2015 Adv. Nat. Sci: Nanosci. Nanotechnol. 6 025002 (8pp) doi:10.1088/2043-6262/6/2/025002
Vietnam Academy of Science and Technology Advances in Natural Sciences: Nanoscience and Nanotechnology
6,6-ionene-stabilized gold nanoparticles: synthesis, characterization and prospects of use
Vladimir V Apyari, Anna N Ioutsi, Viktoriya V Arkhipova, Stanislava G Dmitrienko and Elena N Shapovalova
Chemistry Department, Lomonosov Moscow State University, Leninskie gory 1/3, 119991 Moscow, Russia
Received 18 November 2014
Accepted for publication 18 December 2014
Published 2 February 2015
Abstract
Synthesis of new polycation-stabilized gold nanoparticles was performed by the borohydride approach.
6,6-ionene was used as a polycationic stabilizer. The influence of several factors such as the acidity, the Au:ionene
ratio and the concentration of sodium borohydride during the synthesis were investigated. The key role of the amount
of HCl added during the synthesis on the behaviour of the products in time was shown. Optimal conditions for the
synthesis were chosen. The prepared gold nanoparticles were characterized by surface plasmon resonance band at 520 nm.
Their average diameter is 16 nm. Apart from spherical (73%), there were pentagonal (14%), hexagonal (7%), triangular
(4%) and cylindrical (2%) nanoparticles. These particles were stable in solution for at least four months. Aggregative
stability of the nanoparticles in the presence of different inorganic anions was tested. It was shown that the most
aggregative effect was produced by sulfate, which reveals the prospects for use of ionene-stabilized gold nanoparticles
for the selective detection of sulfate. These nanoparticles can be easily adsorbed on hydrophobic C8-silica, which
may offer the way to new solid-phase nanomaterials for recovering and separation.
1. Introduction Recently, considerable interest of researchers working in various branches of science has been
drawn to nanoparticles and other nano-objects of different kinds. A great part of the research work has been done
in respect of gold nanoparticles (NPs) owing to their relatively high stability and unique properties which can be
utilized in various applications [1–9]. Data on the application of gold NPs in the electrochemical and bioelectrochemical
analysis [10, 11], chromatographic and electrophoretic methods [12, 13], chemical and immunosensors [14–18] have been published.
Many different types of stabilizers/modifiers have been used for preparation of NPs with peculiar properties.
Oligonucleotides, DNA, peptides, various thio-compounds such as cysteine, cysteamine, mercaptoacetic acid, antigenes,
amino-compounds are among them [19–21]. The nature of stabilizing agent is an important factor, not only influencing stability
of the synthesized NPs but also considerably affecting their properties. That is why the search for new substances prospective
as the stabilizers or modifiers of NPs, synthesis and characterization of the new NPs is still of great importance.
Ionenes are among such interesting substances. These are linear anion-exchange polymers described by a general formula (scheme 1).
Scheme 1. n,m-ionene.
These compounds were suggested as effective stationary phases in chromatography and capillary electrophoresis [22– 24].
The presence of positively charged quaternary nitrogen atoms in molecules of ionenes in combination with the ability
to form poly-layers with citrate-capped gold NPs [25] stimulated the interest in these polymers as effective stabilizers
of gold NPs.
Herein, we firstly present the method of synthesis of 6,6ionene-stabilized gold NPs, discuss several their characteristics
and probable ways of further application.
2. Materials and methods
2.1. Materials Hydrogen tetrachloroaurate, sodium borohydride, hexamethylenediamine, 1,6-dibromohexane, N,N-dimethylformamide,
acetone, acetonitrile, sodium hydroxide, hydrochloric acid, sodium sulfate, bromide, chloride, perchlorate, chlorate, fluoride,
nitrate, phosphate and hydrocarbonate, ethylenediaminetetraacetate disodium salt (EDTA) were used. All chemicals were
at least of analytical grade. Silasorb S and silasorb C8 (10 µm, Lachema, Czech Republic) were used for investigation
of NPs sorption properties.
2.2. Instrumentation Absorption spectra of the solutions were recorded by SF-103 spectrophotometer (Akvilon, Russia),
pH was measured by Ekspert 001 ion meter (Ekoniks, Russia). Microphotographs of the samples were recorded, using transmission
electron microscope LEO912 AB OMEGA (Carl Zeiss, Germany). Batch mode adsorption studies were carried out using LOIP
LS-120 shaker. Centrifugation was performed on ELMI CM50 centrifuge.
2.3.
Synthesis
of
6,6-ionene
6,6-ionene
(poly[N,N,-dimethylhexamethyleneimine
hydrobromide])
was
synthesized
according
to
the
following
procedure.
Equimolar solutions of N,N,N’,N’-tetramethylhexamethylenediamine and 1,6-dibromohexane
were
mixed
in
N,N-dimethylformamide
up
to
the
final
concentration
of
each substance of 1 mol L-1.
The mixture was stirred using a magnetic stirrer at room temperature until sedimentation of the polymer was completed.
End point of the reaction was controlled visually by the volume of the sediment. The mixture was poured at stirring into
20-fold volume of water-free acetone. The sediment was filtered under suction and washed doubly with 10 mL of acetone.
The product was dried under the vacuum of 10-2 mm Hg.
The
resulting
powder
was
characterized
by
13C
and
1H
nmR
spectra.
They
are
represented
in
figure
1,
from
which
one
can
conclude
that
only
four
types
of
carbon
atoms
(at
21.74;
25.05;
50.28
and
63.84
ppm)
consisting
in
a
mole
ratio
of
1:1
to
each
other
were
present
in
the
structure
of
the
polymer.
This
fact
corresponds
to
the
presumed
formula
containing
three
couples
of
carbon
atoms
in
the
symmetric
positions
in
the
main
polymer
chain
and
a
couple
of
side
methyl
groups.
In
1H
nmR
spectrum,
there
are
four
strong
signals
(at
1.45;
1.80;
3.07
and
3.32
ppm,
with
the
integrals
of
2:2:3:2).
The
singlet
at
3.07
ppm
corresponds
to
two
methyls,
whereas
the
other
signals
belong
to
methylenes.
These
data
also
prove
the
structure
given
above.
2.4.
Synthesis
of
6,6-ionene-stabilized
gold
nanoparticles
Gold
nanoparticles
stabilized
with
6,6-ionene
(NPs)
were
prepared
by
reducing
metal
salt
precursor
(hydrogen
tetrachloroaurate,
HAuCl4) in the presence of 6,6-ionene by sodium borohydride.
Briefly, a 0.05 g portion of 6,6-ionene was placed in a round bottom bulb, dissolved in 18.5 mL of deionized water;
6.5mL of 0.1molL-1 HCl was injected at stirring. After that, 25 mL of a solution containing 1.25 mL of
1% HAuCl4 was introduced dropwise into the bulb. The brown mixture was stirred for 15 min. Then a solution
of 0.025 g NaBH4 in 50 mL of water was added dropwise into the bulb with vigorous stirring. The color of
the mixture changed to ruby. The solution was stirred for 30 min and kept for 24 h to achieve recrystallization and
complete stabilization of NPs. The concentration of NPs in final solution was 70 µgmL-1 (0.35 mmol L-1 in terms of gold).
The prepared gold NPs were characterized with UV–vis absorption spectra, transmission electron microscopy (TEM) and electron
diffraction (ED).
2.5.
Studies
of
NPs
sorption
on
hydrophobic
and
hydrophilic
silica
To
investigate
sorption
properties
of
NPs
the
weighted
0.05
g
portion
of
silica
was
taken.
Silasorb
S
was
activated
with
5
mL
of
0.001
mol
L-1
solution
of
NaOH.
Then
it
was
separated
by
decantation
and
washed
twice
with
distilled
water.
0.5
mL
of
acetonitrile
was
added
to
improve
wettability
of
silasorb
C8.
Batch
mode
adsorption
studies
were
carried
out
by
agitation
of
the
sorbents
with
10
mL
of
a
solution
of
NPs
of
desired
concentration
at
25
°C
for
15
min.
The
concentration
of
Au
was
varied
in
the
range
of
0.009–0.120
mmol
L-1.
The
solution
and
adsorbent
were
separated
by
centrifugation
at
200
rpm
for
5
min
and
the
concentration
of
residual
NPs
was
determined
spectrophotometrically
at
527
nm,
using
a
preliminary
constructed
calibration
curve.
The
amount
of
adsorbed
nanoparticles
was
evaluated
by
the
difference
of
their
content
in
the
solution
before
and
after sorption.
Figure 1. 13C (a) and 1H (b) NMR spectra of 6,6-ionene.
3.
Results and discussion
3.1.
Synthesis
of
6,6-ionene-stabilized
gold
nanoparticles
Due
to
the
presence
of
well
localized
positive
charges
on
the
quaternary
amino-groups
of
ionenes,
these
polymers
are
characterized
by
strong
anion-exchange
ability;
long
saturated
hydrocarbon
chain’s
inserts
impart
conformation
labiality
to
the
polymer
molecules
as
well
as
producing
some
sterical
effects
when
interacting
with
other
molecules
or
particles.
These
properties
seem
to
suggest
that
6,6-ionene
appear
to
be
a
good
stabilizer
for
gold
NPs.
The
ionene-stabilized
gold
NPs
are
characterized
by
the
surface
plasmon
resonance
(SPR)
band
at
520
nm
in
water
solution,
which
is
typical
for
gold
NPs
of
10–25nm
in
diameter.
Influence
of
several
important
parameters
during
the
synthesis
of
NPs
was
investigated.
3.1.1.
Acidity.
Acidity
remarkably
influences
the
synthesis
of
NPs.
It
was
adjusted
by
hydrochloric
acid
within
the
range
of
0
to
20
mmol
L-1.
The
resulting
absorption
spectra
of
the
solution
after
synthesis
are
shown
in
figure
2(a).
One
can
see
that
no
pronounced
SPR
band
was
observed
at
the
HCl
concentration
less
than 4 mmol
L-1.
There
was
a
well-shaped
SPR
band
of
NPs
at
the
HCl
concentration
of
7–10
mmol
L-1.
At
the
concentration
of
HCl
of
20
mmol
L-1,
the
SPR
band
of
individual
NPs
was
accompanied
with
a
band
of
the
aggregates
at
650
nm.
The
dependence
of
A650/A520
ratio
(which
characterizes
an
aggregative
state
of
NPs)
on
the
concentration
of
HCl
is
represented
in
figure
2(b).
It
has
a
minimum
in
the
range
of
7–10
mmol
L-1
of
HCl.
The
left
part
of
the
curve
correlates
well
with
pH
of
the
solution
(shown
with
a
dotted
line)
that
proves
a
key
role
of
H+
ions
in
stabilization
of
NPs.
Destabilization
of
NPs
at
the
high
concentrations
of
HCl,
which
appears
as
the
increase
of
A650/A520 aggregative ratio, does not seem to be conditioned by H+
ions.
It may be ascribed to the influence of Cl.
Figure 2. (a)
Absorption spectra of ionene-stabilized gold NPs synthesized at 0 (curve 1), 2 (curve 2), 4 (curve 3), 7 (curve 4),
10 (curve 5) and 20 mmol L-1 HCl (curve 6);
(b) Dependence of A650/A520 ratio (solid line) and pH of the solution (dotted line)
on the concentration of HCl.
Figure 3.
The A650/A520 ratio and the SPR band intensity at 520 nm of ionene-stabilized gold NPs
depending on the Au:ionene ratio.
.
Being an anion, Cl. must interact with positively charged NPs causing their electrostatic destabilization. The concentration
of HCl of 7 mmol L-1 was chosen as the optimal one and used in all further experiments.
3.1.2.
The
Au:ionene
ratio.
The
Au:ionene
ratio
is
another
important
factor
affecting
not
only
yield
of
NPs
but
also
their
stability.
Figure
3
represents
the
influence
of
Au:ionene
ratio
on
both
these
characteristics.
It
can
be
seen
that
the
highest
SPR
band
intensities
at
520
nm
were
observed
at
the
Au:
ionene
ratio
more
than
1:3,
whereas
the
A650/A520
ratio
was
minimal
at
1:1.5–1:3
of
Au:ionene.
Thus
the
ratio
1:3
was
considered
to
be
optimal
and
used
in
further
experiments.
Figure 4. Dependence of the SPR band intensity at 520 nm of ionene-stabilized gold NPs on the concentration of NaBH4.
3.1.3.
The concentration of a reductant. This can be considered as one of the most important factors influencing the yield of a reduction product.
We used NaBH4 as a reductant during the synthesis of ionene-stabilized gold NPs. The dependence of the SPR band intensity
of NPs on the concentration of sodium borohydride is depicted in figure 4. According to it the maximum yield of NPs is
achieved at concentrations higher than 5 mmol L-1.
Figure 5. TEM image (a), histogram of size distribution (b), shapes (c) and ED pattern (d) of ionene-stabilized gold NPs.
3.2.
Characterization
of
6,6-ionene-stabilized
gold
nanoparticles
The
presence
of
NPs
in
the
solution
was
proved
by
electron
microscopy.
As
can
be
seen
from
microphotography
(figure
5(a))
and
histogram
of
size
distribution
(figure
5(b)),
NPs
had
the
average
diameter
of
16
nm.
Close
consideration
of
the
TEM
images
shows
that
NPs
are
polymorphic:
spherical
(73%),
pentagonal
(14%),
hexagonal
(7%),
triangular
(4%)
and
cylindrical
(2%)
nanoparticles
were
present
in
the
sample
(figure
5(c)).
Electron
diffraction
pattern
of
the particles (figure 5(d)) confirmed that these nanoparticles were gold NPs.
3.2.1.
Time
stability.
The
stability
of
ionene-stabilized
gold
NPs
in
time
has
been
investigated.
It
is
interesting
that
NPs
synthesized
in
the
presence
of
the
different
concentrations
of
HCl
had
different
behavior
in
time.
This
becomes
clear
from
figure
6.
The
NPs
samples
obtained
at
the
low
(4
mmol
L-1
HCl)
and
high
(20
mmol
L-1
HCl)
acidity
are
both
instable.
But these kinds of instability are quite different.
Figure 6. Absorption spectra of ionene-stabilized gold NPs prepared in 4 mmol L-1 (a), 7
mmol L-1 (b) and 20 mmol L-1 (c) HCl in 1h (curve 1), 5 h (curve 2), 25 h (curve 3), 78 h
(curve 4) and 4 months (curve 4*) after the synthesis, and the colors corresponding to the marginal situations.
At the low acidity (figure 6(a)), the absorption spectrum of NPs recorded in 1 h after the synthesis had no
pronounced SPR band of NPs at 520 nm but contained a long-wave tail at . > 1100 nm. Keeping the solution for
several hours resulted in decreasing the long-wave tail and increasing two bands—at 520 and 650 nm, which
corresponded
to
individual
NPs
and
their
aggregates,
respectively.
Color
of
the
solution
changed
from
gray
to
blue.
The
presence
in
the
spectra
of
an
almost
isobestic
point
at
750
nm
may
prove
that
the
total
amount
of
nanogold
in
the
solution
is
not
changed.
These
facts
seem
to
be
attributed
to
re-crystallization
processes
with
participation
of
NPs
taking
place
in
the
system.
At
the
high
acidity
(figure
6(c)),
one
could
observe
a
quite
pronounced
band
of
NPs
and
their
aggregates
1
h
after
the
synthesis
which
then
was
decreased.
It
may
correspond
to
enlargement
and
sedimentation
of
the
NPs
aggregates.
Color of the solution was changed from blue to light gray.
The NPs prepared at 7 mmol L-1 HCl were stable for at least four months (figure 6(b)).
3.2.2.
Aggregative
stability.
The
aggregative
stability
of
ionene-stabilized
gold
NPs
in
the
presence
of
different
inorganic
anions
and
EDTA
has
been
estimated.
Aggregation
of
NPs
was
estimated
as
A650/A520
aggregative
ratio
at
the
two
different
concentrations
of
anions
of
0.25
and
1.0
mg
mL-1.
The
results
are
represented
in
figure
7.
One
can
see
that
the most aggregative effect was produced with sulfate followed by EDTA and nitrate. This fact
may be explained by the decreased polarizability of these anions. In the case of sulfate, this is
accompanied by its increased charge, which is equal to .2 in the wide pH range. It imparts to this anion
the additional ability to produce the aggregation of NPs, owing to 6 formation
of
crosslinking
between
them.
All
these
facts
make
ionene-stabilized
gold
NPs
prospective
for
the
selective
detection
of
sulfate.
Figure7. Influence of the nature of an added anion on aggregative stability of ionene-stabilized gold NPs
at the 0.25 and 1mgmL-1 concentration level.
Added anion: dihydrophosphate (curve 1), perchlorate (curve 2), chlorate (curve 3), hydrocarbonate (curve 4),
fluoride (curve 5), bromide (curve 6), chloride (curve 7), nitrate (curve 8), EDTA (curve 9), sulfate (curve 10).
Figure8. Isotherm of [NPs] adsorption on silasorb C8. mSilasorb = 0.05 g, V = 10 mL.
3.2.3.
Adsorption
properties.
The
adsorption
of
ionenestabilized
gold
NPs
from
aqueous
solution
on
the
hydrophilic
sorbent
silasorb
S
and
the
hydrophobic
sorbent
silasorb
C8
has
been
studied.
It
was
shown
that
the
sorption
on
silasorb
C8
could
be
described
by
the
classical
Langmuir
isotherm
of
monomolecular
adsorption
(figure
8).
The
maximum
adsorbed
amount
of
NPs
was
5 mmol Au g-1. The adsorption of NPs on silasorb S was significantly lower (<1 mmol Au g-1). The isotherm had irregular
form, which could be a result of multilayer adsorption. Thus, the ionenestabilized gold nanoparticles have certain hydrophobic–
hydrophilic balance that can be utilized in the synthesis of new adsorbents for recovering and separation of moieties of different polarity.
4.
Conclusion It has been stated that 6,6-ionene polycation is a promising stabilizer for the synthesis of gold NPs. The optimal
conditions of the synthesis have been evaluated; they were achieved in 7 mmol L-1 HCl, at 1:3 Au:ionene mol ration,
in 5 mmol L-1 solution of NaBH4. The important influence has been revealed of the concentration of
HCl during the synthesis on the stability and behavior of an ‘NPs– their aggregates’ system. NPs prepared under the optimal
conditions are characterized by the average diameter of 16 nm and an SPR band at 520 nm, being stable for at least four months.
Aggregation of NPs under the influence of anions has been shown to depend on the nature of the anion. Sulfate causes the highest
aggregative
effect,
which
is
prospective
for
its
selective
detection.
Good
sorption
ability
of
NPs
regarding
modified
silica
sorbents
offers
the
way
for
their
application
in
synthesis
of
new
solid-phase
nanomaterials
for
recovering
and
separation.
Acknowledgments
The work was financially supported by the Russian Foundation for Basic Research (grants N 13-03-00100, 14-0331109).
We also thank MSU Joint Use Center and Dr Sergey S Abramchuk for recording TEM images and ED patterns of the samples.
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Scheme 1. n,m-ionene.
Figure 6. Absorption spectra of ionene-stabilized gold NPs prepared in 4 mmol L-1 (a), 7
mmol L-1 (b) and 20 mmol L-1 (c) HCl in 1h (curve 1), 5 h (curve 2), 25 h (curve 3), 78 h
(curve 4) and 4 months (curve 4*) after the synthesis, and the colors corresponding to the marginal situations.
![Isotherm of [NPs] adsorption on silasorb C8. mSilasorb = 0.05 g, V = 10 mL.](clipboard08.jpg "Isotherm of [NPs] adsorption on silasorb C8. mSilasorb = 0.05 g, V = 10 mL.")