One pot green synthesis of polyaniline coated gold nanorods and its applications

Article abstract of DOI:10.1039/c4ra12080a

Morphology controlled high aspect ratio worm-like polyaniline (PANI) layer coated Au-nanostructures (Au/PANI) have been successively synthesized by in situ polymerisation techniques using aniline as a monomer with HAuCl₄ as an oxidising agent in the absence and presence of citric acid (CA). Synthesized composites were characterized by HRTEM, FESEM, XRD, XPS, UV-vis and FTIR study. Spherical morphologies are seen in the absence of CA and Au nanoparticles are coated by a PANI thin layer with ～30 nm thickness. In the presence of CA, as well as depending upon the CA to aniline molar ratio, morphology varies from irregular assembly to regular fiber to spherical-like nanostructures. The nanostructures show fibrous morphology with an average diameter of ～100 nm and lengths of more than 5 μm when CA to aniline molar ratios are 1.0 and 0.2. When the ratios are 2.0 and 0.1, the nanostructure represents the granularlike morphology. Nanofiber formation takes place by the assembly of the CA capped tiny Au-nanorods in the presence of aniline during the polymerisation, and all Au-nanorods are finely coated by a PANI thin layer of ～5 nm thickness. Importantly, fibrous Au/PANI nanostructures show superior catalytic activity compared to spherical/irregular Au/PANI nanostructures towards the reduction of toxic aromatic nitro compounds like 4-nitrophenol (4-NP), 4-nitroaniline (4-NA), 2,4-dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (2,4,6-TNP). This is because of the coating thickness of PANI over Au-nanoparticles in fibrous Au/PANI nanostructures as well as enhanced surface area.

Full text of DOI:10.1039/c4ra12080a

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DOI: 10.1039/C4RA12080A
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One Pot Green Synthesis of Polyaniline Coated Gold Nanorods
and its Applications
Sanjoy Mondal,a Utpal Rana,a Rama Ranjan Bhattacharjeeb and Sudip Malika*
a Polymer Science Unit, Indian Association for the Cultivation of Science, 2A & 2B Raja S.C. Mullick
Road., Jadavpur, Kolkata –700032, India.
bPSG Institute of Advanced Studies, Avinashi Road, Peelamedu, Coimbatore, Tamil Nadu 641004,
India.

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DOI: 10.1039/C4RA12080A
Journal Name
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ARTICLE
One Pot Green Synthesis of Polyaniline Coated
Gold Nanorods and its Applications
Cite this: DOI: 10.1039/x0xx00000x
Sanjoy Mondal,a Utpal Rana,a Rama Ranjan Bhattacharjeeb and Sudip Malika*
Received 00th January 2012,
Accepted 00th January 2012
Morphology controlled high aspect ratio worm like polyaniline (PANI) layer coated Auꢀnanostructures
(Au/PANI) have been successively synthesized by in-situ polymerisation techniques using aniline as a
monomer with HAuCl4 as an oxidising agent in absence and presence of citric acid (CA). Synthesized
composites were characterized by HRTEM, FESEM, XRD, XPS, UVꢀVis and FTIR study. Spherical
morphologies are seen in absence of CA and Auꢀnanoparticles are coated by PANI thin layer with
thickness ~30 nm. In presence of CA as well as depending upon CA to aniline molar ratio, morphology
varies from irregular assemble to regular fiber to spherical like nanostructures. The nanostructures show
DOI: 10.1039/x0xx00000x
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fibrous morphology with an average diameter of ~100 nm and lengths of more than 5
ꢀm when CA to
aniline molar ratios are 1.0 and 0.2. When the ratios are 2.0 and 0.1, the nanostructure represents the
granular like morphology. Nanofiber formation takes place by the assembly of the CA capped tiny Auꢀ
nanorods in presence of aniline during the polymerisation and all Auꢀnanorods are finely coated by PANI
thin layer of thickness ~5 nm. Importantly, fibrous Au/PANI nanostructures show superior catalytic
activity than spherical/irregular Au/PANI nanostructures towards toxic aromatic nitro compounds
reduction like 4ꢀnitrophenol (4ꢀNP), 4ꢀnitroaniline (4ꢀNA), 2,4ꢀdinitrophenol (2,4ꢀDNP) and 2,4,6ꢀ
trinitrophenol (2,4,6ꢀTNP). It is because of the coating thickness of PANI over on Auꢀnanoparticles in
fibrous Au/PANI nanostructures as well as enhanced surface area.
Introduction
single step reaction, b) high percentage of yield and c) no need
to step by step purification.15
In recent years fabrication of nanostructures with controlled
morphology is challenging to scientists because dimensionality
plays an important role for determining material properties.
Oneꢀdimensional (1D) nanostructure including nanorods,
fibers, wires and tubes have attracted more attention due to
their unique optical, electronic properties resulting great
attention in many potentials applications like field effective
transistor,1,2 quantum wire laser,3 electronic memory devises,4,5
catalyst6,7 etc. Making of nanostructure material contain
electrically conducting polymers and metal nanoparticles are
attractive field of research nowadays due to their important
physical properties and potential application in molecular
memory device, batteries, display devices, bioꢀchemistry8, etc.
Among all conductive polymer PANI is the most popular
conductive polymeric materials9,10 due to its easy synthesis,
good environmental stability,11 low cost of the raw/starting
materials, large range of conductivity, high thermal stability
etc. Among the metal nanoparticles, Auꢀnanoparticles has been
extensively studied in the scientific community due to its
exclusive catalytic12ꢀ14 and unique optical properties.13 In-situ
technique is one of the best techniques for synthesis because: a)
Formation of Au/PANI nanostructures by soft template
method17ꢀ19 is going through a series of step such as chemical
reactions, nucleation and precipitation. Each and every step is
influenced by many experimental parameter like external
materials (additives/ dopant), concentration of starting and any
externally applied materials, reaction temperature, reaction time
etc. Actually nucleation step is a vital role for formation of
nanosphere and nanofiber of PANI in solution state.20ꢀ21
Homogeneous nucleation reveals the nanofiber morphology of
PANI in bulk solutions while heterogeneous nucleation leads to
irregular or/and nanosphere like morphology of PANI.20 The
aspect ratio of Au/PANI nanostructures also depend on the
concentration of controlling agent, reaction temperature, pH
and reaction time.10,16
Here we report a simple solution mixing in-situ method for
Au/PANI nanostructure synthesis at room temperature using
HAuCl4, CA and aniline. HAuCl4 acts as an oxidising agent as
well as proton donor to PANI and CA acts as a capping agent
for Auꢀnanoparticles synthesis as well as secondary dopant for
PANI (ES). The morphology of the nanostructure has tune by
changes the molar ratio of CA to aniline. Nanostructures will be
the regular nanofibers (worm) like morphology when CA to
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J. Name., 2013, 00, 1-3 | 1

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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA12080A
aniline molar ratio 1.0 to 0.2 and irregular agglomerated An aqueous solution of HAuCl4 (1 mM, 10 mL) was taken in a
morphology for 2.0 and 0.1. The mechanism of the culture tube then added 100 µL of distilled aniline ( to maintain
nanostructure formation also established from HRTEM studies HAuCl4 to aniline molar ratio = 1:1) with constant stirring for
and result showed that CA play an important role for worm like 30 min then keep it in dark place without stirring for 12 hs to
nanostructure formation. Optimise the reaction condition for get spherical like composite called Au/PANI@0.0 composite.
nanofibers formation by time dependent UVꢀVis and HRTEM
Synthesis of CA stabilized Au seed solution
studies. The synthesized nanostructure showed good catalytic
efficiency and recyclability to several nitro compounds In the typical experiment 10 mL of 1 mM CA aqueous solution
reduction in aqueous medium. In addition the catalytic was mixed with 1 mM HAuCl4 aqueous solution (v/v=1:1) in a
efficiency of worm like nanofibers is much more than that of culture tube equipped with a magnetic stirrer at room
o
the agglomerated structures under identical condition.
temperature (25 C). After 1 h solution colour changed from
greenish yellow to transparent pinkish violate, this solution
(Auꢀ60) actually considered as a active seed solution for
Au/PANI formation.
Experimental Details
Synthesis of Au/PANI@2.0, Au/PANI@1.0, Au/PANI@0.2 and
Au/PANI@0.1 composite:
Materials
o
Aniline was distilled under reduced pressure and store at 5 C
in dark place and Sodium borohydride (NaBH4) purchased from
Merck chemicals. Au(III) chloride trihydrate (HAuCl4, 3H2O,
ACS reagent, ≥49.0% Au basis) and citric acid (ACS reagent,
≥99.5%) were purchased from SigmaꢀAldrich, and used without
further purification. The nitro aromatics compounds (4ꢀ
nitrophenol, 2,4ꢀdinitrophenol, 2,4,6ꢀtrinitrophenol and 4ꢀ
nitroaniline) were purchased from Loba Chemicals, Mumbai.
All solutions were prepared in deionised water (18 MΩ cm,
Millipore Mili Q water).
To making Au/PANI nanofiber, 20 mL Auꢀ60 aqueous solution
was taken in a culture tube then different amount (in µL) of
distilled aniline monomer was added (Table S1) to Auꢀ60
aqueous solution with constant stirring until dissolving the
aniline at room temperature (25 oC). After adding
aniline,solution colour changed pinkish violate to deep blue to
black immediately indicating Au/PANI composite started to
form. The reaction mixture was kept in dark place without
stirring for 12 hs. Finally products were collected by
centrifugation at rpm 8000 and washed with water several times
o
finally dry under vacuum at 60 C temperature to obtain a dark
Instruments
powder of Au/PANIꢀcomposites.
To observe the surface morphology of composites, little amount
of prepared composites dispersed in Mili Q water then drop
casted on a glass cover slip and dried, at room temperature. We
used JEOL, JSM 6700F instrument, operating at 5 kV. To
reduce the surface potential, samples were coated with platinum
for 90 s due to accumulation of electrostatic charge.
The electron microscopy of Au/PANI nanostructure were
carried out by using HRTEM (JEOL, 2010EX), image were
taken using a CCD at an accelerating voltage of 200 kV.
Samples were spreaded over on a 200 mesh Cuꢀgrid coated
with a holey carbon support film.
Catalytic activity towards nitro compound reduction
For monitoring the reduction process, prepared yellow
aqueous solution of aromatic nitro phenols compounds (4ꢀ
nitrophenol, 4ꢀnitroaniline, 2,4 diꢀnitrophenol, 2,4,6ꢀtriꢀnitro
phenols) of 1 mM and 3 mM concentration of ice cold NaBH4
solution. Then in a quartz UV cell of 1 cm path length took 2
mL of water and added 100µL of aromatic nitrophenols
compounds, water dispersed Au/PANI composites subsequently
added Au/PANI (∼0.3 wt %) as a catalyst then immediately
added 100 µL of 3 mM NaBH4 solution. Now solution was
quickly subjected to UVꢀVis measurement for monitoring
reduction processes.6ꢀ7,22
UVꢀVis spectra of Au/PANI samples were recorded by using a
HewlettꢀPackard UVꢀVis spectrophotometer (model 8453) in a
1.0 cm path length quartz cell.
XRD measurements were performed by using a Bruker AXS
diffractometer (D8 advance) using CuKα radiation (λ=1.54 Å),
a generator voltage of 40 kV and a current of 40 mA. The
scanning rang of the samples 2θ=15ꢀ85o the scan rate was
1s/step with a step width of 0.02.
The FTIR spectra were carried out in a FTIRꢀ8400S instrument
(Shimadzu) using KBr pellets of the samples.
XPS was performed using a focused monochromatized MgꢀKα
Xꢀray source (1253.6 eV) in Omicron NanoꢀTechnology 0571
XPS instrument.
Results and Discussion
Morphological Studies
Nanostructures of Au/PANI were prepared with different
molar ratio of HAuCl4 to aniline and were checked by FESEM
observations (Fig.1). Spherical morphology of Au/PANI@0.0
(in absence of CA) was clearly seen in Fig.1a at equal molar
ratio of HAuCl4 to aniline. Whereas in presence of CA and at
the equal molar ratio of HAuCl4 to aniline, wormꢀlike nanofiber
morphology was observed. Important observation signified that
there were tremendous effects of CA on the formation of
Au/PANI nanostructures. It was also seen that morphology of
Au/PANI depended on the molar ratio of CA to aniline. Fibrous
Synthesis of Au/PANI@0.0 composite:
2 | J. Name., 2012, 00, 1-3
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