Interfacial peroxidase-like catalytic activity of surface-immobilized cobalt phthalocyanine on multiwall carbon nanotubes

Abstract

The rapid diffusional mass transfer process (DMTP) always results in a highly efficient reaction. Herein, cobalt phthalocyanine (CoPc) was covalently anchored on to multiwall carbon nanotubes (MWCNTs) by an easy and moderate one-step deamination method to obtain the catalyst MWCNT-immobilized CoPc (CoPc-MWCNT). The interfacial peroxidase-like catalytic activity of CoPc-MWCNTs is described for controllable H₂O₂ activation. According to the experimental results and density functional theory calculations, we can be confident that high-valent cobalt-oxo intermediates are formed during the H₂O₂ activation. Such active species are anchored and exposed on the surface of MWCNTs, shortening the DMTP and enhancing the resistance of CoPc-MWCNTs to oxidative decay. The introduction of linear alkylbenzene sulphonates (LAS) facilitates the catalytic H₂O₂ activation by CoPc-MWCNTs, and at the same time, CoPc-MWCNTs could maintain a high and sustained catalytic activity because of the specific hydrophobic interactions between the long-chain alkyl group of LAS and the π-conjugated surface of the MWCNTs.

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1 Introduction

Hydrogen peroxide (H₂O₂) is an environmentally friendly oxidant used for organic synthesis, bleaching processes in the paper and textile industries, wastewater treatment and for various disinfection applications.^1–3 Recently, much attention has been devoted to the selective activation of H₂O₂ by catalytic systems based on coordination complex catalysts.^4,5 Strategies to develop these catalysts for oxidation reactions are focused on the introduction of supports to improve their performance and stability.^6,7 Typically, the introduction of the supports does not change the reaction mechanism in essence; however, a major challenge lies in precisely controlling the H₂O₂ activation to generate the requisite active intermediates (such as hydroxyl radicals (˙OH) and metal-based oxidants), which also determines the activity and durability of the complex catalysts, even the utilization of H₂O₂.^8–10 In contrast to synthetic catalysts, enzymes are able to achieve excellent selectivity through specific interaction with the substrate and protein environment around the active sites or cofactors.¹¹ In some metalloporphyrin-based enzymatic reaction systems, the reaction channels are determined by the fifth ligands. For example, horseradish peroxidase and catalase present corresponding peroxidase-like and catalase-like activities in the presence of the imidazole ligand and phenolate ligand, respectively.^12,13 In addition, the protein backbone of enzymes also plays a role in protecting the active sites and isolating them from each other to avoid their self-oxidation.

Recently, controllable H₂O₂ activation has been achieved by employing the cellulose matrix to mimic the protein backbone of enzymes, and the reaction channel of H₂O₂ activation was changed from the hemolytic cleavage of the peroxide O–O bond with the generation of an ˙OH radical into a heterolytic cleavage without radical production in the presence of the fifth ligands.¹⁴ However, during the common enzymatic reactions, one of the most important steps is the diffusional mass transfer process (DMTP), where substrates and products must transfer across a boundary to get to the active sites. Therefore, catalytic efficiency could be enhanced by shortening the DMTP course, and if the active sites were in regions where substrates could easily gain access to, in order to react with the generated active species. For the artificial enzyme-like catalysts, so long as the generated active species are anchored without free movement to attack each other, the active sites could be anchored on to the surface of catalysts to shorten the DMTP.

Carbon nanotubes (CNTs) have received increasing scientific interest because of their unusual conjugated structure, high chemical stability and electrical conductivity, and have been extensively employed for the immobilization of enzymes and catalysts.^15–17 In our previous studies, we confirmed that the presence of multiwall carbon nanotubes (MWCNTs) could greatly enhance the catalytic activity of cobalt phthalocyanine (CoPc, an enzyme mimic catalyst) for H₂O₂ activation because of their excellent electrical properties.^18,19 However, the enhanced activity for substrate oxidation (corresponding to a peroxidase-like activity), is also accompanied by an increasing disproportionation of H₂O₂, which decomposes to form water and oxygen (corresponding to a catalase-like activity). Typically, the catalase-like process and peroxidase-like process are in competition during the activation of H₂O₂.²⁰ How to precisely control the reactive channel of H₂O₂ activation towards the peroxidase-like process remains a challenge for the artificial enzyme-like catalysts.

Herein, we report a bioinspired catalytic system based on multiwall carbon nanotube-bonded cobalt phthalocyanine (CoPc-MWCNTs), which was prepared by a moderate one-step deamination method, thus minimizing damage to the conjugated structure of the MWCNTs. Linear alkylbenzene sulphonate (LAS – one of the most widespread surfactants in industrial and domestic wastewater) was employed as the fifth ligand to axially coordinate with the central cobalt ion of CoPc and to help cleave the O–O bond of H₂O₂ heterolytically to achieve the peroxidase-like process. The roles of LAS in the catalytic performance of CoPc-MWCNTs for H₂O₂ activation has been investigated in detail. The possible intermediates generated during the catalytic activation of H₂O₂ were studied by the electron paramagnetic resonance (EPR) spin-trap technique, electrochemistry methods and with density functional theory (DFT) calculations. Our results demonstrate the feasibility of anchoring the active sites on to the surface of MWCNTs, and that LAS not only manifests the role of the fifth ligand, but also makes a great contribution to preventing the agglomeration of the catalyst.

2 Experimental section

2.1 Materials

MWCNTs (lot no. 035NF) and CoPc were supplied by Tokyo Chemical Industry Co., Ltd. Cobalt tetraaminophthalocyanine (CoTAPc) was laboratory-made.¹⁸ The isoamyl nitrite was purchased from Aladdin Industrial Inc. C.I. Acid Red 1 (AR1, Fig. S1†), and sodium 4-ethylbenzenesulphonate (C₂-LAS) was purchased from Acros and was used without further purification. Sodium linear-dodecylbenzenesulphonate (C₁₂-LAS) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. The spin trap reagent 5,5-dimethyl-pyrroline-oxide (DMPO) was obtained from Tokyo Kasei Kogyo Co., Ltd. All other reagents were of analytical grade and used without further purification.

2.2 Catalyst preparation

MWCNTs (1.0 g) were dispersed in DMSO solution with vigorous stirring at 85 °C. Then the DMSO solution of CoTAPc (0.05 g) and isoamyl nitrite were dropped in successively. The reaction was stirred and kept at 85 °C for 12 h. The cooled reaction solution was centrifuged and washed with DMF and ultrapure water, and the centrifugation – wash process was repeated several times to remove ungrafted phthalocyanines. After the deamination reaction between CoTAPc and the MWCNTs, the CoPc-MWCNTs were obtained after drying under vacuum. This method was inspired by ref. 21, and the detailed synthesis is shown in Fig. S2.†

2.3 Catalytic activity experiment

AR1 was employed as a model to investigate the catalytic activity of CoPc-MWCNTs for H₂O₂ activation, and the concentration changes of AR1 were proportional to its absorbance in the UV-vis spectrum. CoPc-MWCNTs (0.2 g L⁻¹) in aqueous solution experienced ultrasonic dispersion 30 min in advance, and H₂O₂ (0.01 M) was added lastly to start the reaction. The repeated catalytic oxidation was operated ten times, in which the known concentrations of AR1 and H₂O₂ were added to ensure the same initial concentration of the substrate and oxidant.

2.4 Analysis

The bonding configurations were investigated using X-ray photoelectron spectroscopy (XPS) on a Thermo Scientific K-Alpha spectrometer (monochromatic Al Kα, 1486.6 eV). Co K-edge X-ray Absorption Near Edge Structure (XANES) spectra and extended X-ray adsorption fine structure (EXAFS) spectra were obtained on the BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF). The storage ring was operated at 3.5 GeV and 241.6 mA. A Si (1 1 1) double-crystal monochromator was used to minimize the harmonics. The spectra were collected at room temperature in the transmission mode with an energy resolution of 0.3 eV. The transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100 microscope. The cobalt content in CoPc-MWCNTs was measured using microwave-assisted digestion-flame atomic absorption spectrometry (Thermo Sollar M6), and the calculated content of CoPc in CoPc-MWCNTs was found to be 44.4 × 10⁻⁶ mol g⁻¹. The UV-vis absorption spectra were obtained on a Hitachi U-3010 spectrophotometer. Cyclic voltammograms (CV) were recorded on an IM6ex (Zahner, Germany) electrochemical workstation using a saturated calomel electrode (SCE) as the reference electrode and Pt wire as the counter electrode. The catalyst powders were dispersed in Nafion solution and sonicated for 60 min. Then the working electrode was prepared by dropping the catalyst solutions onto the carbon fibre paper. Cyclic voltammetry was performed in the LAS and Na₂SO₄ solutions (pH 7.55, 0.01 mol L⁻¹) at room temperature at a scanning rate of 100 mV s⁻¹, and the solutions were flushed with dry nitrogen to remove oxygen from the solutions before the electrochemical experiments. DMPO was employed as the spin-trapper for the radicals in the EPR experiments on a Bruker A300 spectrometer, which was used to record the EPR signals of DMPO, with the settings as follows: centre field, 3518 G; sweep width, 80 G; microwave frequency, 9.88 GHz; modulation frequency, 100 kHz; power, 20 mW.

3 Results and discussion

3.1 Characterization

A deamination reaction was used to prepare the catalyst of CoPc-MWCNTs. The TEM images (see Fig. 1) showed an obvious change on the wall of the MWCNTs after the deamination reaction, indicating that CoPc had been anchored on the surfaces of the MWCNTs. XPS and XANES analyses were performed to probe the bonding configuration of the CoPc-MWCNTs. As shown in Fig. 2, the Co 2p and N1s peaks of CoTAPc were observed in the spectrum of CoPc-MWCNTs, but these had not been observed in MWCNTs, suggesting that CoPc had been successfully anchored on the MWCNTs. From the high resolution Co 2p spectra shown in Fig. S3,† it can be seen that the peaks occurring at 795.3 eV and 779.8 eV are assigned to Co 2p_1/2 and Co 2p_3/2 in the spectrum of CoTAPc, and that also they cannot be found in MWCNTs. When CoTAPc was anchored on the MWCNTs by the deamination reaction, the peaks of Co 2p_1/2 and Co 2p_3/2 increased to 796.4 eV and 780.7 eV, respectively. This is due to the removal of the electron-donating amino groups (which is evidenced by the disappeared N 1s peak of the amino groups in CoPc-MWCNTs, as shown in Fig. S4†), leading to a reduced electron density on the CoPc ring. Thus the electron shield to the inner electrons of the central Co will be weakened, increasing the electron binding energy between Co 2p_1/2 and Co 2p_3/2. For the same reasons, the N 1s peaks corresponding to aza-bridging and pyrrole nitrogen atoms increased from 398.1 eV in CoTAPc to 398.9 eV in CoPc-MWCNTs. As can be seen from the results of the UV-vis spectra (see Fig. S5†), the blue-shift (from 725 nm to 680 nm) of the characteristic Q-band of CoTAPc in the presence of isoamyl nitrite clearly confirms that the deamination process has been achieved. The obvious CoPc absorption at 680 nm was observed in the spectrum of CoPc-MWCNTs. Moreover, there is nearly no difference in the C1s peak from MWCNTs and from CoPc-MWCNTs (see the inset of Fig. 2), which indicates that the surface structure of the MWCNTs has not been destroyed. Accordingly, we can conclude that CoPc has been anchored on the MWCNTs by removing the amino substituents on the CoTAPc. Importantly, such direct bonding between CoPc and MWCNTs results in only minimal damage to the conjugated structure of the MWCNTs. Also, the results of the TGA experiment (see Fig. S6†) indicate that the direct bonding between CoPc and the MWCNTs enhances the stability both of them.

Fig. 1 TEM images of MWCNTs (a and b) and CoPc-MWCNTs (c and d).

Fig. 2 XPS spectra of MWCNTs, CoTAPc and CoPc-MWCNTs.

To understand the electronic structure of the Co ion in the catalysts, XANES characterizations were performed. In Fig. 3a, the first and second peaks were respectively assigned to a dipole forbidden 1s → 3d transition and a shakedown satellite 1s → 4p_z transition, where the 1s → 4p_z transition is a fingerprint of the Co–N₄ structure.²² A clear decline of the 1s → 4p_z transition is observed for CoPc-MWCNTs in comparison with CoPc and CoTAPc, indicating that the π-electron conjugation of the CoPc macrocycles has been changed by covalently bonding to the MWCNTs, which has a strong influence on the coordination between the Co ions and other ligands, including the H₂O₂. It can also be seen in the EXAFS spectra (see Fig. 3b) that the atomic distance R between the Co ion and the nearest neighbouring N is slightly decreased, resulting in a change in the coordination environment of the central cobalt ion. In addition, the 1s → 3d transition shown in the XANES spectra of CoPc-MWCNTs may be because the symmetrical structure of CoPc changes into an inverted symmetry after being covalently bonded with MWCNTs. Furthermore, such results could not be observed in the spectra of the mixture of CoPc and MWCNTs.²³

Fig. 3 XANES (a) and EXAFS (b) spectra of CoPc, CoTAPc and CoPc-MWCNTs.

3.2 Catalytic activity for H₂O₂ activation

AR1 was employed as a model to investigate the catalytic activity of CoPc-MWCNTs for H₂O₂ activation. Fig. 4 shows that CoPc-MWCNTs exhibited a high catalytic activity for activating H₂O₂ to oxidize AR1 in the presence of C₁₂-LAS, whereas such a high activity for H₂O₂ activation was not been observed in the system of CoPc-MWCNTs without C₁₂-LAS or the system of C₁₂-LAS without CoPc-MWCNTs. Although a high catalytic activity for H₂O₂ activation had been presented in the previous CoTAPc-MWCNTs system without LAS, it was essential to maintain the high concentration of H₂O₂ (0.1 M) throughout the reaction, due to the competition between the catalase-like and peroxidase-like processes, and as most of the H₂O₂ is consumed by decomposing into water and oxygen.¹⁸ However, in the presence of C₁₂-LAS, CoPc-MWCNTs exhibit a high activity even with a low concentration of H₂O₂ (0.01 M), indicating that a different reaction channel of H₂O₂ activation is achieved towards the peroxidase-like process. Compared with the catalytic system based on cellulose fibre – bonded CoTAPc,¹⁴ an enhanced activity was achieved under the same conditions by anchoring the CoPc on to the surface of MWCNTs, because of the shorter course of DMTP. Moreover, CoPc-MWCNTs still exhibited high catalytic activity for H₂O₂ activation in the presence of C₁₂-LAS in every run during the successive oxidation of AR1 (see Fig. 5), together with a high catalytic performance at different temperatures (see Fig. S7†), suggesting that CoPc-MWCNTs perform with sufficient stability and are resistant to oxidative decay during the H₂O₂ activation. Accordingly, we hypothesize that the generated active intermediates might be protected or isolated from each other, thereby minimizing the oxidation possibility of the active sites.

Fig. 4 Concentration changes of AR1 (5 × 10⁻⁵ M) under different conditions ([CoPc-MWCNTs] = 0.2 g L⁻¹ (containing 8.88 × 10⁻⁶ M of CoPc), [C₁₂-LAS] = 5 × 10⁻³ M, [H₂O₂] = 0.01 M, pH 7.55, 50 °C).

Fig. 5 The cyclic oxidation of AR1 (5 × 10⁻⁵ M) ([CoPc-MWCNTs] = 0.2 g L⁻¹ (containing 8.88 × 10⁻⁶ M of CoPc), [C₁₂-LAS] = 5 × 10⁻³ M, [H₂O₂] = 0.01 M, pH 7.55, 50 °C).

To further investigate the role of the fifth ligand of LAS in H₂O₂ activation catalyzed by CoPc-MWCNTs, the catalytic oxidations were carried out under the same conditions in the presence of C₂-LAS. As shown in Fig. 6, the catalytic activity of CoPc-MWCNTs for H₂O₂ activation was enhanced in the presence of C₂-LAS, with such enhanced activity depending on the number of C atoms in the alkyl group of LAS, suggesting that the hydrophobic chains of LAS play a noticeable role in this catalytic system. It is generally known that the agglomeration or π–π stacking of CNTs due to the strong van der Waals forces limits their full utilization, but CNTs could be linearly wrapped along the nanotube by compounds with chain-like and conjugated structures to improve their dispersion.^24–26 Thus, with a linear long-chain and benzene ring structure, C₁₂-LAS has a better ability to aid the dispersion of CoPc-MWCNTs, resulting in the higher activity of CoPc-MWCNTs compared to the short-chain C₂-LAS system. However, the dispersion improvement of CoPc-MWCNTs is part of the reason for the activity enhancement, because the catalytic activity of CoPc-MWCNTs is also improved in the presence of C₂-LAS, which cannot improve the dispersion of CoPc-MWCNTs. Cyclic voltammetry (CV) experiments were employed to investigate the role of C₁₂-LAS during H₂O₂ activation catalyzed by CoPc-MWCNTs, in which the electrolytes were C₁₂-LAS and Na₂SO₄, respectively. For the electrochemical oxidation of H₂O₂ catalyzed by CoPc, the possible mechanism is as follows:^27,28

Co^IIPc → Co^IIIPc + e⁻

Co^IIIPc + 1/2H₂O₂ → Co^IIPc + 1/2O₂ + H⁺

Fig. 6 Catalytic oxidation of AR1 (5 × 10⁻⁵ M) by CoPc-MWCNTs activating H₂O₂ (0.01 M) with or without LAS ([CoPc-MWCNTs] = 0.2 g L⁻¹ (containing 8.88 × 10⁻⁶ M of CoPc), [LAS] = 5 × 10⁻³ M, pH 7.55, 50 °C).

It can be seen from Fig. 7 that the electrocatalytic oxidation of H₂O₂ starts from 0.26 V (in 0.01 M C₁₂-LAS solution) and 0.47 V (in 0.01 M Na₂SO₄ solution). The more negative oxidation potential shown in Fig. 7 indicates that C₁₂-LAS promotes the oxidation of H₂O₂ catalyzed by CoPc-MWCNTs. This enhancement in the oxidation peak of H₂O₂ might be attributed to the influence of C₁₂-LAS on the electron transfer process that occurs from Co^IIPc-MWCNTs to Co^IIIPc-MWCNTs.

Fig. 7 Cyclic voltammograms of CoPc-MWCNTs with H₂O₂ (0.01 M) in aqueous solution of 0.01 M C₁₂-LAS (blue) and 0.01 M Na₂SO₄ (red) at pH 7.55.

Moreover, the catalytic activity of CoPc-MWCNTs increased with the increasing C₁₂-LAS concentration (see Fig. 8), but, such activity then dramatically declined when the C₁₂-LAS concentration increased to 20 mM. As shown in the inset image of Fig. 8, after the aqueous solutions of CoPc-MWCNTs with different concentrations of C₁₂-LAS were stood for 10 days, an obvious precipitation of the catalysts was observed in the solution without C₁₂-LAS or with 20 mM C₁₂-LAS. Thus, the reduced activity with 20 mM C₁₂-LAS might be due to the fact that part of the CoPc-MWCNTs is tightly wrapped around by the coil of C₁₂-LAS, resulting in it being difficult for substrates and H₂O₂ to approach the catalytic active sites. Consequently, we can confirm that the SO₃⁻ group of the aryl sulphonates in LAS plays a vitally important role in changing the reaction channel of H₂O₂ activation catalyzed by such supported CoPc, and that the linear long-chain structure of C₁₂-LAS contributes to improving the dispersion of CoPc-MWCNTs.

Fig. 8 Effect of C₁₂-LAS concentration on the catalytic oxidation of AR1 (5 × 10⁻⁵ M) ([CoPc-MWCNTs] = 0.2 g L⁻¹, pH 7.55, 50 °C). Inset: image of the solution of CoPc-MWCNTs with different concentrations of C₁₂-LAS after standing for 10 days.

In order to investigate the peroxidase-like activity of CoPc-MWCNTs in the presence of C₁₂-LAS, the catalytic oxidation of AR1 was carried out at different AR1 and H₂O₂ concentrations, respectively. The results of Fig. 9 suggest the catalytic system with CoPc-MWCNTs and C₁₂-LAS exhibits a character of enzymatic reaction during the activation of H₂O₂. According to the Michaelis–Menten mode, the main kinetic constants, V_max and K_cat (Table 1), were 0.572 μM s⁻¹ and 0.064 s⁻¹. Additionally, although the higher concentration of H₂O₂ maintained the better catalytic activity (see Fig. S8†), AR1 could be completely oxidized with a lower concentration of H₂O₂ as long as the reaction time was long enough. The improved utilization of H₂O₂ suggests that the reaction channel of H₂O₂ activation catalyzed by CoPc-MWCNTs in the presence of C₁₂-LAS ocurred in a peroxidase-like process, and the disproportionation of H₂O₂ was thus significantly minimized.

Fig. 9 Lineweaver–Burk plots for the peroxidase activity of CoPc-MWCNTs activating H₂O₂, with AR1 as the substrate.

Table 1 Kinetic constants of CoPc-MWCNTs activating H₂O₂

Km (mM)	Vmax (μM s^−1)	Kcat (s^−1)	Kcat/Km (s^−1 mM^−1)
0.106	0.572	0.064	0.607

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3.3 Analysis of the catalytic mechanism

According to the above results, the introduction of LAS dramatically enhanced the catalytic activity of CoPc-MWCNTs for H₂O₂ activation towards the peroxidase-like process. And the non-˙OH/˙OOH mechanism was evidenced by the EPR experiment results (see Fig. S9†), indicating the heterolytic cleavage of the peroxide O–O bond.^8,10,29 Together with the results from Fig. 7, the SO₃⁻ group of the aryl sulphonates in C₁₂-LAS determines the reaction channel of H₂O₂ activation catalyzed by CoPc-MWCNTs. As an enzyme mimic catalyst, CoPc has very strong analogies in its structure and physicochemical properties with metalloporphyrins,³⁰ which have two axial coordination sites on a central cobalt ion. During the activation of H₂O₂ catalyzed by CoPc-MWCNTs, one of the axial positions of the cobalt ion is coordinated by H₂O₂, and the other axial position is occupied by C₁₂-LAS. It is recognized that the O atom from SO₃⁻ group of aryl sulphonates could coordinate with the divalent transition metal atoms which lie on an inversion centre and that are coordinated by four N atoms.^31–34 A similar coordination behaviour could occur in the system with CoPc-MWCNTs and C₁₂-LAS because of the Co–N₄ macrocycles in CoPc, and the heterolytic cleavage of the peroxide O–O bond could occur alongside the formation of the high valent cobalt-oxo oxidant. Such active intermediates have been characterized in iron phthalocyanine systems for activating H₂O₂ or other peroxides by the heterolytic cleavage of O–O bond without ˙OH production.^35–37

Furthermore, according to the results of the DFT calculations using graphite as a coronene-like planar sheet to model the CoPc-MWCNTs (see Fig. S10†), the calculated spin density distribution was found to be in good agreement with the experimental results. The results of the calculated bond distances in Table S1† confirm the formation of Co^IV=O, and that the bond distance of the cobalt-oxo is shorter than the Co–O (SO₃⁻) of LAS (C₂-LAS and C₁₂-LAS). Without the fifth ligand of LAS, spin delocalization occurs onto the MWCNTs and Co ions (see Fig. 10a), which agree with our previous results that the active sites on MWCNTs could participate in the oxidation of a substrate.¹⁹ With C₂-LAS or C₁₂-LAS, the spin delocalization occurs onto the central Co ions (see Fig. 10b and c). We could confirm that the high-valent MWCNT-CoPc-oxo intermediates were formed in the presence of LAS and could afford the powerful active species to oxidize substrates by the peroxidase-like process. Also, these active species are covalently anchored on the surface of MWCNTs, facilitating the enhanced activity by shortening the DMTP. Moreover, the hydrophobic interaction between MWCNTs and the alkyl long-chain of C₁₂-LAS could improve the dispersion of CoPc-MWCNTs, ensuring that the catalytic oxidation occurs in a good suspension system.

Fig. 10 DFT-calculated spin density distributions of the generated intermediates at the B3LYP/6-31G level of theory in the catalytic system of CoPc-MWCNTs for activating H₂O₂ (a) without LAS, and with (b) C₂-LAS and (c) C₁₂-LAS, respectively.

4 Conclusions

We describe a bioinspired catalytic system for H₂O₂ activation based on CoPc-MWCNTs and a fifth ligand. CoPc-MWCNTs were prepared by a one-step deamination reaction under moderate conditions. The reaction channel of H₂O₂ activation catalyzed by CoPc-MWCNTs was controlled in the presence of LAS, facilitating the peroxidase-like process and improving the utilization of H₂O₂. The generated high-valent cobalt-oxo intermediates were bonded on the surface of MWCNTs, reducing the transfer resistance of substrates. This study attempted to construct an enzyme-mimic system using MWCNTs to support the catalytic entity, and the introduction of a fifth ligand to mimic the functions of cofactors in natural enzymes. This work not only successfully develops a source of inspiration to design catalysts of artificial enzymes for practical applications, but also reveals further insights into the approach to improve the overall catalytic performance of traditional catalysts by employing CNTs as supports, thus helping to realize catalytic reactions that were not possible in the past. We expect that CNT-based enzyme-mimic catalysts with unique properties and functions will attract increasing research interest and will create new opportunities in various research fields of functional materials.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51133006, 51302246 and 51103133), Textile Vision Science & Education Fund, 521 Talent Project of ZSTU, and Zhejiang Provincial Natural Science Foundation of China (no. LY14E030013 and LY14E030015).

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Footnote

† Electronic supplementary information (ESI) available: The XPS Co1s data, UV-vis spectra, raman spectra, dynamic thermogravimetric analytical results, the EPR spectra, the calculated bond lengths for the energy minimized DFT models and the catalytic performance of CoPc-MWCNTs under other different conditions. See DOI: 10.1039/c4ra15306e

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