High-Performance Electrochemical Catalysts ... - ACS Publications

Oct 6, 2015 - and Quan Yuan*,‡. †. School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China. ‡...
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High-Performance Electrochemical Catalysts Based on ThreeDimensional Porous Architecture with Conductive Interconnected Networks Dong Wang,†,‡,§ Jie Wang,‡,§ Zi-en Liu,‡ Xiangdong Yang,‡ Xiaoxia Hu,‡ Jinqi Deng,‡ Nianjun Yang,†,⊥ Qijin Wan,*,† and Quan Yuan*,‡ †

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China ⊥ Institute of Materials Engineering, University of Siegen, Siegen 57076, Germany ‡

S Supporting Information *

ABSTRACT: The electrochemical applications of traditional carbon nanomaterials such as carbon nanotubes (CNTs) and graphene (G) powders are significantly impeded by their poor three-dimensional (3D) conductivity and lack of hierarchical porous structure. Here, we have constructed a 3D highly conductive CNTs networks and further combined it with mesoporous carbon (mC) for the creation of a core−shell structured (CNT@mC) composite sponge that featured 3D conductivity and hierarchical porous structure. In the composite sponge, interconnected CNTs efficiently eliminates the contact resistance and the hierarchical pores significantly facilitate the mass transport. The electron transfer rates, electroactive surface area and catalytic activity of the CNT@mC composite sponge based catalysts were tested in the direct methanol fuel cells (DMFCs) and electrochemical sensors. In DMFCs, the Pd nanoparticles deposited CNT@mC showed significantly improved catalytic activity and methanol oxidization current. As for amperometric sensing of endocrine disrupting compounds (EDCs), CNT@mC-based catalyst gave a liner range from 10 nM to 1 mM for bisphenol A (BPA) detection and showed great promise for simultaneous detection of multiple EDCs. BPA recovery from environmental water further indicated the potential practical applications of the sensor for BPA detection. Finally, the electrochemical performance of CNT@mC were also investigated in impedimetric sensors. Good selectivity was obtained in impedimetric sensing of BPA and the detection limit was measured to be 0.3 nM. This study highlighted the exceptional electrochemical properties of the CNT@mC composite sponge enabled by its 3D conductivity and hierarchical porous structure. The strategy described may further pave a way for the creation of novel functional materials through integrating multiple superior properties into a single nanostructure for future clean energy technologies and environmental monitoring systems. KEYWORDS: graphene, carbon nanotube, mesoporous carbon, aptamer, electroanalysis

1. INTRODUCTION

requirements and confront considerable challenges, such as poor three-dimensional (3D) conductivity and lack of porous structure. On the one hand, even though CNTs and G show superior conductivity, such excellent property can only proceed along the axis of CNTs or the plane of G because of their onedimensional (1D) and two-dimensional (2D) structures, respectively. These 1D and 2D conductivity result in strong contact resistance between CNTs or G nanosheets, which significantly decrease the efficiency for electron transport.24−26

Because of their exceptional conductivity, catalytic activity, chemical stability, and mechanical properties, sp2 carbon atom bonded carbon nanomaterials,1,2 especially carbon nanotubes (CNTs) and graphene (G),3,4 have attracted enormous attention from a wide range of scientific communities and have been widely applied in areas from energy storage5−11 to electrocatalysis12,13 to electrochemical sensing14,15 in the past few decades. Among all of the applications, developing electrochemical catalysts based on CNTs and G is one of the most popular research areas. These catalysts have played critical roles in fuel cells16−19 and electroanalytical investigations,20−23 with encouraging achievements made. However, with the rapid growth of requirements for the performance of electrochemical catalysts, CNTs and G powders can no longer live up to such © XXXX American Chemical Society

Special Issue: Electrochemical Applications of Carbon Nanomaterials and Interfaces Received: September 4, 2015 Accepted: September 29, 2015

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DOI: 10.1021/acsami.5b08294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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mC/Au) and high sensitivity were obtained toward BPA due to its highly conductive networks. The CNT@mC composite sponge shows better electrochemical properties in DMFCs and electrochemical sensing tests than CNT or G powders, and the strategy described in this study may shed new light on the construction of functional materials with multiple superior properties for clean energy technologies and environmental monitoring systems.

On the other hand, ideal carbon nanomaterials-based catalysts should possess hierarchical porous structure, in which micropores provide large space sites for electron transfer; mesopores afford efficient mass transport pathways for the reactants to diffuse to the inner surface of the catalysts, and macropores serve as reaction solution reservoirs to shorten the diffusion distances.27−29 It is obvious that neither CNTs powders nor G nanosheets possesses such hierarchical porous structure and thus they cannot achieve idea electrochemical performance. Therefore, it is highly urgent to develop novel carbon nanomaterials with superior 3D conductivity and hierarchical porous structure for the creation of high-performance electrochemical catalysts. Construction of interconnected 3D nanostructure is one of the most efficient strategies to achieve 3D conductivity. In recent years, seamless 3D sp2 carbon atom-bonded nanomaterials such as foam-like graphene30,31 and CNT/G hybrid materials32,33 have been reported.34 Among them, CNT sponges35 have attracted wide attention and been applied in areas such as supercapacitors36,37 and fuel cell electrodes38 because of their excellent electric and mechanical properties.39 In a CNT sponge, the junctions between CNTs are chemically bonded by sp2-based carbon atoms, efficiently eliminating the contact resistance confronted by CNTs powder. In addition, the crisscrossed CNTs in the sponge leads to the formation of abundant macropores, which can act as the solution reservoirs to minimize the diffusion distances for reactants as we stated above. As for the hierarchical porous structure, introducing mesoporous materials40−42 into CNT sponge is an effective approach in consideration the finely tuned pores of mesoporous materials.43,44 Accordingly, combining the 3D conductivity of CNT sponge with the highly porous structure of mesoporous materials is a promising strategy for the preparation of high-performance electrochemical catalysts. Herein, we have constructed a mesoporous carbon (designated as mC)-coated CNT core−shell structured (designated as CNT@mC) composite sponge and investigated its electrochemical properties by conducting direct methanol fuel cells (DMFCs) and electrochemical sensing tests. Because of the structural interconnectivities, CNT@mC composite sponge possess highly conductive pathways for electrons. Moreover, previous studies have reported that mC also contains micropores in addition to mesopores due to the burnoff of noncarbon elements such as O, N, and H during carbonization.45 Thus, hierarchical pores ranging from microto macro-pores exists in the CNT@mC composite sponge and such hierarchical porous structure can allow for efficient electron transfer, mass transport, and minimized diffusion distance. In DMFCs tests, the CNT@mC composite sponge was deposited with Pd nanoparticles (designated as CNT@ mC/Pd) and utilized for the catalytic oxidization of methanol in alkaline media. It was found that CNT@mC/Pd-based halfcell reaction showed higher and more stable methanol oxidation currents than CNTs and G powders. Furthermore, CNT@mC was also deposited with Pt nanoparticles (designated as CNT@mC/Pt) and the electrochemical performance of CNT@mC/Pt was tested by amperometric sensing of endocrine disrupting compounds (EDCs) bisphenol A (BPA). A wide liner range was obtained on CNT@mC/Pt modified electrode and BPA recovery experiments indicated the potential applications of CNT@mC based catalysts for EDCs detection. Finally, impedimetric sensor was constructed with gold nanoparticle-deposited CNT@mC (designated as CNT@

2. EXPERIMENTAL SECTION 2.1. Preparation of CNT@mC Composite Sponge. The composite sponge was fabricated with a two-step strategy. First, CNT sponge prepared with a previously reported protocol35 was coated with a layer of mesoporous silica (designated as CNT@mSiO2) via the classic sol−gel method, then the mesoporous silica layer served as the template for the preparation of mC via chemical vapor deposition (CVD). Specifically, 0.15 g of cetyltrimethylammonium bromide (CTAB), 0.5 mL of ammonia, and 10 mL of deionized water were added into 40 mL of ethanol to form a homogeneous solution. Then 0.1 g of tetraethyl orthosilicate (TEOS) was quickly added into 6 mL of the above homogeneous solution and the mixture was instantly dipped into 30 mg of CNT sponge under shaking. This process was repeated for 3 times at intervals of 1 h. Then the sponges were allowed to immerse in the solution and reacted for another 2 h at 50 °C. Finally, the CNT@mSiO2 sponge was obtained by extraction of CTAB with ethanol. After that, the as-prepared CNT@mSiO2 sponge was placed in a CVD furnace and heated to 750 °C for several minutes with flow rates of argon at 300 mL min−1 and acetylene at 15 mL min−1. Then the silica template was etched in hydrofluoric acid (HF, 10%) solution for 12 h and the final CNT@mC composite sponge was obtained. 2.2. Preparation of CNT@mC/Pd, CNT@mC/Pt, and CNT@ mC/Au. The deposition of noble metal nanoparticles on CNT@mC composite sponge was conducted according to previously reported stragtegy.46 Take the preparation of CNT@mC/Pd as an example. Typically, the CNT@mC composite sponge was fixed with an electrode clip and immersed into 0.1 mM K2PdCl4 solution for 0.5 h under stirring. Then the deposition of Pd nanoparticles on the composite sponge was carried out by cyclic voltammetry scanning from 0.5 V to −0.6 V at scan rate of 100 mV s−1 for 30 cycles. The amounts of the deposited GNPs can be fine-controlled through changing the cycle number in electrodeposition process. The CNT@ mC/Pt and CNT@mC/Au were prepared in a similar manner. Cyclic voltammetry scanning from 1.2 V to −0.4 V in 0.1 mM K2PtCl4 solution was employed for preparation of CNT@mC/Pt, and cyclic voltammetry scanning from 1.4 V to −0.5 V in 0.1 mM HAuCl4 solution was employed for preparation of CNT@mC/Au. 2.3. Preparation of CNT@mC/Pd, CNT@mC/Pt, and CNT@ mC/Au Modified Electrodes. Prior to modification, the glass carbon electrode was thoroughly polished with alumina slurries. After that, the electrode was sonicated in ethanol and deionized water for 5 min respectively to remove the substances adsorbed on the surface of the electrode. Take the preparation of CNT@mC/Pd-modified glass carbon electrode (designated as GCE/CNT@mC/Pd) as an example. The CNT@mC/Pd (1 mg) was dispersed in 1 mL of chitosan solution (0.2%) under ultrasonic treatment. Then 50 μL of the above CNT@ mC/Pd suspension was dropped on the surface of glass carbon electrode and dried in air. The CNT@mC/Pt modified glass carbon electrode (designated as GCE/CNT@mC/Pt) and CNT@mC/Aumodified glass carbon electrode (designated as GCE/CNT@mC/Au) were prepared with the same method. 2.4. Preparation of the BPA Binding Aptamer-Functionalized GCE/CNT@mC/Au Electrode. BPA binding aptamers (designated as Apt) were immobilized on the surface of GCE/ CNT@mC/Au (designated as GCE/CNT@mC/Au-Apt) through the reaction between Au nanoparticles and thiol group labeled at the 5′ end of the aptamer. Briefly, the CNT@mC/Au electrodes were immersion in 100 μL aptamer solution (1 μM) at room temperature for 12 h, then the electrode was washed with PBS buffer (10 mM, pH B

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ACS Applied Materials & Interfaces 7) several times. After that, the electrode was immersed 6-mercapto-1hexanol (MCH) solution (1 mM) for 30 min to block the uncovered sites on Au nanoparticles and was further washed with PBS buffer sufficiently.

of the CNT sponge is sustained in the above preparation process, suggesting that the highly conductive pathways for electrons and charge are well-preserved in the CNT@mC composite sponge. N2 adsorption/desorption measurements (Figure 1e) show that the surface area of CNT@mC composite sponge is much higher than that of original CNT sponge, which not only indicates the successful coating of CNTs with mC but also suggests that abundant accessible active site to reactants and electrons exist in CNT@mC composite sponge. 3.2. CNT@mC/Pd as Anode Catalyst for DMFCs. 3.2.1. Construction of GCE/CNT@mC/Pd. The DMFCs is one of the most promising power sources for portable electronics due to their high energy density, easy handling and low operation temperatures.47 The energy densities of DMFCs are closely related to the catalysts employed, and developing novel functional catalyst for DMFCs has long attracted considerable attention.48,49 The electrochemical performance of the CNT@mC composite sponge in DMFCs has been investigated. The oxidation of methanol on GCE/ CNT@mC/Pd is illustrated in Figure 2a. Methanol in the alkaline solution diffuses to the surface of palladium nanoparticles deposited on CNT@mC and further be oxidized. Figure 2b, c shows typical SEM and high-resolution trans-

3. RESULTS AND DISCUSSION 3.1. Construction of CNT@mC Composite Sponge. A two-step strategy is employed to prepare the CNT@mC composite sponge, as illustrated in Figure 1a. The original CNT

Figure 1. (a) Schematic illustration of the construction of CNT@mC composite sponge. TEM images of (b) CNT sponge, (c) CNT@ mSiO2 sponge, and (d) CNT@mC composite sponge. (e) N2 adsorption−desorption isotherms of the original CNT sponge and the CNT@mC composite sponge.

sponge is coated with a layer of mesoporous silica (designated as CNT@mSiO2) with a sol−gel method by using CTAB as the surfactant and TEOS as the silicon source. The mC layer is further deposited on CNTs surface by employing the mSiO2 as the template via a CVD approach, in which acetylene (C2H2) gas served as the carbon source and silica template was removed with HF solution. Transmission electron microscopy (TEM), scanning electron microscope (SEM), and N 2 adsorption/desorption measurements were adopted to characterize the preparation process. TEM images (Figure 1b and Figure S1) and SEM images (Figure S2) clearly showed the interconnected CNTs in the sponge and the average diameter of CNTs was measured to be about 30 nm. Figure 1c presents the TEM images of CNT@mSiO2 sponge and a layer of mesoporous of mSiO2 (ca. 5 nm) was clearly visualized at higher magnifications (Figure S3). Besides, the 3D structure of the sponge is retained and the color of the sponge turned to gray (Figure S4), indicating the successful coating of mSiO2 on CNTs. After carbon deposition and etching of mSiO2 template, a rough surface morphology is observed (Figure 1d and Figure S5) for CNT@mC and the composite sponge turned back to black again (Figure S6). It is noteworthy that the 3D structure

Figure 2. (a) Schematic representation of the methanol oxidization on GCE/CNT@mC/Pd. (b) SEM image of CNT@mC/Pd. (c) HRTEM image of the Pd nanoparticles attached to the surface of CNT@mC. (d) Cyclic voltammograms and (e) Nyquist plots of 5.0 mM [Fe(CN)6]3−/4− on GCE/CNT@mC/Pd, GCE/G/Pd, GCE/CNT/ Pd, GCE/Pd and bare glass carbon electrode (GCE) in 0.1 M KCl solution. The scan rate is 100 mV s−1. The Nyquist plots were recorded at open circuit potentials. C

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currents on the above electrodes are measured to be 177.4, 144.6, 123.9, and 25.3 μA, respectively, which indicates that GCE/CNT@mC/Pd possesses significantly enhanced catalytic efficiency for methanol oxidation compared with the CNT or G powders. The kinetics of methanol oxidation on GCE/CNT@ mC/Pd was further investigated by studying the effects of the scan rate (v) of CV on the oxidation reaction. The dependence of the forward anodic peak current on the scan rate for methanol oxidation in the first region is presented in Figure 3b. The oxidation current shows good linear relationship with v1/2 at scan rates from 20 to 240 mV s−1 (Figure 3c), suggesting that methanol oxidation on GCE/CNT@mC/Pd is controlled by the diffusion of methanol or its intermediates.50 Chronoamperometry is known to be an effective technique to assess the catalytic activity and stability of the electrochemical catalysts. The typical current density−time responses of the above catalysts for methanol oxidation are shown in Figure 3d. In principle, a gradual current decay will be observed before the steady current status is reached due to the formation of Pd oxides or hydroxides and adsorbed intermediates in methanol oxidation reaction. The methanol oxidation current obtained at GCE/CNT@mC/Pd is evidently higher than that of the other three catalysts. Moreover, in the steady state region, GCE/ CNT@mC/Pd shows very slow current decay, which may be ascribed to the fact that the interconnected CNTs in the composite sponge is not vulnerable to peeling off during the oxidization reaction. The above results therefore indicate that the CNT@mC composite sponge-based catalysts possess superior catalytic activity and better stability than CNTs or G powder-based catalysts, which was due to the interconnected conductive networks and the highly porous structure of the CNT@mC composite sponge. 3.3. CNT@mC/Pt as Catalyst for Amperometric Sensors. 3.3.1. Construction of GCE/CNT@mC/Pt. Electrochemical sensors are promising analytical tools and have been widely applied in areas such as clinical diagnostics and environmental analysis due to their high selectivity, good sensitivity and easy operation.51,52 Currently, much attention has been paid to the development of nanomaterials for signal amplification in electrochemical sensors.53 The CNT@mC composite sponge was deposited with Pt nanoparticles and further utilized for the construction of amperometric sensors. SEM and HRTEM images in Figure 4a, b clearly show that Pt nanoparticles are uniformly dispersed on the surface of CNT@ mC and the size distribution of Pt nanoparticles is 1.9−3.7 nm with an average diameter of about 3 nm. The electrochemical behavior of [Fe(CN)6]3−/4− at the GCE/CNT@mC/Pt, GCE/ G/Pt, GCE/CNT/Pt, and GCE/Pt (Pt nanoparticles directly deposited on the electrode) was tested and the results were presented in Figure 4c. The reversible redox peak currents at GCE/CNT@mC/Pt is much higher than that at the GCE/G/ Pt, GCE/CNT/Pt and GCE/Pt, indicating its larger apparent electroactive surface area (Aapp). The Aapp values of the above four electrodes were determined with the Randles−Sevcik equation54

mission electron microscopy (HRTEM) images of the CNT@ mC/Pd composite catalyst. It is observed that Pd nanoparticles were formed uniformly on the surface of CNT@mC and welldispersed Pd nanoparticles can be seen in any microregions in the sponge. In addition, the size distribution of Pd nanoparticles was measured to be 1.5−4.8 nm with an average diameter of about 3 nm. Next, the electrochemical behavior of [Fe(CN)6]3−/4− at the GCE/CNT@mC/Pd was examined. For comparison, Pd-nanoparticle-deposited G-modified electrode (designated as GCE/G/Pd), Pd-nanoparticle-deposited CNTmodified electrode (designated as GCE/CNT/Pd), and Pdnanoparticle-deposited electrode (designated as GCE/Pd) were also tested. Figure 2d shows cyclic voltammograms (CVs) recorded in a 0.1 M KCl aqueous solution containing 5 mM [Fe(CN)6]3−/4−. The reversible redox peak currents at GCE/ CNT@mC/Pd was larger than that at the GCE/G/Pd, GCE/ CNT/Pd, or GCE/Pd, indicating a significantly increased apparent electroactive surface area in CNT@mC/Pd composite catalyst due to the large surface area and highly porous structure of the composite sponge. The Nyquist plots of faradic impedance spectra at the different electrodes are shown in Figure 2e. The diameter of the semicircle in the Nyquist plot is directly correlated with the electron transfer resistance of the corresponding electrode and the GCE/CNT@mC/Pd shows the fastest electron transfer rate, which may be ascribed to the 3D highly conductive pathways in the CNT@mC composite sponge. These results therefore demonstrate that CNT@mC composite sponge possesses larger electroactive surface area and faster electron transfer rate than CNT and G powders. 3.2.2. Methanol Oxidation on GCE/CNT@mC/Pd. Figure 3a presents the CVs of GCE/CNT@mC/Pd, GCE/G/Pd, GCE/ CNT/Pd and GCE/Pd in 0.1 M NaOH containing 0.1 M methanol at a scan rate of 100 mV s−1. The methanol oxidation

Figure 3. (a) Cyclic voltammograms of 0.1 M methanol on GCE/ CNT@mC/Pd, GCE/G/Pd, GCE/CNT/Pd, GCE/Pd, and bare GCE. The supporting electrolyte was 0.1 M NaOH and the scan rate was 100 mV s−1. (b) Cyclic voltammograms of 0.1 M methanol on GCE/CNT@mC/Pd at different scan rates. (c) The corresponding dependence of the peak current at GCE/CNT@mC/Pd on the square root of the potential scan rate (v1/2). (d) Chronoamperometric curves of GCE/CNT@mC/Pd, GCE/G/Pd, GCE/CNT/Pd, and GCE/Pd in 0.1 M NaOH solution containing 0.1 M methanol.

Ip = (2.69 × 105)n3/2A appD1/2Cv1/2

(1)

where n is the number of electrons transferred (n = 1), D the diffusion coefficient of [Fe(CN)6]3−/4− (D = 6.70 × 10−6 cm2 s−1), C the concentration of [Fe(CN)6]3−/4− (mol mL−1), and ν the scan rate (V s−1). The Aapp values are calculated to be 0.15, 0.11, 0.10, and 0.09 cm2 for the GCE/CNT@mC/Pt, D

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Figure 5. (a) Cyclic voltammograms of 1.0 mM BPA on GCE/CNT@ mC/Pd, GCE/G/Pd, GCE/CNT/Pd, GCE/Pd, and bare GCE in PBS solution at a scan rate of 100 mV s−1. (b) Different pulse voltammograms of BPA with different concentrations on GCE/ CNT@mC/Pd in PBS solution at a scan rate of 100 mV s−1. (c) Linear relationship between peak currents and concentrations of BPA. (d) Chronoamperometric curves of GCE/CNT@mC/Pt, GCE/G/Pt, GCE/CNT/Pt, and GCE/Pt in PBS containing 0.5 mM BPA.

Figure 4. (a) SEM image of the CNT@mC/Pt composite catalyst. (b) HRTEM image of the Pt nanoparticles attached to the surface of CNT@mC. (c) Cyclic voltammograms and (d) Nyquist plots of 5.0 mM [Fe(CN)6]3−/4− on GCE/CNT@mC/Pd, GCE/G/Pd, GCE/ CNT/Pd, GCE/Pd, and bare GCE in 0.1 M KCl solution. The scan rate is 100 mV s−1. The Nyquist plots were recorded at open circuit potentials.

regression equation is determined to be Ip,a/μA = 5.94(c/mM) − 0.07 with a coefficient of R = 0.998, and the detection limit is calculated to be 3.3 nM (S/N = 3) base on the above results. In addition, chronoamperometry was also employed to evaluate the stability of GCE/CNT@mC/Pt. The typical current density−time responses of the above catalysts in the detection of BPA is shown in Figure 5d and the stability of GCE/CNT@ mC/Pt is evidently higher than that CNTs and G powders based catalysts. Furthermore, the electrochemical behavior of the mixture of BPA and other EDCs on GCE/CNT@mC/Pt were also investigated (Figure S17). The oxidization peaks of BPA and other tested EDCs occur at different potentials, suggesting the promise of the CNT@mC composite sponge based catalyst for simultaneous detection of multiple EDCs. 3.3.3. Recovery of BPA in Environmental Water Samples. The GCE/CNT@mC/Pt was used to detect BPA in environmental water samples to evaluate its potential practical application. Water samples from the East Lake in Wuhan City were collected and no BPA was detected in the samples. Different amounts of BPA were further spiked into the above environmental water and the resultant samples were analyzed with the GCE/CNT@mC/Pt by differential pulse voltammetry (DPV). As shown in Table 1, the values of recovery range from

GCE/G/Pt, GCE/CNT/Pt, and GCE/Pt, respectively. It is obvious that GCE/CNT@mC/Pt possesses the largest apparent electroactive surface area, which can be ascribed to the large surface area provided by the hierarchical porous structure of the CNT@mC composite sponge. The Nyquist plots of faradic impedance spectra (Figure 4d) further suggests that GCE/CNT@mC/Pt shows much lower electron transfer resistance than CNT and G powders. 3.3.2. GCE/CNT@mC/Pt for Amperometric Sensing of EDCs. To verify the feasibility of the GCE/CNT@mC/Pt for electrochemical sensing, we constructed a detection system toward the EDCs. EDCs are chemical compounds that possess similar functions of hormones in the endocrine system and are capable of disrupting the normal physiological function of endogenous hormones.55 In this study, BPA is chosen as the model analyte because of its pronounced perniciousness and widespread usage in plastic products in the past few years.56 The electrochemical detection of BPA was performed by cyclic voltammetry in 1.0 mM BPA and the results are shown in Figure 5a. Compared to the GCE/G/Pt, GCE/CNT/Pt, and GCE/Pt, an enhancement in the oxidization peak current of BPA is observed at GCE/CNT@mC/Pt, suggesting the much larger effective surface area of GCE/CNT@mC/Pt due to the hierarchical porous structure of CNT@mC composite sponge. Furthermore, the detection of BPA with different concentrations at GCE/CNT@mC/Pt was performed under the optimized pH value (Figure S15) and amounts of Pt loading (Figure S16). As shown in Figure 5b, a well-defined anodic peak is observed at about 0.5 V (vs SCE), and the peak current increases with the increase of BPA concentration. Moreover, the peak current (Ip,a) has good linear relationship with the concentration (c) of BPA in a wide range of 10 to 1 × 106 nM (Figure 5c), suggesting its great flexibility for detection of BPA with concentrations ranging from nM to mM. The linear

Table 1. Recovery of BPA in Environmental Water Samples

E

spiked (μM)

found (μM)

recovery (%)

0.05 1.00 2.00 5.00 10.00 20.00 50.00 100.00

0.47 0.98 2.04 5.26 10.58 21.02 53.64 108.23

94 98 102 105 106 105 107 108

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nm. The CVs (Figure 6d) and Nyquist plots (Figure 6e) indicate that GCE/CNT@mC/Au possesses the larger electroactive surface area and lower electron transfer resistance than GCE/G/Au and GCE/CNT/Au. 3.4.2. Impedimetric Sensing of BPA. The impedimetric response of GCE/CNT@mC/Au-Apt toward BPA were investigated and the results are shown in Figure 7a, b. For

94.0% to 108%, suggesting that the CNT@mC composite sponge based electrochemical sensor is effective and reliable for BPA detection in environmental water samples. 3.4. CNT@mC/Au for the Construction of Impedimetric Sensors. 3.4.1. Construction of Impedimetric Sensor. In addition to amperometric sensors, the CNT@mC composite sponge can also be utilize for the preparation of impedimetric sensors in consideration of the fast electron transfer rate on its 3D highly conductive pathways and large surface area. The schematic illustration of the preparation and mechanism of the impedimetric sensor is shown in Figure 6a. The sensor is

Figure 7. (a) Nyquist plots obtained at GCE/CNT@mC/Au-Apt ((A) without BPA, (A′) with BPA), GCE/G/Au-Apt ((B) without BPA, (B′) with BPA), GCE/CNT/Au-Apt ((C) without BPA, (C′) with BPA) and GCE/Au-Apt ((D) without BPA, (D′) with BPA). (b) Corresponding changes of electron transfer resistance ((R0 − R)/R0) on the above tested electrodes. (c) Nyquist plots obtained at GCE/ CNT@mC/Au-Apt in the presence of BPA with different concentrations. (d) Dependence of changes of electron transfer resistance ((R0 − R)/R0) on the concentration of BPA. Note: R0 and R are the electron transfer resistance of GCE/CNT@mC/Au-Apt in the absence and presence of BPA, respectively. Tris-HCl buffer (10 mM, pH 7.4) containing 2.0 mM [Fe(CN)6]3−/4−and 0.1 M KCl was utilized throughout the impedimetric sensing of BPA.

comparison, BPA binding aptamer functionalized GCE/G/Au (designated as GCE/G/Au-Apt), GCE/CNT/Au (designated as GCE/CNT/Au-Apt) and GCE/Au (designated as GCE/AuApt)were also examined. The GCE/CNT@mC/Au-Apt displays the largest decrease of the electron transfer resistance in the presence of BPA, which may be ascribed to the superior conductivity of CNT@mC composite sponge. The quantitative detection of BPA with GCE/CNT@mC/Au-Apt was conducted under the optimized amounts of Au loading (Figure S19) and interaction time (Figure S20). Figure 7c shows the Nyquist plots of faradic impedance spectra of GCE/CNT@ mC/Au-Apt in the presence of BPA with different concentrations. The diameter of the semicircle in the Nyquist plots decreases gradually with the increase of BPA concentration, suggesting the efficient recognition of BPA by the aptamers at the surface of the electrode.60−62 Figure 7d shows the dependence of the changes of electron transfer resistance on the concentration of BPA. It is apparent that the changes of electron transfer resistance has a fine linear relationship with the concentrations of BPA from 0 to 7 nM. Furthermore, the detection limit for BPA is calculated to be 0.33 nM (S/N = 3) and such high sensitivity may be attributed to the highly

Figure 6. (a) Schematic representation of the detection of BPA with the impedimetric sensor. (b) SEM images of the CNT@mC/Au architecture. (c) HRTEM image of the Au nanoparticles attached to the surface of CNT@mC. (d) Cyclic voltammograms and (e) Nyquist plots of 2.0 mM [Fe(CN)6]3−/4− on GCE/CNT@mC/Au, GCE/G/ Au, GCE/CNT/Au, GCE/Au, and bare glass carbon electrode (GCE) in 0.1 M KCl solution. The scan rate is 100 mV s−1. The Nyquist plots were recorded at open circuit potentials.

constructed by self-assembling aptamers (designated as Apt) on the surface of GCE/CNT@mC/Au. The subsequent introduction of analyte (BPA was still utilized as the model analyte) induces the formation of binding pockets of BPA binding aptamer, which decreases the electron transfer resistance of the impedimetric sensor.57−59 The CNT@mC/Au was first prepared and characterized with SEM before coating on the surface of glass carbon electrode. SEM and HRTEM images (Figure 6b, c) show that Au nanoparticles are well dispersed on the surface of CNT@mC with an average diameter of about 10 F

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ACS Applied Materials & Interfaces conductive networks and large accessible surface areas for electrons transfer in the CNT@mC composite sponge. In addition, to assess the selectivity of GCE/CNT@mC/AuApt, the response of the electrode to the analogue of BPA (diethylstilbestrol, m-dihydroxybenzene, hydroquinone, and catechol) were checked. As shown in Figure 8, the changes

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Additional information, including TEM characterization, SEM characterization, X-ray photoelectron spectroscopy characterization, cyclic voltammograms, detailed chemicals and instruments (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions §

D.W. and J.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21275113, 21201133, 51272186, and 21422105) and the Fundamental Research Funds for the Central Universities (2014203020205).

Figure 8. (a) Nyquist plots obtained at GCE/CNT@mC/Au-Apt in Tris-HCl buffer (control), diethystilbestrol solution (A), m-dihydroxybenzene solution (B), hydroquinone solution (C), catechol solution (D), and BPA solution (E). (b) Corresponding changes of electron transfer resistance obtained at the GCE/CNT@mC/Au-Apt in the presence of BPA and its analogues. Note: R0 and R are the electron transfer resistance of GCE/CNT@mC/Au-Apt in the absence and presence of BPA, respectively.

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of electron transfer resistance for BPA is much higher than that of the tested analogues. The good selectivity of GCE/CNT@ mC/Au-Apt can be ascribed to the fact that aptamers folds into unique binding pockets to accommodate the molecular structures of their targets.63−65

4. CONCLUSIONS In this work, we have highlighted the superior electrochemical properties of the 3D interconnected CNT@mC composite sponge with hierarchical porous structure. The seamless feature of the composite sponge efficiently eliminates the contact resistance and provided highly conductive pathways for electron transfer. The abundant hierarchical pores in the composite sponge can facilitate mass transport and offered large accessible surface area for reactants and electrons. For serving as the anode catalyst for DMFCs, the GCE/CNT@mC/Pd shows enhanced menthol oxidization currents and good electrochemical stability. In the amperometric sensing of EDCs, the CNT@mC/Pt displayed large apparent electroactive surface area and the linear range for BPA detection ranges from 10 nM to 1 mM. More importantly, the oxidization peaks of BPA and other EDCs on GCE/CNT@mC/Pt modified electrode occur at different potentials, which indicates the promise of GCE/CNT@mC/Pt for simultaneous detection of multiple EDCs. Furthermore, an impedimetric sensor was constructed based on the CNT@mC composite sponge. The impedimetric sensor shows good selectivity toward BPA and the detection limit is measured to be 0.33 nM. The outstanding electrochemical performance of the CNT@mC composite sponge has been clearly demonstrates in this study, and this study further suggests the promise of our design to be used for creating other functional materials for renewable energy technologies and environmental monitoring system.

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REFERENCES

(1) Mao, X.; Rutledge, G. C.; Hatton, T. A. Nanocarbon-based Electrochemical Systems for Sensing, Electrocatalysis, and Energy Storage. Nano Today 2014, 9, 405−432. (2) Zhai, Y.; Dou, Y.; Zhao, D.; Fulvio, P. F.; Mayes, R. T.; Dai, S. Carbon Materials for Chemical Capacitive Energy Storage. Adv. Mater. 2011, 23, 4828−4850. (3) Huang, X.; Qi, X.; Boey, F.; Zhang, H. Graphene-based Composites. Chem. Soc. Rev. 2012, 41, 666−686. (4) Sun, X.; Sun, H.; Li, H.; Peng, H. Developing Polymer Composite Materials: Carbon Nanotubes or Graphene? Adv. Mater. 2013, 25, 5153−5176. (5) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (6) Sun, G.; Liu, J.; Zhang, X.; Wang, X.; Li, H.; Yu, Y.; Huang, W.; Zhang, H.; Chen, P. Fabrication of Ultralong Hybrid Microfibers from Nanosheets of Reduced Graphene Oxide and Transition-Metal Dichalcogenides and Their Application as Supercapacitors. Angew. Chem., Int. Ed. 2014, 53, 12576−12580. (7) Li, Y.; Zhou, W.; Wang, H.; Xie, L.; Liang, Y.; Wei, F.; Idrobo, J. C.; Pennycook, S. J.; Dai, H. An Oxygen Reduction Electrocatalyst based on Carbon Nanotube-Graphene Complexes. Nat. Nanotechnol. 2012, 7, 394−400. (8) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444−452. (9) Ai, W.; Jiang, J.; Zhu, J.; Fan, Z.; Wang, Y.; Zhang, H.; Huang, W.; Yu, T. Supramolecular Polymerization Promoted In Situ Fabrication of Nitrogen-Doped Porous Graphene Sheets as Anode Materials for Li-Ion Batteries. Adv. Energy Mater. 2015, 5, DOI: 10.1002/aenm.201500559. (10) Ai, W.; Luo, Z.; Jiang, J.; Zhu, J.; Du, Z.; Fan, Z.; Xie, L.; Zhang, H.; Huang, W.; Yu, T. Nitrogen and Sulfur Codoped Graphene: Multifunctional Electrode Materials for High-Performance Li-Ion Batteries and Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 6186−6192. (11) Kucinskis, G.; Bajars, G.; Kleperis, J. Graphene in Lithium Ion Battery Cathode Materials: A review. J. Power Sources 2013, 240, 66− 79. (12) Liu, X.; Antonietti, M. Moderating Black Powder Chemistry for the Synthesis of Doped and Highly Porous Graphene Nanoplatelets and Their Use in Electrocatalysis. Adv. Mater. 2013, 25, 6284−6290. (13) Mackiewicz, N.; Surendran, G.; Remita, H.; Keita, B.; Zhang, G.; Nadjo, L.; Hagege, A.; Doris, E.; Mioskowski, C. Supramolecular SelfAssembly of Amphiphiles on Carbon Nanotubes: A Versatile Strategy

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DOI: 10.1021/acsami.5b08294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces for the Construction of CNT/Metal Nanohybrids, Application to Electrocatalysis. J. Am. Chem. Soc. 2008, 130, 8110−8111. (14) Agui, L.; Yanez-Sedeno, P.; Pingarron, J. M. Role of Carbon Nanotubes in Electroanalytical Chemistry: A Review. Anal. Chim. Acta 2008, 622, 11−47. (15) Yang, W.; Ratinac, K. R.; Ringer, S. P.; Thordarson, P.; Gooding, J. J.; Braet, F. Carbon Nanomaterials in Biosensors: Should You Use Nanotubes or Graphene? Angew. Chem., Int. Ed. 2010, 49, 2114−2138. (16) Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D. W.; Baek, J. B.; Dai, L. BCN Graphene as Efficient Metal-Free Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem., Int. Ed. 2012, 51, 4209− 4212. (17) Yin, H.; Tang, H.; Wang, D.; Gao, Y.; Tang, Z. Facile Synthesis of Surfactant-Free Au Cluster/Graphene Hybrids for High-Performance Oxygen Reduction Reaction. ACS Nano 2012, 6, 8288−8297. (18) Zhang, L. S.; Jiang, L.-Y.; Yan, H. J.; Wang, W. D.; Wang, W.; Song, W. G.; Guo, Y. G.; Wan, L. J. Mono Dispersed SnO2 Nanoparticles on Both Sides of Single Layer Graphene Sheets as Anode Materials in Li-Ion Batteries. J. Mater. Chem. 2010, 20, 5462− 5467. (19) Zheng, S. F.; Hu, J. S.; Zhong, L. S.; Wan, L. J.; Song, W. G. In Situ One-Step Method for Preparing Carbon Nanotubes and Pt Composite Catalysts and Their Performance for Methanol Oxidation. J. Phys. Chem. C 2007, 111, 11174−11179. (20) Baptista, F. R.; Belhout, S. A.; Giordani, S.; Quinn, S. J. Recent Developments in Carbon Nanomaterial Sensors. Chem. Soc. Rev. 2015, 44, 4433−4453. (21) Liu, Y.; Dong, X.; Chen, P. Biological and Chemical Sensors Based on Graphene Materials. Chem. Soc. Rev. 2012, 41, 2283−2307. (22) Wan, Q.; Cai, H.; Liu, Y.; Song, H.; Liao, H.; Liu, S.; Yang, N. Graphene Nanoplatelets: Electrochemical Properties and Applications for Oxidation of Endocrine-Disrupting Chemicals. Chem. - Eur. J. 2013, 19, 3483−3489. (23) Wu, S.; He, Q.; Tan, C.; Wang, Y.; Zhang, H. Graphene-Based Electrochemical Sensors. Small 2013, 9, 1160−1172. (24) Buldum, A.; Lu, J. P. Contact Resistance between Carbon Nanotubes. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 161403−161406. (25) Franklin, A. D.; Chen, Z. Length Scaling of Carbon Nanotube Transistors. Nat. Nanotechnol. 2010, 5, 858−862. (26) Franklin, A. D.; Farmer, D. B.; Haensch, W. Defining and Overcoming the Contact Resistance Challenge in Scaled Carbon Nanotube Transistors. ACS Nano 2014, 8, 7333−7339. (27) Chmiola, J.; Yushin, G.; Gogotsi, Y.; Portet, C.; Simon, P.; Taberna, P. L. Anomalous Increase in Carbon Capacitance at Pore Sizes Less than 1 Nanometer. Science 2006, 313, 1760−1763. (28) Wang, D. W.; Li, F.; Liu, M.; Lu, G. Q.; Cheng, H. M. 3D Aperiodic Hierarchical Porous Graphitic Carbon Material for HighRate Electrochemical Capacitive Energy Storage. Angew. Chem. 2008, 120, 379−382. (29) Xing, W.; Qiao, S.; Ding, R.; Li, F.; Lu, G.; Yan, Z.; Cheng, H. Superior Electric Double Layer Capacitors Using Ordered Mesoporous Carbons. Carbon 2006, 44, 216−224. (30) Cao, X.; Yin, Z.; Zhang, H. Three-Dimensional Graphene Materials: Preparation, Structures and Application in Supercapacitors. Energy Environ. Sci. 2014, 7, 1850−1865. (31) Chao, D.; Xia, X.; Liu, J.; Fan, Z.; Ng, C. F.; Lin, J.; Zhang, H.; Shen, Z. X.; Fan, H. J. A V2O5/Conductive-Polymer Core/Shell Nanobelt Array on Three-Dimensional Graphite Foam: A High-Rate, Ultrastable, and Freestanding Cathode for Lithium-Ion Batteries. Adv. Mater. 2014, 26, 5794−5800. (32) Komori, K.; Terse-Thakoor, T.; Mulchandani, A. Bioelectrochemistry of Heme Peptide at Seamless Three-Dimensional Carbon Nanotubes/Graphene Hybrid Films for Highly Sensitive Electrochemical Biosensing. ACS Appl. Mater. Interfaces 2015, 7, 3647−3654. (33) Zhu, Y.; Li, L.; Zhang, C.; Casillas, G.; Sun, Z.; Yan, Z.; Ruan, G.; Peng, Z.; Raji, A. R.; Kittrell, C.; Hauge, R. H.; Tour, J. M. A

Seamless Three-Dimensional Carbon Nanotube Graphene Hybrid Material. Nat. Commun. 2012, 3, 1225−1231. (34) Wang, X.; Zhang, Y.; Zhi, C.; Wang, X.; Tang, D.; Xu, Y.; Weng, Q.; Jiang, X.; Mitome, M.; Golberg, D.; Bando, Y. Three-Dimensional Strutted Graphene Grown by Substrate-Free Sugar Blowing for HighPower-Density Supercapacitors. Nat. Commun. 2013, 4, 2905−2912. (35) Gui, X.; Wei, J.; Wang, K.; Cao, A.; Zhu, H.; Jia, Y.; Shu, Q.; Wu, D. Carbon Nanotube Sponges. Adv. Mater. 2010, 22, 617−621. (36) Chen, W.; Rakhi, R. B.; Hu, L.; Xie, X.; Cui, Y.; Alshareef, H. N. High-Performance Nanostructured Supercapacitors on A Sponge. Nano Lett. 2011, 11, 5165−5172. (37) Li, P.; Yang, Y.; Shi, E.; Shen, Q.; Shang, Y.; Wu, S.; Wei, J.; Wang, K.; Zhu, H.; Yuan, Q.; Cao, A.; Wu, D. Core-Double-Shell, Carbon Nanotube@Polypyrrole@MnO(2) Sponge as Freestanding, Compressible Supercapacitor Electrode. ACS Appl. Mater. Interfaces 2014, 6, 5228−5234. (38) Xie, X.; Ye, M.; Hu, L.; Liu, N.; McDonough, J. R.; Chen, W.; Alshareef, H. N.; Criddle, C. S.; Cui, Y. Carbon Nanotube-Coated Macroporous Sponge for Microbial Fuel Cell Electrodes. Energy Environ. Sci. 2012, 5, 5265−5270. (39) Gui, X.; Cao, A.; Wei, J.; Li, H.; Jia, Y.; Li, Z.; Fan, L.; Wang, K.; Zhu, H.; Wu, D. Soft, Highly Conductive Nanotube Sponges and Composites with Controlled Compressibility. ACS Nano 2010, 4, 2320−2326. (40) Deng, Y.; Wei, J.; Sun, Z.; Zhao, D. Large-Pore Ordered Mesoporous Materials Templated from Non-Pluronic Amphiphilic Block Copolymers. Chem. Soc. Rev. 2013, 42, 4054−4070. (41) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 2821−2860. (42) Gu, D.; Bongard, H.; Meng, Y.; Miyasaka, K.; Terasaki, O.; Zhang, F.; Deng, Y.; Wu, Z.; Feng, D.; Fang, Y.; et al. Growth of Single-Crystal Mesoporous Carbons with Im3̅ m Symmetry. Chem. Mater. 2010, 22, 4828−4833. (43) Linares, N.; Silvestre-Albero, A. M.; Serrano, E.; SilvestreAlbero, J.; Garcia-Martinez, J. Mesoporous Materials for Clean Energy Technologies. Chem. Soc. Rev. 2014, 43, 7681−7717. (44) Wei, J.; Zhou, D.; Sun, Z.; Deng, Y.; Xia, Y.; Zhao, D. A Controllable Synthesis of Rich Nitrogen-Doped Ordered Mesoporous Carbon for CO2 Capture and Supercapacitors. Adv. Funct. Mater. 2013, 23, 2322−2328. (45) Fu, R. W.; Li, Z. H.; Liang, Y. R.; Feng, L.; Fei, X.; Wu, D. C. Hierarchical Porous Carbons: Design, Preparation, and Performance in Energy Storage. New Carbon Mater. 2011, 26, 171−179. (46) Benvidi, A.; Dehghani-Firouzabadi, A.; Mazloum-Ardakani, M.; Mirjalili, B. B. F.; Zare, R. Electrochemical Deposition of Gold Nanoparticles on Reduced Graphene Oxide Modified Glassy Carbon Electrode for Simultaneous Determination of Levodopa, Uric Acid and Folic Acid. J. Electroanal. Chem. 2015, 736, 22−29. (47) Aricò, A. S.; Srinivasan, S.; Antonucci, V. DMFCs: From Fundamental Aspects to Technology Development. Fuel Cells 2001, 1, 133−161. (48) Tiwari, J. N.; Tiwari, R. N.; Singh, G.; Kim, K. S. Recent Progress in the Development of Anode and Cathode Catalysts for Direct Methanol Fuel Cells. Nano Energy 2013, 2, 553−578. (49) Kakati, N.; Maiti, J.; Lee, S. H.; Jee, S. H.; Viswanathan, B.; Yoon, Y. S. Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We Have Any Alternative for Pt or Pt-Ru? Chem. Rev. 2014, 114, 12397−12429. (50) Wang, Y.; Zhang, D.; Tang, M.; Xu, S.; Li, M. Electrocatalysis of Gold Nanoparticles/Layered Double Hydroxides Nanocomposites toward Methanol Electro-Oxidation in Alkaline Medium. Electrochim. Acta 2010, 55, 4045−4049. (51) Shao, Y.; Wang, J.; Wu, H.; Liu, J.; Aksay, I. A.; Lin, Y. Graphene Based Electrochemical Sensors and Biosensors: A Review. Electroanalysis 2010, 22, 1027−1036. (52) Bakker, E. Electrochemical Sensors. Anal. Chem. 2004, 76, 3285−3298. H

DOI: 10.1021/acsami.5b08294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Forum Article

ACS Applied Materials & Interfaces (53) Luo, X.; Morrin, A.; Killard, A. J.; Smyth, M. R. Application of Nanoparticles in Electrochemical Sensors and Biosensors. Electroanalysis 2006, 18, 319−326. (54) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed; Wiley: New York, 1980. (55) Colborn, T.; vom Saal, F. S.; Soto, A. M. Developmental Effects of Endocrine-Disrupting Chemicals in Wildlife and Humans. Environmental Impact Assessment Review 1994, 14, 469−489. (56) Food and Agriculture Organization of the United Nations. Bisphenol A (BPA)Current State of Knowledge and Future Actions by WHO and FAO; World Health Organization: Geneva, 2009. (57) Famulok, M.; Mayer, G. Aptamer Modules as Sensors and Detectors. Acc. Chem. Res. 2011, 44, 1349−1358. (58) Ping, J.; Zhou, Y.; Wu, Y.; Papper, V.; Boujday, S.; Marks, R. S.; Steele, T. W. Recent Advances in Aptasensors Based on Graphene and Graphene-Like Nanomaterials. Biosens. Bioelectron. 2015, 64, 373−385. (59) Peng, Y.; Zhang, D.; Li, Y.; Qi, H.; Gao, Q.; Zhang, C. LabelFree and Sensitive Faradic Impedance Aptasensor for the Determination of Lysozyme Based on Target-Induced Aptamer Displacement. Biosens. Bioelectron. 2009, 25, 94−99. (60) Fang, X.; Tan, W. Aptamers Generated from Cell-SELEX for Molecular Medicine: a Chemical Biology Approach. Acc. Chem. Res. 2009, 43, 48−57. (61) Li, D.; Song, S.; Fan, C. Target-Responsive Structural Switching for Nucleic Acid-Based Sensors. Acc. Chem. Res. 2010, 43, 631−641. (62) Jiang, Y.; Liu, N.; Guo, W.; Xia, F.; Jiang, L. Highly-Efficient Gating of Solid-State Nanochannels by DNA Supersandwich Structure Containing ATP Aptamers: A Nanofluidic IMPLICATION Logic Device. J. Am. Chem. Soc. 2012, 134, 15395−15401. (63) Liang, H.; Zhang, X.-B.; Lv, Y.; Gong, L.; Wang, R.; Zhu, X.; Yang, R.; Tan, W. Functional DNA-Containing Nanomaterials: Cellular Applications in Biosensing, Imaging, and Targeted Therapy. Acc. Chem. Res. 2014, 47, 1891−1901. (64) Song, S.; Wang, L.; Li, J.; Fan, C.; Zhao, J. Aptamer-Based Biosensors. TrAC, Trends Anal. Chem. 2008, 27, 108−117. (65) Tan, W.; Donovan, M. J.; Jiang, J. Aptamers from Cell-Based Selection for Bioanalytical Applications. Chem. Rev. 2013, 113, 2842− 2862.

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DOI: 10.1021/acsami.5b08294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX