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Plasma-Enhanced Synthesis of Thin Fluoropolymer Layers with Low Raman and Fluorescence Backgrounds Hongquan Jiang,‡,§ M. K. Jantan,§ S. Manolache,‡ F. S. Denes,†,‡ and M. G. Lagally*,§ Department of Biological Systems Engineering, College of Engineering-Center for Plasma-Aided Manufacturing, and Department of Materials Science and Engineering, UniVersity of Wisconsin- Madison, Madison, Wisconsin 53706 ReceiVed May 5, 2008 Radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD) provides a promising way to deposit extremely hydrophobic, highly adherent nanometer- to micrometer-thick films with thermal stability, a low coefficient of friction, a low dielectric constant, and a low value of surface energy. We describe the synthesis of these fluorinated thin films using hexafluoropropene as starting material and discuss their properties. These coatings, applied to stainless steel, provide ideal substrates for Raman spectroscopy, when extremely low backgrounds are required. Raman spectroscopy measurements of a low-concentration protein film are used to demonstrate sensitivity and level of detectability.
Introduction Because it can be a powerful probe of structure and function in biochemical systems, Raman spectroscopy is becoming firmly established as an essential method in the life sciences.1–5 The vibrational fingerprint information in Raman spectra6 that is unique to a material provides an added dimension of information not available in other methods and may be used to distinguish compounds of different primary, secondary, or tertiary structure and for the identification and quantification of mixtures. In particular, for compounds of proteomic importance, such as peptides, proteins, and sugars, Raman spectroscopy may offer information not readily obtainable from other methods.7 In general, such information is obtained with surface-based Raman spectroscopy, in which a droplet of fluid is deposited on a surface and is subsequently analyzed. The Raman effect is inherently weak, making the characterization of analytes at low concentrations difficult and subject to the influence of background and noise.8,9 Whereas Raman sensitivity can in some cases be improved via surface enhancement, (SERS, surface enhanced Raman scattering) that approach is still not widely possible. Thus the development of surfaces that limit background for biochemical Raman spectroscopy studies is of considerable current inter* To whom correspondence should be addressed: Phone: (608) 263-2078. E-mail:
[email protected]. † Department of Biological Systems Engineering. ‡ College of Engineering-Center for Plasma-Aided Manufacturing. § Department of Materials Science and Engineering.
(1) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27(4), 241–250. (2) Schut, T. C. B.; Puppels, G. J.; Kraan, Y. M.; Greve, J.; VanderMaas, L. L. J.; Figdor, C. G. Int. J. Cancer 1997, 74(1), 20–25. (3) Kajiwara, K.; Franks, F.; Echlin, P.; Greer, A. L. Pharm. Res. 1999, 16(9), 1441–1448. (4) Carden, A.; Rajachar, R. M.; Morris, M. D.; Kohn, D. H. Calcif. Tissue Int. 2003, 72(2), 166–175. (5) Zhang, D. M.; Xie, Y.; Mrozek, M. F.; Ortiz, C.; Davisson, V. J.; BenAmotz, D. Anal. Chem. 2003, 75(21), 5703–5709. (6) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26(2), 163–166. (7) Laurent, G.; Felidj, N.; Aubard, J.; Levi, G.; Krenn, J. R.; Hohenau, A.; Schider, G.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2005, 71, (4). (8) Arutyunyan, N. R.; Brandt, N. N.; Chikishev, A. Y.; Lebedenko, S. I.; Parashchuk, O. D.; Razzhivin, A. P. Fluctuation and Noise Lett. 2005, 5(2), L233–L241. (9) Ramos, P. M.; Ruisanchez, I. J. Raman Spectrosc. 2005, 36(9), 848–856.
est.10–12 We report here on a simple and inexpensive way to provide substrates with extremely low background signal for a range of wavelengths useful in Raman spectroscopy studies. Investigations using Raman spectroscopy of biological samples usually require their deposition onto hydrophobic macromolecular substrates. The low surface energy of these materials prevents the spread of water-based droplets on the surface of the substrate and thus effectively concentrates the analyte within the sampling volume of the instrument. In fact, a hydrophobic surface can lead to a significant analyte concentration enhancement via the coffee ring effect.5,11–13 These concentration-enhanced regions of a spot are used for Raman measurements in order to maximize the signal. It would, of course, be much better to obtain Raman signals without resorting to such concentration enhancement, or signal enhancement via SERS, with the ultimate goal the observation of single covalently attached monolayers of biochemical molecules in individual spots of a bioarray. For this objective, substrates with very low Raman and fluorescent background are essential. Recently, very thin (e.g., 50 nm) Teflon spin-coated on stainless steel was shown to provide low Raman background.5,10–12 Modified perfluorinated soluble macromolecular structures have also been considered for spin-coating applications.14 Spin coated films have several difficulties, most notably structural instability upon washing or sonication. Radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD) provides a promising way to deposit extremely hydrophobic nanometer- to micrometer-thick films. By taking advantage of the plasma-induced fragmentation of low-molecular-weight perfluorinated-hydrocarbon volatile starting materials and the subsequent recombination of the charged and neutral fragments on the surfaces that confine the plasma, it is possible to make chemically inert thin films. These films (10) Zhang, D. M.; Mrozek, M. F.; Xie, Y.; Ben-Amotz, D. Appl. Spectrosc. 2004, 58(8), 929–933. (11) Xie, Y.; Zhang, D. M.; Jarori, G. K.; Davisson, V. J.; Ben-Amotz, D. Anal. Biochem. 2004, 332(1), 116–121. (12) Zhang, D. M.; Ortiz, C.; Xie, Y.; Davisson, V. J.; Ben-Amotz, D. Spectrochim. Acta, Part A 2005, 61(3), 471–475. (13) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389(6653), 827–829. (14) Denes, F.; Hua, Z. Q.; Simonsick, W. J.; Aaserud, D. J. J. Appl. Polym. Sci. 1999, 71(10), 1627–1639.
10.1021/la801396k CCC: $40.75 2008 American Chemical Society Published on Web 07/16/2008
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exhibit thermal stability, a low coefficient of friction, a low dielectric constant, and a low value of surface energy. Furthermore they adhere very strongly to the substrate. We describe here the synthesis of these thin films and discuss their properties. In particular we emphasize the quality of these coatings in providing very low Raman background and low fluorescence. These coatings, applied to stainless steel and other materials, provide ideal substrates for Raman spectroscopy when extremely low backgrounds are required. Figure 1. MS-induced molecular fragmentation of hexafluoropropene.
Experimental Section Materials. Argon and oxygen were purchased from Liquid Carbonic Corp., and hexafluoropropene (HFP) from Aldrich Chemical Co. Mirror polished stainless steel (SS 304#10) sheets were obtained from Medallion Metals. All SS substrates were cut into 3 by 1 in. slides. Precleaned 3 by 1 in. plain microscope glass slides were obtained from Fisher Scientific. All glass slides were cleaned with Piranha solution for 10 min before the deposition. Methods. All plasma-enhanced deposition reactions were carried out in a cylindrical, stainless-steel, capacitively coupled, RF plasma reactor (disk-shaped stainless-steel electrodes; electrode diameter 20 cm; gap 3 cm) described earlier,15 equipped with a 40 kHz power supply. Ultrasonically (Sonicor SC-101TH, Sonicor Instrument Corporation, Copiague, NY) cleaned stainless-steel (or glass) substrates were placed symmetrically onto the lower electrode and oxygen plasma cleaned for 5 min (150 mTorr, 200 W, flow rate 10 sccm). The reactor was then evacuated to its base pressure and HFP vapor was introduced under a controlled flow rate of ∼6 sccm. A continuouswave (CW) plasma was ignited and sustained using the following experimental parameters: pressure in the reactor: 150 mTorr, RFpower dissipated to the electrodes: 100 W, plasma deposition times: 5 s, 10 s, 30 s, 1 min, 2.5 min, 4 min, and 6 min. The relative atomic concentrations of carbon and fluorine in the surfaces of plasma-modified stainless-steel substrates were evaluated by X-ray photoelectron spectroscopy (XPS). The spectra were acquired using a Perkin-Elmer Physical Electronics 5400 small-area XPS system (Mg source; 15 kV; 300 W; takeoff angle 45°). Carbon and fluorine relative atomic concentrations were evaluated using the C 1s and F 1s lines, and the nonequivalent positions of carbon linkages were analyzed. Film thicknesses were determined using an Alphastep 200 Surface Profiler (KLA-Tencor Instruments, San Jose, California) with 400 µm step size at a speed of 1 µm/s. A Horiba Jobin-Yvon LabRAM ARAMIS Raman confocal microscope was used to collect the Raman spectra. A 50 × objective was used to focus 18.5 mW of He-Ne (632.8 nm), 170 mW of YAG (532 nm), and 420 mW of GaAs (785 nm) laser light onto the sample surface with a spot size of about 1 µm. In this spot, the actual laser power on the samples is 6.9 mW for HeNe (632.8 nm), 17.9 mW for YAG (532 nm), and 58.4 mW for GaAs (785 nm) light. The hole size is 500 µm and does not affect the laser intensity if it is more than 193 µm. Fluorescence was measured on a SPEX Fluorolog 1680 0.22 mm Double Spectrometer with slit size of 2.9 mm. Water contact angle (CA) measurements were performed using an OCA 15 Research Grade Video-Based Optical Contact Angle instrument (Future Digital Scientific Corp, Bethpage, NY). The staticCA evaluation procedure recommended in the operation manual was used. Sessile drop amounts were 1 µL and the flow rate from the syringe was 0.5 µL/s. For each sample, five or more CA measurements were performed at different locations of the specific sample surface. Samples of chicken ovalbumin were prepared for Raman investigation according to the following procedure: 1 µL of 3.5 × 10-3 M (3.5 nmol), 7 × 10-6 M (7 pmol), and 7 × 10-7 M (0.7 pmol) COVA were spotted onto plasma-coated stainless-steel and Fisher (15) Cruz-Barba, L. E.; Manolache, S.; Denes, F. Langmuir 2002, 18(24), 9393–9400.
Scientific glass slides. All spots had the same diameter, ∼1.2 mm. The 3.5 × 10-3 M COVA spot thickness was 100 µm. In 7 × 10-6 M COVA spots, the presence of a 28 µm wide coffee ring13 with varying thickness (500-800 nm) was observed. The 7 × 10-7 M COVA spots showed coffee ring thicknesses of ∼105 nm, as measured with the Alphastep 200 Surface Profiler. Raman spectra for the comparison of backgrounds produced by our fluorinated films and by commercial Tienta substrates were collected using a 600 g mm-1 grating, 500-µm-hole and a spectral integration time of 100 s with 532, 633, and 785 nm laser excitation. The Raman spectra for COVA were recorded for 200 s and 2 h with 633 nm laser excitation and a 500-µm-hole 1800-line grating.
Results and Discussion Electron impact ionization and fragmentation in nonequilibrium plasmas generate mono- and multifunctional, reactive, charged (ions of either polarity) and neutral (free radicals) species with energies comparable to common bond energies. Recombination of these species on and with the surface layers (covalent bonding) will result in the deposition of macromolecular, solid-state networks on the surfaces that confine the plasma (i.e., the electrodes). The advantages of CW plasma technology in comparison to modulated discharges is that it requires a less complex power supply, and requires shorter total deposition time. We have earlier successfully used16,17 mass-spectroscopic (MS)-electron impact fragmentation as a representation of plasmaelectron induced fragmentation. We have shown that the positiveion composition in the mass spectra of a particular molecule subject to impact fragmentation allows one to predict the chemical structure of the nascent macromolecular layers. MS-induced molecular fragmentation of hexafluoropropene (Figure 1) indicates that C3F5 (131 Da) and CF3 (69 Da) are the predominant cation fragments. Lower concentrations of CF (31 Da), CF2 (50 Da), C2F3 (81 Da), and C2F4 (100 Da) are also formed. The recombination and polymerization of these fragments will result in the formation of a perfluorinated, very hydrophobic structure. Figure 2 shows an XPS spectrum of a CW-plasma-deposited fluorinated-hydrocarbon film. The spectrum shows effectively only C and F. Oxygen is absent to the level of measurement uncertainty. The other peaks are F KLL Auger lines. A curve fitted high-resolution spectrum of the carbon region of the XPS measurement is shown in Figure 3. The dominance of -CFx(x >1) components is obvious. The presence of C*-(CFx)4, C*F-(CFx)3, C*F2-(CFx)2, and C*F3-CFx peaks suggests that the thin film is a C-F base fluorinated-hydrocarbon block copolymer.18,19 The absence of a significant CdC peak in the (16) Shamamian, V. A.; Hinshelwood, D. D.; Guerin, D. C.; Denes, F. S.; Manolache, S. J. Photopolym. Sci. Technol 2001, 14(1), 91–100. (17) Wu, Y. J. G.; , A. J.; Jen, J.; Manolache, S.; Denes, F.; Timmons, R. B. Plasmas Polym. 2001, 6(3), 123–144. (18) Cruz-Barba, L. E.; Manolache, S.; Denes, F. Polym. Bull. 2003, 50(5-6), 381–387. (19) Beamson, G.; Briggs, D., High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database; John Wiley and Sons: Chichester, 1992; p 295.
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Figure 2. XPS survey of fluorinated film (40 nm) deposited on stainless steel from HFP plasma. Only C 1s and F 1s were observed. O (532 eV) is absent within the level of noise in the spectrum. Figure 5. Raman background spectra of various thicknesses of fluorinated film deposited from HFP plasma; excitation: 633 nm. Integration time: 50 s.
Figure 3. High-resolution XPS C1s spectrum of fluorinated film (40 nm) deposited from HFP plasma. Peak 1 is related to the Mg source.
Figure 6. Raman background at 1000 cm-1 versus thickness of HFP plasma deposited film. The background rises as a quadratic, excitation: 633 nm. Integration time: 50 s.
Figure 4. Thickness versus deposition time for the perfluorinated films deposited from HFP plasma. Measurements made with an Alphastep 200 Surface Profiler instrument.
spectrum suggests that the plasma deposited film has a high hydrophobic character. The deposited-film thickness can be controlled by the plasmaon time (χ) while keeping other parameters constant. The film thickness shows a linear relationship with deposition time (Figure 4). At 100 W, 150 mTorr, and 6 sccm flow of HFP vapor, the film thickness y varies with time as y (nm) ) 35χ (min). Typical thicknesses of films plasma deposited for 5 and 10 s were 3 and 6 nm, respectively. It is noteworthy that low- (3 nm) and high(200 nm) thickness films exhibit practically identical highresolution XPS spectra, suggesting that film-thickness-related
structural (chemical and morphological) modifications of the nascent films are absent. Figure 5 shows Raman intensities from the plasma deposited films in the 500 cm-1 to 6000 cm-1 range as a function of film thickness, using 633 nm excitation. The Raman signal exhibits a nonlinear dependence on thickness. Peak intensities shift from higher to lower wave numbers as the film thickness increases (effectively a peak at 1500 cm-1 begins to grow). Figure 6 shows intensity values at 1000 cm-1 derived from Figure 5 as a function of deposited-film thickness. The increase is quadratic with thickness. It is likely that fluorescence contributes to these signals. The nonlinear behavior of fluorescence of polymeric films with respect to film thickness has been noted20–22 and interpreted in terms of electronic, molecular, and glass transition changes in thin layers. Changes in the maximum emission angle,23 modification of the apparent quantum yields for excited molecules near dielectric interfaces,24 and the influence of reabsorption on films25 can also contribute to a superlinear behavior of the Raman background signal. (20) Antonel, P. S.; Andrade, E. M.; Molina, F. V. Electrochim. Acta 2004, 49(22-23), 3687–3692. (21) Ellison, C. J.; Kim, S. D.; Hall, D. B.; Torkelson, J. M. Eur. Phys. J. E 2002, 8(2), 155–166. (22) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120(46), 11864– 11873. (23) Fernandez, J. E.; Rubio, M. Nucl. Instrum. Methods Phys. Res., Sect. A 1989, 280(2-3), 539–545. (24) Shu, Q. Q.; Hansma, P. K. Thin Solid Films 2001, 384(1), 76–84. (25) Birks, J. B. Mol. Cryst. Liq. Cryst. 1974, 28(1-2), 117–29.
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Figure 7. Comparison of Raman background spectra of thin films deposited from HFP plasma with commercially available sample (Tienta Sciences); excitation: 633 nm; integration time: 50 s.
Figure 8. Comparison of Raman background spectra of thin films deposited from HFP plasma with commercially available sample (Tienta Sciences); excitation: 532 nm; integration time: 50 s.
Figure 9. Comparison of Raman background spectra of thin films deposited from HFP plasma with commercially available sample (Tienta Sciences); excitation, 785 nm; integration time, 50 s.
Comparisons of Raman background intensities from clean stainless steel, stainless steel coated with 3 and 6 nm perfluorinated hydrocarbon films, and a commercially available Teflon-coated stainless-steel substrate10 (Tienta Sciences) are shown in Figures 7–9, respectively, for 532, 632, and 785 nm excitation wavelengths. For all of these thicknesses (and somewhat thicker, not shown), the plasma-coated stainless steel has lower Raman background intensity values than the Tienta substrates (spin coated
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film, 50 nm). The Raman intensities from the substrates with a 3 nm plasma deposited film are, in fact, very close to the intensities of unmodified stainless steel, yet these substrates are nevertheless very hydrophobic, as described below. Intensities recorded at the longer 785 nm excitation wavelength indicate (Figure 9) practically identical Raman intensity profiles for unmodified stainless steel, stainless steel with 3 and 6 nm films, and the Tienta substrate with the spin coated film. Fluorescence measurements show only the presence of the instrument (SPEX Fluorolog 1680) background, whether the stainless steel was coated or not. We investigated the water wettability of our perfluorinated films by contact angle measurements using the sessile drop (1 µL) technique in ambient laboratory conditions. All the water contact angle measurements were done in five seconds in order to avoid droplet evaporation effects. The water contact angle is in the range of 115° ( 6°, independent of the film thickness and of time after preparation of the film. The adhesion to stainless steel of plasma deposited perfluorinated films was tested by exposing substrates coated with 3 and 6 nm thick perfluorinated films to four different environments: overnight in water, 5 min of water/ultrasound, 5 min in hot water, and under a strong hot-water jet. The plasma deposited films show continued excellent film adhesion and good stabilities after the ultrasound and hot-water exposures, with a slight diminishing of the contact angle from 115° ( 5° to 101° ( 5°. However, the hot-water jet removes the deposited layers after 30 s, due probably to mechanical abrasion of the very thin films. We believe this stability is caused by the covalent bonding and cross-linking of the fluorinated fragments with the substrate. Our plasma deposited films can readily be reused after washing off protein analyte spots with DI water. In contrast, the spin-coated Teflon (Tienta Sciences) films are totally exfoliated from the substrate surface already by a gentle washing. Even drying a droplet of protein solution deposited on the spin-coated commercial film destroys the film. The company does not recommend reusing the film (see company Web site). The fluorinate polymer segments in the spin coating solution simply lie on the substrate without any covalent bonding and do not cross-link to the substrate or to each other, making the spincoated film very fragile. On the other hand, the molecular fragments resulting from electron-impact-induced fragmentation occurring in a plasma have dimensions much smaller (from several Å to less than 1 Å) than the substrate roughness values (pores, hill/valley formations, etc.) and accordingly plasma species will diffuse into the cavity structure and become by recombination with other arriving fragments larger molecules (macromolecular networks) that will not be able to leave the cavities. Obviously, this mechanism will produce a significantly better adhesion than the solution deposition of fluorinate polymers, including Teflonlike layers. In addition, owing to the high reactivity of plasma species, direct interactions between the species and surfacelayer molecules of the substrate are promoted. This covalent attachment increases even more the adhesion to the substrate of plasma-generated macromolecular networks. The fluorinated film also was deposited on Piranha cleaned Fisher glass slides. Because of its transparency and hence greater excitation volume for Raman measurements, the glass slide introduces more background than stainless steel. The film has the same good adhesion as on stainless steel. Without piranha cleaning, the film does not have as good adhesion on the glass slide. The reason probably is a contamination layer on the glass slide that cannot be removed with just sonication.
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Figure 10. Raman spectra of ovalbumin films (100 µm and 510 nm) deposited on stainless steel coated with 3 nm fluorinated film; excitation: 633 nm; integration time: 200 s. Note the vertical scale is counts/s. The increase in background is due to the thicker COVA.
Figure 11. Raman spectra of 0 (substrate background), 105, and 510 nm COVA films deposited on stainless steel coated with 3nm fluorinated film; excitation: 633 nm; integration time: 200 s. The background is essentially all from the COVA: a thicker deposited COVA layer produces higher background. The substrate background curve here is not consistent with that in Figure 7 because a different instrumental condition (grating) was used here.
Chicken ovalbumin (COVA) was used as a sample material to test the effect of background on very low Raman signals from a protein. COVA has a characteristic Raman peak around 1000 cm-1. We deposited 100 µm (about 53.4 fmol probed quantity), 510 nm (about 0.14 fmol probed quantity), and 105 nm (∼0.03 fmol probed quantity) COVA films on stainless steel coated with a 3 nm perfluorinated hydrocarbon film. We measured Raman spectra under a 500-µm-hole 1800-line grating with 633 nm laser excitation for 200 s exposure time (Figures 10 and 11). Figure 10 compares the Raman signals from the 510 nm and 100 µm COVA films. The increasing background with thicker COVA layers is generated by fluorescence from the COVA. The substrate background becomes important only for very thin COVA. The signal is much smaller for the 510 nm COVA film, but even here the background is dominated by COVA fluorescence. As Figure 11 shows, the substrate contributes only ∼25% of the background for the 510 nm COVA film, and increases to ∼50% for the 105 nm COVA layer. The Raman signal from the 105 nm COVA film is barely detectable with a 200 s acquisition time. However, as shown in Figure 12, an increase of the integration time to 2 h allows a clear COVA peak to be measured, without apparent damage to the
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Figure 12. Effect of signal averaging. Raman spectra of 105 nm ovalbumin films deposited on stainless steel coated with 3nm fluorinated film; excitation: 633 nm with 200 s and 2 h integration times. Note that the background (CPS) does not change while the signal-to-noise ratio improves dramatically. The substrate is reusable after this measurement.
Figure 13. Raman backgrounds of stainless steel and a glass slide coated with 3nm fluorinated film; excitation: 633 nm with 200 s integration time.
sample. As anticipated, the signal-to-noise ratio improves significantly while the background stays constant (note that counts/ sec are recorded, so that the integration time is factored out). Long integration time may be a practical way to increase the signal-to-noise ratio and expand the detectable range of protein concentrations. It is clear that when the substrate background is large, its effect on the Raman spectra will be such that it will mask small signals, as background generates its own noise, which adds to the overall noise. We can demonstrate the effect using glass substrates, for which added background is introduced by the substrate itself, something that is more or less absent with stainless steel (see Figures 5 and 9). The glass will introduce 70-120 CPS background for settings at which the stainless steel substrate produces 5-10 CPS (Figure 13). In addition, we tested reuse (washing and redeposition) of our plasma coated substrates with the 510 and 105 nm COVA films and compared with commercially available substrates. Using our substrates, the spectrum is always the same, while the Tienta slides could be used only once. We therefore confirm our washing experiments described earlier, and demonstrate that any stresses or chemical interactions generated by depositing the protein did not affect the quality of the substrate coating. We also tested the level of background for the same HFP coatings on Au, Ag, and Cu substrates. The stainless steel has
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the lowest background (for example, 20% HCl cleaned Cu showed about 10-20 CPS higher background than stainless steel but not as high as glass) and so is best for Raman and other measurements based on optical detection. However, the HFP-based coatings on gold, silver, and copper, as well as on wood fibers and textiles showed the same good adhesion and the same high water contact angle as described above for stainless steel and glass.
Conclusions We have developed a process using a hexafluoropropene (HFP) continuous-wave (CW) plasma to deposit highly hydrophobic perfluorinated hydrocarbon polymer films on stainless steel and glass for the purpose of developing ultralow-background substrates for Raman spectroscopy. The films exhibit very low Raman background and low fluorescence background at excitation wavelengths that cover the spectrum of Raman response of organic materials. These coatings can be very thin, as little as 3 nm, without losing performance. High-resolution XPS indicates the presence of a branched and/or cross-linked macromolecular network based on CF, CF2, and CF3 units. The very hydrophobic nature of the HFP-based coatings allows fluid sample depositions that ensure the generation of intense Raman signals. The extremely low background assures highest sensitivity in conventional Raman spectroscopy of biological samples. The film adhesion is
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extremely strong both on stainless steel and on glass and the mechanical stability is exceptional, making long exposure times or signal averaging via scanning viable possibilities. The Raman background and fluorescence characteristics, the adhesion to the substrate, and the stability of the HFP plasmadeposited layers are superior to those characteristics in commercially available substrates. The mechanical stability and adhesion to the substrate of our plasma deposited films additionally make them reusable multiple times, in contrast to only once for current commercial products. We expect that substrates prepared in this manner (especially with very thin coatings) will play an important role in future Raman spectroscopy investigations and may increase opportunities for the straightforward and inexpensive analysis of very low-concentration biological samples. But, because it is possible to make coatings in this manner that are arbitrarily thick, as well as to pattern the coatings, it is likely that many other applications for these coatings, deposited on a variety of substrates, will arise. Acknowledgment. This work was supported by DOE. We acknowledge Dr. Christopher Hunt for training in fluorescence measurements and Dr. Alexander Kvit for training in Raman measurements. LA801396K
