Gold nanodots self-assembled polyelectrolyte film as reusable catalyst for reduction of nitroaromatics

Article abstract of DOI:10.1007/s12039-017-1405-0

Separation of homogeneous catalyst from the reaction mixture is a crucial and difficult process in any catalytic process. To address this issue, a new class of multifunctional catalyst in the form of film was developed using a facile approach to enjoy the advantages of homogeneous catalyst with the versatility of heterogeneous catalyst. To achieve the same, methionine-capped gold nanodots (AuNDs) were self-assembled on a cationic polyelectrolyte modified glass plate for the catalytic reduction of nitro functional groups in the presence of olefinic double bond at mild conditions. Separation of this reusable catalytic film from the reaction mixture is very simple and advantageous when compared to the currently available and conventional catalytic systems. Kinetics of nitro reduction was monitored using absorption spectroscopy and the product formation was confirmed by ¹H and 13CNMR analyses. Prepared AuNDs catalyst was characterized using UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), cyclic voltammetry and atomic force microscopy (AFM) techniques. Graphical Abstract: SYNOPSIS?Colloidal gold nanoparticles are efficient catalysts for organic reactions. But the removal of homogeneous gold colloids from the reaction mixture is very difficult. To address this issue, gold nanodots were synthesized and self-assembled over polyelectrolyte film to form catalytic plates. Removal of these reusable catalytic plates from the reaction mixture is facile.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1405-0

J. Chem. Sci. (2018) 130:4
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-017-1405-0
REGULAR ARTICLE
Gold nanodots self-assembled polyelectrolyte film as reusable
catalyst for reduction of nitroaromatics
∗
PERUMAL VISWANATHAN and RAMASAMY RAMARAJ
School of Chemistry, Centre for Photoelectrochemistry, Madurai Kamaraj University, Madurai,
Tamilnadu 625 021, India
E-mail: ramarajr@yahoo.com
MS received 11 August 2017; revised 8 November 2017; accepted 9 November 2017
Abstract. Separation of homogeneous catalyst from the reaction mixture is a crucial and difficult process in any
catalytic process. To address this issue, a new class of multifunctional catalyst in the form of film was developed
using a facile approach to enjoy the advantages of homogeneous catalyst with the versatility of heterogeneous
catalyst. To achieve the same, methionine-capped gold nanodots (AuNDs) were self-assembled on a cationic
polyelectrolyte modified glass plate for the catalytic reduction of nitro functional groups in the presence of
olefinic double bond at mild conditions. Separation of this reusable catalytic film from the reaction mixture is very
simple and advantageous when compared to the currently available and conventional catalytic systems. Kinetics
of nitro reduction was monitored using absorption spectroscopy and the product formation was confirmed by
1
13
H and C NMR analyses. Prepared AuNDs catalyst was characterized using UV-Vis spectroscopy, X-ray
photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high-resolution transmission electron microscopy
HRTEM), cyclic voltammetry and atomic force microscopy (AFM) techniques.
(
Keywords. Methionine; Au nanodots; catalytic films; nitroaromatics reduction.
1
. Introduction
the problems associated with energy and environment
across the globe. Thus, a lot of attention has been given
to the development of new catalysts that combine the
practical advantages of conventional heterogeneous cat-
alysts with the versatility of homogeneous catalysts.
In recent years, the issues regarding science and tech-
nology of catalysis have been effectively addressed
by nanoscience and great strides have been achieved
Catalysis has been an inevitable field of chemical
research ever since its utility was recognized by research
laboratories and chemical industries. Heterogeneous
catalysts have received a great amount of attention both
from scientific and industrial perspectives. Since 90% of
chemical manufacturing processes in industries rely on
heterogeneous catalysis, it has a significant impact on
2
through novel nanomaterial catalysts. In this regard,
1
world economy. Though heterogeneous catalytic sys-
gold nanoparticles (NPs) have had a significant impact
on catalysis and have widely been employed for various
tems were adopted by industries, these are not perfect for
industries as these lead to high operating temperatures,
low selectivity, environmental damage, uneconomical
cost and a complex process is required for reactivation
of the catalyst. Conversely, homogeneous catalytic sys-
tems offer high activity and selectivity at relatively mild
conditions, but the primary limitation of this catalytic
system is the separation of the catalyst from the reaction
mixtures that is usually critical for an industrial process.
These facts created pressure on the scientific commu-
nity to invent alternate catalytic systems, which address
3
catalytic applications. Moreover, designing a single
catalytic system for multi-purpose catalytic applications
is also an impressive development, which often sur-
passes the conventional catalytic systems employed in
industries and scientific laboratories.
Aromatic amines are widely employed in the synthe-
sis of pharmaceuticals, polymers, dyes and cosmetics.
Transition metal-catalysed reduction of nitroaromat-
ics into their corresponding aromatic amines is the
traditional method used to prepare these industrially
4
important compounds. Pt, Pd, Au, Ag, Fe O @Ni and
3
4
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-017-1405-0) contains
supplementary material, which is available to authorized users.

4
Page 2 of 10
J. Chem. Sci. (2018) 130:4
Ni@Ag NPs are the few reported catalytic materials for 2.2 Preparation of methionine capped AuNDs
the reduction of nitroaromatics, but the major problems
5
0
mL of 0.01 M methionine was mixed with 4.8 mL water and
.2 mL of 0.5 M KOH and stirred for 10 min. 0.1 mL of 0.1
associated with these catalytic systems are high H pres-
2
sures, longer reaction times, lower turnover numbers,
5
M HAuCl4 was added to the above solution and mixed well
for 2 min. To this stirring mixture, 0.6 mL of 0.05 M ice-cold
NaBH4 was added slowly and the stirring was continued for
recyclability and high reaction temperatures. Though
the noble metal NPs-decorated magnetic metal oxides
show the good activity and recyclability, preparation
and preservation of these catalytic systems seem to
be critical in practice. Hence, the designing of new
catalytic systems with features like operational simplic-
3
h at room temperature. The immediate formation of brown
color confirmed the successful formation of AuNDs.
By adopting the same procedure, KOH concentration was
varied (0, 5, 10, 15 and 20 mM) in the preparation of various
ity, simple hydrogen sources, efficient conversion with AuNDs. A black precipitate was observed in the absence and
selectivity and low cost is beneficial for the industry and in the presence of 20 mM and above the concentration of
researchers.
KOH. Moreover, 5, 10and15mMKOHledtotheformationof
stable AuNDs and the AuNDs were represented as M-Au-K5,
M-Au-K10 and M-Au-K15 (The subscripts 5, 10 and 15 denote
the concentration of KOH used in the synthesis).
In the present work, sub-5 nm methionine capped Au
nanodots (AuNDs) were prepared and successfully
self-assembled onto cationic polyelectrolyte, poly(acryl
amide-co-diallyldimethylammoniumchloride) (PADA),
a modified glass plate for the catalytic reduction of
nitroaromatics. This AuNDs-modified glass plate was
successfully re-used for catalytic reduction reactions.
The prepared catalytic film was found to be stable for
more than two months at room temperature. This opens
up the possibility of commercialization of this catalytic
system.
2.3 Preparation of catalytic plate for nitroaromatic
reduction
2
AuNDs coated glass plates with the area of 1.2 cm (0.8 cm ×
1.5 cm) were utilised for the catalytic test studies of reduction
of 4-nitrophenol upon the addition of NaBH4. The protocol
adopted for the fabrication of AuNDs modified glass plates
is as follows: Initially, 86 μL of 0.1% PADA was drop casted
2
on the glass plate area of 1.2 cm , and allowed to dry at room
temperature for 3 h. Then the PADA modified glass plate was
dipped in methionine-capped AuNDs solutions for 90 min
to obtain AuNDs self-assembled catalytic plates. For labo-
ratory scale synthesis of functionalized anilines, glass plate
2
. Experimental
2.1 Materials and methods
2
with the area of 6.25 cm (2.5 cm × 2.5 cm) was modified
Chloroauric acid (HAuCl4) and poly(acrylamide-co-
diallyldimethylammonium chloride) (PADA), were received
from Sigma-Aldrich. DL-methionine, 4-nitrobenzoic acid
and 4-nitroaniline were obtained from the SRL and all other
chemicals with analytical grade were received from Merck.
All glassware were thoroughly cleaned with aqua regia (1:3
HNO3/HCl v/v) (Caution: A qua regia is a powerful oxidiz-
ing agent and it should be handled with extreme care.) and
rinsed extensively with distilled water before use. UV–Visible
absorption spectra were recorded using Agilent Technologies
with PADA and AuNDs. In this preparation, glass plates with
the dimension of 2.5 cm × 7.5 cm were taken and 450 μL
2
of PADA was drop casted on the glass plate area of 6.25 cm
(
2.5 cm × 2.5 cm) and dried for 3 h and then dipped in AuNDs
solution for 90 min to obtain the AuNPs self-assembled cat-
alytic plates. Dipping time of more than 90 min does not
improve the catalytic activity. Figure 1 shows the schematic
representation of the preparation of catalytic films.
8453 spectrophotometer using a 1 cm path length quartz cell
and the samples for absorption studies were prepared by dilut-
ing 0.6 mL of as-synthesized Au colloidal solution in 1.4 mL
of water. High-resolution transmission electron microscopy
PADA
of
(HRTEM) andselected areaelectrondiffraction(SAED) anal-
casting
yses were conducted in a Technai instrument operated at 200
kV. The specimen for the HRTEM analysis was prepared by
dropping the colloidal solution onto carbon-coated copper
grid, and then dried at room temperature. XPS studies were
performed using ULVAC-PHI 5000 versa Probe instrument.
XRD patterns were recorded with a Rigaku diffractometer
using Cu Kα radiation (λ = 1.54 Å). NMR spectrum was
recorded on Bruker 300 MHz instrument using CDCl3 and
Negaꢀvely
charged
AuNDs
Posiꢀvely
charged
PADA
AuNDs
plate
DMSO as solvents and the chemical shift values are reported Figure 1. Schematic representation of the fabrication of
as δ values (ppm) with reference to tetramethylsilane (TMS). AuNDs self-assembled onto PADA-modified glass plate.

J. Chem. Sci. (2018) 130:4
.4 Catalytic test reaction
.1 mL of 2 mM 4-nitrophenol solution was added to 1.4
mL water followed by 0.5 mL of 0.05 M NaBH4. To the
above mixture, 25 μL of as-synthesized AuNDs was added
and the progress of reaction was monitored using UV-Vis
spectroscopy. To test the catalytic activity of AuNDs mod-
ified glass plates (catalytic plates), to the above mentioned
Page 3 of 10
4
2
2.0
1.5
0
1.0
c
b
4
-nitrophenol and NaBH4 mixture, instead of AuNDs solu-
2
0.5
a
d
tion, catalytic plate of 1.2 cm area was immersed to start the
reaction and the progress of reaction was monitored using
UV-Vis spectroscopy.
0.0
3
00 400 500 600 700 800 900
2
.5 Laboratory scale synthesis of functionalized
Wavelength / nm
anilines
Figure 2. UV-Vis absorption spectra obtained for
0
.5mmolofnitroaromaticcompoundswasdissolvedin30mL
M-Au-K5 (a), M-Au-K10 (b), M-Au-K (c) NDs and
15
−
ofsolvent(H2OorTHF)and10mLof0.05Mice-coldNaBH4 the mixture of methionine, KOH and AuCl (d).
4
solution under stirring condition. The catalytic plate (area =
2
.5 cm × 2.5 cm) was then immersed in the above mentioned
solution and the stirring was continued at room temperature (20 mM and above) of KOH creates the unfavourable
for at least 3 h and the reaction was monitored by using TLC. condition for the stabilisation of AuNDs due to kinetic
After completion of reaction, the catalytic plate was removed
instability. In this context, optimum concentration (5–15
from the reaction mixture. When the water was employed as
mM) of KOH has led to the stable formation of AuNDs.
solvent, the reaction mixture was neutralized using diluted
In basic condition, methionine will exist in the depro-
HCl and the product was extracted using excess ethyl acetate
tonated form, i.e., the carboxylic acid group present
from aqueous medium and the dry sodium sulphate was added
in the methionine will lose its proton and will form
to organic extract and filtered. The filtrate was concentrated
the carboxylate anion at the solution pH of ∼10.5 and
under reduced pressure to get the product. For the case of
THF solvent, after completion of reaction, the solvent was
evaporated under reduced pressure and then the water and
ethyl acetate was added to the obtained crude and then the
successfully stabilizes the AuNDs through electrostatic
stabilization mechanism. Moreover, so formed methio-
nine capped AuNDs were negatively charged in nature
usual procedure adopted for water solvent was followed to due to the deprotonated form of methionine. In order
get the product. NMR analyses were carried out using CDCl3 to find the optimum concentration of Au for catalysis,
−
and the results are presented in supporting information.
AuCl concentration was varied by keeping the KOH
4
(
10 mM) and methionine (5 mM) concentrations con-
stant. UV-Vis absorption spectra obtained for M-Au-K5,
M-Au-K , and M-Au-K NDs are shown in Figure 2.
10
15
3
. Results and Discussion
The absorption spectrum obtained for the mixture of
−
methionine, KOH and AuCl has no absorption in the
3.1 Characterisation of AuNDs
4
wavelength range of 400 to 550 nm (Figure 2d), whereas
AuNDs were prepared using sodium borohydride as a all M-Au-K NDs shows the broad absorption in this
reducing agent in the presence of methionine and base region is due to the presence of Au nanostructures. The
(KOH). In the present investigation, KOH was found to observed feeble SPR absorbance for AuNDs may be
have a pronounced effect on AuNDs formation, stability due to the chemisorption of methionine over AuNDs
and their catalytic activity. In the AuNDs preparation, surface. electronic mobility of surface electrons of gold
KOH concentration was varied as 5, 10 and 15 mM and nanoparticles will dramatically decrease when the thiol-
formed AuNDs were named as M-Au-K , M-Au-K , functionalized organic molecules chemisorbed on the
5
10
and M-Au-K , respectively. However, a black precipi- Au NPs surface through the formation of localized Au-
15
tatewasobtainedintheabsenceandinthepresenceof20 S bonds. Since methionine has multifunctional groups,
mM and above concentrations of KOH. In the absence the attachment of other heteroatoms like nitrogen to the
of KOH, methionine will exist as zwitterions in aque- Au surface may cause the disorders in Au-S bond as
6
oussolutionandtheelectrostaticinteractionbetweenthe reported in tiopronin capped gold nanoparticles. Such
methionine molecules led to the aggregation of formed disorders in the Au-S bonding arrangement is related to
methionine capped AuNDs, whereas high concentration the surface potential fluctuations that preclude collective

4
Page 4 of 10
J. Chem. Sci. (2018) 130:4
motion of electrons, decreasing the mobility and damp-
To get a better insight into the reduction of gold ions in
ing the SPR feature.6,7 Similar absorption features were the presence of methionine and their chemical environ-
obtained for the AuNPs prepared using various concen- ment, XPS spectrum was recorded for M-Au-K NDs.
10
trations of Au precursor (Figure 3). These absorption Figure 4 shows the core-level XPS profile of Au, N, S
studies confirm the successful formation of AuNSs in and C. The Au4f7/2 and Au4f5/2 peaks were observed
the presence of methionine.
at the binding energies of 83.3 and 87 eV, respec-
tively, with a binding energy difference of 3.7 eV. This
confirms the successful reduction of Au ions and the
presence of metallic gold in the M-Au-K10 NDs sam-
ple. Compared to the binding energy of bulk metallic
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
a) 0.5 mM Au
b) 1 mM Au
c) 1.5 Au
0
8,9
0
gold (Au = 83.8 to 84.0 eV), the Au in the pre-
pared AuNDs sample showed a small shift to lower
energy (83.3 eV). The interaction of methionine (N and
d
c
d) 2 mM Au
0
S functional groups) with Au may slightly shift the Au
peak. The core-level XPS spectrum of sulphur (S2p
region) showed the doublet with a branching ratio of
2:1 and the splitting energy difference of 1.2 eV. The
two peaks observed at 163.3 and 164.5 eV correspond
b
a
to S2p3/2 and S2p , respectively, is the characteristic
1/2
XPS signal of thioether sulphur. Another peak observed
3
00 400 500 600 700 800 900 1000
10
at 162 eV is due to the interaction of sulphur with Au.
Wavelength / nm
The core-level XPS data of nitrogen shows two peaks
at 399.5 and 401.5 eV. The peak at 399.5 eV corre-
sponds to neutral state nitrogen in methionine capped
AuNDs and the peak at 401.5 eV corresponds to the
Figure 3. UV-Vis absorption spectra obtained for methion-
ine-capped AuNPs prepared from various concentrations of
−
AuCl , where, [Methionine] = 5mM, and [KOH] = 10 mM.
4
Au4f7/2
Au 4f
S 2p
Sp3/2
Au4f5/2
Sp1/2
S-Au
8
2
84
86
88
90
92
158
160
162
164
166
168
Binding energy / eV
Binding Energy / eV
N 1s
C 1s
C-C / C-H
NH2
N-Au
C-N / C-S
C=O
3
95
400
405
410
282
285
288
Binding energy / eV
Binding energy / eV
Figure 4. Core level XPS data obtained for the constituent elements of Meth-Au-K10.

J. Chem. Sci. (2018) 130:4
Page 5 of 10
4
interaction of nitrogen atom to Au. Generally, in the N showed spherical particles with less than 5 nm in size.
s regime, the observed peak at higher binding energy The concentration of KOH employed in the preparation
401.5 eV) is attributed to the positively charged amine ofAuNDswasfoundtoaffecttheparticlesize;M-Au-K5
group whereas in the present experimental condition, and M-Au-K NDs had average particle size of 3.7 nm
1
(
15
as the solution pH is around 10.5, there is no possibil- and M-Au-K having average particle size of 2.6 nm.
10
ity for amine being protonated. On the other hand, the The histogram of M-Au-K and M-Au-K showed that
5
15
Au4f signals shifted to the lower binding energies in size of the majority of the AuNDs formed is around 4
comparison to pure Au (84 eV). A similar negative shift nm and the histogram of M-Au-K shows that size of
10
in binding energy was already reported in Au and Pt most of the particles is around 3 nm. Hence, 10 mM
NPs bonded with amines.1 Hence, the higher bind- KOH favours the formation of smaller size AuNDs in
ing energy (401.5 eV) in N 1s region can be accounted the presence of methionine. Figure 6 shows the HRTEM
1,12
for the interaction of amine groups with Au. The XPS images obtained for AuNPs prepared using 0.5 mM and
−
spectrum of C 1s region was fitted to three components 2 mM AuCl concentrations in the presence of 5 mM
4
that correspond to the carbon atom bonded to carbon methionine and 10 mM KOH. Less than 5 nm AuNDs
or hydrogen (285 eV), carbon atom bonded to sulphur were formed when the Au concentration was 0.5 mM,
and/or oxygen (285.6 eV) and carboxylic acid group whereas, 10 nm anisotropic AuNPs were formed with 2
(
285.5 eV) present in the methionine molecule. From mM Au precursor. In addition, ring patterns observed in
the XPS results, it is confirmed that prepared AuNDs the SAED analysis confirm the polycrystalline nature
sample contains methionine and metallic state Au. Fur- of AuNDs. XRD analyses were carried out to study
ther, it confirms that the formed AuNDs were stabilized the crystalline nature of the prepared AuNDs. Figure 7
by both nitrogen and sulphur functional groups present shows the XRD pattern obtained for the M-Au-K5,
in the methionine.
M-Au-K10 and M-Au-K . Sharp diffraction peaks of
15
To evaluate the actual size and shape of M-Au-K NDs, XRD data confirm the formation of highly crystalline
the HRTEM images were recorded (Figure 5). AuNDs AuNDs. The diffraction peaks obtained for AuNDs at 2θ
1
6
A
E
I
B
F
J
C
G
K
D
12
8
4
0
2
3
4
5
6
7
Particle Size / nm
50
H
40
30
20
10
0
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Particle Size / nm
20
L
15
10
5
2
3
4
5
6
7
Particle Size / nm
Figure 5. HRTEM images, SAED pattern and particle size distribution histogram of M-Au-K (A–D),
5
M-Au-K10(E–H), M-Au-K15 (I–L) NDs.

4
Page 6 of 10
J. Chem. Sci. (2018) 130:4
crystallizations are 0.29, 0.70 and 0.56 for M-Au-K5,
M-Au-K10 and M-Au-K , respectively. Therefore, as
15
observed in catalytic tests, the higher catalytic activity of
M-Au-K is attributed to a higher ratio of (311)/(111).
10
3.2 Catalytic reduction of nitroaromatics
To test the catalytic activity of the prepared AuNDs,
reduction of 4-Nitrophenol (4-NP) in the presence
of NaBH was adopted as the model reaction. UV-
4
Vis absorption spectroscopy was used to monitor the
progress of catalytic reduction of 4-NP. Addition of
NaBH4 to 4-NP solution readily produces the
4
-nitrophenolate ion, which showed the strong absorp-
tion at 400 nm. Without a catalyst, the peak around 400
nm remained unaltered for a long time indicating that
there was no reduction reaction. Whereas, in the pres-
ence of a catalyst (AuNDs), a successive decrease in the
intensity of absorption peak at 400 nm was observed
with time and simultaneously, a new peak started grow-
ing at 302 nm, corresponding to 4-aminophenol (Figure
S1 in Supplementary Information), which confirms the
conversion of nitro to the amine. The mechanistic path-
way for the reduction of aromatic nitrocompounds to
the corresponding aromatic amine compounds has been
reported to follow either a direct route or a condensa-
Figure 6. HRTEM images obtained for AuNPs prepared
using 0.5 mM Au (A–B) and 2 mM Au (C–D).
(111)
(200)
(311)
15
tion route. In the most possible direct pathway, both
M-Au-K5
−
BH ions and nitroaomatics would get adsorbed on the
4
−
catalyst surface first, where BH ions will act as the
4
hydrogen source, to accomplish the hydrogenation of
nitroaomatics on the surface of the metal NPs. Hence,
−
BH ions could charge the metal NPs surface and the
4
16
M-Au-K10
charged metal NPs could act as the electron source.
The aromatic nitrocompound is first reduced to the
nitroso compound and then quickly to the corresponding
hydroxylamine compound. The hydroxylamine com-
pound was finally reduced to the aromatic amine.17 As
the concentration of 4-NP is very low when compared
to NaBH , the reaction follows the pseudo-first-order
kinetics and the rate constant (k) values are calculated
from the slope of ln(A) versus time. In the catalytic
4
M-Au-K15
3
0
40
50
60
70
80
reduction of 4-NP in, the presence of NaBH , complete
reduction of 4-NP was observed at 180 and 200 s at
4
2
θ / Degree
M-Au-K and M-Au-K catalysts respectively, whereas
Figure 7. XRD pattern obtained for M-Au-K NDs.
5
15
M-Au-K took only 120 s for the same. The rate con-
10
stants obtained for M-Au-K , M-Au-K , and M-Au-K
5
10
15
◦
◦
◦
valuesof38.2 , 44.3 and77.8 areassignedtothe(111), NDs catalysts for the reduction of 4-NP are 1.599
−
2
−1
−2 −1
−2 −1
(
200) and (311) crystal planes of face-centred cubic × 10 s , 2.982 × 10
s
and 1.518 × 10 s ,
Au crystals, respectively. Moreover, high index facets respectively, and the M-Au-K NDs was found to
10
could enhance the catalytic activity of nanomaterials have a better catalytic activity compared to other two
as observed by many researchers.1 Hence, the pres- AuNDs. Table 1 compares the catalytic performance
ence of high intensity (311) plane is the key factor for of the present AuNDs with reported results towards
enhanced catalysis. The intensity ratios of (311)/(111) the reduction of 4-NP. The TOF values calculated for
3,14

J. Chem. Sci. (2018) 130:4
Page 7 of 10
4
Table 1. Comparison of catalytic activity of present AuNDs with reported results.
kapp(s 1
−
)
References
Catalyst material
Catalyst
NaBH4
(mol %)
equivalent
−
3
18
19
20
2
D graphene oxide/SiO2–Au
–
1.25
0.5
3.2
20
–
500
300
400
88
100
100
75
1333
147
44
44
125
17 × 10
6.1 × 10
9.5 × 10
–
−3
3
CTAB-Au NPs
Au@DHBC NPs
PGMA@PAH@Au NPs
Methyl-imidazolium-based ionic polymer-AuNPs
PAMAM-AuNPs
PPI-AuNPs
rCD-AuNPs
CeO2-AuNPs
Au@PVP
−
2
1
−
2
22
23
24
25
26
27
28
29
3.3 × 10
−
3
0.10
0.10
5.2 × 10
–
7.91 × 10
1.47 × 10
9.75 × 10
1.28 × 10
1.02 × 10
4.65 × 10
11.7 × 10
2.98 × 10
−
2
−5
−4
−
−
−
−
−
2
2
3
3
2
1.56
17.6
17.6
12.5
Cyclodextrin-AuNPs
Polyaniline-AuNPs
M − Au − K10 NDs
Present catalyst
2
.0
2.0
1.5
1.0
0.5
2
.0
a
b
c
0
min
1.5
1.0
0.5
0.0
0 min
1.5
0 min
1
min
1 min
2 min
Interval
Interval
Interval
1.0
1
0 min
6 min
16 min
0.5
0.0
300
400
500
300
400
500
300
400
500
Wavelength / nm
Wavelength / nm
Wavelength / nm
Figure 8. Catalytic reduction of 4-NP in the presence of NaBH4 using catalytic plates prepared by M-Au-K (a), M-Au-K10
5
(
b) and M-Au-K15 (c) NDs.
the reduction of 4-NP using M-Au-K M-Au-K and for the preparation of catalytic plates are described in
5
,
10
−1
M-Au-K NDs colloids are 0.044, 0.066 and 0.040 s , the Section 2. PADA is positively charged and the pre-
15
respectively.
pared AuNDs are negatively charged in nature, hence,
To find the optimum concentration of Au for catal- it is obvious that when the PADA modified glass plate
ysis, catalytic reduction studies were also performed was dipped in AuNDs solution, AuNDs self-assembled
for the AuNPs prepared using various concentration of over the PADA through electrostatic interaction. Self-
Au (0.5, 1, 1.5 and 2 mM) (Figure S2). Among the assembly of AuNDs on to the PADA modified glass
different Au concentrations, 1 mM Au was found to plates was confirmed by recording AFM images (Fig-
be the optimum concentration of Au for catalysis and ure S3). The AFM images of PADA modified glass
no increment in catalytic activity was observed at the plate showed the smooth and plain surface whereas,
higher Au concentrations (>1 mM). However, in labo- AuNDs self-assembled on PADA modified glass plates
ratory scale reactions, removal of such homogeneous clearlyshowedthedensedecorationofsphericalAuNDs
catalyst (AuNDs colloids) from the reaction mixture over PADA surface. Among the three catalytic plates
and reusability of the catalyst remains a challenge. prepared using M-Au-K , M-Au-K and M-Au-K15
5
10
Hence, to overcome these issues, catalytic plates were NDs, best catalytic activity was observed at M-Au-K10
designed using the so formed AuNDs over glass plates towards the reduction of 4-NP (Figure 8b) and the cat-
with the aid of PADA and successfully applied for cat- alytic plate was reused for five consecutive catalytic
alytic reductions. To understand the catalytic ability of cycles without considerable loss in activity (Figure S4
M-Au-K NDs self-assembled on PADA modified glass in SI). Hence, the M-Au-K NDs self-assembled on the
1
0
2
2
plates, 0.8 × 1.5 cm (area = 1.2 cm ) catalytic plates PADA modified large area (2.5 × 2.5 cm) glass plate was
were subjected to catalytic tests (Figure 8). Protocols chosen to carry out the laboratory scale (0.5 mmol batch)

4
Page 8 of 10
J. Chem. Sci. (2018) 130:4
reductions of different nitroaromatic compounds into Table 2. Reduction of various nitroaromatic compounds.
their corresponding amino compounds. When the cat-
alytic performance of as-synthesized colloids of AuNDs
and AuNDs self-assembled onto catalytic plates were
compared, the catalytic plates showed lower catalytic
activity. This may be due to the low amount of AuNDs
loading in the PADA film when compared to the amount
of Au in as-synthesized AuNDs colloids.
Generally, the reduction of nitroaromatics into their
corresponding amines was accomplished under harsh
conditions including strong acids and various metals
like lead and tin. These reactions suffer from severity
present in safety and environmental issues associated
30
with their applicability in scale-up processes. To over-
come these issues, several modified protocols were
reported in recent years and among the various cat-
alytic systems, noble metal NPs catalysed reduction of
nitroaromatics have received great interest due to their
excellentactivity. Especially, catalyticAuNPsdecorated
membranes are considered to be advantageous for the
31
reduction of nitroaromatics, in the presence of NaBH4.
Selective reduction of nitro groups in the presence of
other reducible functional groups is the most important
reactiontosynthesisfunctionalizedanilinesasindustrial
intermediates for a variety of specific and fine chemi-
32
cals. Conventional Pt group of catalysts hydrogenate
both olefinic and carbonyl functions while reducing
13C NMR spectra (Figure S5–S16 in Supplementary
Information).
the nitro group in the presence of H . Hence, in the
2
search of catalyst for the purpose of selective reduc-
tion of nitroaromatics in the presence of olefinic bond,
reduction of nitro-substituted chalcones were examined
and it was found that the present AuNDs catalyst film
selectively hydrogenated the nitro groups in the pres-
ence of olefinic double bond at room temperature and
also reduced the carbonyl group of chalcone together
with nitro group (Table 1 (2e and 2f)), while leaving
the olefinic double bond unaltered. Hence, the present
catalyst film selectively reduces the carbonyl group
of chalcones (α, β-unsaturated carbonyl system) with-
out affecting olefinic double bond to produce allylic
alcohols that are recognized as important intermedi-
ates in Agrochemistry, Pharmaceutical and Fragrance
Chemistry. Such selectivity is ascribed to the forma-
tion of charged hydrogen species (H and H ) on the
metal catalyst surfaces. These charged hydrogen species
are active towards the reduction of polar groups like
nitro and keto but they are unable to reduce non-polar
olefinic bonds. In addition to this, nitro group reduc-
tions were accomplished successfully in the presence of
carboxylic acid, chloro, methoxy, hydroxyl and amine
groups. Table 2 shows the reduction of different nitro
4
. Conclusions
A facile, single-step preparation of methionine-capped
AuNDs and their successful self-assembly on PADA-
modified glass plate was demonstrated for nitroaromat-
ics reduction application. Moreover, it was observed that
it is very easy to remove the present catalytic plates from
thereactionmixtureandthecatalyticplatesarestablefor
two months at laboratory conditions. Therefore, these
catalytic plates have the advantages of homogeneous
catalysts even though these are heterogeneous catalysts.
Further, these catalytic plates would serve as excellent
alternative for noble metal NPs-decorated metal oxide
and other heterogeneous nanocatalysts in terms recov-
ery and reusability of catalyst.
+
−
Supplementary Information (SI)
Catalytic test reactions of 4-NP reductions in the presence
of AuNDs catalyst, recycle test, H NMR and C NMR data
of reduced products obtained in laboratory scale synthesis
1
13
compounds using the M-Au-K catalyst. Product for- (Table 2) and Figures S1-S16 are given in the Supplementary
10
1
mation was confirmed by recording the H NMR and Information, available at www.ias.ac.in/chemsci.

J. Chem. Sci. (2018) 130:4
Page 9 of 10
4
Acknowledgements
high-index facets and high electro-oxidation activity Sci-
ence 316 732
5. Viswanathan P and Ramaraj R 2016 Polyelectrolyte
assisted synthesis and enhanced catalysis of silver
nanoparticles: electrocatalytic reduction of hydrogen
peroxide and catalytic reduction of 4-nitroaniline J. Mol.
Catal. A Chem. 424 128
RR acknowledges the financial support received from the
CSIR-Emeritus Scientist Scheme, New Delhi. PV is the recip-
ient of Senior Research Fellowship under UGC-BSR scheme.
1
1
6. Henglein A and Lilie J 1981 Storage of electrons in
aqueous solution: the rates of chemical charging and dis-
chargingthecolloidalsilvermicroelectrodeJ. Am. Chem.
Soc. 103 1059
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