Nanocomposite Metal/Poly(Ethylene Oxide)-like Plasma Polymer Films and Their Properties

Transcription

1 WDS'10 Proceedings of Contributed Papers, Part III, 19 24, ISBN MATFYZPRESS Nanocomposite Metal/Poly(Ethylene Oxide)-like Plasma Polymer Films and Their Properties D. Arzhakov, A. Artemenko, I. Gordeev, A. Choukourov, D. Slavinska, H. Biederman Charles University in Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. This study presents a review of deposition of nanocomposite metal/poly(ethylene oxide)-like plasma polymer thin films. Current state-of-the-art is discussed and the first experimental results on deposition of Au/ Poly(ethylene oxide)- like plasma polymers by simultaneous vacuum thermal degradation of polyethylene oxide (PEO) and RF magnetron sputtering of gold are presented. Introduction Organic-inorganic composites are of growing interest because of their unique properties and numerous potential applications such as enhancement of conductivity [1,2], toughness [3], optical activity [4,5], catalytic activity [6], chemical selectivity [7,8] etc. Since the inorganic component in these materials is very often present in a form of inclusions with a nanometer scale the resulted materials are referred to as nanocomposites. In the last decades, a new class of nanocomposites based on plasma polymers has emerged where an inorganic component (usually a metal) is added to a thin film of a plasma polymer. It has been known for many years that an electrical discharge in organic vapors may lead to the formation of deposits on the surfaces adjacent to plasma. For some time these coatings have been considered as unwanted by-products [9]. Since then the area of plasma polymerization has become well recognized as an important part of material science. Plasma polymers have been suggested for application in many fields of science, particularly chemistry, physics and in the recent years in biology. Admixing metallic nanoparticles to plasma polymers may further broaden useful properties and applicability of such systems. Several basic deposition techniques can be used for preparation of metal/plasma polymer nanocomposites: 1) Simultaneous plasma polymerization and sputter-etching of a metal using an RF discharge; 2) Simultaneous plasma polymerization of an organic gas and evaporation of a metal; 3) Plasma polymerization of metal organic compounds 4) Sputtering from composite metal/polymer target 5) Dc planar magnetron sputtering and simultaneous plasma polymerization The above mentioned methods are described in detail in [10,11] The methods involving sputtering have the drawback of the so-called target poisoning when the excitation electrode becomes covered by carbonaceous deposits and the metal emission from the target is limited. When sputtering the composite components from the two independent targets the so-called cross-contamination effect may occur. State-of-the-art PEO is a linear polymer with the general backbone structure of [CH2 O CH2]. PEO is one of the most frequently used water-soluble polymers for biomedical applications because of its high water solubility and chain flexibility. The high degree of hydration and flexibility confers it as protein resistant (non-fouling), biocompatible, and nonimmunogenic. Therefore, PEO is widely used for biomedical application as drug delivery agent which allows to increase water-solubility of poorly soluble drugs and thereby to increase the efficiency of medicines in a human body [12].PEO is also able to prevent microbial adhesion and to inhibit bacterial growth. Several methods exist for coupling PEO molecules to surfaces. For example, simple physical adsorption, radiation and chemical crosslinking methods, and self-assemlend monolayers [13] have been utilized with a different extent of success. Recently, glow discharge-based methods have been given a thorough consideration for fabrication of PEO-like plasma polymers [14-18]. In these methods, low molecular weight molecules of a monomer are activated, usually by an electron impact 19

2 and/or UV photons, in a glow discharge and resultant radicals consequently participate in plasma polymerization reactions creating a three-dimensional cross-linked polymeric network on the substrate. The advantages of plasma polymerization include precise control of the chemical composition and cross-linking density, good adhesion to the substrate, pinhole-free character of even ultra-thin films, possibility to treat almost any kind of solid surfaces, etc. Diversity of active species and randomness of radical mediated plasma polymerization reactions may result in significant difference in chemical composition between precursor and plasma polymer. This may become disadvantageous, especially in applications where good retention of the monomer s structure is required as is the case of PEO-like plasma polymers. It has been generally recognized that <70% retention of the ether structure is necessary to ensure the non-fouling properties of such plasma polymers. Using an RF glow discharge, Lopez et al. [14] demonstrated that plasma-induced polymerization of tetraethylene glycol dimethylether (tetraglyme) generates surfaces having high short-term resistance to adsorption of biomolecules. Lately, Beyer et al. [15] showed that plasma polymerization of triethylene glycol monoallyl ether generates surfaces which are highly efficient with respect to prevention of protein adsorption. Johnston et al. [16,17] demonstrated that continuous wave plasma deposition of cyclic ethers can also lead to surfaces with effective non-fouling properties. More recently, Choukourov et al. [18] prepared non-fouling PEO-like plasma polymers by vacuum thermal degradation of conventional polyethylene oxide. This method was shown to be effective in deposition of non-fouling and nonthrombogenic plasma polymers. The modern studies in this field show that the use of plasma deposited process allow to obtain of surfaces with non-fouing properties, which are coating more than 80% of ether group. Several research groups have also focused their attention on plasma deposition of nanocomposite Metal/PEO-like plasma polymers [19-22], but the published results are not too many. All of these works investigated Ag/PEO-like plasma polymers with motivation to combine the non- fouling properties of PEO with the anti-bacterial properties of silver. In the works [19-21] the nanocomposites were deposited by magnetron sputtering of silver in a gas phase containing an ether-bearing precursor (glycol dimethyl ether). The DC magnetron power of 20 W was kept constant. The working pressure was varied in the range Pa, and the target-tosubstrate distance ranged from 60 to 120 nm. It was found that the variation of the working pressure had a strong impact on preferential facet and size of nanocrystalline silver embedded in PEO. Silver crystallites grew preferentially in (111) crystallographic orientation and their size was about 5 nm at a low pressure. The increase of the working pressure to 2.0 Pa led to obtaining the Ag nanoparticles with the size of about 20 nm and to loss of preferential growth in the facet of (111). It was also observed that the content of the C-O-C groups was significantly lower (17.6%) in the case of silver cosputtering as compared to plasma polymerization of glycol dimethyl ether without silver co-sputtering (45.0%). In the work [22], Ag-сontaining PEO-like coatings were deposited by Radio Frequency Glow Discharge fed with vapors of diethyleneglycol-dimethyl-ether and argon. The RF cathode made of silver was connected as a live sputtering electrode whereas Ag/PEO-like coatings were deposited at the opposite ground electrode. The input power was varied in the range 5-30 W at 50mTorr of pressure. In order to deposit PEO-like coatings without silver, some discharges were run at the low RF power (<5W) and Ar flow (<7.5 sccm) and at high pressure (400mTorr). The results similar to above mentioned were obtained. The Ag nanoparticles with the size of about 4 nm were produced at 50 mtorr pressure. Running the discharge at higher powers led to the increase of the silver content in the films as well as to the increase of fragmentation of the initial monomer. As a result, the decrease of PEO character and the increase of the hydrocarbon content in the polymeric phase were detected. The more than 50% retention of the PEO character was observed only at the low (<2%) Ag content in the nanocomposite whereas the retention decreased to 15% at higher silver loading. Our study aims at expansion of the family of nanocomposite metal/peo-like plasma polymers and focuses at deposition and basic characterization of nanocomposite Au/PEO-like plasma polymers. The use of gold particles for therapeutic applications for animals and humans has already been documented. For many years, intramuscular gold injections and more recently, orally administered gold preparations (Auranofin), have been used to reduce inflammation and pain for joint trauma and rheumatic diseases [23]. Gold nanoparticles are also used as biomolecule targeting agents. In general, 20

3 metal nanoparticles possess strong biological activity and induce an immune reaction, i. e. recognition and elimination of foreign elements from the body. This is accomplished by coating a foreign material with specific proteins that permit phagocytosis. Rapid clearance of intravenously injected colloidal carrier systems including nanoparticles from blood circulation by the tissues of the mononuclear phagocyte system (MPS) is the major obstacle to their delivery to organs or cells. Different strategies have been proposed to prolong the clearance time of nanoparticles, most of them are based on the modification of the hydrophobic particle surface by physical adsorption of a hydrophilic polymer. One of the most commonly used polymers for nanoparticle hydrophilization is PEG/PEO which masks the foreign nanopartcile from recognition by MPS [24]. PEGylation is usually performed in a liquid phase with all shortcomings inherent to wet chemistry. We propose to use vacuum methods including magnetron sputtering and vacuum evaporation for fabrication of Au/PEO-like nanocomposite systems with the possible use in biomedical applications. Experimental Au/PEO-like plasma polymers were prepared by simultaneous vacuum thermal degradation of polyethylene oxide and RF magnetron sputtering of gold. PEO granules were loaded into a copper crucible, which rested on two molybdenum stripes heated by electric current. The crucible was placed 4 cm above a magnetron with a gold target. A radio frequency (Dressler Ceasar 13.56MHz) generator was used to deliver power to the magnetron. A quartz crystal microbalance (QCM) was placed in the plane with the substrates 10 cm above the crucible to control the deposition rate. The entire arrangement resided in a vacuum chamber brought by rotary and diffusion pumps to a base pressure of 1х10-3 Pa. The experiments were performed with argon used as a gas to initiate the plasma. The pressure of 1 Pa and 5 cm 3 (STP) min -1 flow rate were used. For obtaining of coatings with different content of Au, the deposition rate of PEO was varied by varying the temperature of the crucible whereas the power at the magnetron was maintained constant at 10 W. This allowed adding a controllable amount of gold into the nanocomposites. Volume fraction of gold, also called filling factor f, was determined according to equation 1: VAu d Au f = *100% = *100% (1) Vcomp dcomp, where V Au and V comp are volume of gold and total volume of nanocomposite, respectively. Given that QCM crystals of the same working area were used, the equation 1 can be re-written in terms of thickness or deposition rate of gold, d Au, and of nanocomposite, d comp. Since QCM showed constant increase of mass for all the experiments, the deposition rates were determined simply by relating the thickness of the films (measured after the deposition by ellipsometry) to the time of deposition. The Figure 1 shows the schematic deposition system and Table 1 summarizes the typical deposition parameters. Figure 1. Experimental setup: Q quartz crystal microbalance, OES optical emission spectroscopy; MS mass spectrometry; T crucible with PEO; M magnetron; G gold target. 21

4 Table 1. The typical deposition parameters for deposition of nanocomposite Au/PEO-like plasma polymers Matter Power, W QCM frequency shift, Hz/min Deposition rate, nm/min Filling factor of gold, % Au Au+PEO % % % % ,7% Results and discussion Figure 3 shows the C1s XPS of Au/PEO-like plasma polymer deposited by simultaneous vacuum thermal degradation of polyethylene oxide and RF magnetron sputtering of gold with two different fillings of Au in the nanocomposite. According to data given in the literature, this peak was fitted with four components and the ether groups were located at ev. As can be seen, the shape of the C1s peak changes with increasing content of gold in the films: the relative fraction of the C-O-C species decreases from 82 to 48% as the content of gold increases from 5.0% to 38.5%. The loss of the PEO character in the films with the high gold content (the low evaporation rate of PEO) is attributed to the increased specific power of the discharge, i. e. the power per mass unit of the precursor. Such behavior is analogous to that observed in the case of Ag/PEO-like plasma polymers by Chen et al. [19-21] and Favia et al. [22]. The structure of the nanocomposites was observed by TEM for the 30 nm thick films deposited on copper meshes covered with carbon foil, as shown in Fig 2. The film with 38.5% of gold is characterized by the channel-like structure where gold nanoparticles coalesce to form continuous channels separated by PEO-like plasma polymer. Those channels appear consisting of individual nanoparticles with the size in the range of 3-8 nm merged together. In the case of the low content of Au in the polymer matrix, clusters have a broader size distribution in the range 3-40 nm. It is also clear that individual gold nanoparticles without any sign of agglomeration are uniformly distributed in the PEO-like matrix. The deposited nanocomposite Au/PEO-like plasma polymers were found to be optically active inthe visible ranhe. The UV-Vis transmission spectra given in Fig. 4 show the absorption peaks which are due to a surfsce plasmon resonance (SPR) effect. The SPR effect appears when collective resonant oscillations of electronic gas in metals (plasmons) are excited upon interaction with electromagnetic field. The SPR peaks are seen for both of the samples. For the film with the low gold content, the maximal absorption detected at 530 nm. The increase of gold content to 38.5% leads to the red shift of the absorption peak at 570 nm, All the plasmon absorption bands are very broad which reflects a wide size distribution of nanoparticles usually observed for nanocomposites prepared by magnetron sputtering. a) b) Figure 2. TEM images of Au/PEO-like nanocomposites prepared with different evaporation rate of PEO at: (a) 38.5% gold content and (b) 5.0% gold content. 22

5 Fill factor = 38,5% C-C/C-H 37% C-O-C 48% C=O 6% O-C=O 9% Fill Factor =5% C-C/C-H 9% C-O-C 82% C=O 7% O-C=O 2% CPS 9500 CPS Binding Energy (ev) Binding Energy (ev) (a) (b) Figure 3. The C1s XP spectra of Au/PEO-like nanocomposite prepared with different evaporation rate of PEO at: (a) 38.5 vol. % gold content and (b) 5.0 vol. % gold content ,0% Au Transmittance, % ,5% Au Wavelength, nm Figure 4. UV-Vis transmittance of the nanocomposite Au/PEO-like plasma polymers prepared with different evaporation rate of PEO. Conclusions Metal/plasma polymer nanocomposites are perspective candidates for use in various applications including biomedicine and optics. Adjusting the plasma parameters allows significant variation of resultant properties hardly attained by non-plasma methods. In this work, nanocomposite Au/PEO-like plasma polymers were prepared by simultaneous vacuum thermal degradation of polyethylene oxide and RF magnetron sputtering of gold. This method allows obtaining nanocomposites with more than 80% retention of the PEO character and with different filling by gold. Adjustment of extent of crosslinking of the polymeric phase by variation of specific power may produce the films of different swelling/solubility and therefore with controllable release of gold nanoparticles. Acknowledgements. The work was supported by the grants GACR 202/08/8158 and SVV References [1] Coronado E., Galan-Mascaros J.R., Gomez-Garcia C.J. and Laukhin V., Nature 408, 447 (2000). [2] Croce F., Appetecchi G.B., Persi L. and Scrosati B., Nature 394, 456 (1998). [3] Pinnavaia T.J., Science 220, 365 (1983). [4] Wang Y. and Herron N., Science 273, 632 (1996). 23

6 [5] Winiarz J.G., Zhang L.M., Lal M., Friend C.S. and Prasad P.N., J. Am. Chem. Soc. 121, 5287 (1999). [6] Sidorov S.N., J. Am. Chem. Soc. 123, (2001). [7] Merkel T.C., Freeman B.D., Spontak R.J., He Z., Pinnau I., Meakin P. and Hill A.J., Science 296, 519 (2002). [8] Joly C., Smaihi M., Porcar L. and Noble R.D., Chem. Mater. 11, 2331 (1999). [9] Biederman H., Osada Y., Plasma Polymerization Processes, Elsevier, Amsterdam, (1992) [10] D Agostino R., Plasma Deposition, Treatment and Etching of Polymers, Academic Press, New York, (1990) [11] Biederman H., Plasma Polymer Films, Imperial College Press, London, (2004). [12] Mahato R.I., Biomaterials for delivery and targeting of proteins and nucleic acids. Boca Raton London New York Washington, D.C. CRC Press.,(2005). [13] Yuliang J. Wu, Richard B. Timmons, James S. Jen, Frank E. Molock, Colloids and Surfaces B: Biointerface 18, , (2000) [14] Lopez G.P., Ratner B.D., Tidwell C.D., Haycox L.L., Rapoza R.J., Horbett T.A., J. Biomed. Mater. Res. 26, , (1992) [15] Beyer D., Knoll W., Ringsdorf H., Wang J.H., Timmons R.B., Sluka P., J. Biomed. Mater. Res , (1997) [16] Johnston E.E., Ratner B.D., Bryers J.D., Polym. Mater. Sci. Eng. 77, 577, (1997) [17] Johnston E.E., Ratner B.D., Mater. Res. Soc. Abstracts, Fall Meeting, Boston, MA, 464, (1998). [18] Choukourov A. Polonskyi O, Hanus J., Kousal J., Grinevich A, Slavinska D., Biederman H., Plasma Process. Polym., 6, 21 24, (2009) [19] Yue L., Zhou M., Chen Q., Weng J., Zhang Y., Vacuum 83, , (2009). [20] Chen Q., Zhou M., Fu Y., Weng J., Zhang Y., Yue L., Xie F., Huo C., Surface &Coating Techology 202, , (2008) [21] Chen Q., Yue L., Xie F., Zhou M., Fu Y., Zhang Y., Wend J., J. Phys. Chem C,112, , (2008). [22] Favia P., Vulpio M., Marino R., d Agostino R., Mota R. P., Catalano M., Plasmas and Polymers, Vol 5 1,2000 [23] Nanoparticles and Nanodevices in Biological Applications, S. Bellucci ed., the INFN lectures, vol. I, Springer Verlag Berlin Heidelberg, 2009 [24] Nanopartcile Technology for Drug Delivery, R. B. Gupta, U. B. Kompella Eds., Taylor & Francis Group, New York,