Plasma-polymerized HMDSO coatings to adjust the ... - Springer Link

J Nanopart Res (2013) 15:2080 DOI 10.1007/s11051-013-2080-9

RESEARCH PAPER

Plasma-polymerized HMDSO coatings to adjust the silver ion release properties of Ag/polymer nanocomposites N. Alissawi • T. Peter • T. Strunskus • C. Ebbert G. Grundmeier • F. Faupel

•

Received: 15 July 2013 / Accepted: 15 October 2013 / Published online: 24 October 2013 Ó Springer Science+Business Media Dordrecht 2013

Abstract In the current work, we study the silver ion release potential and the water uptake through a SiOxCyHz-polymer which is grown from the precursor hexamethyldisiloxane (HMDSO) in radiofrequency (RF) plasma. These layers were deposited on top of two dimensional (2D) ensembles of silver nanoparticles (AgNPs) with nominal thickness of 2 nm on a 20 nm RF-sputtered polytetrafluoroethylene (PTFE) thin film. The composition of the plasma-polymerized HMDSO barriers was varied by changing the oxygen flow during the polymerization process and their thickness was varied as well. Morphology and optical properties of the nanocomposites were investigated using transmission electron microscopy (TEM) and UV–Visible spectroscopy (UV–Vis), respectively. The concentration of the silver ions released from the nanocomposites after immersion in water for several time intervals was measured using inductively coupled plasma mass spectrometry (ICP-MS). Contact angle analysis and electrochemical impedance spectroscopy (EIS) measurements were also performed

N. Alissawi T. Peter T. Strunskus F. Faupel (&) Institute for Materials Science, Chair for Multicomponent Materials, Faculty of Engineering, Christian-AlbrechtsUniversity of Kiel, Kaiser Str. 2, 24143 Kiel, Germany e-mail: [email protected] C. Ebbert G. Grundmeier Institute for Polymer Materials and Processes, Faculty of Natural Science, University of Paderborn, Warburger Str. 100, 33098 Paderborn, Germany

and results show a strong dependence of the coatings properties and their water uptake on the oxygen content in the coating films and their thickness. Plasma polymerization with increasing the oxygen flow leads to the formation of more hydrophilic thin films with a higher Ag ion release potential. Increasing the thickness of the coatings reduced the amount of the released ions and the rate of the release process was slowed down. This indicates that by tailoring the structure and the thickness of the plasma-polymerized coating films, one can tune the silver ion release properties of Ag/ polymer nanocomposites. Keywords AgNPs Silver ion release Plasma-polymerized HMDSO RF-sputtered PTFE PVD ICP-MS Water diffusion EIS Antimicrobial nanostructure

Introduction The growing interest in controlling of the silver ions released from silver containing nanocomposites into the environment is motivated by the prospect of the expand and rapid development of materials that combine silver nanoparticles (AgNPs) as antimicrobial agent with organic or silicon-based materials to stabilize the NPs and to inhibit the microbial colonization and biofilms formation for biological and medical applications (Radheshkumar and Mu¨nstedt 2006; Zaporojtchenko et al. 2006; Faupel et al. 2007;

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Sharma et al. 2009; Monteiro et al. 2009; Liu et al. 2010a; Vasilev et al. 2010a; Marambio-Jones and Hoek 2010; Sotiriou and Pratsinis 2010; Zaporojtchenko et al. 2010; Liu et al. 2010b; Faupel et al. 2010; Chakravadhanula et al. 2011; Alissawi et al. 2012; Hrkac et al. 2013; Alissawi et al. 2013). According to many researches, AgNPs release ions only in the presence of oxygen and protons (Damm et al. 2007; Rai et al. 2009; Liu and Hurt 2010; Liu et al. 2010b; Lee et al. 2012). Thus, one of the approaches to tune the silver ion release process is by embedding or coating the AgNPs in a matrix as the water diffusion through the matrices or the coatings is crucial for the silver ion release (Damm et al. 2007; Vasilev et al. 2010a, b; Ko¨rner et al. 2010; Yliniemi et al. 2012; Alissawi et al. 2012). HMDSO plasma-polymerized films have been recognized as good candidates for protective coatings on metals and other substrates. Plasma-polymerized films are generally amorphous, pinhole-free, highly crosslinked and are therefore insoluble, thermally stable, chemically inert, and mechanically tough. Furthermore, they are very adhesive to different substrates (Vautrin-Ul et al. 2000; Morent et al. 2009a, b). An important example of plasma polymerization is the deposition of polymer-like or quartz-like thin films with varying concentration of hydrocarbons (SiOxCyHz) from siloxane monomers. Hexamethyldisiloxane (HMDSO, C6H18Si2O); a silicone– oxygen–silicone core with three methyl groups attached to each silicone; is often preferred as precursor since it is generally sufficiently volatile near room temperature, relatively non-toxic and nonflammable, cheap, and available from commercial sources, an addition to its highly organic character as well as its high vapor pressure (Morent et al. 2009a, b). The chemical composition of the HMDSO plasmapolymerized thin films can be adjusted over a wide range from polymer-like properties with various applications as adhesion interfaces to glass-like films with applications corrosion protective layers or antiscratch coatings on plastic substrates, coatings for biocompatible materials, and low-k dielectric layers for microelectronic applications (Vautrin-Ul et al. 2000; Morent et al. 2009a, b; Saulou et al. 2009; Scha¨fer et al. 2011). Messina et al. (Vautrin-Ul et al. 2000) have found that different HMDSO/O2 ratios in the mixture provide varying degree of corrosion protection and the best protection was obtained for

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J Nanopart Res (2013) 15:2080

the plasma-polymerized coating from HMDSO 80 % content precursor. Morent et al. (2009a, b) prepared HMDSO plasma-polymerized films onto polyethylene terephthalate (PET) films using argon and different argon/air mixtures as carrier gases and found that the chemical and physical properties of the obtained coatings can be controlled by varying the operation conditions. When pure argon is used as working gas, the film is polymeric, while with increasing air content, the deposition rate decreases and a gradual change is observed from organic coatings to inorganic, quartz like deposits and the surface of the deposited films becomes rougher. These dense SiOx coatings generally exhibit high barrier properties, while pure HMDSO derived films might be of importance for selective permeation. Further studies making use of these protective properties of HMDSO plasma-polymerized films in combination with AgNPs were also considered. For example, Saulou et al. (2009) prepared composite thin films containing silver nanoclusters embedded in an organosilicon matrix were deposited by plasma-enhanced chemical vapor deposition (PECVD) onto stainless steel in order to prevent microbial adhesion combining the anti-adhesive potentialities of the organic matrix with the broad-spectrum antimicrobial properties of silver which was assumed to be related to Ag? progressive release from the embedded nanoparticles into the surrounding medium and was confirmed by ICP-MS measurements. Scha¨fer et al. (2011) have also demonstrated the potential of organosilicon films produced in an atmospheric pressure PECVD in applications in antimicrobial coatings. They deposited SiOx films with an atmospheric pressure dielectric barrier discharge using Ar, O2 and different precursor gases (HMDSO and silane) and studied the influence of the discharge operation parameters on the chemical composition of the deposited films. The barrier properties of the films and their capability to transport metal ions through the films lead to applications in antimicrobial coatings that gradually, over time release ions under wet conditions. Previously, we have investigated the Ag ion release properties of Ag/PTFE nanocomposites by the usage of well defined model systems consisting of 2D silver nanoparticle ensembles which are either directly accessible or covered by polymer layers of well defined thickness and composition (Yliniemi et al. 2012; Alissawi et al. 2012). This nanocomposite system was prepared by thermal evaporation


Page 3 of 12 Table 1 Composition of HMDSO plasma-polymerized layers depending on O2 flow conditions (only Si, C, and O; H content unknown) Element

Fig. 1 Schematic of the prepared samples of HMDSO/Ag/ PTFE nanocomposites

deposition of AgNPs with different nominal thickness on top of a highly crosslinked RF-sputtered PTFE thin film or sandwiched between two PTFE polymer films. Results showed a correlation between changes in the surface plasmon resonance, composites’ morphology and the kinetics of Ag ion release. A strong dependence of the silver ion release on the particles’ size and content was also observed and that a polymer barrier stabilizes the NPs and can be applied to control the Ag ion release rate. In this work, we continue with the same Ag/PTFE model system as the deposition and the growth of the AgNPs on the PTFE polymer surface were already investigated and only the second layer on top of the NPs were replaced by HMDSO plasma-polymerized films with tailoring their composition and thickness for better understanding of the coatings properties effect on the release process.

Experimental Preparation AgNPs of nominal thickness of 2 nm were deposited on top of 20 nm RF-sputtered PTFE thin films on quartz glass (1 cm2), silicone (0.25 cm2) and carboncoated copper TEM grids. All experiments were carried out in vacuum chambers as explained in our previous work (Alissawi et al. 2012). These samples of Ag/PTFE nanocomposite were then mounted in a vacuum chamber designed in our chair, where we have a plasma–polymer source to deposit barriers with different thicknesses and composition by plasma polymerization of the HMDSO monomer (see Fig. 1 in Peter et al. 2013). A schematic representation of the prepared samples is shown in Fig. 1.

0 sccm O2 (%)

10 sccm O2 (%)

20 sccm O2 (%)

Si

20.4

20.4

20.3

C

54.3

29.7

23.6

O

25.3

49.9

56.1

The precursor for plasma polymerization (HMDSO) is kept in liquid form in an evacuated glass bottle in an ethanol bath. Using a temperature controller the bath is kept at a temperature between -30 and 10 °C. A stop valve is mounted connecting the bottle to the pipe that leads to the chamber. At the end of the pipe a sensitive needle valve is used to control the flow rate of HMDSO and an electronically controlled pulse valve is used to inject the HMDSO in pulses. The nozzle of the precursor injection system is mounted in about 15 mm distance to the substrate holder. Oxygen (O2) and argon (Ar) from two separate flow controllers are merged with the HMDSO flow behind the pulse valve before injection. Ar is injected in order to achieve the desired plasma polymerization conditions and O2 is injected to chemically modify the matrix. This enables full control over the atmosphere for plasma polymerization. This mixture of HMDSO, Ar, and O2 is then injected through the nozzle into the deposition chamber in front of the magnetron (thin film consulting), which is powered using an RF-generator (CESAR 136, 600 W, Dressler) and a matching circuit (VM 1000 A, 1 kW, Dressler). The most stable RF-discharge condition was observed with a pulsed injection of the HMDSO at 5 Hz and an opening time of 20 ms (Peter et al. 2013; Peter 2013). The deposition of the HMDSO coatings was done at low base pressure (about 10-6 mbar) and using an RFgenerator with 10 W power. The pulse injected HMDSO precursor is mixed with oxygen and argon gases. This mixture of HMDSO, Ar, and O2 is then injected through the nozzle into the plasma. A flow rate of 105 sccm (standard cubic centimeter per minute) of Ar leads to a pressure of 8.8 9 10-3 mbar in the main chamber, to which a partial pressure of 3 9 10-4 mbar of HMDSO and 1.2 9 10-3 mbar of O2 are added. Different matrices were prepared by changing the O2 content, which was realized by varying the O2 flow rate from 0 to 20 sccm. The composition of the resulting

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layers, as determined by X-ray photoelectron spectroscopy, is summarized in Table 1. The thickness of the coatings was also varied by changing the deposition time. More details about the plasma polymerization chamber and the preparation parameters and the composition of the resulting HMDSO films were investigated by Peter et al. (2013; Peter 2013). Characterization The morphology of the nanocomposites prepared on carbon-coated copper TEM grids was examined before and after immersion in water using a Tecnai F30 G2 ST (FEI) transmission electron microscope operating at 300 kV using a bright field (BF) contrast. The optical properties of the nanocomposites prepared on quartz glass were recorded using a PerkinElmer Lambda 900 UV–Vis/NIR spectrophotometer from 300 to 800 nm with 2-nm resolution. A 20 nm PTFE on quartz sample was used as a reference sample and all the UV–Vis plots are presented in this paper after linear background correction. Due to the small size of the NPs in our model system, the scattering was neglected and only absorption was considered. Contact angle measurements were conducted on a dataphysics contact angle system and recorded with a Teli CCD camera. Drops of deionized (DI) water (*1 lL) were used to determine contact angles. For silver ion release measurements the samples were immersed in 10 ml air saturated DI water (pH = 7, r = 0.06 lS/cm at T = 13 °C) in small bottles made of high density polyethylene (HDPE) at room temperature for different time intervals. The released silver ion concentration in water was checked by ICP-MS. In this work an Agilent 7500cs instrument was used with liquid sample introduction by a PFA micro-nebulizer. All results are blank-subtracted averages of three replicate measurements. The analytical error as estimated from replicate sample measurements was typically \2 % RSD for Ag. Electrochemical impedance spectroscopy (EIS) measurements were performed to investigate the barrier properties and water uptake of plasma-polymerized HDMSO thin films deposited on conductive ITO substrates. The measurements were done in a special cell described elsewhere (Yliniemi et al. 2012). As electrolyte a 0.2 M phosphate buffer with a pH of 7.5 was used. The plasma-polymerized HDMSO thin film deposited on ITO substrate was connected as working electrode.

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Fig. 2 BF-TEM image of 2 nm Ag/PTFE nanocomposite sample coated by 20 nm plasma-polymerized HMDSO film

Reference and counter electrodes consisted of a platinum wire. The EIS measurements were done with a Gamry Potentiostat Reference 600 in a time period ranging from 0 to 4,068 min.

Results and discussion Morphology and optical properties The BF-TEM micrograph of the sandwiched sample of 20 nm plasma-polymerized HMDSO barrier on top of a 2 nm nominal thickness AgNPs on 20 nm PTFE film deposited on TEM grid is given in Fig. 2. It shows that the average particle size of the NPs is around 5 nm and some aggregates can also be seen. The particles are with almost a uniform shape and are well distributed on the surface of the polymer film following the same model (Zaporojtchenko et al. 2000; Alissawi et al. 2012). Some fringes can be seen in the image which could be due to structural disorders such as twinning or stacking faults, or due to superpositioning of multiple grains. The absorbance spectra of several samples coated by HMDSO plasma-polymerized films prepared with various O2 flows were measured by UV–Vis spectroscopy as shown in Fig. 3. By adding a barrier with no


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0.24

no barrier 0 O2

0.20

20 sccm O2 10 sccm O2

Abs. (a.u.)

0.16

gradient (0 then 10 sccm O2)

0.12 0.08 0.04 0.00 300

400

500

600

700

800

λ (nm)

Fig. 3 UV–Vis spectra of 2 nm Ag/PTFE nanocomposites coated by 20 nm HMDSO plasma-polymerized film of different O2 content and compared to a sample without a barrier

oxygen, the absorbance maximum increases due to the change in the dielectric constant of the surroundings of the nanocomposites (Kreibig and Vollmer 1995; Kelly et al. 2003). While other barriers of the same thickness but with more oxygen content, show a shift and a damping in the SPR peak that can be explained by the oxidation that AgNPs encountered due to the increase in the O2 flow (Kuzma et al. 2012). In order to prove this, a plasma-polymerized HMDSO barrier was prepared with gradient in the oxygen flow, by sputtering the HMDSO without mixing it with O2 at the beginning and after 5 s, 10 sccm of O2 was added. The optical spectrum of the nanocomposite with this kind of barrier showed a peak at the same position as that for the sample without a barrier but with higher absorbance due to the dielectric change effect. Therefore, and due to the damping in the absorbance maxima of the nanocomposites with increasing the O2 flow during the deposition of the coatings, the optical properties were not investigated after immersion of samples in water and the ion release behavior was only studied by measuring the Ag ion concentration in water. Silver ion release studies In contact with water, the samples release Ag ions over a certain period of time, due to the oxidation of the Ag atoms at the surface of the NPs to Ag ions and the diffusion of the ions through the interface to the sample surface and then into the water. TEM images showing the changes in the morphology of the nanocomposites

before and after immersion in water for 7 days are given in Fig. 4a, b. The nanoparticles density looks smaller in the TEM image after 7 days of immersion of the samples in water and the interparticle distance increased as a result of the reduction of the initial Ag content after immersion in water because they were oxidized and ions were released into the water. No growth of large particles took place as the NPs are stabilized by the top layer of plasma-polymerized HMDSO film which prevents the agglomeration. Samples of 2 nm AgNPs/PTFE covered with 20 nm plasma-polymerized HMDSO layers of different O2 contents and samples without a barrier were then immersed in water for 7 days and the cumulative concentration of the released Ag ions was directly measured by ICP-MS and shown in Fig. 5. Since the Ag content and the morphology of the AgNPs are the same for all samples, then the strong difference in the Ag ion release kinetics after 7 days of immersion of the samples in water proves that the properties of the barrier layer play a role in the Ag ion release process. First of all, if we compare between the nanocomposites without HMDSO coatings and the other samples with coatings, it is clear that the nanocomposites without coating released the highest amount of Ag ions as water has a direct contact with the surface of the nanoparticles so they oxidize and dissolute into the water. By adding the plasma-polymerized HMDSO coatings on top of the NPs, a fall in the release has been noticed. However, there were significant differences in the amount of the released ions from the coated nanocomposites despite the fact that the coatings are all of the same thickness of 20 nm. The samples with plasma-polymerized HMDSO barrier of no oxygen, released the minimum amount of Ag ions compared to the ones with more O2. As the oxygen flow was raised from 0 to 20 sccm, the amount of the released ions was increasing as we can see for samples with plasmapolymerized HMDSO coatings with 20 sccm O2. Lamendola et al. (1997) have found that when HMDSO deposition occurs in an oxygen-free environment, no C and CH radicals are in the gas phase and there is a low degree of monomer fragmentation leading to the condensation of HMDSO-like heavy fragments, while when oxygen is utilized, the C, CH, and Si radicals are consumed leading to the deposition of a silica-like coatings. Not only the chemical structure is affected, but also the film morphology as AFM results obtained by Morent et al. (2009b) showed

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20 18

Table 2 Contact angle measurements for plasma-polymerized HMDSO with various O2 content

16

O2 flow

0 sccm

10 sccm

20 sccm

14

Contact angle

101.43°

88.15°

80.20°

12 10 8 6 4

+

Ag released concentration after 7 days in water ( μg/l)

Fig. 4 BF-TEM images of 2 nm Ag/PTFE nanocomposite sample coated by 20 nm plasma-polymerized HMDSO before (a) and after 7 days in water (b)

2 0

0 sccm

0 then 10 sccm 10 sccm

20 sccm

no Barrier

20 nm plasma polymerized HMDSO coatings with various O2 content

Fig. 5 The cumulative concentration of Ag ions released from 2 nm AgNPs/PTFE without a barrier or with 20 nm plasmapolymerized HMDSO barriers of different O2 content after 7 days of immersion in water

that with increasing the air content in the plasma phase, the roughness of the deposited films increases most likely due to the etching of the deposited films by oxygen atoms present in the discharge. This indicates that the oxygen addition during the plasma polymerization process has changed the structure of the plasma-polymerized HMDSO coatings and influenced their water penetration ability, and thus the silver ion release potential of the nanocomposites. Despite the fact that ions diffusion in polymer matrices is much faster than in silica-like ones, we see

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here that the silver ion release rate is higher for the silica-like coatings, which indicates that the silver ion diffusion out of the sample is not the diffusion rate limiting step and that the water diffusion in the matrix is the dominant process. Besides, we can see that the oxidation of the AgNPs during the plasma polymerization process in an oxygen-rich atmosphere did not have an influence on the release potential. To check the wettability of the plasma-polymerized HMDSO films by water, contact angle measurements were performed for 20 nm HMDSO films deposited on Si substrate and Table 2 shows the average contact angle of water for HMDSO films prepared with various oxygen flows. It can be seen that for plasma-polymerized HMDSO films with no oxygen flow, the water contact angle is larger than 90°, which means that the solid surface is hydrophobic. By increasing the oxygen flow from 0 to 20 sccm, the value of the water contact angle decreases and becomes smaller than 90°, indicating that the water tendency to wet the polymer surface increases and the solid surface is considered hydrophilic. These results confirm our conclusions that the oxygen flow during the deposition of the plasmapolymerized HMDSO films affects their chemical

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16

normalized cumulative Ag release


14 12

0.4


+

Cumulative Ag ion release concentration (μg/l)


10 8 6 4 2 0

0.3

0.2

0.1

0.0 0

2

4

6

8

10

12

14

16

Immersion time (days)

Fig. 6 The cumulative concentration of Ag ions released from Ag/PTFE nanocomposites coated by plasma-polymerized HMDSO barrier (20 nm) of different O2 content versus immersion time in water

structure and their interaction with water. This means that by increasing the oxygen flow during the plasma polymerization process, the chemical structure of the plasma-polymerized HMDSO matrix tends to change from hydrophobic polymer-like coatings to hydrophilic ceramic-like ones. For comparison of the kinetics of the Ag ion release from the nanocomposites, samples prepared with same barrier thickness but with different oxygen flow were immersed in water for longer time and the concentration of the released Ag ions was measured at several time periods as shown in Fig. 6. One can see that the sample with 10 sccm O2 plasma-polymerized HMDSO barrier releases much fewer amount of Ag ions than the one with 20 sccm O2 plasma-polymerized HMDSO barriers. Moreover, the Ag ion concentration in water increases up to a saturation value with time where it gets slower after 7 days. In contrast, the Ag ion concentration in water released from nanocomposites with a plasma-polymerized HMDSO barrier with more O2 content increases continuously with time. The amount released by this nanocomposite sample is about three times higher than that released from the one with plasma-polymerized HMDSO barrier of lower O2 content, even though they are containing the same amount of silver. For further study of the kinetics behavior, the released Ag ion amount was normalized to the initial content of AgNPs and plotted versus the square root of

0

200

400

600

800

1000

1200

sqrt of immersion time (sec^1/2)

Fig. 7 The normalized cumulative concentration of Ag ions released from Ag/PTFE nanocomposites coated by plasmapolymerized HMDSO barriers (20 nm) of different O2 contents versus the square root of the immersion time in water

the immersion time in Fig. 7. The initial amount of AgNPs was found by immersion of the samples for several days in (4 %) HNO3 acid, and after that the ion concentration was measured by ICP-MS. To check if there is any Ag left after that, the samples were checked by XPS and no signal from Ag was detected. Plasma-polymerized HMDSO films with more O2 content are more hydrophilic, with almost a linear diffusion behavior, so water diffuses through it (polymer is plasticized by the water). The deviation from the linear diffusion behavior could be explained by the effect of swelling that may occur when the water molecules diffuse into the matrix, which results in a directional transport of water molecules. The released ion concentration increases with time and it took about 1 day for water to diffuse completely from the surface of the barrier till it reaches the interface with the NPs when the Ag ion release process increases with time and these ions diffuse through the polymer into the surrounding immersion water. On the other hand, the samples covered with plasmapolymerized HMDSO films with low O2 content, seems to be rather more hydrophobic. The release kinetics rate was very slow at the beginning and the absolute released amount is much less than the other barrier type, with a linear relation only till the 7th day then the Ag ion release slows down. The difference in the release behavior with immersion time can be explained by the kinetics of the water uptake of the polymer barrier (Damm et al. 2007;

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Fig. 8 Impedance and phase angle versus freq. at different exposure time, 20 nm plasma-polymerized HMDSO, 10 sccm O2

Yliniemi et al. 2008; Damm et al. 2008). Therefore, electrochemical impedance measurements were performed and each of the Figs. 8, 9, and 10 contains the absolute value of impedance (|Z|) and phase angle (u) as a function of frequency (f) measured over a long time scale (0–4,068 min) for ITO substrates covered with 20 nm film of plasma-polymerized HMDSO with 10 or 20 sccm oxygen. As can be seen, for both systems there is a decrease in |Z| value around frequency of 0.1–10 Hz as a function of exposure time. Figures 8 and 9 (or Fig. 10 comparing the 2 types of barriers) show a decrease in the impedance and an increase in the phase angle as a function of exposure time to water. This is related to the polymer water uptake since the water uptake of polymer films increases the capacitance of the coating. Hence, the capacitance of the coating increases together with water uptake, and thus it can be estimated from EIS data at high frequency values (f) using the absolute value of impedance (|Z|) (Yliniemi et al. 2012), C ¼ 1=2pf jZ j

ð1Þ

Here the calculations are done for 100 Hz, where the impedance measurement shows a phase minimum. The EIS measurements for the sample 20 nm plasma-polymerized HMDSO with 10 sccm O2 over the whole exposure time can be seen in Fig. 8. The decrease in impedance and the increase of the phase angle during the measurement could be a hint for water ingress in the plasma-polymerized HMDSO layer.

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These changes could also be observed for the sample 20 nm plasma-polymerized HMDSO with 20 sccm O2, but not in such a drastic way (Fig. 9). For this sample the absolute value of impedance is lower than that for the sample shown in Fig. 8. In Fig. 10, the first run 0 min and the last run after 4,068 min of exposure to the electrolyte are shown for the samples treated with 10 and 20 sccm O2. In direct comparison it can be seen that in the beginning of the EIS measurement, the sample treated with 10 sccm O2 shows higher barrier properties than the sample with 20 sccm O2. From these results it can be concluded that the HMDSO treated with 10 sccm O2 seems to have better barrier properties than the sample treated with 20 sccm O2 at the beginning of exposure to the electrolyte. After a time period of 4,068 min, the impedance of the two samples decreases nearly to the same value indicating that the water has permeated through the plasma-polymerized HMDSO layer. The coating capacitance (Cp) was calculated for 100 Hz using Eq. (1) and the water uptake was also plotted. Changes in the coating capacitance (Cp) and water ingress into the polymer as a function of exposure time to the solution are shown in Figs. 11 and 12 for the sample with 20 sccm O2 and Figs. 13 and 14 for the sample with 10 sccm O2. Once again, it can be seen that the sample treated with 10 sccm O2 shows higher barrier properties than the sample with 20 sccm O2 at the beginning of exposure to the electrolyte and it can be assumed that after 2,500 s the


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Fig. 9 Impedance and phase angle versus freq. at different exposure time, 20 nm plasma-polymerized HMDSO, 20 sccm O2

Fig. 10 EIS spectra of the two plasma-polymerized HMDSO samples with 10 sccm O2 and 20 sccm O2 in direct comparison after 0 and 4,068 min

limit of water uptake is reached. The further increase in the coating capacitance (Fig. 13) could be caused by delamination of the plasma-polymerized HMDSO layer from the ITO substrate. In this case, the final value of water uptake is reached after 2,500 s of exposure. Thus, the water uptake of the sample with 10 sccm O2 shall be *2 % (Fig. 14). When these figures are compared to the increase in the released Ag ion concentration (Fig. 7), a relation between these two processes can be seen; AgNPs dissolution starts when the solution enters the barrier film and reaches

the surface of the particles. At the long exposure times one can also detect a saturation point of the water uptake. To study the effect of the thickness of the plasmapolymerized HMDSO barrier, samples of Ag/PTFE nanocomposites were covered by plasma-polymerized HMDSO barriers of similar oxygen content (20 sccm) but two different barrier thickness (20 and 40 nm). Samples were then immersed in water and the cumulative concentration of the released Ag ions in water was measured by ICP-MS as shown in Fig. 15.

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Fig. 11 Capacity over exposure time for the sample with 20 sccm O2


Fig. 14 Water uptake over exposure time to phosphate buffer for the sample with 10 sccm O2 16 40 nm 20 nm

12 10

+

cumulative Ag release (μg/l)

14

8 6 4 2 0

Fig. 12 Water uptake over exposure time to phosphate buffer for the sample with 20 sccm O2

0

2

4

6

8

10

12

14

Immersion time (days) Fig. 15 Cumulative Ag? released from Ag/PTFE nanocomposites coated by plasma-polymerized HMDSO barriers prepared with same O2 flow (20 sccm) but with two different thicknesses

Fig. 13 Capacity over exposure time for the sample with 10 sccm O2

One notices that the concentration of the ions in water gets less as the thickness of the barrier increases. The main reason is that as the thickness of the top layer

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increases, the water penetration through the barrier into the surface of the NPs gets slower. For the samples with 40 nm barrier thickness, water uptake is as high as for the samples with 20 nm barrier since both coatings were prepared with the same oxygen flow of 20 sccm, so they are both highly hydrophilic. However, the water mobility was to some extent retarded by increasing the thickness of the coatings. This implies that the diffusion of water molecules through the coatings must be taken into account to explain the silver ion release potential as we have shown previously for the Ag/PTFE nanocomposites coated with barriers of RF-sputtered PTFE films (Yliniemi et al. 2012).


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Conclusions

References

The silver ion release kinetics and the water uptake properties of plasma-polymerized HMDSO coatings were investigated. The technique of plasma polymerization of the HMDSO precursor was used to deposit thin films on top of Ag/PTFE nanocomposites that consist of 2D ensembles of AgNPs with nominal thickness of 2 nm deposited on a 20-nm-sputtered PTFE thin film. The nature of the plasma-polymerized HMDSO barriers was varied by varying the oxygen flow during the deposition process from 0 to 20 sccm and the thickness of the films was varied as well. A strong relation has been found between the oxygen flow and the barrier properties of the plasma-polymerized HMDSO films. ICP-MS results showed that the silver ion release potential was depending strongly on the oxygen content in the plasma-polymerized HMDSO films as the released amount of Ag ions and the kinetics of the Ag ion release process increased by increasing the oxygen flow. Besides, the contact angle analysis was used to investigate the effect of the oxygen flow on the resulting structure of the plasmapolymerized HMDSO coatings. Plasma polymerization with a high oxygen flow leads to the formation of hydrophilic thin films. In contrast, thin films obtained without oxygen flow have a polymeric chemical structure and are therefore highly hydrophobic making them exhibit high barrier properties. EIS results showed also a water diffusion dependence on the oxygen content in the coating films. From this point of view, the capability of controlling the film composition by varying the deposition conditions open interesting perspectives. Changing the thickness of the barrier showed also another way of tuning the release of silver ions from the nanocomposites. By increasing the thickness of the coatings, the amount of the released ions decreases and the rate of the release process slows down.

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Acknowledgments We acknowledge the financial support of the German Research Foundation (Deutsche Forschungsgemeinschaft; DFG) under Grant Number Fa 234/20-1 and TR 24 project B 13. The authors are grateful for Dipl.-Ing. S. Rehders for constructing the deposition chambers and for Dipl.-Ing. R. Kloth for his expertize in solving technical problems. Thanks to Dr. U. Schu¨rmann for performing the TEM measurements. Moreover, we would like to thank Dr. D. Garbe-Scho¨nberg and Dipl.-Ing. U. Westernstro¨er for their help in the ICP-MS measurements.

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