Fabrication of luminescent cds nanoparticles on short-peptide-based hydrogel nanofibers: Tuning of Optoelectronic Properties

Article abstract of DOI:10.1002/chem.200900149

The pH-induced self-assembly of three synthetic tripeptides in water medium is used to immobilize luminescent CdS nanoparticles. These peptides form a nanofibrillar network structure upon gelation in aqueous medium at basic pH values (pH 11.013.0), and th

Full text of DOI:10.1002/chem.200900149

DOI: 10.1002/chem.200900149
Fabrication of Luminescent CdS Nanoparticles on Short-Peptide-Based
Hydrogel Nanofibers: Tuning of Optoelectronic Properties
Goutam Palui, Jayanta Nanda, Sudipta Ray, and Arindam Banerjee*[a]
Dedicated to Professor P. Balaram on the occasion of his 60th birthday
Abstract: The pH-induced self-assem-
bly of three synthetic tripeptides in
water medium is used to immobilize lu-
minescent CdS nanoparticles. These
peptides form a nanofibrillar network
structure upon gelation in aqueous
medium at basic pH values (pH 11.0–
13.0), and the fabrication of CdS nano-
particles on the gel nanofiber confers
the luminescent property to these gels.
Atomic force microscopy, field-emis-
sion scanning electron microscopy, and
high-resolution transmission electron
microscopy clearly reveal the presence
of CdS nanoparticles in a well-defined
array on the gel nanofibers. This is a
convenient way to make organic nano-
fiber–inorganic nanoparticle hybrid
nanocomposite systems. The size of the
CdS nanoparticles remains almost
same before and after deposition on
the gel nanofiber. Photoluminescence
(PL) measurement of the CdS nano-
particles upon deposition on the gel
nanofibers shows significant blue
a
shift in the emission spectrum of the
nanoparticles, and there is a considera-
ble change in the PL gap energy of the
CdS nanoparticles after immobilization
on different gel nanofibrils. This find-
ing suggests that the optoelectronic
properties of CdS nanoparticles can be
tuned upon deposition on gel nanofib-
ers without changing the size of the
nanoparticles.
Keywords: gels
nanoparticles
hybrid composites · peptides
·
luminescence
·
·
organic–inorganic
Introduction
ramics.[9] However, the controlled organization of nanoparti-
cles on surfaces in three-dimensional geometries still re-
mains a very challenging task.
During the past several years, metal nanoparticles have at-
tracted continuing interest due to their fascinating proper-
ties and potential uses in electronics,[1] optics,[2] catalysts,[3]
sensors,[4] and other fields.[5] There is a genuine need to fab-
ricate nanostructures with high regularity to tune various
properties of nanomaterials. So, the spatial organization of
inorganic nanoparticles in a well-defined array has received
considerable attention. Many procedures are available in
the literature for depositing inorganic nanoparticles in an or-
dered array on surfaces through size-selective precipitation
techniques,[6] attaching nanoparticles to synthetic self-assem-
bled organic molecules/biopolymers,[7] or incorporating
nanoparticles into the solid structures of polymers[8] or ce-
Self-assembled nanostructured organic materials provide
a key for creating the controlled spatial arrangements of the
inorganic nanoparticles. An increasing number of efforts
have been reported on constructing various types of organ-
ic–inorganic hybrid nanomaterials.[10] Previously, Reches and
Gazit reported that the Alzheimerꢀs b-amyloid diphenylala-
nine core recognition motif could self-assemble into discrete
and extraordinarily stiff nanotubes in aqueous solution[11]
and these nanotubes could serve as a casting mold for pro-
ducing silver nanowires.[12] By using the self-assembly of
peptides in solution, Matsui et al.[13] reported the organiza-
tion of water-soluble metal nanocrystals (Au/Cu/Ag/Pt) into
an ordered array on bolaamphiphilic nanotubes functional-
ized by histidine-rich peptides. Low-molecular-weight gela-
tors can be self-assembled into different types of superstruc-
tures, such as fibers, rods, and ribbons, through various weak
noncovalent interactions[14] and they can act as a good tem-
plate for the fabrication of such inorganic nanoparticles.
There are several examples of the formation of metal nano-
particles in gel-phase materials.[15] Previously, Willner and
co-workers reported Au nanoparticle–hydrogel composites
[a] G. Palui, J. Nanda, S. Ray, Dr. A. Banerjee
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700 032 (India)
Fax : (+91)33-2473-2805
E-mail: bcab@iacs.res.in
arindam.bolpur@yahoo.co.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900149.
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FULL PAPER
based on polyacrylamide gels and showed their electronic
property.[16] A recent report includes the incorporation of
Au nanoparticles within the gel-phase material that is ob-
tained from the self-assembled oligo(p-phenylenevinylene)
system.[17] However, reports on tuning of the optical proper-
ty of CdS nanocrystals by immobilizing them in the gel-
phase material is rare.[18] In an earlier report, Stupp and co-
workers showed the two-dimensional array of CdS nanopar-
ticles on helical nanofibers obtained from an organogel.[19]
In another report, McPherson and co-workers demonstrated
the spatial compartmentalization of CdS nanoparticles on
self-assembled organogel-based nanofibers.[20] Recently, Li
and his group reported the immobilization of quantum dots
(CdSeS) within the dipeptide (diphenylalanine)-based orga-
nogel network.[21] However, there is no report on the immo-
bilization and fabrication of CdS nanoparticles on nanofib-
ers obtained from a short-peptide-based hydrogel network.
The creation of a bio-nanofiber–CdS nanoparticle hybrid
nanocomposite system under mild conditions still remains a
challenging task. Herein, we present synthetic self-assem-
bling tripeptide-based pH-sensitive hydrogels and their use
for immobilizing luminescent CdS nanoparticles within the
gel-phase material by incorporation into definite arrays on
the gel-nanofiber structures. In addition, the optoelectronic
properties of CdS nanoparticles can be nicely tuned by
fixing them on the surfaces of gel nanofibers without chang-
ing the size of the nanoparticles significantly before or after
deposition.
basic aqueous solution (pH 11.0–13.0) at a temperature
range of 65–758C, and when they are allowed to cool to
room temperature gels are obtained. Table 1 shows the gela-
Table 1. Gelation properties of peptides 1–3 in water.
Hydrogels ob-
tained from
Minimum gel concentra- Range of pH in which
tion [% w/v]
gels are stable
peptide 1
peptide 2
peptide 3
2.83
2.93
1.96
11.1–12.7
11.3–12.8
11.5–13.0
tion efficiency of these peptides in water medium. The sol–
gel transition temperatures (Tgel) for all three peptides at
different concentrations (% w/v) are plotted against differ-
ent gelator concentrations (Figure S10, see Supporting Infor-
mation), which shows that the Tgel values of peptidic hydro-
gels increase with increasing concentration (% w/v) of the
gelator peptides until the typical concentration regime is
reached.[23] This concentration regime is termed a “plateau
region”, at which the formation of the gel network is essen-
tially complete and addition of further gelator is no longer
able to enhance the material property by forming its aggre-
gate. On the other hand, it indicates that the gelation of the
solvent approaches saturation.
FTIR study: To understand the mechanism governing the
self-assembly of these hydrogels, a Fourier transform infra-
red (FTIR) experiment was carried out. The FTIR spectra
of the xerogels (Figure S11, see Supporting Information) ob-
tained from short peptides 1–3 show only the major band at
3330–3340 cmÀ1 that is a characteristic feature of hydrogen-
bonded NH stretching. No band was observed around
3400 cmÀ1 for all peptides, which indicates the involvement
of all peptide NH groups in hydrogen bonding. The pres-
ence of the C=O stretching bands (amide I) at 1633.6,
1638.8, and 1639.4 cmÀ1 and NH bending (amide II) peaks
at 1525.6, 1535.8, and 1525.5 cmÀ1 for peptides 1–3, respec-
tively, suggests that a b-sheet conformation is present for all
tripeptides in their corresponding dried gel state.[24] A weak
band observed at 1689.5–1691.5 cmÀ1 is indicative of an anti-
parallel b-sheet arrangement in their self-assembled gel
state.[25]
Results and Discussion
In the course of our investigation of the self-assembling be-
havior of short synthetic peptides,[15g,22] we synthesized three
water-soluble peptides 1–3 with a common chemical struc-
Wide-angle X-ray diffraction study: A wide-angle X-ray
scattering (WAXS) experiment was carried out to obtain in-
formation about the molecular packing of self-assembled
peptides in the gel state. However, heavy scattering of water
molecules prevents information about the molecular packing
of these tripeptides from being obtained in their wet gel
states. Table 2 shows the d-spacing values of the xerogels ob-
tained from all three peptidic hydrogels. It is observed that
the xerogel obtained from peptide 1 (Figure S12, see Sup-
porting Information) provides a sharp peak at 2q=7.918 cor-
responding to a d spacing of 11.2 ꢂ accompanied by another
peak corresponding to a d spacing of 4.9 ꢂ (2q=18.258).
This result indicates the presence of a b-sheet structure with
tural feature Boc-Phe-X-Phe-OH (Boc=tert-butoxycarbon-
yl; X=Val, Leu or Phe). Incidentally, all three peptides self-
associate to form pH-sensitive thermoreversible hydrogels
at basic pH values (pH 11.0–13.0). Due to the stability of
the gel-phase material in the above-mentioned pH range,
the gel nanofibers were used for immobilizing luminescent
CdS nanoparticles within their gel network structure.
Thermal stability: All three synthetic tripeptides form a gel
in aqueous medium. These peptides are readily soluble in
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A. Banerjee et al.
Table 2. Major peaks in the WAXS pattern for the xerogels obtained
from peptides 1–3.
Hydrogels ob-
tained from
d spacing [ꢂ]
peptide 1
30.44, 18.24, 11.17, 8.19, 7.48, 5.29, 4.86, 3.45, 3.05,
2.97, 2.46, 2.17, 2.01, 1.95, 1.91, 1.87
30.21, 20.86, 11.60, 7.65, 7.03, 5.72, 4.94, 4.27, 3.83,
3.54, 3.34, 3.16, 2.97, 2.83, 2.37, 2.26, 2.17, 2.04, 1.99,
1.95, 1.91, 1.88
peptide 2
peptide 3
28.94, 11.54, 5.86, 4.81, 3.43, 2.96, 2.74, 2.63, 2.28,
2.21, 2.05, 1.92
an antiparallel alignment in the dried gel state.[26] A peak
around 2q=25.788 (d=3.45 ꢂ) is characteristic of the p–p
stacking distance. So, the WAXS study supports the FTIR
results about the structure of the assembled gel state. In the
short-angle region, WAXS shows two sharp peaks: one at
2q=4.88 corresponding to a d-spacing value of 18.1 ꢂ,
which is nearly equal to the calculated length of the mole-
cule (15.5 ꢂ), and the other at 2q=2.98 corresponding to a
d-spacing value of 30.44 ꢂ. This result indicates that two
molecules are present along the b axis in the unit cell. Simi-
lar observations were noticed for the xerogels obtained
from peptides 2 and 3 (Figures S13 and S14, see Supporting
Information). A tentative model representing how the gel
fibers are formed from the gelator molecule is illustrated in
Figure 1 and provides the structural insight.
Figure 2. a–c) FESEM images of hydrogels (3% w/v) obtained from pep-
tides 1–3, respectively. Insets: the rodlike morphology of the nanofibrils
(scale bars: 1 mm).
tides have unidirectional intermolecular interactions be-
tween themselves to form such a nanoscale architecture.[27]
Energy-dispersive X-ray (EDX) analysis of the gel nanofib-
ers present in the selected area shows the presence of the
sodium ion; this peak appeared due to gel formation of the
corresponding peptides in basic
medium containing sodium hy-
droxide (Figure S15, see Sup-
porting Information). Figure 3
shows the HRTEM images of a
very dilute solution of the gel-
phase material obtained from
peptides 1–3. The widths of the
fibrils are 35–60 nm for peptide
1, 50–92 nm for peptide 2, and
40–90 nm for peptide 3. The
widths of the gel fibers mea-
sured by using TEM are much
smaller than those of the corre-
sponding gel fibers obtained
from FESEM images. This indi-
cates that small fibrils are at-
tached together to give wider
fibers.
Figure 1. Schematic representation illustrating a tentative model for molecular packing of the gel-phase mate-
rial obtained from peptide 1.
TEM and SEM study: To gain insight into the morphology
of these hydrogels, field-emission scanning electron micro-
scopy (FESEM) and high-resolution transmission electron
microscopy (HRTEM) studies were carried out for xerogels
obtained from three peptidic hydrogels. The FESEM images
(Figure 2) of the xerogels of all these tripeptides show their
nanofibrillar assembly. The fibrils are 80–100 nm in width
for peptide 1, 110–150 nm for peptide 2, and 100–120 nm for
peptide 3 and the fibrils are several hundreds of microme-
ters long. This indicates that all the reported gelator pep-
Fabrication by CdS nanoparticles
Preparation of CdS nanoparticles: These peptide-based pH-
sensitive thermoreversible hydrogels are stable within the
basic pH range of 11.0–13.0, which makes them interesting
candidates for immobilizing important inorganic nanoparti-
cles, such as semiconductor CdS nanoparticles, within the
gel network.[28] A mixture of Cd
ACTHNUGTRNEG(UN NO3)2 (1.9 mmol) and l-
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Luminescent Nanoparticles on Hydrogel Nanofibers
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spectra, which indicates that the b-sheet structure is also
present even after the deposition of CdS nanoparticles on
the gel fibers. Along with these peaks, another peak in the
range of 1689.5–1691.5 cmÀ1 was also observed. This finding
reveals that the antiparallel arrangement is retained in the
gel state even after mineralization. Here, van der Waals in-
teraction between the cysteine-capped nanoparticles and gel
fibers play a role in the ordered orientation of CdS nanopar-
ticles on the gel nanofiber.
Moreover, polarizing optical microscopic images also
demonstrate that the fibrillar morphologies for all peptidic
hydrogels remain unaltered by the mineralization process
(Figure S20, see Supporting Information). It is evident from
the micrographs obtained from the xerogels that all peptides
form a fibrillar network in their self-assembled state, and no
morphological changes are observed due to the fabrication
of CdS nanoparticles on gel fibers. CdS nanoparticles depos-
ited on gel nanofibers appeared green under the cross-polar-
izer due to the luminescent nature of this inorganic nanopar-
ticle.
Figure 3. a–c) HRTEM images of dilute solutions of hydrogels (concen-
tration: 1.41ꢃ10À4 m) obtained from peptides 1–3, respectively.
To obtain visual images of the mineralized gels, FESEM,
AFM, and HRTEM experiments were carried out with the
corresponding xerogels. Figure 4 shows an FESEM image of
cysteine (4.6 mmol) was dissolved in HPLC-quality water
(100 mL). The pH of the solution was adjusted to 11.2–11.8
and argon gas was bubbled through the solution to remove
the dissolve oxygen. Then Na2S was added to the solution,
which was heated for 30 min to obtain the CdS nanoparticle
solution. The absorption peak at 367.9 nm in the UV/Vis ab-
sorption spectrum (Figure S16, see Supporting Information)
suggests the formation of CdS nanoparticles. The size distri-
bution histogram of the CdS nanoparticles observed in the
TEM experiment indicates that the particle sizes are in the
range of 6–14 nm (Figures S17 and S18, see Supporting In-
formation). This colloidal solution was used for the prepara-
tion of the hybrid nanocomposite materials.
Figure 4. FESEM image of hydrogel obtained from peptide 1 (3% w/v)
after mineralization with CdS nanoparticles.
Incorporation of CdS nanoparticles on gel nanofibers: Each
peptide (1–3, 15.0 mg) was placed in basic aqueous solution
(450 mL, pH 11.0–13.0). Then the CdS nanoparticle solution
(50 mL) was added and the resulting mixture was heated to
758C to give a homogeneous solution (concentration of CdS
nanoparticles: 1.9ꢃ10À3 m), which was cooled to room tem-
perature. A yellowish hybrid gel-phase material was ob-
tained due to the incorporation of CdS nanoparticles into
the gel-phase material (3.0% w/v). As a result of the nonco-
valent interaction between the self-assembled gelator mole-
cules and CdS nanoparticles within the gel network, the fab-
rication process leads to an ordered orientation of semicon-
ducting nanoparticles on the gel nanofibers, and the ob-
tained gels show the photoluminescence (PL) property.
The FTIR spectra of the xerogels of the hydrogel–CdS
nanocomposite materials constructed from peptides 1–3 ex-
hibit the characteristic stretching and bending frequencies in
their self-assembled gel state (Figure S19, see Supporting In-
formation). It is observed that the bands at 3300–3340 cmÀ1
(NH stretching), 1637.5–1639.4 cmÀ1 (C=O stretching), and
1525.5–1535.8 cmÀ1 (NH bending) are also present in the IR
the mineralized gel obtained from peptide 1. The image
clearly shows that the morphological feature of the gel-
phase material is conserved during the fabrication process.
It is observed that the CdS nanoparticles are oriented in a
well-ordered array on the gel nanofibrils. The width of the
gel nanofibril does not alter upon mineralization with CdS
nanoparticles. The sizes of the CdS nanoparticles are in the
range of 8–15 nm after mineralization. Similar observations
were made for the other two gel-phase materials (data not
shown). EDX analysis shows the presence of CdS nanoparti-
cles on the surface of gel fibers (Figure S21, see Supporting
Information).
AFM experiments were performed to observe the miner-
alization by CdS nanoparticles within the three-dimensional
gel nanofiber. Figure 5 shows AFM images of the mineral-
ized gels obtained from peptides 1–3. The results indicate
that the nanoparticles are deposited on the fibers in a well-
defined array for all three peptidic hydrogels.
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(CdS) nanoparticle nanocomposite systems with peptide-
based hydrogels.
PL study: It is interesting to study the optical properties of
the CdS nanoparticles before and after fabrication on gel
nanofibers and to examine whether there is any change in
their optical property upon deposition in a well-defined
array on nanofibers. Figure 7 exhibits the PL emission spec-
Figure 5. a)–c) AFM images of hydrogels (3% w/v) after mineralization
with CdS nanoparticles obtained from peptides 1–3, respectively (scale
bars: 100 nm).
HRTEM images show the mineralization of CdS nanopar-
ticles on cross-linking gel nanofibers obtained from peptide
1 (Figure 6), and it is observed that the particles are orient-
Figure 7. PL spectrum of the CdS nanoparticle solution used for minerali-
zation and its two Gaussian-fit subbands, obtained before deposition of
CdS on the gel nanofibers. The peak positions of subbands I and II are
located at about 477 and 517 nm, respectively.
trum of the CdS nanoparticles before the mineralization
process at room temperature. The PL spectrum of the nano-
particle solution used for mineralization (50 mL of the stock
solution was diluted to 500 mL with water to maintain a sim-
ilar concentration of the CdS nanoparticles in the gel-phase
material) shows two emission peaks when excited at 367 nm.
The 477 nm PL peak position of subband I suggests that the
CdS nanoparticles in the sample exhibit properties in the
quantum confinement region, which is attributed to the
band-edge emission. The 517 nm PL peak position of sub-
band II is attributed to the emission of surface defects
caused by sulfur vacancies and/or sulfur dangling bonds.[29]
The band at 477 nm exhibits the PL gap energy for these
CdS nanoparticles of 2.60 eV.
Figure 6. HRTEM images of the hydrogel obtained from peptide 1 after
mineralization with CdS nanoparticles. Inset: Diffraction pattern of the
nanocomposite material observed under HRTEM.
PL spectra of the mineralized gel-phase materials ob-
tained from self-assembling peptides 1–3 and CdS nanopar-
ticles were recorded for thin films of these mineralized gels.
The PL experiment was carried out for wet gel and xerogels
(3% w/v) of CdS nanoparticles with the respective peptides.
For this purpose, the hydrogel–CdS nanocomposite materi-
als of the respective peptides were placed on glass slides
and dried in a vacuum for two days to prepare thin films of
the xerogels. Figure 8 represents the PL excitation (PLE)
spectra of the hydrogel–CdS nanocomposite system (wet
gel/dried gel) and the lex,max values obtained from the spec-
tra are listed in Table 3. The PL spectra of the hybrid wet
gel materials containing CdS nanoparticles exhibit emission
maxima at 428.41, 431.54, and 429.46 nm after excitation at
376 nm of the gel material obtained from peptides 1–3, re-
spectively. The corresponding PL gap energies of the nano-
ed in well-defined arrays. This result also indicates that
there is almost perfect, controlled deposition of CdS nano-
particles on the surface of the gel nanofibers as there are
hardly any particles present outside the nanofibers. Figure 6
indicates that the sizes of the mineralized semiconducting
nanoparticles are in the range of 6–14 nm. EDX analysis
shows peaks for elemental cadmium and sulfur, which
proves the presence of CdS nanoparticles on gel fibers (Fig-
ure S22, see Supporting Information). Similar observations
were also observed for the other two peptidic hydrogels
(Figures S23 and S24, see Supporting Information).
So, inspection of the FESEM, AFM, and HRTEM images
suggests that fabrication of CdS nanoparticles occurs in a
well-ordered array on gel nanofibers. This finding may have
some implications for creating new nanofiber–inorganic
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which means all surface defects have been removed during
the fabrication process and as a result, the lower energy
state situated below the conduction band vanishes. Thus, the
defect-related emission could be quenched completely by
fabrication. At the same time, the intensity of the PL in-
creases by 100 to 250 times after mineralization on gel
fibers. A blue shift in the peak position was also observed
for all hydrogel-based nanofiber–CdS nanoparticle nano-
composite systems. The increase of PL gap energy due to
the immobilization of CdS nanoparticles on the surface of
the nanofibers obtained from the peptide molecule is be-
lieved to arise from an interfacial phase between the two
species as mentioned above. This explanation is based on
the fact that there is no change in CdS nanoparticle size and
hydrophobic interaction occurs between the peptide mole-
cules and CdS nanoparticles; as a result, a 45–53 nm
(Table 3) blue shift of the peak position of subband I of the
nanocomposite has been observed.[30] This method of fabri-
cation reveals that the optoelectronic properties of CdS
nanoparticles can be tuned after their deposition on gel
nanofibers in a well-defined array.
Figure 8. PLE spectra of hydrogels obtained from a) peptide 1, b) peptide
2, and c) peptide 3 (from bottom to top; solid line: xerogel, dashed line:
wet gel).
Table 3. PLE, PL, and Eg data for the CdS–hydrogel nanocomposite
system.
Hydrogel–CdS nanocom- PLE (lex,max
)
PL gap
energy (Eg)
[eV]
lem,max in the
thin film ob-
tained from
gels [nm]
posite system obtained
from
[nm]
Generally, the optoelectronic property of CdS nanoparti-
cles can be tuned by manipulating their size in the quantum
confinement region. However, in this study we were able to
tune the PL gap energy of the CdS nanoparticles not by
changing their size, but by fixing these nanoparticles on gel
nanofibers.
wet xerogel wet xerogel wet
xerogel
gel
gel
gel
peptide 1
peptide 2
peptide 3
376 375
376 380
376 373
2.89 2.91
2.87 2.90
2.89 2.92
428.41 425.07
431.54 427.46
429.48 424.38
To investigate the optical and luminescence properties of
the hybrid materials doped with semiconducting (CdS)
nanoparticles, confocal microscopy images of all the xero-
gels were obtained with a Leica laser scanning confocal mi-
croscope (Leica Confocal System TCS SP2) equipped with
an ArKr laser. Under the microscope at 63ꢃ magnification,
the xerogels showed green PL upon excitation (Figure 10)
near the UV absorption maximum of the CdS nanoparticles.
Due to controlled deposition of luminescent CdS nanoparti-
composite systems were found to be 2.87 to 2.89 eV; these
values were calculated from their corresponding emission
maxima.
The PL spectra of the xerogel thin films obtained from
peptides 1–3 also exhibit emission maxima at 425.07, 427.46,
and 424.38 nm after excitation at 375, 380, and 373 nm, re-
spectively. The corresponding PL gap energies were calcu-
lated to be 2.90–2.92 eV (see Table 3).
Here, it should be noted that for all the hybrid nanomate-
rials obtained from hydrogel–CdS nanocomposite systems
(Figure 9), subband II vanishes in the emission spectra,
Figure 10. a) Photographs i)–iii) illustrating the incorporation of CdS
nanoparticles into the gels obtained from peptides 1, 2 and 3, respective-
ly. b)–d) Confocal microscopy images of xerogels obtained from CdS-
doped hydrogels of peptides 1–3, respectively (scale bars: 300 mm).
Figure 9. PL spectra of hydrogels obtained from a) peptide 1, b) peptide
2, and c) peptide 3 (from bottom to top; solid line: xerogel, dashed line:
wet gel).
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Atomic force microscopy: The morphologies of the reported hydrogels
and hydrogel–CdS nanocomposites were investigated by AFM. A small
amount of gel (at its minimum gelation concentration) of the correspond-
ing compounds was placed on a microscope cover glass and then dried by
slow evaporation. The material was then allowed to dry in a vacuum at
308C for two days. Images were taken with an AutoProbe CP base unit,
di CP-II instrument, model no. AP-0100.
cles on gel fibers, green fluorescence occurred in definite
arrays to give the appearance of small green threads. The
absence of the green coloration for the whole gel material
suggests that deposition of CdS nanoparticles occurs strictly
on the gel fibers, not on the outside of the fibers.
UV/Vis spectroscopy: UV/Vis absorption spectra were recorded on a
Hewlett–Packard (Model 8453) UV/Vis spectrophotometer (Varian Cary
50 Bio).
Conclusion
Optical polarizing microscopy: Small amounts of gel-phase material ob-
tained from peptides at the minimum gel concentration (3.0% w/v) were
placed on a glass microscope slide and examined under an optical polar-
izing microscope (Leica DM 1000) The material was visualized at 40ꢃ
magnification with crossed polarizers. Composite materials obtained
from the corresponding peptides were also observed under similar condi-
tions.
We have demonstrated the formation pH-responsive ther-
moreversible hydrogels from self-assembling short synthetic
peptides and the immobilization of luminescent CdS nano-
particles within the gel-nanofiber network by fabricating the
CdS nanoparticles in a definite array on the nanofibrillar
structures. This is a convenient method to obtain a hybrid
organic nanofiber–inorganic nanoparticle nanocomposite
system under mild conditions. The blue shift in the emission
spectra and the change in the PL gap energy of the CdS
nanoparticles after their deposition on the gel nanofibers
opens up an opportunity to tune the optoelectronic property
of functional nanoparticles, such as CdS nanocrystals, upon
immobilization on the nanofibrillar gel network.
PL spectroscopy: PL spectroscopic measurements of the CdS solution
and gel materials (wet gel and xerogels) obtained from hydrogel–nano-
particle nanocomposite systems were carried out with a Horiba Jobin
Yvon Fluoromax 3 instrument. The CdS solution was placed in a quartz
cell of path length 1 cm and excited at 375 nm with a wavelength incre-
ment of 1 nm and an integration time of 0.5 s. Both the wet gel and xero-
gels were placed in a quartz holder and the spectra were obtained as
above.
Experimental Section
Acknowledgements
Materials: All l-amino acids, 1-hydroxybenzotriazole, dicyclohexylcarbo-
diimide, and di-tert-butyl pyrocarbonate were purchased from Sigma
Chemicals.
J.N. wishes to acknowledge the CSIR, New Delhi, India, for financial as-
sistance. Thanks are also due to the partial support from Nanoscience
and Technology Initiatives, DST, Government of India, New Delhi. We
also thank Prof. D. Chakraborty for useful discussions.
Synthesis: The peptides were synthesized by conventional solution-phase
methods through a racemization-free fragment condensation strategy.[31]
The synthesis and characterization of all the reported peptides are re-
ported in the Supporting Information.
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Published online: June 19, 2009
Chem. Eur. J. 2009, 15, 6902 – 6909
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6909