Graphene Oxide Directed One-Step Synthesis of Flowerlike Graphene

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Graphene Oxide Directed One-step Synthesis of Flower-like Graphene@HKUST-1 for Enzyme-free Detection of Hydrogen Peroxide in Biological Samples Qingxiang Wang, Yizhen Yang, Feng Gao, Jiancong Ni, Yanhui Zhang, and Zhenyu Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11965 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 10, 2016

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ACS Applied Materials & Interfaces

Graphene Oxide Directed One-step Synthesis of Flower-like Graphene@HKUST-1 for Enzyme-free Detection of Hydrogen Peroxide in Biological Samples Qingxiang Wang†*, Yizhen Yang†, Feng Gao†, Jiancong Ni†‡, Yanhui Zhang†, Zhenyu Lin‡ †College

of Chemistry and Environment, Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology, Minnan Normal University, Zhangzhou 363000, P. R. China ‡Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, Fuzhou University, Fuzhou 350116, China

ABSTRACT: A novel metal-organic framework (MOF)-based electroactive nanocomposite containing graphene fragments and HKUST-1 was synthesized via a facile

one-step

solvothermal

method

using

graphene

oxide

(GO),

benzene-1,3,5-tricarboxylic acid (BTC), and copper nitrate (Cu(NO3)2) as the raw materials. The morphology and structure characterization revealed that the GO could induce the transformation of HKUST-1 from octahedral structure to the hierarchical flower shape as an effective structure-directing agent. Also it is interesting to find out that the GO was torn into small fragments to participate in the formation of HKUST-1 and then transformed into the reduction form during the solvothermal reaction process, which dramatically increased the surface area, electronic conductivity and redox-activity of the material. Electrochemical assays showed that the synergy of graphene and HKUST-1 in the nanocomposite leaded to high electrocatalysis, fast response and excellent selectivity toward the reduction of hydrogen peroxide (H2O2). Based on these remarkable advantages, satisfactory results were obtained when the nanocomposite was used as a sensing material for electrochemical determination of H2O2 in the complex biological samples such as human serum and living Raw 264.7 cell fluids. KEYWORDS: Graphene oxide, Metal-organic framework, HKUST-1, Non-enzymatic biosensor, H2O2.

1. INTRODUCTION

1

ACS Paragon Plus Environment


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Hydrogen peroxide (H2O2) has been increasingly recognized as an important small molecule mediator for physiology, aging, cell activation and disease in living organisms.1-4 However, the research development of H2O2 analytical tools has not kept pace with the biological and medical insights into it. Despite various techniques including spectrometry,5 colorimetry6 and positron emission tomography7 have been developed for the detection of H2O2 over the past two decades, a universal solution that is fast, affordable, selective and sensitive has remained elusive. One of the most promising schemes is electrochemical transduction because it offers high sensitivity, excellent selectivity, rapid response, easy handling and low cost.8,9 Enzyme-based electrochemical biosensors are widely used tools for the detection of H2O2 since their inherent high selectivity and effectiveness. However, this type of sensors still have some serious disadvantages such as the enzyme denaturation, variability of biological components, environmental instability, and complicated immobilization procedures. These drawbacks greatly limit their practical application. To overcome these disadvantages, lots of functional materials such as noble metal composite,9 transition metal oxide/sulfide,10,11 and carbonanceous substance

12,13

have

been exploited as non-enzymatic electrocatalysts for the electrochemical detection of H2O2. Nevertheless, the conventional non-enzymatic sensors usually suffer from defects of low anti-interference ability since the non-enzymatic catalysts can also catalyze the other electroactive substances that co-existed with H2O2 in biological samples, such as ascorbic acid (AA), uric acid (UA) and some carbohydrate compounds. So, the development of new electrochemical sensing materials to improve the analytical performance of non-enzymatic H2O2 sensors remains an urgent need in the community. Metal-organic frameworks (MOFs) are novel functional materials with the ultra-high specific surface area, open metal sites, patulous framework structures and tunable functionalities.14 As a result of these fascinating structures and properties, the MOFs show versatile application in the fields of gas separation,15 catalysis16 and energy-carrier storage.17 In addition to these applications, MOFs have also been regarded as a promising candidate of electrochemical sensor materials owing to their 2


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fascinating structure characteristics.18,19 For example, the high porosity and large surface area are believed to be helpful for MOFs to concentrate more analytes, which can produce the stronger signal intensity. The pore size can be easily tuned to capture the desired analytes with appropriate size and configuration, thereby improving the selectivity of the sensor. However, the use of the single-component MOFs as electrode materials for electrochemical sensor construction usually results in narrow linear range, low sensitivity, and insufficient stability due to their low electronic conductivity, poor electrocatalytic ability and inferior mechanical properties. In contrast, MOF-based composite materials that incorporate other functional species bearing high electronic conductivity and mechanical strength into the MOFs host matrices can overcome the defects of single-component MOFs, because the composites usually combine the merits of both MOFs and guest materials. To date, many novel MOF-based composites have been synthesized for electrochemical sensing application in the past years. For example, Zhan et al.20 prepared a ZnO@ZIF-8 nanocomposite and used it for the selective detection of ascorbic acid (AA) and hydrogen peroxide (H2O2). Xu’s group synthesized prepared a core-shell heterostructure of platinum nanoparticles (PtNPs)@UiO-66. The electrochemical assays showed that the composite presented a remarkable electrocatalytic activity with a good anti-interference performance for the detection of H2O2.21 However, these reported MOF composites are usually synthesized through multiple steps including pre-preparation and purification of core parts and the post-growth of MOF shell around the core. Such a complicated procedure not only increases the time and labor consumption of the experiments, but also possibly suppresses the function of the inner conductive core. Therefore, it is still of great concern to explore facile strategies for the synthesis of novel MOF composites bearing unique structures and properties, to broaden and deepen the electrochemical application of the MOF-based materials. Inspired

by

this

requirement,

a

novel

hierarchical

flower-shaped

graphene@HKUST-1 (Hong Kong University of Science and Technology-1) heterostructure was synthesized by a very simple one-step solvothermal method, without the need of pre-synthesis of core part and the post-growth of MOF shell as 3



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reported in traditional methods20, 21. More importantly, it is found that the pristine material of graphene oxide (GO) can act as a morphology and structure-directing agent to effectively induce the transformation of HKUST-1 MOF from octacheral structure to the hierarchical flower shape. Also during the solvothermal reaction process, the GO was torn into small fragments to participate in the formation of HKUST-1 and then changed to the reduction form. Thus, through such a facile morphology and structure regulation strategy by GO, the obtained composite presented better performance than the single-component analog of HKUST-1 and graphene, which includes the larger surface area due to the hierarchical flower-like structure, the faster mass transport rate owing to the larger pore size, and the higher electroactivity by the highly conductive graphene fragments. All these characteristics also beneficial for the composite material to be acted as a high-performance electrochemical sensing material. And the results proved that when the material was utilized as a novel non-enzymatic electrocatalyst for H2O2 sensing analysis, significantly enhanced electrocatalysis toward the reduction of H2O2 versus pure HKUST-1 or graphene was achieved (Scheme 1). Also satisfactory results were obtained when the composite was used for sensing detection of H2O2 in the real samples of serum and living cell fluids, suggesting the material owned high sensitivity and excellent anti-interference ability in complex biological environments. The established method presents a facile way to control the large-scale synthesis of novel highly electroactive MOF-based materials for the electrochemical sensing application. Scheme 1

2. EXPERIMENTAL SECTION 2.1 Reagents and Apparatus. Benzene-1,3,5-tricarboxylic acid (BTC), dopamine (DA) and chitosan (CS) were purchased from Aladdin Reagent Co., Ltd. (China). Copper nitrate trihydrate, glucose (Glu), concentrated sulfuric acid (98% H2SO4), potassium permanganate, sodium nitrate, graphite, 30% H2O2, HCl and ethanol were obtained from Xilong Chemical Co., Ltd. (China). 25 mM phosphate buffered solution (PBS, pH 7.0) was purchased from Shanghai KangYi Instruments Co., Ltd. 4


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(China). The test solution of H2O2 was prepared freshly daily before use. Raw 264.7 cells (mouse leukemic monocyte macrophage cell) were supplied by Jiangsu KeyGEN BioTECH

Co.,

Ltd

3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate

(China). (CHAPS)

was

purchased from TCI Development Co., Ltd. (China). The human serum sample was provided by Zhangzhou Affiliated Hospital of Fujian Medical University. The other chemicals were of analytical reagent and used without further purification. All solutions were prepared with deionized water. All the electrochemical experiments were performed in N2-saturated 25 mM PBS (pH 7.0). Fourier transform infrared (FT-IR) spectroscopy was recorded on NICOLET iS 10 spectrometer (USA). The Raman spectra were collected at LabRAM Aramis (France). Scanning electron microscope (SEM) was recorded on JSM-60-10LA (Japan). Transmission electron microscopy (TEM) was determined on Tecnai G2 F20 (USA). Nitrogen adsorption-desorption isotherms were carried out on Belsorp-MAX (USA). The X-Ray powder Diffraction (XRD) pattern was performed using a Rigaku D/MAX-RB diffractometer (Japan). Electrochemical measurements were measured on the CHI 6043E electrochemical analyzer (China) with a conventional three-electrode system: a modified glassy carbon disk electrode (GCE) with the geometric area of 0.031 cm2 was used as the working electrode, Ag/AgCl (3 M KCl) electrode as the reference electrode and Pt wire as the counter electrode. 2.2 Preparation of SGO@HKUST-1 Composite. Graphene oxide (GO) was prepared according to the classical Hummer's method with minor modification.22 The detailed procedures were given in Supporting Information. The composite of solvothermal-reduced GO (SGO) and HKUST-1 (SGO@HKUST-1) was prepared by a simple one-step solvothermal method. In brief, 0.545 g (2.25 mmol) of Cu(NO3)2·3H2O dissolved in 7.5 mL deionized water was mixed with 7.5 mL of ethanol containing 0.264 g (1.25 mmol) BTC. The mixture was ultrasonicated for 15 min to obtain a homogenous solution. Then different amounts (0 mg, 0.5 mg, 1.5 mg or 2.0 mg) of GO was added into above mixture. After stirring for 30 min, the homogenous solution was transferred to a Teflon-lined stainless steel reactor and 5



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heated at 120 °C for 24 h. After natural cooling to room temperature, the precipitates were carefully collected by centrifugation, and then washed with ethanol and deionized water for several times, finally dried at 80 °C for 10 h in vacuum to obtain the products. As a control, the SGO and HKUST-1 were also prepared by the above solvothermal method under the same conditions. 2.3 Fabrication of the SGO@HKUST-1 Modified Electrode. Prior to modification, a bare GCE was sequentially polished with 1.0, 0.3 and 0.05 µm alumina slurry, and then washed ultrasonically in water and ethanol, respectively for five minutes. To prepare the modified electrode, 1 mg of SGO@HKUST-1 was dispersed into 1 mL deionized water with ultrasonication for 10 min to give a homogeneous suspension. Followed by, 100 µL of prepared SGO@HKUST-1 suspension was added into 100 µL 1.0% acetic acid solution containing 0.3 wt% CS, and then ultrasonicated for 30 min under 80 W to obtain a well-dispersed solution. Thereafter, 10 µL of the prepared dispersion was dropped on the cleaned GCE and dried

in

the

air.

After

rinsing

with

water,

the

modified

electrode

(CS-SGO@HKUST-1/GCE) was achieved. For comparison, the CS-HKUST-1/GCE and CS-SGO/GCE were prepared by the same way. 2.4 Detection of H2O2 Released from Living Cells. Firstly, the Raw 264.7 cells were cultured and seeded according to the previous method.23 Then the cells were separated from the culture medium by centrifugation at 1300 rpm for 5 min and washed with 25 mM PBS (pH 7.0) for three times. Followed by, ~5×105 (estimated by cell counter) washed Raw 264.7 cells were added into the 25 mM deoxygenated PBS for electrochemical measurements. The amperometric (I-t) responses were recorded at -0.40 V vs. Ag/AgCl, and after a steady-state background current was obtained, 0.3 µM CHAPS was added into the system for real sample measurements.

3. RESULTS AND DISCUSSION 3.1 Characterization of SGO@HKUST-1 Nanocomposite. HKUST-1 is one of the first reported MOFs that constituted by the Cu2+ knot and BTC linker. As a classical MOF, it has received great interesting in experimental and theoretical 6


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researches due to its easy preparation process and unique chemical and physical features.26 In this work, the HKUST-1 was also selected as the raw material to prepare the MOF nanocomposite. Figure 1 depicts the scanning electron microscope (SEM) images of the synthesized SGO (A), HKUST-1 (B) and their composites (C-E). Figure 1A shows that the SGO obtained from pure GO after solvothermal treatment at 120 °C for 24 h still exhibits a typical layered structure (Figure 1A) that similar to pure graphene.24 The wrinkled texture in the image further indicates that the SGO keeps the structure feature of graphene. The SEM micrograph of HKUST-1 (Figure 1B) reveals that the synthesized MOF products have well-defined octahedral geometry with regular facets, clear edges and sharp corners, which are the two essential crystallographic characteristics of self-limitation and symmetry. These features are consistent with the morphologies of HKUST-1 reported in the literature.25 The enlarged SEM image in a single HKUST-1 particle (Figure 1B, inset) shows that the HKUST-1 particles has a smooth surface, demonstrating high purity of the sample. It is interesting that when GO was present during the synthesis of HKUST-1, some flower-like particles appeare. And the amount of these flower-like particles increased with the increase of GO content in the reaction solution (Figure 1C–E). When GO reached 0.13 mg mL−1, almost all of the octahedral HKUST-1 particles were changed to the hierarchical flower-like structure. These results clearly indicate that the GO can act as an effective structure-directing agent to induce the transformation of HKUST-1 from original octahedral structure to the hierarchical flower-like shape. It can be also imaged that the morphology change will lead to the larger effective surface area and the higher mass transport mass. In addition, it is found that the obtained flower-like particles had close size with that of the original HKUST-1, and each particle is consisted of four groups of stacking sheet pedals, corresponding to the four facets of half of a HKUST-1. This result suggests that the basic framework unit of HKUST-1 has not been changed by the introduction of GO. Figure 1

7



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Figure 2A and 2B display the transmission electron microscopy (TEM) images of a single HKUST-1 and SGO@HKUST-1 particle, respectively. Through comparison, it can be clearly observed that both HKUST-1 and SGO@HKUST-1 have regular hexagons, which testifies that these two MOF-based products have the similar basic framework as discussed in the SEM characterization. But the edge of the SGO@HKUST-1 hexagon is much coarser than that of the pure HKUST-1, which might be caused by the irregular petal edges of the composite material. The crystalline natures of HKUST-1 and SGO@HKUST-1 were further characterized by the selected area electron diffractions (SAED), and the results are showed in insets of Figure 2A and 2B. It is found that both of these two samples have some concentric diffraction rings, indicating that both of them have the polycrystalline structure. In addition, it can be observed that the diffraction rings of SGO@HKUST-1 are much clearer than those of the single HKUST-1, which is plausibly caused by the higher thickness of the HKUST-1 crystal, but the thin flakes on the flower like SGO@HKUST-1. Figure 2C and 2D show the low and high-resolution TEM images of one flake peeled off from SGO@HKUST-1, respectively. In the low-resolution TEM image, it is observed that some irregular white regions are distributed on the flake. The high-resolution TEM image further shows that these white regions have clear lattice fringes. The interplanar distance was estimated to be 0.32 nm, which is very close to that of graphene nanosheet (0.35 nm).27 Moreover, this interplanar spacing (0.32 nm) is obviously smaller than that of GO (0.90 nm),28 indicating that the oxygen-containing functional groups have been removed, i.e., the GO had been reduced to graphene in the composite via the facile one-step solvothermal process. So, from these characterization, we believe that during the solvothermal synthesis, the GO was torn into the small fragments, and then participated in the formation of HKUST-1 framework through coordinating with Cu2+ ions. On the other hand, under the solvothermal condiction, the assemblied GO fragments were changed to the reduction form. The similar reduction process of GO through the solvothermal condition has also been reported in literature. 29 Figure 2 8


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Figure 3A reveals the powder X-ray diffraction (XRD) patterns of SGO (i), HKUST-1 (ii), and SGO@HKUST-1 (iii). Pure SGO has no peak at 2θ = 11.11° corresponding to the (001) reflection of GO, conforming that the GO had been transformed to the reduced form by the solvothermal reaction.30 The diffraction peaks corresponding to the face-centered cubic structure of HKUST-1 agree well with that in literature,31 highlighting the formation of the HKUST-1 MOF material. The XRD pattern of SGO@HKUST-1 shows that all of the peaks are consistent with those of HKUST-1, indicating that HKUST-1 maintains its crystal structure in the flower-like composite. In addition, the synthesized nanocomposite material does not exhibit any peaks corresponding to CuO or Cu2O,32 confirming that the Cu2O and CuO byproducts were not formed during the reaction. Figure 3

The Raman signatures of SGO (i), HKUST-1 (ii) and SGO@HKUST-1 (iii) are shown in Figure 3B. The typical features in the SGO spectrum are the G band at 1602 cm−1 assigned to the E2g phonon of C sp2 atoms and the D band at 1326 cm−1 corresponding to the breathing mode of κ-point phonons with A1g symmetry.33 For HKUST-1, the peak located at 500 cm−1 is ascribed to the vibration of the Cu-O coordination bond. The bands at 742 cm−1 and 828 cm−1 are related to the out-of-plane ring (C-H) bending vibrations of the BTC ligand. The C=C vibration from the benzene ring of BTC is located at 1005 cm−1 and 1613 cm−1. All these major characteristic peaks also appeared in the SGO@HKUST-1 spectrum. Also, the G and D bands ascribing to the characteristic peaks of graphene are also visible in the spectrum of the composite, which further confirms that graphene and HKUST-1 are both present in the SGO@HKUST-1 composite. The samples were further characterized by Fourier transform infrared (FT-IR) spectroscopy in Figure 3C. For SGO (curve i), no band corresponds to the characteristic absorption of C=O, -OH, and C-O-C (epoxy), confirming that GO has been successfully reduced through the solvothermal treatment.34 The absorption bands 9



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between 1300 cm−1 and 1700 cm−1 on the spectrum of HKUST-1 (curve ii) are attributed to the absorption of BTC. An obvious absorption peak at 500 cm−1 can be ascribed to the Cu-O stretching modes, which indicates the metal-organic coordination state of the HKUST-1.26 For SGO@HKUST-1 composite (curve iii), all absorption bands of free HKUST-1 MOF are present, testifying that the coordination polymerization situation of HKUST-1 is maintained in the composite. Figure 3D shows the textural characterization of samples by N2 adsorption isotherms at 77 K. It is evident that all samples exhibit typical I type curve, suggesting that the samples are microporous materials. But the SGO exhibits a significantly low and negligible porosity (curve a). Compared with SGO, the HKUST-1 materials have much better N2 adsorption capacity, particularly below a pressure of P/P0 =0.1, which indicates that the HKUST-1 has larger specific surface area. Also, the SGO@HKUST-1 exhibits a hysteresis loop at the P/P0 range of 0.29-0.99. The hysteresis loop of the SGO@HKUST-1 resembles the H4 type (based on the IUPAC classification). This phenomenon might be due to the presence of narrow slit-like pores between HKUST-1 crystals and SGO fragments. The pore size distributions of SGO, HKUST-1, and SGO@HKUST-1 were evaluated from the desorption branch using the BJH model. Table S1 lists the detailed data of textural parameters of the three samples. As seen, the SGO@HKUST-1 shows the largest pore volume and average pore diameter, indicating that the composite is more valuable for adsorption and mass transport of analyte than the single-component counterparts of HKUST-1 and graphene, when used as electrochemical sensing material. 3.2 Electrochemical Behaviors of SGO@HKUST-1 and its Electrocatalytic Reduction towards H2O2. Figure S1 in Supporting Information shows the CVs of CS-SGO/GCE, CS-HKUST-1/GCE and CS-SGO@HKUST-1/GCE in 25 mM PBS (pH 7.0) within the potential range from -0.6 V to +0.6 V. It can be seen that not any Faradic response was observed on CS-SGO/GCE, indicating that the CS-SGO film was

electrochemically

inactive

over

this

potential

range.

However,

the

CS-HKUST-1/GCE shows an obvious anodic peak at +0.004 V and two adjacent reduction peaks at -0.25 V and -0.31 V, respectively. From the characteristic of the 10


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two reduction peaks, their electron transfer numbers were both calculated to be one (see the detailed calculation in Supporting Information), which suggested that the MOF material performed two-step single-electron reduction process (Cu(II)/Cu(I) and Cu(I)/Cu(0)) on the electrode surface. For CS-SGO@HKUST-1/GCE, all the redox peaks from HKUST-1 are still visible, testifying the presence of HKUST-1 in the composite and keeps its electroactivity. In addition, it is found that all the peak intensities of CS-SGO@HKUST-1 are significantly enhanced as compared with CS-HKUST-1 film, suggesting that the SGO fragments intercalated in the composite effectively promoted the electron communication kinetics of HKUST-1 as highly conductive channels. And this conclusion was further testified by cyclic voltammetry and electrochemical impedance spectroscopy using [Fe(CN)6]3-/4- as electrochemical probe (Figure S2, see the detailed discussion in Supporting Information). These results also demonstrate that the inferior electron conductivity as a common defect of MOFs can be facilely resolved through doping highly conductive carbon material into the framework of the MOFs by a simple one-step reaction process. The electrocatalytic capacity of SGO@HKUST-1 was then evaluated by CV and amperometric current-time (I-t) curve technologies using H2O2 as the analyte. Figure 4

shows

the

CVs

of

CS-SGO/GCE

(A),

CS-HKUST-1/GCE

(B),

and

CS-SGO@HKUST-1/GCE (C) in PBS without and with 0.2 mM H2O2 and their comparison of peak currents (D). For CS-SGO/GCE, it is found that only slight increase of the cathodic current happens at -0.6 V in CV when 0.2 mM H2O2 was added into the PBS buffer solution, which indicates that the film of CS-SGO has limited

electrocatalytic

activity

toward

H2O2

reduction.

However,

when

CS-HKUST-1/GCE was applied, the oxidation peak on the electrode decreases upon addition of H2O2. The reduction peak of Cu(II)/Cu(I) does not significantly change, but the reduction peak for Cu(I)/Cu(0) displays an obvious increase. These changes suggest that the HKUST-1 has electrocatalysis activity for the irreversible reduction of H2O2 through its reduction transition state (Cu(I)-HKUST-1), and the catalytic mechanism can be illustrated by the following scheme: 11



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Cu(I)-HKUST-1 + e−→Cu(0)-HKUST-1

Page 12 of 33

(1)

(at the electrode surface) Cu(0)-HKUST-1 + 1/2H2O2→OH− + Cu(I)-HKUST-1

(2)

(within the sensing film) Figure 4

When CS-SGO@HKUST-1/GCE was applied to the detection of H2O2, it is observed that the film of CS-SGO@HKUST-1 shows similar electrochemical changes with CS-HKUST-1 after reaction with H2O2. However, the catalytic current corresponding to Cu(I)/Cu(0) couple on CS-SGO@HKUST-1/GCE (5.43 µA) is significantly larger than that at CS-HKUST-1/GCE (1.88 µA), suggesting that the SGO fragments in composite leads to the higher eleccatalytic activity than HKUST-1, due to its higher electron conductivity. Figure 5

Based on above CV analysis, the electrochemical kinetic and electrocatalytic parameter of CS-SGO@HKUST-1 towards H2O2 were further investigated by chronoamperometry

(CA).

Figure

5A

shows

the

CA

curves

of

CS-SGO@HKUST-1/GCE in PBS (pH 7.0) with increasing concentrations (from 0 to 1.0 mM) of H2O2 at a constant potential of −0.4 V. The time evolution of the semi-infinite linear diffusion-controlled current can be described by Cottrell’s equation: Icat = nFAD1/2C0π−1/2t−1/2

(3)

where Icat is the current of CS-SGO@HKUST-1/GCE in presence of H2O2, n the number of electrons transferred, F the Faraday constant, A the geometric area of the electrode, C0 the substrate concentration, D the diffusion coefficient, and t the elapsed time. Figure 5B shows the plots of Icat versus t−1/2 derived from the CA curves. Thus, according to Cottrell’s equation and the slopes of the Icat-t−1/2 curves at each H2O2 concentration, the average value of D for H2O2 in this system was determined to be 4.5 × 10−5 cm2 s−1, which agrees with the values reported in the literature.35 12


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At intermediate time points, the current was dominated by the rate of the electrode towards the electrocatalytic reduction of H2O2. Thus, the catalytic rate constant (Kcat) can be calculated according to the following equation:36 Icat/IL = (πKcatC0t)1/2

(4)

where Icat and IL are the currents of CS-SGO@HKUST-1/GCE in presence and absence of H2O2, respectively, and Kcat the catalytic rate constant. Clearly, plotting the Icat/IL versus t1/2 produces straight lines (Figure 5C). From the slopes of Icat/IL - t1/2, the average value of Kcat was calculated to be 1.74 × 105 M−1 s−1, which is larger than the previously reported values on Cu@N-Chit-G/GCE37 and MnO2/nafion/Pt electrode,38 confirming that the CS-SGO@HKUST-1 has a greater electrocatalytic activity for the reduction of H2O2. 3.3 Chronoamperometric Detection of H2O2. The applied potential (Ea) in chronoamperometry has been reported to have a great influence on the sensitivity, stability and selectivity of sensors.39 Therefore, in this work, the applied potential of the biosensor for the detection of H2O2 was also optimized. Figure 5D shows the I-t curves of CS-SGO@HKUST-1/GCE in PBS upon successive addition of H2O2 under various applied potentials (Ea). The figure shows that when the applied potential is changed from −0.20 V to −0.40 V, the slope of the obtained I-t curves increases gradually, suggesting a continuous increase in the sensitivity of the sensor. Nonetheless, when the applied potential is further increased to −0.45 V, the slope increased slightly; And meanwhile, the I-t curve becomes unstable likely due to interference from the dissolved O2 in the solution.23 Therefore, −0.40 V is used for all further studies.

Figure 6

Figure 6A presents a typical real-time amperometric I-t curve using CS-SGO@HKUST-1/GCE with subsequent injection of various concentrations of H2O2 into 25 mM PBS (pH 7.0) solution. The inset a in the figure shows that even in the low concentration (1 µM) of H2O2, the sensor also shows obvious enhancement in 13



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catalytic current, confirming the high electrocatalytic ability of the nanocomposite. Inset b of the figure further reveals that after addition of H2O2, the response achieved a maximum steady-state current within 4 s (inset b), suggesting that the sensor has ultrafast response for the electro-reduction of H2O2, which can be attributed to the rapid

adsorption,

penetration,

and

electrochemical

reaction

of

H2O2

on

SGO@HKUST-1 layer by its the large pore volume and high electrocatalytic activity. The corresponding calibration curve of catalytic current (I) versus H2O2 concentration (C) is shown in Figure 6B. From the curve, two linear curves were obtained over H2O2 concentration ranging from 1.0 µM to 0.86 mM and from 0.86 mM to 5.6 mM, respectively, with the following regression equations: I/µA =4.25C/mM −2.45×10−4 (r = 0.999); I/µA =8.71C/mM −4.60 (r= 0.997). It is found that the sensor has the higher sensitivity (135.4 µA cm−2 mM−1) at the high concentration region than that (277.4 µA cm−2 mM−1) at the low concentration region. According to literatures,40,41 the different sensitivities may be ascribed to the different electrochemical kinetics of H2O2 at the electrode surface under different H2O2 concentration regions. At the concentration region from 1.0 µM to 0.86 mM, the electro-reduction process of H2O2 is controlled by the combination of H2O2 adsorption and catalytic activation on the electrode surface, resulting in a relatively low sensitivity. However, the H2O2 concentration is increased to the region from 0.86 mM to 5.6 mM, the electrocatalytic activation of H2O2 on the sensing film becomes the only rate-determinating step, which therefore results in the promotion of the analytical sensitivity. It is of note that when the H2O2 concentration is larger than 5.6 mM, the catalytic currents show slight enhancement with the increase of the H2O2 concentration. This can be explained by the adsorption saturation of H2O2 on the active sites of the sensing material. In addition, the limit of detection (LOD) at three times the standard deviation of the average blank value (LOD = 3 × RSD/Slope) was estimated to be 0.49 µM. The analytical performance of our sensor was also compared to some recently published reports, and the results are listed in Table S2 of Supporting Information. From the 14


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table, it can be seen that our sensor exhibits superior performance in terms of linear range, LOD, and response time for H2O2 measurements. We speculated that the outstanding performance is attributed to the following reasons: (1) the abundant MOF pores with appropriate size facilitates the transport of H2O2 molecules through the electrolyte/electrode interface. (2) The SGO loaded on MOF contributes to excellent electronic conductivity and the high catalytic current for H2O2 electro-reduction. (3) Large surface are arising from its hiro structure improves the absorption amount of H2O2 the sensing interface. 3.4 Reproducibility and Stability. The stability and reproducibility are important parameters to evaluate the performance of the sensors. The proposed sensor based on the CS-SGO@HKUST-1 was stored in air at ambient conditions for 15 days, the current response to 0.2 mM of H2O2 was recorded every 3 days. The data shows that the proposed sensor maintains at least 97% of the initial response (Figure 6C), indicating that the sensor is highly stable. The relative standard deviation (RSD) of the current response to H2O2 sensing was less than 2.8% for five tests for the same electrode, suggesting good reproducibility of the sensor. For five independent sensors that prepared by the same way, an RSD value of 3.1% is obtained, indicating the proposed method was reliable. These results reveals that the fabricated enzyme-free sensor based on CS-SGO@HKUST-1 exhibits excellent stability, repeadity and reproducibility for H2O2 detection. 3.5 Interference Studies. In order to assess the selectivity of the developed sensor, the influence of some species possibly coexisted with H2O2 on the response of the sensor was also investigated. Figure 6D showes the interference tests conducted in PBS (7.0) with continuous additions of 0.02 mM H2O2, 0.02 mM glucose (glu), 0.02 mM ascorbic acid (AA), 0.02 mM dopamine (DA), 0.02 mM uric acid (UA) and 0.02 mM H2O2 at the applied potential of -0.4 V. It can be seen that the response current increases from 0.16 µA to 0.24 µA by addition of 0.02 mM H2O2. However, negligible current response is generated by addition of other substances in comparison with the addition of H2O2. Such an excellent selectivity of the developed sensor can be explained the synergic effect of the material feature and the test conditions: first, 15



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the small pore size of the sensing material of SGO@HKUST-1 allows the effective permeation of H2O2 into the sensing film, rather than the DA, Glu, AA and UA with big molecules size; Second, the redox potential of Cu(I)-HKUST-1/Cu(0)-HKUST-1 just matches with reduction process of H2O2 via the reaction of Cu(0)-HKUST-1 + 1/2H2O2→OH− + Cu(I)-HKUST-1. However, the redox potentials of the above-mentioned interferents are much higher than the reduction potential of Cu(I)-HKUST-1/Cu(0)-HKUST-1, to trigger the electrocatalytic reduction of H2O2 ; Finally, the selected applied potential (-0.40 V) is also much lower than the redox potentials of the interferents, avoiding the generation of their interfering response from the interferents. The sound selectivity also means that the developed sensor is promising to be used for the highly selective detection of H2O2 in real samples.

3.6 Real Sample Analysis. Detection of H2O2 in Human Serum Samples. The CS-SGO@HKUST-1/GCE was further utilized to measure H2O2 in human serum using standard additions. In brief, the serum was diluted with 10 mL PBS solution (pH 7.0). Subsequently, a standard solution of H2O2 was injected into the electrolyte solution

and

analyzed

with

the

CS-SGO@HKUST-1/GCE.

The

resulting

amperometric curve is shown in Figure 7A. The H2O2 content in human serum sample was calculated using a regression equation (Figure 6B). The recovery values for four injection are between 97% and 101.2% indicating that this approach is feasible for H2O2 detection in complex serum sample. Figure 7

Detection of H2O2 Released from Living Raw 264.7 cells. The as-fabricated electrode was also used to detect H2O2 released from living cells using the Raw 264.7 cell line as a model. Figure 7B depicts the amperometric response of CS-SGO@HKUST-1/GCE in PBS (pH 7.0). The PBS solution with 5.0 × 105 Raw 264.7

cells

but

without

addition

of

3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonat (CHAPS) (curve a) or PBS blank solution without Raw 264.7 cell after addition of CHAPS (curve b) shows 16


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no obvious current change. However, when 0.3 µM CHAPS was injected into PBS solution containing 5.0 × 105 Raw 264.7 cells, the current increases significantly (curve c), demonstrating that electrode is capable of acting a sensitive sensor for real-time monitoring of H2O2 released from living cells. In addition, from the steady-state catalytic current value of 5.7 nA upon injection of CHAPS, the H2O2 concentration in the cell solution was calculated to be 1.4 µM according to the standard working curve presented in Figure 6B. Knowing that we used 5.0 × 105 cells, the amount of H2O2 released from each cell was calculated to be about 2.8 pM, which agrees well with previously reported literature.41 Therefore, we conclude that our sensing platform based on a flower-like SGO@HKUST-1 nanocomposite can be effectively used for the determination of extracellular H2O2, showing promising application of the sensor.

4. CONCLUSION In summary, a unique hierarchical flower-like SGO@HKUST-1 nanocomposite was prepared through a facile one-step solvothermal method. The characterization results from SEM, TEM, XRD, FT-IR, etc. demonstrated that the GO played a critical role in controlling the morphology and constitution of the product. Meanwhile, the GO itself was torn into fragments to participate in the coordination assembly of HKUST-1, and GO fragments were reduced to graphene. Compared with the original HKUST-1, the synthesized flower-like SGO@HKUST-1 not only had pore volume and pore size, but also presented more intense redox-activity. Electrochemical assays showed that the SGO@HKUST-1 nanocomposite exhibited outstanding performance for nonenzymatic H2O2 detection with a rapid response (< 4 s), a wide linear range from 1.0 µM to 5.6 mM, and a low detection limit of 0.49 µM. In addition, the SGO@HKUST-1/GCE demonstrated excellent anti-interference ability against common interfering species such as UA, AA, and Glu. These high analytical performances are attributed to the synergic effects of SGO with the high conductivity and HKUST-1 MOF with suitable structure and electrochemical response. This study complemented the research of non-enzymatic sensors and opens a new avenue for the 17



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synthesis of functional MOF-based composites with important electrochemical sensing applications.

ASSOCIATED CONTENT * Supporting Information Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Tel: +86-596-2591445. Fax: +86-596-2520035.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is supported by the National Natural Science Foundation of China (No. 21275127), NSFC for Excellent Youth Scholars of China (No. 21222506), and Education-Science Research Project for Young and Middle-aged Teachers of Fujian (No. JA15305).

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Scheme 1 Illustration of the preparation of SGO@HKUST-1 and its application as a non-enzymatic electrocatalyst for H2O2.

25



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Figure 1 SEM images of SGO (A), HKUST-1 (B), SGO@HKUST-1 (C-E: addition of GO during synthesis was 0.03 mg mL−1, 0.10 mg mL−1, and 0.13 mg mL−1, respectively).

26


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Figure 2 TEM images of HKUST-1 (A), SGO@HKUST-1 (B), SAED pattern of HKUST-1 (inset of A) and SGO@HKUST-1 (inset of B). Low magnification TEM image of SGO@HKUST-1 (C). HRTEM image of SGO@HKUST-1 (D).

27



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Figure 3 XRD patterns (A), Raman spectra (B), FT-IR spectra (C) and and Brunauer-Emmett-Teller

(BET)

(D)

for

SGO

(i),

SGO@HKUST-1 (iii).

28


HKUST-1

(ii),

and

Page 29 of 33

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Figure

4

CVs

of

CS-SGO/GCE

(A),

CS-HKUST-1/GCE

(B)

and

CS-SGO@HKUST-1/GCE (C) in 25 mM PBS (pH 7.0) containing 0 and 0.2 mM H2O2. (D) Bar graph of reduction peak currents (Ip) of different electrodes in 25 mM PBS (pH 7.0) without and with (ii) 0.2 mM H2O2.

29



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Figure 5 (A) Chronoamperograms of CS-SGO@HKUST-1/GCE in the absence and presence of various H2O2 concentrations in PBS (pH 7.0) recorded at −0.4 V. (B) The plot of Icat vs. t−1/2. (C) The plot of Icat/IL vs. t1/2. (D) Chronoamperometric responses of the CS-SGO@HKUST-1/GCE upon successive addition of 2.0 mM H2O2 in 10 mL of PBS (pH 7.0) at various potentials.

30


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a

b

Figure 6 (A) Chronoamperometric responses of the CS-SGO@HKUST-1/GCE upon successive additions of H2O2 at the applied potential of −0.40 V. Inset a is the amplified I-t curve in the low concentration region of H2O2. Inset b is response time of the

sensor

upon

addition

of

H2O2.

(B)

Calibration

plots

for

the

CS-SGO@HKUST-1/GCE with H2O2 concentrations of 1.0 µM to 0.86 mM and 0.86 mM to 5.6 mM. The inset reveals an amplified calibration curve with H2O2 concentrations ranging from 1.0 µM to 0.86 mM. (C) Variation of the CS-SGO@HKUST-1/GCE response currents in the presence of 0.2 mM H2O2 in PBS at pH 7.0 tested every 3 days over 15 days. (D) Chronoamperometric responses of the CS-SGO@HKUST-1/GCE after successive addition of 0.02 mM H2O2, 0.02 mM glucose (Glu), 0.02 mM ascorbic acid (AA), 0.02 mM dopamine (DA), 0.02 mM uric acid (UA) and 0.02 mM H2O2 into PBS (7.0) at −0.40 V.

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Figure 7 (A) Chronoamperometric responses recorded at CS-SGO@HKUST-1/GCE upon addition of serum followed by successive additions of 0.02 mM H2O2 in PBS (pH 7.0). (B) Chronoamperometric responses of CS-SGO@HKUST-1/GCE upon addition of 0.3 µM CHAPS with (curve c) and without (curve b) Raw 264.7 cells as well as Raw 264.7 cells without CHAPS (curve a) in PBS (pH 7.0) at −0.40 V.

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Graphene Oxide Directed One-Step Synthesis of Flowerlike Graphene

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