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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Three-Dimensional Dendrite Cu−Co/Reduced Graphene Oxide Architectures on a Disposable Pencil Graphite Electrode as an Electrochemical Sensor for Nonenzymatic Glucose Detection K. Justice Babu,† Sunirmal Sheet,‡ Yang Soo Lee,‡ and G. Gnana kumar*,† †

Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai−625021, Tamil Nadu, India Department of Forest Science and Technology, College of Agriculture and Life Sciences, Chonbuk National University, 567 Baekje-daero, Jeonju-si−54896, Jeollabuk-do, Republic of Korea

‡

S Supporting Information *

ABSTRACT: Three-dimensional (3D) copper−cobalt (Cu−Co)/reduced graphene oxide (rGO) hierarchical architectures are electrochemically deposited over a pencil graphite electrode (PGE), and the modified PGE is directly exploited as a binder-free and disposable electrode for high performance nonenzymatic glucose sensors. The morphological features substantiate that the Cu−Co nanostructures display a 3D, open, porous, interconnected network architecture, in which the rGO layers are tightly pinned among the nanofeelers. Owing to the benefits of dendrite architectures and optimized composition, Cu−Co/rGO/PGE demonstrates better glucose oxidation behavior under alkaline conditions. Being a nonenzymatic glucose sensor, Cu−Co/rGO/PGE demonstrates excellent gluose detection properties with considerable chloride poisoning resistance. The excellent analytical performance of Cu−Co/rGO/PGE comprehends its application in human serum samples. Thus, this report paves constructive opportunities for the development of disposable, environmentally benign, binder-free, cost-efficient, and scalable 3D electrodes, which may be beneficial for the development of economically viable nonenzymatic glucose sensor devices. KEYWORDS: Dendrite, Electrodeposition, Nonenzymatic glucose sensor, Pencil graphite electrode, Porous structure

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INTRODUCTION Glucose is deemed as a significant nutrient for mankind and is widely responsible for the provision of energy for all of kind of physiological activities.1 However, the sustainability of type 1 and type 2 diabetes metabolic disorders, associated with the glucose level fluctuation in human system, leads to a number of hazardous syndromes including kidney failure, heart disease, blindness, nerve degeneration, death, etc.2 The desire for continuous observation of glucose levels has prompted demand for reliable, rapid, accurate, and sensitive glucose detection devices.3 The aforementioned prerequisites are remarkably satisfied by the nonenzymatic electrochemical glucose sensors (NEGS) owing to their long-term stability, high sensitivity, good selectivity, and freedom from pH, temperature, and humidity influences; these properties make them reliable sensing devices, preferable to conventional glucose detection systems.4 However, the widespread application of NEGS is impeded by the high cost of the corresponding device, whose cost is dominated by the expensive, conventionally used electrode materials including glassy carbon electrode (GCE),5 gold (Au),6 and platinum (Pt).7 Furthermore, the aforementioned solid electrodes experienced certain hindrances including the tedious electrode polishing, slurry preparation, catalyst loading process, utilization of insulating binder, surface poisoning, etc., which collectively © XXXX American Chemical Society

prevented the large-scale application of NEGS. Henceforth, the development of a cost-efficient, easily disposable, readily available, less corrosive, environmentally benign, and freestanding solid substrate is highly essential for the constructive development of NEGS devices. In this context, pencil graphite electrode (PGE) is deemed as a futuristic current collector in electrochemical sensors owing to its good mechanical stability, wide availability, affordability, disposability, chemical inertness, elevated electrochemical reactivity, renewable surface, low background current, and ease of modification.8 The modification of PGE with metal nanocatalysts is extremely logical for the enhancement of NEGS performance. Among the catalytic metal probes exploited for NEGS, copper (Cu) nanostructures have received remarkable consideration due to its low band gap energies (1.4−2.3 eV), impressive catalytic activity, lower overpotential, biocompatibility, and affordability.9 Pourbeyram et al., modified PGE with Cu nanoparticles by using a dip coating technique and observed a detection limit of 0.44 μM toward nonenzymatic glucose sensing.10 Nevertheless, the sensing properties detailed in this literature are not evaluated as Received: September 17, 2017 Revised: November 24, 2017

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DOI: 10.1021/acssuschemeng.7b03314 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of (a−c) Cu/PGE, (d and e) Cu−Co/PGE, and (f−h) Cu−Co/rGO/PGE. White arrows indicate the void interspaces, and yellow solid and dotted lines indicate, respectively, the primary and secondary trunks of dendrites and rGO.

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per the standard procedures, which authenticated the nonreliability of a constructed system. Furthermore, a weak adhesion between the PGE and Cu nanoparticles lead to a severe agglomeration, lower surface stability, and surface poisoning of catalytic probes, which collectively limited the NEGS performance. Apart from the above effort, no significant research activities were achieved on Cu nanostructures modified PGE applicable for NEGS. The structural stability, selectivity, anticorrosive property, and electrochemical activity of Cu nanostructures could be enhanced further via its alloy pattern with Co and composite formation with the reduced graphene oxide (rGO).11 The graphene oxide (GO) reduction and decoration of metal nanostructures over its surface are the twostep processes engaged in the rGO based metal composite synthesis. These include the following: time-consuming complex synthesis steps, requirement of volatile solvents, toxic organic reagents and stabilizers, high temperature, and incomplete removal of oxygenated species, toxic surface, large volume of contaminant residues, etc.12 Furthermore, the direct growth of rGO nanocomposites on PGE remains stochastic and highly complicated. Though a number of efforts including chemical vapor deposition,13 drop drying,14 hydrothermal processes,15 thermal driven attachment,16 and dip coating10 were practiced for the modification of PGE with metal nanostructures applicable for the nonenzymatic detection of various analytes, significant efforts have not been devoted on the rGO based metal nanostructures/PGE for NEGS. Furthermore, the aforementioned techniques lead to structural instability of nanomaterials, uncontrolled catalyst loading, poor and unstable contact between the catalyst and electrode surface, which collectively restricted the efficiency and durability of NEGS, urging the exploration of an effectual alternative technique to address the aforesaid challenges. Thus, the straightforward green approach toward the deposition of catalytic probes on PGE is desirable for the construction of NEGS. Accordingly, we report the rational development of Cu−Co/rGO/PGE to outfit the adverse effects of existing NEGS catalytic probes and electrodes and address the challenges of Cu nanostructures toward nonenzymatic glucose oxidation kinetics via its hybridization with Co and rGO nanostructures.

EXPERIMENTAL SECTION

Materials. Copper(II) chloride pentahydrate (CuCl2·5H2O, AR, ≥97%), cobalt(II) chloride hexahydrate (CoCl2·6H2O, ≥98%), ammonium chloride (NH4Cl, AR, ≥99%), potassium chloride (KCl, AR, ≥99.5%), sodium hydroxide (NaOH (pellet), AR, ≥ 98%), glucose (GLU, ≥99.5%), fructose (Fru, AR, ≥99%), maltose (Mal, AR, ≥99%), acetaminophen (AP, AR, ≥99%), uric acid (UA, HPLC, ≥99%), lactose (Lac, AR, ≥99%), sucrose (Suc, AR, ≥99%), galactose (Gal, AR, ≥99%), mannose (Man, AR, ≥99%), citric acid (CA, AR, ≥99.5%), sodium chloride (NaCl, AR, ≥99.5%), xylose (Xyl, AR, ≥99%), ascorbic acid (AA, AR, ≥99.5%), dopamine (DA, AR, ≥99%), and urea (U, AR, ≥99%) were received from Sigma-Aldrich and employed with no any additional refinement. Different grade pencil graphite leads (2H, H, HB, B, 2B, and 4B, 0.5 mm diameter) purchased from a local store were polished with emery paper and washed with ethanol and deionized water and dried in a vacuum before use. Electrochemical Deposition of 3D Cu/PGE, Cu−Co/PGE, and Cu−Co/rGO/PGE. GO sheets were prepared using the modified Hummer’s method as reported elsewhere.17 The electrical contact of PGE was established by soldering Cu wire into the reverse side of PGE, and the geometric electrode area was maintained to be 15.90 mm2. The electrochemical pretreatment of PGE was performed with an amperometric technique in 1 M NH4Cl at 2.0 V vs Ag/AgCl for 100 s. To electrodeposite Cu−Co/rGO over PGE, the pretreated PGE was placed in a mixture of GO (1 mg mL−1) and Cu2+−Co2+ (0.05:0.05 M) in 1 M NH4Cl and −1.2 V vs Ag/AgCl was supplied to PGE for 30 s. The mass ratio maintained among the Cu2+, Co2+, and GO for the preparation of Cu−Co/rGO nanostructures on PGE is 1.0:1.0:0.5. Cu/ PGE and Cu−Co/PGE were fabricated by adopting the aforementioned similar electrodeposition procedure, respectively, in Cu2+ (0.1 M) and Cu2+−Co2+ (0.05:0.05 M) solution. The electrodeposited PGEs were washed and dried for 12 h, and the mass loading maintained for the Cu/ Cu−Co/Cu−Co/rGO nanostructures on PGE was 0.5 mg cm−2. By maintaining the electrodeposition time, mass ratio among the different components, and similar electrochemical conditions, the identical mass loading was maintained in all of the fabricated PGEs. Materials Characterizations. The JEOL-JSM5410LV scanning electron microscopy (SEM), JEOL JEM-2010 transmission electron microscopy (TEM), Rigaku D/MAX-2200 X-ray diffractometer (XRD), JY-HR800 Raman spectroscopy, and Thermo Scientific-Multilab 2000 X-ray photoelectron spectroscopy (XPS) were used to characterize the prepared nanostructures. Electrochemical Characterizations. The PGE's electrodeposition and its electrochemical characterizations were attained with CHI-650E electrochemical workstation. A single compartment with threeB

DOI: 10.1021/acssuschemeng.7b03314 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 2. TEM images of (a) Cu−Co, (b and c) Cu−Co/rGO (yellow dotted circles specify the rGO), and (d) HR-TEM image of Cu−Co/rGO nanostructures (inset) corresponding SAED pattern.

Figure 3. Schematic illustration of the Cu−Co/rGO dendrite formation mechanism. electrode cell equipped with Ag/AgCl (3 M KCl) reference electrode, platinum wire counter electrode, and unmodified/modified PGE working electrode was used for the electrochemical experiments.

may be beneficial for the effectual diffusion and retention of an analyte, respectively, in the outer and inner surface of PGE. The alloy formation of Cu with Co does not collapse the 3D porous structure as evidenced from the morphological images of Cu− Co/PGE (Figure 1d). The magnified SEM image of Cu−Co/ PGE (Figure 1e) reveals that the developed porous networks are comprised of a number of dendrites in all directions associated with the pore walls, which generate the self-supported architecture. The Cu−Co dendrite structures are composed of the distinct primary and secondary trunks, and branches are distributed on the both sides of secondary trunk. The interlaced dendritic feelers at the nanoscale are observed on both sides of the secondary trunk with the smooth and regular contours. The mean length and diameter of the primary trunk of as-prepared dendrite structures are found, respectively, to be 1.7 and 0.23 μm. Furthermore, the lengths of secondary trunks vary from 0.2 to 0.8

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RESULTS AND DISCUSSION Morphological Characterization of Cu−Co/rGO Nanostructures on PGE. The bare PGE exhibits a relatively compact structure with a rough surface, and its mean diameter is found to be 0.5 mm (Figure S1). After the electrodeposition process for 30 s, the immersed PGE surface is homogeneously enveloped with Cu nanostructures (Figure 1a and b). The as-deposited Cu nanostructures exhibit the strongly interconnected three-dimensional (3D) porous networks, and the mean diameter of pores existing on the outermost PGE surface is found to be ∼75−100 μm (Figure 1a−c). The average diameter of underlying pores is decreased with an increase in the contact surface of PGE, which C

DOI: 10.1021/acssuschemeng.7b03314 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 4. (a) XRD patterns of (i) Cu, (ii) Cu−Co, and (iii) Cu−Co/rGO. (b) Raman spectra of (i) GO and (ii) Cu−Co/rGO nanostructures. (c) XPS full scan spectrum of Cu−Co/rGO.

μm, and their mean diameter is found to be 0.12 μm (Figures 1e and 2a). The few layers of rGO flakes are tightly pinned among the nanofeelers of Cu−Co dendrite structures for Cu−Co/rGO composite (Figures 1f−h and 2b and c). The composite formation of rGO with Cu−Co significantly reduces the length and diameter of primary and secondary trunks and the mean length and diameter of primary trunks are found, respectively, to be 1.4 and 0.17 μm owing to the decreased exposure/utilization of Cu2+−Co2+ ions with the inclusion of GO. The electrostatic interaction between the Cu2+−Co2+ ions and negatively charged GO sheets lead to the formation of the Cu2+−Co2+/GO complex. At the initial stage of electrodeposition, Cu2+−Co2+/ GO complex was electrochemically reduced into Cu−Co/rGO nuclei on PGE surface. Upon the continuation of an electrodeposition time, the resultant Cu−Co/rGO nuclei were developed into the nanonodules via anisotropic growth.18 It was achieved through the concentration gradients of Cu2+, Co2+, and GO in the solution via the diffusion-limited growth process, which served as nucleation sites for the subsequent growth of Cu−Co/rGO nanostructures. The nanonodules tended to link together to form main trunks, which were perpendicular to the PGE surface. With the further progression of reaction time, the successive new nanonodule growth started on the main trunk, yielding a secondary trunk. The subsequent growth of nanofeelers on both of its sides enables the formation of a highly branched 3D dendrite structure. Under the high voltage electrodeposition process, vigorous hydrogen bubbles evolved over the PGE surface. With the formation and growth of these hydrogen bubbles, the simultaneous electrochemical reduction of Cu2+−Co2+ ions and GO was achieved in the interval spaces of hydrogen bubbles,19 yielding the 3D Cu−Co/rGO dendrite architectures, and the corresponding growth mechanism is schematically illustrated in Figure 3. The HR-TEM image of Cu−Co/rGO (Figure 2d) exhibits the orderly arranged lattice

fringes for Cu−Co nanostructures with d spacing of 0.209 nm, matching with the (111) plane of cubic structure. The corresponding selected area electron diffraction (SAED) pattern (inset Figure 2d) reveals rings and clear spots with random arrangement, specifying the polycrystalline structure of Cu−Co/ rGO. The EDAX pattern of PGE reveals the presence of C (96.4 at %) and O (3.6 at %) elements (Figure S2a). The EDAX pattern of Cu/PGE shows the existence of Cu (95.6 at %), C (3.5 at %), and O (0.9 at %) elements (Figure S2b), whereas Cu−Co/PGE exhibits the Cu (48.6 at %), Co (47.4 at %), C (3.2 at %), and O (0.8 at %) elements, matching with the 1:1 ratio of Cu and Co maintained in the preparation protocol (Figure S2c). The composite formation of rGO with Cu−Co nanostructures is confirmed from the subsistence of Cu (38.2 at %), Co (38.8 at %), C (17.7 at %), and O (5.3 at %) elements in Cu−Co/rGO/ PGE (Figure S2d). The increased at % of C and O elements ensures the presence of rGO content in Cu−Co/rGO/PGE, and the observed results clearly specify the compositional ratio maintained among the Cu, Co, and rGO is 1.0:1.0:0.5. XRD Studies. The face centered cubic (fcc) structure of Cu nanostructures is explored with its diffraction patterns (JCPDSFile No. 04-0836) (Figure 4a(i)).20 The aforementioned significant diffraction patterns of Cu are slightly shifted for Cu−Co alloy structure (Figure 4a(ii)), and the indistinguishable reflection planes observed for Cu and Cu−Co nanostructures are attributed to the close lattice structures of Cu (3.615 Å) and Co (3.544 Å).21 Along with the characteristic Cu−Co reflection planes, Cu−Co/rGO composite displays the (002) reflection plane of rGO sheets,17 ensuring the composite formation of rGO with Cu−Co nanostructures (Figure 4a(iii)). Raman Studies. GO sheets show the carbonaceous Raman bands at 1350 and 1590 cm−1 (Figure 4b(i)), ascribing, respectively, to the D (k point phonons of Ag symmetry) and D

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Figure 5. (a) CVs of studied PGEs in 0.1 M NaOH at 20 mV s−1, (b) activation mechanism of Cu−Co/rGO/PGE with NaOH solution, (c) CVs of Cu− Co/rGO/PGE in 0.1 M NaOH with diverse scan rates, and (d) calibration plot of Ip vs v1/2.

Figure 6. (a) CVs of PGEs with 5 mM glucose in 0.1 M NaOH at 20 mV s−1, (b) glucose electrooxidation mechanism at Cu−Co/rGO/PGE, (c) CVs of Cu−Co/rGO/PGE with respective of glucose concentration in 0.1 M NaOH at 20 mV s−1, and (d) CVs of Cu−Co/rGO/PGE with 5 mM glucose in 0.1 M NaOH as a function of scan rate.

G bands (E2g phonon of sp2 carbon).22 The ID/IG ratio of GO sheets is found to be 0.8, which is significantly increased to 1.2 for Cu−Co/rGO (Figure 4b(ii)), specifying the increased structural disorders of the resultant nanocomposite.17

XPS Studies. The wide-scan XPS survey spectrum of Cu− Co/rGO reveals the C 1s, O 1s, Cu 2p, and Co 2p peaks (Figure 4c). The core level XPS spectrum of Cu 2p exhibits distinct peaks at 932.4 and 952.8 eV, matching, respectively, with Cu 2p3/2 and E

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nanofeelers on the secondary trunks not only provides robust stability for the prepared dendrite architectures but also affords the continuous electron conduction channels for the enhanced electron transfer rate, which collectively improves the reaction kinetics toward glucose electrooxidation.18,19 Furthermore, the synergistic effects obtained with the combination of Cu−Co alloy structures improve the electrocatalytic oxidation of glucose further.25 The state of the art Cu−Co/rGO/PGE demonstrates a maximum Ipa of 1.6 mA, greater than those of Cu/PGE and Cu− Co/PGE, demonstrating the impact of rGO toward glucose electrooxidation. rGO effectively reduces the solid-state diffusion distance and provides low resistance paths for the easier penetration of an analyte.26−32 The interfacial surface area of Cu−Co/PGE is maximized with rGO that facilitates a heterogeneous electron transfer capability through the interconnected network of rGO with Cu−Co dendrites. The protective layers of rGO on Cu−Co effectively suppress the ion transportation behavior at Cu−Co/rGO/PGE, facilitating the electron transportation behavior. The extended number of homogeneously exposed Cu(III)/Co(IV) centers effectively oxidize glucose into gluconolactone, and the corresponding glucose electrooxidation mechanism at Cu−Co/rGO/PGE is schematically illustrated in Figure 6b. The considerable conductivity, chemical inertness, and adequate mechanical strength of pencil leads are the desired characteristics for use as host sensor substrates for NEGS applications. In general, the pencil lead is mainly composed of a mixture of graphite, clay, and wax, and the grades of leads are categorized on the basis of their composition.33 The “H” and “B” grade pencils containing, respectively, the higher content of clay and graphite demonstrate the maximum hardness and blackness.33,34 To understand the influence of graphite content in the pencil leads toward the electrochemical oxidation of glucose, Cu−Co/rGO nanostructures were elecrodeposited over the diverse grade pencils, and their CV performances were monitored with 5 mM glucose under an alkaline medium at 20 mV s−1 (Figure S5). From the observed results it is clear that the grades of pencil exhibited a certain influence for glucose electrooxidation. The lower graphite content in 2H and H resulted in the high resistivity of a system, which yielded lower glucose oxidation responses than those of other studied pencil grades. The high graphite content of B grade pencils positively influenced the glucose oxidation performance. However, B grade pencils demonstrated the lower glucose oxidation performance in comparison with the HB pencil owing to the presence of a lower level of oxygenated species on their surfaces.33 The existence of optimal oxygen functionalities in HB pencils increases the considerable adhesive characteristics of the nanostructures on its lead surface, facilitating the interfacial area.33 Furthermore, the higher number of edge plane defects of the HB pencil improves the rapid and electrochemically active charge transfer rates, leading to a better glucose electrooxidation performance than those of other studied pencil grades, which indicates that the HB pencil should be used for subsequent NEGS studies. Figure 6c reveals the CV responses of Cu−Co/rGO/PGE with the influence of different concentrations of glucose ranging from 1 to 5 mM under an alkaline medium at 20 mV s−1. A gradual increment in glucose concentration provides an apparent linear increase in the Ipa, indicating the good electrocatalytic activity of Cu−Co/rGO/PGE’s glucose oxidation with no fouling consequence.

Cu 2p1/2 peaks of Cu (Figure S3(i)). The absence of satellite peaks at 935 and 955 eV confirms the existence of Cu(0).23 The Co 2p spectrum displays two discrete peaks at 796.4 and 778 eV, respectively, matching with Co 2p1/2 and Co 2p3/2 peaks, specifying the Co(0) state (Figure S3(ii)).24 The existence of graphitic carbon in Cu−Co/rGO composite is ensured from the C 1s peak found at 284.5 eV, ascribing to the C−C bonding (Figure S3(iii)). The weaker peaks associate with the oxygenated carbons reveals the reduced state of GO sheets, ensuring the structural confirmation of Cu−Co/rGO composite.22 Electrochemical Behavior of Bare PGE, Cu/PGE, Cu− Co/PGE, and Cu−Co/rGO/PGE. The electrochemical behavior of studied PGEs were investigated with a cyclic voltammetry (CV) technique, and the voltammograms obtained in 0.1 M NaOH at 20 mV s−1 are given in Figure 5a. No obvious redox peaks are observed at bare PGE, representing the inert behavior of PGE. By contrast, Cu/PGE exhibits a significant redox peak at −0.128 and +0.56 V vs Ag/AgCl, matching, respectively, with Cu(III)/Cu(II) redox couple (Figure S4).20 The well-defined redox peaks are obtained for Cu−Co/PGE at +0.2 and +0.15 V vs Ag/AgCl. With alkaline conditions, Cu(0) and Co(0) are changed, respectively, into Cu(II) and Co(II) and are electrochemically oxidized to Cu(III) and Co(III) at forward sweep; Co(III) is additionally oxidized to Co(IV). Under the backward sweep, Cu(III)/Co(IV) is reduced, respectively, into Cu(II)/ Co(III).25 Although a similar redox profile is observed at Cu− Co/rGO/PGE, the increased redox peak currents are noticed in comparison with PGE/Cu and Cu−Co/PGE, ascribing to the high surface energy of Cu−Co/rGO composite via the active rGO and the relevant redox couples formation mechanism is schematically illustrated in Figure 5b. The redox peak currents observe with respective of scan rate at Cu−Co/rGO/PGE are linear with the square root of 10−100 mV s−1 scan rate (v1/2) (Figure 5c and d), specifying that the involved electrochemical reaction is a diffusion directed progression. Electrooxidation of Glucose at Bare PGE, Cu/PGE, Cu− Co/PGE, and Cu−Co/rGO/PGE. The electrocatalytic activities of studied PGEs toward glucose oxidation were evaluated by measuring voltammograms with 5 mM glucose under aklaine medium at 20 mV s−1 (Figure 6a). PGE does not display any obvious redox peak toward glucose electrooxidation, specifying the absence of electrocatalytic active sites in PGE. In contrast, Cu/PGE reveals a significant oxidation peak, which is ascribed to the Cu(III)/Cu(II) redox couple’s involvement toward glucose electrooxidation.20 The Cu−Co alloy formation on PGE increases the glucose electrooxidation as evidence from the increased Ipa at 0.44 V vs Ag/AgCl. Cu−Co alloy generates Cu(III)/Co(IV) centers under alkaline medium, which significantly catalyzes the glucose into gluconolactone via electrooxidation. The 3D Cu−Co dendrite structures on PGE are comprised of the porous scaffolds, capacious interparticle and interbranch interspaces, and empty spaces in pore walls, facilitating the effective adsorption and diffusion of glucose. The intrinsic surface properties of dendrite nanostructures including the atomic steps/corners, extended number of high angle edges, and acute tips rationally construct the maximum active sites over the surface of catalysts.18 The diffused analyte efficiently approaches the catalytic active sites of 3D dendrite networks, which is due to incredible surface improvement consequence, influential adsorption capacity of alloy nanostructures, and the tunable pore structures. The strong interconnection among the primary and secondary trunks and interlaced F

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Figure 7. (a) Cu−Co/rGO/PGE’s CVs with 5 mM glucose under various pH levels at 20 mV s−1 and (b) the corresponding plot of pH vs Ipa.

Figure 8. (a) Cu−Co/rGO/PGE’s amperometric behavior upon the successive injection of glucose at 0.4 V vs Ag/AgCl. (inset) Amperometric behavior of Cu−Co/rGO/PGE toward 1−100 μM glucose. (b) Calibration plot of Cu−Co/rGO/PGE amperometric responses with respective of glucose concentration. (c) Interference test of Cu−Co/rGO/PGE with the successive addition of 0.1 mM interfering species and 1 mM glucose in 0.1 M NaOH at 0.4 V vs Ag/AgCl. (d) Chloride poisoning test: Cu−Co/rGO/PGE’s amperometric behavior with 0.1 M NaCl in 0.1 M NaOH with the succeeding glucose inclusion at 0.4 V vs Ag/AgCl.

The influences of pH toward the electrooxidation of glucose at Cu−Co/rGO/PGE were probed with broad pH levels of 5−14 (Figure 7a). A significant oxidation profile at Cu−Co/rGO/PGE is not observed at pH 5 (Figure 7a and b). Meanwhile, the glucose electrooxidation is enhanced with a raised pH (7−13), and high glucose electrooxidation is surveyed at pH 13. The formation of a maximum number of higher oxidized species including Cu(II)/Cu(III) and Co(II)/Co(IV) are facilitated only at high pH level that significantly increases the electrooxidation of glucose at pH 13.25 A further increment in pH level (>13) leads to a decrement in Ipa at Cu−Co/rGO/PGE owing to reference electrode’s erosion.20 The electrochemical impedance spectra (EIS) spectra of studied PGEs were evaluated in 5 mM glucose under an alkaline medium (Figure S7). All of the EIS spectra exhibited the semicircle and linear portions, respectively, at higher and lower

Cu−Co/rGO/PGE’s CV behaviors with 5 mM glucose under an alkaline medium respective of scan rate are provided in Figure 6d. The Ipa associated with the electrooxidation of glucose is progressively improved by means of a raised scan rate along with a positive shift in Epa. Two methods such as ip vs ν1/2 and log ip vs log ν plots, are exploited to determine whether the electrode reactions are controlled by adsorption or diffusion related processes.35−37 The Ipa of gluose oxidation is linear with ν1/2 (Figure S6a), specifying that the electrocatalytic process is a diffusion directed progression. The log ip vs log ν plot was used to additionally confirm the above process. From the log Ipa vs log ν plots, a linearity following the regression fitted equation of log Ipa (mA) = 0.631 log v (mV s−1) + 0.640 (R = 0.996) was obtained (Figure S6b). The calculated slope value of 0.63 toward glucose oxidation specifies the simultaneous diffusion and adsorption processes. G

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ACS Sustainable Chemistry & Engineering

interfering species lead to a minimal variation in the amperometric current response in comparison with glucose. The iso-electric point of Cu−Co in higher alkaline solution (pH 13) is observed to be ∼8−9.5, indicating the negative charges of Cu−Co alloy structures.20,40 Under higher pH (pH = 13), UA and AA demonstrate negative charges due to the loss of protons that affords robust repelling against the negatively charged Cu− Co that limits the elctrooxidation response of UA and AA at Cu− Co/rGO/PGE.20,41 Cu−Co/rGO/PGE demonstrates excellent anti-interference activity aligned with general reducing of sugars in human blood. The interference results demonstrate the excellent selectivity of as-fabricated Cu−Co/rGO/PGE toward glucose electrooxidation, which is due to the affirmative surface of Cu−Co/rGO/PGE and a lower applied potential. The broader opportunities of as-fabricated NEGS is realized with the chloride poisoning test by measuring the amperometric behavior of Cu−Co/rGO/PGE with 0.1 M NaCl under alkaline medium with the succeeding inclusion of various glucose concentration at 0.4 V vs Ag/AgCl. The Cl− ions do not cause any possible interference effect toward the amperometric behavior of Cu−Co/rGO/PGE as substantiate from the unchanged current variation, implying an excellent high resistance of Cu−Co/rGO/PGE toward the high concentration of Cl− poisoning (Figure 8d and corresponding inset). Stability and Reproducibility. The electrochemical stability of Cu−Co/rGO/PGE is comprehended by measuring the current−time response for 60 days under the presence of 2 mM glucose under alkaline medium at 0.4 V vs Ag/AgCl. Cu− Co/rGO/PGE could retain 93.1% of incipient sensitivity at 60th day of operation (Figure S8), amplifying the exceptional stability of fabricated sensor. The reproducibility of eight identically fabricated Cu−Co/rGO/PGEs was determined by analyzing its amperometric responses under the identical conditions and the corresponding performance demonstrates the relative standard deviation (RSD) of 3.4%, authenticating an excellent reproducibility of fabricated sensors. Cu−Co/rGO/PGE displays an RSD of 3.1% for ten repeated experiments, endorsing the excellent repeatability. Real Sample Analysis. The analytical consistency of asfabricated Cu−Co/rGO/PGE in real sample was scrutinized with human blood serum samples and its analytical performances were suitably compared with the commercial gluconometer (ACCU-CHEK-Active glucometer). A human serum sample was gathered from a healthy helper, and a known concentration of glucose was spiked into a diluted serum sample. The relevant amperometric i−t behavior was evaluated at 0.4 V vs Ag/AgCl. Recoveries and RSD for the aforesaid real samples at Cu−Co/ rGO/PGE are measured, respectively, in the range of 98.0− 101.6% and 2.22−2.98% (Table 1), which are comparable to the commercial gluconometer, authenticating its realization in real sample analysis.

frequencies, matching with the electron transfer and diffusion limited processes.38 The Nyquist plot of bare PGE displayed a semicircle with the charge-transfer resistance (Rct) of 1527 Ω. The electrodeposition of nanostructures over the PGE markedly lowered the Rct of a system. Specifically, Cu−Co/rGO/PGE demonstrated the lower Rct of 298 Ω, which is relatively lower than those of Cu/PGE (865 Ω) and Cu−Co/PGE (425 Ω). The tightly pinned rGO flakes among the nanofeelers of Cu−Co dendrite structures provided the continuous electron conduction channels. This reduces the Rct of Cu−Co/rGO/PGE and maximizes the glucose electrooxidation process with the improved interfacial electron transfer rates. Amperometric Behavior of Cu−Co/rGO/PGE. Cu−Co/ rGO/PGE’s amperometric glucose oxidation behavior was measured with the succeeding inclusion of glucose with its increasing concentration in alkaline medium at +0.4 V vs Ag/ AgCl (Figure 8a and corresponding inset). Cu−Co/rGO/PGE exhibits the well-defined stair caselike amperograms, and an obvious stepwise current increment with the successively increasing concentrations of glucose is observed. The steady state current reaches a dynamic equilibrium within 5 s, signifying the high electron transfer behavior of Cu−Co/rGO/PGE. It is also clear that the response current is linearly scaled with the glucose concentration ranging from 1 μM to 4 mM (Figure 8b). Furthermore, Cu−Co/rGO/PGE demonstrates a elevated sensitivity of 0.24 mA mM−1 cm−2 toward glucose electrooxidation in the company of a lower detection limit of 0.15 μM. Thus, the Cu−Co/rGO/PGE outperforms the sensitivity, detection limit, and linear range of existing NEGSs (Table S1). Furthermore, the developed Cu−Co/rGO/PGE sensor effectively overcomes the time, cost, and energy related constraints of the conventional NEGS electrodes including the laborious polishing, catalyst loading, and casting of an electroinactive binder processes.39 The single step and facile electrodeposition strategy addressed in this effort simultaneously reduced the Cu2+−Co2+ ions and GO into Cu−Co/rGO nanocomposite at 30 s and tackled the typical requirements of conventional metal/ rGO nanocomposite synthesis techniques such as multiple steps, toxic reducing agents, volatile solvents, organic linkers, surfactants, and elevated temperature.17 Furthermore, the difficulties in controlling the growth and formation of dendrite nanostructures and their composite formation with rGO sheets without altering the morphological features of bare metal dendrites are effectively tackled with the proposed strategy. Furthermore, the elevated contact resistance exists in common sensors via utilization of an electroinactive binder is overwhelmed with the directly grown Cu−Co/rGO nanostructures on PGE, which enhances the interfacial contact area, consequently promoting NEGS performance. Thus, the reported Cu−Co/rGO/PGE effectively tackles the limitations of existing conventional glucose sensor probes and provides new avenues in the development of time- and cost-efficient, binder-free, miniaturized, and disposable NEGSs. Interference Study of Cu−Co/rGO/PGE. The ability to discriminating the interfering effects generated by the endogenous species, which have common electroactivities with that of a target analyte is one of the significant analytical dynamics for NEGS. Hence, the selectivity of as-fabricated Cu− Co/rGO/PGE was evaluated with the amperometric technique against the consecutive injections of 1 mM glucose and 0.1 mM common physiological interferences including AA, CA, UA, AP, DA, NaCl, and U under alkaline medium at 0.4 V vs Ag/AgCl (Figure 8c). From the amperometric responses it is clear that the

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CONCLUSIONS A facile and single-step electrodeposition strategy is developed for the preparation of 3D Cu−Co/rGO dendrite architectures on PGE using the hydrogen templating approach. The robust, interconnected, and open porous frameworks of Cu−Co/rGO are endowed with high conductivity, large exposed area, and unique pore properties that increase the glucose penetration and fast ion and electron transportation, which collectively enhance the NEGS performance. Furthermore, the fabricated sensor demonstrates the specific sensing of glucose and establishes its relevance in real samples. Thus this study complements the facile, H

DOI: 10.1021/acssuschemeng.7b03314 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

(4) Si, P.; Huang, Y.; Wang, T.; Ma, J. Nanomaterials for Electrochemical Non-Enzymatic Glucose Biosensors. RSC Adv. 2013, 3, 3487−3502. (5) Ye, C.; Zhong, X.; Chai, Y.; Yuan, R. Sensing Glucose based on its Affinity for Concanavalin A on A Glassy Carbon Electrode Modified with A C60 Fullerene Nanocomposite. Microchim. Acta 2015, 182, 2215−2221. (6) Ciftci, H.; Alver, E.; Celik, F.; Metin, A. U.; Tamer, U. NonEnzymatic Sensing of Glucose Using A Glassy Carbon Electrode Modified with Gold Nanoparticles Coated with Polyethyleneimine and 3-Aminophenylboronic Acid. Microchim. Acta 2016, 183, 1479−1486. (7) Chen, C.; Xie, Q.; Yang, D.; Xiao, H.; Fu, Y.; Tan, Y.; Yao, S. Recent Advances in Electrochemical Glucose Biosensors: A Review. RSC Adv. 2013, 3, 4473−4491. (8) Akanda, Md. R.; Sohail, M.; Aziz, Md. A.; Kawde, A. N. Recent Advances in Nanomaterial-Modified Pencil Graphite Electrodes for Electroanalysis. Electroanalysis 2016, 28, 408. (9) Gawande, M. B.; Goswami, A.; Felpin, F. X.; Asefa, T.; Huang, X.; Silva, R.; Zou, X.; Zboril, R.; Varma, R. S. Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 2016, 116, 3722−3811. (10) Pourbeyram, S.; Mehdizadeh, K. Nonenzymatic Glucose Sensor Based on Disposable Pencil Graphite Electrode Modified by Copper Nanoparticles. J. Food Drug Anal. 2016, 24, 894−902. (11) Wang, L.; Lu, X.; Ye, Y.; Sun, L.; Song, Y. Nickel-Cobalt Nanostructures Coated Reduced Graphene Oxide Nanocomposite Electrode for Nonenzymatic Glucose Biosensing. Electrochim. Acta 2013, 114, 484−493. (12) Chua, C. K.; Pumera, M. Chemical Reduction of Graphene Oxide: A Synthetic Chemistry Viewpoint. Chem. Soc. Rev. 2014, 43, 291−312. (13) Mathur, S.; Erdem, A.; Cavelius, C.; Barth, S.; Altmayer, J. Amplified Electrochemical DNA-Sensing of Nanostructured Metal Oxide Films Deposited on Disposable Graphite Electrodes Functionalized by Chemical Vapor Deposition. Sens. Actuators, B 2009, 136, 432−437. (14) Zhang, J.; Zheng, J. An Enzyme-Free Hydrogen Peroxide Sensor based on Ag/FeOOH Nanocomposites. Anal. Methods 2015, 7, 1788− 1793. (15) Congur, G.; Ates, E. S.; Afal, A.; Unalan, H. E.; Erdem, A. Zinc Oxide Nanowire Decorated Single-Use Electrodes for Electrochemical DNA Detection. J. Am. Ceram. Soc. 2015, 98, 663−668. (16) Aziz, Md. A.; Kawde, A. N. Nanomolar Amperometric Sensing of Hydrogen Peroxide using A Graphite Pencil Electrode Modified with Palladium Nanoparticles. Microchim. Acta 2013, 180, 837−843. (17) Gnana kumar, G.; Babu, K. J.; Nahm, K. S.; Hwang, Y. J. A Facile One-Pot Green Synthesis of Reduced Graphene Oxide and its Composites for Nonenzymatic Hydrogen Peroxide Sensor Applications. RSC Adv. 2014, 4, 7944−7951. (18) Cui, X.; Xiao, P.; Wang, J.; Zhou, M.; Guo, W.; Yang, Y.; He, Y.; Wang, Z.; Zhang, Y.; Lin, Z.; Yang, Y. Highly Branched Metal Alloy Networks with Superior Activities for the Methanol Oxidation Reaction. Angew. Chem. 2017, 129, 4559−4564. (19) Plowman, B. J.; Jones, L. A.; Bhargava, S. K. Building With Bubbles: The Formation of High Surface Area Honeycomb-like Films via Hydrogen Bubble Templated Electrodeposition. Chem. Commun. 2015, 51, 4331−4346. (20) Raj kumar, T.; Babu, K. J.; Yoo, D. J.; Kim, A. R.; Gnana kumar, G. Binder Free and Free-Standing Electrospun Membrane Architecture for Sensitive and Selective Non-Enzymatic Glucose Sensors. RSC Adv. 2015, 5, 41457−41467. (21) Pattanaik, G. R.; Kashyap, S. C.; Pandya, D. K. Structure and giant magnetoresistance in electrodeposited granular Cu-Co films. J. Magn. Magn. Mater. 2000, 219, 309−316. (22) Karthikeyan, C.; Ramachandran, K.; Sheet, S.; Yoo, D. J.; Lee, Y. S.; Satish kumar, Y.; Kim, A. R.; Gnana kumar, G. Pigeon Excreta Mediated Synthesis of RGO/CuFe2O4 Nanocomposite and its Catalytic Activity Toward Sensitive and Selective Hydrogen Peroxide Detection. ACS Sustainable Chem. Eng. 2017, 5, 4897−4905.

Table 1. Electrochemical Detection of Glucose in Human Serum Samples at Cu−Co/rGO/PGE glucose concentration original serum sample (mM)

diluted serum sample (mM)

9.6 mM (173 mg dL−1)

3.0

glucose found (mM) glucose added (μM)

glucometera

proposed method

RSDb (%)

recovery (%)

0 200 400 600 800

3.05 3.22 3.41 3.64 3.82

2.94 3.16 3.44 3.58 3.86

2.84 2.56 2.72 2.22 2.98

98.0 98.8 101.2 99.4 101.6

a

Determined by an ACCU-CHEK active glucometer. standard deviation of four measurements.

b

Relative

template-, and binder-free, one-pot preparation, and cost- and time-efficient strategy toward the development of disposable PGE based nonenzymatic probes, which opens up the promising opportunities and realization in NEGS.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03314. Figures S1−S6, SEM image of PGE, EDAX patterns, XPS core level spectra, CV of Cu/PGE in 0.1 M NaOH, plot of Ipa vs Cu−Co/rGO modified pencil grade, calibration plot of Ip vs v1/2 and log Ip vs log v, electrochemical impedance spectra, stability profile of Cu−Co/rGO/PGE, and Table S1, comparative profile of the electrochemical performances of enzyme-free glucose sensors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 91-9585752997 (G.G.k.). ORCID

G. Gnana kumar: 0000-0001-7011-3498 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by University Grants Commission (UGC), New Delhi, India, major Project Grant No. MRPMAJOR-CHEM-2013-36681. K.J.B. thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the award of a Senior Research Fellowship (09/201/0413/ 2016-EMR-I). This research was supported by the Buan Regional Innovation System (Buan RIS) Grant no: 1301002789 funded by CBNU Buan Regional Innovation System.

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J

DOI: 10.1021/acssuschemeng.7b03314 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX