Cu₂O/CuO@rGO heterostructure derived from metal–organic-frameworks as an advanced electrocatalyst for non-enzymatic electrochemical H₂O₂ sensor

Abstract

In this work, a hybrid heterostructure comprising well-dispersed Cu₂O/CuO particles and reduced graphene oxide (rGO) is synthesized by calcinating a mixture of MOFs-118 and GO nanosheets in nitrogen atmosphere to improve the sensitivity and selectivity of H₂O₂ sensors. Thanks to the splendid electrocatalytic activity of the CuO/Cu₂O heterostructure nanoparticles, the good catalytic performance and conductivity of surrounding C-matrix derived from MOFs-118, and the high electronic conductivity and large surface area of rGO, the electrochemical performance of the Cu₂O/CuO@rGO modified glass carbon electrode (GCE) for the oxidation of hydrogen peroxide (H₂O₂) are studied and exhibit a high sensitivity of 431.65 μA cm⁻² mM⁻¹, a low detection limit of 0.71 μM, an extended linear range from 1.5 μM to 11.0 mM as well as a good selectivity and stability. All these results demonstrated that this novel heterostructure composite would be a competitive candidate for the non-enzymatic H₂O₂ sensing.

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Introduction

The rapid and quantitative detection of hydrogen peroxide (H₂O₂) is of tremendous importance in many fields such as health care, clinical diagnosis, pharmaceuticals, environmental monitoring and food industry because of its intermediate role in many biological or environmental reactions.^1–5 Up to now, many types of sensors have been employed in the measurement of H₂O₂ based on fluoroimmunoassay, electrochemical technique, chemiluminescence, etc.^6–11 Among all of these methods, the electrochemical technique has shown much superiority for further development due to its simplicity, fast detection, low cost and high sensitivity.¹² In recent years, many researchers have turned their attention to the non-enzymatic H₂O₂ electrochemical sensor in view of the bad stability and reproducibility of enzymatic sensors.^13–17 So far, a variety of functional materials such as noble metals (Au, Pd, Pt, etc.), carbon-based materials, and transition metal and their sulfides or oxides have been fabricated and applied to the detection of H₂O₂ due to their excellent electrocatalytic performance.¹⁸ But unfortunately, the above materials suffered either unsatisfactory sensitivity or high cost defects, which restricted their practical application in H₂O₂ detection. Therefore, it is greatly imperative to construct a new and effective electrochemical active material which possesses high sensitivity, reliability and low cost for the detection of H₂O₂.

Metal–organic frameworks (MOFs) are a new class of microporous materials which are fabricated by bridging metal ions or clusters with organic ligands via coordination interactions.¹⁹ Endowed with the diverse spatial organization of organic ligands with inorganic nodes, MOFs contain a great number of structural topologies and varied porosities with different channels or cavities.^20,21 Over the past decade, the excellent properties of MOFs, such as the high surface areas, ordered and tailorable porosities, makes them a competitive candidate as gas storage and separation,^22,23 drug delivery,²⁴ catalysis or optoelectronics. In addition, MOFs and their composites have also been applied in the field of electrochemistry including fuel cells,²⁵ rechargeable Li-ion batteries and electrochemical sensors.

Recently, the MOF-derived composites which are featured by the good conductivity, high surface area and rapid mass transport property have been developed as the active materials for electrochemical application.²⁶ For example, Xu et al. synthesized high-content nitrogen-decorated nanoporous carbons by employing ZIF-8 as the precursor and template via a calcination method, finding good electrocatalytic activity for the oxygen reduction reactions (ORR).²⁷ Zhang et al. prepared nano/micro Co₃O₄ materials by calcinating a MOF precursor of cobalt(bdc)₂ in the inert gas, getting enhanced specific capacitance for the supercapacitor.²⁸ And more recently, an anthill-like Cu@carbon nanocomposite was prepared by utilizing a Cu-based MOF (HKUST-1) as the precursor and exhibited good non-enzymatic electrochemical sensing performance for the detection of H₂O₂ due to the favorable catalytic activity and chemical stability of copper and copper oxides.²⁹ However, the MOF-derived materials are usually aggregated during the calcination process, indicating their decreased electrochemical activity. Therefore, the preparation of MOF-derived composites with good dispersity and splendid electrochemical sensing performance is pretty desirable. In recent years, the graphene which has an ultrathin thickness and honeycomb crystal lattice structure has been considered as an excellent supporting material to avoid the aggregation of metal or metal oxide nanoparticles because of its large surface area, excellent conductivity and stability. So it would be a feasible route that calcinate the mixture of graphene oxide (GO) and MOFs to fabricate the sensing materials which are not stacked but have excellent electrochemical performance for the detection of H₂O₂.

Herein, we reported an enzyme-free electrochemical sensor based on the MOF-derived Cu₂O/CuO@rGO composite which was synthesized by calcinating the mixture of MOF-118 and GO in nitrogen atmosphere at 600 °C (denoted as Cu-MOFs/GO-600). Inspired by the superior electrocatalytic activity of the well-dispersed Cu₂O/CuO nanoparticles and the high electronic conductivity of the rGO, the as-prepared Cu-MOFs/GO-600 modified electrode exhibited the enhanced sensitivity and extended linear range as well as a good stability and reproducibility in the detection of H₂O₂. It provided a simple route to fabricate the non-enzymatic electrochemical sensor with enhanced sensing performance.

Experimental details

Chemicals and reagents

4,4′-Biphenyldicarboxylic acid (BPDC, 98%) was obtained from Tianjin Heowns Biochem LLC. Cu(NO₃)₂·3H₂O, pyridine, methylalcohol, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), cyclohexane and hydrogen peroxide (30% H₂O₂) were all purchased from Xilong Chemical Industry Incorporated Co. Ltd. All reagents with no special mention were analytical grade and were used directly without further purification. Ultra-pure water was used as the solvent to prepare different solutions.

Preparation of MOFs-118 powders

The pure MOFs-118 nanoparticles (denoted as Cu-MOFs) were prepared according to the previous report with some modification.³⁰ Firstly, Cu(NO₃)₂·3H₂O (0.134 g, 0.55 mmol), BPDC (0.135 g, 0.55 mmol) and 7.5 mL DMA were mixed in a 20 mL glass tube. After sonicating the mixture for 10 minutes, 2.5 mL methylalcohol and 0.25 mL pyridine were added, respectively. Next, the glass tube was sealed and heated up to 95 °C at a heating rate of 1 °C min⁻¹. Kept this temperature for 60 h and then cooled to room temperature at a heating rate of 1 °C min⁻¹. The as-formed blue block-shaped crystals were washed three times with DMF and cyclohexane, respectively. After that, the products were dried in an oven at 70 °C for 12 h and finally they were grinded to blue powders.

Preparation of Cu-MOFs/GO composite

Typically, the GO was prepared from graphite powders using the Hummers method with a little modification.^31,32 The Cu-MOFs/GO composite was prepared by mixing the as-prepared Cu-MOFs and GO in a glass tube with the mass ratios of 10 : 1. Next, several milliliter alcohol was injected and the suspension was mixed uniformly by sonicating for 1 h. After that, the glass tube was placed in a water bath at a temperature of 70 °C with strong magnetic stirring until the alcohol solvent was completely evaporated. Finally, the obtained samples were grinded to powders.

Preparation of Cu-MOFs/GO-400, Cu-MOFs/GO-600 and Cu-MOFs/GO-800 powders.

The above prepared Cu-MOFs/GO samples were heated up to 400 °C, 600 °C or 800 °C at a heating rate of 3 °C min⁻¹ in a tube furnace under a slow nitrogen flow, and then kept this temperature for 3 h, and finally cooled to room temperature at a heating rate of 3 °C min⁻¹. The obtained products were grinded to powders and denoted as Cu-MOFs/GO-400, Cu-MOFs/GO-600 and Cu-MOFs/GO-800, respectively.

Preparation of the working electrode

The glass carbon electrode (GCE) was carefully polished with 0.05 μm alumina slurry to a mirror-like surface and subsequently cleaned with deionized water and ethanol by sonication for two minutes, respectively. The drop-casting method was employed to fabricate the different sample modified GCE (Cu-MOFs, Cu-MOFs/GO, Cu-MOFs/GO-400, Cu-MOFs/GO-600 and Cu-MOFs/GO-800). Typically, 20 μL sample suspension, which was prepared by sonicating the mixture of 1 mg sample powders and 1 mL ethanol for several minutes, was dropped onto the GCE surface and dried in air. And then 3 μL Nafion solution with the concentration of 0.5 wt% was covered on the surface of sample powders and dried in air to obtain the Cu-MOF-GCE, Cu-MOFs/GO-GCE, Cu-MOFs/GO-400-GCE, Cu-MOFs/GO-600-GCE and Cu-MOFs/GO-800-GCE, respectively. To study the effect of mass loading on the electrochemical performance of samples, 10 μL, 20 μL and 30 μL Cu-MOFs/GO-600 suspension (corresponds to 10 μg, 20 μg and 30 μg samples, respectively) was used for the preparation of different electrode.

Electrochemical measurement

The CHI760E electrochemical workstation was employed to perform all electrochemical experiments with a conventional three-electrode system in which the GCEs modified by different samples were used as working electrode, Hg/HgO electrode as reference electrode, platinum foil as auxiliary electrode. The electrolyte was 0.1 M NaOH solution. Before the electrochemical measurements, all of the prepared GCEs were activated by cyclic voltammograms (CV) for 9 cycles in the potential range of −1.5 V to 0.6 V at 20 mV s⁻¹. 0.1 M KCl with 5 mM K₃Fe(CN)₆/K₄Fe(CN)₆ (1 : 1) solution was employed to perform the electrochemical impedance spectroscopy (EIS) in the frequency of 0.1 Hz to 10 kHz and estimate the effective surface area of working electrode by cyclic voltammetry.

Characterizations

The crystal structure data were obtained from X-ray powder diffraction (XRD, a Rigaku D/Max-2400 diffractometer, Japan; monochromated Cu Kα radiation, k = 1.548 Å; 40.0 kV, 60.0 mA). Transmission electron microscopy (TEM) micrographs were collected by a FEI Tecnai G2F30 microscope.

Results and discussion

Firstly, the influence of the different annealed temperature on the current response of Cu-MOFs/GO samples was studied. As shown in Fig. S1,† the strongest current response belonged to the Cu-MOFs/GO-600 sample. In addition, the electroactive surface areas of these samples prepared at different annealing temperature were estimated by cyclic voltammetry using ferricyanide as a redox probe. As shown in Fig. S2,† the good linear relationships between peak current and the square root of the scan rate revealed good electrochemical reversibility of the active materials. At the same time, the electroactive surface areas were calculated to be 2.98 cm², 4.36 cm² and 4.02 cm² for the Cu-MOFs/GO-400, Cu-MOFs/GO-600 and Cu-MOFs/GO-800 samples, respectively, based on the Randles–Sevcik equation:

I_p = (2.69 × 10⁵)n^3/2AD₀^1/2C₀v^1/2

where I_p (A) is the peak current, v (V s⁻¹) is the scan rate, A (cm²) is the area of the electrode, n, D₀ (cm² s⁻¹) and C₀ (mol cm⁻³) are constants. Obviously, the Cu-MOFs/GO-600 sample have the largest effective surface area. Therefore, the annealed temperature was determined to be 600 °C.

X-ray powder diffraction (XRD) measurement was employed to investigate the crystal structure of the as-prepared samples. As shown in Fig. 1, the peak of GO sample (curve a) at about 2θ = 7.5° can be assigned to the (001) reflection of stacked GO sheets, which demonstrated the successful preparation of GO sheets.³³ The as-prepared Cu-MOFs sample showed a series of diffraction peaks (curve b) which could be in good agreement with the simulated XRD pattern of MOF-118 according to the reported crystal structure data.³⁴ After hybridizing GO sheets with the Cu-MOFs, the primary characteristic peaks of MOFs-118 can still be observed in curve c and yet the peaks at about 7° and 10° became a bit obtuse. This change can be ascribed to the existence of GO sheets in Cu-MOFs/GO sample. Next, this Cu-MOFs/GO precursor was calcinated at 600 °C in N₂-atmosphere to fabricate Cu-MOFs/GO-600 sample and the XRD pattern was labeled in curve d. The presented diffraction peaks at 2θ values of 29.56°, 36.42°, 42.31°, 61.37°, 73.52° and 77.37° can match well with the (110), (111), (200), (220), (311) and (322) planes of cubic Cu₂O, and the peaks located at 35.56° and 38.75° were assigned to the (−111) and (111) planes of monoclinic CuO. The formation of copper oxides may originate from the oxidation in calcination process by the oxygenated functional groups which existed in GO sheets, such as the epoxy, hydroxyl or carboxyl. And simultaneously, the initial GO sheets would be reduced to rGO,^35,36 so a weak diffraction peak at 2θ = 26° which could be assigned to the (002) reflection of rGO could be observed in the XRD pattern of Cu-MOFs/GO-600 sample. Except for the Cu-MOFs/GO-600, the XRD pattern of Cu-MOFs/GO-400 sample also presented Cu₂O and CuO diffraction peaks, indicating a structure damage of the MOF-118 at 400 °C (Fig. S3a†). However, when the annealing temperature was further increased to 800 °C, only the Cu₂O peaks could be observed, which should result from the higher thermodynamic stability of Cu₂O than CuO at high temperature (Fig. S3b†). In addition, the graphite peak also became more obviously in the Cu-MOFs/GO-800 sample, demonstrating the enhanced graphitization at high pyrolysis temperature.

Fig. 1 XRD patterns of (a) GO, (b) Cu-MOFs, (c) Cu-MOFs/GO, (d) Cu-MOFs/GO-600 samples.

The morphology and structure of the as-prepared samples were further studied by transmission electron microscopy (TEM). Fig. 2A depicted a bamboo-leaf-like nanostructure with a width of ∼30 nm and length of ∼100 nm for the Cu-MOFs. And this distinctive structure could be observed more clearly from the sample of Cu-MOFs/GO in which the Cu-MOFs were well dispersed on the GO nanosheets (Fig. 2B), indicating a good hybrid between GO and Cu-MOFs. Fig. 2C presented the TEM image of Cu-MOFs/GO-600 sample. It can be obviously observed that the bamboo-leaf-like configuration has evolved into spherical particles after a post-annealing treatment, and these particles were well anchored on rGO nanosheets with a diameter of 100–200 nm. The rGO in Cu-MOFs/GO-600 exhibited plentiful wrinkles, which resulted from the graphitization of GO sheets in calcination process. Obviously, the rGO nanosheets could effectively avoid the aggregation of particles, which was able to provide a high catalytic performance for the subsequent H₂O₂ detection. The high-resolution transmission electron microscopy (HRTEM) image of the presented particles was shown in Fig. 2D, it recorded three different interplanar spacing of 0.303 nm, 0.242 nm and 0.278 nm, respectively, which could correspond to the (110) and (111) planes of cubic Cu₂O and (110) plane of monoclinic CuO. And we can observe the formation of heterojunction of Cu₂O and CuO in Fig. 2D. The TEM images of Cu-MOFs/GO-400 and Cu-MOFs/GO-800 samples were presented in Fig. S4.† A few of well small particles could be observed in Fig. S4A,† which indicated the thermal decomposition of Cu-MOFs has already started at 400 °C. As shown in Fig. S4B,† when the temperature was heated to 800 °C, the diameter of spherical particles was about 300 nm, resulting from the further growth of small particles at high pyrolysis temperature.

Fig. 2 TEM images of Cu-MOFs (A), Cu-MOFs/GO (B), Cu-MOFs/GO-600 (C) samples and the HRTEM image (D) of Cu-MOFs/GO-600 sample.

The cyclic voltammograms (CVs) of GCE decorated by different samples were conducted in 0.1 M NaOH solution with the absence and presence of 2 mM H₂O₂ at a can rate of 20 mV s⁻¹. As shown in Fig. 3A, the anodic current of Cu-MOFs and Cu-MOFs/GO modified GCEs can be negligible in the absence of H₂O₂. By contrast, a broad reduction peak at about 0.65 V vs. Hg/HgO can be obviously found for the Cu-MOFs/GO-600 modified electrode, which corresponded to the Cu²⁺/Cu³⁺ redox couple according to the previous report.^37,38 When 2 mM H₂O₂ was added into the electrolyte, there was a small increase of the oxidation current for the Cu-MOFs and Cu-MOFs/GO modified GCEs, however, a distinct current response in an extended potential range was observed for the Cu-MOFs/GO-600-GCE, which was nearly two times higher than that of Cu-MOFs or Cu-MOFs/GO modified electrodes. According to the previous reports, the oxidation mechanism can be proposed as follows:

2Cu(I) + H₂O₂ + 2OH⁻ → 2Cu(0) + 2H₂O + O₂ (1)

Cu(II) + H₂O₂ + 2OH⁻ → Cu(0) + 2H₂O + O₂ (2)

Cu(II) → Cu(III) + e⁻ (3)

Cu(III) + H₂O₂ + 2OH⁻ → Cu(II) + 2H₂O + O₂ (4)

Fig. 3 (A) Cyclic voltammetry curves (CVs) of the GCE modified by Cu-MOFs (a), Cu-MOFs/GO (b) and Cu-MOFs/GO-600, respectively (c) in the absence (dotted line) and presence (solid line) of 2 mM H₂O₂; (B) the EIS of GCE modified by Cu-MOFs (a), Cu-MOFs/GO (b) and Cu-MOFs/GO-600 (c). Inset: the magnification graph of high frequency region.

Accordingly, both of Cu(I) and Cu(II) from the modified electrode could oxidize H₂O₂ through reactions 1 and 2 to O₂.³⁹ As the potential shifted to more positive values, Cu(II) would be oxidized to Cu(III), and it can also participate in the electro-oxidation process of the H₂O₂ (eqn (3) and (4)).²⁹ Thereby, there was a remarkable oxidation current response in an extended potential range for the Cu-MOFs/GO-600-GCE from Fig. 3A. In addition, the Cu-MOFs/GO-600 modified GCE exhibited a small onset potential of 0.05 V, which demonstrated its superior electrocatalytic kinetics for the oxidation of H₂O₂. The superior electrocatalytic performance of Cu-MOFs/GO-600 modified electrode can be attributed to the synergistic effect of rGO and the CuO/Cu₂O heterogeneous nanoparticles. Here, the rGO nanosheets provided large specific surface area, high conductivity and the CuO/Cu₂O heterojunction structures offered facile transport pathways for the electrons due to their different electron work functions in the electrochemical process.⁴⁰ This can be further verified by the electrochemical impedance spectroscopy (EIS) as shown in Fig. 3B. A big semicircle at high frequency region in the Nyquist plot, which corresponded to a kinetic-controlled process,⁴¹ could be found in curve a. It represented a high electron charge transfer resistance (R_ct) for the Cu-MOFs-GCE, which indicated an inefficient electron transfer process at the electrode interface.⁴² After hybrid with the GO nanosheets, the R_ct of the modified electrode decreased dramatically as shown in curve b. However, an obvious decrease of the R_ct was observed from the curve c when the Cu-MOFs/GO sample was annealed in the nitrogen atmosphere. This result demonstrated that both hybrid with GO nanosheets and calcination in the nitrogen atmosphere could enhance the conductivity of the as-prepared composite and thus were conducive to improving electrochemical performance of the Cu-MOFs/GO-600-GCE for the hydrogen peroxide sensing.⁴³

Considering that the applied potential have an important effect on the sensing performance of samples for the detection of H₂O₂, so the optimization experiments were performed at different potentials through a typical I–t technique. As shown in Fig. S5,† the current response of the GCE modified by Cu-MOFs/GO-600 was obviously enhanced with the change of applied potential from 0.4 V to 0.5 V. But when the applied potential was further increased from 0.5 V to 0.8 V, the amount of variation of the current density was small and the noisy signals became larger. Therefore, 0.5 V (0.42 V vs. Ag/AgCl) was selected as the optimum potential for the following experiments. Moreover, the amperometric response of the Cu-MOFs/GO-600 modified electrode at 0.50 V with different mass loading was shown in Fig. S6.† According to Fig. S6,† the maximum current response was obtained when the mass loading was 20 μg. Thus the mass loading of 20 μg was used in the subsequent experiments.

The correlation between the current response and H₂O₂ concentration for different modified GCEs were studied by the I–t technique in which various concentration of H₂O₂ were successively injected to the electrolyte solution with a strongly magnetic stirring. As shown in Fig. 4A, upon each addition of H₂O₂, all of these modified electrodes can obtain 95% of steady-state current within 1 s, indicating their very fast electrocatalytic kinetics to the H₂O₂ oxidation. However, the current response of the Cu-MOFs/GO-600-GCE for the same concentration of H₂O₂ solution was obviously larger than that of the Cu-MOFs and Cu-MOFs/GO modified GCE. The corresponding calibration curves displayed the linear correlations of the current response and H₂O₂ concentration for the Cu-MOFs, Cu-MOFs/GO and Cu-MOFs/GO-600 modified GCEs (Fig. 4B). The sensitivity of the Cu-MOFs/GO-600-GCE (431.65 μA cm⁻² mM⁻¹) was almost two times higher than that of the Cu-MOFs/GO-GCE (296.83 μA cm⁻² mM⁻¹) or the Cu-MOFs-GCE (211.40 μA cm⁻² mM⁻¹). Moreover, it presented a more extended linear range (1.5 μM to 11.0 mM, 0.997) than that of the Cu-MOFs/GO-GCE (2.0 μM to 7.0 mM, 0.996) or the Cu-MOFs-GCE (1.0 μM to 5.0 mM, 0.996). And the detection limit of the Cu-MOFs/GO-600-GCE was evaluated as 0.71 μM (S/N = 3). Importantly, the as-prepared Cu-MOFs/GO-600-GCE presented a good superiority relative to the sensitivity, liner range and detection limit in comparison with the previously reported nanomaterial-based non-enzymatic H₂O₂ sensors (Table 1). These obtained results demonstrated that the as-prepared Cu-MOFs/GO-600 composite had a superior electrocatalytic performance for the oxidation of H₂O₂.

Fig. 4 (A) Amperometric responses of the Cu-MOFs (a), Cu-MOFs/GO (b) and Cu-MOFs/GO-600 (c) modified GCEs with the successive addition of different concentration of H₂O₂ at 0.5 V. Inset: a partial magnification of the current response toward a low concentration of H₂O₂ solution. (B) The corresponding calibration curves of the Cu-MOFs (a), Cu-MOFs/GO (b) and Cu-MOFs/GO-600 (c) modified GCEs obtained from (A).

Table 1 Comparison of the performances of the as-developed nonenzymatic H₂O₂ sensor with other reported nanomaterial-based H₂O₂ sensors

Materials	Sensitivity	Linear range (μM)	Detection limit (μM)	Response time (s)	Ref.
a CuO@Cu2O-NWs/PVA, CuO@Cu2O nanowires/poly(vinyl alcohol).b CuS/CS, CuS nanoparticles/chitosan.
Grass-like CuO	119.35 μA mM^−1	10–300	5	<3	43
Heart/dumbbell-like CuO	72.9 μA mM^−1	10–900	4	<3	44
CuO@Cu2O-NWs/PVA^a	39.5 μA cm^−2 mM^−1	1–3000	0.35	<5	45
Cu2O/graphene nanosheets	300 μA cm^−2 mM^−1	300–7800	20.8	<7	46
Cubic Cu2O	25 μA cm^−2 mM^−1	500–8500	1.61	<2	47
CuO–SiNWs	22.27 μA mM^−1	100–13 180	1.6	—	29
CuS/CS^b	36.4 μA mM^−1	1–100	0.3	—	48
Cu-MOFs/GO-600	431.65 μA cm^−2 mM^−1	1.5–11 000	0.71	<1	This work

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The anti-interference ability towards possible co-existed species was important for the electrochemical sensors. As shown in Fig. 5, the interference currents relative to the ascorbic acid (AA), uric acid (UA), glucose, fructose or lactose were almost negligible compared to that of H₂O₂, which indicated the good selectivity of the Cu-MOFs/GO-600-GCE.

Fig. 5 Amperometric responses of Cu-MOFs/GO-600 modified GCE upon successive addition of 1 mM H₂O₂, 0.1 mM AA, 0.1 mM UA, 0.1 mM glucose, 0.1 mM fructose, 0.1 mM lactose and 1 mM H₂O₂ in 0.1 M NaOH.

The reproducibility of the Cu-MOFs/GO-600 modified GCE was examined by repetitive measurements of 1 mM H₂O₂ using one electrode, and the relative standard deviation (RSD) was evaluated to 2.51% (n = 4), indicating a strong anti-poisoning ability of the Cu-MOFs/GO-600 to the oxidation products. Moreover, four Cu-MOFs/GO-600 modified GCEs were prepared by the same method, and the RSD of the current responses was evaluated to 3.56%, which demonstrated the proposed method was reliable.

The long-term stability of the Cu-MOFs/GO-600-GCE was studied by continuously detecting 1 mM H₂O₂ every two days within a period of 15 days. As shown in Fig. 6A, the current response towards the equivalent concentration of H₂O₂ was almost not decreased after the continuous tests, demonstrating the splendid stability and durability for the Cu-MOFs/GO-600 modified GCE. Besides, this electrode was stored in a refrigerator at 4 °C for three weeks, and the current response maintained almost 100% of its initial value (Fig. 6B), it suggested the good long-term storage stability of the Cu-MOFs/GO-600-GCE for H₂O₂ detection.

Fig. 6 The long-term stability of the Cu-MOFs/GO-600 modified GCE through detecting 1 mM H₂O₂ every 2 days over 15 days (A), and its long-term storage stability through storing in refrigerator and detecting 1 mM H₂O₂ every one week (B).

Conclusion

In summary, the Cu₂O/CuO@rGO composites were successfully synthesized through one-step thermal transformation route of the mixture Cu-MOFs and GO in nitrogen atmosphere at 600 °C. The Cu-MOFs/GO-600 modified GCE was applied to the detection of H₂O₂ and presented encouraging electrochemical performances, including high sensitivity of 431.65 μA cm⁻² mM⁻¹, extended linear range of 1.5 μM to 11.0 mM, low detection limit of 0.71 μM as well as a good selectivity, stability and reproducibility, because of the synergistic effect of the CuO and Cu₂O heterostructure, good catalytic performance and conductivity of surrounding C-matrix derived from MOFs-118, the good conductivity and dispersity of rGO substrate. All of the results demonstrated a broad application prospect for the as-prepared CuO/Cu₂O@rGO composites and it provided a simple method to enhance the sensing performance of the enzyme-free electrochemical sensors.

Acknowledgements

This work was supported by grants from Natural Science Foundation of China (NNSFC no. 21673105, 21503102), the Fundamental Research Funds for the Central University (lzujbky-2016-K09, lzujbky-2016-k02), the Science and Technology Program of Gansu Province of China (145RJZA176) and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education (LZUMMM2016008).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23551d

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