Magnetically Recoverable Gold Nanorods as a Novel Catalyst for the Facile Reduction of Nitroarenes Under Aqueous Conditions

Article abstract of DOI:10.1007/s10562-016-1921-4

Abstract: In this work, cysteine-functionalized Fe₃O₄@Carbon magnetic nanoparticles were used for the synthesis of gold nanorods. Fe₃O₄@C nanoparticles were first prepared by synthesis of Fe₃O₄magnetic nanoparticles (MNPs), and then carbon-coated MNPs (Fe₃O₄@C) were synthesized by glucose carbonization using a hydrothermal method. Finally, the gold NRs were loaded on the modified surface of Fe₃O₄@C MNPs. The designed magnetically recoverable gold nanorods, after full characterization by FTIR, SEM, TEM, TGA, VSM, XRD, and ICP-OES, were applied to the reduction of nitroarenes. The Fe₃O₄@C@Cys–Au nanorods showed higher performance than Fe₃O₄@C@Cys–Au nanospheres in a selective facile reduction of nitroarenes to the corresponding aminoarenes in aqueous medium at room temperature using NaBH₄. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1921-4

Catal Lett
DOI 10.1007/s10562-016-1921-4
Magnetically Recoverable Gold Nanorods as a Novel Catalyst
for the Facile Reduction of Nitroarenes Under Aqueous
Conditions
Kamran Lamei1 · Hossein Eshghi1 · Mehdi Bakavoli1 · Sadegh Rostamnia2
Received: 16 September 2016 / Accepted: 13 November 2016
© Springer Science+Business Media New York 2016
Abstract In this work, cysteine-functionalized Fe3O4@
Carbon magnetic nanoparticles were used for the synthe-
sis of gold nanorods. Fe3O4@C nanoparticles were irst
prepared by synthesis of Fe3O4 magnetic nanoparticles
(MNPs), and then carbon-coated MNPs (Fe3O4@C) were
synthesized by glucose carbonization using a hydrother-
mal method. Finally, the gold NRs were loaded on the
modiied surface of Fe3O4@C MNPs. The designed mag-
netically recoverable gold nanorods, after full characteri-
zation by FTIR, SEM, TEM, TGA, VSM, XRD, and ICP-
OES, were applied to the reduction of nitroarenes. The
Fe3O4@C@Cys–Au nanorods showed higher performance
than Fe3O4@C@Cys–Au nanospheres in a selective facile
reduction of nitroarenes to the corresponding aminoarenes
in aqueous medium at room temperature using NaBH4.
Graphical Abstract
Keywords Gold nanorods · Cysteine-modiied
Fe3O4@C · Ultrafast reduction of nitroarenes · Aqueous
mild condition
1 Introduction
The catalytic behavior of noble metal nanoparticles has
been explored in various areas of organic synthesis [1,
2], including the reduction of nitroarenes [3], CO oxida-
tion [4], etc [5]. The catalytic activity of noble metal
NPs is mainly dependent on their size, shape, crystallin-
ity, and the surface state since the potential binding sites
at the surface of NPs affect the catalytic properties. The
catalytic properties of Au [6, 7], Pt [8], Cu [9], Pd [10],
and etc. NPs with a spherical shape have been studied.
Among these noble metal NPs, Au NPs are the most
studied stable metal NPs catalysis of organic reactions
by supported gold nanoparticles [11–17]. Recently, the
high-yielding synthesis of monodisperse gold nanorods
has been developed, but only a few papers have reported
on their applications. Gold nanorods (Au NRs) with dif-
ferent aspect ratio (length/width) can absorb and scatter
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1921-4) contains supplementary
material, which is available to authorized users.
* Hossein Eshghi
heshghi@um.ac.ir
1
Department of Chemistry, Faculty of Science, Ferdowsi
University of Mashhad, 91775-1436 Mashhad, Iran
2
Organic and Nano Group (ONG), Department of Chemistry,
Faculty of Science, University of Maragheh, Maragheh, Iran
1 3

K. Lamei et al.
a wide range of laser light wavelengths, which makes
them suitable materials for photothermal bacterial lysis
[18]. Also, as a consequence of the presence of sharp
edges and corners, the number of active surface sites in
Au NRs is very high compared with spherical Au NPs,
and the small molecules could easily adsorb on the Au
surface, which led to excellent catalytic activities in
many cases [19]. Other applications, such as sensing,
drug and gene delivery, bioimaging, photothermal ther-
apy, and optical storage have been reported [20]. Ami-
noarenes are versatile valuable intermediates widely
used in the preparation of dyes, polymers, pharmaceu-
ticals, pigments, agrochemicals and cosmetics [21–26].
The reduction of nitroarenes catalyzed by noble metal
NPs is an important synthetic method to produce ami-
noarenes [27, 28]. Given reactions occur on the sur-
face of the NPs it is important to maximize the avail-
able noble metal atoms, by dispersion onto supports
which increases availability and avoids agglomeration.
Surface modification with various functional groups
such as thiols also, affects the catalytic properties. The
strong electronic interactions between thiol groups and
NPs prevent Au NPs from aggregating and deactivation.
Amphiphilic ionic liquid terminated gold nanorods were
found to be effective for the reduction of nitro com-
pounds [29].
Scheme 1 FCC–Au NRs catalyzed reduction of nitroarenes
2 Experimental
2.1 Materials
Hydrogen tetrachloroaurate, L-cysteine, iron(III) chloride
hexahydrate, sodium acetate, trisodium citrate, glucose,
cetyltrimethylammoniumbromide, ethylene glycol, etha-
nol, nitric acid and other compounds were obtained from
Sigma-Aldrich in analytical grade and used without further
puriication.
In addition to monometallic catalytic systems, gold
NPs supported on TiO2 [30], CeO2 [31], Al2O3 [32], car-
bon nanotube [33], graphene oxide [34], polymers [35]
and zeolites [36], have been extensively explored for the
hydrogenation of nitroarenes to aminoarenes. Also, a
wide range of organic reactions were catalyzed by sup-
ported gold nanoparticles [37].
2.2 Synthesis of Gold Nanorods
Gold nanorods were prepared using a seedless growth
method in aqueous solution, comparable to the method pre-
viously reported by El-Sayed et al. [41]. An aqueous solu-
tion of HAuCl4 (5.0 ml; 1.0 mM) was added to an aque-
ous solution of cetyltrimethylammonium bromide (5.0 ml;
0.2 M) at 25 °C. Then, AgNO3 (270 µL; 4.0 mM, aq.)
was added and the solution was gently shaken. HCl (8.0
µL, 37%) was introduced (pH of 1–1.15). Then, ascor-
bic acid (70 µL, 78.8 mM) was added to the solution with
gentle shaking until the solution was clear. Immediately
afterwards, ice-cold aqueous solution of NaBH4 (15 µL;
0.01 M) was injected to the unstirred solution and allowed
to react for 6 h. The solution was centrifuged for 30 min at
10,000 rpm and the supernatant was removed and the pre-
cipitate was redispersed in water for further use.
Fe3O4 magnetic NPs have been employed as catalyst
supports, due to their easy recovery by using an exter-
nal magnetic ield. Their insoluble and super paramag-
netic nature make it possible to realize various reac-
tions and reduce the capital and operational costs [38,
39]. Glucose as a carbon source is one of the best shells
for protecting Fe3O4 MNPs from aggregation, and acts
as a suitable support for a variety of organic linkers and
NPs owing to the numerous surface-OH groups [40].
By modiication of the carbon surface, a catalyst with
tunable activity and favorable functionalities can be
obtained.
In most of the above reports, researchers used only
spherical metal NPs to study their catalytic eiciency.
In this present report we demonstrate the eiciency of a
novel designed gold nanorods-loaded Fe3O4@Carbon@
Cysteine core–shell nanocatalyst (FCC–Au NRs) on the
fast catalytic reduction of aromatic nitro-compounds
(Scheme 1).
2.3 Synthesis of Fe3O4 Magnetic Nanoparticles (MNPs)
Fe3O4 Magnetic nanoparticles were prepared by a hydro-
thermal technique as previously reported [42]. Briely,
FeCl3·6H2O (1.6 g, 6 mmol), sodium acetate (4.0 g,
48.8 mmol) and (1.5 g, 5.8 mmol) trisodium citrate were
dissolved in ethylene glycol (75 ml). After vigorous stirring
1 3

Magnetically Recoverable Gold Nanorods as a Novel Catalyst for the Facile Reduction of…
for 1 h, the resulting yellow solution was transferred into
a Telon-lined stainless-steel autoclave with a capacity of
100 ml, then sealed and heated to 200 °C. After reaction
for 12 h, the autoclave was cooled to room temperature.
The obtained black Fe3O4 MNPs were collected with a
magnet and washed several times with water and ethanol,
separately. Finally, the product was dried under vacuum at
50 °C for 12 h.
pellets (USA). A Hitachi U-3010 UV–Visible spectropho-
tometer with a 1 cm path length quartz cuvette was used
to identify the change in concentration over a wavelength
range from 220 to 600 nm. To exhibit the magnetic behav-
ior of the catalyst the Vibrating Sample Magnetometer
(VSM, 7400 Lake Shore, USA) was used.
2.7 Kinetics Study
2.4 Synthesis of Fe3O4@C@Cysteine Nanoparticles
As a probe reaction the reduction of 4-nitrophenol (4-NP)
to 4-aminophenol (4-AP) by NaBH4 was performed to
studying the kinetics and the catalytic activity of FCC–Au
NRs, in a quartz cuvette of UV–Visible spectrophotometer.
The efect of concentration of NaBH4, amount of catalyst,
and initial concentration on the rate of reduction was stud-
ied by using diferent quantities (12.5, 25, 50 and 100 mM
NaBH4 and 0.005, 0.01, and 0.02 mol% FCC–Au NRs).
The Fe3O4 powder (0.1 g) was dispersed in HNO3 solu-
tion (5 ml, 0.1 M) with ultrasonic sound for 10 min, and
then washed with deionized water. The post-treated Fe3O4
nanoparticles were redispersed in 0.5 M glucose solution
(30 ml, aq.) and ultrasonicated for another 10 min. Next,
the suspension was transferred into a Telon-lined autoclave
(50 ml capacity) and kept at 180 °C for 6 h and then cooled
to room temperature. The resulting Fe3O4@C nanoparti-
cles were separated with the help of a magnet, washed with
deionized water (50 ml) and ethanol (50 ml) three times
respectively, and dried at 50 °C for 10 h [35]. In the next
step, of Fe3O4@C (0.1 g) and cysteine (0.4 g) in deionized
water (28 ml) was mixed and sonicated for 10 min. The
mixture was transferred into a Telon lined autoclave and
kept at 150 °C for 3 h and then cooled to room temperature.
The magnetic Fe3O4@C@Cysteine nanoparticles were col-
lected with a magnet and washed several times with water
(50 ml) and ethanol (50 ml) three times respectively, and
dried at 50 °C [43].
2.8 Catalytic Reduction of 4-Nitrophenol
For investigating the catalytic performance of FCC–Au
NRs, the reduction of 4-nitrophenol with NaBH4 was car-
ried out under various conditions. 4-Nitrophenol (30–110
µL, 5 mM) and NaBH4 (0.2 ml, 12.5–100 mM, aq.) solu-
tions were added into deionized H2O in a quartz cuvette
to get the 3 ml total reaction mixture. The solution color
turned to bright yellow rapidly. Subsequently, 0.005, 0.01,
and 0.02 mol% of FCC–Au NRs was added to start the
reaction. The change in 4-nitrophenol concentration that is
proportional to the intensity of absorption peak at 400 nm
was monitored by UV–Vis spectroscopy as a function of
time. After completion of the reduction, the solution color
was changed from yellow to colorless.
2.5 Synthesis of Fe3O4@C@Cysteine–Au Nanorods
(FCC–Au NRs)
Fe3O4@C@Cysteine (0.1 g) were sonicated in deion-
ized water (5 ml) for 10 min and then stirred with colloi-
dal solution of Au nanorods (5 ml) for 1 h. Finally, novel
Fe3O4@C@Cysteine–Au nanorods nanoparticles were col-
lected with a magnet and washed three times with ethanol
(50 ml).
3 Results and Discussion
3.1 Characterization of Fe3O4@C@Cys–Au NRs
Based on our preliminary work on the synthesis of transi-
tion metals-based nanocatalysts [44–46] the Fe3O4 MNPs
were prepared by a hydrothermal technique, coating a layer
of carbon on the surface of the Fe3O4 MNPs and then func-
tionalization of the carbonaceous shell with cysteine. The
Fe3O4@Carbon@Cysteine was then loaded with the gold
nanorods. Finally, the FCC–Au NRs were studied in the
reduction of nitroarenes to the corresponding aminoarenes.
A schematic pathway for the synthesis of FCC–Au NRs is
shown in Scheme 2.
2.6 Characterization of Fe3O4@C@Au Magnetic
Nanoparticles
For morphology and size study of the samples TEM and
SEM experiments were conducted on a Leo 912AB micro-
scope (Germany) operated at 120 kV and Leo 1450VP
(Germany), respectively. Powder XRD patterns were
recorded on a Bruker D4 X-ray difractometer with Ni-
iltered Cu KR radiation (40 kV, 30 mA). The percentage
weight loss of shell and cysteine was studied using TGA,
(Mettler Toledo LF, Switzerland). FT-IR spectra were col-
lected on a Nicolet Fourier spectrophotometer, using KBr
The prepared magnetically green Au NRs loaded nano-
catalyst has been characterized with several methods.
These techniques conirmed that the Au NRs are well dis-
persed on the surface of the solid support and its structure
1 3

K. Lamei et al.
then loaded on cysteine modiied surface of FC MNPs in
Fig. 1e. This image showed that Au NRs were well dis-
persed on surface of carbonaceous shell. The content of Au
NRs in the sample was conirmed by inductively coupled
plasma/optical electron microscopy (ICP-OES, see sup-
porting information). The amount of Au NRs loaded onto
the Fe3O4@C@Cys was 2.981 wt%. For FCC–Au NRs,
after the 5th recycle no signiicant change was observed
in the catalyst structure, as shown in Fig. 1f. The FCC–Au
nanospheres were also prepared to evaluate the catalytic
activity of supported Au nanorods and nanospheres. TEM
image of FCC–Au nanospheres consisted of round-shaped
particles (Fig. 1g).
To examine the thermal stability of the FCC–Au NRs
the thermogravimetric (TG) analysis curve of Fe3O4@C@
Cys MNPs was used (see supporting information S-1). The
FC MNPs showed a four-step weight loss in the tempera-
ture region of 30–800 °C. The initial weight loss up to 250
°C was probably due to the removal of surface adsorbed
water, while the weight loss at 250–500 °C could be attrib-
uted mainly to the evaporation and subsequent decompo-
sition of cysteine bound to the carbonaceous shell and
amorphous carbon coating. The Fe3O4 MNPs can transfer
to Fe2O3 and the weight will increase when the tempera-
ture is higher than 500 °C. The last weight loss from 500 to
600 °C may be a response to decomposition of the residual
carbon.
Figure 2 shows the X-ray difraction patterns of the
Fe3O4 MNPs and FCC–Au NRs. According to the JCPDS
card no. 19-0629, six difraction peaks appearing in Fig. 2a
located at 30.3°, 35.5°, 43.3°, 53.4°, 57.2°, and 62.7° can be
assigned to difraction of the Fe3O4 crystal with an inverse
spinel structure from the (220), (311), (400), (422), (511),
and (440). No other peak can be found in Fig. 2a, indicat-
ing that the spheres are pure Fe3O4 crystalline phase. Also,
According to the JCPDS card no. 04-0784, three additional
difraction peaks at 38.1°, 44.4°, and 64.7° appearing in
Fig. 2b correspond to the (111), (200), and (311) planes of
the gold crystal that conirm the presence of Au NRs. Due
to the heavy atom efect of Au the characteristic peaks of
Fe3O4 becomes weak in Fig. 2b. These results show that
the phase of Fe3O4 MNPs has no change after the high-
temperature carbon coating reaction. Calculations with the
Debye–Scherrer equation (d=0.89λ/Bcosθ) for the ive
strongest peaks (220, 311, 400, 511 and 440), gave mean
grain sizes of 15.5 nm for the Fe3O4 nanoparticles.
Scheme 2 A schematic pathway for synthesis of FCC–Au NRs for
catalytic reduction of nitroarenes
is retained after immobilization on the modiied carbon
coated Fe3O4 MNPs.
As shown in Fig. 1 the morphology of the FCC–Au NRs
was characterized with TEM. The Fe3O4 MNPs are approx-
imately multifaceted cuboid particles with average diam-
eters of about 40 nm (Fig. 1a). As shown in Fig. 1b, in con-
trast to the Fig. 1a, the Fe3O4@Carbon (FC MNPs) show
a Fe3O4 MNPs within a carbonaceous shell that elemental
analysis (Fig. 1h) conirm this elemental composition. Fig-
ure 1c is TEM image of cysteine modiied FC MNPs which
conirm that the structure of FC MNPs unchanged after
chemical modiication. Figure 1d shows the TEM image of
the synthesized Au NRs, the UV–Vis spectra of Au NRs
was showed a strong long-wavelength peak from the longi-
tudinal oscillation of electrons around 760 nm and a weak
short-wavelength peak around 515 nm from the transverse
electronic oscillation. These results are in agreement with
the Au NRs prepared previously [34]. The Au NRs was
Figure 3 shows the FTIR spectrum of the FCC–Au
NRs. In spectrum 3a, uncoated Fe3O4 MNPs show a broad
peak around 580 cm−1 due to the stretching of Fe–O
bond. The characteristic peaks at 1706 and 1615 cm−1
in spectrum 3b correspond to C=O and C=C vibrations,
respectively, which is a result of the aromatization of
glucose during hydrothermal treatment. The broad peak
1 3

Magnetically Recoverable Gold Nanorods as a Novel Catalyst for the Facile Reduction of…
Fig. 1 TEM images of a Fe3O4 MNPs, b FC MNPs, c FCC MNPs, d The Au NRs e FCC–Au NRs, and f recycled FCC–Au NRs, g TEM image
of FCC–Au nanospheres, h The EDS image of FC MNPs
at 3430 cm−1 implies the existence of residual hydroxyl
groups. Also, the peaks in the range 1000–1300 cm−1,
which include the C–OH stretching and O–H bending
vibrations, imply the existence of numerous hydrophilic
groups and the incomplete carbonization of glucose. The
hydrophilic groups can endow Fe3O4@C nanoparticles
with excellent aqueous dispersibility and stability. In 3d
the spectrum of L-cysteine shows its characteristic peaks
at 1610 and 1397 cm−1 due to the asymmetric and sym-
3450 cm−1 in comparison with spectrum 3b and appear-
ance of a very weak peak at 2542 cm−1 due to the pres-
ence of SH groups, conirms the MNPs are functionalized
with cysteine.
The magnetic hysteresis loops of the samples are dis-
played in Fig. 4. The Fe3O4 and Fe3O4@C@Cys–Au NRs
exhibit super paramagnetic behavior at room temperature.
The magnetic saturation (Ms) value of the Fe3O4 and FCC–
Au NRs is 59.64 and 32.46 emu/g, respectively. That the
Ms value of FCC–Au NRs is much lower than that of the
Fe3O4 MNPs can be attributed to the carbon shell for the
disordered structure at interfaces providing less magnetic
moment per unit mass than that of ferromagnetic core
regions, which leads to a decrease in the Ms. The mag-
netic separability of the MNPs was tested in water by plac-
ing a magnet near the glass bottle; the black particles were
attracted toward the magnet within 30 s (inset of Fig. 4).
+
metric stretching of COO−. A very broad peak of NH3
stretch is observed between 3000 and 3500 cm−1. A
weak peak at 2542 cm−1 is due to the presence of S–H
groups of cysteine. Because of the low concentration
of surface-linked cysteine molecules, the spectrum of
cysteine-coated MNPs (Fig. 3c) showed an inconsider-
able change in characteristic peaks of Fe3O4@C MNPs.
For instance, the increase of the broad peak around
1 3

K. Lamei et al.
Fig. 2 XRD patterns of a
Fe3O4 and b FCC–Au NRs
80
60
a
40
b
20
0
-20
-40
-60
-80
-20000
-10000
0
10000
20000
Applied field (Oe)
Fig. 4 Magnetization hysteresis loops measured at room tempera-
tures: a Fe3O4, b Fe3O4@C@Cys–Au NRs. The inset shows the pho-
tographs of the Fe3O4@C in aqueous solution
Fig. 3 FTIR spectra of a Fe3O4, b Fe3O4@C, c Fe3O4@C@Cys, d
cysteine
of the reactions. The 4-NP solution shows the original
absorption peak at 317 nm which shifts to 400 nm due to
the formation of 4-nitrophenolate ions after addition of
NaBH4 (see supporting information S-1 to S-7). Even after
5 h, the reaction does not proceed in the absence of FCC–
Au NRs. In the presence of FCC–Au NRs, the peak inten-
sity of 4-nitrophenolate at 400 nm signiicantly decreases
as the reaction proceeds. Concurrently, a new absorption
peak at 298 nm increases, due to the formation of 4-ami-
nophenol, as shown in Fig. 5a. For further investigation
of FCC–Au NRs catalytic eiciency, the reduction of 1,
2-DNB was tested. As shown in Fig. 5b, this catalyst can
reduce selectively only one nitro group of 1, 2-DNB after
These results demonstrate that the core–shell particles pos-
sess magnetic properties.
3.2 Reduction of 4-Nitrophenol Catalyzed by FCC–Au
NRs
After the successful preparation and characterization of the
FCC–Au NRs, catalytic performance of this nanocatalyst
was explored in the reduction of 4-nitrophenol (4-NP) and
1,2-dinitrobenzene (1,2-DNB) as model reactions in the
presence of NaBH4 as the reductant. The time-dependent
UV–Vis spectral changes were used to monitor the progress
1 3

Magnetically Recoverable Gold Nanorods as a Novel Catalyst for the Facile Reduction of…
a -4
12.5 mM
25 mM
50 mM
100 mM
-3
-2
-1
0
y = -0.0533x - 0.3067
y = -0.0217x - 0.0081
y = -0.0047x + 0.2052
y = -0.0037x + 0.4384
1
0
200
400
Time (sec)
600
800
b
0.06
0.05
0.04
0.03
0.02
0.01
R² = 0.9902
0
0
20
40
60
80
100
[NaBH4]
Fig. 5 UV–Vis absorption spectra of the reduction of a 4-NP, and
b 1,2-DNB with FCC–Au NRs (0.2 ml NaBH4 100 mM, 0.01 mol%
cat.)
Fig. 6 a The ln (Ct/C0) versus the reaction time for the reduction of
4-NP over FCC–Au NRs (0.01 mol% Au) at 25 °C. C0 and Ct is the
absorption peak at 400 nm initially and at time t. b The concentration
of NaBH4 versus the reaction rate
120 s. The new peaks for this reduction product appear at
415 and 288 nm in the presence of NaBH4 as the reaction
proceeds. The easy adsorption of 4-NP molecules onto the
Au NRs surfaces causes the high eiciency of FCC–Au
NRs in reduction reaction [47].
4-nitrophenolate ions reached completion at 50–100 mM
NaBH4. The kobs for 4-nitrophenol reduction increases
from 3.9 × 10−3 s−1 at 12.5 mM NaBH4 to 6.8 × 10−3
s−1, 2.17 × 10−2 s−1, and 5.3 × 10−2 s−1 at 25, 50, and
100 mM NaBH4, respectively. As shown in Fig. 6b, a lin-
ear relationship between NaBH4 concentration and kobs
(R2 = 0.99) was observed. These results clearly show that
100 mM NaBH4 is optimal for the reduction of 4-nitro-
phenol catalyzed with FCC–Au NRs.
As shown in Fig. 6, linear relationships of ln (Ct/C0) ver-
sus reaction time are observed, indicating that the reduction
of 4-NP follows pseudo-irst order kinetics. The rate con-
stant k is calculated to be 5.3 × 10−2 s−1 (0.01 mol% cata-
lyst, 80 µl of 5 mM 4-NP and 0.2 ml NaBH4 100 mM). The
rate constant and the catalytic eiciency of FCC–Au NRs
are higher than previously reported values of 4-NP reduc-
tion reaction catalyzed with Au-α-Fe2O3 hetero-nanoparti-
cles and Au–Fe3O4 heterostructures [43, 48].
3.4 Efect of Catalyst Amount on 4-Nitrophenol
Reduction
3.3 Efect of NaBH4 Concentration on 4-NP Reduction
In a further study, the efect of catalyst amount on the
yield and rate of 4-nitrophenol was examined. As shown
in Fig. 7, the kobs for 4-nitrophenol reduction increases
from 1.6 × 10−2 s−1 at 0.005 mol% catalyst to 5.3 × 10−2
and 5.5 × 10−2 s−1 at 0.01 and 0.02 mol% catalyst,
respectively. Due to the small diference in kobs for 0.01
and 0.02 mol% of catalyst, based on these results the opti-
mal amount of FCC–Au NRs is 0.01 mol% for 1 mmol of
4-NP.
In this study, to optimize the reaction conditions for
the reduction of nitroarenes to the corresponding ami-
noarenes, the efect of concentration of NaBH4 on the
reduction of 4-nitrophenol was irst examined (Fig. 6a).
The yield of 4-aminophenol increases upon increas-
ing NaBH4 concentrations, with complete conver-
sion to 4-aminophenol observed within 16 min when
12.5 mM NaBH4 are added. After 60 s, the reduction of
1 3

K. Lamei et al.
0.005 mol%
0.01 mol%
0.02 mol%
Table 1 Reduction of nitro compounds into amines with NaBH4 cat-
alyzed by FCC–Au NRs
-4
-3
-2
-1
0
y = -0.0533x - 0.3067
y = -0.055x - 0.3167
Entry
Timec
(sec)
Nitroarenea
Productb
y = -0.0161x - 0.5486
OH
OH
1
2
3
60
NH2
NH2
NO2
NO2
0
50
100
150
200
Time (sec)
210
210
Fig. 7 Efect of catalyst amount on 4-NP reduction with 0.2 ml
NaBH4 100 mM
NH2
NH2
NC
NC
30-80 ul
90 ul
100 ul
110 ul
-5
-4
-3
-2
-1
0
y = -0.0164x - 0.5053
NH2
NO2
y = -0.0533x - 0.3067
y = -0.0508x - 0.2917
CHO
CH2OH
y = -0.0133x - 0.5667
4
120
NH2
NO2
CO2H
CO2H
0
50
100
150
Time (sec)
200
250
300
5
6
7
8
9
300
160
60
NO2
NH2
Fig. 8 The efect of initial 4-NP concentration on reduction reaction
rate
NH2
NO2
OMe
OMe
FCA-Au nanospheres
2.5
CH3
CH3
2
1.5
0 sec
460 sec
NO2
NH2
1
0.5
Cl
Cl
NO2
NH2
80
0
250
300
350
400
450
500
λ (nm)
NHNH2
NHNH2
Fig. 9 UV–Vis absorption spectra of the reduction of 4-NP, with
FCC–Au nanospheres (0.2 ml NaBH4 100 mM, 0.01 mol% cat.)
240
NH2
NO2
3.5 Efect of Initial Concentration on 4-Nitrophenol
Reduction
OH
OH
NO2
NO2
10
90
The efect of initial concentration of 4-NP on the reduc-
tion rate was examined. Figure 8 indicate that the kobs
for 4-nitrophenol reduction is 5.3 × 10−2 s−1 for 80 µl of
NH2
NO2
1 3

Magnetically Recoverable Gold Nanorods as a Novel Catalyst for the Facile Reduction of…
Table 1 (continued)
to occur. From the UV–Vis plots in Figs. 5a and 9, it is fur-
ther revealed that for the FCC–Au nanorods (rod-shaped
particles of size 40 nm) within 60 s reaction reaches maxi-
mum conversion level, whereas for FCC–Au nanospheres
(~5 nm, Fig. 1g) it reaches after 460 s reaction time. This
result suggests that Au nanorods is more eicient catalyst
than Au nanospheres.
OH
OH
NH2
NH2
11
180
NH2
NO2
NH2
NH2
Fenger et al. synthesized CTAB-stabilized Au NPs with
various sizes and concluded that 13 nm Au NPs show
a higher catalytic activity than 3 nm Au NPs. During the
catalytic process, the 4-NP molecules are adsorbed and
lie in a lat, parallel orientation on the surface of the Au
NP. Therefore, the 4-NP molecules needs more space at
the Au NP surface for the adsorption [47]. Also, the num-
ber of active surface sites in anisotropic Au NPs (such as
Au nanorods) is very high compared with spherical Au
NPs, and the small molecules could easily adsorb on the
Au surface. Hence, the anisotropic Au NPs often shows
excellent catalytic activities [19]. Rashid et al. concluded
that the rate constants of polygonal Au NP-catalyzed 4-NP
reduction reactions are a few hundred times higher than for
Turke-vich spherical Au NPs [49]. A recent report revealed
that the high-index facets both accept electrons and emit-
ted electrons faster than that of low-index facets. Thus, the
higher-index facets function was more eicient in 4-NP cat-
alytic reduction [50].
12
60
NH2
NO2
NH2
NH2
NH2
NO2
13
14
15
16
60
NH2
NO2
NO2
NO2
120
120
120
140
NO2
NO2
NH2
NO2
H
N
H
N
N
N
NO2
NH2
3.6 Catalytic Performance of the FCC–Au NRs
for the Reduction of Various Nitroarenes.
H
N
H
N
17
To assess the eiciency of the FCC–Au NRs catalytic sys-
tem for the reduction of the nitro group, various nitroarenes
possessing both electron-withdrawing and electron-
donating substituents in the aromatic ring were studied.
4-Nitrophenol was reduced to the corresponding aniline
in 60 s (Table 1, entry1). Unsubstituted nitrobenzene was
reduced to aniline in 210 s (Table 1, entry 2). The FCC–
Au NRs chemoselectively reduced 4-nitro-2-cyano-aniline,
and 4-nitrobenzoic acid to 4-amino-2-cyano-aniline, and
4-aminobenzoic acid, respectively (Table 1, entries 3, and
5), but in the case of 4-nitrobenzaldehyde (Table 1, entry
4) both nitro and -CHO groups reduced to provide 4-amin-
obenzylalcohol. Nitroarenes with electron-donating groups
such as 2-nitroanisole and 4-nitrotoluene were also reduced
to the corresponding amines in 160 and 60 s, respectively
(Table 1, entries 6 and 7). For dinitro compounds, i.e. 1,
2-dinitrobenzene, 1, 3-dinitrobenzene, and 2, 4-dinitrophe-
nol (Table 1, entries 10, 14, and 15) one nitro group was
selectively reduced even after 60 min. No dehalogena-
tion of the product was observed for 2-nitrochlorobenzene
(Table 1, entry 8). 4-Nitrophenylhydrazine and 4-nitro-
2-aminophenol were also reduced to the corresponding
anilines (Table 1, entries 9 and 11). 2-Nitroaniline and
N
N
O2N
H2N
aNitroarene (80 µL, 5 mM) and NaBH4 (0.2 ml, 100 mM) were added
into deionized H2O in a quartz cuvette to get the 3 ml total reaction
mixture. Subsequently, 0.01 mol% of FCC–Au NRs was added to
start the reaction
bThe change in nitroarene concentration was monitored by UV–Vis
spectroscopy as a function of time (see supporting information)
CThis time is related to the disappearance of the peak of the nitro
group in UV–Vis spectroscopy (100% conversion of nitro group to
amine group)
4-NP (5 mM) after 60 s. The kobs decreases to 5 × 10−2,
1.6 × 10−2 and 1.3 × 10−2 s−1 at 90, 100 and 110 µl 4-NP,
respectively. Based on these results the optimal volume of
4-NP is 80 µl (5 mM).
To comparison the catalytic activities of Au nano-
structures with spherical and rods shapes, the reduction
of 4-NP in optimized reaction conditions was used as the
model reaction. The FCC MNPs were synthesized and Au
nanospheres were generated onto the FCA MNPs (see sup-
porting information S-1). Both FCC–Au nanospheres and
FCC–Au nanorods acting as eicient catalyst for reduction
1 3

K. Lamei et al.
-4
-3
-2
-1
0
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