NVP encapsulated gold nanoclusters by in situ

NVP encapsulated gold nanoclusters by in situ polymerization of monomer
M. Boazbou Newmai, Abhitosh Kedia, and Pandian Senthil Kumar
Citation: AIP Conference Proceedings 1591, 600 (2014); doi: 10.1063/1.4872688
View online: http://dx.doi.org/10.1063/1.4872688
View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1591?ver=pdfcov
Published by the AIP Publishing
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NVP Encapsulated Gold Nanoclusters by In Situ
Polymerization of Monomer
M. Boazbou Newmai*, Abhitosh Kedia and Pandian Senthil Kumar
Department of Physics & Astrophysics, University of Delhi, Delhi-110007, India
*E-mail: [email protected]
Abstract. We propose a simple versatile environmental friendly approach for the synthesis of stable gold nanoclusters via
utilizing N-vinyl pyrrolidone (NVP) both as a solvent and reaction medium in systematically reducing the metal precursor,
chloroauric acid, further radically oxidizing itself to form oligomers which simultaneously encapsulates the as-formed gold
nanoclusters, the evidence of which can be clearly seen from TEM and other molecular spectroscopic measurements as
illustrated herewith.
Keywords: Nanoclusters, oligomerization.
were mixed in appropriate amounts at ambient
conditions, simultaneous reduction of metal salt along
with the radical oxidation of the monomer takes place,
resulting in the intuitive in situ formation of gold
nanoclusters encapsulated with NVP oligomers.
INTRODUCTION
Over the past decade, worldwide researchers have
developed various novel methods for the synthesis of
stable gold nanoclusters with exciting physico-chemical
properties as well as variety of potential applications in
the field of optoelectronics, catalysis1,2 ,fuel cell
technology3, medical science4 plasmonics and
nanophotonics. The most widely adopted synthesis
route is the etching-based strategy, which sequentially
etches large gold nanoparticles to form small gold
nanoclusters simply by applying suitable thiols,
biomolecules, or multivalent polymers on the
nanoparticle surfaces5. The only drawback of this
method is their complicated multistep time consuming
procedure involving expensive chemicals. A single
step synthesis of gold nanoclusters at ambient room
temperature is still very scarce6.
The size/shape selective synthesis and stabilization of
metal nanoparticles by the versatile amphilic polymer,
poly vinyl pyrrolidone (PVP) encourage us to utilize its
stable monomer, N-vinyl-2-pyrrolidone (NVP) for the
reduction of metal ions under ambient conditions
without any external energy sources. Also, it is for the
first time to the best of our knowledge that a monomer
NVP has been effectively utilized both as a reaction
medium as well as a solvent for the in situ formation of
oligomer coated metal nanoclusters.
In this present work, we have used HAuCl4 as the
metal precusor and the organic monomer, (N-VinylPyrrolidone) as the solvent as well as reaction medium
by itself. When the monomer and the metal precursor
EXPERIMENTAL SECTION
Hydrochloroauric acid (HAuCl4.3H2O), N-Vinyl–
Pyrrolidinone purchased from Sigma- Aldrich and used
without further purification. For a typical synthesis,
Chloroauric acid (HAuCl4 ) of different concentration
is added to 5mL of NVP solution under continuous
stirring at room temperature. On addition of HAuCl4
the colour of the sample changes from yellow to green
within 5-10min, the reduction kinetic increases with
increase in HAuCl4 concentration. The change in color
from yellow to green indicates the formation of Au
nanoparticles.
CHARACTERIZATION:
All the as-prepared gold nanostructure samples were
characterized by UV-visible optical spectroscopy using
Thermo Scientific Evolution 300 spectrophotometer,
TEM with a Tecnai G2 system operated at 300kV,
FTIR with Perkin Elmer Spectrum RX1, GCMS.
RESULTS AND DISCUSSION
Simple addition of different ratios of NVP and
chloroauric acid visually changes the color of the
Solid State Physics
AIP Conf. Proc. 1591, 600-602 (2014); doi: 10.1063/1.4872688
© 2014 AIP Publishing LLC 978-0-7354-1225-5/$30.00
600
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reaction medium from pale yellow to colorless and then
green within 10minutes confirming the stable formation
of small gold nanoclusters, as illustrated from the
optical absorption spectra and the corresponding TEM
images (figure 1c). The absorption spectra shows a
small hump like structure peaked at around 400nm
(quantifying the size/shape independence of the as-
formed small gold nanoclusters) and a sharp peak at
630nm(fig 1a) arising due to the formation of interconnected chain like gold nanostructures essentially
capped with the oligomerized VP molecules (as shown
by the TEM images for 2mM gold samples). The
absence of the SPR peak at around 520nm strongly
identifies
with
the
formation
of
FIGURE 1. Absorption spectra for different concentrations of HAuCl4(a) The time kinetics of FTIR spectra for 2mM
concentration of HAuCl4 (b) TEM image of 2mM HAuCl4 (c) GCMS Image (d).
particles less than 5nm in diameter. The extend of
polymerization and the increase in chain
length/molecular weight of the monomer strongly
depends on the concentration of the Au ions used in
the reaction mixture, as they only provide the essential
nucleophilic water molecules necessary for the radical
oligomerization of the monomers in the absence of any
other external initiators, leading to the their
simultaneous reduction resulting in the inherent
formation of gold nanoclusters. The FTIR spectra (fig
1b) of the pristine NVP clearly shows a vibrational
band at 1620cm-1 corresponding to its C=C bond,
which starts disappearing after the addition of
chloroauric acid and the band at 1670cm-1 assigned to
the C=O bond kinetically red-shifts and stabilizes at
1690cm-1, which is the characteristic vibrational
signature of the polymer PVP, delineating the fact that
oligomerization of NVP has taken place. The GCMS
spectra (fig 1d) showing intense peaks at 224,
334,444,925 m/z etc. further quantitatively confirms
the increase in the molecular weight of the resultant
NVP oligomer.
CONCLUSION
Small gold nanoclusters were effectively synthesized
by a simple versatile single step colloidal process at
room temperature via in situ radical oxidized
oligomerization of the NVP monomer, the procedure
of which can be further extended to other noble metals
like Pt, Ag etc. with ease. The molecular spectroscopy
and mass spectra measurements quantify the intuitive
in situ oligomerization of the monomer and further
work is currently going on in our research group in
terms of identifying the various relevant parameters
such as the monomer to metal ratio, surrounding
reaction environment etc. for the controlled
601
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simultaneous synthesis of metal nanostructures capped
with different chain length oligomers.
ACKNOWLEDGMENTS
The authors thank M.Tech. Nanoscience and USIC,
University of Delhi for materials characterization and
IGIB, New Delhi for GCMS measurements. AK and
MBN gratefully acknowledges the CSIR for NET-SRF
and UGC for JRF fellowships.
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