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Exquisite Enzyme–Fenton Biomimetic Catalysts for Hydroxyl Radical Production by Mimicking an Enzyme Cascade Qi Zhang, Shuo Chen, Hua Wang, and Hongtao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18690 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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ACS Applied Materials & Interfaces

Exquisite Enzyme–Fenton Biomimetic Catalysts for Hydroxyl Radical Production by Mimicking an Enzyme Cascade Qi Zhang1, Shuo Chen1*, Hua Wang2 and Hongtao Yu1

1

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of

Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China. 2

School of Fisheries and Life Science, Dalian Ocean University, Dalian 116023, China.

Email: [email protected]

Keywords: Au nanoparticles, α-FeOOH, porous carbon, enzyme-Fenton, enzyme-cascade

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ABSTRACT Hydrogen peroxide (H2O2) is a key reactant in Fenton process. As a byproduct of enzymatic reaction, H2O2 can be obtained via the catalytical oxidation of glucose using glucose oxidase in the presence of O2. Another oxidation product (gluconic acid) can suitably adjust the microenvironmental pH contributing to the Fe3+/Fe2+ cycle in the Fenton reaction. Enzymes are extremely efficient at catalyzing a variety of reactions with high catalytic activity, substrate specificity and yields in living organisms. Inspired by the multiple functions of natural multienzyme systems, an exquisite nanozyme-modified α-FeOOH/porous carbon (PC) biomimetic catalyst constructed by in situ growth of glucose oxidase-mimicking Au nanoparticles and crystallization of adsorbed ferric ions within carboxyl into hierarchically PC is developed as an efficient enzyme-Fenton catalyst. The products (H2O2, ~4.07 mmol•L-1) of the first enzymatic reaction are immediately used as substrates for the second Fenton-like reaction to generate the valuable •OH (~96.84 µmol•L-1), thus mimicking an enzyme-cascade pathway. α-FeOOH nanocrystals, attached by C–O–Fe bondings, are encapsulated into the mesoporous PC frameworks facilitating the electron transfer between α-FeOOH and porous carbon support and greatly suppressing iron leaching. This study paves a new avenue to design biomimetic enzyme-based Fenton-catalysts mimicking a nature system for •OH production.


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INTRODUCTION In nature, generation of reactive oxygen species (ROSs), ranging from life sciences to environmental chemistry, is an extremely important process. Among various ROSs, hydroxyl radicals (•OH), as a powerful reactive radical, has been applied widely, for example, soil remediation, water treatment and also participate many physiological processes.1−4 Fenton reaction, which has been studied for about 100 years, is the most conventional way for the production of •OH.5 Particularly plenty of •OH can be produced by Fenton reaction (Fe3+/Fe2+ pair and H2O2) as below:6,7 Fe2+ + H2 O2 → Fe3+ + •OH + OH –

(1)

Fe3+ + H2 O2 → Fe2+ + ∙OOH + H +

(2)

However, either homogeneous Fenton reaction or heterogeneous Fenton-like process is dependence on a suitable acid aqueous-phase condition (pH = 2.0–5.0) for the effective reactivity,8,9 because Fe3+ tend to transform into other complex forms under strong acid or alkaline conditions as follow: strong acid condition: Fe3+ + 6H2 O ↔ Fe(H2 O)3+ 6

(3)

alkaline condition: Fe3+ ↔ FeOH2+ ↔ Fe2 (OH)4+ 2 ↔ other polynclear species ↔ Fe2 O3 ∙nH2 O(s)

(4)

Generally, conventional Fenton reaction is faced with poor Fe3+/Fe2+ cycle, extra H2O2 addition, and narrow working pH range, which leads to the significant iron residue related second pollution, high cost for H2O2 transport and storage, and low activities at neutral or alkaline condition, thus limiting the wide application of Fenton.10 To overcome these disadvantages, the various Fenton-like reactions have been developed such as photoFenton. The photo-Fenton is an attractive method for organic pollutant degradation. In photoFenton process, H2O2 is photo-generated in situ from O2 reduction on photocatalyst surface, avoiding the extra addition of H2O2.11 Unfortunately, the photo-Fenton suffers from the issues 3 ACS Paragon Plus Environment


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of factitious regulating pH and low solar energy utilization, which increase greatly the treatment cost. Especially, the low H2O2 yield is also a primary challenge for the photoFenton. Therefore, the key for Fenton-like reaction is conceiving an efficient Fenton-catalyst to meet the high-efficiency H2O2 generation and low iron leaching simultaneously. Natural enzyme, as an exquisite biocatalyst, can accelerate the biochemical reaction rate up to 1019 times in living organisms.12 In natural system, enzyme cascade reaction, theoretically the reaction product of one type of enzyme as a substrate for another enzyme, is a significant strategy to improve the catalytic performance of enzymatic reaction.13 In order to mimic the complex functionality of natural systems for cascade reactions, a sort of traditional way is by integrating different materials together to fabricate the nanocomposites.14 However, some problems greatly impede their applications, such as their high costs in preparation and purification, sensitivity for environmental conditions and poor operational stability.15,16 To circumvent aforementioned limitations, great efforts have been used to develop the artificial enzymes (i.e. “nanozymes”) for mimicking the functions of natural enzymes. Nanozyme is a all-important branch of biomimetic chemistry from the natural inspiration to mimic the essential and universal laws of natural enzymes. Compared with natural enzymes, nanozymes have some favorable respects, including low cost, long-term storage, high stability, robustness in harsh environments and quantity production.17–19 Recently, because of their excellent optical-electrical performances, versatile biofunctionalization availability and good biocompatibility, Au nanoparticles (AuNPs) have been widely studied in nanotechnology.20 Small AuNPs (3.6 nm) are able to catalytically oxidize glucose and generate gluconic acid (GA) and H2O2 in a “green” approach, similar to that of the natural glucose oxidase. The catalytic reaction is shown as the following equation:21 AuNPs

Glucose + O2 + H2 O $%%& Gluconic acid + H2 O2


(5)

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H2O2, from the oxidation of glucose using glucose oxidase-mimicking AuNPs in the presence of O2, is a key reactant in Fenton reaction. One of the characteristics of enzymatic reaction is high efficiency, ensuring the continuous supply of H2O2 in situ. Besides, by virtue of the glucose oxidation, increase of GA concentration leads to a decline in the environmental pH in favor of the Fe3+/Fe2+ cycle in Fenton process. Inert porous materials such as hierarchically porous carbon (PC) with large pore volume as well as high surface area are commonly used as supports for increasing the dispersion of active sites and preventing metal ion leaching.22 The PC materials used in advanced oxidation process have the advantages of high surface area with controllable surface chemistry, high chemical and thermal stability and easy metal recovery.23 Meanwhile, the hierarchically porous structure with abundant micro-, meso-, or even macropores not only endows this material with continuous electron pathway, but also facilitates mass transport (H2O2) owing to shortening diffusion pathways.24,25 Most importantly, the mesoporous structure and defect sites of PC are introduced through the carbonization at H2 atmosphere and capillary pressure.26 Both defects and mesoporous can act as the adsorption sites of the ferric ion and suppress iron leaching. Based on the characteristics of AuNPs catalytic reaction, incorporating nanozymes (AuNPs) into Fenton-like reaction is a novel strategy to construct an exquisite enzyme-Fenton system for •OH production. Inspired by natural enzyme-cascade, a novel biomimetic catalyst first successfully prepared by in situ growth of glucose oxidase-mimicking AuNPs and crystallization of adsorbed ferric ions within carboxyl into functionalized porous carbon (APC) is developed as an enzyme-Fenton catalyst. The biomimetic design endowed such catalytic system with more activated interfacial catalytic sites as the first enzymatic reaction occurred in close (nanoscale) proximity to the second Fenton-like reaction, so products (H2O2) of the first reaction can be used immediately as substrates for the second reaction to generate •OH. Furthermore, GA are able to achieve the self-regulation of 5 ACS Paragon Plus Environment


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microenvironmental pH in real-time. Under the electro-assistance (i.e. –0.15 V), the efficient electron transfer between α-FeOOH and PC by C–O–Fe bonding facilitates support for the cycle of PC≡Fe(III)/PC≡Fe(II).

RESULTS AND DISCUSSION Scheme 1 illustrates that the Au/α-FeOOH–APC catalyst was prepared by the hydrothermal

treatment

in

situ of

Fe3+/APC

and

the

growth

of

AuNPs. The

electrostatic interaction of Fe3+, AuNPs and APC with plenty of carboxyl groups induced the confining of iron species and AuNPs within the mesoporous frameworks of APC. FT–IR spectra (Figure S1) of the APC displays the intense band at around 1720 cm−1 corresponds to carboxylic groups on the surface of the APC compared with the PC before wet oxidation.10 The other peaks at 1629, 1574 and 1411–1026 cm-1 are assigned to the stretching vibration of C═O, C═C, and C–O bonds, respectively. Figure 1a shows the XRD pattern of samples, the diffraction peaks of Au/α-FeOOH– APC catalyst are in accordance with typical goethite crystalline phase (JCPDS file: 00–029– 713). The Au/α-FeOOH–APC catalyst exhibits the goethite crystals grown on APC surface with the characteristic acicular (15–45 nm along b direction) and elongated (80–240 nm) along the crystallographic c direction (Figure 1b). Au/α-FeOOH–APC with the twining of goethite crystals is similar to the pure α-FeOOH. Meanwhile, two weak peaks in Au/αFeOOH–APC at about 25° and 44° were assigned to the (002) and (100)/(101) planes of carbons, respectively. The diffraction peaks at 38.1°, 44.3°, 64.5° and 77.6° corresponding to the (111), (200), (220) and (311) crystal planes of Au (JCPDS 4–0783) are also observed in the Au/α-FeOOH–APC catalyst. Some small nanocrystal spindles of α-FeOOH exhibit average crystal size of 100 nm grown on the surface of APC as diaplayed in Figure 1b. The obtained Au/α-FeOOH–APC sample was brownish red in color without visible observation of yellow indicating the uniform dispersion of AuNPs and α-FeOOH on APC 6 ACS Paragon Plus Environment

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(Figure S2). Although AuNPs are not obviously observed from the SEM image, which was due to the high surface area and high-density mesoporous structure of APC. EDX spectrum of Au/α-FeOOH–APC catalyst confirms the presence of the AuNPs. Meanwhile, Fe, C and O elements are also observed in the sample (Figure 1c). The uniform dispersion of Fe and Au elements in Au/α-FeOOH–APC (Inset in Figure 1c) verified a homogeneous distribution of closely interactional AuNPs, α-FeOOH and APC (see the TEM image in Figure 1d). As shown in Figure 1d, nanospindles with sizes around 100 nm were embeded on the mesoporous carbon which was in conformity with the small α-FeOOH nanocrystal spindles obtained on SEM image. Meanwhile, Figure 1d also presents that the α-FeOOH and AuNPs were encapsulated within mesopores of APC, thus suppressing the metal leaching. Actually, iron leaching issue for the Fenton reaction pays close attention to for practical application.27 It was only 0.02 mg•L-1 for the soluble iron ion concentration after 120 min reaction at pH of 3 for the Au/α-FeOOH–APC under the electro-assistance, which is much lower than the legal limit of the European Union (2.0 mg•L-1).10 However, the concentration of soluble iron ion was up to 1.89 mg•L-1 under the same conditions for the hybrid catalyst of AuNPs, αFeOOH and APC, indicating the much lower iron leaching for Au/α-FeOOH–APC is due to the strong interaction of α-FeOOH and APC. It is below detection limit for AuNPs leaching from Au/α-FeOOH–APC. Moreover, Figure 1d and Figure S3 also exhibit that the AuNPs are uniformly encapsulated into the APC substrate. The high-resolution TEM image of the nanospindles shows a lattice spacing of 0.74 nm, corresponding to the α-FeOOH (110) plane (Figure 1e). The AuNPs was highly crystalline, with distinct lattice fringes. As displayed in Figure 1f, the lattice spacing of approximately 0.23 nm conformed to the lattice of the Au (111) plane (JCPDS 04-0784).28 Besides, the size of AuNPs falls within the range of 13.2 ± 1.3 nm by the measurement using Zetasizer Nano ZSP (Malven nano-ZS90), which is favorable to the catalytic activity of AuNPs.21 Meanwhile, the AuNPs were characterized with



TEM (Figure S4), which showed that the spherical AuNPs with average diameter of 13 nm in accordance with the abovementioned results. The surface area and porosity play an important role the catalytic performance of catalysts, which are examined by BET. Figure 2a presents that the specific BET surface area and total pore volume (TPV) of the biocatalyst greatly decreased to 186 m2•g-1 and 0.17 cm3•g-1, respectively, which were much smaller than pure APC supports (BET 311 m2•g-1, TPV 0.35 cm3•g-1). Above results indicated that AuNPs and α-FeOOH nanocrystals occupied the voids of pores on APC substrate in agreement with the TEM results. The pore size distribution derived using the non-local density functional theory method (Figure 2b) reveals both micropores (~1.4 nm) and mesopores (~3.5 nm) for the APC. And Au/α-FeOOH–APC has also similar microporous and mesoporous structures with pore size mainly distributed at 1.0–10 nm. It also indicated that the APC support still retained the ordered mesoporous structure after immobilization of α-FeOOH treated at 100 °C in favour of H2O2 transport. However, the contents of micro- and mesopores declined for the Au/α-FeOOH–APC compared to the APC from Figure 2b. The Raman spectrum (Figure 2c) shows two peaks located at 1320 cm-1 (D band) and 1585 cm-1 (G band). The intensity ratio of D band to G band (ID/IG) was 1.06 for APC, slightly higher than that for PC (1.05). The excellent catalytic performance of Au/α-FeOOH–APC can be attributed to following factors: 1) it has a high content of sp3-C bonds and defects, which can act as active sites for metal ions (iron and AuNPs) and O2 adsorption;26 2) its porous structure and high surface area offer the abundant reactive sites, leading to a large electroactive surface area of Fe3+/Fe2+ cycle; 3) its hierarchical pores are beneficial to minimizing the diffusion resistance of mass transport, favoring the fast emission of H2O2. The ID/IG value of Au/α-FeOOH–APC decreased to 0.98 because of the introduction of AuNPs and α-FeOOH. XPS was applied to investigate the electronic structures and the interactions between the APC supports and Au/α-FeOOH. The wide spectrum of photoelectron peaks in Figure 3a 8 ACS Paragon Plus Environment

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show the presence of C 1s, O 1s, Au 4f and Fe 2p. The high resolution XPS spectrum of Fe 2p core-level reveals two photoelectron peaks at 711.7 eV (Fe 2p 3/2) together with a satellite peak at 719.9 and 725.3 eV (Fe 2p 1/2) along with a shakeup satellite at 733.6 eV (insets in Figure 3a), in accordance with previous report of α-FeOOH.29 Figure 3b shows the high resolution O 1s peaks of the Au/α-FeOOH–APC catalysts. The deconvolution of O 1s peaks of the catalysts involves five different peaks distributing to the oxygen in carboxyl (534.3−535.4 eV), hydroxyl, ether, carbonyl (533.1−533.8 eV), Fe–O–H (531.8 eV), Fe–O–C (531.2 eV), and Fe–O–Fe (529.8 eV), respectively.30 It has been reported that the C–O–Fe formation by coal gasification reaction of heat treatment of Fe(NO3)3-impregnated coal and H2O.31 Figure 3c presents that five different C 1s deconvolution peaks including the C═C sp2 (284.6 eV), C–C sp3 (285.1 eV), C−OH and/or C–O–C (286.7 eV), C═O (288.1 eV), and O– C═O (289.0 eV), which indicates the presence of various carbon groups.32 The above results implied

the

Au/α-FeOOH–APC has

plentiful

surface

Fe–O–H

and

the

strong

interaction between carbon support and α-FeOOH via C–O–Fe attributing to the in situ transformation

process

of

Fe3+/APC

to

Au/α-FeOOH–APC

catalysts

upon

hydrothermal treatment. Moreover, the XPS spectrum of Au 4f from AuNPs (Figure 3d) can be assigned to Au 4f7/2 (84.0 eV) and Au 4f5/2 (87.8 eV), respectively, which indicated the reduction of Au3+ to Au0.28 For this enzyme-Fenton reaction, the enzyme activity of nanozyme (i.e. AuNPs) was extremely important for the H2O2 production. We evaluated the glucose oxidasemimicking activity of AuNPs encapsulated into APC substrate (Au–APC) and studied the mechanism of this reaction occurring at the nanoscale surface. The catalytic activity of Au– APC under electro-assistance, as manifested by measuring the H2O2 yields, was dependent on the AuNPs loading, leading to a sigmoidalshaped curve (Figure 4a). When the concentration of AuNPs reached ~6 nM, the reaction turnover increased remarkably, and then attained a plateau at ~10 nM. Therefore, the Au/α-FeOOH–APC catalysts are synthesized using the ~10 9 ACS Paragon Plus Environment


nM AuNPs solution to obtain an optimal catalytic activity. The catalytic activity of AuNPs could be thoroughly hindered as soon as the AuNPs surface on APC substrate was passivated using the thiolated DNA, which results in essentially no color change as shown in Figure 4b. This means that the catalytic activity of AuNPs originates from Au atoms at the nanoscale surface. On the basis of Luo's reports,21 the catalysis of AuNPs followed a typical MichaelisMenten behavior. As summarized in Table S1, the Michaelis-Menten constant (Km) of Au– APC was 7.54 mM with electro-assistance, which was slightly higher than that of AuNPs (6.97 mM) and glucose oxidase (4.87 mM),21 The result indicates that electro-assisted Au– APC has a slightly lower affinity with glucose compared to AuNPs and glucose oxidase. However, the catalytic constants (Kcat) for electro-assisted Au–APC were 1.43 and 2.73-fold larger than that of AuNPs solution and glucose oxidase, implying higher reaction rate. According to previous reports,21 the gluconic acid can be assayed by reaction with hydroxylamine and subsequent complex with Fe3+, which led to an orange-red complex hydroxamate-Fe. When hydroxylamine and Fe3+ were simultaneously added to the glucose solution oxidized by electro-assisted Au–APC, it can be found that the color of the solution turned orange-red (Figure 4c), which confirmed that gluconic acid was indeed produced in this AuNPs-catalyzed reaction.33 Meanwhile, this conclusion was also proved visually by the measurement of pH in solution using laboratory pH meter. As illustrated in Figure 4d, a pH value of 3.25 is observed at 30.5 °C, which was favorable to the Fenton reaction since either classic Fenton reaction or other modes of Fenton process largely relied on a suitable acid aqueous-phase medium (pH = 2–5.0) to realize efficient reactivity.11 To further verify the experimental data, the theoretical value of pH was also calculated on the basis of the following equations: 'H + ( )

–K α +,K 2α + 4cK α (6) 2 AuNPs

C6 H12 O6 + O2 + H2 O $%%& C6 H12 O7 + H2 O2 (7) 10 ACS Paragon Plus Environment

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where 01 is the dissociation constant of gluconic acid (01 = 2.5 × 10-4); 2 is the concentration of gluconic acid (2 = 1.3× 10-3 mol•L-1). According to equations, the theoretical pH value is about 3.33 according to the calculation. The theoretical data of pH basically conforms to the experimental data, meanwhile, the reaction system of pH 3 was optimal in many Fenton processes.34,35 In traditional Fenton process, the H2O2 is one of the important reactants in the reactions as shown in equations 1 and 2. It is well-known that the performance of Fenton reaction for contaminant degradation is highly dependent on the amount of H2O2. In present system, the H2O2 was mainly produced via a route of glucose oxidation with AuNPs under aerobic conditions. This was verified by measuring the amount of H2O2 yields over sample under the different conditions (Figure S5). As known, the temperature (T) and pH are two important factors for the activity of glucose oxidase-mimicking AuNPs.21 The AuNPs exhibited superior pH, thermal, and storage stability to natural enzyme glucose oxidase. Herein, the concentration of H2O2 enzyme-generated on Au–APC was examined (pH 1−5, T 30−70 °C). As illustrated in Figure 5b, the average H2O2 production rate of Au–APC is 1.06−2.03 mM•h1

at 30 to 70 °C under pH 3, indicating high H2O2 yield was obtained on Au–APC with

electro-assistance compared with photo-Fenton. It can be found that the Au–APC has an optimal pH (~3) for H2O2 yields. Meanwhile, Au–APC was also highly active for H2O2 enzyme-generation at pH 1 and 5 (Figure 5a and c), with an average H2O2 production rate of 0.40−0.89 mM•h-1 at pH 1 and 0.66−1.25 mM•h-1 at pH 5 (30 to 70 °C, 120 min). The superior performance of Au–APC for H2O2 production was originated from the high catalytic activity of nanozyme coupling with electrochemistry. The accumulated H2O2 concentration was dependent on T under the same pH value. In addition, Figure 5 presents that the H2O2 yields at the applied temperature of 70 °C was only slightly higher than that of 50 °C at the same pH. Considering the energy savings, the following enzyme-Fenton experiments were performed at a temperature of 50 °C unless otherwise specified. In the enzyme-Fenton 11 ACS Paragon Plus Environment


biocatalytic system, it is also favorable to activating the molecules because of the interaction between the polar molecules and electromagnetic fields. The activated polar organic molecules (glucose) are able to effectively react with oxygen in close proximity, thus accelerating the H2O2 generation. Moreover, the Au atoms at the surface of nanoscale AuNPs were effectively activated under the electro-assistance, thus exposing the more active sites to participate in the enzymatic reaction. The catalytic activity of AuNPs was largely dependent on the activity of Au atoms at the nanoscale surface.21 To verify the hypotheses, the comparison of Au–APC with and without electro-assistance and AuNPs solution for H2O2 production were also carried out, as displayed in Figure 5d. The H2O2 yields with Au–APC were about 3.73 mmol•L-1 within 120 min at 50 °C and pH 3 under the electro-assistance, which was approximately 5.5 and 10.9 times higher than that of AuNPs solution and Au–APC in absence of the electro-assistance, respectively. Hence, the presence of electro-assistance accounts for the high activity of Au–APC. Meanwhile, it was in accordance with the result of catalytic constant (Table S1). To evidence the conversion of H2O2 into •OH over Au/α-FeOOH–APC under electroassistance, the EPR measurements were performed. As shown in Figure 6a, the DMPO spintrapping EPR spectra under the electro-assistance condition appears four typical DMPO–OH adducts with intensity of 1:2:2:1 in the Au/α-FeOOH–APC system, which were much stronger for the electro-assisted Au/α-FeOOH–APC than that with hybrid AuNPs, α-FeOOH and APC. This can be attributed to the high H2O2 production rate (Figure 5d) thanks to the activated AuNPs under the electro-assistance, thus increased the corresponding intensity of DMPO–OH adduct signal. Meanwhile, the α-FeOOH–APC composite did not show the DMPO–OH adduct typical signals in Figure 6a, indicating that the H2O2 in this enzymeFenton system was only produced by AuNPs catalytical oxidation of glucose. In addition, the DMPO–OH adduct signals were not observed from the Au–APC composite under the electroassistance, indicating that H2O2 can not convert into the •OH radicals in absence of Fe2+ ions. 12 ACS Paragon Plus Environment

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The Au/α-FeOOH–APC without the electro-assistance exhibited a very weak DMPO–OH adduct signal. For the Au/α-FeOOH–APC, APC surface with plenty of oxygen containing groups can act as the electron-transfer mediator. Herein, surface oxygen groups with reducibility can promote the cycle of ≡Fe(III)/≡Fe(II).10 As shown in Figure S6, the contents of micro- and mesoporous structures gradually decreased with the increase of loaded Fe3+, which was disadvantageous to the H2O2 transport. Hence, the amount of Fe3+ loading has a crucial effect on the performance of Au/α-FeOOH– APC. Figure 6b reveals the different •OH yields over each Au/α-FeOOH–APC catalyst as a function of Fe3+ loading. The Au/α-FeOOH–APC catalyst showed a hump-like catalytic activity depending on the loading amount of Fe3+ (with 10.68 wt% as an optimal amount). This phenomenon can be reasonably explained as that the increment of Fe3+ loading will promote the utilization of electron but also suppress the electron transfer efficiency on APC (detailed mechanism study will be introduced in the next section), and thus, it should appear an optimal balance between these two contradictory factors at a certain loading amount of Fe3+. Note that, H2O2 can be produced by electroreduction of O2 in electro-Fenton process, however, the oxidation of glucose by AuNPs needs the participation of O2. Therefore, a suitable voltage value, enough to reduce Fe3+ but not to reduce O2, was vital for our enzymeFenton system. As displayed in Figure S7a and b, the initial potential of O2 reduction was at – 0.2 V, whereas the peak potential for Fe3+ reduction was about at 0.1 V for the Au/α-FeOOH– APC. Comparison of •OH production using Au/α-FeOOH–APC under different potentials was also carried out to optimize the potential value, as shown in Figure 6c. More than 90 µmol•L-1 of •OH was obtained at –0.15 V, which was higher than that of the potential value of –0.1 V. The yield of •OH radicals at the applied potential of –0.2 V was higher compared to the potential of –0.15 V under present experimental conditions. Based on Figure S7a, –0.2 V is the initial potential of O2 electroreduction, hence this potential would be adverse to eliminating the effect of electro-generated H2O2 in present work. Meanwhile, it increased to 13 ACS Paragon Plus Environment


129.56 µmol•L-1 within 120 min once the potential of –0.4 V was applied, which could be attributable to the increase of H2O2 production rate owing to the electroreduction of O2. Based on the aforementioned results, the highest •OH yields were obtained at –0.15 V in absence of O2 electroreduction, thus –0.15 V was selected for the following experiments. Efficient generation of •OH radicals plays an essential role in boosting advanced oxidation process, thus, the •OH yields are a key for the Fenton reaction. Figure 6d shows the change of •OH concentration with time profiles for the different catalysts. The •OH can hardly be produced without AuNPs, indicating the electro-assisted α-FeOOH–APC was unable to catalyze the oxidation of glucose to produce H2O2. Additionally, Au–APC was inactive for •OH production even though under electro–assistance because of the absence of the Fe2+ ions in conformity to the results of Figure 6a. Figure 6d presents that the highest •OH yields reach ~96.84 µmol•L-1 within 120 min by Au/α-FeOOH–APC under the electroassistance (–0.15 V), which is 2.2 times that of hybrid AuNPs, α-FeOOH and APC (44.28 µmol•L-1). The result was also in good consistent with the case of EPR spectra. Herein, H2O2 products of the first reaction from the oxidation of glucose over AuNPs can be used immediately as substrates for the second reaction (Fe2+ + H2 O2 → Fe3+ + •OH + OH– ) to generate •OH and finally Fe3+ can be also transformed efficiently into Fe2+ by the electroreduction realizing the Fe3+/Fe2+ cycle, thereby constituting a highly efficient enzymecascade pathway. To evaluate the stability and reusability properties of the Au/α-FeOOH–APC catalyst, four cycles of consecutive tests for •OH generation in the presence of electro-assistance were executed under the given same conditions, and the results are revealed in Figure S8a. There was only a limited loss of catalytic activity during the cycle tests, and the results of all the four cycles showed more than 90 µmol•L-1 of •OH yields within 120 min, while the •OH yields of FeSO4 homogeneous Fenton only reaches about 40 µmol•L-1 (Figure S8b). The reason could be the limited regeneration of Fe2+ and H2O2. The above results indicated that 14 ACS Paragon Plus Environment

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the Au/α-FeOOH–APC catalysts possess sufficient stability for •OH generation in the electroassisted enzyme-Fenton reaction. To make a comparison, the reused Au/α-FeOOH–APC catalyst was also characterized by XRD and XPS. No obvious change is observed on XRD pattern (Figure S9), and no impurities are present in Fe 2p XPS spectrum (Figure S10a). The high-resolution O 1s spectrum shows an obvious shoulder around 533.0 eV for reused Au/α-FeOOH-APC compared with the fresh one (Figure S10b), which corresponds to the oxygen atoms in carboxylic (534.3−535.4 eV) and hydroxyl, ether, carbonyl (533.1−533.8 eV). All these results reveal that the relative contents of O atoms from oxygen functional groups of carbon surface increase obviously after enzyme-Fenton reaction under electro-assistance, which should be related to the interfacial activation, that is, carbon surface oxidation during the Fenton reaction. The surface oxygen concentration of fresh Au/α-FeOOH–APC catalyst was 37.5% based on the calculation of XPS and it increases to 57.4% in the reused one. The TPV of the reused Au/α-FeOOH–APC catalyst almost keep as same as the fresh catalyst except for the slight decrease of BET from 186 to 182 m2•g-1 (Table S2). According to all above-mentioned results and discussions, a schematic illustration of electro-assisted enzyme-Fenton process on the Au/α-FeOOH–APC catalyst was proposed in Scheme 2, which helps to easily understand the high-efficiency generation of active radicals via mimicking an enzyme-cascade pathway. Briefly, benefiting from the activation of surface Au atoms under the electro-assistance, AuNPs efficiently catalyze the oxidation of glucose with oxygen, and H2O2 and gluconic acid are generated by route 1. The APC≡Fe(III) extract the electrons under the electro-assistance to form the APC≡Fe(III)/APC≡Fe(II) redox cycle (route 2). H2O2 produced by route 1 was catalyzed into •OH through APC≡Fe(II) in route 3, at the meantime, APC≡Fe(II) was oxidized into APC≡Fe(III) (route 4). Accordingly, a mimicking enzyme-cascade reaction is constituted by the H2O2 (first reaction products) immediately acted as substrates for the second reaction forming the •OH radicals. 15 ACS Paragon Plus Environment


The efficiency of •OH production is highly dependent on the catalysts for H2O2 synthesis in Fenton reaction. In our enzyme-Fenton system, the high H2O2 yields can be ascribed to two aspects. First, the Au atomsat the surface of nanoscale AuNPs can be effectively activated by electro-assistance, thus plentiful reactive sites on the AuNPs participated in the oxidation of glucose. Second, carbon atoms on the edges of porous carbons with unpaired electron are able to effectively adsorb O2 molecule, which contributes to the oxidation of glucose on Au atom surface.11 The cycle of Fe2+/Fe3+ is another rate limiting step in the enzyme-cascade reaction, which determines the generating rate of oxidizing species •OH. The strong α-FeOOH–APC interactions with C−O−Fe bonds facilitate the electron transfer between α-FeOOH and APC support for the redox cycle of APC≡Fe(III)/APC≡Fe(II) pair.10

CONCLUSION For the first time, we propose a novel enzyme-Fenton system that is synthesized by incorporating the glucose oxidase-mimicking AuNPs and the α-FeOOH mineral with porous carbon via in situ crystallization of adsorbed ferric ions within carboxyl group functionalized porous carbon. In this system, the AuNPs-catalyzed glucose oxidation generates H2O2 (~4.07 mmol•L-1) that induces the reaction of Fe2+ and H2O2 generating the valuable •OH radicals (~96.84 µmol•L-1) by electro-assistance, thus constituting an artificial enzymatic cascade reaction for mimicking the high catalytic activity of natural enzymes. Meanwhile, it is also the first time to report the incorporation of AuNPs into porous carbon further activating Fenton reaction, which can catalyze the enzymatic cascade reaction without any natural enzymes. Moreover, gluconic acid from the glucose oxidation catalyzed by AuNPs is able to suitably adjust the microenvironmental pH promoting the Fe3+/Fe2+ cycle. The α-FeOOH nanocrystals confined in mesoporous carbon frameworks accompanying with surface attached α-FeOOH microcrystals have catalyst-support interaction by C–O–Fe bonds which not only facilitate the high production efficiency of •OH radicals and also suppresses the iron leaching.


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This work may open up a new avenue for the development of novel artificial enzyme-Fenton mimicking the complex nature system to produce •OH radicals and exploring a promising application in biocatalysis coupling advanced oxidation.

EXPERIMENTAL Preparation of α-FeOOH–APC: To obtain the hydrophilic functionalized PC (APC) materials, PC materials were treated by a wet oxidation method.36 α-FeOOH–APC was fabricated using a hydrothermal method. Typically, 0.5 g of hydrophilic APC, 4 mmol of FeCl3·6H2O and 20 mmol of urea were mixed in 40 mL pure water (Millipore, 18 MΩ•cm) under assistance of ultrasonification. After ultrasonication for 30 min, the mixed solution was transferred into a 50 mL Telfonlined autoclave, followed by the hydrothermal reaction at 100 °C for 12 h. Finally, the obtained powders were washed with water and ethanol for several times, then freeze-dried overnight, named as α-FeOOH–APC. Preparation of Au/α-FeOOH–APC: AuNPs were fabricated using the method of HAuCl4 citrate reduction.37 Briefly, 1 mM HAuCl4 was added into the 250 mL aqueous solution with vigorous stirring in a roundbottom flask accompanying with a reflux condenser. Subsequently, trisodium citrate (38.8 mM, 25 mL) was added into the mixed solution, and then the aforementioned solution was boiled for another 15 min. Finally, the solution was stirred continuously until the room temperature. The AuNPs sizes were able to be obtained by TEM. Then, α-FeOOH–APC composites were immersed into AuNPs solution for 4 h at 30 °C, denoted as Au/α-FeOOH–APC. Similarly, APC was dipped in a solution of AuNPs for same time and temperature to obtain the Au–APC composites. Characterization: The morphology of Au/α-FeOOH–APC was obtained by scanning electron microscopy (SEM, Quanta 200 FEG) and transmission electron microscopy (TEM, JEM-2100F). The chemical composition was determined through energy dispersive X-ray spectroscopy (EDX). The specific surface area was measured using the Quantachrome



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Autosorb-1 MP and calculated by using the Brunauer-Emmett-Teller (BET) equation. The crystallinity of the samples was carried out by X-ray diffraction (XRD, EMPYREAN, PANalytical) using a diffractometer with Co Kα radiation. The Raman spectra were obtained using a Renishaw Micro-Raman System 2000 Spectrometer. Fourier transform infrared spectrum (FTIR) of the sample was obtained on KBr pellets on a spectrometer (Nicolet, Madison). X-ray photoelectron spectroscopy (XPS, VG ESCALAB250) was performed to detect the elementary composition of samples. Analytical Methods: The H2O2 concentration was determined by flow-injection chemiluminescence method.38 Typically, 0.65 mM of luminol solution was composed of 0.1 M of Na2CO3 (adjusting pH to 10.15 using 2 M of HCl and placing for 24 h), and then CoCl2 (0.06 mM of Co2+) was added into above solution. The luminol solution and sample were injected in chemiluminescence system (MIP–B) simultaneously through the flow injection apparatus (IFISD). Finally, the chemiluminescent signal was recorded to determine H2O2 concentration. A high performance liquid chromatography (HPLC) fluorescence detection method for the quantitative determination of •OH radicals was developed, based on trapping •OH radicals with dimethyl sulfoxide (DMSO) to generate formaldehyde.39 Formaldehyde then reacted with 1, 3-cyclohexanedione and ammonia to produce a derivative of formaldehyde, which was analyzed by HPLC with fluorescence detector (FLD). The trapping reaction of •OH radical is shown in following equations (8–10). 1 mol of formaldehyde is generated from 2 mol of •OH radicals reacting with DMSO.40

(8)

(9)


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(10)

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional experimental section for the synthesis. Additional Figures (S1–S11) and Tables (S1–S2).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (51478075), the Program of Introducing Talents of Discipline to Universities (B13012), programme for Changjiang Scholars and Innovative Research Team in University (IRT_13R05), and the Fundamental Research Funds for the Central Universities (DUT16TD02).

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Scheme 1. Illustrative procedure for synthesis of the Au/α-FeOOH–APC catalyst.

(c)

Au

O C

Fe

α-FeOOH

Au

APC edge

Fe Fe

(d)

Au

AuNPs

Au

100 nm



Figure 1. (a) XRD patterns and (b) SEM image of Au/α-FeOOH–APC; (c) EDX spectrum, Inset: elemental mapping of Au and Fe; (d–f) TEM images of Au/α-FeOOH–APC. The yellow arrows in (d) denote a large amount of AuNPs.

Figure 2. N2 sorption isotherms (a) and pore size distributions (b) of the APC before and after the Au/α-FeOOH modification; (c) Raman spectra of PC, APC and Au/α-FeOOH–APC.


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Figure 3. Wide scan XPS spectra (a) and high resolution XPS curve fits of O 1s and C 1s (b, c) for the Au/α-FeOOH–APC, respectively; (d) XPS spectra of Au 4f of the Au/α-FeOOH–APC; Inset in Figure (a) shows the high resolution XPS spectra of Fe 2p of the Au/α-FeOOH–APC.

Figure 4. (a) Catalytic activities of Au–APC under different AuNPs concentration; (b) Comparison of the catalytic activity of DNA-modified Au–APC (i) and “naked” Au–APC (ii); (c) Au–APC catalyze



the oxidation of glucose to gluconic acid, producing a red colored product. (i) Au–APC in the absence of glucose, (ii) Au–APC in the presence of glucose, and (iii) glucose oxidase in the presence of glucose; (d) pH value after glucose oxidation by Au–APC.

Figure 5. Concentrations of H2O2 produced on Au–APC as a function of catalysis time at 30~70 °C with (a) pH 1, (b) pH 3 and (c) pH 5; (d) Comparison of H2O2 yields within 120 min by Au–APC with (O) and without (∆) electro-assistance and AuNPs solution (). Error bars represent the standard deviations of duplicate experiments.


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Figure 6. (a) EPR spectral changes of the DMPO−OH adduct under various conditions; Change of •OH concentration over Au/α-FeOOH–APC as a function of Fe3+ loading (b) and potential (c); (d) Time profiles of •OH concentration at 50 °C with –0.15 V under different conditions. (♣: Au/αFeOOH–APC with electro-assistance; ♦: hybrid AuNPs, α-FeOOH and APC with electro-assistance; ♥: Au/α-FeOOH–APC without electro-assistance; ♠: α-FeOOH–APC with electro-assistance; ■: Au– APC with electro-assistance). Error bars represent the standard deviations of duplicate experiments.



Scheme 2. Schematic illustration of enzyme-cascade promoted enzyme-Fenton by electro-assistance over Au/α-FeOOH–APC catalyst.


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