Nitrogen-Doped Graphene as a Robust Scaffold for the Homogeneous

Research Article pubs.acs.org/journal/ascecg

Nitrogen-Doped Graphene as a Robust Scaffold for the Homogeneous Deposition of Copper Nanostructures: A Nonenzymatic Disposable Glucose Sensor N. S. K. Gowthaman, M. Amal Raj,† and S. Abraham John* Centre for Nanoscience and Nanotechnology, Department of Chemistry, The Gandhigram Rural Institute, Gandhigram, 624 302 Dindigul, India S Supporting Information *

ABSTRACT: The attachment of nitrogen-doped graphene (NG) on glassy carbon electrode (GCE) followed by electrodeposition of copper nanostructures (CuNSs) is described in this paper. Nitrogen-doped graphene oxide (N-GO) was prepared by intercalating melamine into graphene oxide (GO) by sonication. The doping of nitrogen was confirmed from the characteristic peaks at 285.3 and 399 eV in the XPS corresponding to the C−N bond and nitrogen, respectively. The presence of amine groups on the N-GO was exploited to attach them on GCE via Michael’s reaction. Subsequently, N-GO was electrochemically reduced to form NG by reducing the oxygen functionalities present on the NGO. Then, the CuNSs on the NG modified electrode was prepared by electrodeposition at various applied potentials with different deposition times. The homogeneous deposition of cubic, spherical, quasidendritic, and dendritic NS at the applied potentials of 0, −0.10, −0.30, and −0.40 V, respectively, was evidenced from scanning electron microscopy (SEM) studies. The surface energy of the system can be reduced by the intercalated nitrogen in the graphene layer via doping. Hence, the NG layers with large surface area act as a robust scaffold for the homogeneous deposition of CuNSs. Further, the electrocatalytic activity of the NG-CuNSs modified GCE toward glucose oxidation was studied. In a comparison with NG and CuNSs, the NG-CuNSs exhibited 2-fold higher oxidation current. Further, it was found that the electrocatalytic activity of the composite electrode depends on the shape of the CuNSs. Among the different CuNSs, the NG-dendritic CuNSs electrode exhibited higher electrocatalytic activity. Finally, the practical applicability of the present sensor was demonstrated by fabricating NG-dendritic CuNSs on screen printed carbon electrode for the determination of glucose in human blood serum and urine samples. KEYWORDS: Nitrogen-doped graphene, Copper nanostructures, Electrodeposition, Scanning electron microscopy, Electrocatalytic activity, Glucose sensor, Screen printed carbon electrode

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INTRODUCTION An incredible scientific and technological attraction to graphene has been witnessed in recent years due to its unique electronic, optical, and chemical properties.1 The graphene acts as a receptor to adsorb molecules in terms of π−π interactions and charge transfer which leads to construction of highly sensitive electrochemical sensors for several biomolecules.1,2 It is an open structured two-dimensional plenary sheet, which is distinct from the rolled structured carbon nanotubes, and thus, it could be effectively exploited for catalysis.3,4 Generally, graphene oxide (GO), prepared by Staudenmaier and Hummer’s methods,5−7 has been used as a precursor for the synthesis of graphene. The GO can be reduced by both chemical and electrochemical methods using reducing agents and applying more negative potential, respectively, to retain its aromatic backbone.2,8−12 It has been shown that the electrochemically reduced graphene sheets have higher conductivity © 2017 American Chemical Society

than that of the chemically reduced graphene because of the higher oxygen/carbon ratio of the former than the latter.10,13 Graphene was fabricated on different electrodes by various approaches for the construction of sensor devices which include electrodeposition of GO,3,13 drop-casting of graphene, sprayand spin-coating, Langmuir−Blodgett processes, layer-by-layer assembly,3 and drop-casting of GO and its subsequent electrochemical reduction.13,14 Although graphene films were successfully fabricated by the above strategies, the thickness and uniformity cannot be controlled by these methods. The selfassembly method can be used to attach nanomaterials successfully on solid substrates.15 Few researchers have reported the attachment of graphene on different solid Received: October 5, 2016 Revised: December 25, 2016 Published: January 9, 2017 1648

DOI: 10.1021/acssuschemeng.6b02390 ACS Sustainable Chem. Eng. 2017, 5, 1648−1658

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Representations of (A) Synthesis of N-GO and (B) Fabrication of NG-CuNSs on GCE

substrates by a self-assembly method.16−19 Recently, selfassembly of GO on GCE through 1,6-hexadiamine SAM was reported from our group.18−20 Copper and copper oxide materials have received enormous attention owing to their excellent electrocatalytic activity toward glucose oxidation besides their low cost and significant catalytic activities.21−23 In contrast to other carbon materials, graphene possesses massive surface area and immense electrical conductivity. Therefore, it is desirable to deposit copper nanostructures (CuNSs) on graphene sheets for the construction of an electrochemical glucose sensor.24−26 It is expected that the synergistic catalytic activity of the composite electrode stems from the combination of huge electrochemically active surface area and high conductivity of the carbon materials besides high catalytic activity of CuNSs.24−26 Doping of nitrogen in graphene not only leads to significant changes in the semiconducting property27,28 but also leads to the formation of composite materials by interacting with transition metals.29−31 Further, the surface energy of the system can be reduced by the doped nitrogen atoms ingrained in the graphene lattice. Hence, N-doped graphene acts as a strong platform for the uniform dispersion of metal nanoparticles.29−32 Recently, nitrogen-doped graphene−copper composite materials for different applications were reported.29−32 Zhou et al. reported an efficient oxygen reduction catalyst based on a copper oxide/ N-doped reduced graphene oxide (N-rGO) nanocomposite with a low temperature (90°) aqueous process (mixture of aqueous GO and NH3).29 Zhang et al. constructed an N-doped graphene (NG) support for few-layer graphene shells incorporated in copper nanoparticles via in situ chemical vapor deposition for SERS applications.30 Kumar et al. studied the interaction of the [Cu(bpy)2(H2O)2] complex with NG for photocatalytic reduction of CO2.31 Jiang et al. demonstrated the copper nanoparticles-decorated NG by annealing of GO, copper nitrate, and glycine at 500 °C as a nonenzymatic glucose sensor.32 Since the reported methods are based on thermal annealing and a hydrothermal process, they require more time and also have high cost. Herein, we have reported an economically viable, less time-consuming synthesis of N-GO via intercalation of melamine with GO by sonication for the first time. Moreover, the electrocatalytic activity with respect to copper morphology in the nanocomposite is not yet reported. Thus, the present study aims to deposit different copper

nanostructures on NG modified glassy carbon electrode (GCE) and study the effect of CuNS morphology on glucose oxidation. In the present study, NG films were fabricated on GCE by attaching the N-GO through electrostatic interaction. Subsequently, the N-GO was electroctrochemically reduced to form NG. The different shapes of CuNSs were electrodeposited on the NG modified GCE at +0.20, +0.10, 0.0, −0.10, −0.20, −0.30, −0.40, −0.50, and −0.60 V. The resulting electrodes were characterized by different spectroscopic, microscopic, and electrochemical techniques. Further, the oxidation of glucose was studied for different shapes of CuNSs modified NG electrodes. Interestingly, the dendritic CuNSs modified NG electrode shows superior electrocatalytic activity toward the oxidation of glucose in contrast to other shapes.

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EXPERIMENTAL SECTION

Preparation of N-GO. The GO33 suspension (1 mg/1 mL water) was exfoliated in basic medium (pH 9) by adding sodium carbonate and sonicated for 30 min for the complete exfoliation of GO. Then, a mixture of 15 mM melamine and the GO suspension was sonicated for 1 h to intercalate melamine with GO. It is expected that the amine functionality of melamine was attached to GO layers via nucleophilic addition to the epoxy functionality of GO. The melamine intercalated GO is referred to as N-GO (Scheme 1A). For the SEM analysis, the as-synthesized N-GO suspension was centrifuged, and the resulting powder was dried in a stream of N2 gas before measurement. Fabrication of GCE with N-GO and NG. The GCE was cleaned according to the reported procedure.18 For the fabrication of N-GO on GCE, the GCE was immersed into a solution of exfoliated N-GO for 12 h. The N-GO was attached on GCE through Michael’s reaction. This electrode is designated as GCE/N-GO. Then, the resultant electrode was electrochemically cycled between 0 and −1.3 V in 0.2 M PB solution (pH 7) for the fabrication of NG.20,34 This electrode is designated as GCE/NG (Scheme 1B). Electrodeposition of Copper Nanostructures on GC/NG Electrode. The CuNSs were electrodeposited on NG modified GC electrode using 0.01 M CuSO4 in 0.1 M H2SO4 at the applied potentials of +0.20, +0.10, 0, −0.10, −0.20, −0.30, −0.40, −0.50, and −0.60 V (vs Ag/AgCl) for 400 s. This electrode is represented as GCE/NG-CuNSs (Scheme 1B). To compare the performance of the NG/CuNSs electrode with electrochemically reduced graphene oxide (ERGO), ERGO was fabricated on GCE using the same procedure. However, instead of taking GO with melamine, the GO was initially assembled on the GCE using 1,6-hexanediamine as a linker. The GC/ GO electrode was then electrochemically reduced to form ERGO on GCE. 1649


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ACS Sustainable Chemistry & Engineering For Raman, XPS, XRD, SEM, and EDAX measurements, indium− tin−oxide (ITO) was used whereas GC plate was used for ATR-FT-IR measurements. For the determination of glucose in real samples, NGdendritic CuNSs were fabricated on screen printed carbon electrode.

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RESULTS AND DISCUSSION Fabrication of N-GO on GCE and Its Electrochemical Reduction. The N-GO was attached on GCE through the Michael’s nucleophilic addition in which the attachment of free amine groups of N-GO on olefinic double bonds of GCE.35,36 The oxygen functionalities including epoxide, carboxyl, and hydroxyl on the surface of N-GO were electrochemically reduced by cycling the GC/N-GO electrode between 0 and −1.30 V for 15 cycles, and therefore, the aromatic lattice of graphene was retained (Figure 1). The observed reduction peak Figure 2. Raman spectra obtained for (a) N-GO and (b) NG modified substrates.

bands appeared at 1340 and 1581 cm−1, respectively.27 The extent of defects due to vacancies, distortion, and edges in carbon materials can be usually identified from the ratio of the intensities of D and G bands (ID/IG). From Figure 2, the ID/IG value was found to be 0.97 and 1.25 for N-GO and NG, respectively. The larger ID/IG (1.25) value clearly reveals the greater sp2 character as well as the decline in the average size of the sp2 domain due to electrochemical reduction. Moreover, it confirms that the electrochemical reduction of N-GO not only leads to alteration in its structure but also introduces some structural defects.37 The obtained results reveal the successful formation of N-GO and NG. From the D and G band intensities, the average crystalline size of the N-GO and NG modified substrates was calculated using eq 1.38

Figure 1. Electrochemical reduction of electrostatically assembled NGO at GCE (15 cycles) in 0.2 M PB solution (pH 7.2) at a scan rate of 100 mV s−1.

⎛ ID ⎞−1 4 La = (2.4 × 10−10)λlaser ⎜ ⎟ ⎝ IG ⎠

at −1.15 V in the first cycle corresponds to the reduction of oxygen functionalities, and it was drastically decreased in the next cycle and completely vanished after six cycles. This indicates the regeneration of the sp2 backbone of graphene after the electrochemical reduction of N-GO. Characterization by ATR-FT-IR Spectroscopy. The attachment of N-GO followed by its electrochemical reduction was confirmed by ATR-FT-IR spectroscopy. Figure S1 exhibits the ATR-FT-IR spectra obtained for GC/N-GO and GC/NG substrates. The N-GO modified substrate shows the peaks at 1080 (ν(CO)), 1180 (ν(CN)), 1420 (CO stretching of carboxylate groups), 1540 (NH bending), 1618 (ν(CC)), 1680 (CO stretching of amide), 3130 (ν(CH stretching)), 3320 (ν(NH stretching)), and 3590 cm−1 (ν(OH)). After electrochemical reduction the peaks at 1080 (ν(CO)), 1420 (CO stretching of carboxylate groups), and 3590 cm−1 (ν(OH)) disappeared indicating the removal of oxygen functionalities present in the N-GO substrate. The peak assignments are shown in Table S1. The above results confirm the successful fabrication of NG. Characterization by Raman Spectroscopy. Raman spectroscopy yields information about the defects and ordered and disordered structures.3 Figure 2 shows the Raman spectra of N-GO and NG modified substrates. The N-GO fabricated substrate displays D and G bands at 1349 and 1590 cm−1, respectively (curve a). A noticeable increase in intensity of the D band when compared to the G band was observed after the electrochemical reduction of N-GO (curve b). The D and G

(1)

Here, La and λ represent the average crystalline size and the laser wavelength (532 nm), respectively. The average crystalline size was found to be 19.80 and 15.38 nm for N-GO and NG modified substrates, respectively. The obtained results suggest that the average crystalline size of N-GO was decreased after electrochemical reduction. Characterization of NG Modified Substrate by SEM and XPS. SEM images of N-GO and NG modified substrates are shown in Figure 3. The N-GO film modified substrate shows a homogeneously ordered structure (Figure 3a) whereas NG shows very thin isolated layers which are nothing but a few layers of graphene (Figure 3b). Further, the NG layers appeared as folded into a typical crumpled structure after the electrochemical reduction of N-GO. It is thought that aggregation of GO takes place during electrochemical reduction due to the increased π−π interaction between the graphene layers.1 The grainy and heterogeneous nature of NG films might be due to high density of edge planes of graphene layers. The fabrication of NG on GCE was further characterized by XPS. Figure 4A shows the XPS survey spectrum of the NG modified substrate. This shows an asymmetric peak at 284 eV along with peaks at the higher binding energy side. The observed peaks at 284, 531, and 399 eV correspond to the C 1s peak of sp2 carbon, the O 1s spectrum of various oxygen functionalities, and the amine functionality of the melamine intercalated with the GO, respectively. Figure 4B shows the C 1650


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Figure 3. SEM images obtained for (a) N-GO and (b) NG modified substrates.

Figure 4. (A) XPS survey spectrum, and (B) C 1s, (C) O 1s, and (D) N 1s spectra obtained for NG modified substrate.

nitrogen atoms in NG substrate were characterized from N 1s spectra. The N 1s spectrum of NG substrate can be deconvoluted into four peaks at 398.2, 399.2, 401.1, and 402.6 eV (Figure 4D). In general, the pyridine-like and pyrrolelike nitrogens provide the π-conjugated system with a pair of πelectrons in the GO layers, which appeared at 398.2 and 399.2 eV, respectively. Carbon atoms present on the GO layers were replaced with nitrogen atoms in the form of “graphitic” nitrogen.27 The corresponding peak for graphitic nitrogen is

1s spectra of the NG fabricated substrate. The C 1s spectra show three peaks at 284.5, 285.3, and 288.1 eV corresponding to the sp2 carbon, CN bond, and CO bond, respectively. The peak at 285.3 eV is due to the CN bond, which confirms the intercalation of melamine into GO. Further, the intensity of the sp2 carbon signal was higher than that for the the oxygencontaining functional groups (Figure 4C). This suggests that the electrochemical reduction of N-GO results in the removal of oxygen functionalities.10 The bonding configurations of 1651


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Figure 5. SEM images obtained for (a) CuNPs and (b) NG-CuNPs modified substrates at an applied potential of −0.1 V.

were grown like quasidendrites at an applied potential of −0.30 V (Figure 6f). The size of each branch of the dendrites is about 200 nm. Further, when the potential is switched to −0.40 V, well-defined dendrites were grown on the surface of the NG with a size of 50 nm for each branch (Figure 6g). Here, homogeneous as well as heterogeneous nucleation may take place concomitantly. For heterogeneous nucleation, the initially deposited particles act as seeds to direct the formation of dendritic morphology whereas the formation of dendrites is a consequence of attachment of primary particles in the homogeneous nucleation.39 Recently, Xie et al. reported one step electrodeposition of the r-GO/Cu composite film and studied the morphology of the composite film by varying the applied potential. They observed the formation of nanograins and pine tree leaf nanostructures at −1.20 and −0.40 V, respectively.40 While switching the applied potentials to −0.50 and −0.60 V, the well-defined dendrites were aggregated (Figure 6h,i. This may be due to overgrowth of dendritic CuNSs. As it can be seen from Figure 6, the primary morphology of the deposited CuNSs was found to be cubic. While switching the applied potential to be more negative, the particle size was decreased monotonically which occurs in concert with a corresponding increase in the particle number density. Obviously, a more negative applied potential on the working electrode generates more nucleation sites, which is attributed to the increase in the particle number density. In other words, the applied potential via the Nernst equation directs the relative surface electroactive species concentration. Further, the deposition of Cu on NG film follows the instantaneous nucleation mechanism followed by diffusion-limited growth.41,42 The formation of different CuNSs over NG film is based on the nucleation mechanism that follows: (i) Cu2+ is reduced to form Cu at an appropriate applied potential and formed nuclei after a long duration, leading to the regular spherical particles. (ii) The spherical particles tend to join together to form a one-dimensional root, and subsequently the nucleation is initiated on the active sites from the root, resulting in the branched growth of dendritic NSs. (iii) With a further increase of the applied potential, the existing nuclei will grow onto the aggregated CuNPs, and the free nanoparticles diffused

expected to appear in the region 400.8−401.3 eV. The peak at the region 402.3−402.9 eV is due to oxidized nitrogen.27,28 The deconvoluted N 1s spectra of the NG substrate show peaks at 401.1 and 402.6 eV in the higher binding energy region, corresponding to graphitic nitrogen and the oxidized nitrogen, respectively. This result is well-supported with the earlier reports and confirms the doping of nitrogen via intercalation of melamine to the graphene layers.27−29 Characterization of NG-CuNS Modified Substrate by SEM and XPS. Figure 5a shows the SEM image obtained for electrodeposition of copper on ITO surface at an applied potential of −0.10 V for 400 s. When copper was deposited directly on the ITO surface, agglomerated spherical particles with size 400 nm were formed. On the other hand, deposition of copper at the NG modified ITO surface leads to the formation of uniform and isotropic CuNPs under identical conditions (Figure 5b). Interestingly, the growth and size of CuNPs was controlled by NG layers. It has been already reported that the homogeneous deposition of metal nanoparticles on the NG film was achieved in the presence of doped nitrogen atoms because it reduces the surface energy of the graphene lattice.29−32 Hence, the N-doped graphene acts as a strong platform for the homogeneous deposition of CuNPs. The size of the deposited CuNPs was in the range of 100 nm. It seems that the CuNPs were not only deposited on the surface of NG layers but also intercalated into the graphene layers. Thus, the CuNPs deposited on NG modified electrode are expected to act as a better catalyst for electrocatalytic applications. Further, the applied potential of copper deposition on NG film will affect the morphology of the CuNSs which in turn will also affect the electrocatalytic application. Thus, the effect of applied potential of CuNSs deposition on the NG film was studied in detail. The electrodeposition of Cu on NG film was also carried out by varying the applied potentials. When the potential was varied from 0.0 to +0.20 V, deposition rate is very low, and only a few spherical particles were observed (Figure 6a,b). On the other hand, cubic CuNSs species were formed on the NG surface at 0 V (Figure 6c), and spherical CuNSs species were deposited at −0.10 V (Figure 6d). The spherical CuNSs were aggregated when the potential was switched to −0.10 V (Figure 6e). The density of the spherical CuNSs is low, and few of them 1652


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Figure 6. SEM images obtained for CuNSs deposited NG modified substrate at different applied potentials: (a) +0.20, (b) +0.10, (c) 0, (d) −0.10, (e) −0.20, (f) −0.30, (g) −0.40, (h) −0.50, and (i) −0.60 V.

S2b). With a further increase of the electrolysis time to 400 s, more uniform dendritic structures were grown on the NG film, and the dendritic coverage on the NG film was very high (Figure S2c). The dendrites were grown as an irregular shape and were highly dense, and the size of the dendritic branch was increased to 600 nm while increasing the deposition time to 600 s (Figure S2d). This is due to the aggregation of dendrites. The obtained results reveal that 400 s is ideal for the deposition of well-defined dendritic CuNSs. The dendritic CuNSs modified NG film was further characterized by XPS. Figure 7A shows the XPS survey spectrum of the NG-dendritic CuNSs modified substrate. The peak at 932 eV corresponding to the Cu2p region along with the C 1s, O 1s, and N 1s peaks confirmed the deposition of CuNSs at the NG film. Moreover, XPS is used to find out the nature of

frequently toward the aggregate and were further immobilized, forming a large aggregate. It has been already demonstrated that dendritic CuNSs act as a better electrocatalyst than other structures.42 This is due to the porous and hierarchical nature of the dendritic CuNSs. Hence, the condition from which dendrites were formed is optimized for further studies. The time dependent growth of dendritic CuNSs on the NG film was monitored by SEM, with the applied potential of −0.40 V kept as constant. At an electrolysis time of 100 s, the density of the deposited NS is very low, and most of the particles obtained are spherical crystals with an average size of 100 nm without any dendritic morphology (Figure S2a). When the deposition time was increased to 200 s, the nonuniform quasidendritic structures with the average branch size of 200 nm were formed (Figure 1653


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Figure 7. (A) XPS survey spectrum and (B) deconvoluted Cu 2p spectrum obtained for NG-CuNSs modified substrate.

[RS(C − RCT)] (inset, Figure S5) was employed to fit the impedance data, where RS is the solution resistance and C is the capacitance. The semicircle obtained from the Nyquist plot can be used to calculate the charge transfer resistance (RCT). The bare GCE shows a large semicircle of about 27.06 kΩ (curve a). When it is fabricated with an ERGO film, the RCT value of 24.12 kΩ was obtained (curve b), whereas the GCE/NG exhibits 22.64 kΩ (curve c). The decrease in the RCT value is attributed to the high surface area provided by the NG film. Interestingly, when CuNSs were deposited on the ERGO and NG film, the RCT value was drastically decreased to 1.84 and 1.44 kΩ, respectively (curves d and e). The higher conductivity of the NG-CuNSs is attributed to the doping of N on the graphene lattice, which in turn reduces the surface energy to favor the deposition of CuNSs on its lattice. These results reveal that the NG acts as a strong platform for the uniform dispersion of CuNSs. The obtained RS, C, and RCT values were given in Table S2. The heterogeneous electron transfer rate constant (ket) was calculated using eq 2.43

copper on the NG modified substrate. The samples were kept under inert atmosphere to avoid copper oxidation. The Cu 2p spectra of NG-dendritic CuNS substrate show peaks at 932.5 and 952.8 eV corresponding to 2p3/2 and 2p1/2 signals of elemental Cu, respectively (Figure 7B). The absence of the strong satellite peaks at 935 and 955 eV confirmed the absence of Cu2+. It is known that Cu+ has the same binding energy as Cu(0). It is distinguished from Cu(0) by the presence of two weak satellite peaks at 939−945 eV. As shown in Figure 7B, the weak satellite peak at 939−945 eV was absent. This confirms the absence of Cu+. The above XPS results suggest that the copper present on the NG substrate is in the form of Cu(0). Characterization by XRD and EDAX. Further, the dendritic CuNSs modified NG film was characterized by XRD and EDAX analysis. Figure S3 shows the XRD patterns of NG and NG-dendritic CuNSs. Figure S3a shows a broad peak around 26.0° corresponding to the crystalline nature of graphene. The peaks at 42.8°, 49°, and 73.7° are the characteristics of (111), (200), and (220) planes of Cu (Figure S3b) (JCPDS 85-1326). The peaks obtained at 30.57°, 35.32°, 49.58°, and 50.85° corresponded to ITO substrate (JCPDS 894599). The obtained XRD results confirm the successful formation of NG and dendritic CuNSs. The doping of nitrogen via intercalation of melamine to the GO and deposition of dendritic CuNSs on NG film was further confirmed by EDAX analysis. Figure S4 shows the EDAX spectra obtained for NGO, NG, and NG-dendritic CuNSs modified substrates. N-GO shows the peaks at 0.27 and 0.53 keV correspond to carbon and oxygen, respectively. The peak corresponding to nitrogen at 0.39 keV confirms the doping of nitrogen to GO (Figure S4A). It was found that the oxygen intensity was decreased after the electrochemical reduction (Figure S4B), and the presence of nitrogen confirms the fabrication of NG on the electrode surface. The characteristic peaks at 0.92 and 8.0 keV show the electrochemically deposited Cu (Figure S4C). These results suggest that nitrogen was doped to the GO and the CuNSs were successfully deposited on NG film. Characterization of GC/NG-CuNSs Electrode by EIS and Chronocoulometry. The conducting nature of the NGCuNSs modified electrode was examined by an electrical impedance spectroscopy (EIS) study. Figure S5 shows the Nyquist plots obtained for bare GCE, ERGO, and NG fabricated GCE and CuNSs deposited graphene and NG films in 1 mM [Ru(NH3)6]3+ containing 0.2 M PB solution (pH 3) from 0.01 to 100 000 Hz. A Randles circuit model

ket =

RT n F AR CTC 0 2 2

(2)

Here, RCT is charge transfer resistance, A and C0 represent the electrode area (0.07 cm2) and the concentration of the redox couple in the bulk solution (1 mM), respectively, and n refers to the number of electrons transferred per molecule of the redox probe (n = 1 for [Ru(NH3)6]3+). The rest of the symbols have their usual representations. The calculated ket values are 1.40 × 10−4, 1.57 × 10−4, 1.68 × 10−4, 2.06 × 10−3, and 2.63 × 10−3 cm s−1 for bare GCE, ERGO, NG, ERGO/CuNSs, and NG/CuNSs modified electrodes, respectively. The obtained higher ket value for GCE/NG/CuNSs reveals that the electron transfer reaction was faster at this electrode compared to that for the graphene/CuNSs electrode. Further, the electrochemically active surface area (A) of the ERGO, NG, and CuNSs fabricated ERGO and NG electrodes was calculated from the Anson equation, eq 3,44,45 as follows: a A= (3) 2nFCD1/2π 1/2 Here, a is the slope obtained from the Anson plot (Q vs t1/2), which is directly obtained from the chronocoulometric experiment, n represents the number of electrons transferred, F is Faraday’s constant, and C and D represent the concentration of the redox couple in the bulk solution (1 1654


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ACS Sustainable Chemistry & Engineering mM) and diffusion coefficient (8.4 × 10−6 cm2 s−1), respectively. The electrochemically active surface area was found to be 0.018, 0.022, 0.561, and 0.781 cm2 for ERGO, NG, ERGO/CuNSs, and NG/CuNS fabricated GCEs, respectively. The lone pair electron present on nitrogen interacts with the deposited CuNSs to form the composite material. Moreover, the surface energy of the system is expected to be reduced by the doped nitrogen atoms ingrained in the graphene lattice, and hence, it provides a higher electrochemically active surface area.27−31 Electrocatalytic Oxidation of Glucose. It has been wellestablished that graphene and CuNPs exhibit good electrical conductivity,24−26,30−32 and therefore, the electrodes modified with them can be effectively utilized for electrocatalytic applications. In this study, the electrocatalytic activity of CuNSs deposited NG film was investigated using glucose as a probe. Figure 8 shows the CVs obtained for 1.0 mM glucose at

The effect of morphology of the different CuNSs on the oxidation of glucose was examined at the NG film fabricated electrode. Figure S6 shows the CVs of 1.0 mM glucose at cubic, spherical, quasidendritic, and dendritic CuNSs modified GC/ NG electrodes in 0.1 M NaOH. The cubic CuNSs (Eapp = 0 V) shows an anodic peak at +0.52 V (curve a) whereas the spherical CuNSs (Eapp = −0.10 V) show oxidation at +0.54 V with enhanced current (curve b). When the electrode was modified with quasidendritic CuNSs (Eapp = −0.30 V), the oxidation potential was shifted toward a less positive potential, but the oxidation current was decreased compared to those for the spherical and cubic NSs, which is due to less population of dendritic CuNSs over the NG film (curve c). The dendritic CuNSs formed on the NG electrode (Eapp = −0.40 V) exhibited glucose oxidation at a less positive potential with enhanced oxidation current in contrast to cubic, spherical, and quasidendritic CuNSs (curve d). This shows glucose oxidation at +0.42 V with a 2-fold increase in current when compared to cubic nanostructures. The enhanced electrocatalytic activity is attributed to the porous nature of dendritic CuNSs which serves a large specific surface area along with abundant active sites and sharp edges. The obtained results clearly indicated that the dendritic CuNSs showed excellent electrocatalytic activity in contrast to those of other CuNSs. The effect of scan rate on the glucose oxidation at GC/NGdendritic CuNSs electrode in 0.1 M NaOH was also investigated. The anodic peak current (Ipa) due to glucose oxidation increased linearly with scan rates from 10 to 150 mV s−1 (Figure S7). The plot of anodic peak current against the square root of scan rate shows good linearity (R2 = 0.9977; Figure S7, inset), indicating that glucose oxidation at the GC/ NG-dendritic CuNSs electrode is a diffusion controlled process. Figure 9 shows the DPVs for each addition of 20 μM glucose at the GC/NG-dendritic CuNSs electrode in 0.1 M NaOH.

Figure 8. CVs obtained for 1 mM glucose at (a) bare, (b) NG, (c) CuNPs, (d) ERGO-CuNPs, and (e) NG-CuNPs modified GCE in 0.1 M NaOH at a scan rate of 50 mV s−1.

bare GC, GC/NG, GC/CuNPs, GC/ERGO-CuNPs, and GC/ NG-CuNPs electrodes in 0.1 M NaOH at 50 mV s−1. As shown in Figure 8, neither the bare GC (curve a) nor the NG modified GC (curve b) electrode shows a response for glucose oxidation. In the anodic scan, the ramp in current starting at about +0.30 V with a shoulder oxidation hump at +0.55 V due to glucose oxidation was observed at the GC/CuNPs electrode (curve c). This suggests the essential role of CuNPs in the electrochemical oxidation of glucose. On the other hand, the GC/ ERGO-CuNPs electrode exhibited a noticeable increase in current with 80 mV less positive potential shift (curve d). The observed higher electrocatalytic activity is due to the huge surface area possessed by ERGO, confirming the important role of the graphene sheets toward glucose oxidation, besides CuNPs. Interestingly, when the CuNPs were deposited on NG film, the glucose oxidation current was substantially increased 2-fold with 60 mV less positive potential (curve e). The above results clearly depict that the GC/NG-CuNPs electrode is a superior electrocatalyst for glucose oxidation when compared to that of GC/CuNPs and GC/ERGO/CuNPs electrodes. This is mainly due to not only the huge surface area provided by NG but also the higher conductivity and fast electron transfer rate offered by the NG film, as evidenced from EIS and chronocoulometry.

Figure 9. DPVs obtained for each 20 μM addition of glucose at the GC/NG-dendritic CuNSs electrode in 0.1 M NaOH. Inset: Calibration plot for oxidation current vs concentration of glucose.

With addition of 20 μM glucose, the GC/NG-dendritic CuNSs electrode exhibits the oxidation peak at +0.39 V. Further, addition of glucose in the range 20−400 μM increases the oxidation peak current of glucose linearly with a correlation coefficient of 0.9930 (Figure 9, inset). The performance of GC/ NG-dendritic CuNSs electrode was further evaluated by amperometry. Figure 10A exhibits an i−t curve for glucose at the GC/NG-dendritic CuNSs electrode in a stirred 0.1 M 1655


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Figure 10. Amperometric i−t curves obtained for (A) each 2 μM addition of glucose and (B) addition of (a) 0.0005, (b) 0.001, (c) 0.005, (d) 0.01, (e) 0.05, (f) 0.1, (g) 0.2, (h) 0.5, (i) 1.0, (j) 2.0, and (k) 5.0 mM of glucose at the time interval of 50 s at the GC/NG-dendritic CuNSs electrode in 0.1 M NaOH (Eapp = +0.50 V). Insets A and B show calibration plots obtained for current vs concentration of glucose.

Table 1. Comparison of Sensitivity, Linear Range, and Limit of Detection of Glucose Obtained from the Present Modified Electrode with the Reported Modified Electrodes sample no. 1 2 3 4 5 6 7 8 9 10

electrodes a

b

Nafion/Cu-NG /GCE Cu-GENTFsc/GCE graphene/pectin-CuNPsd/GCE NAe/NiONFf-rGOg/GCE graphene/CuNPs graphene/CuNPs Cu2O/NiOxh/graphene oxide/GCE CuO/graphene/GCE pencil graphite/CuNPs NG-CuNS/GCE

medium

sensitivity (μA mM−1 cm−2)

NaOH KOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH NaOH

48.13 11.3 35.2 36 607 285 1065 1467.5 1848

linear range (mM)

detection limit (μM)

ref

0.004−4.5 0.9−11 0.001−1 0.002−0.6 up to 4.5 0.005−1.4 0.002−0.87 0.001−8.0 1.0−100 0.0005−5.0

1.3 1.0 0.35 0.77 0.5 0.2 0.4 1.0 0.44 0.014

32 46 47 48 49 50 51 52 53 this work

a Nitrogen-doped graphene/copper nanoparticles. bGlassy carbon electrode. cCopper-graphene nanoflowers. dCopper nanoparticles eNafion. fNickel oxide nanofibers. gReduced graphene oxide. hNickel oxide.

Ca2+, (d) Mg2+, (e) K+, (f) Na+, (g) SO42−, and (h) Cl− and 0.1 mM (i) urea, (j) oxalic acid, (k) uric acid, and (l) ascorbic acid to the same solution does not provoke the current response, and it again responded upon addition of 5 μM glucose (b). These results suggest that it is possible to determine 2 μM glucose even in the presence of a 100-fold excess of common interferents. In order to examine the real applicability of the present sensor, the NG-dendritic CuNSs was fabricated on screen printed carbon electrode (SPCE) with the same procedure adopted for the GC electrode. The SPC/NG-dendritic CuNSs electrode was utilized to determine glucose in human blood serum and urine samples. The SPCE/NG-dendritic CuNSs does not show any response in the absence of blood serum in 0.1 M NaOH (Figure S9A, curve a), and in the presence of serum it shows the oxidation peak at +0.45 V. The obtained peak may be due to the oxidation of glucose present in the serum sample (Figure S9A, curve b). Further, the commercial glucose was spiked to the serum solution. The addition of 100 μM glucose to the serum sample shows the enhanced oxidation peak current at +0.45 V which confirms that it is due to glucose oxidation (Figure S9A, curves c and d). The modified electrode was also used to determine glucose in human urine samples (Figure S9B). The proposed method exhibits good recoveries for the determination of glucose in human blood serum and urine samples (Table S3). Thus, the SPC/NG-dendritic CuNSs electrode can be effectively utilized to determine glucose in real

NaOH solution with an applied potential of +0.50 V. The GC/ NG-dendritic CuNSs electrode shows current response for the initial addition of 2 μM glucose. Further addition of 2 μM glucose in each step increases the current response with an interval of 50 s and reaches saturation in 3 s. The dependence of the response current with respect to the concentration of glucose was linear from 2 to 22 μM with a correlation coefficient of 0.9989 (Figure 10A, inset). Further, glucose was determined in a wide range of concentration. While the concentration increases from 500 nM to 5.0 mM, the amperometric current response was increased linearly (Figure 10B) at an applied potential of +0.50 V. The limit of detection for glucose is 14.4 × 10−9 mol/L (S/N = 3), and the sensitivity is 1848 μA mM−1 cm−2. The performance of the NG-CuNSs modified electrode toward the oxidation of glucose is compared with the earlier reports32,39,46−53 and is given in Table 1. Upon comparison with other glucose sensors (Table 1), the GC/NGdendritic CuNSs electrode displays good linearity in a wide range of glucose concentrations and a lowest limit of detection. Further, we have studied the effect of interferents on the determination of glucose by adding the urea, oxalic acid, and common metal ions such as Na+, Mg2+, and Ca2+. The i−t curve for glucose at the GC/NG-dendritic CuNSs electrode in the presence of interferents in a stirred 0.1 M NaOH is shown in Figure S8. The initial current response was increased for the addition of 2 μM glucose (a), and a linear current increment was observed for further additions. The addition of 0.5 mM (c) 1656



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samples. The present method of electrode fabrication is simple, and anyone can easily determine the glucose level in blood serum and urine using SPCE/NG-dendritic CuNSs.

CONCLUSIONS In the present work, fabrication of nitrogen-doped grapheneCuNSs composite film (NG-CuNS) on GCE was achieved by electrochemical reduction of preassembled N-GO followed by electrodeposition of copper. The N-GO was prepared by intercalating melamine into GO and attaching these species on a GCE via Michael’s reaction. The aromatic backbone of graphene was retained by the electrochemical reduction of oxygen functionalities of N-GO to form NG. Further, nitrogen doping via the intercalation of melamine with GO was confirmed by XPS. Then, the CuNSs on an NG modified electrode was prepared by electrodepositing Cu at various applied potentials with different deposition times, and these materials were characterized by XRD, XPS, and SEM. The SEM images showed that the deposited cubic, spherical, quasidendritic, and dendritic NSs at the applied potentials of 0, −0.10, −0.30, and −0.40 V, respectively, on the NG film were uniform. The electrocatalytic activity of the GC/NG-CuNSs electrode toward glucose oxidation depends on the shape of the CuNSs. Among the different CuNSs, the NG-dendritic CuNSs electrode showed higher oxidation current for glucose upon comparison with other CuNSs. Finally, the NG-dendritic CuNSs were fabricated on a screen printed carbon electrode and were successfully utilized to determine glucose in human blood serum and urine samples. The facile and economically viable fabrication of the present composite material makes it an ideal candidate for glucose analysis. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02390. Additional experimental and characterization details (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. in. Phone: +91 451 245 2371. Fax: + 91 451 245 3031. ORCID

S. Abraham John: 0000-0002-9358-9998 Present Address

† Department of Chemistry, Loyola College, Chennai 600 034, Tamil Nadu, India.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S.K.G. thanks the University Grants Commission (UGC), New Delhi, India, for the award of a Meritorious Student Fellowship (F. 7-225/2009(BSR) dated 15.01.2013). Financial support from the Department of Biotechnology (BT/ PR10372/PFN/20/904/2013), New Delhi, India, is gratefully acknowledged. The authors thank Dr. T. G. Satheesh Babu and Amrita Vishwa Vidyapeetham, Coimbatore, for providing screen printed carbon electrodes. 1657


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