polyaniline composite film: Application to sensitive electrochemical detection of dobutamine

Sensors and Actuators B 196 (2014) 582–588

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

In-situ electro-polymerization of graphene nanoribbon/polyaniline composite film: Application to sensitive electrochemical detection of dobutamine Elham Asadian a , Saeed Shahrokhian a,b,∗ , Azam Iraji zad a,c , Effat Jokar a a

Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran 14588-89694, Iran Department of Chemistry, Sharif University of Technology, Tehran 11155-9516, Iran c Department of Physics, Sharif University of Technology, Tehran 14588-89694, Iran b

a r t i c l e

i n f o

Article history: Received 24 October 2013 Received in revised form 1 February 2014 Accepted 13 February 2014 Available online 22 February 2014 Keywords: Unzipped carbon nanotubes Solvothermal route In situ electro-polymerization Graphene nanoribbons/polyaniline composite Dobutamine

a b s t r a c t The present paper demonstrates the capability of narrow graphene nanoribbons (GNRs) in constructing new sensing platforms. Graphene nanoribbons have been synthesized via a simple solvothermal route through unzipping of carbon nanotubes, which was confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy analysis. These narrow carbon sheets were used to form a composite film by in-situ electro-polymerization with aniline. The produced graphene nanoribbon/polyaniline (GNR/PANI) composite film showed impressive performance in electrochemical determination of dobutamine (DBT). Under optimal conditions, in comparison to bare glassy carbon electrode a significant increase in peak current was observed on the surface of GNR/PANI modified glassy carbon electrode (up to 10 times), which is ascribed to the higher specific surface area induced by GNRs in combination with the electrocatalytic effect of polyaniline layer. We believe that such a composite film has a great potential in different applications including sensors, supercapacitors and etc. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Fabrication of rapid, selective, sensitive and cost effective sensing platforms, especially based on carbon nanomaterials, has attracted a great interest [1–3]. Although a considerable part of researches has been allocated to carbon nanotubes in the last decades [4–6], fascinating properties of graphene, single-layered graphite sheet, brought it very rapidly in the focus of attention in the realm of carbon nanostructures due to its outstanding electrical, mechanical and chemical properties [7–9]. In spite of the fact that graphene has potential applications in sensors [10], composites [11] and energy storage devices such as supercapacitors [12], the application of graphene in the field of nanoelectronic, especially field-effect transistors (FET), is limited due to the lack of band gap in it. In other words, the major challenges for graphene transistors include opening a sizeable and well-defined band gap in it [8]. In order to overcome this problem and open up a band gap in such

∗ Corresponding author. Fax: +98 21 66012983. E-mail address: [email protected] (S. Shahrokhian). http://dx.doi.org/10.1016/j.snb.2014.02.049 0925-4005/© 2014 Elsevier B.V. All rights reserved.

a 2D carbon sheet, one way is to reduce the width of graphene nanosheets [13]. These thin elongated strips of graphene, named graphene nanoribbons (GNRs), display a band gap as a function of ribbon widths meaning by decreasing the ribbon width, the band gap increases gradually and the ribbons transform from semimetals to semiconductors [14,15]. Several approaches have been developed to obtain GNRs including lithographic patterning [16], chemical vapor deposition (CVD) [17] and chemically derived techniques [18]. Very recently, a new method based on lengthwise unwrapping of multi-walled carbon nanotubes (MWCNTs) side walls has been introduced for the synthesis of GNRs [19–25]. This technique will allow the production of narrow GNRs with controllable widths and given edge configurations with respect to the diameter and chirality of the nanotubes. Various physical approaches have been proposed in the literature to synthesis GNRs from carbon nanotubes (CNTs) such as plasma etching [21], electrically unwrapping CNTs [22] and anisotropic etching by metallic nanoparticles [25]. Although these methods give us the opportunity of accurate control over the GNR widths, they all need high-tech instrumentations and cannot produce GNRs in a large scale.

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

In the present research we use a simple yet effective chemical method for synthesis of GNRs from MWCNTs through a solvothermal route. The oxidation reaction is performed in a sealed autoclave in the presence of KMnO4 as an oxidizing agent in an acidic medium (H2 SO4 :HNO3 , 3:1 v/v). The key is to use the created high pressure in a sealed autoclave vessel as a result of high temperature to split pristine MWCNTs. As a result the c c cleavage initiated at structural defects, but subsequent unzipping only take place in the bonds having enough strain. On the other hand, due to the high pressure produced in the medium, the time of reaction diminishes dramatically in comparison with the other chemical techniques. It is noteworthy to mention that, the GNRs prepared by chemical methods is not a good candidate for electronic applications due to the structural defects and\or holes produced in the basal plane which resulted in the loss of electronic properties in the GNR sheets. In other words, the produced GNRs by chemical routes still contain residual defect sites due to the incomplete re-aromatization (even after intense reduction) and hence, their electronic conductivity is not as high as that observed in their counterparts produced by physical methods. Meanwhile, the produced graphene nanoribbons in this way are great candidates for electrochemical purposes since they have numerous edge atoms (especially in comparison with CNTs) which act as active electron transfer sites for fast electrochemical reactions. On the other hand, nanocomposites of graphene/polyaniline have attracted considerable attention in different areas including solar cells [26], supercapacitors [27,28] and sensors [29] particularly electrochemical biosensors [30,31]. In comparison to graphene sheets these composites are more suitable for electrode modification, show more compatibility for bio-functionalization and improve the sensitivity of the biosensors. In order to investigate the capability of the produced GNRs in construction of composite film for sensing applications, we fabricate graphene nanoribbon/polyaniline film through in-situ electro-polymerization on the surface of glassy carbon electrode (GNR/PANI-GCE) in acidic medium. As a model, the obtained composite film was used for the electrochemical investigation of dobutamine, an important catecholamine, which is a drug acts on the sympathetic nervous system and used for the treatment of heart failure and cardiogenic shock and yet there are only a few articles around its electrochemical determination [32–34]. Zhang introduced an adsorptive stripping voltammetry method for the determination of DBT on poly(acridine orange) film modified glassy carbon electrode with the detection limit of 2.0 × 10−9 M for 200 s open circuit accumulation [32]. Pletnev and co-workers used screen-printed electrodes modified with carbon paste that consisted of graphite powder dispersed in ionic liquids (IL) for electrochemical determination of catecholamines including dobutamine [33]. Recently, Wei et al. developed a modified glassy carbon electrode based on a magnesium oxide microflowers–nafion composite film for electrochemical oxidation of DBT with a detection limit of 0.092 ␮M [34]. Here, we used an electrochemical polymerization process for fabrication of a uniformly distributed composite layer of graphene nanoribbons/polyaniline film on the surface of glassy carbon electrode for electrochemical determination of dobutamine. Our results showed a considerable enhancement in the electrochemical performance of the modified electrode: the anodic peak current increased dramatically (up to 10 times) and a slight decrease in peak potential observed on the surface of GNR/PANI-GCE. We ascribed these results to the high surface area introduced by GNRs as well as electrocatalytic behavior of polyaniline. The results obtained for clinical samples are in good agreement with experimental ones revealed that the prepared sensing composite layer with this method can be used in electrochemical determination of chatecholamines such as dobutamine.

583

2. Experimental details 2.1. Synthesis of graphene oxide nanoribbons In order to synthesis graphene nanoribbons, 50 mg MWCNTs (OD: 30–50 nm, L: 1–10 ␮m, purity >95 wt%, PlasmaChem GmbH, Germany) were suspended in 16 mL of concentrated acid mixture (H2 SO4 :HNO3 , 3:1 v/v) and stirred for 6 h. KMnO4 (300 mg) was then added and the mixture was transferred to an autoclave. The sealed autoclave was heated in an oven to 120 ◦ C for 1 h and after that allowed the mixture to cool to room temperature. Then, the reaction mixture was poured onto 50 mL of iced DI water containing 2.5 mL H2 O2 (30%). The resulting light brown colored graphene oxide nanoribbons (GONRs) precipitate was collected on a 200 nm pore size poly-tetrafluoroethylene (PTFE) membrane. The product was washed with DI water until a neutral pH level was achieved and allowed to dry under vacuum at 50 ◦ C for 6 h. 2.2. Preparation of graphene nanoribbon/polyaniline composite film GNR/PANI composite film was synthesized based on the method proposed by Huang et al. for graphene sheets [35]. Briefly, 5 mg of graphene oxide nanoribbons (GONRs) were dispersed in 5 mL of H2 SO4 (1 M) and then 50 ␮L aniline monomer was added to this mixture. The reaction mixture was stirred for 30 min allowing electrostatic adsorption to occur between positively charged monomer and negatively charged GONR. After that, the dispersion was centrifuged and rinsed with DI water. The resulting mixture was re-dispersed in 5 mL DI water and sonicated until a homogeneous dispersion (1 mg mL−1 ) resulted. The GNR/PANI composite film was synthesized through a one-step electro-polymerization process. For this aim, 5 ␮L of GNR/aniline suspension was casted on the surface of a glassy carbon electrode and the potential scanned between −1.3 and +0.8 V (versus Ag/AgCl) at a scan rate of 50 mV s−1 for a total of 10 cycles. During the scanning process graphene oxide nanoribbons was electrochemically reduced at cathodic potentials, while aniline monomer polymerized at anodic potentials on the surface of graphene sheets. 2.3. Materials characterization The morphologies of GNRs and GNR/PANI composite film were characterized with a field emission scanning electron microscope (Zeiss, IGMA VP) and a transmission electron microscope (Philips CM10HT-100KV). The Raman spectra were obtained with a Senterra Raman microscope (Bruker Optics Inc., Germany) equipped with a 785 nm He–Ne laser as an excitation source. X-ray powder diffraction (XRD) spectra were recorded on an X’Pert MPD (Philips) ˚ radiation. instrument using CuK␣ (1.5405 A) An ABB-Bomem MB-100FT-IR spectrophotometer was used for recording FTIR spectra. Cyclic voltammetry (CV) analysis were performed on an Autolab PGSTAT 302-potentiostats. A conventional three-electrode system was used for all electrochemical measurements: a glassy carbon electrode (GCE, modified or unmodified) with a diameter of 2 mm as the working electrode, Ag/AgCl as the reference electrode, and a platinum wire as the auxiliary electrode. 3. Results and discussion 3.1. Synthesis of graphene nanoribbons To obtain optimum conditions for unzipping pristine MWCNTs, the effect of reaction conditions that is the amount of KMnO4 ,

584

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

Fig. 1. (a) FESEM micrographs of partially unzipped CNTs and (b) stacked graphene nanoribbons, (c, d) TEM images of CNTs after the cutting process.

phonons activated by defect-induced double resonance scattering. The ratio between the D and G bands is a good estimation of the quality of carbon nanostructures. As can be seen in Fig. 2 for MWCNTs, there is an intense G band around 1585 cm−1 and a weaker D band in 1345 cm−1 . These results suggest the high quality of the starting material (pristine MWCNTs) as confirmed by low ID /IG ratio (0.28). As

MWCNTs GONRs

Intensity (a.u.)

time and temperature on the final product was studied. We performed the unzipping process at different temperatures including 100, 120 and 150 ◦ C. The reaction was allowed to proceed for different periods (30 min and 1 h) with 600 wt% and 800 wt% of KMnO4 and the morphology of the products were studied by SEM. When MWCNTs were exposed to 600 wt% KMnO4 at 100 ◦ C for 30 min, no changes in morphology of the nanotubes were observed. This is due to the fact that the time of reaction was not sufficient for the unzipping process to lead to GNRs and also the temperature was too low for providing enough pressure in the medium. On the other hand, choosing a stricter oxidation condition for the splitting process (800 wt% KMnO4 at 150 ◦ C for 1 h) caused severe damages in MWCNTs. Hence, by changing the reaction parameters, the optimum oxidation condition for the unzipping process was finally set as follows; 600 wt% KMnO4 at 120 ◦ C for 1 h. Fig. 1a depicts FESEM image of partially unzipped carbon nanotubes. As can be seen, the oxidative process in the acidic medium as well as the presence of high pressure in the autoclave resulted in splitting sites, which indicated with arrows. By progressing the unwrapping process, CNTs were completely unzipped and graphene nanoribbons were obtained as shown in Fig. 1b. Moreover, the ribbon structure can be seen in the TEM images, which confirm the longitudinal unwrapping of CNTs (Fig. 1c and d). Raman spectroscopy is a powerful and effective technique to characterize the composition of carbon nanostructures and also quantitatively measuring the thickness and oxidation state of carbon nanosheets such as graphene. Generally there are two characteristic peaks in the Raman spectra of carbon-based materials: the graphite structure-derived G band (∼1580 cm−1 ) corresponds to E2g phonon vibrations of sp2 carbon atoms, while the defectinduced D band (∼1360 cm−1 ) relates to the transverse optical

1000

1200

1400

1600

1800

2000

Raman Shift (cm-1) Fig. 2. Raman spectra (785 nm excitation) of pristine MWCNTs (solid line) and the produced GONRs (dashed line).

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

MWCNTs GONRs

(002)

(b)

585

Intensity (a.u.)

(a)

(100)

0

10

20

30

40

50

2theta (°) Fig. 3. Comparison of XRD patterns for (a) MWCNT and (b) GONRs. The correspond˚ respectively. ing d-spacing for MWCNT and GONRs are 3.41 A˚ and 8.65 A,

3.2. Electro-polymerization of GNR/PANI composite film In order to investigate the applicability of the prepared graphene nanoribbons in constructing sensing platforms, we synthesized a thin layer of GNR/PANI composite film by a one step electro-polymerization process. The electrochemical procedure was carried out in acidic condition (1 M H2 SO4 ) on the surface of a glassy carbon electrode pre-coated with 5 ␮L of GONRs-aniline suspension and by potential cycling between −1.3 and +0.8 V (versus Ag/AgCl) at 50 mV s−1 . Fig. 4 shows the cyclic voltammograms for the in-situ electropolymerization process. As can be seen, during the consecutive scans of potential, GONRs started to reduce at negative potentials. This is crucial for obtaining a conductive composite film since the as synthesized graphene oxide nanoribbons (GONRs) exhibited very low conductance due to the oxygen-containing functionalities which hamper their electronic conductivity. In other words, the oxidative solvothermal process used for unzipping CNTs introduce some oxygen functionalities such as carboxyls, carbonyls, hydroxyls and epoxide groups at the basal plane and the edge sites. These functional groups disrupt the ␲–␲ conjugation system and hence, the electronic conductivity of the produced ribbons is very low in its oxide state which is similar to that observed for graphene oxide (GO) sheets. In order to regenerate the ␲–␲ conjugation system and restore the electronic conductivity, one has to remove the oxygen functionalities from the edge and basal plane of the

produced GONRs. This can be obtained by chemical reduction using reducing agents (such as hydrazine) or by electrochemical means. Herein, we use an electrochemical approach to reduce graphene oxide nanoribbons and improve the electronic conductivity of the produced composite layer. On the other hand, the oxidation of aniline monomers took place at anodic potentials, which led to the formation of a polymeric film on the surface of graphene nanosheets (Fig. S1). The results are consistent with that reported previously [35]. The final result is formation of a composite layer of GNR/PANI on the surface of glassy carbon electrode as shown in the SEM image (inset in Fig. 4). Fig. 5 depicts the FTIR spectra of GONRs and GNR/PANI composite film. The FTIR spectrum of GO shows an intense peak at 1627 cm−1 corresponds to aromatic C=C. The characteristic peaks of oxygen functional groups appeared at 1724, 1233 and 1063 cm−1 due to the C=O stretching vibrations, epoxy C–O and alkoxy C–O groups, respectively. On the other hand, the broad band peak observed at 3426 cm−1 related to O–H stretching mode. In the case of GNR/PANI composite film, two characteristic peaks of stretching vibrations of quinono and benzene group appeared at 1460 and 1540 cm−1 . Also the intense peaks at 1190 and 1243 cm−1 are attributed to C–N stretching vibrations which together confirm the presence of PANI in the composite layer.

GONR

Transmittance % (a.u.)

expected, after the oxidation process the intensity of Raman G band decreased by 60% along with a considerable enhancement in D band intensity as well as the disorder induced peak broadening observed for D band (the ID /IG ratio increased to 1.4). These observations are attributed to the production of numerous edge sites in GONRs, which act as defects. We also performed powder XRD analysis (Fig. 3) for further study of the structural transformation from MWCNT to narrow GNRs. The graphitic (0 0 2) peak relates to the interlayer distance, appeared in 26.20 ◦ C in the case of pristine MWCNT represents the ˚ By oxidizing the MWCNT with KMnO4 in an d-spacing of 3.41 A. acidic medium, the (0 0 2) peak shifted to the lower 2␪◦ values and ˚ is observed at 10.20 ◦ C, corresponding to the d-spacing of 8.65 A. This broad peak is attributed to the produced exfoliated layers after the unzipping process, which is very similar to the XRD pattern observed in graphene oxide.

Fig. 4. Cyclic voltammograms of the electro-polymerization process obtained in a 1 M H2 SO4 solution at the scan rate of 50 mV s−1 (Inset: SEM image of GNR/PANI composite film on the surface of GCE).

GNR/PANI

(b)

(a) 400

900

1400

1900

2400

2900

3400

Wavenumber (cm-1) Fig. 5. FTIR spectra of (a) GONRs and (b) GNR/PANI composite film.

3900

586

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

120

120 BGCE GNRs/GCE PANI/GCE GNR/PANI/GCE

100 80

d

BGCE CNT/PANI/GCE GNR/PANI/GCE

100

(c)

80

b 60

I (µA)

I (µA)

60 40

c 20

(b) 40

a 20

0 -20

(a)

0

-40 -0.4

-0.2

0.0

0.2

0.4

0.6

-20 -0.4

E (V vs. Ag/AgCl) Fig. 6. Cyclic voltammogramms of 1 mM DBT on the surface of (a) BGCE, (b) GNRs/GCE, (c) PANI/GCE and (d) GNR/PANI/GCE in PBS (pH = 7.0), scan rate was 100 mV s−1 .

3.3. Electrochemical measurements The potential application of the prepared GNR/PANI composite layer as a sensing platform was analyzed using dobutamine (DBT) as an electroactive species. To compare the electrochemical behavior of DBT on the surface of various electrodes in order to get a better insight about surface properties, we modified the glassy carbon electrode (GCE) with different modifier films. Fig. 6 depicts cyclic voltammograms obtained in phosphate buffer solution (PBS) of pH 7.0 containing 1 mM DBT on the surface of different electrodes including bare glassy carbon electrode (BGCE), GCE coated with 5 ␮L of graphene nanoribbons (GNRs/GCE), GCE coated with 5 ␮L of aniline monomer, which electro-polymerized by the same method (PANI/GCE) and finally GCE coated with a composite layer of graphene nanoribbon/polyaniline (GNR/PANI/GCE) recorded at the scan rate of 100 mV s−1 . As expected, the anodic peak current of DBT on the surface of GNR/PANI/GCE increased dramatically from 9 ␮A on the surface of BGCE to 91 ␮A in the case of modified electrode associated with a negative shift in the peak potential (∼65 mV). The resulting increase in the anodic peak current on the surface of GCE coated with nanoribbons (Fig. 6b) is attributed to the larger specific surface area and promoted conductivity introduced by graphene sheets. On the other hand, the electrocatalytic activity of polyaniline toward oxidation of DBT caused a slight increase in peak current along with a noticeable shift in the peak potential (Fig. 6c). The synergetic effect of these two parameters resulted in a considerable enhancement in the electrochemical performance of the modified electrode toward the electro-oxidation of DBT. Moreover, to demonstrate the advantage of GNR/PANI composite film in electrochemical determination of DBT over CNTs the same procedure was applied for modifying the GCE with CNT/PANI composite layer. As clearly can be seen in Fig. 7, the observed anodic peak current for DBT on the surface of GNR/PANI/GCE is much higher than CNT/PANI/GCE (more than 2 order of magnitude). This result was expected since the number of edge atoms serve as reactive sites for electron transfer are extremely augmented in the case of GNRs in comparison to CNTs. In other words, electron

-0.2

0.0

0.2

0.4

0.6

E (V vs. Ag/AgCl) Fig. 7. Cyclic voltammogramms of 1 mM DBT on the surface of (a) BGCE, (b) CNT/PANI/GCE and (c) GNR/PANI/GCE in PBS (pH = 7.0), scan rate was 100 mV s−1 .

transfer activity reported for CNTs are attributed to the tube open ends which consist of dangling bonds. Since the number of these atoms increased upon the unzipping process, GNRs exhibit longer edge plane structure and hence the electron transfer rate increased dramatically in these materials. 3.4. Effect of film thickness and pretreatment time In order to optimize the parameters of the electropolymerization process, the effect of film thickness and pretreatment time was investigated on the surface of GCE during the modification procedure with composite layer. Then, to study the effect of each parameter in the performance of the modified electrode, the electrochemical responses of DBT (1 mM) on the surface of different electrodes in PBS (pH = 7) were recorded and compared to BGCE. The thickness of the composite layer has an effective impact on the electrochemical responses of the modified electrode. Since the electron transfer should take place at the interface of a solid phase (the electrode) and a liquid phase (the electrolyte), there is a critical film thickness that should be optimized to obtain the best performance for the modified electrode. In order to investigate the effect of film thickness on the anodic peak current of DBT, different amount of modifier suspension including 1, 3, 5 and 7 ␮L is casted on the active surface of the GCE. With increasing the suspension drop size from 1 ␮L to 5 ␮L, the accessible specific surface area increased, which resulted in an enhancement in the anodic peak current (Fig. 8b–d). By further increase the volume of modifier suspension to 7 ␮L, the anodic peak current decreased which rose from the increased electrical resistance of the layer and sluggish mass transfer process between DBT and the electrode surface. Thus, a 5 ␮L volume of GONRs/aniline suspension was choose to acquire the optimum composite layer thickness. We also investigated the effect of pretreatment time on the final electrical conductivity of the prepared composite film. In our initial studies, not only we did not observe an enhancement in the anodic peak current of DBT, but also the capacitive current increased dramatically on the surface of the modified electrode (Fig. 8b). We ascribed these observations to the presence of GONRs in the electro-polymerized composite film. On the other word, the

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

587

120

d BGCE Without pretreatment 100sec 300sec

100

80

c

b

I (µA)

60

40

a

20

0

-20

-40 Fig. 8. Effect of drop size of GNR/aniline suspension on the electrochemical response of DBT (1 mM) in PBS (pH = 7.0): (a) BGCE, (b) 1 ␮L, (c) 3 ␮L, (d) 5 ␮L and (e) 7 ␮L of suspension (inset: Plot of anodic peak current versus volume of modifier suspension).

amount of oxygen-containing functional groups in the structure of graphene oxide nanoribbons were such high that even after 10 cycles of electro-polymerization the ribbons were not reduced completely and the disrupted sp2 network in the oxidized structure of GONRs prevented easy electron conductance which blocked the electrode surface toward an effective electron transfer between DBT and the electrode surface. Thus, we subjected the GCE coated with GONRs/aniline film to different pretreatment time at –1.3 V in 1 M H2 SO4 . As can be seen in Fig. 9, by applying 100 s pretreatment to the electrode at the potential of –1.3 V a significant increase in the peak current observed. Ever more increase in the peak current was obtained after 300 s pretreatment under the same condition (Fig. 9d). Meanwhile, further increase in the pretreatment time has no considerable effect on the response signal. These results revealed that the majority of oxygen functionalities were removed during the pretreatment time. From these observations 300 s pretreatment time was used as the optimum condition for the preparation of the modified electrode. Fig. 10a shows the calibration curve of corresponding LSV responses from 1 × 10−7 to 1 × 10−5 M of DBT, revealing a linear

-60 -0.4

-0.2

0.0

0.2

0.4

0.6

E (V vs. Ag/AgCl) Fig. 9. Cyclic voltamogramms of 1 mM DDBT in PBS (pH = 7) obtained on BGCE (a), and GNR/PANI/GCE with different pretreatment times at −1.3 V in 1 M H2 SO4 : (b) 0 s, (c) 100 s and (d) 300 s.

range with a correlation coefficient of R2 = 0.9985. These results indicate that the prepared modified electrode based on GNR/PANI composite film can be used satisfactorily in constructing practical sensing platforms due to its simple synthesis route, high electrical conductivity and electro-catalytic activity. Moreover, in order to demonstrate the applicability of the modified electrode in pharmaceutical and clinical determinations, different amounts of dobutamine (Dobutamex, Exir pharmaceutical Co.) are spiked in a human synthetic serum and the corresponding LSVs are recorded (Fig. 10b). The related calibration curves of the anodic peak current versus the concentration of DBT in the range of 1–100 ␮M are depicted in Fig. 10c. The average peak current observed in the case of serum is lower than that of PBS which is resulted from its more complex matrix however, the slope follow the same trend which indicate that the proposed sensing platform can be applied for sensitive detection of DBT in practical applications.

Fig. 10. (a) Linear sweep voltamogramms of PBS (pH = 7.0) containing various concentrations of DBT and (b) synthetic human serum spiked with various concentrations of DBT at scan rates of 100 mV s−1 . (c) Calibration curves of peak current versus concentration of DBT in buffer solution (solid circle) and in the human serum sample (hollow circle).

588

E. Asadian et al. / Sensors and Actuators B 196 (2014) 582–588

4. Conclusions Graphene nanoribbon/polyaniline composite film was prepared in a two-step procedure: firstly, graphene oxide nanoribbons were synthesized through a solvothermal process and then a thin composite layer of graphene nanoribbon/polyaniline film was fabricated via in-situ electro-polymerization in an acidic medium. The prepared composite film was used as an electrode modifier in constructing a sensing platform which was applied for electrochemical investigation of dobutamine as an important electroactive species. The obtained results revealed a considerable enhancement in the electrochemical performance of the modified electrode toward DBT. Satisfactory results also obtained for determination of DBT in human blood serum samples injected with different amounts of analyte which make the modified electrode very useful in construction of a simple device for the determination of DBT in pharmaceutical and clinical preparations. Acknowledgments The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran, Iran. 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.snb.2014.02.049. References [1] K. Wang, Q. Liu, L. Dai, J. Yan, C. Ju, B. Qiu, X. Wu, A highly sensitive and rapid organophosphate biosensor based on enhancement of CdS–decorated graphene nanocomposite, Anal. Chim. Acta 695 (2011) 84–88. [2] C. Lu, H. Yang, C. Zhu, X. Chen, G. Chen, A graphene platform for sensing biomolecules, Angew. Chem. Int. Ed. 48 (2009) 4785–4787. [3] S. Liu, J. Tiana, L. Wanga, Y. Luoa, W. Lua, X. Suna, Self-assembled graphene platelet–glucose oxidase nanostructures for glucose biosensing, Biosens. Bioelectron. 26 (2011) 4491–4496. [4] J.M. Schnorr, T.M. Swager, Emerging applications of carbon nanotubes, Chem. Mater. 23 (2011) 646–657. [5] M. Valcárcel, S. Cárdenas, B.M. Simonet, Role of carbon nanotubes in analytical science, Anal. Chem. 79 (2007) 4788–4797. [6] N. Karousis, N. Tagmatarchis, D. Tasis, Current progress on the chemical modification of carbon nanotubes, Chem. Rev. 110 (2010) 5366–5397. [7] M. Pumera, Graphene in biosensing, Mater. Today 14 (2011) 308–315. [8] F. Schwierz, Graphene transistors, Nature Nanotech. 5 (2010) 487–496. [9] H.A. Becerril, J. Mao, Z. Liu, R.M. Stoltenberg, Z. Bao, Y. Chen, Evaluation of solution-processed reduced graphene oxide films as transparent conductors, ACS Nano 2 (2008) 463–470. [10] C.H. Lu, H.H. Yang, C.L. Zhu, X. Chen, G.N. Chen, A graphene platform for sensing biomolecules, Angew. Chem. Int. Ed. 48 (2009) 4785–4787. [11] Z. Tang, H. Kang, Z. Shen, B. Guo, L. Zhang, D. Jia, Grafting of polyester onto graphene for electrically and thermally conductive composites, Macromolecules 45 (2012) 3444–3451. [12] H. Gómez, M.K. Ram, F. Alvi, P. Villalba, E. Stefanakos, A. Kumar, Graphene conducting polymer nanocomposite as novel electrode for supercapacitors, J. Power Sources 196 (2011) 4102–4108. [13] J. Bai, Y. Huang, Fabrication and electrical properties of graphene nanoribbons, Mater. Sci. Eng. 70 (2010) 341–353. [14] Y. Lin, V. Perebeinos, Z. Chen, P. Avouris, Electrical observation of subband formation in graphene nanoribbons, Phys. Rev. B 78 (2008) 1614–1709. [15] D. Gunlycke, H.M. Lawler, C.T. White, Room temperature ballistic transport in narrow graphene strips, Phys. Rev. B 75 (2007) 1–5. [16] Y.S. Shin, J.Y. Son, M.H. Jo, Y.H. Shin, H.M. Jang, High-mobility graphene nanoribbons prepared using polystyrene dip-pen nanolithography, J. Am. Chem. Soc. 133 (2011) 5623–5625. [17] J. Campos-Delgado, Y.A. Kim, T. Hayashi, A. Morelos-Gómez, M. Hofmann, H. Muramatsu, Thermal stability studies of CVD-grown graphene nanoribbons: defect annealing and loop formation, Chem. Phys. Lett. 469 (2009) 177–182. [18] J. Lu, J.X. Yang, J. Wang, A. Lim, S. Wang, K.P. Loh, One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids, ACS Nano 3 (2009) 2367–2375.

[19] A.G. Cano-Marquez, F.J. Rodriguez-Macıas, J. Campos-Delgado, C.G. EspinosaGonzalez, F. Tristan-Lopez, D. Ramirez-Gonzalez, Ex-MWCNTs graphene sheets and ribbons produced by lithium intercalation and exfoliation of carbon nanotubes, Nano Lett. 9 (2009) 1527–1533. [20] V. Kosynkin, A.L. Higginbotham, A. Sinitskii, J.R. Lomeda, A. Dimiev, B.K. Price, Longitudinal unzipping of carbon nanotubes to form graphene nanoribbon, Nature 478 (2009) 872–877. [21] L. Jiao, L. Zhang, L. Ding, J. Liu, H. Dai, Aligned graphene nanoribbons and crossbars from unzipped carbon nanotubes, Nano Res. 3 (2010) 387–394. [22] K. Kim, A. Sussman, A. Zettl, Graphene nanoribbons obtained by electrically unwrapping carbon nanotubes, ACS Nano. 4 (2010) 1362–1366. [23] D.B. Shinde, J. Debgupta, A. Kushwaha, M. Aslam, V.K.J. Pillai, Electrochemical unzipping of multi-walled carbon nanotubes for facile synthesis of high-quality graphene nanoribbons, J. Am. Chem. Soc. 133 (2011) 4168–4171. [24] S.R. Dhakate, N. Chauhan, S. Sharma, R.B. Mathur, The production of multi-layer graphene nanoribbons from thermally reduced unzipped multi-walled carbon nanotubes, Carbon 49 (2011) 4170–4178. [25] L.C. Campos, V.R. Manfrinato, J.D. Sanchez-Yamagishi, J. Kong, P. Jarillo-Herrero, Anisotropic etching and nanoribbon formation in single-layer graphene, Nano Lett. 9 (2009) 2600–2604. [26] H. Niu, S. Zhang, Q. Ma, S. Qin, L. Wan, J. Xu, S. Miao, Dye-sensitized solar cells based on flower-shaped ␣-Fe2 O3 as a photoanode and reduced graphene oxide–polyaniline composite as a counter electrode, RSC Adv. 3 (2013) 17228–17235. [27] P. Xiong, H. Huang, X. Wang, Design and synthesis of ternary cobalt ferrite/graphene/polyaniline hierarchical nanocomposites for high-performance supercapacitors, J. Power Sources 245 (2014) 937–946. [28] S. Giri, D., Ghosh, C. K. Das, Growth of vertically aligned tunable polyaniline on graphene/ZrO2 nanocomposites for supercapacitor energy-storage application, Adv. Funct. Mater., Article in press. [29] P.Z. Liu, X.W. Hu, C.J. Mao, H.L. Niu, J.M. Song, B.K. Jin, S.Y. Zhang, Electrochemiluminescence immunosensor based on graphene oxide nanosheets/polyaniline nanowires/CdSe quantum dots nanocomposites for ultrasensitive determination of human interleukin-6, Electrochim. Acta 113 (2013) 176–180. [30] N. Ruecha, R. Rangkupan, N. Rodthongkum, O. Chailapakul, Novel paperbased cholesterol biosensor using graphene/polyvinylpyrrolidone/polyaniline nanocomposite, Biosens. Bioelectron. 52 (2014) 13–19. [31] Q. Xu, S.X. Gu, L. Jin, Y.E. Zhou, Z. Yang, W. Wang, X. Hu, Graphene/ polyaniline/gold nanoparticles nanocomposite for the direct electron transfer of glucose oxidase and glucose biosensing, Sens. Actuators B 190 (2014) 562–569. [32] Y. Zhang, Voltammetric behavior of dobutamine at poly(acridine orange) film modified electrode and its determination by adsorptive stripping voltammetry, Anal. Lett. 37 (2007) 2031–2042. [33] D.V. Chernyshov, N.V. Shvedene, E.R. Antipova, I.V. Pletnev, Ionic liquid-based miniature electrochemical sensors for the voltammetric determination of catecholamines, Anal. Chim. Acta 621 (2008) 178–184. [34] Y. Ling, Q. Huang, D. Feng, X. Li, Y. Wei, Electrochemical oxidation of dobutamine on a magnesium oxide microflowers–nafion composite film modified glassy carbon electrode, Anal. Methods 5 (2013) 4580–4584. [35] X. Feng, R. Li, Y. Ma, R. Chen, N. Shi, Q. Fan, W. Huang, One-step electrochemical synthesis of graphene/polyaniline composite film and its applications, Adv. Funct. Mater. 21 (2011) 2989–2996.

Biographies Elham Asadian obtained her bachelor of Chemistry from Kharazmi University in 2006. Subsequently, she undertook her master studies at the Sharif University of Technology with Prof. Saeed Shahrokhian on chemically modified electrodes based on conducting polymers and carbon nanotubes in clinical and pharmaceutical applications in 2009. Now, she is a PhD student in Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology. Her dissertation is about synthesis of graphene nanoribbons from unzipped carbon nanotubes and their application in design and fabrication of biosensors. She is working under the supervision of Prof. Shahrokhian and Prof. Azam Iraji zad. Saeed Shahrokhian, received his BS degree in 1990, MS degree in 1994 and PhD degree in 1999 from Isfahan University, Isfahan, Iran. At present, he is a Professor of analytical chemistry at the Sharif University of Technology, Tehran, Iran. Azam Iraji zad graduated from Sussex University in 1990 with a PhD in Physics working with Dr. M. Hardiman on growth and conductivity of ultra-thin of Ag films on Ge(1 0 0). She spent one year as a postdoctoral researcher in surface physics, Physics & Astronomy Deptartment, Sussex University from 1990 to 1991. Now she is a Professor in the Physics Department and Head of the Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology. Her main interests are surface science and nanophysics. E. Jokar received her BSc and MSc degrees in Chemistry and Analytical Chemistry from the Sharif University of Technology. She is currently a PhD student at INST.