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Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection
Accepted Manuscript Title: Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection Author: Arshid Numan Muhammad Mehmood Shahid Fatin Saiha Omar K. Ramesh S. Ramesh PII: DOI: Reference:
S0925-4005(16)31155-8 http://dx.doi.org/doi:10.1016/j.snb.2016.07.111 SNB 20609
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
28-4-2016 1-7-2016 21-7-2016
Please cite this article as: Arshid Numan, Muhammad Mehmood Shahid, Fatin Saiha Omar, K.Ramesh, S.Ramesh, Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.07.111 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection Arshid Numan#, Muhammad Mehmood Shahid, Fatin Saiha Omar, K. Ramesh, S. Ramesh* Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Corresponding author: *
[email protected], Tel.: +60–3–7967–4391, Fax: +60–3–7967–4146
#
[email protected]
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Highlights: Co3O4 nanograin-decorated rGO composite was fabricated via hydrothermal route. rGO-Co3O4 nanograin composite was used for modification of electrode. rGO-Co3O4 nanograins showed excellent catalytic performance towards dopamine detection rGO-Co3O4 nanocomposite selectively detected dopamine in the presence of interfering species
Abstract A sensitive and selective detection of dopamine (DA) by a sensor based on cobalt oxide (Co3O4) nanograin-decorated reduced graphene oxide (rGO) composite modified glassy carbon electrode (GCE) is reported. The rGO-Co3O4 nanograin composites are synthesized by a facile hydrothermal route and optimized by varying the contents of rGO (5.7, 7.4, 9.1 and 10.7 wt%, denoted as C1, C2, C3 and C4 respectively). The crystallinity of the composite is examined by X-ray diffraction (XRD). Raman spectrum revealed the successful reduction of graphene oxide (GO) into rGO. The surface morphology through field emission scanning electron microscopy (FESEM) revealed that the granular-shaped Co3O4 are decorated on rGO matrix with an average particle size of ~ 35 nm. For electrochemical oxidation of DA, glassy carbon electrode (GCE) is modified with nanocomposites. Cyclic voltammetric results show that C3 modified GCE exhibit enhanced electrocatalytic performance in terms of oxidation potential and peak current in comparison to those of bare GCE, Co3O4 nanograins, C1, C2 and C4 modified GCE. The choronoamperometric studies indicate that C3 modified GCE
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exhibit a low detection limit of (S/N = 3) 0.277 μM in the linear range of 1 to 30 μM. In addition, C3 demonstrates good selectivity towards the detection of DA in the presence of a 100-fold higher concentration of ascorbic acid, glucose and uric acid as the interfering species. The electrochemical sensing studies suggest that 3D rGO-Co3O4 nanograins endow excellent catalytic activity, high selectivity and sensitivity towards DA. Keywords: Cobalt oxide; Nanograins; Dopamine; Electrochemical study; Amperometric detection
1. Introduction In recent years, neurotransmitters have received considerable attention owing to their vital role in cardiovascular, renal, hormonal and central nervous system [1]. Among various neurotransmitters, dopamine (DA) belongs to the catecholamine and phenethylamine families, which is a naturally occurring hormone present in the human brain. It influences the motivated behaviors such as attention span, neuroplasticity and regulates cognition as well as emotions in our body. In the nervous system, neurotransmitters are the agents for transition of messages between neurons, hence, a disturbance in dopaminergic neurotransmission leads to neurological disorder which results in parkinson disease [2,3] and schizophrenia [4-8]. Therefore, detection and accurate quantification of DA in biological fluids are the subjects of great importance. Several analytical detection approaches; high performance liquid chromatography (HPLC) [9], fluorometry [10], spectrophotometry [11], chemiluminescence capillary electrophoresis and electrochemical methods [12] have been employed for the sensitive detection of DA. However electrochemical sensing methods have drawn considerable interest due to their prominent advantages such as simplicity, high sensitivity,
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reproducibility, cost effectiveness and non-destructive quantitative detection [13,14,15]. Moreover DA is highly electroactive and easily gets oxidised, so electrochemical methods are highly preferred for its quantitative determination [16]. The major challenge in electrochemical detection of DA is the presence of main interfering molecules, such as uric acid (UA) and ascorbic acid (AA) in high concentration in biological tissues [17,18]. Conventional unmodified electrodes are incapable of selectively detecting DA owing to the sluggish electron transfer rate. In addition to this, fouling effect of electrode surface by the formation of oxidation products affects the selectivity and reproducibility of the electrode. To resolve these problems, the surface of electrode is required to be modified with a material having excellent electrocatalytic activity so that it can oxidise DA selectively without interacting with other coexisting analytes [19]. Recently, various materials have been used to modify the electrode surfaces chemically such as conductive polymers [20,21], metal oxide nanoparticles [22] and carbon based nanocomposites [23]. Among carbon based materials, graphene, a monolayer of carbon atoms with two dimensional (2D) hexagonal honeycomb lattice structure, has attracted enormous interest owing to its unique electrical, mechanical and chemical properties [24,25]. The sp2 hybridization of carbon atoms, extraordinary electrochemical surface area (2630 m2 g-1), ballistic conductivity (106 S cm-1), unique electronic configuration, and wide electrochemical window makes it an ideal candidate for electrochemical sensing applications [26,27,28]. The oxygen containing functional groups at the basal plan of GO boost heterogeneous electron transfer kinetics between biomolecule and electrode substrate during electrochemical detection which results in exceptional sensing performance [29,30]. On the other hand, metal oxides with a large surface area to volume ratio have excellent biocompatibitly, outstanding electrocatalytic activities and rapid electron transfer kinetics, which make them suitable for electrochemical sensing, heterogeneous catalysts and
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energy storage devices [31,32,33]. Among the various transition metal oxides, cobalt oxide (Co3O4) emerged as a shining star on the horizon of metal oxide catalyst owing to its simple preparation method, excellent chemical durability, promising ratio of surface atoms and diverse morphologies [34]. It is a p-type semiconductor material which has a cubic spinel structure with both direct and indirect band gaps of 2.10 eV and 1.60 eV, respectively [35]. Moreover, Co3O4 consists of positively polar termination with two Co2+, two Co3+, and four O2−, and negatively polar termination with two Co3+ and four O2−. The Co3O4 crystal contains magnetic Co2+ distributed on tetrahedral sites and non-magnetic Co3+ on octahedral sites [36]. These diverse polar sites of Co3O4 crystal together with high surface area to volume ratio and excellent catalytic activity can not only help in detecting the charges but also provide rapid charge transfer kinetics on the surface of electrode. Due to these properties, unaided Co3O4 nanostructures have been investigated in many reports for various applications [37]. However, only few efforts have been made to use hybrid of Co3O4 with graphene for electrochemical sensing applications [38,39,40]. Several studies have reported cobalt oxide anchored with graphene sheets which have shown excellent electrocatalytic applications [41,42]. In the view of above facts, we report an electrochemical sensing platform based rGO– Co3O4 nanograins for the detection of DA. To the best of our knowledge, there is no report available on the study of rGO functionalized with cobalt oxide nanoparticles for the electrochemical sensing of DA. One-pot hydrothermal synthetic route was used for synthesis of the rGO–Co3O4 nanocomposite. Hydrothermal route was preferred by virtue of its simplicity and choice of tuning various synthesis parameters (pH, temperature, time and concentration of regents or precursors) [43]. The appropriate amount of powdered reagents together with solvent can be put in a Teflon-lined autoclave and heated from room to elevated temperature and pressure for a fixed time. The electrolyte thermodynamics allows us in
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predicting optimum reaction conditions [44]. The as-synthesised nanocomposites were characterized by XRD, RAMAN and FESEM analysis. For electrochemical studies, rGO– Co3O4 nanocomposites were used to modify the surface of GCE and subsequently used for the oxidation of DA. The rGO–Co3O4 nanograins modified GCE displayed better catalytic performance towards the oxidation of DA as compared to the bare GCE and Co3O4 nanograins modified GCE. The sensitive detection of lower concentration of DA was explored by amperometric i-t curve technique and the LOD was found to be 0.277 μM with signal to noise ratio (S/N) of ratio ~3. Moreover, the nanocomposite modified GCE displayed an excellent selectivity for the detection of DA, even in the existence of 100-fold higher concentration of uric acid, glucose and ascorbic acid.
2. Experimental methods 2.1. Materials Graphite flakes were obtained from Asbury Inc. (USA). Potassium permanganate (KMnO4, 99%), sulphuric acid (H2SO4, 98%), hydrochloric acid (HCl, 35%), phosphoric acid, hydrogen peroxide, urea (CH4N2O, 99.5%), cobalt acetate tetrahydrate (Co (CH3COO)2 4H2O) and ethanol absolute (C2H6O, 99.8%) were purchased from Sigma Aldrich and Merck. The chemicals used in the whole investigation were of analytical grade. Deionised water was used throughout all experiments to prepare the solutions. 2.2. Synthesis of rGO–Co3O4 nanograins The rGO–Co3O4 nanograins were synthesised using one-step hydrothermal process. In a typical synthesis of rGO–Co3O4 nanograins, the GO was prepared by a simplified Hummer's method [45]. 5.7 wt% solution of GO (1 mg/ml) was sonicated for 15 minutes in order to exfoliate the stacked GO sheets and subsequently added in 15 ml of ethanol under
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stirring. Subsequently, 10 ml solution of 1 mmol Co(CH3COO)2.4H2O was added dropwise into the mixture and kept under vigorous stirring for 30 minutes until a homogeneous solution was obtained. After that, 15 ml of urea solution (1mmol) was added drop wise and the mixture was stirred for another 30 minutes. The mixture was then transferred into a 100 ml Teflon-lined stainless steel autoclave and kept in the preheated oven at 150 °C for 5h. Finally, the obtained precipitate of rGO–Co3O4 nanocomposite was washed several times with deionized water and ethanol by centrifugation and dried at 60 °C in a hot air oven. The entire experiment was repeated in the absence of GO to synthesize pure Co3O4 nanograins and with the presence of different contents of GO (7.4, 9.1 and 10.7 wt%,) to synthesize rGO–Co3O4 nanocomposites. The rGO–Co3O4 nanocomposites with GO contents of 5.7, 7.4, 9.1 and 10.7 wt% were denoted as C1, C2, C3 and C4, respectively. 2.3. Electrochemical Studies The rGO–Co3O4 nanograins modified GCE was fabricated by dispersing the rGO– Co3O4 nanograins in deionised water (1 mg/ml) and drop casting 5 μl of this solution on the GCE surface (d = 3 mm) followed by drying at room temperature (25 °C) for 1 h. Prior to the electrode fabrication, the GCE was cleaned electrochemically by potential cycling between 0.2 and 0.6 V in 0.1 M H2SO4 and polished with 0.05 micron alumina slurry. All the electrochemical studies were carried out using potentiostat (multi-channel Autolab, PGSTAT30) with conventional three-electrode system with rGO–Co3O4 nanograins modified GCE as working, Platinum (Pt) wire as counter and silver-silver chloride (Ag/AgCl) as a reference electrode at room temperature. Phosphate buffer solution (PBS) with neutral pH (7.2) was used as supporting electrolyte. 2.4. Preparation of real samples Two healthy male urines were selected as biological samples for real time analysis. Collected samples were stored in refrigerator (at 4 °C) immediately after collection. These urine 7
samples were then filtered and diluted 50 folds in 0.1 M PBS solution (pH 7.2) for further measurements. Standard addition method was employed for the determination of DA in both samples. 2.5. Materials Characterization techniques The surface morphology and particle size investigation of rGO–Co3O4 nanograin composites were studied using JEOL JSM-7600F field emission scanning electron microscope (FESEM). The Raman spectra was determined by Renishaw inVia 2000 system using green laser emitting at 514 nm. The structural crystallite of the nanocomposite was examined by using Philips X’pert X-ray diffractometer with copper Kα radiation (λ=1.5418 nm) at a scan rate of 0.02 degree-s-1.
3. Results and Discussion 3.1. XRD analysis The crystalline nature of GO, rGO, Co3O4, and rGO–Co3O4 nanograins was analysed by recording the XRD patterns, which are shown in Fig. 1. The GO (Fig. 1(a)) exhibited a sharp and intense diffraction peak at the 2θ value of 10° which corresponds to the (001) lattice plane [46]. The sharpness of the peak indicated that the high interlayer distance due to the existence of oxygen containing functional groups on the basal plane of GO [47]. However, the peak of GO disappeared and two broad peaks appeared at the 2θ values of 26.8° and 42.7°, attributed to the (002) and (100) lattice planes of the rGO, respectively (Fig. 1(a inset)). These peaks arise from the graphene sheets which were disorderedly stacked upon hydrothermal treatment, which confirms the successful reduction of GO into rGO [42]. Fig 1(b) shows the XRD patterns of the Co3O4 nanograins and rGO–Co3O4 nanocomposite. The diffraction pattern of pure Co3O4 (shown in fig 1(b inset)) exhibited strong crystalline peaks at 2θ values of 18.1°, 30.9°, 36.4°, 44.2°, 54.9°, 58.6°, and 64.4° which correspond to (111),
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(220), (311), (400), (422), (511), and (440) planes, respectively. The rGO–Co3O4 nanocomposite showed the same diffraction peaks pattern as Co3O4 nanograins, (Fig 1(b, inset)). No other characteristic peaks were observed, suggesting the purity of Co3O4 nanograins embedded rGO matrix. All XRD diffraction peaks for Co3O4 nanograins can be indexed as a cubical crystal phase Co3O4 (space group Fd-3m (227), PDF 96-900-5893) without showing any impurity phase. 3.2. Raman studies Raman spectroscopy is a non-destructive conventional tool which is employed to evaluate the functional groups and structural changes in graphite and graphene based materials. The Raman spectrum of GO (Fig. 2(a)) and rGO (Fig. 2(a, inset)) show two intense and well documented D band peak at 1351 cm-1, and G band peak at 1593 cm-1. The D band vibration is attributed to the sp3 defects produced in the lattice by the breathing mode of k-point photons of A1g symmetry of GO and rGO [48,49]. The G band refers to the vibration of sp2 C–C bond in the hexagonal carbon matrix [50]. The intensity ratio between D and G bands (D/G) is usually employed to specify the average size and degree of structural disorder of the in plane sp2 domains [48]. The D/G ratio of GO was found to be approximately 0.93, which was significantly increased up to 1.05 in rGO, suggesting the formation of partially ordered crystal structures with a decrease in size of in-plane sp2 domains and the removal of oxygen moieties during the reduction of GO [51,52]. However, in rGO–Co3O4 nanocomposite, the D/G ratio decreased to 0.90, as a result of decrease in the sp2 domain size (C–C bond) and the reduction of sp3 to sp2 carbon [48]. Additionally, the defects were observed in rGO matrix due to the removal of oxygen containing functional groups from GO layers [46]. Which confirmed the successful reduction of GO into rGO. Despite this, the 2D band (obtained by the Raman scattering of second order of zone boundary phonons of graphene) is extremely important to differentiate between monolayer,
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double or multi-layer graphene sheets. A slight increment in the intensity of 2D band was observed in rGO than GO, suggesting the formation of exfoliated rGO sheets from the stacked multilayer GO sheets. In fact, during the reduction of GO into rGO, oxygen containing functional groups on GO were reduced, which could result in restacking of graphene sheets. However, at the same time Co3O4 nanograins get intercalated between the graphene sheets and avoid them from restacking by acting as spacer in between the different layers of graphene. The Raman spectrum of the rGO–Co3O4 nanograins remained nearly the same value of D/G for rGO together with 2D band (Fig. 2(b)). The peaks aroused at 192, 482, 517, 614 and 687 cm-1 are attributed to the B1g, Eg, F1
2g,
F2
2g
and A1g modes of Co3O4,
respectively (Fig. 2(b, inset)) [48,53]. 3.3 Morphological characterization of the rGO–Co3O4 nanograins The surface morphologies of the synthesized pure Co3O4 and rGO–Co3O4 nanocomposite were investigated via FESEM. Fig. S1 (a-b) showed the formation of granular structure of pure Co3O4 with an average particle size of 35 nm. Fig S1(b) Co3O4 nanograins were successfully incorporated on to the surface of transparent rGO sheets without affecting their granular structure. There is a clear distinction between pure Co3O4 nanoparticles and in composite with rGO (Fig S1 (a)). Unaided Co3O4 nanograins were highly aggregated, while in rGO–Co3O4 nanograins, rGO significantly reduced the aggregations by providing anchoring sites for Co3O4 nanograins (Fig S1 (b)). The FESEM images (Fig. 3(a-b)) of C1 and C2 showed some aggregations of Co3O4 nanograins. This might be due to the lagging of rGO sheet surfaces which were insufficient to provide nucleation sites for cobalt ions during hydrothermal process. As a result, some particles of Co3O4 grow freely in the phase solution, independent from graphene sheets and tend to aggregate with each other. However, with increasing the GO contents in C3, the aggregation of Co3O4 nanograins was decreased as more sites were available for the 10
deposition of Co3O4 nanograins on GO sheets (Fig. 3(c)). In addition, some of the nanograins get intercalated within rGO sheets (displaying blurred image) while most of the nanograins were well distributed and exposed on rGO surface (displaying sharp images). But on further increasing the GO contents in C4, most of the Co3O4 nanograins sandwiched within the GO sheets and fewer particles were exposed on the top layer of rGO sheets which resulted in reduction of electrochemical surface area (Fig. 3(d)).
4. Electrocatalytic activity of rGO–Co3O4 nanograins modified GCE towards oxidation of DA Cobalt oxide is a well reputed catalyst with its exceptional catalytic activity, whereas rGO being a highly conductive sheet provides a conductive platform to Co3O4 nanograins. Thereby the synergistic effect of rGO and Co3O4 nanograins can enhance the electrocatalysis by improving electron transfer rate and good accessibility between DA and electrode surface. A schematic illustration of the redox electrochemistry of DA at the rGO–Co3O4 nanocomposite modified GCE is depicted in Figure 4. Owing to high electroactivity, DA easily gets oxidized electrocatalytically resulting the formation of dopamine quinone (DAQ) on the surface of rGO–Co3O4 nanocomposite modified GCE. Upon application of potential to the modified electrode, DA was oxidized to DAQ by exchanging 2 electrons and 2 protons with modified electrode and produce faradaic current [54].
(1)
The electrocatalytic performance of rGO–Co3O4 nanograins modified GCE was examined to elucidate the charge transfer behaviour. Fig. 5 displays the cyclic voltammetric responses of the modified electrodes in 0.1 M PBS (pH 7.2) at scan rate of 50 mVs-1 in the presence of 0.5 mM DA. The background current for the bare GCE (black line) showed well11
defined redox peaks with the anodic (Epa) and cathodic (Epc) peaks at 285 mV and 14 mV respectively with the peak difference (∆E) of 271mV. However, a significant shift of oxidation peak to the lower potential with improved peak current was observed in the case of unaided Co3O4 nanorgrains modified GCE, which indicates decent electrocatalytic activity of Co3O4 nanograins towards DA. Further noticeable improvement in redox peak current was observed in C1 and C2 modified GCE. This enhancement in electrochemical behaviour can be ascribed to the presence of electrically conductive platform of rGO that was providing excellent electron transport facility which in turn accelerated the electron kinetics at the electrode/electrolyte interface. However, in case of C4 (blue line) modified GCE, a decrease in redox peak current was observed without any significant shift in peak potentials. It is evident that rGO–Co3O4 nanograins exhibited excellent catalytic activity for DA detection as compared to unaided Co3O4 nanograins and bare GCE. This supports the fact that rGO provided enhanced conductive area for Co3O4 nanograins and contributed effectively in electron promotion during electrocatalytic oxidation of DA. Enhanced electrocataylytic response was observed for C3/GCE (green line) as compared to C1, C2 and C4. It exhibited 3.1 fold higher current and very small redox peak difference (∆E = 69 mV) as compared to bare GCE. The poor response of C1 and C2 to oxidise DA might be due to the aggregation of Co3O4 nanograins caused by low concentration of rGO, which result in the reduction of exposed electrochemical surface area to electrolyte. The less exposure of Co3O4 nanograins in C4 (Fig 3d) on the top surface of rGO was due to the sufficient number of rGO sheets that significantly decrease the density of Co3O4 nanograins. These results are in good agreement with aforementioned FESEM results. The C3 was considered optimized sample for further electrochemical studies. The influence of scan rates on the electrocatalytic response of C3 modified GCE towards DA oxidation was studied at different scan rates (from 25 to 250 mVs-1). It is evident 12
from Fig. 6(a) that by increasing the scan rates, redox peak current also increased linearly with a slight shift in oxidation potential in the positive direction, indicating the occurrence of quasi-reversible reaction. The calibration plot (Fig. 6b) of redox peak currents (Ipa & Ipc) with scan rate showed a linear correlation (R2=0.998 and 0.99) which revealed that the electroxidation of DA at rGO–Co3O4 nanograins modified GCE is an adsorption controlled process [55,56]. The cyclic voltammograms (Fig. 7(a)) were recorded for composite C3 modified GCE at different molar concentrations of DA in 0.1 M of PBS (pH 7.2). Significantly, the oxidation peak current increased linearly with the addition of DA in the range of 0.5 to 5 mM and anodic peak potential underwent a slightly positive shift. The logarithmic plot of peak current (Ipa) and different concentrations DA showed a linear plot with a slope value nearly equal to 1, which confirms that the electrochemical oxidation of DA follows the first order kinetics with respect to DA concentration at the C3 composite modified electrode (Fig. 7 (c)).
5. Amperometric detection of DA The amperometric detection method is an important tool for the detection of low concentrations of bioanalytes and to perform the interference studies. Fig 8(a) depicts the amperometric responses of composite C3 modified GCE on the successive injections of 1 μM of DA in a homogeneously stirred solution of 0.1 MPBS at a fixed time interval of 60 s. It can be seen that C3 modified GCE showed rapid response by significant increase in response current to each injection of DA at a fixed potential of +0.16 V indicating that the C3 modified GCE was effectively stimulated the oxidation of DA in 0.1 MPBS (pH 7.2). The calibration curve (Fig. 8b (inset)) indicates that the C3 modified GCE performs a linear response towards DA with the concentration from 1 to 30 μM.
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The limit of detection (LOD) and limit of quantification (LOQ) for DA can be estimated using the standard deviation of the y-intercepts (SD) and the slope of the regression lines (S) (LOD = 3.3(SD/S) and LOQ = 10(SD/S)) [57]. The LOD and LOQ values for DA using C3 modified GCE were found to be 0.277 μM and 0.924 μM respectively. The sensitivity of C3 modified electrode was 0.389 ± 0.0005 μA μM-1 cm-2. The comparison of the electrochemical sensing performance of the present rGO–Co3O4 nanocomposite modified GCE with some of the reported GCE based electrochemical sensors for the detection of DA is summarised in Table 1. Further, the selectivity of the rGO–Co3O4 nanograins for the detection of DA was examined by injecting three different interfering species; uric acid, glucose and ascorbic acid in the evenly stirred PBS containing DA and the change in the current response was observed. Fig. 9 depicts the recorded amperometric i–t curve response for the consecutive additions of DA and interfering species (sample time of 60 s) in a homogeneously stirred 0.1 MPBS (pH 7.2). First, alternate injections of DA with uric acid, glucose and ascorbic acid were used to record the current response. On each injection of DA current response was observed which was equal to the oxidation current of DA. On the other hand the injections of interfereing species did not gave any current response even with 100 fold higher concentration than DA. Next, three consecutive injections of DA followed by three injections of UA, glucose and AA respectively were used to analyaze the current response. Again interfering species did not show any current response, contrary to the DA which gave current stair on each injection. This confirmed that the addition of interfering species did not contribute any current response even with a 100-fold higher concentration than DA. On the other hand, on every injection of DA in the same solution showed approximately the same magnitude of response current for the oxidation of DA. This phenomenon is due to low oxidation potential (+0.16) which was used in amperometric method for interference studies. 14
The low potential helped in detecting DA selectively even in the presence of high concentration interfering species. These results revealed that the present sensor possesses good selectivity and sensitivity towards the quantitative determination of DA even in the presence of common physiological interfering species.
6. Stability and reproducibility In order to evaluate the performance and reliability of proposed sensor in real application, analytical determination of DA in two urine samples was performed using standard addition method. The analytical results were summarized in table 2. To ascertain the accuracy of the results, different concentrations of DA were added in urine containing PBS solution and detected. The recovery of spiked sample ranged between 98.3 to 103.4%, which indicated the successful application of rGO-Co3O4 nanograins for the detection of DA. For the stability study of proposed sensor, the CV response of C1 modified GCE was recorded for DA in a range between -0.2 and 0.6 V at scan rate of 50 mV s−1. The CV response was recorded on each day for one week. The peak current was considered as a factor for indicating the stability of the modified electrode. Here, I is the current response of fresh sensor and I0 is the current response after storage as shown in Fig. S3. A 10.3 % decrease in the anodic peak current with slight shift in peak potential was observed after one week. This revealed that C1 modified GCE can also withstand being stored in solution for a period of time (one week) and the electrocatalytic performance remain almost unchanged, which confirms that modified electrode is very stable and reproducible for the detection of DA.
7. Conclusion
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In this work, rGO–Co3O4 nanograin composites were synthesized by hydrothermal route and were optimized by varying the contents of rGO such as 5.7, 7.4, 9.1 and 10.7 wt%, which were denoted as C1, C2, C3 and C4 respectively. The crystallinity of rGO–Co3O4 composites is examined by XRD. Raman spectrum revealed the successful reduction of GO into rGO. The surface morphology revealed that the granular shaped Co3O4 were decorated on rGO matrix with the average particle size of ~ 35 nm. For electrochemical oxidation of DA, GCE was modified with nanocomposites and it was found that C3 modified GCE showed enhanced electrocatalytic performance in terms of peak catalytic current and shift in over potential when compared to those of C1, C2 and C4 modified GCE. The LOD was found to be 0.277 μM with S/N ratio of 3 with quantitative limit of 0.924 μM using the amperometric i–t curve technique. In addition, the C3 modified GCE displayed good selectivity for the detection of DA in the presence of a 100-fold higher concentration of uric acid, glucose and ascorbic acid. The sensor studies suggested that 3D rGO–Co3O4 nanocomposites endow excellent catalytic activity, high selectivity and sensitivity towards DA. Acknowledgement This work was supported by the High Impact Research Grant (H-21001-F000046) from Ministry of Education, Malaysia, University of Malaya Research Grant (RP025A-14AFR) and Postgraduate Research Grant (PG119-2015A).
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24
Biographies Arshid Numan obtained his bachelor degree in electronics from Sarhad University of Information Technology, Pakistan, in 2011 and master degree in Electrical Engineering from COMSATS Institute of Information Technology, Pakistan, in 2014. Now he is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. His research area is synthesis and characterization of nanomaterials for Supercapacitor, Electrocatalysis and electrochemical sensor applications.
Muhammad Mehmood shahid obtained his master degree in electronics from Sarhad University of Information Technology, Pakistan, in 2011. Now he is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. His research interests are in the areas of Photocatalysis, Photoelectrochemistry, Electrocatalysis and Sensors.
Fatin Saiha Omar obtained her bachelor and master degree in Physics from University of Malaya, Kuala Lumpur, Malaysia. Now she is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. Her research interest are in the area of synthesis and characterization of nanomaterials for Supercapacitor and Sensors applications.
K. Ramesh obtained his Phd degree from University of Malaya, Kuala Lumpur, Malaysia. Currently he is serving as senior lecturer in department of Physics, University of Malaya. His area of interest is Coating (Corrosion, Antifouling, Natural Colorants, Protective Coatings) Polymer Electrolytes for Supercapacitor Batteries and Electrochemical Sensors applications.
S. Ramesh is currently a professor of material science in department of Physics, University of Malaya, Kuala Lumpur, Malaysia. He published more than 200 research papers in several reputed international journals. His current area of interest is in Polymer Electrolytes (Rechargeable Batteries, Solar Cells, Fuel Cell etc.), Supercapacitor, Electrocatalysis and Electrochemical Sensors applications.
25
Figure Captions Fig. 1 XRD patterns (a) rGO (inset GO), (b) rGO–Co3O4 (inset Co3O4) Fig 2(b) shows the XRD patterns of the Co3O4 nanograins and rGO–Co3O4 nanocomposite. Fig. 2 Raman spectra of (a) rGO sheets (inset: the Raman spectrum of GO sheets) and (b) rGO–Co3O4 nanocomposite (inset: the Raman modes of Co3O4 nanograins) Fig. 3 FESEM images of rGO–Co3O4 nanograins (a) composite C1, (b) composite C2, (c) composite C3 and (d) composite C4. Fig. 4 Schematic representation of electrocatalysis of DA at rGO–Co3O4 nanograins modified GCE. Fig. 5 Cyclic voltammograms obtained for bare GCE, Co3O4 nanograins, C1, C2, C3 and C4 nanocomposite modified GCE for 0.5 mM DA in 0.1 M PBS at a scan rate of 50 mVs-1. Fig. 6 (a) CVs observed for rGO–Co3O4 nangrains (C2) in 0.1 M PBS (pH 7.2) containing 0.1 mM DA at various scan rates (b) The plots of peak current versus the scan rates. Fig. 7 (a) Cyclic voltammograms obtained at the rGO–Co3O4 nanograins modified electrode during the successive addition of different concentrations (b) Plot of Ipa vs molar concentration of DA (c) Plot of log Ipa vs log [molar concentration of DA] Fig. 8 (a) Amperometric i–t curve obtained at rGO–Co3O4 nanocomposite modified GCE for the successive addition of 1 mM DA in 0.1 M PBS (pH 7.2) at an applied potential of +0.16 V with a regular interval of 60 s (b) corresponding calibration plot of current versus concentration of DA. Fig. 9 Amperometric i–t curve obtained at rGO–Co3O4 nanograins modified GCE for the successive addition of DA and each 1 mM of uric acid, glucose and ascorbic acid in 0.1 M PBS (pH 7.2) at a regular interval of 60 s. The applied potential was +0.16 V 26
Fig. 1
27
Fig. 2
28
Fig. 3
29
DA
DAQ ‐2e +2e
GCE‐rGO‐Co3O4
Fig. 4
30
Fig. 5
31
Fig. 6
32
Fig. 7
33
Fig. 8
34
Fig. 9
35
Table 1 A comparison of some of the reported electrochemical sensors for DA detection.
Type of Electrode
Technique
pH
LOD (μM)
LOQ (μM)
Linear range (μM)
Sensitivity (μAμM-1)
Reference
Graphene/SnO2/GCE
DPV
6.8
1
Nil
0-100
Nil
[58]
rGO/TiO2/GCE
DPV
6.5
6
Nil
2-60
Nil
[59]
V2O5@PANI/GCE
CA
Nil
0.39
130
6.6-110
Nil
[60]
Graphene/GCE
DPV
Nil
2.64
Nil
4-100
Nil
[61]
Flower-like Au nanostructurecovered gold electrode (FANE)
DPV
7.4
0.2
Nil
1 – 150
Nil
[62]
ERGO/GCE
DPV
7
0.5
Nil
0.5 – 60
0.482
[63]
(GE/Au/GE/CFE)
DPV
7
0.59
Nil
0.59 –43.95
rGO–Co3O4
CA
7.2
0.277
0.924
0–30
[64]
0.389
Present work
36
Table 2 Determination results of DA by using rGO–Co3O4 nanograins in real urine samples (n = 3). Urine sample
Sample 1
Sample 2
Add concentration (μML-1)
Found Concentration (μML-1)
RSD (%)
Recovery (%) (μML-1)
1μM
1.010
1.48
101
3μM
2.972
1.04
99.1
6μM
5.900
2.58
98.3
1μM
1.034
1.68
103.4
3μM
2.948
1.07
98.3
6μM
6.011
1.17
100.1
37
S0925-4005(16)31155-8 http://dx.doi.org/doi:10.1016/j.snb.2016.07.111 SNB 20609
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
28-4-2016 1-7-2016 21-7-2016
Please cite this article as: Arshid Numan, Muhammad Mehmood Shahid, Fatin Saiha Omar, K.Ramesh, S.Ramesh, Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.07.111 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile fabrication of cobalt oxide nanograin-decorated reduced graphene oxide composite as ultrasensitive platform for dopamine detection Arshid Numan#, Muhammad Mehmood Shahid, Fatin Saiha Omar, K. Ramesh, S. Ramesh* Centre for Ionics University of Malaya, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Corresponding author: *
[email protected], Tel.: +60–3–7967–4391, Fax: +60–3–7967–4146
#
[email protected]
1
Highlights: Co3O4 nanograin-decorated rGO composite was fabricated via hydrothermal route. rGO-Co3O4 nanograin composite was used for modification of electrode. rGO-Co3O4 nanograins showed excellent catalytic performance towards dopamine detection rGO-Co3O4 nanocomposite selectively detected dopamine in the presence of interfering species
Abstract A sensitive and selective detection of dopamine (DA) by a sensor based on cobalt oxide (Co3O4) nanograin-decorated reduced graphene oxide (rGO) composite modified glassy carbon electrode (GCE) is reported. The rGO-Co3O4 nanograin composites are synthesized by a facile hydrothermal route and optimized by varying the contents of rGO (5.7, 7.4, 9.1 and 10.7 wt%, denoted as C1, C2, C3 and C4 respectively). The crystallinity of the composite is examined by X-ray diffraction (XRD). Raman spectrum revealed the successful reduction of graphene oxide (GO) into rGO. The surface morphology through field emission scanning electron microscopy (FESEM) revealed that the granular-shaped Co3O4 are decorated on rGO matrix with an average particle size of ~ 35 nm. For electrochemical oxidation of DA, glassy carbon electrode (GCE) is modified with nanocomposites. Cyclic voltammetric results show that C3 modified GCE exhibit enhanced electrocatalytic performance in terms of oxidation potential and peak current in comparison to those of bare GCE, Co3O4 nanograins, C1, C2 and C4 modified GCE. The choronoamperometric studies indicate that C3 modified GCE
2
exhibit a low detection limit of (S/N = 3) 0.277 μM in the linear range of 1 to 30 μM. In addition, C3 demonstrates good selectivity towards the detection of DA in the presence of a 100-fold higher concentration of ascorbic acid, glucose and uric acid as the interfering species. The electrochemical sensing studies suggest that 3D rGO-Co3O4 nanograins endow excellent catalytic activity, high selectivity and sensitivity towards DA. Keywords: Cobalt oxide; Nanograins; Dopamine; Electrochemical study; Amperometric detection
1. Introduction In recent years, neurotransmitters have received considerable attention owing to their vital role in cardiovascular, renal, hormonal and central nervous system [1]. Among various neurotransmitters, dopamine (DA) belongs to the catecholamine and phenethylamine families, which is a naturally occurring hormone present in the human brain. It influences the motivated behaviors such as attention span, neuroplasticity and regulates cognition as well as emotions in our body. In the nervous system, neurotransmitters are the agents for transition of messages between neurons, hence, a disturbance in dopaminergic neurotransmission leads to neurological disorder which results in parkinson disease [2,3] and schizophrenia [4-8]. Therefore, detection and accurate quantification of DA in biological fluids are the subjects of great importance. Several analytical detection approaches; high performance liquid chromatography (HPLC) [9], fluorometry [10], spectrophotometry [11], chemiluminescence capillary electrophoresis and electrochemical methods [12] have been employed for the sensitive detection of DA. However electrochemical sensing methods have drawn considerable interest due to their prominent advantages such as simplicity, high sensitivity,
3
reproducibility, cost effectiveness and non-destructive quantitative detection [13,14,15]. Moreover DA is highly electroactive and easily gets oxidised, so electrochemical methods are highly preferred for its quantitative determination [16]. The major challenge in electrochemical detection of DA is the presence of main interfering molecules, such as uric acid (UA) and ascorbic acid (AA) in high concentration in biological tissues [17,18]. Conventional unmodified electrodes are incapable of selectively detecting DA owing to the sluggish electron transfer rate. In addition to this, fouling effect of electrode surface by the formation of oxidation products affects the selectivity and reproducibility of the electrode. To resolve these problems, the surface of electrode is required to be modified with a material having excellent electrocatalytic activity so that it can oxidise DA selectively without interacting with other coexisting analytes [19]. Recently, various materials have been used to modify the electrode surfaces chemically such as conductive polymers [20,21], metal oxide nanoparticles [22] and carbon based nanocomposites [23]. Among carbon based materials, graphene, a monolayer of carbon atoms with two dimensional (2D) hexagonal honeycomb lattice structure, has attracted enormous interest owing to its unique electrical, mechanical and chemical properties [24,25]. The sp2 hybridization of carbon atoms, extraordinary electrochemical surface area (2630 m2 g-1), ballistic conductivity (106 S cm-1), unique electronic configuration, and wide electrochemical window makes it an ideal candidate for electrochemical sensing applications [26,27,28]. The oxygen containing functional groups at the basal plan of GO boost heterogeneous electron transfer kinetics between biomolecule and electrode substrate during electrochemical detection which results in exceptional sensing performance [29,30]. On the other hand, metal oxides with a large surface area to volume ratio have excellent biocompatibitly, outstanding electrocatalytic activities and rapid electron transfer kinetics, which make them suitable for electrochemical sensing, heterogeneous catalysts and
4
energy storage devices [31,32,33]. Among the various transition metal oxides, cobalt oxide (Co3O4) emerged as a shining star on the horizon of metal oxide catalyst owing to its simple preparation method, excellent chemical durability, promising ratio of surface atoms and diverse morphologies [34]. It is a p-type semiconductor material which has a cubic spinel structure with both direct and indirect band gaps of 2.10 eV and 1.60 eV, respectively [35]. Moreover, Co3O4 consists of positively polar termination with two Co2+, two Co3+, and four O2−, and negatively polar termination with two Co3+ and four O2−. The Co3O4 crystal contains magnetic Co2+ distributed on tetrahedral sites and non-magnetic Co3+ on octahedral sites [36]. These diverse polar sites of Co3O4 crystal together with high surface area to volume ratio and excellent catalytic activity can not only help in detecting the charges but also provide rapid charge transfer kinetics on the surface of electrode. Due to these properties, unaided Co3O4 nanostructures have been investigated in many reports for various applications [37]. However, only few efforts have been made to use hybrid of Co3O4 with graphene for electrochemical sensing applications [38,39,40]. Several studies have reported cobalt oxide anchored with graphene sheets which have shown excellent electrocatalytic applications [41,42]. In the view of above facts, we report an electrochemical sensing platform based rGO– Co3O4 nanograins for the detection of DA. To the best of our knowledge, there is no report available on the study of rGO functionalized with cobalt oxide nanoparticles for the electrochemical sensing of DA. One-pot hydrothermal synthetic route was used for synthesis of the rGO–Co3O4 nanocomposite. Hydrothermal route was preferred by virtue of its simplicity and choice of tuning various synthesis parameters (pH, temperature, time and concentration of regents or precursors) [43]. The appropriate amount of powdered reagents together with solvent can be put in a Teflon-lined autoclave and heated from room to elevated temperature and pressure for a fixed time. The electrolyte thermodynamics allows us in
5
predicting optimum reaction conditions [44]. The as-synthesised nanocomposites were characterized by XRD, RAMAN and FESEM analysis. For electrochemical studies, rGO– Co3O4 nanocomposites were used to modify the surface of GCE and subsequently used for the oxidation of DA. The rGO–Co3O4 nanograins modified GCE displayed better catalytic performance towards the oxidation of DA as compared to the bare GCE and Co3O4 nanograins modified GCE. The sensitive detection of lower concentration of DA was explored by amperometric i-t curve technique and the LOD was found to be 0.277 μM with signal to noise ratio (S/N) of ratio ~3. Moreover, the nanocomposite modified GCE displayed an excellent selectivity for the detection of DA, even in the existence of 100-fold higher concentration of uric acid, glucose and ascorbic acid.
2. Experimental methods 2.1. Materials Graphite flakes were obtained from Asbury Inc. (USA). Potassium permanganate (KMnO4, 99%), sulphuric acid (H2SO4, 98%), hydrochloric acid (HCl, 35%), phosphoric acid, hydrogen peroxide, urea (CH4N2O, 99.5%), cobalt acetate tetrahydrate (Co (CH3COO)2 4H2O) and ethanol absolute (C2H6O, 99.8%) were purchased from Sigma Aldrich and Merck. The chemicals used in the whole investigation were of analytical grade. Deionised water was used throughout all experiments to prepare the solutions. 2.2. Synthesis of rGO–Co3O4 nanograins The rGO–Co3O4 nanograins were synthesised using one-step hydrothermal process. In a typical synthesis of rGO–Co3O4 nanograins, the GO was prepared by a simplified Hummer's method [45]. 5.7 wt% solution of GO (1 mg/ml) was sonicated for 15 minutes in order to exfoliate the stacked GO sheets and subsequently added in 15 ml of ethanol under
6
stirring. Subsequently, 10 ml solution of 1 mmol Co(CH3COO)2.4H2O was added dropwise into the mixture and kept under vigorous stirring for 30 minutes until a homogeneous solution was obtained. After that, 15 ml of urea solution (1mmol) was added drop wise and the mixture was stirred for another 30 minutes. The mixture was then transferred into a 100 ml Teflon-lined stainless steel autoclave and kept in the preheated oven at 150 °C for 5h. Finally, the obtained precipitate of rGO–Co3O4 nanocomposite was washed several times with deionized water and ethanol by centrifugation and dried at 60 °C in a hot air oven. The entire experiment was repeated in the absence of GO to synthesize pure Co3O4 nanograins and with the presence of different contents of GO (7.4, 9.1 and 10.7 wt%,) to synthesize rGO–Co3O4 nanocomposites. The rGO–Co3O4 nanocomposites with GO contents of 5.7, 7.4, 9.1 and 10.7 wt% were denoted as C1, C2, C3 and C4, respectively. 2.3. Electrochemical Studies The rGO–Co3O4 nanograins modified GCE was fabricated by dispersing the rGO– Co3O4 nanograins in deionised water (1 mg/ml) and drop casting 5 μl of this solution on the GCE surface (d = 3 mm) followed by drying at room temperature (25 °C) for 1 h. Prior to the electrode fabrication, the GCE was cleaned electrochemically by potential cycling between 0.2 and 0.6 V in 0.1 M H2SO4 and polished with 0.05 micron alumina slurry. All the electrochemical studies were carried out using potentiostat (multi-channel Autolab, PGSTAT30) with conventional three-electrode system with rGO–Co3O4 nanograins modified GCE as working, Platinum (Pt) wire as counter and silver-silver chloride (Ag/AgCl) as a reference electrode at room temperature. Phosphate buffer solution (PBS) with neutral pH (7.2) was used as supporting electrolyte. 2.4. Preparation of real samples Two healthy male urines were selected as biological samples for real time analysis. Collected samples were stored in refrigerator (at 4 °C) immediately after collection. These urine 7
samples were then filtered and diluted 50 folds in 0.1 M PBS solution (pH 7.2) for further measurements. Standard addition method was employed for the determination of DA in both samples. 2.5. Materials Characterization techniques The surface morphology and particle size investigation of rGO–Co3O4 nanograin composites were studied using JEOL JSM-7600F field emission scanning electron microscope (FESEM). The Raman spectra was determined by Renishaw inVia 2000 system using green laser emitting at 514 nm. The structural crystallite of the nanocomposite was examined by using Philips X’pert X-ray diffractometer with copper Kα radiation (λ=1.5418 nm) at a scan rate of 0.02 degree-s-1.
3. Results and Discussion 3.1. XRD analysis The crystalline nature of GO, rGO, Co3O4, and rGO–Co3O4 nanograins was analysed by recording the XRD patterns, which are shown in Fig. 1. The GO (Fig. 1(a)) exhibited a sharp and intense diffraction peak at the 2θ value of 10° which corresponds to the (001) lattice plane [46]. The sharpness of the peak indicated that the high interlayer distance due to the existence of oxygen containing functional groups on the basal plane of GO [47]. However, the peak of GO disappeared and two broad peaks appeared at the 2θ values of 26.8° and 42.7°, attributed to the (002) and (100) lattice planes of the rGO, respectively (Fig. 1(a inset)). These peaks arise from the graphene sheets which were disorderedly stacked upon hydrothermal treatment, which confirms the successful reduction of GO into rGO [42]. Fig 1(b) shows the XRD patterns of the Co3O4 nanograins and rGO–Co3O4 nanocomposite. The diffraction pattern of pure Co3O4 (shown in fig 1(b inset)) exhibited strong crystalline peaks at 2θ values of 18.1°, 30.9°, 36.4°, 44.2°, 54.9°, 58.6°, and 64.4° which correspond to (111),
8
(220), (311), (400), (422), (511), and (440) planes, respectively. The rGO–Co3O4 nanocomposite showed the same diffraction peaks pattern as Co3O4 nanograins, (Fig 1(b, inset)). No other characteristic peaks were observed, suggesting the purity of Co3O4 nanograins embedded rGO matrix. All XRD diffraction peaks for Co3O4 nanograins can be indexed as a cubical crystal phase Co3O4 (space group Fd-3m (227), PDF 96-900-5893) without showing any impurity phase. 3.2. Raman studies Raman spectroscopy is a non-destructive conventional tool which is employed to evaluate the functional groups and structural changes in graphite and graphene based materials. The Raman spectrum of GO (Fig. 2(a)) and rGO (Fig. 2(a, inset)) show two intense and well documented D band peak at 1351 cm-1, and G band peak at 1593 cm-1. The D band vibration is attributed to the sp3 defects produced in the lattice by the breathing mode of k-point photons of A1g symmetry of GO and rGO [48,49]. The G band refers to the vibration of sp2 C–C bond in the hexagonal carbon matrix [50]. The intensity ratio between D and G bands (D/G) is usually employed to specify the average size and degree of structural disorder of the in plane sp2 domains [48]. The D/G ratio of GO was found to be approximately 0.93, which was significantly increased up to 1.05 in rGO, suggesting the formation of partially ordered crystal structures with a decrease in size of in-plane sp2 domains and the removal of oxygen moieties during the reduction of GO [51,52]. However, in rGO–Co3O4 nanocomposite, the D/G ratio decreased to 0.90, as a result of decrease in the sp2 domain size (C–C bond) and the reduction of sp3 to sp2 carbon [48]. Additionally, the defects were observed in rGO matrix due to the removal of oxygen containing functional groups from GO layers [46]. Which confirmed the successful reduction of GO into rGO. Despite this, the 2D band (obtained by the Raman scattering of second order of zone boundary phonons of graphene) is extremely important to differentiate between monolayer,
9
double or multi-layer graphene sheets. A slight increment in the intensity of 2D band was observed in rGO than GO, suggesting the formation of exfoliated rGO sheets from the stacked multilayer GO sheets. In fact, during the reduction of GO into rGO, oxygen containing functional groups on GO were reduced, which could result in restacking of graphene sheets. However, at the same time Co3O4 nanograins get intercalated between the graphene sheets and avoid them from restacking by acting as spacer in between the different layers of graphene. The Raman spectrum of the rGO–Co3O4 nanograins remained nearly the same value of D/G for rGO together with 2D band (Fig. 2(b)). The peaks aroused at 192, 482, 517, 614 and 687 cm-1 are attributed to the B1g, Eg, F1
2g,
F2
2g
and A1g modes of Co3O4,
respectively (Fig. 2(b, inset)) [48,53]. 3.3 Morphological characterization of the rGO–Co3O4 nanograins The surface morphologies of the synthesized pure Co3O4 and rGO–Co3O4 nanocomposite were investigated via FESEM. Fig. S1 (a-b) showed the formation of granular structure of pure Co3O4 with an average particle size of 35 nm. Fig S1(b) Co3O4 nanograins were successfully incorporated on to the surface of transparent rGO sheets without affecting their granular structure. There is a clear distinction between pure Co3O4 nanoparticles and in composite with rGO (Fig S1 (a)). Unaided Co3O4 nanograins were highly aggregated, while in rGO–Co3O4 nanograins, rGO significantly reduced the aggregations by providing anchoring sites for Co3O4 nanograins (Fig S1 (b)). The FESEM images (Fig. 3(a-b)) of C1 and C2 showed some aggregations of Co3O4 nanograins. This might be due to the lagging of rGO sheet surfaces which were insufficient to provide nucleation sites for cobalt ions during hydrothermal process. As a result, some particles of Co3O4 grow freely in the phase solution, independent from graphene sheets and tend to aggregate with each other. However, with increasing the GO contents in C3, the aggregation of Co3O4 nanograins was decreased as more sites were available for the 10
deposition of Co3O4 nanograins on GO sheets (Fig. 3(c)). In addition, some of the nanograins get intercalated within rGO sheets (displaying blurred image) while most of the nanograins were well distributed and exposed on rGO surface (displaying sharp images). But on further increasing the GO contents in C4, most of the Co3O4 nanograins sandwiched within the GO sheets and fewer particles were exposed on the top layer of rGO sheets which resulted in reduction of electrochemical surface area (Fig. 3(d)).
4. Electrocatalytic activity of rGO–Co3O4 nanograins modified GCE towards oxidation of DA Cobalt oxide is a well reputed catalyst with its exceptional catalytic activity, whereas rGO being a highly conductive sheet provides a conductive platform to Co3O4 nanograins. Thereby the synergistic effect of rGO and Co3O4 nanograins can enhance the electrocatalysis by improving electron transfer rate and good accessibility between DA and electrode surface. A schematic illustration of the redox electrochemistry of DA at the rGO–Co3O4 nanocomposite modified GCE is depicted in Figure 4. Owing to high electroactivity, DA easily gets oxidized electrocatalytically resulting the formation of dopamine quinone (DAQ) on the surface of rGO–Co3O4 nanocomposite modified GCE. Upon application of potential to the modified electrode, DA was oxidized to DAQ by exchanging 2 electrons and 2 protons with modified electrode and produce faradaic current [54].
(1)
The electrocatalytic performance of rGO–Co3O4 nanograins modified GCE was examined to elucidate the charge transfer behaviour. Fig. 5 displays the cyclic voltammetric responses of the modified electrodes in 0.1 M PBS (pH 7.2) at scan rate of 50 mVs-1 in the presence of 0.5 mM DA. The background current for the bare GCE (black line) showed well11
defined redox peaks with the anodic (Epa) and cathodic (Epc) peaks at 285 mV and 14 mV respectively with the peak difference (∆E) of 271mV. However, a significant shift of oxidation peak to the lower potential with improved peak current was observed in the case of unaided Co3O4 nanorgrains modified GCE, which indicates decent electrocatalytic activity of Co3O4 nanograins towards DA. Further noticeable improvement in redox peak current was observed in C1 and C2 modified GCE. This enhancement in electrochemical behaviour can be ascribed to the presence of electrically conductive platform of rGO that was providing excellent electron transport facility which in turn accelerated the electron kinetics at the electrode/electrolyte interface. However, in case of C4 (blue line) modified GCE, a decrease in redox peak current was observed without any significant shift in peak potentials. It is evident that rGO–Co3O4 nanograins exhibited excellent catalytic activity for DA detection as compared to unaided Co3O4 nanograins and bare GCE. This supports the fact that rGO provided enhanced conductive area for Co3O4 nanograins and contributed effectively in electron promotion during electrocatalytic oxidation of DA. Enhanced electrocataylytic response was observed for C3/GCE (green line) as compared to C1, C2 and C4. It exhibited 3.1 fold higher current and very small redox peak difference (∆E = 69 mV) as compared to bare GCE. The poor response of C1 and C2 to oxidise DA might be due to the aggregation of Co3O4 nanograins caused by low concentration of rGO, which result in the reduction of exposed electrochemical surface area to electrolyte. The less exposure of Co3O4 nanograins in C4 (Fig 3d) on the top surface of rGO was due to the sufficient number of rGO sheets that significantly decrease the density of Co3O4 nanograins. These results are in good agreement with aforementioned FESEM results. The C3 was considered optimized sample for further electrochemical studies. The influence of scan rates on the electrocatalytic response of C3 modified GCE towards DA oxidation was studied at different scan rates (from 25 to 250 mVs-1). It is evident 12
from Fig. 6(a) that by increasing the scan rates, redox peak current also increased linearly with a slight shift in oxidation potential in the positive direction, indicating the occurrence of quasi-reversible reaction. The calibration plot (Fig. 6b) of redox peak currents (Ipa & Ipc) with scan rate showed a linear correlation (R2=0.998 and 0.99) which revealed that the electroxidation of DA at rGO–Co3O4 nanograins modified GCE is an adsorption controlled process [55,56]. The cyclic voltammograms (Fig. 7(a)) were recorded for composite C3 modified GCE at different molar concentrations of DA in 0.1 M of PBS (pH 7.2). Significantly, the oxidation peak current increased linearly with the addition of DA in the range of 0.5 to 5 mM and anodic peak potential underwent a slightly positive shift. The logarithmic plot of peak current (Ipa) and different concentrations DA showed a linear plot with a slope value nearly equal to 1, which confirms that the electrochemical oxidation of DA follows the first order kinetics with respect to DA concentration at the C3 composite modified electrode (Fig. 7 (c)).
5. Amperometric detection of DA The amperometric detection method is an important tool for the detection of low concentrations of bioanalytes and to perform the interference studies. Fig 8(a) depicts the amperometric responses of composite C3 modified GCE on the successive injections of 1 μM of DA in a homogeneously stirred solution of 0.1 MPBS at a fixed time interval of 60 s. It can be seen that C3 modified GCE showed rapid response by significant increase in response current to each injection of DA at a fixed potential of +0.16 V indicating that the C3 modified GCE was effectively stimulated the oxidation of DA in 0.1 MPBS (pH 7.2). The calibration curve (Fig. 8b (inset)) indicates that the C3 modified GCE performs a linear response towards DA with the concentration from 1 to 30 μM.
13
The limit of detection (LOD) and limit of quantification (LOQ) for DA can be estimated using the standard deviation of the y-intercepts (SD) and the slope of the regression lines (S) (LOD = 3.3(SD/S) and LOQ = 10(SD/S)) [57]. The LOD and LOQ values for DA using C3 modified GCE were found to be 0.277 μM and 0.924 μM respectively. The sensitivity of C3 modified electrode was 0.389 ± 0.0005 μA μM-1 cm-2. The comparison of the electrochemical sensing performance of the present rGO–Co3O4 nanocomposite modified GCE with some of the reported GCE based electrochemical sensors for the detection of DA is summarised in Table 1. Further, the selectivity of the rGO–Co3O4 nanograins for the detection of DA was examined by injecting three different interfering species; uric acid, glucose and ascorbic acid in the evenly stirred PBS containing DA and the change in the current response was observed. Fig. 9 depicts the recorded amperometric i–t curve response for the consecutive additions of DA and interfering species (sample time of 60 s) in a homogeneously stirred 0.1 MPBS (pH 7.2). First, alternate injections of DA with uric acid, glucose and ascorbic acid were used to record the current response. On each injection of DA current response was observed which was equal to the oxidation current of DA. On the other hand the injections of interfereing species did not gave any current response even with 100 fold higher concentration than DA. Next, three consecutive injections of DA followed by three injections of UA, glucose and AA respectively were used to analyaze the current response. Again interfering species did not show any current response, contrary to the DA which gave current stair on each injection. This confirmed that the addition of interfering species did not contribute any current response even with a 100-fold higher concentration than DA. On the other hand, on every injection of DA in the same solution showed approximately the same magnitude of response current for the oxidation of DA. This phenomenon is due to low oxidation potential (+0.16) which was used in amperometric method for interference studies. 14
The low potential helped in detecting DA selectively even in the presence of high concentration interfering species. These results revealed that the present sensor possesses good selectivity and sensitivity towards the quantitative determination of DA even in the presence of common physiological interfering species.
6. Stability and reproducibility In order to evaluate the performance and reliability of proposed sensor in real application, analytical determination of DA in two urine samples was performed using standard addition method. The analytical results were summarized in table 2. To ascertain the accuracy of the results, different concentrations of DA were added in urine containing PBS solution and detected. The recovery of spiked sample ranged between 98.3 to 103.4%, which indicated the successful application of rGO-Co3O4 nanograins for the detection of DA. For the stability study of proposed sensor, the CV response of C1 modified GCE was recorded for DA in a range between -0.2 and 0.6 V at scan rate of 50 mV s−1. The CV response was recorded on each day for one week. The peak current was considered as a factor for indicating the stability of the modified electrode. Here, I is the current response of fresh sensor and I0 is the current response after storage as shown in Fig. S3. A 10.3 % decrease in the anodic peak current with slight shift in peak potential was observed after one week. This revealed that C1 modified GCE can also withstand being stored in solution for a period of time (one week) and the electrocatalytic performance remain almost unchanged, which confirms that modified electrode is very stable and reproducible for the detection of DA.
7. Conclusion
15
In this work, rGO–Co3O4 nanograin composites were synthesized by hydrothermal route and were optimized by varying the contents of rGO such as 5.7, 7.4, 9.1 and 10.7 wt%, which were denoted as C1, C2, C3 and C4 respectively. The crystallinity of rGO–Co3O4 composites is examined by XRD. Raman spectrum revealed the successful reduction of GO into rGO. The surface morphology revealed that the granular shaped Co3O4 were decorated on rGO matrix with the average particle size of ~ 35 nm. For electrochemical oxidation of DA, GCE was modified with nanocomposites and it was found that C3 modified GCE showed enhanced electrocatalytic performance in terms of peak catalytic current and shift in over potential when compared to those of C1, C2 and C4 modified GCE. The LOD was found to be 0.277 μM with S/N ratio of 3 with quantitative limit of 0.924 μM using the amperometric i–t curve technique. In addition, the C3 modified GCE displayed good selectivity for the detection of DA in the presence of a 100-fold higher concentration of uric acid, glucose and ascorbic acid. The sensor studies suggested that 3D rGO–Co3O4 nanocomposites endow excellent catalytic activity, high selectivity and sensitivity towards DA. Acknowledgement This work was supported by the High Impact Research Grant (H-21001-F000046) from Ministry of Education, Malaysia, University of Malaya Research Grant (RP025A-14AFR) and Postgraduate Research Grant (PG119-2015A).
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Biographies Arshid Numan obtained his bachelor degree in electronics from Sarhad University of Information Technology, Pakistan, in 2011 and master degree in Electrical Engineering from COMSATS Institute of Information Technology, Pakistan, in 2014. Now he is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. His research area is synthesis and characterization of nanomaterials for Supercapacitor, Electrocatalysis and electrochemical sensor applications.
Muhammad Mehmood shahid obtained his master degree in electronics from Sarhad University of Information Technology, Pakistan, in 2011. Now he is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. His research interests are in the areas of Photocatalysis, Photoelectrochemistry, Electrocatalysis and Sensors.
Fatin Saiha Omar obtained her bachelor and master degree in Physics from University of Malaya, Kuala Lumpur, Malaysia. Now she is a doctoral student of material science in University Malaya, Kuala Lumpur, Malaysia. Her research interest are in the area of synthesis and characterization of nanomaterials for Supercapacitor and Sensors applications.
K. Ramesh obtained his Phd degree from University of Malaya, Kuala Lumpur, Malaysia. Currently he is serving as senior lecturer in department of Physics, University of Malaya. His area of interest is Coating (Corrosion, Antifouling, Natural Colorants, Protective Coatings) Polymer Electrolytes for Supercapacitor Batteries and Electrochemical Sensors applications.
S. Ramesh is currently a professor of material science in department of Physics, University of Malaya, Kuala Lumpur, Malaysia. He published more than 200 research papers in several reputed international journals. His current area of interest is in Polymer Electrolytes (Rechargeable Batteries, Solar Cells, Fuel Cell etc.), Supercapacitor, Electrocatalysis and Electrochemical Sensors applications.
25
Figure Captions Fig. 1 XRD patterns (a) rGO (inset GO), (b) rGO–Co3O4 (inset Co3O4) Fig 2(b) shows the XRD patterns of the Co3O4 nanograins and rGO–Co3O4 nanocomposite. Fig. 2 Raman spectra of (a) rGO sheets (inset: the Raman spectrum of GO sheets) and (b) rGO–Co3O4 nanocomposite (inset: the Raman modes of Co3O4 nanograins) Fig. 3 FESEM images of rGO–Co3O4 nanograins (a) composite C1, (b) composite C2, (c) composite C3 and (d) composite C4. Fig. 4 Schematic representation of electrocatalysis of DA at rGO–Co3O4 nanograins modified GCE. Fig. 5 Cyclic voltammograms obtained for bare GCE, Co3O4 nanograins, C1, C2, C3 and C4 nanocomposite modified GCE for 0.5 mM DA in 0.1 M PBS at a scan rate of 50 mVs-1. Fig. 6 (a) CVs observed for rGO–Co3O4 nangrains (C2) in 0.1 M PBS (pH 7.2) containing 0.1 mM DA at various scan rates (b) The plots of peak current versus the scan rates. Fig. 7 (a) Cyclic voltammograms obtained at the rGO–Co3O4 nanograins modified electrode during the successive addition of different concentrations (b) Plot of Ipa vs molar concentration of DA (c) Plot of log Ipa vs log [molar concentration of DA] Fig. 8 (a) Amperometric i–t curve obtained at rGO–Co3O4 nanocomposite modified GCE for the successive addition of 1 mM DA in 0.1 M PBS (pH 7.2) at an applied potential of +0.16 V with a regular interval of 60 s (b) corresponding calibration plot of current versus concentration of DA. Fig. 9 Amperometric i–t curve obtained at rGO–Co3O4 nanograins modified GCE for the successive addition of DA and each 1 mM of uric acid, glucose and ascorbic acid in 0.1 M PBS (pH 7.2) at a regular interval of 60 s. The applied potential was +0.16 V 26
Fig. 1
27
Fig. 2
28
Fig. 3
29
DA
DAQ ‐2e +2e
GCE‐rGO‐Co3O4
Fig. 4
30
Fig. 5
31
Fig. 6
32
Fig. 7
33
Fig. 8
34
Fig. 9
35
Table 1 A comparison of some of the reported electrochemical sensors for DA detection.
Type of Electrode
Technique
pH
LOD (μM)
LOQ (μM)
Linear range (μM)
Sensitivity (μAμM-1)
Reference
Graphene/SnO2/GCE
DPV
6.8
1
Nil
0-100
Nil
[58]
rGO/TiO2/GCE
DPV
6.5
6
Nil
2-60
Nil
[59]
V2O5@PANI/GCE
CA
Nil
0.39
130
6.6-110
Nil
[60]
Graphene/GCE
DPV
Nil
2.64
Nil
4-100
Nil
[61]
Flower-like Au nanostructurecovered gold electrode (FANE)
DPV
7.4
0.2
Nil
1 – 150
Nil
[62]
ERGO/GCE
DPV
7
0.5
Nil
0.5 – 60
0.482
[63]
(GE/Au/GE/CFE)
DPV
7
0.59
Nil
0.59 –43.95
rGO–Co3O4
CA
7.2
0.277
0.924
0–30
[64]
0.389
Present work
36
Table 2 Determination results of DA by using rGO–Co3O4 nanograins in real urine samples (n = 3). Urine sample
Sample 1
Sample 2
Add concentration (μML-1)
Found Concentration (μML-1)
RSD (%)
Recovery (%) (μML-1)
1μM
1.010
1.48
101
3μM
2.972
1.04
99.1
6μM
5.900
2.58
98.3
1μM
1.034
1.68
103.4
3μM
2.948
1.07
98.3
6μM
6.011
1.17
100.1
37