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Field-assisted synthesis of SERS-active silver nanoparticles using conductingpolymers
Ping Xu,*ab Sea-Ho Jeon,a Nathan H. Mack,a Stephen K. Doorn,a Darrick J. Williams,a Xijiang Hanb
and Hsing-Lin Wang*a
Received 10th February 2010, Accepted 29th March 2010
DOI: 10.1039/c0nr00106f
A gradient of novel silver nanostructures with widely varying sizes and morphologies is fabricated
on a single conducting polyanilinegraphite (P-G) membrane with the assistance of an external
electric field. It is believed that the formation of such a silver gradient is a synergetic consequence
of the generation of a silver ion concentration gradient along with an electrokinetic flow of silver
ions in the field-assisted model, which greatly influences the nucleation and growth mechanism of
Ag particles on the P-G membrane. The produced silver dendrites, flowers and microspheres, with
sharp edges, intersections and bifurcations, all present strong surface enhanced Raman
spectroscopy (SERS) responses toward an organic target molecule, mercaptobenzoic acid (MBA).
This facile field-assisted synthesis of Ag nanoparticles via chemical reduction presents an
alternative approach to nanomaterial fabrication, which can yield a wide range of unique
structures with enhanced optical properties that were previously inaccessible by other syntheticroutes.
Introduction
Conducting polymers have been the subject of numerous inves-
tigations due to their potential in electronic and optical devices,
such as light-emitting diodes, sensors, batteries, and electro-
chemical supercapacitors.17 Specifically, polyaniline (PANI) has
emerged as the prime candidate for commercial applications due
to its lower price tag, ease of processing and environmental
stability.810 It has long been recognized that certain metal ions
having a reduction potential higher than that of a conducting
polymer can be reduced by the conducting polymer to form zero-valent metals.11,12 Recently, we have shown that a variety of
experimental parameters, such as dopants, metal ion solution
concentration, as well as the structure and composition of the
conducting polymer, can profoundly influence the overall shape
and size of the electrolessly deposited metal nanostructures.7,1315
It was also found that platinum and palladium nanostructures
chemically deposited on a PANI membrane were highly efficient
catalysts for regioselective hydrosilylation reactions and selective
hydrogenation of alkynes and cinnamaldehyde.16,17 The palla-
dium nanoparticles supported on polyaniline nanofibers have
been used as active catalysts for Suzuki coupling between aryl
chlorides and phenylboronic acid, and for phenol formation
from aryl halides and potassium hydroxide in water and air. 18 Itcan be imagined that nanocomposites consisting of metal
nanostructures and conducting polymers will have even broader
applicability in many other areas. One such application involves
generating materials with dramatically enhanced surface Raman
spectroscopy (SERS) sensitivities to form the basis of a rapid and
reliable biological and chemical sensing platform. SERS is widely
used to amplify Raman scattering signals of absorbed molecules
on a nanostructured metal surface. The origins of this enhance-
ment are primarily due to the strong light-induced electric field,
which strongly depends on the roughness/morphology of the
metal particles.19 Structures with sharp edges, intersections, and
bifurcations typically exhibit extremely strong signal enhance-
ments,2023 and therefore, construction of metal nanoparticles
into well-defined dimensionalities and shapes is of great interest
for SERS applications.2426
Here, we introduce the facile synthesis of nanostructured silver
gradients with varying morphologies on a PANI-graphite (P-G)
membrane with the assistance of an external electric field. In the
absence of an electric field, only silver microspheres assembled
from smaller silver nanosheets are obtained. A possible mecha-
nism for the generation of such silver nanostructures with
different morphologies is also proposed. The deposited silver
nanostructure gradient on the P-G membrane can be used as an
effective SERS substrate for detecting surface absorbed organic
molecules.
Experimental
Materials
Polyaniline (PANI) emeraldine base (EB) powder was obtained
from Aldrich. Graphite (99.9% Nanostructured & Amorphous
Materials Inc. Los Alamos, NM), N-methyl-2-pyrrolidone
(NMP, 99% Aldrich), heptamethyleneimine (HPMI, 98% Acros),
AgNO3 (99.9999% Aldrich), citric acid (99.9% Fisher) and
mercaptobenzoic acid (MBA, Aldrich 90%) were used as
received.
aC-PCS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.E-mail: [email protected] of Chemistry, Harbin Institute of Technology, Harbin150001, China. E-mail: [email protected]
Electronic supplementary information (ESI) available: EDAX, XRD,and SEM images. See DOI: 10.1039/c0nr00106f
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Fabrication of P-G membranes
The P-G membranes are produced by employing a phase inver-
sion method using water as the coagulation bath.16,17 Here a 5
wt% mixture of graphite and polyaniline is used as the active
material. In a typical experiment, 1.0925 g PANI (EB) powder
and 0.0575 g graphite powder were mixed in a 12 ml Teflon vial.
Then, 4.14 g NMP and 0.747 g HPMI were added. The mixture
was stirred for 0.51 h to form a homogeneous solution, followedby being poured onto a glass substrate and spread into a wet film
using a gardeners blade (Pompano Beach, FL) with a controlled
thickness. The wet film was then immersed into a water bath for
24 h, after which the resulting solid membrane was spontane-
ously delaminated from the glass substrate. The membrane was
then dried at room temperature for 12 h before doping in 0.25 M
citric acid for 3 days.
Preparation of Ag nanostructures on P-G membrane
For preparation of silver gradients on P-G membrane, a direct
current (DC) electric field was firstly applied to the membrane
(see the ESI). Then, one drop of 25 mM AgNO3 solution (1 mL)
was deposited on the membrane. One minute later, the
membrane was repeatedly rinsed with distilled water to remove
silver nitrate residue, followed by air drying for 2 h. To investi-
gate the silver structures produced on P-G membranes without
an electric field, a P-G membrane was immersed in an AgNO 3solution of varying concentrations for one minute, then rinsed
with distilled water and dried in air.
Characterization
Scanning electron microscopic (SEM) images were taken on an
FEI Inspect F SEM to study the morphologies of the silver
nanoparticles. The elemental composition was analyzed by
energy-dispersive X-ray spectroscopy (EDX). X-Ray diffraction(XRD) measurements were carried out on a Rigka Ultima III
diffractometer that uses fine line sealed Cu-Ka tube (l 1.5406A) X-rays. Transmission electron microscopic (TEM) and high
resolution TEM (HR-TEM) images were measured on a JEOL
3000F TEM. TEM samples were prepared by scratching the Ag
structures off of the PANI membrane onto a carbon-coated
copper grid. The metal-supported P-G membrane was immersed
in an MBA ethanol solution (3 mg/5 ml) for 15 min and then
rinsed in fresh ethanol prior to the surface-enhanced Raman
spectroscopy (SERS) measurements. The SERS spectra were
recorded on a Kaiser Raman spectrometer through a 20/0.50
microscope objective, coupled to a liquid nitrogen-cooled
charge-coupled device (CCD) detector (wavelength: 785 nm).
The incident laser power was kept at 1 mW and total accumu-
lation times of 10 s were employed.
Results and discussion
The P-G membrane was fabricated according to a slightly
modified literature procedure, where graphite was added to the
PANI powder mixture prior to membrane casting in order to
enhance its overall electrical conductivity. In a typical silver
nanostructure synthesis, a direct current (DC) voltage was first
applied to the P-G membrane. Then, 1 mL of 25 mM AgNO3
solution was dropped on to the P-G membrane that had previ-
ously been doped with citric acid, where a gradient of silver
particles spontaneously formed within 60 s (Fig. 1a). The
elemental composition of all the structures was confirmed by
energy-dispersive X-ray spectroscopic (EDX) analysis to be pure
silver and not silver salt. The XRD pattern of the structures on
the P-G membrane further verifies the presence of silver metals,
with the diffraction peaks at 2q 38.04, 44.20, 64.40, 77.32 and
81.52 corresponding to the (111), (200), (220), (311) and (222)crystal planes, respectively, of face-centered cubic silver (JCPDS
65-2871). The calculated intensity ratio between the (111) and
(200) is much higher than that of bulk silver, indicating that these
silver structures mainly grow along the [111] crystal plane (see the
ESI). The broad peak centered at 2q 24 in the XRD pattern
is ascribed to the amorphous polyaniline and graphite in the P-G
membrane substrate.15,27,28
The morphologies of the silver gradients produced on the P-G
membrane can be divided into five general areas (G1G5), where
each contains distinct Ag particle morphologies, as shown in
a series of scanning electron microscopic (SEM) images (Fig. 1b
f). Structures found on the P-G membrane near the negative
electrode (G1) exhibited highly branched, dendritic silvermorphologies up to 20 mm in length (Fig. 1b). The branches of
the dendritic structures are uniform with a diameter of approx-
imately 100 nm, a stark contrast from silver dendrites prepared
Fig. 1 (a) Macroscopic image of Ag gradient produced on a P-G
membrane under an electric field of 20 V for 1 min; and magnified SEM
images of Ag nanostructures from different parts on the P-G membrane:
(b)(f) correspond to the produced silver structures in regions G1G5
respectively.
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via chemical reduction of Ag+ ions by a zinc or copper plate,29,30
and electrochemical deposition on Ni/Cu electrodes at a poten-
tial of2.0 V,31 which have highly branched feather-like struc-
tures. Silver structures found slightly further from the negative
electrode (G2) consisted of a mixture of shorter dendrite
(compared to those at G1) and flower-like silver structures. The
silver flowers here have similar features to the dendrites in G1,
and presumably act as nucleation sites for their subsequent
longer dendrite growth. Moving further from the negative elec-trode (G3), flower-like structures become dominant, which
consist of larger three dimensional assemblies of Ag nanosheets
with thicknesses of approximately 70 nm, quite different from
those obtained by an electrochemical approach on a Pt film. 32
This trend continues with increasing distance from the negative
electrode, where uniformly smaller flower-like silver structures
are observed (G4). At the opposite side of the P-G membrane,
near the positive electrode, there is a region where no silver
deposition is observed (white frame in Fig. 1a), presumably due
to electrostatic repulsion of Ag ions by the electric field. Inter-
estingly, at the area between Ag deposition and bare PANI
surface (G5), silver microspheres are formed, which are
comprised of many densely packed 50 nm thick nanosheets. Itshould be noted that similar silver gradients are formed using
a variety of electric field potentials that range from 10 to 100 V
(see the ESI). At voltages higher than 200 V, the P-G membrane
breaks down, and voltages lower than 10 V cannot induce the
formation of silver gradients.
The transmission electron microscopic (TEM) images of the
Ag dendrites, flowers and microspheres are shown in Fig. 2. A
high-resolution TEM (HR-TEM) image of the Ag dendrites and
the corresponding selected area electron diffraction (SAED)
pattern are shown in Fig. 2b, which indicate that the marked area
is dominated by single crystalline growth along the [111] crystal
plane. Similarly, the HR-TEM image and SAED pattern of the
Ag flowers (Fig. 2d) also indicate single crystalline morphologiesalong the [111] crystal plane. From the TEM image of the Ag
microspheres (Fig. 2e), it is apparent that they are comprised of
Ag nanosheets, and HR-TEM and SAED analyses of the marked
area (Fig. 2f) show favorable growth along the [111] crystal
plane. A dihedral angle of 70.1 can be distinguished, which is
close to the theoretical value of 70.5.33 However, at the junction
of two Ag nanosheets, the d spacing was measured to be 0.209
nm, close to the theoretical value of [100] d spacing, 0.204 nm.
This may explain why even though Ag dendrites, flowers and
microspheres are almost exclusively single crystalline along the
[111] plane, a minute (200) diffraction peak can still be seen in the
XRD pattern.
The formation of such silver nanoparticle gradients on the P-Gmembranes is easily understood as an electrophoretic effect
resulting from field driven movement of the Ag+ ions toward the
negative electrode. The resulting Ag+ concentration gradient
proceeds to have direct impacts on the observed Ag nano-
structures. In the absence of graphite loading, no Ag gradients
can be formed, as the conductivity of the membrane is so low that
no differential electric field can be generated, thus motion of the
Ag+ ions cannot be induced. The concentration gradient formed
in the presence of an electric field is believed to be responsible for
the widely varying growth of nanostructured silver, whose
morphology is concentration dependent. However, it was found
that the concentration gradient alone does not produce all of the
varying silver structures seen in Fig. 1. Simply varying the Ag ion
concentrationin the absence of an electric fieldtends to yield
only Ag microspheres constructed of even finer Ag nano-
structures (Fig. 3). Yet, skeleton-like Ag structures were
produced when the P-G membrane was immersed in 5 mM
AgNO3 aqueous solution, indicating that to some extent,
morphological control of Ag structure can be realized by varying
the silver ion concentration. Increasing AgNO3 concentration
beyond 10 mM results in Ag microspheres composed of finer
sheet-like structures. With the increase in the concentration, thesize of the microspheres grew larger, with the sheet-like structures
becoming finer and more densely packed. Of note, however, is
that no concentration-related dendritic growth is observed in
these static (non-field driven) conditions. As previously demon-
strated, sheet-like structure growth of silver is an intrinsic char-
acteristic of citric acid-doped PANI.13
The above analyses demonstrate the effect of concentration
gradient and electrokinetic flow on the formation of nano-
structured Ag on a P-G membrane. As schematically illustrated
in Fig. 4, we hypothesize that the relative nucleation and growth
mechanisms of these Ag structures under static and field driven
Fig. 2 TEM and HR-TEM images of Ag dendrites (a, b), flowers (c, d)
and microspheres (e, f). HR-TEM images were taken from the marked
area shown in TEM images. Selected area electron diffraction (SAED)
patterns are inset in HR-TEM images.
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conditions are dramatically impacted by the movement of Ag+
ions. In the static case, Ag+ ions simply diffuse vertically from the
bulk solution down to the membrane surface. Upon interacting
with an active nucleation site on the PANI (an electron donor),
the Ag+ ion is reduced to form zero-valent Ag (Fig. 4a). Once
a metallic nucleation particle is formed, subsequent growth is
directed by the diffusion kinetics of the Ag+ ions in the bulk
solution. Static growth of Ag on the P-G membrane in this
regime results in microspheres with fine sheet-like substructures.
Conversely, when an electric field is applied to the P-G
membrane, the positively charged Ag+
ions are driven toward thenegative electrode, forming a Ag+ ion concentration gradient
(Fig. 4b). Here, the motion of Ag+ ions is a combination of
a vertical diffusion towards the surface and lateral electrokinetic
flow towards the cathode. When these Ag+ ions driven by elec-
trokinetic flow interact with the P-G membrane (or a nucleated
Ag particle on the membrane), the lateral movement of Ag ions is
believed to alter the growth mechanism of the Ag structures to
one that favors unidirectional dendritic growth as compared to
static (omnidirectional) growth. These effects are manifested as
dramatic changes in the Ag morphology as a function of distancefrom the cathode, showing a novel approach to Ag nano-
structure generation that was previously synthetically inacces-
sible.
The highly branched, dendritic silver structures promise to
have numerous SERS applications, as they contain ample
features consistent with electromagnetic hot spots necessary
for efficient SERS excitation. The relative SERS activities of
different silver gradient structures were measured from a self-
assembled monolayer of mercaptobenzoic acid (MBA) on the
metal surface using a 785 nm Raman apparatus in a normal
incidence, backscatter configuration. During the measurement,
the beam was focused on the Ag structures from different areas
through the microscope. MBA is suitable for SERS analysis as itreadily adsorbs to Au and Ag surfaces, and has intense benzene
ring stretches at 1075 cm1 and 1580 cm1, making it readily
identifiable in SERS spectra.34 The spectra taken on the various
metal morphologies are shown in Fig. 5. MBA has weak elec-
tronic interactions with nanostructured silver surfaces and does
not absorb at the Raman excitation frequency,21 thus the SERS
mechanism mainly results from the electromagnetic enhance-
ment associated with the Ag nanostructured surfaces. Notably,
one can see that all three structure regimes (dendrites, flowers,
and spheres) exhibit relatively strong SERS signals. Ag dendrites
and flowers have a relatively stronger SERS response as
compared to that of the microspheres, presumably due to the
differences in morphology and relative surface areas.35,36 Thesedata are consistent with a previous report where Au nanoflowers
exhibit strong SERS responses, attributed to the substantially
Fig. 4 Schematic illustration of the motion behaviors of silver ions
toward the P-G membrane surface and the evolution of the silver struc-
tures under different conditions: (a) without an external electric field; (b)
with a proper external electric field.
Fig. 5 SERS spectra of mercaptobenzoic acid (MBA) absorbed on (a)
silver dendrites, (b) silver flowers and (c) silver microspheres.
Fig. 3 SEM images of Ag structures prepared by immersing the P-G
membrane (without applying any electric field) in (a) 5 mM, (b) 10 mM,
(c) 25 mM and (d) 100 mM AgNO3 aqueous solution for 1 min. Scale bar:
1 mm.
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enhanced local electromagnetic fields generated by their unique
surface topography.37 We believe strongly overlapping electric
fields are present in these interstitial sites of Ag flowers and
dendrites, which result in hot spots along with corresponding
intense SERS activity.38 For the densely packed Ag micro-
spheres, the nanocavities formed between the neighboring
nanosheet structures may support electromagnetic enhance-
ments; however, the densely packed structure also reduces the
total surface area accessible to the adsorbed molecules, therebylimiting the number of SERS-active molecules (self-assembled
monolayer) on the metal surface, resulting in a relatively weaker
SERS response. Here, the Ag nanostructures fabricated on P-G
membranes by the field-assisted approach show comparable
SERS effect to some reported Ag films.39,40
Conclusions
In summary, we have demonstrated a field-assisted fabrication
method capable of preparing a wide range of nanostructured
silver on conducting polyanilinegraphite (P-G) membranes with
various sizes and morphologies. In the absence of an external
electric field, only silver microspheres that consist of denselypacked nanosheets are obtained. Our electric field-assisted model
suggests that the generation of a Ag+ concentration gradient
along with an electrokinetic flow of Ag+ ions dramatically
impacts the nucleation and growth of Ag particles on the P-G
membrane and leads to a wide range of Ag nanostructures that
are not readily available via other synthetic routes. Silver
dendrites, flowers and microspheres present morphologies
consistent with electromagnetic hot spots, resulting in strong
SERS responses. This work demonstrates the feasibility of using
Ag nanoparticle-decorated P-G membranes as highly efficient
SERS substrates via an extremely facile fabrication method-
ology. The field-assisted synthesis of Ag nanoparticles presents
an alternative approach to nanomaterial fabrication, whichyields a wide range of unique structures with enhanced optical
properties that were previous inaccessible.
Acknowledgements
PX thanks the support from the Joint Educational Ph.D.
Program of the Chinese Scholarship Council (CSC) and NSFC
(No. 20776032). HLW acknowledges the financial support from
Laboratory Directed Research and Development (LDRD) fund
under the auspices of DOE, BES Office of Science, and the
National Nanotechnology Enterprise Development Center
(NNEDC). This work was performed in part at the Center for
Integrated Nanotechnologies (CINT), at Los Alamos National
Laboratory (Contract DE-AC52-06NA25396) and Sandia
National Laboratories (Contract DE-AC04-94AL85000).
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