Design and development of Co3O4/NiO composite nanofibers for the application of highly sensitive and selective non-enzymatic glucose sensors

Article abstract of DOI:10.1039/c5ra11739a

Cobaltosic oxide/nickel oxide (Co₃O₄/NiO) composite nanofibers were synthesized via an electrospinning technique and their electrocatalytic activities toward non-enzymatic glucose sensors were evaluated in detail. The Co₃O₄/NiO composite exhibited the homogeneously distributed nanofibers with high porosity, effective inter connectivity and an extended number of conducting channels with an average diameter of 160 nm. The diffraction patterns depicted the face centred cubic crystalline structure of Co₃O₄/NiO nanofibers and the purity of the composite nanofibers was further ensured by using FT-IR and UV-vis spectroscopic analyses. The electrocatalytic performances of prepared nanofibers toward the oxidation of glucose was determined by cyclic voltammetry and amperometry techniques and the experimental results showed that the Co₃O₄/NiO composite nanofibers exhibited a maximum electrooxidation toward glucose, owing to the synergistic effect of Co₃O₄ and NiO. The electrospun Co₃O₄/NiO nanofibers exhibited a detection limit of 0.17 μM, a wide linear range of 1 μM to 9.055 mM and a high sensitivity of 2477 μA mM^-1 cm^-2. The nanofibers have also exhibited favorable properties such as good selectivity, reproducibility, durability and real sample analysis, which ensured its potential applications in the clinical diagnosis of diabetes.

Full text of DOI:10.1039/c5ra11739a

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Title : Design and development of Co3O4/NiO composite nanofibers for the application of highly
sensitive and selective nonꢀenzymatic glucose sensors
Authors and Affiliations :
Author I :
Name
: R. Ramachandran
Affiliation : Department of Physics and Nanotechnology, SRM University,
Kattankulathurꢀ603203,Tamil Nadu, India.
Author II :
Name
: K. Ramachandran
Affiliation : Department of Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Maduraiꢀ625021, India
Author III :
Name
: Geo George Philip
Affiliation : Nanotechnology Research centre, SRM University, Kattankulathurꢀ603203,
Chennai, Tamil Nadu, India.
Author IV :
Name
: Rasu Ramachandran
Affiliation : Department of Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Maduraiꢀ625021, India
Author V:
Name
: Helen Annal Therese
Affiliation : Nanotechnology Research centre, SRM University, Kattankulathurꢀ603203,
Chennai, Tamil Nadu, India.
Author VI :
Name
: G.Gnana kumar
Affiliation : Department of Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Maduraiꢀ625021, India
Corresponding Authors :
(I) Name
: Helen Annal Therese
Affiliation : Nanotechnology Research centre, SRM University, Kattankulathurꢀ603203,
Chennai, Tamil Nadu, India.
Eꢀmail
: helen.a@ktr.srmuniv.ac.in
II) Name
: G.Gnana kumar
Affiliation : Department of Physical Chemistry, School of Chemistry,
Madurai Kamaraj University, Maduraiꢀ625021, India
Tel.No
: 91ꢀ95857572997; Eꢀmail : kumarg2006@gmail.com
1

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Design and development of Co3O4/NiO composite nanofibers for the
application of highly sensitive and selective non-enzymatic glucose sensors
R. Ramachandran,a K. Ramachandran,a Geo George Philip,b Rasu Ramachandran,a
Helen Annal Therese,*b and G. Gnana kumar*c
a Department of Physics and Nanotechnology, SRM University, Kattankulathurꢀ603 203,Tamil
Nadu, India.
b
Nanotechnology Research centre, SRM University, Kattankulathurꢀ603 203, Chennai, Tamil
Nadu, India.
c
Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University,
Madurai ꢀ625 021, Tamil Nadu, India.
Abstract
The cobaltosic oxide/nickel oxide (Co3O4/NiO) composite nanofibers were synthesized via
electrospinning technique and its electrocatalaytic activities toward the nonꢀenzymatic glucose
sensors were evaluated in detail. Co3O4/NiO composite exhibited homogeneously distributed
nanofibers with high porosity, effective inter connectivity and an extended number of conducting
channels with an average diameter of 160 nm. The diffraction patterns depicted the face centred
cubic crystalline structure of the Co3O4/NiO nanofibers and the purity of composite nanofibers
was further ensured by using FTꢀIR and UVꢀvis spectroscopic analyses. The electrocatalytic
performances of prepared nanofibers toward the oxidation of glucose was determined by cyclic
voltammetry and amperometry techniques and the experimental results showed that the
Co3O4/NiO composite nanofibers exhibited a maximum electrooxidation toward glucose, owing
to the synergistic effect of Co3O4 and NiO. The electrospun Co3O4/NiO nanofibers exhibited a
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detection limit of 0.17 µM, a wide linear range of 1 µM ꢀ 9.055 mM and a high sensitivity of
2477 µA mMꢀ1cmꢀ2. The nanofibers have also exhibited favorable properties such as good
selectivity, reproducibility, durability and real sample analysis, which ensured its potential
applications in the clinical diagnosis of diabetes.
Key words: conductive channels, electrospinning, nanofibers, oxidation, porosity.
* Corresponding authors:
* Helen Annal Therese: e.mail: helen.a@ktr.srmuniv.ac.in.
* G. Gnana kumar: eꢀmail: kumarg2006@gmail.com; Tel. No.: 919585752997.
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1. Introduction
Diabetes is one of the most widespread diseases that affect the human population globally. Three
hundred and eighty two million people are affected worldwide1 and it causes 1.5 to 5.1 million
deaths yearly, making it the 8th leading cause of death.2 This pressing situation calls for the
development of cheap, reliable and portable glucose sensors. Currently enzyme based/enzymatic
glucose sensors are widely available on the market. Despite their widespread availability, there
are some critical issues with enzymatic glucose sensors. The enzymatic glucose sensors are cost
prohibitive, with prices reaching nearly 100 dollars per sensor.3 Apart from this, enzymatic
glucose sensors suffer from poor reproducibility, thermal and chemical instabilities and
susceptibility to enzyme poisoning molecules.4,5 The aforementioned serious problems with
enzymatic glucose sensors have facilitated the need for alternate glucose sensors. A number of
various methods such as colorimetry,6 fluorescent spectroscopy7 and other optical methods8 have
been proposed; among which electrochemical nonꢀenzymatic glucose sensors3 stand out. The
affordable cost, low detection limits and high sensitivity of electrochemical based nonꢀenzymatic
glucose sensors favored their extensive applications.9,10 Hence, a large number of materials
including precious11ꢀ13 and nonꢀprecious metal14,15 and metal oxides16,17 have been tested for their
suitability as nonꢀenzymatic glucose sensor probes. Among these various metal and metal
oxides, nickel (Ni) and nickel oxide (NiO) modified electrodes have shown very high sensitivity,
low detection limits and good linear response toward glucose oxidation,18ꢀ20 owing to its
constructive properties such as large specific surface area, excellent electrical conductivity and
better electrocatalytic activity. It also exhibits the advantage of being low cost due to the relative
abundance of Ni in the earth’s crust and NiO has shown to be superior over Ni due to the latter’s
tendency to get oxidized in air, which can drastically reduce its shelf life. However, pure NiO is
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inevitably suffered by its poor surface stability under the oxidation conditions, leading to the
poor sensitivity and selectivity.21 To effectively tackle this adverse impact, NiO based
composites have received more attention, in which the impact of cobaltosic oxide (Co3O4) is
highly vibrant.22 If NiO is complexed with Co3O4, the mechanical, chemical and electrochemical
properties and structural stability of host material could be enhanced. Hence, the aforementioned
metal oxide composites in the form of various morphologies such as nanorods23, nanofibers24 and
as composites25 have been shown to exhibit prompt sensitivity and selectivity toward glucose in
the alkaline media.
One dimensional nanostructures such as nanofibers and nanorods have been shown to be
effective sensor probes due to their larger surface to volume ratio and high porosity.26,27 One of
the best methods to synthesize one dimensional porous nanostructures is the electrospinning
technique. Electrospinning technique has the advantages of low cost, high yield and good
reproducibility and it is a facile technique to produce porous and large surface to volume ratio
nanofibers in large quantities.28ꢀ30 Elecrospinning at the laboratory level can be easily scaled up
to the industrial scale to meet the commercial requirements for the manufacture of nonꢀ
enzymatic glucose sensors as opposed to conventional laboratory techniques such as solꢀgel and
hydrothermal methods. In general, the electrochemical quantification of glucose at prepared
nanostructures is initiated by the adsorption, diffusion and followed by the oxidation at the
catalyst’s surface. The maximum electrooxidation is initiated by the effectual adsorption and
diffusion processes, which can be stimulated only at the extended cavities/porosities of the
prepared catalysts. The high porosity of electrospun nanofibers helps in trapping the analytes
without any significant diffusion resistance, leading to an increase in the response to
electroactive species,31 ensuring good sensitivity of the material toward glucose. To utilize the
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advantages of nanofibers and metal oxide composites, Co3O4/NiO composite nanofibers have
been developed and its influences toward the electrooxidation of glucose under various
electrochemical regimes and conditions have been analyzed and reported in this research effort.
2. Experimental methods
2.1. Materials
Polyvinyl alcohol (PVA) (MW = 40,000 g molꢀ1), nickel (II) acetate tetrahydrate
(Ni(CH3COO)2.4H2O), cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O), glacial acetic acid,
N,Nꢀdimethylformamide (DMF), glucose, urea (U), uric acid (UA), ascorbic acid
(AA), dopamine (DA), acetaminophen (AP) and sodium chloride (NaCl) were obtained from
SigmaꢀAldrich and used without any further purification. All of these reagents were of analytical
grade and used as received.
2.2. Preparation of PVA solution
For the preparation of 15 wt % PVA solution, an appropriate amount of PVA was dissolved in
10 ml of deꢀionized water and the solution was stirred for 2 h at room temperature.
2.3. Preparation of (Ni(CH3COO)2.4H2O) solution
1M Ni(CH3COO)2.4H2O solution was added into 0.3 ml glacial acetic acid and magnetically
stirred at room temperature. The glacial acetic acid was added to improve the conductivity of the
electrospinning solution.
2.4. Preparation of (Co(NO3)2.6H2O) solution
1 M Co(NO3)2.6H2O solution was added into 0.3 ml glacial acetic acid and stirred at room
temperature.
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2.5. Preparation of PVA/(Co(NO3)2.6H2O)/(Ni(CH3COO)2.4H2O) and
PVA/(Ni(CH3COO)2.4H2O) solution
The equal quantities of 1 M Co(NO3)2.6H2O and 1 M Ni(CH3COO)2.4H2O solution were
magnetically stirred for 30 min and one part of the Co(NO3)2.6H2O/Ni(CH3COO)2.4H2O
solution was gradually added to the two parts of PVA solution and magnetically stirred for 30
min at room temperature to obtain a homogeneous solution. The PVA/Ni(CH3COO)2.4H2O
solution was prepared according to the same method.
2.6. Fabrication of electrospun nanofiber matrix
The prepared PVA/Co(NO3)2.6H2O/Ni(CH3COO)2.4H2O or PVA/Ni(CH3COO)2.4H2O solution
was transferred to the syringe. The distance from the needle to the drum collector was fixed at
18 cm and a voltage of 20 kV was applied to the tip of the nozzle. The flow rate of the syringe
pump was set at 0.5 ml hꢀ1. The nanofibres were collected on the rotating drum collector set at a
speed of 800 to 900 rpm. The electrospun nanofiber membrane was then collected and annealed
in a muffle furnace to obtain the Co3O4/NiO composite nanofibers. The electrospun nanofiber
o
o
membrane was annealed to 500 C at a controlled rate of 2 C minꢀ1 and the slow heating rate is
critical to the proper formation of Co3O4/NiO nanofibers. The similar method was used for the
synthesis of bare NiO nanofibers.
2.7. Modification of electrode
Before the surface modification of glassy carbon electrode (GCE) (3mm diameter), GCE was
surface polished with 1.0, 0.3 and 0.05 ꢁm alumina powder and then washed with deꢀionized
water, followed by sonication in ethanol and deionized water and dried at room temperature. The
prepared nanofibers (2 mg mlꢀ1) were dispersed in a mixture of 0.02 ml Nafion and 0.18 ml DMF
and the mixture was sonicated for 60 min to obtain a homogeneous dispersion. Then 10 ꢁl of the
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mixture was dropped onto the surface of polished GCE and dried at room temperature. The NiO
and Co3O4/NiO nanofibers modified GCEs are represented as NiO/GCE and Co3O4/NiO/GCE,
respectively.
2.8. Characterizations
The surface morphologies and average diameter of the nanofibers were investigated with a Field
Emission ꢀ Scanning Electron Microscope (FEꢀSEM, FEG Quanta 200, FEI) coupled with
Energy Dispersive XꢀRay Spectroscopy analyzer (EDAX, Bruker) and Transmission Electron
Microscope (TEM, JEOL 2010). The crystal structure of prepared nanofibers was investigated
with a XꢀRay Diffractometer (XRD, Xpert Pro, Panalytical). The Fourier transform infrared
(FTꢀIR) transmission spectra of prepared nanofibers were obtained by employing a BRUKER
αꢀAlphaꢀT Spectrometer with a SiC radiation source. The UVꢀvis absorption spectra of prepared
nanofibers were acquired by using Metrohm (AnalyticalꢀSpecordꢀ220) spectroscopy.
2.9. Electrochemical characterizations
The electrochemical behavior of synthesized nanofibers were investigated by using a three
electrode system, which consisted of a GCE, Ag/AgCl (1M KCl) and Pt wire as a working,
reference and counter electrode, respectively. The electrochemical studies were carried out by
using a CHIꢀ650D electrochemical workstation at room temperature and the cyclic
voltammograms (CV) and amperometric current responses were recorded by using the three
electrode system. The electrocatalytic activity of modified electrodes was recorded in a solution
containing 0.1 M NaOH at a scan rate of 20 mV sꢀ1 in the absence and presence of 1mM glucose
by using cyclic voltammetry. The amperometric i–t measurements were carried out in a saturated
0.1 M NaOH solution with the successive addition of glucose solutions of different
concentrations at an applied potential of 0.50 V vs. Ag/AgCl. The electroanalytical performances
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of six bare/modified GCEs realized under the same conditions were evaluated and all of the
electroanalytical performances were repeated for 4 times on each bare/modified GCEs and the
average values were reported.
3. Results and Discussion
3.1. Morphological analysis
The
surface
morphological
properties
of
PVA/Ni(CH3COO)2.4H2O,
PVA/
Ni(CH3COO)2.4H2O/Co(NO3)2.6H2O, NiO and Co3O4/NiO nanofibers are provided in Fig. 1.
The asꢀspun PVA/Ni(CH3COO)2.4H2O (Fig. 1a) and PVA/Ni(CH3COO)2.4H2O/Co(NO3)2.6H2O
nanofibers (Fig. 1b) exhibited smooth surfaces with the average fiber diameter of 325 nm and
o
hundreds of micrometers in length. Even after the calcination process at 500 C, the calcined
samples maintained the nanofibrous morphologies. The average diameter of annealed fibers has
shrunk to 160 nm for the bare NiO (Fig. 1c, d) and Co3O4/NiO nanofibers (Fig. 1e, f) and the
reduction in the size of annealed nanofibers is attributed to the loss of polymer component during
the annealing process. However, there were no significant changes observed in the length of
prepared nanofibers. Furthermore, the smooth surface of fiber morphology was completely
collapsed for the bare NiO (Fig. 1c,d) and Co3O4/NiO nanofibers (Fig. 1e,f), leading to the
formation of a rough surface. The annealed nanofibers were composed of numerous nanograins
with the relatively uniform distribution and average diameter of 25 nm. Both the bare NiO and
Co3O4/NiO composite nanofibers exhibited a porous three dimensional network structure, which
is expected to pave ways for the facile electron transportation for the excellent catalytic activity.
The morphological properties of Co3O4/NiO composite nanofibers were further analyzed by
using TEM (Fig. 1g) and it is clear that the resultant fiber is composed of uniformly
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interconnected nanoparticles with an average diameter of 25 nm and the strong interconnection
among the nanoparticles constructed the nanofiber morphology.
The existence of carbon and polymer residues in the nanofibers may interfere with the
glucose sensing process. Hence, the aforementioned traces in bare NiO and Co3O4/NiO
composite nanofibers were evaluated by using EDAX and the corresponding EDAX patterns are
provided in Fig. 2. The presence of Ni and O elements in NiO nanofibers (Fig. 2a) and the
existence of Ni, Co and O elements in Co3O4/NiO composite (Fig. 2b) ensured the chemical
constituents of corresponding nanofibers. Although traces of carbon were found in both the NiO
and Co3O4/NiO composite nanofibers, the traces are present in low amounts and can be
considered as negligible.
3.2. Structural analysis
The diffraction patterns of prepared nanofibers are provided in Fig. 3 and NiO nanofibers
o
exhibited the characteristic diffraction peaks at 38.2, 43.8, 63.2, 75.8 and 79.3 (Fig. 3a), which
represent the (111), (200), (220), (311) and (222), respectively, reflection planes of a face
centered single phase cubic NiO with the Fmꢀ3m space group.31ꢀ33 The diffraction patterns of
electrospun composite nanofibers showed the formation of two separate phases ꢀ Co3O4 and NiO
as illustrated in Fig. 3b and the reflection planes of Co3O4 and NiO correspond to the cubic
crystal system with Fdꢀ3m (JCPDS no. 01ꢀ078ꢀ1969) and Fmꢀ3m space groups (JCPDS no. 01ꢀ
071ꢀ1179), respectively.34,35 The peak broadening observed in the diffraction patterns specified
the nanocrystalline structure of the grains in NiO and Co3O4/NiO composite nanofibers and the
observed crystallite sizes are well matched with the morphological images. No other impurity
phases observed from the XRD analysis ensured the structural purity of the prepared samples.
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The structural characterizations of NiO and Co3O4/NiO composite nanofibers were further
evaluated by using FTꢀIR and the corresponding spectra are provided in Fig. 4. The formation of
NiO nanofibers (Fig. 4a) were ensured from the characteristic NiꢀO stretching bands observed at
424, 451, 480 and 502 cmꢀ1.36ꢀ38 The spectrum corresponding to Co3O4/NiO composite
nanofibers (Fig. 4b) exhibited all the NiꢀO (420ꢀ500 cmꢀ1) stretching vibrations along with the
new additional bands at 565 and 658 cmꢀ1. These additional bands correspond to the CoꢀO
stretching vibrations of Co3+ and Co2+ present at the octahedral and tetrahedral sites of Co3O4
spinel, respectively. 39, 40.
The UVꢀvis spectroscopic analysis of NiO and Co3O4/NiO composite nanofibers were carried
out over the wavelength of 200 ꢀ 800 nm and the obtained UVꢀvis spectra are shown in Fig. 5. A
strong absorption band was observed at 320 nm (Inset: Fig. 5a) for the NiO nanofibers, owing to
the electronic transition from the valence band to the conduction band of NiO.41 The UVꢀvis
spectrum correspond to Co3O4/NiO composite nanofibers (Inset: Fig. 5b) exhibited a band at
320 nm, due to the Ni2+ transition. In addition, the two intra band transitions were observed in
700ꢀ850 nm and 450ꢀ600 nm ranges, which could be attributed to the lower and higher band gap
charge transfer processes of Co3+ and Co2+ respectively.42 From the tauc plot drawn for both the
nanofibers, it is clear that the band gap of Co3O4/NiO composite nanofibers is reduced to 2.19 eV
from that of bare NiO nanofibers (3.87 eV).
3.3. Electrochemical performances
The electrochemical performances of bare GCE, NiO/GCE and Co3O4/NiO/GCE were
characterized by using CV technique in 0.1 M NaOH in the potential range of ꢀ0.2 to 0.7 V at a
scan rate of 20 mV sꢀ1 (Fig. 6a). The voltammograms obtained at bare GCE revealed that no
noticeable redox behavior was observed, indicating that bare GCE was potentially silent under
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the alkaline medium. However, the well defined redox peaks were obtained at both
Co3O4/NiO/GCE and NiO/GCE, owing to the presence of metal catalysts. The wellꢀdefined
Ni2+/Ni3+ redox couple was obtained at NiO/GCE under an alkaline solution with the anodic
(Epa) and cathodic (Epc) peak potential of +0.51 and +0.42 V vs. Ag/AgCl, respectively,43, 44
indicating that the prepared NiO nanofibers exhibited significant electrocatalytic activity in the
alkaline medium and the involved electrochemical reaction is prescribed as follows:
NiO + OH− ↔ NiOOH + e− …………………… (1)
The redox peak currents were further increased at Co3O4/NiO/GCE, owing to the higher
catalytic activity of composite nanofibers through the synergetic effects of Co3O4 and NiO.45, 46
The generation of complex species at Co3O4/NiO/GCE under alkaline conditions is identified
from the Epa observed at 0.46 V vs. Ag/AgCl , which is intermediate between the NiO and
Co3O4 anodic peak potentials. After the absorption of OHꢀ ions, both Ni2+ and Co2+ were
transformed into Ni3+ and Co3+ respectively, and Co3+ was further oxidized into Co4+ at higher
potentials. The Epc observed at 0.47 V vs. Ag/AgCl (Fig. 6a) is related with the reduction of
Ni3+and Co4+ formed in the positive scan and the involved reactions at Co3O4/NiO/GCE are
given below:
NiO + OH− ↔ NiOOH + e− ……………………. (2)
Co3O4 + OH− + H2O ↔ 3CoOOH + e− …………………. (3)
CoOOH + OH− ↔ CoO2+H2O+ e− …………………….. (4)
Fig. 6b shows the voltammograms of Co3O4/NiO/GCE obtained in 0.1 M NaOH as a
function of scan rate. It is clear that the anodic and cathodic peak responses were enhanced with
the increasing scan rates ranging from 10 ꢀ 100 mV sꢀ1 in 0.1 M NaOH. The anodic and cathodic
peaks were shifted to positive and negative values, respectively; with the corresponding
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increment in the scan rate. It is calculated that the Ip is proportional to the square root of scan
rate with the high correlation coefficient (R) of 0.993 and 0.996 (Inset: Fig. 6b) for Ipa and Ipc,
respectively; indicating that a diffusionꢀcontrolled quasiꢀreversible electrochemical reaction was
occurred at Co3O4/NiO/GCE.
3.4. Electrocatalytic oxidation of glucose at GCE, NiO/GCE and Co3O4/NiO/GCE
The electroꢀoxidation of glucose at bare GCE, NiO/GCE and Co3O4/NiO/GCE were examined
by using CV technique under 1 mM glucose in 0.1 M NaOH at a scan rate of 20 mV sꢀ1 and the
obtained voltammograms are given in Fig. 7a. The bare GCE demonstrated negligible catalytic
effects toward the glucose electroꢀoxidation in 0.1M NaOH in the potential window from ꢀ0.2 to
+0.7 V. However, NiO/GCE has exhibited the prompt Ipa of 47 µA at +0.53 V vs. Ag/AgCl,
which is ascribed to the electroꢀoxidation of glucose. It is well established that the observed Ipa
is attributed to the electroꢀoxidation of glucose with the participation of Ni3+ centres in
0.1 M NaOH solution and the involved electrochemical reaction is given as follows:
2NiOOH + glucose → 2NiO + gluconolactone + H2O……….(5)
The electroxidation of glucose was further enhanced at Co3O4/NiO/GCE as evidenced from
the increased Ipa of 74 µA at 0.47 V vs. Ag/AgCl, indicating the irreversible maximum
electrooxidation of glucose. The increased Ipa and positively shifted Epa at Co3O4/NiO/GCE
than that of NiO/GCE are purely attributed to the effective participation of Ni3+/Co4+ centers of
composite nanofibers generated at an alkaline medium.47 The high porosity and well interꢀ
connected nanofibers were observed for the Co3O4/NiO nanofibers. The free mobility of
electrons throughout the length of nanofibers and extended number of electron transfer tunnels
increased the delocalization of charges that enhanced the conductivity of prepared Co3O4/NiO
composite nanofibers. The cavities among the nanofibers and high porosity on the individual
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nanofibers facilitated the rapid adsorption and followed by the efficient diffusion of an anlayte
glucose. The highly adsorbed glucose allowed the access of analytes to the catalytic active sites
and facilitated the intimate contact between them with the minimal diffusion resistance. When
the analyte glucose was rapidly diffused into the surface of electrode, it is oxidized in to
gluconolactone by the participation of Ni3+/Co4+ centres at Co3O4/NiO/GCE, leading to the
increased Ipa under an alkaline solution. The increased electrooxidation of glucose observed at
Co3O4/NiO/GCE is ascribed to the synergetic effects of Co3O4 and NiO nanostructures that
promoted the electrical conductivity and number of active sites. By the collective efforts afforded
by the nanofibers and composite technologies, the maximum electroxidation of glucose was
observed at Co3O4/NiO/GCE and the involved reaction mechanism is schematically illustrated in
Fig. 7b.
Fig. 8a depicts the CVs of Co3O4/NiO/GCE using different concentration of glucose in
0.1M NaOH at a scan rate of 20 mV sꢀ1. The addition of glucose from 1 to 4 mM caused the
obvious increase of Ipa with increasing glucose concentration without the potential shift. This
phenomenon indicates the irreversible electrocatalysis of glucose under the positive scan and the
catalytic conversion of Ni3+ and Co4+ to Ni2+ and Co3+, respectively, under the negative scan.
This result has exhibited the excellent electrocatalytic ability of Co3O4/NiO/GCE in glucose
oxidation and guarantees its amperometric glucose sensor applications.
Furthermore, the electrokinetics involved in Co3O4/NiO/GCE was evaluated through
different scan rates. Fig. 8b depicts the CVs of Co3O4/NiO/GCE at different scan rates
(10ꢀ100 mV sꢀ1) using 1mM glucose in 0.1M NaOH. The Epa and Epc were slightly shifted to
positive and negative values, respectively, with the corresponding increase in the scan rates.
Moreover, the obtained Ipa and Ipc were linearly correlated to the square root of scan rates
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(Inset: Fig. 8b) with the high correlation coefficient (R) of 0.992 (Ipa) and 0.995 (Ipc)
respectively, implying that the reaction is a typical diffusion controlled electrochemical process.
3.5. Amperometric detection of glucose at Co3O4/NiO/GCE
Amperometric analysis is an extremely attractive technique, which is reflected by their fast
response time toward the oxidation of glucose in the electrochemical sensor applications to
evaluate the high sensitivity and detection limit toward glucose. The amperometric study was
carried at Co3O4/NiO/GCE with the successive stepꢀwise addition of different concentrations of
glucose in 0.1M NaOH at an applied potential of 0.5 V vs. Ag/AgCl (Fig. 9a). When the different
concentration of glucose was added at a regular time intervals of 50 s into the alkaline medium, a
rapid current increment was observed. The fast response and high values of the wellꢀdefined
steadyꢀstate current response of nonꢀenzymatic glucose sensor response toward glucose were
observed. The steady state current has reached a fast response time of 6 s, owing to the excellent
electrocatalytic activity, number of active sites, good electron mobility properties and inter
connective network of Co3O4/NiO nanofibers. Fig. 9b shows the calibration curve of the
electrochemical response of Co3O4/NiO/GCE, which exhibited a wide linear range of glucose
concentrations ranging from 1 µM to 9.055 mM with a high correlation coefficient of
0.998 (n=27). The fabricated Co3O4/NiO/GCE exhibited the rapid response time, lower detection
limit and high sensitivity of 5 s, 0.17 µM (S/N=3) and 2477 µA Mmꢀ1cmꢀ2, respectively. The
obtained high sensitivity and low detection limit have been ascribed to the excellent
electrocatalytic activity of the asꢀprepared Co3O4/NiO nanofibers. The highly porous and three
dimensional uniform network of Co3O4/NiO nanofibers exhibited the large number of active
sites. The porous three dimensional network has also permitted the easy access of glucose to the
electrocatalytic active sites with the minimal diffusion resistance. The Co3O4/NiO nanofibers
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showed superior electrocatlytic activity toward nonꢀenzymatic glucose sensing in comparison
with the previous literatures as given in Table 1.48ꢀ59
3.6. Stability and reproducibility studies
To investigate the reproducibility of proposed system, six Co3O4/NiO/GCEs were fabricated
under the similar conditions and CV experiments were performed under the presence of 1 mM
glucose. A relative standard deviation (RSD) of 3.32 % was obtained for a series of
Co3O4/NiO/GCEs, specifying the good reproducibility of fabricated sensors. The operational
stability of fabricated sensors was also monitored through the electroꢀoxidation of glucose for
every 2 days for 30 days and Co3O4/NiO/GCE was stored at inverted beaker at room
temperature. It was found that the Co3O4/NiO/GCE retained about 94.8 % of its original response
to glucose oxidation after a month of storage, which indicates the good stability of the fabricated
sensor system.
3.7. Interference studies
Interference test is the most significant technique in analytical methods, owing to the capability
to distinguish the interfering species from the analyte, which have same physiological
environment in the biosensor. U, UA, AA, DA, AP and NaCl are commonly used interfering
species in the nonꢀenzymatic electrochemical glucose sensors. Fig. 10 reveals the amperometric
response at Co3O4/NiO/GCE toward the addition of 1 mM glucose and 1 mM interfering species
such as U, UA, AA, DA, AP and NaCl in 0.1 M NaOH at an applied potential of 0.5 V
vs. Ag/AgCl. It is clear that Co3O4/NiO/GCE has not exhibited any appreciable current response
toward U, UA, AA, AP and NaCl. Although Co3O4/NiO/GCE has exhibited a minimal current
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response toward DA, the obtained current response is negligible in comparison with glucose.
Thus the proposed sensor is accountable for good selectivity toward the detection of glucose.
3.8. Real sample analysis
The control of diabetic related diseases is purely dependent upon the tight monitoring of glucose
in blood serum. Hence, the practical applicability of fabricated Co3O4/NiO/GCE was evaluated
with the 20 µl of human blood serum in 10 ml of 0.1 M NaOH (pHꢀ13) solution and its
amperometric current responses were monitored at an applied potential of 0.5 V vs. Ag/AgCl.
The known concentrations of glucose were successively added into the blood serum and its
corresponding electrochemical responses were noticed. The fabricated sensor exhibited an
excellent recovery in the range of 97 – 104 % with the 2.67 – 3.36 % relative standard deviation
(Table 2), which guaranteed the potential applications of proposed system in real samples.
The commercially available glucose monitoring systems are available at a cost of 60 to 100
US$, which additionally requires the enzyme coated strips for evaluating the blood glucose
levels and a strip costs between 0.45 to 1.0 US$. Diabetic patients, who are prescribed insulin,
need to check their blood glucose levels anywhere between 3 to 10 times in a day; constructing
the blood glucose monitoring an expensive affair.60 In contrast, the nonꢀenzymatic sensor
reported in this study requires approximately 30 µg of Co3O4/NiO nanofibers and the maximum
cost of the synthesis of nanofibers including high purity materials, electricity costs for
electrospinning and calcination processes furnish a grand total of 0.15 US$ per strip and the
fabricated system can be reused for a couple of times without a drop in performance, which
construct the feasible applications of proposed sensors at a large scale
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4. Conclusion
The three dimensional network of Co3O4/NiO composite nanofibers were synthesized via
electrospinning and subsequent annealing processes. The electrospun nanofibers were highly
porous, interꢀconnected and homogeneously distributed as seen from the morphological analysis
and the prepared nanofibers were successively evaluated for its suitability as the nonꢀenzymatic
glucose sensor probes by using cyclic voltammetry and amperometric techniques. Co3O4/NiO
composite exhibited the superior electrooxidation toward glucose than that of bare NiO
nanofibers and exhibited high sensitivity, wide linear range with low detection limit and has not
been affected by the interferences. It has also demonstrated the high stability, excellent
reproducibility and real sample analysis applications. The good analytical performance, low cost
and facile preparation technique of the prepared electrospun composite Co3O4/NiO nanofibers
pave the ways for simple, effective and highly sensitive glucose biosensors. The fundamental
issues provided in this report will be useful for the construction of onꢀsite glucose sensors and
thereby the diabetes can be controlled for the benefit of a healthy society.
Acknowledgment :
This work was supported by Council of Scientific and Industrial Research (CSIR), New Delhi,
India, major Project Grant No.02(0060)/12/EMRꢀII.
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Fig. 1 SEM images of (a) as spun PVA/(Ni(CH3COO)2.4H2O) nanofibers, (b) as spun
PVA/(Co(NO3)2.6H2O)/(Ni(CH3COO)2.4H2O) nanofibers, (c,d) annealed bare NiO nanofibers
(e,f) annealed Co3O4/NiO composite nanofibers and (g) TEM image of annealed
Co3O4/NiO composite nanofibers.
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Fig. 2 EDAX patterns of (a) bare NiO and (b) Co3O4/NiO composite nanofibers.
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Fig. 3 XRD patterns of (a) bare NiO and (b) Co3O4/NiO composite nanofibers.
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Fig. 4 FTꢀIR spectra of pure (a) NiO and (b) Co3O4/NiO composite nanofibers.
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Fig. 5 The optical band gap energy of (a) bare NiO (Inset: UVꢀvis absorption spectrum
of bare NiO nanofibers) and (b) Co3O4/NiO nanofibers (Inset: UVꢀvis absorption spectrum of
Co3O4/NiO nanofibers).
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Fig. 6 CVs of (a) studied electrodes in 0.1 M NaOH at a scan rate of 20 mV sꢀ1 and (b)
Co3O4/NiO/GCE in 0.1 M NaOH as a function of scan rate ranging from 10 to 100 mV sꢀ1 (Inset:
calibration plot of current vs square root of scan rate).
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Fig. 7 (a) CVs of studied electrodes under the presence of 1 mM glucose in 0.1 M NaOH at a
scan rate of 20 mV sꢀ1 and (b) Mechanism proposed for the electrocatalytic oxidation of glucose
at Co3O4/NiO nanofibers.
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