Synthesis of anisotropic gold nanoparticles and their catalytic activities of breaking azo bond in sudan-1

Article abstract of DOI:10.1016/j.molliq.2014.06.032

Here, we report the high yield synthesis of different shaped gold nano particles (AuNPs) through seed mediated growth process in aqueous medium. Shape dependent surface plasmon resonance (SPR) and the gradual shift of longitudinal SPR band are observed with increasing anisotropy of AuNPs. Structural changes during the growth processes are observed by transmission electronic microscopy (TEM). Here, spherical nano seeds are transformed into rice and dendrimer shaped nanoparticles in the subsequent four stage growth processes. Gold nanoparticles have been demonstrated to be very efficient catalysts for the cleavage of NN of sudan-1 and it is investigated by monitoring the reduction of sudan-1 in the presence of excess NaBH₄. It has been observed that the reaction is first order with respect to the concentration of sudan-1 and the catalytic efficiency of dendrimer shaped AuNPs is higher than the other as synthesized AuNPs.

Full text of DOI:10.1016/j.molliq.2014.06.032

MOLLIQ-04329; No of Pages 8
Journal of Molecular Liquids xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis of anisotropic gold nanoparticles and their catalytic activities of
breaking azo bond in sudan-1
⁎
Gobinda Prasad Sahoo, Dipak Kumar Bhui, Debasish Das, Ajay Misra
Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapore 721 102, WB, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Here, we report the high yield synthesis of different shaped gold nano particles (AuNPs) through seed mediated
growth process in aqueous medium. Shape dependent surface plasmon resonance (SPR) and the gradual shift of
longitudinal SPR band are observed with increasing anisotropy of AuNPs. Structural changes during the growth
processes are observed by transmission electronic microscopy (TEM). Here, spherical nano seeds are transformed
into rice and dendrimer shaped nanoparticles in the subsequent four stage growth processes. Gold nanoparticles
have been demonstrated to be very efficient catalysts for the cleavage of N_N of sudan-1 and it is investigated by
monitoring the reduction of sudan-1 in the presence of excess NaBH4. It has been observed that the reaction is
first order with respect to the concentration of sudan-1 and the catalytic efficiency of dendrimer shaped
AuNPs is higher than the other as synthesized AuNPs.
Received 7 February 2014
Received in revised form 26 June 2014
Accepted 27 June 2014
Available online xxxx
Keywords:
Gold nanoparticles
Seed growth method
TEM
Catalysis
Sudan-1
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
producing size tunable nanomaterials in the solution phase. Morpholo-
gy controlled synthesis of metal nanoparticles requires the successful
Morphology controlled synthesis of nanoparticles is of great signifi-
cance due to its optical [1], electronic [2] and magnetic [3] properties
which depend strongly on the size and shape of the nanomaterials.
Noble metal nanoparticles exhibit localized surface plasmon resonances
resulting in strong optical extinction at visible wavelengths. With
increasing anisotropy of AuNPs, it demonstrates two plasmon
resonance absorption bands: one is around 510 nm, resulting from
transverse plasmon resonance and the other absorption band in the
visible or NIR range is due to the longitudinal plasmon resonance.
Shape controlled gold nanoparticles have outstanding applications in a
variety of fields, such as photonics [4–6], surface enhance Raman
scattering (SERS) [7,8] and catalysis [9,10] in material chemistry
research.
Over the last decade, many efficient synthetic approaches were
developed that allow precise control over gold nanostructures.
Although there were several reports focused on gold nano particles,
shape controlled wet chemical synthesis of gold nano particle like
rods [11,12], cubes [13], star [14], plates [15,16], wires [17], triangles
[18], and nanoribbons [19] is limited. Solution phase approach provides
convenient and reproducible routes for the fabrication of nanoparticles
with controlled size and shape. The resulting nanoparticles are not only
precisely tuned but also to easily dispersed in organic or aqueous media
for numerous potential applications in biological systems. The seed-
mediated growth technique is one of the most promising methods for
utilization of a template such as CTAB [20], SDS [21], polyvinyl
pyrrolidone (PVP) [22], and poly ethylene glycol (PEG) [23] in the
growth solution.
Azo dyes are the largest and most versatile class of dyes, which are
currently used in dyeing various materials such as textiles, leather,
plastics, and cosmetics. During these dyeing processes some amount
of dye are released into sewage treatment systems or to the environ-
ment. In particular, soluble reactive dyes are being released into the
environment. The recalcitrance of the azo dyes to biological degradative
processes results in severe contamination of the rivers and ground
water in those areas having high growth of dye industries. The current
state of the art for the treatment of waste waters containing dyes are
physicochemical techniques, such as adsorption, precipitation, chemical
oxidation, photo degradation, or membrane filtration. One of the
important applications of noble metal nanoparticles is their use as
promising heterogeneous catalysts for a variety of chemical reactions
like oxidation, reduction, hydrogenation and dye removal [24,25].
Heterogeneous catalysis that benefits from easy removal of catalyst
materials and possible use of high temperatures suffered for a long
time from lack of selectivity and understanding of the mechanistic
aspects is indispensable for parameter improvements. Despite the
above mentioned promising attributes, broad applications of nanoparti-
cles as catalysts like molecule synthesis or degradation and removal of
toxic hazardous materials from water and environment have yet to
find much attention. Keeping the above problem in mind, we have de-
cided to take the catalytic property of gold nanoparticles for the cleav-
age of azo bond in sudan-1.
⁎
Corresponding author. Fax: +91 3222 275329.
E-mail addresses: ajaymsr123@gmail.com, ajay@mail.vidyasagar.ac.in (A. Misra).
http://dx.doi.org/10.1016/j.molliq.2014.06.032
0167-7322/© 2014 Elsevier B.V. All rights reserved.
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Here, we propose a simple, facile, efficient, and economical route for
2.3. Studies of catalytic activity
high yield synthesis of anisotropic gold nanoparticles through seed-
mediated growth approach. A small volume of gold seed solution was
added to a first growth solution. Successive transfer of the nano hydro-
sol to the next growth solution results in formation of a dendrimer
shaped structure of AuNPs after four successive steps. In this work, we
also investigated AuNP catalyzed degradation of sudan-1 in the
presence of NaBH4. The as synthesized AuNPs show good catalytic
properties and can be easily separated by centrifuging from the solution
for recycling. The kinetic data indicate that dendrimer shaped AuNPs are
catalytically more active than the other gold nanoparticles.
Reduction of sudan-1 by NaBH4 in the presence of gold nano hydro-
sol was carried out to examine the catalytic activity of the AuNPs.
Catalytic activity of different shaped AuNPs was carried out using the
following procedure. Nanoparticles were precipitated out by centrifuga-
tion and then re-dispersed in water, prior to using it as catalyst. 0.1 mL,
10 mM sudan-1 in ethanol was added to a solution containing 10 mL 1:1
ethanol water mixture and 0.1 mL 1.0 M NaBH4. Then, 0.5 mL re-
disperse gold hydrosol was added to the above mixture with constant
stirring. The color of the solution changes gradually from reddish-
yellow to transparent as the reaction proceeded. The reaction mixture
was extracted with ethyl acetate at room temperature, and the organic
layers were combined and dried with anhydrous Na2SO4. After
completion of the reaction, products were monitored by TLC
(thin-layer chromatography). Purification by column chromatography
(silica gel/petroleum ether and ethyl acetate) gave the desired
products which were confirmed by FT-IR and 1H NMR spectroscopy.
Main reaction products were identified as 1-amino-2-naphthol and
aniline.
2. Experimental procedures
2.1. Materials
Chloroauric acid tetra hydrate (HAuCl4.4H2O), sodium dodecyl
sulphate (SDS), cetyltrimethyl ammonium bromide(CTAB), and L-
ascorbic acid were obtained from Merck India Ltd. Silver nitrate
(AgNO3) was purchased from Sigma-Aldrich Chemical Corp. Sodium
borohydride (NaBH4), sudan-1, acetone and cyclohexane were
purchased from S. D. Fine Chemicals. All the chemicals were of analytical
grade and used without further purification. Ethanol was purchased
from Bengal chem. Pvt. Ltd and was doubly distilled before used for
experiment. Doubly distilled water was used throughout the experi-
ment. All the glassware were cleaned by freshly prepared aquaregia
and rinsed with double distilled water prior to the experiments.
UV–Vis absorption study was done to record the change in
absorbance at a time interval of 3 min. A controlled experiment without
AuNPs was also carried out using mixtures of NaBH4 and sudan-1 and
no change in the UV–Vis spectra of sudan-1 with time was observed.
2.4. Characterizations
UV–Vis absorption spectra were measured using a Shimadzu UV-
1601 spectrophotometer. The morphology and size of Gold nanoparti-
cles were investigated using JEOL-JEM-2100 transmission electron
microscopy (TEM). Samples for TEM study were prepared by placing a
drop of gold hydrosol onto a carbon film supported on a copper grid
followed by solvent evaporation under vacuum. FT-IR measurements
were done using a Perkin Elmer (Spectrum RX1) spectrophotometer
with the KBr disk technique. 1H NMR (300 MHz) spectra were recorded
on a BRUKER-AC 300 MHz spectrometer. Chemical shifts are reported in
ppm from tetra methyl silane as the internal standard, with the solvent
resonance (deutero chloroform: 7.26 ppm).
2.2. Synthesis of gold nanostructures
2.2.1. Preparation of SDS capped gold seed
10 mL aqueous solution containing 0.1 mM HAuCl4.4H2O and 1 mM
SDS was prepared in a round bottom flask. The initial yellowish solution
slowly turned into reddish-violet with addition of freshly prepared
0.2 mL 0.1(M) NaBH4 under ice cooled condition. The above gold nano-
particle solution was used as seeds (sample A) after 2 h for the subse-
quent growth processes.
3. Results and discussion
2.2.2. Growth process
3.1. UV–Vis study
20 mL of growth solution containing 0.25 mM HAuCl4.4H2O, 0.1 M
cetyltrimethyl ammonium bromide (CTAB), 0.2 mL acetone and
0.32 mL cyclohexane was prepared in a 50 mL conical flask. 0.02 mL,
0.1 mM silver nitrate (AgNO3) and 0.1 mL of 0.1 M freshly prepared
ascorbic acid were added to the above growth solution with constant
stirring. The orange color of gold salt in the growth solution disappeared
after the addition of ascorbic acid and this change of color was due to the
reduction of Au3+ to Au+. Now, the growth solution was divided into
four parts in four different 25 mL stopper conical flask containing
5 mL growth solution each. They were labeled as B, C, D and E. 1.0 mL
of the seed solution (sample-A) was added to the growth solution la-
beled B (step 1). Rapid development of red color in sample-B indicates
the reduction of Au+ to Au0. After 30 s, 1.0 mL of sample-B was added
to the growth solution C (step 2). Solution labeled sample-C turns violet
in color indicating the formation of anisotropic gold nanoparticles.
Again, after 30 s, 1.0 mL of sample-C was added to sample-D (step 3).
In the last step (step 4), 1.0 mL of sample-D was added to sample-E.
Sample-E became blue in color and it was due to the formation of highly
anisotropic gold nanoparticles. Each solution of gold hydrosol was
centrifuged for 10 min at a speed of 8000 rpm to precipitate out the
particles from the solution and then re-dispersed in 5 mL doubly
distilled water by sonication. The re-dispersed solution was used for
further experiment. A schematic presentation of different steps of the
above synthesis procedures is shown in Scheme 1.
Formation and the stability of gold nanoparticles in aqueous
colloidal solution are confirmed by UV–Vis spectral analysis which is
one of the most important tools for characterizing the metal nanoparti-
cles. The absorption behavior arises from localized surface plasmon
resonance (LSPR), which originates from coherent oscillations of
electrons in the conduction band induced by the electromagnetic field.
Fig. 1(A) illustrates that the pink colored gold seed solution has LSPR
maxima at 508 nm. Fig. 1(B–E) shows the UV–Vis extinction spectra
of gold hydrosol obtained using four step seed growth protocols
(sample B–E). The classical electrostatic model predictions of absorp-
tion cross sections for nanospheroids of gold have been demonstrated
to split the dipolar resonance into two bands; the band centered about
508 nm is due to surface plasmon resonance along the transverse
direction and the bands centered at 680–873 nm are referred to as the
longitudinal plasmon absorption [26]. The UV–Vis spectrum of
sample-B (Fig. 1B) is broad with a peak at 508 nm and a small hump
at the long wavelength (606 nm) region. This duel peaks indicate that
AuNPs of sample-B are anisotropic in nature. Both sample-C & sample-
D have a transverse resonance band at 508 nm but a longitudinal SPR
band is shifted more to the red for sample-D (803 nm) than sample-C
(680 nm). These large shifts of longitudinal SPR band indicate that the
aspect ratio of particles of sample-D is higher than that of sample-C.
On the other hand both the transverse (610 nm) and longitudinal
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
3
Scheme 1. Schematic illustration of the steps used to synthesize anisotropic gold nanoparticles; representative colors are observed for different nanoparticle solution. (For interpretation of
the references to color in this figure legend, the reader is referred to the web version of this article.)
(873 nm) band shifted more to the red (Fig. 1E) compared to other
samples and this is due to larger size and greater anisotropy of dendri-
mer shaped AuNPs of sample-E.
3.2. TEM study
TEM images of gold nanoparticles obtained from seed and different
growth solution are shown in Figs. 2 & 3 respectively. These pictures
illustrate an interesting morphological evaluation as the growth
proceeds. Fig. 2A shows TEM images of seed prepared in aqueous SDS
using sodium borohydride as reducing agent at 0 °C. Particles are mostly
spherical in shape with an average diameter of ~10 nm. SAED image
(Fig. 2) suggests that the seed particles are crystalline in nature. Fig. 3-
(i) & (ii) shows the TEM images of sample-B & C with uneven spherical
and rice like structures. Sample-D and E appear highly anisotropic and
dendrimer in shape as shown in Fig. 3-(iii) & (iv). SAED images (inset
of Fig. 3-iv) of the dendrimer shaped particles show the diffraction
pattern from the different crystal plane of face centered cubic (fcc)
gold nanoparticles and the sharp intensity of the circle arising from
different crystal plane suggests that particles are mostly crystalline in
nature.
1.5
1.2
0.9
0.6
A
B
C
D
E
0.3
0.0
3.3. EDX study
500
550
600
650
700
750
800
850
900
wave length (nm)
Energy dispersive X-ray (EDX) spectroscopy analysis suggests that
the signals are coming from pure gold, as shown in Fig. 4. Apart from
gold a strong signal at the position of Cu is also observed and this strong
signal of Cu is coming from copper grid. The same study is performed on
focusing other positions of the grid and the similar signals of Au along
Fig. 1. UV–Vis absorption spectra of (A) gold seed sol and (B–E) the gold sol synthesized in
four different steps through seed mediated growth approach. Inset picture shows the color
of the solutions. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
A
Fig. 2. TEM image (A) of gold seed synthesized in aqueous SDS. SAED image of gold seed particle.
with Cu are observed. It confirms that our synthesized particles are pure
gold particle.
and/or AgBr are deposited on the surface of the gold seeds. Because of
potential deposition or chemisorptions of Ag0 and/or AgBr, defects
and/or islands are produced on the surface of the seeds that provide
active sites for subsequent growth. Gold atoms which are formed due
to the reduction of Au+ by mild reducing ascorbic acid on the seed
surface begin to grow on these active sites of seed particles, yielding
anisotropic shaped particles. The morphology of sample-C, D & E
becomes highly anisotropic due to the use of irregularly faceted parti-
cles i.e. sample-B, C & D respectively as seed. The already nucleating
small gold cluster developed on an otherwise smooth facet, and some
3.4. Effect of foreign ions (Ag+ ion), SDS and CTAB
Ag+ ions are used as catalyst in the growth solution for the aniso-
tropic growth of nanoparticles. Ag+ ions in the growth solution are
reduced to Ag0 slowly in the presence of mild reducing agent ascorbic
acid. Since AgBr is produced due to the presence of high concentration
of bromide ions in the growth solution, a significant amount of Ag0
Fig. 3. TEM images of (i) sample-B, (ii) sample-C, (iii) sample-D and (iv) sample-E.
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
5
Fig. 4. Energy-dispersive X-ray spectrum (EDAX) of sample-E.
small tips and islands are formed on the surfaces of irregular shaped
particles (sample-D). When these irregular shaped particles are added
to the growth solution, these small tips acted as nucleation sites for
the subsequent growth of gold and this results in dendrimer-shaped
(sample-E) particle as shown in Fig. 3-(iv).
The small size AuNP seed can be synthesized in the presence of SDS.
In the absence of SDS larger size AuNPs are formed and those particles
cannot be used for synthesizing highly anisotropic gold nanoparticles.
During growth process, CTAB also plays an important role of template
for the formation of anisotropic gold nanoparticles. CTAB not only acts
as templates but also helps to stabilize the negatively charged Au(III)
and Au(I) species by quantitative binding. In the absence of CTAB,
addition of excess ascorbic acid to the aqueous solution of AuCl−4 instan-
taneously generates small gold nanoparticles at room temperature.
Acetone is used to loosen the micellar frame work and cyclohexane is
necessary for the increased formation of elongated CTAB micelle [27]
which acts as template for anisotropic growth of nanoparticles.
as seed. A key feature of the disproportionation of AuI via 3AuI
→ 2Au0 + AuIII that helps to establish its role during the formation of
nanocrystals is that the disproportionate reaction is catalyzed by metal-
lic gold [28] and is therefore autocatalytic [29].
h
i
h
i
‐
AuIIIC14 ½aq:ꢀCTB AuIIIBr4 ‐micelle
h
i
h
i
‐
−
AuIIIBr4 ½micelleꢀ2e‐from ascorbic acid AuIBr2
h
i
‐
AuIBr2 Au0 from gold nano seedAuIII þ Au0ðnano crstalsÞ
3.6. Catalytic activity of AuNPs
Reduction of azo dye to their corresponding amino derivatives is
industrially important. Normally sudan-1 is not reduced by sodium
borohydride in aqueous or alcoholic solution. Sudan-1 exhibits a strong
absorption peak at 487 nm. Upon addition of NaBH4 and gold nanopar-
ticles as catalyst (i.e., an aqueous dispersion of the different shaped gold
nanoparticles), the absorption peak at 487 nm gradually decreases in
intensity as the reduction proceeds (Fig. 5-i) and a new band centered
at ~355 nm is appeared and it corresponds to the formation of
1-amino-2-naphthol and aniline. Decreasing intensity of the 487 nm
band with time suggests the decreasing concentration of sudan-1 as
the reaction proceeds. However, there is no proportional increase in
the 1-amino-2-naphthol and aniline peak intensity; probably due to
the difference in the molar extinction co-efficient of 1-amino-2-
naphthol and aniline with respect to sudan-1. Since the peak at
487 nm was much stronger than that at 355 nm, kinetics of the reaction
3.5. Effect of ascorbic acid
In CTAB solution, [AuIIICl4]− presents as [AuIIIBr4]− with the bromide
ions originating from the partially dissociated CTAB. [AuIIIBr4]− is
reduced to [AuIBr2]− through a two electron process in the presence
of ascorbic acid and ascorbic acid is oxidized to dehydro ascorbic acid
(a two-electron process). Importantly however, at this stage we find
that AuI ions in the growth solution are not reduced to Au0 at any appre-
ciable rate even when an excess of up to five molar equivalents of ascor-
bic acid is present. But, AuI ions in the growth solution are reduced to
Au0 only upon the addition of previously synthesized Au nanoparticles
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
i
0min
3min
6min
9min
....
....
33min
300
350
400
450
500
550
600
Wave length (nm)
0.4
0.2
ii
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
A
B
C
D
E
0
5
10
15
20
25
30
Time (min)
Fig. 5. (i) Typical UV–Vis spectra for the successive reduction of sudan-1 by NaBH4 in the
presence of AuNP hydrosol of sample-C. (ii) Plots of ln At versus time, t in the presence of
different shaped AuNPs as catalyst (sample A–E) at 25 °C.
Fig. 6. (i) 1H NMR (300 MHz) spectra of 1-amino-2-naphthol. Chemical shifts are reported
in ppm using tetra methyl silane as the internal standard, with the solvent resonance
(deuterochloroform: 7.26 ppm). (ii) Solid state FT-IR spectra of 1-amino-2-naphthol.
were measured by recording the absorbance of 487 nm band with time.
The initial volume of gold hydrosol and the concentrations of sudan-1
and NaBH4 were kept identical for each set for comparison of the
kinetics. UV–Vis spectra exhibit two isobestic point at ~313 nm and
~392 nm respectively (Fig. 5-i), indicating the presence of two or
more species, in the solution. AuNP catalyzed products of the above
reaction mixture were extracted and identified using TLC, FTIR and
NMR spectroscopic analysis. In this case the main reaction product is
1-amino-2-naphthol and aniline. After cooling to room temperature,
the reaction mixture was extracted with ethyl acetate, and the organic
layers were combined and dried with anhydrous Na2SO4. Purification
by column chromatography (silica gel/petroleum ether and ethyl
acetate) gave the desired products which were confirmed by FT-IR
C\N stretching vibration. The product with a melting point of 219–
220 °C showed the NMR peak at δ 5.6 (broad s, aromatic amine, 2H),
10.2 (broad s, phenolic 1H) and 7.1 to 7.9 (m, naphthalene 6H) in the
NMR spectrum in CDCl3 (Fig. 6-i).
The proposed mechanism of the AuNPs catalyzed surface reaction is
shown in Scheme 2. The catalytic reduction proceeds on the surface of
the metal nanoparticles. The AuNPs react with borohydride ions to
form the metal hydride. Concomitantly, sudan-1 that adsorbs onto the
AuNP surface is reduced by hydride ions on the nano surface. The
adsorption/desorption of both reagents on the AuNP surface are fast
and can be modeled in terms of a Langmuir isotherm. To check the
feasibility of reaction, two blank experiments, one with sudan-1 under
identical experimental conditions but no AuNPs and the second one
without NaBH4 were carried out and no significant change in the optical
density of sudan-1 was observed. Thus, AuNPs act as catalyst for the
reduction of sudan-1 by NaBH4. Here the rate determining step is the
cleavage of the adsorbed sudan-1 to 1-amino-2-naphthol and aniline,
and 1H NMR. FT-IR spectra (Fig. 6-ii) show strong band at 817 cm−1
,
1160 cm−1, 1640 cm−1 and 3200 to 3600 cm−1. The broad band at
3200 to 3600 cm−1 is due to \N\H and \OH stretching vibration.
The \N\H vibrations (both symmetric and asymmetric) in aromatic
compounds often overlap with the aromatic \OH absorption, which
also appear in the 3100–3500 cm−1 regions to give broad spectra. An
out-of-plane N\H bending vibration that appears as a broad band at
817 cm−1 and band at 1640 cm−1 that appears for N\H bending
vibrations are present in the product. The band at 1160 cm−1 is due to
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G.P. Sahoo et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
7
NH2
OH
N
Au nanoparticles
EtOH:H2O=1:1; RT; 30Min
N
+
OH
NH2
Proposed Mechanism:
H-
NH
H-
NH
HN
O
N
N
O-
O-
+
NH2
NH2
OH
Scheme 2. Proposed mechanism of N_N bond cleavage of sudan-1 by sodium borohydride in the presence of gold nanoparticles.
which desorbs afterward. Therefore, the rate constants can be evaluated
by first-order kinetics when excess NaBH4 is used. So, the apparent rate
constant ‘Kapp’ is proportional to the total surface area ‘S’ of the nanopar-
ticles [30–32].
gold nanoparticles. The higher catalytic activity of dendrimer shaped
AuNPs is due to the presence of irregular facets which are presumed
to be the reactive site to catalyze the reduction of sudan-1 on the
nanoparticles surface in the presence of NaBH4.
dct
dt
4. Conclusions
−
¼ KappCt ¼ K1SCt
ð1Þ
The morphological, structural, and spectral changes involved in the
seed-mediated growth of gold nanostructures in the presence of CTAB
were systematically investigated. A four step protocol for the synthesis
of AuNPs that allows the morphology of the products to be changed
markedly by varying the number of steps in the seed mediated growth
process. This is a facile and effective method for rapid and large-scale
synthesis of anisotropic AuNPs. It has been observed that anisotropic
AuNPs with attractive optical properties could have promising applica-
tions to surface-enhanced Raman spectroscopy, chemical or biological
sensing, and the fabrication of nano devices. The present method of
synthesis is interesting due to the simplicity of operation, cost-
effectiveness, high throughput, and lack of elaborate equipments. Here
in, we presented the catalytic activities of different shaped gold
nanoparticles for the cleavage of azo bond of sudan-1 to 1-amino-2-
naphthol and aniline. And, it is expected that the gold nano dendrimer
with high catalytic activity will greatly promote their practical
application to eliminate the organic pollutants from wastewater.
Since C0∞ A0 and Ct∞ At
The above rate equation can be rewritten,
lnAt ¼ lnA0‐ Kapp
ꢁ
t
where Ct is the concentration of sudan-1 at time t and K1 is the rate
constant normalized to S, the surface area of nanoparticles. Upon
addition of Au-based catalyst, regardless of the morphology a certain
period of time is required for sudan-1 to adsorb onto the catalyst's
surface before the reaction could be initiated. This is called induction
time t0 in which no reduction takes place. Then the reaction becomes
stationary and follows a first order rate law. This induction time is
responsible for the non-linear nature of lnAt vs. t plot. While analyzing,
we have fitted the data points beyond the induction time with linear
plot. The reaction rate constant using different shaped AuNPs as catalyst
is evaluated by plotting the log At versus time, where At stands for
absorbance at time ‘t’ (Fig. 5-ii). Our calculated reaction rate constants
Acknowledgment
(kapp) at 25 °C are 7.57 × 10−3 s−1, 1.491 × 10−2 s−1, 2.402 × 10−2 s−1
,
7.871 × 10−2 s−1 and 1.292 × 10−1 s−1 for the sample (A to E),
which indicate a higher catalytic activity for the dendrimer shaped
We gratefully acknowledge the financial support received from CSIR
(ref. no. 01 (2443)/10/EMR-II), New Delhi for carrying out this research
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TEM measurements.
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Please cite this article as: G.P. Sahoo, et al., Journal of Molecular Liquids (2014), http://dx.doi.org/10.1016/j.molliq.2014.06.032