Composite of graphene quantum dots and Fe₃O₄ nanoparticles: peroxidase activity and application in phenolic compound removal

Abstract

Graphene quantum dots (GQDs) are graphene sheets with lateral sizes less than 100 nm, and have a higher electron conjugate state and a better dispersion ability in aqueous solution compared to micrometer-sized graphene oxide (GO) sheets. Therefore they can overcome the drawbacks of GO and are an ideal candidate for nano-composites. In this work, composites of GQDs and Fe₃O₄ nanoparticles (NPs) (GQDs/Fe₃O₄) were prepared via a one-step co-precipitation approach. The as-prepared GQDs/Fe₃O₄ composites showed superb peroxidase-like activities, which were much higher than composites of micrometer sized GO and Fe₃O₄ NPs (GO/Fe₃O₄), individual GQDs, and individual Fe₃O₄ NPs. The excellent peroxidase activities of the GQDs/Fe₃O₄ composites can be attributed to the unique properties of GQDs and the synergistic interactions between the GQDs and Fe₃O₄ NPs. The GQDs/Fe₃O₄ composites also exhibited a higher stability and reusability than natural peroxidases. The application of a GQDs/Fe₃O₄ composite as a catalyst for the removal of phenolic compounds from aqueous solutions was explored with nine phenolic compounds, and showed better or comparable removal efficiencies for some phenolic compounds compared to native horseradish peroxidase (HRP) under the same conditions. The extraordinary catalytic performance and physical properties of the as-prepared GQDs/Fe₃O₄ composite render it practically useful for industrial wastewater treatment.

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Introduction

Natural peroxidases, such as horseradish peroxidase (HRP), vanadium haloperoxidase and glutathione peroxidase, are well known catalysts for many oxidation reactions in the presence of a peroxide (H₂O₂ in most cases).^1–3 However, the utilization of natural peroxidases on a large scale remains challenging, mainly because of their high costs and low stabilities. Therefore, the exploration of peroxidase mimic systems with high stability and activity has attracted great attention during the last few decades.^4–6 For instance, Fe₃O₄,^7–9 Fe₂O₃,^10–12 Au,¹³ Pt,¹⁴ Co₃O₄,^15,16 V₂O₅,^17,18 CuO¹⁹ and RuO₂²⁰ nanoparticles (NPs) possess peroxidase-like catalytic activity toward the reduction of H₂O₂, and have been used in various fields such as H₂O₂ detection,²¹ immunoassays,⁷ glucose detection,^22,23 nucleic acids detection^24,25 and cytotoxicity assays.²⁶ Due to the low cost, easy preparation, high thermal stability and reusability in comparison to natural peroxidases, NPs have also been used as catalysts for the oxidation degradation of organic pollutants in water.^27–31 Unfortunately, the peroxidase-like activity of most bare NPs is relatively low.³² Additionally, without proper surface modification, bare NPs agglomerate quickly, and thus their effective surface area and peroxidase-like activity are suppressed.

Single-atomic layered graphene oxide (GO), due to its high electron conjugate state and abundant surface oxygen-containing groups, has showed peroxidase-like activity to catalyze the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) with H₂O₂.³³ More attractively, GO sheets can serve as matrices for templating or anchoring different nanoparticles on them, which can dramatically enhance the properties of the nanoparticles.^34–40 For example, a Au NPs/GO hybrid exhibited a synergetic and switchable peroxidase-like activity in response to specific DNA molecules.³⁵ The GO supported magnetic Fe₃O₄ NPs also showed a relatively stronger peroxidase-like catalytic activity for the reduction of H₂O₂ compared to the NPs alone.^41–43 However, the lateral size of GO sheets generated from graphite oxidation and exfoliation can usually reach several micrometers or even larger, which is sometimes a drawback for using GO composites as catalysts. Graphene quantum dots (GQDs) are graphene sheets with lateral sizes less than 100 nm and have several unique properties, such as a better dispersing capability in aqueous solutions, compared to micrometer-sized GO sheets.^44,45 Actually, we have demonstrated previously that GQDs, prepared through the photo-Fenton reaction of GO, with a defect-free 2D hexagonal lattice structure and enriched periphery carboxylic groups, have a much higher intrinsic peroxidase-like activity compared to micrometer sized GO, and show a greater performance and stability in H₂O₂ detection.^46,47

Herein, we explore the peroxidase-like activity of composites of GQDs with Fe₃O₄ NPs. To this end, we have prepared and fully characterized the composites of GQDs and Fe₃O₄ NPs (GQDs/Fe₃O₄). We have illustrated that the as-obtained GQDs/Fe₃O₄ composites exhibited a much higher peroxidase-like activity than the composites of micrometer sized graphene oxide (GO) and Fe₃O₄ NPs (GO/Fe₃O₄), individual GQDs and Fe₃O₄ NPs. The GQDs/Fe₃O₄ composite with a GQDs to Fe₃O₄ ratio of 1 : 1 (in weight) showed the highest catalytic activity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) with H₂O₂, which was about 22 fold higher than for individual Fe₃O₄ NPs, 25 fold higher than for GQDs, and 9 fold higher than for GO/Fe₃O₄ (1–1). The GQDs/Fe₃O₄ composites were stable over a wide range of temperatures and showed good reusability. In addition, we explored the potential application of GQDs/Fe₃O₄ composites for the removal of phenolic compounds. Although the removal efficiency was slightly lower than that with the same amount of HRP, the high thermal stability, easy-preparation and low cost of the GQDs/Fe₃O₄ composites still show its potential in the treatment of industrial wastewater.

Experimental methods

Materials

Horseradish peroxidase (HRP) was purchased from Majorbio Biotech Company, USA. 4-Aminoantipyrine (4-AAP) was bought from East China Normal University Chemical Factory. Other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd., Shanghai, China. All reagents were used as received.

Fabrication of the GQDs/Fe₃O₄ and GO/Fe₃O₄ composites

GO and GQDs were prepared as described in our previous work.^46,48,49 The GQDs/Fe₃O₄ composites were synthesized through a one-step co-precipitation procedure. In a typical experiment, 67.5 mg (0.25 mmol) of FeCl₃·6H₂O and 34.75 mg (0.125 mmol) of FeSO₄·7H₂O (with a molar ratio of 2 : 1) were dissolved in 2 mL of an acidic aqueous solution with pH = 5, which was adjusted by diluted HCl to decrease the hydrolysis of Fe²⁺. The GQDs (2.9 mg mL⁻¹) were dispersed in water with pH = 12, which was adjusted with ammonium hydroxide. All solutions were maintained in a N₂ atmosphere. Then, the solution of ferrous and ferric salts was added drop-wisely into the solution of GQDs with vigorous stirring at 25 °C in a nitrogen atmosphere. The ratios of GQDs to Fe₃O₄ (theoretically estimated based on the initial amount of GQDs and ferrous and ferric salts) were controlled to be 10/1, 5/1, 2/1, 1/1, 1/2, 1/5, and 1/10 in weight, respectively. After 90 min of reaction, the precipitates were collected by centrifugation and washed with water for 5 times. The solid products were then re-dispersed in distilled water and stored at 4 °C. The as-obtained composites were named as GQDs/Fe₃O₄ 10–1, 5–1, 2–1, 1–1, 1–2, 1–5, and 1–10 accordingly. For comparison, the GO/Fe₃O₄ composites with GO to Fe₃O₄ ratios of 10/1, 5/1, 2/1, 1/1, 1/2, 1/5, and 1/10 were prepared through a similar procedure, and were named as GO/Fe₃O₄ 10–1, 5–1, 2–1, 1–1, 1–2, 1–5, and 1–10 accordingly. Bare Fe₃O₄ NPs were also prepared.

Characterization

Atomic force microscope (AFM) images were taken on a MultiMode Nanoscope V scanning probe microscopy system (Veeco, USA). AFM cantilever tips with a force constant of ∼48 N m⁻¹ and a resonance vibration frequency of ∼330 kHz were used and the scanning rate was set at 1 Hz. Transmission electron microscope (TEM) images were obtained using a JEM-2010 transmission electron microscope (JEOL, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on an Axis Ultra DLD spectrometer (Kratos Analytical, UK) using a monochromated Al Kα source at 15 kV. FTIR spectra were acquired on a Nicolet 6700 FT-IR (Thermo Electron, USA) in the range of 4000–400 cm⁻¹. The samples for the FTIR measurements were prepared by grinding the dried powder with KBr and then compressing them into thin pellets. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Lakeshore 736, USA) at 300 K. Thermogravimetric (TG) analyses were carried out using a thermogravimetric analyzer (Shimadzu DSC-50, Japan) in a 30–1000 °C temperature range at a scan rate of 10 °C min⁻¹ using stainless-steel pans under an air atmosphere. UV-visible absorption spectra were recorded using a Shimadzu UV-2550 (Shimadzu, Japan).

Peroxidase-like activity assay

The peroxidase-like activity of the GQDs/Fe₃O₄ composites was assayed as following: GQDs/Fe₃O₄ (10 μg) was first dispersed in 1 mL of NaAc buffer (0.1 M, pH 3.5) containing 0.9 mM TMB and 5 mM H₂O₂, as substrates, in a 1 mL quartz cuvette. The mixture was measured in time course mode by monitoring the absorbance change at 652 nm for 300 s on a UV-visible spectrophotometer at 30 °C. Similar procedures were also performed for GO/Fe₃O₄, GQDs and Fe₃O₄.

Phenolic compounds removal

The removal of phenolic compounds using GQDs/Fe₃O₄ 1–1 as the catalyst in the presence of H₂O₂ was explored. In a typical experiment, 10 μg of GQDs/Fe₃O₄ 1–1 was added in 1 mL of NaAc buffer (0.1 M, pH 3.5) containing 6 mM of phenol and 30 mM of H₂O₂, and then incubated at 30 °C for 3 h. To compare the peroxidase-like activity of GQDs/Fe₃O₄ 1–1 with that of HRP, 10 μg of HRP was used to perform the same assay.

After 3 h of reaction, the removal efficiency was determined by measuring the residual phenolic compounds present in the supernatant with potassium ferricyanide and 4-aminoantipyrine (4-AAP).⁵⁰ Typically, after centrifugation, 25 μL of the clear supernatant from the reaction was mixed in turn with 775 μL of phosphate buffer (0.1 M, pH 7.0), 100 μL of potassium ferricyanide (83.4 mM in 0.25 M sodium bicarbonate solution), and 100 μL of 4-AAP (20.8 mM in 0.25 M sodium bicarbonate solution). The total volume reached 1 mL. After the color of the mixture had developed completely over 5 minutes, the absorbance was measured at 490 nm against a blank (800 μL of phosphate buffer, 100 μL of potassium ferricyanide, and 100 μL of 4-AAP solution). For the phenolic compounds which cannot produce a colorimetric product under these conditions, such as 2-aminophenol, 1,4-benzenediol and catechol, the absorbances of their characteristic absorption peaks were measured directly.

Results and discussion

Characterization of the GQDs/Fe₃O₄ composites

The morphologies of the GQDs and GO sheets used in this work were characterized using AFM imaging (acquired under tapping mode) and the images are depicted in Fig. S1, ESI.† The average heights of both the GO sheets and the GQDs were ∼1 nm (Fig. S1 inset, ESI†), demonstrating their single atomic layer feature.^48,49 Fig. 1a and b show the morphology of the as-prepared GQDs/Fe₃O₄ 1–1 composite. Differing from the two dimensional layered structures of GQDs and GO, the Fe₃O₄ NPs and GQDs aggregated into three dimensional structures. Most probably, as schematically depicted in Fig. 1c, the Fe₃O₄ NPs are surrounded by GQDs to form composite particles. The sizes of the GQDs/Fe₃O₄ particles reached 30 nm (Fig. 1a). To get an insight into the hyperfine chemical structure, the FTIR spectra of GQDs and GQDs/Fe₃O₄ 1–1 were measured and are shown in Fig. 2a. The appearance of the stretching vibrations peak at 597 cm⁻¹ for Fe–O suggests the presence of iron oxide. Comparably, besides the vibration and deformation peaks of the O–H groups at 3409 cm⁻¹ and 1384 cm⁻¹, the C–O (epoxy) stretching vibration peak at 1227 cm⁻¹, and the C–O (alkoxy) stretching peak at 1032 cm⁻¹, after the formation of the composite, the C=O stretching vibration peak shifted from 1736 to 1624 cm⁻¹, indicating that the periphery carboxylic groups of the GQDs had been converted into carboxylate groups. This result led us to believe that the GQDs bound to the Fe₃O₄ particles through, most probably, Fe–O chemical bonds. Some other new bands at 1387 and 1227 cm⁻¹ may also be attributed to Fe–O bond formation between the carboxyl groups in GQDs and the iron atoms in the Fe₃O₄ NPs.

Fig. 1 AFM images, TEM images, and schematic representations of the structures of the GQDs/Fe₃O₄ 1–1 (a, b, c) and GO/Fe₃O₄ 1–1 (d, e, f) composites, respectively.

Fig. 2 (a) FTIR spectra of the GQDs (1) and the GQDs/Fe₃O₄ 1–1 composite (2); (b) TG curves of the Fe₃O₄ NPs (1), GQDs/Fe₃O₄ 1–1 (2) and the GQDs (3); (c) XPS spectrum of Fe 2p in GQDs/Fe₃O₄ 1–1; (d) hysteresis loop for GQDs/Fe₃O₄ 1–1.

The content of GQDs in the composites is a key issue to understand their structures. Fig. 2b shows the TG curves for the GQDs/Fe₃O₄ 1–1 composite. In an air atmosphere, the mass loss can be mainly attributed to the decomposition of the GQDs. At 1000 °C, the Fe₃O₄ NPs alone retains 91.3% of the initial weight, and the GQDs were totally gone. The GQDs/Fe₃O₄ 1–1 composite retained about 58.2% of the initial weight, implying the actual ratio of GQDs to Fe₃O₄ in the GQDs/Fe₃O₄ 1–1 composite was approximately 1 : 1 in weight, agreeing with the initial amounts of the GQDs and iron precursors. The formation of Fe₃O₄ was also confirmed using X-ray photoelectron spectroscopy (XPS) and magnetic property measurements. Fig. 2c shows two peaks at 711.2 and 725.1 eV, corresponding to 2p_3/2 and 2p_1/2 for Fe binding with oxygen.⁵¹ Additionally, the spectrum does not contain the charge transfer satellites for Fe 2p_3/2 around 720 eV, reflecting the formation of mixed oxides of Fe(II) and Fe(III), such as Fe₃O₄.⁵² The magnetic properties of GQDs/Fe₃O₄ 1–1 were also investigated with a vibrating sample magnetometer. Fig. 2d and S2† depict the magnetization curves of GQDs/Fe₃O₄ 1–1 and bare Fe₃O₄ measured at 300 K. The saturation magnetization values were 1.6 emu g⁻¹ for GQDs/Fe₃O₄ 1–1 and 61.0 emu g⁻¹ for Fe₃O₄. Both curves exhibited no magnetic hysteresis loops revealing their superparamagnetic characteristics. Due to the non-magnetic GQD portion, the saturation supermagnetization of GQDs/Fe₃O₄ 1–1 was much lower than that of Fe₃O₄.

For comparison, we also prepared composites of GQDs/Fe₃O₄ and GO/Fe₃O₄ with different GQDs to Fe₃O₄ and GO to Fe₃O₄ ratios through similar procedures. As shown in Fig. 1d and e, in the GO/Fe₃O₄ composites, the GO sheets with micrometer lateral sizes were well decorated with a large amount of Fe₃O₄ NPs on both sides, which is different from the GQDs/Fe₃O₄ composites. Notably, the as-obtained GQDs/Fe₃O₄ composites exhibited a much better dispensability in water than bare Fe₃O₄ nanoparticles, since the Fe₃O₄ NPs were wrapped by GQDs which have abundant oxygen-containing groups (Fig. 1c). Additionally, as previously mentioned, GQDs and Fe₃O₄ both exhibit peroxidase-like activities.^7,33 Therefore, we foresaw that the GQDs/Fe₃O₄ composites might have high peroxidase-like activities.

Peroxidase-like activities of the GQDs/Fe₃O₄ composites

The peroxidase-like activities of the as-prepared GQDs/Fe₃O₄ composites were evaluated through the catalytic oxidation of TMB in the presence of H₂O₂. The activities were also compared with those of bare GQDs, Fe₃O₄ and GO/Fe₃O₄. As depicted in Fig. S4, ESI,† the color of the reaction solution with the GQDs/Fe₃O₄ 1–1 composite changed more rapidly than with GO/Fe₃O₄ 1–1, GQDs, or Fe₃O₄ as observed by the naked-eye. These qualitative results indicate that the as-obtained GQDs/Fe₃O₄ composite had a much higher peroxidase activity than the others under the same reaction conditions.

The activities of the as-prepared composites were measured quantitatively by following the absorbance intensity of the reaction solutions at 652 nm. Fig. 3a compares the absorbance intensities of the reaction products using Fe₃O₄, GQDs/Fe₃O₄ and GO/Fe₃O₄, with GQDs or GO to Fe₃O₄ ratios of 0.1–10. Obviously, the GQDs/Fe₃O₄ composites were more active than the GO/Fe₃O₄ composites, GQDs, GO and Fe₃O₄. Among the GQDs/Fe₃O₄ composites, the GQDs/Fe₃O₄ 1–1 composite exhibited the highest catalytic activity, which was about 9 fold higher than GO/Fe₃O₄ 1–1, 22 fold higher than bare Fe₃O₄, and 25 fold higher than bare GQDs. The activities of the GQDs/Fe₃O₄ composites were not a simple addition of the activities of the GQDs and Fe₃O₄, there is a synergistic effect. To confirm this hypothesis, the GQDs and the Fe₃O₄ NPs (1 : 1 in weight) were mixed together as a control. As shown in Fig. 3b, the mixture of GQDs and Fe₃O₄ NPs exhibited a much weaker peroxidase-like activity than GQDs/Fe₃O₄ 1–1. The superb peroxidase activity of GQDs/Fe₃O₄ 1–1 suggests that it is possible that during the preparation of GQDs/Fe₃O₄, the oxygen atoms in the carboxylate groups in the GQDs coordinate to iron ions forming a GQDs/Fe₃O₄ conjugate, as concluded from the IR results. The formation of the conjugates makes the electron-transfer from the electron rich GQDs to Fe₃O₄ NPs more efficient. This is consistent with the literature that reports that Fe²⁺ may play a dominant role in the catalytic peroxidase-like activity of Fe₃O₄ NPs.⁷ This might be the partial reason that the as-prepared GQDs/Fe₃O₄ composites have higher peroxidase-like activities compared to the mixture of GQDs and Fe₃O₄ NPs, and the individual GQDs and Fe₃O₄ NPs. In the case of the GO/Fe₃O₄ composites, due to the large lateral size and the surface oxygen-containing groups, the electron conjugate state of GO is worse than that of GQDs, and thus the electron-transfer from GO to the Fe₃O₄ NPs that are anchored on its surface is less efficient (Fig. 1d). The GO/Fe₃O₄ structure also affects its dispensability in aqueous solutions, which may eventually influence its catalysis. Consequently, the GO/Fe₃O₄ composites show a much lower peroxidase-like catalytic activity than the GQDs/Fe₃O₄ composites.

Fig. 3 (a) Comparison of the reaction products of the GQDs/Fe₃O₄ or GO/Fe₃O₄ composites with different weight ratios. The reaction was performed with GQDs/Fe₃O₄ or GO/Fe₃O₄ composites (10 μg), TMB (0.9 mM) and H₂O₂ (5 mM) in 1 mL of NaAc (pH 3.5). The ratio of Fe₃O₄ alone equals 0, while the ratio of GQD or GO alone equals ∝. (b) Time-dependent absorbance changes at 652 nm for the as-prepared composite and its components. (c) The reaction rates of the different materials (10 μg) versus the concentration of H₂O₂ at 30 °C, with TMB (0.9 mM) in 1 mL of NaAc (pH 3.5).

To optimize the peroxidase-like activity of the GQDs/Fe₃O₄ composite, the initial reaction rate versus the H₂O₂ concentration was examined. The same experiments were done with GO/Fe₃O₄, GQDs, and Fe₃O₄ NPs for comparison. As presented in Fig. 3c, the reaction rate using GQDs/Fe₃O₄ 1–1 increased very fast at low H₂O₂ concentrations and reached a plateau when the H₂O₂ concentration was 5 mM. In contrast, with the same H₂O₂ concentrations, all the other materials showed much slower reaction rates. The peroxidase-like activities of these materials were further characterized by the Michaelis–Menten constant (K_m), the maximal reaction velocity (V_max), and the turnover number (K_cat). The values were obtained according to the Lineweaver–Burk equation, and are summarized in Table 1. The K_m value for GQDs/Fe₃O₄ 1–1 is smaller than for the other materials, implying that GQDs/Fe₃O₄ 1–1 has a higher affinity for H₂O₂. The higher V_max value for GQDs/Fe₃O₄ 1–1 indicates the synergistic effect on the peroxidase-like activity between GQDs and Fe₃O₄ NPs. However, the K_cat value for GQDs/Fe₃O₄ 1–1 is lower than that for HRP using the same amount of HRP and GQDs/Fe₃O₄ 1–1. In addition, the GQDs/Fe₃O₄ 1–1 composite shows a better affinity, and a higher V_max with the other substrate TMB (Fig. S5 and Table S1, ESI†).

Table 1 Comparison of the apparent kinetic parameters Michaelis–Menten constant (K_m) and the maximum reaction rate (V_max) of the different materials (H₂O₂ as the substrate). The turnover number (K_cat) was determined with the same amount of the material or HRP

Material	Km (mM)	Vmax (μM min^−1)	Kcat (mM min^−1 g^−1)
GQDs/Fe3O4 1–1	0.46	14.08	1.4 × 10^6
GO/Fe3O4 1–1	1.10	4.42	4.4 × 10^5
GQDs	0.62	1.42	1.4 × 10^5
Fe3O4	0.49	0.79	7.9 × 10^4
HRP	1.13	10.74	3.6 × 10^9

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Though the turnover number for the GQDs/Fe₃O₄ 1–1 composite is lower than for native HRP, as an inorganic composite, GQDs/Fe₃O₄ should have a much better stability. To illustrate its thermal stability, GQDs/Fe₃O₄ 1–1 was incubated for 2 h at different temperatures before the catalytic reaction. As shown in Fig. 4a, the activity of GQDs/Fe₃O₄ 1–1 was found to be well maintained from 20 to 80 °C, and the optimal working temperature was around 50 °C. Fig. 4b shows that GQDs/Fe₃O₄ 1–1 retained ∼60% of its initial activity after being reused more than 10 times. It was found that 64% of the initial activity of GQDs/Fe₃O₄ 1–1 remained after more than 40 days storage at 4 °C (Fig. S6, ESI†). Comparably, more than 60% of the activity of HRP was lost under the same conditions. The high activity and stability, combined with easy preparation and low costs, renders the GQDs/Fe₃O₄ 1–1 composite a potent peroxidase mimic for many applications under harsh conditions.

Fig. 4 (a) Effect of temperature on the peroxidase-like activity of GQDs/Fe₃O₄ 1–1; (b) reusability of the GQDs/Fe₃O₄ 1–1 composite.

Application of GQDs/Fe₃O₄ 1–1 in the removal of phenolic compounds

To explore the application of GQDs/Fe₃O₄ 1–1 in the removal of phenolic compounds from water, nine compounds with different substituent groups (Table 2) were tested. In parallel, for comparison, the oxidation reactions of these phenolic compounds with the same amount of HRP as a catalyst were also conducted. As summarized in Table 2, for phenol, the removal efficiency with the GQDs/Fe₃O₄ 1–1 composite was higher than with HRP. For some phenolic compounds, such as 3-aminophenol and 2-cholorolphenol, the removal efficiencies with GQDs/Fe₃O₄ 1–1 were comparable to using HRP. For the other phenolic compounds tested, the GQDs/Fe₃O₄ 1–1 composite showed lower removal efficiencies compared to HRP. The removal efficiency for phenol with GQDs/Fe₃O₄ 1–1 was dependent on the concentration of H₂O₂ and temperature (Fig. 5). The GQDs/Fe₃O₄ 1–1 composite showed a pronounced removal efficiency for phenol from 20 to 70 °C. The oxidation reaction of the phenolic compounds is determined by both steric effects and inductive effects imparted by the substituent on the phenyl ring. In the case of GQDs/Fe₃O₄ 1–1, we believe steric effects possibly dominate the reaction, since the existence of a substituent decreased the reaction rate, and thus the removal efficiency of phenol was the best. Taking these results together, the easy-preparation, high reusability, improved stability, and low cost of the GQDs/Fe₃O₄ composite give it potential in the treatment of industrial wastewater.

Table 2 Removal efficiencies of phenolic compounds using the same amount of GQDs/Fe₃O₄ 1–1 and HRP

Substrate	Removal efficiency (%)
	GQDs/Fe3O4 1–1	HRP
Phenol	80.3%	67.4%
2-Methoxyphenol	26.3%	97.7%
4-Methoxyphenol	46.7%	96.9%
3-Aminophenol	16.9%	17.3%
2-Aminophenol	47.9%	86.3%
1,4-Benzenediol	41.0%	72.6%
2-Cholorolphenol	14.7%	18.2%
4-Cholorolphenol	21.7%	37.0%
Catechol	13.4%	46.5%

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Fig. 5 The phenol removal efficiency of GQDs/Fe₃O₄ 1–1 versus the concentration of H₂O₂ (a) and temperature (b).

Conclusions

A facile approach for the synthesis of GQDs/Fe₃O₄ composites through co-precipitation was developed. The peroxidase-like activities of the as-obtained GQDs/Fe₃O₄ composites were explored and compared with those of GQDs, Fe₃O₄ nanoparticles, GO/Fe₃O₄ composites, and HRP. It was illustrated that the GQDs/Fe₃O₄ 1–1 composite showed a much higher activity compared to the corresponding GO/Fe₃O₄ composite or its individual components. The high peroxidase-like activity of GQDs/Fe₃O₄ 1–1 can be attributed to the unique properties of GQDs and the synergistic effect of GQDs and Fe₃O₄. Due to its high catalytic activity, good stability and reusability, the application of the GQDs/Fe₃O₄ 1–1 composite for the removal of phenolic compounds from aqueous solutions was explored. We demonstrated that the as-obtained GQDs/Fe₃O₄ 1–1 composite exhibited better or comparable catalytic efficiencies for some phenolic compounds compared to HRP. These results indicate that the GQDs/Fe₃O₄ 1–1 composite is a potent peroxidase mimic that holds great promise for many catalytic applications.

Notes

The authors declare no competing financial interests.

Acknowledgements

We thank the National Science foundation of China (no. 91123011, 90923041, 31070742), the state key laboratory of bioreactor engineering (no. 2060204), 111 Project (no. B07023), the Science and Technology Commission of Shanghai Municipality (no. 11DZ2260600 and 12nm0503500), and the National “863” Program of China (no. 2012AA022603) for the financial support of this work.

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Footnote

† Electronic supplementary information (ESI) available: AFM images of graphene oxide (GO) and graphene quantum dots (GQDs); hysteresis loops of Fe₃O₄; TEM image of Fe₃O₄ NPs; photograph of the products formed after the oxidation of TMB; reaction rates for the different materials versus the concentration of TMB; comparison of the storage stability of the GQDs/Fe₃O₄ 1–1 and HRP; comparison of the apparent kinetic parameters Michaelis constant (K_m) and the maximum reaction rate (V_max) for the different materials (TMB as a substrate). See DOI: 10.1039/c3ra44709j

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