Graphene-Multiwalled Carbon Nanotube Hybrids Synthesized by Gamma Radiations : Application as a Glucose Sensor

Three-dimensional hybrid nanomaterial of graphene-multiwalled carbon nanotubes (G-MWCNTs) was synthesized using gamma rays emitted by a Co source with a dose rate of 3.95Gymin. The products were characterized by fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis), photoluminescence (PL), and micro-Raman spectroscopy, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). FTIR and UV-Vis analysis reveals the formation of hybrid nanomaterial which is confirmed by XRD, micro-Raman analysis, and PL. SEM micrograph depicts the composite structure of graphene layers and MWCNTs, while the TEMmicrograph exhibits graphene layers covered by MWCNTs. The G-MWCNTs hybrid used as electrode for electrochemical studies in K 3 Fe(CN) 6 shows enhancement in electrocatalytic behavior, compared to each individual startingmaterial, therefore, has been applied for amperometric sensing of glucose in alkaline solution and exhibits sensitivity of 12.5 μAmM cm and low detection limit 1.45μM (S/N = 3) in a linear range of 0.1 to 14mM (R = 0.985).


Introduction
Carbon is one of the most abundant elements found in nature existing in various allotropic forms.It has four electrons in its outer valence shell and is a conducting in nature.In the last two decades, novel nanostructures have emerged such as fullerenes, carbon nanotubes, and graphene.The flat monolayer of sp 2 carbon atoms tightly packed in twodimensional honeycomb-like lattice is the building block for graphene.Due to its remarkable mechanical, optical, thermal, electronic, and magnetic [1][2][3][4][5] properties it has been used in various areas of nanosize research such as electronics, batteries, fuel cells, super capacitors, sensors, and biosensors [6][7][8][9][10][11]. Compared to carbon nanotubes, graphene exhibits potential advantages of high surface area, ease of processing, low cost, safety [12], and high purity (absence of transition metals, Fe, Ni, etc.) [13].Thus, it provides a good platform to study the electrocatalytic effects of carbon materials.In fact, graphene oxide (GO) sheets are rich in oxygencontaining functional groups such as alkoxy, hydroxyl, and carboxyl groups [14,15] and, hence, can be well dispersed in polar solvents.However, the formation of reduced GO in aqueous solution results in aggregation of sheets due to hydrophobic nature and induces strong - interactions when the oxygen-containing functional groups are removed during the reduction process [16].Important drawbacks in the synthesis of graphene are difficulty in isolating single layer and its tendency to curl, fold, and corrugate due to the flexibility of sheets [17].The large folds arise during processing [18], whereas smaller ripples tend to be inherent in the structure of isolated layers [19].As a result, graphene shows lower surface area and conductivity compared to the predicted theoretical value.Hence, preparation of individual graphene sheets is rather critical.On the other hand, carbon nanotubes (CNTs), consisting of cylindrical graphene sheets with nanometer diameter, have high mechanical strength and chemical stability, good electrical conductivity, rich optical properties, and high surface area.Therefore, CNTs constitute important materials as well as electrodes for sensing applications [20].Electrical conductivity of CNTs is also affected 2 Journal of Nanotechnology due to the nonuniform contact and discontinuities between all the CNTs in the heterogeneous structure [21], thereby limiting their application as sensors [17,[22][23][24].Therefore, an intelligent and unique graphene-carbon nanotube hybrid film, consisting of CNTs connected perpendicularly to graphene layers, could overcome some of the shortcomings and limitations of individual components [25][26][27][28][29]. Thus, many efforts have been made so as to obtain graphene-carbon nanotube hybrids by different methods of synthesis [25][26][27][28][29][30][31][32].
Gamma () radiations have been used for many years as a competitive method to develop new materials, metal nanoparticles, and their composites [33][34][35].Advantages of this technique over the other methods are self-generation of reducing agent, avoiding the use of toxic reducing agents such as hydrazine, simultaneous control of ion reduction and suppression of undesired oxidation products [33], formation of products under ambient conditions, simplicity, and economic.
In the present paper, an efficient, highly reproducible, and clean route towards synthesis of three-dimensional graphenemultiwalled carbon nanotube (G-MWCNT) hybrids has been reported.The hybridization of GO and acid functional multiwalled carbon nanotubes (f-MWCNTs) was carried out by -ray irradiation of the deoxygenated mixture of GO and MWCNTs in alcohol-water solution (Figure 1).Probable mechanism involving the formation of hybrid has been proposed.The differences observed in the resistance of the hybrid nanomaterials indicate that GO : MWCNTs ratio can further be optimized to improve the interface and nanostructures.Additionally, the electrocatalytic behavior of G-MWCNTs hybrids with different ratios of GO and MWCNTs has been examined.The G-MWCNTs hybrids show an improvement in electrocatalytic properties in comparison to each of the individual starting materials.The highest activity is observed for 1 : 1 hybrid G-MWCNTs.Increase in ratio to 1 : 3 does not show significant difference.Therefore, performance of 1 : 1 G-MWCNTs hybrid has been investigated as an amperometric glucose sensor.

Instrument.
The 60 Co -source was from the Department of Chemistry, University of Pune, supplied by BARC (Bhabha Atomic Research Center), Mumbai, India, with a dose rate of 3.95 Gy min −1 .The solutions were sonicated using a sonicator supplied by IMECO, India (34 KHz frequency, 500 W).
Voltammetric measurements were performed on a Potentiostat/Galvanostat (model 263-A, EG&G USA) controlled by power suite software package and GPIB interface, in a conventional three-electrode system, together with a Pt loop as counter and Ag/AgCl (saturated KCl), as a reference electrode.Bare glassy carbon (GC) and GC modified with GO, -COOH functionalized multiwall carbon nanotubes (f-MWCNTs), and G-MWCNT hybrids electrodes were used as working electrodes.All measurements were performed in air atmosphere and room temperature.Prior to experiments, GC electrodes were polished with 0.3 and 0.05 m alumina and rinsed with copious amount of Milli-Q water.

Synthesis of GO.
GO was synthesized from graphite powder using modified Hummer's method [36].In brief, 1 g of graphite and 0.5 g of sodium nitrate were added in 23 mL of concentrated sulphuric acid under constant stirring.KMnO 4 (3 g) was added gradually to the above solution while keeping the temperature less than 20 ∘ C to prevent overheating and explosion.The mixture was stirred at 35 ∘ C for 1 h and the resulting solution was diluted by adding 500 mL of double distilled water under vigorous stirring.To ensure the completion of reaction with KMnO 4 , the suspension was further treated with 30% H 2 O 2 solution (5 mL).The graphene oxide sheets thus obtained were washed with HCl and H 2 O, followed by filtration and drying.

Synthesis of G-MWCNT Hybrids. G-MWCNT hybrids
were prepared by -irradiation method with three different weight ratios of 3 : 1, 1 : 1, and 1 : 3 (ratio of GO to MWCNTs).Initially, appropriate amount of GO was dispersed in Milli-Q water (1 mg mL −1 of solid) by sonication (1 h) followed by addition of f-MWCNTs and alcohol (methanol, ethanol, isopropanol, or tert-butanol); the mixture was further sonicated for 30 min.The homogenous suspension was transferred into a glass bottle fitted with a septum, bubbled with nitrogen gas for 10 min to remove the dissolved oxygen, and then irradiated at room temperature for different time intervals (12 h-60 h, the corresponding doses being 2.84, 5.70, 11.38, and 14.22 kGy, resp.) at a dose rate of 3.95 Gy min −1 .The product thus obtained was collected, washed with Milli-Q (18 MΩ cm −1 ) water, and dried under vacuum.UV-Vis spectra of samples (S1) indicated significant effect on the extent of reduction of GO + f-MWCNTs; the best conversion was observed for a total dose of 11.38 kGy, and hence further experiments were carried out at a dose of 11.38 kGy.Results for f-MWCNTs, GO, and GO + f-MWCNTs (1 : 1) irradiated at 11.38 kGy in ethanol/water solution are discussed in detail with other systems included wherever necessary.

Preparation of Modified GC
Electrodes.GC electrode (3.0 mm diameter) was polished with 0.3 and 0.05 m alumina slurries to obtain a mirror-like surface and rinsed with Milli-Q water and dried.Fresh dispersions of GO, f-MWCNTs, and G-MWCNTs were prepared by adding 1.0 mg of sample in 1.0 mL of Milli-Q water followed by sonication.From this, 10 L of solution was drop-casted on the precleaned GC disk electrode and dried under vacuum at room temperature to obtain modified GC electrodes.Bare GC and modified electrodes were used for electrochemical measurements.

Characterization Techniques.
Fourier transform infrared (FTIR) spectra of all the samples were recorded on a FTIR spectrophotometer (Thermo Scientific NICOLET 6700) using KBr as the mulling agent.Ultra violet-visible (UV-Vis) and photoluminescence (PL) spectra of the samples were obtained on a UV-Vis spectrophotometer (PerkinElmer LAMBDA 950) and spectrofluorometer (JASCO, FP8300), respectively.X-ray diffraction (XRD) analysis of powders was carried out on a Bruker AXSD-8 advance X-ray diffractometer with monochromatic CuK  radiation ( = 1.5406Å).Data were collected from 10 ∘ to 60 ∘ at a scan rate of 0.1 ∘ min −1 .The morphologies of the samples were observed under scanning electron microscope (SEM, JEOL JSM-6360) and transmission electron microscope (TEM, TECNAT G2 20ULTRA-TWIN, FEI instrument).Suspensions of the samples in Milli-Q water were loaded on grids of 200 mesh size.Micro-Raman spectra of the samples were recorded by using HORIBA Jobin Yvon (France Instrument), at 532 nm laser power 1.7 mW, 100x objective lens and 0.9 NA.HAMEG programmable LCR-Bridge hm8118 was used to measure the resistance of the samples.For measurement of resistance, pellets of the samples (1 cm 2 and 500 m in thickness) were prepared by placing 50 mg of each powder between two iron plates of a die and compressed at a pressure of 78 KN.

Results and Discussion
3.1.Spectroscopic Characterization.The FTIR spectra of GO, f-MWCNTs, GO + f-MWCNTs (physical mixture), and G-MWCNTs are given in Figure 2(a).Spectrum of GO exhibits peaks at 1622, 1730, and 1035 cm −1 corresponding to C=C, C=O, and C-O stretching, respectively.Also, the broad band at 3340 cm −1 is attributed to O-H stretching mode of intercalated water [15].On the other hand, spectrum related to f-MWCNTs (Figure 2(a)) shows two strong peaks at 3500 and 1710 cm −1 representing O-H and C=O stretching, respectively.The spectrum of GO + f-MWCNTs exhibits bands similar as for GO, while, in the case of G-MWCNTs hybrid material (Figure 2(a)), the characteristic absorption bands of GO, f-MWCNTs, and GO + f-MWCNTs [37] are suppressed drastically.However, two new peaks are observed at ∼1450 and ∼1580 cm −1 that can be attributed to the C=C stretching of aromatic ring indicating the formation of G-MWCNTs hybrid.
These results were further confirmed by UV-Vis analysis of the samples (Figure 2(b)).The peak observed for GO at 230 nm (spectrum (A)) represents the - * transitions of C-C aromatic ring, while the shoulder at 308 nm corresponds to - * transition of C=O bond [38,39].In case of f-MWCNTs the absorption peak observed at ∼260 nm (spectrum (B)) is attributable to the - * transitions of f-MWCNTs [40,41].However, the spectrum of G-MWCNTs hybrid (spectrum (C)) shows distinctly different nature compared to GO or f-MWCNTs.This spectrum consists of four peaks at 208, 225, 275, and 281 nm.To understand the origin of these new peaks, UV-Vis spectrum of unirradiated solution of GO + f-MWCNTs (physical mixture in ethanol-water, Figure 2(b) spectrum (D)) and irradiated samples of GO (spectrum (E)) and f-MWCNTs (spectrum (F)) were recorded.From the results, it is observed that, in case of physical mixture, the spectrum shows the features similar to GO.On the contrary, GO after irradiation exhibits a peak at 275 nm, while f-MWCNTs show peaks at 208, 225, and 281 nm.These results justify the formation of hybrid after irradiation (spectrum (C)).The considerable red shift in peaks (230 to 275 for GO and 260 to 281 for f-MWCNTs) in the hybrid sample is due to increase in electron density and structural ordering consistent with the restoration of sp 2 carbon atoms [29,32].Further, the red shift confirms the - stacking interactions between the aromatic basal planes of GO and MWCNTs.Apart from this, the new peaks at 208 and 225 nm can be attributed to the formation of C-H bonds simultaneous to C-C bond breaking [42].
To verify whether the GO nanosheets are electronically coupled to the MWCNTs instead of physical attachment, the photoluminescence (PL) spectra of GO, f-MWCNTs, GO + f-MWCNTs, and G-MWCNTs were obtained at an excitation wavelength of 240 nm and the results are compared in Figure 3.As seen from the figure, the PL spectrum of GO and GO + f-MWCNTs (Figure 3, solid and dotted line) exhibits a strong peak at ∼470 nm that can be correlated to the - * transition of GO.The origin of a sharp emission peak at ∼567 nm can be ascribed to the C-O, C=O, and O=C-OH functionalized groups [43].
Similarly, intense emission is observed for f-MWCNTs (Figure 3 dashed line) [44].On the other hand, in case of G-MWCNTs hybrid significant quenching of the PL spectrum is observed (Figure 3, dashed-dotted line and inset) which could be due to the following reasons: (i) the emission from graphene being quenched due to absorption of electrons by MWCNTs (excellent electron acceptors) [32], (ii) removal of functional groups from GO and f-MWCNTs after irradiation, and (iii) formation of new sp 2 clusters [43] resulting in the blue shift observed in the spectrum (470 nm in GO to 466 nm in G-MWCNTs hybrid).

Crystallographic and Structural
Analysis.The X-ray diffractograms of GO (A), GO + f-MWCNTs (B), f-MWCNTs (C), and G-MWCNTs hybrid (D) are displayed in Figure 4(a).As observed from the figure, GO and GO + f-MWCNTs (Figures 4(A  new structure.It has been reported that, after irradiation and removal of oxygen-containing functional groups, graphene oxide shrinks and the interlayer spacing decreases [45].However, in the present reaction, at the time of reduction (during irradiation) MWCNTs are introduced between the graphene layers, and they avoid stacking up of graphene layers,therefore interlayer spacing does not show significant changes.It is worth noting that the data for GO and f-MWCNTs are in good agreement with the previous reports [28,46].The electronic and crystallographic structures of the samples were evaluated by micro-Raman spectroscopy.
The Raman spectra of carbon materials show three wellknown peaks called D, G, and 2D observed at ∼1340, ∼1540, and ∼2650 cm −1 , respectively.The G peak corresponds to the optical E 2g phonons at the Brillouin zone center and represents the bond stretching of sp 2 carbon pairs in both rings and chains.The D peak corresponds to the breathing mode of sp 2 atoms in aromatic rings indicating the presence of defects [31,47] and 2D band corresponds to second-order dispersion, a characteristic feature of two-phonon double resonance process.Figure 4(b) illustrates the typical micro-Raman spectra of f-MWCNTs, GO, GO + f-MWCNTs, and G-MWCNTs samples obtained at an excitation wavelength of 532 nm.Three distinct peaks are observed in each sample corresponding to D band, G band, and 2D band, respectively, as mentioned earlier.In the Raman spectra the D/G and 2D/D + G intensity ratios ( D / G and  2D / D+G ) are usually used to evaluate the average size of crystalline sp 2 domains and defect densities in graphene sheets [32].However, the  D / G and  2D / D+G ratios of G-MWCNTs hybrid (1.30 and 4.10, resp.) are remarkably greater than those of GO (1.03 and 1.62) and GO + f-MWCNTs (1.06 and 1.62) indicating higher degree of topological disorder and defect density together with higher degree of crystallinity within G-MWCNTs hybrid material in which a larger number of smaller sized graphitic demines are present in new hybrid materials.Also, the width of D and 2D bands for G-MWCNTs (Figure 4(b) inset) is narrower than that of the starting materials (GO and f-MWCNTs) [48].These results indicate the presence of defects caused by irradiation in G-MWCNTs hybrid material which leads to an improved electron transfer as confirmed from the electrochemical measurement discussed in Section 3.5.Further, on radiolysis of water, -ray irradiation decomposes water molecules to both oxidative ( • OH) and reductive (H • and e − ) species (1), whereas alcohols act as • OH radical scavengers and thereby convert into reducing radicals (2) which provides a reducing medium for chemical reaction to occur under -ray irradiation in the absence of oxygen (oxygen support formation of strong oxidative products such as where RH corresponds to methanol, ethanol, isopropanol, and tert-butanol.However, to study the effect of different alcohol structures on the -ray irradiation reaction, a variety of alcohols (methanol, ethanol, isopropanol, and tertbutanol) with 1 : 1 (V/V) ratio with water as a solvent medium were tested.The results revealed that ethanol is the most effective in this reaction system (S2, S3).This can be related to the differences in the rate constants of alcohols for scavenging hydroxyl radicals where ethanol exhibits the highest rate constant in contrast with other alcohols [50].Alcohol radicals react with the oxygen atoms of the functional groups (epoxide, hydroxyl present on GO, or f-MWCNTs) and carbon radicals are formed.Thereafter, the carbon radicals combine with hydrogen radicals present in the system and after decarboxylation reaction C=C bonds are formed in G-MWCNTs.

Morphology.
Morphology of the samples was studied with the help of SEM and TEM (Figures The SEM image of GO (Figure 5(a)) is seen to consist of randomly aggregated wrinkled sheets closely associated with each other.This feature is in agreement with the TEM image of GO (Figure 5(c)).The SEM image of MWCNTs shows dense structure comprised of randomly aggregated tubules (Figure 5(b)).
The SEM images of G-MWCNT hybrids (3 : 1, 1 : 1, and 1 : 3 ratios) are shown in Figure 6 (a1, a2), (b1, b2), and (c1, c2), respectively.As observed from the micrographs, the MWC-NTs are seen to be incorporated into the graphene layers and the surface morphology exhibits a mixed structure with MWCNTs dispersed between different GO sheets.This can be due to the possible interactions between - * hydrophobic region of GO and the side walls of f-MWCNTs.

Electrical Resistance.
The electrical resistance of the samples was measured to evaluate the conductivity of the synthesized nanomaterials and also the connectivity of layers along the edges in case of G-MWCNTs; Figure S4 in Supplementary Material available online at http://dx.doi.org/10.1155/2014/903872 shows the bar diagram of resistance versus sample (GO, f-MWCNTs, and G-MWCNTs 3 : 1, G-MWCNTs 1 : 1, and G-MWCNTs 1 : 3).It can be seen that GO shows the highest resistance (6 kΩ cm −1 ) due to the presence of hydroxyl and carboxyl functional groups.But f-MWCNTs show significantly lower resistance (0.52 kΩ cm −1 ) than GO.However, among the hybrid nanomaterials, the resistance of G-MWCNTs 1 : 1 is significantly lower (∼0.006kΩ cm −1 ) compared to the resistance of G-MWCNT 3 : 1 (3 kΩ cm −1 ).Resistance obtained for G-MWCNT 1 : 3 (∼0.004) is similar as for 1 : 1 ratio.These results reveal that G-MWCNTs 1 : 1 and 1 : 3 ratios are highly conducting in nature which is perhaps due to the strong coupling between graphene sheets and carbon nanotubes [27].
The results of all the above characterizations confirm the formation of new hybrid structure containing grapheme and MWCNTs.

Electrochemical Analysis.
The electrochemical response of G-MWCNT hybrids (1 : 3, 1 : 1, and 3 : 1 ratios), GO, and f-MWCNTs were investigated by using K 3 Fe(CN) 6 as the redox system due to its sensitivity/insensitivity to the electronic properties, surface microstructure, and surface chemistry of carbon electrodes [51].
Figure 8(a) shows the electrochemical response of K 3 Fe (CN) 6 [5 mM] and KCl [0.5 mM] at GC bare, GO, f-MWCNTs, G-MWCNTs (3 : 1), G-MWCNTs (1 : 1), and G-MWCNTs (1 : 3) modified electrodes at a scan rate of 100 mVs −1 .The cyclic voltammogram of K 3 Fe (CN) 6 is apparently improved at G-MWCNTs 1 : 1 and 1 : 3 ratios modified electrode, reflected by the enhancement in the peak currents (  ) and a decrease in the peak to peak potential [ΔEp] compared to that of GO modified electrode.These results signify that the electronic structure and the surface properties of G-MWCNTs modified electrode are advantageous for electron transfer [51,52] which is attributed to the strong coupling between graphene sheets and carbon nanotubes [27].products and is highly reproducible.Oxidizable species, such as ascorbic acid (AA) and uric acid (UA), usually coexist with glucose in biological fluids on different electrodes, particularly on nonenzymatic glucose sensors [53].
The normal physiological level of glucose is 3-8 mM, which is much higher than that of the interfering species, AA, and UA (∼0.1 mM).However, electron transfer rates of the interfering species are highly greater than that of glucose.Consequently, their oxidation currents are comparable to that from highly concentrated glucose.Therefore, the amperometric responses of the modified electrode at +0.6 V in solutions of 0.1 M NaOH for 0.1 mM UA, 0.1 mM AA, and 3 mM glucose were evaluated.For a better comparison, the response current of 3 mM glucose is set as 100%, and the responses of the interfering species are normalized by this value.The current responses obtained for UA and AA were ∼0.10 A and ∼0.12 A, 8.06% and 9.60%, respectively, which are significantly lower as compared to that of glucose (∼1.24 A) (S10).The results show that the interference from UA and AA is negligible.
These results indicate that the selectivity of G-MWCNTs is good enough to be used as a sensor for glucose.The batchto-batch reproducibility was estimated from the current responses of 6 different sensors toward 3.0 mM glucose, and an RSD of 4.89% was obtained, indicative of the reliability of the proposed method.The long-term stability is also a significant parameter for evaluating the performance of the sensor.A response current of 3 mM glucose was determined every two days.The results show that the response current is ∼75.30% of its initial value after 15 days (S11), which indicate that the stability of the sensor is good.

Conclusion
The -ray irradiation method appears to be a clean, simple, and cost-effective method to synthesize highly stable G-MWCNT hybrid nanomaterials.Characterization results proved that the G-MWCNT hybrid shows significant decrease in the self-aggregation and stacking of GO and MWCNTs due to appropriate coupling and interconnected network between MWCNTs and GO layers.This results in an increase in the stability, electrocatalytic activity, and electrical conductivity of hybrid nanomaterial compared to GO and MWCNTs individually.The amperometric sensor based on the G-MWCNTs modified electrode exhibits sensitivity of 12.5 AmM −1 cm −2 and low detection limit 1.45 M, with a linear range from 0.1 mM to 14 mM ( 2 = 0.985) and negligible interference due to the presence of ascorbic acid and uric acid.These improvements in the properties of G-MWCNTs qualify it as an important material to develop electrical as well as electrochemical sensing devices.Therefore, it appears to be promising for the fabrication of sensors based on the G-MWCNTs hybrid for other applications.

Figure 4 :
Figure 4: (a) X-ray diffractograms of GO (A), GO + f-MWCNTs (B), f-MWCNTs (C), and G-MWCNTs (D).(b) Raman spectra of f-MWCNTs, GO + f-MWCNTs, GO, and G-MWCNTs, from the top to the bottom, respectively; (b) inset shows the Raman spectra at an expanded scale to manifest the 2D and D + G bands.