Transmetalation reaction between hydrophobic silver nanoparticles and aqueous chloroaurate ions at the air-water interface

Article abstract of DOI:10.1021/jp0530552

The transmetalation reaction between a sacrificial nanoparticle and more noble metal ions in solution has emerged as a novel method for creating unique hollow and bimetallic nanostructures. In this report, we investigate the possibility of carrying out the transmetalation reaction between hydrophobic silver nanoparticles assembled and constrained at the air-water interface and subphase gold ions. We observe that facile reduction of the subphase gold ions by the sacrificial silver nanoparticles occurs resulting in the formation of elongated gold nanostructures that appear to cross-link the sacrificial silver particles. This transmetalation reaction may be modulated by the insertion of an electrostatic barrier in the form of an ionizable lipid monolayer between the silver nanoparticles and the aqueous gold ions that impacts the gold nanoparticle assembly. Transmetalation reactions between nanoparticles constrained into a close-packed structure and appropriate metal ions could lead to a new strategy for metallic cross-linking of nanoparticles and generation of coatings with promising optoelectonic behavior. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0530552

19620
J. Phys. Chem. B 2005, 109, 19620-19626
Transmetalation Reaction between Hydrophobic Silver Nanoparticles and Aqueous
Chloroaurate Ions at the Air-Water Interface
Renu Pasricha, Anita Swami, and Murali Sastry*
Nanoscience Group, Materials Chemistry DiVision, National Chemical Laboratory, Pune-411 008, India
ReceiVed: June 8, 2005; In Final Form: August 1, 2005
The transmetalation reaction between a sacrificial nanoparticle and more noble metal ions in solution has
emerged as a novel method for creating unique hollow and bimetallic nanostructures. In this report, we
investigate the possibility of carrying out the transmetalation reaction between hydrophobic silver nanoparticles
assembled and constrained at the air-water interface and subphase gold ions. We observe that facile reduction
of the subphase gold ions by the sacrificial silver nanoparticles occurs resulting in the formation of elongated
gold nanostructures that appear to cross-link the sacrificial silver particles. This transmetalation reaction may
be modulated by the insertion of an electrostatic barrier in the form of an ionizable lipid monolayer between
the silver nanoparticles and the aqueous gold ions that impacts the gold nanoparticle assembly. Transmetalation
reactions between nanoparticles constrained into a close-packed structure and appropriate metal ions could
lead to a new strategy for metallic cross-linking of nanoparticles and generation of coatings with promising
optoelectonic behavior.
Introduction
linked by bifunctional bridging linkers such as TBBT (4,4′-
thiobenzenethiol) through hydrogen bonding at the air-water
interface. Recently, we have shown in this laboratory how the
symmetry-breaking nature of the air-water interface can be
exploited to build anisotropic nanostructures by carrying out
the reduction of precursor metal ions preferentially at the
interface.12 This has been achieved by either constraining the
precursor metal ions (formation of flat gold nanostructures by
reduction of a monolayer of hydrophobic gold ions with
anthranilic acid in the subphase12a) or the reducing agent
(formation of flat gold nanosheets by the reduction of gold ions
in the subphase by hexadecylaniline12b/alkylated tyrosine Lang-
muir monolayer12c) to the air-water interface thus confining
the reduction of metal ions strictly to the interface.
In a series of elegant reports, research groups led by Xia13
and Bai14 have investigated the possibility of using the trans-
metalation reaction to modulate the structure of aqueous
nanoparticles. The galvanic exchange reaction between sacri-
ficial nanoparticles (which act as a template) and other suitable
metal ions often results in the formation of hollow nanostruc-
tures13,14 with interesting application potential. Xia and co-
workers have studied in detail the mechanistic aspects of the
transmetalation reaction between sacrificial Ag nanospheres/
nanocubes and Au ions.13 They observe that the plasmon
absorption band of the metal nanostructures following the
replacement reaction can be tuned in the range of 500-1200
nm by simple variation of the experimental conditions.13 Using
Co nanoparticles as sacrificial templates, Bai and co-workers
have shown that the transmetalation reaction with Au and Pt
ions results in the formation of hollow Pt nanospheres14a and
rodlike Au-Pt alloy nanoparticles14b with excellent catalytic
properties.
The synthesis, deposition, and subsequent patterning of thin
films of metal nanoparticles is of great interest because of their
potential for application as building blocks in nanodevices.1 It
is well-known that metal nanoparticles possess unique optical,
chemical, magnetic, and electrical properties, which are different
from the properties in their bulk states.2 Apart from variation
in parameters such as size, shape, and dielectric constant of the
dispersing medium, these properties can be controlled through
their ordered or patterned assembly on suitable substrates.3 This
is because the parameters that influence the collective behavior
of assembled nanostructures can now be manipulated by
chemists with a high degree of control, thus providing a new
initiative to research on cluster-engineered materials. To date,
several techniques have been reported for fabricating patterned
arrays of nanoparticles on solid surfaces that include spin
coating,4 photolithography,5 soft lithography,6 microcontact
printing,7 etc. The Langmuir-Blodgett (LB) technique is a
versatile method, which has been extensively used for generating
ordered monolayer arrays of metallic,8 semiconducting,9 and
polymer nanoparticles10 and their thin film formation. One
variant of the LB technique involves spreading a colloidal
suspension of hydrophobic nanoparticles on water, allowing the
solvent to evaporate, and subsequently transferring the nano-
particle array that forms on the water surface to solid substrates.
However, the role of the air-water interface in the above-
mentioned cases is relatively passive, since it is utilized merely
to organize nanoparticles. An exciting option is to design
experiments wherein the air-water interface is dynamic and
permits reactions involving nanoparticles. Progress in this
direction was made by Chen and co-workers11 who showed that
two-dimensional gold nanoparticle networks could be cross-
An interesting possibility that has not been addressed so far
is to study transmetalation reactions involving nanoparticles
assembled on suitable surfaces/interfaces with appropriate metal
ions. An important motivation for such a study is the possibility
of interconnecting close-packed nanoparticles assembled on
* To whom correspondence should be addressed. E-mail: m.sastry@
ncl.res.in. Current affiliation for Murali Sastry: Chief Scientist, Tata
Chemicals Innovation Centre, Mumbai-400 059, India. E-mail: msastry@
tatachemicals.com.
10.1021/jp0530552 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/29/2005

Silver Nanoparticles and Chloroaurate Ions
J. Phys. Chem. B, Vol. 109, No. 42, 2005 19621
surface/interfaces through transmetalation reactions leading to
electrically conducting structures over large length scales. That
this is indeed a realizable goal is indicated by the rodlike
structures observed by Bai et al in the transmetalation reaction
of Co nanoparticles and Au/Pt ions; the rodlike structures were
explained to arise due to interconnections formed during
transmetalation of linearly assembled Co nanoparticles in
solution.14b In this context, the air-water interface could be an
excellent candidate to follow in real time transmetalation
reactions between hydrophobic nanoparticles assembled in close-
packed structures on water and aqueous metal ions present in
the subphase. In this paper, we discuss our experiments on the
reaction of aqueous chloroaurate ions in the subphase with
hydrophobic silver nanoparticles constrained to the air-water
interface. Consumption of the sacrificial silver nanoparticles
leads to the formation of gold nanostructures at the air-water
interface of interesting morphology. Furthermore, we show that
the transmetalation reaction, and thereby the morphology of
nanostructures formed at the air-water interface, can be
modulated by placing an electrostatic barrier (in the form of
Langmuir monolayers of octadecylamine (ODA, cationic) or
stearic acid (StA, anionic)) between the hydrophobic silver
nanoparticles and the aqueous chloroaurate ions in the subphase.
Presented below are the details of the investigation.
nanoparticle solution in benzene at a concentration of 1
mg/mL was then spread slowly onto the ODA/StA monolayers
in a dropwise fashion followed by 15 min of solvent evaporation.
At different times in the reaction of the ODA-capped silver
nanoparticles with AuCl4- ions in the subphase, the nanoparticle
monolayers were transferred onto carbon-coated copper grids
and quartz slides by the conventional LB method at a controlled
surface pressure of 15 mN/m and a substrate immersion/
withdrawal speed of 10 mm/min. LB films of Ag-ODA were
also formed by nanoparticle organization on deionized water
as the subphase under the conditions mentioned above; this
serves as a point of reference and a control. The quartz substrates
were rendered hydrophobic prior to transfer of the nanopar-
ticles by deposition of three monolayers of lead arachidate onto
the substrates. It is known that metal salts of fatty acids
(cadmium arachidate, lead arachidate, etc.) form stable mono-
layers strongly bound to substrates with oxide layers and
hydrophobization of the support resulted in significantly better
transfer ratios of the nanoparticle monolayers. For the LB films
grown on different substrates, monolayer transfer was observed
during both upward and downward strokes of the substrate at
the close to unity transfer ratio. The transmetalation reaction
between silver nanoparticles and gold ions was also carried out
by first transferring a thick Ag-ODA LB film on quartz (20
ML) followed by immersion in a 10-3 M aqueous HAuCl4
solution.
Experimental Details
UV-vis Spectroscopy. The transmetalation reaction between
ODA-capped Ag nanoparticles and aqueous gold ions at the
air-water interface and in solution was monitored by measuring
the UV-vis absorption spectra of LB films of the nanoparticles
on quartz substrates at different times of the reaction. The spectra
were recorded on a JASCO dual-beam spectrophotometer
(model V-570) operated at a resolution of 1 nm.
Transmission Electron Microscopy (TEM). TEM measure-
ments of the Ag-ODA nanoparticles LB films deposited before
and after transmetalation reaction at the air-water interface onto
carbon-coated copper grids were performed on a JEOL model
1200EX instrument operated at an accelerating voltage at 120
kV. Selected area electron diffraction (SAED) patterns were
recorded from ensembles of particles under conditions of an
operating voltage of 120 kV, camera length of 80 cm, and
insertion of a 120 µm field limiting aperture.
Current-Voltage (I-V) Measurements. The silver nano-
particle film as well as the bimetallic film obtained by
transmetalation reaction were deposited onto quartz substrates
by vertical lifting and were allowed to dry. After thorough
drying, the nanoparticles films were used for I-V measure-
ments. Electrodes of 1 mm width were painted at the opposite
ends of the nanoparticulate films using conducting silver paste
as thick pads to ensure proper electrical contact. The separa-
tion between the electrodes was 10 mm. All I-V measurements
were done using a Keithley 238 High Current Source Measure
Unit. The I -V characteristics were measured in the sweep
mode at a voltage increment of 5 V in the range of -40 to
+40 V.
Chemicals. Silver sulfate (Ag2SO4), chloroauric acid (HAuCl4‚
xH2O), potassium hydroxide (KOH), tyrosine (C9H11NO3),
stearic acid (C18H36O2), and octadecylamine (C18H39N) were
obtained from Aldrich Chemicals and used as received.
(a) Synthesis of hydrophobic Silver Nanoparticles. Silver
nanoparticles capped with ODA (Ag-ODA) were prepared
as described elsewhere.8d In a typical experiment, 10 mL of
10-3 M aqueous silver sulfate solution was taken along with
10 mL of 10-3 M aqueous solution of tyrosine and diluted to
100 mL with deionized water. To this solution, 1 mL of 10-1
M solution of KOH was added, and the mixture was allowed
to boil until the colorless solution turned yellow, indicating the
formation of silver nanoparticles. The aqueous solution of sil-
ver nanoparticles (25 mL) was taken in a beaker, and its pH
was adjusted to 5 using dilute hydrochloric acid (HCl). This
solution was taken in a separating funnel and added to 25 mL
of 10-3 M solution of ODA in chloroform. Vigorous shaking
of the mixture results in the quantitative transfer of silver
nanoparticles from the aqueous to chloroform phase. After the
completion of phase transfer, the organic phase was separated
from the aqueous phase, rotavapped, and washed with ethanol
to remove uncoordinated ODA molecules (if any). The purified
and dried powder of ODA-capped silver nanoparticles could
be dispersed in a range of weakly polar/nonpolar organic
solvents.
(b) Transmetalation Reaction of Hydrophobic Silver
Nanoparticles with Aqueous Chloroaurate Ions at Air-
Water Interface. The transmetalation reaction between ODA-
capped silver nanoparticles and aqueous chloroauric acid
(HAuCl4) was carried out at the air-water interface using a
Nima model 611 LB trough equipped with a Wilhelmy plate
as the surface-pressure sensor at room temperature. In a typical
experiment, Langmuir monolayers of ODA/StA were formed
by spreading a 75 µL solution of ODA/StA in chloroform (1
mg/mL concentration) on the surface of 10-3 M aqueous
HAuCl4 subphase. At least 15 min was allowed for solvent
evaporation, and the monolayer was compressed to a surface
pressure of 15 mN/m. An aliquot of 100 µL of Ag-ODA
Results and Discussion
Parts A and B of Figure 1 show representative TEM images
of a one monolayer (ML) LB film of hydrophobic silver
nanoparticles (Ag-ODA) transferred from deionized water as
the subphase at a surface pressure of 15 mN/m. It is observed
from these images that the hydrophobic silver nanoparticles
assemble into large two-dimensional domains of hexagonal
close-packed nanoparticles on the surface of water. An analysis

19622 J. Phys. Chem. B, Vol. 109, No. 42, 2005
Pasricha et al.
Figure 2. (A) UV-vis absorption spectra recorded from curve 1, 20
ML Ag-ODA LB film from pure water as the subphase and curves 2
and 3, 20 ML LB films of Ag-ODA nanoparticles after 1 h of
transmetalation reaction on the surface of 10-3 M aqueous HAuCl4 in
the presence of StA and ODA monolayers, respectively. Curve 4
corresponds to the UV-vis absorption spectrum recorded from a 20
ML LB film of the Ag-ODA nanoparticles after 1 h of reaction of the
nanoparticles with 10-3 M HAuCl4 solution in the subphase at the air-
water interface. (B) UV-vis absorption spectra recorded from curve
1, 20 ML Ag-ODA LB film from pure water as the subphase (same
as curve 1 in Figure 2A) and curves 2-6, 20 ML Ag-ODA LB film
Figure 1. Representative TEM images of one monolayer of ODA-
capped silver nanoparticles assembled on deionized water (A and B)
and on 10-3 M HAuCl4 solution after 1 h of transmetalation reaction
deposited from pure water as the subphase after immersion in 10-3
M
aqueous HAuCl4 solution for t ) 10 min, 1, 4, 12, and 48 h,
respectively. The inset in the figure shows photographs of a 20 ML
Ag-ODA LB film deposited from pure water as the subphase before
-
with subphase AuCl4 ions (C and D). The TEM images in E and F
correspond to ODA-capped silver nanoparticles assembled onto the
ODA Langmuir monolayer on the surface of 10-3 M HAuCl4 recorded
after 1 h of reaction. The insets in B, D, and F correspond to SAED
patterns recorded from the nanoparticles in the main part of the
respective figures.
(a) and after (b) transmetalation reaction by immersion in 10-3
M
HAuCl4 for 1 h. The pictures correspond to curves 1 and 6 in the figure,
respectively.
irregularly shaped particles that show no ordered packing.
Clearly, the transmetalation reaction has taken place, and the
irregular structures arise due to consumption of silver nanopar-
ticles by the reduced gold ions. Occasionally, rodlike and
triangular structures similar to those observed by us in an earlier
study on 4-hexadecylaniline-mediated reduction of gold ions12b
are seen in the film after transmetalation (structures identified
by arrows in part C and D of Figure 1). Further evidence for
the occurrence of the transmetalation reaction is provided by
the UV-vis spectrum recorded from a 20 ML LB film of the
Ag-ODA nanoparticles deposited 1 h after reaction of the
nanoparticles with subphase AuCl4- ions (curve 4, Figure 2A).
The surface plasmon band characteristic of silver nanoparticles
(curve 1) is completely damped and is now replaced by an
absorption band centered at 588 nm (curve 4). The latter
absorption band is a clear indication of the formation of gold
of the particles in these and other similar images yielded an
average nanoparticle size of 20 ( 6 nm. The silver nanoparticles
are spherical in nature and often show contrast characteristic
of multiply twinned particles (MTPs, Figure 1B). The SAED
pattern recorded from the silver nanoparticles is shown as an
inset in Figure 1B. The diffraction rings indicate that the
particles are polycrystalline and could be indexed on the basis
of the face centered cubic (fcc) structure of silver. A 20 ML
LB film of the silver nanoparticles was transferred from the
water subphase onto a quartz substrate, and the UV-vis
absorption spectrum obtained from this film is shown in Figure
2A (curve 1). A strong absorption band centered at ca. 470 nm
is observed from the multilayer Ag-ODA film. This absorption
is characteristic of excitation of surface plasmon vibrations in
the silver nanoparticles and is responsible for the yellow color
exhibited by these films (inset of Figure 2B, slide “a”). Well-
dispersed silver nanoparticles in water normally show an
absorption at ca. 400 nm. The red shift in this absorption band
in LB films of the nanoparticles indicates assembly of the
nanoparticles15 or anisotropic nanostructures16 and also the fact
that the dielectric properties of the film are significantly different
from those of water. Such shifts have been observed earlier.15,16
The inference that the particles are in an assembled, close-
packed configuration is consistent with the TEM results on
monolayers of the Ag-ODA nanoparticles assembled on water
(parts A and B of Figure 1).
-
nanoparticles by the oxidative reduction of subphase AuCl4
ions by the silver nanoparticles. The SAED pattern recorded
from the monolayer after the transmetalation reaction is shown
as an inset in Figure 1D. The diffraction rings indicate that the
particles are polycrystalline and could be indexed on the basis
of the fcc structure of gold. We hasten to add here that the
crystallography of gold and silver is almost identical, and we
rely on the UV-vis data presented above as an indicator that
the transmetalation reaction has occurred and that gold has been
deposited on the silver structures.
The surface plasmon absorption in spherical, noninteracting
gold nanoparticles occurs at ca. 520 nm in water. The shift to
longer wavelengths observed in the LB films of the silver
nanoparticles post transmetalation is similar to that observed
by Xia and co-workers in their study of transmetalation of silver
nanocubes by gold ions.13 The red shift in the gold surface
plasmon band was explained as arising due to the formation of
Parts C and D of Figure 1 correspond to representative TEM
images of the Ag-ODA nanoparticle Langmuir monolayer
recorded 1 h after reaction with the aqueous HAuCl4 subphase.
It is seen that the morphology of the particles has changed
dramatically; the original close-packed spherical nanoparticulate
structure (parts A and B of Figure 1) is now replaced by

Silver Nanoparticles and Chloroaurate Ions
J. Phys. Chem. B, Vol. 109, No. 42, 2005 19623
a thin shell of gold around the sacrificial silver nanocube core
and under certain experimental conditions, could extend into
the near-infrared region of the electromagnetic spectrum.13 We
believe the red shift in the surface plasmon band of gold
observed in this experiment may also be explained on the basis
of the formation of shells of gold around the spherical silver
core, evidence for which will be provided in subsequent studies
below. The reaction of subphase chloroaurate ions with the silver
nanoparticles at the air-water interface is rapid and is complete
within 15 min of spreading the monolayer (data not shown).
The silver ions formed as a consequence of the transmetalation
reaction are expected to dissolve in the subphase. However, the
concentration of the Ag+ ions was found to be below the
detection limits of atomic absorption spectroscopy.
The rate of reaction of subphase gold ions with the silver
nanoparticles may be modulated by positioning a suitable barrier
between the nanoparticles and the subphase. Since the reaction
involves gold ions, an electrostatic barrier comprised of an
ionizable lipid monolayer would be an excellent candidate. With
this in mind, we have carried out the transmetalation reaction
between hydrophobic Ag nanoparticles and AuCl4- ions in the
subphase in the presence of a cationic (octadecylamine) and an
anionic Langmuir monolayer (stearic acid). One would expect
that the presence of the cationic ODA monolayer would result
Figure 3. I-V characteristics of 60 ML LB film of silver nanoparticles
deposited by vertical lifting on quartz substrate before (triangles) and
after (stars) the transmetalation reaction. The current values in the plot
obtained before the transmetalation reaction (triangles) have been
multiplied by a factor of 5000 to bring the values on the same scale as
those recorded from the film after transmetalation (stars).
-
in an increase in the AuCl4 counterion concentration at the
air-water interface and thus, enhance the rate of reaction of
between dodecylthiolate-monolayer-protected Ag clusters and
gold ions leading to the formation Ag-Au bimetallic nanopar-
ticles has been reported previously by Murray and co-workers.17
Taken together with the findings of this study, it is clear that
the transmetalation reaction can proceed even in the presence
of a barrier to ion transport, the reasons for which need to be
elucidated.
the silver nanoparticles with the gold ions. On the other hand,
-
anionic StA would lead to a reduction of AuCl4 co-ion
concentration at the air-water interface and slow the reaction.
Parts E and F of Figure 1 show representative TEM images
recorded from the hydrophobic Ag nanoparticle Langmuir
monolayer after 1 h of reaction in the presence of an ODA
barrier monolayer. The transmetalation reaction in this experi-
ment leads to the formation of a large percentage of elongated
and twisted wormlike nanostructures. Clearly, the ODA Lang-
muir monolayer does not impede the reaction between the gold
ions and hydrophobic silver nanoparticles and to a large extent,
appears to promote the formation of high aspect ratio nanopar-
ticles most likely due to metallic cross-linking from the
transmetalation reaction. The SAED pattern recorded from the
monolayer after the transmetalation reaction is shown as an inset
in Figure 1F; the diffraction rings could be indexed on the basis
of the fcc structure of gold subject to the caveat mentioned
earlier.
The TEM images shown in parts E and F of Figure 1 indicate
formation of elongated and coiled nanostructures most likely
due to metallic cross-linking of the sacrificial silver nanoparticles
after the transmetalation reaction in the presence of ODA
monolayer; the formation of these interlinked structures was
further confirmed by electrical transport measurements. Figure
3 shows the current vs voltage (I-V) plots of a 60 ML LB
film of silver nanoparticles before (triangles) and after (stars)
the transmetalation reaction. The LB films were deposited on
quartz substrates on which the silver electrodes had been
deposited by vertical lifting. The I-V plot is fairly linear in
both cases, however, the resistance of the film after the
transmetalation reaction reduces by an order of 5 × 103 (please
note that the current values recorded for the LB film before
transmetalation have been multiplied by a factor of 5000 in this
plot). The significant drop in the film resistance after trans-
metalation is a clear indication of the formation of elongated
structures leading to an electrical interlinking of the silver
nanoparticles observed in the TEM images (parts E and F of
Figure 1). The I-V measurements were carried out after
thorough drying of the films in a dehumidified atmosphere at
different sweep rates and different voltage increments. We did
not observe a detectable difference in the I-V plots under these
different experimental conditions and believe, therefore, con-
tributions due to water, an so forth, to the electrical conduction
may be ruled out. However, more detailed studies are required
to fully understand this issue and will be addressed in future
communications.
The UV-vis absorption spectrum recorded from a 20 ML
Ag-ODA nanoparticle film transferred after 1 h of reaction
with aqueous AuCl4- ions in the presence of an ODA Langmuir
monolayer barrier is shown as curve 3 in Figure 2A. Complete
disappearance of the silver surface plasmon band at 470 nm
and appearance of a strong plasmon absorption at 570 nm
indicate the completion of the transmetalation reaction and that
the elongated structures seen in the TEM images (parts E and
F of Figure 1) are due to metallic gold. Attempts were made to
follow the kinetics of reaction in this experiment. These
experiments (TEM measurements of the silver nanoparticle
monolayer at times smaller than 10 min, data not shown) did
not show significant differences in the nanoparticle structure at
reaction times ∼10 min. We note that in these experiments, as
well as in those to be described below, the transmetalation
reaction between the silver nanoparticle surface and aqueous
gold ions takes place even though a hydrophobic sheath of ODA
molecules surrounds the particles and in some of the examples,
an additional barrier to ion transport in the form of ODA and
StA monolayers is present. The galvanic exchange reaction
The reaction between subphase gold ions and Ag-ODA
nanoparticles in the presence of the anionic Langmuir mono-
layer, stearic acid, resulted in the formation of structures that
were significantly different than those obtained with and without

19624 J. Phys. Chem. B, Vol. 109, No. 42, 2005
Pasricha et al.
Figure 5. (A and B) Representative TEM images at different
magnifications of one monolayer of Ag-ODA recorded 1 h after
spreading the monolayer on the surface of pure distilled water (pH
adjusted to 3 by diluted HCl) in the presence of StA. (C and D)
Representative TEM images of Ag-ODA nanoparticles after reaction
with aqueous chloroaurate ions by immersion of a 20 ML Ag-ODA
LB film in gold ion solution for 48 h and thereafter dissolved in
chloroform.
Figure 4. Representative TEM pictures of Ag-ODA nanoparticles
presence of ODA (Figure 2A, curve 3) and StA (Figure 2A,
curve 2) are rather similar although the structure of the
assemblies is so different deserves comment. The similarity in
the UV-vis spectra in these experiments suggests that the
assembly of the nanoparticles (honeycomb in the case of StA
and no regular assembly of ODA barriers) does not play a crucial
role in determining the optical properties of the film. In other
words, interparticle interaction via plasmon coupling is negli-
gible, and to a large extent, the film properties are determined
by the plasmon frequency of the Ag-core-Au-shell bimetallic
nanoparticle structure.
as a function of time of the reaction with 10-3 M subphase AuCl4
-
ions in the presence of StA as a barrier: A-D, times of reaction t )
15 min, 1, 2, and 3 h, respectively, and E and F, representative TEM
images at different magnification recorded from Ag-ODA nanopar-
-
ticles after 3 h of reaction with subphase AuCl4 ions in the presence
of the StA barrier.
ODA as a barrier. Due to the slowing down of the reaction for
the electrostatic reasons discussed above, the kinetics of reaction
could also be followed. Parts A-D of Figure 4 show represen-
tative TEM images recorded from the Ag-ODA nanoparticles
after 15 min, 1, 2, and 3 h of reaction, respectively, with 10-3
M aqueous HAuCl4 in the subphase in the presence of StA as
a barrier. It is observed that at the beginning of the reaction,
the particles assemble into rudimentary open, ringlike structures
(parts A and B of Figure 4) that then close up to form
honeycomb-like nanoparticle patterns toward the completion of
the reaction (parts C and D of Figure 4). The low and high
magnification images of the Ag-ODA nanoparticles after 3 h
of reaction illustrate this point bettersthe honeycomb-like
patterns are seen to be composed of nanoparticles of dimensions
10-18 nm along the periphery while the gaps in the structures
are devoid of nanoparticles. The assembly of nanoparticles along
the periphery of the honeycomb pattern is truly long range and
extends well up to 80-100 µm in length. Unlike in the previous
cases (reaction of Ag-ODA nanoparticles directly with sub-
phase gold ions and in the presence of ODA Langmuir
monolayer), elongated, coiled structures are not seen in this case.
The only evidence that the transmetalation reaction has taken
place is from the optical properties of a 20 ML LB film of these
Ag-ODA nanoparticles transferred onto quartz (curve 2, Figure
2A). The UV-vis absorption spectrum clearly shows a strong
absorption band at 578 nm that is characteristic of gold
nanoparticles. The SAED pattern recorded from the reacted gold
nanoparticles in the presence of StA is shown in the inset of
Figure 4A. The ring pattern suggests that the particles are
polycrystalline and they could be indexed on the basis of the
fcc structure of gold/silver. The fact that the optical properties
of the nanoparticles formed by a transmetalation reaction in the
There could be a number of reasons for the honeycomb
structures formed. One is that the transmetalation reaction
leads to assembly into such a structure while the second
possibility is that there is a separation of the Ag-ODA and
StA phases; the transmetalation reaction then proceeds along
the Ag-ODA rich phases (which in this case, is the honeycomb
pattern). Yet another possibility is that the transmetalation
reaction itself drives the phase separation. To test this hypothesis,
the Ag-ODA nanoparticles were spread on the surface of
deionized water held at pH 3.4 (pH close to that of the 10-3
M
HAuCl4 solution, pH adjusted using dilute HCl) in the presence
of StA and then lifted onto a TEM grid. Parts A and B of Figure
5 show representative TEM images from this monolayer. It is
clear that there is no evidence for a honeycomb-structured
assembly of the silver nanoparticles and in most respects, the
images resemble those recorded from the nanoparticles spread
directly onto water (parts A and B of Figure 1). This control
indicates that the honeycomb assembly of the nanoparticles is
driven by the transmetalation reaction between the silver
-
nanoparticles and the subphase AuCl4 ions. The fact that the
particles in the honeycomb structures are not interconnected
(Figure 4F) suggests that the transmetalation reaction forces the
separation of the StA and Ag-ODA phases into the lovely
honeycomb pattern observed. Thinking about the possibility of
van der Waals or physical interaction between the particles,
conductivity measurements were also done for the LB films
deposited after transmetalation reaction in the presence of the
StA monolayer. However, an increase in the conductivity of

Silver Nanoparticles and Chloroaurate Ions
J. Phys. Chem. B, Vol. 109, No. 42, 2005 19625
the film was negligible (data not shown for brevity) and nowhere
comparable to that obtained in the case of transmetalation in
the presence of the ODA monolayer (Figure 3).
ions and the effusion of Ag+ ions during the transmetalation
reaction thereby limiting the reaction to the stage where only a
thin shell of gold is formed around the silver core. It is known
that a gold shell of 3-5 atomic layers is sufficient to dampen
the surface plasmon vibrations of the underlying silver core
completely2b,c and could thus explain the UV-vis spectroscopy
trends observed.
In conclusion, we have shown that the facile transmetala-
tion reaction between hydrophobic silver nanoparticles and
aqueous chloroaurate ions can be carried out at the air-water
interface. Such reactions with nanoparticles assembled into
close-packed structures at interfaces could lead to metallic cross-
linking of the nanoparticles and synthesis of extended nano-
particulate structures as shown. In the presence of suitable
electrostatic barriers, the assembly of the nanoparticles after
transmetalation reaction leads to interesting honeycomb-like
superstructures. The possibility of connecting nanoparticles in
close proximity via electrical contacts shows potential for
generation of conducting electrodes with nanoparticles as
building blocks and in the design of optical coatings and
chemical/biological sensors.
It would be of interest to study the transmetalation reaction
between LB films of Ag-ODA nanoparticles during immersion
in HAuCl4 solution. The UV-vis absorption spectra recorded
from a 20 ML LB film of Ag-ODA nanoparticles measured
as a function of time of immersion in 10-3 M HAuCl4 solution
is shown in Figure 2B. It is observed that as the transmetalation
reaction proceeds, there is a progressive decrease in the intensity
of the silver plasmon band at 470 nm (curve 2, 10 min of
reaction) that is then followed by the appearance of a separate
and distinct absorption band from metallic gold nanoparticles
at ca. 560 nm after 1 h of reaction. Further immersion in the
chloroaurate ion solution leads to an increase in the gold
plasmon band intensity and a small shift in the peak position to
570 nm (curves 4-6). The reaction is complete after nearly 48
h of reaction and is much slower than the transmetalation
reaction between the silver nanoparticles and gold ions at the
air-water interface. We believe that this difference in reaction
rates is due to the fact that the infusion rate of gold ions and
effusion of Ag+ ions arising from the transmetalation reaction
would be much smaller in the case of the LB films of the
hydrophobic silver nanoparticles during immersion in chloro-
auric acid solution. Such a constraint does not occur at the air-
water interface where the silver nanoparticles fully access the
subphase gold ions with no barrier to metal ion diffusion. The
color of this film turned from yellow (inset of Figure 2B, slide
“a”) to a deep blue upon completion of the reaction with gold
ions (inset of Figure 2B, slide “b”). The blue color indicates
the interparticle surface plasmon coupling in a monolayer of
gold nanoparticles.15 The 20 ML Ag-ODA LB film after 48 h
of immersion in chloroauric acid solution was dissolved in
chloroform, and the nanoparticles were imaged by TEM (parts
C and D of Figure 5). It is observed that the particles are well
separated from one another and that the particles are bigger
(average size ∼50 nm) than the as-prepared silver nanoparticles
(20 ( 6 nm). This may be attributed to the fact that the AgCl
formed after the transmetalation reaction is not able to leach
out fully due to entrapment in the lipid monolayer and stays in
the form of solid AgCl. We do not observe any interconnection
between the nanoparticles possibly because during immersion
of the Ag-ODA LB film in the chloroauric acid solution, the
film swells and leads to considerable separation between the
silver nanoparticles. Transmetalation thereafter may not be
capable of interlinking the nanoparticles, and they merely grow
in size. We note that the UV-vis absorption curves recorded
from the LB films after transmetalation reaction (parts A and
B of Figure 2) show the absorbance at ca. 580 nm, indicative
of the formation of gold nanostructures. Usually it is observed
that the transmetalation reaction between silver nanoparticles
and gold ions in either aqueous13,14 or organic18 medium leads
to the formation of hollow gold nanostructures. However, in
the present study, the UV-vis spectroscopy results (parts A
and B of Figure 2) indicate the formation of either pure solid
gold nanostructures or bimetallic nanostructures (gold-coated
silver cores).19 TEM results support the UV-vis spectroscopy
data showing the presence of both solid gold (parts D and F of
Figure 1 and Figure 4F) and bimetallic Au-Ag (Figure 4F,
particles indicated by arrows) nanostructures. While the exact
reasons for this difference are not understood at this moment,
we believe the ODA-capping layer surrounding the sacrificial
silver nanoparticles may be playing an important role. The ODA
Acknowledgment. A.S. thanks the Council of Scientific and
Industrial Research, Government of India, for a research
fellowship during the initial stages of this work.
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