Palladium nanoparticles modified electrospun CoFe₂O₄ nanotubes with enhanced peroxidase-like activity for colorimetric detection of hydrogen peroxide

Abstract

Herein, we report a simple procedure to decorate small palladium nanoparticles (Pd NPs) on the surface of CoFe₂O₄ nanotubes; the decorated nanotubes possess intrinsic peroxidase-like activity for the sensitive detection of H₂O₂. The CoFe₂O₄ nanotubes are prepared via an electrospinning technique, followed by a calcination process in air. The functionalization of Pd NPs on the surface of CoFe₂O₄ nanotubes was achieved through an in situ reduction process using ascorbic acid (AA) as a reducing agent. The synthesized Pd nanoparticles, which are small in size, are evenly dispersed on the surface of the CoFe₂O₄ nanotubes. The hollow structure of the CoFe₂O₄ nanotubes and the uniform distribution of the Pd nanoparticles enhance peroxidase-like activity toward the catalytic oxidation of the peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The peroxidase-like property of the Pd/CoFe₂O₄ composite nanotubes provides a facile approach for the colorimetric detection of H₂O₂ with a low detection limit. This work provides great potential for Pd/CoFe₂O₄ nanotubes as enzyme-like nanocatalysts to sensitively detect H₂O₂ in biological systems.

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Introduction

Artificial enzyme mimics are materials that possess catalytic activity similar to natural enzymes but that display many advantages over said enzymes, such as superior stability, easy preparation, good recyclability and low cost.^1–3 Over the past few decades, artificial enzyme mimics have been shown to have diverse applications in biosensing, medical diagnosis, food processing and environmental monitoring. Early reports on artificial enzyme mimics mainly include cyclodextrins, porphyrins, dendrimers, biomolecules, and supramolecular complexes.^1–8 Recently, nanomaterial-based artificial enzyme mimics have attracted increasing attention owing to their large surface area, high and tunable catalytic activity, good selectivity, and facile preparation at low cost.^9–11 For example, since the first report of Fe₃O₄ nanoparticles,¹² various other nanomaterials, including metal oxide nanoparticles and nanowires,^13–17 metal and metal alloy nanoparticles,^18–20 carbon nanomaterials,^21–23 chalcogenide nanomaterials,^24–26 polymer nanoparticles,^27–29 and multiferroic nanomaterials,^30–32 have also been shown to possess intrinsic peroxidase-like activity. To enhance peroxidase-like catalytic performance, much study has been performed on the fabrication of nanocomposites with synergistic catalytic effects. For instance, conducting polyaniline (PANI)/Cu₉S₅ nanocomposites exhibited higher catalytic activity as artificial peroxidase mimics than individual PANI and Cu₉S₅ nanoparticles due to their strong interactions and synergistic effect.³³ Other systems, such as Au nanoparticles/graphene composite,³⁴ dumbbell-like Pt₄₈Pd₅₂–Fe₃O₄ nanoparticles,³⁵ Co₃O₄/reduced graphene oxide composite,³⁶ and CuS/graphene nanocomposite³⁷ have also displayed superior peroxidase-like catalytic activity. Therefore, much effort should be devoted to design and prepare new nanocomposites with improved enzyme-like catalytic performance.

Noble metal nanoparticles, especially Au, Pd and Pt, have attracted widespread interest due to their unique optoelectronic properties and large surface area, which display potential applications in catalysis, electronics, energy storage and conversion, sensors, and biomedical research.³⁸ Recently, noble metal nanoparticles have also been shown to possess intrinsic enzyme-like catalytic activity. However, individual noble metal nanoparticles usually aggregate during the catalytic process, limiting their catalytic activity. Therefore, it is necessary to anchor metal nanoparticles on specific supports. Recently, Zhao, Quan and co-workers reported that Au nanoparticles supported on the surface of graphene nanosheets exhibit superior intrinsic peroxidase-like activity over individual Au nanoparticles and graphene nanosheets alone, demonstrating a synergistic coupling effect between these two components.³⁴ Similarly, Au nanoparticles supported on the surface of graphene oxide and carbon nanotubes,^39,40 Au/Pd nanoparticles supported on the surface of graphene,⁴¹ and porous Pt nanoparticles supported on the surface of graphene oxide⁴² have also been found to possess excellent intrinsic peroxidase-like catalytic activities. In addition to carbon-based nanocomposites, metal oxide-based nanocomposites have also been found to exhibit high enzyme-like activity.^43–46 By combining them with other active components, enhanced catalytic activity can also be achieved.

Recently, spinel ferrite compounds such as CoFe₂O₄ and ZnFe₂O₄ nanomaterials have been reported to display intrinsic peroxidase-like activity.^31,32 However, the catalytic activities of pure CoFe₂O₄ and ZnFe₂O₄ nanoparticles as peroxidase mimics are relatively weak. Therefore, GE/CoFe₂O₄ and CNT/ZnFe₂O₄ nanocomposites were prepared as peroxidase mimics, which showed enhanced catalytic activities over independent CoFe₂O₄ and ZnFe₂O₄ nanomaterials.^47,48 In this study, we demonstrate that the modification of noble metal nanoparticles on the surface of CoFe₂O₄ nanotubes provides a new platform as an artificial enzyme for self-activated peroxidase-like catalysis. In addition, the Pd/CoFe₂O₄ composite nanotubes have also been used to construct a colorimetric assay for the detection of H₂O₂ based on their peroxidase-like activity; the assay exhibits high sensitivity and a low detection limit.

Experimental

Chemicals and materials

Poly(vinyl pyrrolidone) (PVP, weight-average molecular weight, M_w = 1 300 000 g mol⁻¹) was purchased from Sigma-Aldrich. Cobalt(II) acetate tetrahydrate (Co(Ac)₂·4H₂O) and 3,3′,5,5′-tetramethylbenzidine (TMB) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Iron nitrate (Fe(NO₃)₃·9H₂O) was purchased from Tianjin East China Reagent Factory. Na₂PdCl₄ was obtained from Aladdin. All other chemicals were of analytical grade and were used without further purification.

Preparation of CoFe₂O₄ nanotubes by electrospinning technique

The CoFe₂O₄ nanotubes were prepared through an electrospinning technique. In a typical procedure, 1.64 g of PVP was dissolved in a mixture containing 11.4 g of ethanol and 7.8 g of N,N′-dimethylformamide (DMF), followed by the addition of 0.50 g of Fe(NO₃)₃·9H₂O and 0.154 g of Co(Ac)₂·4H₂O. A homogeneous mixture solution was obtained after vigorous stirring at room temperature for about 12 h. The as-prepared mixture solution was electrospun using an applied voltage of 15 kV. A piece of aluminum foil was used to collect the PVP/Fe(NO₃)₃/Co(Ac)₂ nanofibers, and the distance between the nozzle and the collector was fixed at about 15 cm. The as-fabricated PVP/Fe(NO₃)₃/Co(Ac)₂ nanofibers were then calcined under air atmosphere at 550 °C for 3 h to remove the PVP matrix and produce the CoFe₂O₄ nanotubes.

Preparation of Pd nanoparticles modified CoFe₂O₄ nanotubes

5 mg of the obtained CoFe₂O₄ nanotubes were dispersed in 8 mL of water, then 45 mg of sodium citrate was added to the above solution. After shaking for several minutes, 2 mL of aqueous solution containing 3 mg of Na₂PdCl₄ was added to the mixture solution. Finally, 0.4 mL of ascorbic acid (AA) with a concentration of 0.1 M was added to the above solution and heated at 100 °C for 2.5 h under mechanical stirring. After cooling to room temperature, the obtained product was separated from the reactive solution by a magnet. After washing several times with distilled water and ethanol, the sample was dried at room temperature. The Pd NPs were also synthesized by a similar procedure in the absence of CoFe₂O₄ nanotubes.

Investigation of the peroxidase-like activity of Pd/CoFe₂O₄ composite nanotubes and H₂O₂ detection

The peroxidase-like activity of the as-prepared Pd/CoFe₂O₄ composite nanotubes was evaluated by testing the catalytic oxidation of TMB by H₂O₂. In a typical experiment, 20 μL of TMB solution (15 mM in dimethylsulfoxide, DMSO) and 20 μL of H₂O₂ (30%) were added to 3 mL of acetate buffer solution (pH = 4.0) in a spectrophotometer cell. Then 20 μL of catalyst suspension (2.81 mg mL⁻¹) was added to the above mixture, and a blue color quickly appeared. The peroxidase-like activity during the catalytic reaction was monitored in time course mode by recording the absorption spectra at 650 nm. For the detection of H₂O₂, the same concentration of TMB (final concentration of 100 μM) in acetate buffer was used. Then H₂O₂ solution with a given concentration was added to the TMB solution. After the addition of the catalyst (20 μL 2.81 mg mL⁻¹), UV-vis absorption spectral measurement was used to monitor the catalytic activity in time course mode. In regard to the monitoring of H₂O₂ concentration, a dose–response curve was obtained by recording the absorption of the spectra at 650 nm in the presence of different concentrations of H₂O₂. Hence, the detection limit can be calculated from the dose–response curve, which is similar to previous reports.^15,37

Characterization

The morphology of the products was determined by scanning electron microscopy (SEM, Nova NanoSEM 450) and transmission electron microscopy (TEM, JEOL JEM-1200 EX) operated at 5.0, 15 and 100 kV, respectively. High-resolution TEM (HRTEM) experiments were performed on a FEI Tecnai G2 F20 electron microscope with an acceleration voltage of 200 kV. X-ray diffraction (XRD) data were collected using an X-ray diffractometer (Empyrean, PANalytical B.V.) based on Cu-Kα radiation. X-ray photoelectron spectra (XPS) were performed on a Thermo Scientific ESCALAB250 instrument. UV-vis spectra were recorded on Shimadzu UV-3101 and 2501 PC spectrometers. The weight percentage of Pd in the Pd/CoFe₂O₄ composite nanotubes was determined by inductively coupled plasma (ICP) atomic emission spectrometric analysis (PerkinElmer OPTIMA 3300DV).

Results and discussion

Morphology and composition

In the present work, CoFe₂O₄ nanotubes were prepared by electrospinning a solution containing Fe(NO₃)₃ and Co(Ac)₂ (as precursors to CoFe₂O₄), PVP (as a polymer carrier), ethanol and DMF (as solvents), followed by a calcination process at 550 °C in air. The as-electrospun PVP/Fe(NO₃)₃/Co(Ac)₂ composite nanofibers were collected in the form of a nonwoven mat on aluminum foil. As shown in Fig. 1a, the PVP/Fe(NO₃)₃/Co(Ac)₂ composite nanofibers show a uniform fiber-like morphology throughout the entire length, and there are no beads or irregular particles in the product. After calcination, the composite nanofibers are converted into CoFe₂O₄ nanotubes with rough surfaces and outer diameters in the range of 50 to 200 nm (Fig. 1b). The hollow structure of the CoFe₂O₄ nanofibers was further investigated by TEM (Fig. 1c and d); the wall thickness of the hollow CoFe₂O₄ nanotubes is about 10 to 20 nm. The formation of the hollow structure of the CoFe₂O₄ nanotubes through the single spinneret and calcination process is likely due to the phase separation between the precursors of CoFe₂O₄ and polymer carrier along with solvent evaporation during the electrospinning process, which is in accordance with previous reports.^49,50

Fig. 1 (a) SEM image of the electrospun PVP/Co(Ac)₂/Fe(NO₃)₃ composite nanofibers; (b) SEM image of the synthesized CoFe₂O₄ nanotubes; (c and d) TEM images of CoFe₂O₄ nanotubes at low and high magnifications.

The functionalization of Pd NPs on the surface of CoFe₂O₄ nanotubes was achieved by placing the CoFe₂O₄ nanotubes in an aqueous reduction bath containing Na₂PdCl₄, sodium citrate and AA at a certain temperature. Fig. 2a and b present typical SEM and HRTEM images of the as-prepared Pd/CoFe₂O₄ composite nanotubes. The highly crystalline nature of the Pd nanoparticles on the surface of the CoFe₂O₄ nanotubes can be observed. The HRTEM image shows that the lattice fringe spacing, measured to be about 0.22 nm, is consistent with the (111) crystallographic plane of face-centered cubic (fcc) Pd,⁵¹ while the lattice fringe spacing of 0.25 nm is attributed to the (311) plane of inverse spinel CoFe₂O₄ with a cubic lattice.⁵² Furthermore, the energy dispersive X-ray (EDX) spectrum also demonstrates the existence of Pd, Fe, Co, O, and C elements in the as-prepared Pd/CoFe₂O₄ composite nanotubes (Fig. 2c). It should be noted that the signals of Cu and Si in the EDX spectrum are attributed to the carbon-coated copper grid and the TEM instrument. This result indicates the formation of Pd nanoparticles on the surface of the CoFe₂O₄ nanotubes. The selective area electron diffraction (SAED) pattern displays several discernible concentric rings, demonstrating the polycrystalline nature of the CoFe₂O₄ nanotubes (Fig. 2d). The element mapping analysis reveals that Pd atoms are distributed on the entire surface of the CoFe₂O₄ nanotubes (Fig. 2e–i). The weight percentage of Pd in the Pd/CoFe₂O₄ composite nanotubes is calculated to be about 7.73%, as determined by ICP measurement.

Fig. 2 (a) SEM image, (b) HRTEM image, (c) EDX spectrum of the synthesized Pd/CoFe₂O₄ nanotubes; (d) SAED pattern of the Pd/CoFe₂O₄ nanotubes; (e) HAADF-STEM pattern and EDX element mapping images of (f) Co–K, (g) Fe–K, (h) O–K and (i) Pd–L in Pd/CoFe₂O₄ nanotubes.

The crystalline structure of the synthesized Pd/CoFe₂O₄ composite nanotubes was also investigated by XRD measurements. As shown in Fig. 3, before the modification of Pd nanoparticles, the XRD pattern of individual CoFe₂O₄ nanotubes can be indexed to inverse spinel CoFe₂O₄ with a cubic lattice (JCPDS # 22-1086).⁵² The diffraction peaks at 2θ = 18.4, 30.3, 35.6, 43.0, 53.5, 57.1, 62.6, and 74.1° can be designed to the (111), (220), (311), (400), (422), (511), (440) and (533) planes. There are no other obvious diffraction peaks in the XRD pattern of CoFe₂O₄ nanotubes, indicating the complete removal of PVP and the good phase purity. After the modification with Pd nanoparticles, another obvious peak appears at about 2θ = 40.3°, which corresponds to the (111) Bragg reflection of the Pd nanoparticles. Using the Scherrer equation, the average crystalline size of the Pd nanoparticles is estimated to be about 8.3 nm.

Fig. 3 XRD patterns of the synthesized (a) CoFe₂O₄ nanotubes and (b) Pd/CoFe₂O₄ nanotubes.

X-ray photoelectron spectra (XPS) were further used to describe the chemical composition and surface electronic state of the as-prepared Pd/CoFe₂O₄ composite nanotubes. The spectrum clearly shows that the characteristic signals of Fe 2p bands are observed at about 712.1 and 725.9 eV, which are attributed to Fe 2p_3/2 and Fe 2p_1/2, respectively (Fig. 4a). This result indicates the presence of Fe³⁺ in the Pd/CoFe₂O₄ composite nanotubes.⁵³ In Fig. 4b, two Co 2p_3/2 and Co 2p_1/2 peaks with binding energies of 781.2 and 796.6 eV are observed, respectively. In addition, two satellite peaks that are related to shake-up peaks of the Co 2p_3/2 and Co 2p_1/2 edges appear at around 786.6 and 804.7 eV, suggesting the presence of Co²⁺ species in the Pd/CoFe₂O₄ composite nanotubes.⁵³ In the O 1s spectrum (Fig. 4c), the higher binding energy band at 532.4 eV is ascribed to hydroxyl groups, while the other peak at about 530.7 eV is related to the lattice oxygen. In addition to Fe, Co and O elements, Pd and C elements are also displayed in the XPS spectrum, among which the signal of C 1s may originate from the carbonaceous contaminant. As shown in Fig. 4d, the characteristic signals for Pd 3d_5/2 and Pd 3d_3/2 appear at 335.9 eV (Pd 3d_5/2) and 341.2 eV (Pd 3d_3/2) in the XPS spectrum; these are attributed to metallic Pd.⁵³ From the XPS spectrum, it can be also concluded that almost no oxidation of Pd is observed by using this reduction strategy, which may be due to the protection of sodium citrate. The existence of Pd, Fe, Co and O elements suggests the successful modification of Pd nanoparticles on the surface of CoFe₂O₄ nanotubes.

Fig. 4 XPS spectra of the synthesized Pd/CoFe₂O₄ nanotubes. (a) Fe 2p, (b) Co 2p, (c) O 1s, and (d) Pd 3d.

Peroxidase-like activity of Pd/CoFe₂O₄ composite nanotubes

It has been reported that spinel ferrite compounds such as CoFe₂O₄ and ZnFe₂O₄ nanomaterials possess intrinsic peroxidase-like activity; however, their catalytic activity is relatively low. By modification with noble metal nanoparticles, it is anticipated that their catalytic performance will be greatly enhanced. In this study, we evaluated the peroxidase-like catalytic activity of the as-prepared Pd/CoFe₂O₄ composite nanotubes using a model reaction involving the oxidation of TMB in the presence of H₂O₂. As shown in Fig. 5a, the synthesized Pd/CoFe₂O₄ composite nanotubes can quickly catalyze the oxidation of TMB in the presence of H₂O₂, which produces a typical blue color in the mixture solution. The UV-vis spectra of TMB solution were used to evaluate the catalytic process, and a maximum absorption band at around 650 nm was observed when we mixed the Pd/CoFe₂O₄ composite nanotubes with TMB and H₂O₂ in acetate buffer for 10 min. This band is related to the charge transfer complex (CTC), which demonstrates the oxidation of TMB. Control experiments were performed to show that no blue color was generated in mixtures of TMB + H₂O₂, TMB + Pd/CoFe₂O₄ composite nanotubes, and H₂O₂ + Pd/CoFe₂O₄ composite nanotubes. Accordingly, no obvious absorption peaks for the oxidation of TMB appear in the UV-vis spectra for these three systems. This result suggests that Pd/CoFe₂O₄ composite nanotubes can catalyze the oxidation of TMB in the presence of H₂O₂.

Fig. 5 (a) UV-vis absorption curves of TMB solutions of different systems (TMB + H₂O₂ + Pd/CoFe₂O₄, TMB + Pd/CoFe₂O₄, H₂O₂ + Pd/CoFe₂O₄, TMB + H₂O₂); the inset shows the corresponding photographs of these systems. (b) Time-dependent absorption changes at 650 nm with the addition of different types of catalysts (CoFe₂O₄ nanotubes, Pd NPs, physical mixture of CoFe₂O₄ nanotubes and Pd NPs, Pd/CoFe₂O₄ composite nanotubes). (c) Dependence of the peroxidase-like activity of the synthesized Pd/CoFe₂O₄ composite nanotubes on the pH values and (d) the corresponding line chart.

In comparison, we have evaluated the catalytic activities of Pd/CoFe₂O₄ composite nanotubes, pure CoFe₂O₄ nanotubes, individual Pd nanoparticles fabricated by the same method, and even the physical mixture of CoFe₂O₄ nanotubes and Pd nanoparticles, respectively. As shown in Fig. 5b, pure CoFe₂O₄ nanotubes and Pd nanoparticles alone show very low peroxidase-like activity. Although the catalytic activity of the physical mixture of CoFe₂O₄ nanotubes and Pd nanoparticles is higher than that of the individual CoFe₂O₄ nanotubes and Pd nanoparticles, it is much lower than that of the Pd/CoFe₂O₄ composite nanotubes. The superior peroxidase-like activity of the Pd/CoFe₂O₄ composite nanotubes is likely due to the small size of the Pd nanoparticles and the synergistic effect between the Pd nanoparticles and the CoFe₂O₄ nanotubes. We have also studied the relationship between the catalytic activity of the Pd/CoFe₂O₄ composite nanotubes and the pH of the solution. As shown in Fig. 5c, the absorbance at 650 nm increases more rapidly at pH 4.0 than at other pH values; thus, the optimal pH for the peroxidase-like catalytic reaction is chosen as 4.0. Accordingly, the line chart of the relative activity at varied pH values is shown in Fig. 5d.

In addition, the recyclability of Pd/CoFe₂O₄ composite nanotube catalysts towards the oxidation of TMB in the presence of H₂O₂ has been investigated. Here, the Pd/CoFe₂O₄ composite nanotube catalyst can be easily separated from the solution after the reaction by using a magnet. As shown in Fig. S1,† it is found that the catalytic activity decreases dramatically after the first cycle. However, the activity does not change much after the following three cycles. The decrease of the catalytic activity may be due to the loss of the catalyst during the recycling process and the adsorption of the product on the surface of the Pd/CoFe₂O₄ composite nanotube catalyst after the reaction. The TEM images in Fig. S2† shows the morphology of the Pd/CoFe₂O₄ composite nanotube catalyst after five cyclic catalytic reactions. It is clearly seen that the tube-like morphology and the size of the nanoparticles does not change greatly after the fifth catalytic reaction. However, it appears that a thin organic layer is adsorbed on the surface of the Pd/CoFe₂O₄ composite nanotubes, which may have led to a decrease in their peroxidase-like catalytic activity.

Performance for H₂O₂ detection

On the basis of the peroxidase-like activity, the accurate determination of H₂O₂ could be achieved by a colorimetric strategy. As the concentration of H₂O₂ is proportion with the intensity of the absorption band at 650 nm corresponding to the oxidation of TMB, H₂O₂ could be quantitatively detected by monitoring the absorption spectrum of the TMB solution after its catalytic oxidation by Pd/CoFe₂O₄ composite nanotubes. Fig. 6a displays the time-dependent absorption changes at 650 nm in the presence of H₂O₂ with various concentrations under optimal conditions (pH = 4.0). It is obvious that the reaction rate increases with increasing H₂O₂ concentration, resulting in an increased absorption at 10 min. The typical H₂O₂ concentration–response curve in time course mode shows that H₂O₂ can be detected in a linear range of 10 to 100 μM. Also, the limit of detection is calculated to be about 1.68 μM at a signal to noise ratio (S/N) of 3. This result indicates that the as-prepared Pd/CoFe₂O₄ composite nanotubes provide potential applications in biosensors and environmental monitoring.

Fig. 6 (a) The time-dependent absorbance changes of TMB solution at 650 nm catalyzed by Pd/CoFe₂O₄ composite nanotubes in the absence or presence of H₂O₂ with varied concentrations in acetate buffer solution (pH = 4.0); (b) the dose–response curve for the detection of H₂O₂. The inset shows the linear calibration plot for H₂O₂, which exhibits the relationship between low concentrations of H₂O₂ and the corresponding absorbance as well as the capacity for the detection of H₂O₂.

Conclusions

In summary, we have successfully fabricated Pd nanoparticles modified CoFe₂O₄ composite nanotubes using a strategy involving electrospinning, calcination, and an in situ reduction process. The small Pd nanoparticles were distributed uniformly on the surface of CoFe₂O₄ nanotubes.

When employed as nanocatalysts for a peroxidase-like reaction, the obtained Pd/CoFe₂O₄ composite nanotubes with unique structures exhibited superior catalytic activity over the individual CoFe₂O₄ nanotubes and Pd NPs alone. Significantly, the obtained Pd/CoFe₂O₄ composite nanotubes with peroxidase-like catalytic activity have been exploited for the colorimetric detection of H₂O₂ with high sensitivity. It is anticipated that this work will be useful for the fabrication of new nanocomposite-based biomimetic catalysts and for extending their applications in biosensing and environmental monitoring.

Acknowledgements

This study was financially supported by research grants from the National Natural Science Foundation of China (51273075, 51473065, 21274052 and 21474043), the Fok Ying Tung Education Foundation of China.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01527a

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