Dipole-dipole plasmon interactions in self-assembly of gold organosol induced by glutathione

Article abstract of DOI:10.1039/b607399a

Assemblies of gold nanoparticles in an organic medium have been synthesized to study the plasmon-plasmon interactions amongst the gold nanoparticles. A pH-sensitive biomolecule, glutathione (GSH), has been introduced as a molecular linker of the 'parent' gold nanoparticles to obtain small nanoparticle aggregates. The optical spectra of gold nanoparticles shifted to the red region indicate dipole-dipole interactions in the gold particle assembly. The aggregates have been characterized by UV-Vis, FTIR, HRTEM and XRD techniques. A controlled method of aggregating gold nanoparticles in organic solvents has been achieved successfully under controlled pH conditions with different concentrations of the molecular linker, GSH. The pH dependent anchoring of GSH onto gold surfaces has been proved beyond doubt to bring about nanoparticle aggregation. the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006.

Full text of DOI:10.1039/b607399a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Dipole–dipole plasmon interactions in self-assembly of gold organosol
induced by glutathione
Soumen Basu, Sudipa Panigrahi, Snigdhamayee Praharaj, Sujit Kumar Ghosh,
Surojit Pande, Subhra Jana and Tarasankar Pal*
Received (in Durham, UK) 25th May 2006, Accepted 20th July 2006
First published as an Advance Article on the web 10th August 2006
DOI: 10.1039/b607399a
Assemblies of gold nanoparticles in an organic medium have been synthesized to study the
plasmon–plasmon interactions amongst the gold nanoparticles. A pH-sensitive biomolecule,
glutathione (GSH), has been introduced as a molecular linker of the ‘parent’ gold nanoparticles
to obtain small nanoparticle aggregates. The optical spectra of gold nanoparticles shifted to the
red region indicate dipole–dipole interactions in the gold particle assembly. The aggregates have
been characterized by UV-Vis, FTIR, HRTEM and XRD techniques. A controlled method of
aggregating gold nanoparticles in organic solvents has been achieved successfully under controlled
pH conditions with different concentrations of the molecular linker, GSH. The pH dependent
anchoring of GSH onto gold surfaces has been proved beyond doubt to bring about nanoparticle
aggregation.
regime exhibit a strong absorption band in the visible region
Introduction
and this is indeed a small particle effect, since it is absent in the
individual atoms as well as in the bulk.15,16 The physical origin
Bottom-up assembly of inorganic nanoparticles is a promising
means of creating advanced materials with superior electronic,
of the light absorption by metal nanoparticles is the coherent
optical, and mechanical properties.1,2 Due to the spherical
oscillation of the conduction band electrons induced by the
symmetry and uniform reactivity of individual nanoparticle
interacting electromagnetic field. The absorption band results
surfaces, the synthesis of small controlled nanoparticle assem-
when the incident photon frequency is resonant with the
blies is a significant challenge. Recently, assembling individual
collective oscillation of the conduction band electrons and is
nanoparticles into ensembles of specific shape and structure
known as the surface plasmon resonance (SPR). The reso-
has become a widely pursued objective.3 The organization and
nance frequency of this SPR is strongly dependent upon the
patterning of inorganic nanoparticles into 2D and 3D func-
size, shape, dielectric properties and local environment of the
nanoparticles.15,17–21 When a cluster of metal nanoparticles
tional structures is a fundamental prerequisite for the assembly
of chemical, optical, magnetic, and electronic devices.4 Several
are placed in close proximity to one another, the interparticle
approaches have been described for the generation of 2D and
coupling effects become very important in studying the particle
3D arrays of metal and semiconductor nanoparticles: (i) by
plasmon resonances, especially the shift of the plasmon reso-
evaporation of the solvent of hydrophobic colloids,5 (ii) by
nant wavelength as a function of particle separation. In
random inclusion of the nanoparticles into gel and glassy
colloidal solutions, the color of Au nanoparticles may range
matrices,6 (iii) by template-mediated synthesis at structured
from red to purple, to blue and almost black, due to the
surfaces in porous protein crystals7 or in bacterial superstruc-
formation of aggregates. Nanoparticle aggregates of gold with
tures, and (iv) through chemical coupling in solution by means
interparticle distances substantially greater than the average
of bifunctional cross-linkers.8,9 An example of the latter is the
particle diameter appear red (with lmax E 520 nm), but as the
interparticle distances in the aggregates decrease to less than
alkanedithiol-directed aggregation of gold nanoparticles.8,9
The aggregation of nanoparticles induced by specific biologi-
approximately the average particle diameter, the color be-
cal agents has attracted significant interest as a self-assembly
comes blue.22–24
process for the construction of complex nanostructures that
The recent development of nanoscience and nanotechno-
exhibit new collective properties. It is interesting to generate
logy involves the synthesis and stabilization of metal
nanoparticles in non-aqueous solvent.25–27 The organic solu-
and study the interparticle interactions in a closely packed
assembly of metal nanoparticles while the particles are held by
tion-based synthesis of metallic nanoparticles possesses certain
weak forces.
With their excellent compatibility with biomolecules10–13
nanoparticle size, monodispersity and chemical nature of the
advantages. It shows a high degree of control over the
and distance-dependent optical absorbance,10,14 colloidal gold
nanoparticle surface. These metal organosols are convenient
nanoparticles have attracted intense scientific and technologi-
objects for the investigation of the physicochemical properties
of metallic nanoparticles.28 Moreover, a wide number of
cal interest. Metallic particles of gold in the nanometre size
reactions are catalyzed in organic media. Nanoparticles in
organic media are becoming an interesting topic in view of
their various applications in optical devices,29,30 optical energy
Department of Chemistry, Indian Institute of Technology, Kharagpur,
721302, India. E-mail: tpal@chem.iitkgp.ernet.in
ꢀc
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transport,31,32 near-field scanning optical microscopy
(NSOM),33,34 surface enhanced Raman scattering spectro-
scopy,35,36 chemical and biological sensors.14,37 The properties
of regularly packed aggregates have been well explored.38 The
interesting colors observed in gold sols have led to extensive
study of their optical spectroscopic properties in an effort to
correlate their behavior under different microenvironmental
conditions. Without the application of organosol systems, the
study often fails to register the impacts of the solvent on the
sol properties. However, regularly packed aggregates have
variable optical properties. If these properties can be con-
trolled, then there might, for example, be applications for the
clusters as the substrates in SERS, as novel pigments or in
spectrally selective coatings for solar glazing.39 Unfortunately,
in general when gold nanoparticles in suspension are aggre-
gated or densified, they precipitate out as mesoporous
sponges, losing their plasmon resonance in the process. Ki-
mura et al. have reported the formation of a three dimensional
superlattice of gold nanoparticles.40 The groups of Mirkin37
and Alivisatos41 have demonstrated the formation of aggre-
gated metal clusters, using DNA as the recognition element in
aqueous medium. Some of the amino acids can induce aggre-
gation of gold nanoparticles.42 Mandal et al. have reported the
aggregation of cysteine-capped silver particles.43
Microscope. One drop of the test solution was placed on a
KBr pellet and was dried under vacuum for 6 h before
analysis. The X-ray diffraction (XRD) pattern was recorded
in an X’pert pro diffractometer with Co (Ka = 1.78 891)
radiation.
Synthesis of gold nanoparticles
Gold nanoparticles capped with tetraoctylammonium bro-
mide (TOAB) in toluene were prepared using the biphasic
reduction procedure. An aqueous solution of chloroauric acid
(5 mL, 2.0 mM) was first extracted into toluene using the
phase transfer catalyst, TOAB (5 mL, 2.0 mM). Addition of
phase transfer reagent to the aqueous phase resulted in swift
ꢂ
movement of the AuCl4 ions to the organic layer. The
solution was diluted by the addition of 15 mL of toluene.
Finally, 2 mg of sodium borohydride was introduced and the
reaction mixture was shaken vꢂigorously. During shaking, at
first, the yellow color of AuCl4 was found to disappear and
the solution became colorless. Then, the solution slowly
turned red to wine red, indicating the formation of gold
nanoparticles. After that the sol showed no further change
in color implying the completion of the reaction. The final
concentration of gold in this experiment was 0.5 mM. The
organosol prepared by the above method was evaporated to
ca. 2 ml in a rotary evaporator resulting in a gold concentra-
tion of 5 mM. An aliquot of 30 mL of this highly concentrated
gold suspension was diluted to 3 mL with THF.
In this paper, we report a simple method for the formation
of gold aggregates in organic solvents i.e., aggregation of a Au
organosol by the modification of gold nanoparticles with pH
sensitive and ionizable thiols, namely, glutathione (GSH)
which is used as a short chain molecular linker because of its
biological and clinical significance. High-resolution transmis-
sion electron microscopy (HRTEM), X-ray diffraction
(XRD), Fourier transform infrared spectroscopy (FTIR) and
absorption spectroscopy have been used to characterize the
assemblies. It has been found that these nanocrystals self-
assemble into monolayers or multilayers after the ligand-
exchange process. The particle size, interparticle distances
and absorption band wavelengths are found to be highly
dependent on the pH of the medium and the concentration
of the GSH.
Making glutathione–Au assemblies
In a typical synthesis, to an aliquot of colloidal gold (3 mL, 50
mM) dispersed in THF, dilute HCl (100 mL, 10 mM) was added
to lower the pH value. Then, an aliquot of GSH solution (30
mL, 10 mM) prepared in THF was added to that deep red
acidic gold colloidal solution. The color changed from red to
blue indicating the formation of the aggregates of gold nano-
particles. The method is reproducible and the gold nanopar-
ticle aggregates in organic solvent prepared by this method
remain stable for a few days.
Results and discussion
Experimental
The formation of gold nanoparticle aggregates was probed by
monitoring the absorption changes at several stages of the
reaction. Fig. 1 shows the absorption spectral features for the
evolution of gold aggregates in THF. The aqueous solution of
Reagents and instruments
All the reagents used were of AR grade and the solvents were
dried. Chloroauric acid (HAuCl4 ꢁ 3H2O) was purchased
from Aldrich. Sodium borohydride (Aldrich) and glutathione
(Aldrich) were used as received.
ꢂ
AuCl4 ions exhibits an absorption peak at 293 nm (trace a)
which can be attributed to the metal-to-ligand charge transfer
(MLCT) band of AuCl4ꢂ complexes.44 The toluenic dispersion
of TOAB shows no characteristic absorption maximum in the
range of 300–700 nm (trace b). Addition of the toluenic
solution of TOAB to the aqueous solution of HAuCl4 caused
the disappearance of the 293 nm absorption band due to
The absorption spectrum of each solution was recorded in a
Spectrascan UV 2600 digital spectrophotometer (Chemito,
India) in a 1 cm well-stoppered quartz cuvette. The spectra
were taken with reference to a solvent in a reference cell in the
double beam spectrophotometer to subtract solvent back-
ground. HRTEM was carried out on a JEOL JEM-2010
UHR transmission electron microscope. Samples were pre-
pared by placing a drop of solution on a carbon coated copper
grid. FTIR Spectral characteristics of the samples were col-
lected in reflectance mode with Nexus 870 Thermo-Nicolet
instrument coupled with a Thermo-Nicolet Continuum FTIR
ꢂ
AuCl4 complexes and a new absorption band appears with
a shoulder at 345 nm (trace c) which can be ascribed to the
formation of an ion-pair [TOA]+[AuCl4]ꢂ, ([TOA]+ = tetra-
octylammonium ion).44 Upon addition of NaBH4 to the
reaction mixture, the yellow color of the solution gradually
disappeared and after a certain time, the solution becamꢂe
completely colorless, indicating the formation of AuO2
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D 4 5r, where D is the center-to-center distance and r the
radius of the particles, but is increasingly important when D is
less than that. Therefore, in the aggregates of gold nanopar-
ticles, it is very difficult to observe the properties of the isolated
sample as interparticle interactions generally overpower the
single-particle property. The color of the solution in such
aggregates depends on nanoparticle spacing. Nanoparticle
aggregates of gold with interparticle distances substantially
greater than the average particle diameter appear red with
lmax E 520 nm, but as the interparticle distances in the
aggregates decrease to less than approximately the average
particle diameter, the color becomes blue. The blue color of
the solution in the present case indicates that the interparticle
distances in the aggregate are obviously less than the average
particle diameter, which is also evident from the TEM studies.
The appearance of a red-shifted band at B580 nm can be
attributed to the surface plasmon resonance of gold organized
into an aggregated structure.
Fig. 1 Absorption spectra of (a) aqueous solution of HAuCl4, (b) the
toluenic dispersion of TOAB, (c) [TOA]+[AuCl4]ꢂ, (d) TOAB-capped
gold nanoparticles in toluene, (e) TOAB-capped gold nanoparticles in
THF and (f) gold aggregates in THF induced by GSH at pH E 4.
We have studied the aggregation of gold organosol with
GSH (containing a-NH2 and -SH groups) in THF for the first
time. The applicability of cysteine (cys) was also tested as the
molecules possess similar functional groups to GSH. It was
seen that both the aggregating species are able to interlink gold
nanoparticles in THF shortening the interparticle distance.
Thus, the ‘field effect’ becomes effective as a result of which a
broad banded peak in the red region is observed. Between the
two aggregating species, cys has shown to be very effective and
prompt in its action. This was confirmed from the ready
appearance of the 580 nm peak (Fig. 2, trace c). This peak
was also observed when GSH was introduced into the system
under identical conditions (temperature, concentration and
pH of the medium) (Fig. 2, trace b). The spectral change
(aggregation) is detectable with a lower concentration of cys
(5 mM) than GSH. To confirm the aggregating behavior
of GSH and cys, we have separately considered the action of
1-dodecanethiol (1-DDT) on the gold organosol in THF. The
stabilizing action of DDT is well documented in literature but
species in the alkaline condition.45 On further shaking, the
appearance of a pinkish tinge within the solution indicates the
onset of the evolution of gold particles. The color of the
solution gradually changed from light pink to red to wine
red upon completion of the reaction. Absorption measurement
of this resulting solution shows a new absorption band with
maximum at 530 nm (trace d), which corresponds to a typical
plasmon band of gold nanoparticles. The metallic particles
dispersed in toluene were dried under vacuum and redispersed
well in tetrahydrofuran (THF) without any sign of aggregation
or precipitation of the particles. Trace e shows the surface
plasmon resonance with a maximum at 521 nm of gold
nanoparticles dispersed in THF. The blue shift of the SPR
on changing the solvent from toluene to THF is due to the
change in refractive index of the medium.46 Upon addition of
GSH solution to the deep red solution of colloidal gold
followed by acidification with dilute HCl, the color of the
solution changed from red to blue. Absorption measurement
of this blue solution shows a red-shifted broad band with a
maximum at B580 nm with an observable decrease in the
absorption intensity (trace f). It is now well documented in the
literature that the assembly of nanoparticles in various states
of aggregation influences the plasmon resonance.15 The ap-
pearance of an absorption maximum at B580 nm indicates
that the particles are assembled into an aggregated structure.
This is attributable to electric dipole–dipole interactions and
coupling between plasmons of neighboring particles in the
aggregates.
The optical properties of small particles in the nanometre
size range are mainly determined by two contributions:47 (i)
the particles acting individually as an isolated sample and (ii)
the collective properties of the whole ensemble. But in the
closely packed aggregates of an ensemble of a large number of
particles (as seen in the present case), the isolated-particle
approximation breaks down and electromagnetic interactions
between the particles become important and affect the surface
plasmon resonance enormously. This effect is negligible if
Fig. 2 Absorption spectra of (a) 1-DDT capped gold nanoparticles in
THF, (b) gold aggregates in THF induced by GSH and (c) gold
aggregates in THF induced by cysteine at pH E 4.
ꢀc
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Fig. 3 Transmission electron micrographs of (A) gold particles dispersed in THF, (B) aggregates of the parent particles after addition of GSH and
(C) high-resolution transmission electron micrographs of an individual GSH capped gold nanocrystal.
its monofunctionality does not allow it to interlink the gold
nanoparticles in question. Hence, no aggregation was de-
tected; only a slight shift (B2 nm) of lmax value was noted
(Fig. 2, trace a).
In a preliminary control experiment, we observed the optical
spectra of a TOAB capped gold colloid in THF and then the
effect of sequential addition of 10 mM HCl solution in the
organosol system. There is almost no shift in the peak position
until the pH value is lowered to B3.5, indicating no aggrega-
tion effects from the added HCl solution. At still lower pH the
particles aggregate due to the decrease in energy barrier by
both lowering the surface potential of the particles and
increasing the ionic strength. The evolution of particle aggre-
gates due to the addition of glutathione is dependent on the
pH of the sol system. Generation of aggregates with a perfect
blue color is observed at pH r 5 with glutathione. The
molecular structure of GSH is shown in Scheme 1. Gluta-
thione is the major cellular antioxidant that plays a crucial role
in maintaining the balance between oxidation and antioxida-
tion. This tripeptide also plays an important part in cellular
detoxification and is required for many aspects of immune
response. It is also an important factor in brain damage,48
glucose homeostasis,49 HIV expression50 and cancer therapy.51
As glutathione consists of amino acids viz. glutamic acid,
cysteine and glycine, its binding mode with gold surfaces is
pH sensitive. In GSH, several possible binding points are
present, which gives it different binding possibilities with gold.
The availability of one or several anchoring points for binding
on the gold surface depends primarily on the pH of the
medium. Some of the anchoring points in glutathione are
the two carboxylic groups in the glutamic acid moiety and
glycine residue, three –NH2 groups in the three amino acids
and the sulfur atom in the cysteine residue. The pKa values
corresponding to the two carboxylic groups have been deter-
mined as 2.56 and 3.5052 and for the cysteine residue as 9.42
Fig. 3 shows the transmission electron micrographs of the
(A) TOAB-capped gold particles dispersed in THF, (B) ag-
gregates of the parent particles after addition of GSH at pH B
4 and (C) high-resolution transmission electron micrographs
of an individual GSH capped gold nanocrystal. The TOAB-
capped gold colloids produce nearly spherical particles with
diameter ranging from 5 to 6 nm which is shown in Fig. 3A.
From the Fig. 3B, it is seen that the spherical and tiny gold
particles with a mean diameter of B 6 nm undergo coalescence
and form a closely packed assembly. In a HRTEM image of
Au-GSH nanocrystals (Fig. 3C), the particles show lattice
images originating from a single crystal. The distances between
the adjacent lattice fringes, measured as 2.36 A, are the
interplanar distances of the Au {111} plane, which are in
excellent agreement with the {111} d-spacing of bulk Au
(2.355 A).
The XRD pattern for the GSH-capped gold aggregates is
shown in Fig. 4. Several peaks are observed, these being at
44.21, 51.51, 76.41, 93.31 and 98.61, which correspond to the
{111}, {200}, {220}, {311} and {222} facets of the fcc crystal
structure of gold, respectively. The observation of diffraction
peaks of gold nanoparticles indicates that these are crystalline
in nature. The broadening of the spectrum is related to the
reduced particle size.
Fig. 4 X-Ray diffraction pattern of gold aggregates.
Scheme 1 Molecular structure of glutathione (GSH).
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average pH value has been given. In this regard, because of the
delocalization of electrons, the other two –NH– moieties of
GSH remain ineffective. The surface coordination of the gold
particles with GSH was also accomplished from the FTIR
spectrum. Fig. 6A shows the FTIR spectrum recorded from
the gold aggregates in the spectral window 2400–2600 cmꢂ1
(curve 1) along with the spectrum recorded from GSH powder
(curve 2) at pH E 4. The prominent –SH vibrational band
centered at ca. 2524 cmꢂ1 is clearly seen in the free GSH
molecules (curve 2) and vanishes upon coordination with
colloidal gold (curve 1). Further evidence for the presence of
surface bound GSH is provided by FTIR measurements of the
gold particles in the spectral window 3200–3500 cmꢂ1 (curve 1,
Fig. 6B). The N–H stretch of the GSH molecules is observed at
3250 cmꢂ1 (curve 2, Fig 6B). But a weak and red shifted (3307
cmꢂ1) peak position (curve 1, Fig. 6B) suggests an interaction
of the N–H group with the gold particles. The position of the
carboxylate stretching vibration in the GSH capped gold
remains unaltered (curve 1, Fig. 6C) at 1713 cmꢂ1 which
suggests that there is no hydrogen bonding.
Fig. 5 UV-visible spectra of gold aggregates in THF at various pH.
for aqueous solutions.53 However, predicting solely from the
pKa values of the different groups as to which binding mode is
expected is not always accurate since the acid–base character
of a certain group can change the binding capability with the
metal surface and the medium.54 It is to be mentioned that in
organosol systems, gold is stabilized sterically. The GSH binds
with the gold surfaces through –SH as well as –NH2 anchoring
points and hence the gold particle aggregates. At lower pH,
linking via the a-amines is activated due to the zwitterionic
species, but linking through –COOH groups is suppressed due
to the undissociated carboxylic groups. However, at inter-
mediate or higher pH, dissociation of the carboxyl group
would be expected to hinder the binding of gold colloids via
the a-amine group, resulting in little or no cross-linking. Fig. 5
shows the optical spectra of gold colloids (50 mM) in THF at
different pH for a particular concentration of GSH solution
(0.1 mM). There is clearly a progressive new color develop-
ment for the organosol system in THF and red shifting of the
surface plasmon resonance band with decreasing pH is ob-
served. At pH E 7, no spectral changes occur but as the pH
decreases then the dipole plasmon resonance shifts to the
longer wavelength. At lower pH, one –SH group of the
cysteine moiety and the primary –NH2 group of the glutamic
acid moiety can bind effectively the gold nanoparticles to form
aggregates which is authenticated from FTIR spectra. It
should be mentioned that in an organic solvent like THF
exact measurement of pH is not possible, and therefore an
The
classical
Derjaguin–Laudau–Verwey–Overbeck
(DLVO) theory55 has been widely employed in colloid science
to study particle–particle interactions, colloidal stability, coa-
gulation and the behavior of the electrolyte solutions. This
theory is based on the idea that pair-wise interaction forces
dominate which arise from the interplay of attractive van der
Waals forces, Fattr and repulsive Coulomb forces, Frep
screened by Debye–Huckel ion clouds. Obviously, the dis-
¨
persed colloid is stable for Frep c Fattr whereas the condition
of Frep { Fattr leads to aggregation. Therefore, in colloidal
solution several factors, such as particle size, surface potential,
and electric double layer influence the stability of nanoparti-
cles and their aggregation. Nanoparticles are stable in solution
due to electrostatic repulsion of their charged surface. Lack of
sufficient surface charge or stabilizing agent will cause the
particles to aggregate or precipitate. In the present experiment,
tetraoctylammonium ions are adsorbed onto the surface of
gold nanoparticles, creating a surface charge that stabilizes the
particles. Addition of glutathione to the TOAB-stabilized gold
nanoparticles disrupts the tetraoctylammonium ionic layers
causing particle aggregation via slow ‘place exchange reac-
tion’. Since there are no species in solution to generate
sufficient surface charge on the gold particles, the particles
will aggregate. Thus, the density of the surface charge and the
surface potential of the particles can be varied over a wide
range. When the surface potential is far away from the
Fig. 6 FTIR spectra of pure glutathione (curve 2) and glutathione induced gold aggregates (curve 1) at pH E 4 in the spectral windows (A)
2400–2600 cmꢂ1, (B) 3200–3500 cmꢂ1 and (C) 1500–1750 cmꢂ1
.
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Fig. 8 Time-dependent UV-visible spectra of gold nanoparticles at
pH E 4 recorded at various times after addition of glutathione
(0.1 mM).
Fig. 7 UV-visible spectra for gold aggregates at different concentra-
tion of glutathione at pH E 4.
potential of zero charge (pzc), the particles are highly dis-
persed and very stable; when the surface potential is near or
equal to the pzc and the electrostatic repulsion cannot over-
come the van der Waals attraction, the nanoparticles become
aggregated. Upon addition of glutathione to a solution of
colloidal gold, one functional group reacts with the gold
nanoparticles while the other functional groups remain ex-
tended from the nanoparticles. Thus, the multifunctional
linker molecules position the colloidal nanoparticles in close
proximity to each other, causing aggregation between the
particles.
in Fig. 7. For the sample with a very low GSH content, i.e.,
[GSH]/[Au(0)] = 0.02, no assembly is observed. When we
increased GSH concentration, the lmax shifted to higher
wavelength region, resulting a change in the morphology from
isolated domains to compact domains.
Large gold nanoparticles (45 nm) exhibit a surface plas-
mon resonance (SPR) that is based on the size of the metal
core and the interparticle spacing (dipolar coupling) between
the nanoparticles. We have monitored the shift in the surface
plasmon resonance of gold nanoparticles as a function of time
to investigate the kinetics of the aggregation process. A
solution of GSH was added to the acidic solution (pH E 4)
of gold particles at room temperature and the spectra were
acquired over different time intervals as shown in Fig. 8. A
distinctive red shift of the dipole plasmon resonance from 520
to 580 nm is observed which indicates that interparticle
spacing decreases with the progress of the place exchange
reaction.
In the present experiment, the TOAB molecules present on
the particle surface provide a steric barrier against aggrega-
tion. The thickness of the electrical double layer is character-
ized by the inverse of Debye length (kꢂ1). For a symmetric
(n : n) electrolyte, the inverse Debye length is given by
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2000e2 n2 NA M
kꢂ1
¼
ekBT
For this study, the system temperature is assumed to be T =
298 K, and the permittivity of toluene at this temperature is
B2.39 ꢃ 10ꢂ10 C2 Nꢂ1
Conclusion
m
ꢂ2. The remaining constants in the
expression for the inverse Debye length are the Boltzmann
constant, kB = 1.381 ꢃ 10ꢂ23 J Kꢂ1, the fundamental electron
We have demonstrated a very simple approach for preparing
gold nanoparticle aggregates by linking individual gold col-
loidal particles in a dispersion. The treatment of TOAB-
stabilized gold nanoparticles with GSH molecules causes their
aggregation. It is possible to control the interparticle distance
in the self-assembled gold nanocrystals over a limited range by
capping with GSH. Upon aggregation, the solution shows a
broad red shifted absorption band around 580 nm, which is
attributed to interparticle plasmon coupling between the
particles. The degree of coupling, and thus the nature of this
band, are influenced by the pH of the colloidal solution and
the concentration of GSH solution. From the FTIR spectra it
was confirmed that GSH binds to the gold surface via a strong
thiolate bond whereas the –NH2 group coordinates with the
gold surface weakly. The aggregate may find application in
optoelectronic devices.
charge, e = 1.602 ꢃ 10ꢂ19 C and Avogadro number, NA
=
6.022 ꢃ 1023 molꢂ1. Since the electrolyte concentration (c) of
the colloidal solution is kept at millimolar (0.1 mM) level, the
inverse Debye length is calculated as 1.78 ꢃ 107 which
indicates that the electrical double layer surrounding the
colloidal particles is large enough to be treated as a constant.
So the surface potential becomes the determining factor for the
stability of the nanoparticles. Upon addition of GSH mole-
cules to the gold colloid, the particle surface becomes neutral
leading to the aggregation of the gold particles.
To evaluate the morphological effect on the ligand (GSH)
concentration, a set of gold aggregates was prepared by using
GSH at pH E 4 with variable molar ratios of [GSH]/[Au(0)]
from 0 to 20. The optical spectra of these aggregates are shown
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Zhang, Y. Zhu and C. Che, New J. Chem., 2000, 24, 739; (c) S.
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Acknowledgements
The authors are thankful to IIT Kharagpur, CSIR, UGC and
DST New Delhi for financial assistance. We are thankful to
Dr P.V. Satyam of IOP, Bhubaneswar for extending help in
TEM analysis.
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