In situ precipitation of Nickel-hexacyanoferrate within multi-walled carbon nanotube modified electrode and its selective hydrazine electrocatalysis in physiological pH

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Abstract

Hybrid nickel-hexacyanoferrate-functionalized multiwalled carbon nanotube modified glassy carbon electrode (GCE/Ni-NCFe@f-MWCNT) has been prepared using electrodeposited Ni on functionalized MWCNT modified GCE (GCE/Ni@f-MWCNT) as a template and [ Fe ( CN ) 6 ] 3 - as an in-situ chemical precipitant, without any additional linker. Characterization of the GCE/Ni-NCFe@f-MWCNT by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy and cyclic voltammetry (CV) collectively revealed that the functionalized MWCNT is effective to uptake large amount of Ni species and, in turn, substantial quantity of Ni-NCFe units within its internal structure. Cyclic voltammetry of the GCE/Ni-NCFe@f-MWCNT showed non-stoichiometric K+/e and insertion/exertion behavior (∂Epa/∂log [KCl] = 42.9 and (∂Epc/∂log [KCl] = 117 mV/decade) due to some kinetic limitation to the insertion of K+ ion through the hydrophobic basal planes of the hybrid electrode. A quantitative model has been proposed to estimate the influencing components (underlying support, surface functional groups, basel plane, impurities, and insertion into edge-plane defects) in the formation of the hybrid Ni@f-MWCNT and, in turn, the Ni-NCFe@f-MWCNT. Electrocatalytic hydrazine oxidation on the GCE/Ni-NCFe@f-MWCNT showed 33 times enhancement in the current signal over GCE/f-MWNT in a pH 7 phosphate buffer solution. Amperometric it method of hydrazine detection yielded current sensitivity and calibration range of 120.2 μA/μM and 20–200 μM, respectively. The hybrid GCE/Ni-NCFe@f-MWCNT material is tolerable to other co-existing interferences such as oxalic acid, citric acid and nitrite. Finally, three different water real sample analyses were successfully demonstrated with appreciable recovery values.

Research highlights

► In-situ chemical precipitation of nickel-hexacyanoferrate within multiwalled carbon nanotubes. ► Ni-NCFe@f-MWCNT characterization by XRD, XPS, FE-SEM, EDAX, TEM, CV. ► ‘Edge-plane hole defects and oxygen functionality’ mechanistic model for formation of Ni-NCFe@f-MWCNT. ► Highly stable, sensitive and selective amperometric sensing of hydrazine in physiological solutions.

Introduction

Hybrid carbon nanotube-inorganic/organic molecular moieties possessing new class of functional materials with synergistic effect of the individuals have gained tremendous interest in recent timings owing to their exceptional physical and chemical properties including stabilizing of unusual intermediate species of chemical, biochemical and biomedical interest with enhanced performance towards applications [1], [2], [3]. Nickel-hexacyanoferrate (Ni-NCFe) is a mixed-valent inorganic polymer analogue from Prussian blue (PB) family [4], having zeolite type of macro-molecular crystal structures [4], [5], useful for various technological applications [6], [7], [8], including electro-chemical and -biochemical sensing of small molecules viz., hydrazine [9], [10], hydrogen peroxide [11], [12], dopamine [13], [14], nitric oxide [15] and reduced nicotinamide adenine dinucleotide (NADH) [16], [17], which in turn led to dehydrogenase enzyme-coupled biosensors [18]. Ni-NCFe and other PB analogues have strong alkali metal-ion dependent property and the naked compounds disintegrate at pH > 4 [6], [7], [8], [19], [20], [21], [22]. For unknown reasons, the alkali metal-ions (Na+ or K+) in the buffer solution are not found to be solely useful to maintain the electrical neutrality of the naked materials [6], [7], [8], [19], [20], [21], [22]. In order to solve the problem, extra Na+- and K+-containing electrolyte was added as free ions to the working buffer solution [9], [11], [16], [23]. Meanwhile, nafion, a perfluoro-cation exchange polymer, over layer-coated and sol–gel composed matrixes were reported as dressed Ni-NCFe materials to arrest the disintegration steps, and hence, leading to neutral buffer solution-based electrochemical applications [9], [10], [14]. However, such processes can considerably reduce the out put current signals and increase the internal resistance of the systems. Interestingly, here in we report a hybrid Ni-NCFe/functionalized multi-walled carbon nanotube modified glassy carbon electrode (designated as GCE/Ni-NCFe@f-MWCNT, GCE = glassy carbon electrode and f = functionalized), prepared by cathodically deposited Ni on f-MWCNT modified GCE as a template (GCE/Ni@f-MWCNT) and [ Fe ( CN ) 6 ] 3 - as a precipitant, as a stable material for physiological solution (pH 7 sodium phosphate buffer solution, PBS) and for enhanced electrochemical and electrocatalytic applications.

Conventionally, bulk Ni-metal was used as a source to form thin film of Ni-NCFe using millimolar solution of [ Fe ( CN ) 6 ] 3 - dissolved in ∼0.1 M alkali metal ion, preferably with Na+ ions, either by immersion or by potentiostatic polarization methods [21], [22]. Since the approach uses the reaction associated with corrosion mechanism (Ni2+ ion formation) to form a new interface, it may plug-off certain portions of the underlying metal along with the Ni-NCFe flaking into the solution. Because of these problems this preparation approach was less commonly used for application purpose in the literature [16]. On the other hand, no such complications were reported in several other extended methods in which fine and nano Ni0, Ni2+ or [ Fe ( CN ) 6 ] 3 - species were incorporated into matrixes, such as, 3,3-thiodipropionic acid self assembled-Au [23], starbust poly(amidoamine) (PAMAM) dendrimer combined 3-mercaptopropionic acid or 2-aminoethanethiol self assembled Au [24], DNA [25], diamine functionalized paraffin wax chemically immobilized graphite electrodes [13] and carbon ceramic material [10]. However, electrochemical operation solely with neutral buffer solution could be rarely achieved [10].

Recently, few papers reported carbon nanotube (CNT) as matrix to prepare hybrid Ni-NCFe modified electrodes for selective electro-analytical applications, for example, CNT-Polyaniline-Ni-NCFe electrode for electrically switched ion-exchange [26], [12], CNT-Ni-NCFe for glucose and cholesterol biosensor [11]. Note that those reported procedures adopted ex-situ preparation approach, in which, nanoparticles of Ni-NCFe were prepared discreetly by solution phase and/or electrochemical co-deposition procedure, in which a mixture containing { Ni 2 + + [ Fe ( CN ) 6 ] soln 3 - } was first converted to nano Ni-NCFe particles, {Ni-NCFe}nano, and then they were allowed to assemble/adsorb on the CNT (CNTsurface{Ni-NCFe}nano) utilizing the effect of preloaded linker, such as, polyaniline, chitosan and histidine within the matrix [26], [12]. However, good stability could not be achieved with these systems in pH 7 PBS without any free added alkaline metal ions. Meanwhile, ex-situ technique based hybrid CNT-Cobalt hexacyanoferrate (Co-NCFe@CNT) [27], [28] and CNT-Cerium hexacyanoferrate (Ce-NCFe@CNT) [29] complexes were recently prepared and operated in pH 7 PBS [27]. First time in this work, we demonstrate a new in-situ preparation method for novel Ni-NCFe@f-MWCNT hybrid material formation on GCE, through sequential surface modification procedure with GCE/Ni@f-MWCNT and [ Fe ( CN ) 6 ] 3 - , which is also highly amenable for stable electrochemical and electrocatalytic application in pH 7 sodium PBS.

Hydrazine is an important chemical of environmental, industrial and pharmaceutical interest, and reported to be neurotoxin producing carcinogenic and mutagenic effects [30], [31]. Sensitive and selective detection of the compound is of vital importance to address key issues about the hydrazine effect in the respective application settings. Another important reason for choosing hydrazine oxidation as a model reaction here was the following: N2 and 4H+ being the reaction products of hydrazine oxidation, when they are produced within the interphase, more specially at the inter-walls of the Ni-NCFe@f-MWCNT, they may alter the local pH and disturb the internal structure and in turn may influence the stability of the hybrid network [19]. The present investigation with the hybrid Ni-NCFe@f-MWCNT material not only provides an elegant route to synthesize physiologically stable PB analogue, but also to understand the structural and electron-transfer features of the new hybrid CNT-macro-molecular inorganic units, which are least reported in the literature.

Section snippets

Reagents and materials

Mullti-walled carbon nanotube (>90% purity) and single-walled carbon nanotube (∼70% purity) were purchased from Aldrich, USA. Nickel chloride hexahydrate (NiCl2.6H2O) was from Central drug house (P) Ltd, potassium ferricyanide from Merck specialties private Ltd and hydrazine sulphate extrapure from Sisco Research laboratories, India. Other chemicals were of analytical grade, and used as received without further purification. Aqueous solutions were prepared using deionized and alkaline KMnO4

Physicochemical characterization

Powder XRD responses of the f-MWCNT, Ni@f-MWCNT, Ni-NCFe and Ni-NCFe@f-MWCNT showed appreciable peaks indicating the presence of crystalline hybrid nano-materials within the respective matrixes (Fig. 1). For the case of f-MWCNT, apart from the standard 2θ peaks (26 and 42.7°) a peak at low angle, 11.8°, was discreetly noticed (Fig. 1A). The low angle peak is analogous to a recent report by Hae et al. for the hydroxyl, epoxide, and carboxyl functionalized graphite oxide (GO) surface at 12.84°(0 0 

Conclusions

A hybrid nickel-hexacyanoferrate-multiwalled carbon nanotube chemically modified glassy carbon electrode (GCE/Ni-NCFe@f-MWCNT) has been prepared by a new in situ precipitation technique using electrochemically deposited GCE/Ni@f-MWCNT as a template and [ Fe ( CN ) 6 ] 3 - as a precipitant, without any additional interlinking agent. The resulted hybrid GCE/Ni-NCFe@f-MWCNT showed a very high Ni-NCFe active site accumulation and excellent stability in pH 7 sodium PBS. Characterization by XRD, XPS, FESEM,

Acknowledgements

The authors gratefully acknowledge financial support from Department of Science and Technology, India. We also thank Prof. Jyh-Myng Zen for his valuable discussions on this manuscript.

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