Smart oligopeptide gels: In situ formation and stabilization of gold and silver nanoparticles within supramolecular organogel networks

Article abstract of DOI:10.1039/b605498f

Tripeptide with redox active chemical entities based smart organogels have been used for in situ formation and stabilization of gold and silver nanoparticles within the supramolecular gel networks and the gold nanoparticles are aligned in arrays along the gel nanofibers of peptide 1-toluene gels. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b605498f

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Smart oligopeptide gels: in situ formation and stabilization of gold and
silver nanoparticles within supramolecular organogel networks{
Sudipta Ray, Apurba Kumar Das and Arindam Banerjee*
Received (in Cambridge, UK) 18th April 2006, Accepted 19th May 2006
First published as an Advance Article on the web 1st June 2006
DOI: 10.1039/b605498f
hydro/organogelators to prepare and stabilize gold nanoparticles
byinsitureduction.12 However,noneoftheabovereportsregarding
the in situ synthesis of gold and silver nanostructures using
supramolecular gels as a template include short peptide molecules
as gelators. In this report, we present in situ synthesis and
stabilization of different shaped gold and silver nanoparticles
withinthe gel-phase networkusing redox active tyrosine containing
new oligopeptide based supramolecular organogels.
Tripeptide with redox active chemical entities based smart
organogels have been used for in situ formation and stabiliza-
tion of gold and silver nanoparticles within the supramolecular
gel networks and the gold nanoparticles are aligned in arrays
along the gel nanofibers of peptide 1–toluene gels.
Supramolecular organogels have numerous applications including
structure-directing agents for synthesis of nanoporous materials,
templates for assembling nanoparticles, electro-optical display
materials, media for the growth of large organic, inorganic and
macromolecular crystals of high optical quality, and others.1
Tuning the synthesis of nanoscale materials is one of the most
significant challenges faced by modern chemistry. Supramolecular
organogels consisting of multiple entangled fibrillar networks have
been exploited to direct the shape and nanostructures of the
inorganic materials.2 Various inorganic nanostructures can be
obtained using supramolecular organogels as templates. One such
example is the helical nanofibers prepared by Shinkai3 and
coworkers using supramolecular organogels as a structure-direct-
ing agent. Similarly the groups of Hanabusa4 and Stupp5 have
developedtransitionmetalnanotubesandCdSribbonsrespectively
using supramolecular gels as templates. Supramolecular hydrogels
have been employed as a template for synthesizing inorganic
nanotubes.6 Recently, Hanabusa and his coworkers reported the
preparation of helical silica nanostructures using amino acid based
gelators as the structure-directing agent.7 The formation of ‘pearl
necklace’architectureofCdSwithinorganicnanoparticleshavinga
diameter of about 30 nm has been reported by Lu8 and his
coworkers using organogels as a template. However, there are only
a few reports of the use of supramolecular gels for the construction
of gold and silver nanostructures.9–12 Liu et al. have shown the
formation of silver nanohelices using a racemic gelator as a
template.9 Kimura and his coworkers have described self-
organization of gold nanoparticles into a network structure using
thiol-terminated gelators.10 They heated a gelator with octanethiol
stabilizedgoldnanoparticlesandoncoolingfurthergelationcaused
gold nanoparticles to assemble into fibrous aggregates of the gel
network. Recently, Smith and his coworkers have reported gold
nanoparticle synthesis by UV irradiation of a supramolecular
organogel containing HAuCl4 and tetraoctylammonium bro-
mide.11 Very recently, Vemula and John have used urea-containing
One interesting issue is the type of distribution, stability and the
shape of the gold nanoparticles (GNPs) and silver nanoparticles
(SNPs) within the gel phase. These nanoparticles can be distributed
either all over the gel or they can be aligned in a particular array
along the gel micro/nanostructures. It is interesting to fabricate the
aligned arrays of GNPs and SNPs using gel fibers as templates for
possible optical device uses. For these reasons, we have synthesized
a series of terminally protected tyrosine containing new oligopep-
tides 1–313 (Fig. 1) which self-assemble to form gels in various
organic solvents. There are several examples of peptide based
hydrogelators/organogelators,14 but none of these low molecular
weight gelators have been exploited for the in situ synthesis of gold
and silver nanoparticles within the gel-phase network. In gel
forming peptides the presence of tyrosine residue(s) can be used to
reduce Ag+/Au+3 within the gel phase into Ag0/Au0 nanoparticles.
This can promote the in situ formation and stabilization of GNPs
and SNPs within the gel network structures.
The minimum gelation concentrations (MGC) and the results of
gelation tests for these peptide gelators are listed in ESI Table 1.{
Peptides 1 and 2 produce gels in various organic solvents like
benzene, 1,2-dichlorobenzene, toluene, p-xylene, m-xylene, nitro-
benzene and tetralin. Peptide 1 forms a gel in dimethyl sulfoxide
whereas peptide 2 forms a gel in chloroform. Peptide 3 can gelate
solvents like 1,2-dichlorobenzene, nitrobenzene and methanol–
water (1 : 1) solvent. Gel melting temperatures (Tgel) of these
Department of Biological Chemistry, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata 700 032, India.
E-mail: bcab@mahendra.iacs.res.in; Fax: +91-33-2473-2805
{ Electronic supplementary information (ESI) available: Experimental
procedures, spectra, Table 1, FE-SEM and TEM images. See DOI:
10.1039/b605498f
Fig. 1 Schematic representation of tripeptides 1, 2 and 3 showing their
chemical structures.
2816 | Chem. Commun., 2006, 2816–2818
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peptide gelators were analyzed by the inverted test tube method.
The morphology of these gelator peptides were characterized by
transmission electron microscopy (TEM) and field emission
scanning electron microscopy (FE-SEM). The SEM images of
the xerogels obtained from peptide 1 in toluene show entangled
nanofibrillar networks with an average diameter of 180 nm (ESI
Fig. S7a{). The TEM images of the xerogels prepared from gels of
peptide 2 in toluene and of peptide 3 in methanol–water (1 : 1)
(1% w/v) reveal a 3D network of nanofibrillar structure (ESI
Fig. S7b and Fig. S7c{).
Fig. 2 TEM image indicating the definite alignment of gold nanopar-
ticles along a gel nanofiber obtained from the peptide 1–toluene gel.
To study the self-assembling behavior of these gel-forming
peptides, FT-IR and temperature dependent 1H NMR experi-
ments were done. Peptide 2 forms a gel in CHCl3. So, temperature
oxidation of tyrosine residues of peptide 3. After 1 h a gold
nanoparticle embedded bluish violet colored gel was formed. A
surface plasmon band around 551 nm was observed in the UV-vis
spectrum indicating the formation of gold nanoparticles11 (ESI
Fig. S9b{). The following method was used for the in situ
preparation of silver nanoparticles in tyrosine containing peptide
3 gels. 1 mg of AgNO3 was added into the methanol–water gel of
peptide 3 and the mixture was heated above 50 uC until the
appearance of a clear solution. Upon slow cooling the color of the
transparent solution changed from light yellow to strong yellow
within 1 h indicating the formation of silver nanoparticles via
oxidation of tyrosine residues of the tripeptide 3 and then the silver
nanoparticle embedded gel was formed within a few minutes. A
surface plasmon band around 363 nm suggests the existence of
silver nanoparticles (ESI Fig. S9c{).
Transmission electron microscopy was used to characterize the
gold and silver nanoparticles embedded gels. A small piece of the
gel of a particular peptide in its respective solvent was placed on a
carbon-coated copper grid (300 mesh) and it was allowed to dry
under reduced pressure at room temperature for two days. Fig. 2
shows that gold nanoparticles (with an average diameter of 50–
80 nm) were aligned in a definite array along the gel nanofiber
obtained from peptide 1–toluene gel. Gold nanoparticles (with an
average diameter of 15–20 nm) in the toluene gel-phase network of
peptide 2 were visualized by TEM (Fig. 3). Fig. 4a clearly
illustrates the presence of gold nanoparticles within the gel network
structure of peptide 3. Fig. 4b shows the TEM image of individual
gold nanoparticles with different morphologies including spherical
and hexagonal with various particle sizes ranging from 15 nm to
40 nm. Previous reports include the synthesis of hexagonal shaped
gold nanoparticles of various sizes using different chemical and
biological methods.16 However, none of these methods include the
1
dependent H NMR chemical shifts of peptide 2 gel in CDCl3
(1% w/v) have been recorded from the temperature 25 uC (gel
state) to 75 uC (solution state) (ESI Fig. S8{). Significant changes
in chemical shifts have been observed for all amide NHs. These
results indicate that all amide NHs are involved in intermolecular
hydrogen bonding to form the gel state. The FT-IR spectrum of a
toluene gel formed by the peptide 2 shows absorption bands at
3298 and 1649 cm21 which can be assigned as intermolecularly
hydrogen bonded N–H and CLO stretching vibrations respectively
(ESI Fig. S14{).
Previous reports have demonstrated that gold and silver
nanoparticles have been synthesized by tyrosine, alkylated tyrosine
and tyrosine based peptides.15 Incorporating the redox active
tyrosine residue into the gel forming tripeptides, we want to
explore the in situ synthesis of gold and silver nanostructures
within the gel-phase network.
The following method has been used for the synthesis of gold
nanoparticles in peptides 1– and 2–toluene gels. Toluene (5 mL)
and tricaprylylmethylammonium bromide (70 mL) were mixed in a
beaker and the mixture was continued to stir for half an hour. An
aqueous yellow colored solution of HAuCl4 (20 mg in 2 mL) was
added to it for transferring the chloroaurate to the toluene layer.
The toluene layer was then separated. Gelator peptide 1 (10 mg) or
2 (10 mg) was added in 800 mL of toluene chloroaurate solution
and it was heated above 100 uC to produce a homogeneous
solution. Upon cooling a stable gel has been formed and the yellow
coloration was not changed for several days. Triethylamine (5 mL)
was added into the toluene gel and it was heated in an oil bath
above 100 uC to produce a clear solution. Then the yellow color
was rapidly changed to a colorless solution and it was turned into
a violet solution within a few minutes indicating the formation of
gold nanoparticles via the oxidation of tyrosine residues of peptide
1 or 2. Upon slow cooling, after 1 h a gold nanoparticle embedded
violet colored gel was formed. The presence of a surface plasmon
band around 548 nm suggests the existence of gold nanoparticles
(ESI Fig. S9a{). Peptide 3 forms a stable gel in methanol–water
(1 : 1) within the pH range from 7 to 11. The gelator peptide 3
(20 mg) was added in 400 mL of methanol–water (1 : 1) solution at
pH 10 and it was heated until the appearance of a clear solution.
Upon cooling below room temperature the complete volume of
the respective solvent was immobilized and a stable gel was
formed. 1 mg of HAuCl4 was added into the gel and the mixture
was heated above 50 uC to dissolve. Upon cooling, the yellow
color of the solution gradually changed to a colorless solution
[Au(III) to Au(I)] and then it turned into bluish violet color,
indicating the formation of gold nanoparticles [Au(0)] via
Fig. 3 (a) TEM image of gold nanoparticles formed within the gel
network structure of peptide 2–toluene gel. (b) TEM picture of the in situ
formed gold nanoparticles at a higher magnification.
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Chem. Commun., 2006, 2816–2818 | 2817

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nanoparticles along the nanostructured gel fibers of peptide
1-toluene gels. Such materials may be important for the future
development of nanostructured advanced materials from the
conjugates of tyrosine containing oligopeptide based smart gels
and Au/Ag nanoparticles, which may open up applications in the
promising field of supramolecular devices. Gels containing useful
chemical entities like tyrosine make themselves suitable for the
applications, such as in situ formation of gold and silver
nanoparticles with different shapes and sizes without any reducing
and stabilizing agents and this can be utilized to make smart gels
containing useful functionalities for the in situ preparation and
stabilization of metal nanoparticles within the gel network
structure in future.
Fig. 4 (a) TEM image of gold nanoparticles within the gel network
structure of peptide 3–methanol–water (1 : 1) gel and (b) magnified TEM
image of gold nanoparticles showing hexagonal and spherical morphology.
We acknowledge the DST, New Delhi, India for financial
assistance Project No (SR/S5/OC-29/2003). S. Ray and A. K. Das
wish to acknowledge the CSIR, New Delhi, India. We gratefully
acknowledge the Nanoscience and technology initiative of
Department of Science and Technology of Govt. of India, New
Delhi for using the TEM facility.
Notes and references
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Fig. 5 (a) TEM image of silver nanoparticles embedded in a spherical
sponge-like gel network structure of peptide 3–methanol–water (1 : 1) gel
and (b) TEM image of these silver nanoparticles at higher magnification.
formation of hexagonal gold nanoparticles within the gel phase.
Fig. 5 shows the TEM images of silver nanoparticles (with an
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gel of peptide 3. A characteristic EDX profile for silver nano-
particles is given in ESI Fig. S12c.{ From X-ray diffraction studies
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S13{). In order to understand the role of the tyrosine residue in the
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tripeptide Boc–Ala-Phe-Ala–OMe (AFA) without any tyrosine
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nanoparticles within the gel network. This result convincingly
demonstrates that the tyrosine residue has a definite role for the
preparation of gold/silver nanoparticles by in situ reduction.
We have successfully demonstrated the in situ preparation of
gold and silver nanoparticles into a gel network structure using
tyrosine containing oligopeptide based organogelators. The
tyrosine residue(s) of gelator peptides have been successfully
utilized to reduce Au+3/Ag+ into colloid Au0/Ag0 nanoparticles and
after the reduction, the gelator peptides retain their gelation
properties intact. Hence, these nascent metal nanoparticles are
trapped and stabilized within the supramolecular gel-phase
network. This is an wonderful demonstration of the exploitation
of tyrosine containing gel-forming oligopeptides for the in situ
preparation of gold and silver nanoparticles and their concomitant
stabilization within the supramolecular assemblies. Another
remarkable feature is that the alignment of in situ prepared gold
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This journal is ß The Royal Society of Chemistry 2006