22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Antibacterial silver containing nanocomposites prepared by gas aggregation
source of nanoparticles combined with PE-CVD
A. Kuzminova1, O. Kylián1, J. Beranová2, A. Shukurov1, O. Polonskyi3 and H. Biederman1
1
Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic
2
Charles University in Prague, Faculty of Science, Prague, Czech Republic
3
Christian-Albrechts-University at Kiel, Institute of Material Science, Faculty of Engineering, Kiel, Germany
Abstract: Silver containing nanocomposites were prepared by gas aggregation source of
Ag nanoparticles combined with plasma enhanced chemical vapor deposition performed in
HMDSO/O 2 working gas mixtures. It is shown that Ag+ ion release and related
antibacterial character of deposited coatings may be adjusted by amount of silver
nanoparticles in the films as well as by chemical structure of matrix material.
Keywords: PE-CVD, HMDSO, Ag nanoparticles, nanocomposites, antibacterial coatings
1. Introduction
Nanocomposite materials based on metal nanoparticles
(NPs) embedded in a dielectric matrix are employed in
wide range of applications such as production of
decorative coatings, (bio)sensing, fabrication of high
frequency magnetic components, photovoltaic materials
or full cells. In addition, increasing resistance of certain
bacterial strains to common antibiotics triggered off
research focused on the possibility of employing metalcontaining nanocomposites as antibacterial coatings that
should prevent bacterial adhesion to the surface or kill
bacteria when they come in contact with a surface. From
this point of view the most interesting and studied are
silver based nanocomposites as Ag is for centuries known
as potent bactericidal agent [1]. Although the exact
bactericidal mechanism of silver is still not fully
understood, it is supposed that the antibacterial nature of
silver is predominantly connected with its ability to
release silver ions that may subsequently interact with
thiol groups of vital enzymes, suppress replication ability
of bacterial DNA, induce generation of reactive oxygen
species that provoke oxidative stress or irreversibly
damage cell membranes of pathogenic organisms and
hence inhibit their growth (e.g., [2, 3]).
Various techniques based on chemical synthesis have
been established to produce Ag/polymer nanocomposites.
However, the use of these techniques is often rather
problematic since they are typically multi-step and timeconsuming. In addition, these techniques require solvents
and thus do not comply with the demands on eco-friendly
process.
Therefore increasing attention is paid to
techniques based on plasma technologies, such as cosputtering from two magnetrons [4] or combination of Ag
sputtering with plasma enhanced chemical vapor
deposition (PE-CVD) [5-7] that were employed for
production of Ag/PEO, Ag/PTFE, Ag/Si:C:O:H and
Ag/C:H:O antibacterial coatings. However, also these
techniques pose certain limitations: since the Ag NPs are
formed as the result of diffusion and aggregation of
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atomic silver in growing matrix of plasma polymer, the
size and distribution of formed NPs is strongly influenced
by matrix material, which makes it rather difficult to
control independently the size and amount of NPs and
properties of matrix.
In this study we introduce an alternative method, which
is based on combination of PE-CVD process used for
production of matrix material with deposition of Ag
nanoparticles by means of a gas aggregation source of Ag
nanoparticles.
The main attention is devoted to
characterization of nanocomposites produced in this way
as well as evaluation of their antibacterial character.
2. Experimental details
Deposition system depicted in Fig. 1a was used for the
preparation of thin films of plasma polymerized
hexamethyldisiloxane (HMDSO). It consisted of high
vacuum chamber pumped by diffusion and rotary pumps,
RF planar electrode operated at frequency 13.56 MHz and
load-lock system for introduction of samples into the
deposition chamber. The precursor HMDSO (Sigma) was
thermally stabilized and vaporized outside the apparatus.
In order to evaluate the effect of chemical composition of
matrix material on antibacterial properties, the HMDSO
was used either alone or in a mixture with oxygen
(HMDSO/O 2 ratios up to 1:60). Pressure during the
depositions was 4 Pa and applied RF power during the
depositions was 40 W.
Ag nanoparticles were prepared by means of gas
aggregation source of nanoparticles based on 3-inch
planar magnetron equipped with Ag target described in
more details in [8]. The magnetron was placed into water
cooled gas aggregation chamber ending in an orifice
(2 mm in diameter), which separated aggregation zone
from the main deposition chamber. Magnetron was
powered by DC power supply operated in constant current
mode (0.1 A). Working gas was argon and its pressure in
the aggregation chamber was 30 Pa. Deposition time of
Ag NPs was 2 min.
1
(sterilized by UV light for 30 min from each side) was
placed onto the inoculated agar surface (the coated side
facing down). Glass coated with films of plasma
polymerized HMDSO and SiO x served as negative
controls. After overnight incubation at 37 °C, the plates
were photographed and the inhibition zones (clear zones
where bacteria did not grew) around discs were measured.
3. Results
It was found that thin films deposited using pure
HMDSO (denoted as pHMDSO) have the structure
typical for plasma polymerized organic films with high
atomic concentration of carbon and relatively low O and
Si atomic concentrations. As can be seen in Table 1,
addition of oxygen to HMDSO causes, in agreement with
previous studies [9], a dramatic increase of atomic
concentration of oxygen at the expense of carbon. For
O 2 :HMDSO ratios above 20:1 the deposited coatings
exhibit chemical structure close to stoichiometric SiO 2 .
As demonstrated in previous study [10], changes in
chemical structure of prepared films is accompanied by
changes in their water contact angle that was found to
decrease from 100° measured for samples deposited in
pure HMDSO down to 15° for samples for whose
production was employed O 2 :HMDSO mixture 60:1. On
the other hand, no significant changes were observed in
the morphology of samples deposited using different
working gas mixtures: all samples were smooth with
RMS roughness below 1 nm.
Fig. 1. Experimental set-up used for deposition of a) thin
films and b) Ag nanoparticles.
Various techniques were employed for the
characterization of prepared thin films of plasma
polymers, Ag nanoparticles and their nanocomposites.
The thickness of films was determined by means of
spectroscopic ellipsometry (Woolam M-2000DI). The
chemical composition of deposited coatings was analyzed
by X-Ray Photoelectron Spectroscopy (XPS, Phoibos
100, Specs) using an Al Kα X-ray source (1486.6 eV,
Specs). Wettability of prepared samples was evaluated by
means of a sessile water drop method. Morphology of
samples was determined by means of SEM (Tescan Mira
II) and AFM (Quesant Q-scope 350). Optical properties
of prepared nanocomposites were studied by UV-Vis
spectrophotometer (Hitachi U-2900) in the spectral range
325-900 nm. The ion release kinetics was monitored by
MS-ICP (NexION® 300 ICP-MS System).
For evaluation of antibacterial character of prepared
nanocomposites Gram negative bacterium E. coli, strain
K12 (laboratory stock) was employed. Bacteria were
cultivated in Luria broth (LB) at 37 °C until
mid-exponential phase of growth (optical density at 450
nm ca. 0.5). The culture was then diluted 1:10 in sterile
distilled water and 1 ml was placed on the surface of LB
agar plate. After all liquid soaked into the agar, a glass
disc coated by silver containing nanocomposites
2
Table 1. Surface elemental composition of samples as
determined by XPS.
O 2 :HMDSO
ratio
0
1
5
10
20
60
C
[%]
56.6
42.6
25.3
7.5
3.5
4.2
O
[%]
16.3
29.6
43.5
57.9
60.2
63.6
Si
[%]
27.1
28.1
31.2
34.6
36.3
32.2
The next step was deposition of Ag nanoparticle films.
As can be seen in Fig. 2, where an example of Ag NPs
deposited on Si wafer coated with 20 nm thick film of
SiO x together with corresponding histogram of diameter
of nanoparticles is presented, the mean size of NPs is
14 ± 5 nm and they form sub-monolayer.
The Ag/pHMDSO and Ag/SiO x nanocomposites were
subsequently deposited on glass in the form of sandwich
structure by sequential deposition of layers of plasma
polymers and Ag nanoparticles. The first and the last
layers were always pHMDSO or SiO x . The thickness of
pHMDSO and SiO x inter-layers was kept constant and
equal to 7-10 nm in this study, which is lower than the
mean lateral distance of NPs deposited in a single step.
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1 layer
2 µultilayers
5 µultilayers
SiOx
Because of this the coatings may be still considered as
nanocomposites with randomly dispersed Ag NPs rather
than multi-layered sandwich structures. As can be seen in
Fig. 3 increasing number of layers of Ag nanoparticles
leads to enhancement of the intensity of anomalous
absorption peak of silver, which confirms increasing
amount of NPs in the coating.
0,6
Increasing number
of interlayers of Ag NPs
Absorbance [a.u.]
0,5
Glass
SiOx
1 intelayer of Ag NPs
2 intelayers of Ag NPs
5 intelayer of Ag NPs
0,4
0,3
0,2
pHMDSO
102
10-1
10-2
10-2
2 hours
144 hours
10-1
72 hours
100
24 hours
100
144 hours
101
72 hours
101
24 hours
Fig. 2. SEM image of Ag nanoparticles deposited onto
SiO x film and corresponding size histogram of NPs.
103
102
2 hours
Silver ion release [µg/l]
103
Tiµe in water
Fig. 4. Ag ion release from samples immersed in water
SiO x based nanocomposites as compared to pHMDSO
ones. This assumption was confirmed by preliminary
antibacterial tests. Fig. 5 shows representative sections of
photographs of glass discs placed on LB agar surface
inoculated with E. coli after overnight incubation.
Distinct differences in antibacterial properties were
observed in dependence on used matrix material as well
as in dependence on the number of Ag inter-layers.
Whereas for Ag/pHMDSO nanocomposites only an
indication of formation of inhibition zone was detected, in
case of Ag/SiO x nanocomposites the inhibition zone was
clearly visible and its diameter increased with increasing
content of Ag nanoparticles.
0,1
0,0
400
500
600
700
800
900
Wavelength [nm]
Fig. 3. UV/Vis spectra of nanocomposites Ag/SiO x with
different number of interlayers of Ag NPs.
For
comparison also UV/Vis spectrum of uncoated glass is
presented.
As mentioned in the introduction, antibacterial nature of
Ag based nanocomposites is connected with their ability
to release Ag+ ions. In order to quantify ion release in
aqueous environment, MS-ICP measurements were
performed. It was observed that the amount of released
Ag+ ions and its kinetics was strongly influenced by the
amount of Ag NPs in the coating as well as by the
chemical composition of the matrix material. As it was
expected, the amount of released silver ions gradually
increases with the time for which the samples were
immersed in water and with the amount of Ag NPs
present in the nanocomposites. Moreover, the ion release
was much higher for SiO x -based nanocomposites as
compared to Ag NPs embedded in pHMDSO as can be
seen in Fig. 4.
According to MS-IPC results it can be expected that
higher antibacterial effect will be exhibited in case of
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Fig. 5. Sections of photographs of glass discs placed on
the LB agar surface inoculated with E. coli, after
overnight incubation. The diameter of discs was 12 mm.
4. Conclusion
Nanocomposites containing Ag clusters embedded in
hydrophobic (pHMDSO) and hydrophilic (SiO x )
matrixes, respectively, were prepared by PE-CVD method
combined with deposition of Ag nanoparticles by gas
aggregation source. It was found that both the amount of
Ag NPs and chemical structure of the matrix influence the
capability of produced nanocomposites to release
Ag+ ions in aqueous environment. This is important result
from point of view of fabrication of antibacterial coatings
with tunable rate of silver ion release.
3
5. Acknowledgments
This research has been supported by the Czech Science
Foundation through the Project 13-09853S. Authors
would like to thank R. Abel and A. Matthiessen for
MS-ICP measurements.
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