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3D Network and 2D Paper of Reduced Graphene Oxide/ Cu2O Composite for Electrochemical Sensing of Hydrogen Peroxide Chunfeng Cheng, Chunmei Zhang, Xiaohui Gao, Zhihua Zhuang, Cheng Du, and Wei Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04070 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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Analytical Chemistry

3D Network and 2D Paper of Reduced Graphene Oxide/ Cu2O Composite for Electrochemical Sensing of Hydrogen Peroxide Chunfeng Cheng, †, ‡ Chunmei Zhang, †, ‡ Xiaohui Gao, †, ‡ Zhihua Zhuang, †, § Cheng Du†, § and Wei Chen*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China ‡

University of Chinese Academy of Sciences, Beijing 100039, China

§

University of Science and Technology of China, Hefei 230029, Anhui, China

*Corresponding author, E-mail: [email protected]

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ABSTRACT In this study, two-dimensional (2D) and three-dimensional (3D) freestanding reduced graphene oxide-supported Cu2O composites (Cu2O-rGO) were synthesized via simple and cost-efficient hydrothermal and filtration strategies. The structural characterizations clearly showed that highly porous 3D graphene aerogel supported Cu2O microcrystals (3D Cu2O-GA) have been successfully synthesized and the Cu2O microcrystals are uniformly assembled in the 3D GA. Meanwhile, paper-like 2D reduced graphene oxide-supported Cu2O nanocrystals (2D Cu2O-rGO) have also been prepared by a filtration process. It was found that the products prepared from different precursors and methods exhibited different sensing performances for H2O2 detection. The electrochemical measurements demonstrated that the 3D Cu2O-GA has high electrocatalytic activity for the H2O2 reduction and excellent sensing performance for the electrochemical detection of H2O2 with a detection limit of 0.37 µM and a linear detection range from 1.0 µM to 1.47 mM. Meanwhile, the 2D Cu2O-rGO structure also showed good electrochemical sensing performance toward H2O2 detection with a much wider linear response over the concentration range from 5.0 µM to 10.56 mM. Compared to the previously reported sensing materials, the as-obtained 2D and 3D Cu2O-rGO materials exhibited higher electrochemical sensing properties toward the detection of H2O2 with high sensitivity and selectivity. The 2D and 3D Cu2O-rGO composites also exhibited high sensing performance for the real-time detection of H2O2 in human serum. The present study indicates that 2D and 3D graphene-Cu2O composites have promising applications in the fabrication of non-enzymatic electrochemical sensing devices.

Keywords: Graphene; copper oxide; freestanding; hydrogen peroxide; electrochemical sensor

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INTRODUCTION The accurate detection of H2O2 has attracted much attention because H2O2 has important applications in various fields, such as food, pharmaceutical, catalysis, gas sensors, and environmental analyses, lithium ion batteries, solar energy.1-4 Therefore, developing low-cost, simple, fast and sensitive methods for monitoring concentration level of H2O2 is of practical significance for both industry and academia. To date, many kinds of techniques, like chromatography,5 chemiluminescence,6 fluorescence,7 spectrophotometry,8 colorimetry,9 and titrimetry

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have been used for the quantification of H2O2. Compared with the above

detection techniques, electrochemical method is considered to be a more efficient method with the advantages of good selectivity, high-sensitivity, low-cost, and straightforward manipulation. Although enzyme-based electrochemical sensors exhibit obvious advantages of high detection performance, the main drawbacks, including complicated immobilization procedure, high cost of enzymes, susceptibility to temperature and pH value and the insufficient stability originated from the nature of enzymes, limit their extensive applications.11 Therefore, a growing interest in the development of enzyme-free H2O2 sensors with peroxidase-like activity for detecting hydrogen peroxide has been aroused. In recent years, with the fast development of nanotechnology, various nanostructured materials have been successfully used to construct enzyme-free H2O2 sensors, such as noble metal nanocrystals,12,

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transition metal oxide

nanowires,14 carbon nanotubes15 and graphene-based nanostructures.16-18 Among of them, noble metal-based nanomaterials, such as Pt,19 Pd,20 Ag,21 and Au,22 usually exhibit high performances for the electrochemical detection of H2O2. However, the high cost, poor tolerance to poisonous reaction intermediates and rare storage of noble metals have greatly limited their commercial applications. In addition，harsh experimental conditions,23, 24 structure- and property-controlled fabrication of nano- or micro-devices for practical non-enzymatic detection of H2O2 in vivo or in vitro are still challenging. 3



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Therefore, it is necessary to develop sensitive, reliable, but low-cost electrochemical sensing materials for the hydrogen peroxide detection. Non-precious-metal catalysts (NPMCs) are considered as possible substitutes to precious metals for enzyme-free H2O2 sensors due to their highly catalytic activity and good stability. Up to now, a variety of NPMCs such as Co3O4,25, 26 Fe3O4,27 Cu2O,28 MnO2 29 have already been successfully applied to the electrochemical detection of H2O2. Meanwhile, as a novel carbon material, graphene was first obtained by a manual mechanical cleavage of graphite with Scotch tape in 2004,30 and since then a wide range of techniques have been developed for synthesizing graphene-based materials. Two dimensional (2D) graphene with sp2-bonded carbon atoms arranged in a honeycomb crystal lattice has attracted tremendous attention due to its outstanding physical and chemical properties, including single-atom thickness, high electron mobility, unprecedented pliability and impermeability, large theoretical specific surface area and so on.31,

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Up to now, various

graphene-supported metal oxides have been applied in electrochemical sensors and electrocatalysis. Metal oxides, particularly transition metal oxides have some exceptional physicochemical properties, such as high alkaline corrosion resistance in electrochemical environments, multiple oxidation states, and distinct crystalline structures. Among the transition metal oxides, Cu2O has been used as promising candidates for electrochemical sensors, owing to its narrow band gap (2-2.2 eV) with suitable energy level position,33 non-toxicity and low cost. Cu2O can be potentially applied in photo-splitting of water,34 solar energy conversion,35 photocatalysis,36 lithium-ion batteries,37 and gas sensing,38 etc. For example, Zhang at al. synthesized Cu2O-graphene hybrid nanomaterials by an ultrasound-assisted approach and the as-obtained anode material exhibited enhanced lithium ion battery performance.39 Deng at al. reported the synthesis of reduced graphene oxide (rGO)-conjugated Cu2O nanowires and the composite exhibited high sensing sensitivity toward NO2 detection at room temperature.40 In addition, by taking advantaging of the high 4


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catalytic activities, carbon-based nanocomposites have also been used as effective and cheap catalysts for removal of organic pollutants and other photocatalytic reactions. 41-43 Based on the advantages of graphene and Cu2O discussed above, in this work, we report freestanding three dimensional graphene aerogel-supported Cu2O microcrystals (3D Cu2O-GA) and two dimensional reduced graphene oxides-supported Cu2O paper (2D Cu2O-rGO) synthesized by two facile and cost-efficient methods and their applications in electrochemical detection of H2O2. As for the 3D Cu2O-GA, without any surfactant, the composite was synthesized via a hydrothermal and lyophilization two-step strategy. The electron microscopy characterizations clearly showed the uniform dispersion of Cu2O microparticles in the 3D GA. By taking advantages of the large surface area and high electronic conductivity of graphene and high electrocatalytic activity of Cu2O microparticles, the fabricated 3D Cu2O-GA exhibited excellent sensing performance for the electrochemical detection of H2O2. Similar to the previously reported method by Ruoff’s group,44 here, 2D Cu2O-rGO with well-ordered macroscopic structure has been successfully fabricated by a vacuum filtration of GO sheets colloidal dispersion through a microporous filtering membrane. The thickness of the obtained freestanding 2D Cu2O-rGO is tunable ranging from 1 to 30 µm. The ultimate tensile stress and strain of GO papers are obviously higher than those of graphite foils. The as-obtained 2D Cu2O-rGO also exhibited good electrocatalytic performance toward the electro-reduction of H2O2. Compared to the previously reported sensing materials, the present work focuses on the fabrication of freestanding and robust 2D and 3D graphene-based macroscopic materials through cost-effective and facile ways with controlled structure and layers, which makes graphene-based macroscopic assemblies attractive candidates for a wide range of practical applications. Meanwhile, compared with other synthetic methods, the two syntheses presented here have the following advantages. First, both hydrothermal reaction and vacuum filtration are simple and cost-efficient and might 5



be applied to industrial-scale synthesis. Second, the morphology, the reduction extent and the properties of the 2D and 3D materials can be controlled by selecting appropriate solvent, manipulating temperature and the concentration of chemicals. Last, the as-obtained 2D and 3D nanomaterials have potential applications as nano-devices in chemical sensors, water purification, flexible substrates, and energy etc.

Experimental section Chemicals Graphite powders (C, ≥98.0%), copper (II) chloride (CuCl2·2H2O, A.R. grade, ≥99.0%) were purchased from Tianjin Guangfu Delicacy Chemical Research Institute. Copper (II) acetate monohydrate (Cu(CH3COO)2·H2O, A.R. grade, ≥99.0%-102.0%), sodium nitrate (NaNO3, A.R. grade, ≥99.0%), .

hydroxylamine hydrochloride (NH2OH HCl, A.R. grade, ≥98.5%). Sodium dodecyl sulfate (SDS, C.P. grade, ≥85.0%) were purchased from Xilong Chemical Reagent. Potassium permanganate (KMnO4, A.R. grade, ≥99.8%), hydrogen peroxide (H2O2, A.R. grade, ≥30%), sulfuric acid (H2SO4, A.R. grade, 95%-98%), sodium hydroxide (NaOH, A.R. grade, ≥96.0%) were obtained from Beijing Chemical Reagent. Water was supplied by a Water Purifier Nanopure water system (18.3 MΩ cm). Human blood serum was obtained from Affiliated Hospital of Northeast Normal University in Changchun, China. And all the chemicals were of analytical grade and used as received. Ultrapure N2 (≥99.999%) was purchased from the Changchun Juyang gas Limited Liability Company. Synthesis of three dimensional graphene aerogel-supported Cu2O microcrystals (3D Cu2O-GA) Graphene oxide (GO) was prepared from graphite powder using a modified Hummers and Offeman’s method.45-47 Typically, GO (7.3 mg/mL) suspension was first ultrasonicated for 30 min to form a homogeneous solution. 0.1595 g Cu(CH3COO)2·H2O dissolved in 10 mL deionized water was transferred into 20 mL Teflon-lined stainless steel autoclave under constant stirring. 6 mL of the prepared GO solution 6


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was then added into the Teflon-lined stainless steel autoclave under vigorous stirring for an hour. After that, the Teflon-lined stainless steel vessel was heated at 180 °C for 16 h and then cooled down to room temperature. After washed by pure water several times, the obtained composite was further freeze-dried under -50 °C. Finally, the 3D freestanding product of Cu2O-GA was collected for further use. For comparison, with the similar process, a pure graphene aerogel (3D GA) and Cu2O microparticles were also synthesized separately. Synthesis of the 2D reduced grapheme oxide-supported Cu2O paper (2D Cu2O-rGO) Cu2O nanocubes were firstly synthesized according to the previously reported procedure.36 Typically, 1.0 mL of 0.1 M CuCl2 solution (light blue) and 0.1756 g of SDS were added into 18 mL deionized water under vigorous stirring. After the SDS powder was dissolved, the sample vial was placed in an oil bath at 32-34 °C. After that, 0.36 mL of 1.0 M NaOH solution was introduced and the color of the solution changed into deep blue immediately, representing the formation of Cu(OH)2 precipitate. Next, 0.8 mL of 0.1 M hydroxyl ammonium hydrochloride was quickly injected into the solution and the color became brown yellow (or orange) and turbid rapidly. The mixture was stirred for another 30 min for nanocrystal growth and then centrifuged at 5000 rpm for 5 min. After removing the supernatant solution, the precipitate was washed with 6 mL of a 1:1 volume ratio of water and ethanol. The obtained composite was dried in vacuum oven at 50 °C for 4 h. GO (6.0 mg/mL) suspension was sonicated for 30 min to form a homogeneous solution. The sample of Cu2O-rGO composite with a mass ratio of 1:2 (Cu2O: GO) was obtained by the simple filtering and annealing (300 °C, 1 h) processes. Material characterization The morphologies of the as-prepared materials were characterized using a XL30 ESEMFEG scanning electron microscope (SEM) operating at an accelerating voltage of 20 kV. The powder X-ray diffraction 7



(XRD) of the products were carried out on a PW1700 Powder diffractometer, using Cu Kα radiation with a Ni filter (λ=0.154059 nm at 20 kV). X-ray photoelectron spectroscopy (XPS) measurements were performed by using a VG Thermo ESCALAB 250 spectrometer (VG Scientific) operated at 120 W. The binding energies were calibrated against the carbon 1s line. Raman spectra were collected with a Renishaw 2000 equipped by an Ar+ ion laser giving the excitation line of 514.5 nm and an air-cooling charge-coupled device (CCD) as the detector (Reinshaw Co.U.K.). Electrochemical measurements Before each experiment, a glassy carbon electrode (GCE: 3.0 mm in diameter) was polished with alumina slurries (Al2O3, 0.05 mm in diameter) on a polishing cloth in order to obtain a mirror finish, followed by ultrasonication in ethanol and pure water for 10 min, successively. For preparing a catalyst-coated working electrode, the catalyst of 3D Cu2O-GA was dispersed in a mixture of solvents containing water, Nafion (5 wt%) and isopropyl alcohol (Vwater: Visopropyl alcohol: VNafion= 4: 1: 0.025) to form a 2 mg/mL suspension by ultrasonication for a few seconds. Once the ink formed homogeneously, 5 µL of the ink was dropped on the surface of the clean GCE with a micropipette and then dried at room temperature. The mass loading of Cu2O-GA on the GCE is 10 µg. To facilitate electrochemical measurements for 2D Cu2O-rGO, the Cu2O-rGO paper was cut into square pieces (0. 2 cm × 0.2 cm) as freestanding working electrode material with an active area of 0.04 cm2 and the cut Cu2O-rGO paper was adhered to the surface of GC by using Nafion as adhesive agent (the mass loading≈50 µg). All electrochemical experiments were carried out at room temperature. Electrochemical measurements were carried out on a computer-controlled CHI 660 D electrochemical workstation (Chenhua Instrument, Shanghai, China) with a conventional three-electrode system, by using a Pt plate (5 mm × 5 mm × 0.2 mm) as the counter electrode and the Ag/AgCl (saturated KCl) as reference 8


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electrode. All electrode potentials given in this report were referred to the same Ag/AgCl reference. The working electrodes were first activated with cyclic voltammetry (CV) in N2-saturated 0.1 mol/L PBS solution (pH 7.0) until a steady CV was acquired. To examine the sensing performances of the materials for the detection of hydrogen peroxide, different concentrations of hydrogen peroxide stock solution were added into the N2-saturated 0.1 mol/L PBS solution, and the CVs were recorded at a scan rate of 100 mV/s. The amperometric current-time (i-t) curves were measured at -0.4 V for 4,000 s in 0.1 mol/L PBS saturated with N2 gas. Amperometry was used to monitor the current change during successive injections at 50 s intervals that resulted of an increment of H2O2 concentration.

RESULTS AND DISCUSSION Synthesis and characterization of 3D graphene aerogel-supported Cu2O microcrystals (3D Cu2O-GA) and 2D reduced graphene oxide-supported Cu2O paper (2D Cu2O-rGO-P) In the current study, simple and cost-efficient hydrothermal and filtration strategies were used to prepare two different 2D and 3D freestanding reduced graphene oxide-supported Cu2O composites, as shown in Figure 1A and C. The overall preparation procedures of the 3D Cu2O-GA and 2D Cu2O-rGO composites are schematically illustrated in Figure 1B and D. During the hydrothermal treatment for the preparation of 3D Cu2O-GA, the color of the solution changed from blue-black to transparent and only a 3D column-like freestanding cylindrical structure was produced in the bottom of the Teflon-lined stainless steel autoclave, as shown in Figure 1A, indicating the formation of 3D Cu2O-rGO composite. During the hydrothermal reduction, graphene oxide serves as not only a stabilizing agent to control the morphology of Cu2O microcrystals but also a structure directing agent to induce the synthesis of Cu2O microcrystals with low oxidation states. From the control experiments shown in Figure S1 A, B and S2 (Supporting Information), with the absence of graphene oxides, only Cu2O-CuO hollow microspheres were produced, implying the 9



important role of graphene as structure and component directing agent. Different from other amorphous or one-dimensional (1D) carbon materials, graphene sheets can be easily self-assembled into 3D networks. 3D graphene aerogel with interconnected porous structure has outstanding structural advantages, such as

high porosity, ultra low mass density, large surface area, and fast mass transport kinetics. Therefore, with 3D graphene aerogel as electrocatalyst support, the extremely high specific surface area can not only offer a large number of accessible open pores to ions in solution but also provide abundant active sites for the growth and anchoring of functional metal oxide materials with high mass loading and high dispersity. Owning to the above advantages and the synergistic effect between metal oxide and 3D graphene aerogel, graphene-based composites are expected to have improved electroanalytical performance with high detection sensitivity, selectivity, and fast amperometric response. As illustrated in Figure 1D, 2D freestanding Cu2O-rGO-P with a sandwich-like structure has also been prepared. In order to assemble GO sheets into well-ordered macroscopic structure, a facile filtration method was used. First, the two components of the as-prepared Cu2O nanocubes and graphene oxide were mixed and then transferred to a vacuum filter. After the fabrication of Cu2O-GO paper by vacuum filtration, an annealing reduction process was applied to transform Cu2O-GO paper to Cu2O-rGO paper. Owing to the unique properties, for example, good biocompatibility, high flexibility, high conductivity and robust mechanical strength, graphene-based papers have been used as promising electrochemical sensing platforms for the detection of bacteria,48 organophosphates,49 small molecules,50 and even viruses.51 Here, the Cu2O-GO paper was investigated as an electrochemical sensor for H2O2 detection. The morphologies of the 3D Cu2O-GA and 2D Cu2O-rGO-P were examined by SEM and TEM. Figure 2A and B show the SEM images of the as-synthesized 3D Cu2O-GA composite at different magnifications. It can be seen that the produced metal oxide microcrystals with a cubic shape are uniformly dispersed on 10


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the surface and inside of the 3D GA, revealing that 3D GA serves here as good substrate for the growth of Cu2O microrystals. In comparison, 3D GA with the absence of copper salt precursor (Figure S1C, D) was also synthesized. We can see that the prepared 3D GA still shows the high interconnected macro-porous structure mainly by virtue of hydrophobic effect, electrostatic interaction, hydrogen bonding, and π-π interaction. The morphology and structure of the 2D Cu2O-rGO paper was characterized by TEM and SEM. Figure 2C and D show the TEM and SEM images, respectively, of the 2D Cu2O-rGO paper after high-temperature annealing. It can be seen that the prepared composite shows a paper-like morphology with smoother surface compared to the 3D Cu2O-GA. Meanwhile, after annealing treatment under nitrogen atmosphere, the produced Cu2O nanocrystals are uniformly dispersed on the surface and in the inner of the composite film. To examine the effect of annealing treatment, the morphology of the as-synthesized Cu2O-rGO paper was also characterized by SEM (Figure 3A and B). By comparing Figure 2C and Figure 3B, one can see that the shape of the Cu2O nanocrystals changed from nanocubes to nanospheres after the high-temperature annealing treatment. Such phenomenon has also been observed for preparing other metal nanocrystals. Meanwhile, it should be noted that the post-annealing process can significantly influence the size of the Cu2O nanocrystals. From the size distribution histograms shown in Figure 2D and Figure 3B insets, after the high-temperature annealing, the average particle size of Cu2O decreased from 258.3 nm to 54.6 nm. On the other hand, there appear a few of Cu2O nanocrystal aggregates on the surface of rGO matrix after the calcinations process (Figure 2D), which may be from the Oswald ripening during the annealing in order to reducing the surface energy. Moreover, from the cross-section SEM images shown in Figure 3C and D, the Cu2O-rGO paper after annealing appears to be thicker than the as-prepared Cu2O-GO paper. It is mainly because during the heat treatment of Cu2O-GO paper, the high temperature may lead to the decomposition 11



of the oxygen-containing functional groups attached on the graphene oxide surface into gases which can expand into the spaces between the stacked layers, resulting in the increase of the thickness approximately from 1 to 2 µm. The composition and the distribution of the elements in the 2D composite were also examined by SEM-mapping measurements. From Figure 3E-H, it can be seen that the elements of Cu, C, and O are uniformly distributed in the 2D Cu2O-rGO-P. The freestanding 3D Cu2O-GA and 2D Cu2O-rGO-P composites were further characterized by XRD. As shown in Figure 4A, the XRD patterns demonstrate that both of the 3D GA-Cu2O and 2D Cu2O-rGO-P composites show the strong diffraction signals from Cu2O phase (space group C2/c; a0=4.685Å, b0 =3.426 Å, c0=5.130 Å, β=99.54°; JCPDS 05-0667). The strong diffraction peaks can be indexed to the (110), (111), (220), (311) planes of cubic Cu2O crystal phase. It should be noted that no diffraction peaks from impurities, such as Cu and CuO, can be observed in the XRD spectra, indicating the pure Cu2O phase of the as-synthesized oxides. These structural characterizations clearly indicate that by using different preparation methods and different metal precursors, 3D and 2D graphene-supported Cu2O composites have been successfully synthesized. Figure 4B shows the Raman spectra of the prepared GO, 3D Cu2O-GA, 2D Cu2O-GO-P and 2D Cu2O-rGO-P. One can see that in all the spectra, there are two distinct scattering peaks at ~1346 and ~1584 cm-1, corresponding to the D and G bands of carbon, respectively. The intensity ratio of the D and G bands (ID/IG) from the 3D Cu2O-GA composite (1.25) is obviously larger than that of GO (0.78), suggesting that after the hydrothermal reduction most of the oxygenated groups on GO surface have been successfully removed. Similar result was obtained from the 2D Cu2O-rGO-P composite. Similarly, after the annealing treatment, the ID/IG of the Cu2O-rGO-P (1.55) increases compared to the Cu2O-GO-P (1.39), indicating that the heat treatment can also remove the oxygenated groups of GO. Meanwhile, the larger ID/IG ratios from 12


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the present 2D and 3D composites than that of GO suggests the decrease in the sp2 carbon domain and the increase of defect concentration caused by the loading and insertion of Cu2O micro- and nano-crystals in the graphene sheets. The composition of the freestanding 3D Cu2O-GA and 2D Cu2O-rGO-P composites and the oxidation states of Cu and C were further investigated by XPS. Figure 5A shows the XPS survey spectra of the two samples in the full range of 0-1200 eV, clearly indicating the presence of Cu, O and C without other impurities. The deconvoluted C1s spectra of the 2D Cu2O-rGO-P before and after annealing treatment are shown in Figure 5C and D, respectively. The peaks at 288.7, 296.7 and 284.6 eV correspond to O-C=O, C-OH and C-C/C=C, respectively. Compared to the pristine 2D Cu2O-GO-P (Figure 5C), the peak intensities of C-O and C=O peaks in the 2D Cu2O-rGO-P (Figure 5D) show obvious decrease. Such result indicates that after the thermal reduction, most of the oxygen-containing groups in GO have been reduced, which is consistent with the above Raman result. Similar result was also obtained for the 3D Cu2O-GA (Figure S3A, B Supporting Information), suggesting the removal of oxygenated groups during the hydrothermal process. Figure 5B and Figure S3C show the Cu 2p XPS spectra of the freestanding 2D Cu2O-rGO-P and 3D Cu2O-GA composites. The peak components at 932.3 eV and 952.1 eV correspond to the Cu 2p3/2 and Cu 2p1/2 of Cu+, respectively. Actually, it is very difficult to differentiate Cu+ and Cu0 from the XPS features since the binding energies of Cu+ and Cu0 are very close with only a difference of 0.1-0.2 eV.52 However, above XRD results (Figure 4A) clearly demonstrate the formation of Cu2O crystals with no presence of metallic copper in the two prepared samples, and thus the Cu 2p XPS peaks can be assigned to Cu2O. Meanwhile, the peaks at 934.3 eV (Cu 2p3/2) and 954.1 eV (Cu 2p1/2) are attributed to the CuO/Cu(OH)2 (Cu2+) in Cu2O-rGO. The existence of CuO/Cu(OH)2 can also be confirmed by the shake-up satellite peaks at 943.0 eV (Figure 5B and Figure S3C). Shake-up peaks are usually observed when the 13



outgoing photoelectrons simultaneously interact with a valence electron and excite to a higher energy level. These peaks often appear around 9-10 eV higher than the main peaks. Combined with the XRD result, the detected XPS signals of CuO/Cu(OH)2 (Cu2+) on the surface of the prepared samples are reasonable because XPS can only detect the components within the upper 5 nm thickness on the surface, while the XRD can detect the components in micrometer-order thickness. The presence of tiny amount of CuO/Cu(OH)2 could be due to the surface oxidation of Cu2O crystals when the samples were exposed in humid air. Electrochemical sensing performances of the 2D and 3D Cu2O-rGO composites for H2O2 detection

To evaluate the electric conductivity of the as-fabricated nanocomposites, electrochemical impedance spectroscopy (EIS) measurements were first carried out. In a typical Nyquist plot, the semicircle diameter reflects the charge transfer resistance (Rct) at the interface of electrode and electrolyte. As displayed in Figure S4, both of the Cu2O-rGO-P and Cu2O-GA composites show similar semicircle diameters to that of glassy carbon electrode, indicating the high electron transfer efficiency of the obtained composites. The electrocatalytic properties of the as-prepared freestanding 3D Cu2O-GA composite and 2D Cu2O-rGO paper for the reduction of H2O2 were first studied by cyclic voltammetry (CV). Figure 6 A and B shows the CVs of the 3D Cu2O-GA and 2D Cu2O-rGO-P with the absence and presence of different concentrations of H2O2. Figure S5 shows the CVs of bare GC electrode, and GC modified by 3D graphene aerogel (3D GA), rGO-paper (rGO-P), 3D Cu2O-GA and 2D Cu2O-rGO-P in N2-saturated 0.1 M PBS (pH 7.0) solution with the absence and presence of H2O2. It can be see that for the bare GC electrode, only a small background current can be observed in the buffer, and with the addition of H2O2 in the electrolyte, the current density shows almost no obvious change. Such result indicates that bare GC electrode has negligible catalytic activity for the reduction of H2O2. As shown in Figure S5A and C, compared to the bare GC, the 3D 14


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Cu2O-GA and 2D Cu2O-rGO-P showed much larger double layer charging current, which can be ascribed to the large electric double layer capacitance of the 2D and 3D graphene structure. Meanwhile, there is a strong redox couple in the CV curves of the two 2D and 3D composites. The cathodic and anodic peak currents can be ascribed to the reduction of CuO to Cu2O and the oxidation of Cu2O to CuO. Upon the addition of H2O2 in the electrolyte, an obvious increase of the cathodic peak currents, as well as a small shift of the cathodic peak potentials can be observed, indicating the high catalytic activities of the 3D Cu2O-GA and 2D Cu2O-rGO-P for the reduction of H2O2. For comparison, as shown in Figure S5B and D, there is only small increase of reduction current, suggesting the low catalytic activities of 3D GA and rGO-P without the loading of Cu2O crystals for the H2O2 reduction. On the other hand, it is interesting that by comparing the CV curves in Figure 6A and B and the calibration curves in Figure 6E, the 3D Cu2O-GA and 2D Cu2O-rGO-P showed different sensing sensitivity to the H2O2 concentration, that is the 3D composite exhibits higher sensing sensitivity than the 2D one. The different electrocatalytic performances of the two samples can be due to the following reasons. First, as a highly porous and excellent conductive material, 3D graphene aerogel with interconnected porous structure and high specific surface area can enhance the electron transfer to target H2O2 molecule and improve the mass diffusion. Second, the highly dispersed Cu2O particles in the 3D pore structure can provide large surface area, and thus more exposed active sites toward the electrochemical reduction of H2O2. In contrast, for the 2D Cu2O-rGO paper, although the post-heating treatment can effectively increase the distance between the graphene layers, it is still difficult for the Cu2O nanoparticles between the graphene layers to efficiently contact with the target H2O2 molecules, which is also reflected by the longer response time to the H2O2 compared with the 3D Cu2O-GA in the amperometric tests.

15



Figure 6 C and D shows the typical amperometric responses of the 3D Cu2O-GA and 2D Cu2O-rGO-P to the successive addition of H2O2 into the stirred PBS solution at an applied potential of -0.4 V (around the peak potential of the H2O2 reduction). It is obvious that with the increase of H2O2 concentration the cathodic current increases apparently and exhibits a short response time, indicating a rapid response and high sensitivity to H2O2. From the corresponding calibration curves presented in Figure 6E, both of the 3D Cu2O-GA and 2D Cu2O-rGO-P exhibit excellent sensing activities toward H2O2. From the fitting lines in Figure 6E, a linear detection range from 1 to 1471 µM (correlation coefficient: R2=0.999) was obtained from the as-prepared 3D Cu2O-GA. With the same correlation coefficient, the as-prepared 2D Cu2O-rGO-P shows a linear detection range from 5.0 µM to 10.56 mM. Meanwhile, based on the signal-to-noise ratio of 3 (S/N=3), the limits of detection (LOD) were calculated to be 0.37 and 3.78 µM for the 3D Cu2O-GA and 2D Cu2O-rGO-P, respectively. Table 1 and Table 2 summarize the analytical performances of nanomaterials and the graphene-based papers or films for the detection of H2O2. It can be seen that the obtained detection limits or linear ranges of the present Cu2O-rGO are lower or wider than those of other reported sensing materials. It is well known that ascorbic acid (AA), uric acid (UA), dopamine (DA) and NaCl may interfere the electrochemical detection of H2O2, and the anti-interference ability against these interferents is also an important factor for the practical application of the present Cu2O-rGO sensing materials. To evaluate the selectivity of the Cu2O-rGO for the electrochemical detection of H2O2, the steady-state amperometric responses of the sensing materials were examined with addition of the different analytes in electrolyte. As shown in Figure 6D, the addition of AA (0.2 mM), UA (0.2 mM), DA (0.2 mM) and NaCl (0.2 mM) does

not cause any cathodic signal, while a significant current response is observed after the addition of 0.1 mM of H2O2. These results indicate that the co-existence of small amount of NaCl, AA, UA and DA does not 16


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cause interference for the detection of H2O2, and the excellent anti-interference ability makes both of the 3D Cu2O-GA and 2D Cu2O-rGO-P ideal sensing materials in trace electrochemical detection of H2O2.

To explore the practical application of the synthesized 3D Cu2O-GA and 2D Cu2O-rGO-P as H2O2 sensors, the freestanding Cu2O-rGO probes were applied to perform real-time detection of H2O2 in human serum. The human serum was first saturated by ultrapure N2 to remove the residual oxygen in serum which can influence the current signal to some extent. Then, certain concentration of serum and standard concentrations of H2O2 were added into 10 mL N2-saturated 0.1 M PBS solution for three times with an interval of 100 s. As shown in Figure 7, upon the injection of serum and H2O2, an increased cathodic current can be observed immediately and both the 3D Cu2O-GA and 2D Cu2O-rGO-P show quick and sensitive responses with the addition of serum and H2O2. These results demonstrate that the 3D Cu2O-GA and 2D Cu2O-rGO-P are sensitive to the H2O2 even in a biological environment. The concentration of H2O2 is determined by using the regression equation (shown in Figure 6E) and the results are tabulated in Table S1 and S2. It can be seen that for both 3D Cu2O-GA and 2D Cu2O-rGO-P, the mean recoveries are close to 100%. Besides, the sensing materials also showed good reproducibility for the H2O2 detection in human serum (Figure S6). Upon the addition of serum and H2O2 in the solution, three similar experimental results were obtained for each sensing material. In addition, in order to further verify the reliability of the proposed biosensors for real sample analysis, the sensing performance of the present 2D Cu2O-rGO-P was also evaluated by the determination of H2O2 in disinfectants.53 Figure S7 shows the amperometric curve of 2D Cu2O-rGO-P in N2-saturated PBS with the presence of the disinfectants. In comparison with the classical potassium permanganate titration method,54 the results determined by the 2D Cu2O-rGO-P are in satisfactory agreement (Table S3). These experiments demonstrate that the freestanding Cu2O-rGO

17



composites are reliable electrochemical probes for sensitive and selective detection of H2O2 in vivo or in vitro.

Conclusion We report here simple and cost-efficient hydrothermal and filtration strategies for preparing 3D Cu2O-GA and 2D Cu2O-rGO-P composites. The SEM, TEM, XRD and XPS characterizations clearly showed the formation of highly porous 3D GA-Cu2O structure and the 2D paper-like Cu2O-rGO composite. By using the Cu2O-rGO composites as electrochemical sensing platforms, their electrochemical sensing properties for the detection of H2O2 were studied by different electrochemical techniques. The electrochemical measurements showed that both of the 2D and 3D graphene-supported Cu2O crystals have excellent sensing performances for H2O2 detection. Compared with the previously reported sensing materials, the present 3D Cu2O-GA exhibited larger and more sensitive current response and lower detection limit toward H2O2, and with the 2D Cu2O-rGO-P as sensing material, a wider linear range was obtained. Moreover, both of the 2D and 3D composites exhibited excellent sensing selectivity and sensitivity for H2O2 detection in the presence of high concentrations of DA, AA, UA and NaCl interferents. In addition, the freestanding Cu2O-rGO composites have also been successfully used for the detection of hydrogen peroxide in real human serum. The present study indicates that the 2D/3D freestanding non-precious-metal catalysts/graphene composites represent a class of promising electrochemical sensing materials for practical non-enzymatic detection of H2O2 in vivo or in vitro.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected] (W. Chen) ORCID Wei Chen: 0000-0001-5700-0114

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Plan (2016YFA0203200), the National Natural Science Foundation of China (Nos. 21575134, 21633008, 21773224 and 21405149) and K. C. Wong Education Foundation. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.00. SEM images, XRD and XPS spectra, electrochemical impedance plots, cyclic voltammograms, and the practical application for real-time detection of H2O2 in human serum. (PDF)

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A

C

B

D

Figure 1 Photographs of the 3D Cu2O-GA obtained after freeze-drying (A) and 2D Cu2O-rGO-P obtained by filtration (C). Schematic diagrams of the synthesis of 3D Cu2O-GA composite (B) and 2D Cu2O-rGO-P (D).

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Figure 2 (A, B) Representative SEM images of the freestanding 3D Cu2O-GA at different magnifications. (C, D) Representative TEM and SEM images of the 2D freestanding Cu2O-rGO paper after high-temperature annealing.

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A

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Figure 3 (A, B) Representative SEM micrographs of the as-prepared 2D Cu2O-rGO paper without thermal annealing treatment at different magnifications. Inset in B shows the size distribution histogram of the Cu nanocubes dispersed in the rGO paper. (C) and (D) show the SEM images of the 2D Cu2O-rGO paper before and after thermal annealing, respectively. (E-H) Cross-sectional SEM elemental mapping images of the 2D Cu2O-rGO paper: (F) C; (G) O and (H) Cu.

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Figure 4 (A) XRD patterns of the as-synthesized freestanding 3D Cu2O-GA and 2D Cu2O-rGO-P composites. For comparison, the bulk XRD data of Cu2O (black bars, space group: C2/c, JCPDS 05-6676) are also shown. (B) Raman spectra of the prepared GO, 3D Cu2O-GA, 2D Cu2O-GO-P and 2D Cu2O-rGO-P composites.

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Cu2p1/2 Cu2p3/2

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Figure 5 (A) XPS survey spectra of the 2D Cu2O-rGO-P and 3D Cu2O-GA composites in the range of 0-1200 eV. (B) XPS of Cu 2p from the 2D Cu2O-rGO-P composite. XPS of C1s from the 2D Cu2O-rGO-P before (C) and after (D) annealing treatment.

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Figure 6 Cyclic voltammograms of GC electrode modified with 3D Cu2O-GA (A), 2D Cu2O-rGO-P (B) upon addition of different concentrations of H2O2 in 0.1 M PBS (pH=7.0), potential scan rate 100 mV/s. Amperometric curves of 3D Cu2O-GA/GC (C) and 2D Cu2O-rGO-P/GC (D) with successive addition of H2O2 in N2-saturated 0.1 M PBS (pH=7.0) at the applied potential of -0.4 V under constant stirring. The insets in (C) and (D) show the magnified cathodic current response to the low concentrations of H2O2. (E) The corresponding calibration curves of the 3D Cu2O-GA and 2D Cu2O-rGO-P towards H2O2 detection. The error bars represent the standard deviation of three separate measurements on the electrodes. (F) Amperometic responses of the 3D Cu2O-GA to the addition of different analytes in 0.1 M PBS (pH=7.0): 0.1 mM H2O2, 0.2 mM DA, 0.2 mM AA, 0.2 mM UA and 0.2 mM NaCl at -0.4 V. 29



2D Cu2O-rGO-P 3D Cu2O-GA

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-2

J (µA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

H2O2

-40 -60

serum

-80 -100

0

100

200

300

400

500

Time (s) Figure 7 Amperometric responses of the 3D Cu2O-GA and 2D Cu2O-rGO-P upon the addition of human serum sample followed by successive addition of certain amount of H2O2 for three times in 10 ml 0.1 M PBS (pH = 7.0).

30


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Table 1 Comparison of the Analytical Performances Based on the Present nanomaterials and the Previously Reported Sensing Systems (Non-Paper- or Film-Structured Materials) for H2O2 Detection Material Porous Cu2O

0.0015-1.5

Reference

LOD/µ µM 1.5

55

Talanta. 2011,85, 1260-1264

Electrochem. Commun. 2009, 11, 812-815

hierarchical Cu2O

0.0001-0.022

0.0039

56

core-shell CuO

0.01-5.5

10

57

J. Phys. Chem. C. 2010, 114, 9645-9650

Ag-Cu2O

0.002-1.3

0.7

58

Electroanal. 2016, 28, 477-483

0.3-3.3

3.3

28

Biosens. Bioelectron. 2013, 45, 206-212

0.002-0.15

1.03

59

CrystEngComm. 2012, 14, 6639-6646

8.4

60

Sens. Actuators. B. 2015, 206, 735-743 Electrochim.Acta. 2013, 104, 439- 447

Graphene-Cu2O Mesocrystalline Cu2O a

CQDs-Cu2O

a

Linear range/mM

0.02-4.3

Ag nanowire

0.1-3.1

29.2

61

Nanoporous Au

0.01-8.0

3.26

62

Electrochimica Acta. 2011, 56, 4657-4662

MnO2-GO

0.0005-0.6

0.8

54

Talanta. 2010, 82, 1637-1641 Electrochem.Commun. 2011,13,1131-1134 Talanta. 2010, 82, 340-347

Graphene-Pt

0.002-0.71

0.5

63

ZnO/nano-Ag/particles

0.008-0.983

0.9

64

3D Cu2O-GA

0.001-1.47

0. 37

The present work

2D Cu2O-rGO-P

0.005-10.56

3.78

The present work

CQDs-Cu2O: Carbon quantum dots coated on the octahedral Cu2O surface.

Table 2 Comparison of the Analytical Performances Based on the Present nanomaterials and the Previously Reported Paper- or Film-Structured Materials for H2O2 Detection Material

Linear range/mM

LOD/

Reference

µM AuNPs/Graphene paper AuNPs/porous grapheme film

0.005-8.6 0.0005-4.9

2

65

ACS. Nano. 2012, 6 (1), 100-110

0.1

66

Langmuir. 2012, 28, 9885-9892 Bioelectrochemistry. 2016, 109, 87-94

Au@PB NPs Graphene paper

0.001-0.03

0.1

67

AgNCs/Graphene paper

0. 02-10

3

68

Electrochimica. Acta. 2013, 89, 222-228

0-2.5

0.2

69

Nanoscale. 2014, 6, 4264-4274 Adv.Funct.Mater. 2012, 22, 2487-2494

PtNPs/Graphene film PtNPs/MnO2 /grapheme paper

0.0002-0.0133

1

70

PBNPs/ grapheme paper

0.005-0.6

1.5

71

Electrochimica. Acta. 2013, 89, 454-460

PBNPs/ grapheme paper

1-7

5

72

Adv.Funct.Mater. 2013, 23, 5297-5306

73

Electroanal. 2011, 23 (4), 900-906

HRP/Graphene film

0.0035-0.329

1.7

3D Cu2O-GA

0.001-1.47

0. 37

The present work

2D Cu2O-rGO-P

0.005-10.56

3.78

The present work

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Cu2O

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