Langmuir 2003, 19, 5475-5483
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Silicone Transfer during Microcontact Printing Karin Glasma¨star,* Julie Gold, Ann-Sofie Andersson, Duncan S. Sutherland, and Bengt Kasemo Department of Applied Physics, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden Received September 16, 2002. In Final Form: April 14, 2003 Microcontact printing (µCP) is a widely used method to make miniaturized patterns on surfaces. In this work, the issue of the possible transfer of stamp material from the stamp to the substrate during stamping was addressed. Poly(dimethylsiloxane) was used to stamp Milli-Q water or buffer on substrates of SiOx, TiO2, and Au. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to measure and characterize the substrate before and after stamping to detect the possible transfer of stamp-related material to the substrates. Both the XPS and the ToF-SIMS analyses show that silicone-related material is transferred from flat stamps and that even more material is transferred from patterned stamps. Interestingly, a UV/ozone treatment (essentially oxidation of the surface) of the stamps before inking and stamping significantly reduces the silicone transfer. Two application examples are used to illustrate the importance of silicone transfer to the substrates during µCP: water condensation patterns and supported lipid bilayer formation.
Introduction Microcontact printing (µCP), or soft lithography, was introduced by Kumar et al. in the early 1990s1,2 as a means for fast and low-cost pattern transfer onto gold films supported on silicon substrates. The technique has since become widely used to pattern surfaces for a variety of research and technical applications.1-6 During µCP, an elastomeric stamp, having relief structures covered (inked) with molecules, is brought into contact with a solid substrate and then removed, leaving a pattern of the “ink” in the areas where the stamp was in contact with the substrate. The conformal contact between the stamp and the surface is ascertained by the interfacial forces and flexibility of the stamp material. Depending on the intended application, the ink used for printing varies from, for example, alkanethiols1-3 to proteins,4-6 and the substrate varies from glass5,6 to metal surfaces.1-4,7 The most common surfaces are gold, glass/SiO2, and silver. Despite the broad variation of inks and substrates, there is one common feature: the stamp is made of an elastomer, usually poly(dimethylsiloxane) (PDMS). PDMS is a hydrophobic polymer known to be very dynamic in terms of the mobility of the polymer chains at the surface.8 This mobility enables hydrophobic or more hydrophilic side groups to be exposed at the surface depending on the environment. The surface of PDMS can be modified by UV/ozone treatment, creating a thin (2030-nm) glassy layer on the surface.9 Such thin glasslike * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (2) Kumar, A.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 1498. (3) Biebuyck, H. A.; Larsen, N. B.; Delamarche, E.; Michel, B. IBM J. Res. Dev. 1997, 41, 159. (4) Tan, J. L.; Tien, J.; Chen, C. S. Langmuir 2002, 18, 519. (5) Wheeler, B. C.; Branch, D. W.; Corey, J. M.; Weyhenmeyer, J. A.; Brewer, G. J. Proceedings of the 19th International Conference-IEEE/ EMBS; Chicago, IL, 1997. (6) Kung, L. A.; Kam, L.; Hovis, J. S.; Boxer, S. G. Langmuir 2000, 16, 6773. (7) Xia, Y.; Kim, E.; Whitesides, G. M. J. Electrochem. Soc. 1996, 143, 1070. (8) Mark, J. E.; Allock, H. R.; West, R. In Inorganic polymers; Prentice Hall Inc.: Englewood Cliffs, NJ, 1992, 141.
layers function as a diffusion barrier and are reported to be quite brittle10,11 compared to bulk PDMS. The use of hydrophobic inks such as alkanethiols in µCP do not normally require any pretreatment of the stamps.2,12 However, if the ink is hydrophilic, for example, water-soluble proteins,13-15 phospholipids,16 or amines,17 the stamp i often oxidized.13,17 The oxidized stamp is claimed to have a higher protein adsorption capability13 and to give higher coupling yields when stamping amines17 than the untreated stamp. A matter of concern, in some situations where µCP is used, is the risk of the transfer of molecules or fragments from the stamp material to the patterned surface. In the case of PDMS, these fragments could be low-molecularweight residues, molecules, molecular fragments, etc. Such transfer could compromise the end result by contaminating the surface, changing the wetting and adhesion properties. There are surprisingly few studies of these aspects in the literature. A few publications report PDMS contamination on the printed substrates,18-20 but these studies were not aiming at studying the transfer process as such or a solution to the problem. In one of these publications,18 a slightly different process than the standard µCP procedure was used (1-6 bar overpressure during the printing of thiols). PDMS was detected, by X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption (9) Ouyang, M.; Yuan, C.; Muisener, R. J.; Boulares, A.; Koberstein, J. T. Chem. Mater. 2000, 12, 1591. (10) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (11) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991. (12) Delamarche, E.; Schmid, H.; Bietsch, A.; Larsen, N. B.; Rothuizen, H.; Michel, B.; Biebuyck, H. J. Phys. Chem. B 1998, 102, 3324. (13) St. John, P. M.; Davis, R.; Cady, N.; Czajka, J.; Batt, C. A.; Craighead, H. G. Anal. Chem. 1998, 70, 1108. (14) Kam, L.; Boxer, S. G. J. Biomed. Mater. Res. 2001, 55, 487. (15) James, C. D.; Davis, R. C.; Kam, L.; Craighead, H. G.; Isaacson, M.; Turner, J. N.; Shain, W. Langmuir 1998, 14, 741. (16) Hovis, J. S.; Boxer, S. G. Langmuir 2001, 17, 3400. (17) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055. (18) Bo¨hm, I.; Lampert, A.; Buck, M.; Eisert, F.; Grunze, M. Appl. Surf. Sci. 1999, 141, 237. (19) Yang, Z. P.; Belu, A. M.; Liebmann-Vinson, A.; Sugg, H.; Chilkoti, A. Langmuir 2000, 16, 7482. (20) Jud, P. P. Semester thesis. ETH Zu¨rich, Zu¨rich, Switzerland, 2001.
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spectroscopy, as a contamination on the printed surface. In another report, Yang et al.19 printed biotinylated aminoterminated oligoethyleneglycol on activated poly(ethylene terephthalate) (PET) surfaces, using stamps treated by oxygen plasma for 10 min. They found silicone in the regions where the stamp had been in contact with the substrate (by time-of-flight secondary ion mass spectrometry, ToF-SIMS). In the third report, nonoxidized stamps were used to print poly(ethylene glycol)-peptide derivatives with flat stamps on TiO2, where the transfer of an Si-containing species to the substrate was shown with XPS.20 To the best of our knowledge, there is only one previous study in the literature, by Graham et al., that examines in any detail the issue of PDMS being transferred to the substrate during the µCP procedure.21 They demonstrate PDMS contamination on gold by XPS [Si(2s)] and ToF-SIMS (positive and negative spectra) after the µCP of thiols. The study was carried out using flat stamps. The authors propose a 1-week-long cleaning process for the stamps to lower the amount of transfer to below the detection limit of XPS. This treatment was suggested to remove low-molecular-weight fragments. There are several pending questions regarding the possible transfer of molecules/fragments from PDMS stamps to the substrate. To what extent does the amount of the transferred contaminant material depend on the pretreatment of the stamp or material onto which the pattern is stamped? Does the type of ink molecules used or size of the pattern affect the amount being transferred? Does the transfer influence the quality of the stamped patterns? Such questions, some of them arising during an application of µCP in our laboratory, motivated us to make a more detailed study of this issue, focusing on the role of the treatment of the stamp preceding inking and stamping. Questions regarding the influence of the type of ink molecules (e.g., competitive adsorption) were not addressed in this work. We used XPS and ToF-SIMS to measure the amount and character of PDMS transfer from flat and patterned stamps to substrate surfaces of gold, TiO2, and SiOx. The µCP process was performed using water or buffer as the ink and without external pressure. We explored whether a UV/ozone pretreatment of the PDMS stamp could reduce the amount of transferred material. The choice of SiOx as one of the substrates was motivated by the many applications of µCP on glass substrates. TiO2 and gold surfaces were chosen for two reasons. First, there was a desire to have more than one surface (chemistry) for comparative purposes, and second, XPS detection of Sicontaining molecules (transferred from PDMS) is much easier on a surface not containing Si as an element. Materials and Methods Surface Preparation. All substrates for µCP were prepared from (100)-oriented polished silicon wafers (Siltronix, Archamps, France). Wafers protected by a sacrificial poly(methyl methacrylate) (PMMA) layer were partially cut using a diamond saw (Loadpoint Microace 3+). PMMA was then removed by sequential ultrasonic agitation in acetone, 2-propanol, and Milli-Q water. Thin films of TiO2 and Au were deposited on some of the wafers by electron-beam-induced thermal evaporation (AVAC HVC-600). The TiO2 surfaces were prepared by first evaporating a 40-nmthick film, which was then oxidized to saturation by active oxygen treatment (O2 RF plasma: 0.5 Torr; 200 W; 120 s).22 The gold (21) Graham, D. J.; Price, D. D.; Ratner, B. D. Langmuir 2002, 18, 1518. (22) Aronsson, B. O.; Lausmaa, J.; Kasemo, B. J. Biomed. Mater. Res. 1997, 35, 49.
Glasma¨ star et al.
Figure 1. Schematic of the procedure of µCP used in this work. A stamp is cast from a master. The stamp is soaked in ink (Milli-Q or PBS) for 1 min, rinsed (Milli-Q only or PBS followed by Milli-Q), and dried in a stream of N2. The stamp is “rolled” over the sample and left in contact for 3 min before it is rolled off. No external pressure is applied. coatings were produced by first covering the surface with a thin adhesion layer of Ti, followed by the evaporation of 30 or 40 nm of Au. Uncoated silicon wafer substrates (with thin native oxide layers of ∼1-2 nm) were used as the model SiO2 substrates, hereafter referred to as SiOx. The individual samples were released from the wafer by breaking along the partially cut lines. Before use, all the samples were blown with nitrogen gas to remove particulate contaminations, which was followed by exposure to UV/ozone for 5 min to remove hydrocarbon contaminations. µCP. Stamps for µCP were formed by curing PDMS (Sylgard 184, Dow Corning, Brussels, or Rhodorsil RTV 1556 A+B, Sikema AB, Stockholm) against a master (room temperature, >24 h). The Sylgard 184 was chosen because it is the most common PDMS in the µCP community. The master for the flat stamps was an Au-coated silicon wafer. Patterned masters (lines or squares) were made by standard photolithography techniques. The line patterns were 5-µm-wide lines (in contact during stamping) separated by 15-µm-wide spaces. The square patterns were a grid of 10-µm-wide, elevated lines. The squares between the lines were 10 by 10 µm and were not in contact with the surface during stamping. The µCP procedure used is illustrated in Figure 1. The stamps were used either as prepared or after UV/ozone treatment for 10 min. Because the prime interest in this work was to investigate the possible transfer of material from the PDMS stamp during the patterning of water-soluble molecules, such as proteins, the ink chosen should be representative of the bulk solutions used in such µCP processes. The two model “inks” were Milli-Q water and phosphate buffer saline (PBS, Sigma-Aldrich). XPS Analysis. All the samples for the XPS analysis were mounted in the vacuum chamber within 20 min after preparation (i.e., after stamping or surface pretreatment). All the XPS spectra were recorded using monochromatic Al KR radiation, with a takeoff angle of 45° from the sample surface normal (PHI 5500C MultiTechnique system). The survey spectra were recorded at a pass energy of 187.85 eV, and detailed spectra of the individual elements were recorded at 5.85 eV. The survey spectra from two spots on each sample were used for the calculation of their atomic composition at the surface, and one detailed spectrum was recorded to obtain chemical-shift information (at least two samples). The X-ray-beam spot diameter was about 1 mm. In one experiment, flat stamps (untreated or UV/ozone pretreated) were covered with a thin Al foil (approximately 13µm thick), which had a square (5 × 5 mm) hole. A gold sample was positioned on top of the foil for 3 min, ensuring noncontact between the stamp and the surface but allowing possible vapors from the PDMS stamp to reach the gold surface. This experiment was performed to control any transfer of molecules from the PDMS stamp to the substrate via the gas phase, that is, from the regions of the stamp that are not in direct contact with the substrate.
Silicone Transfer
Langmuir, Vol. 19, No. 13, 2003 5477 Table 1. Atomic Composition by XPS Survey Spectra of PDMS Stampsa PDMS sample
C(1s)
O(1s)
Si(2p)
untreated treated
42.8 ( 0.3 36.4 ( 0.1
31.4 ( 1.0 38.5 ( 0.5
25.8 ( 0.7 25.1 ( 0.6
a Mean values are noted in percent plus or minus the standard deviation from two data points per sample type. PDMS was Sylgard 184 elastomer, untreated or UV/ozone-treated (10 min).
nitrobenz-2-oxa-1,3-diazol-4-yl)amino]dodecanoyl-1-hexadecanoylsn-glycero-3-phosphocholine (NBD-HPC; Molecular Probes, U.S.A.). Vesicles produced by this method have an average diameter of 40-60 nm (by dynamic light scattering). Glass coverslips (Merck) were cleaned by sequential ultrasonic agitation in acetone, 2-propanol, and Milli-Q water, dried, and UV/ozonetreated for 10 min. The cleaned coverslips were then stamped and subsequently incubated in a solution of vesicles (30 µL in 1 mL of buffer) for 10 min, rinsed with buffer, and imaged using an Axioplan 2 Imaging epifluorescence microscope (Zeiss).
Figure 2. High-resolution XPS Si(2p) spectra from PDMS stamps (Sylgard 184 elastomer), untreated or UV/ozone-treated. The curves are displaced along the y axis for clarity and are normalized with respect to the total signal. The spectra were recorded using an electron gun to compensate for the charging and were later shifted to C(1s) ) 284.4 eV for a charging reference. The arrow points at a shoulder, indicating the presence of more oxidized species. The XPS analysis of PDMS (Sylgard 184 elastomer) stamps were performed using equipment and settings the same as those described previously, with the addition of an electron flood gun to compensate for the charging. The electron gun was adjusted so that the O(1s) signal of PDMS was at about 532 eV. A result of the charging neutralization process was that the total intensity varied between the samples. Therefore, we chose to normalize the curves of Figure 2 with respect to the total signal. The spectra shown in Figures 2 and 3 were corrected for charging effects by shifting the peaks as follows: PDMS spectra were shifted so that the binding energy (BE) for C(1s) was 284.4 eV; spectra from the Au samples were shifted to Au(4f7/2) ) 84.0 eV; spectra from the SiOx samples were shifted to Si(2p3/2) ) 99.3 eV; and spectra from the TiO2 samples were shifted to Ti(2p3/2) ) 458.8 eV. ToF-SIMS Analysis. The ToF-SIMS analyses were performed using a ToF-SIMS IV (ION-TOF GmbH) system in static mode,23 that is, the ion dose was below 1013 ions/cm2. Gallium ions (25 kV; 1 pA) were used as the primary beam. Both high mass resolution spectra (m/∆m ) 7000) and high lateral resolution images (
