Nano Research
Nano Res
DOI
10.1007/s12274-014-0617-6
Interconnected 1D Co3O4 nanowires on reduced
graphene oxide for enzymeless H2O2 detection
Lingjun Kong,1 Zhiyu Ren,1 () Nannan Zheng,2 Shichao Du,1 Jun Wu,1 Jingling Tang,2 and Honggang
Fu1 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0617-6
http://www.thenanoresearch.com on October 22, 2014
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1
TABLE OF CONTENTS (TOC)
Interconnected 1D Co3O4 nanowires on reduced
graphene oxide for enzymeless H2O2 detection
Lingjun Kong, Zhiyu Ren, Nannan Zheng, Shichao Du,
Jun Wu, Jingling Tang, and Honggang Fu*
1
Heilongjiang University, China
2
Harbin Medical University, China.
Interconnected
1D
Co3O4
nanowires,
assembled
with
small
nanoparticles, are designed and synthesized on reduced graphene oxide
via a simple solvothermal method. Owing to the synergistic effects of
abundant active sites, the orientation transmission of electrons, and the
unimpeded pathways for matter diffusion, the hybrids exhibit excellent
enzymeless H2O2 detection performance, and are used in the
monitoring H2O2 generated from liver cancer HepG2 cells.
1
Nano Res
DOI (automatically inserted by the publisher)
Research Article
Interconnected 1D Co3O4 nanowires on reduced graphene oxide
for enzymeless H2O2 detection
Lingjun Kong,1 Zhiyu Ren,1 () Nannan Zheng,2 Shichao Du,1 Jun Wu,1 Jingling Tang,2 and Honggang Fu1
( )
1
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, School of
Chemistry and Materials Science, Heilongjiang University, Harbin 150080 P. R. China, Tel.: +86 451 8660 4330, Fax: +86 451
8666 1259, E–mail: [email protected]; [email protected]
2 Department of Pharmaceutics, School of Pharmacy, Harbin Medical University, Harbin, 150086 P. R. China.
Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)
© Tsinghua University Press and Springer–Verlag Berlin Heidelberg 2013
ABSTRACT
Enzymeless hydrogen peroxide (H2O2) detection with high sensitivity and excellent selectivity are desirable for
clinical diagnosis. Herein, one–dimensional Co3O4 nanowires were successfully constructed on reduced
graphene oxide (rGO) via a simple hydrothermal procedure and consequent thermal treatment. These Co 3O4
nanowires, assembled by small nanoparticles, are interlaced with one another and make spider web-like
structure on rGO. The formation of Co3O4–rGO hybrids is attributed to the structure–directing and anchoring
role of DDA and GO, respectively. Such unique structure possesses abundant active sites, the orientation
transmission of electrons, and the unimpeded pathways for matter diffusion, and then endows Co 3O4–rGO
hybrids with excellent electrocatalytic performance. As a result, the obtained Co3O4–rGO hybrids can serve as
an efficient electrochemical catalyst for H2O2 oxidation and sensitive detection. Under the physiological
condition, the oxidation current of H2O2 is linear to its concentration from 0.015 mM to 0.675 mM with
sensitivity of 1.14 mA﹒mM–1﹒cm–2 and a low detection limit of 2.4 μM. Furthermore, the low potential (–0.19
V) and the good selectivity make Co 3O4–rGO hybrids suitable for monitoring H2O2 generated from liver cancer
HepG2 cells. Therefore, it is promising as a non–enzymatic sensor to achieve real–time quantitative detection of
H2O2 in biological application.
KEYWORDS
Interconnected nanowires, Co3O4–rGO hybrids, synergistic effect, electrocatalysis, enzymeless H2O2 detection,
————————————
Address correspondence to H. Fu, [email protected], [email protected]
2
1
Introduction
Electrocatalytic detection of hydrogen peroxide
(H2O2) has attracted much interest over past years
for the construction of abiotic catalyzed fuel cells [1],
the oxygen evolution reaction [2], especially for the
development of H2O2 sensing in biological body
[3–5]. The role of H2O2 in the process of human life
activities is closely related to its content. Real–time
detection of H2O2 in vivo not only can help to reveal
the inherent law between small molecules and
human disease, but also can accurately diagnose
diseases, and monitor the progress of the disease
[6].
Commonly, electrochemical H2O2 sensors with
enzymes detect the substrate with specificity, high
efficiency, and high sensitivity in certain
physiological conditions [5, 7]. But, because of the
characters of high cost, difficulty for recycle, and
being disturbed easily by external environment, the
practical application of enzyme–based H2O2 sensors
has been greatly limited. Therefore, considerable
attention has been focused on direct electrocatalytic
oxidation of H2O2 at metal–based electrodes for
enzyme–free sensing with fast response, high
stability and reproducibility [8, 9].
Noble metal [10] and its alloys [1] as attractive
materials for H2O2 sensing have been extensively
studied. The electron configuration determines its
inherent good catalytic performance. The highest
proportion of d–band in noble metal can participate
in coordination, and then the specific ligand
structure can be formed with the carrier or reaction
to improve its catalytic activity. Despite of that, the
high cost, as the primary factor, is still one of the
unresolved issues on the development in sensors, so
people attempt to replace noble metal for cutting
cost [11–13]. C. Wang et al found Au–Fe3O4 NPs are
more active than either single component [14].
Dumbbell–like PtPd−Fe3O4 nanoparticles, X. Sun et
al. preparated, show better performance in H2O2
electrochemical detection [15]. In this case, Fe3O4
not only greatly enhances the catalytic properties of
PtPd particles because of the electron transfer
between two different components, but also reduces
the cost of electrode materials. People may wonder
whether noble metals could be substituted
completely by transition metal oxides.
Recent studies present evidences that many kinds
of transition metal oxides themselves also have
certain catalytic activity, such as MnO2, NiO, TiO2
and so on [16–19]. Among them, Co3O4 shows
excellent electrocatalytic performance toward H2O2
[20–22]. However, the current detection results are
not as good as being expected, which is mainly
attributed to its single morphology and poor
conductivity [23]. In term of morphology, most of
the reported Co3O4 for H2O2 detection are
nanoparticles. Although they have large surface
area for active sites to be exposed, the directional
confinement for rapid electronic transmission is
limited. For the intrinsic characteristic-poor
conductivity, the most effective solution is to
introduce the conductive agent [24, 25]. Thus, if we
could design a novel hybrid to overcome these
problems synchronously, it could provide good
opportunities for the development of H2O2 sensors.
As is well known, the one–dimensional (1D)
nanostructures, like nanowires [26] and nanotubes
[27], have outstanding performance in the
orientation transmission of electrons. However, in
general, 1D structure grows vertically on the
substrate by seed-induced strategy, resulting in the
uniform structure. Although it can cause the
directional transmission electron, its lower surface
area is not conducive to electrocatalysis [28, 29]. So,
it may easily be conceived, that, if 1D Co 3O4
nanowires assembled with nanoparticles, it may
simultaneously possess the large surface area and
active sites for catalysis, and the directional
confinement for rapid electronic transmission.
Further, if such anticipated Co3O4 nanowires could
be interlaced with each other, the spider web-like
structure could provide the unimpeded pathways
for matter diffusion, which can further improve the
electrochemical performance [30]. On the other
hand, to improve the conductivity, the appropriate
conductive agent cannot be ignored. Recently, it has
been found that reduced graphene oxide (rGO) acts
as a two–dimensional (2D) conductive template to
assemble 1D nanostructures for three–dimensional
(3D) conductive networks, and suppresses the
volume change and agglomeration of 1D
nanostructures [31, 32]. The unique capabilities of
rGO could inspire the synthesis of interconnected
1D Co3O4 nanowires for H2O2 detection. In this case,
rGO could be used as both the conductive agent [31]
and the structure–directing agent for 3D network.
Based on the above design, we fabricated
interconnected 1D Co3O4 nanowires on rGO
(Co3O4–rGO hybrids) via simple solvothermal
reaction and consequent thermal treatment,
utilizing dodecylamine (DDA) and GO as
structure–directing agent and anchoring agent,
respectively (as shown in Scheme 1). And, the Co3O4
nanowires assembled with small nanoparticles,
which are attributed to encircling DDA protectors
3
for suppressing the undesirable grain growth. The
high catalytic activity of Co3O4–rGO hybrids for
H2O2 was come true through this strategy, which
increased the active sits of electrochemistry
obviously, and accelerated the transport of electrons.
The obtained Co3O4–rGO hybrids were also used to
detect H2O2 released from liver cancer HepG2 cells.
Teflon–lined stainless autoclave, and then kept at 180
ºC for 12 h. After the autoclave cooled down to room
temperature, the precursor product was collected,
washed, and dried, which is denoted as CoG–p.
Then, Co3O4–rGO hybrids (CoG–x) was obtained by
thermal–treatment CoG–p at 300 ºC in air for 2 h. As
control, the samples with different dosage ratio and
different solvothermal reaction time were all
prepared. Meanwhile, rGO decorated with Co3O4
nanoparticles (Co3O4/rGO) and pristine Co3O4 are
prepared. The detailed experimental parameters
were displayed in Table 1.
Table 1. The experimental parameters of the synthesized
Co3O4–rGO hybrids.
Samples
Scheme 1 Schematic illustration of the synthesis procedure for
Co3O4–rGO hybrids and their electrocatalytic mechanism.
2
2.1
Experimental
Initial dosage ratio of (mol ratio)
Co
DDA
GO
CoG–1
3
6
4
CoG–2
1
2
4
CoG–3
2
4
4
CoG–4
4
8
4
Co3O4/rGO
3
0
4
Co3O4
3
6
0
Material synthesis
Synthesis of graphene oxide (GO). GO sheets were
synthesized from expandible graphite flakes by a
modified Hummers method [33]. Briefly, expandible
graphite (2.0 g) was combined with 50 mL
concentrated sulfuric acid in a 250 mL beaker under
vigorous agitation at room temperature. Afterwards,
sodium nitrate (2.0 g) and potassium permanganate
(6.0 g) were slowly poured into the beaker in a
sequence. Next, the mixture was heated at 28 ºC for
24 h. After that, 80 mL of distilled water was added
into the solution. 5 min later, 20 mL of 30% H 2O2 was
dropped into the reaction system. Finally, the
product was washed with hydrochloric acid solution
and then washed three times with water. The
resulting solid was obtained by the low temperature
drying.
Synthesis of Co3O4–rGO hybrids. In a typical
procedure, 0.218 g of Co(NO3)2·6H2O and 0.375 g of
GO were firstly dispersed in 20 mL of deionized
water by ultrasonication, and then 15 mL ethanol
with 0.52 g DDA was slowly added dropwise into
this mixed solution with vigorous stirring. The
solution was then transferred into a 50 mL
2.2
Characterization
The morphology of the products was analyzed
using scanning electron microscopy (SEM, Hitachi
S–4800) with an acceleration voltage of 15 kV and
transmission electron microscopy (TEM, JEOL
JEM–3010) with an acceleration voltage of 300 kV.
Carbon–coated copper grids were used as sample
holders for TEM analysis. X–ray photoelectron
spectroscopy (XPS) analysis was performed with
AXIS UL TRA DLD. X–ray diffraction (XRD) patterns
were obtained by a Bruker D8 Advance
diffractometer by using CuKα radiation (λ=1.5406 Å).
The Raman spectra were acquired using a Raman
spectrophotometer (HR800, HORIBA Jobin Yvon
Company) excited by a laser with 457.9 nm
wavelength.
Nitrogen
adsorption–desorption
isotherms at 77 K were collected on Micromeritics
Tristar Ⅱ3020 nitrogen adsorption apparatus. The
Brunauer–Emmett–Teller (BET) equation was used to
calculate
the
specific
surface
area.
Thermogravimetric (TG) and differential scanning
calorimetryanalysis (DSC) were performed on TA
4
Q600 under a stream of air at a heating rate of
10 °C·min–1. The Fourier transform infrared (FT–IR)
spectra were recorded on a Nicolet IS10. The
bright–field microscopy image was taken by the
confocal laser scanning microscopy (Olympus).
2.3
Electrochemical measurements
Electrochemical experiments were performed in a
conventional three–electrode system, using a
saturated calomel electrode (SCE) and a Pt sheet as
the reference and the counter electrode, respectively.
A 0.4 cm diameter glass carbon (GC) used as
working electrode was polished with 30 nm Al 2O3
paste, followed by washing with water and alcohol.
Five milligrams of catalyst mixed with 50 µ L of 5
wt % Naﬁon ionomer was dispersed in 0.1 mL of
alcohol solution. After the catalyst ink was sonicated
for 0.5 h, a volume of the ink was evenly dropped
onto the well–polished GC electrode surface and
allowed to dry at ambient temperature, denoted as
CoG–x–GC electrode. To evaluate the activity of the
support catalyst in relation to H2O2 biosensor, a 0.10
M Phosphate Buﬀer Saline (PBS, pH 7.4) and H2O2
(30% vol) solution was used for electrochemical
measurements. Electrochemical activity and stability
of catalysts to H2O2 were tested with a BAS 100B
electrochemical workstation. Initially, electrode
potentials were cycled between two potential limits
until two perfectly overlapping, subsequent
voltammograms were obtained. Electrochemical
impedance spectra (EIS) were performed with
AUTOLAB electrochemical workstation in 0.1 M KCl
containing 2 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1)
mixture at room temperature.
2.4 Detection of extracellular release of H2O2 from
liver cancer HepG2 cells
Human liver cancer HepG2 cells were obtained
from Harbin Medical University. The cells were
maintained in a culture medium consisting of
Dulbecco’s modiﬁed minimum essential medium
(37 °C, 5% CO2) and subcultured every 3 days. After
growing to 80% confluence, HepG2 cells were
washed three times with PBS (pH 7.4), and the cell
number was estimated by a hemocytometer. 5.0 mL
PBS was added for the detection of the flux of H2O2
releasing from cancer HepG2 cells. The CoG–1/GC
electrode was biased at –0.19 V (vs. SCE). After a
steady background was obtained, 0.2 μM
N–formylmethionyl–leucyl–phenylalanine
(fMLP)
was added, and the response current was recorded at
37 °C.
3
Results and Discussion
3.1 Structure and morphology of Co3O4–rGO
hybrids
According to the design strategy, CoG–p was
synthesized by solvothermal reaction at 180 °C for 12
h. In the FT–IR spectrum of CoG–p (Figure S1), the
characteristic peaks at 2927 cm–1 is attributed to C–H
asymmetric stretching vibration of the hydrophobic
chains of DDA, and the absorption at 1647 cm–1 and
1489 cm–1 are assigned to the vibration of –NH2. Such
results imply Co coordinated with DDA through
sharing the electron pair of the –NH2 group.
According to the TG and DSC analysis (Figure S2),
further thermal treatment was carried out to remove
DDA. Obviously, after calcination at 300 ºC, the
absorption peaks of the coordination complex
between Co and DDA are either weakened or
completely lost in the FT–IR spectrum of CoG–1
(Figure S1). Simultaneously, two distinct peaks at 567
cm–1 and 660 cm–1 appear, assigned to the vibration
of Co–O bond [34]. The results indicate that most of
DDA decompose after calcination, and the
coordination complex converts to Co3O4. In addition,
the oxygen–containing functional peak at 1382 cm–1
(C–O) disappears for the sample CoG–1 [35]. In
contrast, the skeletal vibration of the graphene sheets
at 1570 cm–1 emerges, further indicating that GO is
reduced during the solvothermal reaction [36].
To validate the crystallographical structure, CoG–1
was characterized by XRD (shown in Figure 1a).
Compared with the XRD pattern of CoG–p, six
high–intensity crystal peaks at 2θ = 31.2°, 36.8°, 44.7°,
55.8°, 59.3°, and 65.1° can be perfectly indexed as
(200), (311), (400), (422), (511) and (440), respectively.
They are accord with the standard date of
spinel–phased Co3O4 crystal (JCPDS NO. 43–1003),
further indicating that the conversion of structure
5
from the amorphous to high crystallinity Co3O4 takes
place during calcination process. However, less
percentage and lower crystallinity of rGO makes its
diffraction peaks so weak. Figure 1b shows the
Raman spectra of CoG-1 and CoG–p. Apparently, the
narrow, strong and not overlapped D-band (at 1354
cm-1), G-band (at 1580 cm−1), and the intense 2D-band
(at 2712cm−1), which are the symbols of the graphitic
carbon, can be observed in the Raman spectrum of
CoG-1. And, the ID/IG of CoG-1 is about 0.778, which
is smaller than that of CoG-p (0.937). Moreover,
compared to the Raman spectrum of CoG-p, four
characteristic peaks from crystalline Co3O4
(corresponding to Eg, F2g1, F2g2, and A1g modes)
become more distinct in the spectrum of CoG-1.
These demonstrate that the crystallinity of Co3O4
improves and that GO is further reduced during
thermal treatment, which consists with XRD analysis.
All these results provide the stronger evidence for
the coexistence of Co3O4 and rGO in the hybrids.
characteristic of Co3O4, attributed to surface
hydroxide species from exposure to air. For the C 1s
XPS of CoG–1, the main peak centered at about 284.6
eV originates from the graphitic sp2 carbon atoms
(C=C), whereas the weaker ones arise from the
oxygenated carbons at ca. 285.7 eV (C–O), 287.9 eV
(C=O), and 289.0 eV (O=C–O), suggesting that GO
has been well deoxygenated to form rGO [37].
However, although DDA contains nitrogen element,
N1s peak cannot be observed in the XPS spectrum of
CoG-1 (Figure S3), implying that there is no nitrogen
element doped in rGO during both hydrothermal
procedure and thermal treatment.
Figure 2 Typical SEM (a–b), TEM (c–d), and HRTEM image
(e) of CoG–1. The inset of (e) is the according SAED.
Figure 1 (a) XRD patterns of CoG–1 and CoG–p; (b) Raman
spectrum of CoG – 1 and CoG – p; (c) and (d) the
high–resolution Co 2p and C 1s XPS spectra of CoG–1,
respectively.
In order to confirm the surface chemical
compositions and valence states of CoG–1, XPS is
performed and shown in Figure 1(c–d). In the Co 2p
XPS of CoG–1, accompanying weaker satellite peaks,
two major peaks at 795.6 and 780.2 eV are
corresponded to 2p1/2 and 2p3/2 spinorbit components,
respectively (Figure 1c). Moreover, the Co 2p3/2 peak
at 780.2 eV is more intense. These are the typical
The size, morphology, and structure of CoG–1
were also characterized by SEM and TEM. Obviously,
rGO is fully and uniformly covered by nanowire–like
Co3O4. These Co3O4 nanowires are interlaced with
one another, making spider web-like structure. The
high–magnification TEM image reveals that Co 3O4
nanowires are about 20–30 nm in diameter and
several micrometers in length. The nanowires are
composed of numerous nanoparticles with the
average size of 5 nm (Figure 2d, Figure S4 and S5).
The maintaining of these small nanoparticles during
the thermal–treatment process is mainly due to the
encircling DDA, which can effectively suppress the
undesirable grain growth [38]. The obtained loose
and interconnected structure, containing multiple
6
dimensions (0D, 1D and 2D), not only provides a
large accessible surface area (BET surface area : 140.6
m2 ﹒g, Figure S6) and more active sites, but also
facilitates the charge transportation and matter
diffusion, indicating the better electrocatalytic
performance. The HRTEM image of CoG–1 shows
three lattice fringes of 0.24 nm, 0.14 nm, and 0.29 nm
corresponding to Co3O4 (311), (440) and (220)
respectively. The corresponding selected area
electron diffraction (SAED) (inset image of Figure 2e)
illustrates that the hybrids are polycrystalline
structure, including Co3O4 and rGO.
3.2 Formation mechanism of Co3O4–rGO hybrids
To investigate the formation details of hybrids, we
carried out the synthesis of Co3O4–rGO hybrids
under different condications. From Figure S7, it can
be seen that, when Co3O4–rGO hybrids are
synthesized without DDA, Co3O4 nanoparticles with
larger size (about 20 nm) are anchored on the surface
of rGO by the strong electrostatic interaction
between Co2+ and GO. When Co3O4–rGO hybrids are
synthesized without GO, sheet-like Co3O4 can be
obtained, assembled by Co3O4 nanocrystals whose
size is similar to that aggregated as nanowires
(Figure S7b and S7d). In the case, the complexes of
Co(DDA)n2+ were formed first, due to the
coordination of Co2+ and NH2 headgroup of DDA.
Then, Co(DDA)n2+ further assemble with each other
driven by Van der Wals force of the encircling long
alkyl chains.
Above all, both DDA and GO play key roles in the
whole synthetic process of Co3O4–rGO hybrids. Thus,
we propose that DDA palys the structure-directing
role for the formation of the interconnected 1D
nanowire, and rGO provides sites for anchoring
Co3O4 nanowires by the electrostatic interaction, as
illustrated in Scheme 2 [39]. In the reaction, part of
Co(DDA)n2+ formed at the beginning of reaction (or
DDA) could anchor on the surface of GO by the
electrostatic interaction between Co(DDA)n2+ (or
DDA) and the oxygen-containing groups of GO
(-COOH or -OH). However, due to the large steric
exclusion, the surface of GO cannot be occupied
completely, as shown in Figure S8a. With
prolongation of reaction, the adjacent Co(DDA) n2+ (or
DDA) on GO further assemble with each other by
Van der Wals force among the long alkyl chains, and
which could even assemble with the isolated
Co(DDA)n2+ and DDA forming spider web-like
micelles (Figure S8b and S8c). Most important of all,
such micelles bind strongly on GO by the
electrostatic interaction. When the solvothermal time
is extended, the spider web-like micelles are further
aggregation, resulting in thicker and shorter Co3O4
nanorods (Figure S8d).
In addition, Co3O4 amount also plays an important
role in determining the shapes of Co3O4–rGO hybrids.
With increasing Co3O4 amount from 50 wt% to 78
wt% calculated by TG curves (as shown in S9 and
Table S1), the morphology of Co3O4 on rGO changes
from nanoparticles, interconnected nanowires to
nanorods (Figure S10). However, in any case, the
encircling DDA may evolve to the amorphous layer
to prevent the agglomeration and the undesirable
grain growth of Co3O4 nanocrystals.
Scheme 2 Schematic illustration of the proposed formation
mechanism for Co3O4-rGO hybrids.
3.3 Electrocatalytic activity of Co3O4–rGO hybrids
to H2O2
7
The electrocatalytic activity of CoG–1/GC
electrode towards H2O2 was investigated. PBS
solution (0.10 M, pH 7.4) was used for
electrochemical measurements, because it is able to
mediate enzymeless detection of H2O2. Figure 3a
shows the typical cyclic voltammograms (CVs) of
CoG–1/GC, Co3O4/GC, rGO/GC, and Co3O4/rGO/GC
electrodes in PBS solution with 0.10 mM H2O2.
Obviously, CoG–1/GC electrode generates an
obvious couple of high redox peaks at –0.19 V in
comparison with the bare rGO/GC, Co3O4/GC, and
Co3O4/rGO/GC electrodes. The pair of well–defined
redox peaks associated with the oxidation of H2O2,
illustrating the synergistic effect between Co3O4
nanowires and rGO enhance the electrochemical
activity for H2O2. According to previous reports, the
electrocatalytic oxidation of H2O2 on CoG–1 should
undergo the following reaction [40]:
2Co(Ⅲ)+ H2O2+ 2OH– → 2Co(Ⅱ) + O2 + 2H2O
To further clarify the advantage of Co3O4–rGO
hybrids in electrocatalytic activity, electrochemical
impedance spectra (EIS) were carried out (Figure. 3b).
The electrocatalytic activity of CoG–1 hybrids is
better than those of the corresponding pristine rGO,
Co3O4, and Co3O4/rGO resulting from the lower Rct
and Rs of CoG–1. These mean that the electron
transfer resistance and the ion diffusion impedance
are both reduced dramatically when rGO is
introduced. Meanwhile, the synergic effect between
rGO and Co3O4 can further enhance the
electrochemical performance. One the other hand,
spider web-like Co3O4 nanowires on rGO, in contrast
to Co3O4/rGO, could supply more efficient interfacial
active sites for electrocatalysis, and promote the
orientation transmission of electrons. As a result, it is
expected that the fill factor of CoG–1/GC electrodes
could be remarkably improved and further ensure a
promising performance. This is corroborated by the
electrochemical performance as mentioned above in
Figure 3a.
Figure 3 (a) CVs of CoG–1/GC, rGO/GC, Co3O4/GC, and
Co3O4/rGO/GC electrodes in 0.1 M PBS (pH 7.4) solution with
0.1 mM H2O2, scan rate: 50 mV﹒s –1. (b) Nyquist plots of
CoG–1/GC, rGO/GC, Co3O4/GC, and Co3O4/rGO/GC electrodes
in 0.1 M KCl containing 2 mM [Fe(CN)6 ]3/4–.
Figure 4a shows the CVs of the CoG–1/GC
electrode at different scan rates, which can explore
the reaction kinetics process. Both of the redox peak
currents increase linearly as the scan rate grow from
25 to 200 mV ﹒ s–1, suggesting that it is a
surface–controlled electrochemical process (Figure
4b) [41]. Such dependence is also consistent with the
fast charge propagation in CoG–1/GC electrode.
Apparently, the interconnected rGO, and 1D Co3O4
nanowire assembled with nanoparticles seem to
support charge transport within the hybrids. In other
words, the unimpeded distribution of charge to
Co3O4–rGO matrix is feasible.
Figure 4 (a) CVs of CoG–1/GC electrode in 0.10 M PBS (pH
7.4) solution with 0.1 mM H2O2 measured at different scan rates
(25–200 mV∙s–1). (b) The calibration plots between the anodic
(black line) and cathodic (red line) peak currents and the scan
rate (Ipa = 0.1075 V + 0.0057, R2 = 0.9964; Ipc = –0.2264 V –
0.0069, R2 = 0.9989; Ipa, Ipc, and V are anodic, cathodic peak
current and scan rate, respectively). (c) CVs of CoG–1/GC
electrode in 0.1 M PBS solution with different H2O2
concentrations (0–0.7 mM). (d) I–t curve of CoG–1/GC electrode
(holding at –0.19 V vs. SCE) to the successive addition of H2O2
in 0.1 M PBS. The inset is the calibration linear relationship of
8
currents versus H2O2 concentration.
Also, a series of CVs were recorded on CoG–1/GC
electrode at various concentrations of H2O2. With the
increase of H2O2 concentration, the reduction current
at –0.19 V increases (Figure 4c). Therefore, –0.19 V
(vs. SCE) is used as the detection voltage for
evaluating the H2O2 sensitivity of CoG–1/GC electrode
upon the successive addition of H2O2 with stirring
constantly. Figure 4d shows the typical I–t curve of
electrochemical response versus successive addition
of different H2O2 around the CoG–1/GC electrode.
Staircase curve can be seen that the electrochemical
response as the concentration of H2O2 increased. The
current starts to become more obvious when
micromole level of H2O2 is added and quickly
reaches a stable value. Current response time is
about 5s. To value the CoG–1 activity in the
non–enzymatic reaction with H2O2 as substrate, the
under inset of Figure 4d depicts the calibration curve
of CoG–1/GC electrode obtained from the
current–time plot, showing a well–defined typical
behavior of a catalytic reaction with a linear range
from 0.015 mM to 0.675 mM. A current plateau is
observed when the H2O2 concentration is higher than
0.675 mM. A linear regression equation of I (mA) =
0.04 (mA﹒mM–1) ×C (mM) + 0.106, (n = 3, R2 = 0.9998)
is thus derived from the calibration curve, revealing
that the H2O2 sensor has a detection limit as low as
2.4 μM (S/N = 3). The relatively smaller linear range,
however, is compensated by the remarkably
enhanced sensitivity and the low response peak
potential (–0.19 V). Through linear fitting, its
sensitivity up to 1.14 mA ﹒ mM–1 ﹒ cm–2, which
greatly exceeds the sensitivity of most previously
reported graphene or cobalt oxide/hydroxide–based
H2O2 detection (Table S2).
The low response potential also indicates that
Co3O4–rGO hybrids possess excellent selectivity for
H2O2 [42]. As shown in Figure 5, the addition of
interferents, e.g. 0.05 mM of urea, uric acid (UA),
L–cysteine (L–Cys), dopamine (DA) and glucose (Glc)
according to the order in PBS (pH 7.4) solution
containing 0.1 mM of H2O2 gives rise to the
negligible current response, while a significant
current response is observed for the subsequent
addition of 0.1 mM of H2O2. Compare the response
current of H2O2 after adding interferents with before,
it is indicated that the interferents had no impact on
the H2O2 detection performance. The stability and
repeatability of Co3O4-rGO hybrids have also been
evaluated by CVs measured at different cycles. As
shown in Figure S11, the current response of the
hundredth cycle is almost the same as that of first
cycle. It indicates the superior stability and accuracy
of Co3O4-rGO hybrids.
Figure 5 Amperometric response to the addition of different
analytes to 50 mL of electrolyte (0.1 M PBS), UA = uric acid,
L–Cys = L–cysteine, DA = dopamine, Glc = glucose.
The influence of Co3O4 content on the
electrocatalytic performance of the hybrids was
further investigated by cyclic voltammetry (as shown
in Figure S12). It can be concluded that CoG–1
exhibits the lowest reduction peak current and
relatively small peak potential, suggesting that
CoG–1 possesses the better electrochemicial
performance for H2O2 and has more value for
practical biological application.
3.4 Detection of extracellular release of H2O2 from
liver cancer HepG2 cells
Detection of H2O2 in cell level is constrained by
many factors, such as small scale of cells, the low
content of H2O2 cellular, the lack of effective capture
H2O2 probe and so on. The proven methods on
cellular H2O2 detection have been reported.
Especially, the electrochemical method (such as
chronoamperometry) has high time resolution and
its curve can reflect the intracellular H2O2 release
process when cells are stimulated by drugs and
environment [42, 43]. So, the sensitive CoG–1 probe
is applied to perform real–time detection of H2O2.
9
HepG2 cells are employed as model cells to
demonstrate the excellent electrochemical detection
ability of CoG–1. Under the stimulation of
N–formylmethionyl–leucyl–phenylalanine (fMLP),
HepG2 cells can rapid release H2O2, which is good
for authenticity. The cells were observed by a
confocal laser scanning microscopy, as shown in
Figure 6a. After growing, the HepG2 cells can attain
to 80% confluence. After a period of stability, 0.2 μM
of fMLP is added into the electrolyte. We can see
from Figure 6b, an obvious increase in current can be
observed. The current decreases to a stable level
within 350 s, indicating that H2O2 is either consumed
or diffused away from the electrode surface. To
prove the current signal is from materials response to
H2O2, catalase is applied to consume the release H2O2.
With adding 500 U ﹒ ml–1 catalase, the current
decreases to background level afterwards. As a
contrast test, control wells containing no HepG2 cells
do not generate any signal response to the addition
of fMLP or catalase. This observation substantially
demonstrates that the H2O2 electrochemical
biosensor based on CoG–1/GC electrode establishes a
sensitive, reliable analysis for the routine
determination of H2O2 released by live cells and
potential to be useful for further physiological and
pathological investigations.
nanoparticles, benefits from the structure–directing
and anchoring role of DDA and GO, respectively.
The Co3O4–rGO hybrids exhibited excellent
electrocatalytic activity and selectivity for H2O2
oxidation. The enhanced effectiveness was ascribed
to the interconnected structure, and the synergistic
effect of Co3O4 and rGO, in favor of the large
accessible surface area, more active sites and the
orientation transmission of electrons, as well as the
unimpeded pathways for matter diffusion. In
addition, the highly active Co3O4–rGO hybrids can
be used for real–time detection of H2O2 released from
HepG2 cells, implying they are suitable for biological
and biomedical application, such as the intra– and
extracellular H2O2 detection.
Acknowledgements
We gratefully acknowledge the support of the Key
Program Projects of the National Natural Science
Foundation of China (No. 21031001), the National
Natural Science Foundation of China (No. 51102082,
91122018, 21371053), the Cultivation Fund of the Key
Scientific and Technical Innovation Project, Ministry
of Education of China (No. 708029), Program for
Innovative Research Team in University (IRT–1237),
Youth Foundation of Heilongjiang Province of China
(QC2013C009).
Electronic Supplementary Material: Supplementary
material such as IR, TG, SEM image and additional
data, is available in the online version of this article
at http://dx.doi.org/10.1007/s12274–***–****–*
(automatically inserted by the publisher).
References
Figure 6 (a) The bright–field microscopy image of HepG2 cells.
(b) Amperometric response of CoG–1/GC electrode in 0.1 M
–
PBS (pH 7.4) with the addition of 0.2 µm fMLP and 300 U mL 1
catalase in the absence and present of HepG2 cells.
4
Conclusions
In summary, we have demonstrated the fabrication
and application of Co3O4–rGO hybrids for H2O2
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Electronic Supplementary Material
Interconnected 1D Co3O4 nanowires on reduced graphene
oxide for enzymeless H2O2 detection
Lingjun Kong,1 Zhiyu Ren,1 () Nannan Zheng,2 Shichao Du,1 Jun Wu,1 Jingling Tang,2 and Honggang Fu1
( )
1
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People's Republic of China, Heilongjiang
University, Harbin 150080 P. R. China, Tel.: +86 451 8660 4330, Fax: +86 451 8666 1259, E–mail: [email protected]
2 Department of Pharmaceutics, School of Pharmacy, Harbin Medical University, Harbin, 150086 P. R. China.
Supporting information to DOI 10.1007/s12274–****–****–* (automatically inserted by the publisher)
————————————
Address correspondence to H. Fu, [email protected], [email protected]
13
Results and disccussion
Figure S1 FT–IR spectra of CoG–p and CoG–1.
The decomposition temperature of DDA was measured by TG analysis under air atmosphere. As
shown in Figure S2a, TG curve of CoG–p presents a great loss of weight about 36.6% from 200 to
400 ºC sharply, indicating the thermal decomposition of DDA and GE in air atmosphere. Meanwhile,
two obvious exothermic peaks at 230 °C and 390 °C appear in the corresponding DSC curve. In
contrary, only an exothermic peak at 420 °C can be observed in the DSC curve of pure GO, attributed
to the decomposition of GO. So, 300 °C is identified as the calcination temperature to ensure the
existent of GE.
Figure S2 TG and DSC curves of CoG–p (a) and GO (b) measured under air atmosphere.
14
Figure S3 XPS spectrum of CoG-1.
Figure S4 High-magnification SEM image of CoG–1.
Figure S5 TEM images of CoG–1. a) The original image of Figure 2d, and b) the high-magnification image.
15
Figure S6 N2 adsorption–desorption isotherms of CoG–1.
Figure S7 SEM and TEM images of samples synthesized at 180 ℃ for 12 h, (a, c) Co3O4/rGO (without DDA) and
(b, d) Co3O4 (without GO).
16
Figure S8 SEM images of Co3O4-rGO hybrids synthesized at 180 ℃ for different reaction time, (a–d) 6h, 9 h, 12 h,
and 15 h, respectively.
Figure S9 TG curves of CoG–2, CoG–3, CoG–1, CoG–4, and rGO.
Table S1 The experimental parameters of the synthesized Co3O4–rGO hybrids.
Samples
Co3O4 content calculated from TG analyses (%)
CoG–1
72
CoG–2
50
CoG–3
58
CoG–4
78
17
Figure S10 SEM images of Co3O4-rGO hybrids synthesized with different ratio of Co3O4 and rGO, (a–d) CoG–2,
CoG–3, CoG–1, and CoG–4, respectively.
Table S2. Comparison of the analytical performance of different H2O2 biosensors.
Detection limit
Sensitivity
(μM)
(mA mM–1 cm–2)
(mM)
–0.19[a]
2.46
1.00
0.02–0.43
[1]
Ag@C@Ag/GCE
–0.56[a]
23
––––
0.07–10
[2]
GNPs/GN–CS/GCE
–0.40[b]
1.6
––––
0.005–35
[3]
GN–Co3O4 NPs/GCE
–0.48[a]
0.06
1.14
0.0002–0.211
[4]
CoOOH nanosheets
0.10[a]
40
0.099
–––1.6
[5]
Cu2O/GE
–0.40[a]
20.8
––––
0.3–7.8
[6]
RGO/Fe3O4 /Au
–0.30[b]
3.2
––––
0.1–6.0
[7]
Electrode materials
Potential (V)
Co3O4/MWCNTs
Linear range
Ref.
Co3O4 nanostructures
–0.77[a]
––––
––––
0–1.7
[8]
spinel CoMnOx (SCM)
–0.65[a]
15
––––
0.1–25
[9]
GN–AuNPs/GCE
–0.40[a]
6.0
––––
0.020–0.280
[10]
HRP–AuNP–PANi
–0.25[b]
0.3
0.1178
–––0.36
[11]
PtPd − Fe3O4
–0.25[a]
0.005
––––
Pt–MnO2/rGOP
–0.15[a]
1.0
0.1295
0.002–13.33
[13]
CoG–1
–0.19[b]
2.4
1.14
0.01–0.675
This work
2*10–5—1*10–4—1*10–3
[12]
[a] Potential vs. Ag/AgCl; [b] Potential vs. SCE.
18
Figure S11 CVs of CoG-1/GC electrodes in 0.1 mol﹒L–1 PBS with 0.1 mM H2O2 solution obtained at the first
cycle, fifth cycle and hundredth cycle, respectively.
Figure S12 CVs of CoG–1/GC, CoG–2/GC, CoG–3/GC, and CoG–4/GC electrodes in 0.1 mol﹒L–1 PBS with 0.1
mM H2O2 solution, scan rate: 50 mV﹒s−1
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20