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Author's personal copy
Electrochimica Acta 97 (2013) 398–403
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Ultrafine palladium nanoparticles grown on graphene nanosheets for enhanced
electrochemical sensing of hydrogen peroxide
Xiao-mei Chen a,c,∗ , Zhi-xiong Cai d , Zhi-yong Huang a , Munetaka Oyama c , Ya-qi Jiang b , Xi Chen b,∗∗
a
College of Biological Engineering, Jimei University, Xiamen 361021, China
State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China
c
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
d
Ministry of Education Key Laboratory of Analysis and Detection Technology for Food Safety, Fuzhou University, Fuzhou 350002, China
b
a r t i c l e
i n f o
Article history:
Received 6 December 2012
Received in revised form 31 January 2013
Accepted 8 February 2013
Available online 4 March 2013
Keywords:
Graphene nanosheets
Palladium nanoparticles
Hydrogen peroxide
Amperometric
Enzymeless
a b s t r a c t
A nonenzymatic electrochemical method was developed for hydrogen peroxide (H2 O2 ) detection using
an electrode modified with palladium nanoparticles (PdNPs)-graphene nanosheets (PdNPGNs). Ultrafine PdNPs were homogeneously modified on graphene nanosheets through a facile spontaneous redox
reaction and characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy
and X-ray photoelectron spectroscopy technique. Based on the voltammetric and amperometric results,
the PdNPGNs-modified glassy carbon electrode (PdNPGNs-GCE) demonstrated direct and mediatorless
responses to H2 O2 at a low potential. The analytical performances of the PdNPGNs-GCE toward H2 O2
reduction was evaluated in the linear response range from 0.1 ␮M to 1.0 mM, with a detection limit
(S/N = 3) of 0.05 ␮M. The PdNPGNs-GCE showed excellent resistance toward poisoning from such interfering species as ascorbic acid, dopamine and glucose. Furthermore, the electrochemical sensor presented
good characteristics in terms of stability and reproducibility, promising the applicability of this sensor in
practical analysis.
© 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Hydrogen peroxide (H2 O2 ), a well-known oxidizing agent, is not
only a by-product of a large number of oxidase enzymes, but also
an essential mediator in food, pharmaceutical, clinical, industrial
and environmental analysis [1,2]. In living organisms, H2 O2 is a
signaling molecule to regulate various biological processes, such
as vascular remodeling, immune cell activation and root growth
[3]. As a result, there is an ever-growing demand to create high
sensitivity, high reliability, rapid, recyclable and low cost H2 O2
sensors. Electrochemical H2 O2 sensors, especially nonenzymatic
amperometric biosensors, hold a leading position among various
biosensors.
Recently, the interest in these enzymeless sensors has been
centered on the efforts to find a breakthrough in the electrocatalysts [4–7]. Taking the advantages of catalytic activities,
efficient electron transfer rate, and large specific surface area,
nanomaterials, especially noble metal nanoparticles modified electrodes usually exhibit high electrocatalytic activities toward H2 O2
∗ Corresponding author. Tel.: +86 592 6181910; fax: +86 592 6180470.
∗∗ Corresponding author. Tel.: +86 592 2184530; fax: +86 592 2184530.
E-mail addresses: [email protected] (X.-m. Chen), [email protected]
(X. Chen).
0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.02.047
reduction [8–10]. Among these nanomaterials, palladium nanoparticles (PdNPs) are one of the most efficient catalysts, especially in
the formation of the C C bond and chemical transformations such
as hydrogenation, hydrodechlorination, carbonylation and oxidation [11,12]. Furthermore, the abundance of Pd on the earth is at
least fifty times more than that of Pt [13], which has raised the
interest for intensive research on PdNPs in areas of electrocatalysis. In the sensing of H2 O2 , PdNPs show much enhanced catalytic
activity for H2 O2 reduction with the H2 O2 detection limit reaching
0.1 ␮M level [14]. Despite this increased activity and sensitivity,
probes made from these PdNPs are still not sensitive enough for
the determination of H2 O2 in some special biological systems.
Recent studies have indicated that the size and the distribution of PdNPs play a vital role in the catalytic ability for H2 O2 ,
and the matrix for the preparation of PdNPs is also very important
[15,16]. This gives a hint that the sensitivity of H2 O2 sensing can
be improved greatly by PdNPs with ultrafine size and well dispersion on some proper materials. Moreover, as a famous supporting
material, graphene nanosheets (GNs) possess a theoretical surface
area of 2630 m2 g−1 , which surpasses that of graphite (10 m2 g−1 ),
and is two folds larger than that of carbon nanotubes (1315 m2 g−1 )
[17,18]. Therefore, GNs have received considerable interest for
immobilizing nanoparticles on them [19–21].
In our previous study, we discovered a facile method to synthesize high active PdNPs on graphene oxide (GO) nanosheets by the
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X.-m. Chen et al. / Electrochimica Acta 97 (2013) 398–403
galvanic cell effect between PdCl4 2− and GO [22]. Previous research
based on first-principles calculations indicates that Pd could interact with and bind more strongly to GNs because more interaction
states and transmission channels are generated between them,
moreover, Pd tends to grow into three-dimensional structures on
GNs surfaces [23–25]. This provides a hint that GNs could be an
ideal substrate for growing and anchoring PdNPs. In the further
research, we found that ultrafine PdNPs can be easily prepared on
the GNs by spontaneous redox between PdCl6 2− and GNs. Taking
the advantage of the ultrafine, well-dispersed, high-yield, “clean”
surface (template-free preparation) of PdNPs and the large surface area, high conductivity of GNs, the nano-composite revealed
an unusually high electrocatalytic activity and excellent performance for H2 O2 determination. Under optimized conditions, the
linear response of current to H2 O2 concentration was valid in the
range from 0.1 ␮M to 1.0 mM, with a detection limit (S/N = 3) of
0.05 ␮M, which is much sensitive than most of the other H2 O2 sensors. Moreover, the detection was carried out in phosphate buffer
solution (PBS) with the pH of 7.4, which is helpful to better understand the effects of H2 O2 in biological environment. To the best
of our knowledge, this is the first time that a PdNPGNs modified
electrode has been employed in the development of nonenzymatic
H2 O2 biosensors. This result demonstrated that the applications of
a PdNPGNs modified electrode was feasible in H2 O2 detection and
indicated a breakthrough in nonenzymatic sensors using Pd-based
electrodes.
2. Experimental
2.1. Materials
K2 PdCl6 was purchased from Wako Pure Chemicals, Co. Ltd.
(Japan); the graphite powder was from Lvyin Co. (China); 5% Nafion
ethanol solution and Pd black were from Aldrich Chem. Co. (USA);
rod glassy carbon electrodes (GCEs) and Pd electrode were from
BAS Co. Ltd. (Japan). All other reagents were of analytical grade
399
and used without further purification. The pure water for solution
preparation was from a Millipore Autopure WR600A system (USA).
2.2. Instrumentation
Morphologies and crystal structures of PdNPGNs observed by
TEM and high-resolution TEM (HRTEM) were performed on a
JEM-2100 transmission electron microscopy with an acceleration
voltage of 200 kV. All TEM samples were prepared by depositing a
drop of diluted suspension in ethanol on a copper grid coated with
carbon film. Energy dispersive X-ray spectroscopy (EDX) analysis
was used to identify the elemental composition of the complex.
Electronic binding energies of Pd3d were measured by X-ray photoelectron spectroscopy (XPS) analysis which was performed on a
PHI Quantum 2000 Scanning ESCA Microprobe with a monochromatized microfocused Al X-ray source. All the binding energies
were calibrated by C 1s as reference energy (C 1s = 284.6 eV).
Electrochemical measurements were performed with a CHI 830
Electrochemical Analyzer. A conventional three-electrode system
included a GCE coated with PdNPGNs film, a silver auxiliary electrode and a saturated calomel reference electrode (SCE).
2.3. Procedures
GO was prepared according to a modified Hummer’s method
[26,27]. For the reduction of GO, 50 mg as-synthesized GO was dispersed in 100 mL water to obtain a yellow-brown aqueous solution
with the aid of ultrasonication. GNs were achieved by heating the
GO aqueous solution in an oil bath at 100 ◦ C for 24 h [28].
In a typical synthesis of ultrafine PdNPs on GNs, homogeneous
GNs suspension (5 mL 0.5 mg mL−1 ) and K2 PdCl6 (0.5 mL 10 mM)
aqueous solution was kept in a vial under vigorous stirring for
60 min at 30 ◦ C. Then, the reaction mixture was washed with pure
water and centrifuged to remove the remaining reagents. Before
the preparation of the catalysts modified GCE, the GCE was polished
with 1, 0.3 and 0.05 ␮m ␣-Al2 O3 , sequentially. For coating the GC
Fig. 1. Representative TEM (A) and HRTEM (B) images of PdNPGNs composite. The EDX (C) and Pd3d XPS spectra (D) of the as-synthesized PdNPGNs composite.
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electrode surface with PdNPGNs nano-composite, we use Nafion
to consolidate the modification. 20 ␮L of PdNPGNs suspension was
dispersed in 20 ␮L 0.5% Nafion ethanol solution, then deposited
5 ␮L of PdNPGNs/Nafion on the polished GC electrode and dried in
the air for 4 h at room temperature.
3. Results and discussion
3.1. Characterization of PdNPGNs composite
The PdNPGNs composite could be obtained easily by the magic
mixture of GNs and Pd precursor at 30 ◦ C for 1 h. Fig. 1A and B
shows the representative TEM images of the product at different
magnifications. Low-magnification TEM images (Fig. 1A) show that
high-yield and ultrafine PdNPs are uniformly dispersed on GNs surface. The magnified image (Fig. 1B) shows that the average size of
these PdNPs was about 2 nm. Fig. 1C shows a typical EDX analysis
of the prepared PdNPGNs composite, in which obvious Pd peaks
could be found, suggesting that PdNPs were successful attached on
the GNs surface. XPS patterns of the resulting PdNPGNs composite
(Fig. 1D) show significant Pd3d signals corresponding to the binding
energy of Pd0 , which further supported the conclusion that PdNPs
were effectively assembled on the GNs surface.
3.2. Electrocatalytic reduction of H2 O2
The CV method was used to compare and investigate the electrochemical behavior of the prepared PdNPGNs/Nafion-GCE. Fig. 2A
presents the CV responses on a PdNPGNs/Nafion modified electrode
in 0.1 M PBS solution (pH 7.4) with and without 5 mM H2 O2 . When
no H2 O2 was added, during the positive potential scan, the major
redox process included an anodic peak at 0.20 V, corresponding
to the formation of palladium oxides and a single sharp cathodic
peak at about 0.03 V, corresponding to the reformation of a clean
surface of Pd0 [29]. With the addition of H2 O2 , the currents of
the modified electrode changed dramatically with an increase of
the reduction peak current at −0.31 V, indicating that a catalytic
reaction occurred on the electrode. Previous studies have pointed
out that in the presence of H2 O2 , the Pd surface becomes oxidized to Pd(OH) during the positive scan and this involves the
chemisorptions of H2 O2 molecules on its surface. It is then catalytically transformed to the reduced surface during the cathodic scan
[30,31]. In the experiment, Pd played a key role in the reduction of
H2 O2 on PdNPGNs/Nafion-GCE (see more details in Fig. 2). Considering this, H2 O2 tend to be reduced on the PdNPGNs surface similar
with that on Pd electrode.
According to the deduction, the status of Pd can be changed
rapidly between oxidized and reduced states, and thereby the
adherence of oxidized ascorbic acid or dopamine on the electrode surface can be avoided. Moreover, the PdNPGNs/Nafion-GCE
presented a very low potential for H2 O2 reduction (−0.31 V),
thus enabling low potential amperometric detection. This indicates that the proposed sensor is potentially free from other
interferences.
We further investigated the effect of different electrode
materials on H2 O2 reduction. Fig. 2B compares the voltammograms obtained from the reduction of H2 O2 by a bare GCE, a
GNs/Nafion-GCE, a Pd electrode, a Pd black/Nafion electrode and a
PdGNs/Nafion-GCE. As can be seen from their CV curves, no obvious response can be observed at the bare GCE, and there is only
a quite weak current peak at the GNs/GCE. In contrast, the Pd
electrode, Pd black/Nafion electrode and the PdNPGNs/Nafion-GCE
exhibit a variously reduction peak associated with the reduction of
H2 O2 . For the PdNPGNs/Nafion-GCE, although the reduction peak
potential is more negative than that of the Pd electrode and Pd
Fig. 2. (A) CV curves of the PdNPGNs/Nafion-GCE in the PBS solution with and without 5 mM H2 O2 . (B) CV curves in PBS–H2 O2 solution on different electrodes. The
inset enlarge CV curves on bare-GCE and GNs-GCE. Scan rate: 40 mV s−1 ; PBS: 0.1 M,
pH = 7.4; H2 O2 : 5 mM.
black/Nafion electrode, the peak current is about 7.5 folds and
2.3 folds larger than that obtained from the other two electrodes,
respectively. These results clearly demonstrate that PdNPGNs has
much higher catalysis toward the reduction of H2 O2 , and can
remarkable enhance the sensitivity and selectivity in H2 O2 sensing.
The superior electrocatalytic activity of the advanced material can
be attributed to three factors: (i) the combination of GNs with
well dispersive ultrafine PdNPs provides larger electrochemically
active surface area, which facilitates the adsorption of detection
H2 O2 ; (ii) the “clean” surfaces of the PdNPs offer more active
sites in the electro-oxidation; (iii) the high coverage of PdNPs
and high conductivity of GNs, which can accelerate the electron
transfer toward reduction of H2 O2 . These results illustrate that
the PdNPGNs/Nafion-GCE present the best catalytic performance
among the four electrodes tested.
3.3. Performance of the electrochemical sensor for H2 O2 detection
To improve the performance of the non-enzymatic H2 O2 sensor, several factors such as the applied potential and pH value of
electrolyte solution were optimized.
It is well known that the applied potential strongly affects
the amperometric response of a sensor. We have systemically
investigated the impact of applied potentials on the amperometric response on the PdNPGNs/Nafion-GCE toward reduction
of H2 O2 . Amperometric response was performed by varying the
potential between −0.4 V and 0.0 V in the presence of 0.05 mM
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Fig. 3. The effect of applied potential (A) and solution pH (B) to the chronoamperomertry response of 0.05 mM H2 O2 on the PdNPGNs/Nafion-GCE.
H2 O2 . As shown in Fig. 3A, the current response increased gradually from −0.4 V to −0.25 V, and reached the maximum at
−0.25 V, then decreased at more positive potentials than −0.25 V.
Taking the sensitivity and the signal/noise ratio into consideration, −0.25 V (vs. SCE) was chosen as the optimum applied
potential.
The effect of the buffer solution pH on the sensor was examined. Fig. 3B shows the amperometric responses of the sensor in
0.1 M PBS with different pH from 6.0 to 8.0 in the presence of
0.05 mM H2 O2 . It is seen that the current toward the reduction of
H2 O2 increased with increasing pH value and reached a maximum
at a pH value of 7.4. Then, the current decreased as pH was further
increased. Therefore, a PBS at pH 7.4 was selected as the supporting
electrolyte in the further studies.
Under optimal experimental conditions, PdNPGNs/Nafion-GCE
was applied in the determination of H2 O2 . Fig. 4A shows a typical
current–time plot of the PdNPGNs/Nafion-GCE on successive step
changes of H2 O2 concentration into stirring PBS (0.1 M, pH 7.4) at an
applied potential of −0.25 V. With the addition of H2 O2 the stirring
buffer solution, the current responds rapidly increased to the substrates and could achieve 95% of the steady-state current within 3
s, which is faster than that of most other reports [32–36]. The linear
relationship between the catalytic currents and the concentrations
of H2 O2 is shown in Fig. 4B. The catalytic current increased with
the increase of H2 O2 from 0.1 to 1000 ␮M with a detection limit
(S/N = 3) of 0.05 ␮M. Taking the advantage of the ultrafine, welldispersed, high-yield, “clean” surface of PdNPs and the large surface
401
Fig. 4. (A) Chronoamperometry curves of the PdNPGNs/Nafion-GCE with the successive addition of 0.1, 0.5, 1.0, 2.5, and 50 ␮M H2 O2 . Inset is the enlarge view of the
chronoamperometry curve with the H2 O2 from 0.1 to 3 ␮M and 3 to 20.5 ␮M. (B) The
calibration curve of H2 O2 concentration and currents. The inset shows a close look
of H2 O2 concentration from 0.1 to 10 ␮M. Applied potential: −0.25 V; PBS: 0.1 M,
pH = 7.4.
area, high conductivity of GNs, the PdNPGNs/Nafion-GCE revealed
excellent performance for the H2 O2 determination. The advantages
of the proposed sensor could also be supported by comparison with
other H2 O2 sensors as shown in Table 1 [37–44].
As it is well known that, some co-existing electroactive species
in nature will affect the sensor responses. So, we have monitored
the response of the sensor toward each 0.1 mM of ascorbic acid
(AA), glucose and dopamine (DA) additions. The working potential
was hold at −0.25 V. Each addition of the electroactive interfering
species brought out hardly discernible current response, whereas
notable response was observed for 0.05 mM H2 O2 (Fig. 5). These
results suggest that the interfering effect caused by these electroactive species is quite negligible, validating the highly selective
detection of H2 O2 at the composite film.
The long-term storage and operational stability of the electrode is essential for the continuous monitoring of H2 O2 . The
stability of the present electrode was examined using the same
PdNPGNs/Nafion-GCE for 10 repetitive measurements in successive additions of 0.05 mM H2 O2 at −0.25 V. The relative standard
deviation was 3.2%, confirming that the as-synthesized electrode
for H2 O2 sensing is stable. The long-term storage was evaluated by
measuring its sensitivity to H2 O2 over a week period. The sensor
was stored in air and the sensitivity was tested each day. The result
demonstrated that the sensitivity was 92% of its initial sensitivity
after 7 days. These results indicate that the PdNPGNs/Nafion-GCE
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Table 1
Comparison of different H2 O2 sensors in terms of LOD and linear range.
Electrode
b
P2 W17 Fe-PdNP/GCE
AgNP-TiO2 /GCE
PtNP/ITO
PdNP-MCNs/GCEc
PdNP-TiO2 /GCE
PtNP-MnO2 /rGOPd
Pt48 Pd52 -Fe3 O4 -carbon/GCE
PdNP-CNF/CPEe
PdNP-GNs/GCE
a
b
c
d
e
Linear range (␮mol L−1 )
LODa (␮mol L−1 )
References
1.5–3.9 × 10
1 × 102 to 6 × 104
5-8 × 102
7.5–1.0 × 104
–
2–1.3 × 104
0.1–2.0; 2.0–1.4 × 104
0.2–2.0 × 104
0.1–1.0 × 104
1.0
1.70
0.4
1.0
3.81
1.0
0.005
0.2
0.05
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
This work
3
LOD: the limit of detection.
P2 W17 Fe-PdNPs: K7 P2 W17 O61 (FeOH2 )·8H2 O-palladinum nanoparticles.
PdNPs-MCNs: palladinumnanoparticles-mesoporouscarbonnanospheres.
PtNPs-MnO2 /rGOP: PtNPs-MnO2 /reduced graphene oxide paper.
PdNPs-CNF/CPE: palladinumnanoparticles-carbonnanofibers/carbonpasteelectrode.
No. 21175112) and the Science and Technology Planning Project of
Fujian Province, China (2012Y0052), which are gratefully acknowledged.
References
Fig. 5. Chronoamperometry curves of the PdNPGNs/Nafion-GCE exposed to H2 O2
(0.05 mM), AA (0.1 mM), DA (0.1 mM) and glucose (0.1 mM). Applied potential:
−0.25 V; PBS: 0.1 M, pH = 7.4.
displays good long-term storage ability and excellent stability,
promising a desirable H2 O2 sensor for most routine analyses.
4. Conclusions
In summary, this study proposed a novel and high-effective nonenzymatic H2 O2 sensor based on the PdNPGNs/Nafion modified
electrode. Comparing with other PdNPs, the superior electrocatalytic activity of the advanced material can be attributed to the
ultrafine size and “clean” surface of the PdNPs, which may offer
more active sites in the electro-catalysis and large surface area, high
conductivity of GNs, which can improve the performance of the
electrochemical sensor. Owing to these, the PdNPGNs/Nafion modified electrode exhibit distinctly good performance toward H2 O2
reduction with higher catalytic current and more stable response
compared to the major of the other electrodes. It is the first time to
construct PdNPGNs in non-enzymatic H2 O2 sensing. This approach
provides a new facile route to construct effective H2 O2 sensors, and
may provide a promising way to detect other biologically important
compounds. Further experiments, such as the practical application
of this novel sensor and the construction of non-enzymatic labels
for ultrasensitive bioanalysis, are underway.
Acknowledgements
Chen XM thanks the Japan Society for the Promotion of Science (JSPS) for the fellowship. This research work was financially
supported by National Natural Science Foundation of China (NSFC,
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