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Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of π–π and hydrogen bonding interaction
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Journal of Photochemistry and Photobiology A: Chemistry xxx (2017) xxx–xxx
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction Sumin Wang, Lei Yang, Qiguan Wang* , Yaru Fan, Jiayin Shang, Shenbao Qiu, Jinhua Li, Wenzhi Zhang, Xinming Wu Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China
A R T I C L E I N F O
Article history: Received 30 June 2017 Received in revised form 15 August 2017 Accepted 11 September 2017 Available online xxx Keywords: Graphene LBL film Self-assembly Quadruple hydrogen bonds p–p interactions Electrochemistry
A B S T R A C T
In this paper an organic molecule labeled as UPPY with both p–p interaction unit of pyrene (PY) and hydrogen bonding unit of urediopyrimidinone (UP) was employed as binding module to link the layer-bylayer graphene film. Firstly, UPPY was anchored on the surface of thermal reduced graphene (trGO) with the aids of p–p interactions forming trGO-UPPY. The trGO-UPPY showed different morphology in CHCl3 and N,N-dimethylformamide (DMF) due to the different hydrogen binding mode. Then by the synergism of hydrogen bonding and p–p interactions offered by UPPY, multilayer film of trGO-UPPY was prepared through layer-by-layer technique. The electron-transfer resistance (Ret) of trGO-UPPY/ITO decreased from 53 V of bare ITO to 27 V. In addition, trGO-UPPY/ITO electrodes exhibited enhanced electrochemical activity toward dopamine (DA). Compared with bare ITO, the oxidation peak current of DA on rGO-UPPY/ITO can be enhanced about 100 times than that of bare ITO. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The supramolecular self-assembly afford researchers ideal bottom-up approaches for fabricating novel functional materials and devices with desirable architectures and properties. Design of novel building blocks and assembly strategies is of great importance for materials to self-assembly at nanometer scale. As the zero-dimensional (0D) and one dimensional (1D) functional units, dendrimers [1,2], nanoparticles [3,4], fullerenes [5,6], carbon nanotubes [7,8] have been widely used as the favorite nanoscale building blocks in the supramolecular chemistry. By means of its single-atom thickness and unprecedentedly large lateral surface, graphene has recently become a novel kind of two-dimensional (2D) building blocks [9,10]. By using supramolecular self-assembly methods, not only the solubility and conductivity of the graphene have been enhanced, but also complicated graphene based structures such as 2D multilayer film [11,12], three-dimensional (3D) ball [13,14] and hydrogels [15,16] exhibiting novel collective physiochemical properties have also been realized, which significantly expands the practical applications of graphene. Molecular recognition through noncovalent interactions is the fundamental of supramolecular self-assembly. Because graphene
* Corresponding author. E-mail address: [email protected] (Q. Wang).
has the large lateral dimension, for the achievement of stable graphene assemblies, those noncovalent interactions with high binding strength are usually needed to overcome the stacking of graphene sheets. Pyrene (PY) derivatives based p–p interactions which have strong affinity with the graphene layer by physical adsorption on the conjugate structure of graphene, would be a potential driving force for graphene assembly [17–19]. In addition, ureidopyrimidinone (UP) quadruple AADD (A: hydrogen bond acceptor, D: hydrogen bond donor) hydrogen bonding unit possessing very strong binding strength (107 M1 in CHCl3 or toluene) [20], is also a good candidate in the self-assembly of graphene [21]. Cooperation of noncovalent multi-interactions would be an effective way to fabricate graphene assemblies with desirable architecture and functionality [22–24]. Through hydrogen bonding and p–p interactions, Cheng et al. prepared graphene oxide (GO) hydrogels and organogels by supramolecular self-assembly from an amphiphilic molecule, which has a polarcarbohydrate head group attached to a pyrene group [25]. In our previous work, by designing and employing a self-assembly precursor of UP terminated pyrene derivatives (labeled as UPPY) (Fig. 1a), a novel hybrid showing graphene-wrapped MWNT morphology with enhanced electrochemical sensitivity was fabricated thanks to the cooperation of the hydrogen bonding and p–p interaction. Benefiting from the quadruple AADD hydrogen bonding arrays of
http://dx.doi.org/10.1016/j.jphotochem.2017.09.023 1010-6030/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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Fig. 1. Supramolecular self-assembly scheme of trGO-UPPY LBL multilayer film from the module of UPPY.
UP, UPPYUPPY homodimer could be formed (Fig. 1a). Simultaneously, the PY unit could also form strong p–p interaction with nano-carbon material. In this article we want to show that as a binding module, UPPY has universal adaptability to the selfassembly of nano-carbon material due to the contained p–p and hydrogen bonding unit [26]. The Layer-by-Layer (LBL) self-assembly method, initially reported by Decher and Hong is one of the most convenient techniques for fabricating films with finely controlled [27]. LBL films of graphene linked by noncovalent interactions such as electrostatic, hydrogen bonding supplied by the polymer for instance poly (vinyl alcohol), polyacrylic acid and ionic liquid modified on the graphene layer have been prepared [28–34]. Herein, UPPY unit was further employed to drive the supramolecular self-assembly of graphene film by LBL technique. Firstly, the UPPY was anchored on the surface of thermal reduced graphene (trGO) with the aids of p–p interactions forming trGO-UPPY (Fig. 1b left). Then by the layer-by-layer technique, a 2D graphene multilayer film of trGO-UPPY showing enhanced electrocatalytic properties to the oxidation of dopamine was self-assembled by the synergism of hydrogen bonding and p–p interactions offered by UPPY (Fig. 1b, right).
at room temperature affording a stable black dispersion. After filtrated and thoroughly washed with CHCl3, black powder of trGOUPPY was prepared. 2.3. Preparation of trGO-UPPY LBL Films Firstly, a cleaned quartz or ITO slide that had been ultrasonicated in a piranha solution (mixture of 98% H2SO4 and 30% H2O2, v/v 7:3) for 30 min was dipped in the PEI solution of H2O (1.5 mg/mL) at pH = 7.0 for 1 h. After it was rinsed with water and dried with nitrogen flow, the PEI modified substrate was dipped into the 1-pyrenebutyric acid solution of DMF for 15 min, rinsed and dried, leading to the modification of the substrate by 1pyrenebutyric acid. Then the 1-pyrenebutyric acid modified substrate was immersed in the suspension of trGO-UPPY in DMF (0.1 mg/mL) for 15 min. After dried by N2, the substrates adsorbed by one layer of trGO-UPPY were dipped again into the trGO-UPPY dispersion to obtain the second trGO-UPPY layer. The LBL multilayer films of (trGO-UPPY)n with desirable layer number (n) could be prepared by repetition of such steps (Fig. 1b, right). 2.4. Instrumentation
2. Experimental section 2.1. Regents Graphene oxide (GO) was prepared from natural graphite by the modified Hummers procedure [35,36]. Thermal reduced graphene oxide (trGO) was prepared by thermal reduction of GO under the Ar atmosphere at 600 C for 2 h [37]. UPPY (Fig. 1a) was prepared according to our previous reported methods [26]. Poly(ethyleneimine) (PEI) with the average molecular weight of 50,000 by GPC was purchased from Aldrich. During electrochemical experimental process, Milli-Q water (18 MV/cm) was used. N,NDimethylformamide (DMF) of HPLC grade was used for selfassembly process. All other reagents and chemicals were used as received unless otherwise noted. 2.2. Synthesis of trGO-UPPY Mixture of trGO (2 mg) and UPPY (6 mg) in CHCl3 (20 mL) was subjected to ultrasonication, and then it was allowed to stir for 6 h
UV–vis spectra were measured by a Shimadzu 1901 UV–vis spectrophotometer. 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 XRD6000 X-ray diffractometer. The Cu Ka line (l = 1.5451 nm) from a sealed tube with a copper anode was used as a source of radiation. Cyclic Voltammetry (CV) was carried out in a conventional threeelectrode system with the LBL trGO-UPPY multilayer films assembled on ITO glass as working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt wire as the counter electrode, using a 0.1 M phosphate buffer solution (PBS, pH = 7.0) containing 0.1 M KCl as electrolyte solution. The working electrode area is 0.25 cm2, which was always kept immersed in the electrolyte solution during the data collection.
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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3. Results and discussion 3.1. Synthesis and characterization of trGO-UPPY As depicted in Fig. 1, the engineering design behind the formation of supramolecular film of graphene is the establishment of organic molecule UPPY composed of both a planar conjugated pyrene ring and a ureidopyrimidinon (Fig. 1a). The pyrene group in UPPY can take strong p–p interactions with rGO, thus the UP units were attached to the surface of graphene forming trGO-UPPY (Fig. 1b, left). Driven by the strong AADD hydrogen bonding interactions between the UP units of UPPY, the LBL trGO-UPPY films could be fabricated (Fig. 1b, right). Fig. 2a presents the UV–vis spectra of trGO-UPPY, trGO and UPPY determined in CHCl3. The trGO exhibited featureless absorption in the range of 275–450 nm (green line). Two absorption peaks centered at 326 and 342 nm as the characteristic absorption peaks of UPPY with slight blue shift compared to the pure UPPY (blue line) were observed in the spectrum of trGO-UPPY (red line), clearly confirming the presence of UPPY on the trGO. The blue shift of the absorption peaks of UPPY in trGO-UPPY can be understood as the result of the p–p and hydrogen bonding interactions between the graphene sheets with UPPY [38]. The XRD pattern of trGO-UPPY, rGO and UPPY is compared in Fig. 2b. For trGO (green line) a broad peak centered at 2u = 26.3 attributed to the typical diffraction peak (002) of graphite is observed. The UPPY showed several sharp peaks in the range of 10– 30 , indicating the crystalline state of UPPY. However, the sharp peaks for UPPY disappeared in the XRD pattern of trGO-UPPY (red line), indicating the spatial arrangement of UPPY was varied in trGO-UPPY under the p–p and hydrogen bonding interactions with trGO [26]. The elemental composition of trGO-UPPY was confirmed by XPS survey scans (Fig. 2c). For trGO only C1s and weak O1s peaks
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appeared at around 284.5 and 533.3 eV in the XPS spectra. In the spectra of trGO-UPPY, besides the C1s and O1s features, N1s peak at around 400.0 eV could also be observed, showing the presence of UPPY in trGO-UPPY. By the XPS analysis, the content of nitrogen, oxygen and carbon atom in trGO-UPPY could be calculated as 1.10 atom%, 7.64 atom% and 91.26 atom% respectively, by which the amount of UPPY in trGO-UPPY could be calculated as 2.2 104 mol/g. The association behavior between UPPY and trGO in different solvents such as CHCl3 and DMF was analyzed by TEM. Different from the smooth and thin layer structure of raw trGO material (Fig. 2d), in CHCl3, the graphene surface in trGO-UPPY was heavily wrinkled and folded (Fig. 2e). This is because the different sites on one layer of rGO were linked together by the strong quadruple hydrogen bonding interactions between the absorbed UPPY units (Fig. 2d inset). As comparison, in DMF negligible wrinkle was found on the graphene layer of trGO-UPPY, because quadruple hydrogen bonds were inhibited by DMF. 3.2. Fabrication and characterization of LBL trGO-UPPY multilayer films To verify the contribution of UPPY to the self-assembly of graphene with such unit, the LBL self-assembly of the trGO-UPPY was carried out as depicted in Fig. 1b (right). For preparation of trGO-UPPY LBL films, the surface of the substrate should be firstly modified with pyrene groups, which could link with trGO-UPPY through p–p interactions. By dipping the PEI modified substrate into the pyrenebutyric acid solution, pyrene groups were absorbed on the substrate by the electrostatic interactions (Fig. 1b, right). The peak with the highest intensity attributed to the sp2 hybridized carbons of pyrene group appeared at 284.5 eV in the C1s XPS spectrum (Fig. S1), and the UV–vis spectrum (Fig. S2) showed the characteristic absorption band of pyrene group at 278, 333 and
Fig. 2. UV–vis (a), XRD (b) and XPS (c) spectra of trGO-UPPY, trGO and UPPY. TEM images of trGO/CHCl3 (d), trGO-UPPY/CHCl3 (e) and trGO-UPPY/DMF (f). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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Fig. 3. (a) UV–vis absorption spectra of trGO-UPPY multilayers on the quartz slide with the increasing of number of layers, from bottom to up the layer number is 3, 6, 9 and 12 respectively. Inset: the absorbance at 281 nm with the increasing number of layers. (b) XPS N1s spectra of the 5-layer trGO-UPPY multilayer film.
348 nm, exhibiting the successful attachment of pyrene groups on the slide. Subsequently, dipping the substrate modified by pyrene groups into the trGO-UPPY/DMF suspension, one layer of trGO-UPPY could be absorbed on it. Then by repeating deposition of the above substrate in trGO-UPP/DMF, multilayer films of trGO-UPPY will be formed with the help of hydrogen bonding and p–p interactions. Here the polar solvent of DMF was more favorable for the LBL assembly process than the apolar solvent of CHCl3, because in DMF the typical quadruple hydrogen bonding interactions among trGOUPPY could be inhibited by the interactions between the polar solvent of DMF and the ureidopyrimidinone units, which could not only enhance the dispersity of trGO-UPPY but also make the trGOUPPY clustering unbundled to incorporate with the quadruple hydrogen bonding modules on the substrate [39]. With the successfully LBL assembling of trGO-UPPY on quartz slide, two obvious absorption bands at 281 nm and 347 nm
attributed to the UPPY (Fig. 3a) were observed on the UV–vis spectrum of the LBL film. Meanwhile, featureless absorption ascribed to the graphene transition in the range of 400–700 nm (Fig. 3a) was also observed. Furthermore, the absorption value at 281 nm was linearly increased with the increase of the number of layers (Fig. 3a, inset). This indicates a stepwise growth in thickness for the trGO-UPPY LBL films. XPS analysis proved the presence of hydrogen bonding interactions in the linkage of trGO-UPPY LBL films. The XPS N1s spectrum of the 5-layer trGO-UPPY multilayer film could be deconvoluted into three component peaks centered at 400.4, 399.0 and 399.9 eV (Fig. 3b). The peak at 400.4 eV was attributed to the nitrogen of N¼in UPPY acting as hydrogen acceptor in hydrogen bonds [40]. A higher binding energy was found relative to the reported value of 397.1 eV because of the charge transfer to the adjacent hydrogen [39]. The two peaks at 399.0 and 399.9 eV were ascribed to nitrogen of NH C¼O participating the formation of
Fig. 4. SEM images of the surface of pyrene group functionalized quartz slide (a) as well as the 1-layer (b), 9-layer (c) and 15-layer (d) LBL trGO-UPPY films on quartz slides; (e) CVs of (trGO-UPPY)n multilayer film in 1 mM Fe(CN)64 and 0.1 M KCl with the scan rate of 50 mV s1. (f) Nyquist plots of blank ITO and the (trGO-UPPY)9 multilayer film modified electrode in 1 mM Fe(CN)64.
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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Fig. 5. (A) CVs of 1 105 M DA on (trGO-UPPY)9/ITO and bare ITO at scan rate of 50 mV s1. (B) CVs of (trGO-UPPY)9/ITO with different concentrations of DA at scan rate of 50 mV s1, (a) 6 106, (b) 8 106, (c) 1 105, (d) 1.2 105, (e) 2 105, (f) 4 105, (g) 6 105, (h) 8 105, (i) 1 104, (j) 2 104, (k) 3 104, (l) 5 104, (m) 7 104 and (n) 1 103 M; Inset: The linear relationship of peak current (Ip) to the concentration of DA in the range of 6 106–1 103 M. (C) Amperometric curve of DA on (trGOUPPY)9/ITO working electrode at 0.30 V. Linear relationship of response current to the concentration of DA for amperometric curve in the range of 0.2 mM to 4 mM (D) and 4 mM to 140 mM (E) respectively.
hydrogen bond and the free one respectively [41]. When behaving as a hydrogen donor, binding energy of nitrogen in NH C¼O should be decreased because of charge transfer from the adjacent hydrogen [39]. The surface morphologies of the trGO-UPPY LBL films were studied by SEM (Fig. 4a–d). After the quartz slide was treated with pyrenebutyric acid, some small dots (as denoted by arrows) were scattered on the slide indicating the successful deposition of pyrene groups (Fig. 4a). After 1-layer of trGO-UPPY was absorbed on the pyrene terminated slide by the p–p interaction, the pyrenebutyric dots were covered by the thin, transparent and wrinkled layer of graphene (Fig. 4b). With the increase of deposited layers, more densely distributed graphene sheets were exhibited in the case of the 9-layer (Fig. 4c) and 15-layer (Fig. 4d) trGO-UPPY films due to more trGO sheets were absorbed on the slide by the cooperation of hydrogen bonding and p–p interactions. In the case of 15-layer trGO-UPPY films (Fig. 4d), graphene aggregates were found because of the excessive adsorption of graphene sheets. The inherent electrochemistry of (trGO-UPPY)n multilayer films was investigated by using the Fe(CN)64/3 as redox probe. As seen from Fig. 4e, with the layer number of (trGO-UPPY)n increased from 3, 5 and 7 to 9, the oxidation peak current (Ip) of Fe3+/Fe2+ was enhanced from 1.25, 1.30 and 1.37 to 1.85 mA, due to the increase of the surface electrode area with the increased loading of trGOUPPY. Simultaneously, the peak-to-peak separation between the oxidation and reduction peak decreased from 646, 563 and 498 to 350 mV with the increasing layer number of (trGO-UPPY)n, indicating the barrier to electron transfer decreased [42,43]. Meanwhile, the cathodic and anodic redox peak potentials of Fe3
+ /Fe2+ were slightly shifted to more negative and positive values gradually, as a result of the enhanced electron transfer properties of the modified electrode attributed to the more trGO-UPPY deposited on the electrode. In addition, the Ip decreased when the layer number was more than 9 (Fig. S3), because the significant aggregation of graphene layers formed as shown in the SEM images (Fig. 4d). Furthermore, the electrochemical impedance spectroscopy (EIS) of bare ITO and the 9-layer of trGO-UPPY film modified ITO electrode ((trGO-UPPY)9/ITO) was compared in Fig. 4f. The semicircular part obtained at higher frequency range was characteristic of electron transfer process. The diameter of the semicircle is equal to the electron-transfer resistance (Ret). After assembled with 9-layer of trGO-UPPY films, the Ret decreased from 53 V of bare ITO to 27 V, revealing that (trGO-UPPY)9 multilayer could form high electron conduction pathways between the electrode and electrolyte, thus it was speculated that trGO-UPPY/ ITO electrode should be an excellent electrochemical sensor for electroactive compounds.
3.3. Electrocatalysis and detection of dopamine by trGO-UPPY/ITO electrode Dopamine (DA) is one of the important neurotransmitters in mammalian central nervous system. Understanding the electrochemical properties and quantifying the presence of DA in human body fluids is important [44]. As a proof of concept, the electrocatalytic performance of trGO-UPPY LBL films to DA was examined. Fig. 5A showed the CVs of bare ITO and (trGO-UPPY)9/
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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ITO electrodes toward DA (1 105 M) in PBS (pH = 7.0) buffer solution. As shown in Fig. 5A, the bare ITO has no obvious CV response to DA, whereas the CV on the (trGO-UPPY)9/ITO displayed an obvious cathodic peak at 0.30 V, corresponding to the electrooxidation of DA to dopaminequinone at the electrode surface [44]. Notably, the reduction peak current of DA on (trGO-UPPY)9/ITO (490 nA) is 100 times higher than that obtained at the bare ITO. The enhanced catalytic properties of trGO-UPPY/ITO to DA could be ascribed to the increased surface area and decreased electrontransfer resistance. Additionally, the p–p interactions between the aromatic structure of trGO-UPPY and DA molecules could noncovalently bond DA to the modified working electrodes, which could also enhance the response of the electrode to DA [44]. The CVs for different concentrations of DA at (trGO-UPPY)9/ITO is shown in Fig. 5B. Upon addition of DA, a remarkable increase in the oxidation current was observed (Fig. 5B). The reduction current is increased linearly as the DA concentration increased from 6 106 to 1 103 M (inset, Fig. 5B). It follows the equation as IDA (nA/cm2) = 720 + 155CDA (mM), where IDA is redox peak current, and CDA is the concentration of DA. This indicates the possible potential of trGO-UPPY/ITO as a sensor for the determination of dopamine in wide range of 106 M–103 M. Furthermore, the amperometric response of (trGO-UPPY)9/ITO to the addition of DA at 0.30 V was investigated in PBS solution. As shown in Fig. 5C, sensitive and rapid response was observed on (trGO-UPPY)9/ITO electrode when DA concentration was varied from 0.2 mM, 2 mM to 20 mM. As CDA ranges from 0.2 mM to 4 mM, the amperometric signals of DA follows the linear regression equation of IDA (nA/cm2) = 36 + 151CDA (mM) (Fig. 5D) with a correlation coefficient of 0.999. When the concentration of CDA changed from 4 mM to 140 mM, the amperometric signals of DA follows linear equation of IDA (nA/cm2) = 460 + 250CDA (mM) (Fig. 5E). Therefore, the rGO-UPPY-MWNT/GC offered a reasonable linear response and high sensitivity of 150–250 nA/mM cm2 toward DA. The linear range and sensitivity are comparable with the reported value [45,46]. 4. Conclusion By using the binding module of UPPY that containing pyrene group and pyrimidinone group simultaneously, the LBL graphene film with high coverage, decreased electron transfer resistant and high electrocatalytic performance toward dopamine could be assembled. This work showed that having two kinds of noncovalent binding unit, UPPY could be a versatile module to drive the supramolecular self-assembly of nano-carbon material. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21772152); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Natural Science Foundation of Shaanxi Province (No. 2015JM5224). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2017.09.023. Reference: [1] Y. Liu, C. Yu, H. Jin, B. Jiang, X. Zhu, Y. Zhou, Z. Lu, D. Yan, A supramolecular janus hyperbranched polymer and its photoresponsive self-assembly of vesicles with narrow size distribution, J. Am. Chem. Soc. 135 (2013) 4765–4770.
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Journal of Photochemistry and Photobiology A: Chemistry xxx (2017) xxx–xxx
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction Sumin Wang, Lei Yang, Qiguan Wang* , Yaru Fan, Jiayin Shang, Shenbao Qiu, Jinhua Li, Wenzhi Zhang, Xinming Wu Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710021, China
A R T I C L E I N F O
Article history: Received 30 June 2017 Received in revised form 15 August 2017 Accepted 11 September 2017 Available online xxx Keywords: Graphene LBL film Self-assembly Quadruple hydrogen bonds p–p interactions Electrochemistry
A B S T R A C T
In this paper an organic molecule labeled as UPPY with both p–p interaction unit of pyrene (PY) and hydrogen bonding unit of urediopyrimidinone (UP) was employed as binding module to link the layer-bylayer graphene film. Firstly, UPPY was anchored on the surface of thermal reduced graphene (trGO) with the aids of p–p interactions forming trGO-UPPY. The trGO-UPPY showed different morphology in CHCl3 and N,N-dimethylformamide (DMF) due to the different hydrogen binding mode. Then by the synergism of hydrogen bonding and p–p interactions offered by UPPY, multilayer film of trGO-UPPY was prepared through layer-by-layer technique. The electron-transfer resistance (Ret) of trGO-UPPY/ITO decreased from 53 V of bare ITO to 27 V. In addition, trGO-UPPY/ITO electrodes exhibited enhanced electrochemical activity toward dopamine (DA). Compared with bare ITO, the oxidation peak current of DA on rGO-UPPY/ITO can be enhanced about 100 times than that of bare ITO. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The supramolecular self-assembly afford researchers ideal bottom-up approaches for fabricating novel functional materials and devices with desirable architectures and properties. Design of novel building blocks and assembly strategies is of great importance for materials to self-assembly at nanometer scale. As the zero-dimensional (0D) and one dimensional (1D) functional units, dendrimers [1,2], nanoparticles [3,4], fullerenes [5,6], carbon nanotubes [7,8] have been widely used as the favorite nanoscale building blocks in the supramolecular chemistry. By means of its single-atom thickness and unprecedentedly large lateral surface, graphene has recently become a novel kind of two-dimensional (2D) building blocks [9,10]. By using supramolecular self-assembly methods, not only the solubility and conductivity of the graphene have been enhanced, but also complicated graphene based structures such as 2D multilayer film [11,12], three-dimensional (3D) ball [13,14] and hydrogels [15,16] exhibiting novel collective physiochemical properties have also been realized, which significantly expands the practical applications of graphene. Molecular recognition through noncovalent interactions is the fundamental of supramolecular self-assembly. Because graphene
* Corresponding author. E-mail address: [email protected] (Q. Wang).
has the large lateral dimension, for the achievement of stable graphene assemblies, those noncovalent interactions with high binding strength are usually needed to overcome the stacking of graphene sheets. Pyrene (PY) derivatives based p–p interactions which have strong affinity with the graphene layer by physical adsorption on the conjugate structure of graphene, would be a potential driving force for graphene assembly [17–19]. In addition, ureidopyrimidinone (UP) quadruple AADD (A: hydrogen bond acceptor, D: hydrogen bond donor) hydrogen bonding unit possessing very strong binding strength (107 M1 in CHCl3 or toluene) [20], is also a good candidate in the self-assembly of graphene [21]. Cooperation of noncovalent multi-interactions would be an effective way to fabricate graphene assemblies with desirable architecture and functionality [22–24]. Through hydrogen bonding and p–p interactions, Cheng et al. prepared graphene oxide (GO) hydrogels and organogels by supramolecular self-assembly from an amphiphilic molecule, which has a polarcarbohydrate head group attached to a pyrene group [25]. In our previous work, by designing and employing a self-assembly precursor of UP terminated pyrene derivatives (labeled as UPPY) (Fig. 1a), a novel hybrid showing graphene-wrapped MWNT morphology with enhanced electrochemical sensitivity was fabricated thanks to the cooperation of the hydrogen bonding and p–p interaction. Benefiting from the quadruple AADD hydrogen bonding arrays of
http://dx.doi.org/10.1016/j.jphotochem.2017.09.023 1010-6030/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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Fig. 1. Supramolecular self-assembly scheme of trGO-UPPY LBL multilayer film from the module of UPPY.
UP, UPPYUPPY homodimer could be formed (Fig. 1a). Simultaneously, the PY unit could also form strong p–p interaction with nano-carbon material. In this article we want to show that as a binding module, UPPY has universal adaptability to the selfassembly of nano-carbon material due to the contained p–p and hydrogen bonding unit [26]. The Layer-by-Layer (LBL) self-assembly method, initially reported by Decher and Hong is one of the most convenient techniques for fabricating films with finely controlled [27]. LBL films of graphene linked by noncovalent interactions such as electrostatic, hydrogen bonding supplied by the polymer for instance poly (vinyl alcohol), polyacrylic acid and ionic liquid modified on the graphene layer have been prepared [28–34]. Herein, UPPY unit was further employed to drive the supramolecular self-assembly of graphene film by LBL technique. Firstly, the UPPY was anchored on the surface of thermal reduced graphene (trGO) with the aids of p–p interactions forming trGO-UPPY (Fig. 1b left). Then by the layer-by-layer technique, a 2D graphene multilayer film of trGO-UPPY showing enhanced electrocatalytic properties to the oxidation of dopamine was self-assembled by the synergism of hydrogen bonding and p–p interactions offered by UPPY (Fig. 1b, right).
at room temperature affording a stable black dispersion. After filtrated and thoroughly washed with CHCl3, black powder of trGOUPPY was prepared. 2.3. Preparation of trGO-UPPY LBL Films Firstly, a cleaned quartz or ITO slide that had been ultrasonicated in a piranha solution (mixture of 98% H2SO4 and 30% H2O2, v/v 7:3) for 30 min was dipped in the PEI solution of H2O (1.5 mg/mL) at pH = 7.0 for 1 h. After it was rinsed with water and dried with nitrogen flow, the PEI modified substrate was dipped into the 1-pyrenebutyric acid solution of DMF for 15 min, rinsed and dried, leading to the modification of the substrate by 1pyrenebutyric acid. Then the 1-pyrenebutyric acid modified substrate was immersed in the suspension of trGO-UPPY in DMF (0.1 mg/mL) for 15 min. After dried by N2, the substrates adsorbed by one layer of trGO-UPPY were dipped again into the trGO-UPPY dispersion to obtain the second trGO-UPPY layer. The LBL multilayer films of (trGO-UPPY)n with desirable layer number (n) could be prepared by repetition of such steps (Fig. 1b, right). 2.4. Instrumentation
2. Experimental section 2.1. Regents Graphene oxide (GO) was prepared from natural graphite by the modified Hummers procedure [35,36]. Thermal reduced graphene oxide (trGO) was prepared by thermal reduction of GO under the Ar atmosphere at 600 C for 2 h [37]. UPPY (Fig. 1a) was prepared according to our previous reported methods [26]. Poly(ethyleneimine) (PEI) with the average molecular weight of 50,000 by GPC was purchased from Aldrich. During electrochemical experimental process, Milli-Q water (18 MV/cm) was used. N,NDimethylformamide (DMF) of HPLC grade was used for selfassembly process. All other reagents and chemicals were used as received unless otherwise noted. 2.2. Synthesis of trGO-UPPY Mixture of trGO (2 mg) and UPPY (6 mg) in CHCl3 (20 mL) was subjected to ultrasonication, and then it was allowed to stir for 6 h
UV–vis spectra were measured by a Shimadzu 1901 UV–vis spectrophotometer. 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 XRD6000 X-ray diffractometer. The Cu Ka line (l = 1.5451 nm) from a sealed tube with a copper anode was used as a source of radiation. Cyclic Voltammetry (CV) was carried out in a conventional threeelectrode system with the LBL trGO-UPPY multilayer films assembled on ITO glass as working electrode, a saturated calomel electrode (SCE) as reference electrode and a Pt wire as the counter electrode, using a 0.1 M phosphate buffer solution (PBS, pH = 7.0) containing 0.1 M KCl as electrolyte solution. The working electrode area is 0.25 cm2, which was always kept immersed in the electrolyte solution during the data collection.
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3. Results and discussion 3.1. Synthesis and characterization of trGO-UPPY As depicted in Fig. 1, the engineering design behind the formation of supramolecular film of graphene is the establishment of organic molecule UPPY composed of both a planar conjugated pyrene ring and a ureidopyrimidinon (Fig. 1a). The pyrene group in UPPY can take strong p–p interactions with rGO, thus the UP units were attached to the surface of graphene forming trGO-UPPY (Fig. 1b, left). Driven by the strong AADD hydrogen bonding interactions between the UP units of UPPY, the LBL trGO-UPPY films could be fabricated (Fig. 1b, right). Fig. 2a presents the UV–vis spectra of trGO-UPPY, trGO and UPPY determined in CHCl3. The trGO exhibited featureless absorption in the range of 275–450 nm (green line). Two absorption peaks centered at 326 and 342 nm as the characteristic absorption peaks of UPPY with slight blue shift compared to the pure UPPY (blue line) were observed in the spectrum of trGO-UPPY (red line), clearly confirming the presence of UPPY on the trGO. The blue shift of the absorption peaks of UPPY in trGO-UPPY can be understood as the result of the p–p and hydrogen bonding interactions between the graphene sheets with UPPY [38]. The XRD pattern of trGO-UPPY, rGO and UPPY is compared in Fig. 2b. For trGO (green line) a broad peak centered at 2u = 26.3 attributed to the typical diffraction peak (002) of graphite is observed. The UPPY showed several sharp peaks in the range of 10– 30 , indicating the crystalline state of UPPY. However, the sharp peaks for UPPY disappeared in the XRD pattern of trGO-UPPY (red line), indicating the spatial arrangement of UPPY was varied in trGO-UPPY under the p–p and hydrogen bonding interactions with trGO [26]. The elemental composition of trGO-UPPY was confirmed by XPS survey scans (Fig. 2c). For trGO only C1s and weak O1s peaks
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appeared at around 284.5 and 533.3 eV in the XPS spectra. In the spectra of trGO-UPPY, besides the C1s and O1s features, N1s peak at around 400.0 eV could also be observed, showing the presence of UPPY in trGO-UPPY. By the XPS analysis, the content of nitrogen, oxygen and carbon atom in trGO-UPPY could be calculated as 1.10 atom%, 7.64 atom% and 91.26 atom% respectively, by which the amount of UPPY in trGO-UPPY could be calculated as 2.2 104 mol/g. The association behavior between UPPY and trGO in different solvents such as CHCl3 and DMF was analyzed by TEM. Different from the smooth and thin layer structure of raw trGO material (Fig. 2d), in CHCl3, the graphene surface in trGO-UPPY was heavily wrinkled and folded (Fig. 2e). This is because the different sites on one layer of rGO were linked together by the strong quadruple hydrogen bonding interactions between the absorbed UPPY units (Fig. 2d inset). As comparison, in DMF negligible wrinkle was found on the graphene layer of trGO-UPPY, because quadruple hydrogen bonds were inhibited by DMF. 3.2. Fabrication and characterization of LBL trGO-UPPY multilayer films To verify the contribution of UPPY to the self-assembly of graphene with such unit, the LBL self-assembly of the trGO-UPPY was carried out as depicted in Fig. 1b (right). For preparation of trGO-UPPY LBL films, the surface of the substrate should be firstly modified with pyrene groups, which could link with trGO-UPPY through p–p interactions. By dipping the PEI modified substrate into the pyrenebutyric acid solution, pyrene groups were absorbed on the substrate by the electrostatic interactions (Fig. 1b, right). The peak with the highest intensity attributed to the sp2 hybridized carbons of pyrene group appeared at 284.5 eV in the C1s XPS spectrum (Fig. S1), and the UV–vis spectrum (Fig. S2) showed the characteristic absorption band of pyrene group at 278, 333 and
Fig. 2. UV–vis (a), XRD (b) and XPS (c) spectra of trGO-UPPY, trGO and UPPY. TEM images of trGO/CHCl3 (d), trGO-UPPY/CHCl3 (e) and trGO-UPPY/DMF (f). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
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Fig. 3. (a) UV–vis absorption spectra of trGO-UPPY multilayers on the quartz slide with the increasing of number of layers, from bottom to up the layer number is 3, 6, 9 and 12 respectively. Inset: the absorbance at 281 nm with the increasing number of layers. (b) XPS N1s spectra of the 5-layer trGO-UPPY multilayer film.
348 nm, exhibiting the successful attachment of pyrene groups on the slide. Subsequently, dipping the substrate modified by pyrene groups into the trGO-UPPY/DMF suspension, one layer of trGO-UPPY could be absorbed on it. Then by repeating deposition of the above substrate in trGO-UPP/DMF, multilayer films of trGO-UPPY will be formed with the help of hydrogen bonding and p–p interactions. Here the polar solvent of DMF was more favorable for the LBL assembly process than the apolar solvent of CHCl3, because in DMF the typical quadruple hydrogen bonding interactions among trGOUPPY could be inhibited by the interactions between the polar solvent of DMF and the ureidopyrimidinone units, which could not only enhance the dispersity of trGO-UPPY but also make the trGOUPPY clustering unbundled to incorporate with the quadruple hydrogen bonding modules on the substrate [39]. With the successfully LBL assembling of trGO-UPPY on quartz slide, two obvious absorption bands at 281 nm and 347 nm
attributed to the UPPY (Fig. 3a) were observed on the UV–vis spectrum of the LBL film. Meanwhile, featureless absorption ascribed to the graphene transition in the range of 400–700 nm (Fig. 3a) was also observed. Furthermore, the absorption value at 281 nm was linearly increased with the increase of the number of layers (Fig. 3a, inset). This indicates a stepwise growth in thickness for the trGO-UPPY LBL films. XPS analysis proved the presence of hydrogen bonding interactions in the linkage of trGO-UPPY LBL films. The XPS N1s spectrum of the 5-layer trGO-UPPY multilayer film could be deconvoluted into three component peaks centered at 400.4, 399.0 and 399.9 eV (Fig. 3b). The peak at 400.4 eV was attributed to the nitrogen of N¼in UPPY acting as hydrogen acceptor in hydrogen bonds [40]. A higher binding energy was found relative to the reported value of 397.1 eV because of the charge transfer to the adjacent hydrogen [39]. The two peaks at 399.0 and 399.9 eV were ascribed to nitrogen of NH C¼O participating the formation of
Fig. 4. SEM images of the surface of pyrene group functionalized quartz slide (a) as well as the 1-layer (b), 9-layer (c) and 15-layer (d) LBL trGO-UPPY films on quartz slides; (e) CVs of (trGO-UPPY)n multilayer film in 1 mM Fe(CN)64 and 0.1 M KCl with the scan rate of 50 mV s1. (f) Nyquist plots of blank ITO and the (trGO-UPPY)9 multilayer film modified electrode in 1 mM Fe(CN)64.
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Fig. 5. (A) CVs of 1 105 M DA on (trGO-UPPY)9/ITO and bare ITO at scan rate of 50 mV s1. (B) CVs of (trGO-UPPY)9/ITO with different concentrations of DA at scan rate of 50 mV s1, (a) 6 106, (b) 8 106, (c) 1 105, (d) 1.2 105, (e) 2 105, (f) 4 105, (g) 6 105, (h) 8 105, (i) 1 104, (j) 2 104, (k) 3 104, (l) 5 104, (m) 7 104 and (n) 1 103 M; Inset: The linear relationship of peak current (Ip) to the concentration of DA in the range of 6 106–1 103 M. (C) Amperometric curve of DA on (trGOUPPY)9/ITO working electrode at 0.30 V. Linear relationship of response current to the concentration of DA for amperometric curve in the range of 0.2 mM to 4 mM (D) and 4 mM to 140 mM (E) respectively.
hydrogen bond and the free one respectively [41]. When behaving as a hydrogen donor, binding energy of nitrogen in NH C¼O should be decreased because of charge transfer from the adjacent hydrogen [39]. The surface morphologies of the trGO-UPPY LBL films were studied by SEM (Fig. 4a–d). After the quartz slide was treated with pyrenebutyric acid, some small dots (as denoted by arrows) were scattered on the slide indicating the successful deposition of pyrene groups (Fig. 4a). After 1-layer of trGO-UPPY was absorbed on the pyrene terminated slide by the p–p interaction, the pyrenebutyric dots were covered by the thin, transparent and wrinkled layer of graphene (Fig. 4b). With the increase of deposited layers, more densely distributed graphene sheets were exhibited in the case of the 9-layer (Fig. 4c) and 15-layer (Fig. 4d) trGO-UPPY films due to more trGO sheets were absorbed on the slide by the cooperation of hydrogen bonding and p–p interactions. In the case of 15-layer trGO-UPPY films (Fig. 4d), graphene aggregates were found because of the excessive adsorption of graphene sheets. The inherent electrochemistry of (trGO-UPPY)n multilayer films was investigated by using the Fe(CN)64/3 as redox probe. As seen from Fig. 4e, with the layer number of (trGO-UPPY)n increased from 3, 5 and 7 to 9, the oxidation peak current (Ip) of Fe3+/Fe2+ was enhanced from 1.25, 1.30 and 1.37 to 1.85 mA, due to the increase of the surface electrode area with the increased loading of trGOUPPY. Simultaneously, the peak-to-peak separation between the oxidation and reduction peak decreased from 646, 563 and 498 to 350 mV with the increasing layer number of (trGO-UPPY)n, indicating the barrier to electron transfer decreased [42,43]. Meanwhile, the cathodic and anodic redox peak potentials of Fe3
+ /Fe2+ were slightly shifted to more negative and positive values gradually, as a result of the enhanced electron transfer properties of the modified electrode attributed to the more trGO-UPPY deposited on the electrode. In addition, the Ip decreased when the layer number was more than 9 (Fig. S3), because the significant aggregation of graphene layers formed as shown in the SEM images (Fig. 4d). Furthermore, the electrochemical impedance spectroscopy (EIS) of bare ITO and the 9-layer of trGO-UPPY film modified ITO electrode ((trGO-UPPY)9/ITO) was compared in Fig. 4f. The semicircular part obtained at higher frequency range was characteristic of electron transfer process. The diameter of the semicircle is equal to the electron-transfer resistance (Ret). After assembled with 9-layer of trGO-UPPY films, the Ret decreased from 53 V of bare ITO to 27 V, revealing that (trGO-UPPY)9 multilayer could form high electron conduction pathways between the electrode and electrolyte, thus it was speculated that trGO-UPPY/ ITO electrode should be an excellent electrochemical sensor for electroactive compounds.
3.3. Electrocatalysis and detection of dopamine by trGO-UPPY/ITO electrode Dopamine (DA) is one of the important neurotransmitters in mammalian central nervous system. Understanding the electrochemical properties and quantifying the presence of DA in human body fluids is important [44]. As a proof of concept, the electrocatalytic performance of trGO-UPPY LBL films to DA was examined. Fig. 5A showed the CVs of bare ITO and (trGO-UPPY)9/
Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023
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ITO electrodes toward DA (1 105 M) in PBS (pH = 7.0) buffer solution. As shown in Fig. 5A, the bare ITO has no obvious CV response to DA, whereas the CV on the (trGO-UPPY)9/ITO displayed an obvious cathodic peak at 0.30 V, corresponding to the electrooxidation of DA to dopaminequinone at the electrode surface [44]. Notably, the reduction peak current of DA on (trGO-UPPY)9/ITO (490 nA) is 100 times higher than that obtained at the bare ITO. The enhanced catalytic properties of trGO-UPPY/ITO to DA could be ascribed to the increased surface area and decreased electrontransfer resistance. Additionally, the p–p interactions between the aromatic structure of trGO-UPPY and DA molecules could noncovalently bond DA to the modified working electrodes, which could also enhance the response of the electrode to DA [44]. The CVs for different concentrations of DA at (trGO-UPPY)9/ITO is shown in Fig. 5B. Upon addition of DA, a remarkable increase in the oxidation current was observed (Fig. 5B). The reduction current is increased linearly as the DA concentration increased from 6 106 to 1 103 M (inset, Fig. 5B). It follows the equation as IDA (nA/cm2) = 720 + 155CDA (mM), where IDA is redox peak current, and CDA is the concentration of DA. This indicates the possible potential of trGO-UPPY/ITO as a sensor for the determination of dopamine in wide range of 106 M–103 M. Furthermore, the amperometric response of (trGO-UPPY)9/ITO to the addition of DA at 0.30 V was investigated in PBS solution. As shown in Fig. 5C, sensitive and rapid response was observed on (trGO-UPPY)9/ITO electrode when DA concentration was varied from 0.2 mM, 2 mM to 20 mM. As CDA ranges from 0.2 mM to 4 mM, the amperometric signals of DA follows the linear regression equation of IDA (nA/cm2) = 36 + 151CDA (mM) (Fig. 5D) with a correlation coefficient of 0.999. When the concentration of CDA changed from 4 mM to 140 mM, the amperometric signals of DA follows linear equation of IDA (nA/cm2) = 460 + 250CDA (mM) (Fig. 5E). Therefore, the rGO-UPPY-MWNT/GC offered a reasonable linear response and high sensitivity of 150–250 nA/mM cm2 toward DA. The linear range and sensitivity are comparable with the reported value [45,46]. 4. Conclusion By using the binding module of UPPY that containing pyrene group and pyrimidinone group simultaneously, the LBL graphene film with high coverage, decreased electron transfer resistant and high electrocatalytic performance toward dopamine could be assembled. This work showed that having two kinds of noncovalent binding unit, UPPY could be a versatile module to drive the supramolecular self-assembly of nano-carbon material. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 21772152); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry; the Natural Science Foundation of Shaanxi Province (No. 2015JM5224). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. jphotochem.2017.09.023. Reference: [1] Y. Liu, C. Yu, H. Jin, B. Jiang, X. Zhu, Y. Zhou, Z. Lu, D. Yan, A supramolecular janus hyperbranched polymer and its photoresponsive self-assembly of vesicles with narrow size distribution, J. Am. Chem. Soc. 135 (2013) 4765–4770.
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Please cite this article in press as: S. Wang, et al., Supramolecular self-assembly of layer-by-layer graphene film driven by the synergism of p–p and hydrogen bonding interaction, J. Photochem. Photobiol. A: Chem. (2017), http://dx.doi.org/10.1016/j.jphotochem.2017.09.023