22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Nitrogen and oxygen rich plasma polymer films with tuneable surface charge
S. Babaei and P.-L. Girard-Lauriault
Department of Chemical Engineering, McGill University, Montréal, H3A 2B2, Canada
Abstract: Plasma polymer films containing tuneable concentrations of N and O functional
groups were deposited in a low-pressure glow discharge reactor by plasma polymerization
of binary gas mixtures of a hydrocarbon (ethylene or butadiene) and a heteroatom source
gas (ammonia or carbon dioxide). The deposition parameters controlled the composition of
the coatings and, in turn, the surface charge between 26 mV and -28 mV.
Keywords: plasma polymer, nitrogen rich, oxygen rich, surface charge
1. Introduction
Synthetic polymers play an important role in disease
management and health care. Their applications range
from coatings for tablets or capsules of pharmaceutical
preparations to devices such as contact lenses, vascular
prostheses, or biosensors [1]. Most of these polymers
suffer from limited interaction capabilities with cells and
biomolecules due to their intrinsically low surface energy.
In particular, they lack the ability to trigger selective
interactions towards particular groups of cells and
biomolecules [2]. In order to improve the performance of
polymer based medical devices, a modification of their
surface chemistry and morphology can be applied. One
frequently proposed strategy is the coating of a surface
with a plasma deposited organic macromolecule (plasma
polymer) containing a desirable chemical composition
and morphology allowing some control on the
interactions of the surface with surrounding biomolecules
and living cells [3]. These interactions are governed by
the physico-chemical properties of the films such as
chemical functional groups, surface energy, roughness
and surface charge, and will lead to interaction that can be
classified as either physical (adsorption), or chemical
(covalent bonding) or a combination of both, e.g.,
electrostatic attraction of a bio-molecule to an oppositelycharged surface [4]. While most previous works on the
effect of plasma polymerized coating’s chemistry have
focussed on chemical functional groups [5], the
importance of surface charge in protein surface
interactions has been established [6, 7].
In previous work, we have demonstrated that nitrogen
rich plasma polymer films could be made either adherent
or non-adherent to specific cell types by adjusting the
surface primary amine content [8]. The amount of amines
required to stimulate the adhesion of cells was found to be
cell type dependant thereby demonstrating that such films
could be used to create surfaces with selective cell
adhesion properties. A longer term objective is to
understand how the surface chemistry and surface charge
of N and O rich plasma polymer coatings influence the
competitive protein adhesion and, in turn, cell adhesion,
proliferation and differentiation. As a first step, we are
developing and investigating plasma polymer coatings
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with well characterized surface chemistry and surface
charge.
In this work, an array of low-pressure plasma copolymerized films, prepared from binary gas mixtures of
ethylene (C 2 H 4 )/ammonia (NH 3 ), butadiene (C 4 H 6 )
/ammonia, ethylene/carbon dioxide (CO 2 ) and
butadiene/carbon dioxide were produced in order to
deposit either nitrogen rich or oxygen rich coatings with
adjustable chemical composition and surface charge. The
coatings are analysed to determine
the chemical
composition (elements and functional groups), evaluated
by X-ray photoelectron spectroscopy (XPS) and chemical
derivatization and the surface charge, evaluated by the
measurement of zeta-potential using an electro-kinetic
analyser (EKA), of plasma polymer films deposited in
various conditions.
2. Experimental Method
2.1. Deposition of Plasma Polymer Films
A low pressure capacitively coupled radio-frequency
glow discharge in gas mixtures containing a source of
carbon and a source of heteroatom, was used to produce
the plasma polymerized coatings. The coatings are
hereafter referred to as PPM:N and PPM:O where “PP”
stands for Plasma Polymerized ,"M" refers to
hydrocarbon source gas, either “E” (ethylene) or “B”
(butadiene) and "N" and "O" stand for nitrogen and
oxygen, containing films, respectively. The films were
deposited on 500 µm thick (100) p-type silicon wafer
substrates which were cut to small pieces of 4.5 cm × 3
cm. The substrates were sonicated first in deionized water
and then in isopropanol for 10 minutes before being dried
under a nitrogen flow. The depositions were performed in
a cylindrical stainless steel vacuum chamber (20 cm in
diameter and 50 cm in height) equipped with a disc
shaped powered electrode (Ø=10 cm) onto which the
samples were placed. A showerhead gas distributer,
positioned 4 cm above the powered electrode also served
as grounded electrode. The chamber is evacuated using a
combination of rotary-vane and turbomolecular pumps to
high vacuum before the processes mixtures are introduced
via mass-flow controllers. The gas flow ratio, defined as
R= (flow of heteroatom source gas)/(flow of hydrocarbon
1
source gas) was the main experimental parameter varied
in order to control film composition. Table 1 contains a
description of the various experimental parameters used
in the present study. The pressure was maintained
constant at 80 Pa during deposition runs by a throttling
gate valve coupled to a capacitance pressure gauge. An
automatic impedance matching network is used to
generate the capacitively-coupled radio frequency (RF,
13.56 MHz) discharge at a power of 20 W. A typical
deposition run lasted about 10 min to deposit coatings
around 200 nm thick.
Table 1. Gas flow ratio of different plasma polymers
Plasma
Polymer
R
PPE:N
0.25≤R≤0.75
PPB:N
1≤R≤3
PPE:O
1≤R≤4
PPB:O
8≤R≤12
Hydrocarbon flow
rate
Ethylene kept
constant at 20 sccm
Butadiene kept
constant at 15 sccm
Ethylene kept
constant at 5 sccm
Butadiene kept
constant at 4 sccm
Heteroatom
gas source
Ammonia
Ammonia
Carbon dioxide
Carbon dioxide
2.2. Stability
The stability of the coatings was evaluated by
immersing them in deionized water for 24 hours and
measuring the thickness loss of the coatings. The
thickness of the samples was determined by measuring of
the depth of a grove produced by scratching the surface
with a needle using a Dektak profilometer.
2.3. X-ray Photoelectron Spectroscopy
XPS analyses were performed in a Thermo Scientific
K-AlphaTM instrument using monochromatic Al K ɑ
radiation, around 22 hours after deposition. Survey
spectra were acquired at a pass energy of 160 eV and
corrections were done by referencing all peaks with
respect to C 1s signal at a binding energy of 285.0 eV. The
atomic concentrations, calculated using 2.3.16 PR 1.6
Casa XPS, are used to evaluate surface composition. The
relative sensitivity factor (R.S.F.) values for carbon,
nitrogen, oxygen and fluorine are 1, 1.8, 2.93, and 4.43,
respectively.
2.4. Derivatization of Amine Groups
The surface concentration of primary amines, [-NH 2 ],
were determined according to the method developed by
Favia et al.[9], using the derivitization reaction of 4(trifluoromethyl)benzaldehyde (TFBA) vapor with
surface amine groups. Each covalently linked TFBA
molecule incorporates three fluorine atoms and the amine
concentration can be deducted from the fluorine content
of derivatized surfaces determined by XPS.
It is noteworthy that recently published work has shown
that secondary reactions of TFBA with other functional
groups, such as imines, were possible and that therefore,
the derivatization results can be considered as general
measures of nucleophilic reactivity rather than an
evaluation of primary amines [10]. However, surface
2
charge results presented show that a significant portion of
the N groups present on the surface induce a surface
charge in physiological pH and the assumption that a
significant portion of the derivatized groups are likely to
be amines is reasonable. A small glass container (150
cm3) equipped with an Ar inlet and a gas outlet was used
to carry out the reaction. The container was filled with
glass beads (diameter = 2 mm) up to height of 1 cm and a
small amount of TFBA was subsequently added. The
samples, positioned on a microscope glass slide, were
placed on the container with glass beads. In order to
reduce the chance of any reaction with the oxygen, the
container was purged with argon before being incubated
at 45˚ C for 3 hours. The surface concentration of primary
amines was derived from equation using atomic
concentrations obtained by XPS (1);
[𝑁𝑁2 ]𝑢 = [𝑁]𝑢 ×
[𝐹]𝑑
3[𝑁]𝑑
× 100
(1)
where the subscripts u and d stand for underivitized and
derivitized samples, respectively.
2.5. Derivitization of Carboxylic Acid Groups
The surface concentration of carboxylic acid groups,
[-COOH], was determined by surface derivatization with
toluidine blue (TBO)[3]. The samples were immersed in
5ml of TBO solution (15 mg/l) containing 0.1mM sodium
hydroxide (NaOH) and then incubated at 40˚C for 1 hour.
The samples were then washed with fresh 0.1mM NaOH
to remove free TBO and were immersed in 5ml of an
aqueous acetic acid solution (50%v/v) and incubated at
40˚C for 0.5 hour to release bound TBO. The
concentration of bound and released TBO was determined
by measurement of the solution’s transmission at 630 nm
using a UV-VIS spectrometer. The samples’ surface
carboxyl group concentration was calculated by assuming
that each [-COOH] group links to a single TBO molecule.
2.6. Electro Kinetic Analyser
The surface charge of coatings was measured by using
an electro-kinetic analyser (EKA) with clamping cell, 20
hours after deposition. The device operates on streaming
voltage mode. A 10mM sodium chloride solution was
used as electrolyte and sodium bicarbonate, hydrochloric
acid and sodium hydroxide were used to adjust the
electrolyte pH to 7.4.
3. Results and Discussion
The XPS-derived compositions, evaluated in terms of
[N] and [NH 2 ], of PPE:N and PPB:N for different values
of flow ratios are shown in Fig. 1. Both nitrogen content
and amine group concentration rise nearly linearly with
increasing flow ratio [11], but more rapidly for PPE:N
[12]. It is now well established that plasma polymer films
with high densities of functional groups can be unstable
(soluble) in aqueous media [13], something that is
incompatible with surface charge measurements.
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Considering this, only PPE:N and PPB:N coatings
deposited in the flow ratio ranges of 0.25 – 0.75 and 1 -3,
respectively, were selected for this study. These films
exhibited a thickness loss of less than 5% after immersion
for 24h in aqueous media. Films deposited at lower flow
ratios had a nitrogen contents lower than the practical
threshold for the purpose of this study.
In line with the results of nitrogen-rich coatings, the [O]
and [-COOH] concentration of PPE:O and PPB:O at
different flow ratios also show an almost linear increasing
trend (Fig. 2). The same criterion for stability than the one
used for nitrogen rich coatings was used in choosing the
deposition conditions of oxygen rich coatings.
Fig. 3 shows the surface charge of PPE:N, PPB:N,
PPE:O and PPB:O at different flow ratios.
Fig 1. Nitrogen content and amine concentration of
PPE:N and PPB:N
Fig 2. Oxygen content and carboxylic acid concentration
of PPE:O and PPB:O
Surface charge arises from the dissociation of the
surface functional groups. Therefore, the surface charge is
a function of the degree of ionization and pH of the
aqueous solution. Away from their isoelectric point –the
pH above or below which a particular functional group
carries a net charge– surfaces containing amine or
carboxyl groups become positively and negatively
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charged, respectively. In accordance with this, all
coatings part of the present study are expected to exhibit a
surface charge at physiological pH, i.e., 7.4. The nitrogen
rich films acquire positive surface charge due to existence
of amine groups and in the same fashion, carboxyl groups
in oxygen-rich coatings result in negative surface charges.
In agreement with Fig. 1 and Fig. 2, the absolute value of
surface charge increases by increasing the flow ratio in
both the cases of nitrogen and oxygen rich coatings, due
to more abundant amine and carboxylic groups,
respectively.
Possible explanations for rather different behaviours
between coatings deposited with ethylene and butadiene
can be discussed as follow. It can be suggested that, C 4 H 6
as precursor produces a more densely cross-linked
structure than C 2 H 4 , since it contains two double bond
and consequently, more free radicals are created due to
bombardment of charged particles and photons, which
yields a more cross-linked coating [12]. Even if butadiene
leads to more stable coatings compared to ethylene as a
precursor, the later leads to sharper changes in surface
chemistry in comparison to butadiene. It is established
that the difference in the chemical structure of a
hydrocarbon can directly impact the physiochemical
properties of the resulting plasma polymer [12]. This
effect is more pronounced in the case when mild plasma
conditions prevail, such as in the present study.
4. Conclusion
We have shown that by adjusting the precursor gas
composition in a RF capacitively coupled glow discharge,
it is possible to control the surface charge of plasma
polymer films from positive to negative values (from -28
to +26 mV) when immersed in an aqueous environment at
physiological pH, 7.4. Ammonia as a heteroatom source
gas along with ethylene and butadiene as hydrocarbons
produces amine groups which result in positive surface
charge when dissociated in water. On the other hand,
carbon dioxide as heteroatom source gas produces
carboxyl groups which result in negative surface charge
when dissociated in water. Increasing the flow ratio
results in an increase in the concentration of the above
mentioned functional groups which, in-turn, increases the
absolute value of the surface charge when the functional
groups are dissociated in aqueous environment.
It has been previously established that some control
over the selectivity of surfaces towards proteins can be
gained by varying the surface charge and surface
chemistry. It is therefore expected that the coatings
presented and characterized in this study will be useful in
the development of surfaces which will allow control over
protein and cell adhesion when exposed to various
biological environments.
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[8] P. L. Girard‐Lauriault, F. Truica‐Marasescu, A. Petit,
H. T. Wang, P. Desjardins, J. Antoniou, et al.,
Macromolecular bioscience, 9, 911 (2009).
[9] P. Favia, M. Stendardo, and R. d'Agostino, Plasmas
and Polymers, 1, 91 (1996).
[10] C. P. Klages, Z. Khosravi, and A. Hinze, Plasma
Processes and Polymers, 10, 307 (2013).
[11] J.-C. Ruiz, P.-L. Girard-Lauriault, F. TruicaMarasescu, and M. R. Wertheimer, Radiation Physics and
Chemistry, 79, 310 (2010).
[12] A. Contreras-García and M. R. Wertheimer, Plasma
Chemistry and Plasma Processing, 33, 147 (2013).
[13] J.-C. Ruiz, A. St-Georges-Robillard, C. Thérésy, S.
Lerouge, and M. R. Wertheimer, Plasma Processes and
Polymers, 7, 737 (2010).
Fig. 3. Surface charge of a) PPE:N, PPB:N and b)
PPE:O , PPB:O
5. Acknowledgment
The authors would like to acknowledge McGill
Engineering Doctoral Award (MEDA), Fonds de
recherche du Québec (FQRNT) and Natural Sciences and
Engineering Research Council of Canada (NSERC) for
the funding.
6. References
[1] K. Bazaka, M. V. Jacob, R. J. Crawford, and E. P.
Ivanova, Acta biomaterialia, 7, 2015 (2011).
[2] M. R. Wertheimer, L. Martinu, J. Klemberg-Sapieha,
and G. Czeremuszkin, Materials engineering- New York-,
14, 139 1999.
[3] M. J. Garcia‐Fernandez, L. Martinez‐Calvo, J. C.
Ruiz, M. R. Wertheimer, A. Concheiro, and C. Alvarez‐
Lorenzo, Plasma Processes and Polymers, 9, 540 (2012).
[4] M. R. Wertheimer, A. St-Georges-Robillard, S.
Lerouge, F. Mwale, B. Elkin, C. Oehr, et al., Japanese
Journal of Applied Physics, 51, 11PJ04 (2012).
[5] P. L. Girard‐Lauriault, F. Mwale, M. Iordanova, C.
Demers, P. Desjardins, and M. R. Wertheimer, Plasma
Processes and Polymers, 2, 263 (2005).
[6] M. Dargahi, V. Nelea, A. Mousa, S. Omanovic, and
M. T. Kaartinen, RSC Advances, 4, 47769 (2014).
[7] M. Dargahi and S. Omanovic, Colloids and Surfaces
B: Biointerfaces, 116, 383 (2014).
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