Nickel oxide decorated MoS2 nanosheet-based non-enzymatic sensor for the selective detection of glucose

Article abstract of DOI:10.1039/c9ra09318d

Understanding blood glucose levels in our body can be a key part in identifying and diagnosing prediabetes. Herein, nickel oxide (NiO) decorated molybdenum disulfide (MoS₂) nanosheets have been synthesized via a hydrothermal process to develop a non-enzymatic sensor for the detection of glucose. The surface morphology of the NiO/MoS₂ nanocomposite was comprehensively investigated by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer-Emmett-Teller (BET) analysis. The electro-catalytic activity of the as-prepared NiO/MoS₂ nanocomposite towards glucose oxidation was investigated by cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and amperometry in 0.1 M NaOH. The NiO/MoS₂ nanocomposite-based sensor showed outstanding electrocatalytic activity for the direct electro-oxidation of glucose due to it having more catalytic active sites, good conductivity, excellent electron transport and high specific surface area. Meanwhile, the NiO/MoS₂ modified glassy carbon electrode (GCE) showed a linear range of glucose detection from 0.01 to 10 mM by amperometry at 0.55 V. The effect of other common interferent molecules on the electrode response was also tested using alanine, l-cysteine, fructose, hydrogen peroxide, lactose, uric acid, dopamine and ascorbic acid. These molecules did not interfere in the detection of glucose. Moreover, this NiO/MoS₂/GCE sensor offered rapid response (2 s) and a wide linear range with a detection limit of 1.62 μM for glucose. The reproducibility, repeatability and stability of the sensor were also evaluated. The real application of the sensor was tested in a blood serum sample in the absence and presence of spiked glucose and its recovery values (96.1 to 99.8%) indicated that this method can be successfully applied to detect glucose in real samples.

Full text of DOI:10.1039/c9ra09318d

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PAPER
Nickel oxide decorated MoS2 nanosheet-based
non-enzymatic sensor for the selective detection of
glucose†
Cite this: RSC Adv., 2020, 10, 643
Gayathri Jeevanandham,‡ R. Jerome,‡ N. Murugan, M. Preethika,
*
Kumaran Vediappan
and Ashok K. Sundramoorthy
Understanding blood glucose levels in our body can be a key part in identifying and diagnosing prediabetes.
Herein, nickel oxide (NiO) decorated molybdenum disulfide (MoS2) nanosheets have been synthesized via
a hydrothermal process to develop a non-enzymatic sensor for the detection of glucose. The surface
morphology of the NiO/MoS2 nanocomposite was comprehensively investigated by field-emission
scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM),
powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller
(BET) analysis. The electro-catalytic activity of the as-prepared NiO/MoS2 nanocomposite towards
glucose oxidation was investigated by cyclic voltammetry, electrochemical impedance spectroscopy
(EIS) and amperometry in 0.1
M NaOH. The NiO/MoS2 nanocomposite-based sensor showed
outstanding electrocatalytic activity for the direct electro-oxidation of glucose due to it having more
catalytic active sites, good conductivity, excellent electron transport and high specific surface area.
Meanwhile, the NiO/MoS2 modified glassy carbon electrode (GCE) showed a linear range of glucose
detection from 0.01 to 10 mM by amperometry at 0.55 V. The effect of other common interferent
molecules on the electrode response was also tested using alanine, L-cysteine, fructose, hydrogen
peroxide, lactose, uric acid, dopamine and ascorbic acid. These molecules did not interfere in the
detection of glucose. Moreover, this NiO/MoS2/GCE sensor offered rapid response (2 s) and a wide linear
range with a detection limit of 1.62 mM for glucose. The reproducibility, repeatability and stability of the
sensor were also evaluated. The real application of the sensor was tested in a blood serum sample in the
absence and presence of spiked glucose and its recovery values (96.1 to 99.8%) indicated that this
method can be successfully applied to detect glucose in real samples.
Received 9th November 2019
Accepted 11th December 2019
DOI: 10.1039/c9ra09318d
rsc.li/rsc-advances
concentration in biological samples (on an everyday basis) such
as blood, urine, etc. Generally, conventional methods such as
uorometry,5 colorimetric,6 spectro-photometric,7 optical,8
1. Introduction
Glucose is one of the important metabolic intermediate
components that can be used to generate energy in the human
body. The normal glucose concentration in human blood is in
the range of 4.4–6.6 mM. If the glucose levels exceed above or
below the normal range, this may lead to metabolic disorders
(e.g., diabetes). Glucose levels below 2.8 mM cause hypogly-
cemia.1,2 The number of adults affected by type 2 diabetes is
expected to increase to 511 million in 2030.3 Specically, as
stated by WHO, India had about 69.2 million people with dia-
betes by 2015, which is also expected to rise over the next 12
years.4 So, it is critically important to accurately detect glucose
acoustic9 and uorescence10 methods have been used to detect
glucose. Meanwhile, electrochemical sensors have been re-
ported, with high selectivity and sensitivity to detect glucose.11,12
Specically, electrochemical biosensors based on glucose
dehydrogenase and glucose oxidase (GOD) have been exten-
sively studied due to their high selectivity and sensitivity
towards glucose in medical research.13,14 However, there are
some critical disadvantages associated with the preparation of
enzymatic biosensors. For example, instability of enzymes
(GOD) on the sensor surface due to environmental factors (pH
and temperature), which can deteriorate the reproducibility and
reliability of sensors.15 In addition, GOD and glucose dehydro-
genase enzymes are relatively expensive. To overcome these
issues, non-enzymatic glucose sensors have been developed for
practical applications.16–18 To develop a non-enzymatic glucose
sensor, a large number of transition metal (Pt, Ni, Au, Co, Ru,
Cu, In), transition metal oxide (NiO, WO3, RuO2)19,20 and metal
Department of Chemistry, SRM Institute of Science and Technology,
Kattankulathur-603 203, Tamil Nadu, India. E-mail: ashokkus@srmist.edu.in;
ashok.sundramoorthy@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09318d
‡ Both authors contributed equally.
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alloys (Au–Cu, Au–Ni, Pt–Pd, Ni–Cr, etc.) have been used as
electrocatalysts to selectively oxidize glucose to glucolactone.21,22
These catalytic materials showed high electroactivity toward
carbohydrates and glucose.23–25 Among them, nickel oxide (NiO)
is one of the promising electrocatalysts to use to prepare a non-
enzymatic glucose sensor,26,27 due to its stable redox activity
(Ni2+/Ni3+), nontoxicity and low cost.28 Because of its unique
properties, various NiO nanostructures have been synthesized,
such as nanoowers,29 nanorods,30 nanobers,31 nano-
particles,32 nanoplates,33 nanoakes,34 nanosheets35 and hollow
porous materials,36 for various applications.
2. Experimental
2.1 Chemicals and reagents
Sodium molybdate dihydrate (Na2MoO4$2H2O), thiourea
(H2NCSNH2) and nickel nitrate hexahydrate Ni(NO3)2$6H2O
were purchased from Sisco Research Laboratories Pvt. Ltd.
(SRL), Maharashtra, India. Sodium hydroxide (NaOH) was
purchased from Thermo Fisher Scientic Company (India) and
glucose (C6H12O6) was purchased from Loba Chemie Pvt Ltd,
Maharashtra, India. All of the purchased chemicals were of
analytical grade. The distilled water was obtained from Milli-Q
(18.2 MU cm @ 25 ꢀ 2 ꢁC) water system. The blood serum
was obtained from the SRM Medical College Hospital and
Research Centre, Kattankulathur, Tamil Nadu. This experiment
was approved by the ethics committee at SRM-IST (no. 002/HYC/
IEC/2018).
On the other hand, layered two dimensional (2D) transition
metal dichalcogenides (TMDs) such as molybdenum disulphide
(MoS2),37–40 tungsten disulphide (WS2) and graphene41 have
been used to fabricate electrochemical sensors.42 As an impor-
tant candidate, MoS2 consisting of a S–Mo–S bonded trilayered
structure43,44 (with a band gap in the range of 1.2 to 1.8 eV) has
shown high catalytic activity, surface area and electrical
conductivity.21,45 It was found that MoS2 shows multi-fold
enhancement in the sensitivity of electrochemical sensor
devices compared to graphene.46 The spacing between the
neighbouring layers of MoS2 (0.62 nm)47 is larger than that in
graphene (0.35 nm),48 which helps to improve the performance
of the electrochemical sensors49 due to the exposed edges
similar to in functionalized graphene sheets.50 In order to
further improve the catalytic activity of MoS2, various nano-
particles, metal oxides and polymers have been incorporated in
layered materials.51,52 For example, MoS2-based nano-
2.2 Apparatus and equipment
All electrochemical measurements were performed on an elec-
trochemical workstation (Model: CHI-760E), Austin, TX, USA.
Electrochemical measurements were carried out using a stan-
dard three-electrode system with a modied glassy carbon
electrode (NiO/MoS2/GCE) as a working electrode, Ag/AgCl (3 M
KCl) as the reference electrode and platinum wire as the counter
electrode. The UV-Vis was performed using a Carry 5000, Agilent
Spectrophotometer. Powder X-ray diffraction (PXRD) spectra
were recorded using a PXRD diffractometer (X'pert PXRD
system, Malvern Panalytical India) with CuKa radiation (l ¼
0.15406 nm). The particle size and morphology of the nano-
composite were determined by FE-SEM with an attached energy-
dispersive X-ray (EDX) spectrometer (for elemental analysis)
(FEI Quanta FEG 200) at an accelerating voltage of 20 kV. HR-
TEM was carried out using a JEM-2100 Plus, Jeol, operating at
200 kV. The XPS was carried out on a PHI VersaProbe III
Scanning XPS Microprobe, (Physical Electronics, USA). BET data
was obtained using a Model BELSORP Max; Make-Microtrac
BEL, Japan.
53
composites such as Cu/MoS2 (linear range of detection from
0 to 4 mM), MoS2–TiO2/Au,54 MoS2-graphene,55 CuS/MoS2,56
MoS2–Au/Pt/GCE57 (from 10 mM to 3 mM), MoS2–g-C3N4,58 NiO/
multi-walled CNT,59 CuNi/C,60 Pt/Ni/MoS2 61 and nickel/copper/
carbon nanotubes62 have been reported to detect glucose. These
methods have some disadvantages, such as the use of expensive
noble metal catalysts (Au, Pt), bimetallic catalysts and carbon
nanotubes, which are relatively expensive. In order to decrease
the cost of sensors with improved selectivity and sensitivity, new
and simple sensor systems are still in demand for future
glucose sensor applications.
In the present work, for the rst time, hydrothermally NiO
nanoparticles were synthesized on MoS2 nanosheets. The as-
prepared, NiO decorated MoS2 nanocomposite was compre-
hensively characterized by UV-Vis, XRD, HR-TEM, FE-SEM,
energy dispersive X-ray spectroscopy (EDX), XPS and BET anal-
ysis. Moreover, the electrochemical properties of the NiO/MoS2
hybrid lm coated GCE were investigated by cyclic voltammetry
(CV), electrochemical impedance spectroscopy (EIS) and
amperometric analysis. This NiO/MoS2/GCE sensor showed
high electrocatalytic activity towards glucose from 0.01 to
10 mM. The limit of detection (LOD) was found to be 1.62 mM.
Herein, we have attempted to use MoS2 layers as a support
material to form a composite with NiO. This study revealed that
because of a higher loading of NiO, enhanced catalytic activity
2.3 Synthesis of MoS2
MoS2 was synthesized using a hydrothermal method. Briey,
2.2 g of Na2MoO4$2H2O and 2.0 g of H2NCSNH2 were rst dis-
solved in 80 mL of distilled water and vigorously stirred for
30 min at room temperature. Then, the mixture was transferred
to a 100 mL autoclave made of Teon-lined stainless steel and
maintained at 200 ꢁC for 18 h in a hot air oven. Finally, the black
precipitate (MoS2) was collected by ltration, followed by
washing with water and absolute ethanol several times. The
ꢁ
obtained MoS2 precipitate was dried in a hot air oven at 70 C
for 12 h.21,63
2.4 Synthesis of the NiO/MoS2 nanocomposite
was observed. It was also used to detect glucose in blood serum 0.06 g of MoS2 powder was dispersed in water (20 mL) and 0.03 g
with high selectivity. We believe our study could help others to of Ni (NO3)2$6H2O was also dissolved in 20 mL of water sepa-
use this nanocomposite to make commercially valuable glucose rately. Then, both solutions were stirred continuously for
sensors.
30 min using a magnetic stirrer. Aer that, the Ni (NO3)2$6H2O
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solution was added dropwise into the MoS2 dispersion under
continuous stirring for 10 min. The pH of the mixed solution
was adjusted to 11 by adding 25% NH3 under constant stirring
at room temperature. The mixture was then transferred to
a 100 mL Teon-lined stainless steel autoclave and was then
heated to and maintained at 120 ꢁC for 2 h. Aer that, the
autoclave temperature was allowed to cool down and the solu-
tion was collected and centrifuged at 10 500 rpm for 15 min.
Finally, the black precipitate (NiO/MoS2) was collected by
ltration and washed with water and absolute ethanol several
ꢁ
times. Later, it was dried at 70 C for 12 h (Scheme 1).
2.5 Preparation of a NiO/MoS2/GCE-based sensor
The GCE was polished on a micro cloth using an alumina slurry
with different particle sizes (of 1, 0.3, and 0.05 mm) and then
washed with distilled water, followed by ethanol for few min to
obtain a mirror-like surface. Aer that, the NiO/MoS2 nano-
composite (5 mg) powder was dispersed in 2 mL of water by
bath sonication for 30 min. The as-obtained NiO/MoS2 disper-
sion was then allowed to stand at room temperature for 15 min
to settle out bulk particles. Later, the supernatant solution was
collected for further use. Finally, 10 mL of the NiO/MoS2
supernatant solution was drop cast onto the surface of the GCE
and dried at 50 ꢁC in a hot air oven to obtain a NiO/MoS2
modied GCE. For comparison, MoS2/GCE and NiO/GCE were
also prepared under the same conditions.
Fig. 1 UV-Vis spectra of (i) MoS2 and (ii) NiO/MoS2 nanocomposite
dispersions (the inset shows an enlarged portion of the spectrum of
the MoS2).
to a higher wavelength (674 to 676 nm). This may be due to the
interaction between the MoS2 nanosheets and NiO nano-
particles present in the nanocomposite.
The crystal structure of the MoS2 and NiO/MoS2 nano-
composite were also studied using PXRD (Fig. 2). The PXRD
spectrum of MoS2 shows peaks at 14.15, 33.43, 39.95 and 58.63ꢁ
(curve a). However, for the NiO/MoS2 nanocomposite, major
peaks were observed at 2q ¼ 13.80, 32.23, 33.11 (NiO), 39.26,
48.25 (NiO) and 58.78ꢁ. The PXRD peaks at 13.80, 32.23, 39.26
and 58.78ꢁ can be attributed to the planes of the hexagonal 2H-
MoS2 phase (JCPDS no. 73-1508) (Fig. 2, curve b),47 which
conrms the formation of MoS2. In addition, two peaks
observed at 2q angles of 48.25ꢁ (200) and 33.11ꢁ (111) belong to
NiO planes corresponding to the cubic structure (JCPDS card no
#47-1049).66 It is clear that the NiO nanoparticles were
successfully decorated on the surface of MoS2.
3. Results and discussion
3.1 UV-Vis, PXRD, FE-SEM and HR-TEM analysis
UV-Vis spectra of the MoS2 and NiO/MoS2 nanocomposite
dispersions are shown in Fig. 1. It can be seen that the strong
absorption peaks of MoS2 appeared at around 620 and 674 nm,
which is in agreement with the MoS2 nanosheets reported
elsewhere (red curve i).64 The UV-Vis spectrum of the NiO/MoS2
nanocomposite also shows three absorption bands at around
320, 617 and 676 nm. The main absorption peak observed at
320 nm indicates the presence of NiO nanoparticles in the
nanocomposite, as shown in Fig. 1 (black curve ii).65 It is worth
noting that one of the MoS2 absorption peaks is shied to
a lower wavelength (620 to 617 nm) and another band is shied
Next, the surface morphologies of MoS2 and NiO/MoS2
nanocomposite were studied by FE-SEM. Fig. 3a shows the FE-
SEM image of MoS2 layers illustrating the nanosheet-like
structure of MoS2.47 Fig. 3b shows the surface morphology of
Scheme 1 Schematic representation of the synthesis of the NiO/MoS2 nanocomposite via a hydrothermal process.
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MoS2 conrmed the presence of Mo and S without any other
impurities (Fig. 3c). Similarly, Fig. 3d exhibits the EDX spectrum
of the NiO/MoS2 nanocomposite, which conrms the presence
of Mo, S, Ni and O. The high magnication FE-SEM images of
the MoS2 and NiO/MoS2 nanocomposite are also shown in
(Fig. S1a and b†). This FE-SEM and EDX analysis proved the
successful modication of MoS2 layers with NiO.
Furthermore, the nanocomposite was analysed by HR-TEM.
Fig. 4a shows agglomerated MoS2 nanosheets in the bulk
materials. Interestingly, the decoration of NiO particles on MoS2
sheets helps to form a uniform nanocomposite lm without
aggregation, as shown in Fig. 4b. So, it was conrmed that NiO
nanoparticles assist in the prevention of the agglomeration of
the MoS2 nanosheets. Next, the lattice fringes of the NiO/MoS2
nanocomposite were measured, where the lattice spaces of both
NiO nanoparticles (lattice space ¼ 0.253 nm)67 and MoS2 sheets
(lattice space ¼ ꢂ0.669 nm)47 were observed (Fig. 4c). Further-
more, the size distribution of NiO nanoparticles was analysed
by FE-SEM. The particle sizes of NiO varied from ꢂ38 to 72 nm
on the MoS2 nanosheets (Fig. 4d and S2†).
Fig. 2 PXRD spectra of the (a) MoS2 powder and (b) NiO/MoS2
nanocomposite.
3.2 XPS analysis
the NiO/MoS2 nanocomposite where NiO nanoparticles are
incorporated into the nanocomposite, as it conrmed by EDX
(Fig. 3c and d). Elemental and chemical composition analysis of
The chemical composition and binding energies of NiO/MoS2
were investigated by XPS. Fig. 5a and b show the survey
Fig. 3 FE-SEM images of the (a) MoS2 and (b) NiO/MoS2 nanocomposite. EDX spectra of the (c) MoS2 and (d) NiO/MoS2 nanocomposite.
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Fig. 4 HR-TEM images of the (a) MoS2 and (b and d) NiO/MoS2 nanocomposite. (c) High-resolution HR-TEM image showing the lattice spaces of
NiO/MoS2.
spectrum and atomic percentages of the NiO/MoS2 nano- corresponding pore-size width distribution for MoS2 (Fig. S3a
composite. The major elements such as Mo, S, Ni, and O are and b†) and the composite (Fig. S3c and d†). The specic
present (Fig. 5b). Fig. 5c shows the main peaks of Mo 3d at surface area and mean pore diameters of both MoS2 and NiO/
binding energies of 226.3, 228.4, 229.3, 232.4, 232.6 and MoS2 composite were determined (Fig. S3b and d†) as 3.57 and
235.8 eV, respectively. Two characteristic peaks for 3d5/2 and 15.17 m2 gꢃ1, respectively. The NiO/MoS2 composite exhibits
3d3/2 are located at 229.3 and 232.4 eV, indicating the domi- a typical type (IV) isotherm pattern.72 The average pore sizes of
nance of Mo4+. The contribution from the peaks centered at the MoS2 and NiO/MoS2 nanocomposite are in the range of 5 to
228.4 eV is assigned to Mo5+ and the doublets at 232.4 and 25 nm and 8 to 30 nm, respectively.73 It is clear that larger pore
235.8 eV correspond to the higher oxidation states of 3d5/2 and width diameters were observed for the NiO/MoS2 composite,
3d3/2 in Mo6+.68,69 As shown in Fig. 5d, the high-resolution which suggests the presence of more catalytically active surfaces
spectrum of S 2p in NiO/MoS2 can be tted into ve peaks, for enhanced electrocatalytic activity.
with two major peaks at 161.9 and 164.1 eV, which are assigned
to S 2p3/2 and S 2p1/2 of the MoS2 phase. In the Ni 2p spectrum,
the three peaks located at 855.2, 857.1 and 861.2 eV, corre-
sponding to NiO, Ni2O3 and Ni(OH)2, respectively70 (Fig. 5e).
3.3 Electrochemical properties of the NiO/MoS2
nanocomposite and electro-oxidation of glucose
Similarly, the O 1s spectrum can be deconvoluted into three
peaks with binding energies of 530.5, 531.8 and 532.9 eV, which
may be attributed to Ni–O–C, NiO and O]C bonds, as reported
previously (Fig. 5f).69,71
To ascertain the surface areas of the NiO/MoS2 nano-
composite and MoS2, BET analysis was carried out. Fig. S3†
shows the N2 adsorption–desorption isotherms and the
A freshly prepared NiO/MoS2/GCE was used to record cyclic
voltammograms (CVs) in 0.1 M NaOH (Fig. 6, curve a), showing
a well-dened redox peak for the NiO decorated on MoS2. The
formal potential (E0 ¼ Epa + Epc/2) of NiO/MoS2/GCE was found
0
to be +0.46 V, which is closer to the reported value for NiO in
0.1 M NaOH.74 The peak to peak separation (DEp ¼ Epa ꢃ Epc)
was also calculated as 70 mV, which indicates a highly reversible
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Fig. 5 XPS spectra of NiO/MoS2. (a) Survey spectrum and (b) pie chart showing the atomic percentages of elements present in NiO/MoS2. (c)
High-resolution spectra of Mo 3d, (d) S 2p, (e) Ni 2p and (f) O 1s.
electron transfer reaction. Next, CVs were also recorded using peak current of NiO/MoS2/GCE was increased at 0.55 V, with
the above modied electrodes in the presence of 50 mM of a small decrease in the cathodic current at 0.43 V (Fig. 6, curve
glucose in 0.1 M NaOH solution. As can be seen, the anodic b). This result indicates that the NiO/MoS2 nanocomposite
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Fig. 7 CVs of NiO/MoS2/GCE in the presence of different concen-
trations of glucose from 50 to 300 mM in 0.1 M NaOH. Scan rate ¼
Fig. 6 CVs of (a) NiO/MoS2/GCE (blank), (b) NiO/MoS2/GCE (with 50
mM of glucose), (c) NiO/GCE (blank), (d) NiO/GCE (with 50 mM of
glucose), (e) MoS2/GCE (blank), (f) MoS2/GCE (with 50 mM of glucose)
and (g) bare-GCE (with 50 mM of glucose) were recorded in 0.1 M
50 mV sꢃ1
.
NaOH. Scan rate ¼ 50 mV sꢃ1
.
the nanocomposite due to the presence of MoS2 (Fig. 6, curve a).
Similarly, the electro-oxidation of glucose (50 mM) on NiO/GCE
was also studied under the same conditions. It was found that
NiO/GCE shows a very small oxidation peak for 50 mM glucose at
0.5 V, with a lower peak current (Fig. 6, curve d). This experiment
further suggested that the observed enhanced electrocatalytic
activity of the nanocomposite (NiO/MoS2/GCE) is due to the
synergistic effect between NiO and MoS2. In the same manner,
CVs were recorded using MoS2/GCE in the absence and presence
of 50 mM of glucose. It was found that MoS2/GCE did not show any
well-dened oxidation peak for glucose except for changes in the
background current (Fig. 6, curves e and f). As expected, there were
no reduction or oxidation peaks observed at the bare GCE for
glucose (curve g). The above results conrm that MoS2 plays an
important role in the enhancement of catalytic activity for glucose.
Furthermore, EIS was used to measure the changes in elec-
trode resistance aer modication with various materials. Fig. S4†
shows the Nyquist plots of the modied electrodes obtained,
featuring a semicircle and linear part at higher and lower
frequencies, which correspond to the limited and diffusion-
limited electron transfer processes.77 Nyquist plots typically illus-
trate the modied electrode charge transfer resistance (Rct), which
can be determined from the diameter of the semicircles. It was
shows enhanced glucose oxidation at the surface of the sensor.
The electrochemical glucose oxidation reaction may follow an
irreversible process, as given in eqn (1)–(3)74–76 (Scheme 2). The
electrode reaction of Ni(II) to Ni(III) is a quasi-reversible process
that is favored in solution containing negatively charged
hydroxide ions. Aer the electrocatalytic oxidation of glucose,
the oxidized form of the nanocomposite was reduced back to its
initial form, as given in eqn (2).74
NiO + OHꢃ / NiO(OH) + eꢃ
Ni(OH)2 + OHꢃ $ NiO(OH) + H2O + eꢃ
NiO(OH) + glucose / Ni(OH)2 + glucolactone
(1)
(2)
(3)
As a control experiment, NiO/GCE (without MoS2) was
prepared and used to record CVs in 0.1 M NaOH (Fig. 6, curve c).
As shown, NiO/GCE exhibited an ill-dened redox peak at E0 of
0
0.44 V with low anodic and cathodic peak currents (Ipa ¼ ꢃ0.0439
mA and Ipc ¼ 0.0238 mA) compared to those of NiO/MoS2/GCE,
which proves that there is enhanced electrochemical activity for
Scheme 2 Electrocatalytic oxidation mechanism of glucose at the NiO/MoS2/GCE.
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Fig. 8 (a) Amperometric curve recorded using the NiO/MoS2/GCE with successive additions of glucose from 0.01 to 10 mM at an applied
potential of 0.55 V. The solution was stirred at a rate of 1200 rpm. (b) The obtained linear calibration plot of glucose concentrations vs. oxidation
currents with error bars (n ¼ 3).
found that the NiO/MoS2/GCE (87.18 U) has a low Rct compared to mM of glucose from 20 to 200 mV sꢃ1. Fig. S7a† shows that both
NiO/GCE (109.42 U), MoS2/GCE (113.07 U), and bare/GCE (119.42 the anodic and cathodic peak currents increased with scan rate,
U) (Fig. S4†). These EIS results corroborated our hypothesis that indicating that glucose oxidation on NiO/MoS2 may follow
the conductivity of the nanocomposite was increased due to a surface-controlled oxidation process.79,80 A good linear rela-
existing favorable interactions between NiO/MoS2 and the tionship between scan rate and anodic/cathodic (Ipa/Ipc) peak
electrolyte.
currents were obtained with correlation coefficient (R2) values of
The effect of pH on the glucose oxidation was also studied. It 0.9911 and 0.9832 (Fig. S7b†).
was found that NiO/MoS2/GCE shows a high catalytic current for
Next, various concentrations of glucose were tested on the
glucose (50 mM) at pH 13 (0.1 M NaOH) compared to other NiO/MoS2/GCE sensor. CVs of NiO/MoS2/GCE were recorded with
studied electrolytes (pH 11, 9, 7.4, 5 and 3) (Fig. S5†). Further- subsequent additions of glucose from 50 to 300 mM (Fig. 7). The
more, the oxidation potential of glucose was also shied to oxidation peak currents of the sensor were increased linearly with
a higher positive voltage (+0.86 V) at pH 7.4 and the catalytic glucose concentration. When the concentrations of glucose
current decreased (Fig. S5†). So, we selected 0.1 M NaOH as increased from 50 to 300 mM, the oxidation potential of the
a supporting electrolyte to detect glucose.78 It is worth noting that sensor was slightly shied to a more positive value, which may be
the oxidation potential and catalytic currents of glucose (50 mM) due to a diffusion controlled mass transfer process, so a high
were also shied according to the pH of the electrolyte (Fig. S6a voltage has to be applied to carry out glucose oxidation81 (Fig. 7).
and b†). A lower oxidation potential with a high catalytic current
was observed at pH 13, which was found to be the optimum pH.
3.5 Optimization of catalyst loading on GCE
Various amounts of NiO/MoS2 were placed on the GCE surface to
nd out the optimum loading of the material for effective glucose
3.4 Effect of scan rate and linear range of glucose detection
The effect of scan rate on the glucose oxidation current was oxidation. For this purpose, GCE was coated with different
investigated using NiO/MoS2/GCE in 0.1 M NaOH containing 50 volumes of NiO/MoS2 dispersion (stock ¼ 2.5 mg mLꢃ1), such as
Table 1 Linear range of detection, LOD, applied potential and electrolyte used in the present method compared with other reported glucose
sensorsa
Working electrode
Applied potential (V)
Electrolyte
Linear range (mM)
Detection limit (mM)
Reference
Ni(OH)2/CILE
NiO-GR/GCE
0.55
0.35
0.50
0.6
0.4
0.7
0.5 M NaOH
0.2 M NaOH
0.1 M NaOH
0.1 M NaOH
1 M KOH
0.1 M NaOH
0.1 M NaOH
0.1 M NaOH
0.05–23
0.02–11.2
0.1–1.7
0.004–4.8
0.01–0.75
0.01–4
6
5
4
2
2.5
1
0.5
1.62
87
88
89
90
91
78
92
Ti/TiO2 NTA/Ni
Ni-NPs/TiO2NTs
Nanostructured a-Ni(OH)2/FTO
Cu2O/MoS2/GCE
Cu/GNs
0.6
0.55
0–4.5
0.01–10
NiO/MoS2/GCE
This work
a
CILE ¼ carbon ionic liquid electrode. Ti/TiO2 NTA/Ni ¼ Ti/TiO2 nanotube array/Ni composite electrode. Ni-NPs/TiO2 NTs ¼ Ni nanoparticle loaded
TiO2 nanotube arrays. GNs ¼ graphene nanosheets. NiO-GR/GCE ¼ nickel oxide and graphene nanocomposite modied carbon electrode. FTO ¼
uorine-doped tin oxide electrode.
650 | RSC Adv., 2020, 10, 643–654
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10 mL (25 mg), 20 mL (50 mg), 30 mL (75 mg) and 40 mL (100 mg), as glucose due to their overlapping oxidation potentials.83,84 It was
shown in Fig. S8a.† The GCE coated with 10 mL (25 mg) of NiO/ reported that the concentrations of these interferent molecules
MoS2 showed a higher catalytic current for 50 mM of glucose. But, are about 30 times lower than the glucose levels in human blood
further increasing the amount of catalyst did not improve the (4.4–6.6 mM).78 As shown in Fig. 9, aer the addition of each
catalytic current of glucose (Fig. S8b†). Moreover, the oxidation interferent, the NiO/MoS2/GCE sensor did not show any
potential of glucose also shied to a more positive voltage, maybe signicant current response at 0.55 V. Interestingly, the NiO/
due to thick lm formation, which is not favorable for the inter- MoS2/GCE sensor responded well to both additions of glucose
action between glucose and NiO/MoS2/GCE (Fig. S8c†). From this (2 mM) into the same solution. This experiment suggested that
study, the optimal catalyst loading was found to be 10 mL (25 mg) of NiO/MoS2/GCE can be used for the selective detection of glucose
NiO/MoS2 on the GCE surface.
without any signicant interference. Furthermore, in order to
nd out the concentrations of interferent compounds which
will start to affect the sensor response, we carried out amper-
ometry in the presence of 2 mM glucose followed by additions of
interferent compounds (0.01, 0.1, 0.5 and 1 mM) in the same
electrolyte at a constant potential of 0.55 V (Fig. S9†). It was
found that alanine and dopamine only start to show small
interferent currents when used above 0.1 and 0.5 mM, respec-
tively (Fig. S9†). However, L-cysteine, hydrogen peroxide, fruc-
tose, lactose, ascorbic acid and uric acid did not interfere up to
the studied concentrations (Fig. S9†).
Next, the repeatability and stability of the NiO/MoS2/GCE
sensor were also tested by cyclic voltammetry in the presence of
50 mM of glucose in 0.1 M NaOH. The CVs were recorded using
a NiO/MoS2/GCE sensor in freshly prepared 0.1 M NaOH con-
taining 50 mM glucose in the time interval of 0 to 8 h (Fig. 10a
and b). The relative standard deviation (RSD) was estimated as
3.02 for the ve measurements. The obtained bar graph shows
that NiO/MoS2/GCE had good repeatability even aer 8 h of
repeated use (note: glucose oxidation CVs were recorded in time
intervals of 0, 2, 4, 6 and 8 h, not continuously). The stability of
the NiO/MoS2/GCE was also tested by recording repetitive CVs
up to 20 cycles (Fig. 10c and d). As can be seen, the redox peak
currents of NiO/MoS2/GCE did not decrease signicantly with
3.6 Amperometric detection of glucose
Amperometry can be used to detect an analyte of interest at
a particular applied potential with high sensitivity. Fig. 8a
shows amperometric current responses of the NiO/MoS2/GCE
sensor with successive additions of glucose from 0.01 to 10 mM
at an applied voltage of 0.55 V in 10 mL of 0.1 M NaOH. During
this experiment, electrolyte was constantly stirred using
a magnetic pellet at 1200 rpm. Each addition of glucose was
made into the electrochemical cell in time intervals of 50 s.
Upon consecutive additions of glucose to the electrochemical
cell, a gradual increase in the steady-state current was
observed.77 This shows that the NiO/MoS2/GCE sensor has
excellent catalytic activity and sensitivity towards glucose.
Fig. 8b shows the obtained calibration plot for glucose using
the NiO/MoS2/GCE-based sensor. This amperometric detection
was repeated three times using a same modied electrode.
From the obtained amperometric curve the mean and standard
deviation were calculated, as given in the error bars in Fig. 8b.
The calibration graph shows a good linear relationship between
the concentrations of the glucose and oxidation currents from
0.01 to 10 mM with a linear correlation coefficient of (R2) 0.9828.
The limit of detection (LOD) for glucose was estimated using
the following formula: LOD ¼ 3 ꢄ standard deviation (SD)/slope
value (S).82 The SD of the blank current (without glucose) was
estimated as 1.1547 ꢄ 10ꢃ8 A and the slope value was estimated as
0.214 ꢄ 10ꢃ7 A mMꢃ1. The calculated LOD was 1.62 mM, which is
comparable to some of the reported methods (Table 1). Further-
more, the linear range, LOD, and applied potential used for
glucose oxidation are compared with other reported methods in
Table 1. It seems that a wide linear range of detection is possible
using the proposed method. The observed improvement in the
analytical performance of the NiO/MoS2/GCE-based sensor may
be due to the favorable electron transfer between the glucose and
electrode surface. In addition, the excellent electrical conductivity
and higher surface area of the nanocomposite were also found to
be responsible for the improved electrocatalytic activity.
3.7 Interference, reproducibility and repeatability studies of
NiO/MoS2/GCE
A newly prepared NiO/MoS2/GCE sensor was tested with other
interferent biomolecules such as alanine, L-cysteine, hydrogen
peroxide, fructose, lactose, uric acid, dopamine and ascorbic
acid, which are commonly present alongside glucose in bio-
logical systems. Some of these important electroactive biomol-
Fig. 9 Amperometric response recorded using a NiO/MoS2/GCE in
0.1 M NaOH in the presence of glucose (2 mM), alanine (1 mM), L-
cysteine (1 mM), hydrogen peroxide (1 mM), fructose (1 mM), lactose (1
mM), uric acid (1 mM), dopamine (1 mM), ascorbic acid (1 mM) and
glucose (2 mM). The rotation rate was 1200 rpm and the applied
ecules may affect the direct electrochemical oxidation of potential used was 0.55 V.
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Fig. 10 (a) CVs of a NiO/MoS2/GCE were recorded in 0.1 M NaOH containing 50 mM of glucose at different time intervals of 0, 2, 4, 6, and 8 h and
(b) the corresponding bar diagram displays the changes in current vs. time. (c) CVs were recorded continuously for 20 cycles using a NiO/MoS2/
GCE in 0.1 M NaOH at a scan rate of 50 mV sꢃ1. (d) The bar graph shows the changes in the current percentages according to the number of
potential cycles.
the number of cycles. This indicates the strong attachment of
the NiO/MoS2 on the surface of GCE.
3.8 Real sample analysis
To test the real-world application of the sensor, NiO/MoS2/GCE
was used to detect glucose in blood serum by amperometry. The
amperometry response of the NiO/MoS2/GCE was recorded with
successive stepwise additions of (10, 20 and 30 mM) glucose in
10 mL of 0.1 M NaOH containing 10 mL of blood serum. The total
glucose concentration in the blood serum was found to be 4.37 ꢀ
0.07 mM using the HbA1c technique (conducted by a private
medical lab).85 Furthermore, three different glucose concentra-
tions (0.01, 0.02 and 0.03 mM) were spiked into the NaOH/serum
Furthermore, the NiO/MoS2/GCE sensor was used to record
CVs in 0.1 M NaOH (fresh solution) aer glucose oxidation was
carried out (Fig. S10†). It was found that the used NiO/MoS2/
GCE sensor again showed a reversible redox peak of NiO with
almost the same peak currents as compared to before glucose
oxidation (Fig. S10,† curves a–c) even aer repeated use. This
study further conrmed that there was no electrode fouling, so
this sensor can be used for multiple measurements.
Table 2 Electrochemical detection of various concentrations of glucose (spiked) in blood serum samples using NiO/MoS2/GCE as a non-
enzymatic sensor
Glucose concentration
(mM)
Glucose added
(mM)
S. no.
Samples
Glucose founda (mM)
RSD
Recovery%
1
2
3
4
Human blood serum
4.4
4.4
4.4
4.4
—
4.37 ꢀ 0.07
4.41 ꢀ 0.017
4.42 ꢀ 0.021
4.43 ꢀ 0.014
1.50
3.19
3.40
1.87
—
Human blood serum spiked with glucose
Human blood serum spiked with glucose
Human blood serum spiked with glucose
0.010
0.020
0.030
97.8
96.1
99.8
a
Mean value ꢀ standard deviation for n ¼ 3.
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solution. The spiked glucose concentrations were estimated by
amperometry and their recovery values were calculated.86 We
selected this spike-and-recovery method to test the sensor
applicability and efficiency in recovery analysis. This real sample
8 M.-S. Steiner, A. Duerkop and O. S. Woleis, Chem. Soc. Rev.,
2011, 40, 4805–4839.
9 J. Luo, P. Luo, M. Xie, K. Du, B. Zhao, F. Pan, P. Fan, F. Zeng,
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sensor shows good reproducibility and recovery rate (96.1–99.8%)
170.
for the detection of glucose in real sample analysis.
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4. Conclusions
In summary, a NiO/MoS2 nanocomposite was synthesized success-
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wavelengths. FE-SEM, PXRD, EDX, XPS and HR-TEM revealed that 17 F. Gao, F. Zhou, Y. Yao, Y. Zhang, L. Du, D. Geng and
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NiO/MoS2/GCE sensor is highly stable and can be used for multiple 21 K.-J. Huang, L. Wang, Y.-J. Liu, Y.-M. Liu, H.-B. Wang, T. Gan
measurements. Real sample analysis was also carried out in blood
serum with successive additions of glucose. The recovery analysis
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14034.
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of glucose in blood serum. We envisage that this NiO/MoS2 3502.
nanocomposite-based sensor can be used for the selective detection 23 C. Barrera, I. Zhukov, E. Villagra, F. Bedioui, M. A. Paez,
´
of glucose in biological and medical samples.
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Conflicts of interest
There are no conicts to declare.
25 M. d. S. M. Quintino, H. Winnischofer, M. Nakamura,
K. Araki, H. E. Toma and L. Angnes, Anal. Chim. Acta,
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26 M.-S. Wu, R.-Y. Ji and Y.-R. Zheng, Electrochim. Acta, 2014,
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Acknowledgements
G. J. thanks the SRM IST for a Junior Research Fellowship.
27 J. Yoon, E. Lee, D. Lee, T.-S. Oh, Y. S. Yoon and D.-J. Kim, J.
Electrochem. Soc., 2017, 164, B558–B560.
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