World Journal of Nano Science and Engineering, 2011, 1, 51-61
doi:10.4236/wjnse.2011.12008 Published Online June 2011 (
Copyright © 2011 SciRes. WJNSE
Studies on the Effect of t h e Capping Materials on the
Spherical Gold Nanoparticles Catalytic Activity
Roshdi Seoudi1,2, Doaa A. Said3
1Spectroscopy Department, Physics Division, National Research Center, Cairo, Egypt
2Department of Physics, College of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
3Physics Department, Faculty of Women, Ain Shams University, Cairo, Egypt
Received March 30, 2011; revised April 11, 2011; accepted April 18, 2011
Size-controlled gold nanoparticles (AuNPs) were prepared in the presence of different capping materials
(sodium citrate, cetyltrimethylammonium bromide (CTAB), and chitosan). The results obtained suggest that
the AuNPs were synthesized with different particle size, which is controlled by changing the molar ratio be-
tween sodium citrate, (CTAB), and chitosan to Au (III). The catalytic activities of the AuNPs with different
capping materials were studied for 4-nitrophenol reduction by NaBH4 as a model reaction. AuNPs with dif-
ferent capping materials is comparable from the value of the apparent rate constant of 4-nitrophenol reduc-
tion (0.6 × 10–3, 1.9 × 10–3, and 2.4 × 10–3 s–1) for sodium citrate, CTAB and chitosan. From the results, it is
concluded that, AuNPs catalyzed the electron transfer process between 4
and nitro compounds with all
the capping materials used AuNPs capped by chitosan were more active for the reduction than the other two.
Keywords: Gold Nanoparticle, Catalytic Activity, TEM, UV-VIS Spectroscopy
1. Introduction
Nanomaterials and its composites have become increa-
singly popular due to their size-specific, unique proper-
ties, and promising a breakthrough in the development of
novel methods in medicine [1,2], and sensor [3,4]. Gold
nanoparticle AuNPs, have received more attention due to
their strong optical absorption in the visible region [5,6],
catalysis properties [7-9], and enhanced sensitivity in
surface-enhanced Raman scattering (SERS) studies [10].
All the above properties are strongly affected by the size
and shape of the AuNPs. Several methods are known and
have been exploited for centuries and there are still many
deficiencies in the development of stable colloids con-
taining AuNPs of various sizes and shapes for precise
application. All synthesis procedures must overcome
thermodynamic principles, which predict that AuNPs
will tend to agglomerate, collapsing the colloid via pre-
cipitation or flocculation of the particles. This tendency
can be hindered by chemical species, which surround the
particles, making particle agglomeration difficult. The
synthesis of AuNPs in aqueous solution is still the gener-
al route. The most popular method for preparing AuNPs
in water used the citrate to reduction of HAuCl4 under
boiling conditions [11]. Colloidal Au nanospheres were
prepared in CTAB [12]. Therefore, diverse approaches
have been developed to the reduction of Au(III) salts in
water [13-17] using different ligands as colloidal particle
stabilizers [18-23]. Until around 1980, gold was not fully
recognized for its catalytic ability. Recent techniques
have shown AuNPs to be highly active, and it has the
distinction of being the most highly active catalyst for
the hydrochlorination of acetylene to produce vinyl chlo-
ride [24]. The complexities of catalyst fouling have given
rise to a number of simulations that attempt to explain
and predict catalytic fouling [25-28]. Gold has many
unexpected properties that prevented earlier discovery of
its catalytic potential. Unlike most other elements, gold
does not have a stable oxide. It also does not follow the
rule of thumb that other catalysts. The atomic radius of
gold is smaller and its most stable state is gold III [29,30].
However, application prospects of conventional chemi-
cally synthesized nanomaterials are often complicated
because of their toxicity, caused by contamination with
chemical precursors or additives during their synthesis
procedures [31]. Recently, much attention has been paid
to chitosan due to its excellent properties such as bio-
compatibility, biodegradability, nontoxicity [32-36]. Chi-
Copyright © 2011 SciRes. WJNSE
tosan is a natural polymer with abundant primary amino
groups in its molecular structure; the structure is shown
in Figure 1.
Chitosan was chosen as a protecting agent in synthesis
of metal nanoparticles. Esumi et al. [37] reported the
formation of gold–chitosan nanocomposites by adsorp-
tion of chitosan molecules on particle surfaces. Ballauff
et al. [38] have shown that it is possible for chitosan to
not only stabilize, but also reduce gold chloride into gold
nanoparticles. AuNPs are highly active and it can be
used to decrease the temperature catalysts in the number
of important reactions such as CO oxidation and propyl-
ene epoxidation [39]. In this work, gold nanoparticle was
prepared with different capping materials (sodium citrate,
CTAB and chitosan) and used for catalysis of the reduc-
tion of 4-nitrophenol (4-NP) to study the effect of cap-
ping on the catalytic reduction which has not been dem-
onstrated earlier.
2. Experimental
Cetlytrimethylammonium, sodium borohydried (NaBH4),
tri-sodium citrate, and hydrogen tetrachloroaurate (III)
were purchased from Sigma-Aldrich. Medium-molecu-
lar-weight chitosan (2-amino-2-deoxy-(1-4)-β-D-glu-
copyranose) with a degree of deacetylation of 100% and,
4-Nitrophenol (reagent grade) was purchased from Fluka.
All aqueous solutions were made with ultra-high-purity
water purified.
2.1. Samples Preparation
2.1.1. Preparation of AuNPs Capped by Sodium
A 20 mL aqueous solution containing 2.5 × 10–4 M
HAuCl4 and 2.5 × 10–4 tri-sodium citrate was mixed in a
conical flask. Next, 0.6 mL of ice cold 0.1 M NaBH4
solution was added to the solution all at once while stir-
ring. The solution turned pink immediately after adding
NaBH4, indication particle formation. The nanoparticle
solutions were used in a catalytic reduction within 12 h
after preparation. Citrate serves only as the capping agent
since it cannot reduce gold salt at room temperature
2.1.2. Preparation of AuNPs Capped by CTAB
AuNPs capped by CTAB was prepared by mixing CTAB
solution (5.0 mL, 0.20 M) with 5.0 mL of 0.00050 M
HAuCl4. To the stirred solution, 0.60 mL of ice-cold
0.010 M NaBH4 was added, which resulted in the forma-
tion of a brownish yellow solution. Vigorous stirring of
the solution was continued for 2 min, then the solution
was stirred, it was kept at 25˚C without further stirring.
2.1.3. Preparati on of Au NPs cap ped by Ch i tos an
A completely dissolved of chitosan solution, 0.04 gm (20
mg/ml) in 1% acetic acid solution was prepared first; due
to the poor solubility of chitosan, the mixture was vor-
texed to completely dissolve it and kept overnight. The
solution was filtered through 30 μm Millipore syringe
filters to remove any impurities before use. 20 ml of
aqueous solution of HAuCl4 (10 mM) was added to a 40
ml chitosan solution under magnetic stirring for two
hours and 8 ml of NaBH4 (0.1 M) freshly prepared was
added drop by drop. The solution turned brown imme-
diately after addition of NaBH4; stirring continued until a
transparent wine-red solution was obtained. The sample
was kept at room temperature. The same procedure was
repeated by adding 10 mM of HAuCl4 to 40 ml of chito-
san nanoparticle.
2.2. Catalytic Reduction of 4-Nitrophenol
The photocatalysis reactions were carried out in standard
quartz cuvette with a 1-cm path length containing the
reaction mixture, 1.4 cm3 of water, and 300 μL of 2 mmol
dm–3 4-nitrophenol were taken. Addition and proper
mixing of 1 cm3 of aqueous 0.03 mol dm–3 sodium bo-
rohyride and 30 μL of gold nanoparticle solution to the
reaction mixtures caused the decrease in the intensity of
the peak of 4-nitrophenol. The progress of the reduction
of 4-NP was monitored in situ using a UV-visible spec-
trophotometer (Oceanoptics HR4000 Cg). The reaction
temperature was held constant at room temperature
Figure 1. Chemical structure of chitosan.
Copyright © 2011 SciRes. WJNSE
(15˚C) to reduce thermal effects on the catalytic rate. The
time for the reduction started and completion of the reac-
tion varies and depends upon the capped of the AuNPs.
3. Results and Discussion
3.1. UV-Visible Spectra
UV-vis absorption spectra of AuNPs capped by sodium
citrate, CTAB, and chitosan are shown in Figures 2-4.
The absorption spectrum of gold nanoparticle capped by
sodium citrate shows a sharp peak at 514 ± 3 nm. The
slow shift of the band position depends on the ratio of the
gold salt and capping materials during the reaction
processes. This peak is due to collective oscillation of the
electrons in the conduction band due to the oscillation
frequency, known as the surface plasmon oscillation. At
resonance the amplitude of the local electric field in the
particle, El is enhanced as compared to the one of the ap-
plied field, Eo. In other words, complex local field factor
fl =El/Eo greater the SPR. The position of the plasmon
band for AuNPs capped by CTAB nearly the same as
sodium citrate. In the other side this band was shifted to
higher wavelength in the case of capped by chitosan and
the change of the plasmon band more clearly than sodium
citrate and CTAB. Chitosan is used as a stabilizing poly-
mer for AuNPs because the dispersed solutions are due to
formation of coordination bonds between Au ions and the
amine and hydroxyl groups of chitosan and this chelation
evenly disperses Au ions which reduced to form dis-
persed AuNPs of relatively uniform size. It is easy to
change the particle size by changing the ratio between Au
salt and chitosan and this change in the particle size is
more clearly than sodium citrate and CTAB. This is may
be due to the chemical reaction between the amine and
hydroxyl group in chitosan and Au (III) more active than
that of sodium citrate and CTAB.
3.2. Transmission Electron Microscope Data
Figure 5(a-f) shows a typical TEM images of gold na-
noparticle capped by sodium citrate, CTAB and chito-
san as well as the frequency % as a function of the aver-
age particle size (in the images). The samples that meas-
ured by TEM was chosen according to the plasmon band
position. The ratio between the Au salt and capping for
these samples was (10:20), (10:5), and (10:20) for so-
dium citrate, CTAB, and chitosan, respectively. The im-
ages indicated that, the gold was prepared with particle
size about 15 nm for all capping and some aggregation
was appeared in AuNPs capped by chitosan.
3.3. Catalytic Reduction of 4-Nitrophenol
To obtained the AuNPs with different capping materials
is an excellent catalyst for hydrogenation reaction, we
Figure 2. UV-vis spectrum of AuNPs prepared with different ratio from gold salt and capping material (sodium citrate).
Copyright © 2011 SciRes. WJNSE
Figure 3. UV-vis spectrum of AuNPs prepared with different ratio from gold salt and capping material (Cetlytrimethylam-
monium bromide).
Figure 4. UV-vis spectrum of AuNPs prepared with different ratio from gold salt and capping material (chitosan).
Copyright © 2011 SciRes. WJNSE
(a) (b)
(c) (d)
(e) (f)
Figure 5. TEM images of AuNPs c apped by sodium citrate (a) and its frequency % as a function of the average particle size
(b), CTAB (c) and its frequency % as a function of the average particle size (d), and chitosan (e) and its frequency % as a
function of the average particle size (f).
Copyright © 2011 SciRes. WJNSE
investigated the reduction of 4-nitrophenol by sodium
borohydride without and with AuNPs. The reaction was
chosen because UV-visible absorbance respond directly
corresponds to the state of the reaction; therefore, rate
constant can be derived from the intensity peaks. Figure
6 shows the ultraviolet–visible spectrum of the 4-NP
without and with NaBH4. From this figure it is clear that;
the two absorption beaks at 226 and 317 nm in the spec-
trum of (4-NP) assigned to *
ππ (from the ring of
phenol) and *
πn (from lone pair of electron in the
oxygen and nitrogen atom). After adding NaBH4, the
band at 317 nm immediately red shifted to 400 nm due to
the generation of 4-nitrophenate anions in the system.
Also, the color of the solution was changed from light
yellow to yellow-green. In the absence of AuNPs as a
catalyst, this peak remained unaltered even after 10 h
Figure 6 and this indicating that; the reducing agent
NaBH4 itself cannot reduce the 4-nitrophenolate ion and
the reaction rates can be assumed to be independent of
borohydride so the reduction is not achievable in the
presence of NaBH4 alone.
After the addition of AuNPs capped by sodium citrate
to the solution and in order to follow the kinetics of the
reduction reaction, the change in the intensity of absorp-
tion of nitrophenolate was monitored using UV–visible
spectrophotometry at regular time intervals. Sodium bo-
rohydried reduces water to hydrogen
42 22
The reduction reaction is carried out by the hydrogen
and involves the production of hydrogen gas seen in the
form of bubbles. There is a concomitant emergence of a
peak at 310 and 230 nm which corresponding to the for-
mation of 4-aminophenol Figure 7. Continuous reduc-
tion in the intensity of the peak at 400 nm shows the
consumption of 4-nitrophenol. The reaction mechanism
can be reasoned by the inherent hydrogen adsorption,
desorption characterics of Au nanoparticle. The AuNPs
shuttle the hydrogen transport between NaBH4 and
4-nitrophenol. The shuttling behaviors can be reasoned
that the AuNPs adsorbs hydrogen from the NaBH4 and
efficiently release during the reduction reaction and
hence AuNPs acts as a hydrogen carrier in this reduction
reaction Scheme 1.
The same behaviors were observed after adding the
AuNPs capped be CTAB Figure 8 and chitosan Figure 9
except that the rate reaction. The reaction was setup in
such a way that pseudo first order rate kinetics can be
used to model the reaction. Pseudo first order law
KA t
 where
. The apparent rate
constant is calculated from the decrease of the peak in-
tensity of nitrophenol at 400 nm (the slope of the curve
in Figure 10 and it is found to be 0.6 × 10–3, 1.9 × 10–3,
Figure 6. UV-vis spectrum of 4-nitrophenol using sodium borohydried as a catalyst at different time.
Copyright © 2011 SciRes. WJNSE
Figure 7. UV-vis spectrum of 4-nitrophenol using sodium borohydried and AuNP s capped by sodium citrate as a catalyst at
different times.
Figure 8. UV-vis spectrum of 4-nitrophenol using sodium borohydried and AuNPs capped by CTAB as a catalyst.
Copyright © 2011 SciRes. WJNSE
Figure 9. UV-vis spectrum of 4-nitrophenol using sodium borohydried and AuNPs capped by chitosan as a catalyst.
Figure 10. UV-Vis absorption changes vs. time in seconds for the disappearance of 4-nitrophenol absorption at 400 nm.
Copyright © 2011 SciRes. WJNSE
hydrogen adsorption
Scheme 1.
Structure 1 of AuNPs capped by chitosan.
and 2.4 × 10–3 S–1 for AuNPs capped by sodium citrate,
CTAB and chitosan, respectively.
It was found that was higher in case of chitosan and it
is demonstrate that the AuNPs capped by chitosan is a
highly effective carrier for catalysis. This seems obvious
when we consider that the mobility of NH and OH
groups on the AuNPs-chitosan increase the surface activ-
ity of gold nanoparticles Structure 1.
5. Conclusions
The rate of the gold nanoparticle reduction of
4-nitrophenol with sodium borohydried is faster in case
of AuNPs capped by chitiosan because the surface activ-
ity is more active in the catalytic activity application. So
that chitosan brush particles will work more effectively
as a template for gold in the water solution compared to
other capping.
6. Acknowledgements
The author expresses their thanks to Professor M. A.
El-Sayed and his group, LDL, Gatech for helping me
during preparation this work.
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