Variation in morphology of gold nanoparticles synthesized by the spontaneous reduction of aqueous chloroaurate ions by alkylated tyrosine at a liquid-liquid and air-water interface

Article abstract of DOI:10.1039/b404391j

We demonstrate the formation of gold nanocrystals of different morphologies using alkylated tyrosine (AT) as a reducing agent at a liquid-liquid and air-water interface. The reduction of aqueous chloroaurate ions occurs in a single step wherein the AT mol

Full text of DOI:10.1039/b404391j

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A R T I C L E
Variation in morphology of gold nanoparticles synthesized by the
spontaneous reduction of aqueous chloroaurate ions by alkylated
tyrosine at a liquid–liquid and air–water interface
Anita Swami,a Ashavani Kumar,a Moneesha D’Costa,b Renu Pasrichaa and
Murali Sastry*a
aMaterials Chemistry Division, National Chemical Laboratory, Pune - 411 008, India.
E-mail: sastry@ems.ncl.res.in
bOrganic Chemistry (Synthesis) Division, National Chemical Laboratory,
Pune - 411 008, India
Received 23rd March 2004, Accepted 28th May 2004
First published as an Advance Article on the web 1st July 2004
We demonstrate the formation of gold nanocrystals of different morphologies using alkylated tyrosine (AT) as
a reducing agent at a liquid–liquid and air–water interface. The reduction of aqueous chloroaurate ions occurs
in a single step wherein the AT molecule plays the multifunctional role of a phase transfer, reducing and
capping agent. Gold nanoparticles formed at the air–water interface are very thin, flat sheet or ribbon-like
nanostructures, which are highly oriented in the (111) direction. On the other hand, reduction of aqueous
chloroaurate ions at a liquid–liquid interface by AT molecules present in the organic phase yielded
nanoparticles having predominantly spherical morphology but with no specific crystallographic orientation. The
difference in morphology of the nanoparticles may be due to the different orientational and translational
degrees of freedom of the AT molecules and gold ions at these two interfaces. The AT-capped gold
nanoparticles were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), X-ray
diffraction (XRD), and nuclear magnetic resonance spectroscopy (1H NMR), while the LB films of flat gold
sheets were also studied by X-ray photoemission spectroscopy (XPS).
4-hexadecylaniline (4-HDA) in the one-step synthesis of gold
nanoparticles in non-polar organic media.15 This molecule
plays the role of a phase transfer agent (enabling transfer of
Introduction
Nanoparticles are well-known for their unique size-dependent
optical and electronic properties1 with potential application in
the field of catalysis,2 as electron microscopy markers,3 and in
gene therapy.4 A number of experimental methods have been
reported for the synthesis of metal nanoparticles over a range
of sizes in both polar5–12 (e.g. water) and non-polar13–15
organic solvents. It is being increasingly recognized that
properties of nanomaterials such as their optical/electronic16,17
and catalytic activity18 depend not only on the size of the
particles but also their shape. Therefore, there is considerable
current interest in developing synthesis methods to control the
morphology of nanoparticles. These methods involve the
synthesis of nanoparticles in the presence of templates such
as porous alumina,19,20 polycarbonate membranes,21 carbon
nanotubes,22,23 aqueous surfactant media24,25 and by using a
seeded growth process.26
As far as metal and semiconductor nanoparticles are con-
cerned, most of the experimental methods reported so far are
based on the synthesis of predominantly rod-like nanoparticles
of silver,27,28 gold,24,29,30 CdSe,31 and tungsten sulfide32 with a
tunable aspect ratio and nanoprisms of silver16/gold33 and
CdS.34 In almost all of these protocols, the morphology of the
nanoparticles has been tailored by reducing the metal ions
externally in the presence of different water-soluble surfactants.
In addition, Xia and co-workers have developed methods
for the synthesis of silver nanowires,35a,b nanocubes,35c and
triangular nanoplates35d through a soft, self-seeding, polyol
process. Further to this they have also established the one-step
approach to the large scale synthesis of gold, platinum and
palladium nanostructures with hollow interiors35e–g by the
replacement reaction between silver nanostructures and an
aqueous precursor metal ion solution. Very recently, we have
demonstrated the multifunctional capability of the molecule
2
AuCl4 ions from water to the non-polar organic solvent),
reducing agent and capping molecule for the gold nanoparticles
thus formed.15 While the nanoparticles were predominantly
spherical in nature, their size could be controlled by varying
the metal ion:4-HDA molar ratio in the biphasic reaction
medium.15 Making use of the built-in anisotropy of the air–
water interface, we thereafter showed that Langmuir mono-
layers of 4-HDA on an aqueous subphase of chloroauric acid
resulted in the formation of nano-sheets/-ribbons of gold at the
air–water interface, the 4-HDA Langmuir monolayer present-
ing a truly two-dimensional reducing interface.36 The next
logical step is the rational design of amphiphiles that possess
polar reducing functional groups that can thereafter be used in
the synthesis of metal nanoparticles by the spontaneous
reduction of the corresponding metal ions both at the interface
between water and a non-polar organic solvent and at the air–
water interface.
Our group37,38 and others39 have identified the amino acids
aspartic acid,37 tryptophan38 and tyrosine39 that are capable of
reducing chloroaurate ions under mild experimental conditions
leading to the formation of stable solutions of gold nano-
particles. In this paper, the first step in the rational design
strategy is taken and illustrated using the reducing amino acid
tyrosine suitably alkylated to render it water-insoluble (see the
schematic, inset of Fig. 7 later). Reaction of the alkylated
tyrosine (AT) with aqueous chloroaurate ions at the interface
between water and an organic solvent bearing the AT mole-
cules both under vigorous stirring conditions leads to electro-
2
static complexation of AuCl4 ions with the protonated
amine groups of AT, phase transfer of the gold ions into the
organic solvent followed by reduction of the metal ions and
formation of highly stable gold nanoparticles [Scheme 1(A)] of
2 6 9 6
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 6 9 6 – 2 7 0 2
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Scheme 1 (A) Diagram showing, step 1: hydrophobization and phase transfer of chloroaurate ions from an aqueous to a chloroform phase by
stirring the aqueous chloroauric acid solution with the solution of alkylated tyrosine (AT) in chloroform; step 2: reduction of chloroaurate ions
(entrapped in AT reverse micelle) by AT and formation of spherical gold nanoparticles capped and stabilized by oxidized AT. (B) Formation of flat
gold nanoribbons/nanosheets at the air–water interface by the spontaneous reduction of chloroaurate ions by the AT Langmuir monolayer (see text
for details).
ca. 42 ¡ 13 nm diameter. The AT molecules may also be
assembled on the surface of aqueous chloroauric acid as the
subphase. The reduced orientational and translational degrees
of freedom experienced by the AT molecules in the Langmuir
monolayer leads to the highly localized reduction of gold
ions electrostatically bound to the interface and the formation
of nanosheets and nanoribbons [Scheme 1(B)] of highly
(111)-oriented gold at the interface. The anisotropic gold
nanostructures may be assembled as multilayers by the
Langmuir–Blodgett method on suitable substrates. An advant-
age of this rational design strategy that we hope to exploit in
forthcoming studies is the additional degree of freedom to
change the size and shape of the reducing headgroup thereby
changing the crystallography of the nucleating metal phase
at the air–water interface. Presented below are details of the
study.
by octadecylamine (0.16 g, 0.59 mmol). The solution was
stirred at room temperature under a nitrogen atmosphere
overnight. TLC indicated formation of the product. The
solvent was removed under vacuum and the residue taken up in
aqueous sodium bicarbonate (15 mL) and extracted with ethyl
acetate (15 mL 6 3). The organic extracts were dried over
sodium sulfate and evaporated to obtain 0.49 g crude product,
which was purified by column chromatography on silica gel to
get 0.28 g pure product (83.5% yield).
1H NMR (CDCl3): d 7.39 (m, 5H), 7.09 (d, 2H, J ~ 8.06 Hz),
6.91 (d, 2H, J ~ 8.06 Hz), 5.63 (br s, 1H), 5.02 (br s, 3H), 4.18
(m, 1H), 3.12 (m, 2H), 2.95 (m, 2H), 1.40 (s, 9H), 1.24 (m, 32H),
0.87 (t, 3H, J ~ 6.59 Hz).
(b) N-(tert-Butyloxycarbonyl)-tyrosinyl octadecylamine. To
a solution of N-(tert-butyloxycarbonyl)-O-(benzyl)-tyrosinyl
octadecylamine (0.09 g, 0.14 mmol) in methanol (5 mL) was
added 10% Pd–C (0.01 g, 11% by weight). The reaction mixture
was hydrogenated in a Parr hydrogenation apparatus for 4 h at
50 psi. TLC examination indicated completion of the reaction.
The catalyst was filtered off and the filtrate was concentrated
under vacuum to obtain N-(tert-butyloxycarbonyl)-tyrosinyl
octadecylamine (0.076 g, 99% yield) as a pure white solid.
1H NMR (CDCl3): d 7.00 (d, 2H, J ~ 8.79 Hz), 6.74 (d, 2H,
J ~ 8.79 Hz), 5.83 (m, 1H), 5.13 (m, 1H), 4.21 (m, 1H), 3.14 (m,
2H), 2.94 (m, 1H), 1.41 (s, 9H), 1.30 (m, 2H), 1.24 (s, 30H), 0.86
(t, 3H, J ~ 6.59 Hz).
Experimental details
A. Materials
The following chemicals were used as obtained: octadecylamine
(C18H39NH2) from Aldrich Chemicals Co.; hydrogen tetra-
chloroaurate trihydrate (chloroauric acid, HAuCl4?3H2O),
chloroform (certified A.R. reagent) and 99% ethanol from
S.D. Fine Chemicals Ltd. and L-tyrosine (C9H11NO3) from
Sisco Research Laboratory.
B. Synthesis of alkylated tyrosine (AT)
(c) Tyrosinyl octadecylamine. The N-(tert-butyloxycarbonyl)-
tyrosinyl octadecylamine (0.08 g, 0.15 mmol) was dissolved in
dry dichloromethane (1 mL). To this, 1 mL of trifluoroacetic
acid was added and stirring was continued at room tempera-
ture for 20 min, when TLC examination indicated completion
of the reaction. The solvents were completely removed by
(a) N-(tert-Butyloxycarbonyl)-O-(benzyl)-tyrosinyl octadecyl-
amine. To a stirred clear solution of N-(tert-butyloxycarbonyl)-O-
(benzyl)-tyrosine (0.20 g, 0.53 mmol) and 1-hydroxybenzotriazole
(HOBt) (0.14 g, 1.036 mmol) in anhydrous DMF (5 mL) was
added diisopropylcarbodiimide (0.1 mL, 0.64 mmol), followed
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 6 9 6 – 2 7 0 2
2 6 9 7

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evaporation under vacuum and co-evaporation with dichloro-
methane. A gummy product (0.08 g, 97% yield) was obtained
which solidified on standing.
1H NMR (CDCl3 1 DMSO-d6): d 6.89 (d, 2H, J ~ 8.75 Hz),
6.62 (d, 2H, J ~ 8.34 Hz), 3.90 (t, 1H, J ~ 7.15 Hz), 2.91–3.04
(m, 2H), 2.90 (d, 2H, J ~ 7.15 Hz), 1.23 (m, 2H), 1.08 (s, 30H),
0.71 (t, 3H, J ~ 6.75 Hz and 7.16 Hz).
out on a VG MicroTech ESCA 3000 instrument at a pressure
of better than 1 6 1029 Torr. The general scan, C 1s and Au 4f
core level spectra were recorded with un-monochromatized
Mg-Ka radiation (photon energy ~ 1253.6 eV) at a pass energy
of 50 eV and an electron takeoff angle (the angle between the
electron emission direction and the surface plane) of 60u. The
overall resolution of measurement is thus ca. 1 eV for the XPS
measurements. The core level spectra were background
corrected using the Shirley algorithm40 and the chemically
distinct species resolved using a non-linear least squares pro-
cedure. The core level binding energies (BEs) were aligned with
the adventitious carbon binding energy of 285 eV.
C. Synthesis of gold nanoparticles at a liquid–liquid interface
In a typical experiment, 50 mL of a 1 6 1024 M aqueous
solution of HAuCl4 was taken in a beaker along with 50 mL
solution of 2 6 1024 M AT in chloroform. Considering AT as
a 2 e2 donor, three equivalents of AT are required for the
2
Results and discussion
reduction of two equivalents of AuCl4 ions and hence the
ratio of AT:Au(III) was maintained at 2:1 where AT molecules
are present in excess. The biphasic mixture was stirred
vigorously on a magnetic stirrer for 6 h and resulted in a
reddish pink color in the organic phase. The chloroform was
removed by rotary evaporation, the sample was washed
repeatedly with ethanol and the gold nanoparticles obtained
as a dry powder. The AT-capped gold nanoparticles could be
readily redispersed in chloroform, hexane and toluene and were
characterized by different techniques.
The inset of Fig. 1A shows a picture of test tubes containing the
biphasic mixtures of aqueous HAuCl4 and AT in chloroform
before (test tube a) and after the reaction for 6 h under
continuous stirring at room temperature (test tube b). The
chloroform phase, which is initially colorless takes on a reddish
pink hue after reaction indicating formation of gold nano-
particles in the organic phase. Fig. 1A shows the UV-vis
spectra recorded from the organic phase as a function of time
of reaction of aqueous chloroaurate ions with AT in chloro-
form. Curves 1, 2, 3 and 4 correspond to the spectra recorded
after 0, 2, 4 and 6 h of reaction, respectively. The increase in the
surface plasmon resonance centered at ca. 535 nm with time
indicates formation of gold nanoparticles in the chloroform
phase and that the density of the particles increases steadily
during this period of reaction. We would like to mention that
after 6 h of reaction, the intensity of the surface plasmon
resonance does not increase further, indicating complete reduc-
tion of the chloroaurate ions in this time interval. Under
stirring conditions, the amine groups in the AT molecules are
expected to be fully protonated leading to electrostatic com-
D. Synthesis of gold nanoparticles at the air–water interface
In a typical experiment, 75 mL of AT in chloroform (con-
centration 1 mg mL21) was spread on an aqueous 1 6 1024
M
HAuCl4 solution in a Nima model 611 LB trough. A standard
Wilhelmy plate was used for surface pressure sensing. Surface
pressure–area (p–A) isotherms were recorded at room tem-
perature at compression and expansion rates of 50 cm2 min21
at different times after spreading of the AT monolayer. After
measurement of p–A isotherms, the AT monolayer was
compressed to a surface pressure of 20 mN m21 and
maintained at this pressure for 36 h. After complete reduction
2
plexation with AuCl4 ions at the water–chloroform interface
2
followed by transfer of AuCl4 ions from the aqueous to the
2
of AuCl4 ions, multilayer films of the AT-reduced gold
2
chloroform phase. We believe that the AuCl4 ions become
entrapped in the hydrophilic wells of the reverse micelle-like
nanoparticles of different thickness were formed by the
Langmuir–Blodgett (LB) technique at a surface pressure of
15 mN m21 and a deposition rate of 20 mm min21 with a
waiting time of 5 min between dips on carbon-coated trans-
mission electron microscopy (TEM) grids, quartz and Si (111)
substrates for TEM, UV-vis spectroscopy, X-ray diffraction
(XRD), and X-ray photoemission spectroscopy (XPS) mea-
surements, respectively. The Si (111) and quartz substrates
were hydrophobized by depositing three monolayers of lead
arachidate prior to transfer of the AT-reduced gold nano-
particle monolayers. The hydrophobization of the support
resulted in better transfer ratios of the nanoparticle mono-
layers. For the LB films grown on different substrates,
monolayer transfer was observed both during the upward
and downward strokes of the substrate at close to unity transfer
ratio.
UV-vis spectroscopy measurements of all the samples were
performed on a Hewlett-Packard HP 8542A diode array
spectrophotometer operated at a resolution of 2 nm. XRD
patterns of the LB films and gold nanoparticles synthesized in
chloroform and solution-cast on glass substrates were recorded
on a Phillips PW 1830 instrument operating at a voltage of
40 kV and a current of 30 mA with Cu-Ka radiation. TEM
measurements on the gold nanoparticle films cast onto carbon-
coated TEM grids were carried out on a JEOL instrument
model 1200EX at an accelerating voltage of 60 kV. Proton
NMR spectra of the solutions of gold nanoparticles synthe-
sized at both liquid–liquid and air–water interfaces were
recorded on a Bruker AC 200 MHz instrument and scanned in
the range 0–15 ppm. For comparison, the proton NMR
spectrum of AT in CDCl3 1 DMSO-d6 was also recorded. XPS
measurements on the gold nanoparticle LB films were carried
Fig. 1 (A) UV-vis spectra of the chloroform phase recorded as a
function of time of reaction of aqueous 1 6 1024 M HAuCl4 solution
with a 2 6 1024 M chloroform solution of AT. Curves 1, 2, 3 and 4 in
the figure correspond to the spectra recorded at time t ~ 0, 2, 4 and 6 h
of reaction, respectively. The inset shows a picture of the test tubes
containing the biphasic mixtures of AT in chloroform (lower phase)
and aqueous HAuCl4 (upper phase) before (test tube a) and after (test
tube b) the reaction. (B) UV-vis spectra of 1 6 1024 M aqueous
HAuCl4 solution after reaction with 2 6 1024 M tyrosine at room
temperature (curve 1) and under boiling conditions (curve 2). Curve 3
corresponds to the UV-vis spectrum recorded from the chloroform
phase after reaction of AT with aqueous AuCl42 ions at a liquid–liquid
interface at room temperature (identical to curve 4 in Fig. 1A).
2 6 9 8
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 6 9 6 – 2 7 0 2

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structures formed by the AT molecules, which enables the
transfer of gold ions from the aqueous to the organic phase
as shown in Scheme 1(A). The gold nanoparticle solutions in
chloroform were extremely stable over time, indicating effective
capping of the nanoparticles by AT.
structure, presumably during evaporation of the solvent,
chloroform. It is observed from this TEM image that although
close-packed, the gold particles are well-separated from each
other with an apparently uniform inter-particle separation.
This indicates that the gold nanoparticles are capped by an
oxidized product of AT that prevents physical contact between
the particles and also provides sufficient hydrophobicity to the
gold nanoparticles to enable their dispersion in chloroform. At
higher magnification, the morphology of the gold nanoparticles
is observed more clearly (Fig. 2B). Fig. 2C is a plot showing the
particle size distribution (PSD) histogram measured from 150
particles of Fig. 2A and other similar images. The solid line in
Fig. 2C is a Gaussian fit to the PSD histogram and this
yielded an average particle size of 42 ¡ 13 nm. Fig. 2D
shows the selected area electron diffraction pattern recorded
from the gold nanoparticles shown in Fig. 2A. A number of
diffuse rings are observed in the electron diffraction pattern
and could be indexed based on the fcc structure of gold. The
presence of diffuse rings suggests that the gold nanoparticles
are polycrystalline.
It would be instructive to compare the rate of reaction of
aqueous gold ions directly with the amino acid tyrosine, which
has been implicated in the reduction of gold ions by silk fibroin
proteins.39 Fig. 1B shows the UV-vis spectra recorded from
1 6 1024 M aqueous chloroauric acid solution after reaction
with 2 6 1024 M tyrosine for 6 h at room temperature (curve 1)
and under boiling conditions (curve 2). For comparison, the
UV-vis spectrum of gold nanoparticles formed in chloroform
by the reaction of aqueous chloroaurate ions with alkylated
tyrosine at room temperature as discussed earlier is shown
(curve 3). The absence of a clear surface plasmon resonance in
the case of the room temperature reduction of gold ions by pure
tyrosine (curve 1) indicates that tyrosine does not readily
2
reduce AuCl4 ions under these conditions. Boiling the gold
ion–tyrosine reaction mixture does lead to the formation of
gold nanoparticles in solution (curve 2) but the presence of a
broad absorption band indicates considerable aggregation of
the gold nanoparticles in solution. A highlight of this part of
the work is the finding that after alkylation, tyrosine is capable
of rapid and facile reduction of gold ions at room temperature
(curve 3). The exact reason for the increase in reducing
capability of tyrosine after alkylation is not fully understood.
As will be shown below, Langmuir monolayers of AT on
aqueous chloroauric acid solution also lead to facile reduction
of gold ions and therefore the role of the solvent, chloroform,
in enhancing the reduction rate of gold ions by AT may be
ruled out. Currently, studies are under way to establish whether
alkylation of tyrosine at different sites leads to a variation in its
reducing power.
Fig. 3A shows the XRD pattern recorded from a drop-
coated film of AT-stabilized gold nanoparticles synthesized in
chloroform on a glass substrate. From the XRD pattern,
prominent Bragg reflections at 2h values of 38.2, 42.2, 66.4 and
79.5u are observed which correspond to the (111), (200), (220)
and (311) Bragg reflections of fcc gold, respectively, and are
thus in agreement with the electron diffraction results discussed
above.41 It is clear from the above measurements that the AT
molecule reduces the AuCl42 ions at the liquid–liquid interface
leading to the formation of stable gold nanoparticles capped by
oxidized AT in chloroform.
The above study shows that AT molecules reduce chlor-
oaurate ions spontaneously and lead to the formation of
hydrophobized gold nanoparticles of nearly spherical shape
dispersible in chloroform. The reducing capability of AT
coupled with the in-built anisotropy of the air–water interface
provides one with the interesting option of using Langmuir
monolayers of AT in the highly localized formation of gold
nanoparticles by the spontaneous reduction of chloroaurate
ions in the subphase. Fig. 4 shows pressure–area (p–A)
isotherms recorded at room temperature and at a compression
and expansion rate of 50 cm2 min21 at time intervals of 0, 2, 6, 10
Drop-coated films of AT-capped gold nanoparticles were
prepared by solvent evaporation on carbon-coated copper
TEM grids and analyzed by transmission electron microscopy
(Fig. 2). At low magnification, a number of highly polydisperse
gold nanoparticles possessing a variety of shapes are observed
(Fig. 2A). The gold nanoparticles assemble into a close-packed
and 20 h after spreading the AT monolayer on the 1 6 1024
M
aqueous HAuCl4 solution as the subphase. The subphase is at a
pH of 3.7 and would thus lead to protonation of the amine
groups in the AT Langmuir monolayer. This would result in
2
electrostatic complexation of AuCl4 ions with the cationic
Fig. 2 (A,B) Representative TEM pictures of AT-reduced gold nano-
particles synthesized in chloroform (see text for details). (C) Particle
size distribution histogram of more than 150 particles shown in images
similar to Fig. 2A. (D) Selected area electron diffraction pattern
recorded from the gold nanoparticles shown in Fig. 2A.
Fig. 3 (A) XRD pattern recorded from a drop-coated film of
AT-reduced gold nanoparticles in chloroform (see text for details).
(B) XRD pattern recorded from a 24 monolayer (ML) LB film of
AT-reduced gold nanoparticles at the air–water interface deposited on
a glass substrate.
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2 6 9 9

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Fig. 4 p–A Isotherms recorded during one compression and expan-
sion cycle of the AT Langmuir monolayer on the surface of the aqueous
HAuCl4 subphase as a function of time of spreading of the monolayer.
The times at which the isotherms were recorded are indicated next to
the respective curves.
Langmuir monolayer in a manner similar to that occurring at
the liquid–liquid interface. A large expansion of the monolayer
2
is observed during the compression from ca. 37 A molecule
21
˚
at time t ~ 0 h to ca. 55 A molecule21 at t ~ 20 h. The liftoff
2
˚
2
area after equilibration of the AuCl4 ions at the air–water
Fig. 5 (A–D) Representative TEM images recorded at different
magnifications from one monolayer of AT-reduced gold nanoparticles
formed at the air–water interface and transferred onto a carbon-coated
TEM grid by vertical lifting.
2
interface (determined to be 55 A molecule21) is considerably
˚
larger than the liftoff area for the uncomplexed AT molecules
(ca. 35 A molecule21), which indicates complexation of
2
˚
2
AuCl4 ions to the charged AT monolayer. It was observed
surrounded by AT molecules) takes place in the chloroform
phase and not at the water–chloroform interface. Thus we
speculate that the dramatic morphological variation of nano-
structures formed at liquid–liquid and air–water interfaces is
due to the different degrees of freedom (orientational and
translational) experienced by the AT molecules and gold ions
at the two interfaces as clearly shown in Scheme 1. The
nanoribbons/nanosheets formed at the air–water interface
could be transferred by the Langmuir–Blodgett method on
glass and quartz substrates for further study by XRD and
UV-vis spectroscopy.
that the AT monolayer collapsed at a surface pressure close to
23 mN m21 and, consequently, the p–A isotherms with time
were recorded to a limiting pressure of 20 mN m21 (Fig. 4).
From the p–A isotherm measurements, a region of reasonable
incompressibility is seen to occur up to surface pressures of ca.
20 mN m21 and, therefore, multilayer films of the AT–Au
complex of different thickness were transferred onto hydro-
phobized quartz, TEM grids and Si (111) substrates for
additional analysis at a surface pressure of ca. 15 mN m21
.
The nature of the gold nanostructures formed at the air–
water interface was studied by recording the TEM images of
a monolayer of the AT-complexed particles after complete
reduction of chloroaurate ions. Fig. 5A–D shows representa-
tive TEM images of 1 ML (monolayer) of the AT–gold
nanofilm recorded 36 h after spreading of the AT monolayer on
the surface of the 1 6 1024 M HAuCl4. It is clear from the
images that a large concentration of flat gold nanoribbons/
nanosheets is obtained at the air–water interface, in contrast to
the spherical gold nanoparticles obtained by the reduction of
gold ions by AT in chloroform. The dark regions observed in
the nanostructures may be due to folding/buckling of the
ribbons/sheets during growth or deposition of the LB film.
Regions of overlap between nanosheets are also observed to
lead to variation in contrast, indirectly suggesting that the
nanosheets are extremely thin. We have observed the formation
of similar kinds of ribbons/sheets in our previous study in
which a hexadecylaniline Langmuir monolayer was used on the
subphase of an aqueous solution of chloroauric acid.36 In case
of the air–water interface, reduction of gold ions is localized to
a quasi two-dimensional surface since the reducing AT mole-
cules are restricted to the interface owing to their amphiphi-
licity. This leads to a high degree of anisotropy in the shape of
the nanostructures formed [Scheme 1(B)]. In the case of the
liquid–liquid interface, initially the AT molecules are concen-
trated at the interface because of their amphiphilicity. However
Fig. 3B shows the XRD pattern recorded from a 24 ML LB
film of the gold nanosheets/nanoribbons transferred onto a
glass substrate. This film shows a very strong gold (111) and
(222) Bragg reflections at 2h ~ 38.2 and 81u, respectively. The
peaks corresponding to the (200) and (220) reflections are very
weak indicating that the growth is highly face-selective. It is
clear from the comparison of Fig. 3A and 3B that highly
oriented growth of the gold nanocrystals occurs in the case of
gold ions reduced at the air–water interface by the AT
Langmuir monolayer. In the case of gold nanoparticle
synthesis at a liquid–liquid interface there is no such specific
orientation. This result is consistent with the TEM results that
show the different morphologies of the gold nanocrystals in
the two experiments. In order to check whether formation of
highly (111)-oriented, flat gold nanosheets/nanoribbons at the
air–water interface is due to epitaxy between the AT Langmuir
monolayer and the nucleating gold nanocrystals, efforts were
made to break the periodicity of the AT Langmuir monolayer
by mixing it with the surfactant, octadecylamine (ODA), in
different ratios. p–A Isotherms of mixtures of AT and ODA in
the molar ratios 1:1 and 1:3 on an aqueous HAuCl4 subphase
did not show any evidence of phase separation and confirms
the breakage in periodicity of the AT Langmuir monolayer.
The nanogold films formed using these mixed monolayers were
transferred onto glass and analyzed by X-ray diffraction. The
XRD patterns of these LB films (formed from AT:ODA mixed
Langmuir monolayers) also showed very strong (111) Bragg
peaks indicating similar face-selective growth in the (111)
direction. Thus, this control experiment suggests that the
2
under vigorous stirring conditions, complexation of AuCl4
ions with AT results in hydrophobization and transfer of
AuCl42 ions from water to the chloroform phase [Scheme 1(A)]
and hence further reduction of gold ions (symmetrically
2 7 0 0
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BE component is attributed to electron emission from Au
metal41 while the smaller, high BE component arises from the
presence of unreduced Au31 ions in the LB film. The XPS
results thus show that a major fraction of the AuCl42 ions are
reduced to metallic Au by the AT Langmuir monolayer. Based
on charge neutrality considerations, it would be expected that
2
one AuCl4 ion would bind to one protonated AT molecule.
However, since the conversion of Au31 to Au is a 3e2 transfer
0
process, it is expected that three equivalents of AT molecules
are required for the reduction of two equivalents of gold ions
2
(considering AT as a 2e2 donor). Hence all the AuCl4 ions
0
bound to the interface would not be fully reduced to Au by the
AT Langmuir monolayer, evidence of which is clearly seen by
2
the presence of the fraction of unreduced AuCl4 ions in the
LB films identified by XPS.
In order to analyse the oxidation product of AT molecules
Fig. 6 (A) UV-vis spectra recorded from LB films of AT-reduced gold
nanosheets of different thicknesses (number of monolayers indicated
next to the respective spectrum) deposited on quartz. The inset is a plot
of the intensity of the surface plasmon resonance maximum
(absorbance at 612 nm) as a function of the number of monolayers
of AT-capped gold nanoparticles in the LB films deposited on quartz.
The solid line is an aid to the eye and has no physical significance.
(B) Au 4f core level spectrum recorded from a 24 ML AT-reduced gold
nanoparticle LB film deposited on a Si (111) substrate and stripped into
two chemically distinct spin–orbit components (see text for details).
2
formed consequent to the reduction of AuCl4 ions, AT-
capped nanoparticles were separated out from the organic
solvent and also from the LB films and redispersed in CDCl3 1
1
DMSO-d6 for H NMR spectroscopic measurements. Curves
1, 2 and 3 in Fig. 7 correspond to the 1H NMR spectra
recorded from pure AT, AT-capped gold nanoparticles formed
at the liquid–liquid interface and LB films of AT-reduced gold
nanostructures formed at the air–water interface dispersed in
CDCl3 1 DMSO-d6, respectively. The NMR spectrum of AT
(curve 1) shows a number of prominent resonances that are
identified in the schematic given in the inset of the figure. The
AT molecule shows chemically shifted protons at 6.8 and
6.6 ppm in curve 1 and are assigned to the two sets of aromatic
protons labeled j and i, respectively, in the schematic. The
chemically shifted peaks centered at 2.9–3.0 ppm, 1.2, 1.0 and
0.7 ppm correspond to the a-CH2, b-CH2, methylene and
methyl protons in the C18 hydrocarbon chain, respectively. In
curves 2 and 3 (AT-reduced gold nanoparticles at the liquid–
liquid and air–water interfaces, respectively) there is a complete
disappearance of the peaks at 6.8, 6.6, 2.9–3.0 and at 1.2 ppm.
Disappearance of signatures of aromatic protons in the
1H NMR spectra indicate polymerization of AT following
formation of highly anisotropic, (111)-oriented flat nanostruc-
tures at the air–water interface is not the consequence of
epitaxy but most probably due to surface energy considerations
[the (111) crystallographic face is the most energetically
favorable crystal face] and stabilization of the nanosheets/
nanoribbons by the oxidized product of AT. The data
pertaining to the control experiment have not been shown
for brevity.
An important advantage of the synthesis and organization of
nanoparticles at the air–water interface is that multilayer
superlattice films of close-packed nanoparticle assemblies may
be deposited by the LB method. Fig. 6A shows the UV-vis
spectra recorded from LB films of the AT–gold nanoribbon
complexes of different thicknesses deposited on quartz sub-
strates. A broad absorption band centered at ca. 610 nm is
observed that increases in intensity with increasing number of
monolayers in the LB film. The absorption at 610 nm is due to
excitation of surface plasmon vibrations in the gold nanopar-
ticles. This absorption band is red shifted and broadened
relative to the narrow absorption band at 534 nm observed for
gold nanoparticles synthesized in chloroform. The shift and
broadening of the resonance in the LB films indicates
considerable aggregation of the particles and is also supported
by the TEM results. The inset of Fig. 6A shows the intensity of
absorption at 610 nm plotted as a function of the number of
monolayers. The monotonic and almost linear increase in the
intensity of the absorption band with film thickness indicates
that similar amounts of the nanogold–AT monolayers are
deposited during each cycle, which may be possibly due to
growth of gold multilayers in a lamellar fashion.
2
the reduction of AuCl4 ions. Oxidative oligomerization/
polymerization of aromatic diamines in the presence of
oxidants such as salts of various transition metals is well-
known in the literature.42 Thus, we believe that the formation
of gold nanoparticles at the liquid–liquid and air–water inter-
faces is accompanied by the polymerization of AT. It is clear
from the NMR results that unreacted AT is not present on
the gold nanoparticles formed at both the liquid–liquid and
A 24 ML AT-capped gold nanoparticle LB film was depo-
sited on a Si (111) substrate and analyzed by XPS. The general
scan spectrum showed the presence of strong C 1s, N 1s and Au
4f core levels with no evidence of impurities. The film was
sufficiently thick and, therefore, no signal was measured from
the substrate (Si 2p core level). The Au 4f core level recorded
from the LB film deposited 36 h after spreading the AT
monolayer on aqueous chloroauric acid solution is shown in
Fig. 6B. The spectrum was background corrected using the
Shirley algorithm40 prior to curve resolution. The Au 4f core
level spectrum recorded from the LB film could be stripped into
two spin–orbit pairs. The two chemically distinct Au 4f7/2
components were centered at binding energies (BEs) of 84.5
and 87.5 eV (labelled 1 and 2 in Fig. 6B, respectively). The low
Fig. 7 1H NMR spectra recorded from pure AT (curve 1); AT-
reduced gold nanoparticles at the liquid–liquid interface synthesized in
chloroform and then dispersed in CDCl3 1 DMSO-d6 (curve 2);
AT-reduced gold nanoparticle LB film after dissolving in CDCl3 1
DMSO-d6 (curve 3). The inset is a schematic of the AT molecule used in
this study identifying the protons that appear in the NMR spectra.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 6 9 6 – 2 7 0 2
2 7 0 1

View Article Online
15 P. R. Selvakannan, S. Mandal, R. Pasricha, S. D. Adyanthaya and
M. Sastry, Chem. Commun., 2002, 1334.
16 R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and
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17 T. Huang and R. W. Murray, J. Phys. Chem. B, 2001, 105, 12 498.
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19 B. R. Martin, D. J. Dermody, B. D. Reiss, M. M. Fang, L. A. Lyon,
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air–water interfaces. At this stage, the extent of polymerization
is not comprehensible from 1H NMR studies. This would
require separation of the polymer from the nanoparticles for
detailed analysis and is currently under study.
In summary, it has been shown that the ability of the amino
acid tyrosine to reduce gold ions can be enhanced by alkyla-
tion. Alkylation of tyrosine enables the use of this multi-
functional molecule in the spontaneous reduction of aqueous
gold ions at both the liquid–liquid and air–water interfaces
with dramatic differences in the morphology of the gold
nanoparticles formed. The rational strategy developed for truly
two-dimensional reduction of metal ions at an interface is
currently being extended to control the crystallography of the
reducing surface and thereby controlling the crystallography of
the nucleating nanoparticle face.
24 K. Esumi, K. Matsuhisa and K. Torigoe, Langmuir, 1995, 11,
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25 Y. Y. Yu, S. S. Chang, C. L. Lee and C. R. C. Wang, J. Phys.
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Acknowledgements
A. S. and A. K. would like to thank the Council for Scientific
and Industrial Research (CSIR), Govt. of India, for research
fellowships. This work was partially funded by a grant from the
Department of Science and Technology (DST), Govt. of India
and is gratefully acknowledged.
29 K. R. Brown, D. G. Walter and M. J. Natan, Chem. Mater., 2000,
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J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 6 9 6 – 2 7 0 2