Fabrication of phosphonate functionalized platinum nanoclusters and their application in hydrogen peroxide sensing in the presence of oxygen

Article abstract of DOI:10.1016/j.electacta.2012.07.005

The water-soluble phosphonate functionalized spherical platinum nanoclusters (Pt-NCs) composed of primary nanoparticles are synthesized by the thermal decomposition of platinum(IV)-complex. The size, morphology and structure of Pt-NCs are analyzed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Attachment of phosphonate groups on Pt surface is confirmed by Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and zeta potential analysis. The phosphonate functionalized Pt-NCs efficiently suppress the oxygen reduction reaction (ORR) while fully preserving the Pt-like activity for the hydrogen peroxid reduction reaction. Importantly, at certain cathodic potential, only reduction reaction of hydrogen peroxide occurs at the Pt-NCs surface. Consequently, the phosphonate functionalized Pt-NCs can be used to fabricate a new kind of hydrogen peroxide electrochemical sensor with high performance in the presence of oxygen. Under optimized experimental conditions, the hydrogen peroxide electrochemical sensor shows fast response (less than 2 s) and wide linear range (1.0 × 10 ^-6-1.0 × 10^-3 M) in the presence of oxygen. The results presented here indicate an important new direction in the quest to design selective catalysts for the fabrication of noble-metal based H ₂O₂ electrochemical sensor.

Full text of DOI:10.1016/j.electacta.2012.07.005

Electrochimica Acta 80 (2012) 233–239
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of phosphonate functionalized platinum nanoclusters and their
application in hydrogen peroxide sensing in the presence of oxygen
a
b
a
a
a,∗
a
a,∗
Jiangfeng Xu , Xingyu Wu , Gengtao Fu , Xinyu Liu , Yu Chen , Yiming Zhou , Yawen Tang
,
Tianhong Lua
a
Jiangsu Key Laboratory of New Power Batteries, College of Chemistry and Materials Science, Nanjing Normal University, 1# Wenyuan Road, Nanjing 210046, PR China
Institute of Hepatobiliary Surgery, The Affiliated DrumTower Hospital, School of Medicine, Nanjing University, 321# Zhongshan Road, Nanjing 210093, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2012
Received in revised form 28 June 2012
Accepted 3 July 2012
Available online 10 July 2012
The water-soluble phosphonate functionalized spherical platinum nanoclusters (Pt-NCs) composed of
primary nanoparticles are synthesized by the thermal decomposition of platinum(IV)-complex. The size,
morphology and structure of Pt-NCs are analyzed by dynamic light scattering (DLS) and transmission
electron microscopy (TEM). Attachment of phosphonate groups on Pt surface is confirmed by Fourier
transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and zeta potential anal-
ysis. The phosphonate functionalized Pt-NCs efficiently suppress the oxygen reduction reaction (ORR)
while fully preserving the Pt-like activity for the hydrogen peroxid reduction reaction. Importantly, at
certain cathodic potential, only reduction reaction of hydrogen peroxide occurs at the Pt-NCs surface.
Consequently, the phosphonate functionalized Pt-NCs can be used to fabricate a new kind of hydrogen
peroxide electrochemical sensor with high performance in the presence of oxygen. Under optimized
experimental conditions, the hydrogen peroxide electrochemical sensor shows fast response (less than
Keywords:
Platinum nanoclusters
Catalyst
Phosphonic acid groups
Hydrogen peroxide sensing
Amperometric response
−6
−3
2
s) and wide linear range (1.0 × 10 –1.0 × 10 M) in the presence of oxygen. The results presented
here indicate an important new direction in the quest to design selective catalysts for the fabrication of
noble-metal based H2O2 electrochemical sensor.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
Among various preparation methods, the solution phase
synthesis is particularly attractive for the synthesis of Pt nanos-
tructures due to the ease of scale-up. For example, the porous
three-dimensional dendritic and polypod-like Pt nanostructures
have been produced by direct thermolytic reduction of trans-
Pt(NH ) Cl in high-boiling-point solvents [17]. Meanwhile, it is
Platinum (Pt) nanostructures have attracted great attention in
the past several decades due to their specific properties in chem-
istry and physics, which are caused by size and surface effects
and are distinct from those of bulk metal or single metal atom,
and due to their wide applications in catalysis [1–3]. Meanwhile,
it has been established that the catalytic reactivity of Pt nanos-
tructures highly depends on their morphology and size due to the
crystal face effect and area effect [4–10], and therefore the design
and synthesis of well-controlled shape and size of Pt nanostruc-
tures are very critical for their applications in catalysis. Recently, Pt
nanostructures with various shapes such as nanoshells, multipods,
nanospheres, nanowires, hollow nanospheres and nanocubes, have
been synthesized by different strategies such as templating, seed-
ing, chemical reduction, organic-sol and thermal decomposition
methods [3,11–16].
3
2
2
well known that the detection and quantitative determination
of hydrogen peroxide (H O ) represents an extremely important
2
2
topic that has relevance pertaining to environmental, pharma-
ceutical, clinical and industrial research [18–21]. Among virous
traditional analytical methods, an free-enzyme based electrochem-
ical determination method possesses many advantages, such as
cost-effectiveness, sensitivity, stability, simple operation and ease
of miniaturization [22,23]. Until now, many Pt-based nanostruc-
tures were proposed to construct H O2 electrochemical sensor
2
owing to their high electrocatalytic efficiency and good stability
[24–27]. Unfortunately, Pt-based nanostructures modified elec-
trodes were not suitable for selective reduction of H O2 in the
2
presence of oxygen due to their high electrocatalytic activity
towards oxygen reduction reaction (ORR), which seriously impeded
their potential applications for in situ determination of hydrogen
peroxide in a number of important areas, such as clinical analysis,
environmental science and industrial production.
∗ Corresponding authors. Tel.: +86 25 85891651; fax: +86 25 83243286.
E-mail addresses: ndchenyu@yahoo.cn (Y. Chen), tangyawen@njnu.edu.cn
Y. Tang).
(
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.07.005

2
34
J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
O
OH
O
OH
5
P
P
A
HO
HO
HO
4
N
N
HO
P
O
P
O
OH
N
3
OH
OH
O
2
P
HO
1
Scheme 1. Structure of diethylene triamine penta(methylene phosphonic acid)
DTPMPA).
(
0
00
0
2
20
3
00
40
Very recent, some very excellent works show surface modifi-
400
60
500
cation of Pt metal can remarkably change their catalytic activity.
For example, Markovic and co-workers found that on the cyanide-
covered Pt(1 1 1) surface activities of the oxygen reduction reaction
can change from a 25-/10-fold increase in the presence of sulfu-
ric/phosphoric acid anions to no effect in the presence of perchloric
acid anions, to a 50-fold decrease in alkaline solutions [28]. Mean-
while, they also found a chemically modified Pt electrode with
a self assembled monolayer of calix arene molecules can selec-
tively block the oxygen reduction reaction, but in such a way
that the hydrogen oxidation reaction proceeds with Pt-like activity
80
600
100
700
800
120
B
[
29]. We report herein a one-pot preparation of the spherical plat-
inum nanoclusters (Pt-NCs) composed of Pt nanoparticles by using
diethylene triamine penta(methylene phosphonic acid) (DTPMPA,
shown in Scheme 1) as the complexing agent, reducing agent
and its oxidation product as stabilizing agent. The as-prepared
phosphonate functionalized Pt-NCs exhibit excellent electrocat-
alytic activity towards reduction of H O , but weak electrocatalytic
2
2
activity towards ORR. Consequently, the Pt-NCs modified electrode
shows good selectivity for H O2 detection in the presence of oxy-
gen.
2
0
20
40
60
80
100
Time / s
2
. Experimental
2
.1. Materials
C
All glassware used was thoroughly cleaned with freshly pre-
pared aqua regia solution (HCl/HNO , 3:1), then washed thoroughly
3
with H O before use. Diethylene triamine penta(methylene phos-
2
phonic acid) (DTPMPA, shown in Scheme 1) was purchased from
Shandong Taihe Water Treatment Co., Ltd. (Shandong, China).
Chloroplatinic acid hexahydrate (H PtCl ·6H O), hydrogen per-
2
6
2
oxide (H O ), sodium hydroxide (NaOH) and hydrochloric acid
2
2
(
(
HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd.
Shanghai, China). The solution pH was adjusted by the addition of
dilute NaOH or HCl solution.
0
50
100 150 200 250 300 350 400
2.2. Preparation of spherical platinum nanoclusters (Pt-NCs)
Time / min
To prepare the Pt-NCs, 2 mL of 0.0386 M H PtCl6 and 2 mL of
.0386 M DTPMPA were added in 36 mL deionized water, and the
2
Fig. 1. (A) Time-dependent UV–vis absorption spectra of a pH 11.0 solution contain-
◦
0
ing 0.0386 M H PtCl and 0.0386 M DTPMPA at 120 C. The spectra are recorded at a
2
6
solution pH was adjusted to 11 using NaOH solution. The resulting
solution was refluxed for 6 h at an oil-bath temperature of 120 C,
time interval of 5 min. Inset is the representative photograph of Pt-NCs. Dependence
of the absorbance value at (B) 260 nm and (C) 600 nm in (A) on time.
◦
and the solution color changed from light yellow to colorless and
finally became brown-black (inset in Fig. 1A). After the reaction, the
obtained Pt-NCs were separated by centrifugation at 20,000 rpm for
1.0 cm quartz cells. The X-ray diffraction (XRD) patterns of the cat-
alysts were obtained with Model D/max-rC X-ray diffractometer
◦
˚
3
0 min, washed several times with water, and then dried at 60 C
using Cu K␣ radiation source (ꢀ = 1.5406 A) and operating at 40 kV
and 100 mA. Transmission electron microscope (TEM) measure-
ments were made on a JEOL 2000 transmission electron microscope
operated at an accelerating voltage of 200 kV. The sample for
TEM characterization was prepared by placing a drop of colloidal
solution on carbon-coated copper grid and dried at room tem-
perature. X-ray photoelectron spectroscopy (XPS) measurements
for 5 h in a vacuum dryer.
2.3. Physical characterization
The UV–vis spectra were recorded at room temperature on
a UV3600 spectrophotometer (Shimadzu, Japan) equipped with

J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
235
were performed with a Thermo VG Scientific ESCALAB 250 spec-
30
25
trometer with a monochromatic Al Ka X-ray source (1486.6 eV
photons), and the vacuum in the analysis chamber was main-
mean=13.8nm
−9
tained at about 10 mbar. The binding energy was calibrated
by means of the C 1s peak energy of 284.6 eV. Fourier trans-
form infrared (FT-IR) measurements were carried out using a
Nicolet 520 SXFTIR spectrometer. The spectra were collected in
2
1
1
0
5
0
5
0
−
1
the wave number range between 400 and 4000 cm
over 128
−1
scans at a resolution of 4 cm . The zeta potential measure-
ment and dynamic light scattering (DLS) analysis were performed
with a Malvern Zetasizer Nano ZS90 analyzer at room tempera-
ture. Electrospray ionization mass spectrum was determined on a
TM
Finnigan LCQ
tion.
mass spectrograph (ESI-MS) in the aqueous solu-
1
0
15
20
25
Cluster diameter / nm
2.4. Electrochemical measurement
Fig. 2. The particle size distribution histogram of Pt-NCs from DLS measurement.
All electrochemical experiments were carried out on a CHI 660 C
electrochemical workstation (CH Instruments, Shanghai, Chenghua
Co.). A standard three-electrode system was used for all electro-
chemical experiments, which consisted of a platinum wire as the
auxiliary electrode, a saturated calomel reference electrode (SCE),
and a Pt-NCs modified glassy carbon electrode (3 mm diameter) as
the working electrode. Rotating disk electrode test was performed
on Gamry’s Rotating Disk Electrode (RDE710) with a glassy carbon
disk (5 mm diameter). All potentials in this study are reported with
respect to the SCE. All of the electrochemical measurements were
carried out at room temperature.
the weak reducibility of aminopolyphosphonates molecules [31].
After 325 min, the localized surface plasmon resonance absorbance
of reaction solution reaches a maximum value and remains almost
IV
invariable with time (Fig. 1A and C), indicating that DTPMPA–Pt
complex has been reduced completely to Pt0 nanoparticles.
Dynamic light scattering (DLS) is a powerful technique to char-
acterize colloidal suspension. Based on the analysis of scattered
light fluctuations caused by the Brownian motion of particles, it
provides size measurements from the nanometers up to a few
microns. A first estimation of particle size of Pt-NCs was achieved
by DLS experiment (Fig. 2). As observed, as-prepared Pt-NCs have
a relatively narrow size distribution with mean particle size of
An evenly distributed suspension of catalyst was prepared by
sonicating the mixture of 4 mg Pt-NCs and 4 mL H O for 30 min,
2
and 2 ␮L the resulting suspension was laid on the surface of the pre-
1
3.8 nm.
The morphology, the size, and the dispersion of Pt-NCs were
2
cleared glassy carbon electrode (3 mm diameter, 0.0706 cm ). After
drying at room temperature, 2.5 ␮L of 5.0 wt% nafion solution was
spread on the surface of the catalyst layer and allowed to dry. The
obtained electrode was denoted as the nafion/Pt-NCs/GC electrode.
The specific metal loading of Pt-NCs on the electrode surface was
further characterized by TEM measurements. A low magnification
image shows as-prepared Pt-NCs are spherical and the average par-
ꢀ
ticle size is about 15.2 nm (Fig. 3A and A ), consistent with DLS
analysis. The structural details of Pt-NCs were revealed by mid-
dle resolution TEM image. Obviously, the Pt-NCs are composed of
small Pt nanoparticles with mean particle size of 3.0 nm (Fig. 3B
−2
about 28 ␮g cm . For comparison, the commercial Pt black (E-TEK
Co., Ltd.) was also been used to fabricate the nafion/Pt-black/GC
electrode by the same preparation method.
ꢀ
and B ). These primary Pt nanoparticles are interconnected to one
another to form larger secondary Pt-NCs. Further high resolution
TEM image and the selected-area electron diffraction (SAED) image
demonstrate that Pt-NCs are polycrystalline in nature with Pt(1 1 1)
lattice fringes of 2.29 A (Fig. 3C and inset).
3
. Results and discussion
3.1. Preparation and characterization of Pt-NCs
It is worth noting that the appropriate concentration of pre-
cursors plays an essential role in the formation of Pt-NCs with
uniform size distribution. For example, the Pt-NCs with uniform
size distribution could not been obtained when the concentration
The Pt-NCs were prepared by refluxing the mixture solution
−3
−3
of 1.93 × 10 M H PtCl6 and 1.93 × 10 M DTPMPA (pH 11) at
2
◦
an oil-bath temperature of 120 C for 6 h. During the process, the
IV
−4
solution color changed from light yellow to colorless and finally
of DTPMPA–Pt complex decreased to 9.65 × 10 M or increased
−
3
became brown-black (inset in Fig. 1A). Considering that the H PtCl6
to 3.86 × 10 M (see TEM images in Fig. 4). However, it is a pity
2
aqueous solution has characteristic absorption behavior, UV–vis
absorption spectroscopy was used to monitor the reaction process
momentarily. In the initial stage of the reaction, the absorption
that the detailed reason is not clear for us.
The surface composition and nature of the Pt-NCs are investi-
gated by XPS measurement (Fig. 5A). The Pt 4f signal of Pt-NCs is
fitted to three pairs of doubles: 71.45, 74.85 eV; 72.94, 76.29 eV;
peaks of H PtCl6 at 200 and 260 nm decrease with increasing
2
0
II
IV
time (Fig. 1A and B). At 40 min, the absorption peaks of H PtCl6
75.2, 78.55 eV, which can be assigned to Pt , Pt O, Pt O2 species,
respectively. By measuring the relative peak areas according to the
2
completely disappear, indicating DTPMPA has completely inter-
IV
0
acted with H PtCl6 to generate the DTPMPA–Pt complex due to
curve-fitting, it is found that the percentage of Pt species in Pt-NCs
2
the strong coordinating capability of DTPMPA. FT-IR (Fig. S1, Sup-
porting information) and ESI-MS (Fig. S2, Supporting information)
measurements further confirm that DTPMPA can interact with
is 78.5%, indicating that Pt species is predominantly in the metallic
state in Pt-NCs. The fact demonstrates that Pt precursor is success-
IV
fully reduced to form metallic Pt in our synthesis. The crystalline
nature of the Pt-NCs was confirmed by recording X-ray diffrac-
tion (XRD) (Fig. 5B). Well-resolved diffraction peaks at 2ꢁ values of
IV
IV
H PtCl , and form DTPMPA–Pt complex with DTPMPA/Pt molar
2
6
ratio = 1:1. With further increasing time, the localized surface
plasmon resonance of Pt nanoparticles (i.e., the very wide UV
absorption in the wavelength range of 800–200 nm) appears at
◦
◦
◦
◦
39.58 , 46.42 , 67.48 and 81.06 are indexed as Pt(1 1 1), Pt(2 0 0),
Pt(2 2 0) and Pt(3 1 1) diffraction peaks, indicating Pt nanoparticles
in Pt-NCs possessed the face centered cubic crystal (fcc) structure.
According to Scherrer equation [32], the average particle size of Pt
5
2 min [12,30] (Fig. 1A and C), indicating that the thermal decom-
IV
position of DTPMPA–Pt complex occurs. This is likely ascribed to

2
36
J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
ꢀ
ꢀ
Fig. 3. (A) Low resolution, (B) middle resolution and (C) high resolution TEM images and corresponding size distribution histograms of (A ) Pt-NCs and (B ) small Pt
nanoparticles in Pt-NCs. The inset in (C) shows the fast Fourier transform selected-area electron diffraction (SAED) image of Pt-NCs.
nanoparticles in Pt-NCs is calculated to be 2.2 nm, which is in good
agreement with the statistical HRTEM measurement result.
It is worth noting that Pt-NCs show an excellent colloidal
stability. For example, no precipitation or flocculation was
observed even after three months (Fig. S3, Supporting information).
Some previous investigations have indicated aminopolyphospho-
nates could be oxidized to small aminophosphonate molecules
such as hydroxymethylene-phosphonic acid, aminomethylene-
phosphonic acid and iminodi(methylene-phosphonic acid) in the
presence of auric acid and ozone [33–35]. Fig. 6 shows FT-IR spec-
trum of as-prepared Pt-NCs. The asymmetric and symmetric CH2
are observed fairly [36,37], indicating that Pt-NCs are covered
with aminophosphonate molecules. Furthermore, the appearance
of N 1s (398.5 eV) and P 2p peaks (133.2 eV) also confirm that
some aminophosphonate molecules adsorb on Pt-NCs surface
(Fig. S4, Supporting information). Compounds with primary amine
functional groups can form stable monolayer on Pt surfaces through
amine as an anchor group [38]. The N 1s peak of Pt-NCs obviously
shifts to lower binding energy (398.5 eV) compared to N 1s peak
of bulk DTPMPA (401.4 eV, data not shown), confirming the for-
mation of the N Pt bond. Based on the experimental observations
and literatures, we affirm small aminophosphonate molecules (i.e.,
the oxidation product of EDTMPA ligand) adsorb on generated Pt-
NCs surface via N Pt bond during the thermal decomposition of
−
1
stretching vibrations peaks (2922 and 2854 cm ), the character-
−1
istic peaks of
phosphonic acid group (the clear broad band at 900–1300 cm
N H (1615 cm ) and the characteristic peak of
−
1
IV
)
DTPMPA–Pt complex.

J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
237
Fig. 4. TEM images of the Pt colloid synthesized by varying concentrations of both H2PtCl6 and DTPMPA to (A) 9.65 × 10− M and (B) 3.86 × 10−3 M.
4
[
111]
A
B
0
Pt
[
200]
4+
Pt
[
220]
[311]
2+
Pt
8
2
80
78
76
74
72
70
68
20
40
2-Theta /º
60
80
Binding Energy / ev
Fig. 5. (A) XPS spectrum of Pt-NCs in the Pt 4f region and (B) XRD pattern of Pt-NCs.
The surface charge of Pt-NCs is characterized by zeta potential
analysis (Fig. 7). With the decrease of the pH from 10.8 to 1.5, the
zeta potentials of Pt-NCs decrease from −48.6 ± 6 to −28.2 ± 4 mV,
suggesting that the negatively charged phosphonic acid groups
are exposed to the outside of Pt-NCs. Meanwhile, zeta potential is
an important parameter that reflects the behavior of colloids. The
higher zeta potential of nanoparticles corresponds to the higher
surface charge density of nanoparticles, which generally make the
nanoparticles suspensions maintain the higher stability due to
electrical repulsion. Thus, the extremely negative zeta potential
of Pt-NCs is responsible for the excellent colloidal stability of Pt-
NCs. Furthermore, the control and quantitative measurements of
the monolayer surface charge are both practical importance and
-25
-30
-35
-40
-45
-50
pKa1
-CH2
-PO H
3
2
pKa2
N-H
1
2
3
4
5
6
7
8
9
10 11
pH
4
000
3000
2000
1000
-1
Wavenumbers / cm
Fig. 7. Dependence of zeta potentials of Pt-NCs on solution pH. (For interpretation
of the references to color in the text, the reader is referred to the web version of the
article.)
Fig. 6. FT-IR spectrum of Pt-NCs.

2
38
J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
1
0
0
.0
.5
.0
A
0
.001 0.009
0.05 0.1
1
0
Pt NCs
A
0.5
0.9
3
E-TEK Pt
0
.094 V
0
.213 V
-1
5
-
-
-
-
-
-
-
-
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.005 0.01
0.09 0.3
-
2
3
0.7
1
7
-
9
mM
-4
-5
-
-
-
6
7
8
4.0
-
0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
1
00
200
300
400
500
600
Potential / V (vs. SCE)
Time / s
B 1
1
.2
.0
1
1
5
0
5
0
5
E-TEK Pt
Pt NCs
B
-
0.8
-
-
-
-
-
-
-
10
15
20
25
30
35
0
0
0
.6
.4
.2
40
-
0.0
0.1
0.0
0.1
0.2
0.3
0
.0
0.2
0.4
0.6
0.8
1.0
Potential / V (vs. SCE)
Concentration / mM
Fig. 8. (A) Background subtracted linear sweep curves of Pt-NCs (black curve) and
Fig. 9. (A) Amperometric responses of the Pt-NCs modified electrode upon succes-
sive additions H2O2 to 8 mL of 0.1 M PBS (pH 7.4) at the applied potential of 0.1 V
under continuous stirring condition. (B) Plot of catalytic current vs. H2O2 concen-
tration.
commercial Pt black (red curve) in the absence (dashed line) and presence (solid
−
1
line) of oxygen in a 0.1 M PBS (pH 7.4) at a scan rate of 50 mV s . (B) Typical steady-
state polarization curves of Pt-NCs (black curve) and commercial Pt black (red curve)
modified electrode in the presence of 5 mM H2O2 in a 0.1 M PBS (pH 7.4) at a scan
−1
rate of 5 mV s and rotation rate of 1600 rpm. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of the article.)
the typical steady-state polarization curves of H O2 at Pt-NCs and
2
−
1
commercial Pt black catalysts at a scan rate of 5 mV s and rota-
tion rate of 1600 rpm. It is observed interestingly that reduction
peak current of H2O2 at Pt-NCs is bigger than that of commercial Pt
black, indicating that aminophosphonate molecules covered on Pt-
NCs surface enhance the electrocatalytic activity of Pt-NCs towards
H2O2 reduction. Considering the possible hydrogen-bonding inter-
action between PO3H2 groups and H2O2 molecule originated from
the particular capacity of hydrogen-bonding of PO3H2 groups, we
presume that the excellent electrocatalytic activity of Pt-NCs for
H2O2 reduction likely relates to the enrichment of H2O2 on Pt-NCs
surface.
scientific interest. The titration curve of the zeta potential for as-
prepared Pt-NCs is plotted as a function of solution pH, as shown
in Fig. 7 (red lines). The pKa is defined as the pH value when half
of the acid is ionized. The values are estimated from the midpoints
of inflection of the titration curves, assuming that half the acids
are deprotonated at that point. According to fitting titration curve,
the apparent pKa1 and pKa2 values of phosponate molecules immo-
bilized on Pt surface are estimated to be 2.72 and 6.38. The wide
pH-sensitive range (ca. pH 2.0–8.0) of the water-soluble phospho-
nate functionalized Pt-NCs may provide fundamentals for their
potential applications in various fields such as wetting, adhesion,
and protein immobilization [39].
3
.3. Amperometric determination of H O at the Pt-NCs modified
2 2
electrode
3.2. Electrocatalytic activity of Pt-NCs
The high electrocatalytic performance makes the Pt-NCs mod-
ified electrode a good electrochemical sensing platform for the
amperometric determination of H O . The electrochemical data
The electrocatalytic activities of Pt-NCs and commercial Pt black
towards ORR are examined by linear sweeping voltammetry in
.1 M PBS solution (pH 7.4) (Fig. 8A). As observed, ORR onset poten-
2
2
0
from Fig. 8A and B show ORR cannot occur at 0.1 V, but the reduc-
tial at Pt-NCs shows a 119 mV shift to more negative potential
compared with the commercial Pt black, in agreement with the
result of ORR in acidic medium (Fig. S5, Supporting information).
ThisindicatesthattheaminophosphonatemoleculescoveredonPt-
NCs surface suppress the electrocatalytic activity of Pt-NCs towards
ORR due to the coverage of surface-active Pt site. Fig. 8B displays
tion reaction of H O2 has occurred at 0.22 V. In order to avoid the
2
interference of ORR, 0.1 V is identified as the optimum applied
potential for the detection of H O . The amperometric response
2
2
of Pt-NCs modified electrode towards H O2 at 0.1 V is illustrated
2
in Fig. 9A. On addition of an aliquot of H O , the Pt-NCs modi-
2
2
fied electrode reaches 95% of the steady-state current in less than

J. Xu et al. / Electrochimica Acta 80 (2012) 233–239
239
2
s, attributing to the high electrocatalytic efficiency of Pt-NCs.
References
Fig. 9B shows the calibration curve of the Pt-NCs modified elec-
trode for H O determination. The electrochemical response shows
[
[
1] S.H. Chen, K. Kimura, Journal of Physical Chemistry B 105 (2001) 5397.
2] W.X. Tu, H.F. Liu, K.Y. Liew, Journal of Colloid and Interface Science 229 (2000)
453.
2
2
−
6
a linear dependence on the concentration of H O from 1.0 × 10
2
2
−3
[3] J.B. Liu, M.W. Zhu, P. Zhan, H. Dong, Y. Dong, X.T. Qu, Y.H. Nie, Z.L. Wang,
Nanotechnology 17 (2006) 4191.
to 1.0 × 10 M with a correlation coefficient of 0.9993. And the
−6
detection limit is 1.0 × 10 M at a signal-to-noise ratio of three.
To characterize the reproducibility of Pt-NCs modified electrode,
repetitive measurements were carried out in solution containing
[
4] Z.Y. Zhou, Z.Z. Huang, D.J. Chen, Q. Wang, N. Tian, S.G. Sun, Angewandte Chemie
International Edition 49 (2010) 411.
[5] C. Wang, H. Daimon, Y. Lee, J. Kim, S.H. Sun, Journal of the American Chemical
Society 129 (2007) 6974.
0
.1 mM H O . Ten successive measurements showed a relative
2 2
[
6] R. Narayanan, M.A. El-Sayed, Journal of the American Chemical Society 126
2004) 7194.
[7] R. Huang, Y.-H. Wen, Z.-Z. Zhu, S.-G. Sun, Journal of Materials Chemistry 21
2011) 11578.
standard deviation of 2.4%, which was indicative of acceptable
detection reproducibility for practical analysis. In addition, the rel-
(
(
ative standard deviation for detection of 0.1 mM H O2 with seven
2
[
[
8] S. Zhang, Y. Shao, G. Yin, Y. Lin, Journal of Materials Chemistry 20 (2010) 2826.
9] N.V. Long, et al., Nanotechnology 21 (2010) 035605.
sensors prepared under the same conditions was 6.3%. When the
Pt-NCs modified electrode was stored at room temperature for
one month, the current response remained 94% of its initial value,
indicating the long-term stability of the proposed Pt-NCs modified
electrode.
[10] M. Min, Y. Park, C. Kwak, A.A. Serov, Electrochimica Acta 55 (2010) 737.
11] X.W. Teng, H. Yang, Nano Letters 5 (2005) 885.
12] N.C. Bigall, T. Hartling, M. Klose, P. Simon, L.M. Eng, A. Eychmuller, Nano Letters
[
[
8
(2008) 4588.
[13] Y. Song, R.M. Garcia, R.M. Dorin, H.R. Wang, Y. Qiu, E.N. Coker, W.A. Steen, J.E.
Miller, J.A. Shelnutt, Nano Letters 7 (2007) 3650.
[
14] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272
4
. Conclusions
(1996) 1924.
[
15] B. Escobar Morales, S.A. Gamboa, U. Pal, R. Guardián, D. Acosta, C. Maga n˜ a, X.
Mathew, International Journal of Hydrogen Energy 35 (2010) 4215.
[16] H.M. Chen, R.-S. Liu, M.-Y. Lo, S.-C. Chang, L.-D. Tsai, Y.-M. Peng, J.-F. Lee, Journal
of Physical Chemistry C 112 (2008) 7522.
In summary, the phosphonate functionalized Pt-NCs with high
colloidal stability are successfully synthesized by using DTPMPA
as the complexing agent, reducing agent and its oxidation product
as stabilizing agent. The phosphonate functionalized Pt-NCs effi-
ciently suppress the oxygen reduction reaction (ORR) while fully
preserving the Pt-like activity for the hydrogen peroxid reduction
reaction. Due to their particular electrocatalytic properties towards
H O reduction and ORR, the phosphonate functionalized Pt-NCs
can be used to construct a new kind of H O2 electrochemical sen-
sor in the presence of oxygen. The results presented here indicate
an important new direction in the quest to design selective cata-
[
17] X.H. Zhong, Y.Y. Feng, I. Lieberwirth, W. Knoll, Chemistry of Materials 18 (2006)
468.
2
[18] M. Zheng, P. Li, C. Yang, H. Zhu, Y. Chen, Y. Tang, Y. Zhou, T. Lu, Analyst 137
(2012) 1182.
19] K. Thandavan, S. Gandhi, S. Sethuraman, J.B.B. Rayappan, U.M. Krishnan, Nano-
technology 22 (2011) 265505.
20] Y. Song, K. Cui, L. Wang, S. Chen, Nanotechnology 20 (2009) 105501.
[21] D. Gligor, A. Maicaneanu, A. Walcarius, Electrochimica Acta 55 (2010) 4050.
22] B. Prieto-Simón, G.S. Armatas, P.J. Pomonis, C.G. Nanos, M.I. Prodromidis, Chem-
istry of Materials 16 (2004) 1026.
23] N. Sun, L. Guan, Z. Shi, N. Li, Z. Gu, Z. Zhu, M. Li, Y. Shao, Analytical Chemistry
78 (2006) 6050.
24] S.J. Guo, D. Wen, Y.M. Zhai, S.J. Dong, E. Wang, ACS Nano 4 (2010) 3959.
25] X.J. Bian, X.F. Lu, E. Jin, L.R. Kong, W.J. Zhang, C. Wang, Talanta 81 (2010) 813.
26] Y. Liu, D.W. Wang, L. Xu, H.Q. Hou, T.Y. You, Biosensors and Bioelectronics 26
(2011) 4585.
27] J. Zhang, M. Oyama, Electrochimica Acta 50 (2004) 85.
28] D. Strmcnik, M. Escudero-Escribano, K. Kodama, V.R. Stamenkovic, A. Cuesta,
N.M. Markovic, Nature Chemistry 2 (2010) 880.
[
[
2
2
[
2
[
[
[
[
lysts for the fabrication of noble-metal based H O electrochemical
2
2
sensor. In the future, the phosphonate functionalized Pt-NCs may
have potential applications in catalysis, biosensor and medicine,
owing to their particular morphology, small particle size, as
well as biocompatibility of PO H -terminated functional interface
[
[
3
2
[
40–43].
[29] B. Genorio, D. Strmcnik, R. Subbaraman, D. Tripkovic, G. Karapetrov, V.R. Sta-
menkovic, S. Pejovnik, N.M. Markovic, Nature Materials 9 (2010) 998.
[
30] Y. Bi, J. Ye, Chemical Communications 46 (2010) 1532.
Acknowledgements
[31] B. Nowack, Water Research 37 (2003) 2533.
[
[
32] J.B. Xu, T.S. Zhao, Z.X. Liang, Journal of Power Sources 185 (2008) 857.
33] L.V. Kurochkina, N.I. Pechurova, N.I. Snezhko, V.I. Spitsyn, Zhurnal Neorganich-
eskoi Khimii 23 (1978) 2676.
The authors are grateful for the financial support of NSFC
21073094 and 21005039), the United Fund of NSFC and Yunnan
(
[34] B. Nowack, A.T. Stone, Environmental Chemistry Letters 1 (2003) 24.
[
35] F. Zhang, Y. Zhou, Y. Chen, Z.W. Shi, Y.W. Tang, T.H. Lu, Journal of Colloid and
Interface Science 351 (2010) 421.
Province (no. U1137602), Natural Science Foundation of Jiangsu
Higher Education Institutions of China (10KJD150004), University
Postgraduate Research and Innovation Project in Jiangsu Province
[
36] T. Sainsbury, D. Fitzmaurice, Chemistry of Materials 16 (2004) 3780.
[37] J.H. Yuan, Y. Chen, H.X. Zha, L.J. Song, C.Y. Li, J.Q. Li, X.H. Xia, Colloids and Surfaces
B 76 (2010) 145.
(
CXLX11 0871), and a Project Funded by the Priority Academic Pro-
[
[
38] B. Wu, N. Zheng, G. Fu, Chemical Communications 47 (2011) 1039.
39] Y. Chen, X.-J. Yang, B. Jin, L.-R. Guo, L.-M. Zheng, X.-H. Xia, Journal of Physical
Chemistry C 113 (2009) 4515.
gram Development of Jiangsu Higher Education Institutions.
[
[
40] Y. Chen, X.J. Yang, L.R. Guo, B. Jin, X.H. Xia, L.M. Zheng, Talanta 78 (2009) 248.
41] Y. Chen, B. Jin, L.R. Guo, X.J. Yang, W. Chen, G. Gu, L.M. Zheng, X.H. Xia, Chemistry
Appendix A. Supplementary data
–
A European Journal 14 (2008) 10727.
42] Y. Chen, X.J. Yang, L.R. Guo, J. Li, X.H. Xia, L.M. Zheng, Analytica Chimica Acta
44 (2009) 83.
[43] Y. Chen, L.R. Guo, W. Chen, X.J. Yang, B. Jin, L.M. Zheng, X.H. Xia, Bioelectro-
chemistry 75 (2009) 26.
[
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.electacta.2012.07.005.
6