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Enhanced Electronic Communication and Electrochemical Sensitivity Benefiting from the Cooperation of Quadruple Hydrogen Bonding and #-# Interactions in Graphene/Multi-Walled Carbon Nanotube Hybrids Qiguan Wang, Su-Min Wang, Jiayin Shang, Shenbao Qiu, Wenzhi Zhang, Xinming Wu, Jinhua Li, Weixing Chen, and Xinhai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11157 • Publication Date (Web): 25 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Enhanced

Electronic

Communication

and

Electrochemical Sensitivity Benefiting from the Cooperation of Quadruple Hydrogen Bonding and ππ Interactions in Graphene/Multi-Walled Carbon Nanotube Hybrids Qiguan Wang†, Sumin Wang†*, Jiayin Shang†, shenbao Qiu†, Wenzhi Zhang†, Xinming Wu†, Jinhua Li†, Weixing Chen†, Xinhai Wang‡ †Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China ‡ School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China KEYWORDS: MWNT and graphene, Quadruple hydrogen bonds, π-π interactions, Electronic communication, Electrochemistry ABSTRACT: By designing a molecule labeled as UPPY with both ureidopyrimidinone (UP) and pyrene (PY) units, the supramolecular self-assembly of multi-walled carbon nanotube (MWNT) and reduced graphene oxide (rGO) was driven by the UP quadruple hydrogen bonding and PY based π-π interactions to form a novel hybrid of rGO-UPPY-MWNT, in which the

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morphology of rGO-wrapped MWNT was found. Bridged by the two kinds of noncovalent bonding, enhanced electronic communication was occurred in rGO-UPPY-MWNT. Also, under the cooperation of UP quadruple hydrogen bonding and PY based π-π interactions, higher electrical conductivity and better charge transfer were observed for rGO-UPPY-MWNT, compared with the rGO-MWNT composite without such noncovalent bonds, and that with just single PY based π-π interaction (rGO-PY-MWNT) or UP quadruple hydrogen bond (rGO-UPMWNT). Specifically, the electrical conductivity of rGO-PY-MWNT hybrids was increased approximately seven-fold, and the interfacial charge transfer resistance was nearly decreased by one order of magnitude compared with rGO-MWNT, rGO-UP-MWNT and rGO-PY-MWNT. Resulted from its excellent electrical conductivity and charge transfer properties, the rGOUPPY-MWNT modified electrode exhibited enhanced electrochemical activity toward dopamine with detect limit as low as 20 nM. 1. Introduction Nanostructured graphitic hybrids containing two-dimensional (2D) graphene and onedimensional (1D) carbon nanotubes (CNTs) have emerged as promising materials for potential applications in electrochemistry such as biosensors, electrocatalysis, supercapacitors and lithiumion batteries because of their superior electrical conductivity, high specific surface area, good carrier mobility.1-4 Various methods have been developed to integrate CNTs with graphene to form new hybrid materials, ranging from chemical vapor deposition of CNTs between graphene or graphene oxide (GO) nanosheets,5-9 to layer-by-layer assembly of oppositely charged graphene and CNTs,10 and simple solution mixing of CNTs with graphene or GO followed by reduction step or with the aids of surfactants.11-15 In those cases, the contact resistance resulted from CNT-CNT or graphene-graphene could be effectively diminished, however, the resistance


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from the loose contact between CNT and graphene still hinder the realization of the full electrochemistry of CNT-graphene hybrids. In order to further decrease the contact resistance between graphene and CNTs, covalent bond has been employed to link the graphene with CNTs. The introduced covalent bonds, acting as the bridge for electron motion, could close the spatial distance and greatly enhance the degree of intimate contact between graphene and CNT, which make the so-prepared hybrids show remarkable electrochemical, optoelectronic and gas sensing properties.16-18 Supramolecular self-assembly, driven by molecular recognition of the noncovalent bonds such as π-π, hydrogen bonding, host-guest, electrostatic and hydrophobic-hydrophilic interactions, has also been recognized as an important, effective, and feasible approach to prepare graphene-CNT hybrids.19-21 By utilizing noncovalent bonds as bridges to link the graphene and CNT, the contact resistance in the assembled hybrids could also be significantly decreased without damaging their inherent conjugation. Gooding’s group reported that the increased intersheet electrical communication and enhanced collective conductivity were found in the assembled graphene sheets linked by a bipyrene terminal molecule via π-π stacking interactions.22 Ureidopyrimidinone (UP) quadruple AADD (A: hydrogen bond acceptor, D: hydrogen bond donor) hydrogen bonding unit first reported by Meijer et al, possessing very strong binding strength (107 M-1 in CHCl3 or toluene),23 could exist as homodimmers in apolar solvents as well as in bulk (Figure 1). Benefited from the intermolecular quadruple AADD arrays and the intramolecular hydrogen bonds (Figure 1), UP binding module is regarded as a good candidate for the self-assembly of nanostructured graphitic carbons.24-26 Linked by strong hydrogen bonding interactions, layer-by-layer and self-supported films of UP functionalized multi-walled carbon nanotube (MWNT) were prepared in our group.25 Utilizing UP based supramolecular


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polymer, selectively bonding of semiconducting single walled carbon nanotube (SWNT) from metallic SWNT was realized by Pochorovski’s group.26 Besides the multiple hydrogen bonding, π-π interactions also show the strong strength, and highly conductive hybrids of CNT, graphene or fullerene with those π-orbital-rich compounds such as pyrene derivatives have been fabricated.27,28 Synthesizing graphene-wrapped CNT structures is another promising approach to decrease the resistance of rGO-CNT hybrids by increasing the junction area between the graphene and CNTs. However, up to now, just GO rather than reduced graphene (rGO) has been used to wrap CNTs from supramolecular self-assembly technique,29,30 probably because it is difficult to introduce enough amounts of strong noncovalent interactions on the surface of rGO to bend the 2D layer for wrapping CNT. To further improve the intimate contact as well as the junction area between graphene and CNT, in this article, by designing UP-pyrene derivatives, a novel hybrid showing graphenewrapped MWNT morphology was fabricated, linked by pyrene-graphitic based π-π interactions and UP quadruple hydrogen bonding interactions (Figure 1). The polyfunctional UP-pyrene selfassembly precursor (labeled as UPPY) has both π-π interaction end (pyrene unit, PY) and hydrogen bonding end (UP). When UPPY was mixed with the MWNT and rGO in apolar solvents, the self-stacking of MWNT and rGO was effectively suppressed under the strong π-π interactions occurred in UPPY/MWNT and UPPY/rGO. In addition, cooperated by the quadruple hydrogen bonding, a stable supramolecular rGO-UPPY-MWNT hybrid was formed bridged by UPPY·UPPY homodimer (Figure 1). The specified supramolecuar hybrid showed good electronic communication, decreased contact resistance and enhanced electrochemical activity due to the cooperation of π-π and quadruple hydrogen bonding interactions.


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Figure 1. Supramolecular self-assembly scheme of rGO-UPPY-MWNT from rGO, UPPY and MWNT. 2. Experimental Section 2.1 Materials All reagents and chemicals were used as received unless otherwise noted. MWNT with the length of 10-30 µm and the external diameter of 20-30 nm were purchased from Chengdu Institute of Organic Chemistry, Chinese Academy of Science. Graphite powder, natural briquetting grade, was purchased from Alfa Aesar. Graphene oxide (GO) was prepared from natural graphite by the modified Hummers procedure.31,32 Reduced graphene oxide (rGO) with the molecular ratio of C/O of 16.5 calculated from the X-ray photoelectron spectra results was prepared by facile thermal reduction of GO under the Ar atmosphere at 600 oC for 2 hour.1 Diphenylphosphoryl azide was purchased from Alfa Aesar. 2-Amino-6-undecylpyrimidin-4(1H)one33 and N-[(butylamino)carbonyl]-6-tridecylisocytosine23 was prepared according to reported


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methods. 1-pyrenebutyric acid was purchased from Aldrich. Toluene was refluxed with Na and redistilled. Chloroform and triethylamine (Et3N) was refluxed with CaH2 and redistilled. During electrochemical experimental process, Milli-Q water (18 MΩ/cm) was used. All other reagents and chemicals were used as received unless otherwise noted. 2.2 Preparation of the supramolecular composites Synthesis of UPPY 1-Pyrenebutyric acid (514.1mg, 1.783 mmol) and Et3N (0.3 mL, 2.15 mmol) in dry toluene (50 mL) were stirred under an argon atmosphere till completely dissolved. Diphenylphosphoryl azide (0.49 mL, 2.27 mmol) was added into the mixture. After heated at 40 o

C for 1 h, and 80 oC for 4 h, 2-amino-6-undecylpyrimidin-4(1H)-one (530 mg, 2.0 mmol) was

added into the mixture and subsequently stirred at 80 oC for another 16 h. Evaporating the solvent, the resultant residue was thoroughly washed with cold methanol, and then was subjected to recrystallization by the mixture of CHCl3 and methanol to afford the product as a white solid in 68% yield. 1H NMR (CDCl3 δ ppm): 12.96 (s, 1H), 11.40 (s, 1H), 10.17 (s, 1H), 8.32(d, J = 8.0 Hz, 2H), 8.11(m, 2H), 8.04(d, J = 8.0 Hz, 2H), 7.97(s, 1H), 7.94(m, 2H),5.74 (s, 1H), 3.51(m, 4H), 2.42(t, J = 8.0 Hz, 2H), 2.25(m, 2H), 1.63(m, 2H), 1.25-1.33 (m, 16H), 0.87 (t, J = 8.0 Hz, 3H); 13C NMR (CDCl3, δ ppm): 130.92, 130.38, 129.30, 128.16, 126.99, 126.88, 126.58, 126.01, 125.23, 124.50, 124.46, 124.30, 124.03, 123.16, 39.58, 32.18, 31.41, 30.73, 30.36, 29.09, 28.98, 28.83, 28.71, 28.43, 26.49, 22.20, 13.64. MS (ESI) m/z: 551 [M+H]+; Anal. Calcd (%) for C35H42N4O2: C 76.33, H 7.69, N 10.17; found: C 76.35, H 7.72, N 10.16. Synthesis of supramolecular rGO-UPPY-MWNT Typically, rGO (2 mg), MWNT (2 mg) and UPPY (6 mg) were dispersed in CHCl3 (20 mL) by ultrasonication, and the mixtures were allowed to stir for 6 h at room temperature. The stable black dispersion was obtained and filtered


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with a polycarbonate membrane (0.22 µm), and then thoroughly washed by CHCl3 to obtain the rGO-UPPY-MWNT which can be redispersed in CHCl3 (0.5 mg/mL) by ultrasonication. Synthesis of reference composites For comparison, composites of rGO-MWNT, rGO-PYMWNT, rGO-UP-MWNT, rGO-UPPY and MWNT-UPPY were prepared as references. Here, the PY is pyrene and the UP denotes N-[(butylamino)carbonyl]-6-tridecylisocytosine (Supporting Information, Figure S1), which is a representative of ureidopyrimidinone compound. The preparative conditions of these composites were similar with rGO-UPPY-MWNT. 2.3 Instrumentation UV–vis spectra were measured by a Shimadzu 1901 UV–vis spectrophotometer. Fluorescent spectra were obtained by a PerkinElmer LS55 Fluorescent spectrophotometer. 1H NMR spectra were recorded on a Bruker Avance dpx 400 MHz instruments using TMS as internal standard. Mass spectra were obtained on Bruker APEX II spectrometers. Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer. Transmission electron microscope (TEM) images were recorded from a JEM2010 instrument. Scanning electron microscope (SEM) images were collected by an S4800 instrument. X-ray photoelectron spectra (XPS) were measured using a PHI 5400 X-ray photoelectron spectrometer. All spectra were calibrated with the C 1s photoemission peak for sp2 hybridized carbons at 284.5 eV. The X-ray diffraction (XRD) patterns of the samples were recorded with a Shimadzu XRD-6000 X-ray diffractometer. The Cu Kα line (λ = 1.5451 nm) from a sealed tube with a copper anode was used as a source of radiation. The room temperature resistance of the samples in compressed pellet form was measured by using a standard four-point probe configuration. All electrochemical experiments were carried out with a CHI660 workstation. 2.4 Electrochemical Measurements


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The electrochemical characterizations were conducted in a conventional three-electrode system. Cyclic voltammetric and amperometric experiments were performed using a 0.1 M phosphate buffer solution (PBS, pH = 7.0) containing 0.1 M KCl as electrolyte solution, a Pt wire as the counter electrode and a Ag/AgCl electrode as reference electrode respectively. A sample-coated glassy carbon (GC) electrode (3.0 mm in diameter) was used as working electrode. For preparing the working electrode, GC disk electrode with the diameter of 3.0 mm was polished with a 0.05µm Al2O3 and washed with water and ultrasonicated in ethanol and water (each for 1 min), dried by N2. Then 3 µL of dispersion of rGO-UPPY-MWNT in CHCl3 (0.5 mg/mL) was dropped on the GC electrode to prepare the rGO-UPPY-MWNT/GC electrode. Under the same condition, reference electrodes of rGO-MWNT/GC, rGO-UP-MWNT/GC and rGO-PY-MWNT/GC could be prepared. Electrochemical impendence spectroscopy (EIS) measurements were performed to determine the interfacial charge-transfer resistance of different electrodes and conducted over the frequency range between 100 kHz to 0.1 Hz in the solution of 2.5 mM K4[Fe(CN)6]/K3[Fe(CN)6] in PBS (pH 7.0 ) using a Pt wire as the counter electrode and a saturated calomel electrode (SCE) as reference electrode at open circuit potential with an ac perturbation of 5 mV. 3. Results and Discussion 3.1 Synthesis and Characterization of rGO-UPPY-MWNT For the supramolecular self-assembly of rGO-UPPY-MWNT, a functional organic molecule labeled as UPPY composed of both a planar conjugated pyrene ring and a ureidopyrimidinone was designed (Figure 1), which exists as homodimer linked by the AADD hydrogen bonding in bulk or apolar solvents. Because each pyrene group at the two ends of UPPY·UPPY homodimers


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can take strong π-π interactions with rGO and/or MWNT, by which many UP units were attached to the surface of rGO and the wall of MWNT, thus rGO and MWNT were finally bonded together to form the superamolecular hybrids linked by the self-complementary quadruple hydrogen bonds (Figure 1) in the UPPY·UPPY homodimers.

Figure 2. TEM images of (a) rGO-UPPY/CHCl3, (b) rGO, (c, d) rGO-UPPY-MWNT/CHCl3, (e) SEM image of rGO-UPPY-MWNT and (f) TEM image of rGO-UPPY-MWNT in DMF. The morphologies of supramolecular rGO-UPPY-MWNT assembly were studied by TEM and SEM images. As shown in Figure 2a, for rGO-UPPY in apolar solvent CHCl3, rGO surface was heavily wrinkled, which is quite different from the smooth and thin layer structure of raw rGO material (Figure 2b). This is due to the strong quadruple hydrogen bonding between UPPY units absorbed on the same layer of rGO, which make the rGO surface folded. For MWNTUPPY/CHCl3 only tubular MWNTs could be observed (Supporting information, Figure S2). As


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sharp contrast, in the case of rGO-UPPY-MWNT in CHCl3 (TEM images, Figure 2c and 2d) the thin layer of rGO was significantly rolled up, by which MWNTs were wrapped. The SEM images of rGO-UPPY-MWNT (Figure 2e) also reflected the similar rGO-wrapped MWNTs (see the arrows in Figure 2e). In order to clarify the key factors for the formation of wrapped morphology in rGO-UPPYMWNT hybrids, as control experiments morphologies of rGO-UPPY-MWNT in N,Ndimethylformamide (DMF) were investigated. As shown in Figure 2f, unlike the case in CHCl3, in DMF neither thin layers of rGO were rolled up nor were wrapped MWNT structures found. It was because in DMF quadruple hydrogen bonding interactions between rGO and MWNT was inhibited influenced by the interactions between polar solvent DMF and ureidopyrimidinone. Furthermore, in the presence of just one kind of interaction such as the rGO-UP-MWNT (only hydrogen bonding, Supporting information, Figure S3) and rGO-PY-MWNT (only π-π interactions, Supporting information, Figure S4), rGO-wrapped MWNTs cannot be found either. These control experiments proved the importance of cooperation between π-π and hydrogen bonding interactions for the fabrication of the specified wrapped structures in rGO-UPPYMWNT. Thanks to the π-π interactions, considerable UPPY molecules could be absorbed on one layer of rGO or on one tube of MWNT so that strong hydrogen bonding interactions between rGO and MWNT are available for bending rGO and wrapping around MWNTs. The wrapping structure was beneficial to increase the junction area between rGO and MWNT.


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25.8

Intensity (a.u.)

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a

26.3

b 19.0 9.6

24.8

11.1

c d

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20

30

40

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2 Theta(degree)

Figure 3. XRD spectra of (a) MWNT, (b) rGO, (c) UPPY and (d) rGO-UPPY-MWNT. Figure 3 exhibits the XRD patterns of rGO-UPPY-MWNT, MWNT, rGO and UPPY. For MWNT (Figure 3a) a broad peak centered at 2θ = 25.8o attributed to the typical diffraction peak (002) of graphite is observed. The rGO (Figure 3b) showed a similar diffraction peak at 26.3o. For UPPY (Figure 3c) sharp peaks appeared at 2θ values of 9.6, 11.1, 19.0 and 24.8o, indicating the crystalline state of UPPY. However, in the XRD pattern of rGO-UPPY-MWNT (Figure 3d), the sharp peaks for UPPY disappeared, indicating the spatial arrangement of UPPY was varied under the strong interactions with rGO and MWNT. In addition, the characteristic peak of MWNT was overlapped and included in the broad peak of rGO in rGO-UPPY-MWNT (Figure 3d), because of the tightly wrapping by rGO, which makes the XRD response of MWNT weakened.34,35 As contrast, in the presence of just hydrogen bonding or π-π interactions like the case of rGO-UP-MWNT or rGO-PY-MWNT (Supporting information, Figure S5), the peak at 2θ = 25.8o of MWNT could be clearly observed.


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(a)

C1s

rGO-UPPY-rGO 392

396

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O1s rGO

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400.4

406

404

402 400 398 Binding Energy (eV)

396

Figure 4. (a) XPS survey scans of rGO-UPPY-MWNT, rGO, and MWNT, (b) The deconvoluted N1s XPS spectrum of rGO-UPPY-MWNT. Figure 4a shows the XPS survey scans of rGO-UPPY-MWNT, rGO and MWNT. The XPS survey scan spectra of MWNT and rGO showed only C1s and weak O1s peaks at around 284.5 and 533.3eV respectively, and the peak attributed to N1s could not be found. For the spectrum of rGO-UPPY-MWNT, besides the C1s and O1s features, N1s peak at around 400.0 eV could also be observed, due to the presence of UPPY in rGO-UPPY-MWNT. Based on the XPS results, the content of nitrogen, oxygen and carbon atom in rGO-UPPY-MWNT was calculated as 1.55


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atom%, 5.45 atom% and 93.00 atom% respectively, by which the amount of UPPY in rGOUPPY-MWNT could be calculated as 3.48×10-4 mol/g. The deconvoluted XPS N1s spectrum of the rGO-UPPY-MWNT is exhibited in Figure 4b, which indicated the presence of strong hydrogen bonding interactions. The peaks at binding energy of 399.3 eV and 400.4 eV are attributed to nitrogen in –NH–C=O and –N= of UPPY respectively. Evidently, a higher binding energy for the N1s of –N= was found relative to the reported value of 397.1 eV,36 because the imine behaves as a hydrogen acceptor for the formation of the quadruple hydrogen bonding, which make the nitrogen atoms more electropositive.25 Meanwhile, the N1s feature of amide in UPPY showed a lower binding energy compared to that of reported 399.9 eV37 because of the formation of hydrogen bonding where nitrogen of amide acts as hydrogen donor.25

0.6

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0.5 Absorbance

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327

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0.0 300 350 400 Wavelength(nm)

450

Figure 5. UV-vis spectra of (a) UPPY, (b) rGO, (c) MWNT and (d) rGO-UPPY-MWNT.


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The UV-vis spectra of rGO-UPPY-MWNT, UPPY, rGO and MWNT were compared in Figure 5. For the pure UPPY, the UV-vis spectrum showed obvious peaks at 276, 327 and 344 nm (Figure 5a). While both rGO and MWNT exhibited featureless absorption in the range of 275450 nm (Figure 5b and 5c). Moreover, the absorption peaks of UPPY could be observed from rGO-UPPY-MWNT (Figure 5d), which confirmed that UPPY had been successfully introduced into the hybrid. Notably, compared with the pure UPPY the absorption peaks of UPPY in rGOUPPY-MWNT became broad, undistinguishable, and red shifted to 279, 329 and 346 nm (Figure 5d), illustrating significant electronic communication among the constituents occurred in rGOUPPY-MWNT.38,39

Absorbance

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Figure 6. (A) UV-vis absorption spectra of rGO-UPPY-MWNT/CHCl3 without (a) and with (b) the presence of CH3OH, the volume ratio of CHCl3 to CH3OH of line b is 4:1; (B) Fluorescent spectra of rGO-UPPY-MWNT/CHCl3 upon titration with CH3OH. From line a to line j, the volume ratio of CH3OH to CHCl3 is increased from 0:100 to 4:1. Ex: 350 nm. The function of hydrogen bonds in rGO-UPPY-MWNT/CHCl3 was further manifested by UV-vis and fluorescent titration experiments. As shown in Figure 6A the characteristic absorption of rGO-UPPY-MWNT/CHCl3 can be significantly changed by introduction of CH3OH. When CH3OH was coexisted with rGO-UPPY-MWNT/CHCl3 (the volume ratio of CHCl3 to CH3OH is 4:1), the peaks at 279, 329 and 346 nm attributed to UPPY became much more distinct (Figure 6A). It is because the hydrogen bonding interaction was disrupted by polar solvent of CH3OH, which leads to the dissociation of rGO-UPPY-MWNT and inhibits the electronic communication. The fluorescent spectrum of rGO-UPPY-MWNT in CHCl3 showed the characteristic emission of UPPY at 380, 398 and 422 nm (Figure 6B(a)). Similar with reported pyrene functionalized graphene (MWNT) systems, the fluorescence of UPPY in rGO-UPPY-MWNT had been quenched because of photoinduced electron transfer and energy transfer between UPPY and rGO (MWNT). However, upon titration with CH3OH, the fluorescent emission intensity of rGOUPPY-MWNT was progressively enhanced (Figure 6B) because photoinduced electron and energy transfer processes was blocked, resulted from the dissociation of supramolecular rGOUPPY-MWNT caused by the broken quadruple hydrogen bonding.40-42 In addition, UV-vis and fluorescent titration of UPPY and rGO-PY-MWNT were carried out as control experiments. No obvious changes could be observed for both UV-vis and fluorescent spectra of UPPY/CHCl3 during addition of CH3OH (Supporting Information Figure S6, S7). This


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excludes the possibility that spectral changes for rGO-UPPY-MWNT/CHCl3 were caused by the bonding of free UPPY with CH3OH. Furthermore, the UV-vis and fluorescent spectra of rGOPY-MWNT/CHCl3 could not be changed by adding CH3OH (Supporting Information Figure S8, S9) because there presents no quadruple hydrogen bonding. These control experiments further proved that cooperated with the π-π interaction, quadruple hydrogen bonds can behave as the bridge linkers between rGO and MWNT, and responsible for the significant electronic communication between the constituents in rGO-UPPY-MWNT. Generally, two major paths may exist for the electronic communication in rGO-UPPY-MWNT hybrids. The first is the intramolecular electronic communication in rGO, UPPY and MWNT respectively. And the latter should be intermolecular electronic communication resulted from either electron hopping from one molecule to another through space, or charge transfer as well as electron coupling via the UPPY·UPPY homodimers (through bond electronic communication).22 For the system of rGO-UPPY-MWNT, the enhancement of electronic communication should be attributed to cooperation of π-π and quadruple hydrogen bonds. Firstly, the noncovalent modification by UPPY make their inner conjugation of rGO and MWNT well undisrupted, which is beneficial for the intramolecular electronic communication. Moreover, under the cooperation of π-π and hydrogen bonding interactions, the distance between rGO and/or MWNT is significantly decreased, which can enhance the through space electronic communication. More importantly, as the introduction of π-π and quadruple hydrogen bonding, the through bond electronic communication between rGO and MWNT can be greatly enhanced due to the increase of electron motion pathways. 3.2 Electrochemical behavior of rGO-UPPY-MWNT


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(A)

(B) 200

12

-1

Electrical conductivity (S cm )

180

d

160

10

140

-Z''/ohm

8 6

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0 rGO-MWNT

rGO-PY-MWNT rGO-UP-MWNT rGO-UPPY-MWNT

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Z'/ohm (D)

(c)

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30 I(µ A)

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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

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E(V)

[Scan rate (mV/sec)]

Figure 7. (A) Electrical conductivity of rGO-MWNT, rGO-PY-MWNT rGO-UP-MWNT and rGO-UPPY-MWNT;

(B)

Nyquist

plot

of

modified

electrodes

in

2.5

mM

K4[Fe(CN)6]/K3[Fe(CN)6] and PBS vs SCE reference electrode (pH 7.0); (C) Cyclic voltammogram of modified electrodes in 1 mM Fe(CN)63−/4− and 0.1 M KCl at 50 mV s-1 vs Ag/AgCl reference electrode; (D) Plot of peak current vs square root of scan rate at modified electrodes. The modified electrodes were (a) rGO-UPPY-MWNT/GC, (b) rGO-UP-MWNT/GC, (c) rGO-PY-MWNT/GC and (d) rGO-MWNT/GC modified glassy carbon electrodes respectively. Above experiments indicate that the cooperated quadruple hydrogen bonds and π-π interactions play a vital role in the formation of rGO-UPPY-MWNT supramolecular network and


17


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Page 18 of 34

the remarkable enhancement of electronic communication. Thus the collective electrical conductivity as well as electrochemical behavior of the rGO-UPPY-MWNT is expected to be improved compared with rGO-MWNT, rGO-UP-MWNT and rGO-PY-MWNT. The electrical conductivity of rGO-MWNT, rGO-PY-MWNT, rGO-UP-MWNT and rGOUPPY-MWNT was compared in Figure 7A. The electrical conductivity of rGO-MWNT was 1.6 S cm-1. It was increased to 2.8 S cm-1 for rGO-PY-MWNT and 3.1 S cm-1 for rGO-UP-MWNT respectively. Surprisingly in the case of rGO-UPPY-MWNT, the electrical conductivity was enhanced to as high as 11.1 S cm-1. A possible explanation is that compared with rGO-MWNT, the PY or UP groups presented in rGO-PY-MWNT or rGO-UP-MWNT could prevent the selfstacking of rGO or MWNT and increase the conductive pathways, thus the electrical conductivity of GO-PY-MWNT and rGO-UP-MWNT was increased. As far as rGO-UPPYMWNT was concerned, the cooperated π-π and hydrogen bonding promoted the electronic communication between rGO and MWNT, which make the electron transfer efficiency of the conductive pathways increased. Simultaneously, the wrapped structures could enlarge the junction area and decrease the contact resistance, thus the electrical conductivity of GO-UPPYMWNT is significantly increased compared with rGO-PY-MWNT or rGO-UP-MWNT. Furthermore, the charge transfer resistance of rGO-MWNT, rGO-UP-MWNT, rGO-PYMWNT and rGO-UPPY-MWNT modified GC electrodes was measured using EIS (Figure 7B). Generally, the semicircle of EIS at high frequencies was characteristic of the charge transfer process and the diameter of the semicircle was equal to the charge transfer resistance (Rct). The rGO-MWNT/GC electrode showed the large charge transfer resistance (Rct =550 Ω, curve d, Figure 7B). However, the Rct decreased to 290 and 270 Ω for rGO-PY-MWNT/GC and rGO-UPMWNT/GC respectively (curve c and b, Figure 7B), Amazingly, when hydrogen bonding and π-


18

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π interaction were both presented in the case of rGO-UPPY-MWNT/GC, the Rct (50 Ω, curve a, Figure 7B) showed just one-tenth value of rGO-MWNT/GC (550 Ω, curve d, Figure 7B). Cyclic voltammetry (CV) was also employed to evaluate the electrochemical performance of the assembled networks. Figure 7C shows the cyclic voltammograms of rGO-UPPY-MWNT, rGO-MWNT, rGO-UP-MWNT and rGO-PY-MWNT modified GC electrodes recorded in 1 mM K4[Fe(CN)6]. The rGO-UPPY-MWNT/GC exhibit well defined oxidation and reduction peaks at 0.26 and 0.15 V vs Ag/AgCl due to Fe3+/Fe2+ redox couples with the peak-to-peak separation (∆Ep) of 110 mV. For the rGO-UP-MWNT, rGO-PY-MWNT and rGO-MWNT modified GC electrodes, the ∆Ep value was increased to 212, 260 and 317mV respectively. Different ΔEp can be used to show the barrier to electron transfer. From Figure 7C, the barrier to electron transfer decreases in the order of rGO-MWNT, rGO-PY-MWNT, rGO-UP-MWNT and rGO-UPPYMWNT. To explain above results, the electroactive surface area (ESA) for the modified electrodes was estimated. For ESA measurements, CVs of modified electrodes in 1 mM Fe(CN)63−/4− at scan rates varying from 10 to 400 mV/s were recorded and a linear relationship between the anodic peak current and the square root of scan rate was obtained (Figure 7D). As a result, the ESA was calculated based on the Randles-Sevcik equation: 47 Ip = 2.69 × 105AD1/2n3/2Cv1/2 where Ip is the peak current (mA), A is the ESA of the electrode investigated (cm2), D is the diffusion coefficient of ferricyanide (7 × 10−6 cm2/s), 47 n is the number of electrons transferred in the redox reaction, C is the concentration of ferricyanide (M), and v is the scan rate (V/s). The ESA value was calculated from the slope of the Ip versus v1/2 linear regression equation (Figure


19


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Page 20 of 34

7D).47 The ESA values for the rGO-UPPY-MWNT, rGO-UP-MWNT, rGO-PY-MWNT and rGO-MWNT modified GC electrodes were calculated as 0.218, 0.167, 0.144 and 0.115 cm2 respectively. Accordingly, the change of the ESA may be the reason for the different behavior in the Rct and ΔEp of the four assembled hybrids. It is noted that the rGO-UPPY-MWNT/GC had just a 1.90 fold increase in the ESA relative to rGO-MWNT, but it exhibited a 10 fold decrease in the Rct. This indicates that besides ESA, there must be other factors contributed to the excellent electrochemical behavior of rGO-UPPY-MWNT/GC, which may be correlated with the increased electrical conductivity and the electronic communication cooperated by π-π and hydrogen bonding. Evidently, the enhanced conductivity and electronic communication along with the increased ESA provided more pathways for the excellent electron transfer. 3.3 Electrocatalysis and amperometric detection of dopamine Based on the excellent electrical conductivity and electrochemical behavior, it was speculated that rGO-UPPY-MWNT/GC electrode should be an excellent electrochemical sensor for electroactive compounds. As a proof of concept, the electrochemical catalytic performance of the rGO-UPPY-MWNT/GC electrode was examined by using dopamine (DA) as a representative, which is of practical importance in chemical, biological, clinical and many other fields.43,44


20

Page 21 of 34

I(mA/cm

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(I/(µA/cm )

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-0.2 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

E(V)

E(V)

Figure 8. Cyclic voltammogram of 1×10–3 mol/L DA on (a) rGO-UPPY-MWNT/GC, (b) rGOPY-MWNT/GC, (c) rGO-UP-MWNT and (d) rGO-MWNT/GC electrode in 0.1M PBS of pH 7.0 at scan rate of 50 mV s-1 vs Ag/AgCl reference electrode in N2 atmosphere. Representative cyclic volatmmograms of DA (1×10-3 mol/L) in PBS buffer solutions on the modified GC electrodes are presented in Figure 8. As shown in Figure 8a, the CV obtained on the rGO-UPPY-MWNT/GC electrode displays a cathodic peak at 0.30V and an anodic peak at 0.02V, assigned to the oxidation of DA (1) to o-dopaminoquinone (2) and the reduction of 2 back to 1 respectively. The peak to peak separation of 1/2 redox couple (∆Ep12) is calculated as 280 mV. At pH 7.0, 2 could undergo intramolecular cyclization to form leucodopaminochrome (3). Due to the oxidation/reduction of 3 to dopaminochrome (4), a redox pair with an anodic peak at -0.38 V and a cathodic peak at -0.21 V attributed to the 3/4 redox couple were observed from Figure 8a, which were not presented at other carbon electrodes such as graphene, fullerene, nanofiber and graphite.45 Differently, the CV scans for modified GC electrodes of rGOMWNT, rGO-PY-MWNT and rGO-UP-MWNT showed much weaker redox peaks with the


21


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Page 22 of 34

larger ∆Ep12 values of about 400 mV (Figure 8b-d). Especially, the redox peaks attributed to 3/4 became more indistinctive than that on the rGO-UPPY-MWNT/GC. On the other hand, the reduction peak current assigned to 1/2 on rGO-UPPY-MWNT/GC (1.07 mA/cm2 Figure 8a) was greatly improved compared with the cases of rGO-MWNT/GC (0.35 mA/cm2, Figure 8d), rGO-PY-MWNT/GC (0.53 mA/cm2, Figure 8c) and rGO-UP-MWNT/GC (0.55 mA/cm2, Figure 8b). The well defined and increased redox peaks for rGO-UPPY-MWNT/GC in comparison with rGO-MWNT, rGO-PY-MWNT and rGO-UP-MWNT modified GC electrodes are mainly resulted from the increase of more efficient electron transfer pathways, due to the increased ESA and the well electrical contact brought by cooperated linkage from π-π and quadruple hydrogen bonding interactions. The CVs for different concentrations of dopamine at rGO-UPPY-MWNT/GC (Figure 9) revealed that upon the addition of dopamine, a remarkable increase in the oxidation current at 0.30 V was observed. The reduction current is increased linearly as the dopamine concentration is increased from 1.6×10–5 to 2.3×10–3 M (inset, Figure 9), which follows the equation as Ip (mA/cm2) = 0.90 Cdopamine (mM) + 0.10, where Ip is redox peak current, and Cdopamine is the concentration of DA. This indicates the possible potential of rGO-UPPY-MWNT/GC as a sensor for the determination of dopamine is in the range of 10-4 M~10-3 M. Moreover, the peak current of DA at 0.30V showed a proportional relation to the scan rate in the range 50~400mV/s (Figure S10, Supporting information), which reveals the oxidation of dopamine on the rGO-UPPY-MWNT/GC electrode should be an adsorption controlled process.


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Page 23 of 34

2.0

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0.0

0.4

0.0 0.4 0.8 1.2 1.6 2.0 2.4

C DA/mM

a

0.0

-0.4 -0.8 -1.2 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 E(V)

Figure 9. Cyclic voltammogram of MWNT-UPPY-rGO/GC with different concentrations of DA. (a) 0, (b) 1.6×10–5, (c) 3.6×10–5, (d) 5.6×10–5, (e) 9.6×10–5, (f)1.96×10–4, (g) 3.96×10–4, (h) 7.96×10–4, (i) 1.3×10–3 and (j) 2.3×10–3 M in 0.1M PBS at pH 7.0 and a scan rate of 50 mV s-1 vs Ag/AgCl reference electrode in N2 atmosphere. Inset: The linear relationship of peak current (Ip) of DA to the concentration of DA (CDA) in the range of 1.6×10–5 to 2.3×10–3 M. Next, the amperometric response on the assembled electrodes at 0.25V to the addition of DA in PBS solution was investigated. As shown in Figure 10A, 10B and 10C, the response of MWNT-UPPY-rGO/GC is sensitive and rapid with the injection of DA varied from 50 nM, 200 nM, 1µM to 40 µM. Furthermore, the amperometric signals of DA at rGO-UPPY-MWNT/GC electrode (Figure 10D(a)) showed linear relationship with the concentration of DA (CDA) within a broad concentration range from 50 nM to 120 µM, which follows the linear regression equation of IDA (nA/cm2) = 930 + 270 CDA (µM) with a correlation coefficient of 0.998. In the wide linear range, a satisfactory sensitivity of 270 nA/µM cm2 is achieved. Interestingly, a remarkably


23


amperometirc response (signal-to-noise ration = 3) was observed even by injecting DA with concentration being as low as 20 nM (inset, Figure 10D). Therefore, the rGO-UPPY-MWNT/GC offered a reasonable linear response and high sensitivity toward DA, and the detection limit was also satisfactory compared with reported carbon material-based electrodes.44, 46-48 In contrast, rGO-PY-MWNT/GC, rGO-UP-MWNT/GC and rGO-MWNT/GC have no similar amperometric responses to the addition of DA in concentration range from 50 nM to 200 nM (Figure 10B), indicating it is invalid for the above three electrodes to detect dopamine at such low concentrations. In addition, compared with rGO-UPPY-MWNT/GC (270 nA/µM cm2, Figure 10D(a)), the linear regression produced low slope for rGO-UP-MWNT/GC (46.1 nA/µM cm2, Figure 10D(b)), rGO-PY-MWNT/GC (56.0 nA/µM cm2, Figure 10D(c)) and rGO-MWNT/GC (50.8 nA/µM cm2, Figure 10D(d)). The excellent amperometric response of rGO-UPPYMWNT/GC could be attributed to the joint effect of increased ESA, enhanced electrical conductivity and charge transport properties resulted from the cooperation of π-π and multiplehydrogen bonding interactions.

20µM 2

20µM

20

15µM 10 1µM

a b c d

(B) 1.2

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I(µ A/cm )

30

2

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a b c d

(A)

I(µ A/cm )

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Page 24 of 34

25µM

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25 I(µA/cm )

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I(µ A/cm )

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12.5µM

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0

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600 Time(s)

700

800

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20

40 60 80 Concentration(µM)

2

100

120

Figure 10. (A) Amperometric curve of DA on (a) rGO-UPPY-MWNT/GC, (b) rGO-UPMWNT/GC, (c) rGO-PY-MWNT/GC and (d) rGO-MWNT/GC; (B-C) The partial magnify picture of (A); (D) Linear relationship of concentration DA on the four modified electrodes. Conclusion In this work, supramolecular hybrids based on rGO and MWNT were successfully selfassembled by quadruple hydrogen bonding and π-π interactions. The cooperating of hydrogen bonding and π-π interactions make rGO rolled up and wrapped around MWNT and lead to the better electronic communication, enhanced electrical conductivity, as well as high electrochemical sensitivity towards electroactive biomaterials. This strategy was not limited to the combination of rGO and MWNT, which can also be adapted to other nano-carbon material and conjugated polymers to prepare novel electrical catalysts. ASSOCIATED CONTENT Supporting Information. structure of N-[(butylamino)carbonyl]-6-tridecylisocytosine; TEM image of MWNT-UPPY, rGO-UP-MWNT and rGO-PY-MWNT; XRD spectra of rGO-PYMWNT, rGO-UP-MWNT and rGO-UPPY-MWNT; UV-vis absorption spectra of UPPY and


25


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rGO-UPPY-MWNT; fluorescent spectra of UPPY and rGO-PY-MWNT; linear relationship of peak current of DA oxidation to scan rate. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: +86 29 86173324. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 21103133; 51502233); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Natural Science Foundation of Shaanxi Province (No. 2015JM5224). REFERENCES (1)

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