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Preparation and Photoinduced Patterning of Azidoformate-Terminated Self-Assembled Monolayers Supavadee Monsathaporn and Franz Effenberger* Institut fu¨ r Organische Chemie, Universita¨ t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany Received July 29, 2004. In Final Form: September 30, 2004 This paper presents a new method for the patterning of self-assembled monolayers (SAMs) using UV light. Azidoformate-terminated SAMs starting from 18-acetoxy-octadecyltrichlorosilane SAMs on silicon, prepared for the first time, are electrophilic and photosensitive, and can be patterned by UV irradiation through a mask. The resulting structured surfaces are still electrophilic and can be reacted with nucleophilic functions, for example, primary amines.
1. Introduction Patterning of self-assembled monolayers (SAMs) has been intensively studied because of the possibility of producing versatile complex surfaces which have a high potential in technological applications. For instance, the micropatterning can be used to prepare organic thin-film transistors and1 light-emitting diodes2 and to control cell distribution.3 To create such features, diverse patterning techniques have been developed. Apart from photolithography,4 which is still the most widely used method, microcontact printing,5 microfluidic network,6 electron beam,7 and scanning probe lithography8 are used. Whereas the attempts in developing microelectronic devices are still directed mainly toward smaller and smaller structures, the interest in devices for biological systems is more centered on the search of structured surfaces with higher selectivity and reactivity for attachment of biomolecules and for the preparation of bioassays.9 Most of the functionalized SAMs applied today are terminated by alkyl and nucleophilic groups (e.g., hydroxyl, amino, carboxylate). The interaction of biomolecules possessing mainly nucleophilic functions with these monolayers, therefore, is normally only physisorption but not chemical bonding, which in many cases results in only loose coupling and nonoriented placement of biomolecules. The covalent coupling of biomolecules to SAMs is usually * Author to whom correspondence should be addressed. Phone: (711) 6854265. Fax: (711) 6854269. E-mail: franz.effenberger@ oc.uni-stuttgart.de. (1) Gooding, J. J.; Praig, V. G.; Hall, E. A. H. Anal. Chem. 1998, 70, 2396. (2) Kagan, C. R.; Breen, T. L.; Kosbar, L. L. Appl. Phys. Lett. 2001, 79, 3536. (3) Singhvi, R.; Kumar, A.; Lopez, G. P.; Stephanopoulos, G. N.; Wang, D. I. C.; Whitesides, G. M. Ingber, D. E. Science 1994, 264, 696. (4) (a) Okasaki, S. J. J. Vac. Sci. Technol., B 1991, 9, 2829. (b) Calvert, J. M. Thin Films 1995, 20, 109. (5) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (6) Delamarche, E.; Bernard, A.; Schmid, H.; Bietsch, A.; Michel, B.; Biebuyck, H. J. Am. Chem. Soc. 1998, 120, 500. (7) (a) Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. (b) Sondag-Huethorst, J. A. M.; Van Helleputte, H. R. J.; Fokkink, L. G. J. Appl. Phys. Lett. 1994, 64, 285. (c) Schmelmer, U.; Jordan, R.; Geyer, W.; Eck, W.; Go¨lzha¨user, A.; Grunze, M.; Ulman, A. Angew. Chem., Int. Ed. 2003, 42, 559. (8) (a) Ross, C. B.; Sun, L.; Crooks, R. M. Langmuir 1993, 9, 632. (b) Lim, J.-H.; Ginger, D. S.; Lee, K.-B.; Heo, J.; Nam, J.-M.; Mirkin, C. A. Angew. Chem., Int. Ed. 2003, 42, 2309. (c) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808. (9) (a) Pirrung, M. C. Angew. Chem., Int. Ed. 2002, 41, 1276. (b) Hyun, J.; Ahn, S. J.; Lee, W. K.; Chilkoti, A.; Zauscher, S. Nano Lett. 2002, 2, 1203.
the method of choice to circumvent this problem if long operational performance is required. For this purpose either the biomolecules or the surface termination has to become electrophilic. Electrophilic polychloromethylstyrene thin films have been prepared, for example, by oxidation of chloromethyl groups to aldehydes10 or by the attachment of a bifunctional electrophilic linker on aminoterminated SAMs.11 Electrophilic biomolecules, for example, enzymes, were obtained, for instance, by chemical oxidation of glucose oxidase (GOx) carbohydrate residues, followed by coupling the resulting “aldehydic” enzyme with amino-terminated SAMs.12 In this paper, we focus on a new approach to create structured electrophilic SAMs as platforms for biosensors. The patterning method relies on the photoinduced reaction of azidoformate-terminated monolayers on silicon for two reasons: first, the azidoformate function is electrophilic and presents a well-established method to covalently attach nucleophilic parts of biomolecules;13 second, azidoformates decompose by UV irradiation, yielding the corresponding nitrenes as reaction intermediates, which can subsequently react to give carbamates by H abstraction from the solvent.14 2. Experimental Section Prior to SAM preparation, the silicon wafers were cleaned by chemical treatment in a freshly prepared mixture of concentrated H2SO4 and 30% H2O2 [70:30 (v/v)] at 80 °C for 1 h, followed by an extensive rinse with Millipore water, and finally blown dry in a stream of nitrogen. The preparation of SAM A (Scheme 1) was performed by immersing the cleaned Si(100) wafer for 18 h in a 0.5% (v/v) solution of 18-acetoxyoctadecyl-1-trichlorosilane (AcO-OTS) in toluene. To remove excess polymer, the samples were rinsed with CH2Cl2 and wiped with a soft 100% cotton cloth which was dipped in CH2Cl2. The acetoxy-terminated SAMs (A in Scheme 1) were immersed in a solution of LiAlH4 in tetrahydrofuran (THF) for 15 min, then into water for 1 min, and finally into 10% HCl for 2 min. Subsequently, the covered wafers were washed by an extensive rinse with Millipore water and blown dry in a stream of nitrogen. The hydroxy-terminated SAMs obtained (B in Scheme 1) were (10) Brandow, S. L.; Chen, M.-S.; Fertig, S. J.; Chrisey, L. A.; Dulcey, C. S.; Dressick, W. J. Chem.sEur. J. 2001, 7, 4495. (11) Chung, C. L.; Camarero, J. A.; Woods, B. W.; Lin, T.; Johnson, J. E.; De Yoreo, J. J. J. Am. Chem. Soc. 2003, 125, 6848. (12) Nakano, K.; Doi, K.; Tamura, K.; Katsumi, Y.; Tazaki, M. Chem. Commun. 2003, 1544. (13) Honda, I.; Shimonishi, Y.; Sakakibara, S. Bull. Chem. Soc. Jpn. 1967, 40, 2415. (14) Kreher, R.; Bockhorn, G. H. Angew. Chem., Int. Ed. Engl. 1964, 3, 589.
10.1021/la048080y CCC: $27.50 © 2004 American Chemical Society Published on Web 10/27/2004
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Scheme 1. Transformation of an Acetoxy-Terminated into an Azidoformate-Terminated SAM and Its Follow-Up Reactions
placed in a cylindrical flask equipped with a gas inlet tube and sealed with a rubber septum. The flask was evacuated and purged with nitrogen. A total of 10 mL of toluene and 1 mL of 20% phosgene solution in toluene were given through syringes. After 18 h the samples were removed from the phosgene solution and rinsed with CH2Cl2. The chloroformate-terminated SAMs (C in Scheme 1) were then used and characterized without further treatment. To prepare azidoformate-terminated SAMs (D in Scheme 1), a fresh solution containing 0.5 g of sodium azide, 15 mL of acetone, and 3 mL of water was prepared in a cylindrical flask equipped with a magnetic stirring bar, and the chloroformate-terminated SAMs C were placed in the flask for 1 h. Then the samples were removed and rinsed with Millipore water. The azidoformate-terminated SAMs D prepared were placed in a 0.01 M solution of stearylamine in CCl4 for 18 h to receive the double layer F (Scheme 1). The wafers were finally thoroughly rinsed with CH2Cl2 and wiped with a soft cotton cloth dipped in CH2Cl2. Transmission Fourier transform infrared (FTIR) spectra were obtained by using a Fourier transform spectrometer (model IFS66V, Bruker). Spectra were run in a dry atmosphere after evacuating the sample chamber with a vacuum pump and purging with dried and filtered air. They were referenced to background spectra previously determined from cleaned silicon samples from the same wafer. Spectra for an individual monolayer in the bilayer sample F were taken by subtracting the spectra of the monolayer sample obtained prior to deposition of the second layer. Contact angles were measured by using a Dataphysics OCA 20 contact angle measurement system at ambient temperature. Reported values are averages of four measurements taken at different points on the surface. An atomic force microscopy (AFM) image of the surface with patterned SAMs was created in contact mode with a Molecular Imaging operating as an atomic force microscope driven by commercial software (Picoscan 5.0). A UV lamp (Heraeus, type TQ150, 150-W mercury tube) was used to perform the photolysis.
3. Results and Discussion Our investigations were carried out starting from AcOOTS, which gives highly ordered monolayers on silicon(100) (SAM A in Scheme 1).15a The synthesis of this longchain compound was performed by reactions of an alkenyl Grignard and a dibromoalkane catalyzed by Li2CuCl4, followed by nucleophilic substitution of the bromo function by potassium acetate and subsequent hydrosilylation.15,16 From the FTIR-absorption data, a coverage of 8799% was calculated for SAM A according to the literature.17 The reduction of the ester-terminated SAM A to the corresponding hydroxy-terminated SAM B was achieved quantitatively with a solution of LiAlH4 in THF.18 Immersing the hydroxy-terminated SAM B in a solution of phosgene in toluene resulted in the formation of the chloroformate-terminated SAM C. The phosgene solution is not difficult to handle when using a needle and carrying out the reaction in a hood. This preparation of an acid chloride functionalized surface was performed in the liquid phase, which is more efficient than that in the gas phase using oxalyl chloride.19 The resulting SAM C is highly electrophilic and reacts easily with nucleophiles.20 With (15) (a) Schu¨tz, M. Dissertation, Universita¨t Stuttgart, Stuttgart, Germany, 2002. (b) Holberg, S. Dissertation, Universita¨t Stuttgart, Stuttgart, Germany, 2000. (c) Bidlingmaier, B. Dissertation, Universita¨t Stuttgart, Stuttgart, Germany, 1999. (16) Johnson, D. K.; Donhoe, J.; Korg, J. Synth. Commun. 1994, 24, 1557. (17) (a) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (b) Effenberger, F.; Go¨tz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462. (18) (a) Pomerantz, M.; Segmu¨ller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132, 153. (b) Tillman, N.; Penner, T. L.; Ulman, A. Langmuir 1989, 5, 101.
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Table 1. Transmission FTIR Data and Relative Coverage of the Differently Terminated SAMs a SAMs: -OSi(CH2)18X A: B: C: D: E: F:
X ) OAc X ) OH X ) OCOCl X ) OCON3 X ) OCONH2 X) OCONH(CH2)17CH3
νas, νs(CH2) [cm-1] 2917.0, 2849.8 2917.2, 2849.7 2918.0, 2850.1 2917.2, 2849.6 2919.1, 2850.4 2917.9, 2850.3
area νas, coverageb ν(CO) νs(CH2) [%] [cm-1] 0.089 93 0.094 65 0.072 32 0.087 09 0.066 96 0.071 41
98 103 79 95 73 78c
1744.5 1773.3 1733.3 1705.2 1690.8
a The letters A-F correspond to the SAMs represented in Scheme 1. b The coverage is based on OTS films (X ) H) with 100% coverage.17 c Measured area and coverage values concern only the second layer and were obtained by subtracting the values of the first layer.
sodium azide in a water and acetone solution, SAM C reacts under chloride exchange to the azidoformateterminated SAM D. The reactions demonstrated in Scheme 1 have been followed and can reliably be characterized by FTIR measurements.17 In Table 1, the carbonyl vibrations of 1773, 1733, and 1705 cm-1 are detected for -OCOCl, -OCON3, and -OCONH2, respectively.21 These values confirm the chemical transformations of the terminal groups. The percentage of coverage is calculated from the νas(CH2) and νs(CH2) IR absorption bands.17 The reproducible lower coverage of SAM C is probably a result of decreasing absorbance of the CH2 group adjacent to -OCOCl. When azidoformate-terminated SAMs D, showing an advancing water contact angle of 69°, are immersed overnight in a solution of stearylamine in CCl4, the contact angle rises to 103°. Ulman reported a typical value for alkyl-terminated SAMs of 113°.22 According to IR data, a quantitative reaction of the electrophilic azidoformateterminated SAM D with the amine to the bilayer F can be deduced (82% yield). We can deduce from the position of the CH2 vibration frequencies (2917.9 and 2850.3 cm-1) that the generated second layer is also well packed. Figure 1 presents a comparison of the FTIR spectra of SAMs from AcO-OTS (spectrum a) reduction with LiAlH4 to the OH-terminated SAM (spectrum b), subsequent phosgenation to the chloroformate-terminated SAM (spectrum c), reaction of the chloroformate-terminated SAM with sodium azide, and the follow-up reaction of the azidoformate-terminated SAM (spectrum d) with stearylamine into the methyl-terminated bilayer SAM (spectrum e). The shift of the carbonyl bands is significant and characteristic for each SAM. Very weak valence vibrations of the azide group at 2181 (νas) and 2141 cm-1 (νs) could be seen (Figure 1d). The disappearance of the azide vibrations and the new strong carbonyl stretching frequency at 1691 cm-1 are supportive of the successful chemical transformation of the terminal group, that is, the formation of the -OCONHC18H37-terminated SAM (Figure 1e). Furthermore, three additional peaks at 3328, 2958, and 1534 cm-1 corresponding to NH-, CH3-, and C(O)-NH- vibrations, respectively, were observed. These IR results are in agreement with the literature.21 (19) Husseini, G. A.; Niederhauser, T. L.; Peacock, J. G.; Vernon, M. R.; Lua, Y.-Y.; Asplund, M. C.; Sevy, E. T.; Linford, M. R. Langmuir 2003, 19, 4856. (20) Monsathaporn, S. Dissertation, Universita¨t Stuttgart, Stuttgart, Germany, 2004. (21) Weidlein, J.; Mu¨ller, U.; Dehnicke, K. Schwingungsspektroskopie, 2. Auflage; Georg Thieme Verlag: Stuttgart, 1988; Chapter 5, 8. (22) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991; Chapter 3.
Figure 1. Transmission FTIR spectra showing the region between 3500 and 1300 cm-1 of various SAMs of -O-Si(CH2)18X on Si(100); X ) (a) OAc, (b) OH, (c) OCOCl, (d) OCON3, and (e) OCONH(CH2)17CH3.
Exposure of SAM D to UV light in the presence of cyclohexane gives exclusively hydrogen abstraction forming the carbamate-terminated SAM E (Scheme 1). The photolysis and thermolysis of azidoformates have been investigated thoroughly.14,23,24 It has been postulated that irradiation of azidoformates gives carbalkoxynitrenes, that is, short-lived intermediates containing electron-deficient nitrogen. A number of investigations present evidence for the formation of these intermediates.23,24 Kreher and Bockhorn reported that azidoformate is reduced into carbamate with a 50-97% yield by photolysis in primary alcohols, for instance, methanol and ethanol.14 This hydrogen abstraction product presumably arises from the triplet nitrene. Its absolute yield can be decreased by compounds that are potential radical inhibitors, for example, nitrobenzene and hydroquinone.23c,b Using tetrachloroethylene as a triplet trap, Breslow et al. were able to reduce the yield of carbamate to 0.9%. Mainly, intra- and intermolecular C-H insertion products are obtained by photolysis and thermolysis in cyclohexane, cyclopentane, 3-methylhexane, or cyclohexene solutions.23a Six-membered rings are the preferred products of longchain alkyl azidoformates caused by intramolecular C-H insertions. N-Cyclohexylurethane, the intermolecular C-H insertion product, is obtained exclusively if cyclohexane as a solvent is used.24b In contrast to the literature,23,24 we found that C-H insertions into the secondary C-H bond of cyclohexane or into the alkyl chains of SAMs did not occur at all by irradiation of SAM D with UV light. The only product we obtained results from hydrogen abstraction whereby the carbamate-terminated SAM E is exclusively formed. This conclusion can be deduced from the FTIR data summarized in Table 1. The highly ordered and well-packed crystal-like structure of SAM D probably inhibits the intramolecular C-H insertion as well as the C-H insertion into cyclohexane. The photolytic behavior of azidoformate-terminated SAMs described so far can be exploited for patterning of SAM D. By irradiation with UV light through a mask in the presence of cyclohexane, the region exposed to light is transformed to the corresponding “unreactive” car(23) (a) Prosser, T. J.; Marcantonio, A. F.; Genge, C. A.; Breslow, D. S. Tetrahedron Lett. 1964, 36, 2483. (b) Breslow, D. S.; Prosser, T. J.; Marcantonio, A. F.; Genge, C. A. J. Am. Chem. Soc. 1967, 89, 2384. (c) Breslow, D. S.; Edwards, E. I. Tetrahedron Lett. 1967, 22, 2123. (24) (a) Breslow, D. S. In Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic Press: New York, 1984; Chapter 10. (b) Lwowski, W. In Nitrenes; Lwowski, W., Ed.; Interscience Publishers: New York, 1970; Chapter 6.
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Scheme 2. Patterning of Azidoformate-Terminated SAM D by UV Irradiation through a Mask for 30 min with 150 W
Figure 2. (a) Optical micrograph of condensed H2O on a patterned SAM on Si(100). The darker region corresponds to the SAM terminated by -OCONH-C17H34CH3, whereas the bright region represents the -OCONH2-terminated SAM. The upper left part shows the employed mask. (b) AFM image of the same sample showing an ending part of the pattern.
bamate, whereas the unexposed area remains as electrophilic azidoformate, which can be subsequently reacted with nucleophiles, for example, stearylamine (Scheme 2). In Figure 2a, patterning of azidoformate-terminated SAMs is demonstrated. The pattern is visualized through an optical microscope after condensing small water droplets on the surface. The darker, nonirradiated regions represent areas terminated by -OCONHC17H34CH3 obtained after irradiation and reaction of the unchanged part with stearylamine. The brightly shining areas are the -OCONH2-terminated parts of the monolayer resulting from H-abstraction of the nitrene intermediate formed by irradiation. The same patterned SAM was imaged by AFM, showing the ending of one of the miniscule structures (Figure 2b). The differences in the interaction between the cantilever tip and the surface give the contrast between regions terminated by -OCONH2 (brighter pixels) and -OCONH-C17H34CH3 (darker pixels). 4. Conclusions For the first time, an azidoformate-terminated SAM on silicon could be prepared by chemical transformations of the known, corresponding acetoxy-terminated SAM. All
reactions occur almost quantitatively and under preservation of a high order of the SAMs. Azidoformateterminated SAMs are electrophilic and photolabile. The generation of patterned electrophilic surfaces is possible by simple photoinduced decomposition of azidoformateterminated SAMs through a mask. This new type of photoinduced patterning requires in contrast to normal photolithographic processes only one step for completion to get a still reactive electrophilic patterned surface. Because the azidoformate function is only negligibly watersensitive, these results encourage for selective, patterned immobilization of biomolecules in aqueous solution. The azidoformate monolayers have been successfully applied to covalently attach GOx and horseradish peroxidase.20 Investigations on this topic are in progress in collaboration with Philippe Allongue et al. at the Ecole Polytechnique, Paris.25 Future work with azidoformate-terminated SAMs will focus on improving surface patterning through more sophisticated masks and optics and studying the reactivity of the patterned and unpatterned surfaces with biomolecules. Acknowledgment. We appreciate the contributions of Steffen Maisch for the optical visualization of patterning and Philippe Allongue and Catherine Henry de Villeneuve for support of the AFM measurements. This work was supported by the Fonds der Chemischen Industrie. Supporting Information Available: Details of the synthesis of 18-acetoxyoctadecyl-1-trichlorosilane 18-acetoxy1-octadecene. This material is available via the Internet at http://pubs.acs.org. LA048080Y (25) Monsathaporn, S.; Effenberger, F.; Allongue, P.; Henry de Villneuve, C. Publication in preparation.
