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Stabilization of Plasma-Polymerized Allylamine Films by Ethanol Extraction Li-Qiang Chu, Wolfgang Knoll, and Renate Fo¨rch* Max-Planck-Institute fu¨r Polymerforschung, Ackermannweg 10, 55128, Mainz, Germany ReceiVed March 8, 2006. In Final Form: April 26, 2006 The effect of ethanol extraction on plasma-polymerized allylamine (PPAA) films was investigated by measuring their thickness change using surface plasmon resonance (SPR) spectroscopy and optical waveguide spectroscopy (OWS). It was found that much of the freshly deposited PPAA films is lost upon ethanol treatment. The decrease in PPAA thickness is related to the plasma input power, the monomer vapor pressure, and the thickness of the deposited films. Despite the relatively high loss in film thickness, the densities of the amine groups in the extracted PPAA are comparable to those of fresh films, as seen by Fourier transform infrared (FT-IR) spectroscopy. The results of this study are of specific importance with respect to the biomedical application of plasma-polymerized functional thin films, in which the stability of a plasma polymer in contact with aqueous media is essential.
Introduction Plasma polymerization of allylamine has attracted considerable interest over the past two decades because of the numerous applications of aminated surfaces in biotechnology. It has been reported that plasma-polymerized allylamine (PPAA) films may be applied as substrates for cell culture1,2 and for the immobilization of biomolecules such as polysaccharides,3 DNA,4,5 and so on. The advantage of the plasma polymerization technique includes good adhesion on a large variety of substrates. It is a one-step, all dry process used to achieve pinhole-free, conformal coatings, and no damage to the bulk substrate occurs. It has been noticed by many researchers that PPAA films are not stable in aqueous environments, which has been associated with the dissociation of low molecular weight materials from the deposited coating.4-8 In biomedical applications, these dissolved small molecules may initiate undesirable side effects, leading to the failure of a biomedical device, for example, in immune responses. The loss of material also results in a reduction of the plasma polymer thickness.6-8 In some cases, the thickness of the PPAA films plays an important role in practical applications. For example, Zhang et al.5 investigated the DNA adsorption on PPAA films and found that the amount of DNA adsorption is thickness dependent. Therefore, it is fundamentally important to stabilize the plasma-polymerized film, as well as determine the extent of material loss in order to control film thickness. In this work, ethanol extraction was employed to remove all soluble, non-cross-linked material from freshly deposited PPAA films.8 The thickness change of the PPAA films upon ethanol extraction was determined by surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS) measurements. To * To whom correspondence should be addressed. Tel: (+49) 6131 379487. Fax: (+49) 6131 379100. E-mail:
[email protected]. (1) Harsch, A.; Calderon, J.; Timmons, R. B.; Gross, G. W. J. Neurosci. Methods 2000, 98, 135-144. (2) Griesser, H. J.; Chatelier, R. C.; Gegenbach, T. R.; Johnson, G.; Steele, J. G. J. Biomater. Sci., Polym. Ed. 1994, 5, 432-456. (3) Dai, L.; St. John, H. A.; Bi, J.; Zientek, P.; Chatelier, R. C.; Griesser, H. J. Surf. Interface Anal. 2000, 29, 46-55. (4) Chen, Q.; Fo¨rch, R.; Knoll, W. Chem. Mater. 2004, 16, 614-620. (5) Zhang, Z.; Chen, Q.; Knoll, W.; Foerch, R.; Holcomb, R.; Roitman, D. Macromolecules 2003, 36, 7689-7694. (6) Zhang, Z.; Chen, Q.; Knoll, W.; Fo¨rch, R. Surf. Coat. Technol. 2003, 174-175, 588-590. (7) Tatoulian, M.; Frederic, B.; Arefi-Khonsari, F.; Amouroux, J.; Bouloussa, O.; Rondelez, F.; Paul, A. J.; Mitchell, R. Plasma Process. Polym. 2005, 2, 38-44. (8) van Os, M. T.; Menges, B.; Foerch, R.; Vancso, G. J.; Knoll, W. Chem. Mater. 1999, 11, 3252-3257.
Figure 1. Schematic of the plasma polymerization system.
evaluate the influence of the plasma conditions on the stability of the deposited films, PPAA films were prepared at different input powers and monomer vapor pressures. The chemistry of the PPAA films before and after extraction as well as the surface morphology were characterized by Fourier transform infrared (FT-IR) and atomic force microscopy (AFM), respectively. Experimental Section Plasma Polymerization. Plasma polymerization was carried out in a home-built inductively coupled radio frequency (rf) (13.56 MHz) plasma reactor (Figure 1). The reaction chamber, enclosed in a Faraday cage, was a Pyrex glass cylindrical tube, 30 cm in length and 10 cm in diameter.8 The reaction chamber was evacuated to 10-3 mbar using a rotary pump. The flow rate of the monomer vapor was controlled by a Kobold floating ball flowmeter. The allylamine monomer (3-amino-1-propene, 99%) was purchased from Sigma-Aldrich (Germany). The monomer was degassed by three freeze-thaw cycles before use, but was not purified further. The plasma power employed in this work ranged from 5 to 150 W. The monomer vapor pressure varied from 0.055 to 0.22 mbar. The deposition time was adjusted to obtain the desired thickness. To remove the soluble part from the PPAA films, the samples were submersed into pure ethanol and shaken for 15 h. After extraction, the films were washed with excess ethanol to remove small molecules adsorbed on the surface, and then dried at T ) 50 °C for 2 h. The substrates used for the SPR and OWS measurements were LaSFN9 glass slides (Hellma Optik, Jena, Germany) coated with approximately 2 nm of Cr and 50 nm of gold, which were thermally evaporated. To ensure optimum adhesion of the plasma polymer to the gold, a monolayer of 1-octadecanethiol (5 mM in ethanol, 10 min immersion) was self-assembled on the gold. The PPAA films for FT-IR measurements were deposited onto 30 × 25 mm glass slides coated with 50 nm of gold. Si wafers were used as the substrate for AFM measurements. FT-IR analysis was carried out using a Nicolet 850 spectrometer operated in the reflective mode. AFM measurements were carried out using a Veeco Dimension 3100 in the tapping mode
10.1021/la0606392 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Figure 2. Comparison of the FT-IR spectra of PPAA films before and after ethanol extraction. Deposition conditions: 5 W, CW, 0.079 mbar. under ambient air. The surface roughness for an area of 2 × 2 µm was obtained using standard AFM software (Nanoscope 6.11r1). SPR and OWS Measurements. The use of SPR or OWS for the characterization of thin films has already been discussed in details elsewhere.9-11 The SPR curve can be fitted using the Fresnel equations obtaining the optical thickness (nd) of the dielectric medium. If the refractive index of the thin film is available, one can determine the geometrical thickness of the film. In contrast to SPR, OWS can distinguish between the refractive index n and the thickness d of the film, provided that at least two optical waveguide modes can be excited. SPR and OWS measurements were carried out with a homebuilt setup based on the Kretschmann configuration.11 The SPR and OWS spectra were recorded against air immediately after plasma polymerization for freshly deposited PPAA films and after ethanol extraction.
Results and Discussion Surface Characterization. FT-IR was employed to identify PPAA film composition before and after ethanol extraction. Figure 2 shows typical FT-IR spectra for a PPAA film deposited at 5 W. The band at 3335 cm-1 indicates the presence of primary and secondary amines. It was found that there is almost no change in the IR bands of the PPAA film upon ethanol extraction. The overall decrease in the band intensity is due to a decrease in film thickness. These data show that the composition of the PPAA film after ethanol extraction has not changed significantly. This was confirmed by XPS data (not shown here), which also showed no measurable change in the composition of the PPAA surface. The roughness (Rq) of the extracted PPAA was below 1 nm (Figure 3). Compared to the fresh PPAA, the roughness of the extracted PPAA was a little higher, which may be due to the loss of some low molecular weight materials. Film Thickness upon Ethanol Extraction. The thicknesses of both fresh and extracted PPAA were measured using SPR or OWS. Four positions were measured on each film. The standard deviation of the measured thickness was less than 2%. Delamination of the film from the gold-coated substrate was not observed in the present study. However, SPR and OWS data show the loss of material from the films leading to a reduction in film thickness. The percentage of remaining film was used as an indication of the stability of PPAA films:
Percentage of remaining film )
dextracted × 100% dfresh
(9) Knoll, W. Annu. ReV. Phys. Chem. 1998, 49, 569-638. (10) Prucker, O.; Christian, S.; Bock, H.; Ru¨he, J.; Frank, C. W.; Knoll, W. Macromol. Chem. Phys. 1998, 199, 1435-1444. (11) Biesalski, M.; Ru¨he, J. Langmuir 2000, 16, 1943-1950.
Figure 3. AFM image of the PPAA films (A) before and (B) after ethanol extraction. Deposition conditions: 5 W, CW, 0.079 mbar.
Here, dfresh and dextracted are the thickness of fresh and extracted PPAA, respectively. Figure 4A shows two OWS spectra of a PPAA film (deposited at 100 W, continuous wave (CW), for 7 min) before and after ethanol extraction. Because there are more than two optical waveguide modes in each spectrum, both the refractive index n and thickness d of the film can be determined from the simulation. For the fresh PPAA, the thickness was found to be d ) 601 nm and refractive index was n ) 1.594, while, for the extracted PPAA, d and n were 470 nm and 1.585, respectively. It is evident that the PPAA film shows a considerable decrease in thickness because of the loss of material during extraction. The decrease in the refractive index n could correspond to a decrease in film density because some soluble part was extracted upon ethanol treatment. To study films of d < 150 nm using SPR, we have to assume that the refractive index of the films in all subsequent work is constant, regardless of film thickness and plasma conditions. The error from this assumption is estimated to be lower than 2% if the deviation of n is less than 0.010. PPAA films of increased thickness were extracted in ethanol for different lengths of time. Figure 4B shows the percentage of remaining film after extraction as a function of time. Clearly, 8 h of ethanol extraction results in a distinct decrease in film thickness that can be associated with the loss of unbonded, low molecular weight material from the freshly deposited films. We assume that this includes all molecular fragments from the surface and the “bulk” of the film. Only small additional changes in thickness were observed after a further 7 h of extraction. Extraction times greater than approximately 15 h showed no further significant changes. Therefore, we conclude that 15 h is sufficient to stabilize the 100 W PPAA film under the present conditions.
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Figure 4. Ethanol extraction treatment of PPAA films deposited using 100 W, CW at different deposition times. (A) Typical s-polarized OWS spectra of PPAA films (d ) 601 nm) before and after ethanol extraction; (B) the percentage of remaining film via extraction time.
The data in Figure 4B also suggests a correlation between the film thickness and the amount of material lost upon extraction. It can be seen that, for films deposited at 100 W, CW, the relative thickness of the remaining materials increased with increasing thickness. For example, 27.7% remained for a freshly deposited film of d ) 47 nm, but 78.0% remained for a d ) 601 nm film. The net thickness decreases for the above films were 34.2 and 132.0 nm, respectively. This suggests that ethanol extraction affects the whole plasma-deposited film and not just the outermost layer. It is well-known that UV irradiation initiates cross-linking within polymer networks. The UV in the plasma may initiate the cross-linking reaction inside the deposited film during the plasma polymerization process such that thicker film will be exposed to UV for a longer time, and hence may have a higher crosslinking density within the film. This would suggest that thicker films resemble a layer-like structure with a vertical chemical gradient, which probably leads to the apparent improved stability observed for thicker films in comparison to thinner films deposited under the same conditions. Plasma power also influences the reaction mechanisms during plasma polymerization, thus effecting the surface chemistry and cross-link density of the deposited films. We investigate the effect of plasma deposition power on the extraction behavior of the PPAA films. For this purpose, we deposited films of approximately the same thickness. From Figure 5A, it can be seen that, as the plasma power was increased from 50 to 100 W and 143.5 W, the percentage of remaining film increased from 30.4 to 42.4% and 57.9%, respectively. This clearly shows that high-power PPAA deposits lose less materials after extraction than low-power plasma deposits. In this case, the high input power results in a high fragmentation of the monomer molecules
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Figure 5. The thickness of PPAA films after ethanol extraction as a function of (A) plasma power and (B) monomer vapor pressure.
and more intense UV irradiation of the film during deposition, consequently leading to a high cross-linking density within the deposited plasma polymer. The monomer pressure employed also affects the stability of the PPAA films upon ethanol extraction. After ethanol extraction for 15 h, the films deposited at low monomer vapor pressure showed higher values for the remaining thickness (69.8%) compared to 19.4% for the film deposited at a high monomer vapor pressure (Figure 5B). This indicates that the PPAA film deposited at high pressure is less stable than that deposited at low monomer vapor pressure. This may originate as a result of two independent processes. On one hand, the process pressure affects the deposition time: the lower the pressure, the lower the deposition rate and the longer the deposition time. The time for film deposition at 0.055 mbar was 132 s, which is longer than the 60 s required for the film deposited at 0.22 mbar. This, in turn, will result in longer UV exposure time for low-pressure deposition, which may lead to higher cross-linking density. On the other hand, at low pressure, the electrons, ions, and radicals have long mean free paths and bombard the surface with high momentum, which may cause surface cross-linking reactions as the film deposits.12 As a result, the films deposited at low monomer pressure may have a higher cross-linking density than films deposited at high monomer pressure. The decrease in PPAA thickness can be ascribed to the loss of low molecular weight materials. It is generally believed that a high cross-linking degree inhibits the loss of materials from the plasma-deposited films. Therefore, there should be a correlation between the decrease in PPAA thickness and the (12) Butoi, C. I.; Mackie. N. M.; Gamble, L. J.; Castner, D. G.; Barnd, J.; Miller, A. M.; Fisher, E. R. Chem. Mater. 2000, 12, 2014-2024.
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degree of cross-linking inside the films. Our present data indicate that plasma conditions, such as plasma power and monomer pressure, are of great importance for the formation of crosslinking in PPAA films, thus affecting the stability of films. The UV irradiation from the plasma is also believed to be an important factor influencing the cross-linking degree within a plasma polymer film. Unfortunately, there is, to date, no reasonable method to quantify the degree of cross-linking inside plasmadeposited films with any kind of resolution. More precise control over the deposition mechanisms and the possibility of perhaps tailoring lateral and vertical chemical gradients within thin films remain a challenging task.
Conclusions The soluble part of PPAA films increases with decreasing thickness, decreasing input power and increasing monomer vapor pressure. We relate this to different reaction mechanisms occurring during the deposition process. Even after extraction, the chemistry of the films was found to be the same, and enough material remains covalently bonded for subsequent reactions. We observe a small roughness increase after ethanol extraction. Ethanol extraction is an easy approach to stabilize the PPAA film with a high retention of functionalities. LA0606392
