Non-enzymatic hydrogen peroxide electrochemical sensor based on a three-dimensional MnO₂ nanosheets/carbon foam composite

Abstract

A new type of three-dimensional electrochemical sensor platform, MnO₂ nanosheets/carbon foam (MnO₂/CF) hybrid nanostructure, was successfully fabricated for the nonenzymatic detection of H₂O₂. The morphology, structure and composition of the 3D nanostructures were systematically characterized by electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Uniform and thin MnO₂ nanosheets can be formed on the carbon foam after a simple chemical process by immersing the carbon foam in a KMnO₄ aqueous solution. The content of MnO₂ in the MnO₂/CF composites was found to play a great role in its sensing performance for H₂O₂ detection. The MnO₂/CF2 sample with a 24.5% MnO₂ content showed the best performance among the studied MnO₂/CF composites. The 3D hierarchical porous structure with thin MnO₂ nanosheets can provide an enhanced electrochemically active surface area, high electrical conductivity and improved analyte diffusion, which makes it a promising electrochemical sensing platform. The electrochemical studies showed that the H₂O₂ sensor based on the MnO₂/CF2 exhibited an excellent detection performance with a wide linear range from 2.5 × 10⁻⁶ to 2.055 × 10⁻³ M and a detection limit of 1.2 × 10⁻⁷ M (S/N = 3). These results demonstrate that such 3D nanocomposites have promising application in electrochemical sensors.

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1. Introduction

H₂O₂ is one of the most important and widely used analytes in electrochemical analysis, since it is a common oxidizing agent or an essential intermediate in biochemical, pharmaceutical, clinical, industrial and environmental fields. The precise and rapid detection of H₂O₂ is of significant importance. Owing to high sensitivity and selectivity, electrochemical sensors based on the electrocatalysis of immobilized enzymes toward H₂O₂ are used widely.^1–3 However, due to the inevitable drawbacks of the enzymatic sensors such as instability, limited lifetime, high cost of enzymes, critical operating situation and complicated immobilization procedure, much effort has been devoted to the direct determination of H₂O₂ at non-enzymatic electrodes.^4–6

Nowadays, metal and transition metal oxide nanoparticles have been applied extensively in fabricating highly efficient non-enzymatic electrochemical sensors.^7–11 Noble metallic nanostructures, like nanostructured Pt, Pd, Au, Ag and their alloys, are excellent candidate materials for the construction of non-enzymatic hydrogen peroxide sensors owing to their superior electrocatalytic activities.^12–17 However, the relatively high price and limited resources restrict their widely application. In recent years, much cheaper transition metal compound nanomaterials including CuO, CuS, Fe₃O₄, MnO₂, MoS₂, NiO and TiO₂, have been applied extensively in fabricating highly efficient non-enzymatic H₂O₂ sensors.^18–28 In particular, manganese dioxide nanomaterials are considered as one of the most appealing inorganic materials and have drawn attention in bioanalytical chemistry,^29,30 especially in electrochemical sensing of H₂O₂, due to its low cost, natural abundance, environmental pollution-free and excellent catalytic activity which can promote H₂O₂ decomposition. Many kinds of MnO₂ nanomaterials have been utilized in the fabrication of H₂O₂ sensors.^31–33 However, due to intrinsic poor electric conductivity of MnO₂ (10⁻⁵–10⁻⁶ S cm⁻¹), researchers tried to combine MnO₂ with various carbon materials like graphite,³⁴ graphene,³⁵ ordered mesoporous carbon,³⁶ carbon nanotube³⁷ and carbon nanofiber³⁸ to enhance the electric conductivity of MnO₂-based sensing materials.

Recently, 3D self-supported materials have been fabricated as platforms for electrochemical and gas sensors by modifying electrochemically active materials on 3D scaffolds, such as carbon/graphene foam, Ni foam and other substrates.^39–48 High detection performance with low detection limit, wide linear range and short response time has been achieved by these 3D material-based electrochemical sensors attributed to the large surface area, high electric conductivity and porous structure of the substrates and the high electrocatalytic performance of the electrochemically active materials. However, there are only a few reports on 3D non-enzymatic and non-noble metal H₂O₂ sensors. Xi et al.⁴⁹ reported that non-enzymatic sensors fabricated by thionine/polydopamine/graphene foams can detect H₂O₂ in a linear range from 0.4 to 660 μM. Zhang et al.⁴⁴ reported that a linear range from 0.38 to 13.46 mM could be reached by a non-enzymatic sensor based on MnO₂/graphene foam. Both of the aforementioned materials showed their applications in non-enzymatic H₂O₂ sensor. Nevertheless, the preparation of graphene foam via chemical vapor deposition (CVD) using Ni foam as sacrificial template is complex, costly and time consuming. The output of graphene foam through CVD is also limited. So it is still a challenge to explore new types of 3D materials with easy preparation processes, large scale productivity and low cost for fabricating highly efficient non-enzymatic H₂O₂ sensors.

In this paper, flexible and porous carbon foam (CF) carbonized from low-cost commercial melamine resin foam was used as skeleton to fabricate non-enzymatic H₂O₂ sensor. After in situ redox reaction with KMnO₄, a layer of MnO₂ nanosheets can grow epitaxially on the surface of carbon foam. The 3D sensors exhibited low detection limit, short response time, and wide linear range toward the detection of H₂O₂ which can be ascribed to the novel structure of MnO₂/CF composites. The porous structure of CF can not only provide the multiple electron paths, but also enable the efficient contact between the electrolyte and electrode surface, resulting in a rapid response to the analyte (H₂O₂). The large surface area of MnO₂ nanosheets offers plenty of contact area with H₂O₂ ensuring the low detection limit. The present study provides a new platform for nonenzymatic detection of H₂O₂.

2. Experimental section

All the reagents used in the experiments were analytical grade and used without further purification. All aqueous solutions were prepared with ultrapure water (18.3 MΩ cm).

2.1. Preparation of carbon foam

Melamine resin foam (MRF, supplied by Puyang Green Universe Chemical Co., Ltd) was carbonized in a tube furnace fixed with a quartz tube with 5 cm inner diameter under the protection of 100 mL min⁻¹ Ar flow. Eight pieces of MRF with a size of 0.5 cm × 3.5 cm × 25 cm were piled up and put in the middle of the tube furnace. Carbonization temperature program was as follows. First, the temperature was raised from ambient temperature to 300 °C in 1 h and kept for 5 minutes. Second, the temperature was further raised to 400 °C in 100 minutes and kept for 5 minutes. Finally, the temperature was increased to 1000 °C in 5 h and kept for 1 h. The furnace was then cooled down to room temperature.

2.2. Fabrication of MnO₂/carbon foam composites (MnO₂/CF)

In order to improve the hydrophilicity of CF, CF was firstly wetted with ethanol and then washed with deionized water three times to remove residual ethanol. Without such a hydrophilicity process, the as-prepared CF can not be wholly infiltrated by KMnO₄ aqueous solution, resulting in nonuniform growth of MnO₂ nanosheets. A series of MnO₂/CF composites were prepared by varying the concentration of KMnO₄ aqueous solution from 0.5, 1, 2 and 3 to 5 mM. The composites were named as MnO₂/CF0.5, MnO₂/CF1, MnO₂/CF2, MnO₂/CF3 and MnO₂/CF5, respectively. In a typical synthesis of MnO₂/CF, 55 mg CF was dipped into 0.2 L KMnO₄ aqueous solution and the reaction was finished when the purple color of KMnO₄ disappeared. The reaction was conducted at 60 °C in a water bath.

2.3. Materials characterization

Morphology of the samples was viewed by scanning electron microscopy (SEM; XL30) and transmission electron microscopy (TEM; Hitachi H-600). High-resolution TEM (HRTEM) images, X-ray energy dispersive spectroscopy (EDS) and the selected area electron diffraction (SAED) were carried out on a JEM-2010(HR) microscope. The crystal structures of the as-prepared products were characterized with an X-ray powder diffractometer (XRD; D/Max 2500 V/PC, Cu-Kα radiation). X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Thermo ESCALAB 250 spectrometer (VG Scientific) operated at 120 W with an energy analyzer working in the pass energy mode at 100.0 eV. An Al Kα line was used as the excitation source. The binding energy was calibrated against the carbon 1s line. The contents of MnO₂ in the composites were calculated from thermogravimetric analysis (TGA, Pyris Diamond TG/DTA) which was carried out under air flow of 60 mL min⁻¹ with a heating rate of 5 °C min⁻¹.

2.4. Preparation of working electrodes and electrochemical measurements

In order to preserve the novel 3D structure of MnO₂/CF composites, the working electrodes for H₂O₂ sensors were fabricated by cutting a piece of corresponding MnO₂/CF composites and pasting it on a graphite plate with conductive carbon adhesive. The graphite plate was connected to a stainless steel wire to facilitate the test. Other faces of the graphite plate were covered by epoxy resin to avoid their contact with solution, except the working face. The active materials were weighed on an electronic balance with the precision of 0.01 mg and the weights are between 0.1 and 0.2 mg. A CHI 660D electrochemical workstation (Shanghai, Chenhua) was used for all the electrochemical and impedance spectroscopy measurements with conventional three-electrode configuration using a graphite plate counter electrode and an Ag/AgCl reference electrode (saturated with KCl (aq.)). The electrochemical sensing of H₂O₂ was carried out in 0.1 M phosphate buffer solution (PBS, pH = 7.4) and the PBS was degassed with N₂ for 20 min before test.

3. Results and discussion

3.1. Synthesis and characterization of materials

The morphology of the 3D nanomaterials was first characterized by electron microscopy. As showed in Fig. 1A, large-scale 3D nanostructures can be obtained with the present method, which is very favorable for the constructing electrochemical sensing platform. It should be noted that the performance of sensors based on nanoparticles may decrease with duty cycle increasing because of the aggregation of particles. The present rigid 3D nanomaterials, however, can provide very stable structure as sensing platform. It can be seen that the white melamine resin foam changed into grey carbon foam after carbonization. The colors of the MnO₂/CF composites become darker and darker with the increase of the concentration of KMnO₄ aqueous solution, which is ascribed to the increase of the thickness of MnO₂ layers (Fig. 2 and Fig. S1†). It is noteworthy that all the as-prepared carbon foam and the MnO₂/CF composites retained the original foam-like structure of melamine resin foam. Fig. 1B–D shows the SEM images of the prepared carbon foam at different magnifications. It can be seen that 3D carbon network with hollow structure was produced through the carbonization of melamine resin foam. As shown in Fig. 1E–G, after the reaction between the carbon foam and KMnO₄ aqueous solution, uniform MnO₂ nanosheet layers were formed on the framework of the carbon foam. The foam-like materials are composed by triangle fibers which interconnected and formed plenty of micron-sized pores leading to network architecture.

Fig. 1 (A) Digital photographs of melamine resin foam (MRF), carbon foam (CF) and the MnO₂/CF composites with different MnO₂ contents. (B–G) SEM images of CF (B–D) and MnO₂/CF3 (E–G) at different magnifications.

Fig. 2 Top view SEM images of the as-prepared 3D porous nanomaterials, (A) CF, (B) MnO₂/CF0.5, (C) MnO₂/CF1, (D) MnO₂/CF2, (E) MnO₂/CF3 and (F) MnO₂/CF5. The insets show the corresponding cross section view SEM images.

The effect of the KMnO₄ concentration on the structure of the MnO₂/CF composites was studied. Fig. 2 shows the SEM images of the MnO₂/CF structures produced from different KMnO₄ concentrations. From Fig. 2A, the carbon foam framework has a very smooth surface. After immersion of carbon foam in different concentrations of KMnO₄, all the surfaces of the 3D skeleton were covered by thin MnO₂ nanosheets. Interestingly, the size of the MnO₂ nanosheet and the thickness of the MnO₂ layers are strongly dependent on the concentration of KMnO₄. As shown in Fig. 2B–F, the MnO₂ nanosheets grow larger and larger with concentration of KMnO₄ below 2 mM. However, when the concentration of KMnO₄ further increases above 2 mM, the size of the produced MnO₂ nanosheets decreases gradually. This phenomenon can be ascribed to the rapid reaction between KMnO₄ and CF and the quick formation of small MnO₂ nucleus at the relatively high concentrations of KMnO₄. From the cross section view SEM images shown in Fig. 2 insets (enlarged images are shown in Fig. S1†), the thickness of the MnO₂ nanosheets layer increases with increasing the KMnO₄ concentration. Therefore, by changing KMnO₄ concentration 3D MnO₂/CF nanocomposites with optimized structure and composites can be constructed.

The crystal structure of the MnO₂ nanosheets was characterized by high-resolution TEM (HRTEM) measurements. As showed in Fig. 3A, the MnO₂ nanosheets have numerous wrinkles and ripples and are only a few nanometers in thickness. Such porous structure can effectively improve the surface/interface area of MnO₂ nanocrystals and the solution diffusion among the interspaces of MnO₂ nanosheets, which is beneficial for the rapid response to analyte. The HRTEM image shown in Fig. 3B reveals that the MnO₂ nanosheets are actually composed of small MnO₂ nanocrystals. From each crystal, well-resolved lattice fringes with interplanar spacing of 0.25 nm can be observed, which can be indexed to the (101) plane of birnessite-type MnO₂.⁵⁰

Fig. 3 (A and B) HRTEM images of the MnO₂ nanosheets stripped from MnO₂/CF0.5 at different magnifications.

The X-ray energy dispersive spectroscopy (EDS) result shown in Fig. S2A† demonstrates that the as-prepared composites mainly contain C, Mn, and O. Trace of K is probably from KMnO₄ since there is always a possibility of potassium ions co-existing in the MnO₂ matrix.⁵¹ The crystalline structure of MnO₂ in the composites was identified by XRD measurements. As shown in Fig. S2B,† for CF, there are two broad diffraction peaks with 2θ around 25° and 44°, which can be ascribed to the (002) and (100) planes of amorphous carbon.⁵² For MnO₂/CF composites, except for the peaks from CF, new diffraction peaks can be observed with 2θ around 12°, 37° and 66°, corresponding to the (001), (111) and (020) diffraction of birnessite-type MnO₂ crystalline phase (JCPDS no. 42-1317).⁵³ Meanwhile, with the increasing of KMnO₄ concentration in the synthesis, the diffraction peaks belonging to MnO₂ became stronger and the intensities of the diffraction peaks from CF decreased, indicating increased MnO₂ content from MnO₂/CF0.5 to MnO₂/CF5. The XPS survey on sample MCF0.5 is shown in Fig. S2C.† The XPS signals of elements C, Mn and O could be seen clearly. Fig. S2D† shows the Mn 2p core-level XPS spectrum. The binding energies of Mn 2p_3/2 and 2p_1/2 located at about 642.1 and 653.8 eV, respectively, with a spin-energy separation of 11.7 eV, suggesting the predominant oxidation state of Mn is +4.⁵⁴ Thermogravimetric analysis (TGA) was then performed to analyze the real composition of the products. From the results showed in Fig. S3,† the weight percents of MnO₂ in MnO₂/CF0.5, MnO₂/CF1, MnO₂/CF2, MnO₂/CF3 and MnO₂/CF5 were calculated to be 7.7%, 10.9%, 24.5%, 47.9% and 66.9%, respectively.

3.2. Electrochemical sensing performance of 3D MnO₂/CF composites for H₂O₂ detection

3.2.1. Electrochemical response of MnO₂/CF electrode to H₂O₂. The electrochemical sensing performance of the MnO₂/CF composites with different size and thickness of MnO₂ nanosheets were evaluated by cyclic voltammetry (CV). Fig. 4A and B show the CV curves of the bare carbon foam (CF) and MnO₂/CF2 composite electrodes with the absence and presence of different concentrations of H₂O₂ in 0.1 M PBS (pH = 7.4), respectively. It can be seen that on the bare CF electrode, with the concentration of H₂O₂ increasing from 0 to 5 mM, the obtained CVs exhibited little change, indicating the negligible electrocatalytic activity of carbon foam for H₂O₂ reduction. Compared to the bare CF, obvious H₂O₂ reduction current can be observed on the MnO₂/CF2 composite electrode and the current increases with the increasing of H₂O₂ concentrations. As shown in Fig. 4C, the reduction current at −0.6 V exhibits a good linear relationship (R² = 0.999) with the H₂O₂ concentration, indicating the outstanding sensitivity of MnO₂ nanosheets to the concentration change of H₂O₂. The cyclic voltammogram tests demonstrate that the response current of MnO₂/CF composite electrodes to H₂O₂ is mainly from the MnO₂ nanosheets and CF only serves as a conductive backbone. Fig. S4† shows the CVs of H₂O₂ reduction on the other MnO₂/CF composite electrodes. Similar to the MnO₂/CF2, all the MnO₂/CF electrodes exhibit electrochemical response to the addition of H₂O₂. Such CV results indicate that the formed MnO₂ nanosheets on the carbon foam have electrocatalytic activity for H₂O₂ reduction and the 3D MnO₂/CF composites could be used as electrochemical sensing platform for H₂O₂ detection.

Fig. 4 CVs of bare CF (A), MnO₂/CF2 (B) in the absence and presence of H₂O₂ in 0.1 M PBS (pH = 7.4) at a scan rate of 0.02 V s⁻¹. (C) The calibration curve of the reduction current dependent on H₂O₂ concentration obtained from (B) at −0.6 V.

By comparing the CVs on the MnO₂/CF composites with different MnO₂ contents, the onset potentials of H₂O₂ reduction are different. For the MnO₂/CF0.5, MnO₂/CF3 and MnO₂/CF5 samples, the onset potentials are close to or more negative than 0.0 V. However, the onset potentials observed from the MnO₂/CF1 and MnO₂/CF2 electrodes are much more positive than 0.0 V (around 0.15 V). Therefore, the MnO₂ content covered on the carbon foam has a large effect on the electrocatalytic activity and the MnO₂/CF1 and MnO₂/CF2 composites exhibit the optimized composition for H₂O₂ reduction. From the SEM images shown in Fig. 2 insets and Fig. S1,† too thin and too thick MnO₂ layers were formed on the MnO₂/CF0.5, MnO₂/CF3 and MnO₂/CF5 composites. However, the MnO₂/CF1 and MnO₂/CF2 exhibit a moderate thickness of MnO₂ layers. Moreover, compared to other composites, the MnO₂/CF1 and MnO₂/CF2 are covered by larger MnO₂ nanosheets (Fig. 2), suggesting the larger pores formed in the two porous structures. The moderate thickness of MnO₂ nanosheets and the large pores in the MnO₂/CF1 and MnO₂/CF2 structures are beneficial for the electrical conductivity and mass transport during electrochemical reactions, resulting in the higher electrochemical performance for H₂O₂ reduction. By comparing Fig. 4B and Fig. S4B,† one can see that current from MnO₂/CF3 is larger than that from MnO₂/CF2 electrode. It should be noted that the currents shown in Fig. 4 and Fig. S4† are not normalized to the mass loading of MnO₂ on the electrodes. In fact, the weights of the MnO₂/CF electrodes increase with the increasing of MnO₂ mass ratio in the MnO₂/CF composites. In order to compare the performances of MnO₂/CF2 and MnO₂/CF3, the CVs of the two electrodes with response currents normalized to the weight of MnO₂ were shown in Fig. S5.† It is clear that the normalized current response from MnO₂/CF2 is larger than that of MnO₂/CF3. Based on above electrochemical results, MnO₂/CF2 is mainly selected in the following studies for H₂O₂ detection.

3.2.2. Amperometric response of the MnO₂/CF electrodes to H₂O₂. In a typical hydrogen peroxide sensing experiment, 10 μL H₂O₂ solution with different concentrations (7.5 mM, 15 mM, 60 mM, 0.15 M, 0.6 M and 1.5 M) was successively injected into the stirring electrolyte solution (0.1 M PBS, pH = 7.4) at room temperature at a potential of −0.45 V. The background current was allowed to decay to a constant value before H₂O₂ solution was added to the cell. It was observed that the MnO₂/CF2 electrode responded quickly to the change of H₂O₂ concentration and reached a steady-state signal within 10 s (Fig. 5A). The corresponding calibration curve for the H₂O₂ detection is shown in Fig. 5B. The sensor displays a linear range from 2.5 μM to 2.06 mM (R = 0.999), and a detection limit of 1.2 × 10⁻⁷ M (signal/noise = 3). For comparison, the amperometric response of the MnO₂/CF5 towards the reduction of H₂O₂ and the corresponding calibration curve are shown in Fig. 5C and D. The linear range for H₂O₂ detection is from 35 μM to 0.56 mM and the detection limit is estimated to be 3.1 × 10⁻⁵ M. By comparing the insets in Fig. 5A and C, at low H₂O₂ concentrations, the MnO₂/CF2 still have sensitive amperometric responses. However, only unresolvable currents were observed on the MnO₂/CF5 composite upon the addition of different concentrations of H₂O₂. Obviously, in comparison with the MnO₂/CF5 composite, the MnO₂/CF2 exhibits a much higher electrochemical sensing performance for H₂O₂ with a wider linear range and lower detection limit. The amperometric responses of the other MnO₂/CF electrodes to H₂O₂ and the corresponding calibration curves are shown in Fig. S6.†

Fig. 5 The amperometric response of the MnO₂/CF2 (A) and MnO₂/CF5 (C) electrodes to H₂O₂, 10 μL 7.5 mM, 15 mM, 60 mM, 0.15 M, 0.6 M and 1.5 M H₂O₂ were added into the PBS at a–f, respectively. Insets in (A) and (C) are the amperometric response from low H₂O₂ concentration range at the corresponding electrodes, respectively. (B and D) The corresponding plots of the response current vs. H₂O₂ concentration obtained from the MnO₂/CF2 (B) and MnO₂/CF5 (D) electrodes, respectively.

To compare the sensitivity of the MnO₂/CF composites for H₂O₂ detection, Fig. 6 shows the plots of current change dependence on the concentration change of hydrogen peroxide at the five electrodes. Obviously, all the MnO₂/CF composites show linear current responses to H₂O₂ concentrations, again suggesting the good response of the MnO₂/CF-based H₂O₂ sensors. Moreover, the largest slope of the plots was obtained from the MnO₂/CF2 electrode, indicating the highest sensitivity of the MnO₂/CF2 sample. The comparison of the detection performance obtained from the 3D MnO₂/CF materials is summarized in Table 1. It can be seen that the sensitivity of the MnO₂/CF electrodes to H₂O₂ increases with MnO₂ content below 24.5% (the MnO₂ content in MnO₂/CF2). Further increase of MnO₂ content above 24.5% leads to decreased sensitivity and narrower linear range. Thus, the MnO₂/CF2 exhibited the best sensing performance for H₂O₂ detection. Such performance change agrees well with the results of electrocatalytic activity obtained from CV measurements (Fig. 4 and S4†). The decreased detection performance with high content of MnO₂ could be mainly attributed to the intrinsic low electric conductivity of the thick MnO₂ layers.

Fig. 6 Current responses of the MnO₂/CF composites to the change of H₂O₂ concentrations. Data were obtained from Fig. 4B and Fig. S4.†

Table 1 Comparison of the performance of various H₂O₂ sensors based on MnO₂/CF composites

Samples	MnO2 wt%	Linear range (μM)	Detection limit (M)	Sensitivity (μA μM^−1)
MnO2/CF0.5	7.7%	1.05 × 10^−5 to 2.06 × 10^−3	1.0 × 10^−6	0.015
MnO2/CF1	10.9%	3.5 × 10^−5 to 3.56 × 10^−3	1.6 × 10^−7	0.020
MnO2/CF2	24.5%	2.5 × 10^−6 to 2.06 × 10^−3	1.2 × 10^−7	0.054
MnO2/CF3	47.9%	3.5 × 10^−5 to 1.06 × 10^−3	2.1 × 10^−6	0.009
MnO2/CF5	66.9%	3.5 × 10^−5 to 5.55 × 10^−4	3.1 × 10^−5	0.012

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To further study the charge transfer resistance of the MnO₂/CF-based H₂O₂ sensors, electrochemical impedance spectroscopy results from different samples were collected and compared, as shown in Fig. S7.† The diameter of the semicircle at high frequency corresponds to the charge transfer resistance (R_ct) at the interface where reaction takes place, involving both ion and electron transfer processes. Note that the R_ct increases with the increase of MnO₂ content in the composites which is ascribed to the intrinsic low electric conductivity of MnO₂. For comparison, the electrochemical sensing performance of MnO₂/CF2 and other reported materials for H₂O₂ detection are listed in Table 2. It can be seen that the present MnO₂/CF2 shows better or at least comparable performance compared to the other electrochemical sensing materials. The good performance of MnO₂/CF2 could be attributed to following reasons. First, the 3D interconnected carbon foam scaffold provides a high conductive backbone for epitaxial growth of MnO₂ nanosheets which favors the electron transfer and thus guarantees the full utilization of MnO₂ nanosheets. Second, the hierarchical porous structure of the hybrid networks, including the micro-sized macropores of carbon foam and the nano-sized interspace among MnO₂ nanosheets, is favorable for electrolyte immersion and diffusion. Finally, the ultrathin and wrinkled structure of MnO₂ nanosheets is beneficial for preventing aggregation of MnO₂ nanosheets and improving their available surface area to analyte. Here, we also compared the detection performance with those of non-electrochemical H₂O₂ sensors. It was found that our results are comparable to those obtained by other non-electrochemical detection methods. For instance, carbon dots derived from β-cyclodextrin are capable of detecting H₂O₂ in the linear range from 2.0 × 10⁻⁶ M to 5.0 × 10⁻⁴ M with the detection limit of 1.0 × 10⁻⁶ M via colorimetric detection method.⁶⁸ In another report,⁶⁹ a TiO₂/SiO₂ composite prepared by a sol–gel route showed a linear response to H₂O₂ concentration ranging from 7.0 × 10⁻⁶ to 7.0 × 10⁻² M by a phosphorescence method.

Table 2 Comparison of the performance of electrochemical H₂O₂ sensors based on various materials

Samples	Linear range (M)	Detection limit (M)	Ref.
Fe3O4/MWCNT	6.0 × 10^−5 to 3.6 × 10^−4	1 × 10^−5	55
CoOOH nanosheets	4.0 × 10^−5 to 1.6 × 10^−3	4 × 10^−5	56
CoOxNPs/ERGO	5.0 × 10^−6 to 1.0 × 10^−3	2 × 10^−7	57
Cu-NPs/PoPD	1.0 × 10^−6 to 1.0 × 10^−3	1 × 10^−7	58
Pd/PEDOT	2.5 × 10^−6 to 1.0 × 10^−3	2.84 × 10^−7	59
Ni(OH)2/MWCNT	1.5 × 10^−6 to 2.5 × 10^−3	6.1 × 10^−7	60
Urchin-like core–shell CuO	1.0 × 10^−5 to 5.55 × 10^−3	—	61
MnO2 microspheres/Nafion	1.0 × 10^−5 to 1.5 × 10^−4	2 × 10^−6	62
MnOx nanoparticles	2.0 × 10^−5 to 1.26 × 10^−3	2 × 10^−5	31
MnO2 nanosheets/chitosan	5.0 × 10^−6 to 3.5 × 10^−3	1.5 × 10^−6	63
Mn-NTA nanowires	5.0 × 10^−6 to 2.5 × 10^−3	2 × 10^−7	64
MnO2 nanorods	1.0 × 10^−6 to 1.5 × 10^−3	1 × 10^−7	65
MnO2/VACNTs	1.2 × 10^−6 to 1.8 × 10^−3	8.0 × 10^−7	37
MnO2/graphene/CNT	1 × 10^−6 to 1.03 × 10^−3	1 × 10^−7	66
MnO2/GO	5 × 10^−6 to 6 × 10^−4	8.0 × 10^−7	67
MnO2/CF2	2.5 × 10^−6 to 2.055 × 10^−3	1.2 × 10^−7	This work

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3.2.3. Interferences and stability. The possible interference of foreign chemicals, which might exist in real samples, was investigated during the amperometric determination of H₂O₂. The interference experiments were performed in 0.1 M PBS (pH = 7.4) at −0.45 V by comparing the amperometric response from H₂O₂ (50 μM) and twofold concentrations of each interfering substance of glucose (Glu), dopamine (DA), ascorbic acid (AA), and uric acid (UA). The amperometric responses of the MnO₂/CF2 towards the different analytes are showed in Fig. 7A. It can be seen that twofold concentration of Glu, DA, AA and UA shows negligible current changes compared to that of H₂O₂. Such result indicates the remarkable anti-interference properties of the MnO₂/CF2-based sensor to Glu, DA, AA and UA. The stability of the MnO₂/CF2 sensor was evaluated by measuring the amperometric response at −0.45 V in 0.1 M PBS. As displayed in Fig. 7B, only a little current change can be observed during 1000 s test period, indicating the high stability of the MnO₂/CF2 sensor.

Fig. 7 (A) Interference experiments of glucose (Glu), dopamine (DA), ascorbic acid (AA), uric acid (UA) for the H₂O₂ sensing on the MnO₂/CF2-based sensor in 0.1 M PBS (pH = 7.4) at −0.45 V. The concentration of analytes are: H₂O₂ 50 μM, Glu 100 μM, DA 100 μM, AA 100 μM and UA 100 μM. (B) Stability test of the MnO₂/CF2-based sensor in 0.1 M PBS (pH = 7.4) at −0.45 V (vs. Ag/AgCl).

4. Conclusions

In summary, a type of nonenzymatic electrochemical sensors based on 3D porous MnO₂/CF composite were fabricated by in situ deposition of MnO₂ nanosheets on the surface of carbon foam. Structural characterizations clearly demonstrated the formation of porous MnO₂ nanosheet layers on the 3D carbon framework. It was found that the size of the MnO₂ nanosheet and the thickness of the MnO₂ composite depend strongly on the content of MnO₂. The sensors fabricated from the 3D MnO₂/CF nanostructures exhibited a MnO₂ content-dependent sensing performance for H₂O₂ detection. The MnO₂/CF2 with 24.5% MnO₂ content exhibited the highest performance among the studied materials with low detection limit, high sensitivity, wide linear range, good selectivity and stability. The excellent sensing properties of the 3D MnO₂/CF hybrids could be attributed to the high conductivity from carbon backbone, improved mass transport from the porous structure and the high electrocatalytic activity of the thin MnO₂ nanosheets. Therefore, the catalytic nature of MnO₂ towards H₂O₂ reduction, combined with the high conductive carbon network make the 3D MnO₂/CF composites hold the promise for the development of nonenzymatic sensor at a low cost.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21275136) and the Natural Science Foundation of Jilin province, China (no. 201215090).

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Footnote

† Electronic supplementary information (ESI) available: Additional structural characterizations and electrochemical measurements. See DOI: 10.1039/c4ra09007a

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