Plasma Polymerization of Amine-Rich Films Aimed at Their Bioapplications

Lenka Zajíčková, Anton Manakhov, Petr Skládal, Marek Eliáš, David Nečas, Jan Čechal Central European Institute of Technology – CEITEC, Brno, Czech Republic 2014 TechCon Featured Article Best Poster Winner

mine coatings are useful for numerous applications including tissue engineering and biosensing. The bio-applications of Aamine thin films, such as a biomolecule immobilization, biosensors or a cell adhesion enhancement require good layer stability in water. However, typically for allylamine or ammonia/ethylene plasma polymerization, increased plasma polymer cross-linking, leading to improved film stability, is achieved at the expense of the amine-group Figure 1. Examples of monomers suitable for the deposition of amine containing plasma polymers: density. Cyclopropylamine (CPA) is a promising non-toxic monomer a) allylamine, b) ethylenediamine, c) cyclopropylamine, d) diaminocyclohexane and e) n-heptylamine. recently used for the deposition of amine-rich thin films. The stability of the CPA plasma polymers depends on power, discharge mode, Plasma Polymerization of Cyclopropylamine monomer flow rate and the discharge configuration. In this paper Although CPA is not toxic and has a high vapor pressure of 32 kPa at it is demonstrated that the deposition of CPA plasma polymers can 25 oC, the investigation of CPA plasma polymerization has been quite be processed on various substrates including sensor electrodes (for limited. It is surprising because very good results have been already biosensing application) and on the nanofibrous meshes (for a tissue obtained concerning the incorporation of NH2 groups in inductively engineering). The thickness loss below 1% combined with high amine coupled radio frequency (RF) discharge [12] and recent experiments concentration (>7at.%) can be achieved at the optimized plasma in a RF capacitively coupled plasma (CCP) reactor showed a very good conditions and therefore these coating have promising future for stability of CPA amine-rich films [13]. The RF capacitively coupled bio-applications. discharges can be used for the plasma polymerization in different configurations providing a variety of conditions at the substrate. Plasma Introduction polymerization is typically carried out at low RF power that does not The preparation of amine-rich surfaces has attracted attention of lead to a high fragmentation of the monomer. A high retention of the numerous researchers due to its great potential applications, such monomer structure also requires limited bombardment of the growing as biomolecule immobilization [1,2], microfiltration membranes film by energetic ions. [3], enzyme electrodes [4], adhesion enhancement [5] or biosensor In this work, two configurations of RF CCP (Figure 2) are compared development [6]. The simplest technique of amine grafting relies on for the deposition of thin films from CPA/Ar gas mixtures. In the surface plasma treatment in nitrogen or ammonia discharges [2, 7]. previously reported CPA/Ar plasma polymerization [13], the substrates However, these techniques lead to unstable functionalization of a thin were placed at the floating potential in a tubular horizontally mounted near surface layer with rather short duration. It has been observed that reactor with the electrodes being tube side flanges. It ensured low the grafted groups almost disappeared after several days of aging in energy of ions accelerated through an adjacent plasma sheath by a air, and the composition of the surface became comparable with the difference between plasma and floating potentials (maximum 15 V untreated layer [8]. [14]). The ion energy was further diminished by collisions in the plasma An alternative efficient way to create amine surfaces is a plasma sheath because of the pressure of 120 Pa. The experimental set-up is polymerization of amine-based monomers. In this case the stability schematically depicted in Figure 2a. The distance between the radio of amine groups can be enhanced and, thus, aging effect reduced [1]. frequency and grounded electrodes was 185 mm and the inner diameter Due to a high reactivity of primary amines the plasma polymerization of the tube, i.e. the diameter of the electrodes, was 80 mm. The RF should be performed at mild conditions, i. e., low power input and power was 20 - 30 W. pulsing the discharge, avoiding oxygen species. The presence of oxygen A vertically oriented set-up of RF CCP, that is often used for reactive contamination can easily oxidize and “de-activate” the amine groups. ion etching (RIE) [14] and for plasma enhanced chemical vapor Thus, the preparation of amine-rich surfaces is more often performed in deposition (PECVD) of the films growing under ion bombardment low pressure discharges. (e.g. diamond like carbon films [15] and protective organosilicon films For many years, allylamine has been the monomer of choice and silicon oxides [16]), is depicted in Figure 2b. The substrates lie on [3,9,10,11] thanks to the presence of vinyl group (Figure 1) that the RF electrode whose potential is the sum of a RF component and a enable free radical polymerization of this amine compound. However, DC self-bias that develops due to a different mobility of electrons and allylamine is a highly toxic flammable chemical compound with a lethal ions in the plasma. The distance between parallel plate electrodes of the vertical reactor is usually a few tens of millimeters. In the investigated dose LD50 equal to 35 mg/kg. Additionally, the stability of amine-rich allylamine plasma polymers is not sufficient for bioapplications. A process of the CPA/Ar polymerization, the interelectrode distance was successful preparation of amine-rich films was achieved for example 55 mm. The diameter of the bottom RF electrode was 420 mm and, using diaminocyclohexane [3] and ethylenediamine [6,4]. Another therefore, a higher RF power, 100 - 250 W, than in the tubular reactor promising approach for the amine-rich coating deposition is the plasma was used. In order to minimize the energy of ions accelerated through polymerization of the isomer of allylamine, cyclopropylamine (CPA). R. the plasma sheath the pressure was 50 Pa. Snyders’ group showed that pulsed plasma polymerization of CPA using The CPA/Ar plasma polymerization in the tubular reactor showed inductively coupled RF discharge lead to more efficient incorporation of that it is advantageous to employ a pulsed wave (PW) mode of the RF amine groups than in the case of allylamine [12]. discharge [13]. The PW mode (Figure 2c) combines plasmachemical processes initiated by energetic electrons (tON) with the plasma off-time

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(tOFF) in which the reactions are governed by existing radicals. It leads, in general, to a higher retention of the monomer structure. Therefore, the plasma conditions are for simplicity described by a mean RF power Pmean calculated as the on-time power PON multiplied by the duty cycle D.C. (1)

The mean power is used in the composite parameter W/F that expresses an average power input per unit of monomer flow. Although the description of the complex plasma polymerization process by this parameter is crutial simplification it proved to be applicable as scaling parameter in some case [17].

Figure 3. Chemical structure of plasma polymerized CPA assessed by a) infrared spectroscopy and b) - c) Figure 2. Two configurations of CCP plasma reactor that are characterized by different potential conditions fitting of XPS N1s signal by three components, NHx at 399.1 eV binding energy (BE), C=N at 398.1 eV and at the substrate: (a) substrate at floating potential and (b) substrate on DC self-biased RF electrode. Graph N-C=O/CǅN at 400.2 eV. The reported films were deposited at optimized conditions in two different in (c) represents schematically main parameters of pulsed RF discharges. configurations of CCP reactor, R2 denotes the set-up drawn schematically in Figure 2b whereas R3 corresponds to set-up in Figure 2a. According to the fitting of X-ray photoelectron spectroscopy (XPS) N1s signal the films deposited in the PW mode of the tubular reactor Water Stability of Amine-Rich Plasma Polymers at optimum W/F contained about 12 at.% of NH groups (Figure x The application of amine-rich thin films for cell adhesion enhancement 3b). A high concentration of amine groups was accompanied by a or immobilization of biomolecules requires high stability of the layers in presence of nitriles (CǅN), isonitriles (CǅN-) and/or conjugated aqueous media and prevention of trapped toxic molecules. The amine- imines (-N=C=N-) identified in the infrared spectra as a double peak rich plasma polymerized allylamine thin films showed a significant at 2245 and 2190 cm-1 (Figure 3a). The deposition in vertical reactor decrease in nitrogen concentration (N/C decreasing from 0.22 to 0.06) confirmed that the PW process can produce films with a higher amount [9, 11] and film thickness loss up to 90 % after the immersion [10]. The of amine groups (Figure 3c) but the comparison of FT-IR spectra of stability can be improved by an increase of plasma power at the expense both amine-rich films revealed less amount of nitriles and conjugated of amine concentration in the films [19]. imines (Figure 3a). The difference between the chemistry of the Recently, it was found that copolymerization of acetylene and NH films deposited in tubular and vertical reactor at different potentials 3 or ethylene and NH leads to the deposition of stable amine coatings requires further clarification. A high amount of nitrile groups can affect 3 (thickness loss 15 % in water) bearing around 5 at.% of amine groups, the antibody immobilization chemistry due to possible nucleophilic if a high flow rate of C H or C H is used [7,19,20]. In order to addition reaction with the participation of nitriles [18]. Moreover, 2 4 2 2 obtain even higher concentration of the amine groups, a high flow the conjugated imines can overcomplicate the analysis of the layer rate of ammonia is necessary and the coatings deposited in these chemistry during the immobilization process. Anyway, both types of conditions suffer from thickness loss (up to 70 %) after immersion the films are suitable for bioapplications because they possess surfaces in water. By plasma polymerization of n-heptylamine it is possible rich of amine groups and they are still sufficiently stable in water as discussed in the next section. continued on page 28

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Plasma Polymerization of Amine-Rich Films albumin (HSA) was attached to the QCM surface via cross-linkage continued from page 27 obtained by intermediate reaction with glutaraldehyde (Figure 5a). The XPS N1s curve fitting confirmed the immobilization of the antibody, to deposit sufficiently stable (thickness loss 15 %) amine coatings as high concentrations of amide environment (N-C=O) as well as + but the concentration of amine groups and nitrogen in such films is the protonated amine groups (NH3 ) attributed to aminoacids were significantly lower compared to the allylamine plasma polymers detected (Figure 5b). (N/C ~ 0.12) [21]. The sensitivity of the sensor with a CPA plasma polymer thickness The CPA plasma polymerization in the tubular reactor (Figure 2a) of 20 nm, was evaluated as the change of the frequency due to the yielded films with various degrees of water stability. However, optimized antigen binding, i.e. approximately 100 Hz for 50 μg/ml HSA, is a very conditions employing PW mode of the discharge resulted in sufficiently promising achievement (Figure 5c). It significantly exceeds responses water-stable polymers rich of amine groups [13]. The film R3, whose obtained using QCMs with the antibody immobilized via a thiol-based chemistry is reported in Figure 3c, exhibited thickness loss about 20 self-assembled monolayer because their signal to HSA was typically % after immersion in water for 48 hours. XPS analyses revealed that around 35 Hz [24] immersion in water induced an oxygen increase by 4-5 at.% at the expense of nitrogen. The XPS N1s curve fitting showed a decrease of the NHx contribution by 4.5 at.% accompanied with an increase of amide (N-C=O) environment by 2.0 at.%. Although, the water stability these films can be considered as promising it is desired to obtain amine-rich films that do not show any changes induced by interaction with water. Recently, the CPA pulsed plasma polymerization was carried out in the reactor depicted in Figure 2b. The film R2, whose chemistry is reported in Figure 3b (NHx environment of 9.7 at.%), exhibited a thickness loss below 1% after water immersion for 216 hours. This is an excellent result compared to previously published behavior of amine- rich films. The NHx environment decrease only by 2.8 at.% while the amide concentration was not affected by the immersion in water. Figure 4 shows the SEM micrographs of amine-rich films after 48 hours in water.

Figure 4. SEM micrographs of amine-rich films after 48 hours immersion in water. The film a)-R3 prepared in the tubular reactor shown in Figure 2a exhibited 20 % thickness loss whereas the film b)-R2 prepared in vertical reactor shown in Figure 2b was completely stable. Sparsely dispersed embedded particulates were observed in as-deposited film b)-R2.

Biosensors Prepared with Help of Plasma Polymerization Figure 5. Development of QCM biosensor : a) scheme of the biosensor functionalization, b)- XPS N1s curve The biomolecule immobilization step is critical for the development of fitting of the surface after antibody immobilization (PP-GA-Ab), c)- the response of the QCM biosensor in any sort of a biosensor. It provides the core of the biosensor and gives the form of frequency change Δf. it its identity. Moreover, the immobilized biomolecule needs to keep its original functionality as required for the function of the biosensor. Modification of Polymer Nanofibers by CPA Plasma Polymers Therefore, care has to be taken to avoid a steric hindering of the Electrospun nanofibers with a high surface area to volume ratio are recognition sites [22]. Antibodies remain by far the most frequently gaining increasing interest because of their potential applications in used biorecognition elements. They offer high affinity and specificity biomedical devices, tissue engineering scaffolds and drug delivery against the target analyte [23]. In order to immobilize biomolecule on carriers [25]. However, an exploitation of nanofiber meshes for these the surface a covalent bond between the surface functional group and purposes requires activation of their surface by reactive groups, such biomolecule (e.g. protein) must be developed. as carboxylic acid, hydroxyl or amine groups. Therefore, the plasma Following the promising results on the plasma polymerization of polymerization of CPA was tested also on the surface of biodegradable CPA in CCP discharges shown in previous sections the films were polycaprolactone (PCL) micro/nanofibers. Fibrous meshes were applied to the development of a quartz crystal microbalance (QCM) prepared by NanospiderTM technology using the laboratory machine immunosensor. The pulsed CPA plasma polymer was deposited onto NS LAB 500S from Elmarco s.r.o. The nanofibers were formed from 50 the QCM gold electrode and then the antibody specific to human serum cm length wire rotating electrode (or roller) and collected on nonwowen

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polypropylene foil. As shown by SEM (Figure 6), the fibers were 1.2 ± 2. M. Ghasemi, M. J. G. Minier, M. Tatoulian, M. M. Chehimi, and F. Arefi-Khonsari, “Ammonia plasma treated polyethylene films for adsorption or covalent immobiliza- 0.4 μm in thickness and mesh pore size was 8 ± 2 μm. tion of trypsin: quantitative correlation between X-ray photoelectron spectroscopy As shown in Figure 6, the uncoated PCL fibers exhibited a high data and enzyme activity.,” J. Phys. Chem. B, vol. 115, no. 34, pp. 10228–38, Sep. water contact angle (above 120o) whereas the deposition of the CPA 2011. 3. M. Müller and C. Oehr, “Plasma aminofunctionalisation of PVDF microfiltration polymers using R2 process led to its significant decrease to 30o. The membranes: comparison of the in plasma modifications with a grafting method XPS analysis revealed that the entire top surface of the nanofibers was using ESCA and an amino-selective fluorescent probe,” Surf. Coatings Technol., vol. coated by the CPA plasma polymer as the elemental and the functional 116–119, pp. 802–807, Sep. 1999. compositions of the surface were very similar to the reported CPA 4. H. Biederman, I. H. Boyaci, P. Bilkova, D. Slavinska, S. Mutlu, J. Zemek, M. Trchova, J. Klimovic, and M. Mutlu, “Characterization of glow-discharge-treated plasma polymerization results (Figure 3b). The concentration of the cellulose acetate membrane surfaces for single-layer enzyme electrode studies,” J. NHx environment determined by the XPS N1s curve fitting exceeded Appl. Polym. Sci., vol. 81, no. 6, pp. 1341–1352, Aug. 2001. 7 at.%. It is well known that surfaces exhibiting low water contact angle 5. J. Borris, M. Thomas, C.-P. Klages, F. Faupel, and V. Zaporojtchenko, “Investigations into Composition and Structure of DBD-Deposited Amino Group and high amine concentration are more favorable for a cell adhesion Containing Polymer Layers,” Plasma Process. Polym., vol. 4, no. 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S1, pp. S70–S74, Jun. 2009. 10. A. Abbas, C. Vivien, B. Bocquet, D. Guillochon, and P. Supiot, “Preparation and Multi-Characterization of Plasma Polymerized Allylamine Films,” Plasma Process. Polym., vol. 6, no. 9, pp. 593–604, Sep. 2009. 11. K. L. Jarvis and P. Majewski, “Influence of film stability and aging of plasma polymerized allylamine coated quartz particles on humic acid removal.,” ACS Appl. Mater. Interfaces, vol. 5, no. 15, pp. 7315–22, Aug. 2013. 12. L. Denis, P. Marsal, Y. Olivier, T. Godfroid, R. Lazzaroni, M. Hecq, J. Cornil, Figure 6. Water contact angles (on left) of uncoated (120o) and plasma coated (30o) PCL nanofibers. SEM and R. Snyders, “Deposition of Functional Organic Thin Films by Pulsed Plasma micrograph (on right) of plasma coated PCL nanofibers. Polymerization: A Joint Theoretical and Experimental Study,” Plasma Process. Polym., p. NA–NA, Dec. 2009. 13. A. Manakhov, L. Zají ková, M. Eliáš, J. echal, J. Pol ák, J. Hnilica, Š. Bittnerová, Conclusion and D. 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Plasma Polymerization of Amine-Rich Films continued from page 29 SAVE THESE DATES APRIL 25-30, 2015 SANTA CLARA TECHCON About the Authors SOCIETY OF VACUUM COATERS Lenka Zajíčková 58th Annual Technical Conference Lenka Zajíčková received her PhD degree in plasma physics Santa Clara Convention Center, CA, USA from Masaryk University, Brno, Czech Republic in 1999. She was a postdoctoral fellow at the Ruhr Universität APRIL 27-30 TECHNICAL PROGRAM Bochum (Germany) and the University of Minnesota in featuring a Symposium on Minneapolis (U.S.). She is currently a leader of the research group Plasma Technologies at Central European Institute of Technology (CEITEC) and associate professor at the Coating Technologies Department of Physical Electronics, Masaryk University in Brno. Her research interests include plasma processing of materials, plasma for the Interconnected Age diagnostics, plasma chemistry, and methods for the characterization of optical ADVANCING MOBILITY, DURABILITY AND and mechanical properties of thin films. Her work in the field of plasma polymers, protective coatings, carbon nanomaterials and magnetic nanoparticles resulted in PERFORMANCE OF MOBILE ELECTRONICS more than 70 scientific papers. + Nine Traditional Technical Sessions Anton Manakhov + Comprehensive Education Program Anton Manakhov is a postdoctoral Marie Curie Fellow + Equipment Exhibit at CEITEC, Masaryk University. He received a B.Sc. and M.Sc. (diploma cum laude) in Chemistry from the Ivanovo + Interactive Networking Events State University of Chemistry and Technology, Russian Federation. After graduation in 2006 he worked for two OCTOBER 10, 2014 years at the Samsung SDI to research plasma display • Abstract Submission Deadline panel technology. Since obtaining his PhD in physics in • Sponsored Student Application Deadline 2012 from University of Namur he is working at CEITEC under BioFibPlas project funded by SoMoPro grant. His current research interests include the use of plasma polymerization for biomedical and biosensing applications, characterization of the surface chemistry and morphology.

For further information, contact Lenka Zajíčková, at [email protected].

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