Graphene Nanoribbon-Supported PtPd Concave Nanocubes for

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Graphene Nanoribbon-Supported PtPd Concave Nanocubes for Electrochemical Detection of TNT with High Sensitivity and Selectivity Ruizhong Zhang, Chia-Liang Sun, Yu-Jen Lu, and Wei Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03390 • Publication Date (Web): 15 Nov 2015 Downloaded from http://pubs.acs.org on November 21, 2015

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Analytical Chemistry

Graphene Nanoribbons-Supported PtPd Concave Nanocubes for Electrochemical Detection of TNT with High Sensitivity and Selectivity Ruizhong Zhang, †, ‡ Chia-Liang Sun, §, # Yu-Jen Lu, ⊥ and Wei Chen*† †

State Key Laboratory of Electroanalytical Chemistry, Changchun institute of Applied

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

University of Chinese Academy of Sciences, Beijing 100039, China

§

Department of Chemical and Materials Engineering, Chang Gung University, Guishan,

Taoyuan 333, Taiwan, ROC #

Biosensor Group, Biomedical Engineering Research Center, Chang Gung University,

Guishan, Taoyuan 333, Taiwan, ROC ⊥

Department of Neurosurgery, Chang Gung Memorial Hospital, No. 5, Fu-Shing Rd.,

Guishan, Taoyuan 333, Taiwan, ROC Corresponding author E-mail: [email protected]

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ABSTRACT In this work, PtPd concave nanocubes (PtPd-rGONRs) anchored on graphene nanoribbons were successfully fabricated through a hydrothermal process. The structural characterizations confirmed that PtPd concave cubes with an average size of around 11 nm have been successfully synthesized and they are uniformly assembled on the surface of rGONRs. The electrochemical measurements demonstrated that the PtPd-rGONRs composite-modified glassy carbon electrode (GCE) shows much enhanced current signals for TNT reduction, which is 4 and 12-folds higher than rGONRs and bare glassy carbon electrode, respectively. The PtPd-rGONRs exhibited a wide linear range for TNT detection from 0.01 to 3 ppm with the sensing limit of 0.8 ppb. Moreover, the PtPd-rGONRs showed excellent detection stability for the determination of TNT. Most importantly, the PtPd-rGONRs-based electrochemical detection platform can be successfully applied to TNT detection in tap water and real lake water samples. The present study indicates that graphene nanoribbon-supported nanocrystals are promising in designing high performance electrochemical sensors for explosives detection.

Keywords: Graphene nanoribbon, PtPd, concave nanocube, TNT, electrochemical sensor, electrocatalyst

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Nitroaromatic compounds, including trinitrotoluene (TNT) and their derivatives, have been widely used in the military and aviation industries, forensic investigations and the preparation of matches and fireworks. However, in addition to the well-known security issues, the wide use of these explosives could also result in harmful effects on our environment and health as toxic and mutagenic substances.1-3 It is therefore imperative to develop novel sensing materials and new methods for rapidly and efficiently detecting ultratrace amounts of nitroaromatic compounds. A variety of methods have been developed and applied to analysis of nitroaromatic compounds, such as chromatography,4 spectrophotometry,5,6 Raman spectrometry,7 immunoassay,8 electrogenerated chemiluminescence9 and electrochemical techniques.10-13 Among these techniques, electrochemical methods have received considerable interests due to their inherent selectivity, low limit of detection, and their potential ability for constructing inexpensive and portable detectors.10,14-16 Selectively electrochemical sensing of nitroaromatic explosives can be achieved based on the redox activities of the nitro groups associated with different explosive materials. It has been found that the electrochemical reduction of nitroaromatic compounds undergoes the successive reduction of a nitro group to a hydroxylamine followed by the further reduction to an amine group.17 However, a bare electrode shows low electrochemical activity for the reduction of nitroaromatic compounds, which leads to the low detection selectivity, sensitivity and stability. In recent years, by taking advantage of the unique properties of nanomaterials, the detection performances of electrochemical sensors have been largely promoted. To date, a lot of novel nanostructured materials have been successfully applied to the construction of electrochemical sensing platforms for the sensitive detection of nitroaromatic compounds. Typical examples are Ag nanoparticles-functionalized graphene,13 amino-functionalized mesoporous silica microspheres,16 Pt nanoparticles ensemble-on-graphene hybrid18 and TiO2/metal nanoparticles,19 etc. All

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these related studies indicated that sensing platforms fabricated from nanomaterials can significantly promote the detection performance for nitroaromatic compounds. Thus, it is highly desirable to design new nanomaterials to further push forward the development of electrochemical sensors for sensitively and selectively detecting ultratrace nitroaromatic compounds. Since discovered by Novoselov and Geim in 2004,20 graphene with single-atom thickness has attracted increasing attention in electrochemistry, such as fuel cells,21 chemical sensors,22 lithium-ion batteries23 and capacitors,24 etc. due to its outstanding physicochemical properties and large surface area (~2630 m2/g). Meanwhile, graphene has also been introduced into nitroaromatic compounds sensors as catalytic component. For example, Wang’s group reported that the electrode modified with ionic liquid-functionalized graphene hybrid can significantly enhance the sensitivity for TNT detection.3 Chen et al. constructed a nitrogen-doped graphene-based sensing platform for sensitively electrochemical detection of TNT with a

low detection limit.25

In another study, Lu et al. reported that

Ag

nanoparticles-functionalized graphene can be used as an enhancing material for nitroaromatic compounds detection.13 Meanwhile, Guo et al. also fabricated a TNT sensor with Pt nanoparticles-assembled graphene and a remarkable increase was achieved in electrochemical sensing performance.18 These studies show that heteroatom doping or noble metal nanoparticles modification for graphene is a kind of promising approach for high-performance detection of trace nitroaromatic compounds. However, to take full advantage of the unique properties of graphene and to exploit the related applications, reliable and cost-efficient techniques are critical for preparing high-yield graphene with high quality. In recent years, graphene nanoribbons (GNRs) have attracted much attention in nanodevices.26,27 In 1996, Nakada et al. predicted theoretically the existence of GNRs for the first time.28 Although GNRs have been prepared by various methods,29-34 there still is a lack of exploration for its wide applications. Furthermore, because of the large surface area, highly

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electrical conductivity and high electrochemical stability in acidic and alkaline electrolytes, graphene nanoribbons have been widely used as a good support for nanoparticles dispersing. Based on the advantages of graphene discussed above, in this study, a new heterostructure of PtPd concave nanocubes anchored on graphene nanoribbons (PtPd-rGONRs) was fabricated by one-pot hydrothermal synthetic method and used as a kind of sensing material for fabrication of electrochemical TNT sensor. To the best of our knowledge, there is still no report previously on the preparation of such types of hybrid nanostructures and their application for the electrochemical detection of TNT. The as-prepared PtPd-rGONRs composite shows large surface area, good conductivity and highly electrocatalytic activity for TNT reduction. The electrochemical sensing system based on PtPd-rGONRs exhibited a sensitive response to TNT with a wide linearity from 0.01 to 3 ppm and a detection limit of 0.8 ppb. The PtPd-rGONRs composite-based detection platform can also be used for the TNT detection in real water systems.

EXPERIMENTAL SECTION Chemicals. Poly (vinyl pyrrolidone) (PVP, Mw≈55000), glycine (C2H5NO2, AR, 99.5-100.5%), potassium hexachloroplatinate (IV) (K2PtCl6, 40%Pt), sodium tetrachloropalladate (II) (Na2PdCl4·3H2O, 99%) were all purchased from Sigma-Aldrich. Disodium hydrogen phosphate dodecahydrate (Na2HPO4·12H2O, A.R., ≥99.0%), sodium dihydrogen phosphate dehydrate (NaH2PO4·2H2O, A.R., ≥99.0%) were obtained from Beijing Chemical Works. Nitrobenzene (NB), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), 2, 4-dinitrotoluene (2, 4-DNT) and 2, 6-dinitrotoluene (2, 6-DNT) and 1, 3, 5-trinitrophenol were obtained from Alfa Aesar. 2, 4, 6-trinitrotoluene (TNT) was supplied by the Institute of Forensic Science, Ministry of Public Security (China), and its stock solution (3 mg/mL) was prepared in acetonitrile. From the initial TNT stock solution, two standard stock solutions (1 mg/mL and 0.1 mg/mL)

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were further prepared by diluting the 3 mg/mL stock solution with acetonitrile. To obtain the different concentrations of TNT solution for electrochemical analysis, a certain amount of the standard stock solutions was added into 10 mL of electrolyte (0.1 M PBS + 0.4 M KCL, pH = 6.5) by a pipettor. All aqueous solutions in this study were prepared by water supplied by a Nanopure water system (18.3 MΩ cm). All the chemicals were used as received without further purification. Synthesis of graphene nanoribbon-supported PtPd concave nanocubes (PtPd-rGONRs). Graphene oxide nanoribbons were prepared by using the method of facile unzipping of multiwalled carbon nanotubes under the help of microwave energy.35 Typically, graphene oxide nanoribbons were dispersed in DMF under sonication for 1 h. Sodium tetrachloropalladate (Na2PdCl4, 5.88 mg), potassium tetrachloroplatinate (K2PtCl6, 9.72 mg), glycine (58 mg) and poly (vinylpyrrolidone) (PVP, 200 mg) were then added to the DMF solution of graphene oxide nanoribbons (GONRs, 10 mL, 0.4 mg/mL). After the ultrasonication about 30 min, the solution was then transferred to a Teflon-lined stainless steel autoclave. The sealed vessel was then heated at 200 °C for 8 h before it was cooled to room temperature. The resulting black colloidal products were precipitated using acetone, separated by centrifugation, and further washed several times with an ethanol-acetone mixture. Finally, the obtained PtPd concave nanocubes supported on reduced graphene oxide nanoribbons (PtPd-rGONRs) were dispersed in water for further use. For comparison, the reduced graphene oxide nanoribbons (rGONRs) were also prepared through the similar process without the addition of metal precursors. Material characterization. The size and morphology of the as-prepared materials were examined by using a Hitachi H-600 transmission electron microscope (TEM) operated at 100 kV. The samples were prepared by dropping a water dispersion of materials onto carbon-coated copper TEM grids using pipettes and dried under ambient conditions. The powder X-ray diffraction (XRD) analysis was performed on a D8

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ADVANCE (Germany) using Cu Kα radiation with a Ni filter (λ=0.154059 nm at 30 kV and 15 mA) to examine the crystallinity of the products. High-resolution TEM (HRTEM) images and the corresponding live fast Fourier transform (FFT), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, and elemental mapping were all carried out on a JEM-2010(HR) microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a VG Thermo ESCALAB 250 spectrometer (VG Scientific, United Kingdom) with Al Kα X-ray radiation as the X-ray source for excitation operated at 120 W. The binding energies were calibrated against the carbon 1s line. Raman spectra were collected with a Renishaw 2000 equipped by an Ar+ ion laser giving the excitation line of 514.5 nm and an air-cooling charge-coupled device (CCD) as the detector (Reinshaw Co.,U.K.). UV-Vis spectroscopy was performed with a Cary spectrophotometer using 1 cm quartz cuvette with a resolution of 2 nm. Electrochemical measurements and detection of nitroaromatic compounds. The electrochemical characterizations were conducted in a conventional three-electrode system, by using a Pt wire as the counter electrode and the Ag/AgCl (saturated KCl) as reference electrode, respectively. Electrochemical measurements of linear adsorption stripping voltammetry (LASV) and current-time (i-t) were performed on a CHI 660D electrochemical workstation (Chenhua Instrument, Shanghai, China). 0.1 M phosphate buffer solution (PBS, pH = 6.5) containing 0.4 M KCl was used as electrolyte solution. A sample-coated glassy carbon electrode (3.0 mm in diameter) was used as working electrode. The GC electrode was first polished carefully with 0.3 and 0.05 µm alumina slurry on a polishing cloth to obtain a mirror finish, and followed by sonication in ultrapure water and ethanol successively. For preparing a working electrode, an electrocatalyst (PtPd-rGONRs or rGONRs) was dispersed in a mixed solution of water, isopropyl alcohol and Nafion solution (5 wt%, Dupont) (Vwater: Visopropyl alcohol: VNafion = 4: 1: 0.025) to form a catalyst ink (2

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mg/mL) with the aid of sonication. Next, 5 µL of the catalyst ink was dropped on the surface of glassy carbon electrode by using a pipette and then dried at ambient condition. Electrochemical impendence spectroscopy (EIS) measurements were performed to determine the interfacial charge-transfer resistance of different electrodes and conducted under an oscillation potential of 5 mV over the frequency range between 0.01 Hz and 1 MHz in the solution of 5 mM K4[Fe(CN)6]/K3[Fe(CN)6] containing 0.1 M KCl. In this work, two steps are included in the detection of nitroaromatic compounds. Firstly, nitroaromatic compounds are preconcentrated from solution to electrode surface under 0.0 V (vs Ag/AgCl) for 150 s. Secondly, electrochemical reduction of the accumulated compounds is measured by using linear sweep voltammetry (LSV) at 50 mV/s.

RESULTS AND DISCUSSION Preparation

and

structure

characterization

of

graphene

nanoribbons-supported

PtPd

(PtPd-rGONRs) concave nanocubes. In this work, a one-pot wet-chemical reduction strategy was employed to synthesize the PtPd-rGONRs in the presence of graphene oxide nanoribbons (GONRs) (see the Experimental section for details). The morphologies of GONRs and PtPd-rGONRs were studied by TEM. Figure 1A and Figure S1A-C show the TEM images of GONRs. Compared with the raw material of multiwalled carbon nanotubes (MWCNTs, ~11 µm in length), the obtained graphene oxide nanoribbons (GONRs) shows a short length of just about 2 µm. Meanwhile, graphene oxide nanoribbon structures can be found on both sides of carbon nanotubes (CNTs), whereas the central cores of nanotubes remained slightly dark and tube-like shape. These results indicate that core-shell heterostructures of MWCNT-GONR can be successfully produced through this microwave unzipping process. Recently, it has been reported that short CNTs have a lot of advantages than longer ones. For example, shorter CNTs have

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a lower electrical resistance and Warburg prefactor, resulting in better rate performance at high current densities for Li-ion batteries.36 Meanwhile, shorter CNTs have larger capacitance and higher surface area compared to longer CNTs in terms of double layer capacitance.37,38 On the other hand, shorter CNTs also showed much better electrochemical performance as catalyst supports in fuel cells.39 More recently, it has been demonstrated that short GONRs exhibited much improved electrochemical sensitivity in detection of analytes owing to the more generated edge structures at both ends.40 Thus we expect that the as-prepared short core-shell heterostructures of MWCNT-GONR can provide large surface area and plenty of active sites when applied for catalysts support. Figure 1 B and C show the TEM of the PtPd-rGONRs composite at different magnifications. One can see that the formed metal nanocrystals are uniformly dispersed on the rGONRs surface and no nanocrystal is dispersed outside the nanoribbons, suggesting that rGONRs serve as good templates for the formation of metal nanocrystals. Based on the size distribution histogram (Figure 1D), the as-synthesized PtPd nanocrystals have a mean size of 10.9 ± 0.2 nm. The formation of rGONRs and PtPd-rGONRs were also studied by UV-Vis spectroscopy. From the UV-Vis absorption spectra shown in Figure S1D, the absorption from π-π* transitions of aromatic C＝C bonds shows a red shift from ~230 nm for GO to ~270 nm for GONRs, and the n-π* transitions of C=O at 301 nm disappears for GONRs, indicating that the as-prepared GONRs have fewer oxygen-containing functional groups than GO. Moreover, after the hydrothermal reaction, the absorption peak at ~270 nm further shifts to ~350 nm, suggesting the formation of reduced graphene oxide nanoribbons (rGONRs). From the XRD spectrum of the PtPd-rGONRs composite (Figure 2A), the sharp and narrow peak at 2θ ≈ 26° is ascribed to the (002) of stacked rGONRs.41,42 This result indicates that GONRs has been reduced to rGONRs during the hydrothermal process in which glycine acts as reducing agent. The peaks at 40.0, 46.6 and 68.1°can be assigned to the (111), (200) and (220) of fcc structured PtPd nanocrystals. Meanwhile, it

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can be seen that the diffraction peaks from the PtPd-rGONRs composite locate between those of the pure Pt (red bars) and Pd (blue bars), suggesting the production of PtPd alloys.43,44 Based on Scherrer’s equation, the diameter of the PtPd nanocrystals is about 10.5 nm calculated from the broadening (200) diffraction peak. Such result agrees well with that from TEM characterization. From above XRD measurements, the PtPd nanocrystals and rGONRs formed simultaneously in the hydrothermal reaction. The composition of the PtPd-rGONRs and chemical states of Pt, Pd, and carbon were further investigated by XPS. The deconvoluted C1s spectrum of PtPd-rGONRs and GONRs are shown in Figure 2B and C, respectively. The three fitted peaks can be ascribed to the carbons in C=C/C-C, C-O (epoxy/ hydroxyl), and C=O (carbonyl/ketone).45-47 In comparison with the XPS spectrum of pristine GONRs (Figure 2C), the C-O and C=O in the PtPd-rGONRs show much decreased peak intensities. Such result suggests that most oxygen-contained groups have been reduced by the hydrothermal treatment. Figure 2D and E show the Pt 4f and Pd 3d XPS spectra of PtPd-rGONRs. It can be seen that Pt 4f photoelectron spectrum is composed of two pairs of doublets: the most intense doublet (at 70.79 and 74.11 eV) is from Pt 4f7/2 and Pt 4f5/2 of metallic Pt, while the weaker doublet (at 71.5 and 75.46 eV) could be assigned to the +2 oxidation state of Pt.48 Similarly, in the Pd XPS spectrum, one doublet (at 335.24 and 340.47 eV) is ascribed to the binding energies of Pd 3d5/2 and Pd 3d3/2 of zerovalent state of the Pd, another doublet (at 336.73 and 341.28 eV) comes from +2 oxidation state of Pd.44,49 Notably, the peak intensities of both metallic Pt and Pd are significantly higher than the peaks corresponding to oxidation states of Pt and Pd, indicating that efficient reduction of PtCl62- and PdCl42- ions in the present reduction process. The nonmetallic Pt and Pd components could be ascribed to the partial surface oxidation of PtPd nanocrystals. In addition, the reduction process of GONRs to rGONRs was also studied by Raman. As shown in Figure 2F, the Raman spectrum of GONRs displays a sharp D band at ~1356 cm-1 and a G band at ~1600 cm-1. For the

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PtPd-rGONRs composite, these two bands blue-shift to ~1375 cm-1 and ~1606 cm-1, suggesting an interaction between rGONRs and PtPd nanocrystals. Moreover, from Figure 2F, the intensity ratio of the D- and G-band from the PtPd-rGONRs obviously increased compared to pristine GONRs (0.77 in GONRs and 0.80 in PtPd-rGONRs), suggesting the removal of oxygenated groups during the hydrothermal process, and the decrease of the in-plane sp2 domains structure of the resulting rGONRs support. The structure of PtPd nanocrystals were further analyzed by high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-disperse X-ray analysis (EDX) measurements. Figure 3A and B show the HRTEM images of PtPd-rGONRs composite, from which, it can be clearly observed that most of the formed PtPd nanocrystals are distinct concave nanocubes and uniformly dispersed on the rGONRs. The HRTEM image of a single PtPd nanocrystal (Figure 3C) viewed along the (100), as demonstrated by the fast Fourier-transform (FFT) pattern (Figure 3C inset), shows that each formed PtPd alloy nanoparticle is single crystal with high-quality fringes. The interplanar distance of lattice fringes was determined to be 0.195 nm, which is ascribed to the (200) distance of fcc palladium and platinum.50 From the elemental maps of Pt and Pd (Figure 3D-G), one can see that both Pt and Pd have an even distribution in each concave nanocube, confirming the alloy structure of PtPd nanocubes. Based on the EDX result (Figure S2, supporting information), the Pt/Pd ratio in the nanocubes was calculated to be 1.21, in accord with the result from XPS measurement (Pt/Pd = 1.45). Electrochemical sensing performance of PtPd-rGONRs for TNT detection. Due to the remarkable physical and chemical properties, graphene nanoribbons (GNRs) have attracted much attention for the development of high performance electrochemical sensors. For instance, Sun et al. found that graphene nanoribbons exhibited enhanced catalytic activity for electrochemical detection of Parkinson’s disease.40 In another work, Pong’s group demonstrated that electrochemical sensing platform fabricated from core-shell

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MWCNT/GONR shows much higher electrocatalytic activity toward ascorbic acid, uric acid and dopamine than those prepared from MWCNT and graphene.35 Although GNRs show obvious advantages for analytical applications, it is very necessary to explore new GNRs-based functional materials with improved electroanalytical performance. Therefore, PtPd-rGONRs composites are expected to have potential applications in electrochemical sensors. Electrochemical impedance spectroscopy (EIS) was carried out to study the electrical conductivity of PtPd-rGONRs -modified electrode. Figure 4A shows the impedance spectra of bare glassy carbon electrode (GCE), rGONRs-modified GCE (rGONRs/GCE) and PtPd-rGONRs -modified GCE (PtPd-rGONRs/GCE) in 5 mM Fe(CN)63-/4- containing 0.1 M KCl. It can be seen that the PtPd-rGONRs/GCE shows much smaller charge-transfer resistance compared to rGONRs and GCE, indicating the improved electron flux and accelerated electron shuttle between the electrolyte and the electrode substrate. Therefore, PtPd-rGONRs can be used for fabricating electrochemical sensors. Previous studies showed that the strong interaction between nitroaromatic compounds and the electron-rich group of nanomaterials has obvious merits for improving the detection sensitivity of nitroaromatic compounds. Figure 4B displays the adsorptive stripping voltammograms (ASVs) obtained from PtPd-rGONRs/GCE, rGONRs/GCE and bare GCE in 0.1 M phosphate buffer solution (PBS) containing 0.4 M KCl with the presence of 3 ppm TNT. On the PtPd-rGONRs/GCE, three well-defined reduction peaks can be observed at -0.33, -0.48 and -0.58 V which could be assigned to the stepwise reduction of three different nitro groups of TNT to the corresponding hydroxylamine and amine groups.1 Importantly, the three reduction peak potentials are all more positive than those from rGONRs/GCE, bare GCE and some previously reported electrode materials (Table 1). It can also be observed that PtPd-rGONRs/GCE exhibits greater cathodic current density from TNT reduction compared to GONRs/GCE and bare GCE. For instance, the reduction current density of the first peak at -0.33 V

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recorded at the PtPd-rGONRs/GCE is about 4 and 12 times, respectively, larger than those at the rGONRs and bare GCE. These results indicate that PtPd-rGONRs exhibit much higher electrocatalytic activity for TNT reduction and therefore such composite can act as a kind of enhancing material for detecting TNT. The influences of different sensing conditions, such as pH, supporting electrolyte and accumulation time were investigated. The electro-reduction of TNT in 0.1 M PBS with different pH was first investigated (Figure S3A). Figure S3B shows that the highest signal-to-background can be obtained for PBS with a pH of 6.5 and therefore PBS with pH of 6.5 is used throughout the following electrochemical measurements. The effect of KCl concentration in buffer solution on electro-reduction current of TNT was also investigated. It can be observed from Figure S3C and D that the reduction current firstly increases with KCl concentration, and then reaches a constant level at concentrations higher than 0.4 M. Therefore, phosphate buffer containing 0.4 M KCl was used as the optimum electrolyte. The accumulation time is another key factor that may affect the TNT detection efficiency. As shown in Figure S3E and F, the reduction current increases rapidly with the time increasing from 0 to 150 s. However, with the accumulation time further increasing, current density shows only a little increase. From Figure S3F, the obtained current-accumulation time plot shows a typical adsorption process. Considering both the sensitivity and required detection time, the optimal accumulation time of 150 s is used for the subsequent analysis. Under the above optimized detection conditions, electro-reduction of TNT catalyzed by PtPd-rGONRs composite was then studied. Figure 4C shows the stripping voltammograms of PtPd-rGONRs/GCE recorded with TNT concentrations changing from 0.01 to 8 ppm. It was found that at low TNT concentrations, only the first peak (at -0.33 V) is detectable. With the increase of TNT concentration, the other two peaks appear sequentially. Therefore, the first peak (at -0.33 V) is the most favorable

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characteristics for TNT detection in low concentration range and it was chosen for subsequent quantitative analysis. On the other hand, the electrochemical measurements show a TNT concentration-dependent electrochemical response on PtPd-rGONRs. Figure 4D displays the dependence of current density on TNT concentration. A high linearity (R2=0.998) was achieved within the concentration increasing from 0.01 to 3 ppm. The linear regression can be expressed as J=451.95 [TNT] + 29.038 (J is the current density at -0.33 V and [TNT] is the concentration of TNT). The limit of detection (LOD) was estimated to be 0.8 ppb on the basis of the signal to noise ratio of 3 (LOD=3SD/S). Figure S4A shows the stripping voltammograms of rGONRs/GCE obtained at different TNT concentrations with a range from 0.05 to 3 ppm. By comparing the voltammograms in Figure 4C and Figure S4A, it can be seen that at a same TNT concentration, the background-subtracted current density obtained from PtPd-rGONRs/GCE is much larger than that from rGONRs/GCE, suggesting the higher catalytic activity of PtPd-rGONRs composite for TNT reduction. From Figure S4B, a linearity in the concentration range from 0.05 to 1.6 ppm and a LOD of 5.6 ppb were obtained for TNT detection on rGONRs/GCE. Such sensing performance is much lower than that from PtPd-rGONRs composite. Moreover, we also compared the sensing performance of PtPd-rGONRs composite with those of other electrochemical sensing materials. As shown in Table 1, the as-prepared PtPd-rGONRs composite has higher detection performance than the electrode materials reported before. These results indicate that PtPd-rGONRs are promising nanostructured materials for the enhanced electrochemical detection of TNT. The enhanced electrocatalytic activity could be partially ascribed to the unpaired electrons in d orbital of high dispersion of PtPd nanocrystals can efficiently coordinate with electron-deficient TNT and thus can enrich TNT molecules through the strong charge transfer interaction. Meanwhile, high capacity for the accumulation of TNT by the one-dimensional planar structure of rGONRs, the highly electronic conductivity of PtPd-rGONRs (as revealed by EIS result), as well as a

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synergistic effect between electroactive PtPd nanocrystals and excellent adsorptive properties of rGONRs can also result in the enhanced sensing performance for TNT detection. Subsequently, the selectivity and stability of the detection system were evaluated. The electrochemical responses to a series of nitroaromatic compounds, including nitrobenzene (NB), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-NT), 2, 4-dinitrotoluene (2, 4-DNT), 2, 6-dinitrotoluene (2, 6-DNT), and 1, 3, 5-trinitrophenol (TNP), were investigated on PtPd-rGONRs. It was observed that the number of cathodic peaks is strongly dependent on the number of nitro groups contained in nitroaromatic compounds. As shown in Figure 5A, the NB, 2-NT and 4-NT exhibit only one reduction current peak at -0.53, -0.57 and -0.55 V, respectively. Meanwhile, 2, 4-DNT and 2, 6-DNT exhibit two reduction peaks (Figure 5B). Figure 5C displays the stripping voltammograms of TNT and TNP on PtPd-rGONRs/GCE. It can be seen that TNP shows four electrochemical reduction peaks, which is significantly different from TNT. It is worthy to note that the characteristic reduction peak at -0.33 V from TNT can be used for the detection of TNT with high selectivity. As shown in Figure 5D, the chronoamperometric curve recorded at -0.33 V demonstrates that the addition of other nitroaromatic compounds could not influence the detection of TNT. This measurement further confirms that the detection platform fabricated from PtPd-rGONRs/GCE has excellent capacity to distinguish TNT from other nitroaromatic explosives, i.e high sensing selectivity for TNT detection. In addition, the stability of the PtPd-rGONRs-based TNT detection platform was assessed using the accelerate durability tests by cycling the as-prepared PtPd-rGONRs/GCE between -0.7 and -0.2 V at 50 mV/s in 0.1 M PBS with 0.4 M KCl (pH=6.5) and 10 ppm TNT. After 1000 continuous potential cycles, the peak potentials of TNT reduction exhibit only a small negative shift of 5 mV, and the TNT reduction current (Figure 5E) can maintain 95% of its initial value, suggesting the satisfied long-term stability of the detection platform. To estimate the practical application of PtPd-rGONRs/GCE in real

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sample analysis, the ultrapure water was replaced by tap and lake water without any pretreatment in the preparation of buffer solution, in which an extra 2 ppm target analyte TNT was added. As shown in Figure 5F, the ASV responses to 2 ppm TNT are almost same for all three samples. The recovery in five parallel measurements ranges from 97.8 to 106.1% for tap water sample and 96.2 to 108.5% for lake water sample, indicating the promising application and good reproducibility of PtPd-rGONRs-based detection platform for real sample determination.

CONCLUSION In summary, a simple one-pot hydrothermal strategy was used for the synthesis of PtPd concave nanocubes anchored on graphene nanoribbons (PtPd-rGONRs). This unique structure combines the large surface area and excellent adsorptive properties of rGONRs with the highly electrocatalytic activity of PtPd nanocrystals for TNT reduction, endowing the PtPd-rGONRs composite enhanced electrochemical detection for TNT. The electrochemical sensing system based on PtPd-rGONRs exhibited a high sensing performance for TNT detection with a linearity between 0.01 to 3 ppm and a detection limit of 0.8 ppb. On the other hand, the PtPd-rGONRs composite exhibited high selectivity, good reproducibility for the determination of TNT. Moreover, the PtPd-rGONRs-based electrochemical sensing platform can be successfully applied to TNT detection in tap and lake water with good results. Due to the facile synthesis, rapid response, low detection limit and wide linearity, such PtPd-rGONRs-based electrochemical sensing platform is suitable for determination of TNT in real application.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21575134 and 21275136). CLS and YJL thank the Chang Gung Memorial Hospital (CMRPD2C0012) for financially

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supporting this research.

Supporting Information Available TEM images of the prepared graphene oxide nanoribbons (GONRs), UV-vis spectra of GONRs and PtPd-rGONRs, EDX of PtPd-rGONRs, the influences of pH, concentration of KCl, and accumulation time on the performance for TNT detection, electrochemical sensing performance of rGONRs for TNT detection. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interest.

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Figure captions Figure 1 (A) TEM image of GONRs. (B, C) Representative TEM micrographs of PtPd nanocrystals supported on rGONRs (PtPd-rGONRs) at different magnifications. (D) The size distribution histogram of PtPd nanocrystals.

Figure 2 (A) XRD pattern of the as-synthesized PtPd-rGONRs composite. For comparison, bulk Pt (red bars) and Pd (blue bars) from the JCPDS are also included. (B, C) XPS of C1s from PtPd-rGONRs (B) and GONRs (C). (D, E) XPS of Pt 4f (D) and Pd 3d (E) from the PtPd-rGONRs composite. (F) Raman spectra of GONRs and PtPd-rGONRs.

Figure 3 (A-C) High-resolution TEM micrographs of the PtPd nanocrystals supported on the rGONRs surface at different magnifications. The inset in (C) shows the FFT pattern of an individual PtPd nanocrystal. (D) The high-angle annular dark-field (HAADF)-STEM image of PtPd-rGONRs, and the corresponding elemental mapping of (E) Pt, (F)Pd, and (G) the overlay.

Figure 4 (A) EIS spectra obtained at PtPd-rGONRs/GCE, rGONRs/GCE and bare GCE in 5 mM K4[Fe(CN)6]/K3[Fe(CN)6] containing 0.1 M KCl. (B) Stripping voltammograms obtained at the PtPd-rGONRs/GCE, rGONRs/GCE and bare GCE in 0.1 M PBS (pH = 6.5) with 0.4 M KCl and 3 ppm of TNT. (C) Stripping voltammograms at the PtPd-rGONRs/GCE with different concentrations of TNT in 0.1 M PBS (pH = 6.5) with 0.4 M KCl. (D) The plot of the linear relationship between peak current density and TNT concentrations at -0.33 V. The error bars represent the standard deviation of three separate measurements.

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Figure 5 ASV of PtPd-rGONRs/GCE in 0.1 M PBS with 0.4 M KCl (pH = 6.5) for 4 ppm of NB, 2-NT, 4-NT (A), 2, 4-DNT and 2, 6-DNT (B), and TNP and TNT (C). (D) The chronoamperometric response obtained at the PtPd-rGONRs/GCE to the successive injection of 1ppm TNT (a), NB (b), 2-NT (c), 4-NT (d), 2, 4-DNT (e) and 2, 6-DNT (f) at a constant potential of -0.33 V. (E) ASVs obtained at PtPd-rGONRs/GCE in 0.1 M PBS with 0.4 M KCl (pH=6.5) and 10 ppm TNT before and after 1000 cycles. (F) The determination of 2 ppm TNT by ASV (scan rate: 50 mV/s, 150 s preconcentration time at 0 V) with different water samples.

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(A)

Page 22 of 28

(B)

100 nm

(C)

(D)

10.9 ± 0.2 nm

Figure 1

22


PtPd-rGONRs Pd: 65-2867 Pt: 65-2868

(111) (200) (220)

20

40

60

68

80

284

288

280

288

76

78

80

Intensity (a.u.)

5/2

Pd 3d3/2

334

292

336

338

340

342

344

346

Binding energy (eV)

(F)

G band D band

Intensity (a.u.)

Sum C-C/C=C C=O C-O

284

74

(E)Pd 3d

Binding energy (eV)

(C)

72

Binding energy (eV)

Sum C-C/C=C C=O C-O

Intensity (a.u.) 280

Pt 4f 5/2

70

2θ θ (degree)

(B)

Pt 4f7/2

(D) Intensity (a.u.)

(A)

Intensity (a.u.)

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


Intensity (a.u.)

Page 23 of 28

PtPd-rGONRs

GONRs

1000 1200 1400 1600 1800 2000

292

Binding energy (eV)

Raman Shift (cm-1)

Figure 2

23



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(A)

(B)

Page 24 of 28

(C)

d200(0.195 nm) (D)

(E)

(F)

(G)

Pd-L

Pt-M

Figure 3

24


Overlay

Page 25 of 28

400

(A)

0

-2

J (µA cm )

-Z'' / ohm

300 200 100

Bare GCE rGONRs PtPd-rGONRs

0 100

200

300

(B)

-400

-800

-1200

0

400

Bare GCE rGONRs PtPd-rGONRs -0.8

Z' / ohm 0

-0.6

-0.4

-0.2

0.0

E (V vs Ag/AgCl) 1200

(C)

-400

-2

J (µ A cm )

-2

J (µ A cm )

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


0 ppm -800

-1200

8 ppm

(D)

900 2

R =0.998

600 300

-1600

0 -0.8

-0.6

-0.4

-0.2

0.0

E (V vs Ag/AgCl)

0

2

4

6

[TNT]/ppm

Figure 4

25


8


(A)

0

(B)-400 J (µ A cm )

-800

-2

-2

J (µ A cm )

-400 -800 -1200

NB 2-NT 4-NT

-1600 -0.8

-0.6

-0.4

-0.2

-1200 -1600

2,4-DNT 2,6-DNT

-2000 0.0

-0.8

-0.6

E (V vs Ag/AgCl)

(C)

(D) 0 -200

b

-2

J (µ A cm )

-2

J (µA cm )

-0.2

a a

-800 -1200

TNT TNP

-1600 -0.8

-0.6

-0.4

-0.2

0.0

cde fa

-400 -600 -800

E (V vs Ag/AgCl)

(E)

-0.4

E (V vs Ag/AgCl)

0

-400

0

200

400

600

800

Time /s

(F)

0

-2

400 µA cm

-400

-2

J (µ A cm )

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

Page 26 of 28

-800

-1200

ultrapure water tap water lake water

initial after 1000 cycles

-1600 -0.8

-0.6

-0.4

-0.2

-0.8

0.0

E (V vs Ag/AgCl)

-0.6

-0.4

-0.2

E (V vs Ag/AgCl)

Figure 5

26


0.0

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Table 1 Comparison of the analytical performances based on the present PtPd-rGONRs composite and the previously reported sensing systems for TNT detection Material

Linear range/ppm

LOD/ppb

VO2

0.1-1

1

Anal. Chem. 2015, 87, 334-337

a

1-70

450

Chem. Commun. 2011, 47, 12494-12496

1

Biosens. Bioelectron. 2014, 58, 85-91

4

Biosens. Bioelectron. 2011, 26, 3475-3481

Ag/CS-G

Pt-Pd NPs/CNTs-rGO

0.0035-0.19

b

0.03-1.5

IL-GNs

Reference

Nitrogen-doped graphene

0.12-2

29.5

Electrochem. Commun. 2012, 16, 30-33

c

0.5-40

300

ACS Nano 2010, 4, 3959-3968

2-5

PNEGHNs

TiO2/nano-Pt particles

200

Adv. Funct. Mater. 2007, 17, 1487-1492

Cu/SWCNT

0.001-2

1

Anal. Chem. 2006, 78, 5504-5512

PtPd-rGONRs

0.01-3

0.8

Present work

a

Ag/CS-G: Ag nanoparticles assembled on carboxylic sodium groups-functionalized graphene; c

liquid-graphene hybrid nanosheets; PNEGHNs: Pt nanoparticle ensemble-on-graphene hybrid nanosheet.

27


b

IL-GNs: ionic


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TOC

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Graphene Nanoribbon-Supported PtPd Concave Nanocubes for

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