J. Phys. Chem. C 2007, 111, 14113-14124
14113
Role of Different Phospholipids in the Synthesis of Pearl-Necklace-Type Gold-Silver
Bimetallic Nanoparticles as Bioconjugate Materials
Mandeep Singh Bakshi,*,†,‡,§,⊥ Fred Possmayer,†,‡ and Nils O. Petersen*,†,‡,|
Department of Obstetrics and Gynaecology, Department of Biochemistry, and Department of Chemistry,
UniVersity of Western Ontario, 339 Windermere Road, London, Ontario, Canada N6A 5A5,
National Institute for Nanotechnology, Edmonton, Alberta, Canada, and Department of Chemistry,
Guru Nanak DeV UniVersity, Amritsar 143005, Punjab, India
ReceiVed: April 12, 2007; In Final Form: July 12, 2007
A seed-growth (S-G) method has been used to synthesize gold (Au) and Au-silver (Ag) bimetallic pearnecklace-type nanoparticles (NP) as bioconjugate materials by using a series of phospho-glycerol (PG) and
phospho-choline (PC) lipids as capping agents. All PG lipids produce a fine pear-necklace arrangement of
Au NP with dimensions between 10 and 20 nm. Addition of Ag converts this arrangement into Au-Ag
bimetallic NP with a diameter essentially close to that of Au NP. Use of PC lipids does not show this
arrangement but promotes a significant anisotropic growth especially in the presence of Ag. This difference
has been attributed to a difference in the capping ability of PG and PC lipids because of their anionic and
zwitterionic nature, respectively. XPS results have demonstrated the presence of adsorbed PG and PC lipids
on Au or Au-Ag bimetallic surfaces in their respective samples. The results also indicate a decrease in the
capping amount of a lipid with an increase in the growth of Au-Ag bimetallic NP. The growth of Au-Ag
bimetallic NP from Au NP has been ascribed to the nucleation of Ag atoms at the {111} facets of Au NP in
the presence of PG lipids, while anisotropic growth is occurring mainly at all other planes of fcc crystal
geometry of Au-Ag bimetallic NP in the presence of PC lipids. It has been concluded that in order to get
fine Au-Ag bimetallic bioconjugate materials capped with PG lipids, one has to use ascorbic acid (AA) as
a weak reducing agent at the end of the S-G reaction sequence so as to give the required time for lipid
molecules to adsorb at the liquid-solid interface.
Introduction
Phospholipids are the essential constituents of pulmonary
surfactants.1 A collective contribution of phospho-glycerol (PG)
and phospho-choline (PC) lipids along with surfactant proteins
helps the lung to acquire minimum surface tension during a
normal breathing process.1 High surface activity is achieved
when lipid molecules form a fluid film of lipid bilayers at airalveolar interfaces. An uninterrupted bilayer is highly surfaceactive and helps the air-alveolar interface to achieve a surface
tension value close to 0 mNm-1 during the lung compression.
Conventional surfactants, however, are also known to have a
high surface activity but surface tension never drops to a value
close to 0 mNm-1 even for a micellar solution.2 The main
difference lies in the membrane-forming ability of lipids.
Phospholipids are known to form a continuous membrane at
an interface,3 whereas conventional surfactants like sodium
dodecyl sulfate or cetyltrimethyl ammonium bromide adsorb
individually. Thus, a continuous membrane of phospholipids is
more surface-active than individually adsorbed conventional
surfactant molecules.
Recent reports4 have suggested that phospholipids also act
as excellent capping agents for the synthesis of noble metal NP.
* Corresponding authors. E-mail: [email protected] and
[email protected].
† Department of Obstetrics and Gynaecology, University of Western
Ontario.
‡ Department of Biochemistry, University of Western Ontario.
§ Department of Chemistry, University of Western Ontario.
| National Institute for Nanotechnology.
⊥ Guru Nanak Dev University.
They provide both steric and charge stabilization to colloidal
metal NP by forming a lipid bilayer on the surface of NP. The
presence of a bilayer thus prevents anisotropic growth and leads
to the formation of 1D monodisperse NP. Not all lipids act as
good capping agents. The capping ability is generated by
electrostatic interactions between the lipid polar head group and
charged NP surface.5 It depends on the nature of the polar head
group, whether it is zwitterionic (PC) or ionic (PG). Anionic
PG lipids are generally sodium salts, whereas PC lipids are
zwitterionic with phospho-choline head groups. An anionic PG
head group is expected to have much-stronger interactions with
a charged metal NP surface in comparison to the zwitterionic
head group of PC. Thus, weak electrostatic interactions show
poor capping ability and consequently leave some crystal planes
uncapped, which become active sites for further nucleation. This
secondary nucleation is very hard to control and consequently
leads to anisotropic growth.6
Bioengineering7 is a fast developing area of fine synergism
between biological molecules and nanometallic surfaces and has
great implications to produce macromolecular machines. The
basis of this arises from a suitable combination between the
two at molecular scale. We have attempted this by synthesizing
Au and Au-Ag bimetallic NP in the presence of bioactive
phospholipids. Recent advances have proven to be a remarkable
success in achieving the formation of nanorods or nanowires.
Murphy and co-workers6a-h have synthesized 1D nanorods
successfully by using a simple and convenient S-G method.
Other groups6i,j as well as our group4f have also reported the
synthesis of Au nanowires. In the present work, we have used
10.1021/jp072862t CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/01/2007
14114 J. Phys. Chem. C, Vol. 111, No. 38, 2007
Bakshi et al.
SCHEME 1 : Molecular Structures of Different PG and PC Phospholipids
a similar S-G method to synthesize lipid-capped Au and AuAg bimetallic NP. Interestingly, this method works very well
for our systems too. A series of PG and PC phospholipids
(Scheme 1) have been selected to evaluate their capping ability
on Au NP and Au-Ag bimetallic NP in order to form
bioconjugate materials. Phospholipid-NP conjugate materials
are foreseen as the basic components for bionanometallic devices
with potential applications in sensors, gene sequencing, and
biocatalysis.
Experimental Section
Materials. Tetrachloroauric acid (HAuCl4), silver nitrate
(AgNO3), sodium borohydride (NaBH4), and trisodium citrate
(Na3Cit) were obtained from Aldrich. 1-palmitoyl-2-oleoyl-snglycero-3-[phospho-rac-(1-glycerol)] (sodium salt) (POPG) (16:
0-18:1), 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (sodium salt) (DPPG) (16:0), 1,2-dimyristoyl-sn-glycero3-[phospho-rac-(1-glycerol)] (sodium salt) (DMPG) (14:0),
1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) (16:
0-18:1), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)
(16:0), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)
(14:0) were procured from Avati Polar Lipids. Ultrapure water
(18 MΩcm) was used for all aqueous preparations.
Preparation of Lipid-Capped Gold Nanoparticles by the
Seed-Growth (S-G) Method. In the S-G method,6a the first
step includes the preparation of a seed solution. First of all, 25
mL of HAuCl4 aqueous solution ([HAuCl4] ) 0.5 mM) was
taken in a screw-capped glass bottle, and Na3Cit was added in
it so as to make a final concentration of [Na3Cit] ) 0.5 mM.
Then, 0.6 mL of aqueous NaBH4 ([NaBH4] ) 0.1 mol dm-3)
solution was added under constant stirring, giving rise to a rubyred color to the final solution, which acts as a seed solution.
The growth solution was prepared by taking 120 µL of lipid (1
mg/mL) in chloroform, in a glass screw-capped tube. The
chloroform was evaporated under the flux of pure N2 leaving a
dried lipid film at the bottom of the tube. In this tube, 5 mL of
pure water was added along with 2-3 small glass beads and it
was vortexed for couple of minutes to completely disperse the
lipid in the aqueous phase. This was followed by the addition
of HAuCl4 so to make [HAuCl4] ) 0.5 mM, which gave a lightyellow color to the solution. Then, 0.5 mL of previously made
seed solution was added in it, and, finally, 0.2 mL of freshly
prepared ascorbic acid (AA) aqueous solution ([AA] ) 0.1 M)
was added. This gave an instant deep-ruby-red color to the
solution. The solution was mixed couple of times by inverting
the bottle and kept in the dark without disturbing it at least for
2 days. The pH of the aqueous NP solution was always close
to neutral in each case.
A similar procedure was adopted to synthesize Au-Ag
bimetallic NP. Here, the addition of AgNO3 was carried out
immediately after the addition of HAuCl4 in the growth solution,
while the rest of the reaction sequence was the same as that
mentioned in the previous paragraph. Three concentrations of
AgNO3, namely, 0.12, 0.25, and 0.5 mM, were used. Purification
of each aqueous sample of lipid-capped NP was carried out by
repeated washing (at least 3 to 4 times) with distilled water
after performing centrifugation at 10 000 rpm for 10 min each
time to remove the maximum uncapped lipid vesicles from the
aqueous phase. It is to be mentioned that no vesicles or
liposomes were observed from TEM studies after the purification
process.
In all of these reactions, an important point to be noted is to
choose whether to use AA or seed at the end of a reaction
sequence because entirely different morphologies of NP appear
in either case. Use of AA at the end only produces a fine pearlnecklace-type arrangement. This point has been discussed in
detail in the Discussion section by taking the example of the
POPG reaction.
Methods. UV-visible spectra of as-prepared solutions were
taken by a UV spectrophotometer (Multiskan Spectrum, model
no. 1500) in the wavelength range of 200-900 nm to determine
the absorbance due to surface plasmon resonance (SPR). The
formation of Au and Ag NP was monitored in the visible
absorption range of ∼530 and ∼410 nm, respectively. The shape
and size of Au and Au-Ag bimetallic NP were characterized
by transmission electron microscopy (TEM). The samples were
prepared by mounting a drop of aqueous NP solution on a
carbon-coated Cu grid and allowed it to dry in air. They were
observed with the help of a Philips CM10 transmission electron
microscope operating at 100 kV. To understand the pearlnecklace morphology of Au-Ag bimetallic NP, high-angle
annular dark-field (HAADF) scanning TEM (with CM20) was
also done. The X-ray diffraction (XRD) patterns were characterized by using Bruker-AXS D8-GADDS with Tsec ) 480. The
chemical composition of some samples containing Au and AuAg bimetallic NP was confirmed with the help of X-ray
photonelectron spectroscopic (XPS) measurements. A portion
of an aqueous sample was placed onto a clean silicon wafer,
Pearl-Necklace-Type Bimetallic Nanoparticles
Figure 1. (a) UV-visible spectra of aqueous gold and gold-silver
bimetallic nanoparticle solutions in the presence of POPG ([POPG] )
0.027 mM). Black arrows indicate the absorbance due to surface
plasmon resonance of nanometallic silver (see details in the text). (b)
Absorbance spectra of identical reactions mentioned in part a except
the addition of seed at the end of the seed-growth reaction. Inset shows
two weak shoulders due to two different kinds of morphologies of Au
NPs (see details in the text).
and then it was put into the introduction chamber of the XPS
instrument. The liquid was then pumped away. The sample was
analyzed by using a Kratos Axis Ultra X-ray photoelectron
spectrometer. XPS can detect all elements except hydrogen and
helium and can probe the surface of the sample to a depth of
7-10 nm. Survey scan analyses were carried out with an
analysis area of 300 × 700 µm.
Results
UV-Vis Measurements. PG-Capped NP. Nanosized Au
particles show absorbance in the UV-visible region due to SPR,
which originates due to the resonance of collective conduction
electrons with incident electromagnetic radiations.8 It provides
a sharp absorbance in the visible region around 520 nm. The
shape of the resonance peak can be qualitatively related to the
nature of NP. Small and uniform-sized NP with a narrow size
distribution gives a sharp absorbance, whereas NP with a wide
size distribution or any kind of aggregation shows a broad
absorbance.8 Ag NP demonstrates this behavior around 410 nm.
Figure 1 shows the UV-visible absorbance of POPG-capped
Au NP in the absence and presence of Ag. The POPG-capped
Au NP in the absence of Ag shows a sharp absorbance at ∼520
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14115
nm (Figure 1a). Addition of AgNO3 at constant lipid concentration has a significant effect on the SPR of Au NP. Increase in
[AgNO3] from 0.12 to 0.5 mM causes a blue shift of ∼40 nm
in the absorbance of Au NP from 520 to 480 nm, while a
shoulder due to the SPR of Ag NP starts appearing at ∼350
nm (shown by arrows). Further increase in the amount of AgNO3
([AgNO3] ) 0.5 mM) makes it quite prominent and red shifts
to 380 nm. It appears that the increase in the amount of AgNO3
induces nucleation of Ag° atoms on the surface of already
available Au NP, thus reducing their SPR.9 This might cause a
blue shift in the absorbance of Au NP and consequently
produces a red shift in the absorbance of Ag NP. It can most
probably be due to some kind of self-association among both
kinds of NP10 in order to form Au-Ag bimetallic NP (we will
come to this point along with TEM images) at the expense of
smaller Ag NP through the Ostwald ripening process.11 Similar
UV-visible spectra were obtained for Au and Au-Ag bimetallic NP when DMPG and DPPG were used as the capping agents
(see the Supporting Information, Figure S1).
A drastic change in the absorbance behavior is observed when
the seed solution is added at the end of the reaction sequence
instead of the AA (see the Experimental Section). Figure 1b
shows the absorbance of an aqueous NP solution obtained from
the identical reactions except adding seed at the end of the
reaction sequence. For pure Au NP, a broad absorbance is
obtained in comparison to the sharp one obtained in Figure 1a
for the same reaction. A close up (inset) indicates that this broad
maximum in fact consists of two shoulders around 535 and 570
nm (indicated by arrows). Both shoulders take the form of broad
maxima around 520 and 620 nm, respectively, in the presence
of [AgNO3] ) 0.25 and 0.5 mM. The first one shows a blue
shift of 15 nm, and the second one red shifts with 50 nm in
comparison to that in the absence of Ag. A blue shift from 535
to 520 nm can be due to the formation of Au-Ag bimetallic
NP9 as demonstrated by a similar NP in Figure 1a, whereas the
red shift in the latter case indicates the presence of some ordered
arrangement.11 Additional rise in the amount of Ag ([AgNO3]
) 0.5 mM) produces only one prominent broad maximum at
∼540 nm with a very-weak hump at ∼420 nm, suggesting the
presence of a large Au-Ag bimetallic NP in an aggregated state.
TEM images will differentiate between the NP obtained by using
different reaction sequences in the next section.
PC-Capped NP. Interestingly, when instead of anionic PG,
corresponding zwitterionic PC lipids (Scheme 1) such as POPC,
DMPC, and DPPC were used, an entirely different behavior of
SPR was observed (see the Supporting Information, Figure S2).
The Au NP synthesized in the presence of all PC lipids show
mainly broad absorbance between 500 and 700 nm when
[AgNO3] ) 0-0.5 mM is used, which indicated the presence
of a large NP with unspecified growth.8e,f
Thus, a marked difference between the SPR of PG- and PCcapped Au or Au-Ag bimetallic NP indicates a fundamental
difference between the capping ability of lipids during their
synthesis. This difference one can easily speculate from the ionic
and zwitterionic nature of PC and PG lipid head groups, which
have to provide charge and steric stabilization10b of colloidal
NP during crystal growth.
TEM Studies. As observed in the previous section that the
capping behavior of PG (Figure 1a) and PC (see the Supporting
Information, Figure S2) lipids produce entirely different kinds
of absorption spectra of Au or Au-Ag bimetallic NP, we have
divided this section into two parts in order to understand that
how the shape and structure of NP is different in the presence
of PG and PC lipids. The first section explains the shape and
14116 J. Phys. Chem. C, Vol. 111, No. 38, 2007
Bakshi et al.
Figure 2. (a) TEM micrographs of POPG-capped gold nanoparticles ([POPG] ) 0.029 mM). Note the POPG bilayer around individual as well as
groups of nanoparticles shown by block arrows. Inset shows a high-resolution image of a single Au NP covered by a 4-nm POPG bilayer. Part b
shows the pearl-necklace type gold-silver bimetallic NP made up of several spindle-shaped particles (indicated by block arrows). Part c shows a
much-smoother arrangement with a coating of POPG bilayer.
Figure 3. Mechanistic point of view of bilayer formation on a citrate-stabilized gold nanoparticle surface.
structure of NP in the presence of PG lipids, and the second
section explains the same in the presence of PC lipids. Apart
from this, the UV-visible spectra also indicate a dramatic
change when AA (Figure 1a) or seed (Figure 1b) is used at the
end of the reaction sequence; therefore, this aspect has also been
explained in detail with TEM studies.
Pearl-Necklace-Type Bimetallic Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14117
Figure 4. (a) High-angle annular dark-field (HAADF) scanning transmission electron microscopic image of a POPG-capped pearl-necklace-type
Au-Ag bimetallic. (b) The corresponding energy-dispersive X-ray spectrum (see details in the text).
PG-Capped NP. POPG as Capping Agent (AA Added Last).
TEM image in the absence of Ag (Figure 2a) mainly shows
spherical POPG capped Au NP with an average size distribution
of 13.9 ( 4.5 nm. The Au NP seems to arrange in a typical
pearl-necklace model.4f A high-resolution image (inset) shows
the presence of a lipid bilayer of ∼4 nm thickness around each
or group of Au NPs (indicated by block arrows). The corresponding lipid membrane studies12 have also reported the
thickness of the lipid bilayer around 4 nm. The bilayer formation
is thought to be achieved due to the following steps (Figure 3).
In the first step, the Na+ counterions of PG lipids interact with
the citrate stabilized negatively charged Au NP surface. This is
followed by electrostatic interactions with anionic lipid molecules to form a monolayer. Because hydrophobic tails cannot
survive in the aqueous phase, they interact with the tails of other
lipid molecules and thus lead to a bilayer arrangement. The
origin of the pearl-necklace arrangement is thought to arise from
the fusion of lipid bilayers of adjoining POPG-capped Au
NPs.13,4f In the presence of [AgNO3] ) 0.12 mM (Figure 2b),
the spherical NPs (15.3 ( 4.9 nm) merge with each other in a
clear pearl-necklace-type arrangement with an average thickness
of 14.5 nm and become much smoother as the amount of AgNO3
reaches 0.5 mM (Figure 2c). Figure 2c also shows that each
arrangement is completely covered with a lipid bilayer. Interestingly, the average size distribution of the Au NPs (Figure 2a)
and the thickness of this arrangement (Figure 2c) are very close
to each other, which probably suggests that fusion among Au
NPs in the presence of Ag forms Au-Ag bimetallic NPs. This
is further proved by the HAADF STEM image (Figure 4a) as
well as its corresponding energy-dispersive X-ray microanalysis
(EDAX) (Figure 4b). One would clearly see many bright discshaped connections (shown by block arrows) made up of Ag
NP probably bound to [111] facets of roughly spherical NP (will
be explained further by XRD later in the Discussion section).
Similarly, because we use DMPG as a capping agent, the
observed morphologies of Au and Au-Ag bimetallic NPs
(Figure 5) are almost identical to those obtained in the presence
of POPG. The spherical Au NPs (11.7 ( 4.7 nm) capped with
DMPG bilayers are arranged in a perfect pearl-necklace model
(Figure 5a) in the absence of Ag and that converts into a fine
arrangement as the amount of Ag increases (Figure 5b and c).
In the case of DPPG (Figure 6), the overall transformation of
Au NPs to a pearl-necklace arrangement essentially follows the
same order as mentioned in the case of POPG and DMPG, but
there are some significant differences as far as the size and shape
of the NPs are concerned. First, the average size distribution of
DPPG-capped spherical Au NPs is equal to 20.3 ( 7.3 nm
(Figure 6a), which is much larger than that obtained for POPG
(13.9 ( 4.5 nm) and DMPG (11.7 ( 4.7 nm) capped Au NP.
Second, the addition of [AgNO3] ) 0.12 mM clearly changes
the spherical Au NPs into polyhedral geometries with size
distributions of 23.7 ( 9.5 nm (Figure 6b). No clear DPPG
bilayer is observed in Figure 6a, whereas broken bilayer is
indeed present around polyhedral NPs (Figure 6b). Increase in
the amount of [AgNO3] ) 0.5 mM transfers the polyhedral
NPs into a pearl-necklace arrangement but with less continuity
and smoothness (Figure 6c). The difference in the morphology
of Au or Au-Ag bimetallic NPs in the presence of DPPG
from POPG and DMPG will be explained in the Discussion
section.
14118 J. Phys. Chem. C, Vol. 111, No. 38, 2007
Bakshi et al.
Figure 5. (a) TEM micrographs of DMPG-capped gold nanoparticles ([DMPG] ) 0.030 mM). (b) Pearl-necklace-type gold-silver bimetallic NP
made up of several spindle-shaped particles. (c) Much-smoother arrangement.
POPG as Capping Agent (Seed Added Last). Entirely different
morphologies of Au and Au-Ag bimetallic NPs are observed
when the seed solution is added at the end of the reaction
sequence. In the absence of Ag, two different kinds of Au NP
morphologies arranged in a clear pearl-necklace model are
observed in the same sample as shown in Figure 7a-c. Figure
7a shows large flower-like Au NP with an average size
distribution of 68.0 ( 11.1 nm (see the Supporting Information,
Figure S3a), whereas Figure 7c contains several groups of
mainly spherical Au NPs with size distributions of 31.9 ( 6.9
nm (see the Supporting Information, Figure S3c). UV-visible
absorbance with two weak shoulders in fact corresponds to these
two different morphologies (Figure 1b, inset). It appears that
the absorbance at ∼570 nm is due to large flower-like NPs,
whereas at ∼535 nm it is due to relatively smaller spherical
NPs. Figure 7b may be an intermediate state between the two.
A close inspection of Figure 7a-c (see the NP in circle in each
case) indicates that there is some fundamental difference
between the capping ability of POPG from one case to another.
Figure 7a suggests anisotropic growth in comparison to predominantly uniform growth in Figure 7c. Recently, some
studies14a,b have reported the synthesis of multibranched starshaped Ag NPs, but a flower-like morphology has never been
observed according to our knowledge. However, almost identical
hyperbranched flower-like Au NPs have been reported recently
by Lou et al.14c They attributed such growth to shape-directing
citrate salt.15 Murphy et al.16 have mentioned that when AA is
added at the end of the S-G reaction the growth of NPs becomes
too slow, but the reaction completes immediately if seed solution
is added at the end. In relevance to a time-dependent adsorption
of phospholipids at immiscible interfaces,17 it seems that the
slow adsorption of POPG on the Au surface is surpassed by
the fast reduction in the later case. This leaves some of the
crystal planes uncapped and consequently leads to anisotropic
growth. On the contrary, slow reduction provides enough time
for POPG to cap the Au surface successfully and thus prevents
any secondary nucleation process.
As we add [AgNO3] ) 0.12 mM, even larger flower-like NPs
(101.2 ( 42.5 nm, see the Supporting Information, Figure S3d)
with greater anisotropic growth are obtained (Figure 7d), which
are connected to other much-smaller dendritic-type NPs (42.7
( 8.3 nm, see the Supporting Information, Figure S3e) (Figure
7e). This sample also shows the presence of small NPs (17.0 (
5.1 nm, see the Supporting Information, Figure S3f) in the form
of the pearl-necklace model (Figure 7f). Two distinct UVvisible absorbances in Figure 1b for this sample are in fact
originating from large and small NPs. Increase in the size of
flower-like NPs from 68.0 (Figure 7a) to 101.2 nm (Figure 7d)
causes a red shift from 570 to 620 nm, whereas the formation
of Au-Ag bimetallic NPs (Figure 7f) brings a blue shift from
535 to 520 nm as observed in Figure 1a. At [AgNO3] ) 0.25
mM, both large flower-like (120.4 ( 46.8 nm, see the
Supporting Information, Figure S3ga) and small dendritic NPs
(50.7 ( 9.0 nm, see the Supporting Information, Figure S3gb)
are present in almost equal amounts (Figure 7ga,b) along with
the presence of a fine pearl-necklace arrangement (Figure 7h).
Further increase in the amount of [AgNO3] ) 0.5 mM reduces
the number of flower-like NPs dramatically. One could see them
converting into groups of small dendritic Au-Ag bimetallic NPs
as shown in encircled regions of Figure 7i. A complete
conversion would produce a clear pear-necklace arrangement
(Figure 7j) of such NPs (58.0 ( 12.4 nm, see the Supporting
Information, Figure S3j) and thus could be the cause of only
one broad absorbance in Figure 1c for this sample.
Pearl-Necklace-Type Bimetallic Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14119
Figure 6. (a) TEM micrographs of DPPG-capped gold nanoparticles ([DPPG] ) 0.028 mM). (b) Pearl-necklace-type gold-silver bimetallic NP
polyhedral nanoparticles. (c) Much-smoother arrangement.
TABLE 1: Shape, Size, and Arrangement of Au and Au-Ag Bimetallic NPs Synthesized in the Presence of Different PG and
PC Phospholipids (AA Added Last)a
POPG
POPC
DMPG
DMPC
DPPG
DPPC
Au +
Ag ) 0 mM
S NP
13.9 ( 6.5 nm
P-N
S NP
10.3 ( 2.4 nm
S NP
11.7 ( 4.7 nm
P-N
P NP
15.3 ( 8.4 nm
S NP
20.3 ( 9.3 nm
P-N
P NP
9.07.3 nm
Au +
Ag ) 0.12 mM
P NP
15.3 ( 7.9 nm
fused P-N
P NP
fused
P NP
13.7 ( 6.7 nm
fused P-N
P NP
fused
P NP
23.7 ( 9.5 nm
fused P-N
P NP
fused
Au +
Ag ) 0.25 mM
P NP
15.8 ( 6.7 nm
fused P-N
P NP
fused
P NP
14.9 ( 7.3 nm
fused P-N
P NP
fused
P NP
24.9 ( 9.1 nm
fused P-N
P NP
fused
Au +
Ag ) 0.5 mM
fused P-N
Th )14.5 6.5 nm
clumps
fused P-N
Th )13.7 5.9 nm
P NP
fused
fused P-N
Th )23.7 ( 8.1 nm
P NP
fused
a
S, spherical; P, polyhedral; P-N, pearl-necklace; Th, thickness.
PC-Capped NP. When POPC is used as a capping agent for
the synthesis of Au NPs; the shape, size, and aggregation
morphology (Figure 8) change dramatically in comparison to
that obtained in the presence of POPG (Figure 2). Figure 8a
shows spherical Au NPs with a size distribution of 10.3 ( 2.4
nm but without any clear pearl-necklace arrangement. Even no
lipid bilayer was observed in this case. As the amount of Ag
increases ([AgNO3] ) 0.25 mM, Figure 8b), the NPs start
merging with each other in a random fashion that subsequently
converts them into large clumpy aggregates at [AgNO3] ) 0.5
mM (Figure 8c). The presence of large aggregates is mainly
responsible for broad UV-vis absorbance (see the Supporting
Information, Figure S2). Using seed at the end of this reaction
sequence gives no flower-like or dendritic NPs. Large polyhedral
14120 J. Phys. Chem. C, Vol. 111, No. 38, 2007
Bakshi et al.
Figure 7. TEM micrographs of flower-shaped (a) and polyhedral (c) gold nanoparticles arranged in a typical pearl-necklace model in the presence
of POPG ([POPG] ) 0.027 mM). The image shown in b is considered to be intermediate between a and c. TEM images of flower-shaped (d),
predominantly dendritic nanoparticle (e), and smoother pearl-necklace-type gold-silver bimetallic NP arrangements in the presence of [AgNO3] )
0.12 mM. Parts g and h show flower-like/dendritic nanoparticles in the presence of [AgNO3] ) 0.25 mM. Part i demonstrates how flower-like
nanoparticles convert into dendritic nanoparticles (j) in the presence of [AgNO3] ) 0.5 mM. All NPs are obtained by using seed at the end of the
reaction sequence (see details in the text for all images).
Au NPs are obtained in the absence of Ag (see the Supporting
Information, Figure S4a) while micrometer-sized Au-Ag
bimetallic plates are observed in the presence of [AgNO3] )
0.5 mM (see the Supporting Information, Figure S4b). Similarly,
when DMPC is used as a capping agent (see the Supporting
Information, Figure S5), the overall morphology of Au and AuAg bimetallic NPs is very much similar to that explained for
POPC. Use of DPPC does not show any ordered morphologies
of Au NPs (see the Supporting Information, Figure S6a),
whereas the addition of Ag produces significant anisotropic
growth (see the Supporting Information, Figure S6b).
Thus, the TEM images fully support the UV-visible results
and demonstrate that all PG lipids used as capping agents herein
produce ordered shapes and structures of Au or Au-Ag
bimetallic NPs when AA is added at the end of the seed-growth
reaction, whereas this is not so when PC lipids are used as
capping agents. All results of TEM studies have been summarized in Table 1 where one can clearly differentiate between
Pearl-Necklace-Type Bimetallic Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14121
Figure 8. (a) TEM micrographs of polyhedral POPC-capped gold nanoparticles ([POPC] ) 0.028 mM). (b) Polyhedral POPC-capped gold-silver
bimetallic nanoparticles with anisotropic growth in the presence of [AgNO3] ) 0.12 mM, and (c) [AgNO3] ) 0.5 mM.
TABLE 2: Binding Energies/eV (I) and Results of XPS Analysis in Atomic Percent (II)
sample
Au
+ (Ag ) 0mM)
Au
+ (Ag ) 0.25mM)
Au
+ (Ag ) 0.5mM)
Au
+ (Ag ) 0mM)
Au
+ (Ag ) 0.25mM)
Au
+ (Ag ) 0.5mM)
a
Au 4f
(I)
(II)
84.15
2.0
84.15
9.6
84.15
7.6
83.95
0.7
83.95
1.4
84.25
2.1
Ag 3d
(I)
368.35
(0.64)a
368.35
(1.05)a
368.05
(1.14)a
368.35
(1.95)a
C 1s
(II)
6.1
8.0
1.6
4.1
(I)
P 2p
(II)
(I)
285.05
(26.2)b
285.75
(4.7)b
285.75
(4.2)b
POPG
52.5 133.15
(0.15)c
45.0 135.25
(0.09)c
31.8 135.25
(0.04)c
283.75
(115)b
283.15
(51.0)b
284.95
(32.0)b
POPC
80.9 131.95
(2.6)c
71.4 131.95
(1.0)c
67.2 134.05
(0.76)c
O 1s
(II)
0.3
0.9
0.3
1.8
1.4
1.6
(I)
Na 1s
(II)
531.5
(19.5)d
532.85
(3.5)d
532.85
(3.25)d
39.1
530.95
(20.0)d
530.35
(13.1)d
531.55
(8.76)d
14.0
33.8
24.7
18.4
18.4
(I)
N 1s
(II)
1069.7
(1.45)e
1071.8
(0.08)e
1071.8
(0.12)e
2.9
1068.5
(0.29)e
1068.8 0.4
(0.29)e
1069.4
(0.19)e
0.2
(I)
(II)
0.8
0.9
400.75
(2.57)f
399.85
(0.71)f
1.8
1.0
0.4
(Ag/Au). b (C/Au). c (P/Au). d (O/Au). e (Na/Au),
the shape and structure of Au or Au-Ag bimetallic NPs
obtained when PG and corresponding PC lipids are used as
capping agents.
XPS Studies. The surface composition of Au NPs is examined
by XPS measurements. Figure 9 shows the XPS spectra of Au
and Au-Ag bimetallic NPs capped with POPG, and similar
spectra for POPC are shown in the Supporting Information,
Figure S7. Table 2 lists the binding energies and atomic percent
values. Each spectrum has been divided into two ranges of
binding energies for the sake of clarity. Figure 9a shows strong
emission peaks due to Au 4f, Au 4d, and Ag 3d electrons,
whereas weak peaks due to Au 4p, Au 4s, Ag 3p, and Ag 3s
are present in Figure 9b. Apart from this, other strong peaks
due to C 1s and O 1s, and weak peaks due to P 2p, are also
visible in each case. As mentioned in the previous sections that
the nature of the lipids significantly influences the morphology
of the NPs, we expect the presence of both kinds of lipids, that
is, PG as well as PC adsorb on the nanometallic surface as
capping agents. In the case of POPG, the emission peaks due
to P 2p (Figure 9a) along with O 1s (Figure 9b) suggest that
PO4- group appears to interact with the positively charged Au
surface (due to the presence of adsorbed Na+ ions on citrate
stabilized Au NP, Figure 3). In the case of POPC, the presence
of both P 2p as well as N 1s emission peaks (see the Supporting
Information, Figure S7) indicates that POPC is interacting with
the Au surface through zwitterionic head groups. Strong C 1s
emission can therefore be due to the presence of hydrocarbon
tails of adsorbed POPG/POPC molecules in each case. To make
14122 J. Phys. Chem. C, Vol. 111, No. 38, 2007
Figure 9. XPS spectra of gold and gold-silver bimetallic nanoparticles
in the presence of POPG.
a qualitative comparison among different samples in the absence
and presence of Ag, we have computed the amount of each
element adsorbed on the Au surface from the respective ratios,
that is, Ag/Au, C/Au, P/Au, O/Au, and Na/Au (Table 2). The
Ag/Au ratio increases as the amount of Ag increases in AuAg bimetallic NPs, but the ratios of rest of the elements decrease
in a similar fashion. This demonstrates that the amount of POPG/
POPC adsorbed on the Au surface actually decreases when Au
NPs turn into Au-Ag bimetallic NPs. This is readily understood
on the basis of two main reasons. First, the addition of Ag would
obviously lead to an increase in the amount of nanometallic
surfaces against the constant amount of POPG/POPC. Second,
the Au-Ag bimetallic surface is not expected to be positively
charged because citrate-stabilized Au NPs would only welcome
the Na+ ions (or ammonium groups in the case of zwitterionic
POPC), whereas neutral Au-Ag bimetallic surfaces should not
have any affinity with charged groups. Thus, the XPS results
clearly demonstrate that Au and Au-Ag bimetallic NPs are
capped with POPG or POPC in their respective samples.
Discussion
All results clearly indicate that two main characteristic
properties of present lipids strongly influence the morphologies
of NPs. The first one is the time-dependence adsorption of lipid
molecules at the liquid-solid interface and the second is the
polarity. A uniform morphology of NPs capped with PG lipids
can be achieved if sufficient time is given for the adsorption of
lipid molecules at the liquid-solid interface under slow growth
Bakshi et al.
conditions. A slow reduction by a weak reducing agent like
AA provides enough time for lipid molecules to adsorb at a
freshly synthesized nanometallic surface. On the contrary, if
the reaction is accelerated by using seed at the end, many crystal
planes remain uncapped and vulnerable for secondary nucleation. This proves that the secondary nucleation process is much
faster than the time taken by lipid molecules to adsorb at the
liquid-solid interface. Therefore, slow reduction by AA is the
most-ideal situation to synthesize Au-Ag bimetallic NPs, which
takes nearly 1 h (see the Supporting Information, Figure S8) to
complete the reaction in comparison to fast reduction. This
provides enough time for lipid molecules to cap the nanometallic
surface and reduce the maximum chances of secondary nucleation. That is why a fine pearl-necklace-type Au-Ag bimetallic
arrangement is obtained for all PG lipids under slow growth
conditions.
Alternatively, all PC lipids promote greater anisotropic growth
rather than PG. This point is directly related to the capping
ability of a lipid molecule. PG lipids are ionic in nature and are
expected to have stronger interactions with electropositive gold
surfaces (because of the adsorbed Na+ ions on the citratestabilized gold surface)4f in comparison to zwitterionic PC lipids
(Scheme 2). This provides relatively less probability for
anisotropic growth in the former case rather than in the latter
case. The stronger adsorption of POPG is very much evident
from the bilayer formation in Figure 2a, whereas no bilayer is
observed in the case of POPC-capped Au NPs (Figure 8a).
Because the amphiphilic property of a stable bilayer is expected
to be the driving force for the formation of pearl-necklace
arrangement due to bilayer fusion,13,4f such an arrangement is
always observed when PG lipids are used as capping agents.
The absence of this arrangement in the case of PC-capped NPs
is simply due to its insufficient capping amount. Apart from
this, the adsorption of POPG molecules are expected mainly
on {100} facets just like that of conventional surfactants such
as SDS and CTAB, and hence would promote the growth on
{111} facets in the presence of Ag18-22 (see the XRD patterns
of Au or Au-Ag bimetallic NP and the variation of relative
intensity in the Supporting Information, Figure S9). On the
contrary, a close-packed bilayer formation in the case of POPC
may not be similar to that of POPG for the following reasons.
First, POPGs have relatively smaller head groups than POPCs
and can therefore better conform to a high curvature where the
head groups occupy smaller areas than the tails.23 Thus,
zwitterionic PC head groups would have much-weaker interactions with charged Au surfaces in comparison to ionic PG head
groups. Second, such interactions may lead to a highly unstable
arrangement on the basis of charge stabilization if the bilayer
arrangement is presumed to be identical to that of the POPGs
(see arrangements a and b in Scheme 2). The zwitterionic head
group is expected to interact with the citrate-stabilized gold
surface through the electropositive ammonium group, but the
same charge repulsions between the adjoining PO4- groups will
not allow an arrangement similar to that of POPGs. Therefore,
the most stable could be the one shown in part c of Scheme 2
where effective packing is possible because the positive charge
on one lipid associates with the negative charge on the
neighboring lipid. This arrangement is mainly responsible for
very-clear crystalline arrays that give spiral structures on airwater interfaces in the case of DPPC.23 This arrangement would
therefore leave some of crystal planes uncapped with PC
molecules and consequently result in anisotropic growth.
As far as a comparison among various PG lipids is concerned,
both POPG and DMPG exhibit almost similar capping abilities
Pearl-Necklace-Type Bimetallic Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 38, 2007 14123
SCHEME 2 : Schematic Arrangement of POPG/POPC Lipid Molecules on the Gold Surface
as evident from the identical morphology of Au and Au-Ag
bimetallic NPs in both cases (Figures 2 and 5, respectively). A
significant change in the shape of DPPG-capped Au NPs in
the presence of [AgNO3] ) 0.12 mM (Figure 6b) indicates a
drastic reduction in the capping ability of DPPG in comparison
to similar cases of POPG- (Figure 2b) and DMPG-capped NP
(Figure 5b). No appropriate explanation can be given at this
point and needs further investigation. But one can speculate on
the basis of the fluidity behavior of the present PG lipids. Both
POPG and DMPG are more-fluid in comparison to DPPG24
because of the presence of unsaturation (one double bond in
the hydrocarbon chain) in the former case and less hydrophobicity (C14) in the latter case. A complete saturation and greater
hydrophobicity (C16) of DPPG with respect to POPG and
DMPG will make it less-fluid. Thus, greater fluidity would
provide easy access for lipid molecules to leave the vesicular
structures for a capping process in comparison to the lesserfluid vesicular structures of DPPG.
Conclusions
The present study concludes that a fine synergism between
phospholipids and noble metal surfaces at the nanoscale level
can be achieved by appropriately selecting both the polarity of
a phospholipid and the reaction conditions. Because of a timedependence adsorption of lipid molecules at the liquid-solid
interface, a proper capping ability can only be achieved under
slow reducing conditions. It has been observed that the addition
of ascorbic acid at the end of the S-G method reduces the
probability of secondary nucleation and thus produces PGcapped fine pearl-necklace-type Au-Ag bimetallic arrangements.
The results also conclude that because of the anionic nature
of PG lipids, they act as wonderful capping agents just like that
of conventional surfactants. Because of their appropriate capping
of {100} crystal planes, the growth is directed at {111} facets
with the results of Au-Ag bimetallic NPs are obtained in each
case. This is not achieved in the case of all PC lipids under
identical growth conditions because of their poor capping ability
that resulted in anisotropic growth of Au-Ag bimetallic NPs.
Thus, the present study is a step forward in synthesizing
organized assemblies of bioconjugate materials made up of
bioactive phospholipids and noble metals (Au or Au-Ag
bimetallic) at the nanoscale level.
Acknowledgment. These studies were supported by Grants
MOP 66406 and FRN 15462 from the Canadian Institutes of
Health Research.
Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
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