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2. Experimental section

2.1. Materials

FeSO₄·7H₂O was purchased from Basf Chemical Co., Ltd. (Tianjin, China).  NaNO₃, C₆H₅Na₃O₇·2H₂O (citric acid), NaOH, dichloromethane (CH₂Cl₂), 30% H₂O₂, glucose, lactose, fructose, sucrose and maltose were obtained from Guangcheng Reagent Co. (Tianjin, China). 3,3′,5,5′-tetramethylbenzidine dihydrochloride (TMB·2HCl) were purchased from Solarbio Co. (Beijing, China). Glucose oxidase (GOx, 10 KU) was obtained from Sigma-Aldrich and stored at -20 ^oC in a refrigerator. All chemicals were of analytical grade and used as received without further purification. 5,10,15-tri(4-chlorophenyl)corrole (Corrole), shown in Fig. 1, was prepared according to the previous literature.³⁵

Fig. 1. The molecular structure of 5,10,15-tri(4-chlorophenyl)corrole.

2.2. Preparation of the Corrole-Fe₃O₄ nanocomposites

The Corrole-Fe₃O₄ nanocomposites were synthesized by a facile two-step method. First, the Fe₃O₄ MNPs were prepared according to a reported procedure with a slight modification.⁴⁸ Typically, a solution containing 4 mmol of NaOH, 1 mmol of C₆H₅Na₃O₇·2H₂O (citric acid, trisodium salt dehydrate), 0.2 mol of NaNO₃ and 19 mL of deionized water was heated to 100 ^oC. After forming a pellucid solution, 1 mL of FeSO₄·7H₂O (2 M) solution was rapidly added into the above mixture (equivalent to 0.10 M Fe²⁺ in the alkali solution), and then the mixed solution was transferred to a Teflon-lined stainless steel autoclave and maintained at 100 ^oC for 1 h. After cooling down to room temperature naturally, the Fe₃O₄ NPs were separated from the resulting solution using a magnet, washed with deionized water for several times and dried at 50 ^oC for 8 h.

Corrole-Fe₃O₄ nanocomposites were prepared as follows: firstly, Corrole (1 mg) was dissolved into CH₂Cl₂ solution (1 mL). Secondly, the as-prepared Fe₃O₄ nanoparticles (30 mg) dispersed into CH₂Cl₂ solution (10 mL) were then added into the above solution. Finally, the mixed solution was stirred vigorously for 3 h. After that, the product was separated by a magnet, washed with CH₂Cl₂ and absolute     ethanol for several times and dried at room temperature.

2.3. Characterization

The crystal structures of the products were measured on a powder X-ray diffractometer (XRD) with a graphite monochromatized Cu Kɑ radiation (D/Max 2500 PC, Rigaku). The operation voltage and current were kept at 40 kV and 40 mA. The morphology of the nanoparticles was obtained by scanning electron microscopy (SEM, JEOL, Japan). Energy dispersive X-ray spectroscopy (EDS) was carried out to identify the elemental composition of the products. The fluorescent spectra were obtained using an F-4600 FLSPECTOROPHOTOMET spectrofluorophotometer (Hitachi High-Tech Science Corporation, Tokyo, Japan). UV-vis absorption spectra were recorded on a MAPADA UV-3200PC spectrophotometer (Shanghai, China).

2.4. Peroxidase-like catalytic activity

Different colorimetric reaction systems included: (a) 1.0 mM TMB, 0.25 M H₂O₂, 0.4 mg mL^-1 Corrole-Fe₃O₄ in 0.2 M acetate buffer (pH 3.8); (b) 1.0 mM TMB, 0.25 M H₂O₂, 0.4 mg mL^-1 Fe₃O₄ in 0.2 M acetate buffer (pH 3.8); (c) 1.0 mM TMB, 0.25 M H₂O₂, in 0.2 M acetate buffer (pH 3.8); (d) 1.0 mM TMB, 0.4 mg mL^-1 Corrole-Fe₃O₄ in 0.2 M acetate buffer (pH 3.8); (e) 1.0 mM TMB in 0.2 M acetate buffer (pH 3.8), respectively.

2.5. Reaction mechanism assay

The kinetic measurements were carried out in a time course by monitoring the absorbance at 652 nm on a UV-vis spectrophotometer. For the kinetic assay of H₂O₂, TMB (1mM) was added in 1.4 mL buffer with various concentrations of H₂O₂. Similarly, H₂O₂ (0.25 M) was added in 1.4 mL buffer with various concentrations of TMB for the kinetic assay of TMB. Unless otherwise stated, experiments were implemented in a buffer solution (0.2 mM, pH 3.8, 2 mL) by using 200 μL of Corrole-Fe₃O₄ as enzyme mimics (4 mg/mL). The kinetic parameters were calculated according to the Lineweaver-Burk plot [49]: 1/ν= (K_m/V_max)(1/[S] + 1/K_m), where ν is the initial velocity, V_max corresponds to the maximal reaction velocity, [S] means the concentration of substrate and K_m represents the Michaelis-Menten constant.

The catalytic mechanism was further explored by the photoluminescence (PL) technique using terephthalic acid as a probe molecule. Terephthalic acid readily reacts with ·OH to produce 2-hydroxyterephthalic acid, a fluorescent product.⁵⁰ The fluorescent spectra were obtained as follows: 200 μL H₂O₂ (0.25 M), 200 μL Corrole-Fe₃O₄ nanocomposites with different concentrations and 200 μL terephthalic acid (0.5 mM) were incubated in 1.4 mL buffer (0.2 mM, pH 3.8) for 1h. Then, the mixture was monitored on a spectrofluorophotometer.

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3. Results and discussion

3.1. Characterization of Corrole-Fe₃O₄ nanocomposites.

Fig. 2 shows XRD patterns of the Fe₃O₄ MNPs and Corrole-Fe₃O₄ nanocomposites, respectively. The strong and sharp peaks at 2θ = 30.2◦, 35.5◦, 43.2^o, 53.4^o, 57.0^o and 62.6^o can be assigned to the (220), (311), (400), (422), (511) and (440) crystalline planes of Fe₃O₄ MNPs, respectively. In addition, these peaks were in well accordance with the standard PDF card (JCPDS card No.75-1610) of Fe₃O₄, indicating that the modification of Corrole did not change the phase of Fe₃O₄ NPs_.

Fig. 2. XRD patterns Fe₃O₄ MNPs (A) and Corrole-Fe₃O₄ MNPs (B), respectively.

Fig. 3 shows the morphologies of Fe₃O₄ MNPs and Corrole-Fe₃O₄ nanocomposites imaged by SEM, respectively. It can be seen from Fig. 3A that the pure Fe₃O₄ MNPs are composed of sphere-like nanoparticles. Meanwhile, slight aggregations were also observed shown in Fig. 3A, due to their magnetic properties. Fig. 3B indicates that Corrole-Fe₃O₄ nanocomposites almost inherited the morphologies of pure Fe₃O₄ MNPs.

Fig. 3. SEM images of (A) Fe₃O₄ NPs and (B) Corrole- Fe₃O₄ nanocomposites, respectively.

The composition of the Corrole-Fe₃O₄ MNPs was further confirmed by means of energy dispersive X-ray analysis (Fig. 4). As shown in Fig. 4, it can be found that the characteristic peaks of C, N, Cl, O and Fe elements deriving from Corrole and Fe₃O₄, respectively, could be detected, indicating the successful preparation of Corrole-Fe₃O₄ MNPs.

Fig. 4. EDX spectra of Corrole-Fe₃O₄ MNPs

The magnetic behavior of the obtained Corrole-Fe₃O₄ nanocomposite was characterized by using a magnet. As shown in Fig. 5, well-dispersed Corrole-Fe₃O₄ nanocomposites could be collected within 3 min when the magnet was put near as-prepared samples. On one hand, the result indicates the Corrole-Fe₃O₄ nanocomposites exhibit excellent magnetic property. On the other hand, benefiting from the magnetic properties, a simple and rapid magnetic separation approach was achieved, showing the possibility of easy recycling for extensive applications such as catalysis.

Fig. 5. The photographs of Corrole-Fe₃O₄ suspension in water before and after exposure to a magnet.

3.2. Peroxidase-like catalytic activity of the Corrole-Fe₃O₄ nanocomposites

The peroxidase-like activity of the as-prepared Corrole-Fe₃O₄ composites was evaluated through the catalysis between a colorless peroxidase substrate, 3,3′,5,5′-tetramethylbenzidine (TMB), and H₂O₂ to generate a colorimetric reaction. Fig. 6 presents the time-dependent absorbance changes of different reaction systems at 652 nm. Obviously, it can be found that there is a remarkable increase in absorbance of Corrole-Fe₃O₄-H₂O₂-TMB system, but very weak increases of Fe₃O₄-H₂O₂-TMB system, and almost no change for the other three systems: H₂O₂-TMB, Corrole-Fe₃O₄-TMB and TMB system, respectively. Furthermore, the corresponding color differences further reveal the enhanced peroxidase-like activity of the Corrole-Fe₃O₄ composites. These results suggested that in the presence of H₂O₂, the system containing Corrole-Fe₃O₄ nanocomposites produced a deeper color change than that of pure Fe₃O₄ alone, indicating that Corrole-Fe₃O₄ nanocomposites have a higher catalytic activity than that of pure Fe₃O₄ (Fig. 6a and 6b). However, in the absence of Corrole-Fe₃O₄ catalysts or H₂O₂, color change of the different solutions (Fig. 6c, 6d and 6e) is negligible, suggesting that both Corrole-Fe₃O₄ catalysts and H₂O₂ were indispensible for the color reaction. Clearly, these results show that Corrole-Fe₃O₄ demonstrated a much higher catalytic activity than that of pure Fe₃O₄. In addition, to highlight the advantages of our sensing material over the previously reported Fe-based nanostructures as catalysts, we made a comparison of the reaction time required for colorimetric visualization, as listed in Table 1. Obviously, Corrole-Fe₃O₄ exhibits the shortest time response for visual observation.

Fig. 6. Time-dependent absorbance evolution at 652 nm of TMB in different reaction systems: (a) TMB + Corrole-Fe₃O₄ + H₂O₂, (b) TMB + Fe₃O₄ + H₂O₂, (c) TMB + H₂O₂, (d) TMB + Corrole-Fe₃O₄, and (e) TMB. The concentrations of TMB, H₂O₂, and Corrole-Fe₃O₄ were 1 mM, 0.25 M, and 0.4 mg mL⁻¹, respectively. The insets represent the corresponding colorimetric photograph.

Table 1 A comparison of the reaction time required for colorimetric visualization using Corrole-Fe₃O₄ nanocomposites and other Fe-based nanostructures as catalysts.

Catalyst	Reaction time	Refs.
CoFe-LDHs	30 min	51
Fe-SBA-15	20 min	52
Fe(III)-polymer nanoparticles	20 min	53
Fe3O4 nanoparticles	10 min	10
GO-Fe3O4	15 min	54
Corrole-Fe3O4	5 min	This work

3.3. Optimization of experimental conditions

As the case of HRP and NPs-based peroxidase mimetics, the catalytic performance of Corrole-Fe₃O₄ is also dependent on pH and temperature (Fig. 7), respectively. The pH-dependent experiments in the range of 1.7-9, shown in Fig. 7A, indicated that the optimal catalytic activity was obtained when pH value was approximately 3.8. The phenomenon was consistent with those of HRP²⁴ and other nanoenzymes,^24,56 implying that the oxidation of TMB proceeds more easily under weakly acidic conditions than neutral conditions and basic conditions.^55,56 Besides, the effect of temperature on the catalytic activity from 26.5 ^oC to 66.5 ^oC was studied. The experimental results showed that the optimal temperature for the Corrole-Fe₃O₄ was 46.5 ^oC, which was slightly lower than those of IONPs (T = 50 ^oC)⁵⁷ and H@M composite (T=50 ^oC).⁵⁸ Thus, the optimal pH and temperature are approximately pH 3.8 and 46.5 ^oC, respectively.

Fig. 7. The influence of buffer pH (A) and incubation temperature (B) on the catalytic activity of the reaction system at 652 nm with the as-synthesized Corrole-Fe₃O₄ nanocomposites as peroxidase mimetics.

3.4. The steady-state kinetic analysis

 The peroxidase-like activity of Corrole-Fe₃O₄ enzyme mimics was further explored by performing steady-state kinetic studies. A series of kinetic data with H₂O₂ and TMB as substrates were acquired by changing one substrate concentration while leaving the other concentration constant. It was found that these kinetic data could fit well to typical Michaelis-Menten curves (Fig. 8A and 8B), which was similar in the kinetic behavior to those of nature HRP enzyme²⁴ and reported peroxidase mimics.^54,59 Michaelis-Menten constant (K_m) and the maximum initial velocity (V_max) listed in Table 2 were calculated by using Lineweaver-Burk double-reciprocal plots (Fig. 8C and 8D). As we all know, K_m is generally used to measure the affinity of a given enzyme toward the substrate: the smaller the value of K_m, the stronger the affinity between the enzyme and the substrate. From Table 2, the K_m value of Corrole-Fe₃O₄ with H₂O₂ as substrate was 1.33 mM, which was much lower than HRP²⁴ and other Fe-based peroxidase mimetics, indicating that Corrole-Fe₃O₄ had a higher affinity for H₂O₂ than that of HRP²⁴ and other Fe-based peroxidase mimetics.^31,59,60 On the other hand, the K_m value of Corrole-Fe₃O₄ with TMB as a substrate was determined to be 0.697 mM, which was much higher than that of HRP and other Fe-based peroxidase mimetics^31,54,60,32 except for ZnFe₂O₄,⁵⁹ suggesting that Corrole-Fe₃O₄ had a lower affinity for TMB than that of HRP²⁴ and other Fe-based peroxidase mimetics,^31,54,60,32 while it had a higher affinity for TMB than that of ZnFe₂O₄.⁵⁹

Fig. 8. Steady-state kinetic assay and catalytic mechanism of the Corrole-Fe₃O₄ nanocomposites. (A) The concentration of TMB was 1 mM and the H₂O₂ concentration was varied. (B) The concentration of H₂O₂ was 0.25 M and the TMB concentration was varied. (C and D) Double reciprocal plots of catalytic activity of Corrole-Fe₃O₄ nanocomposites with the concentration of one substrate (H₂O₂ or TMB) fixed and the other varied.

Table 2 Comparison of the apparent Michaelis-Menten constant (K_m) and maximum reaction rate (V_max).

catalyst	Substrate	Km (mM)	Vmax (10-8 M s-1)	Ref
Corrole-Fe3O4	H2O2	1.33	0.6372	This work
Corrole-Fe3O4 Fe3O4 Fe3O4 ZnFe2O4	TMB H2O2 TMB H2O2	0.697 154 0.098 1.66	4.482 9.78 3.44 7.74	This work 14 14 43
ZnFe2O4 PBMNPs3 PBMNPs3 GO-Fe3O4 GO-Fe3O4 Casein-Fe3O4 Casein-Fe3O4 H2TCPP-Fe3O4 H2TCPP-Fe3O4 HRP HRP	TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB H2O2 TMB	0.85 323.6 0.307 0.71 0.43 4.75 0.021 0.919 0.439 3.70 0.434	13.31 117 106 5.31 13.08 15.9 10.6 1.075 19.08 8.71 10	43 19 19 39 39 44 44 20a 20a 14 14

3.5. Mechanism studies on catalytic reaction

The catalytic mechanism of the peroxidase-like of the Corrole-Fe₃O₄ nanocomposites was assumed to result from the decomposition of H₂O₂ into •OH during the reactions, similar to previous peroxidase mimics.^61-63 This hypothesis was confirmed by signal intensity of the fluorescence spectra. As shown in Fig. 9, it can be clearly seen that signal intensity gradually increased with increasing the amount of Corrole-Fe₃O₄ nanocomposites. In other words, the larger amount of Corrole-Fe₃O₄ NPs, the higher yield of •OH. The result shows that Corrole-Fe₃O₄ NPs have excellent intrinsic peroxidase-like activity and accelerate the decomposition of H₂O₂ to generate a high yield of •OH, consistent with previous works such as H₂TCPP-NiO nanorods⁶⁴ and H₂TCPP-Fe₃O₄ nanoparticles.³²

Fig. 9. Fluorescence spectra of terephthalic acid (TA) in the presence of H₂O₂ and Corrole-Fe₃O₄ with different concentrations. The reaction was performed in 1.4 mL NaAc buffer (0.2 mM, pH 3.8) containing 0.5 mM TA, 0.25 M H₂O₂ and Corrole-Fe₃O₄ with different concentrations, the total volume is 2 mL. The excitation wavelength (EX) is at 315 nm.

3.6. H₂O₂ detection using the Corrole-Fe₃O₄ nanocomposites as peroxidase mimetics

In light of the catalytic activity of Corrole-Fe₃O₄ depends on the concentration of H₂O₂, the system discussed above could be applied to detect H₂O₂. Under optimal conditions, the H₂O₂ concentration-response curve was plotted in Fig. 10A, showing that the response signal gradually increases with increasing H₂O₂ concentration. Fig. 10B manifests the calibration curve of absorbance at 652 nm versus H₂O₂ concentration. A linear range from 10 µM to 100 µM was observed with the limit of detection as low as 3.6 µM. It is found that the detection limit of Corrole-Fe₃O₄ nanocomposites is of the same order of magnitude as that of Fe₃O₄ nanoparticles (3 µM)¹⁰ and Cu-SBA-15 sample (3.7 µM),⁶⁵ lower than that of [FeIII(biuret-amide)] on mesoporous silica (10 µM).⁶⁶

 

Fig. 10. (A) A dose-response curve for H₂O₂ using Corrole-Fe₃O₄ nanocomposites. (B) The linear calibration plot for the detection of H₂O₂ at 652 nm. Error bars represent the standard deviation based on three repeated measurements.

3.7. Glucose detection using the Corrole-Fe₃O₄ nanocomposites as peroxidase mimetics

H₂O₂ is an important by-product involved in many biochemical reactions, such as the glucose oxidase (GOx)-catalyzed oxidation reaction of glucose. Additionally, Glucose detection related to human health levels is of vital importance in common analysis. Based on results of the study, a colorimetric quantitative test of glucose can be realized using the Corrole-Fe₃O₄ nanocomposites combined with GOx. Fig. 11A shows a typical glucose concentration-response curve. It can be found that absorbance signal increases gradually with increasing glucose concentration. The corresponding calibration plot was described in Fig. 11B. The absorbance was linearly related to glucose concentration range from 4 µM to 40 µM. The limit of detection was 25 calculated to be 2.46 µM (S/N=3). The results show that our sensing method presents a reasonable linear response range compared to some reported nanomaterials.^{32,54,59,60,67} More significantly, the limit of detection is lower than Fe-based peroxidase mimetics like Fe₃O₄ magnetic nanoparticles (30 µM),¹⁰ GCNT-Fe₃O₄ nanocomposite (22 µM),⁶⁸ [FeIII(biuret-amide)] on mesoporous silica (10 µM)⁵⁰ and MnSe-g-C₃N₄ peroxidase mimetics (8 µM),⁶⁹ indicating a more sensitivity of Corrole-Fe₃O₄ MNPs.

To examine the selectivity of the present method toward glucose, detection experiments using maltose, sucrose, lactose and fructose as control samples were investigated (Fig. 12). It can be obviously found that the absorbance of these glucose analogues was not obvious even when their concentrations were 5 times as high as glucose. The color difference also can be detected visually from the corresponding photographs (Fig. 12 inset). These results demonstrated that the method developed here has high selectivity toward glucose detection

Fig. 11. (A) A dose-response curve for glucose detection using Corrole-Fe₃O₄ nanocomposites. (B) The linear calibration plot for glucose. Error bars represent the standard deviation based on three repeated measurements.

Fig. 12. Determination of the selectivity of glucose detection (from left to right: blank, 5 mM maltose, 5 mM sucrose, 5 mM lactose, 5 mM fructose and 1 mM glucose). Inset: the color difference with different samples.

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4. Conclusions

In conclusion, the Corrole-Fe₃O₄ nanocomposites were prepared for the first time by a facile two-step approach and developed as an enhanced peroxidase mimetic. Furthermore, the introduction of Corrole could significantly improve peroxidase-like activity of the Fe₃O₄, which could be observed visually from both the absorbance data and photographs before and after modification by Corrole. More importantly, the optimization results indicate the catalytic activity of the Corrole-Fe₃O₄ is sensitive to variations of pH and temperature. The affinity features for H₂O₂ and TMB were applicable to quantitative determination of H₂O₂ and glucose, exhibiting reasonable response range and low limit of detection, respectively. Given the advantages of low cost, easy separation, facial fabrication, fast response and good selectivity, it can be expected that the novel Corrole-Fe₃O₄ nanocomposites as a peroxidase mimetic may broaden the scope of its applications in the field of catalysis and bioassays.

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Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant no. 21271119), Scientific Research Foundation of Shandong University of Science & Technology (Grant No.2015RCJJ018).

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