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Preparation of Hydrophilic Poly(dimethylsiloxane) Stamps by Plasma-Induced Grafting Quanguo He,† Zhengchun Liu,† Pengfeng Xiao,† Rongqing Liang,‡ Nongyue He,*,† and Zuhong Lu*,† Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, China, and Institute of Plasma Physics, Chinese Academy of Science, Hefei 230031, China Received September 16, 2002. In Final Form: April 21, 2003 We report a method to prepare hydrophilic poly(dimethylsiloxane) (PDMS) stamps that could transfer patterns of polar molecules homogeneously in soft lithography. In this method, we demonstrated a twostep procedure by using a mixed gas of argon (Ar) and hydrogen (H2)-based microwave plasma pretreatment to activate PDMS and subsequent acrylonitrile grafting onto PDMS, generating a hydrophilic surface with cyano (-CN) group coatings. The PDMS hydrophilic surface exhibited high affinity for wetting acetonitrile, which is a conventional solvent for DNA synthesis. Acetonitrile droplets possessed contact angles as low as 17 ( 7° on the cyano (-CN) groups grafted PDMS stamps surface. The hydrophilicity was stable and could last for at least 1 month at room temperature. Attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectrum were used to characterize the surface hydrophilization. Such a hydrophilic and inert PDMS stamp showed significance for reactive multilevel fabrications in soft lithography. The possible chemical mechanism for PDMS surface grafting was discussed.
1. Introduction Surface patterning by microstructured poly(dimethylsiloxane) (PDMS) stamps is attracting more and more interest in various areas such as microelectronics, micromechanics, and biological and chemical analytical systems. A conventional PDMS stamp has a hydrophobic surface, which enables nonpolar chemicals to be transferred onto substrates.1-4 However, the intrinsic hydrophobicity of a PDMS stamp surface prevents polar “inks” (i.e., polar chemical solution or polar reactive solution such as etchant) from spontaneous wetting, which leads to the generation of inhomogeneous or defective patterns. Therefore, a hydrophilic surface of PDMS is required to transfer hydrophilic chemicals to form molecular patterns and thus fabricate microdevices and nanodevices. For example, patterning self-assembled monolayers (SAMs),3 patterning ligands on reactive SAMs, and patterning proteins and cells were conveniently implemented in recent years.5-7 There is a tremendous need for hydrophilic PDMS surfaces. By using hydrophilic PDMS stamps, the researchers were able to transfer polar reagents such as nucleic acid,8,9 proteins,7 peptide nucleic acid (PNA), and even polysaccharide, constructing biological molecular * Authors to whom correspondence may be addressed. E-mail:
[email protected];
[email protected]. Fax: 0086-25-7712719, 008625-3619983. Telephone: 0086-25-3619983, 0086-25-3792245. † Southeast University. ‡ Chinese Academy of Science. (1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550-575. (2) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870-5875. (3) Kumar, A.; Biebuyck, H. A. Langmuir 1994, 10, 1498-1511. (4) Xia, Y.; Rogers, J. A.; Paul, K. E. Chem. Rev. 1999, 99, 18231848. (5) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 20552060. (6) Ostuni, E.; Kane, R.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2000, 16, 7811-7819. (7) Ostuni, E.; Chen, C. S.; Ingber, D. E.; Whitesides, G. M. Langmuir 2001, 17, 2828-2834. (8) Xiao, P.; He, N.; He, Q.; Zhang, C.; Wang, Y.; Lu, Z.; Xu, J. Sci. China (B) 2001, 44 (4), 442-448.
patterns or arrays. In our developed technique for onchip synthesis of high-density oligonucleotide microarrays,8-10 four different mononucleotides (5′-DMT-2′deoxynucleoside phosphoramidites of thymidine, N4-isobutyryl-2′-deoxycytosine, N2-isobutyryl-2′-deoxyguanosine, and N6-phenoxyacetyl-2′-deoxy-adenosine; abbreviated T, C, G, and A, respectively) dissolved in polar acetonitrile solutions were consecutively transferred onto a hydroxyl- or amino-tailored (HO-/NH2-) glass slide surface through addressable contact printing reactions by a set of PDMS stamps which had hydrophilic surfaces. Several methods were developed to modify PDMS surfaces. Hollahan and Carlson used oxygen plasma to activate the surface hydroxylation on cured and uncured PDMS.11 Sowell used radio frequency discharge to improve surface bondability of PDMS polymer.12 Cahalan and Verhoeven used grafting polymerization to an oxygenplasma-pretreated PDMS surface.13 Furthermore, Kuhn et al. developed a chemical treatment for plasma-activated PDMS and grafted hydrophilic functionalities onto the activated surface to improve its wettability.14 Whitesides et al. applied plasma oxidation to modify the PDMS stamps surface, generating a functional monolayer by adsorption of alkylsiloxane on the treated PDMS stamp surface, which improved the compatibility between the polar reactive solution and the PDMS surface.1,2,15,16 On the basis of plasma oxidation, Delammache et al. devised a (3(9) Xiao, P.; Lu, Z.; Liu, Z.; He, Q.; He, N. Chin. Sci. Bull. 2002, 47 (14), 1073-1076. (10) Lu, Z.; Zhang, L.; Ma, J.; Xu, C.; Chen, Y. International Patent WO 9951770 (US Patent 6423552), 1999. (11) Hollahan, J. R.; Carlson, G. L. J. Appl. Polym. Sci. 1970, 14, 2499-2508. (12) Sowell, R. R.; Delollis, N. J.; Gregory, H. J.; Montoya, O. J. Adhes. 1972, 4, 15-24. (13) Cahalan, P. T.; Verhoeven, M. US Patent 5229172, 1993. (14) Kuhn, G.; Weidner, St.; Decker, R.; Ghode, A.; Friedrich, J. Surf. Coat. Technol. 1999, 116-119, 796-801. (15) Ferguson, G. S.; Chaudhury, M. K.; Sigal, G. B.; Whitesides, G. M. Science 1991, 253, 776-778. (16) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 12301232.
10.1021/la020785h CCC: $25.00 © 2003 American Chemical Society Published on Web 07/19/2003
Surface Patterning by PDMS Stamps
aminopropyl)triethyoxysilane linkage on oxidized PDMS and attached poly(ethylene glycol) to prepare hydrophilic stamps of amino-terminated surface, which was very useful in printing polar inks such as Pd catalyst for ELD.17 Recently, Hu et al. used ultraviolet grafting to improve the hydrophilicity of PDMS microfluidic devices, and the grafted PDMS microchannels demonstrated better compatibility with aqueous solution and reduced adsorption of peptides.18 The above hydrophilic PDMS had either amino- or hydroxyl-terminated surfaces, which were reactive with mononucleotides used for DNA synthesis. In previous studies, we found that oxygen plasma treatment caused damage (fractures, cracks, etc) to the microstructures of PDMS stamps, which would produce cross-contamination between neighboring patterned sites on glass during the contact printing process. Therefore, DNA synthesis (oligonucleotide microarray fabrication) necessitates both a hydrophilic and an inert (nonreactive to DNA reagents) PDMS stamp surface. During this research, we developed a method using argon (Ar) and hydrogen (H2) mixed gas microwave plasma pretreatment and acrylonitrile grafting to prepare a stable hydrophilic surface for PDMS stamps. PDMS stamps were plasma-treated with Ar and H2 mixed gas, less damage could be introduced to the microstructures, and the original high-resolution patterns kept well. The treated PDMS stamps were immersed in acrylonitrile solution, and hydrophilic functionalities of cyano groups were grafted onto the surface of PDMS stamp. Attenuated total reflection Fourier transform infrared (ATR-FT-IR) and X-ray photoelectron spectroscopy (XPS) characterizations confirmed the presence of cyano (-CN) groups. A possible chemical mechanism of radical-induced grafting was discussed. The treated PDMS stamps have been applied to DNA microarrays synthesis, implying a promising technique for on-chip fabrication of high-density DNA microarrays.8-10 2. Materials and Methods 2.1. Materials. The hydroxyl-terminated prepolymer H-107 (vulcanized silicone rubber, molecular weight (3-6) × 104 g/mol) was purchased from Hangzhou Resin Co., Zhejiang Province, China. The prepolymer, cross-linking agent tetraethoxysilane and initiator dibutyltindidodecanoate were mixed in the manufacturer’s recommended ratio for PDMS preparation. Ar and H2 high-pressure bottled gas (99.9 vol %) were purchased from Hefei Gases Co. The deionized water resistance was 18.4 MΩ, and the water was double distilled. Acrylonitrile and other solvents were purified by distillation and were dehydrated prior to use. Relevant chemicals used, if not further stated, were purchased at least analytical grade and purified accordingly. 2.2. Instruments. Plasma pretreatment was conducted in the microwave-plasma treatment system19 developed by Applied Plasma Research Division, Institute of Plasma Physics, Chinese Academy of Science. The contact angle of the droplets was measured with a Rame-Hart 100R contact angle instrument. ATR-FT-IR spectra were recorded on a Nicolet Nexus 870 FTIR instrument with a DTGS detector at 4 cm-1 resolution, and 64 scans were collected per trace. X-ray photoelectron spectra were measured under a pressure of 1.33 × 10-7 Pa on an ESCA LAB MK2 instrument with Mg KR as source and a step of 0.05. 2.3. PDMS Stamp Preparation. Preparation of a PDMS stamp was described elsewhere.1,8,20 Briefly, a PDMS prepolymer, cross-linking agent, and initiator were carefully mixed in a (17) Donzel, C.; Geissler, M.; Bernard, A.; Wolf, H.; Michel, B.; Hilborn, J.; Delamarche, E. Adv. Mater 2001, 13, 1164-1167. (18) Hu, S.; Ren, X.; Bachman, M.; Sims, C. E.; Li, G. P.; Allbritton, N. Anal. Chem. 2002, 74, 4117-4123. (19) Fang, F.; Liang, R.; Ou, Q.; Sui, Y. Thin Solid Films 2001, 390, 197-201. (20) Zhang, L.; Liu, J.; Lu, Z. Supramol. Sci. 1998, 5, 713-715.
Langmuir, Vol. 19, No. 17, 2003 6983 polyethylene beaker, and degassed under vacuum until no visible bubbles were observed. They were then cast on a silicated glass board and instantly covered with the featured master that was patterned photolithographically to form a sandwich-like arrangement.1,6 The sandwich was cured at room temperature for 24 h to ensure complete cross linking, treated for 30 min at 150 °C, and cured for 2 h at 200 °C. The master board was peeled off to expose the negative-replicated patterns of PDMS stamps affixed on the silicated glass board. To extract PDMS surface contaminants and residual low molecular weight species from the photoresist before microwave plasma treatment, the PDMS surface was bathed with diethyl ether and acetone and rinsed with detergent solution and deionized water in sequence. The surface was then dried under vacuum for 1 h at 100 °C and finally cooled to room temperature for further plasma treatment. 2.4. Microwave-Plasma Treatment and Postchemical Grafting. The microwave-plasma chamber used here was a sealed Pyrex glass cylinder.19 The PDMS stamp board was laid flat so that it was parallel to the annular waveguide slots rings. The treatment procedures were as follows. The chamber was evacuated to a pressure of about 6 Pa and purged with Ar back to atmospheric pressure, followed by evacuation to 6 Pa again. Argon was introduced at a flow rate of 30 standard cubic centimeters per minute (SCCM) to induce plasma, and then hydrogen was gradually adjusted up to a flow rate of 15 SCCM and mixed with argon. A stable plasma discharge remained for 5 min. During the microwave plasma exposure pretreatment, the chamber pressure was kept less than 60-70 Pa, and the output power was 800 W. After plasma exposure, the PDMS stamps board was transferred and immersed into acrylonitrile solution for 4 h. To remove any absorbed impurities after grafting, PDMS stamps were rinsed with the acetone, ethanol, detergent solution, and deionized water successively, followed by drying at 100 °C for 1 h and cooling to room temperature. With these controlled conditions, the distortion of PDMS stamps by solvent swelling effects was minimal.
3. Experimental Results and Discussions 3.1. Contact Angle. Contact angle measurement is a simple and direct method to characterize the hydrophilicity of sample surfaces. The measurements were made through a sessile droplet on the surface of the samples. In our experiments, the deionized water and acetonitrile droplet (each droplet was 5 µL) were delivered onto 10 separate sites on the PDMS surface by a micropipet, and each measurement was done within 30 s at room temperature (25 °C, 65% relative humidity). The contact angles were measured before and after the microwave plasma treatment, as well as before and after postgrafting. For mixed gas plasma pretreated surfaces, two monomers of acrylamide and acrylonitrile were investigated in our preliminary grafting experiments. In acrylamide grafting, we used aqueous solutions of ceric ion and acrylamide with different concentrations to graft the plasma-pretreated PDMS. Acrylamide homopolymerization dominated in the grafting reaction, and a viscous coverage surrounded the PDMS surface. We identified it as a polyacrylamide by toluidine staining.13 By immersing or washing the PDMS in water or polar solvents such as ethanol, we could remove such viscous depositions with ease. The mean of the contact angles for acrylamidegrafted PDMS was significantly larger than those of acrylonitrile grafting, and its hydrophobicity was rapidly recovered as native PDMS sample. We believe that the viscous layer was a physical deposition of polyacrylamide rather than a chemically grafted layer. Besides, whether the amide or imide groups of polyacrylamide were grafted onto PDMS stamps or not, they are reactive to mononucleotides (T, C, G, A) reagents and cause side reactions if using them in microcontacting reactions for DNA synthesis, so the acrylamide grafting method was given up.
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Table 1. Contact Angle Data and Retrogression along Time for Acrylonitrile Grafting PDMS PDMS sample surface
H 2O
CH3CN
original without washing after extraction and washed after plasma treatment without grafting (in an hour) after plasma treatment without grafting (in a week) after plasma treatment and grafted (in an hour) after plasma treatment and grafted (in a week) after plasma treatment and grafted (in a month)
100 ( 5° 90 ( 5° 70 ( 10° 85 ( 5° 28 ( 12° 35 ( 15° 35 ( 15°
90 ( 5° 90 ( 5° 76 ( 8° 84 ( 10° 17 ( 7° 25 ( 10° 23 ( 13°
Figure 1. ATR-FT-IR spectra from PDMS surfaces: 1, the untreated PDMS; 2, the plasma pretreated but hydroquinone quenched; 3, the plasma pretreated and 20% acrylonitrile grafted; 4, the plasma pretreated and 80% acrylonitrile grafted.
The data collected from acrylonitrile grafting are summarized in Table 1. The untreated PDMS exhibited high hydrophobicity due to the two-methyl groups in its main molecular chains. Little change in contact angle values was observed after the sample was extracted and washed in ether. The immediate measurement for the plasma-treated sample without acrylonitrile grafting showed a decrease of 10-20° in contact angle measurements, but its contact angle was almost recovered to its original value in a week. A remarkable improvement on the hydrophilicity was achieved for the plasma-pretreated and grafted PDMS. The contact angle for the fresh grafted PDMS was as low as 17 ( 7° for acetonitrile droplets. The same results were achieved for deionized water and anhydrous ethanol droplets. The results showed that the plasma pretreated and grafted PDMS stamp surfaces exhibit strong affinity for most polar solvents. The retrogression property of hydrophilicity for above samples was also evaluated, as shown in Table 1. The hydrophilicity of the PDMS surface without grafting was not stable and soon resumed to native hydrophobicity in a week. However, hydrophilicity of the grafted PDMS showed very good durability. The small contact angles remained stable for at least a month at room temperature. 3.2. Infrared Spectrum. Unlike acrylamide grafting, we used tetrahydrofuran-solving acrylonitrile as grafting solution in different concentrations. Preliminary grafting experiments of acrylonitrile gave smaller contact angles (24 ( 7°) and more durable hydrophilicity than acrylamide. The acrylonitrile-grafted stamps have been used for further ATR-FT-IR characterizations to determine whether the appropriate chemical groups were present on the surface of samples. In Figure 1, ATR-FT-IR spectra of four different PDMS surfaces were measured: (a) the untreated PDMS surface simply through solvents rinse, extractions, and drying (curve 1); (b) the plasma pretreated PDMS surface grafted from acrylonitrile solution contain-
ing 200 ppm hydroquinone as quencher (curve 2); (c) the surface of the plasma pretreated and grafted from 20% acrylonitrile solution without hydroquinone (curve 3); (d) the surface grafted from 80% acrylonitrile similar to (c) (curve 4). Curves 1-4 possessed absorption peaks of 2963, 1259, 1085, 1014, 864, and 794 cm-l, which correspond to group vibrations of νa(CH3), δ (-CH2-), νa(CH3), ν(CH3-Si), ν(Si-O-Si), ν(Si-C), and ν[Si-(CH3)2]. These are common characteristic peaks of PDMS. Since hydroquinone was used as a quencher in acrylonitrile solution in sample (b) (curve 2), the PDMS sample (b) showed no distinctive difference from the untreated sample (curve 1). It meant that the hydroquinone quencher could scavenge the desired acrylonitrile grafting onto PDMS. Consequently, few hydrophilic groups of acrylonitrile were grafted onto the PDMS surface and there was no apparent hydrophilic change to its surface. However, the grafted samples (curve 3) from the 20% acrylonitrile solution possessed new absorption peaks of 2323 and 2346 cm-1, suggesting that acrylonitrile molecules were successfully grafted onto the plasma-pretreated PDMS surface, and hydrophilic functionalities of cyano groups (-CN) were formed. Tetrahydrofuran is a good solvent for acrylonitrile, and it is also a low polarity and swelling solvent for PDMS. For acrylonitrile grafting, tetrahydrofuran could enhance both ends of acrylonitrile molecules approaching to the PDMS surface. Therefore, both hydrophilic (cyano group, -CN) and hydrophobic (-CHdCH2) groups of acrylonitrile could be grafted onto the PDMS surface, but only outward grafted cyano groups could improve its hydrophilicity. Probably using a more polar concentration solution could help to form more ordered and orientated grafting for outward cyano groups. It was confirmed by curve 4 in Figure 1 (grafted in 80% acrylonitrile solution). Curve 4 exhibited a strong absorption peak of 2247 cm-1, which is due to the presence of more cyano groups (-CN). Also a new absorption peak of 2879 cm-1 in curve 4 in Figure 1 indicated the formation of more methylene groups (-CH2-) on the PDMS surface, while all surfaces contained the same characteristic adsorption of methyl groups (-CH3) at 2963 cm-1 as displayed from curves 1 to 4. Compared to some weak peaks in curve 3, curve 4 indicates a more orientated formation of cyano groups after grafting. Although acrylonitrile possesses a hydrophilic (cyano group, -CN) and a hydrophobic group (-CHdCH2), acrylonitrile molecules might be assembled in an orderly fashion on the PDMS surface of sample d with the hydrophilic cyano groups in an outward orientation while hydrophobic groups are facing the PDMS surface if there is a more polar acrylonitrile presence in the grafting solution. A stronger polarity of grafting solution is favorable for more orientated grafting, and it is understood according to the molecular interactions theory in which molecules of similar polarity attract one another strongly. Curve 3 and curve 4 imply that more acrylonitrile and less tetrahydrofuran in the grafting solution could enhance
Surface Patterning by PDMS Stamps
Langmuir, Vol. 19, No. 17, 2003 6985 Table 2. XPS Data of PDMS Surfaces
surfacea
C1sb (BE, eV)
O1s (BE, eV)
Si2p (BE, eV)
N1s (BE, eV)
1 2 3 4 5
53.33 (285.3) 60.26 (285.4) 61.10 (284.8) 56.10 (284.7) 71.20 (285.1)
25.68 (532.05) 23.94 (532.65) 24.38 (532.15) 29.12 (532.35) 24.75 (532.45)
20.66 (101.45) 15.45 (102.0) 13.89 (101.5) N.A. (101.6) N.A. (101.6)
0.33 (399.5) 0.34 (401.5) 0.64 (401.6) 14.77 (405.8) 4.05 (402.2)
a PDMS surfaces: 1, untreated; 2, plasma pretreated but hydroquinone quenched in acrylonitrile grafting solution; 3, plasma pretreated and 5% acrylonitrile grafted; 4, plasma pretreated and 20% acrylonitrile grafted; 5, plasma pretreated and 80% acrylonitrile grafted (possible orientated grafting). b Surface elemental composition (%).
Figure 2. XPS survey spectra from PDMS surfaces: 1, the untreated; 2, the plasma pretreated but hydroquinone quenched in acrylonitrile solution; 3, the plasma pretreated and 5% acrylonitrile grafted; 4, the plasma pretreated and 20% acrylonitrile grafted; 5, the plasma pretreated and 80% acrylonitrile grafted (possible orientated grafting).
the grafting orderliness, and the polarity of solvents media played an important role in orientated grafting onto PDMS. 3.3. XPS Spectrum. To investigate the possible mechanism of orientated grafting, XPS spectra were recorded from five kinds of PDMS surfaces, i.e., the untreated PDMS surface, the plasma pretreated surface with acrylonitrile grafting solution containing 200 ppm hydroquinone, and the plasma pretreated surfaces with three different acrylonitrile-grafting solutions (5%, 20%, and 80% acrylonitrile, respectively). The related data are summarized in Table 2. XPS spectra are depicted in Figure 2, and high-resolution XPS scanning spectra of nitrogen are shown in Figure 3. Both the untreated PDMS (curve 1 in Figure 2) and the plasma-pretreated PDMS with hydroquinone (curve 2 in Figure 2) did not show apparent nitrogen peaks near the 400 eV binding energy (BE). Their nitrogen elemental analysis (N1s) values were as little as 0.33% and 0.34%, respectively, as listed in Table 2. Such results could be due to the instrumental errors during analysis. Yet the C/O/Si ratio (53.33%:25.68%:20.66%, shown in Table 2) of the untreated PDMS approximates the theoretical ratio 2:1:1 (calculated from the repeat unit -(CH3)2SiO- of PDMS), which implies the reliability of XPS data. The 5% acrylonitrile grafted PDMS (curve 3 in Figure 2) gives a very weak nitrogen peak, and its nitrogen atom percentage increases to 0.64%, but the elemental ratio (C/O/Si) is similar to the quenched sample (in Table 2). The XPS curve 4 in Figure 2 of the 20% acrylonitrile grafting PDMS sample indicates an apparent nitrogen peak at BE 405.8 eV. It has a relative broad peak in high-
Figure 3. High-resolution XPS scan of nitrogen around its is binding energy (Nls) from PDMS surfaces: 4, 20% acrylonitrile grafted; 5, the plasma pretreated and 80% acrylonitrile grafted (the possible orientated grafted).
resolution XPS spectra of nitrogen, as displayed in Figure 3. The PDMS sample grafted in 80% acrylonitrile exhibits a strong nitrogen peak, as shown in curve 5 in Figure 2, and it is a sharp peak at 402.2 eV for high-resolution XPS spectra in curve 5 in Figure 3. However, the former contained a higher nitrogen percentage (14.77%) than the latter (4.05%), although a larger nitrogen percentage does not mean a relatively better orientated grafting (i.e., outward -CN groups). The former elemental ratio (C/O/ N) approximates to 4/2/1 (56.10%:29.12%:14.77%), and its broad peak near N1s indicates a diversified chemical status of nitrogen like cyano, imine group existence on grafted PDMS surface. Its C/N ratio is as high as nearly 4 (56.10%:14.77%). As we know that the C/N ratio could reach 4 only when ideal grafting occurred (as two methyl groups of PDMS repetitive unit -(CH3)2SiO- are grafted two acrylonitrile molecules). The sharp peak at 402.2 eV of the latter indicates the existence of an orientated grafting of simple outward -CN groups, its C/N/O ratio is only 3:1:0.165 (71.2%:24.75%:4.05%). 3.4. Possible Grafting Mechanism. After microwave plasma pretreatment of H2 and Ar mixed gas, the oxygen elemental ratio of PDMS did not increase and carbon did not decrease. But when PDMS was treated with O2, the C/O ratio was reduced due to the depletion in carbon and the richness in oxygen.2,21 For the sample grafted in 80% acrylonitrile solution, the single strong -CN group absorption peak of 2247 cm-1 seen in the ATR-FT-IR and the sharp peak at 402.2 eV in XPS give good crossexamined evidence that an orientated grafting layer had been formed onto the PDMS surface. For the sample (21) Fakes, D. W.; Newton, J. M.; Watts, J. F.; Edgell, M. J. Surf. Interface Anal. 1987, 10, 416-423.
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grafted in 20% acrylonitrile solution, relatively weak absorptions of multiple peaks in ATR-FT-IR correspond to its broad nitrogen peak in XPS, implying a nonorientated grafting and that both hydrophilic and hydrophobic group of acrylonitrile molecules could be grafted onto PDMS surface and, therefore, implying the occurrence of various chemical states for nitrogen. On the basis of the above discussion, we gave the following possible plasmainduced radical grafting mechanism. When oxygen or other etching gas plasma treated the PDMS surface, the plasma would inevitably cause damage such as burning ablation or erosion, surface damage, or stress and cracks.1,2,5,20 The hydrophilicity of the plasmapretreated surfaces without grafting was temporary, so the hydrophobicity was soon recovered.21 There were two typical mechanisms in plasma polymerization and grafting: the free radical (or ionization) mechanism and the competitive polymerization mechanism.22 In addition, according to investigations by Domenico et al., silicone hydride (Si-H) species could be produced after plasma exposure in the presence of hydrogen and inert gas and could be used for the covalent bonding to other grafting molecules.23 However, this mechanism could not explain the results obtained by using different gases for plasma pretreatment, because no distinctive Si-H was detected in the related ATR-FT-IR spectra for the plasma-treated samples seen in Figure 1, curves 2-4 (Si-H group should exhibit adsorptions of a moderate peak ranging 22202120 cm-1 and strong double peaks ranging 1010-700 cm-1). Another reason is that oxygen composition was nearly unchanged even when the carbon composition changed slightly (see XPS surface elemental analysis in Table 2), which was also contrary to the generation of Si-H bonds (if H replaced -CH3 groups in PDMS, elemental C composition would decrease while O increases). The bond energies of Si-O, C-H, Si-H, Si-C, and C-C follow a descending order of 108, 98, 94, 88, and 85 kcal/mol, respectively. For the PDMS molecular structure, there are no C-C bonds and the Si-H bond is highly reactive and unstable. Therefore, the most possible cleavage tends to occur at the lower energy bonds of Si-C and C-H. They could be activated by plasma to generate considerable radical sites on the surface. In the 1970s, it was discovered that there were free radicals on RTV silicone and polyethylene, and no appreciable decay was found after more than a day through electron spin resonance (ESR).12 Since the first ionization potentials of Ar, H2, and H are 15.8, 15.37, and 13.6 eV, respectively, the higher the ionization potential is, the easier it is to excite the radical species by plasma and the more stable it exists. The chemical mechanism of PDMS grafting was accordant to a radical mechanism. The possible radical mechanism of PDMS grafting is schemed in Figure 4. Plasma Ar was first activated, and the activated Ar* in turn induced H2 into active species when H2 was introduced. H2* would split to hydrogen radicals which were absorbed on the PDMS surfaces generating considerable radical-reactive sites. As the PDMS was immersed in acrylonitrile solution, the grafting began. The more polar the solution was, the more hydrophilic -CN groups were grafted in outward orientation. When some quenching agents were added into the grafting solution, the grafting was terminated. Previous investigations had proved that plasma pretreatment of a polymer surface was a very (22) Yasuda, H. Polym. Prepr. 1978, 19, 491-507. (23) Domenico, E. D.; Stewart, M. T.; Urban, M. W. US Patent 5364662, 1994.
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Figure 4. Plasma-induced acrylonitrile grafting scheme: 1, hydrophobic dimethyl groups on untreated PDMS surface; 2, new-formed radical species after Ar and H2 gas plasma treatment; 3, acrylonitrile grafted PDMS surface; 4, hydrophilic surface with outmost cyano (-CN) groups after orientated acrylonitrile grafting onto PDMS.
complicated process accompanied by cross-linking, scission, rearrangements, and so on.21 The elemental analysis of the plasma-pretreated PDMS with hydroquinone solution was different from that of the untreated PDMS surface, which might be due to the rearrangements and interactions with surroundings on plasma-pretreated PDMS surfaces. Contact angle decrease of PDMS without grafting and its hydrophobicity recovery implied the radical decaying and intermolecular rearrangement. According to the mechanism, it is probable that the introduction of acrylonitrile vapor during plasma pretreatment for PDMS could also graft hydrophilic functionalities onto PDMS surfaces. Further investigation is required. 4. Conclusions In soft lithography related techniques, a polar-molecule compatible and a nonreactive PDMS surface is necessary for patterns transfer, because spontaneous spreading of polar reactive solution should be realized on the patterned PDMS surface to effect a microprinting reaction. In this paper, we have prepared stable hydrophilic PDMS stamps grafted with cyano functionalities (-CN groups) by a twostep procedure using H2 and Ar mixed gas plasma pretreatment and post-acrylonitrile grafting. Using these hydrophilic stamps, we have in situ synthesized oligonucleotide microarrays (3′-GGA CTC TCT GAA TCG GAG GA).8,9 Such a method for preparation of hydrophilic PDMS stamps offered an alternative in improving and controlling surface hydrophilicity of a polymer surface. It is expected that the hydrophilic PDMS stamps will find more applications in microfabrication and nanofabrication, especially in those fabrications requiring reactive multilayer patterns. Acknowledgment. The National Natural Science Foundation of China and the High-tech Project of Jiangsu Province, China, supported this work. LA020785H
