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The Boosted Sensor Performance by Surface Modification of Bifunctional rht-Type Metal-Organic Framework with Nanosized Electrochemically Reduced Graphene Oxide Cong Li, Tingting Zhang, Jingyu Zhao, He Liu, Bo Zheng, Yue Gu, Xiaoyi Yan, Yaru Li, Nannan Lu, Zhiquan Zhang, and Guodong Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13788 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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ACS Applied Materials & Interfaces

The Boosted Sensor Performance by Surface Modification of Bifunctional rht-Type Metal-Organic Framework with Nanosized Electrochemically Reduced Graphene Oxide Cong Li, Tingting Zhang, Jingyu Zhao, He Liu, Bo Zheng, Yue Gu, Xiaoyi Yan, Yaru Li, Nannan Lu, Zhiquan Zhang,* and Guodong Feng*

Laboratory of Analytical Chemistry, College of Chemistry, Jilin University, Changchun 130012, China

KEYWORDS: rht-type metal-organic framework, nanosized graphene, electrochemical, synergetic effect, hydrogen peroxide

ABSTRACT: The surface and interface could be designed to enhance properties of electrocatalysts and they are regarded as the key characteristics. This report describes surface modification of a bifunctional rht-type metal-organic framework (MOF, Cu-TDPAT) with nanosized electrochemically reduced graphene oxide (n-ERGO). The hybrid strategy results in a Cu-TDPAT-n-ERGO sensor with sensitive and selective response toward hydrogen peroxide (H2O2). Compared with Cu-TDPAT, Cu-TDPAT-n-ERGO exhibits significantly enhanced electrocatalytic activities, highlighting the importance of n-ERGO in boosting their electrocatalytic activity. The sensor shows a wide linear detection range (4 - 12000 µM), and the detection limit is 0.17 µM (S/N = 3) that is even lower than horseradish peroxidase or recently published noble metal nanomaterials based biosensors. Moreover,

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the sensor displays decent stability, excellent anti-interference performance, and applicability in human serum and urine samples. Such good sensing performance can be explained by the synergetic effect of bifunctional Cu-TDPAT (open metal sites and Lewis basic sites) and n-ERGO (excellent conductive property). It is expected that rht-type MOF-based composites can provide wider application potential for the construction of bioelectronics devices, biofuel cells, and biosensors.

1. INTRODUCTION Hydrogen peroxide (H2O2) with oxidizing and reducing properties has been widely used in pharmaceutical, industrial, and environmental protection areas.1 Also due to high energy density, easy storage, and fast reduction kinetics, H2O2 is investigated as an alternative oxidant in fuel cells.2 Furthermore, as a primary product of human metabolism, H2O2 plays an essential role in regulating biological signaling transduction processes.3 Therefore, the accurate and effective detection of H2O2 is needed. To date, spectrophotometry,4 chemiluminescence,5 titrimetry,6 and electrochemistry7 have been developed to detect H2O2. Among these methods, electrochemical approach is the promising way for the detection of H2O2 in vivo and in vitro as a result of its high selectivity, sensitivity, as well as ease of operation.8 In a general way, for the sake of accelerating the electron transfer between electrode and H2O2, enzyme modified electrodes are employed and industrialized.9 Nevertheless, this kind of biosensors still faces some challenges, such as expensive cost, poor stability, and strict conditions.10 These issues are subsequently circumvented by using nanomaterials including noble metals,11 transition metals,12 metal oxides13 or sulfides,14 and carbon-based materials15 to enhance the electrocatalytic activity toward H2O2 redox reaction. This method is called non-enzymatic electrochemical

sensors. Typically, noble metallic nanoparticles display excellent 2 ACS Paragon Plus Environment

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electrocatalytic ability toward the reduction of H2O2,16 but they are relatively expensive for fabricating electrochemical sensors. It is worthy to develop noble metal free catalysts with the low cost and the enhanced electrocatalytic properties. Metal-organic frameworks (MOFs), assembled by metal ions/clusters and organic ligands, are developed porous crystalline materials.17 Recently, MOFs have been used for gas storage,18 separations,19 catalysis,20 and drug delivery21 on account of their unique properties including large surface areas, a crystalline ordered structure, and tunable pore sizes.22 Specifically, the rht-type MOFs, as a highly successful class of MOFs, comprise highly symmetrical 24-connected faceted polyhedra of linked M2(carboxylate)4 paddlewheel clusters.23 This rht-type MOF encloses truncated tetrahedral, octahedral, and cuboctahedral cages.24 Moreover, they have open metal sites and tunable pore sizes, exhibiting promising applications in various research fields. In addition to the aforementioned applications, applying MOFs to electrochemistry is a comparatively new area of study, and it attracts enormous scientific interest.25 However, in contrast to the richness and variety of MOF structures, only a few MOFs have been reported for electrochemical sensor application by reason of their low electronic conductivity and instability in water medium.26 As is known to all, Cu compounds often possess enzyme-like activities for H2O2 reduction.27 Accordingly, Cu-based MOF can provide a new opportunity for mimetic catalysis. At present, there were only some Cu-based MOFs used for electrochemical sensors, such as Cu-BTC,28 Cu-BTB,29 and Cu-BIB.30 We reasoned that an electro-active rht-type Cu-MOF with excellent stability toward air and water ought to be a competitive candidate in the field of electrochemical H2O2 sensor.



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Previous studies have reported that the hybridization with functional materials might provide an efficient way to improve chemical and physical properties and circumvent drawbacks of MOFs.31 Currently, the MOFs-carbon hybridization exhibited tremendous potential. For instance, Zhang et al. developed Cu-BTC and macroporous carbon composites for electrocatalytic NADH and H2O2.28 Wang and co-workers reported carbon nanotubes-MOF hybrids as an electrochemical pesticides sensor.32 Guo’s group designed synthesis of the H2O2 electrochemical biosensor based on MOF-graphene nanosheets.33 Unfortunately, these composites were subjected to the defects of difficult preparation process and poor dispersibility. Besides, the one-step preparation of MOFs-carbon hybrids can result in different coordination directions between carbon and MOF units, and the entire framework structures would be influenced.34 The surface and interface are regarded as the key characteristics of catalysts. Therefore, designing the surface and interface of electrocatalysts can improve their catalytic properties.35 On the one hand, the surface conditions such as geometry, composition, pore, and architecture structure, play a unique role in the surface adsorption and activation abilities of analytes, as a result, these parameters have a great influence on the activity, selectivity, and stability of the materials.36 And on the other side, interfacial structures determine the charge transfer efficiency because the interface is referred to as the boundary between two domains, and charge transfer is needed to go through the interface when materials contain multiple components.37 In conclusion, preparing electrocatalysts with the designed surface and interface is another development direction for enhancing their catalytic performance.


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In this paper, we put forward and demonstrate an alternative approach to greatly improve the electrochemical sensing performance of rht-type Cu-based MOF (Cu-TDPAT) through modifying its surfaces with nanosized electrochemically reduced graphene oxide (n-ERGO) . The formula of Cu-TDPAT was Cu3(TDPAT)(H2O)3·10H2O·5DMA, in which H6TDPAT was 2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine. Notably, Cu-TDPAT possessed two functionality including open metal sites and Lewis basic sites, which can provide biomimetic catalytic activities and adsorption affinity for H2O2. Moreover, the n-ERGO crossed together on the surface of Cu-TDPAT to form a hierarchical structure, and the interaction between Cu-TDPAT and n-ERGO was attributed to π-π packing and hydrogen bonding, which made n-ERGO be an integral component of Cu-TDPAT framework to enhance charge transport. Therefore, a sensitive and selective H2O2 biosensor was created based on Cu-TDPAT-n-ERGO composites, and the Cu-TDPAT-n-ERGO composites exhibited the current amplification and the electrocatalytic ability. The electrochemical measurements

showed

that

Cu-TDPAT-n-ERGO

exhibited

drastically

enhanced

electrocatalytic activity toward the H2O2 reduction in reference to the parent Cu-TDPAT. The performance of the sensor with respect to applicability in human serum and urine samples, sensitivity, selectivity, stability, linear range, and detection limit was presented and discussed. 2. EXPERIMENTAL SECTION 2.1 Apparatus. Scanning electron microscope (SEM, Hitachi S-4800) was applied to analyze morphology. Transmission electron microscope (TEM, FEI Tecnai F20) was used to monitor energy dispersive X-ray (EDX) element maps. Powder X-ray diffractometer (PXRD, Rigaku D/max 2550) was applied to characterize the crystal structure. The Fourier transform 5 ACS Paragon Plus Environment


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infrared spectrometer (FTIR, FTIR-8700) was employed to investigate functional groups. X-ray photoelectron spectroscopy (XPS, ESCALAB-MKII 250) was applied to characterize the chemical compositions of samples. An electrochemical cell consisting of a glassy carbon electrode (GCE) as the working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE), was employed for the electrochemical measurements with a CHI 660A electrochemical workstation (Shanghai Chenhua). The phosphate buffer solutions (PBS) used for electrochemical experiment were deaerated with nitrogen gas for 20 minutes. 2.2 Chemicals. Graphite powder was supplied by Qingdao Hengrui Industrial, China. Sulfuric acid (H2SO4), potassium permanganate (KMnO4), H2O2, Copper nitrate trihydrate (Cu(NO3)2·3H2O), NaOH, and NaHCO3 were supplied by Beijing Chemical Factory, China. 5-Aminoisophthalic acid, cyanuric chloride, 1,4-dioxane, and HBF4 were purchased from Aladdin Reagent Ltd. (Shanghai, China). Reagents were analytical grade and applied as received. The real samples were provided by First Hospital, Jilin University, Changchun. 2.3 Synthesis of nanosized GO. Nanosized GO (n-GO) can be obtained by a modified published procedure.38 In brief, GO was firstly synthesized according to the modified Hummers method.39 5.0 mg GO was then dispersed in a certain amount of ultrapure water through 4 h ultrasonication process, subsequently, the suspension was centrifuged at 8000 rpm for 30 min to collect the upper phase containing n-GO. 2.4 Preparation of Cu-TDPAT. Cu-TDPAT was synthesized according to the previous report.40 H6TDPAT was prepared firstly. Briefly, 15.2 g 5-aminoisophthalic acid, 5.36 g NaOH, and 8.74 g NaHCO3 were dissolved in 140 mL ultrapure water. The solution was kept 6 ACS Paragon Plus Environment

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on stirring at 0 ◦C for 30 min. Upon drop by drop addition of cyanuric chloride in 1,4-dioxane (3.70 g/70 mL), the obtained mixture maintained heating at 100 ºC for 24 h. Later, the solution was treated with HCl solution to make the resulting solution pH value be 2. The H6TDPAT solid was filtered, washed with ultrapure water, and dried to be collected. Then the fresh prepared H6TDPAT (0.030 g) was mixed with 0.164 g Cu(NO3)2·3H2O dissolved in the mixing solvent of 2 mL DMA, 2 mL DMSO, and 100 µL ultrapure water. And 0.9 mL HBF4 was added into the above mixture. After that, the resulting solution was sealed in a phial and kept at 85 ºC for 3 d. The blue crystals were collected by filteration, and rinsed with DMA. The Cu-TDPAT was thus obtained, and the crystal has a formula of Cu3(TDPAT)(H2O)3·10H2O·5DMA. Finally, 6.0 mg Cu-TDPAT was added to 1.0 mL DMF with ultrasonication for several minutes. 2.5 Preparation of Cu-TDPAT-n-GO composites. The above n-GO aqueous dispersion and 6.0 mg mL−1 Cu-TDPAT suspension were mixed under stirring and ultrasonication, so the Cu-TDPAT-n-GO composites were obtained. 2.6 Sensor fabrication. Before modification, the clean GCE was obtained by polishing with 0.3 and 0.05 µm Al2O3 power, and followed by ultrasonic washing with 1:1 nitric acid, alcohol, and ultrapure water. After the GCE was dried by nitrogen gas, 5 µL Cu-TDPAT-n-GO

suspension

was

coated

on

the

GCE

surface,

forming

Cu-TDPAT-n-GO/GCE. Upon dryness under room temperature, electrochemical reduction of Cu-TDPAT-n-GO/GCE was performed in pH 7.0 PBS at a constant potential of -1.2 V for 20 s. Finally, the Cu-TDPAT-n-ERGO/GCE was carefully rinsed with ultrapure water to remove the residual adsorbed substances. For comparison, n-ERGO/GCE and Cu-TDPAT/GCE were 7 ACS Paragon Plus Environment


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prepared by casting 5 µL 0.5 mg mL−1 n-GO or 3 mg mL−1 Cu-TDPAT on the bare electrode surface, respectively, and employing the same fabrication system. 3. RESULTS AND DISCUSSION 3.1 Characterization of composites The formation of Cu-TDPAT-n-ERGO composites involved a three-step process, as explained in the Experimental Section and shown in Scheme 1. First, Cu-TDPAT was prepared by using H6TDPAT, a Lewis basic sites-rich hexacarboxylate ligand. Interestingly, Cu-TDPAT owns the highest open metal sites density among all know rht-type MOFs. These open metal sites can serve as unique catalytic sites for chemical reactions. Additionally, Cu-TDPAT contains the oxygen atoms with the electron-accepting ability, which can polarize the Cu atom in the framework. Moreover, there are plenty of Lewis basic sites in framework so that Cu-TDPAT could have high adsorption affinity for acidic small molecule. Consequently, Cu-TDPAT should be an attractive candidate for electrochemical applications. Second, by using a simple and fast ultrasonic method, the Cu-TDPAT-n-GO composites were obtained. It is well-known that n-GO possesses high surface area and hydrophilic groups, which is expected to favor the electron transfer as well as the dispersity of Cu-TDPAT. Finally, the Cu-TDPAT-n-ERGO was acquired by an electrochemical treatment. The electo-reduction process can give rise to the composites with greatly enhanced conductivity, stability, and catalytic performance. Based on above points, excellent electrochemical properties for Cu-TDPAT-n-ERGO can be anticipated. These expectations will be further verified by the following characterizations and electrochemical experiments.


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Scheme 1. Schematic presentation for the fabrication of Cu-TDPAT-n-ERGO composites The texture of GO and n-GO (a), Cu-TDPAT (b), and Cu-TDPAT-n-ERGO (c) can be observed on SEM images presented in Figure 1. As shown in Figure 1a, GO was seen as a dense plane of graphene sheets stacked together with some wrinkles. The cause for these wrinkles was that introducing sp3 hybrid carbon disrupted the sp2 carbon sheets during oxidation process.41 By the combination of ultrasonication and centrifugation, n-GO with nanoscale size could be obtained, and the result was similar to that reported (Figure 1a, inset). The octahedral crystals of Cu-TDPAT were obviously observed, and their sizes were around 3 µm, suggesting perfect crystallinity of Cu-TDPAT (Figure 1b). The polyhedral structure endued Cu-TDPAT with large surface area, which made it ideal candidate for electrochemical studies. Furthermore, the rough surface of Cu-TDPAT was visible, which is beneficial for loading the n-GO. From the SEM image of Cu-TDPAT-n-ERGO (Figure 1c), it 9 ACS Paragon Plus Environment


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can be seen that most n-ERGO sheets crossed together on the surface of Cu-TDPAT to form a hierarchical structure, and the interface between the Cu-TDPAT and the n-ERGO was coherent, resulting in efficient electron transfer within the hybrid structure. The n-ERGO with hierarchical structure possessed the higher accessible surface area and conductivity, making contribution to enhance the electrocatalytic performance.42 The EDX mapping images of the Cu-TDPAT-n-ERGO showed additional evidence of the existence and distribution of elements (Figure S1, Supporting Information). In the EDX mapping results, different colors were employed to show different elements. It was clearly seen that the uniform distribution of C, O, and Cu elements, and the EDX mapping images verified that n-ERGO and Cu-TDPAT existed in Cu-TDPAT-n-ERGO composites.

Figure 1. SEM images of a) GO, inset: n-GO. b) Cu-TDPAT. c) Cu-TDPAT-n-ERGO. The crystal structure features of n-GO, Cu-TDPAT, and Cu-TDPAT-n-GO samples were established through PXRD. As depicted in Figure 2a, n-GO exhibited a diffraction peak at 2θ =10.8°, and the d-spacing was calculated to be 8.1 Å, which can be put down to the existence of oxygen-containing groups.43 As for Cu-TDPAT, the positions of the experimental and simulated patterns coincided with each other, testifying the successful synthesis of Cu-TDPAT. The tetragonal crystal system of Cu-TDPAT contained space group: I4/m and lattice parameter: a = 26.860 Å, c = 37.753 Å. Moreover, the water stability of Cu-TDPAT 10 ACS Paragon Plus Environment

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was tested in boiling water for 1 day. The major peaks of the framework remained intact, indicating Cu-TDPAT had exceptional water and thermal stability. This excellent stability of Cu-TDPAT will enable its utilization of various electrochemical applications in aqueous media.

Additionally,

the

characteristic

peaks

of

Cu-TDPAT

still

remained

in

Cu-TDPAT-n-GO. The results manifested that Cu-TDPAT-n-GO maintained crystallinities and underlying topologies after the incorporation of n-GO and ultrasonic treatment.

Figure 2. a) PXRD patterns of n-GO, simulated Cu-TDPAT, experimental Cu-TDPAT, Cu-TDPAT-n-GO, and Cu-TDPAT under hydrothermal conditions for 24 h. b) FT-IR spectra of n-GO, Cu-TDPAT, and Cu-TDPAT-n-GO. FT-IR characterization was further employed to explain the interaction between Cu-TDPAT and n-GO. As displayed in Figure 2b, the n-GO showed the bands at 3419, 1723, 1621, 1362, 1240, and 1080 cm−1, arising from the -OH stretch, C=O stretch, C=C vibrations, C-OH bending vibrations, alkoxy C-O vibrations, and epoxy C-O stretch, respectively.44 In the spectrum of Cu-TDPAT, the peaks appeared at 3413, 1637, 1613, 1418, 1380, 779, 718, 619, and 486 cm−1, which were in accordance with previous report, confirming again the successful synthesis of Cu-TDPAT.40 To be noticed, the spectrum of Cu-TDPAT-n-GO largely resembled that of Cu-TDPAT. Another interesting feature, quite unique for 11 ACS Paragon Plus Environment


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Cu-TDPAT-n-GO, was the disappearance of the peaks at 1240 cm−1 and 1080 cm−1. The reasonable explanation to these changes was that the Cu-TDPAT interacted with n-GO through π-π packing and hydrogen bonding.45 Hence, n-GO can become an integral component of Cu-TDPAT framework to form Cu-TDPAT-n-GO composites, instead of the physically mixed n-GO and Cu-TDPAT crystals, which could result in efficient charge transfer within the hybrid structure.

Figure 3. XPS spectra of Cu-TDPAT-n-ERGO The chemical compositions of Cu-TDPAT-n-ERGO were investigated by XPS, and the corresponding results were shown in Figure 3. The full survey of Cu-TDPAT-n-ERGO displayed peaks for C 1s, O 1s, and Cu 2p. The deconvoluted C 1s XPS spectra of 12 ACS Paragon Plus Environment

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Cu-TDPAT-n-ERGO showed three bands at the position around 284.7, 286.2, and 288.0 eV, which were assigned to C-C, C-O, C=O groups, respectively (Figure 4b).46 The binding energy for O 1s was observed at around 531.6 eV,47 mainly deriving from O(II) ions in crystalline network of the Cu-TDPAT (Figure 4c). Furthermore, the XPS analysis of Cu 2p shown in Figure 4d exhibited two peaks at 935-943 eV and 955-963 eV, assigned to Cu 2p3/2 and Cu 2p1/2 peak, suggesting Cu(II) existed in the Cu-TDPAT-n-ERGO.48 These findings revealed successful decoration of the Cu-TDPAT with n-ERGO. With different modified electrodes in hand, efforts shifted to characterizing the electron transfer properties and the surface features of the different materials. Thus, electrochemical impedance spectroscopy (EIS) measurements were carried out.49 Figure 4a showed Nyquist diagrams of different electrodes in 10 mM [Fe(CN)6]3−/4− and 0.1 M KCl solution. All the EIS shown in Figure 4a consisted of a straight line and a semicircular portion, corresponding to the diffusion-limited and electron transfer-limited process, respectively.50 The semicircle diameter represented the charge transfer resistance (Rct).51 It was found that the Rct of the bare GCE was 483.9 Ω, whereas the Cu-TDPAT/GCE showed the Rct value of 1317 Ω, indicating the poor electrical conductivity of Cu-TDPAT. It was attributed to the intrinsic insulating nature of the carboxylate bonds utilized to form MOFs. Interestingly, the Rct of the Cu-TDPAT-n-GO/GCE decreased relative to the Cu-TDPAT/GCE (995.2 Ω), demonstrating a more convenient process of transferring the probe molecules to the electrode surface. The reasonable explanation may be that the n-GO with good conductibility can serve as charge transfer mediators. In addition, the hydrophilic property of n-GO made the electrode surface, which used to be hydrophobic, much easier to get wetted. Thus, probe molecules can easily 13 ACS Paragon Plus Environment


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accumulate on the electrode surface. After electrochemical treatment, the Rct of the Cu-TDPAT-n-ERGO/GCE was drastically decreased to 124.1 Ω. The much lower Rct further confirmed that Cu-TDPAT-n-ERGO/GCE exhibited faster electron transfer ability, which was benefited from unique structure of Cu-TDPAT-n-ERGO. These important characteristics make Cu-TDPAT-n-ERGO become a competitive candidate for various electrochemical applications especially electrochemical sensors.

Figure 4. a) Nyquist diagrams of GCE, Cu-TDPAT/GCE, Cu-TDPAT-n-GO/GCE, and Cu-TDPAT-n-ERGO/GCE in 0.1 M KCl solution with 10 mM Fe(CN)63-/4-. Inset: the equivalent circuit. Rs: bulk resistance, Rct: charge transfer resistance, Cdl: constant phase element,

Zw:

Warburg.

b)

CVs

of

Cu-TDPAT,

Cu-TDPAT-n-GO/GCE,

and

Cu-TDPAT-n-ERGO/GCE in 0.1 M PBS (pH 7.0), scanning rate = 50 mV s-1. 3.2 Electrocatalytical reduction of H2O2 at modified electrode Based on above characteristics, the Cu-TDPAT-n-ERGO/GCE was preliminarily applied for the electrochemical detection of H2O2. At first, the electrochemical properties of the Cu-TDPAT/GCE, the Cu-TDPAT-n-GO/GCE, and the Cu-TDPAT-n-ERGO/GCE in PBS (pH 7.0) were studied by cyclic voltammetry (CV). Figure 4b manifested that the CV of pure 14 ACS Paragon Plus Environment

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Cu-TDPAT/GCE exhibited two redox peaks, which was derived from the Cu(II) center redox reactions. However, after interaction with n-GO, the anodic and cathodic current of the Cu-TDPAT-n-GO/GCE increased sharply and only one pair of redox peaks can be observed. This was caused by n-GO with the fast electron transfer ability.52 Furthermore, by an electrochemical treatment, the Cu-TDPAT-n-ERGO/GCE presented one pair of enhanced redox peaks that centered at the nearly same potentials as the Cu-TDPAT-n-GO/GCE, indicating that the electrochemical reduction process resulted in the composites with high electrochemical conductivity and electrocatalytic activity. The above results were consistent with the phenomenon observed in EIS curves.

The electrocatalytic activity of the Cu-TDPAT-n-ERGO/GCE toward H2O2 reduction was then investigated. Figure 5 showed the CVs of the GCE (a), the Cu-TDPAT-n-GO/GCE (b), and the Cu-TDPAT-n-ERGO/GCE (c) in pH 7.0 N2-saturated PBS with or without 5 mM H2O2 at a scan rate of 50 mV s-1. For the bare GCE, only a weak reduction peak of H2O2 at 0.72 V was observed as a result of slow electron transfer kinetics of H2O2 reduction process. In contrast, the Cu-TDPAT-n-GO/GCE and the Cu-TDPAT-n-ERGO/GCE exhibited a reduction peak appeared at about - 0.31 V and a remarkable current response, which were 26 times and 40 times improvement in comparison with the bare GCE, respectively, indicating both the Cu-TDPAT-n-GO/GCE and the Cu-TDPAT-n-ERGO/GCE possessed efficient electrocatalytic activity toward the reduction of H2O2. The marked improvement can be explained by the synergistic effect of Cu-TDPAT and n-GO or n-ERGO, and such distinctly different electrocatalytic activities of Cu-TDPAT-n-GO/GCE and Cu-TDPAT-n-ERGO/GCE unambiguously demonstrated that n-GO and n-ERGO did matter. The results showed that the 15 ACS Paragon Plus Environment


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Cu-TDPAT-n-ERGO/GCE was better than the Cu-TDPAT-n-GO/GCE from the viewpoint of their electrochemical responses. This was rationalized by the larger surface area and higher conductivity of n-ERGO with hierarchical structure relatively to n-GO, resulting in the faster electrons transfer involved in the catalytic reaction. Hence Cu-TDPAT-n-ERGO/GCE can sensitively trace the presence of H2O2 electrochemically. For the sake of distinguishing the contribution of individual components and their synergistic effect, electrochemical responses of H2O2 at the n-GO/GCE, the n-ERGO/GCE, and the Cu-TDPAT/GCE were also investigated (Figure S2, Supporting Information). When 5 mM H2O2 was added into the solution (pH 7.0 PBS), the reduction peak current at the n-GO/GCE was increased (3.1 times of bare GCE) at - 0.61 V, which indicated that the n-GO/GCE exhibited appreciable electrocatalytic performance toward H2O2 reduction due to the high surface area. Meanwhile, the peak current at the n-ERGO/GCE was 6.9-fold enhanced in comparison with the bare GCE, and the reduction peak potential was centered at - 0.45 V. The result demonstrated that the n-ERGO films had superior electrocatalytic ability than the n-GO films, and it further confirmed that the Cu-TDPAT-n-ERGO/GCE was better than the Cu-TDPAT-n-GO/GCE for the H2O2 electrocatalytic reduction. Besides, at the Cu-TDPAT/GCE, the reduction peak current was 23 times higher than bare GCE with the peak potential shifting to - 0.88 V, indicating the Cu-TDPAT had the excellent current amplification but relatively weak electron transport ability. The reason for current amplification performance of the Cu-TDPAT/GCE may originate in its adequate porous structure for the pre-concentration of H2O2, thus giving rise to the signal enhancement. The relatively weak electron transport ability of Cu-TDPAT was attributed to two factors. Firstly, the intrinsic insulating nature of the carboxylate bonds 16 ACS Paragon Plus Environment

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utilized to form MOFs resulted in low conductivity, and electron cannot migrate along or access the skeleton of MOFs crystals. Secondly, the micro-sized MOFs usually had poor contacts with the smooth electrode surface, resulting in that the interfacial electron transfer from MOFs to electrode surface become difficult. The above information clearly demonstrated that both n-GO and n-ERGO with good conductibility can play the role of charge transfer mediators to solve the problem of weak electron transport in Cu-TDPAT, boosting the electrocatalytic activity of Cu-TDPAT by the integration of Cu-TDPAT with n-GO and n-ERGO. The outstanding sensing performance of the Cu-TDPAT-n-ERGO/GCE was benefited from three aspects: (1) n-ERGO had large surface area, high conductivity, and preeminent catalytic performance. (2) Cu-TDPAT provided the open metal active sites of Cu(II) that possessed enzyme-like activities with H2O2 and acted as the catalytic center. In addition, Cu-TDPAT was highly porous, which afforded a microenvironment for pre-concentration of H2O2 in the pores,

resulting

in

a

higher

sensing

sensitivity.

(3)

The

hybrid-structured

Cu-TDPAT-n-ERGO served as the current amplifier and the electrocatalyst. There was a synergetic catalytic effect between Cu-TDPAT and n-ERGO. On one hand, Cu-TDPAT could enlarge the surface area of n-ERGO, and on the other, n-ERGO was used as an electron transfer mediator and binder between Cu-TDPAT and electrode to enhance the stability of electrochemical performance and facilitate the electrocatalytic role of Cu-TDPAT for H2O2 reduction. Figure 5d illustrated the CVs of the Cu-TDPAT-n-ERGO/GCE in deaerated PBS containing different content of H2O2. Clearly, the cathodic currents increased with the 17 ACS Paragon Plus Environment


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increase

content

of

H2O2,

demonstrating

good

electrocatalytic

Page 18 of 31

activity

of

the

Cu-TDPAT-n-ERGO/GCE toward H2O2 reduction. In addition, the reduction of H2O2 at the Cu-TDPAT-n-ERGO/GCE began at the same potential of the reduction of Cu(II), and this finding indicated the electrocatalytic reduction of H2O2 took place by the center metallic cation of Cu-TDPAT-n-ERGO. Therefore, the pertinent electrocatalytic mechanism can be proposed as follows: 2Cu(II)-TDPAT-n-ERGO + 2e → 2Cu(I)-TDPAT-n-ERGO

(1)

2Cu(I)-TDPAT-n-ERGO + H2O2 → 2Cu(II)-TDPAT-n-ERGO + 2OH−

(2)

The first step was an electrochemical recognition progress, namely, H2O2 can be absorbed to the pores and surfaces of Cu-TDPAT-n-ERGO. And the second procedure was the electrochemical catalysis of H2O2 by Cu-TDPAT-n-ERGO. In the cathodic pathway Cu(II)-TDPAT-n-ERGO was reduced electrochemically to Cu(I)-TDPAT-n-ERGO, then the H2O2 reduction was catalyzed by Cu(I)-TDPAT-n-ERGO. Simultaneously, the catalytic center Cu(I) was returned to Cu(II) by H2O2. 3.3 Effect of scan rate The impact of scanning rate on the reduction current of 5 mM H2O2 at the Cu-TDPAT-n-ERGO/GCE was detected as well. With reference to Figure 6, we can see that the reduction peak current (Ipc) increased with the scan rates (v) range from 10 to 400 mM s-1, and the linear relationship between Ipc and v was obtained, which was expressed as: Ipc (µA) = -26.0 - 0.106 v ( mV s-1), R = 0.999. These results indicated that in the investigated potential range, the electrochemical reduction of H2O2 at the Cu-TDPAT-n-ERGO/GCE underwent the surface-controlled irreversible process. 18 ACS Paragon Plus Environment

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Figure 5. CVs of bare GCE a), Cu-TDPAT-n-GO/GCE b), and Cu-TDPAT-n-ERGO/GCE c) in pH 7.0 PBS with (red curves) or without (black curves) 5 mM H2O2. Scan rate: 50 mV s-1. d) CVs of Cu-TDPAT-n-ERGO/GCE in pH 7.0 PBS with 0, 2, 5, 10, and 15 mM H2O2, respectively. Scan rate: 50 mV s-1.



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Figure 6. a) CVs obtained for 5 mM H2O2 at the Cu-TDPAT-n-ERGO/GCE in different scan rates (10, 40, 70, 100, 130, 160, 190, 220, 250, 280, 310, 340, 370, 400 mV s-1). b) Dependence of reduction peak currents on scan rates. 3.4 Experimental variables optimization The electrochemical performance of electrodes can be modulated by controlling electrodeposition time. Electrochemical deposition of n-ERGO was performed at a constant potential of - 1.2 V with the deposition time from 10 to 40 s. Figure S3a (Supporting Information) displayed effect of deposition time on the H2O2 reduction peak current. Clearly, the n-ERGO/GCE prepared by the electrochemical deposition for 20 s exhibited the maximum response current, which was ascribed to its optimized microstructure for electrocatalysis. When the electrodeposition time was short, the electrochemical reduction of n-GO was not complete so that the n-ERGO film could not form a hierarchical structure. When the deposition time was prolonged to 30 s and 40 s, the sheets of n-ERGO became thick and denser to result in the stack of n-ERGO layer, which would hinder the electron transfer. Consequently, n-ERGO/GCE prepared with 20 s was selected as the optimum time. In order to select the optimal detection potential, the electrocatalytic H2O2 behavior at the Cu-TDPAT-n-ERGO/GCE was investigated with different applied potentials (from - 0.20 to 0.40 V) by a current-time (i-t) analytical approach (Figure S3b, Supporting Information), and the corresponding calibration plots were displayed in Figure S3c (Supporting Information). Obviously, the maximum response current was observed at - 0.35 V. As shown in Figure 4, it was found that the reduction of H2O2 at the Cu-TDPAT-n-ERGO/GCE was center at - 0.31 V, implying that the reduction of H2O2 at the Cu-TDPAT-n-ERGO/GCE needed - 0.31 V 20 ACS Paragon Plus Environment

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potential or lower to achieve the maximum current. Thus - 0.35 V was the optimal potential for the following studies, considering compatibly high and stable current response. 3.5 Amperometric measurement of H2O2 Figure 7 displayed the amperometric i-t performance of Cu-TDPAT-n-ERGO/GCE to the continuous injection of H2O2 in pH 7.0 PBS at - 0.35 V. After injecting H2O2, Cu-TDPAT-n-ERGO/GCE exhibited strong and rapid current response, benefiting from the high electrocatalytic efficiency of Cu-TDPAT-n-ERGO. As illustrated in Figure 7b, the corresponding calibration curve was linear range from 4 to 12000 µM with regression equation Ipc (µA) = - 1.11 - 0.00841 C (µM) (R = - 0.9996), and the detection limit was 0.17 µM

(S/N

=

3).

In

contrast

with

other

reported

H2O2

sensors

(Table

1),

Cu-TDPAT-n-ERGO/GCE had wider linear range and lower detection limit, especially better than horseradish peroxidase or some other MOF-based biosensors.14, 28, 38, 53-58 It revealed the excellent performance of Cu-TDPAT-n-ERGO/GCE as a promising electrochemical sensor for the detection of H2O2.

Figure 7. a) Amperometric i-t curve of different concentrations of H2O2 in pH 7.0 PBS at Cu-TDPAT-n-ERGO/GCE using operating voltage of - 0.35 V. The concentration of H2O2, in turn, is 4, 7, 10, 40, 70, 100, 200, 400, 600, 800, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 21 ACS Paragon Plus Environment


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4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10000, 10500, 11000, 11500, 12000 µM. b) The linear fit of peak currents to concentrations of H2O2 according to the curves shown in a). Table 1 Comparison of different sensors for the determination of H2O2 Linear range

Detection of limit

Electrode material

Reference (µM)

(µM)

Cu-BTCa/MPCb/GCE

10-11600

3.2

[28]

Cu-BTC/GN/GCE

10-11180

3

[54]

NGc/AgNP/MMEd

5-47

0.56

[38]

GQDse/GCE

1000-10000

20

[55]

OMCNf/GCE

4-40,40-12400

1.52

[56]

Pd-HCNFg/GCE

5-2100

3

[53]

CuS/RGO/GCE

5-1500

0.27

[14]

5.98-35.36

0.48

[57]

Cyt c/MPCEi

20-240

14.6

[58]

Cu-TDPAT-n-ERGO/GCE

4-12000

0.17

This work

HRP/SPEh

a

Cu-BTC = Cu(II) benzene-1,3,5-tricarboxylate

b

MPC = macroporous carbon

c

NG = nanoscale graphene

d

MME = membrane modified electrode

e

GQDs = graphene quantum dots

f

OMCN = ordered mesoporous carbon nitride 22 ACS Paragon Plus Environment

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g

Pd-HCNF = palladium-helical carbon nanofiber

h

HRP/SPE = horseradish peroxidase/screen-printed electrode

i

Cyt c/MPCE = cytochrome c/macroporous active carbon electrode 3.6 Reproducibility, stability, and selectivity The intraassay reproducibility test showed the relative standard deviation (RSD) for six

successive measurements of 100 µM H2O2 with the same Cu-TDPAT-n-ERGO/GCE was 1.82%. In addition, the interassay reproducibility was estimated by monitoring 100 µM H2O2 with five different Cu-TDPAT-n-ERGO electrodes, which were prepared via the same procedure. The RSD of 1.78% was obtained.

Figure 8. a) Operational stability of the Cu-TDPAT-n-ERGO/GCE at operating voltage of 0.35 V in a stirring pH 7.0 PBS with 100 µM H2O2. b) Current response of Cu-TDPAT-n-ERGO/GCE to 20 µM H2O2 with 200 µM Na2HPO4, KH2PO4, LiClO4, CaCl2, and 60 µM AA, UA, DA, glucose, l-proline, l-tyrosine, and l-cysteine. For the stability test, the proposed sensor was used for measuring the response current of 100 µM H2O2 daily. The research found that the current response kept 89% of the original current response after 15 days storage. The operational stability was also examined. The Cu-TDPAT-n-ERGO/GCE was used to determine 2 mM H2O2 for 1600 s continuously, and 23 ACS Paragon Plus Environment


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92% of the initial current was still retained after 1600 s (Figure 8a). This stability and reproducibility manifested the better performance of non-enzymatic Cu-TDPAT-n-ERGO sensor than enzymatic sensors. The effect of probable interferences on the H2O2 response of Cu-TDPAT-n-ERGO/GCE was assessed (Figure 8b). The results concluded that K+, Na+, Li+, Ca2+, Cl−, NO3−, ClO4−, SO42−, HPO42−, H2PO4− in 10-fold excess, as well as 3-fold concentration of glucose, uric acid (UA), ascorbic a cid (AA), l-proline (LP), l-tyrosine (LT), and l-cysteine (LC) did not cause the current change of H2O2, suggesting the excellent selectivity in H2O2 sensing at Cu-TDPAT-n-ERGO/GCE.

Table 2 Detection of H2O2 in human urine and serum samples (n = 5) Sample

Added (µM)

Found (µM)

RSD (%)

Recovery (%)

Urine 1

70.00

68.98

0.69

98.54

Urine 2

110.0

112.0

1.02

101.8

Serum 1

70.00

68.38

0.55

97.68

Serum 2

110.0

113.5

1.19

103.2

3.7 Application of the H2O2 sensor in real samples To prove the practical application of this sensor, the diluted human serum and urine samples were spiked with H2O2 and tested by the standard addition method. The obtained results were shown in Table 2. The recoveries were between 97.68% and 103.2%, indicating the proposed sensor is capable of detecting H2O2 and resisting the interference in real samples.


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Thus, the fabricated sensor based on Cu-TDPAT-n-ERGO hybrid is promising for physiological and pathological study. 4. Conclusions In summary, we introduced an alternative approach to greatly improve the electrochemical sensing performance. This strategy involved the preparation of an original design of the functional material based on the surface modification of bifunctional Cu-TDPAT (open metal sites and Lewis basic sites) with n-ERGO exhibiting the current amplification and electrocatalysis. The proposed sensor showed boosted electrocatalytic activity toward H2O2 reduction, and it exhibited wide linear range, low detection limit, super stability, satisfying reproducibility, excellent anti-interference ability, and the facile synthetic method. These factors indicated marked superiorities of rht-type MOFs in H2O2 detection, opening up new possibilities of enlarging rht-type MOFs-based materials in the electrochemical application field. ASSOCIATED CONTENT

Supporting Information.

EDX elemental maps of Cu-TDPAT-n-ERGO, CV curves of n-GO/GCE, n-ERGO/GCE, and Cu-TDPAT/GCE in 0.1 M PBS (pH 7.0) with or without 5 mM H2O2, optimizing deposition time of graphene, the amperometric curves of the Cu-TDPAT-n-ERGO/GCE for the successive additions of 100 µM H2O2 at different operating voltages, and the corresponding calibration plots for the amperometric i-t curves. AUTHOR INFORMATION 25 ACS Paragon Plus Environment


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Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial support by the National Natural Science Foundation of China (21375045) and Natural Science Foundation of Jilin Province (20130101118JC) are gratefully acknowledged. Project 2016146 is also supported by Graduate Innovation Fund of Jilin University.

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