In situ generation of a high-performance Pd-polypyrrole composite with multi-functional catalytic properties

Article abstract of DOI:10.1039/c3dt53567c

We report on a bottom up approach for the synthesis of a Pd-polypyrrole nanocomposite material. The composite material was characterized by means of different techniques, such as UV-vis, IR, and Raman spectroscopy, which offered information about the chemical structure of the polymer, whereas electron microscopy images provided information regarding the morphology of the composite material and the distribution of the metal particles in the polymer matrix. During the synthesis of the nanocomposite, the Pd nanoparticles act as a catalyst for a model proton-coupled electron transfer reaction. The Pd-polypyrrole nanocomposite material was also used as a catalyst for the electro-catalytic detection of tryptophan, a precursor for some neurotransmitters. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53567c

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In situ generation of a high-performance
Pd-polypyrrole composite with multi-functional
catalytic properties
Cite this: Dalton Trans., 2014, 43,
6396
a
a
b
Meenakshi Choudhary, Rafique Ul Islam, Michael J. Witcomb and
Kaushik Mallick*
a
We report on a bottom up approach for the synthesis of a Pd-polypyrrole nanocomposite material. The
composite material was characterized by means of different techniques, such as UV-vis, IR, and Raman
spectroscopy, which offered information about the chemical structure of the polymer, whereas electron
microscopy images provided information regarding the morphology of the composite material and the
distribution of the metal particles in the polymer matrix. During the synthesis of the nanocomposite, the
Pd nanoparticles act as a catalyst for a model proton-coupled electron transfer reaction. The Pd-poly-
pyrrole nanocomposite material was also used as a catalyst for the electro-catalytic detection of
tryptophan, a precursor for some neurotransmitters.
Received 19th December 2013,
Accepted 7th February 2014
DOI: 10.1039/c3dt53567c
www.rsc.org/dalton
temperature conditions during which the Pd-acetate acts as an
oxidizing agent that facilitates the oxidative polymerization of
1
. Introduction
Metal nanoparticles are of significant interest as they show pyrrole while, on the other hand, the reduction of metal salt
exceptional electronic, optical, and catalytic properties due to produces metal nanoparticles. During the polymerization
their quantum size effects. Nanoparticles are currently under process, the reduction of 4-nitrophenolate and methylene blue
1
intensive study for applications in optical and electronic supports the catalytic effect of a proton-coupled electron trans-
2
3
4
devices, sensors, biomedical applications and as catalysts in fer reaction. The Pd-polypyrrole nanocomposite material was
5
different types of chemical reactions. The main challenge in used also as an electrocatalyst to investigate the electrochemi-
the application of these materials is agglomeration, and that cal recognition of tryptophan by a cyclic voltammetry (CV)
can be overcome through particle stabilization. Polymer stabil- technique.
ized metal nanoparticles have attracted much attention
6
–8
recently as a new research direction in catalysis.
The
polymer matrices serve both as the support as well as the
stabilizer of the nanoparticles, thus providing a mechanism to 2. Experimental section
prevent particle agglomeration. Palladium based catalysts, in
2
.1. Materials
particular nanoscale palladium particles, have recently drawn
enormous attention due to their versatile role in organic
Pyrrole, ammonium persulfate (APS), potassium tetrachloro-
palladate (K PdCl ), 4-nitrophenol, methylene blue (MB),
Fe(CN) ·3H O and tryptophan were purchased from Sigma-
9
–11
2
4
synthesis.
The use of palladium nanoparticles in catalysis
1
2–14
K
4
6
2
is not only industrially important,
but also is scientifically
Aldrich, were of analytical purity, and used without further
purification. Toluene was obtained from Merck, and ultra-pure
water (specific resistivity >17 MΩ cm) was used in this experi-
ment wherever required.
interesting since they provide details of the mechanism of the
reactions.
In the present investigation, we synthesized polypyrrole
encapsulated palladium nanoparticles using an in situ
polymerization and composite formation (IPCF) method. IPCF
is a facile synthesis technique carried out under ambient
2.2. Procedure
4-Nitrophenolate (4NP) was made by adding sodium hydroxide
to a water solution of 4-nitrophenol to secure a final concen-
a
Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park
tration of 10− mol dm . Similarly, methylene blue solution
4
−3
2
b
006, South Africa. E-mail: kaushikm@uj.ac.za
was made by adding ultra-pure water to obtain a final concen-
DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand,
Private Bag 3, WITS 2050, South Africa
tration of 10− mol dm . Water solutions of APS and K PdCl
4
−3
2
4
6396 | Dalton Trans., 2014, 43, 6396–6405
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were made by adding ultra-pure water to obtain a final concen- working electrode using a ‘drop and dry’ method. After each
tration of 10− mol dm
2
−3
.
run, the electrode was washed and an identical amount of new
catalyst was applied as a coating to the electrode for the next
study.
2
.3. Experimental details
a) In a typical experiment, 0.030 g of pyrrole was diluted in
methanol (15 mL) in a conical flask, after which 10 mL of
(
−
2
−3
K
2
PdCl
4
(10 mol dm ) was added slowly to it. A colloidal
3
. Results and discussion
solution appeared at the bottom of the flask. The solution was
allowed then to stand at rest for half an hour. Subsequently, The SEM image (Fig. 1A) shows the chain-like morphology of
TEM and SEM specimens were prepared by pipetting 2 μL of the product consisting of spherical units. The average dia-
the deposited material onto lacey, carbon coated, copper grids meter of a unit was about 100 nm while the chains were a few
and allowed to air dry. A small portion of the product was used micrometres in length. The TEM image (Fig. 1B) shows the
for optical characterization, the remaining fraction being dried uneven surface of the spheres. The higher magnification TEM
under vacuum for 30 min for the XRD and XPS analyses.
images (Fig. 1C and D) show a high density distribution of
(
b) In the first set of catalytic experiments, 1.5 mL of 4NP dark spots. Such images and the stereo images (not shown)
−
4
−3
−2
(
10 mol dm ) was mixed with 60 μL of pyrrole (10 mol clearly indicate that the 2–4 nm diameter nanoparticles are
−
3
dm ) in two quartz cuvettes. To the above solutions, 60 μL encapsulated and homogeneously distributed within and
−2
−3
APS (10 mol dm ) was added in one cuvette and 60 μL throughout the product matrix. The EDX analysis (Fig. 2A)
PdCl
2
K
2
4
(10− mol dm−3) was added to the other cuvette.
obtained from the electron beam being focused onto a dark
(
c) In the second set of experiments, 1.5 mL MB was mixed spot confirms that these spots are palladium. Focusing the
−3
with 0.5 mL of pyrrole (1.0 mol dm ) in two quartz cuvettes. beam between the dark spots yielded no X-ray peaks for palla-
−
3
−3
To the above solutions, 0.5 mL APS (10 mol dm ) was dium. To identify the chemical state of the palladium, X-ray
added in one cuvette and 0.5 mL K PdCl (10 mol dm−3
−
3
)
2
4
photoelectron spectroscopy (XPS) measurements were carried
was added to the other cuvette. The progress of the reactions out. The Pd 3d region of the XPS spectrum of the Pd-polypyr-
was monitored using a spectrophotometer.
role composite is illustrated in Fig. 2B, which reveals the pres-
ence of Pd 3d5/2 and 3d3/2 peaks at binding energies of 335.35
and 340.60 eV, respectively. These binding energy values are in
2
.4. Characterization
1
5
Transmission electron microscopy (TEM) was performed at an accord with those reported for metallic palladium. XPS ana-
accelerating voltage of 197 kV using a Philips CM200 trans- lysis also confirmed that 1.93 wt% Pd was present on the
6
mission electron microscope equipped with a LaB source. An
ultrathin window energy-dispersive X-ray system (EDX) and a
Gatan Imaging Filter (GIF) attached to the TEM were used to
determine the chemical composition of the samples. SEM
studies were performed in a TESCAM VEGAS operating at
2
–10 kV. To prevent possible charging, the samples were
sputter coated with a thin uniform layer of carbon before
viewing. The UV-vis spectra were measured using a Shimadzu
UV-1800 UV-VIS spectrophotometer with a quartz cuvette. IR
spectra were collected utilizing a Shimadzu IRAffinity-1 with a
resolution of 0.5 cm− . Raman spectra were acquired using the
green (514.5 nm) line of an argon ion laser as the excitation
source. Light dispersion was undertaken via the single spectro-
graph stage of a Jobin–Yvon T64000 Raman spectrometer. The
X-ray diffraction (XRD) patterns were recorded on a Shimadzu
XD-3A X-ray diffractometer operating at 20 kV using Cu-Kα
radiation (k = 0.1542 nm). The measurements were performed
over a diffraction angle range of 2θ = 20° to 70°. The XPS
spectra were collected in an ultra-high-vacuum chamber
attached to a Physical Electronics PHI 560 ESCA/SAM system.
Electrochemical measurements were carried out with a mini
Autolab PGSTAT 910 potentiostat connected to a computer. A
three-electrode system was used in the experiment with a bare
electrode and a modified Glossy Carbon Electrode (GCE) as
the working electrode. An Ag/AgCl electrode (saturated KCl)
and a Pt wire electrode were used as the reference and counter
1
Fig. 1 SEM image (A) shows the chain-like morphology of the product
Pd-polypyrrole) with spherical units. The TEM image (B) shows the
uneven surface on the spheres. Higher magnification TEM images (C
(
and D) show the high density distribution of nanoparticles, within the
electrodes, respectively. The catalyst was deposited on the range of 2–4 nm diameter, throughout the matrix.
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The optical behaviour of the product was studied by Raman
and FT-IR spectra analysis. Fig. 3A shows the Raman spectra of
the resulting material prepared by pyrrole and Pd-acetate
using an in situ approach. Two sharp peaks at 1540 and
589 cm− are attributed to the CvC ring stretching and N–H
1
1
−
1
in-plane bending vibrations. A small peak at 1487 cm can be
assigned to N–C stretch vibration. The vibrational peak at
1
1
364 cm− is due to both C–C and CvC stretching. A
−
1
shoulder-like appearance at 1390 cm results from the N–C
1
6
ring stretching mode, while the characteristic vibration band
at 1248 cm− is assigned to a ring deformation mode. The
position of the inter-plane C–H bending and stretching modes
1
is clearly evident at 1155 and 1210 cm− respectively.
1
16,17
Fig. 2 (A) EDX spectrum from the area shown in Fig. 1B. The palladium
peaks are derived from the dark spots, whereas the copper peak comes
from the TEM copper mesh support grid. (B) The palladium 3d XPS spec-
trum, the peaks at binding energies at 335.35 eV for 3d5/2 and 340.60 eV
for 3d3/2 are indicative of metallic palladium. XRD spectra (C), indicates
the highly oriented, face-selective, crystalline character of the Pd-
particles.
surface of the polypyrrole matrix. The crystalline nature of the Fig. 3 In the Raman spectrum (A), the two sharp peaks at 1540 and
palladium nanoparticles was confirmed by electron diffraction 1589 cm− are attributed to both the CvC ring stretching and the N–H
1
in plane bending vibration of the Pd-polypyrrole composite material
on single nanoparticles (not shown) and from XRD measure-
ments (Fig. 2C). The strong (111) Bragg reflection indicates
that the Pd-particles possess a highly oriented, face-selective,
whereas in the FT-IR spectra, (B), the bands of the C–H and the N–H in-
plane deformation vibration are located at 1021 cm− . Spectrum (a) and
1
(b) in (B) represent the FT-IR spectra of the Pd-polypyrrole and polypyr-
crystalline character.
role respectively.
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3
1,32
Fig. 3B, spectra (a), shows a typical FT-IR spectrum of the properties which depend on their size and morphology.
product obtained during the reaction between pyrrole and Pd- The efficiency of the electronic and electrochemical redox pro-
−
1
acetate. The bands above 2500 cm are responsible for the perties makes these classes of nanostructured materials very
aromatic C–H and N–H stretching vibrations. The prominent promising for technological applications. Metal nanoparticles
−
1
band observed above 1700 cm corresponds to formation of a have also been used as a modifier of electrodes for tryptophan
carbonyl group, which is due to the over-oxidized structure of analysis. Silver nanoparticles and a rutin complex film modified
1
8–20
the polypyrrole.
1
The bands occurring at 1469 and graphite electrode have been reported for tryptophan determi-
415 cm− are for the C–N stretching vibration in the ring. The nation. Gold nanoparticles have been much explored as com-
1
33
polypyrrole ring breathing vibration is located in the region ponents for biosensors development due to the capability to
1
−1
1
250 to 1100 cm− . The peak at 1111 cm is attributed to the increase the electronic signal when a biological component is
2
1
breathing vibration of the pyrrole ring. The bands of C–H maintained in contact with the nanostructured surface. On the
and N–H in-plane deformation vibration are located at other hand, silver, platinum, palladium, copper, cobalt and
1
−1
1
021 cm− while the band at 744 cm corresponds to C–H other metals have been extensively explored in biosensor
2
2
−1
34–38
out-of-plane bending vibration. The bands above 2500 cm
Fig. 3B) are responsible for the aromatic C–H and N–H as an electro-catalyst and different methodologies have been
stretching vibrations. The spectroscopic evidence confirms the attempted to incorporate Pd particles into polymer matrices for
formation of polypyrrole. For comparison, we also synthesized fabrication of Pd modified electrodes.
polypyrrole using pyrrole and APS as a precursor and the oxi- In the present work, we have compared the affinity and sen-
development.
Palladium is one of the preferred metals used
(
3
6,39
dizing agent respectively. The product had a similar FTIR spec- sitivity of polypyrrole and Pd-polypyrrole for the electrochemi-
trum, Fig. 3B, spectra (b), that we recorded during the reaction cal detection of tryptophan. Cyclic voltammetry (CV) of
3
−/4−
between pyrrole and Pd-acetate. From reference to the optical electroactive species of [Fe(CN)
6
]
(FCN) has been used
properties and microscopic images, it can be confirmed that a widely to test the barrier kinetics of the interface. The extent of
Pd-polypyrrole nanocomposite was produced during the reac- kinetic hindrance to the electron transfer process increases
tion between pyrrole and Pd-acetate.
with increasing film thickness and decreases with the defect
density of the barrier. Fig. 4A shows the CV responses of
3
.1. Recognition of biomolecules using Pd-polypyrrole as an
0.5 mM of FCN at bare GCE (a), the polypyrrole modified GCE
electrocatalyst
(
b), and a Pd-polypyrrole modified GCE (c) at pH 7.5, operating
−
1
Development of rapid and sensitive methods for the detection at a scan rate of 50 mV s . After modifying the electrode with
of biomolecules is important for both clinical and numerous polypyrrole, a decrease in the redox peak current was observed
non-clinical applications. Tryptophan, 2-amino-3-(1H-indol-3- (curve b), indicating that the polypyrrole acts as an electron
yl)-propionic acid, is an essential amino acid and a precursor and mass transfer blocking layer and thus hinders the
for serotonin, melatonin and niacin. It has been implicated diffusion of ferricyanide toward the electrode surface. In con-
as a possible cause of schizophrenia and early diagnosis is trast, for the Pd-polypyrrole modified electrode, the voltam-
very important for the proper treatment of the patient. It has metric response of ferricyanide (curve c) is restored close to
been reported that spectroscopy, chemiluminescence, electro- that obtained at the bare GCE (curve a). This demonstrates
phoresis, chromatography and electro-analysis methods have that the Pd nanoparticles have been successfully assembled on
2
3
2
4
been used for tryptophan determination.
Among these the electrode surface and provide the necessary conduction
methods, electrochemical techniques have received consider- pathways, while also acting as nanoscale electrodes in promot-
able attention for the detection of biomolecules due to their ing the electron transfer between the analyte (ferricyanide)
sensitivity, simplicity and low cost. However, the electrochemi- and the electrode surface. The decreasing peak height indi-
cal detection of tryptophan at unmodified electrode surfaces is cates the passivation of the electrode surface.
not ideal due to sluggish electron transfer processes and the
Fig. 4B shows the cyclic voltammogram of bare GCE (a) in
2
5
high over-potential for the oxidation of tryptophan. Much the absence and (b) in the presence of the analyte (0.2 mM
effort has been devoted to finding new materials to modify tryptophan) in 0.01 mM phosphate buffer at pH 7.5, utilizing a
electrodes in order to reduce the over-potential of tryptophan. scan rate of 50 mV s− . The figure reveals an enhanced peak
Various modified electrodes have been applied to the determi- current of about ∼2.0 μA in the presence of tryptophan when
nation of the tryptophan concentration in pharmaceutical and only the bare GCE was used as the working electrode. A notice-
1
2
6
27
clinical samples utilizing nafion, butyryl-choline, carbon able current response change was observed when the electrode
2
8
29
nanotubes,
1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone
was modified with polypyrrole in the absence and in the pres-
3
0
and a carbon nanotube–cobalt salophen complex with detec- ence of tryptophan. In Fig. 4B, curve (c) is the voltammogram
tion limits of 1.6 nM, 6.0 × 10 M, 2.0 × 10 M, 1.8 × 10
and 1.0 × 10− M of tryptophan, respectively.
−
7
−8
−6
M
of polypyrrole modified GCE in the absence of tryptophan.
A broad area with an anodic peak position at about 1.15 V
7
Functional nanomaterials with unique physical and chemi- results from the oxidative response of polypyrrole whereas, in
cal properties have provided significant advantages for biologi- the presence of 0.2 mM tryptophan, Fig. 4B, curve (d), the
cal detection. Metallic nanoparticles are interesting materials anodic peak position appears at a lower potential region result-
with respect to their unique electronic and electrocatalytic ing from the oxidation of tryptophan at 0.89 V.
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Fig. 5 Cyclic voltammogram of Pd-polypyrrole modified GCE (a) in the
absence of tryptophan and with the increasing concentration of trypto-
phan, (b) 0.20 mM, (c) 0.40 mM and (d) 0.60 mM in 0.01 mM PBS buffer
solution at pH 7.5 at a scan rate of 50 mV s−
1
.
3.2. Mechanism of formation of the Pd-polypyrrole
composite
Among conducting polymers, polypyrrole and its derivatives
are of particular interest owing to their high conductivity,
stability in the oxidized state and interesting redox properties.
The simplicity of the synthetic procedures and availability of
the initial monomers are also attractive features of polypyrrole.
A wide variety of methods have been applied to the preparation
of polypyrrole by the oxidative polymerization of their
Fig. 4 (A) The cyclic voltammogram of (a) the bare GCE, (b) the poly-
pyrrole modified GCE and (c) the Pd-polypyrrole modified GCE in
40
monomer. Chemical synthesis of polypyrrole has a long
history. Polypyrrole has been prepared by the oxidation of the
3
−/4−
5
.0 mM [Fe(CN)
6
−
]
(FCN) buffer solution at pH 7.5 maintaining a scan
4
1
1
monomer (pyrrole) using ammonium persulfate (APS),
rate of 50 mV s . Cyclic voltammogram (B) is of (a) bare GCE and (c)
4
2,43
polypyrrole modified GCE without analyte, while (b) is of the bare GCE aqueous or anhydrous FeCl
as well as other salts of
as chemical oxidants. In the present
3
44,45
and (d) is of the polypyrrole modified GCE in 0.2 mM tryptophan con-
iron (III) and copper (II)
taining PBS (phosphate buffered saline) (0.01 mM) buffer at pH 7.5 and
work, Pd-acetate was used as the oxidant for the synthesis of
polypyrrole. Oxidative polymerisation of pyrrole to polypyrrole
proceeds via the oxidation of pyrrole to a radical cation, which
subsequently couples with another radical cation to form bipyr-
using a scan rate of 50 mV s−
1
.
Comparing the cyclic voltammogram of the polypyrrole role. This process is then repeated to form longer chains. Each
modified electrode in the absence of tryptophan (Fig. 4B, step of the polymerization is associated with the release of an
−
+
curve (c)) with that of the Pd-polypyrrole modified electrode electron (e ) and a proton (H ) (Scheme 1). The released elec-
4
6
also in the absence of tryptophan (Fig. 5, curve (a)) reveals tron is then used to reduce the Pd ion to form a Pd atom.
similar curves with the exception of ∼3.0 μA enhancement of Subsequently, coalescence of the atoms results in the for-
the anodic peak current for the latter electrode over the mation of clusters which stabilize within the polymer.
former. This implies that the presence of Pd nanoparticles has
improved the charge transfer at the interface.
The CV response of the Pd-polypyrrole modified electrode
in the presence of tryptophan (0.20 mM) (Fig. 5, curve (b))
showed a well separated anodic peak at 0.75 V with a current
value of 28 μA. Increasing the concentration of tryptophan to
0
.40 and 0.60 mM (Fig. 5, curves (c) and (d) respectively)
resulted in an increasing current while maintaining the same
anodic peak position. The results demonstrated the key role
played by the Pd-nanoparticles on the electrocatalytic oxi-
dation of tryptophan.
Scheme 1 Oxidative polymerisation of pyrrole.
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3
.3. Pd nanoparticles as a catalyst for a proton coupled
electron transfer reaction
The electron-transfer step is important in many homogeneous
4
7,48
and heterogeneous reactions.
Electron transfer in associ-
ation with proton transfer is a well-known mechanism in
chemistry, biology as well as in catalysis. The interest in
proton-coupled electron transfer (PCET) reaction results from
the fact that they play a key role in all the processes leading to
energy storage into chemical bonds and thus for all expected
4
9
renewable energy systems based on solar fuels. Hetero-
geneous excited-state PCET is potentially important for
5
0
2
systems involving solar energy conversion. TiO and other
high-band gap semiconductor-based dye-sensitized photo-
electro-synthesis cells are driven by photo-excitation and elec-
tron injection by bound chromophores or by a direct-band-gap
5
0
excitation process. A combined electrochemical and theore-
tical study has suggested that hydrogen production from weak
acids catalyzed by NiFe hydrogenases involves a proton-
5
1
coupled electron transfer step. A mononuclear ruthenium
complex has shown a multiple proton-coupled electron trans-
5
2
fer
towards multi-electron transfer reactions which are
known to be active catalysts for water oxidation. The cobalt–
porphyrin system has been utilized as a catalyst for a hydrogen
evolution reaction followed by an electron transfer (ET) and
proton transfer (PT) pathway mechanism from carboxylic
53
acid. Oxidation of phenols is an extremely important reaction
in many areas of natural and artificial chemistry in which elec-
tron transfer is inevitably associated with proton transfers. A
mechanistic analysis has confirmed that the catalytic oxidation
5
4
of phenols follows a concerted PCET pathway. Oxidation
reactions of alcohols with the Ru(IV)-oxo complexes proceed via
5
5
a concerted proton-coupled electron transfer mechanism. In
Fig. 6 (A) UV-vis spectra of the 4NP (10 mol dm−3) in the presence of
−4
the electron transfer step, there may be a redox potential pyrrole (10− mol dm ) and APS (10 mol dm−3) at different time inter-
2
−3
−2
difference between the donor and the acceptor, which may vals. Incomplete quenching of the absorbance peak (a) at 400 nm in the
4
3
NP spectra took 204 min. The emergence of the peak at about
20 nm, peak (b), is due to the formation of the polymer. (B) (a) UV-vis
spectrum of the 4NP and (b) the UV-vis spectrum of the 4NP in the
restrict or slow down the passage of an electron. An effective
catalyst with an intermediate redox potential value between
the donor–acceptor partners helps the electron transfer and
−2
−3
−2
−3
2 4
presence of pyrrole (10 mol dm ) and K PdCl (10 mol dm ) after
acts as an electron relay system. Metal nanoparticles are well- 2 min. The absorption band at 325 nm results from the π*–π transition
known examples of this type of redox catalyst.
due to the formation of polypyrrole. A shoulder-like feature in the
region of 375–450 nm corresponds to the bipolaron transition.
Fig. 6A shows the UV-vis spectra of 4NP in the presence of
pyrrole and APS at different time intervals. From the figure, it
can be seen that an incomplete quenching of the absorbance
spectra of 4NP at 400 nm, peak (a), took some 204 min. As Fig. 6B, curve (a), is the UV-vis spectra of 4NP, and in the pres-
mentioned earlier, polymerization of pyrrole is an oxidation ence of pyrrole and K PdCl , a complete quenching of the
2
4
process and in the presence of an oxidizing agent (APS), it absorbance peak of 4NP occurred in about 2 min. In this reac-
−
forms a radical cation with the release of an electron (e ) and tion, the palladium (II) salt acts as an oxidizing agent, like
+
45
a proton (H ). The gradual quenching of the 4NP spectra indi- other metal salts, that polymerizes pyrrole to form polypyr-
cates the reduction of the nitro group. The emergence of the role. The released electrons during the process of polymeriz-
peak at about 320 nm, peak (b), is due to the formation of the ation play a dual role in: (i) the formation of a radical-cation
polymer. The reduction mechanism of 4NP involves a single on the nitrogen of the nitro group (first step of the nitro-group
electron transfer (SET) mechanism with the formation of a reduction mechanism, Scheme 2) and (ii) the reduction of the
5
6
radical cation followed by protonation (Scheme 2). The elec-
2 4
K PdCl to form catalytically active palladium nanoparticles.
trons and protons which are generated during the polymeriz- The in situ generated Pd particles then catalyze the reduction
ation reaction participate in the reduction process of 4NP, an of 4NP. On decreasing the concentration of 4NP, as indicated
example of the proton coupled electron transfer reaction. by the UV-vis spectrum at 400 nm, Fig. 6A, a new absorption
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feature at around 420 nm corresponds to the bipolaron tran-
sition, which is a feature of the oxidized state of polypyrrole. A
similar kind of phenomenon also happens in the reduction
process of MB, which involves the receiving of two electrons
associated with two protons (Scheme 3). Fig. 7A, curve (a), and
7B, curve (a), show the UV-vis spectrum of MB with an absorp-
tion intensity peak at 665 nm. After the addition of pyrrole
and APS, the absorbance value decreased due to a dilution
effect as shown in Fig. 7A, curve (b). Initially, a rapid quench-
ing of the dye was observed, but the rate of reaction was drasti-
cally retarded within a short period of time, there being no
significant reduction after an hour. Similarly, Fig. 7B, curve
(
b), is the UV-vis spectrum of MB immediately after the
addition of pyrrole and K PdCl . Within 2 min of the addition,
2
4
there was no characteristic peak for MB. The spectrum,
Fig. 7B, curve (c), is due to the formation of Pd-polypyrrole
composite. For comparison, the UV-vis spectrophotometric
Scheme 2 Proton coupled electron transfer mechanism for the
reduction of 4NP and MB.
peak developed at 320 nm, which results from the π*–π tran-
5
7
sition and is due to the formation of polypyrrole. The inten-
sity of the absorption peak increases, as marked by an upward
direction arrow, indicating the increase of polypyrrole concen-
2 4
tration with time. When K PdCl was used as the oxidant, the
absorption peak of 4NP (Fig. 6B) was completely quenched
and a new absorption band appeared at 320 nm again result-
5
7
ing from the formation of polypyrrole.
A shoulder-like
Fig. 7 (A) (a) UV-vis spectra of the MB (10 mol dm−3) and (b) UV-vis
−4
−
4
−3
−2
Scheme 3 The mechanism of the catalytic electron transfer process.
The metal cluster relays the electron from the donor molecule to the
acceptor molecule. The schematic shows the thermodynamic con-
ditions to be fulfilled when the cluster redox potential is higher than the
donor (D) potential and lower than the acceptor (A) potential. A = dye in
spectra of the MB (10 mol dm ) in the presence of pyrrole (10 mol
−3
−2
−3
dm ) and APS (10 mol dm ). Incomplete quenching of the absor-
bance peak at 665 nm of the MB spectra took 60 min. (B) (a) A UV-vis
−4
−3
spectra of the MB (10 mol dm ) and (b) the UV-vis spectra of the MB
−4
−3
−2
−3
(10 mol dm ) in the presence of pyrrole (10 mol dm ) and K
2
PdCl
(10 mol dm ). From (B), curve (c), the MB spectra can be seen to have
disappeared after 5 min due to the addition of pyrrole and K PdCl
4
−
+
−2
−3
the oxidised form; A = dye in the reduced form. D = donor in the oxi-
0
dised form. D = donor in the reduced form. E is the redox potential.
2
4
.
6402 | Dalton Trans., 2014, 43, 6396–6405
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Paper
transportation of the charged species was accelerated in the
presence of the palladium nanoparticles.
From thermodynamics considerations, the above implies
that the redox potential of the Pd-nanoparticles should be
intermediate between the redox potential of the donor system,
+
0
+
pyrrole-polypyrrole (D/D ) E [D/D ], the lower threshold, and
−
the redox potential of the acceptor system, 4NP and MB (A/A )
0
−
62
E [A/A ], the upper threshold (Scheme 3). The reaction
between these systems, which is slow in the absence of the pal-
ladium, becomes efficient as a result of the electron transfer
through the metal cluster relay. The Pd nanoparticles further
lower the kinetic barrier, which facilitates a faster rate of
0
2+
0
−
reduction. If E [Pd /Pd(nano)] is higher than E [A/A ], as in a
bulk metallic system, the electron is still transformed from D
to Pd(bulk), but thermodynamics do not allow a relay effect to
occur between Pd(bulk) to A and, as a consequence, no catalytic
6
3
effect has been observed. Conversely, if the clusters are very
small, the transfer of electrons is also not feasible due to the
large potential difference. Thus, an effective transfer of elec-
Fig. 8 UV-vis spectra of (a) polypyrrole and (b) the Pd-polypyrrole
composite. In both the spectra, the absorption band in the range of 300
to 350 nm corresponds to the π*–π transition resulting from the for-
mation of polypyrrole. A shoulder-like appearance observed from 415 to trons is only possible when the potential of the cluster is more
0
2+
5
00 nm (indicated by an arrow, in spectrum (a)) corresponds to a bi- positive than that of the electron donor system, i.e., E [Pd /
polaron transition, a characteristic feature for the oxidized state of poly-
pyrrole. A broad absorption band starting from 450 nm to around
0
+
Pd(nano)] > E [D /D]. An effective relay of electrons is possible
between an acceptor and a donor when ideal conditions are
7
50 nm in spectrum (b), can be assigned to a bipolaron transition, which
0
−
0
2+
0
+
established, that is, E [A/A ] < E [Pd /Pd(nano)] > E [D /D].
The generally accepted mechanism of electron transport is
due to successive hopping from molecule to molecule to over-
come the trap caused by intra-molecular vibrations. In the
is a feature of the oxidized state of polypyrrole.
properties of the blank samples of polypyrrole (a) and Pd-poly- hopping mechanism, to evaluate the electron mobility, there
pyrrole (b) in the absence of 4NP and MB, respectively, (Fig. 8) are two rate processes at different spatial scales that have to be
were studied and the spectral features corroborated with the considered. The processes are the electron transfer within
above statements. In both spectra, the absorption band in the molecular dimers and the electron diffusion in organic solids
range of 300 to 350 nm corresponded to the formation of poly- or polymers. The first process can be characterized by an inter-
pyrrole. Both the shoulder-like appearance observed from 415 molecular electron transfer at the atomistic level, while the
to 500 nm (indicated by an arrow, Fig. 8, spectrum a) and a latter can be simulated at the molecular level by the random
6
4
broad absorption band starting from 450 nm to around walk technique. In our first experiment in which APS was
50 nm seen in Fig. 8, spectrum (b), can be assigned to the used as the oxidant, the electron transfer mechanism was
bipolaron transition of the polymer. The position of this through a strongly disordered system (polymer matrix) and is
7
6
5
absorption band depends on various factors such as counter thus considered to be a short range electron transfer. This is
ions, solvent, chemical structure and the morphology of the in contrast to the second experiment where the presence of
5
8
polymer.
PCET reactions are those in which an electron and a proton follow a long-range hopping mechanism. The electron rich
palladium nanoparticles would enable the relay of electrons to
5
9
are transferred together. The particularity of such reactions acceptor system accelerates the ‘proton hopping’ process
is that the electron and the proton are transferred to different through a Grotthuss mechanism and pulls the proton to the
6
0,61
centres.
Sequential transfer of electron and proton path- acceptor.
5
9
ways is lower in energy than concerted pathways. The mul-
tiple site electron–proton transfer reaction is another class of
the PCET reaction in which (a) an ‘electron–proton donor’
transfers ‘electrons and protons’ to spatially separated accep-
4. Conclusions
tors or (b), an ‘electron–proton acceptor’ accepts ‘electrons and In the present work, a metal–polymer composite material was
5
9
protons’ from spatially separated donors. In the present designed using an in situ polymerization and composite for-
work, we have coupled two reaction systems in which the mation approach. Pyrrole and Pd-acetate were used as the pre-
pyrrole polymerization process releases both an electron plus a cursor for the synthesis of a Pd-polypyrrole nanocomposite in
proton, and the reduction of the 4NP (or MB) occurs through which Pd-nanoparticles within the size range of 2–3 nm were
it receiving an electron and a proton, an example of a ‘proton highly dispersed and stabilized in the polypyrrole matrix. The
coupled electron transfer’ reaction. Without metallic nanopar- Pd-polypyrrole composite material was used as an electrocata-
ticles, the reaction rate was found to be slow whereas the lyst for the electrochemical determination of tryptophan, a
This journal is © The Royal Society of Chemistry 2014
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Dalton Transactions
biomolecule and the precursor of different neurotransmitters. 18 I. Rodríguez, B. R. Scharifker and J. Mostany, J. Electroanal.
During the synthesis, Pd nanoparticles acted as a catalyst for Chem., 2000, 491, 117.
the reduction of 4-nitro phenol, a model reaction being an 19 A. Schmid, L. R. Sutton, S. P. Armes, P. S. Bain and
example of a proton coupled electron transfer mechanism.
G. Manfré, Soft Matter, 2009, 5, 407.
2
0 T. W. Lewis, PhD thesis, Department of Chemistry, Univer-
sity of Wollongong, Australia, 1998. http://ro.uow.edu.au/
thesis/1107
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The authors acknowledge financial support from the Research
Committee and the Faculty of Science of the University of
Johannesburg. One of us (MJW) gratefully thanks the Depart-
ment of Science and Technology (DST), and the National
Research Foundation (NRF) of South Africa for financial
support. We gratefully acknowledge the constructive com-
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