Noncovalent functionalization of graphene by CdS nanohybrids for electrochemical applications

Thin Solid Films 568 (2014) 58–62

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Noncovalent functionalization of graphene by CdS nanohybrids for electrochemical applications Li Wang a, Wei Qi a,b,c,d,⁎, Rongxin Su a,b,c,d, Zhimin He a,b,d a

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China d Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, PR China b c

a r t i c l e

i n f o

Article history: Received 21 June 2013 Received in revised form 1 August 2014 Accepted 8 August 2014 Available online 14 August 2014 Keywords: Graphene Cadmium sulfide Nanocomposites Noncovalent functionalization Glucose Biosensor

a b s t r a c t Graphene–CdS (GR–CdS) nanocomposites were synthesized via a noncovalent functionalization process. To retain the intrinsic electronic and mechanical properties of graphene, the pristine graphene was firstly modified with 1-aminopyrene based on a strong π–π bond between the pyrenyl groups and the carbon rings of the graphene. Then the CdS nanocrystals were uniformly grown on the amino-graphene. The GR–CdS nanocomposites were characterized by UV–vis spectroscopy and scanning electron microscopy. A glucose biosensor was then fabricated based on the as-prepared GR–CdS nanocomposite by immobilizing glucose oxidase (GOD) in a chitosan thin film on a glassy carbon electrode. Direct electron transfer between GOD and the electrode was achieved and the biosensor showed good electrocatalytic activity with glucose ranging from 0.5 to 7.5 mM and a sensitivity of 45.4 μA mM−1 cm−2. This work provided a simple and nondestructive functionalization strategy to fabricate graphene-based hybrid nanomaterials and it is expected that this composite film may find more potential applications in biosensors and biocatalysis. © 2014 Elsevier B.V. All rights reserved.

1. Introduction After the discovery of graphene, it has become a sparklingly rising star on the horizon of material science. Due to its superior mechanical, thermal, optical and electronic properties [1–3], graphene exhibits tremendous promise in various nanodevice applications such as batteries, fuel cells, supercapacitors, and sensors [4,5]. Besides, based on its unique geometrical structure and excellent conductivity, recently, biological and electrocatalytic applications of graphene have also started to be developed, particularly for constructing electrochemical biosensors [6]. Recently, to further extend the potential application of graphene, the development of graphene-based hybrid nanostructures such as graphene-inorganic nanocomposites, are attracting more and more interest [7]. These nanoparticulate hybrid systems were derived from the decoration of graphene sheets with inorganic nanoparticles such as metal [8], metal oxides [9] or semiconductor nanoparticles [10] and the nanocomposites can show properties superior to those of their individual constituents based on synergetic effects and thus enhance their performance in various applications.

⁎ Corresponding author at: Chemical Engineering Research Center, Tianjin University, Tianjin 300072, PR China. Tel.: +86 22 2740 7799; fax: +86 22 2740 7599. E-mail address: [email protected] (W. Qi).

http://dx.doi.org/10.1016/j.tsf.2014.08.003 0040-6090/© 2014 Elsevier B.V. All rights reserved.

As the surface of pristine graphene is hydrophobic and tends to be chemically inert, it is necessary to activate the graphitic surface in order to fabricate graphene-based nanocomposites [11]. In most of the researches, graphene nanocomposites are synthesized with graphite oxide and then chemically reduced, because graphite oxide is more hydrophilic compared with pristine graphene due to the oxygen-containing functional groups on both basal planes and the edges of graphite oxide, which can improve the solubility of the nanosheets [12]. However, the main disadvantage of this synthesis process is that the sp2 hybridized structure of the pristine graphene may be destroyed in the graphite oxide and chemical reduction procedure, which tends to degrade the mechanical and electronic performance of graphene [13]. In this paper, we aim to explore a method to fabricate graphene–CdS nanocomposites through a nondestructive synthetic route to protect the intrinsic properties of graphene. As a kind of semiconductor nanomaterial, CdS nanoparticles have been extensively studied due to their unique optical and electronic properties which can be used in the fabrication of electrochemical sensors [14]. Instead of the harsh oxidative approaches, in current study, the surface of pristine graphene was first modified with 1-aminopyrene via a strong π–π stacking mechanism between the pyrenyl groups and the carbon rings of the graphene. These interactions are spontaneous and have led to significant analytical advantages in the case of graphene or carbon nanotubes [15]. Herein, 1-aminopyrene, a bifunctional molecule, was previously reported as a proper interlinker for noncovalent functionalization of

L. Wang et al. / Thin Solid Films 568 (2014) 58–62

carbon nanotubes or graphene by forming a self-assembly layer on the surface of these materials [15–17]. Subsequently, the CdS nanoparticles in situ grew onto the surface of the modified graphene uniformly from CdS precursors. The as-prepared graphene–CdS (GR–CdS) nanocomposites were then used to construct a glucose biosensor, which exhibited improved performances connected with faster electron transfer, high sensitivity and good stability.

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allowed to dry at room temperature. After that, 1 mg of as-prepared GR–CdS nanocomposites was dispersed in 0.5% chitosan (CS) acetic acid solution, and the suspension was mixed thoroughly with 1 mL of GOD solution (10 mg/mL). Next, 10 μL of the resulting solution was cast onto the GC electrode surface using a syringe, and the modified electrode was moved into a refrigerator and kept at 4 °C to dry overnight. The fabricated modified electrode was stored at 4 °C in a refrigerator when not in use.

2. Experimental details 3. Results and discussion 2.1. Chemicals 3.1. Characterization of GR–CdS nanocomposites Glucose oxidase (GOD, E.C. 1.1.3.4, 125 U/mg, from Aspergillus niger) and D-(+)-glucose were purchased from Sigma (St. Louis, USA). Thioacetamide (TAA), 1-aminopyrene and cadmium nitrate tetrahydrate were purchased from Alfa Aesar (Ward Hill, USA). Chitosan (CS) was obtained from TCI (Tokyo, Japan). The graphene was supplied by Nanjing XF Nano Material (Nanjing, China). All other reagents were of analytical grade and used as received without further purification. 2.2. Instrumentation Electrochemical measurements were performed on a LK2005A electrochemical workstation and the cyclic voltammograms range from −1.0 to 0 V. A three-electrode system was employed with a glassy carbon (GC) electrode as working electrode, a platinum foil as counter electrode, and a KCl saturated calomel electrode (SCE) as reference electrode. UV–vis spectra were recorded on a Persee TU-1810 spectrophotometer. The microstructure and surface morphologies of composites were identified by a scanning electron microscope (SEM, HITACHI S-4800) at the acceleration voltage of 5 kV with an energy dispersive X-ray spectrometer (EDS). The morphologies of composites were identified by a JEM-2100 transmission electron microscope (TEM, Tokyo, Japan) operated at 200 kV. 2.3. Synthesis of GR–CdS nanocomposites Briefly, 4 mg pristine graphene and 2 mg 1-aminopyrene were dispersed in 5 mL ethanol. The solution was sonicated for 3 h under a nitrogen atmosphere and stirred overnight to prepare the aminographene. Subsequently, 2 mg of amino-graphene was sonicated in 4 mL tetrahydrofuran (THF) containing 5 mg Cd(NO3)2 for 30 min. Then, 500 μL 0.1 M TAA solution was slowly added into the mixture under vigorous stirring. The solution was kept for 2 h at room temperature under continuous stirring and the GR–CdS nanocomposites were obtained after centrifugation, and washed several times with distilled water. The procedure for the preparation of GR–CdS nanocomposites was schematically shown in Fig. 1. 2.4. Preparation of the modified electrodes Prior to use, the bare GC electrode was firstly polished with 0.3 and 0.05 μm alumina slurry, respectively to obtain a mirror-like surface and ultrasonically cleaned in ethanol and water thoroughly. Then it was

SEM was used to characterize the surface morphologies of GR–CdS nanocomposites. From Fig. 2a, the pristine graphene showed a clean layer and flake-like shape structure. In contrast, for GR–CdS nanocomposites, many spherical nanoparticles were observed with the average diameter of less than 20 nm (Fig. 2b), indicating the formation of CdS nanoparticles with a homogeneous diameter and well distribution onto the surface of graphene. The formation mechanism of the GR–CdS hybrids can be explained as follows: The abundant amino groups of 1-aminopyrene stacked on the graphene can effectively adsorb Cd2 + ions due to the preferential affinity of Cd2 + for the amino group [16], then the CdS nuclei formed on the graphene surfaces and grew based on the interaction of Cd2+ with S2−, the latter being slowly released by the decomposition of TAA. This results in the uniform formation of CdS nanoparticles on graphene surface. For comparison, GR–CdS nanocomposites were also synthesized in the same process except the aid of 1-aminopyrene, and the SEM micrograph showed largely irregular aggregates of CdS particles around the graphene (inset of Fig. 2b). The morphology of GR–CdS nanocomposites was characterized further by TEM (Fig. 2c). In addition, the existence of Cd and S elements of the nanocomposites could be further confirmed by the EDS analysis (Fig. 2d), in which the C and O signals originated from the graphene, and the Cd and S signals were obtained from the CdS. The UV–vis absorption spectroscopy was also used to analyze the GR–CdS nanocomposites dispersed in distilled water with the concentration of 0.5 mg mL−1. As shown in Fig. 3, the absorption peak of graphene was shifted from 266 nm to 239 nm. The change of the surface of the graphene could also partly indicate the formation of the nanocomposites [18]. 3.2. Direct electrochemistry of the GOD on the GR–CdS/CS/GC electrode Fig. 4 presented the cyclic voltammograms (CVs) of the GOD/CS/GC, GOD/GR/CS/GC and GOD/GR–CdS/CS/GC electrodes. A pair of well-defined and nearly symmetric redox peaks was obtained in each GOD modified electrode. The characteristic redox waves were attributed to the GOD, which was characteristic of a reversible electron transfer process involving the redox active center and the flavin adenine dinucleotide (FAD) [19,20]. Thus, it indicated that a direct electron transfer between GOD and electrode surface was achieved. Obviously, the GOD/GR–CdS/CS/GC electrode (Fig. 4, curve a) offered the highest peak current than GOD/GR/CS/GC (Fig. 4, curve b) and GOD/

Fig. 1. Scheme for the preparation of GR–CdS nanocomposites.

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a

b

200 nm

100 nm

100 nm

c

d

100 nm

Fig. 2. SEM of pristine graphene (a), GR–CdS nanocomposites (b) and CdS aggregates grown on the pristine graphene (inset of b); TEM of GR–CdS nanocomposites (c); EDS analysis of GR–CdS nanocomposites (d).

CS/GC (Fig. 4, curve c) electrodes. For the GOD/GR–CdS/CS/GC electrode, the formal potential (E°′), calculated by averaging the cathodic and anodic peak potential, was approximately −0.475 V, which was close to the standard electrode potential of GOD [21], suggesting that the GOD molecules retained their bioactivity after immobilization on the electrode. The peak potential separation (ΔEp) of GOD/GR–CdS/CS/GC electrode was about 50 mV. This result revealed that the existence of the GR–CdS nanocomposites played an important role in promoting the electron exchange between the electroactive center of GOD and

the electrode. The reasons could be summarized as follows. Graphene contains sp2 hybridized carbon atoms packed into a dense honeycomb crystal structure and is characterized by high surface area and excellent electric conductivity, which can accelerate the direct electron communication. The introduction of CdS, a kind of semiconductor nanocrystals, could greatly increase the electrode surface area. Furthermore, it is thought that the nanometric edges of CdS nanoparticles could penetrate slightly into the GOD, which could decrease the distance between the redox center of GOD and the electrode, and thus promote the electron 6

a b

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c I / µA

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0

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E / V (vs. SCE) Fig. 3. UV–Vis spectra of graphene (black), CdS (red) and GR–CdS nanocomposites (blue) suspensions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. CVs of the GOD/GR–CdS/CS/GC (a), the GOD/GR/CS/GC (b), and the GOD/CS/GC (c) electrodes in a N2-saturated phosphate buffer solution (0.1 M, pH = 7) at a scan rate of 100 mV s−1.

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3.3. Electrocatalytic properties of the modified electrode towards glucose Fig. 7 showed the CVs of GOD/GR–CdS/CS/GC electrode in an O 2 -saturated 0.1 M PBS solution with various concentrations of glucose. It has been reported that as the substrate of GOD, glucose can be catalyzed to form gluconolactone by GOD in the O2-saturated condition, and this biocatalytic reaction involves reduction of the flavin group (FAD) embedded in the GOD to give the reduced form of the GOD (FADH2), followed by reoxidation of the flavin by molecular oxygen to regenerate the oxidized form of the GOD (FAD). As a result, the addition of glucose in the solution can result in the increase of the reduced form of GOD and the consumption of oxygen, and the peak current

15 I p / µA

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E / V (vs. SCE) Fig. 6. CVs of the GOD/GR–CdS/CS/GC electrode in a N2-saturated phosphate buffer solution (0.1 M) with different pH values from 4.0 to 8.0 at a scan rate of 100 mV s−1.

originating from the reduction of O2 decreased with the addition of the glucose. Based on this, a glucose biosensor was further developed [28]. It was clearly seen that the reduction peak current of dissolved oxygen decreased with successive addition of glucose, which was consistent with the reports in literatures [29,30]. The reason was that the reaction between GOD and glucose leaded to the consumption of oxygen. Based on this, the concentration of glucose could be determined. The decrease of peak currents increased linearly with the glucose concentration from 0.5 to 7.5 mM with a correlation coefficient (R2) of 0.9975 (Fig. 7, inset). The sensitivity was 45.4 μA mM−1 cm−2, and the detection limit was 19.0 μM at a signal-to-noise ratio of 3. These results showed that the GOD/GR–CdS/CS/GC electrode had a better performance in terms of sensitivity and detection limit compared with the glucose sensors based on graphene-based nanocomposites synthesized from graphene oxide [31–33], which indicated that the noncovalent functionalization strategy towards graphene could effectively protect electronic properties of graphene for electrochemical application. When the glucose concentration exceeded 8.0 mM, the catalytic peak currents reached a platform, which could be well described by the Michaelis–Menten kinetics. The apparent Michaelis–Menten constant (Kapp m), which gives an indication of the affinity between enzyme and substrate, can be calculated using the electrochemical version of the Lineweaver–Burk equation [22]. 1/Iss = 1/Imax + Kapp m / (Imax × C). The Kapp m for the GOD/GR–CdS/CS/GC electrode was

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20 mV s

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E o'/ V

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transfer [14]. Besides, the GR–CdS nanocomposites are fabricated via noncovalently chemical assembly process instead of traditionally aggressive reactions, so the intrinsically electronic and mechanical properties of the components are protected and a significantly synergistic effect in the hybrids can be achieved for excellent electrochemical behavior. The influence of scan rate on the cyclic voltammetric performance of GOD/GR-CdS/CS/GC electrode was investigated. With the increase of scan rate, the redox peak current and the peak separation increased simultaneously, as shown in Fig. 5. Both anodic and cathodic peak currents increased linearly with scan rates from 20 to 300 mV/s (inset of Fig. 5). These characteristics suggested that the redox reaction of GOD at the GR-CdS/CS modified electrode was a surface-controlled electrochemical process [22,23]. According to the Laviron's equation [24], the electron transfer coefficient α was evaluated to be 0.47 and the apparent heterogeneous electron transfer rate constant (ks) was estimated to be 3.71 s−1. Here, the ks value of GOD/GR–CdS/CS/GC electrode was higher than that of GOD/GR/CS/GC electrode (3.46 s−1), GOD/CS/GC electrode (2.62 s−1) and Nafion/nanoCdS/carbon ionic liquid electrode (0.291 s−1) [25]. The higher ks further implied that the GR–CdS modified electrode was more beneficial to the direct electrochemistry of GOD. Cyclic voltammograms of the GOD/GR–CdS/CS/GC electrode in 0.1 M PBS with different pH values were shown in Fig. 6. Both the anodic and cathodic peak potentials shifted negatively with increasing pH from 4.0 to 8.0. The maximum current was achieved at pH of 7.0, which was close to the optimum pH observed for the GOD [26]. In addition, the formal potential showed a linear response to pH with the linear regression equation: E°′ = − 0.0592 pH − 0.05 (r2 = 0.9934) (inset of Fig. 6), the slope was − 59.2 mV/pH, which was close to the theoretical value of − 58.6 mV/pH for a reversible, two-proton coupled with two-electron redox reaction process [27].

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E / V (vs. SCE) Fig. 5. CVs of the GOD/GR-CdS/CS/GC electrode in a N2-saturated phosphate buffer solution (0.1 M, pH = 7) at various scan rates. The scan rates (from inner to outer) were 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280 and 300 mV s−1, respectively. Inset: plot of the peak current Ip vs. scan rate.

-0.2

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E / V (vs. SCE) Fig. 7. CVs of the GOD/GR–CdS/CS/GC electrode in an O2-saturated phosphate buffer solution (0.1 M, pH = 7) containing different amounts of glucose from 0 to 9.5 mM at a scan rate of 100 mV s−1. Inset: plot of the decrease of reduction peak current (ΔIp) vs. the concentration of glucose.

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calculated to be 3.43 mM. This value was much smaller than some reported values [34,35], which indicated that the GOD/GR–CdS/CS/GC electrode possessed a higher enzymatic activity and higher affinity to glucose. The reproducibility of the biosensor was investigated for 3 mM glucose sensing. With a series of 6 experiments, the relative standard deviation of 5.3% was achieved. The stability of electrode was also investigated by examining the cyclic voltammetric peak currents of GOD after scanning for 50 cycles. There was nearly no obvious decrease of the voltammetric response on GOD/GR–CdS/CS/GC electrode. When the GOD/GR–CdS/CS/GC electrode was stored at 4 °C for two weeks, the current response to 2 mM glucose remained 92.3% of its original value, indicating good stability of the electrode. 4. Conclusions In summary, the GR–CdS nanocomposites have been synthesized via a facile noncovalent functionalization process. With the aid of 1-aminopyrene, active amine groups were introduced which could provide nucleation centers on the surface of graphene and facilitate the uniform growth of CdS nanoparticles. Based on the electrochemical behavior, a sensor is developed for the determination of glucose with good analysis performance. This work provided a nondestructive and simple functionalization method to fabricate graphene-based hybrid nanomaterials, which had a great potential for applications in various fields, such as electrochemical sensing, catalytic reaction, and super capacitors. Acknowledgment This work was supported by the Natural Science Foundation of China (51173128, 31071509, 20976125), the 863 Program of China (2012AA06A303, 2013AA102204); the Ministry of Science and Technology of China (2012YQ090194), the Beiyang Young Scholar of Tianjin University (2012) and the Program of Introducing Talents of Discipline to Universities of China (No. B06006). References [1] A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183. [2] R. Negishi, H. Hirano, Y. Ohno, K. Maehashi, K. Matsumoto, Y. Kobayashi, Layer-by-layer growth of graphene layers on graphene substrates by chemical vapor deposition, Thin Solid Films 519 (2011) 6447. [3] S. Mitra, S. Banerjee, D. Chakravorty, Tunneling conduction in graphene/(poly)vinyl alcohol composite, J. Appl. Phys. 113 (2013) 154314. [4] V. Kiisk, T. Kahro, J. Kozlova, L. Matisen, H. Alles, Nanosecond laser treatment of graphene, Appl. Surf. Sci. 276 (2013) 133. [5] K. Vinodgopal, B. Neppolian, N. Salleh, I.V. Lightcap, F. Grieser, M. Ashokkumar, T.T. Ding, P.V. Kamat, Dual-frequency ultrasound for designing two dimensional catalyst surface: reduced graphene oxide-Pt composite, Colloids Surf. A Physicochem. Eng. Asp. 409 (2012) 81. [6] S. Pruneanu, F. Pogacean, A.R. Biris, M. Coros, F. Watanabe, E. Dervishi, A.S. Biris, Electro-catalytic properties of graphene composites containing gold or silver nanoparticles, Electrochim. Acta 89 (2013) 246. [7] A. Marlinda, N. Huang, M. Muhamad, M. An'amt, B. Chang, N. Yusoff, I. Harrison, H. Lim, C. Chia, S.V. Kumar, Highly efficient preparation of ZnO nanorods decorated reduced graphene oxide nanocomposites, Mater. Lett. 80 (2012) 9. [8] Y. Fang, S. Guo, C. Zhu, Y. Zhai, E. Wang, Self-assembly of cationic polyelectrolytefunctionalized graphene nanosheets and gold nanoparticles: a two-dimensional heterostructure for hydrogen peroxide sensing, Langmuir 26 (2010) 11277.

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