Sensors and Actuators B 267 (2018) 93–103
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journal homepage: www.elsevier.com/locate/snb
Electrochemical non-enzymatic glucose sensor based on hierarchical
3D Co3 O4 /Ni heterostructure electrode for pushing sensitivity
boundary to a new limit
Hongyan Xu a , Chengkai Xia a , Siyan Wang a , Feng Han a , Mohammad Karbalaei Akbari b ,
Zhenyin Hai b , Serge Zhuiykov a,b,∗
a
b
Department of Materials Science and Engineering, North University of China, Taiyuan 030051, PR China
Ghent University Global Campus, 119 Songdomunhwa-ro, Yeonsu-gu, Incheon 21985, South Korea
a r t i c l e
i n f o
Article history:
Received 8 January 2018
Received in revised form 3 April 2018
Accepted 4 April 2018
Available online 5 April 2018
Keywords:
Non-enzymatic glucose detection
Electrochemical biosensor
Co3 O4 /Ni heterostructure
Sensitivity
a b s t r a c t
In this study, 3D Co3 O4 /Ni heterostructures were synthesized on the porous Ni substrate by hydrothermal method for electrochemical non-enzymatic glucose detection. Both structure and morphology of
fabricated 3D Co3 O4 /Ni heterostructures were characterized by X-ray diffraction (XRD), Fourier transformed infrared (FTIR) and scanning electron microscopy (SEM) techniques. The results showed that
the pure Co3 O4 cluster nanofibers with the diameter of ca. 50 nm have grown layer by layer on the 3D
Ni substrate. They provided high surface-to-volume ratio resulting in the high specific surface area of
17,886 m2 /g. The electrochemical non-enzymatic sensor based on fabricated 3D Co3 O4 /Ni heterostructures enabled the highest sensitivity of 13,855 ␮A mM−1 cm−2 towards glucose reported-to-date. This
sensor also exhibited fast response time (less than 1 s), detection limit as low as 1 ␮M (S/N = 3) and excellent selectivity to glucose at the presence of ascorbic acid, uric acid, sodium chloride, dopamine, cysteine
and d-fructose.
© 2018 Elsevier B.V. All rights reserved.
1. Introduction
Modern medical diagnostic techniques for the glucose presence
in the patient’s blood provide unique requirements towards the
rapid and accurate glucose measurement. These demands have
recently extended not only for diabetic patients, but also for other
non-medical area of interests including the pharmaceutical analysis, food field, biotechnology, and environment monitoring [1–5].
In this regard the development of ultra-sensitive, accurate, inexpensive and fast glucose sensor with the superior selectivity is
very important [6,7]. Notwithstanding the great progress during last decade in fabrication of various types of glucose sensing
devices, including glucose detection by electronic, optical, acoustic,
transdermal and fluorescent technologies [8–15], these analytical methods are usually confronted with several problems. For
instance, the interference of coexisting cationic or anionic species,
tedious detection procedures, long assay time and expensive equipment [16]. On the contrary, the electrochemical sensing technology
∗ Corresponding author at: Ghent University Global
Songdomunhwa-ro, Yeonsu-gu, Incheon 21985, South Korea.
E-mail address: serge.zhuiykov@ugent.be (S. Zhuiykov).
https://doi.org/10.1016/j.snb.2018.04.023
0925-4005/© 2018 Elsevier B.V. All rights reserved.
Campus,
119
has been widely developed for the glucose detection owing to
such advantages as very high sensitivity, simple operation, rapid
response and cost effectiveness [17,18]. However, most of the
enzyme-based glucose biosensors can be seriously affected by environmental conditions such as temperature, pH value, humidity and
toxic chemicals, which could be limiting factors for applications of
enzymatic biosensors [19,20]. In addition, the enzymatic biosensors often suffer from the intrinsic drawbacks such as complicated
immobilization procedures, poor reproducibility, chemical instability, and high cost [21]. Therefore, it is still vital to develop a
simple, highly sensitive and selective, enzyme-free glucose sensor
[22].
Recently, noble metal and various metal oxide based nanomaterials have been developed as non-enzymatic glucose sensors
considering that the electrode material is the most important factor
in determining the sensing performance of non-enzymatic sensors
[23–37]. Among these nanomaterials, cobalt oxide (Co3 O4 ) possesses several advantages such as biological compatibility, wide
band gap, high stability and low cost [38]. That is why it has been
widely explored so far in electrochemical catalysis, supercapacitors and various electrochemical sensors [28,39–44]. Thus, due to
its good reproducibility and selectivity, Co3 O4 has also been utilized in the development of non-enzymatic glucose detectors [45].
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H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
For example, early in 2010 electrospun fabricated Co3 O4 nanofibers
were reported [46], which exhibited a fast response time (<7 s) and
a reasonable sensitivity (36.25 ␮A mM−1 cm−2 ) towards glucose.
Co3 O4 nanotubes were also synthesized by the low temperature
aqueous chemical growth method and demonstrated higher sensitivity of 1089 ␮A mM−1 cm−2 and linear range of 5–40 ␮M [47].
Hence, a variety of fabrication techniques including electrospinning, templated strategy and hydrothermal method have been
developed, which subsequently resulted in various reported morphologies of the Co3 O4 -based nanomaterials such as nanoparticles
[48–50], nanofibers [51,52], nanowires [53], nanotubes [54–56],
nanoporous [57,58], nanoflowers [59] and nanosheets [60].
Furthermore, the electrochemical performance of nanostructured Co3 O4 for the selective glucose detection can be improved
by combining it with other nanomaterials [29]. Recently, laminated 3D-KSCs/Co3 O4 nanoclusters composite fabricated on the
three-dimensional kenaf stem-derived carbon (3D-KSCs) as a substrate showed a high sensitivity of 1377 ␮A mM−1 cm−2 and a wide
linear range of 0.088–7 mM [61]. Graphite/Co3 O4 electrodes have
also exhibited a good performance with a wide linear range of
0.5–16.5 mM due to the improvement of their electrical conductivity [4]. Moreover, by taking the advantages of heterojunctions,
TiO2 /Co3 O4 acicular nanotube arrays fabricated on the fluorine-tin
oxide (FTO) substrate have also demonstrated a high sensitivity of 2008.82 ␮A mM−1 cm−2 within the wide linear range of
0.01–3.0 mM [16].
On the other hand, porous Ni substrate represents an ideal electrode substrate owing to its extensively developed 3D network
structure, high mechanical strength, and high conductivity. It was
recently demonstrated that the Co3 O4 nanostructures could grow
directly on the Ni foam [62]. This approach reduced the system
resistance generated by the adhesive during the electrode assembled by in-situ growth of working materials on the conductive
substrate. At the same time, porous Ni substrate due to its large
specific surface area and fast electronic transfer also contributes to
the improvement in the electrochemical performance [63]. Thus,
some of non-enzyme glucose sensors recently shown one of the
highest sensitivities (12,970 ␮A mM−1 cm−2 ) reported to date [63].
Therefore, in order to boost the recent advancements in sensitivity,
in this study, Co3 O4 hierarchical nanofiber clusters were directly
synthesized on the Ni porous substrate to form 3D Co3 O4 /Ni heterostructure sensing electrode. The electrochemical properties of
the fabricated 3D Co3 O4 /Ni heterostructure electrode were comprehensively investigated thereafter for the non-enzymatic glucose
detection. To the best of our knowledge, the obtained sensitivity
(13,855 ␮A mM−1 cm−2 ) for such 3D Co3 O4 /Ni heterostructure is
the superior glucose sensitivity among the other non-enzymatic
glucose sensors reported to date. It allowed to reach completely
new boundary in the rapid glucose detection and confirmed that the
hierarchical 3D Co3 O4 /Ni heterostructure is indeed a very promising active material for fabrication of non-enzymatic glucose sensors
with excellent sensing capabilities.
2. Experimental section
2.1. Preparation of the Co3 O4 /Ni heterostructure electrode
Cobalt nitrate hexahydrate Co(NO3 )2 ·6H2 O, urea CO(NH2 )2 ,
ammonium fluoride NH4 F, d-(+)-glucose, ascorbic acid (AA), cysteine (Cys), d-fructose (D-F), sodium chloride (NaCl), Dopamine
(DA) and uric acid (UA) were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China) without further treatment.
These chemicals were chosen to imitate the real blood conditions
[5]. Porous Ni substrate (1 mm thick) was purchased from Taiyuan
Lizhiyuan Battery Sales Division (Taiyuan, China). Glucose solution
was prepared before each experiment by dissolving d-(+)-glucose
in deionized water. Deionized water was used throughout all experiments.
3D Co3 O4 /Ni heterojunction sensing electrode was synthesized
by two-step hydrothermal method, schematically presented in
Fig. 1. The first step is to synthesize the precursor layer on the
surface of the Ni substrateand the second one is to fabricate
heterostructure 3D Co3 O4 /Ni sensing electrode with high surfaceto-volume ratio by calcination treatment.
The detailed synthesis processes are as follows: (1) The Ni substrate was cut into pieces with the size of 30 × 10 mm and was
Fig. 1. Schematic diagram of the fabricating process for development of Co3 O4 /Ni heterostructure (For interpretation of the references to color in the text, the reader is
referred to the web version of this article.).
H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
95
sonicated in acetone with subsequent treatment in hydrochloric
acid and deionized water for 10 min. (2) After the cleaning process,
each piece of Ni substrate was placed into 50 mL Teflon lined stainless steel autoclave filled with 30 mL aqueous solution containing
1 mmol Co(NO3 )2 ·6H2 O, 2 mmol CO(NH2 )2 and 5 mmol NH4 F. (3)
The autoclave was put into an oven at 120 ◦ C for 12 h. During this
hydrothermal treatment, Co2 (OH)2 CO3 precursor precipitated on
the porous 3D Ni substrate. The formed heterostructures were
cooled down to the room temperature with their subsequent
removal from the solution. They were sonicated again in the deionized water for 20 min. (4) After that the developed precursor/Ni
heterostructures were placed into a tube furnace and annealed at
450 ◦ C for 3 h. Visual analysis of the fabricated 3DCo3 O4 /Ni heterostructures has confirmed the transformation of their pink color
into the black one.
2.2. Characterzation of heterostructures
The fabricated heterostructures were characterized by D/maxrB X-ray diffraction (XRD) system with a CuK␣ radiation
(␭ = 0.15406 nm). Surface morphologies of the samples and the
elements analysis were studied using Hitachi SU-5000 scanning
electron microscope (SEM) operated at voltage of 20 kV and
equipped with Energy-dispersive X-ray spectroscopy (EDX). The
specific surface areas and the pore-size distribution of the samples were obtained from the results of N2 adsorption–desorption
isotherms at 77 K after using Brunauer-Emmet-Teller (BET)
and Barrett-Joyner-Halenda (BJH) (JW-BK122F, China). Fourier
transformed infrared (FTIR) spectra were obtained on a FTIR8400S spectrophotometer (Shimazu, Japan) in the range of
3000–500 cm−1 . The thermal decomposition behavior of the precursor, scraped from the Ni substrate, was also analyzed by the
thermogravimetric analyzer (Setaram Labsys Evo, France) with a
flow rate of nitrogen of 20 mL/min and the heating rate of 2 ◦ C/min.
The sensing performance towards glucose concentration was
carried out on the CHI 660E electrochemical workstation (Shanghai,
China). This workstation was employed to perform all electrochemical measurements including cyclic voltammetry (CV) and
amperometric experiments (I-t). CVs were recorded for the different glucose concentrations in a stationary solution, whereas the
I-t measurements were performed under agitation with a magnetic mixer. In all these experiments, a common three-electrode
system was used with 3D Co3 O4 /Ni heterostructure electrode,
platinum sheet and saturated calomel electrode (SCE) acting as
working electrode, auxiliary electrode and reference electrode,
respectively. All electrochemical measurements were conducted
in 1.0 M NaOH aqueous solution at room temperature with different concentration of glucose to mimic the in vitro test of the blood
glucose. Electrochemical impedance spectroscopy (EIS) tests were
carried out between 0.01 Hz–100 KHz in 0.1 M NaOH solution on a
PGSTAT128N Autolab electrochemical workstation. The selectivity
of the fabricated 3D Co3 O4 /Ni heterostructures against AA, Cys, D-F,
NaCl, DA and UA was also investigated. Considering that the ratio of
glucose and interfering agents in human serum is more that 30:1 at
a physiological level [63], the selectivity measurements were conducted in 1 M NaOH solution by a successive injections of 0.1 mM
and 0.01 mM of other interfering agents at 0.5 V.
3. Results and discussion
3.1. Characterization of 3D Co3 O4 /Ni heterostructures
Fig. 2 depicts the XRD patterns of the as-prepared precursor/Ni
substrate and the finally obtained 3D Co3 O4 /Ni heterostructure.
Three strong diffraction peaks of Ni (JCPDS no. 03-1051) were
Fig. 2. XRD patterns of as-synthesized precursor/Ni substrate (a) and 3D Co3 O4 /Ni
heterostructures (b).
observed at 2␪ = 44.83◦ , 52.23◦ and 57.01◦ in both Fig. 2a and b. In
addition, several characteristic peaks at 2␪ = 28.16◦ , 29.08◦ , 30.71◦ ,
38.78◦ and 32.35◦ were clearly observed in Fig. 2a, which could
be well indexed to the (400), (401), (012), (501) crystal planes
of Co2 (OH)2 CO3 (JCPDS no. 48-0083) and (104) crystal planes of
CoCO3 (JCPDS no. 01-1020), respectively [44]. The XRD pattern, presented in Fig. 2b, provided several characteristic peaks of Co3 O4 at
2␪ = 19.0◦ , 31.27◦ , 36.85◦ , 49.08◦ and 55.65◦ , which are well indexed
to the (111), (220), (311), (331) and (422) crystal planes of Co3 O4
(JCPDS no. 43-1003), respectively [44]. The XRD analysis indicates
that the black powder on the nickel foam is pure Co3 O4 .
The measured FTIR spectra of both the precursor/Ni substrate and 3D Co3 O4 /Ni heterostructures are presented in Fig. 3a
and b, respectively. Specifically, the O H stretching vibration
at 3570 cm−1 and 3420 cm−1 and the OCO2 stretching vibration at 1647 cm−1 are observed in Fig. 3a. The absorption peaks
at 1469 cm−1 and 733 cm−1 originated from the vibration of
CO3 2− . The absorption peaks in perturbation area at 987 cm−1 and
511 cm−1 are attributed to the stretching vibration of Co O and
Co OH, respectively. In addition to XRD data, these results confirmed the development of Co2 (OH)2 CO3 and CoCO3 on the porous
Ni substrate. Moreover, the characteristic adsorption peaks at
668 cm−1 and 579 cm−1 presented in Fig. 3b were assigned to
Co O and the peaks at 2344 cm−1 , 1653 cm−1 , and 1552 cm−1 were
attributed to H2 O and CO2 . This indicates the complete transformation of pure Co3 O4 from the Co2 (OH)2 CO3 precursor, as
demonstrated in the XRD pattern.
Fig. 4 shows the thermal gravimetric analysis (TGA) curve of
precursor Co2 (OH)2 CO3 with evident weight-loss stages. The first
weight loss occurred from 250 to 350 ◦ C and resulted from the
decomposition of Co2 (OH)2 CO3 into Co3 O4 , and the second weight
loss was observed from 350 to 430 ◦ C. It was caused by the conversion of CoCO3 into Co3 O4 . These data also reaffirmed the existence
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H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
Fig. 3. FTIR spectra for as-synthesized Co2 (OH)2 CO3 /Ni (a) and 3D Co3 O4 /Ni heterostructures (b).
precipitated on its surface (Fig. 5b). Fig. 5c and d shows the
morphologies of Co3 O4 nanofiber clusters. These images clearly
displayed the morphology transformation of the Co3 O4 from its
precursor during the annealing process. Noteworthy, the Co3 O4
flower-like clusters grew layer by layer on the Ni substrate (Fig. 5d)
and these flower-like clusters are formed by the agglomeration of
a large number of the needle-like nanofibers with the diameter of
approximately 50 nm (Fig. 5e). Further SEM investigation of these
needle-like nanostructures at the higher magnification revealed
that the developed Co3 O4 nanofibers possess loose pores apparently caused by the escape of H2 O and CO2 during the annealing
process (Fig. 5f). Moreover, they consist of various nano-grains
with the average size of approximately 10–20 nm and have high
surface-to-volume ratio, which is absolutely essential for ensuring
high sensitivity of the sensor [64,65]. In addition, Fig. 5g shows the
EDX spectrum of the developed 3D Co3 O4 /Ni heterostructure. The
spot was chosen from Fig. 5d. The EDX spectra also confirmed the
presence of Co, O, and Ni (the substrate) only in the 3D Co3 O4 /Ni
heterostructure. No impurity peaks were observed.
Fig. 6 provides information about the nitrogen adsorptiondesorption isotherms and BJH curves of the synthesized
Co2 (OH)2 CO3 /Ni and 3D Co3 O4 /Ni heterostructures. The specific surface areas of both the Co2 (OH)2 CO3 /Ni and 3D Co3 O4 /Ni
heterostructures calculated from Fig. 6a were 8544 m2 /g and
17,886 m2 /g, respectively. It can be clearly seen from the pore
size distribution (Fig. 6b) that the pore diameters of Co2 (OH)2 CO3
surface are mainly concentrated within 2–10 nm range whereas
the 3D Co3 O4 /Ni heterostructure has pore size distribution in
wider range of 2–30 nm. Consequently, considering these data
combined with the SEM analysis, it can be concluded that the
release of CO2 and H2 O during annealing process caused fabrication of unique nanostructures with a large number of pores and
significantly enhanced the specific surface area of the 3D Co3 O4 /Ni
heterostructure. In fact, large mesoporous size is beneficial to the
electrons transfer and the high surface-to-volume ratio provided
more active sites for the electrochemical reaction. Thus, this
unique nanostructure can ensure substantial positive effect on the
glucose detection.
3.2. Electrochemical behavior of 3D Co3 O4 /Ni heterostructure
electrode
Fig. 4. TGA measurements of Co2 (OH)2 CO3 powder.
of Co2 (OH)2 CO3 and CoCO3 in the precursor. Therefore, during
the heating process the Co2 (OH)2 CO3 initially reacted with O2 in
air at the temperature of 250–350 ◦ C to produce Co3 O4 , CO2 and
H2 O. Then as the temperature increased further, the CoCO3 was
subsequently oxidized by O2 to Co3 O4 and CO2 at 350–430 ◦ C in
accordance with Eqs. (1) and (2). After 430 ◦ C, the weight was kept
constant, which indicated the full conversion of the precursor to
Co3 O4 [44]. Based on TGA results, 450 ◦ C was subsequently chosen
to be the annealing temperature in this study.
3Co2 (OH)2 CO3 + O2 = 2Co3 O4 + 3CO2 + 3H2 O
(1)
6CoCO3 + O2 = 2Co3 O4 + 6H2 O
(2)
SEM images of the different samples of porous Ni substrate,
Co2 (OH)2 CO3 precursor precipitated on the Ni substrate and 3D
Co3 O4 nanofiber clusters grown on Ni substrate are summarized in
Fig. 5. Compared to the smooth and flat surface of the porous Ni
substrate (Fig. 5a), Co2 (OH)2 CO3 was homogeneously and evenly
CVs of electrochemical response of glucose sensor based on
3D Co3 O4 /Ni heterostructure electrode were recorded at different scan rate from 10 to 80 mV s−1 in 0.1 M NaOH solution. Two
pairs of redox peaks caused by the electrochemical redox reactions
between the Co3 O4 and OH− ions are clearly presented in the CV
curves for the 3D Co3 O4 /Ni heterostructure (Fig. 7a). One pair at the
lower potential is caused by the reversible transition between the
Co3 O4 and CoOOH (Eq. (3)) and the other pair at the higher potential is aroused by the redox reaction between CoOOH and CoO2
(Eq. (4)). The closed CV curves indicated rapid and reversible redox
reactions of the active materials (Eqs. (3) and (4)). As the scan rate
increased from 10 mV s−1 to 100 mV s−1 , the anodic peak shows
a notable positive shift, the cathodic peak shifts negatively. This
indicates that the reactions of Eqs. (3) and (4) are quasi-reversible
[1,3,4,9,16,66].
In Fig. 7b, the peak current is proportional to the square root
of scan rate, which indicated that the redox reaction of the 3D
Co3 O4 /Ni heterostructure is controlled by the diffusion process of
the glucose from the bulk solution/viscous layer interface to the
viscous layer/Co3 O4 surface interface [55]. Fig. 7c illustrates the CV
curves for the 3D Co3 O4 /Ni heterostructure obtained before and
after addition of 0.4 mmol glucose at the scan rate of 20 mV s−1 .
Significant increase in the response current was observed upon the
glucose addition to the solution, which reaffirmed that the fabri-
H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
97
Fig. 5. SEM images of the morphologies of porous Ni substrate (a), Co2 (OH)2 CO3 precipitated on its surface at the low magnification (b), various Co3 O4 nanofiber clusters (c)
and (d), respectively, needle-like Co3 O4 nanofiber (e), high SEM magnification of the Co3 O4 nanofibers (f) and EDX measurement of fabricated 3D Co3 O4 /Ni heterostructure.
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H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
cated 3D Co3 O4 /Ni heterotructure responded well to the increase of
glucose concentration. In alkaline solution, Co3 O4 is firstly oxidized
to CoOOH at the lower potential about 0.2 V vs SCE and then to CoO2
at the higher potential, as shown in Eqs. (3) and (4), respectively.
According to Eq. (5), when the glucose is present in the reaction
system, it is oxidized to the gluconolactone under the catalysis of
hierarchical CoO2 . With the reaction processing, CoO2 consumption and the CoOOH accumulation accelerated forward (Eq. (4))
and backward reactions (Eq. (3)), causing the increasing of the oxidization current at the higher potential and reduction current at
the lower potential, respectively [67]. As a result, an excellent current response to the presence of glucose was observed, as shown in
Fig. 7c. Similar mechanism was suggested for the glucose detection
by Pt- and CuO-based sensing electrodes [68,69].
Co3 O4 + OH− + H2 O ↔ 3CoOOH + e−
−
CoOOH + OH ↔ CoO2 + H2 O + e−
(3)
(4)
2CoO2 + C6 H12 O6 (glucose) → 2CoOOH
+ C6 H10 O6 (gluconolactone)
Fig. 6. Nitrogen adsorption-desorption isotherms (a) and BJH (b) curves for
Co2 (OH)2 CO3 /Ni and 3D Co3 O4 /Ni heterostructures.
(5)
Furthermore, EIS was performed to understand electrooxidation of glucose on 3D Co3 O4 /Ni heterostructure electrode.
Nyquist plot of the electrode measured at 0.5 V, where the electrooxidation of glucose occurs, in the frequency range of 0.01–105 Hz
in 0.1 M NaOH solution with 0–1 mM glucose as shown in Fig. 7d.
The impedance plot exhibited a small semicircle in the highfrequency region and a straight line in the low-frequency region. It
Fig. 7. CV curves for 3D Co3 O4 /Ni heterostructure at the different scan rates (a). The peak currents dependence on the square root of the scan rate (b). CV curves for fabricated
3D Co3 O4 /Ni heterostructure before and after glucose injection (c). Nyquist plots in the frequency range 0.01–105 Hz in 0.1 M NaOH solution at the absence and presence of
1 mM of glucose. Randles equivalent circuit model is shown in insert.
H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
99
the sample and the interval between two measurements was ∼4 h.
There was no significant deviation among the three testing results,
which suggested good reproducibility of the 3D Co3 O4 /Ni heterostructure based glucose sensor. It can be seen that the fabricated
3D Co3 O4 /Ni heterostructure sensing electrode has a wide linear
response range up to 3.6 mM of glucose with the related coefficient
of 0.9956. When the concentration of glucose in 1 M NaOH solution is lower than 3.6 mM, the reactions on the sensor are totally
controlled by diffusion process and the amperometric response
increases linearly with the increasing of glucose concentration. The
linear relationship between the current density and the measured
glucose concentration can be calculated by the following equation:
I(A) = 0.01425 + 0.02771·C (mM). Considering that S is the slope of
the linear fitting equation and A is the specific surface area of the
developed 3D Co3 O4 /Ni heterostructure sensing electrode, the sensor’s sensitivity was calculated by the following equation:
Sensitivity = S/A
Fig. 8. Amperometric responses of the glucose sensor based on 3D Co3 O4 /Ni heterostructure sensing electrode to the different glucose concentrations at the room
temperature (a). Insert is the sensor response time to the change of glucose concentration. The measured data and the calibration curve of the sensor’s amperometric
response to the glucose concentration (b). Each concentration was repeated three
times and the error bars indicate the deviation.
was observed that Rct at higher frequency region for 3D Co3 O4 /Ni
heterostructure electrode is decreased after adding glucose to
the solution, suggesting the higher electron transfer from the
electrolyte solution to the current collector, hence causing the
captivating sensing performance. Moreover, the straight line in
the low-frequency region denoted Warburg impedance, which
indicated the diffusion controlled process in the electrochemical
process [70]. The EIS results effectively support the analysis of CVs
in the aspect of dynamics CVs at various scan rates were recorded in
alkaline solution to judge the kinetics of 3D Co3 O4 /Ni heterostructure electrode (Fig. 7a). As depicted in Fig. 7b, the anodic and
cathodic peak currents are proportional to the square root of scan
rates, demonstrating a typical diffusion-controlled dynamic process. Furthermore, no significant positive/negative shift is observed
for anodic/cathodic peak, implying unimpeded diffusion kinetics
originated from the heterostructure electrode.
Fig. 8a shows the representative amperometric responses of
the glucose sensor based on 3D Co3 O4 /Ni heterostructure sensing electrode to the different glucose concentrations at the room
temperature. The sensing potential was determined to be 0.5 V vs.
SCE from the CV curves. The insert depicts the sensor’s amperometric response time of less than 1 s to the injection of 0.20 mM of
glucose to the solution. Clear linear relationship between the measured glucose concentration and the sensor’s current was recorded
at the low measuring concentration range. The measured data and
the calibration curve of the sensor’s amperometric response to
the glucose concentration are plotted in Fig. 8b. The chronoamperometry measurements were repeated at least three times on
(6)
very
high
glucose
sensitivity
of
Remarkably,
13,855 ␮A mM−1 cm−2 was obtained. As concentration of glucose increased higher than 3.6 mM, the reactions are controlled
by both diffusion process and charge-transfer process, and the
appropriate amperometric response increases slowly and deviates
from the linear rule. With further increasing of glucose concentration, the charge-transfer process becomes dominated, and
when the glucose concentration reaches to the saturation of the
sensor, the reactions are totally controlled by the charge-transfer
process, and the corresponding amperometric response becomes
constant. In addition, a limit of detection (LOD) of 1 ␮M (S/N = 3)
was also interpreted from Fig. 8. The electrochemical properties of
Co3 O4 -based glucose sensors at the different NaOH concentrations
(0.1 M, 0.3 M, 0.7 M, 1.0 M, 3.0 M) were tested and the results are
presented in Supplementary section (Fig. S1). In 0.1–3.0 M NaOH
solutions, 3D Co3 O4 /Ni heterostructure-based sensor works well
and the amperometric response increases with the increasing of
NaOH concentration.
Table 1 summarizes the main presented characteristics of the
different non-enzymatic glucose sensors reported to date in comparison with the obtained characteristics of the glucose sensor
based on fabricated 3D Co3 O4 /Ni heterostructure sensing electrode in this work. It appear to indicate that the developed glucose
sensor based on 3D Co3 O4 /Ni heterostructure sensing electrode
exhibits the highest sensitivity of 13,855 ␮A mM−1 cm−2 at the
room temperature, fast response time of less than 1 s and wide linear response range to glucose among all sensors reported to date.
These excellent characteristics could be attributed to two main factors. One is peculiar morphology of the fabricated 3D Co3 O4 /Ni
heterostructure with very high surface-to-volume ratio and a large
number of meso-pores. This hierarchical structure provided high
specific surface area and abundant catalytic sites for electrochemical reactions. The other factor is the low internal resistance of
the sensing electrode stipulated by in-situ growth of the active
Co3 O4 nanofibers on 3D porous Ni substrate, which can ultimately
improve the electrons flow rate during redox reaction [59,77,78].
The high sensitivity and fast response to glucose were mainly
due to the high specific surface area and mesoporous size of the
fabricated 3D Co3 O4 /Ni heterostructure electrode. The redox reaction on the 3D Co3 O4 /Ni heterostructure is a diffusion-controlled
process, i.e., the electrochemical reaction is controlled by the concentration of reactants on the electrode. According to the Fick’s law,
the diffusion can be represented by the following equation:
ds = −DF
dc
· dt
dx
(7)
where dt is the diffusion time, ds is the diffusion amount of reactant in dt time, F the diffusion area, dx is the surface state, dc/dx
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H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
Table 1
Comparison of the sensor’s characteristics of glucose sensor based on 3D Co3 O4 /Ni heterostructure in this work with other reported electrodes for glucose detection.
Samples
Linear range (mM)
Sensitivity (␮A mM−1 cm−2 )
Detection limit (␮M)
Applied potential (V)
Response time (s)
Reference
Hierarchical Co3 O4 /Ni
Co3 O4 /3D-KSCs
NiCo2 O4 /rGO
Co3 O4 /Zn
Co3 O4 UNSs -Ni(OH)2
Co3 O4 UHMSA
Co3 O4 nanosheets
Nest-like Co3 O4
TiO2 /Co3 O4 ANTAs
Co3 O4 nanofibers
Co3 O4 @graphene
Ni@PSI
Co3 N NW/TM
Cu/CNT
NA/NiONF-rGO/GCE
GF/Co3 O4 -NPs/GCE
Ni(OH)2 NPs/Ni foam
CoOOH nanosheet
Co–CuNP
NCoNWs
Mn3 O4 NPs/N-GR
HAC/NiO nanocomposite
Co3 O4 NF/GOH
Ni NF
Ni(OH)2 /TiO2
BaTiO3 film
CuO NS/CC
Co3 O4 /ZnOp-n junction
NiO nanosheet
Ni NPs/TiO2 NWAs
NiO-HAC
GCE-Graphene-Ni
MoS2 -NiCo2 O4
NiCo2 O4 /ECF
GOx/PEDOT-MSs-PtNPs/SPE
Co-CoO-Co3 O4
Sn:Co3 O4
Au@Cu2 O
Au NCs/GCE
Fe@Pt/C
Pt0.7 Co0.3 /C
POT-Au/Cu-G
NiO/Pt/ERGO/GCE
0.04–3.6
0.088–7
0.04–1.28
0.005–0.62
0.005–0.04
0.1–5
Up to 0.31
0.02–0.4
0.01–3.0
Up to 2.04
0.02–8
0.002–5
0.0001–2.5
0.0005–1.8
0.002–0.60
Up to 0.2115
Up to 6.0
0.003–1.109
Up to12
0.005–0.57
0.0025–0.5295
0.005–4.793
0.25–10
0.005–0.7
0.03–14
0.0001–1
Up to 1
0.01–5
0.001–0.4
0.001–7
0.01–3.3
0.001–1.15
0.001–1.6
0.005–19.175
0.1–10
0.002–6.06
0.0006–5.5
0.05–2.0
1–9
1–16
0.10–14.20
1–30
0.002–5.66
13,285
1377
2082.57
193
1089
102.77
12,970
929
2008.82
36.25
628
21.73
3325.6
602.04
1100
1140
1950.3
526.8
4651
300.8
1011
1721.5
492.8
2370
192
23.79
4901.96
116.64
1138
50.97
199.86
2213
1748.58
1947.2
116.25
949.3
921
715
2131
11.75
73.60
37
668.2
1.0
26
2.0
2.0
1.08
1.84
0.058
1.0
0.3396
0.97
0.038
0.2
0.05
0.1
0.77
0.06
0.16
1.37
0.6
5.0
1.0
0.055
–
5.0
8.0
7.941
1.0
1.38
0.18
0.18
1
0.1
0.152
1.5
1.55
0.58
0.1
18
100
300
30
0.027
0.2
+0.50
+0.50
+0.55
+0.52
+0.35
+0.35
+0.50
+0.60
+0.50
+0.59
+0.55
+0.55
+0.
+0.55
+0.
−0.48
+0.45
+0.52
+0.60
+0.60
+0.90
+0.50
+0.62
+0.55
+0.50
+0.50
+0.55
+0.25
+0.60
+0.40
+0.55
+0.60
+0.418
+0.55
+0.60
+0.55
+0.55
–
+0.23
−0.15
−0.10
+0.1
+0.60
<1
–
5
<7
5
3
<10
0.5
<5
<7
4
<3
–
–
–
–
–
–
–
–
–
–
<8
–
<1
–
–
2
–
–
5
1
–
–
5–7
<1
<4
–
–
–
–
–
2.5
This work
[61]
[71]
[29]
[47]
[59]
[63]
[72]
[55]
[46]
[57]
[1]
[73]
[2]
[3]
[4]
[5]
[7]
[8]
[9]
[10]
[11]
[28]
[12]
[16]
[17]
[18]
[45]
[20]
[30]
[32]
[33]
[35]
[36]
[74]
[75]
[76]
[21]
[23]
[24]
[25]
[26]
[27]
the concentration gradient, D the diffusion coefficient, the negative sign indicates that the direction of flow is from high to low
concentration.
At a certain temperature with the particular glucose concentration, D and dc/dx are constant, and F crucial for the concentration
of reactants. The higher diffusion area will result in more reactants participating into the reaction. The fabricated 3D Co3 O4 /Ni
heterostructure electrode with high specific surface area caused
by hierarchical Co3 O4 /Ni nanofiber clusters on the 3D Ni substrate
greatly improved the diffusion area, and consequently, the diffusion velocity of reactant (ds/dt) is increased. With the help of wide
migration of electrons channel caused by the meso-pores, high
and fast current density is generated. Therefore, the fabricated 3D
Co3 O4 /Ni heterostructure electrode was able to provide high sensitivity and fast response to glucose.
When the glucose concentration was higher than 3.6 mM, the
slope of the fitting curve is significantly reduced, indicating a
weaker sensitivity of the 3D Co3 O4 /Ni heterostructure electrode
to the higher glucose concentration. This is caused by the different electrochemical kinetics. When the concentration of glucose in
alkaline solution is increased, the glucose diffusion rate increased
gradually and exceeded its consumption rate at the electrode surface, and as a result, the electrochemical reaction rate is gradually
limited by the rate of charge flow. Thus, the response current cannot increase linearly at the higher glucose concentration. Therefore,
it can be concluded that the preparation of the sensing electrode
with high surface-to-volume ratio is one of the key factors for high
performance of the non-enzymatic glucose sensor [63].
Considering that the selectivity of any glucose sensor is also one
of the main sensing characteristics, the fabricated sensor based on
3D Co3 O4 /Ni heterostructure sensing electrode was investigated
against the AA, UA, NaCl, DA, Cys and D-F. According to the Ref. [63],
the ratio of glucose and interferences in the human serum is more
than 30:1 at a physiological level. Thus, in order to demonstrate the
selectivity of the glucose sensor in human blood, the higher concentration ratio for glucose and follow interferences should be used for
the interference test. Therefore, the experiments were conducted
in 1 M NaOH solution by the successive following injections of
0.1 mM glucose and 0.01 mM of the different interfering substances
at 0.50 V. Fig. 9 represents the sensor’s selectivity measurements at
the room temperature. The typical amperometric response of the
sensor based on the developed 3D Co3 O4 /Ni heterostructure sensing electrode is observed by injection of 0.1 mM glucose into the
test solution, as depicted in Fig. 9. After that, all interfering agents
were subsequently injected into the text solution. Nevertheless,
very negligible sensor’s response was observed upon their injection
confirming the fact that the 3D Co3 O4 /Ni heterostructure sensing
electrode has an excellent stability against all interfering agents. In
alkaline solution, there are two redox reactions between the Co3 O4
and CoOOH (Eq. (3)) and between CoOOH and CoO2 (Eq. (4)) as
H. Xu et al. / Sensors and Actuators B 267 (2018) 93–103
101
Acknowledgments
This project was supported by the Key Program Project of Shanxi
Province of China (No. 201703D421008), National Natural Science
Foundation of China (No. 61501408). S.Z. acknowledges the support
from the “100 Talents Program” of Shanxi province, P.R. China.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.snb.2018.04.023.
References
Fig. 9. Amperometric response curve of the glucose sensor based on 3D Co3 O4 /Ni
heterostructure sensing electrode to the addition of interfering agents: AA, DA, D-F,
NaCl, Cys, UA and glucose in the text solution.
discussed above. Glucose can be oxidized by CoO2 to gluconolactone (Eq. (5)), which accelerates the forward reaction of Eq. (4), and
consequently, increases anodic current at the applied potential of
+0.5 V. Although many of the above-mentioned disruptors are also
electrochemically active, they do not react with Co3 O4 , CoO2 and
CoOOH (as proved in Fig. 9), indicating excellent anti-interference
performance. The outstanding selectivity could be attributed to the
electrostatic repelling effect between 3D Co3 O4 /Ni heterostructure
sensing electrode and interfering species. 3D Co3 O4 /Ni heterostructure sensing electrode would be negatively charged in 0.1 M NaOH
because the pH of electrolyte is above the isoelectric point of Co3 O4
[79]. In addition, the major interfering species (AA and UA) are
easy to lose protons in alkaline solution and possess a negatively
charged shell [80]. The electrostatic repulsion between the interferent and 3D Co3 O4 /Ni heterostructure sensing electrode leads
to improved selectivity. Furthermore, the negatively charged shell
allows penetration and diffusion of glucose. Consequently, the electrostatic repulsion between the shell of interfering agent and the
3D Co3 O4 /Ni heterostructure electrode would lead to the improved
selectivity. Hence, the presence of these interfering agents has no
effect on the glucose detection. The results obtained confirmed that
the developed glucose sensor based on 3D Co3 O4 /Ni heterostructure sensing electrode in this work has high selectivity towards
glucose.
4. Conclusion
3D Co3 O4 /Ni heterostructures were synthesized on the
porous Ni substrate by hydrothermal method for electrochemical
non-enzymatic glucose measurement. Materials characterization
techniques have revealed that the fabricated 3D Co3 O4 /Ni heterostructures consist of pure Co3 O4 cluster nanofibers with
diameter of approximately 50 nm, which have been grown layer
by layer on the porous 3D Ni substrate. They provided high
surface-to-volume ratio resulting in the high specific surface area
of 17,886 m2 /g. The electrochemical non-enzymatic sensor based
on fabricated 3D Co3 O4 /Ni heterostructures exhibited the highest
sensitivity of 13,855 ␮A mM−1 cm−2 towards glucose among the
non-enzymatic glucose sensors reported-to-date. This sensor has
also demonstrated fast response time (less than 1 s), detection limit
as low as 1 ␮M (S/N = 3) and an excellent selectivity to glucose at
the presence of ascorbic acid, uric acid, sodium chloride, dopamine,
cysteine and d-fructose. Thus, current investigation confirms that
the 3D Co3 O4 /Ni heterostructure is very promising material for
high-performance non-enzymatic glucose sensors.
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Biographies
Hongyan Xu received her Ph.D. degree from North University of China in 2009.
She worked as a short-term researcher at Technische Universität Berlin and the
University of Oxford. She is currently a Professor in School of Materials Science and
Engineering, North University of China. Her research interests focus on the synthesis
of functional nanomaterials and their optical and electro-chemical applications.
Chengkai Xia received the bachelor’s degree from the North University of China
in 2016. He is currently working toward his master degree at School of Materials
Science and Engineering of the North University of China. His research focuses on
nano metal oxides and their electrochemical behavior.
Siyan Wang received her Ph.D. degree from University of Chinese Academy of Sciences, China in 2016. She is now an assistant professor in School of Materials Science
and Engineering, North University of China. Her major research interests are material development and device design for photocatalytic and photoelectrochemical
water splitting using solar energy.
103
Feng Han received his bachelor’s degree from the North University of China in 2017.
He is now a master student in the School of Materials Science and Engineering, North
University of China. His main research field is nanomaterials and their versatile
applications.
Mohammad Karbalaei Akbari received his M.Sc. degree in Materials Science and
Engineering from the Semnan Province University, Iran in 2012. He was recipient of
several financial supports by Iran nanotechnology initiative council for his activities
in nanoscience. Mohammad is currently working toward his Ph.D. degree at the
Department of Applied Analytical and Physical Chemistry of the Ghent University
Global Campus, Korea. His recent research activities in the Environmental & Energy
Research Center focus on Atomic Layer Deposition of metal oxide semiconductors
and two-dimensional nano-materials.
Zhenyin Hai received his M.Sc. degree from the North University of China in 2016.
He is currently working toward his Ph.D. degree at the Department of Applied Analytical & Physical Chemistry of the Ghent University Global Campus, Korea. His
main researches focus on two dimensional materials, nanostructured semiconductor oxides and their versatile applications.
Serge Zhuiykov received his Ph.D. in Materials Science and Engineering in 1991.
He has more than 26 years of combined academic and industrial experience working at the different universities in Australia, Japan, Korea and Europe and industrial
environments. Since 2015 he is a Senior Full Professor at the Department of Applied
Analytical & Physical Chemistry of the Ghent University Global Campus, Korea and a
Director of Environmental & Energy Research Center. His research interests lie in the
area of the development, design and fabrication of new two-dimensional nanomaterials for solid-state environmental sensors and other advanced functional devices.
He has published more than 230 peer-reviewed scientific publications including 3
monographs in 2007, 2014 and 2018, respectively. He is a recipient of the 2007, 2011,
2013 Australian Academy of Science/Japan Society for Promotion of Science and
2010 Australian Government Endeavour Executive Awards for his work on nanomaterials for chemical sensors. In 2017, he was selected as one of recipients very
prestigious “100 Talents” Program of the Shanxi Province, P.R. China.