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Article

Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes

by
Junqi Zha
1,†,
Wugao Wu
1,†,
Peng Xie
1,
Honghua Han
1,
Zheng Fang
2,*,
Yantao Chen
1,* and
Zhongfan Jia
3,*
1
Shenzhen Key Laboratory of Environmental Chemistry and Ecological Remediation, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, China
2
State Key Laboratory of New Textile Materials & Advanced Processing Technology, School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, China
3
Institute for Nanoscale Science and Technology, College of Science and Engineering, Flinders University, Bedford Park, SA 5042, Australia
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(6), 614; https://doi.org/10.3390/catal12060614
Submission received: 20 April 2022 / Revised: 25 May 2022 / Accepted: 29 May 2022 / Published: 3 June 2022
(This article belongs to the Special Issue Polymer-Based Artificial Enzymes)
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Abstract

:
Peroxidase-like nanozymes are nanoscale materials that can closely mimic the activity of natural peroxidase for a range of oxidation reactions. Surface coating with polymer nanogels has been considered to prevent the aggregation of nanozymes. For a long time, the understanding of polymer coating has been largely limited to its stabilization effect on the nanozyme in aqueous media, while little is known about how polymer coating plays a role in interaction with substrates and primary oxidants to dictate the catalytic process. This work reported a facile sequential modification of Fe3O4 nanoparticles to polyacrylamide coated nanozymes, and as low as 112 mg/L samples with only 5 mg/L Fe3O4 could nearly quantitatively (99%) remove a library of organic dyes with either H2O2 or Na2S2O8 as primary oxidants. The catalytic results and molecular simulation provide both experimental and computational evidence that the hydrogen bonding interaction between the reactant and nanozymes is key for the high local concentration hence catalytic efficiency. We envision that this work, for the first time, provides some insights into the role of polymer coating in enhancing the catalytic activity of nanozyme apart from the well-known water dispersity effect.

1. Introduction

Advanced oxidation processes (AOPs), producing highly oxidative hydroxyl radicals that react with persistent organic pollutants, have become a popular technique for wastewater remediation [1]. Among various AOPs, the Fenton process, in which Fe2+ triggers a decomposition of H2O2 into hydroxyl radicals, has attracted significant attention due to its simplicity, diverse applications, and easy operation of the reaction system [2]. Fe2+ could be replaced by Fe3+ or other transition metal ions and result in a similar process. Fenton or Fenton-like reactions produce a high concentration of hydroxyl radicals when using hydrogen peroxide as the primary oxidant [3]. These highly reactive hydroxyl radicals are responsible for further reactions involving a radical process, such as radical polymerization and pollutant degradation. However, the traditional Fenton process shows a low efficiency in generating hydroxyl radicals from hydrogen peroxide (<30%), therefore requiring the addition of a large amount of H2O2 (30~6000 mM) and Fe2+ (18~410 mM) [4]. While a high concentration of Fe2+ can easily form an iron slurry, leading to the toxicity of the catalyst, the system also incurs a high cost.
Nanozymes are nanomaterials that display enzyme-like reactivity but can overcome the limitations of natural enzymes, including high cost and poor stability upon changing the reaction and storage environment [5]. Recently, using metallic compounds as nanozymes represents a viable strategy to remove organic pollutants [6]. Firstly reported by Yan and coworkers in 2007, the ferromagnetic Fe3O4 nanoparticles have shown intrinsic peroxidase-like catalytic activity that could activate hydrogen peroxide and produce hydroxyl radicals, leading to the oxidization of multiple substrates [7]. Their high specific surface area and excellent stability make nanoparticles expose much more active sides during the reaction and thus can simultaneously transfer more electrons in the catalysis process. Other than Fe3O4 nanoparticles, many other metal nanoparticles, such as FeS [8], CuFe2O4 [9], CeO2 [10], MnO2 [11], CdS [12], and CuO-Fe3O4 [13] have demonstrated a similar peroxidase-like activity as Fe3O4. These nanozymes can efficiently oxidize and remove organic pollutants when mixed with hydrogen peroxide [14], peroxymonosulfate [9,15], or persulfate [8,16,17]. Interestingly, the Fe3O4 nanozyme can activate both hydrogen peroxide and peroxymonosulfate and persulfate [18]. Among the available non-noble metal oxide or sulfide nanoparticles, Fe3O4 nanoparticles stand out as an ideal catalyst due to their minimum environmental footprint, cost-effectiveness, easy access and simple separation and recycling by an external magnetic field. Unfortunately, the organic pollutants and oxidized products can also bind to the surface of nanozymes, and block the active sites, leading to the aggregation of nanoparticles and the drastic attenuation of its enzyme-like activity.
Surface modification is a potential method to inhibit aggregation and maintain the stability of Fe3O4 nanoparticles [19]. For example, Fe3O4 coated with dopamine could afford nanoparticles with better water dispersibility than the naked Fe3O4 nanoparticles [20]. Polydopamine has also been coated on the surface of Fe3O4 microspheres to assist the immobilization of Ag nanoparticles on the surface of Fe3O4 microspheres, resulting in fast adsorption of a model dye methylene blue (MB) [21]. Glutathione-coated Fe3O4 nanoparticles exhibited enhanced peroxidase-like activity for oxidizing 2,4-dichlorophenol [14]. Moreover, with the help of molecularly imprinted polymers, Liu and collaborators recently created substrate-binding pockets on the surface of Fe3O4 nanozymes, showing a nearly 100-fold enhancement in the substrate selectivity compared to the bare Fe3O4 [22]. While these studies provide evidence that surface modification of nanozymes with polymers could prevent aggregation and improve substrate selectivity, little is known about how the polymer layers interact with organic substrates, hence promoting the catalytic process.
In this work, we proposed a facile method for preparing monodisperse Fe3O4 nanozymes with excellent water dispersity via the classic dopamine coating and in situ polymer encapsulation. The prepared samples showed enhanced peroxidase-like activity and achieved the purpose of removing organic dyes. Importantly, we successfully employed molecular simulation methods to study the interaction and diffusion of substrates into the polymer coating, demonstrating a coordination effect of organic dyes, persulfates, and amino groups from polymers. This provided some important insights into the selection of appropriate polymers for nanozyme modification toward disparate application scenarios.

2. Results

2.1. Design and Synthesis of Fe3O4@Gel

Here, the oleic acid-coated nanoparticles were referred to as OA-Fe3O4. The alkane chains of OA are helpful for Fe3O4 nanoparticles to be dispersed in non-polar solvents, such as n-hexane. On the other hand, dopamine (DA) could assist Fe3O4 NPs dispersion in polar solvents, such as methanol or water. In this work, we coated Fe3O4 nanoparticles with dopamine through a phase-transfer process from the n-hexane phase to the methanol phase (Figure 1B). The obtained nanoparticle in methanol is referred to as DA-Fe3O4. DA has also been used to directly coat the Fe3O4 nanoparticles in an alkaline solution [21,23]. The driving force for such a phase-transfer process was attributed to the strong interaction with Fe3O4 exerted by DA. The hydroxyl groups and amide groups of dopamine endow hydrophilic to the surface of Fe3O4 nanoparticles and ensure their aqueous dispersibility. The averaged hydrodynamic diameter of DA-Fe3O4 was about 24.4 nm, as determined by DLS in Figure 2A. However, serious aggregation and precipitation of DA-Fe3O4 still appeared after three-day storage in an aqueous solution (pH = 7) as the naked Fe3O4 (see Figure 1C).
To tackle this problem, we further performed a chemical modification to encapsulate DA-Fe3O4 nanoparticles into a thin polymer layer. The amino groups of dopamine coating on DA-Fe3O4 NPs were chemically modified with acryloyl groups and then copolymerized with AAM monomer. The polymerization formed a porous hydrogel layer that covered DA-Fe3O4 NPs. Hereafter, the Fe3O4 embedded nanogel/nanocapsule is referred to as Fe3O4@Gel. As shown in Figure 1C, Fe3O4@Gel was stable during three-day storage in an aqueous solution, and its hydrodynamic diameter was about 78.8 nm (see Figure 2A). We can imagine that the aggregation of Fe3O4 nanoparticles should be effectively inhibited after polymeric encapsulation. Notably, the corresponding peaks for DA-Fe3O4 totally disappeared in the diameter distribution of Fe3O4@Gel, indicating that all DA-Fe3O4 nanoparticles should be encapsulated into polymeric nanogel.

2.2. Physical Characterization of Samples

Except for the hydrodynamic diameter in an aqueous solution, more characterizations were carried out for the three Fe3O4 samples. In the X-ray diffraction (XRD) pattern of naked Fe3O4 and DA-Fe3O4 (see Figure 2B), Miller indices with strong response peaks were 220, 311, 400, 440, and 511, respectively. Such patterns were well-matched with the standard Fe3O4 (JCPDS no. 19-0629) [14], indicating high purity and good crystalline morphology for the surface-modified DA-Fe3O4.
The vibration sample magnetometer behavior of naked Fe3O4 and Fe3O4@Gel were measured. As shown in Figure 2C, the saturation magnetic induction of naked Fe3O4 was 72.8 emu/g, and slowly decreased to 64.9 emu/g after encapsulation in polymer nanogel but was still greater than 16.3 emu/g [24]. Such larger values verified that dopamine modification and polymeric encapsulation exerted little influence on the magnetic property of Fe3O4 nanoparticles.
The functional groups of the surface-modified Fe3O4 samples have also been characterized by FT-IR spectroscopy (Figure 2D). The Fe-O stretching vibration was near 588 cm−1, a typical absorption band for Fe3O4. Compared with naked Fe3O4, some new absorption bands appeared in the infrared spectrum of DA-Fe3O4. The narrow peak located at 1488 cm−1 was attributed to the C=C vibration of the benzene ring, and the peak located at 1297 cm−1 can be attributed to the C=O stretching of the phenolic hydroxyl group validating the successful preparation of DA-Fe3O4 [20]. For the sample Fe3O4@Gel, the broad peak occurring between 3500 and 3200 cm−1 corresponds to hydrogen-bonded O-H stretching. The increased intensity of the peak at 1629 cm−1 indicated the collective signals from the aromatic ring of DA, N-H bends, and C=O stretches of amide bonds of the polymer layer.
The dried sample of Fe3O4@Gel was further characterized by TEM. As shown in Figure 3, the diameter of Fe3O4@Gel was estimated between 10 nm and 20 nm. The high-resolution TEM image in Figure 3C displays a crystal lattice fringe with about 0.25 nm, fitting well with the cubic structure Fe3O4 (311) planes [14,25]. As the dried polymer nanocapsule formed a thin film around the Fe3O4 surface and resulted in a slight aggregation of the samples. Careful observation reveals that the Fe3O4@Gel samples presented a core-shell morphology with the inner Fe3O4 core (dark grey) and the outer polymer layer (light grey). Moreover, the elemental mapping (Figure S1 in Supplementary Materials) demonstrates that the Fe elements were in the core area while the N elements were in the shell area, unambiguously indicating the core-shell structure of Fe3O4@Gel nanoparticles.

2.3. Peroxidase-Mimicking Activity of Fe3O4@Gel

The Fe3O4 nanoparticles could catalytically oxidize peroxidase substrates in company with H2O2, exhibiting a peroxidase-like activity as firstly reported by the Yan group [7]. In this work, the peroxidase-like activities of modified Fe3O4 samples were evaluated according to the catalytic oxidation of TMB, a peroxidase chromogenic substrate. As shown in Figure 4A, DA-Fe3O4 exhibited improved peroxidase-mimicking activity, ca. 2.7-times of naked Fe3O4 nanoparticles. Since the naked Fe3O4 nanoparticles were prepared without oleic acid or dopamine, their lower catalytic degradation ability suggested that dopamine was conducive to the preparation of water-dispersed Fe3O4 nanoparticles, which could effectively enhance its catalytic performance. After polymer encapsulation, the enzymatic performance was further increased to ca. 3.1 times of naked Fe3O4 nanoparticles. As a comparison, the polymeric nanogel exhibited no significant enzymatic activity. Intuitively, the polymer nanogel seems to prevent the diffusion of organic substrates to the nanozyme centers. However, the results suggested that the permeability of the porous polymeric layer is good enough to allow the free diffusion of substrates and hydrogen peroxide molecules. In our previous work on encapsulating horseradish peroxidase (HRP), the low Michaelis-Menten constant and high catalytic efficiency validated the good permeability for the free diffusion of substrate molecules into the enzyme active site [26,27].

2.4. Efficient Removal of Organic Dyes with H2O2

Next, we explored the effect of polymeric encapsulation on the catalytic performance of Fe3O4 on organic dyes. Indigo carmine (IC) was chosen as another target for catalytic oxidation. As shown in Figure 4B, in the IC/H2O2 mixed solution, IC can be rapidly degraded with the addition of Fe3O4@Gel; the removal efficiency was as high as 57.6% after 15 min and 99% after 180 min. In comparison, there is almost no IC degradation after the addition of naked Fe3O4, DA-Fe3O4, or nanogel without Fe3O4. As aforementioned, the peroxidase-mimicking activity of DA-Fe3O4 and Fe3O4@Gel were very similar when oxidating TMB. In our opinion, this can be rationalized by the adsorption effect between the polymeric nanogel and dyes. The nanogel is composed of polyacrylamide, which is rich in amide groups. Each IC molecule contains two sulfonic acid groups and thus could be adsorbed into the nanogel through hydrogen bonding. As shown in Figure 5A, the increased local concentration of IC inside the gel increased the oxidation reaction rate.
Next, the catalytic performance was evaluated with various concentrations of H2O2 and Fe3O4@Gel. Increasing H2O2 could enhance the removal efficiency of IC, and the enhancement became less intense when H2O2 was above 20 mM, indicating that the local concentration of H2O2 inside the nanocapsules approached saturation (see Figure S2A). Similarly, the saturation concentration of Fe3O4@Gel was [Fe3O4] = 5 mg/L, and double loading of Fe3O4@Gel only increased the removal efficiency from 79.7% to 93.8% (see Figure S2B). In the following studies, the dosage of Fe3O4@Gel was fixed at 112 mg/L, which is equivalent to [Fe3O4] = 5.0 mg/L.
The magnetic property of Fe3O4@Gel could also be used in the effective collection of nanozymes under external magnetic fields (Figure 5). Reuse experiments were performed to evaluate the catalytic sustainability of Fe3O4@Gel. It was revealed that more than 85% of IC could still be removed in the third run. Considering the mass loss in recirculation, we can speculate that Fe3O4@Gel nanoparticles could be a good choice in the continuous treatment of wastewater. In contrast, the catalytic performance of the Fe3O4 nanoparticles recycled three times is only 40% of the fresh sample when oxidizing acetaminophen [28].

2.5. Efficient Removal of Organic Dyes with Na2S2O8

Other than H2O2, Na2S2O8 can also supply free radicals in the advanced oxidation process. Here, another catalytic oxidation system was constructed based on Na2S2O8 in company with Fe3O4@Gel. As shown in Figure 6, IC can be fully removed by both Fe3O4@Gel/H2O2 and Fe3O4@Gel/Na2S2O8, while a remarkable difference is displayed in the oxidation of other dyes, i.e., AO7, DR81, and AZO. The oxidation performance of Fe3O4/Na2S2O8 was significantly higher than that of Fe3O4/H2O2. This may be attributed to the longer half-life of sulfate radicals (•SO4−), suppling a higher probability of encountering and oxidizing dye molecules before quenching. Nevertheless, the polymer encapsulation significantly enhanced the catalytic oxidation performance of the Fe3O4 nanozyme, regardless of the primary oxidants H2O2 or Na2S2O8. Especially when oxidizing AZO, the removal efficiency was improved by 77%. Considering that all dyes in this work (i.e., IC, AO7, DR81, and AZO) contain sulfonic acid groups, the polymeric nanocapsules should be able to adsorb these dyes, thereby improving their catalytic oxidation efficiency.
The catalytic performance was also evaluated with different loading of Na2S2O8 and Fe3O4@Gel, and various pH. As shown in Figure S3, Fe3O4@Gel showed the best degradation efficiency of AO7 when pH = 7; the degradation was slightly weakened in the alkaline environment, and was only 20% in the acidic environment, especially at pH = 4. Furthermore, Figure S4 shows that increasing Na2S2O8 could also enhance the removal efficiency of AO7, indicating that the local saturation concentration of Na2S2O8 inside the nanocapsules should be about 10 mM. Similarly, the plateau concentration of S2O8−2 was close to that of H2O2 when using Fe3O4@Gel. Notably, Fenton or Fenton-like systems often used a different design of the catalysts and reaction conditions, which could be difficult for a direct comparison. As such, we have listed the catalysis performance of previous work, from which one could have a better understanding of the pros and cons of each system (Table S1).

2.6. Identification of Primary Reactive Oxidants

The catalytic degradation is usually through a radical mechanism. To identify the types of radicals in our systems, we performed EPR experiments by adding the spin-trapping agent DMPO. It is known that the Fenton reaction generates hydroxyl radicals (•OH) from H2O2, whereas the •SO4− is produced from the decomposition of Na2S2O8. As illustrated in Figure 7B, the characteristic quartet peak with an intensity ratio of 1:2:2:1 indicated large quantities of DMPO-OH adducts in the Fe3O4@Gel/H2O2 system. In contrast, the obvious signals of both DMPO-OH (1:2:2:1) and DMPO-SO4 (1:1:1:1:1:1) appeared for the Fe3O4@Gel/Na2S2O8 system, suggesting an in situ conversion of •SO4− to •OH [16,28].
The highly reactive hydroxyl radicals are supposed to initiate the degradation reaction of the substrates. Deactivation of quenching these radicals with more radical-reactive species should then inhibit the degradation reaction. To prove this hypothesis, we took the Fe3O4@Gel/H2O2 system as an example by adding tert-butanol (TBA) as a typical hydroxyl radical (•OH) scavenger. As depicted in Figure 7A, the concentration of IC decreased quickly, and the removal efficiency was as high as 73% after 150 min. As the amount of TBA increased from 1 mM to 8 mM, the degradation process of IC was significantly inhibited, and the removal efficiency decreased from 65% to 11%. These data agreed with the fact that hydroxyl radicals are direct oxidants to the substrates, and almost all the hydroxyl radicals were scavenged when the concentration of TBA exceeds 5 mM [29].

2.7. An Adsorption Mechanism as Revealed by Molecular Simulation

Our previous work [30] revealed that the molecular interactions between the PAM nanogel and the dye (AZO) enable the dye molecules to be efficiently absorbed into the nanogel interior. In this work, IC was chosen as the mode dye, and acrylamide tetramers were used to represent the PAM nanocapsule. Using molecular simulations, we compared the intermolecular interactions existing in two typical systems, i.e., the PAM/H2O2/IC system and the PAM/Na2S2O8/IC system. The three components in the simulated system were first randomly placed inside the simulation box and then aggregated, driven by molecular interactions. The number of atomic contacts between every two components was used to track the aggregating process.
As presented in Figure 8, the number of atomic contacts between dye and H2O2/Na2S2O8 was the most among all the formed contacts, which increased and then approached a plateau after about 20 ns. This means that the simulated system reached the thermodynamic equilibrium after 20 ns. Several insights could be obtained from the simulation results. In the PAM/H2O2/IC system, the number of atomic contacts between PAM and IC increased rapidly and reached its equilibrium value of 5960 after 15 ns, indicating a strong interaction between PAM and IC. In other words, the interaction between H2O2 and PAM was much weaker, and the number of associated atomic contacts was only 2041. Given that the large number of atomic contacts formed between IC and H2O2 was ca. 15,023, H2O2 should also be largely absorbed into the nanocapsule. As illustrated in Figure 9, IC was the key in the adsorption process, which can interact with PAM and H2O2 to promote aggregation of the three components, increasing the local concentration of IC and H2O2 inside the nanocapsule. Considering that the concentrations of organic pollutants are very low (<1000 ppm) in wastewater, hydroxyl radicals cannot capture the organic pollutants during their short lifetime (ca. 1 μs) [31] via free diffusion. In the PAM nanocapsule, the locally concentrated dye and H2O2 were endowed with increased possibilities to be oxidized by hydroxyl radicals.
In the PAM/Na2S2O8/IC system, the number of atomic contacts between Na2S2O8 and IC increased rapidly and reached its plateau value (ca. 22,257) after 20 ns, indicating that the stronger intermolecular interactions were formed between the secondary ammonia of IC and Na2S2O8, such as hydrogen bonding. In contrast, the number of atomic contacts between PAM and IC was only 3154, which is about one-half of that in the PAM/H2O2/IC system. Considering that the number of atomic contacts between PAM and Na2S2O8 was 4025, significantly larger than that between PAM and H2O2 in the PAM/H2O2/IC system, we suppose that Na2S2O8 formed stronger hydrogen bonding with the primary ammonia of PAM, thus decreased the adsorption between PAM and IC. The three components in the PAM/Na2S2O8/IC system can interact with each other, which is conducive to aggregation or adsorption. In contrast, H2O2 in the PAM/H2O2/IC system requires the assistance of IC to enter the nanocapsules. This also rationalizes the stronger oxidizing and removing efficiency of Na2S2O8 on organic dyes in Figure 6.

3. Materials and Methods

3.1. Materials

Ferrous chloride tetrahydrate (FeCl2·4H2O, 99.95%), ferric chloride (FeCl3·6H2O, 99%), Oleic acid (OA, 99%) acrylamide monomer (AAM), N-acryloxysuccinimide (NAS), ammonium persulfate (APS), tetramethylethylenediamine (TEMED), 3,3’,5,5’-Tetramethylbenzidine dihydrochloride (TMB), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), hydrogen peroxide (30%) and sodium persulfate (Na2S2O8) were obtained from Sigma-Aldrich Co. (Shanghai, China) Ammonia water (NH3·H2O, 25–28%), dopamine hydrochloride and all dyes (Indigo carmine, IC; Acid orange 7, AO7; Azophloxine, AZO; Direct Red 81, DR81) were supplied by Aladdin Co. (Shanghai, China) Other chemicals were of analytical grade and were used without further purification.

3.2. Preparation of Fe3O4 Nanoparticles

3.2.1. Naked Fe3O4 NP

Fe3O4 nanoparticles were prepared by a co-precipitation method with minor modifications from previous reports [32,33]. Briefly, FeCl3·6H2O (2.5 mmol), and FeCl2·4H2O (1.25 mmol) were dissolved in deoxygenated water (5 mL) to get a homogenous mixture. The mixture was slowly dropped into ammonia water (2%, 25 mL, 90 °C) under vigorous stirring (1500 rpm). After that, oleic acid (0.5 mL) was slowly added to the solution and stirred for at least 10 min. The blackish precipitates were gathered with a permanent magnet, washed by deionized water and anhydrous ethanol several times, and then homogenously dispersed in n-hexane (10 mL). Hereafter, the Fe3O4 nanoparticles coated with oleic acid were denoted as OA-Fe3O4 NP in this work.

3.2.2. DA-Fe3O4 NP

Dopamine was used to modify the surface of Fe3O4 by a phase-transfer method. Typically, 0.64 mL of Fe3O4 (0.08 mmol) in n-hexane was mixed with 2 mL of dopamine (0.8 mmol) in anhydrous methanol and then stirred at 1500 rpm for 1 h. The dopamine coated Fe3O4 NPs were transferred from the non-polar organic phase to the polar methanol phase, and then gathered and washed with deionized water several times. The obtained Fe3O4 NPs coated with dopamine could be well dispersed in water, which was denoted as DA-Fe3O4 NPs hereafter.

3.3. Encapsulation of Fe3O4 Nanoparticles

To synthesize the Fe3O4-embedded nanogel, DA-Fe3O4 NPs were firstly modified to introduce acryloyl groups by reacting surface amide groups with N-acryloxysuccinimide (NAS). The reaction was performed by dissolving DA-Fe3O4 (8 mM, 100 μL) into 1 mL deoxygenated water under magnetic stirring (1500 rpm), followed by a dropwise addition of 20 μl NAS DMSO solution (20 mg/mL). The reaction lasted for two hours at room temperature. Subsequently, the acrylated DA-Fe3O4 was mixed with a 40 mg AAM aqueous solution (2.5 mL). The Fe3O4-embedded nanogel was synthesized via in situ free radical polymerization with the addition of 4 mg APS as initiator and 15 µL TEMED as a stabilizer. After stirring for another two hours, the reaction mixture was concentrated and then washed with deionized water several times.

3.4. Characterization of Fe3O4 Nanoparticles

The morphology of Fe3O4 nanoparticles was characterized with a Talos F200× transmission electron microscope (TEM; Thermo Fischer, Waltham, MA, USA). Fourier transform infrared spectrometer NICOLET IS10 (FT-IR; Thermo Fischer, USA) was used to detect the chemical structures of naked Fe3O4, DA- Fe3O4, and Fe3O4@Gel nanoparticles. X-ray diffraction (XRD) analysis was obtained in an Empyrean X-ray powder diffractometer (PANalytical,Almelo, Holland). Magnetic properties of Bare Fe3O4 and Fe3O4@Gel nanoparticles were examined with a Physical Property Measurement System X-MaxN50 (Quantum Design, San Diego, California ,USA). The hydrodynamic diameters of samples were measured using a Zeta Sizer Nano S90 (Malvern, Malvern,UK) after sonicating the aqueous samples for 30 min. Electron paramagnetic resonance (EPR) experiments were performed with DMPO as a spin-trapping agent on an EMX-E spectrometer (Bruker, Karlsruhe, Germany). All the spectroscopic measurements were conducted using a 96-well plate in an Epoch2 microplate reader (BioTek, Winooski, VT, USA).

3.5. Peroxidase-Mimic Activity Assessment

The peroxidase-mimic activity of the samples was measured via a luminescence method [30,34]. Briefly, the peroxidase substrates TMB was used as the subject of the catalytic oxidations, and the oxidated TMB could be detected according to the absorbance at 650 nm. The TMB/H2O2 system was prepared by mixing TMB (10 μL, 10 mg/mL), H2O2 (64 μL, 30%), and acetic acid-sodium acetate buffer solution (800 μL, pH 3.6). The oxidation reaction was started by adding nanozyme sample (200 µL, [Fe3O4] = 74 μg/mL) with stirring at 800 rpm and 30 °C. The color change was recorded for 15 min. The catalytic efficiency can be judged by the rate of forming oxidated TMB.

3.6. Batch Experiments for Dye Degradation

The typical process of using Fe3O4@Gel to treat organic dyes in the reactor was as follows: Dyes (200 µL, 5 mg/mL) and H2O2 (100 μL, 3%) were added sequentially in PBS buffer solutions (pH = 7.0) with stirring at 800 rpm and 30 °C. Then, the reaction was initiated by the addition of the Fe3O4@Gel suspension (200 µL, [Fe3O4] = 74 μg/mL) and the final volume was 5 mL. Each degradation process was performed with three replicates. Organic dyes were also degraded by the Fe3O4/Na2S2O8 system to degrade organic dyes, and the concentration of Na2S2O8 in the reactor was 10 mM or 0.024 mg/mL. Other conditions remained unchanged from the Fe3O4/H2O2 system.
Concentrations of dyes in the degradation process were determined by detecting the absorbance of supernatant. The characteristic absorption peaks of dyes IC, DA81, AO7, and AZO are 610, 510, 484, and 508 nm, respectively. The removal efficiency (D, %) of dye could be determined as D% = C0/Ct × 100%, where C0 is the initial concentration of dye, and Ct represents the concentration after elapsed time t.

3.7. Molecular Modeling and Computer Simulations

The interaction modes at molecular level among nanocapsule, organic dye, and H2O2/Na2S2O8 were explored based on all-atom molecular dynamics (AAMD) simulations. As verified in our previous work [30], the repeating unit of polyacrylamide could represent the nanocapsule material to form interactions with other components. Gauss view software (version 5.0, Wallingford, CT, USA) was used to construct the structures of acrylamide tetramers, indigo carmine, H2O2, and Na2S2O8, and Automated Topology Builder (ATB) [35] was used to produce the associated topologies. Molecular interactions between each other were described by the GROMOS force field (version 54A8) [36], and water molecules were represented by the classical TIP3P model [37]. The simulated system was composed of 10 acrylamide tetramers, 50 indigo carmine, 200 H2O2, or Na2S2O8, these ratios were set following the experimental feeding ratios. To make the solution electrically neutral, sodium ion (Na+) and chloride ion (Cl−) were added. As the size of the simulation box is 6 nm × 6 nm × 6 nm, periodic boundary conditions were applied in the x-, y-, and z-directions to approximate an infinite environment.
All AAMD simulations were conducted in the isothermal-isobaric (NPT) ensemble using the GROMACS simulation package (version 2018.4) [38] at the National Supercomputing Center in Shenzhen. The simulations were performed with a 2-fs time step at 1 bar and 310 K, and the last 20-ns trajectory was used for data analysis. The PyMOL suite (Schrödinger Co.) [39] was used to visualize the simulated systems. Other details of the simulation can be found in our previous work [30]. The number of atomic contacts between each component of the simulated system was counted via the gmx mindist utility of GROMACS; one atomic contact occurs when the distance between any two atoms in different groups is less than 0.6 nm.

4. Conclusions

We reported a facile method to prepare uniformly dispersed polymer-coated Fe3O4 nanozymes. The surface modification of Fe3O4 NPs with dopamine allows further functionalization with acryloyl groups. Followed by an in situ polymerization, Fe3O4 NPs were encapsulated in polyacrylamide nanogel. In comparison with the naked Fe3O4 NPs, the polymer-coated nanoparticles displayed an enhanced catalytic removal efficiency to a library of organic dyes when using either H2O2 or Na2S2O8 as primary oxidants. In this study, most dyes were oxidatively removed using as little as 5 mg/L of PAM-coated Fe3O4. The molecular simulation revealed that the coordination interactions of organic dyes, primary oxidants, and the amide groups of polymer nanocapsules could be the key to the enhanced activity of nanozymes. Moreover, the higher removal efficiency using Na2S2O8 as the primary oxidant was attributed to the long life of the •SO4− radicals and the stronger interaction between Na2S2O8 and the amide groups of PAM, which allows gradual conversion from sulfate radicals to hydroxyl radicals. As such, the PAM-coated Fe3O4 in the presence of Na2S2O8 demonstrated systematically higher (up to above 90%) catalytic efficiency than those using H2O2 (mostly below 10%). While the small particle size resulted in much higher catalytic efficiency with lower catalyst loading, it resulted in the difficulty of catalyst recycling. Future work will focus on immobilizing these nanogels on the micron size carriers for better recyclability. Overall, this work provided insights into the understanding of why specific polymer coating layers could overall improve the catalytic efficiency and may guide the further design of polymer-based nanozyme in different catalytic applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12060614/s1, Figure S1: The corresponding elemental mapping pattern of the dried Fe3O4@Gel; Figure S2: The catalytic degradation of IC using Fe3O4@Gel as a nanozyme with various concentrations of H2O2 (A) and Fe3O4@Gel (B); Figure S3: Influence of pH on the Fe3O4@Gel-mediated oxidation of dye pollutant; Figure S4: Influence of various initial parameters on the Fe3O4@Gel-mediated oxidation of dye pollutant: (A) Na2S2O8 dosage, (B) Fe3O4@Gel loading; Table S1: A comparison of catalysis performance between this work and literature that used similar catalysis systems. Refs [40,41,42,43,44] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, Y.C. and Z.J.; methodology, J.Z. and Z.F.; validation, J.Z., W.W. and P.X.; investigation, J.Z. and H.H.; data curation, J.Z.; writing—original draft preparation, W.W. and Y.C.; writing—review and editing, J.Z. and Z.F.; project administration, Y.C.; funding acquisition, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Natural Science Foundation of China, grant number 22173061.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Schematic illustration for preparing the water-dispersible Fe3O4 nanoparticles via polymeric encapsulation (Fe3O4@Gel). (B) Nanoparticle samples dispersed in n-hexane and methanol, respectively (OA-Fe3O4 & DA-Fe3O4); (C) Nanoparticle samples freshly prepared and after three days of storage in an aqueous solution.
Figure 1. (A) Schematic illustration for preparing the water-dispersible Fe3O4 nanoparticles via polymeric encapsulation (Fe3O4@Gel). (B) Nanoparticle samples dispersed in n-hexane and methanol, respectively (OA-Fe3O4 & DA-Fe3O4); (C) Nanoparticle samples freshly prepared and after three days of storage in an aqueous solution.
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Figure 2. Physical characterization of the naked Fe3O4, DA−Fe3O4, and Fe3O4@Gel. (A) Size distribution measured by DLS; (B) XRD patterns; (C) Magnetization curves; (D) FT−IR spectra.
Figure 2. Physical characterization of the naked Fe3O4, DA−Fe3O4, and Fe3O4@Gel. (A) Size distribution measured by DLS; (B) XRD patterns; (C) Magnetization curves; (D) FT−IR spectra.
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Figure 3. TEM images of Fe3O4@Gel nanoparticles (A,B) and the enlarged nanoparticles showing the polymer layer (C) with an average diameter of 12.0 ± 2.8 nm. The accelerated voltage was 80 kV.
Figure 3. TEM images of Fe3O4@Gel nanoparticles (A,B) and the enlarged nanoparticles showing the polymer layer (C) with an average diameter of 12.0 ± 2.8 nm. The accelerated voltage was 80 kV.
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Figure 4. Peroxidase mimic activities of Fe3O4 samples based on the oxidation of TMB (A) and IC (B). Reaction conditions: [H2O2]0 = 20 mM, [Fe3O4] = 5 mg/L, [TMB]0 = 200 mg/L, [IC]0 = 200 mg/L, pH = 7, and T = 30 °C.
Figure 4. Peroxidase mimic activities of Fe3O4 samples based on the oxidation of TMB (A) and IC (B). Reaction conditions: [H2O2]0 = 20 mM, [Fe3O4] = 5 mg/L, [TMB]0 = 200 mg/L, [IC]0 = 200 mg/L, pH = 7, and T = 30 °C.
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Figure 5. (A) Reusability of Fe3O4@Gel as illustrated by sketch map. (B) Variation of removal efficiency of IC when catalyzed by the reused Fe3O4@Gel. Reaction conditions: [H2O2] = 20 mM, [Fe3O4] = 5 mg/L, [IC] = 200 mg/L, pH = 7, and T = 30 °C.
Figure 5. (A) Reusability of Fe3O4@Gel as illustrated by sketch map. (B) Variation of removal efficiency of IC when catalyzed by the reused Fe3O4@Gel. Reaction conditions: [H2O2] = 20 mM, [Fe3O4] = 5 mg/L, [IC] = 200 mg/L, pH = 7, and T = 30 °C.
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Figure 6. Comparison of the catalytic performance between the Fe3O4/H2O2/dye system and the Fe3O4/Na2S2O8/dye system. (A) Removal efficiencies as functions of elapsed time; (B) Removal efficiencies of dyes after oxidized for two hours. Reaction parameters: [Na2S2O8]0 = 10 mM, [H2O2]0 = 10 mM, [Fe3O4] = 5.0 mg/L, [dye]0 = 100 mg/L, pH = 7.0, and T = 30 °C.
Figure 6. Comparison of the catalytic performance between the Fe3O4/H2O2/dye system and the Fe3O4/Na2S2O8/dye system. (A) Removal efficiencies as functions of elapsed time; (B) Removal efficiencies of dyes after oxidized for two hours. Reaction parameters: [Na2S2O8]0 = 10 mM, [H2O2]0 = 10 mM, [Fe3O4] = 5.0 mg/L, [dye]0 = 100 mg/L, pH = 7.0, and T = 30 °C.
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Figure 7. (A) Effect of TBA on the degradation of IC in the Fe3O4@Gel/H2O2 system; (B) EPR spectrum of DMPO-•OH adduct (♦) and DMPO-•SO4− adduct (∗) in the Fe3O4@Gel/H2O2 and Fe3O4@Gel/Na2S2O8 systems, respectively.
Figure 7. (A) Effect of TBA on the degradation of IC in the Fe3O4@Gel/H2O2 system; (B) EPR spectrum of DMPO-•OH adduct (♦) and DMPO-•SO4− adduct (∗) in the Fe3O4@Gel/H2O2 and Fe3O4@Gel/Na2S2O8 systems, respectively.
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Figure 8. Time evolution of the simulated system as revealed by two order parameters. They are the number of atomic contacts (A) and hydrogen bonds (B) between different components, i.e., polymeric gel (PAM), dye (IC), and H2O2.
Figure 8. Time evolution of the simulated system as revealed by two order parameters. They are the number of atomic contacts (A) and hydrogen bonds (B) between different components, i.e., polymeric gel (PAM), dye (IC), and H2O2.
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Figure 9. A proposed mechanism for the enhanced removal of dye pollutants via a combination of adsorption and catalytic oxidation. The yellow dashed line denoted the hydrogen bonds formed between PAM, dye, and H2O2.
Figure 9. A proposed mechanism for the enhanced removal of dye pollutants via a combination of adsorption and catalytic oxidation. The yellow dashed line denoted the hydrogen bonds formed between PAM, dye, and H2O2.
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MDPI and ACS Style

Zha, J.; Wu, W.; Xie, P.; Han, H.; Fang, Z.; Chen, Y.; Jia, Z. Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes. Catalysts 2022, 12, 614. https://doi.org/10.3390/catal12060614

AMA Style

Zha J, Wu W, Xie P, Han H, Fang Z, Chen Y, Jia Z. Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes. Catalysts. 2022; 12(6):614. https://doi.org/10.3390/catal12060614

Chicago/Turabian Style

Zha, Junqi, Wugao Wu, Peng Xie, Honghua Han, Zheng Fang, Yantao Chen, and Zhongfan Jia. 2022. "Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes" Catalysts 12, no. 6: 614. https://doi.org/10.3390/catal12060614

APA Style

Zha, J., Wu, W., Xie, P., Han, H., Fang, Z., Chen, Y., & Jia, Z. (2022). Polymeric Nanocapsule Enhances the Peroxidase-like Activity of Fe3O4 Nanozyme for Removing Organic Dyes. Catalysts, 12(6), 614. https://doi.org/10.3390/catal12060614

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