Hierarchical 3-Dimensional Nickel–Iron Nanosheet Arrays on Carbon Fiber Paper as a Novel Electrode...

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Cite this: DOI: 10.1039/c5nr06802a

Received 1st October 2015,Accepted 26th October 2015

DOI: 10.1039/c5nr06802awww.rsc.org/nanoscale

Hierarchical 3-dimensional nickel – iron nanosheetarrays on carbon ber paper as a novel electrodefor non-enzymatic glucose sensing †Palanisamy Kannan, a,c Thandavarayan Maiyalagan, b Enrico Marsili,* c Srabanti Ghosh, d

Joanna Niedziolka-Jönsson* a and Martin Jönsson-Niedziolka* a

Three-dimensional nickel – iron (3-D/Ni – Fe) nanostructures are exciting candidates for various appli-cations because they produce more reaction-active sites than 1-D and 2-D nanostructured materials andexhibit attractive optical, electrical and catalytic properties. In this work, freestanding 3-D/Ni – Fe inter-connected hierarchical nanosheets, hierarchical nanospheres, and porous nanospheres are directly grownon a exible carbon ber paper (CFP) substrate by a single-step hydrothermal process. Among the nano-structures, 3-D/Ni – Fe interconnected hierarchical nanosheets show excellent electrochemical propertiesbecause of its high conductivity, large speci c active surface area, and mesopores on its walls ( vide infra ).The 3-D/Ni – Fe hierarchical nanosheet array modi ed CFP substrate is further explored as a novel elec-trode for electrochemical non-enzymatic glucose sensor application. The 3-D/Ni – Fe hierarchicalnanosheet arrays exhibit signi cant catalytic activity towards the electrochemical oxidation of glucose, ascompared to the 3-D/Ni – Fe hierarchical nanospheres, and porous nanospheres. The 3-D/Ni – Fe hierarchi-cal nanosheet arrays can access a large amount of glucose molecules on their surface (mesopore walls)for an e ffi cient electrocatalytic oxidation process. Moreover, 3-D/Ni – Fe hierarchical nanosheet arraysshowed higher sensitivity (7.90 μA μM

− 1 cm − 2 ) with wide linear glucose concentration ranging from0.05 µM to 0.2 mM, and the low detection limit (LOD) of 0.031 µM (S/N = 3) is achieved by the ampero-metry method. Further, the 3-D/Ni – Fe hierarchical nanosheet array modi ed CFP electrode can bedemonstrated to have excellent selectivity towards the detection of glucose in the presence of 500-fold

excess of major important interferents. All these results indicate that 3-D/Ni – Fe hierarchical nanosheetarrays are promising candidates for non-enzymatic glucose sensing.

1. IntroductionHierarchical nanostructures assembled by low-dimensionalbuilding blocks including nanoparticles,1,2 nanorods,3,4 andnanoplates5,6 often exhibit significant morphology and/or sizedependent properties.7–11 Considered as an important func-tional nanomaterial, nickel– iron (Ni– Fe) nanostructuresexhibit extraordinary electrical, catalytic, and magnetic

properties and have potential technological applications inelectromagnetic shielding, absorbing materials, catalysis, andsensors.10,12 –14 Great attention has been focused on thefabrication of Ni– Fe micro-/nanostructures by using variousphysical and chemical methods.10,15 –18 For instance, low dimensional Ni– Fe nanostructures including nanochains10and nanorods15,19 have been synthesized by hydrothermal andcalcination methods. Three dimensional (3-D) Ni– Fe nano-structures such as flowers,18 dendrites,20 and Ni– Fe layered

double hydroxide films8

have also been reported. The 3-D Ni–

Fe hierarchical nanostructures are preferable for various tech-nological applications because they produce more reaction-active sites than 1-D and 2-D nanostructured materials andexhibit attractive optical, electronic, magnetic, and catalyticproperties.7,18

The formation of hierarchical nanostructures is a self-assembly process, in which the ordered superstructures areformed by small building blocks, i.e., 0-D nanoparticles,1-dimensional nanorods or nanowires, and 2-D nano-flakes.21 –23 Moreover, it is well known that templates and/or

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr06802a

a Institute of Physical Chemistry, Polish Academy of Sciences, 44/52 ul. Kasprzaka,01-224 Warsaw, Poland. E-mail: [email protected], [email protected]; Fax: +48223433333; Tel: +4822333433130b School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK cSingapore Centre on Environmental Life Sciences Engineering (SCELSE), Nanyang Technological University, 60 Nanyang Drive, SBS-01N-27, Singapore. E-mail: [email protected]; Fax: +65-6515-6751; Tel: +65-6592-7895d Department of Chemical, Biological and Macromolecular Sciences, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata-700098, India

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shape modifiers and controllers are usually used to preparesuch hierarchical and complexed micro/nanostructures.24,25 Inaddition, hierarchically nanostructured materials have alsobeen fabricated by physically constrained crystal growth withthe assistance of templates,26,27 which need to be completely removed after synthesis. Template-free methods, including solution phase reduction,28 chemical vapour deposition,29thermal decomposition,30 electrochemical deposition,31 andhydrothermal methods,32 were widely explored to synthesizehierarchical nanostructured materials. Among them, thehydrothermal method has received huge attention because of its simplicity, and suitability for large-scale production, whichoff ers abundant control parameters and can be used for elec-trode modifications. Although diff erent nanoarchitectureshave been successfully prepared by the hydrothermal method(vide supra), we show that it is possible to directly grow 3-DNi– Fe hierarchical nanosheet arrays, hierarchical nanospheres,and porous nanospheres through the hydrothermal process ona flexible substrate.

To accurately detect glucose is of immense scientific tech-

nological importance for clinical diagnostics in diabetescontrol and analytical applications in biotechnology, environ-mental pollution control, and the food industry.33 –37 Sinceenzyme based amperometric sensors have significant draw-backs in stability because of the intrinsic nature of enzymes,38 –41 some methods have been developed to deter-mine the glucose concentration without using them.42 –46However, because of the surface poisoning from the adsorbedintermediates and chloride, most of the non-enzymaticsensors have distinct problems with low sensitivity and poorselectivity.47 –49 Interferential eff ects from endogenous speciesincluding ascorbic acid, uric acid, and dopamine cannot betotally eliminated. As a result, a major consideration in practi-cal non-enzymatic glucose sensing is focused on fabricating high-performance devices using resourceful transition-metalcatalysts.50 –59 Now, diabetes mellitus is a public healthproblem, and this metabolic disorder results from insulindeficiency and hyperglycemia and is reflected by blood glucoseconcentrations higher or lower than the normal range of 80– 120 mg dL− 1 (4.4– 6.6 mM). Good glycemic control is thecornerstone for proper diabetes management. Hence, it is per-tinent to explore and develop non-enzymatic sensors with ahigh sensitivity and a low detection limit to achieve convincing clinical measurements.

In this work, three-dimensional nickel– iron (3-D/Ni– Fe)

interconnected hierarchical nanosheets, hierarchical nano-spheres, and porous nanospheres are directly grown on flexiblecarbon fiber paper (CFP) collectors by a single-step hydro-thermal process. Our approach utilizes small amounts of Femetal as a doping agent for growing various morphologies. It is supposed that the ratio of urea and metal salts can assist the formation of hierarchical nanostructures, which is con-firmed by the experimental results (vide infra). Further, westudy the enzyme-free electrochemical sensing of glucose as amodel system by using 3-D/Ni– Fe nanostructures. Interestingly,3-D/Ni– Fe hierarchical nanosheet arrays resulted in a remark-

able enhancement in the electrocatalytic activity of glucose oxidation, reusability, and stability because of its highconductivity, large specific surface area and mesopores on its walls. The 3-D/Ni– Fe hierarchical nanospheres and porousnanospheres were also tested under similar experimental con-ditions used for comparing the electrochemical activity. The3-D/Ni– Fe hierarchical nanosheet arrays showed higher sensi-tivity (7.90 μ A μM− 1 cm− 2) with wide linear response of glucoseconcentration ranging from 0.05 µM to 0.2 mM, and the lowdetection limit (LOD) of 0.031 µM (S/N = 3) is achieved by tamperometry method. These results show the potential of these materials for use as electrodes for ultrasensitive electro-chemical experiments. We anticipate that they can be used inmany diff erent systems in the future.

2. Experimental section2.1. Materials

FeCl2·4H2O and Ni(NO3)2·6H2O, NaOH were obtained fromSigma-Aldrich Chemical Company. Urea (>99%) was receivfrom Merck Company. The nitric acid (HNO3) solution wasobtained from Aldrich Company. All other chemicals used inthis investigation were of analytical grade. All the solutions were prepared with Millipore water (18 mΩ) obtained from aMillipore system. Argon (99.99%) was supplied by MultaCompany, Poland.

2.2. Pre-treatment of CFP

The CFP substrate was washed repeatedly with 0.1 M HNO3solution, and then subjected to 5 min ultra-sonication treat-ment in solvents such as acetone, ethanol and water, respectively. Next, the CFP substrate was rinsed with Millipore watefollowed by drying with a stream of argon gas. Finally, theabove pre-treated CFP substrate was used as collectors for preparing various morphologies of 3-D/Ni– Fe nanostructures.

2.3. Synthesis of 3-D/Ni –Fe hierarchical nanosheet array nanostructures

The typical synthesis of 3-D/Ni– Fe hierarchical nanosheet arrays on carbon fiber paper can be explained as follows (optimized ratio as 2:1): 2 mM of FeCl2·4H2O and 1 mM of Ni(NO3)2·6H2O were added to 20 mL of Millipore water, andthen stirred for 15 min to obtain a homogeneous solution.Thereafter, 25 mM of urea was added quickly to the reaction

mixture, and then the reaction mixture was stirred again for10 minutes; next, the above reaction mixture was transferred toa Teflon-lined stainless steel autoclave vessel. Finally, the pretreated CFP substrate was placed in the reaction mixture, andthen continuously heated at 90° C for 6 h. After completion othe reaction, the CFP was carefully removed from the reaction vessel, and then thoroughly washed with Millipore water, followed by drying at 40 °C for 8– 12 h for further studies. Theother 3-D/Ni– Fe nanostructures such as hierarchical nano-spheres and porous nanospheres were obtained by repeating the above procedure by changing the Ni– Fe ratio to 1 : 1, and

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1 : 2, respectively. The mass loading for all samples of Ni– Fenanostructures on the CFP substrate was calculated as∼ 0.42 mg cm− 2. The 3-D/Ni– Fe hierarchical nanostructuresgrown over the CFP substrate can be used as a working electrode for electrochemical non-enzymatic glucose sensorexperiments.

2.4. Instrumentation

The morphologies of Ni– Fe nanostructures were characterizedby field emission-scanning electron microscopy (FE-SEM JEOL JSM-6700F). High-resolution transmission electron microscopy (HRTEM) was carried out using a JEOL JEM 3010 instrument with an acceleration voltage of 200 kV. The samples wereplaced on a carbon-coated copper grid for morphology andsize analyses. The crystallographic information was obtainedby the powder X-ray diff raction technique (XRD, ShimadzuXRD-6000, Ni filtered CuKα ( λ = 1.54 Å) radiation operating at 30 kV/40 mA). The specific surface area of the nanostructures was determined using the classic BET method (the Brunauer–Emmett – Teller isotherm). The BET isotherm is the basis fordetermining the extent of nitrogen adsorption on a givensurface. XPS analysis was performed on a VG ESCALAB MK II with an Mg-Kα (1253.6 eV) achromatic X-ray source. Nitrogenadsorption data were collected using a Quantachrome auto-sorb automated gas sorption system. The electrochemicalmeasurements were performed with a two compartment cell with a CFP (exposed geometric area = 0.16 cm2), a Pt wire, and Ag/AgCl (3 M KCl) as the working, auxiliary and referenceelectrodes, respectively. Cyclic voltammograms were recorded

using a computer controlled Autolab PGSTAT30 (Metrohm Autolab) electrochemical analyser. All electrochemical experments were carried out under an argon atmosphere.

3. Results and discussion3.1 Synthesis and characterization of 3-D/Ni –Fe hierarchicalnanostructures on CFP

The 3-D/Ni– Fe hierarchical nanosheet arrays with high density could be easily grown on CFP by a facile and one step hydrothermal route. The CFP was chosen here because of its finegroove-like skeleton and high porosity, which helped toincrease the active surface area of nanostructures.60 –62 Themorphology of as-synthesized 3-D/Ni– Fe hierarchicalnanosheet arrays/CFP was characterized by the FE-SEM technique, as shown in Fig. 1. It can be seen that the smart 3-D/Ni– Fe hierarchical nanosheet arrays were quite uniform,upright, outward, densely packed and well aligned, hierarchi-cal wall-like nanosheets grown on the surface of the CFP substrate and were ultrathin, which is favorable for the fullutilization of active materials (Fig. 1A – D) for technologicalapplications. Such hierarchical wall-like nanosheets were com-posed of several individual nano-walls with the lengths of 5– 7µm (Fig. 1A). Each wall appeared to grow from the samorigin/center of the CFP substrate. This indicates that the hier-archical wall-like 3-D/Ni– Fe bimetallic nanosheets were suc-cessfully synthesized by the surfactant-free hydrothermalreduction method. Interestingly, from the enlarged views

Fig. 1 FE-SEM images of interconnected hierarchical 3-D/Ni – Fe nanosheets (A), and the corresponding di ff erent enlarged views (B, C), and closeup view of a single nanosheet array showing the interconnected nano “ wall block ” nanostructure (D).

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(Fig. 1B and C), the nanosheets are interconnected with eachother to form a wall-like nanostructure, which might possessbetter mechanical strength and durability. Further, these crys-talline nanosheets well-retained their hierarchical nanosheet morphology without any calcination/annealing process. Asshown in the high-magnification FE-SEM image (Fig. 1D), theinterconnected nanosheets form a highly open and meso-porous structure. Thus, a large specific (active) surface of 3-D/Ni– Fe hierarchical nanosheets is highly accessible by the elec-trolyte/analyte when it is employed as an electrode material forelectrochemical sensing.

It can be observed from Fig. 2 that in situ growth of hier-archical Ni– Fe nanosheet arrays resulted in 3-D interconnectedand oriented nanoarchitectures, the length is more than∼ 60 µm (Fig. 2A), which can help overcome the problem of low conductivity in conventional nanosheet arrays. Further, it hasalready been pointed out that numerous 3-D/Ni– Fe hierarchi-cal nanosheets were grown uniformly on the CFP surface.Importantly, wall-like 3-D/Ni– Fe hierarchical nanosheet arrayscannot be destroyed into discrete walls even under ultra-sonication for a long period of time (20 min), indicating that

the complexed nanostructures are actually integrated and not made up of loosely grown nano-walls through magnetic dipoleinteractions. The 3-D/Ni– Fe hierarchical nanosheet arrays weresynthesized using nickel nitrate and iron chloride as metalsources and urea as the precipitant under hydrothermal con-ditions at 90 °C (see the Experimental section). At this temp-erature, urea was decomposed into ammonia, the reason forcreating an alkaline environment, and carbonate, which servedas an intercalated anion. Now, Ni2+ and Fe3+ cations releasedfrom the reaction mixture are simultaneously reduced tometallic atoms, and then spontaneously formed 3-D/Ni– Fe

bimetallic nanoparticle nuclei. The formed nuclei (Ni cations)occupy the center of the octahedral arrangement to grow intoa small sheet-like structure, with OH− groups forming the ver-tices of octahedra; then the octahedra share edges to formtwo-dimensional sheets, which rely on the hydrogen bondingbetween the hydroxyl groups of adjacent sheets. Next, thesheets can further grow three-dimensional network-like sheetsby self-assembly and then reconstruction as well as a furthergrowth step process. The grown hierarchical Ni– Fe nanosheetscharacteristics will potentially exhibit superior performance inthe electrochemical non-enzymatic glucose sensor (see below)During the reaction, the urea not only acts as the stabilizing orcomplexing regent but also plays a key role in the formation othe hierarchical morphology. Because of the urea, thenanosheets becomes flexible,63 and can be wrinkled and bent, which will help nanosheets connect with each other and leadsto the formation of ordered arrays (Scheme 1-1).

The synergistic eff ect of Fe in the reaction mixture alsohighly influences the formation of 3-D/Ni– Fe hierarchicalnanosheet arrays under these conditions, i.e., Fe acted as anucleating agent inducing the reduction of Ni at this (90 °C)

reaction temperature and assists the formation of such hier-archical 3-D/Ni– Fe nanostructures. As a result, sheet-like Ni– Fehierarchical nanostructures formed, and simultaneously, rapidrelease of water molecules during the conversion processresulted in the generation of mesopores over the surface of nanosheets (Fig. 2B), which resulted in the growth (in situ) of mesoporous 3-D/Ni– Fe hierarchical nanosheets on the CFPsurface (Scheme 1-1). The distance between nanosheets was∼ 250 nm and the film thickness was approximately 50– 100 nm(Fig. 2B). To conclude, a yellowish-brown film was coated othe surface of CFP (Fig. 2C, ii), indicating the successfu

Fig. 2 FE-SEM images of extended views of interconnected 3-D hierarchical nanosheets (A), and view of mesopores in the nanosheets (B), and

photographs (C) of carbon ber paper before (i) and after (ii) growing the Ni – Fe hierarchical nanosheet arrays.

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growth of 3-D/Ni– Fe hierarchical nanosheet arrays. The massesof all the Ni– Fe nanostructures were determined by cutting theCFP with grown samples into smaller pieces with the diameterof 15 mm. Then CFP with grown samples and CFP were weighed with a high-precision analytical balance (Sartorius,max. weight 5100 mg, d = 0.001 mg) from which the exact mass of the samples was then determined. The loading density of the 3-D/Ni– Fe active material is calculated to be0.42 mg cm− 2 (vide supra).

Next we used the Ni– Fe ratio of 1 : 1 for preparation; inter-estingly we observed that 3-D/Ni– Fe has a hierarchical nano-

sphere morphology. The FE-SEM image obviously shows that the as prepared 3-D/Ni– Fe nanoparticles have a hierarchicalnanospheres (flower-like) morphology (Fig. 3A and B). The par-ticles are uniform in size, ∼ 400 nm (Fig. 3B). The inset of Fig. 3B shows that the hierarchical nanospheres with a flower-like architecture are assembled by several interconnected tiny sheet-like petals with a smooth surface and the thickness ca.150 nm. Such hierarchical nanosphere architectures follow astepwise growth mechanism; at first, the generation of tiny crystalline nanoparticle nuclei occurs in a reaction mixture(vide infra) and subsequently, site-specific anisotropic growth

occurred due to a non-symmetrical assembly process(Scheme 1-2). Then, the particles continue to grow as strikinghierarchical nanospheres (inset of Fig. 3B) with few nanoleaves sharing their originating center through coarsening,also known as Ostwald ripening (Scheme 1-2).64,65

On the other hand, for the Ni– Fe ratio of 1 : 2, the nano-particle nuclei undergo random aggregation, followed bygrowth of interconnected porous nanosphere particles (Fig. 3Aand B) due to nanoparticle-face attraction through van der Waals forces (Scheme 1-3). Further, CO2 decomposed fromCO(NH2)2 can also act as soft template to induce the verity of

3-D/Ni– Fe nanostructure morphologies, since no other surfac-tant, templates and emulsions were used in this work (videsupra ). As can be seen from the enlarged view (Fig. 3B aninset), the porous nanospheres were uniform with a size of ∼ 300 nm (Fig. 3B). Importantly, no calcination treatmentprocess was required in this work for preparing hierarchical Ni–Fe nanostructures. It should be noted here that this procedureestablishes a universal protocol for the synthesis of 3-D/Ni– Fehierarchical nanostructures without adopting diff erent strat-egies, reaction conditions, or injection of foreign reagents(vide supra ). Changing the ratio of the stabilizer and the pre-

Scheme 1 Schematic representation of the formation of freestanding 3-D/Ni – Fe interconnected hierarchical nanosheets (1), hierarchical nano-spheres (2), and porous nanospheres (3) on a exible carbon ber paper (CFP) substrate.

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cursor or vice versa resulted in the formation of 3-D/Ni– Fenanostructures of diff erent morphologies (Fig. S1 in the ESI†).

The HRTEM images (Fig. 4; image A) show that the nano-structure of Ni– Fe grown on CFP is composed of a well-definednanosheet morphology with the length of 1 μm; the Ni– Fenanosheet off ers abundant catalytic sites, which contributes tothe high electrocatalytic activity of the composite material. TheHRTEM image of the Ni– Fe hierarchical nanosheet (image B)shows mesoporous architectures. A large amount of well-dis-tributed pores (on the surface) can be clearly seen, which is well-consistent with the results observed from FESEM. Thehierarchical Ni– Fe nanosheet together with the mesoporousstructure provides a large specific surface area, which was esti-mated by the Brunauer– Emmett – Teller (BET) method66 to be337.9 m2 g − 1, much higher than those for hierarchical Ni– Fenanospheres (211.3 m2 g − 1) and porous Ni– Fe nanospheres(136.4 m2 g − 1), respectively (Fig. S2†). The improved surfacearea allows for the rapid access of electrolyte ions, providesabundant active sites for the electrocatalytic reactions, and

facilitates fast electron transport between the electrodematerials and the analytes. The close up view of the HRTEMimage (image C) clearly shows that the interplanar spacing of 0.14 nm corresponds to the (111) crystalline plane. Further, aselected area electron diff raction (SAED) pattern analysis of hierarchical 3-D Ni– Fe nanosheets showed several well-definedrings with discrete dots in the diff raction pattern illustrating the crystalline nature of the sample (image D). This SAEDdiff raction pattern was well aligned with the XRD spectra,suggesting the crystalline nature of hierarchical Ni– Fe nano-structures. Next, the Ni– Fe hierarchical nanospheres grown on

CFP were composed of ultrathin nanoflakelets with a dimen-sion of ∼ 400 nm (image E), and close assemblies of Ni– Feporous nanospheres (image F) with a size of ∼ 300 nm werefurther confirmed by HRTEM analysis. The results obtainedfrom HRTEM were highly consistent with the FESEM analysis

The phase structure and purity of the as-synthesized 3-D/Ni– Fe samples were examined by the XRD method. Fig. shows the XRD pattern of all Ni– Fe samples. It can be seenfrom Fig. 5 that all of the diff raction peaks can be indexed topure nanocrystalline structures. The XRD profile of the 3-D/Ni–Fe hierarchical nanosheet arrays (Fig. 5A), hierarchical nanospheres (Fig. 5B), and porous nanospheres (Fig. 5C) on theCFP substrate shows that the peaks at 43.1, 53.3, 62.7, 76.479.6, and 96.0 could be indexed as the (111), (200), (220(311), (222), and (400) crystalline planes of the fcc (face cetered-cubic) Ni phase (JCPDS No. 04-0850), respectively.1,67 –69The peak corresponding to the (111) plane is more intensethan the other planes. The ratio between the intensities of the(200) and (111) diff raction peaks is lower (0.51) than the usual

value (0.60), suggesting that the (111) plane is the predominant orientation. Interestingly the ratios of the relative peak intensities of the high index planes (311) and (222) are higher(0.62 versus 0.42) and (0.67 versus 0.55) than the standard values (dotted box in Fig. 5A). This observation reveals that th3-D/Ni– Fe hierarchical nanosheets were more abundant inhigh index facets than 3-D/Ni– Fe hierarchical nanospheres(Fig. 5B) and 3-D/Ni– Fe porous nanospheres (Fig. 5C). Theabsence of NiO peaks mainly at 37.2 and 44.3 corresponds tothe (111) and (220) planes, indicating that no oxide product was observed in the samples. Furthermore, the diff raction

Fig. 3 FE-SEM images of hierarchical 3-D/Ni – Fe nanospheres (A), porous 3-D/Ni – Fe nanospheres (C) and the corresponding enlarged views (B, D),and insets showing the close up view of hierarchical nanosphere (B) and porous nanosphere (D) morphologies.

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peaks at 35.4, 53.1 and 62.5 can be indexed as the (311), (511),and (440) planes corresponding to the Fe component in all theNi– Fe nanostructures.70 Their XRD patterns diff er only in therelative intensities of given crystalline planes. The intensity of Fe peaks gets better (Fig. 5C) when the mole ratio 1 : 2 (Ni– Fe)is used for the synthesis of 3-D/Ni– Fe porous nanospheresrather than equal (Fig. 5B) or lower (Fig. 5A) mole ratios.

It has also been reported that the high concentration of theFe precursor (in the case of the 1 : 2 ratio) resulted in the for-mation of oxide phases,68 though in our synthetic protocol wehave not observed any oxide phases. Except for Ni– Fe phases,the small broad diff raction peak observed at 26.5 inFig. 5(A – C) corresponds to the (002) plane of the graphiticcarbon of the CFP substrate.71 No additional peaks apart fromthe carbon fiber substrate were observed; this indicates that high-purity 3-D/Ni– Fe nanostructures were obtained during the synthesis. The strong, sharp peaks and very low back-grounds reveal that the as-synthesized Ni– Fe bimetallic nano-particles had a high degree of crystallinity (vide supra ). As wediscussed, Fe acted as a nucleating agent during the reactionand assisted the formation of highly crystalline 3-D/Ni– Fe

nanostructures.X-ray photoelectron spectroscopy (XPS) is a powerful tool toidentify the elements’ states in bulk material.72 By the analysisof binding energy (BE) values, we have confirmed the nature of Ni and Fe metals in the 3-D/Ni– Fe hierarchical nanostructures.Core-level high-resolution XPS spectra of Ni– Fe hierarchicalnanosheets exhibit two peaks present at 855.4 and 873.1 eV with a spin-energy separation of 17.7 eV, corresponding toNi 2p3/2 and Ni 2p1/2 , respectively (Fig. S3A †).73 The binding energy of the Ni 2p3/2 peaks is diff erent from that in otherreports of NiO (853.7 eV), NiS (853.1 eV) and Ni (852.6 eV),74,75

which reveals that the Ni exist in the form of divalent. Nexthe high-resolution Fe 2p spectrum (Fig. S3B†) shows two dis-tinct peaks with binding energies of about 707.8 eV forFe 2p3/2 and 722.4 eV for Fe 2p1/2 .76 The shakeup satellite peak present at ∼ 860– 870 eV and ∼ 712– 720 eV is characteristic of Ni2+ and Fe3+ ions, respectively, in hierarchical Ni– Fe nano-structures. It also shows that the synthesized Ni– Fe nano-structures contain both Fe3+ and Fe2+ ions, but it is obviousthat the amount of Fe3+ is higher than the amount of the Fe2+

ions. Generally, the Fe3+ and Fe2+ ions are found in its latticedue to its inverse spinel structure. The XPS result confirmsthat the Fe exists as Fe2+/Fe3+ and Ni as Ni2+ in the preparedsamples. The chemical composition (atomic percent) of the as-prepared 3-D/Ni– Fe hierarchical nanosheets was determinedby energy-dispersive X-ray (EDX) spectrometry as shown Fig. S3C.† The only detectable elements are iron, nickel,carbon, and oxygen. While the carbon and oxygen should arisefrom the carbon fiber paper, and oxidation from the surface of the samples, the corresponding elementary analysis revealsthat the atomic ratio (%) of Ni to Fe is 63.3 : 31.2, which is veclose to the initial set ratio of Fe : Ni = 2 : 1. The atomic ratio

(%) of other morphologies such as 3-D/Ni–

Fe hierarchicalnanospheres and 3-D/Ni– Fe porous nanospheres were47.7 : 46.9% and 32.4 : 63.1%, respectively, which is consisten with their ratios 1 : 1 and 1 : 2. The above EDS results matched with the XRD pattern analysis, which revealed the crystallinitof the as-prepared hierarchical Ni– Fe nanostructures.

3.2 Electrochemical behaviour of the 3-D/Ni –Fe hierarchicalnanostructures on CFP electrode

To take advantage of its (3-D/Ni– Fe hierarchical nanosheet arrays) mesopores and large specific (active) surface area

Fig. 4 High-resolution TEM images of hierarchical Ni–

Fe nanosheets with di ff erent magni cations (A–

C), and the corresponding SAED pattern(image D). Images E and F represent hierarchical Ni – Fe nanospheres and porous Ni – Fe nanospheres, respectively.

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(Fig. S2†), we decided to study enzyme-free electrochemicalsensing of glucose as a model system. The electrocatalytic be-havior of 3-D/Ni– Fe hierarchical nanostructures towards

glucose oxidation was studied by cyclic voltammetry (CV) in0.1 M NaOH solution. As shown in Fig. 6A, a couple of sensi-tive redox peaks for the 3-D/Ni– Fe hierarchical nanosheet array CFP electrode is observed at potentials of 0.43 and 0.58 V (Fig. 6A, curve a), which could be mainly attributed to theNi(II)/Ni(III) redox couple in alkaline medium.77,78 It has beenknown that when nickel is immersed in alkaline solutions,spontaneous dissolution of the metal occurs followed by theformation of Ni(OH)2 and finally NiOOH (eqn (1) and (2)).79

Niþ 2OH ! NiOþ H2Oþ 2e ðorÞNiþ 2OH ! NiðOHÞ2 þ 2e

ð1Þ

NiOþ OH ! NiOOHþ e ðorÞNiðOHÞ2 þ OH $ NiOOHþ H2Oþ e

ð2Þ

Furthermore, the electrochemical signal of the 3-D/Ni– Fehierarchical nanosheet array CFP electrode is much largerthan that of the bare CFP electrode (Fig. 6A, curve b),suggesting that the former electrode is highly suitable for elec-trochemical detection. Next, on the injection of 0.25 mMglucose, the sharp anodic peak current recorded from the3-D/Ni– Fe hierarchical nanosheet array CFP electrode is signifi-cantly higher than that from the same electrode without

glucose. In addition, the behaviour of the cathodic peak current is slightly increased due to the glucose oxidation(Fig. 6A, curve c). These experimental results suggest the exce

lent electrocatalytic properties of 3-D/Ni– Fe hierarchicalnanosheet arrays toward the oxidation of glucose. The oxidation of glucose at the 3-D/Ni– Fe hierarchical nanosheet array CFP electrode was electrocatalyzed by the NiO(OH)/Ni(OH2redox couple, according to the following reactions (eqn (3)and (4)):80

NiðOHÞ2 þ OH $ NiOOHþ H2Oþ e ð3Þ

NiOOHþ glucose! NiðOHÞ2 þ glucolactone ð4Þ

The significant increase of the anodic peak current contri-butes to the excellent electrocatalytic activity of Ni(OH)2 in the

3-D/Ni– Fe hierarchical nanosheet arrays on glucose oxidation, which is accompanied by the oxidation of Ni2+ to Ni3+ , as ineqn (3). In addition, because of the absorption of glucose andthe oxidized intermediates on the active sites in mesopores of the 3-D/Ni– Fe hierarchical nanosheet array based electrode,the anodic peak potential shifts to a little positive direction.81It is also ascribed to the diff usion limitation of glucose at theelectrode surface;67 these results are consistent with the pre- vious Ni based literature.67,80,81

Further, Fe acted as a synergistic catalyst to enhance theelectrocatalytic oxidation of glucose in the bimetallic Ni– Fe

Fig. 5 XRD patterns of as-synthesized 3-D/Ni – Fe hierarchical nanosheet (A), hierarchical Ni – Fe nanosphere (B), and porous Ni – Fe nanosphere (C)morphologies.

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based nanoparticle system.1,2,8 Importantly, the increased con-centration of Fe content in the bimetallic system has increasedthe overall electrocatalytic reaction to some extent;1,2,8 howeverin our case the morphology of Ni– Fe nanostructures has also

played an important role. We further conducted chronoam-perometric measurements to evaluate the catalytic rate con-stant (k cat ); when the oxidation current is dominated by therate of the electrocatalytic reaction, the catalytic current ( I cat )can be written as eqn (5).

I cat = I L ¼ π1=2ðk cat C 0t Þ1=2 ð5Þ

where I cat and I L are the currents in the presence and absenceof glucose, respectively; k cat is the catalytic rate constant (M− 1 s− 1), C 0 the bulk concentration (M) of glucose, and t the

elapsed time (s). Based on the slope of I cat / I L vs. t 1/2 , the k cat value is calculated to be 1.80 × 103 M− 1 s− 1 taking the glucoseconcentration C 0 = 0.25 mM into account for the 3-D/Ni– Fehierarchical nanosheet array CFP electrode, while the k cat

obtained on the bare CFP electrode is calculated to be 0.31 ×103 M− 1 s− 1. Fig. 6B displays the CVs of the 3-D/Ni– Fe hierarchi-cal nanosheet array CFP electrode in 0.1 M NaOH containingdiff erent concentrations of glucose (from 0.05 to 0.5 mM) at ascan rate of 50 mV s− 1. Upon successive additions of glucose, aremarkable current and potential increase of the anodic peak is obviously observed; this indicates that the 3-D/Ni– Fe hier-archical nanosheet array CFP electrode has good sensitivityFig. 6C shows the eff ect of the scan rate on glucose oxidationat the 3-D/Ni– Fe hierarchical nanosheet array CFP electrode in0.1 M NaOH containing 0.25 mM glucose. It can be seen tha

Fig. 6 (A) CVs of the 3-D/Ni – Fe hierarchical nanosheet array CFP electrode in 0.1 M NaOH solution with (a) and without (b) 0.25 mM glucose. TheCVs obtained for the bare CFP electrode in 0.1 M NaOH containing 0.25 mM glucose for comparison (c). (B) CVs of the 3-D/Ni – Fe hierarchicalnanosheet array CFP electrode in a 0.1 M NaOH solution containing di ff erent concentrations of glucose. The scan rate of 50 mV s − 1 is used for theabove (A) and (B) experiments. (C) CVs of the 3-D/Ni – Fe hierarchical nanosheet array CFP electrode in 0.25 mM glucose at various scan rates, andthe inset shows plots of peak currents versus the square root of the scan rate. (D) CVs of 3-D/Ni – Fe hierarchical nanosheet array CFP (a), 3-D/Ni – Fehierarchical nanosphere CFP (b), and 3-D/Ni – Fe porous nanosphere CFP (c) electrodes in 0.1 M NaOH solution containing 0.25 mM glucose.

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the anodic peak shows a slightly positive shift, and the catho-dic peak shifts negatively with the increase in scan rate. Theseresults indicate that the redox reaction of Ni(OH)2 on thesurface of carbon fiber is rapid and reversible. Moreover, theanodic peak potential shifts positively, suggesting that there isa kinetic limitation in the reaction of glucose oxidation.44 Theinset of Fig. 6C shows that both anodic and cathodic peak cur-rents from the 3-D/Ni– Fe hierarchical nanosheet array CFPelectrode linearly scale with the square root of the scan rate,indicating that the redox reaction of Ni(OH)2 at the CFPsurface is a typical diff usion-controlled electrochemicalprocess.82 To compare the electrochemical performance of the3-D/Ni– Fe hierarchical nanosheet array based CFP electrode,CVs of the 3-D/Ni– Fe hierarchical nanosphere CFP and 3-D/Ni–Fe porous nanosphere CFP electrodes were measured in 0.1 MNaOH electrolyte containing 0.25 mM glucose at a scan rate of 50 mV s− 1, and the corresponding CV is shown in Fig. 6D. TheCV analysis shows three peaks at 0.57, 0.60, and 0.62 V for 3-D/Ni– Fe interconnected hierarchical nanosheets, Ni– Fe nano-spheres and porous Ni– Fe nanospheres, respectively. The 3-D/

Ni–

Fe interconnected hierarchical nanosheets show the lowest overpotential and the highest oxidation current, 310 µA vs. 260and 230 µA for Ni– Fe nanospheres and porous Ni– Fe nano-spheres, respectively. It can be seen that the 3-D/Ni– Fe hier-archical nanosheet array CFP electrode (Fig. 6D, curve a)showed enhanced electrocatalytic response towards the oxi-dation of glucose in contrast to the 3-D/Ni– Fe hierarchicalnanosphere CFP (Fig. 6D, curve b), and 3-D/Ni– Fe porousnanosphere CFP (Fig. 6D, curve c) electrodes due to a largespecific (active) surface area, and the interconnected wall-likeconfiguration of nanosheets provided an ideal platform foraccessing a large amount of glucose molecules. Overall, theobserved higher electrocatalytic activity of 3-D/Ni– Fe hierarchi-cal nanosheet arrays can be explained as: (i) the nanostructureof 3-D/Ni– Fe hierarchical nanosheet arrays off ers a largespecific (active) surface area; (ii) the interconnected wall-likeconfiguration and good electronic conductivity of 3-D/Ni– Fenanosheets could support and provide reliable electrical con-nections towards the glucose molecules; (iii) their compati-bility, i.e. 3-D/Ni– Fe nanosheets can be directly grown on thecurrent collector (CFP), makes them self-supporting nano-structures, (iv) 3-D/Ni– Fe hierarchical nanosheets as activematerials possess high electrochemical activity and contributeconsiderably compared to the other morphologies.

3.3 Application of the 3-D/Ni–

Fe hierarchical nanosheet array CFP electrode in non-enzymatic glucose sensing

The simple fabrication procedure makes the as-prepared elec-trode an eff ective platform for the amperometric determi-nation of glucose. Considering that the applied potential canstrongly aff ect the amperometric response of biosensors, wehave systemically investigated the impact of applied potentialon the amperometric response at the hierarchical 3-D/Ni– Fenanosheet array CFP electrode to glucose. The results indicatethat with the applied potential shifting from 0.60 to 0.70 V (in0.05 V steps), the steady-state current increases and reaches a

maximum at 0.65 V; at the same time, applied potentials of0.60 and 0.70 V can also cause a notable enhancement of thecurrent response per addition of glucose. Importantly, thesmaller background current and lower noise observed at anoptimized applied potential due to the glucose oxidationprocess was effi ciently completed at this potential, and thusthe applied potential of 0.65 V was selected for glucose detection. Fig. 7A shows the amperometric response of the 3-D/Ni–

Fe hierarchical nanosheet array CFP electrode towards the successive stepwise additions of glucose to a homogeneouslystirred 0.1 M NaOH solution at a working potential of0.65 V. A steep increase in current responses was obtainedafter each addition of 0.5 μM glucose solution, and a steady-state current was achieved within 2 s, indicating that the 3-D/Ni– Fe hierarchical nanosheet array CFP electrode exhibitsextremely sensitive and rapid response characteristics. Thismight be due to the fact that 3-D interconnected mesopores inthe walls provide low resistance and promote electron transferto reduce the response time.67 The corresponding calibrationcurve presented in Fig. 7B is linear over a concentration range

from 0.5 μM to 10.5 μM glucose with a correlation coeffi

cient of 0.9981. The sensitivity of the hierarchical 3-D/Ni– Fenanosheet array CFP electrode is calculated to be7.90 μ A μM− 1 cm− 2. It was further noted that the current response increases accordingly when the concentration of glucose continuously increases up to 200 μM (Fig. 7C) with acorrelation coeffi cient of 0.9902 (Fig. 7D), indicating that allactive sites of the 3-D/Ni– Fe hierarchical nanosheet array CFPelectrode were highly sensitive at higher concentration ofglucose. Based on a signal-to-noise ratio of 3 (S/N = 3), thdetection limit (LOD) was 0.031 μM calculated from the stan-dard deviation of amperometric baseline currents, suggesting the very good sensitivity of the 3-D/Ni– Fe hierarchicalnanosheet array CFP electrode.

In order to evaluate the performance of the sensor, a com-parison of 3-D/Ni– Fe hierarchical nanosphere CFP and 3-D/Ni– Fe porous nanosphere CFP electrodes towards the non-enzymatic glucose sensor is shown in ESI Fig. S4A.† It can beobserved that the sensitivities of 3-D/Ni– Fe hierarchical nano-sphere CFP (curve a) and 3-D/Ni– Fe porous nanosphere(curve b) CFP electrodes were 3.10, and 2.60 μ A μM− 1 cm− 2,respectively. We consider that the excellent electrocatalyticactivity of the 3-D/Ni– Fe hierarchical nanosheet array CFP elec-trode should be attributed to the fact that the 3-D inter-connected wall-like nanostructures with abundant mesopores

and holes expanding from the surface to the bottom not only facilitate transport of glucose molecules through the electro-lyte/electrode interface, but also allow them to come intocontact with more active surfaces (vide supra). Table S1† sum-marizes the non-enzymatic glucose detection performance with diff erent Ni- or Fe-based electrodes reported so far. It canbe concluded that our 3-D/Ni– Fe hierarchical nanosheet array CFP electrode possesses a higher sensitivity and a larger linearrange toward glucose detection. These superiorities wereattributed to the fact that the 3-D/Ni– Fe hierarchical nanosheet arrays combined the synergistic eff ects from Fe, which results

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in high electrocatalytic activity and the electrical network formed directly on the CFP surface. Moreover, well dispersedNi– Fe nanosheets on the surface of CFP can favour the easy access of glucose to nanosheets, which significantly enhancedthe electrochemical response towards glucose oxidation. Werecorded the amperometric responses of 0.5 μM glucose (stepsa, b, g, k, l, and m), 250 μM of ascorbic acid (AA, step c), dopa-mine (DA, step d), uric acid (UA, step e), acetaminophen (AP,step f), fructose (step h), galactose (step i), and lactose (step j)in homogeneously stirred 0.1 M NaOH solution, as shown inFig. S4B.† It was found that 3-D/Ni– Fe hierarchical nanosheet arrays provide remarkable responses only for glucoseoxidation, and there is no obvious amperometric current response for interfering species as compared to glucose, agree-ing well with the Ni based electrodes.53,83

In order to verify the reliability of the non-enzymaticglucose sensor for routine analysis, the sensor was employedto determine glucose in blood serum samples. A commercialOne-Touch Ultra glucometer (Johnson & Johnson Medical, Ltd) was used as the standard tool to check the reliability in hospi-tals. In the current clinical practice, plasma rather than wholeblood was used to determine the concentration of glucose. Thelevels of glucose in plasma are usually 10– 15% higher thanglucose in the whole blood. In other words, [glucose]blood =[glucose]plasma /1.14.84 Through conversion, the results arelisted in Tables S2 and S3.† It can be found that the values

of glucose in human whole blood with the proposed sensing electrode compared favourably to results obtained from thehospital. In general, the proposed method provided decreasedanalysis volume and an inexpensive and portable detectionplatform. More importantly, it has been shown to off er anaccurate determination of glucose in real samples without anyseparation procedure.

4. Conclusions We demonstrated the direct growth of 3-D/Ni– Fe intercon-nected hierarchical nanosheets, hierarchical nanospheres, andporous nanospheres on a flexible CFP substrate by a single-step hydrothermal process. Among the nanostructures, 3-D/Ni– Fe interconnected hierarchical nanosheets showed excel-

lent electrochemical properties because of their high conduc-tivity, large specific surface area and mesopores on their wallsThe 3-D/Ni– Fe hierarchical nanosheet array modified CFP sub-strate was successfully explored as a novel electrode for electrochemical non-enzymatic glucose sensor application. The 3-D/Ni– Fe hierarchical nanosheet arrays exhibit significant improvement in the electrochemical oxidation of glucose,as compared to the 3-D/Ni– Fe hierarchical nanospheres, andporous nanospheres. Moreover, 3-D/Ni– Fe hierarchicalnanosheet arrays possessed wide linear glucose concentrationranging from 0.05 µM to 200 µM, higher sensitivity

Fig. 7 Amperometric response of the 3-D/Ni – Fe hierarchical nanosheet array CFP electrode at an applied potential of 0.65 V in 0.1 M NaOH withthe dropwise addition of 0.5 μM glucose (A), and the corresponding calibration plot of the obtained current response vs. glucose concentration (B).Typical amperometric response of the 3-D/Ni – Fe hierarchical nanosheet array CFP electrode at 0.65 V with the successive additions of glucose upto 200 μM in 0.1 M NaOH (C) and the corresponding calibration plot of the obtained current response vs. glucose concentration (D).

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(7.90 μ A μM− 1 cm− 2) and the lowest detection limit (LOD) of 0.031 µM (S/N = 3). Further, the 3-D/Ni– Fe hierarchicalnanosheet array modified CFP electrode can be demonstratedto have excellent selectivity towards the detection of glucose inthe presence of 500-fold excess of major important inter-ferents. All these results indicate that 3-D/Ni– Fe hierarchicalnanosheet arrays are a promising candidate for non-enzymaticglucose sensing.

AcknowledgementsPalanisamy Kannan thanks the NanOtechnology Biomaterialsand aLternative Energy Source for ERA Integration [FP7-REGPOT-CT-2011-285949-NOBLESSE] project for financialsupport.

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