Fabrication of a non-enzymatic Ni(ii) loaded ZSM-5 nanozeolite and multi-walled carbon nanotubes paste electrode as a glucose electrochemical sensor

Article abstract of DOI:10.1039/c5ra20622g

Effective electro-oxidation of glucose is critically important in developing analytical sensors and carbohydrate-based fuel cells. In this study, a template-free ZSM-5 nanozeolite was synthesized hydrothermally with spherical particle diameters of 40-60 nm, as characterized by scanning electron microscopy. Then, a carbon paste electrode (CPE) was modified by multi-walled carbon nanotubes (MWCNTs), ZSM-5 nanozeolite and Ni²⁺ ions (Ni-MW-ZSM-5/CPE). Electrochemical studies of this electrode were performed using cyclic voltammetry which exhibits the redox behavior of the Ni(iii)/Ni(ii) couple in alkaline medium. This modified electrode was used as an anode for the electrocatalytic oxidation of glucose in 0.1 mol L^-1 NaOH solution. The results confirmed that ZSM-5 nanozeolite at the surface of the CPE improved the catalytic efficiency of the dispersed nickel ions toward glucose oxidation. The values of electron transfer coefficient, electrode surface coverage and charge-transfer rate constant for Ni(iii)/Ni(ii) redox couple were found to be 0.65, 4.04 × 10^-8 mol cm^-2 and 0.184 s^-1, respectively. Also, the diffusion coefficient and the mean value of the catalytic rate constant for glucose and redox sites of the electrode were found to be 1.66 × 10^-4 cm² s^-1 and 1.136 × 10⁸ cm³ mol^-1 s^-1, respectively. The sensor showed an acceptable linear range of 0.5-6.1 mM with a detection limit of 0.14 mM (S/N = 3) by cyclic voltammetry technique. Moreover, differential pulsed voltammetry method revealed a linear range of 0.0001-0.01 mM with a detection limit of 3.5 × 10^-5 mM. Based on the results, the fabricated electrode (Ni-MW-ZSM-5/CPE) showed good catalytic activity, good stability, high sensitivity and reproducibility.

Full text of DOI:10.1039/c5ra20622g

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Fabrication of a non-enzymatic Ni(II) loaded ZSM-5
nanozeolite and multi-walled carbon nanotubes
paste electrode as a glucose electrochemical
sensor†
Cite this: RSC Adv., 2015, 5, 105707
Seyed Karim Hassaninejad-Darzi*
Effective electro-oxidation of glucose is critically important in developing analytical sensors and
carbohydrate-based fuel cells. In this study, a template-free ZSM-5 nanozeolite was synthesized
hydrothermally with spherical particle diameters of 40–60 nm, as characterized by scanning electron
microscopy. Then, a carbon paste electrode (CPE) was modified by multi-walled carbon nanotubes
2+
(MWCNTs), ZSM-5 nanozeolite and Ni
ions (Ni–MW–ZSM-5/CPE). Electrochemical studies of this
electrode were performed using cyclic voltammetry which exhibits the redox behavior of the Ni(III)/Ni(II)
couple in alkaline medium. This modified electrode was used as an anode for the electrocatalytic
ꢀ
1
oxidation of glucose in 0.1 mol L NaOH solution. The results confirmed that ZSM-5 nanozeolite at the
surface of the CPE improved the catalytic efficiency of the dispersed nickel ions toward glucose
oxidation. The values of electron transfer coefficient, electrode surface coverage and charge-transfer
rate constant for Ni(III)/Ni(II) redox couple were found to be 0.65, 4.04 ꢁ 10ꢀ mol cm and 0.184 s
8
ꢀ2
ꢀ1
,
respectively. Also, the diffusion coefficient and the mean value of the catalytic rate constant for glucose
ꢀ
4
2
ꢀ1
8
3
ꢀ1 ꢀ1
and redox sites of the electrode were found to be 1.66 ꢁ 10 cm
s
and 1.136 ꢁ 10 cm mol
s ,
respectively. The sensor showed an acceptable linear range of 0.5–6.1 mM with a detection limit of 0.14
mM (S/N ¼ 3) by cyclic voltammetry technique. Moreover, differential pulsed voltammetry method
revealed a linear range of 0.0001–0.01 mM with a detection limit of 3.5 ꢁ 10 mM. Based on the
results, the fabricated electrode (Ni–MW–ZSM-5/CPE) showed good catalytic activity, good stability,
high sensitivity and reproducibility.
Received 5th October 2015
Accepted 7th December 2015
ꢀ5
DOI: 10.1039/c5ra20622g
www.rsc.org/advances
glucose. Glucose oxidase (GOx) is one of the famous enzymes
1
. Introduction
for detection of glucose in solution due to its being inexpensive,
4
Glucose is the primary energy source of the body. The level of stable and of practical use. Application of GOx in biosensors
glucose in the blood has been applied for diagnosis of diabetes. has also brought about several problems. For example, activity
Besides the need for monitoring and determining of glucose in of GOx can be easily affected by temperature, pH, humidity and
5
the case of diabetes patients, it is also necessary for non- toxic chemicals. Also, a complicated procedure, including
diabetic critical care patients in order to control glucose adsorption,
cross-linking,
entrapment,
and
electro-
1
levels. Oxidation of most carbohydrates can be easily detected polymerization, is required for the immobilization of the
by electrochemical methods. However, their electrochemical enzyme on the solid electrode and this may decrease the activity
5
,6
oxidation is not easy, as it requires a large overpotential on of the GOx.
conventional electrodes. Glucose is a keen metabolite for living
2
Non-enzyme electrochemistry-based method has several
organisms, especially in the case of patients suffering from advantages over enzyme-based methods for glucose detection in
3
diabetes. Since Clark and Lyons reported the rst enzyme terms of stability, simplicity, reproducibility, cost, and the fact
7
electrode in 1962, and other electrochemical biosensors have that it is free of oxygen limitation. Considering these respects,
later been employed successfully for the determination of the enzymeless glucose sensor is an attractive alternative tech-
nique. Actually, many efforts have been tried for the determi-
nation of glucose without enzyme, gold nanoparticles onto
Research Laboratory of Analytical & Organic Chemistry, Department of Chemistry,
8
9
modied glassy carbon electrode, CuO nanosheets electrode,
Faculty of Science, Babol University of Technology, Shariati Av., P.O. Box: 484,
10
Babol, Iran, 47148-71167. E-mail: hassaninejad@nit.ac.ir; Fax: +98 1132334203; CuO/ZnO composite nanoarrays, magnetic copper ferrite onto
1
1
12,13
Tel: +98 1132334203
multiwalled carbon nanotubes, nickel electrode,
hierar-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra20622g
14
chical Cu–Co–Ni electrode, and Pt nanoparticles encapsulated
1
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15
in carbon microspheres. Also, modied carbon paste elec-
2
. Experimental
16,17
trodes
and palladium nanoparticles deposited on gra-
18
2.1. Reagents and materials
phene were proposed for determination of glucose without
19
enzyme. Velarde et al. studied oxidation of D-glucose with Tetraethylorthosilicate (TEOS), sodium aluminate (NaAlO2),
hydrogen peroxide as oxidant over several titanium-containing sodium hydroxide, glucose and NiCl $6H O were of analytical
2
2
NaY zeolites. It is important to develop a nonenzymatic reagent grade and were purchased from Merck Company and
sensor that has high sensitivity and stability, and is interference used without further purication. Graphite powder and paraffin
ꢀ
3
free, for the determination of glucose by the method of elec- oil (density 0.88 g cm ) as the binding agent (both from Dae-
trochemical oxidation.
jung company) were used for preparing the pastes. Multi-walled
Production of modied electrodes for electrocatalysis has carbon nanotubes (MWCNTs) more than 95 wt% purity was
20–22
been extensively developed and investigated by researchers.
purchased from Iranian Nanomaterials Pioneers Company.
Modied electrodes provides an excellent way to accelerate Also double distilled water was used throughout the
charge transfer processes, decrease the over potentials as well as experiment.
to increase the intensity of the corresponding voltammetric
2
0,23,24
responses.
Zeolites and nanozeolites are ordered porous
25
2.2. Preparation of organic template free ZSM-5 nanozeolite
crystalline materials with wide practical applications. They
have high surface areas with strongly organized microporous ZSM-5 nanozeolite has been synthesized using tetraethylortho-
channel systems which is an advantage compared to other silicate (TEOS) and sodium aluminate (NaAlO ) as a silica and
2
25
classical support materials of interest for fuel cell technology.
aluminum sources, respectively. In a typical synthesis, sodium
One of the most representative articial zeolites, ZSM-5 zeolite, aluminate was dissolved in NaOH solution under stirring, fol-
has been widely used in industry as catalysts, adsorbents, ion lowed by successive addition of double distilled water. Then,
26
exchangers and zeolite membranes. Also, zeolites and nano- TEOS was added to the above mixture and stirred until a homo-
zeolites have been so far utilized in zeolite modied electrodes geneous gel was obtained. The gel was transferred into a Teon
27
ꢂ
(ZMEs) and applied in electrocatalysis reaction.
lined stainless steel autoclave and heated at 180 C for 24 h under
In the recent years, multi-walled carbon nanotubes static condition. Aer this procedure, the product was separated
(
MWCNTs) have received great attention as nanomaterials for by centrifugation (15 000 rpm), washed several times with double
ꢂ
ꢂ
the fabrication of electronic devices because of its surprising distilled water, dried overnight at 90 C and calcinated at 550 C
physical and electrical properties such as highly elastic for 5 h in air. The molar ratio of the above reactants was as
modules, high tensile strength, good chemical stability, large follows: 1.0Al O : 30SiO : 3.3Na O : 1350H O.
35
2
3
2
2
2
surface-to-volume ratio, and high thermal and electrical
2
8,29
conductivity.
a novel platform for fabricating chemical sensors, and can be
utilized to help electron-transfer between the fabricated elec- XRD pattern was recorded by X-ray diffractometer instrument
As an electrode material, MWCNTs provide
2
.3. Apparatus and characterization
30,31
ꢀ
trode and the electroactive species.
The surprising electro- (XRD, GBC-MMA) with Be ltered Cu K radiation (1.5418 A)
a
chemical features of MWCNTs make them appropriate for operating at 35.4 kV and 28 mA. The scanning range of 2q was
ꢂ
ꢂ
usage in faradaic processes (e.g., the fast electron transfer set between 5 and 50 with scan rate of 0.05 degree per
kinetics of MWCNTs, due to the presence of edge plane graphite second. The morphology of the synthesized zeolite was
sites within the walls and at the ends of MWCNTs) or in non- investigated with scanning electron microscopy (SEM, VEGA2-
faradaic processes such as the large changes in conductance TESCAN) and transmission electron microscopy (TEM, JSM-
due to the presence of the cloud of electrons surrounding their 1400 transmission electron microscope, JEOL Japan, with an
walls, can accept or withdraw charges from or to molecules in accelerating voltage of 100 kV). The Bruker FT-IR spectrometer
32
their near chemical environment.
(Vector 22) was utilized for recording of Fourier transform
Nickel is a relatively abundant and low cost material that is infrared (FT-IR) spectrum at room temperature. The particle
utilized widely in numerous industrial applications. It is well size distributions of synthesized zeolite was determined using
established that Ni can be used as a catalyst based on its surface laser particle sizer Analysette 22 NanoTec plus (Fritsch GmbH,
oxidation properties and has long-term stability in alkaline solu- Germany) caused by the solid material suspended in distilled
25,26
33,34
tions.
to glucolactone (two hydrogen are liberated in this process) can be ultrasound bath with incorporated stirrer. The electro-
catalyzed by the NiOOH/Ni(OH) redox couple at Ni electrode or chemical experiments were performed using a potentiostat/
modied electrode with nickel in the alkaline medium.
galvanostat (SAMA500 electroanalyzer system, Isfahan, Iran)
According to the literatures,
the oxidation of glucose water. Disintegration of the solid powders was supported by an
2
In respect of literature survey, no ZSM-5 nanozeolites and with a voltammetry cell in a three electrodes conguration at
MWCNTs were employed for the modication of CPE for elec- 25 ꢃ 1 ꢂC. The Ag|AgCl|KCl (3 M) and platinum wire (Azar
trocatalytic oxidation of glucose. In this study, ZSM-5 nano- Electrode Co., Iran) were used as reference and auxiliary
zeolite was synthesized and characterized by different electrodes, respectively. The CPE, modied CPE with ZSM-5
techniques. Then, this nanozeolite was utilized for modication nanozeolites and modied CPE with ZSM-5 nanozeolites and
of CPE and applied for electrocatalytic oxidation of glucose in MWCNTs were used as the working electrode in the electro-
the alkaline medium.
chemical experiments.
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.4. Preparation of the working electrodes
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2
The SEM image of the crystalline phase provides useful
approach to the determination of size and morphology of the
obtained crystals. SEM image of synthesized ZSM-5 nanozeolite
is illustrated in Fig. 2, which indicates the formation of spher-
ical nanosized particle with diameter between 60–90 nm.
Similar result was obtained from TEM image. Fig. 3 displays
histograms of particle size distributions of synthesized ZSM-5
nanozeolite according to the Mie theory. The average particle
Typically to prepare MW–ZSM-5/CPE, 0.05 g of ZSM-5 nano-
zeolite (25 wt%) and 0.02 g of MWCNTs (10 wt%) were mixed
with 130 mg of graphite powder and then, paraffin oil was
blended with the mixture in a mortar by hand mixing for 30 min
until a uniformly wetted paste was obtained. This paste was
packed into the end of a glass tube (ca. 0.35 cm i.d. and 10 cm
long) and the copper wire was used for electrical contact. A new
surface was achieved by pushing an excess of the paste out of
the tube and polishing with a weighing paper. For comparison,
unmodied CPE (bare CPE), MW/CPE and ZSM-5/CPE were also
prepared in the same mentioned method.
3. Results and discussion
3.1. Characterization of ZSM-5 nanozeolite
XRD powder pattern of synthesized ZSM-5 nanozeolite is pre-
sented in Fig. 1. The crystallization products matched the
characteristic peaks of ZSM-5 nanozeolite at 2q values of 7.9,
26,36
8.9, 23.2 and 24.5 degrees with the reference sample
indi-
cated that pure phase of ZSM-5 nanozeolite was prepared. It
must be emphasized that when TEOS was used as the silicon
source, it can be hydrolyzed and produce alcohol which was
proved to have structure directing effect on the formation of
35
ZSM-5 nanozeolite. The crystallite size (D
c
) of synthesized
ZSM-5 nanozeolite was also calculated using Debye–Scherrer
37
equation:
Kl
D
c
¼
(1)
b cos q
where K is the shape factor, which usually takes a value about
.9, l is the wavelength of the X-ray source used in XRD (0.15418
0
nm), b is the breadth of the observed diffraction line at its half-
intensity maximum in radian and q is the main Bragg peak
angle. According to the obtained results from XRD pattern, b is
ꢂ
0
.115 degree at 2q ¼ 7.98 . The crystallite size (D ) of ZSM-5
c
nanozeolite was found to be 69.3 nm.
Fig.
1 The representation XRD pattern of synthesized ZSM-5 Fig. 2 The representation (a) SEM and (b) TEM images of synthesized
nanozeolite.
ZSM-5 nanozeolite.
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3.2. Electrochemistry of fabricated electrodes
Potassium ferricyanide (K Fe(CN) ) was selected as a probe to
4
6
evaluate the performance of the fabricated bare CPE, MW/CPE,
ZSM-5/CPE and MW–ZSM-5/CPE electrodes using CV tech-
nique. Fig. 5 illustrates the typical CVs of the electrochemical
oxidation of K
4
6
Fe(CN) at the surface of these electrodes in the
1
0 mM of K Fe(CN)
4
6
and 0.1 M KCl solution. It is obvious that
the electron transfer rate was slow, with a peak-to-peak sepa-
ration (DE ) of 240 mV at the CPE indicating an irreversible
p
electron transfer process. Meanwhile, the MW/CPE, ZSM-5/CPE
and MW–ZSM-5/CPE displayed a well-shaped cyclic response for
3
ꢀ
4ꢀ
the Fe(CN)
and 150 mV, respectively. These are a quasi-reversible system
because DE is greater than 59 mV that expected for a reversible
system. As can be seen, the oxidation and reduction currents for
6
/Fe(CN)
6
p
redox couple with a DE of 130, 170
Fig. 3 Particle size distribution of ZSM-5 nanozeolite.
p
3
ꢀ
4ꢀ
the Fe(CN)6 /Fe(CN)6 redox couple is higher than that at the
surface of MW/CPE and ZSM-5/CPE electrodes. Therefore, the
fabricated MW–ZSM-5/CPE electrodes was selected for further
experiments. The obtained result from CV of MW–ZSM-5/CPE in
various buffered solutions with different pH value does not
show any shi in the anodic peak potentials for oxidation of
sizes of ZSM-5 nanozeolites are between 45 and 130 nm and the
mean particle size is about 87 nm, corresponding to cumulative
volume frequency of 84%.
FTIR spectroscopy is a very useful technique for obtaining
vibrational information about the species in materials. Fig. 4
illustrates FT-IR spectrum of as-synthesized ZSM-5 nanozeolite.
K
4
Fe(CN)
behavior of the Fe(CN)
ZSM-5/CPE electrode is not dependent on the solution pH.
6
. It can be concluded that the electrochemical
ꢀ1
The bands located at 1070–1230 cm are characteristic of SiO
tetrahedron units. The absorption bands at 1230 and 550 cm
4
1
3ꢀ
4ꢀ
6
/Fe(CN)
6
redox couple in the MW–
ꢀ
40
supply information on the difference between ZSM-5 nano-
zeolite and other types of zeolites. The external asymmetric
ꢀ
1
stretching vibration near 1230 cm is due to the presence of
structures containing four chains of four-member rings
between SiO and AlO tetrahedral of ZSM-5 structure and is
3
.3. Electrochemical behavior of fabricated electrodes in
alkaline solution
4
4
38
Inset (a) in Fig. 6 shows CVs of bare CPE and MW–ZSM-5/CPE in
0.1 M NaOH solution at the potential range from 0.2 to 0.7 V vs.
a structure sensitive IR band of ZSM-5 zeolite. The bands near
100 cm and 800 cm are due to the internal asymmetric
stretching and external symmetric stretching of external link-
ages, respectively. Also, the band at about 550 cm is attributed
to a structure-sensitive vibration caused by the double four-
member rings of the external linkages and the band near 450
ꢀ
1
ꢀ1
1
ꢀ1
Ag|AgCl|KCl (3 M) and potential sweep rate of 20 mV s . As can
be seen, no current was obtained with these electrodes and the
background current for MW–ZSM-5/CPE is larger than that at
bare CPE due to high surface area. Also, Fig. S1 (in the ESI†)
shows CVs of MW/CPE and ZSM-5/CPE in 0.1 M NaOH solution.
No current was obtained with these electrodes and the back-
ground current for ZSM-5/CPE is larger than that at MW/CPE.
The inclination of the base line for the voltammogram of
ꢀ
1
ꢀ
1
cm is ascribed to the Si–O and Al–O bending vibration due to
3
9
the formation of only ZSM-5 crystal. The broad bands at 2800–
ꢀ
1
ꢀ1
3
700 cm (centered at 3450 cm ) and band at about 1630
ꢀ
1
cm are attributed to the stretching and bending vibration of
water molecules adsorbed on hydroxyl groups, respectively.
26
Fig. 5 The cyclic voltammograms of (a) bare CPE, (b) ZSM-5/CPE, (c)
4 6
MW/CPE and (d) MW–ZSM-5/CPE in the presence of 10 mM K Fe(CN)
ꢀ1
solution at a scan rate of 20 mV s and pH of 7.0 in 0.1 M KCl as
supporting electrolyte before immersion of these electrode in 0.5 M
Fig. 4 The FT-IR spectrum of synthesized ZSM-5 nanozeolite.
2
NiCl solution.
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Fig. 6 The cyclic voltammograms of the Ni/CPE and Ni–MW–ZSM-5/CPE in the absence (a and c) and presence (b and d) of 0.005 M glucose in
ꢀ1
0
.1 M NaOH at scan rate of 20 mV s , respectively. The inset (a) shows cyclic voltammograms of the bare CPE and MW–ZSM-5/CPE in 0.1 M
NaOH before immersion in 0.5 M NiCl solution. The inset (b) displays Tafel plot derived from the rising part of the curve d.
2
ZSM-5/CPE electrode can be attributed to the additional resis- surfaces of Ni/CPE, Ni–MW/CPE and Ni–ZSM-5/CPE, respec-
tance obtained from the presence of ZSM-5 nanozeolite and the tively. Also, the Ni/CPE, Ni–MW/CPE, Ni–ZSM-5/CPE and Ni–
oxidation–reduction process of oxygen species in the molecular MW–ZSM-5/CPE displayed a well-shaped cyclic response for the
4
1
sieve layer.
Ni(OH) /NiOOH redox couple with DE of 150, 120, 140 and 130
2 p
2
+
In order to incorporate Ni ions at the surface of the CPE mV, respectively. As can be seen in Fig. S2,† the presence of the
and MW–ZSM-5/CPE electrodes, these electrodes were MWCNTs in CPE could enhance the peak currents and decrease
immersed in a well-stirred aqueous solution of 0.5 M NiCl
0 min at 150 rpm and then washed completely with distilled advantages of Ni–MW/CPE had been explained by the good
water to remove the surface adsorbed species. Fig. 6c demon- electrical conductivity, high chemical stability and high surface
2
for the oxidation potential by decreasing the overpotential. The
1
45
strates CV of Ni–MW–ZSM-5/CPE (MW–ZSM-5/CPE that area and fast electron transfer rate of MWCNTs. Also, the peak
immersed in the 0.5 M NiCl ) in 0.1 M NaOH solution and scan current of Ni(OH) oxidation at the surface of Ni–ZSM-5/CPE is
2
2
ꢀ
1
rate of 20 mV s . It can be inferred that the electrochemical much greater than that at bare CPE (see curves a in Fig. S3† and
behavior of Ni–MW–ZSM-5/CPE modied electrode in alkaline 6). It can be expected that diffusion of Ni ions in ZSM-5
solution is similar to that of Ni anode.
are ascribed to the oxidation of Ni(OH)
ZSM-5/electrolyte to NiOOH and reduction of NiOOH to Ni(OH)
2
+
4
0,42,43
These redox waves nanozeolite is much faster owing to their coordination to the
2
at interface of the MW– nanozeolite framework, the bigger cages and channels and fast
2
+
23,46
2
migration of Ni ions from the cages.
These results
ꢀ
1
at scan rate of 20 mV s with a peak potential of 0.54 and 0.41 V conrmed that the presence of both MWCNTs and ZSM-5 on the
vs. Ag|AgCl|KCl (3 M), respectively.
42,43
The broad cathodic peak fabrication of Ni–MW–ZSM-5/CPE had a great inuence on
can be ascribed to the phase transformation of b-NiOOH to g- improving the oxidation currents and decreasing the
NiOOH due to slow, irreversible overcharging during cycling, overpotential.
and the corresponding reduction to a-Ni(OH) . This phenom-
enon originally reported by Bode et al.
The electrocatalytic oxidation of the glucose was investigated
at the surface of Ni/CPE and Ni–MW–ZSM-5/CPE in 0.1 M NaOH
2
44
Aer stabilization of nickel species on the surface of the solution. Curves b and d in Fig. 6 display the CVs for electro-
electrodes, the anodic and cathodic peak currents for Ni–MW– catalytic oxidation of the glucose in the surfaces of Ni/CPE and
ZSM-5/CPE were much greater than those at Ni/CPE (see Fig. 6a Ni–MW–ZSM-5/CPE electrodes in 0.1 M NaOH solution and
ꢀ1
and c) and were greater than those at Ni–MW/CPE and Ni–ZSM- 0.005 M glucose at the scan rate of 20 mV s , respectively. In
/CPE (see curves a in Fig. S2 and S3†). It can be realized that the the presence of glucose, an increase in current was observed at
5
peak current of Ni(OH) oxidation at the surface of Ni–MW– the surface of Ni–MW–ZSM-5/CPE but no signicant variation
2
ZSM-5/CPE is about 2.8, 2.3 and 1.5-fold greater than that at the in the current intensity was observed at the surface of Ni/CPE.
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For comparison, the Ni–MW/CPE and Ni–ZSM-5/CPE modied electrodes (5, 7.5, 10 and 12.5 wt% of MWCNTs together with 25
electrodes were utilized to 0.005 M glucose oxidation in 0.1 M wt% of ZSM-5 nanozeolite with respect to the graphite powder)
ꢀ
1
NaOH solution at the scan rate of 20 mV s (see curves b in were prepared and studied for their voltammetric signal under
Fig. S2 and S3†). The anodic current densities for electro- identical conditions in 0.005 M glucose at 0.1 M NaOH solution
oxidation of glucose at the surface of Ni–MW/CPE and Ni–ZSM- (see Fig. S5 in ESI†). The result showed that with increasing the
ꢀ
2
5
/CPE were obtained to be 0.98 and 1.2 mA cm , respectively. amount of MWCNTs 10 wt%, the oxidation peak current for
The Ni–ZSM-5/CPE displayed a similar oxidation peak for glucose was increased and then began to level off. Therefore, we
glucose to the Ni–MW–ZSM-5/CPE, meanwhile the catalytic selected these conditions (MWCNTs 10 wt% and ZSM-5 nano-
oxidation current for the Ni–ZSM-5/CPE modied electrode was zeolite 25 wt% with respect to the graphite powder) for prepa-
much smaller than that at the surface of the Ni–MW–ZSM-5/ ration of modied electrode.
CPE. For example, the anodic current densities at 0.005 M
glucose for Ni–MW–ZSM-5/CPE and Ni–ZSM-5/CPE are 4.3 and
3
2
.4. Effect of scan rate on the NiOOH to Ni(OH) conversion
ꢀ2
1.2 mA cm , respectively. This may be due to the unique
The electrochemical behavior of Ni–MW–ZSM-5/CPE was
studied in 0.1 M NaOH at various scan rates. Fig. 7 displays the
CVs of the Ni–MW–ZSM-5/CPE electrode in 0.1 M NaOH at
various scan rates from 5 to 500 mV s . With increasing of scan
rate, the anodic and cathodic currents were enhanced and the
anodic peaks moved to more positive potentials and the
cathodic peaks shied to more negative potential values. This
result indicates a limitation in the charge-transfer kinetics
which is due to chemical interactions between the electrolyte
ions and the modied electrode.
according to a theory dened by Laviron for the linear potential
sweep voltammetric response in the case of surface conned
electroactive species at the small concentrations. The expres-
sions for peak to peak separation of DE > 0.2/n V, where n is the
structure and good electrical properties of MWCNTs, which
could help to increase the electrical conductivity of catalysts
28,29
materials.
As a result, this improvement in the current
ꢀ
1
densities demonstrates that incorporation of both MWCNTs
and ZSM-5 onto a carbon paste electrode enhances the elec-
trochemical signal of glucose oxidation.
It can be observed clearly in Fig. 6d that the oxidation of
glucose gives rise to a typical electrocatalytic response, with an
increase in the anodic peak current and together with
a decrease in the cathodic peak current. The oxidation potential
of glucose is observed at 0.63 V vs. Ag|AgCl|KCl (3 M) that is
49,50
This observation is possible
2
+
3+
positive than the potential observed for Ni to Ni transition at
the Ni–MW–ZSM-5/CPE in the absence of glucose (i.e. 0.54 V).
This recommends an interaction between the glucose and the
51
p
number of exchanged electrons, can be written as the following
equations:

lm redox sites conned at the electrode surface.
In order to obtain information about the rate determining
ꢀ
ꢁ
step, a Tafel plot was illustrated for Ni–MW–ZSM-5/CPE using
the data derived from the raising part of the current–voltage of
curve d in Fig. 6. This part of voltammogram, known as Tafel
region, is affected by electron transfer kinetics between the
0
1 ꢀ a
E
pa ¼ E þ X ln
(2)
(3)
m
h i
a
0
Epc ¼ E þ Y ln
47
m
glucose and the modied electrode. Inset (b) in Fig. 6 illus-
trates the plot of log I vs. E in the presence of 0.005 M glucose
ꢂ
ꢃ
p
ꢀ
p
RT
1
log k
s
¼ a logð1 ꢀ aÞ þ ð1 ꢀ aÞlog a ꢀ log
at scan rate of 20 mV s in 0.1 M NaOH solution on the surface
nFy
of Ni–MW–ZSM-5/CPE. The Tafel slope is equal to n(1 ꢀ a)F/
ꢂ
ꢃ
að1 ꢀ aÞnFDE
p
2.303RT which comes up to 5.0669 V per decade, which indi-
ꢀ
(4)
2
:303RT
cates that transfer coefficient (a) for electrooxidation of glucose
is about 0.85.
where X ¼ RT/(1 ꢀ a)nF, Y ¼ RT/anF, m ¼ (RT/F)(k
s
/ny), Epa and
The electrocatalytic behavior of the modied electrodes with
Epc are the anodic and cathodic peak potentials, respectively.
ZSM-5 nanozeolite ratio of 5, 10, 15, 20, 25 and 30 wt% together Also, a, k and y are the electron-transfer coefficient, apparent
s
with 10 wt% of MWCNTs with respect to the graphite powder charge-transfer rate constant and scan rate, respectively. From
were studied by CV technique in 0.005 M glucose at 0.1 M of these expressions, the a can be calculated by measuring the
NaOH solution (see Fig. S4 in ESI†). It was experimentally variation of the peak potential with respect to the scan rate and
determined that higher anodic current of glucose oxidation is ks can be determined for electron transfer between the electrode
observed when 25% of nanozeolite was used. It is suggested that and the surface layer by measuring the E values.
p
with increasing the nanozeolite over than 25% in the modied
The inset (a) in Fig. 7 displays the plots of the variation of E
p
electrode, the resistance of the modied electrode may increase with respect to the logarithm y in the ranges of 0.100–0.500 V
ꢀ
1
due to low conductivity of ZSM-5 nanozeolite and a decrease in
s
for both anodic and cathodic peaks in 0.1 M NaOH onto Ni–
glucose oxidation current is distinguished. Also, at lower ratios MW–ZSM-5/CPE. It can be seen that E is proportional to log y at
p
ꢀ1
51
of nanozeolite in Ni–MW–ZSM-5/CPE, the amount of available y > 0.100 V s as demonstrated by Laviron. From inset (a) in
pores for Ni insertion decrease and available NiOOH active Fig. 7 and eqn (2) and (3), the value of anodic electron transfer
2
+
site is reduced for glucose oxidation which ultimately cause coefficient (a) is found to be 0.65. This results show that the rate
2
5,48
a low current density.
The amount of MWCNTs in Ni–MW– limiting steps for cathodic and anodic might not be the same
48
ZSM-5/CPE had a signicant effect on the anodic oxidation step. According to the eqn (4), the mean value of charge-
current of glucose. Four different modied carbon paste transfer rate constant (k ) is calculated to be 0.184 s .
s
ꢀ1
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Fig. 7 The cyclic voltammograms of the Ni–MW–ZSM-5/CPE in 0.1 M NaOH at various scan rates from inner to outer: 0.005, 0.010, 0.015,
ꢀ
1
0
.025, 0.040, 0.060, 0.080, 0.100, 0.150, 0.200, 0.250, 0.300, 0.350, 0.400, 0.450 and 0.500 V s . The inset (a) shows the plot of E
p
vs. log y for
CVs at scan rates 0.100–0.500 for anodic peaks a and cathodic peaks b. The inset (b) shows the dependency of Ipa and Ipc on lower values of y
ꢀ1
(
0.005–0.080 V s ) for anodic peaks a and cathodic peaks b.
The inset (b) in Fig. 7 shows the plots of anodic and cathodic this process contributed directly to the electrocatalytic oxida-
0
peak currents for oxidation–reduction of the NiOOH/Ni(OH)
2
tion of glucose with electrochemical catalytic (EC ) mechanism.
33
redox couple versus scan rate on the surface of Ni–MW–ZSM-5/ According to the literature, the oxidation of glucose to gluco-
CPE at low values from 0.005 to 0.008 V s in 0.1 M NaOH nolactone (two hydrogen are liberated in this process) can be
ꢀ1
solution. This dependence is probably due to electrochemical catalyzed by the NiOOH/Ni(OH) redox couple in the alkaline
2
activity of immobilized redox species at the surface of modied medium. When glucose diffuses from the bulk solution to the
electrode. The electrode surface coverage (G*) can be calculated surface of the electrode, it is quickly oxidized to gluconolactone
from the linear part of the plot and using the following equation by the NiOOH species on the surface of modied electrode.
which corresponds to reversible process with adsorbed Therefore, the amount of NiOOH species decreases due to its
4
7
species.
chemical reaction with glucose. Simply, the electrocatalytic
oxidation mechanism of glucose at Ni–MW–ZSM-5/CPE can be
(5) described by the following equations that is the basis for the
2
2
n F AG*y
I
p
¼
4RT
fabrication of a nonenzymatic sensor for electrochemical
detection of glucose:
where I
p
, n, A and G* are the peak current, the number of
electrons involved in the reaction (n ¼ 1), the surface area of the
2
ꢀ
electrode (0.096 cm ) and the surface coverage of the redox
[(Ni(OH) –MW–ZSM-5/CPE)] + OH
2
%
ꢀ
species, respectively. The total surface coverage of the immo-
[(NiOOH–MW–ZSM-5/CPE)] + H O + e (E)
2
ꢀ8
bilized active species is found to be about 4.04 ꢁ 10 mol
ꢀ
2
cm , considering the mean of both anodic and cathodic [(NiOOH–MW–ZSM-5/CPE)] + glucose /
0
[(Ni(OH) –MW–ZSM-5/CPE)] + glucolactone (C )
2
currents. Also, the amount of loaded nickel onto the Ni–MW–
ZSM-5/CPE lm is about 0.228 mg.
3.5. Electrocatalytic oxidation of glucose at the surface of
Ni–MW–ZSM-5/CPE
3.6. Effect of scan rate on the oxidation of glucose
As can be seen in Fig. 6c and d, the enhanced anodic peak was The dependence of the glucose electrocatalytic oxidation
appeared around 0.63 V vs. Ag/AgCl/KCl (3 M); meanwhile, current on the scan rate of the potential (y) under the optimal
ꢀ
1
cathodic peak current around 0.41 V was decreased aer addi- conditions was investigated in the range of 10–300 mV s at the
tion of glucose. It can be deduced that the over potential for surface of Ni–MW–ZSM-5/CPE. Fig. 8a displays CVs of Ni–MW–
glucose electrooxidation was decreased in the surface of ZSM-5/CPE in the presence of 0.005 M glucose in 0.1 M NaOH.
modied electrode. Also, the anodic current for glucose oxida- The anodic peak currents (I ) increased and the anodic peak
pa
tion is higher than that for Ni(OH)
2
/NiOOH conversion in the potentials shied to more positive directions by increasing the
same scan rate. It can be concluded that the applied modier in scan rate. It can be proposed that a kinetic limitation was
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Fig. 8 (a) The cyclic voltammograms of the Ni–MW–ZSM-5/CPE in the presence of 0.005 M glucose in 0.1 M NaOH at various scan rates from
ꢀ1
1/2
inner to outer: 0.010, 0.025, 0.050, 0.075, 0.100, 0.150, 0.200 and 0.300 V s . (b) Variation of Jpa vs. y . (c) The plot of log Ipa vs. log y and (d) the
plot of Ipa/y vs. y.
1/2
occurred in the reaction between the redox sites of the glucose 3.7. Chronoamperometric studies
49,50
and Ni–MW–ZSM-5/CPE at high scan rate.
The increase in the
The electrocatalytic oxidation of the glucose at the surface of
modied electrode was also considered by chronoamperometry
method. Fig. 9 illustrates chronoamperometric measurements
for glucose at the Ni–MW–ZSM-5/CPE. It shows the current–
time proles obtained at a working electrode potential of 650
mV vs. Ag|AgCl|KCl (3 M) for various concentrations of glucose
peak current with the y can be considered for adsorption or
diffusion control of the process. Fig. 8b shows the plot of anodic
1
/2
current density (J ) versus square root of scan rate (y ) that
pa
obtained from CVs of Ni–MW–ZSM-5/CPE in 0.005 M glucose and
1
/
0
.1 M NaOH solution. As can be observed, the plot of Jpa versus y
2
1/2
was found to be linear with equation of Jpa (mA) ¼ 0.4542y (mV
(0, 0.147, 0.476 and 1.67 mM). It was found that the observed
ꢀ
1 1/2
2
s
)
+ 2.0091 and R ¼ 0.9976, meanwhile, the plot of Jpa
current from chronoamperograms was in good agreement with
the detected current from cyclic voltammetry experiment and
the current increases as the glucose concentration increases
against y did not show a linear curve (see Fig. S6 in ESI†). From
this observation, it can be realized that this process is diffusion-
47,48
controlled process rather than surface-controlled process.
(curves b–d). This result supports our conclusion about the
From the theoretical point of view, a slope of 0.5 or 1.0 is
catalytic role of NiOOH for oxidation of glucose that glucose
expected for the plot of log Ipa vs. log y under diffusion- or
oxidation starts directly aer the formation of the rst amount
42
adsorption-controlled process, respectively. A linear depen-
dence is observed between log Ipa and log y at the surface of Ni–
MW–ZSM-5/CPE in the glucose oxidation (see Fig. 8c). From
linear section, the slope of 0.3099 is found close to the theo-
retically predicted value of 0.5 for a purely diffusion-controlled
current. However, the contribution of a kinetic limitation to the
overall process cause to the small alteration with theoretical
54
of NiOOH on the surface of electrode.
An exponential behavior of achieved I–t curves shows that
a diffusion-controlled process has occurred according to Cot-
trell equation. From the chronoamperometric study, the diffu-
sion coefficient of glucose, D, was determined in aqueous
55
solution by using Cottrell equation as below:
4
8,52
value.
Fig. 8d displays a plot of scan rate normalized current
1/2 ꢀ1/2 ꢀ1/2
I ¼ nFACD
p
t
(6)
1
/2
(
I /y ) vs. logarithm scan rate (log y). The characteristic shape
pa
0
of an electrochemical catalytic (EC ) mechanism is detected where F is the faraday number, A is the area of the electrode, C is
which express that the electrode reaction is coupled with an the known concentration of glucose and D is the apparent
53
irreversible follow up chemical step. Also, at higher potential diffusion coefficient. Inset (a) in Fig. 9 demonstrates experi-
ꢀ
1/2
scan rates, the cathodic peak current relating to Ni(III) reduction mental plots of I vs. t
for 0.147 mM of glucose at the surface
0
50
to Ni(II) appears which conrms EC mechanism.
of Ni–MW–ZSM-5/CPE. The same curves were plotted for all
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Fig. 9 Main panel: chronoamperograms obtained at Ni–MW–ZSM-5/CPE in 0.1 M NaOH solution in the a absence and presence of b 0.147, c
ꢀ1/2
0
.476 and d 1.67 mM glucose, (the potential step was 0.65 V vs. Ag/AgCl/KCl (3 M)). The inset (a) displays plot of I vs. t
, derived from the data of
chronoamperogram b. The inset (b) illustrates plot of the slope of the straight lines against glucose concentration. The inset (c) shows plots of
1/2
I
L
cat/I vs. t , obtained from chronoamperograms b–d.
concentrations and then the slopes of the resulting straight 3.8. Effects of glucose concentration and calibration curves
lines were plotted vs. the glucose concentration (see inset (b) in
Fig. 9). From the slope of the resulting plots and using the
Fig. 10 illustrates the effect of glucose concentration on its
electrooxidation current onto Ni–MW–ZSM-5/CPE at scan rate
Cottrell equation, the mean value of the D was obtained to be
ꢀ1
of 25 mV s . It is clearly observed that the anodic peak current
ꢀ
2
4
2
ꢀ1
ꢀ1
1
0
.66 ꢁ 10 cm s (with n ¼ 2, F ¼ 96 485 C mol and A ¼
increased with increasing of glucose up to the concentration of
56
.096 cm ).
Chronoamperometry can be applied for the evaluation of the
6.1 mM (see Fig. 10a and b). In the concentrations above 6.1
mM, no remarkable increase in the anodic peak current was
observed. It can be specied that this effect may be due to the
saturation of active sites and/or poisoning the electrode surface
with adsorbed intermediates. Thus, 6.1 mM of glucose repre-
sented the optimum concentration aer which the adsorption
of the oxidation products at the surface of electrode may cause
the stoppage of further oxidation. The dependence of the
anodic peak current on the glucose concentration is shown in
the Fig. 10c. As can be seen, the plot of peak current versus
glucose concentration consists of two linear regions with
different slopes, corresponding to two different substrate
concentration ranges. The decrease in sensitivity (slope) in the
catalytic rate constant (kcat) of the electrocatalytic oxidation of
glucose on the active sites of the modied electrode according
by the following equation:
49
I
cat
1=2
1=2
¼
p
ðkcat
C
0
tÞ
(7)
I
L
where, Icat and I
L
are the currents in the presence and absence of
is the catalytic rate
glucose, respectively. The symbol k
cat
3
ꢀ1 ꢀ1
constant (cm mol
s ), C0 is the bulk concentration of
ꢀ3
glucose (mol cm ) and t is the elapsed time (s). Inset (c) in
Fig. 9 shows the experimental plots with the best ts for the
different concentrations of glucose. From the slopes of the Icat
/
58
1
/2
second linear range is caused by kinetic limitations. From
analysis of these data, we expected that the detection limit (S/N
L
I versus t for all concentrations, the mean value of kcat was
8
3
ꢀ1 ꢀ1
calculated to be 1.136 ꢁ 10 cm mol
s . Comparison of the
¼
3) in the lower linear range was calculated to be 0.14 mM.
Since differential pulsed voltammetry (DPV) has a much
estimated kcat with other kcat values in the literature is per-
formed and presented in Table 1.
1
4,16,56,57
higher current sensitivity and better resolution than CV, DPV was
investigated under the optimized conditions for determination of
glucose. Fig. 11 displays typical DPVs with increasing glucose
concentration leading to a linear increase in the peak current.
Results indicated that this sensor gave a good linear response to
the glucose concentration with two glucose concentration ranges:
Table 1 Comparison of the catalytic rate constant of Ni–MW–ZSM-5/
CPE for oxidation of glucose with some of the previously reported
work in 0.1 M NaOH
3
ꢀ1 ꢀ1
Electrode
k
cat (cm mol
s
)
Ref.
ꢀ
4
ꢀ3
1
ꢁ 10 to 1 ꢁ 10 mM which could be tted to the regression
6
Cu–Co–Ni/carbon nanobers/GCE
Ni(II)–Qu–MWCNT–IL–PE
2.2 ꢁ 10
14
15
51
52
2
6
equation of J
p
¼ 1194.9Cglucose + 3.4944 (R ¼ 0.9975, Cglucose is in
ꢀ3 ꢀ2
1.91 ꢁ 10
7
6
8
mM), and 1 ꢁ 10 to 1 ꢁ 10 mM which could be tted to the
Cu-based metal–organic framework 1.554 ꢁ 10
2
Ni–POAP/CPE
Ni–MW–ZSM-5/CPE
1.8 ꢁ 10
regression equation of J
p
¼ 126.49Cglucose + 4.593 (R ¼ 0.9979,
1.136 ꢁ 10
This work
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Fig. 10 The cyclic voltammograms of the Ni–MW–ZSM-5/CPE in 0.1 M NaOH solution with various concentrations of glucose: (a) 0.0–3.4 mM
ꢀ1
and (b) 3.8–6.1 mM at scan rate of 20 mV s . (c) The plot of electrocatalytic current density (J) vs. glucose concentration.
MWCNTs and ZSM-5 nanozeolites. This value is much lower than
those reported in enzymeless glucose sensor in the literature (see
Table 2) and CV method in this work.
3.9. Reproducibility and stability of the Ni–MW–ZSM-5/CPE
The reproducibility and stability of the fabricated electrode were
evaluated via the comparison of the currents of different elec-
trodes using CV technique. The anodic current of ve modied
Ni–MW–ZSM-5/CPE electrodes to 0.005 M glucose was tested
independently, and the relative standard deviation (RSD) was
3.26%. A reproducible current response with a RSD of 2.62%
was observed for 5 successive assays of 0.005 M glucose. The
long-term stability was explored by measuring a glucose solu-
tion intermittently, and the electrode was stored at room
temperature when it is not in use. Aer 1 and 3 months, the
electrode response to electrocatalytic oxidation of glucose
retains 93% and 87% of initial value, respectively.
Comparisons of fabricated sensor with other nonenzymatic
glucose sensor are done and results are listed in Table 2. As can
be seen in this table, the proposed glucose sensor shows
good sensing performance in terms of LOD and sensitivity.
In
works,
comparison
with
some
previously
reported
1
0,14,16,17,34,56,59–63
it seems that Ni–MW–ZSM-5/CPE can act
Fig. 11 (a) Differential pulse voltammograms of Ni–MW–ZSM-5/CPE
in 0.1 M NaOH solution containing different concentrations of glucose
as an efficient electrocatalyst in glucose oxidation process. It
from inner to outer: 0, 0.0001, 0.0005, 0.0008, 0.001, 0.003, 0.0055, must be noted that the sensitivity of the fabricated electrode is
ꢀ
1
0
.0075 and 0.01 mM (the scan rate: 50 mV s , step height: 5 mV, step
higher than that of previous works (see Table 2).
.10. Interferences and blood serum sample measurement
width: 1 s, pulse width: 200 ms, pulse height: 50 mV, and sample
period: 50 ms). (b) The plot of electrocatalytic current density (J) as
a function of glucose concentrations.
3
Because the electrochemical oxidation of NiOOH to glucose has
no special selective response, the effects of some interfering
Cglucose is in mM). The detection limit (S/N ¼ 3) in the lower linear substances on the glucose biosensor were investigated in 0.1 M
ꢀ
5
range was calculated to be 3.5 ꢁ 10 mM which may be attrib- NaOH solution. The interference compounds were dopamine
uted to the high efficiency of the electron transfer mediator (DA), ascorbic acid (AA) and urea acid (UA) in view of a possible
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Table 2 Comparison of the various nonenzymatic glucose sensors based on different electrodes
Applied potential
(V) vs. Ag/AgCl/KCl (3 M) Sensitivity (mA mM cm
ꢀ1
ꢀ2
Glucose sensors
)
Linear range
LOD (mM) Ref.
Cu–Co–Ni/carbon nanobers/GCE
Ni(II)–Qu–MWCNT–IL–PE
Ni/Al-LDH/chitosan/GCE
Cu-based metal–organic framework
Cu nanocluster/MWCNT/GCE
Dimethylglycoxime/Cu (DMG–CuNPs) 0.69
CuO/modied Cu electrode
Ni/carbon nanober paste electrode
Ordered mesoporous CPE
0.55
0.48
0.48
0.59
0.65
104.68
36.9 (mA mM
0.182 (mA mM
—
251.38
700
2792.64
420.4
10.81
10 mM to 4.30 mM 3.05
14
15
24
51
54
55
56
57
58
ꢀ
ꢀ
1
)
5 mM to 2.8 mM
10 mM to 10 mM
0.2–8.0 mM
1.0
1
)
10.0
29.8
0.21
0.5
0.7–3.5 mM
1 mM to 5.0 mM
0.8 mM to 2.2 mM
2 mM to 2.5 mM
0.5–2.5 mM
0.50
0.60
0.45
0.8
1.0
20.0
2.5–5 mM
CuO nanorods/graphite
0.60
0.55
0.63
371.43
202
510.3
4 mM to 8 mM
50 mM to 23 mM
0.5–6.1 mM
4.0
6.0
140
0.035
59
60
Nanoscale Ni(OH)
2
modied CILE
Ni–MW–ZSM-5/CPE
This work
1194.9
0.1–10 mM
application of the developed biosensor to the analysis of glucose preparation and regeneration, stable response and very low
in human serum. Since the normal physiological level of ohmic resistance.
glucose in the human blood is about 30 times of AA, DA and
64
UA, the interference effect of 20 mM AA, DA and UA were
investigated on the CV response of 600 mM of glucose. At this
level, the current response for such electroactive interfering
species to that of glucose by the sensor was below 5%. There-
fore, good selectivity for glucose can be achieved with the
fabricated sensor.
In order to observe whether there is no matrix effect in blood,
the fabricated sensor was used to determine glucose in blood
serum samples with CV method. When considering the normal
and clinical range of blood sugar (4–6 mM) in human blood
serum, the analytical applicability of the Ni–MW–ZSM-5/CPE
biosensor for analysis of glucose in human blood developed
in this work proves promising. Therefore, the Ni–MW–ZSM-5/
CPE shows a sufficient selectivity to be used for determination
of glucose at high or low levels in blood.
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