Morphology-dependent electrochemical sensing properties of manganese dioxide-graphene oxide hybrid for guaiacol and vanillin

Article abstract of DOI:10.1016/j.electacta.2014.09.116

Various morphologies of manganese dioxide (MnO₂) electrocatalysts, including nanoflowers, nanorods, nanotubes, nanoplates, nanowires and microspheres were prepared via facile hydrothermal synthesis and precipitation methods. By simply grinding

Full text of DOI:10.1016/j.electacta.2014.09.116

Electrochimica Acta 147 (2014) 157–166
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Morphology–dependent electrochemical sensing properties of
manganese dioxide–graphene oxide hybrid for guaiacol and vanillin
*
Tian Gan , Zhaoxia Shi, Yaping Deng, Junyong Sun, Haibo Wang
College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 11 July 2014
Received in revised form 20 September 2014
Accepted 24 September 2014
Available online 28 September 2014
Various morphologies of manganese dioxide (MnO2) electrocatalysts, including nanoflowers, nanorods,
nanotubes, nanoplates, nanowires and microspheres were prepared via facile hydrothermal synthesis
and precipitation methods. By simply grinding with graphene oxide (GO), MnO2 could be readily
dissolved in water with high solubility and stability. The structures and electrochemical performances of
these as–prepared MnO2–GO hybrids were fully characterized by various techniques, and the properties
were found to be strongly dependent on morphology. As sensing materials for the simultaneous
determination of guaiacol and vanillin for the first time, the nanoflowers–like MnO2, coupled with GO,
exhibited relatively high sensitivity. The enhanced electrocatalytic activity was ascribed to the high
purity, good crystallinity, and unique porous microstructure, which were favorable for transfer of
electrons. These results may provide valuable insights for the development of nanostructured modified
electrodes for next–generation high–performance electrochemical sensors.
Keywords:
Morphology–dependent
manganese dioxide
Graphene oxide
guaiacol
vanillin
ã 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Therefore, there is an interest in developing a simple, sensitive and
rapid method such as electrochemical sensor for the determina-
Guaiacol and vanillin are two kinds of phenolic compounds that
widely distribute in various plants and also be found in common
foods and plant origin [1], which definitely have positive effect on
human health because of their anti–allergic, anti–artherogenic,
anti–inflammatory, antioxidant, antimutagens and antimicrobial
activity [2,3]. In some cases, they are essential for the overall food
flavour perception, but they become undesirable when their
concentration exceed certain limits, leading to typical phenolic
off–flavour [4]. More than that, if large amounts of these flavor
enhancers are ingested, they cause headaches, nausea and
vomiting, and can affect liver and kidney functions [5]. Conse-
quently, it is important to determine the contents of guaiacol and
vanillin in foods. A number of commonly used analytical methods
are available for qualitative and quantitative determination of
guaiacol and vanillin such as spectrophotometry [6], liquid
chromatography [7], capillary electrophoresis [8] and gas chro-
matography–mass spectroscopy [9]. However, these methods
require time–consuming sample preparation, expensive and
unportable equipment, and a skilled person to operate [10].
Nevertheless, the electrochemical sensors have advantages such as
low–cost instrumentation, fast analysis, and improved sensitivity.
tion of guaiacol and vanillin. Up to our knowledge, there is no
report about the simultaneous determination of guaiacol and
vanillin using electrochemical techniques.
Manganese dioxide (MnO2) is one of the most important
functional metal oxides not only due to its good electrochemical
properties, low toxicity, lowcost, relative abundance but also for its
wide applications. In the past few decades, with the advances of
nanoscience, the controlled synthesis of MnO2 nano–and micro-
crystals with desired shapes had fuelled fundamental studies, due
to their morphology had an important impact on the surface area,
active site and ion kinetics of materials, which could effectively
influence its electrochemical performance [11]. Therefore, it is
necessary to investigate the morphology–depending electrochem-
ical performance of various MnO2 nanostructures. Until now,
several types of morphologies have been made available through
studies including porous [12], rod [13], tube [14], plate [15], wire
[16], and sphere [17] MnO2. These studies show that nanostruc-
tured MnO2 leads to the improvement of its electrochemical
capacity [11]. However, the poor electrical conductivity and
dispersibility of MnO2 have limited its applications in electro-
chemical sensing.
It is well known that graphene oxide (GO), the derivant of
graphene, not only acts as a support for nanocrystals but also as a
conductive material for electron transfer in composite catalyst
systems [18,19]. In fact, with a large number of hydrophilic
* Corresponding author. Tel.: +86-376-6390702; fax: +86-376-6390597.
E-mail address: gantianxynu@163.com (T. Gan).
http://dx.doi.org/10.1016/j.electacta.2014.09.116
0013-4686/ã 2014 Elsevier Ltd. All rights reserved.

158
T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
Fig. 1. SEM images of MnO2 with nanoflowers (a,b), nanorods (c), nanotubes (d), nanoplates (e), nanowires (f) and microspheres (g,h) morphology.
oxygen–containing functional groups on a hydrophobic basal
plane, GO also behaves as an amphiphilic macromolecule [20].
With such structural features, GO can be utilized as a special kind
of surfactant to enhance the dispersion of MnO2 in water.
150 ꢁC prior to nitrogen adsorption measurements. The BET surface
area was determined by a multipoint BET method using the
adsorption data in the relative pressure (P/P0) range of 0.05–0.3.
An adsorption isotherm was used to determine the pore size
distributionbytheBarret–Joyner–Halenda(BJH)method, assuming
a cylindrical pore model. The nitrogen adsorption volume at the
relative pressure (P/P0) of 0.990 was used to determine the pore
volume (PV) and average pore size (APS). The number of surface
catalytic active sites per unit surface area (Ns) was quantified by
methanol chemisorption in a Cahn TGA microbalance (Model TG–
131) coupled with a PC for temperatureand weight monitoring[21].
TheMnO2 basedcatalystswereexposedtoamixtureof2000 ppmof
methanol vapor in He at 100 ꢁC that generated a stable monolayer of
surface methoxy species. The amount of adsorbed surface methoxy
species was determined gravimetrically.
Hereby, MnO2 catalysts with different morphologies including
nanoflowers, nanorods, nanotubes, nanoplates, nanowires and
microspheres were prepared by facile hydrothermal synthesis and
precipitation methods and then loaded on GO layers. The
electrochemical activities of guaiacol and vanillin on MnO2–GO
nanocomposites modified electrode were tested in this study. The
reasons for different electrocatalytic activities of MnO2 correlated
with different morphologies and for largely improved electro-
catalytic activity of MnO2–GO were explored. The following
electrochemical experiment showed that the MnO2 with nano-
flowers morphology had the most sensitive determination signals
for the simultaneous determination of guaiacol and vanillin, which
could be attributed to the big surface area and high active sites of
2.2. Synthesis of manganese dioxide–graphene oxide
MnO2 nanoflowers. Therefore,
a novel and highly sensitive
electrochemical method for simultaneous determination of
guaiacol and vanillin was proposed.
2.2.1. Manganese dioxide nanoflowers
In a typical procedure [12], 20 mL water containing 0.396 g of
Mn(NO3)2 6H2O and 0.1 g of triblock copolymer P123 was slowly
ꢀ
2. Experimental
added dropwise into 20 mL of 0.1 M KMnO4 with vigorous stirring
at 100 ꢁC. After that, the precipitate was quickly collected by
filtration, washed with water and absolute ethanol for six cycles,
and finally dried at 60 ꢁC for 6 h.
2.1. Chemicals and apparatus
Mn(NO3)2, KMnO4, MnCl2
ꢀ
4H2O, Mn(CH3COO)2 4H2O, MnSO4,
ꢀ
P123, sodium dodecylbenzenesulfonate (SDBS), urea, oxone
monopersulfate compound, AgNO3, (NH4)2S2O8, K2S2O8, P2O5
and graphite powder were purchased from Aldrich Chemicals
with analytical purity. All chemicals were used as received without
further purification. The water used was re–distilled.
2.2.2. Manganese dioxide nanorods
0.1 M MnSO4 and 0.1 M KMnO4 were firstly dissolved in 30 mL
water [13]. To this 1 mL of H2SO4 (60% concentration) was added
under vigorous stirring conditions for 30 min. The prepared
solution was transferred to a 100 mL Teflon–coated container
and autoclaved at 150 ꢁC for 30 min. The resultant solution was
centrifuged for 30 min at 8000 rpm and the obtained precipitate
was thoroughly washed using water and dried at 60 ꢁC for 6 h.
Electrochemical measurements were performed on a CHI 660 E
electrochemical workstation (CH Instruments Inc. Shanghai). A
conventional three–electrode system, consisting of a MnO2–GO
modifiedglassycarbonelectrode(GCE),asaturatedcalomelreference
electrode (SCE) and a platinum wire auxiliary electrode, was
employed. Field emission scanning electron microscopy (SEM) was
conductedwitha HitachiS–4800 microscope (Japan). Fieldemission
transmission electron microscopy (TEM) images were measured
usingaTecnaiG220S–Twinmicroscope(FEICompany,Netherlands).
Raman spectra were recorded using a micro–Raman spectrome-
ter (Witech CRM200, the excitation wavelength at 532 nm). Fourier
transform infrared spectroscopy (FTIR) was measured on a Bruker–
Tensor 27 IR spectrophotometer. Powder X–ray diffraction (XRD)
analyses were carried out with a Bruker D8 Advance X–ray
2.2.3. Manganese dioxide nanotubes
MnO2 nanotubes were prepared by a hydrothermal method [14].
0.45 g of KMnO4 and 1.0 mL of concentrated hydrochloric acid were
added to 40 mL of water and kept stirring for 20 min. Then the
Table 1
The specific areas and pore characteristics of the MnO2 with different morphologies
Samples
SBET (m2 gꢂ1
)
APS (nm)
PV (cm3 gꢂ1
)
MnO2 nanoflowers
MnO2 nanorods
MnO2 nanotubes
MnO2 nanoplates
MnO2 nanowires
MnO2 microspheres
207
179
122
94
87
86
3.4
3.6
7.1
4.8
11.5
27.7
0.51
0.50
0.32
0.17
0.13
0.11
diffractometer with Cu–K radiation (
l
= 1.5406 Å). The Brunauer–
a
Emmett–Teller(BET)surfacearea(SBET)oftheMnO2 was analyzedby
nitrogen adsorption at 77 K in a Micromeritics ASAP 2020 nitrogen
adsorption apparatus (U.S.A.). All the samples were degassed at

T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
159
Fig. 2. (A): DPVs of 1.0
mM guaiacol and vanillin on GCE modified by GO–MnO2 nanocomposites with different morphologies in pH 1.81 B–R buffer. Accumulation
time = 2 min, amplitude = 50 mV, pulse width = 0.2 s, pulse period = 0.5 s. (B): Oxidation peak current change of guaiacol and vanillin on different GCE surfaces.
mixture was transferred to a 50 mLTeflon–lined autoclave. Then the
autoclave was kept at 110 ꢁC for 10 h. After cooling down to room
temperature, the brown precipitation was collected by centrifuga-
tionat8000 rpmfor10 min. Theproductwas washedwithwater and
ethanol for six cycles and then dried at 60 ꢁC for 10 h.
and maintained at 120 ꢁC for 12 h. After the reaction was
completed, the resulting black solid product was filtered, washed
with water and dried at 120 ꢁC for 3 h.
2.2.6. Manganese dioxide microspheres
MnO2 microspheres were synthesized through catalytic oxida-
2.2.4. Manganese dioxide nanoplates
tion method [17]. 2 g Mn(CH3COO)2 4H2O, 15 g oxone monopersul-
ꢀ
In order to prepare MnCO3 precursor [15], 2 mmol MnCl2
ꢀ4H2O
fate compound and 5 mL AgNO3 solution (0.06 M) were mixed
together in 200 mL of water under room temperature. Then, 10 mL
concentrated sulfuric acid was added into the solution. After the
homogenous solution stood for 36h, the resulting black precipitate
was washedwithwater forsixtimesandthendriedat105 ꢁCfor24 h.
and 3 mmol urea were added into 45 mL of 3 mmol SDBS water
solution sequentially under stirring. After another 30 min, the
solution was transferred into a 50 mL Teflon–lined autoclave and
kept at 110 ꢁC for 12 h. The autoclave was cooled naturally to room
temperature. The precipitates were centrifuged, washed with water
and ethanol for six cycles and dried at 60 ꢁC for 10 h. Finally, the
MnCO3 was calcined at 400 ꢁC for 4 h to obtain MnO2 nanoplates.
2.2.7. Manganese dioxide–GO
GO was synthesized according to a modified Hummer’s method
mainly including two approaches [22]. The first process was the
preoxidation of graphite using thermic concentrated H2SO4,
K2S2O8, and P2O5. The second one was the further oxidation of
the preoxidized graphite with iced concentrated H2SO4 and
KMnO4. Followed by washing with 10% HCl solution and water,
GO could be obtained after being dried.
2.2.5. Manganese dioxide nanowires
MnO2 nanowires were synthesized according to the report [16].
0.008 mol MnSO4 H2O and an equal amount of (NH4)2S2O8 were
ꢀ
put into 35 mL water at room temperature to form a homogeneous
solution, which was then transferred into a Teflon–lined autoclave
Fig. 3. TEM images of GO (a), MF (b,c) and MFG (d,e,f).

160
T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
Fig. 4. Raman spectra (A), FTIR spectra (B) and XRD pattern (C) of MFG.
MnO2 with differentmorphologies were respectively mixed with
different morphologies. The well–shaped morphologies indicate
that MnO2 nanoflowers, nanorods, nanotubes, nanoplates, nano-
GO with a mass ratio of 1:5 in an agate mortar and ground for 2 h by
hand with no others to add, producing a dark brown MnO2–GO
colloidal powder with uniform color that was readily dissolved in
water by vigorously ultrasonic treatment to produce 2 mg mLꢂ1
suspension.Nofiltrationorwashingwasneededinthewholeprocess.
wires and microspheres are successfully prepared. Image
a
confirms the large–scale production of flower shaped nano-
structures, which are composed of interconnected nanoflakes,
showing a porous nature. The pore size between the sheets is
observed in a wide range of 6–32 nm (b). The thickness of the
nanoflakes is found to be about 6 nm. These indicate that specific
surface area of the nanoflowers should be great. SEM result in
image c reveals that the nanorods–shaped structure of MnO2 is
uniform, which has a diameter of 15–34 nm and a length of
80–210 nm. The tubes–like MnO2 is synthesized successfully and
shown in image d. The obtained MnO2 nanotubes have smooth
surfaces with an outer diameter of about 90 nm. Fig. 1e shows the
typical morphology of MnO2 nanoplates. Hexagonal plates with
somewhat smooth surface and lots of fragments can be seen from
the image. In detail, the average thickness and diameter of the
2.3. Electrode preparation
A GCE (diameter of 3 mm) was firstly polished with 0.05
alumina slurry on silk, and then washed with ethanol/water (1:1,
V/V) and water in an ultrasonic bath, each for 1 min. 10.0 L of the
mm
m
obtained MnO2–GO colloid solution was coated onto the surface of
GCE and allowed to dry under an infrared lamp in the air.
2.4. Analytical procedure
Unless otherwise stated, a Britton–Robinson (B–R) buffer of pH
1.81 was used as the supporting electrolyte for electrochemical
measurements. After 5 mL of pH 1.81 B–R buffer was placed in the
electrochemical cell, the required volume of guaiacol and vanillin
standard solution was added by a micropipette. Finally, the
differential pulse voltammograms were recorded after 2 min of
accumulation at 0.1 V. The oxidation peak currents at 0.68 V and
0.86 V were measured for guaiacol and vanillin, respectively.
nanopaltes are ꢃ200 nm and ꢃ4
mm, respectively. Furthermore,
the building nanoplates are randomly inclined or perpendicular to
the substrate. By contrast, the nanowire size of MnO2 in image f is
about 200 nm thick and about 9 mm long. Fig. 1g indicates that the
hierarchically assembled MnO2 spheres are multi–scale organized
nanoarchitecture. The diameter of the MnO2 spheres ranges from
2 mm to 3 mm, indicating the microstructure of the synthesized
material. The high–resolution SEM image in Fig. 1h shows that the
prepared MnO2 sphere possesses dense nanostructure on its
surface, constructing a hierarchical dandelion–like structure.
Furthermore, the BET surface area, pore volume and average
pore size of MnO2 catalysts were also investigated and summa-
rized in Table 1. It is found that the MnO2 nanoflowers show the
highest surface area which is decreased by changing morphology
to nanorods. The surface area of MnO2 nanorods is higher than that
of MnO2 with other morphologies. The data in Table 1 also suggest
3. Results and discussion
3.1. Morphology–dependent electrochemical activity of manganese
dioxide–graphene oxide
SEM images of the samples reflect the idea about the
morphology of samples. Fig. 1 shows clear images of MnO2 with
Fig. 5. (A): DPVs of 1.0 mM guaiacol and vanillin on GCE (a), GO/GCE (b), MF/GCE (c) and MFG/GCE (d). Curve e corresponds to DPV curve of MFG/GCE in pH 1.81 B–R buffer.
Other conditions are as in Fig. 2. (B): Nyquist diagram (Z00 vs. Z0) for the EIS measurements in the presence of 5 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) in 0.1 M KCl at different
electrodes.

T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
161
Fig. 6. The cyclic voltammograms of 50.0
on oxidation peak current (a,b) and peak potential (a’,b’) of guaiacol and vanillin, respectively.
m
M guaiacol (A) and vanillin (B) on MFG/GCE in pH 1.81 B–R buffer at scan rates from 50 to 300 mV sꢂ1. Inset: the effects of scan rate
the pore diameters of all the catalysts were between 3.4 nm and
27.7 nm. The strategic configuration of the nanoflowers–type
MnO2 is reckoned to achieve a high electrochemical performance.
For comparison purposes, MnO2 with different morphologies
and GO nanocomposites were used to the simultaneous determi-
rough surfaces and abundant pores, the specific surface area of
nanoflowers–like MnO2 increases dramatically, as Table 1 indi-
cates. It is well known that large specific surface areas provide
more active sites and absorbed more analytes. Moreover, these
pores also allow the electrons to transit inside their interior pore
channels, which would improve electrocatalytic activity [23]. Take
the selectivity and sensitivity for the simultaneous determination
of guaiacol and vanillin into consideration, MnO2 nanoflowers and
MnO2 nanoflowers–GO hybrid, which are respectively defined as
MF and MFG for simplification, were selected as sensing materials
in the following work.
nation of 1.0 mM guaiacol and vanillin in pH 1.81 B–R buffer
through differential pulse voltammetry (DPV) technique, and the
results were presented in Fig. 2A. The increased oxidation peak
heights in the DPVs denote increased electrocatalytic activity. On
the GCE modified with GO and microspheres, nanowires, nano-
plates or nanotubes–like MnO2, a very small oxidation peak
belonging to guaiacol and an evident oxidation peak attributing to
vanillin can be observed, indicating the electron transfer rate of
guaiacol is very slow on MnO2 with these shapes. However, two
oxidation peaks of guaiacol and vanillin are independent and
evident on MnO2 nanorods–GO modified electrode, indicative of
high selectivity. The highest electrocatalytic activity in this series
of nanocomposites is observed using MnO2 nanoflowers–GO
modified electrode. The sharp anodic current peaks appear at
0.68 V and 0.86 V for guaiacol and vanillin, respectively. The
oxidation current change of guaiacol and vanillin on different GCE
surfaces is shown in Fig. 2B. Hence, there is a strong evidence that
the electrochemical sensing properties of GO–MnO2 samples can
be tailored by their morphologies. From SEM images d–g in Fig.1, it
can be seen that lots of amorphous impurities and fragments
present among MnO2 crystals, in contrast, regular nanoflowers and
nanorods provide high purity, good crystallinity, which are
favorable for reducing the probability of the recombination of
electrons and thus reduce the chemical energy barrier. Addition-
ally, the nanoflowers–like MnO2 in Fig. 1a shows good dispersity
and no obvious agglomeration is observed, plus the significantly
3.2. Characterization of manganese dioxide nanoflowers–graphene
oxide nanocomposite
The structural information of MFG was further confirmed
through TEM. As expected, the lamellate and nanosheets GO is
successful prepared (Fig. 3a). Fig. 3b shows the 3D hierarchical
structure of the MF nanostructures. It can be seen that MF
nanostructures is made up of many thin nanosheets, and a large
amount of irregular pores are randomly distributed in the nano-
sheetsofMF. According toTEM imagesofd and e inFig. 3, the crystals
of MF are covered and wrapped by ultrathin GO layers, which may
expose hydrophilicgroupsoutsideandenhancethe dispersion ofMF
in water. Furthermore, a high–resolution TEM image (Fig. 3c)
recorded from a free nanosheet of MF gives clear lattice fringes arise
from MnO2 crystallites, which is different from the image in Fig. 3f,
because the fuzzy contrast which appears on the outer surface of
MnO2 crystallites originates from the amorphous GO.
Since Raman spectroscopy is a useful technique for distinguish-
ing the order and crystalline structures of GO layers and oxide–
based materials, we studied the Raman spectra of different
samples, as shown in Fig. 4A. At 200–800 cmꢂ1 regions, the
spectra show Raman vibrations of MF. The band around 642 cmꢂ1
can be attributed to the symmetric stretching vibration (Mn–O) of
the MnO6 groups, while the peak at 572 cmꢂ1 can be due to the
Mn–O stretching in the basal plane of the MnO6 sheet [24]. So the
MF may be birnessite [25]. The well–known Raman peaks for GO
structures are D and G bands at around 1350 and 1585 cmꢂ1
,
corresponding to the presence of defects/disorders and
well–defined sp2 graphitized structure [26], respectively. So,
greater ID/IG peak intensity ratio indicates higher defects and
disorders in hexagonal carbon–type structure of graphene. The
ID/IG ratio for GO does not change after the loading of MF crystals,
confirming the attachment of MF nanostructures on GO sheets
with almost no disorders and defects, this is an advantage for
the MFG in charge for electrons transportation [27]. However, the
weakened bands attributing to MF on MFG may be ascribed to the
low density of MF with the introduction of GO.
Scheme 1. Oxidation mechanism of guaiacol and vanillin on MFG/GCE.

162
T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
Table 2
determination. However, the peak currents at the MFG/GCE (curve
d) are remarkably enhanced. Noteworthy is the biggest potential
difference (i.e. 180 mV) on MFG/GCE, rendering this particular
composite is more selective for the simultaneous determination of
guaiacol and vanillin. Besides, the DPV curve of MFG/GCE in pH
1.81 B–R buffer is featureless, showing the attribution of the two
oxidation peaks to guaiacol and vanillin, respectively.
Number of surface active sites of MF and MFG in different electrolytes and with
different mass ratios
Samples
Ns (m )
mol mꢂ2
MF
pH 1.81 B–R buffer
3.8
7.0
4.4
3.2
3.2
4.7
7.0
6.4
6.2
pH 1.81 B–R buffer (mass ratio 1:5)
pH 7.00 B–R buffer (mass ratio 1:5)
pH 11.92 B–R buffer (mass ratio 1:5)
Mass ratio 1:7 (pH 1.81 B–R buffer)
Mass ratio 1:6 (pH 1.81 B–R buffer)
Mass ratio 1:5 (pH 1.81 B–R buffer)
Mass ratio 1:4 (pH 1.81 B–R buffer)
Mass ratio 1:3 (pH 1.81 B–R buffer)
Electrochemical impedance spectroscopy (EIS) has been
employed to study the interfacial properties of the modified
electrodes. Nyquist plots were obtained for bare GCE, GO/GCE,
MF/GCE and MFG/GCE in the frequency range from 1 to 105 Hz and
represented in Fig. 5B. The impedance spectra were recorded using
a redox couple of 5 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) in 0.1 M KCl as
an electrochemical probe. The information regarding electron
transfer kinetics and diffusion characteristics can be obtained from
the shape of the impedance spectrum. The semicircle portion and
linear portion of the impedance spectra respectively correspond to
the electron transfer limiting and diffusion limiting electrochemi-
cal processes, respectively. And the charge transfer resistance (Rct)
refers to the semicircle diameter [31]. A semicircle of about
3110 ohm in diameter for bare GCE indicates a slow electron
transfer process. After modification of MF or GO on GCE surface,
the semicircle diameter decreases to 1167 and 1050 ohm,
respectively. The remarkable decrease of semicircle diameter
(i.e., 311 ohm) can be obtained on MFG/GCE because lots of active
sites occurred, suggesting that a significant acceleration of the Fe
(CN)63–/4–redox reaction occurred on the surface of MFG film
modified GCE. The results prove that the MFG nanocomposite on
the GCE surface can reduce the electron transfer resistance to the
flow of electrons, confirming the results obtained from Fig. 5A.
MFG
The FTIR spectra of GO and MFG samples are shown in Fig. 4B.
The spectra of GO confirms the presence of C–O–H (nO–H at
3407 cmꢂ1), COOH (nC=O at 1721 and 1407 cmꢂ1) and C–O–C (nC–O
at 1225 cmꢂ1). The band at 1628 cmꢂ1 is due to the deformation of
the O–H bond in water. The peaks locate at 1056 and 1096 cmꢂ1 are
due to C–O (i.e., hydroxyl, ether) stretching vibrations [28]. In the
case of MFG, the main characteristic peaks of GO still remain and a
new peak appears at 520 cmꢂ1, which arises from the stretching
vibration of the Mn–O bonds [29].
XRD pattern of the resulting MFG is shown in Fig. 4C. The
diffraction peak at around 2u
= 10 ꢁ, corresponding to the (001)
reflection of GO, which overlaps with the (001) basal reflection of
MF. The diffraction peak at 2
reflection of MF, while two other peaks are indexed as the (20l/11l)
and (02l/31l) diffraction bands, respectively [30]. The strategic
configuration of the MF is reckoned to achieve a high electro-
chemical performance.
ꢁ
u
= 24 is resulted from (002) basal
3.3. Enhancement effect of manganese dioxide nanoflowers–graphene
3.4. Reaction mechanism of guaiacol and vanillin
oxide hybrid
The oxidation peak currents of 50.0 mM guaiacol and vanillin on
In order to understand the enhanced electrocatalytic activity of
MFG hybrid, the DPV behaviors of a mixture of 1.0 mM guaiacol and
the MFG film modified electrode were also examined when
changing the scan rate in the range of 50–300 mV sꢂ1 using cyclic
voltammetry (CV) technique (Fig. 6). The results show that the
vanillin at different electrodes were compared in pH 1.81 B–R
buffer. As shown in Fig. 5A, at bare GCE (curve a), two small anodic
peaks are obtained with a potential difference of about 120 mV.
After modification of the GCE by GO, the peak potential shifts
negatively but only one anodic peak is observed (curve b), the peak
potentials of guaiacol and vanillin are indistinguishable, indicating
poor selectivity and sensitivity. In contrast, two oxidation peaks
corresponding to guaiacol and vanillin are observed on MF
modified GCE (curve c). The oxidation peak currents on MF/GCE
are obvious but weaker and wider, the sensitivity is poor for further
oxidation peak currents (Ip,
increase with scan rate (
, mV sꢂ1) according to the regression
equations: Ip = 3.976 + 0.136
(r2 = 0.996) (inset a of Fig. 6A) and
Ip =–6.933 + 1.370
(r2 = 0.998) (inset b of Fig. 6B), indicating
mA) of guaiacol and vanillin linearly
n
n
n
adsorption–controlled electrode processes of guaiacol and vanillin
on MFG/GCE. Meanwhile, the oxidation peak potentials (Ep, V) of
guaiacol and vanillin positively shift with scan rate (inset a’ of
Fig. 6A and inset b’ of Fig. 6B), and the relationship between Ep and
n
is in accordance with the following equation [32]:
Fig. 7. DPVs at MFG/GCE in (A) 1.0
m
M vanillin and different concentrations of guaiacol: 0.03, 0.05, 0.08, 0.1, 0.3, 0.5, 0.8 and 1.0
mM, and (B) 1.0
mM guaiacol and different
concentrations of vanillin: 0.03, 0.1, 0.3, 0.5, 0.8,1.0, 3.0, 5.0, and 8.0
as in Fig. 2.
mM. The insets show the calibration plots of guaiacol and vanillin versus peak current. Other conditions are

T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
163
Table 3
Comparison of the analytical performances of this work with the reports
Modified electrode
Linear range (
m
mol dmꢂ3
)
Detection limit (
m
mol dmꢂ3
)
Ref.
Guaiacol
Vanillin
Guaiacol
Vanillin
Laccase/screen printed electrode
PEI/Au/laccase/GCE
laccase or tyrosinase/GCE
Reduced graphene oxide/GCE
AuPd–graphene/GCE
Boron–doped diamond electrode
Au–Ag/GCE
Arginine/graphene/GCE
MFG/GCE
0.1ꢃ500
0.79ꢃ170
1ꢃ10
0.05
0.03
0.11
0.2
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
0.5ꢃ500
0.1ꢃ7.0 and 10ꢃ40
3.3ꢃ98
0.02
0.16
0.04
0.332
0.0015
0.2ꢃ50
2ꢃ100
0.03ꢃ30
0.03ꢃ8
0.0013
This work
RT RTk0
RT
Ep ¼ E00
þ
ln
þ
anF
lnv
(1)
0.1 M NaOH, HCl, H2SO4, HClO4 solution were examined by CV.
It is found that the oxidation peaks of guaiacol and vanillin are
distinguishable in B–R buffer, and the biggest peak currents for
them can be obtained when the pH value is 1.81. So pH 1.81 B–R
buffer is chosen as the electrolyte for the electrochemical oxidation
of guaiacol and vanillin.
Next, the effect of the mass ratio between MF and GO on the
determination of guaiacol and vanillin was studied. It is found that
the MFG is difficult to fully disperse in water when the mass ratio of
MF vs. GO exceeds 1:3, which may be caused by the weak solubility
of MF in water. So the mass ratios between MF and GO including
1:7, 1:6, 1:5, 1:4 and 1:3 were investigated, and the highest peak
could be obtained when the mass ratio is 1:5. As discussed before,
the MF has definitely catalytic effect on the oxidation of guaiacol
and vanillin, and GO acts as the support and electron transfer
media, so both of them are indispensable. Here, MF was mixed
with GO according to the mass ratio of 1:5 for better determination
sensitivity.
a
nF
a
nF
where k0 is the standard rate constant of the surface reaction,
0
E0 is the formal potential,
of guaiacol and vanillin, and other symbols have their usual
meanings. According to Eq. (1), the plot of Ep vs. ln has a good
linear relationship, from which n can be determined from the
slop (i.e., 0.0285 for guaiacol and 0.0215 for vanillin). Assuming
a is transfer coefficient of the oxidation
n
a
a
= 0.5, the value of n ꢄ 2 is calculated for guaiacol and vanillin.
Therefore, both of the oxidation of guaiacol and vanillin are two–
electron reaction, as demonstrated in Scheme 1.
3.5. Simultaneous determination of guaiacol and vanillin
The conditions of the electrolyte, such as types of electrolyte
and solution pH, are major factors that affect the response of
guaiacol and vanillin on the MFG/GCE. Therefore, the electro-
In order to further testify the catalytic activity of MFG in
different electrolytes and with different mass ratios, a methanol
chemisorption technique was conducted to estimate the number of
surface catalytic active sites per unit surface area (Ns) (Table 2). The
chemical responses of 1.0 mM guaiacol and vanillin at the MFG/GCE
in different supporting electrolytes such as 0.2 M PBS (pH 5.8–8.0),
0.2 M sodium acetate–acetic acid buffer (pH 2.6–5.8), boric
acid–borax buffer (pH 7.4–9.0), B–R buffer (1.81–11.92) and
Fig. 8. Differential pulse voltammograms at MFG/GCE in food sample (black curves), food sample with first standard addition (red curves) and food sample with second
standard addition (blue curves). Other conditions are as in Fig. 2.

164
T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
active surface site density of MF is found to have an increase from
3.8
mol mꢂ2 to 7.0 mol mꢂ2 after combining with GO, further
Na+, Zn2+, Ca2+; 1500–fold concentration of Cu2+; 2000–fold
concentration of citric acid, sucrose, glucose, tartaric acid;
1000–fold concentration of glycine; 100–fold concentration of
uric acid; 80–fold concentration of dopamine, L–dopa, and 50–fold
concentration of ascorbic acid (peak current change <8%).
For simultaneous and quantitative determination of guaiacol
and vanillin, DPV curves at different concentrations of guaiacol
were recorded in Fig. 7A, where vanillin concentration was kept at
m
m
indicating the contribution of GO in this electrochemical system.
And the Ns value is the biggest for MFG with mass ratio of 1:5 under
acidic environment, which is in well accordance with the above
discussion about the effects of electrolyte and mass ratio on
determination sensitivity of guaiacol and vanillin.
The influence of amount of MFG suspension on the oxidation
peak current of guaiacol and vanillin was investigated. When
gradually improving the volume of MFG suspension from 0 to
1.0
m
M. The inset shows that the peak current varies linearly with
guaiacol concentration between 0.03 and 1
m
M with r2 = 0.995.
10
m
L, the oxidation peak currents of guaiacol and vanillin greatly
Importantly, the anodic peak current of vanillin is almost
uninfluenced by the increase of guaiacol concentration, suggesting
that oxidations of guaiacol and vanillin at the MFG/GCE are
independent of each other. With the DPV technique the detection
increase. During this period, the accumulation efficiency of
MFG/GCE obviously enhances, resulting in remarkable oxidation
peak current enhancement of guaiacol and vanillin. However, the
oxidation peak currents decrease slightly with further improving
limit of guaiacol is 1.3 nM in the presence of 1.0 mM vanillin
the amount of MFG suspension up to 15
blocking of too thick film for electrons transfer. In order to shorten
m
L, maybe due to the
interference (S/N = 3). Fig. 7B presents DPV responses at different
concentrations of vanillin while guaiacol is kept constant at
the time of solvent evaporation and to achieve high sensitivity,
1.0 mM. Similar to the scenario in Fig. 7A, the anodic peak current of
10
m
L MFG suspension is used to modify the GCE surface.
guaiacol stays almost constant as vanillin concentration is
increased gradually, further confirming that this modified elec-
trode can be employed for simultaneous determination of guaiacol
and vanillin. The inset in Fig. 7B illustrates that the peak current
increases linearly with vanillin concentration between 0.03 and
For further improving the sensitivity, accumulation was
employed when detecting guaiacol and vanillin. It is found that
the oxidation signals of guaiacol and vanillin obviously increase
with positive shift of accumulation potential from–0.3 to 0.1 V. As
further moving the accumulation potential to 0.5 V, the oxidation
peak currents of guaiacol and vanillin decrease. To achieve high
sensitivity and excellent oxidation shape, the accumulation was
performed at 0.1 V. The influences of accumulation time on the
oxidation signals of guaiacol and vanillin were further studied.
When improving the accumulation time form 0 to 2 min, the
oxidation peak currents of guaiacol and vanillin on MFG film
modified GCE greatly enhance. The remarkable signal enhance-
ment indicates that accumulation is feasible to improve the
detection sensitivity. Longer accumulation time than 2 min does
not enhance the oxidation peak currents of guaiacol and vanillin
obviously, suggesting that the amount of guaiacol and vanillin tend
to a limiting value. Considering sensitivity and efficiency, 2 –min
accumulation is employed.
8 m
M with r2 = 0.995. In the presence of guaiacol, the low limit is
1.5 nM for vanillin (S/N = 3). Although a number of guaiacol or
vanillin sensing composites have been reported in recent years
[33–40], it is significant that MFG used in our study can realize the
direct determination of guaiacol without the help of expensive
enzyme, so it is more stable under robust sampling conditions, and
the lowest detection limits for guaiacol and vanillin were achieved
compared with the reported electrochemical methods (Table 3).
Moreover, this is the first report for the simultaneous determina-
tion of guaiacol and vanillin using electrochemical method as far as
our knowledge.
3.6. Practical application
In this work, we found that the MFG modified GCE was
unqualified for the successive measurements because the oxida-
tion peak currents of guaiacol and vanillin decreased continuously.
Thus, it was just employed for the single measurement. The
reproducibility between multiple MFG/GCEs was then tested by
The MFG modified GCE was used in several food samples
including biscuit, jelly, chocolate and juice to evaluate its practical
application. The samples were purchased from local market.
Thereinto, the juice sample was used directly, but biscuit, jelly and
chocolate samples were used after pretreatment. For biscuit and
chocolate samples, 10 g of each was ground to powder in an agate
mortar, respectively, and then stirred with 15 mL absolute ethanol
for 30 min. For jelly sample, 4 g of it was dissolved into 10 mL
mixture of pH 1.81 B–R buffer and 1% ethanol, which was shook
vigorously for 10 min. The filtrates of these three solutions were
collected after vacuum filtered and diluted in 50 mL volumetric
parallel determination of the oxidation peak currents of 1.0 mM
guaiacol and vanillin. The relative standard deviation (RSD) is 3.2%
and 3.7% for ten MFG/GCEs, respectively, indicative of excellent
fabrication reproducibility and detection precision.
The potential interferences for the detection of guaiacol and
vanillin were studied. Under the optimized conditions, the
oxidation peak currents of guaiacol and vanillin were individually
measured in the presence of different concentrations of interfer-
ents and the peak current change was then checked. No influence
flasks using water, respectively. Upon addition of 100 mL sample
solution into 5.0 mL pH 1.81 B–R buffer, the DPV curves were
recorded from 0.3 to 1.1 V after 2 –min accumulation at 0.1 V (black
curves in Fig. 8), it can be seen that there is only one oxidation peak
attributing to vanillin in biscuit and chocolate samples, indicating
on the detection of 1.0
mM guaiacol and vanillin is found after the
addition of 2000–fold concentration of NO3–, K+, Fe3+, Al3+, Mg2+
,
Table
4
Detection and recovery of guaiacol and vanillin in biscuit, jelly, chocolate and juice samples
Sample
Detection
Recovery test
This method (nmol dmꢂ3
)
RSD (%)
Spiked (nmol dmꢂ3
)
Found (nmol dmꢂ3
)
Recovery (%)
Biscuit
Jelly
guaiacol
vanillin
guaiacol
vanillin
guaiacol
vanillin
guaiacol
vanillin
—
—
100.0
100.0
200.0
200.0
300.0
300.0
200.0
200.0
91.64
102.3
189.6
197.4
277.8
285.9
207.6
195.7
91.64
102.3
94.80
98.97
92.55
95.26
103.8
97.83
38.0
12.0
122
—
3.8
3.9
2.6
—
chocolate
Juice
85.0
32.5
—
4.2
1.2
—

T. Gan et al. / Electrochimica Acta 147 (2014) 157–166
165
the existence of individual vanillin in these two food samples.
However, guaiacol and vanillin coexist in jelly sample because two
oxidation peaks locating at about 0.66 V and 0.86 V appear.
Regarding to the juice sample, there are two peaks at the potential
of 0.69 V and 0.97 V, which correspond to the respective oxidation
of guaiacol and certain interferent in complicated real sample. Each
sample is determined by three times, and the RSD is lower than
4.5%, revealing good precision. The contents of guaiacol and
vanillin are obtained by the standard addition method (red curves
in Fig. 8), and the results are listed in Table 4. In addition, the
recovery of guaiacol and vanillin standard is performed to testify
the accuracy of this method. In order to achieve this, different
guaiacol and vanillin standard solutions are further added into
these four samples, respectively, as shown in blue curves in Fig. 8.
The value of recovery ranges from 91.64% to 103.8%, suggesting that
this method is effective and reliable.
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synthesis and precipitation methods. When they were loaded on
GO layers and then used as sensing materials for the simultaneous
determination of guaiacol and vanillin, the MnO2 with morphology
of porous nanoflowers showed relatively high sensitivity. The
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Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 61201091 and No. 21305119).
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