Modification of Porous Silicon Surfaces with Activated Ester

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Modification of Porous Silicon Surfaces with Activated Ester Monolayers James T. C. Wojtyk, Kim A. Morin, Rabah Boukherroub, and Danial D. M. Wayner* Steacie Instiute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada Received September 27, 2001. In Final Form: April 9, 2002 A series of activated esters of undecylenic acid with activated ester end groups (3-nitrophenyl, N-succinimidyl, S-ethyl, and 2,2,2-trifluoroethyl) have been synthesized and covalently linked to a porous silicon (PSi) surface via a thermal hydrosilylation reaction. Diffuse reflectance infrared Fourier transform (DRIFT) or transmission FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize the monolayers and to follow the course of reactions with primary amines in aqueous solution. It was found that the reaction of the 3-nitrophenyl undecylenate with the PSi surface resulted in significant oxidation of the surface. A control reaction with nitrobenzene confirmed that the arylnitro group was responsible for the surface oxidation and grafting of the nitroaromatic on the surface via a Si-O-N linkage. The N-succinimidyl (NHS) and thioethyl activated ester-modified surfaces formed monolayers with no apparent oxidation of the PSi surface. However, we find that the hydrophobic end groups prevent wetting of the porous structure and further reaction is not possible. Reactions of these groups could be achieved by the addition of 25% (v/v) of ethanol to the aqueous solution of amine, electrochemical partial oxidation of the surface, or preparation of mixed monolayers consisting of mixtures of undecylenic acid and the activated undecylenate ester in a ratio of 5:1 or higher. Under these conditions the NHS group was displaced by ethanolamine or n-butylamine. As expected, exposure of primary amines to the activated S-ethyl thioester did not result in any reaction. However, amide bond formation was observed upon reaction of the methyl ester of cysteine illustrating specificity of this activating group toward cysteine-terminated proteins.

Introduction The possibility of using well-established microfabrication methods for the integration of diverse chemical and biochemical functionality into microelectronics platforms for applications in genomics and proteomics1-5 or for the construction of libraries of complex molecules6-8 has naturally led to research efforts aimed at immobilizing biomolecules on semiconductor surfaces. While established solid phase synthesis protocols provide a starting point for the development of new surface chemistry, there are some key differences in the criteria for optimizing semiconductor surface chemistry suitable for the fabrication of a viable device. Many of the criteria are similar. Chemical approaches to functional group manipulation must allow for the covalent immobilization of biomolecules under physiological conditions that are both reproducible and predictable. Immobilization of biomolecules on semiconductor surfaces imposes other important criteria if one wishes to retain or manipulate the electronic properties of the semiconductor itself. Surface attachment cannot generate electronic defects at the semiconductor/organic interface which tend to pin the Fermi energy and degrade the performance of the device. The surfaces must remain protected against defect formation (usually oxidation) in biological or other aqueous media where binding experiments are carried out. Finally, approaches should allow

spatial control of the surface chemistry at least on the micrometer scale but eventually at the molecular level. Porous silicon (PSi) is a well-characterized substrate that has been widely investigated for a number of electronics, optoelectronics, and sensor applications.9-11 The porous layer is a large surface area matrix that is prepared by chemical and/or electrochemical etching of single-crystal Si wafer in HF-based solutions. One of the benefits in using porous silicon as a biosensor substrate is the ability to control the size of the pores by choosing the appropriate etching parameters. Recently, Sailor and co-workers developed a sensitive label-free biosensor that exploits the optical properties of the porous silicon layer for transduction of biomolecular interactions.12-14 The immobilization of proteins on the surface was a five step synthetic procedure in which biotinylated bovine serum albumin was initially covalently linked to an oxidized PSi surface. The biotinylated proteins were then immobilized on an intervening streptavedin layer. The detection limit of these sensors has been estimated, experimentally12,14 and theoretically,15 to be on the order of ∼1-10 ng/mm2 of porous material. Sailor and co-workers oxidize the PSi prior to immobilization to make use of straightforward methods to immobilize molecules on silica surfaces. However, the recent development of a number of approaches to im-

(1) Wildt, R. M. T. d.; Mundy, C. R.; Gorick, B. D.; Tomlinson, I. M. Nat. Biotechnol. 2000, 18, 989. (2) MacBeath, G.; Schreiber, S. L. Science 2000, 289, 1760. (3) Chee, M.; Yang, R.; Hubbell, E.; Berno, A.; Huang, X. C.; Stern, D.; Winkler, J.; Lockhart, D. J.; Morris, M. S.; Fodor, S. P. A. Science 1996, 274, 610. (4) Lockhart, D. J.; Winzeler, E. A. Nature 2000, 405, 827. (5) Young, R. A. Cell 2000, 102, 9. (6) Crews, C. M.; Spliftgerber, U. Trends Biochem. Sci. 1999, 24, 317. (7) Bishop, A.; Buzko, O.; Heyeck-Dumas, S.; Jung, I.; Kraybill, B.; Liu, Y.; Shah, K.; Ulrich, S.; Witucki, L.; Yang, F.; Zhang, C.; Shokat, K. M. Annu. Rev. Biophys. Biomol. Struct. 2000, 29, 577. (8) Schreiber, S. L. Bioorg. Med. Chem. 1998, 6, 1127.

(9) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (10) Canham, L. T. Ed. Properties of Porous Silicon; INSPEC: London, 1997; Vol. 18. (11) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. Appl. Phys. 1997, 82, 909. (12) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925. (13) Janshoff, A.; Dancil, K.-P. S.; Steinem, C.; Greiner, D. P.; Lin, V. S.-Y.; Gurtner, C.; Motesharei, K.; Sailor, M. J.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 12108. (14) Lin, V. S.-Y.; Motesharei, K.; Dancil, K.-P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840. (15) Caide, X.; Boukherroub, R.; Wojtyk, J. T. C.; Wayner, D. D. M.; Luong, J. H. T. Langmuir 2002, 18, 4165.

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mobilize organic molecules directly on the as anodized, hydrogen-terminated surface may obviate the need to carry out this step.16-27 This can only be true if the organically modified PSi surfaces are stable in the aqueous environments where the chemistry and sensing take place. Freshly etched PSi is hydrogen-terminated and rapidly oxidized in aqueous media. However, it is possible to modify the PSi-H surface with a monolayer thick organic film by thermal,28-30 photochemical,31 or catalyzed reaction32-34 with alkenes, alkynes, and aldehydes. These organically modified surfaces are extremely stable in aqueous environments. Canham et al. examined the stability of some of these derivatized PSi surfaces in simulated biological fluids and concluded that the organic monolayer dramatically increased its stability and reduced the corrosion rate.35 While the reaction with n-alkenes provides a simple route for the passivation of the silicon surface, the incorporation of biomolecules into the monolayers is not straightforward since the terminal-methyl group is essentially inert. Chidsey and co-workers used a radical initiated process to generate sulfonyl chloride groups on the surface which reacted with a wide range of amines to form sulfonamides.20 Boukherroub and Wayner,22 Sudholdter and co-workers,24 and Smith and co-workers36 reacted ester terminated alkenes with Si(111)H to form an alkyl monolayer which was able to react with a variety of standard chemical reagents that allowed functional group interconversion. For example, hydrolysis of an esterterminated surface in dilute acid led to a carboxylic acid modified surface that could be coupled to biologically relevant molecules using approaches developed for solid phase synthesis37 or for aqueous peptide synthesis.38,39 (16) Cicero, R. L.; Wagner, P.; Linford, M. R.; Hawker, C. J.; Waymouth, R. M.; Chidsey, C. E. D. Polym. Prepr. 1997, 38, 904-905. (17) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (18) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (19) Terry, J.; Linford, M. R.; Wirgen, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. App. Phys. Lett. 1997, 71, 1056. (20) Wagner, P.; Nock, S.; Spudich, J. A.; Volkmuth, W. D.; Chu, S.; Cicero, R. L.; Wade, C. P.; Linford, M. R.; Chidsey, C. E. D. J. Struct. Biol. 1997, 119, 189-201. (21) Boukherroub, R.; Bensebaa, F.; Morin, S.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (22) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (23) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429. (24) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; Maas, J. H. v. d.; Jeu, W. H. d.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759-1768. (25) Buriak, J. M. Adv. Mater. 1999, 11, 265. (26) Buriak, J. M. Chem. Commun. 1999, 1051. (27) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 2462-2464. (28) Song, J. H.; Sailor, M. J. J. Am. Chem. Soc. 1998, 120, 2376. (29) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683. (30) Boukherroub, R.; Morin, S.; Wayner, D. D. M.; Bensebaa, F.; Sproule, G. I.; Baribeau, J.-M.; Lockwood, D. J. Chem. Mater 2001, 13, 2002. (31) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 3257. (32) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 13391340. (33) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491. (34) Stewart, M. P.; Buriak, J. M. Adv. Mater. 2000, 12, 859. (35) Canham, L. T.; Reeves, C. L.; Newey, J. P.; Houlton, M. R.; Cox., T. I.; Buriak, J. M.; Stewart, M. P. Adv. Mater. 1999, 11, 1505. (36) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (37) Seneci, P. Solid-Phase Synthesis and Combinatorial Techniques; John Wiley & Sons: New York, 2000. (38) Bodanszky, M.; Bodanszky, A. Practice of Peptide Synthesis, 2nd rev. ed.; Springer-Verlag: Berlin, 1994.

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In this paper, we report a simple two step procedure that introduces an activated ester directly onto porous silicon surface. These activated esters are shown to be reactive with amines in aqueous solutions and provide a potentially new route for the immobilization of biomolecules. Suitable reagents satisfy three criteria: (1) an n-alkene terminus which reacts with hydride-terminated porous silicon surfaces under conditions which do not lead to competitive formation of detectable amounts of oxide; (2) a methylene tether which chemically protects the silicon surface and separates the surface and the biomolecule; (3) a terminal activated ester group (AG) which is susceptible to reaction with biomolecules in aqueous media. Activated esters I-III have been synthesized and covalently attached to a porous silicon surface under thermal conditions.30

Experimental Section Materials. All chemicals were reagent grade or higher and used as received unless otherwise specified. MilliQ water (18MΩ) was used for all experiments. 1-Decene, n-butylamine, undecylenic acid, 2,2,2-trifluoroethanol, 3-nitrophenol, N-hydroxysuccinimide, ethanethiol, n-butylamine, 4-(dimethylamino)pyridine (DMAP), dithiothreitol (DTT), 1,1,2-trichloroethane (TCE), and 1,3-diisopropylcarbodiimide (DIC) were all available from Aldrich. The methyl esters of L-alanine (Me-Ala) and L-cysteine (Me-Cys) were purchased from Sigma. All other solvents were obtained from VWR and distilled under an inert atmosphere. Tetrahydrofuran (THF) was passed through an activated column of alumina and refluxed over Na for 24 h prior to distillation. n-Heptane was washed with concentrated H2SO4, water, and 10% Na2CO3 before distillation from Na. N,NDimethylformamide (DMF) was distilled under reduced pressure over CaH2. Phosphate-buffered saline (PBS) solutions were obtained from Aldrich, and the pH was adjusted with a 0.1 M KOH solution. Synthesis of 3-Nitrophenyl Undecylenate (I). In a 100 mL round-bottom flask equipped with a magnetic stirrer and a nitrogen inlet was placed undecylenic acid (1.00 g, 5.43 mmol) and 3-nitrophenol (0.83 g, 6.00 mmol). The mixture was dissolved in 10 mL of dry THF and cooled to 0 °C before a 10 mL THF solution of DIC (0.75 g, 6.00 mmol) and DMAP (0.073 g, 0.60 mmol) was added dropwise via a syringe over a 10 min period. The mixture was allowed to warm gradually to room temperature and stirred at this temperature for 18 h. The urea byproduct was filtered off and the filtrate concentrated to produce a crude solid that was recrystallized from EtOAc to produce a yellow solid, yield ) 1.24 g, 73%. 1H NMR (400.1 MHz) in CDCl3: δ 8.13 (d, 1H, J ) 7.5 Hz), 8.03 (s, 1H), 7.55 (t, 1H, J ) 8.3), 7.44 (d, 1H, J ) 7.5 Hz), 5.80 (m, 1H), 4.96 (m, 2H), 2.40 (t, 2H), 2.05 (m, 2H), 1.76 (m, 2H), 1.20-1.50 (m, 10H). 13C NMR (100.1 MHz, CDCl3): δ 172.1, 158.5, 151.5, 149.5, 139.1, 130.4, 128.5, 122.6, 114, 34.6, 34.1, 29.8, 29.7, 29.6, 29.3, 25.1. IR (CCl4, cm-1): 3023 (vinyl H), 2928, 2853 (CsH), 1765 (CdO), 1610 (CdC), 1532 (NO2), 1351 (NO2), 1211 (CsO). Synthesis of N-Succinimidyl Undecylenate (II). In a 100 mL round-bottom flask equipped with a magnetic stirrer and a nitrogen inlet were placed undecylenic acid (1.00 g, 5.43 mmol) and N-hydroxysuccinimide (0.69 g, 6.00 mmol). The mixture was dissolved in 5 mL of dry DMF and cooled to 0 °C before a 5 mL DMF solution of DIC (0.75 g, 6.00 mmol) and DMAP (0.073 g, 0.60 mmol) was added dropwise via a syringe over a 10 min (39) Bodanszky, M. Int. J. Pept. Protein Res. 1985, 25, 449.

Modification of Porous Silicon Surfaces period. The mixture was allowed to warm to room temperature and stirred for 18 h at this temperature. The urea byproduct was filtered off and the filtrate concentrated to produce a crude solid. Flash chromatography of the mixture using dichloromethane as eluant and concentration of the collected fractions produced a white solid, 1.57 g, 98%. 1H NMR (400.1 MHz) in DMSO-d6: δ 5.80 (m, 1H), 4.96 (m, 2H), 2.85 (br s, 4H), 2.60 (t, 2H, J ) 7.5 Hz), 2.05 (m, 2H), 1.75 (m, 2H), 1.20-1.50 (m, 10H). 13C NMR (100.1 MHz, CDCl3): δ 169.6, 169.1, 139.6, 114.6, 34.6, 31.4, 29.6, 29.4, 29.3, 29.2, 29.1, 25.9, 24.9. IR (CCl4, cm-1): 3077 (vinyl H), 2928 (CsH), 2853 (CsH), 1819 (CdO), 1791 (CdO), 1749 (Cd O), 1640 (CdC), 1462 (CH2), 1160 (CsO). Synthesis of S-Ethyl Undecylenate (III). In a 100 mL round-bottom flask equipped with a magnetic stirrer and a nitrogen inlet were placed undecylenic acid (1.00 g, 5.43 mmol) and ethanethiol (0.37 g, 6.00 mmol). The mixture was dissolved in 10 mL of dry THF and cooled to 0 °C before a 10 mL THF solution of DIC (0.75 g, 6.00 mmol) and DMAP (0.073 g, 0.60 mmol) was added dropwise via a syringe over a 10 min period. The mixture was stirred for 18 h at 0 °C. The urea byproduct was filtered off and the filtrate concentrated to produce a crude product that was loaded onto a column of silica gel. Flash chromatography of the mixture using chloroform as the eluant and concentration of the collected fractions produced a colorless oil, yield ) 1.07 g, 82%. 1H NMR (400.1 MHz) in CDCl3: δ 5.80 (m, 1H), 4.96(m, 2H), 2.88 (q, 2H, J ) 7.5 Hz), 2.54 (t, 2H, J ) 7.5 Hz), 2.05 (m, 2H), 1.68 (m, 2H), 1.20-1.50 (m, 10H). 13C NMR (100.1 MHz) in CDCl3: δ 195.8, 139.7, 114.1, 34.5, 33.8, 30.6, 29.3, 29.2, 29.1, 28.9, 24.7, 14.8. IR (cm-1): 3077 (vinyl H), 2974 (C-H), 2928 (C-H), 2856 (CsH), 1692 (CdO), 1640 (CdC). Surface Characterization. Diffuse reflectance infrared Fourier transform (DRIFT) and transmission FTIR spectra were recorded using a Nicolet MAGNA-IR 860 spectrometer at 2 cm-1 resolution. The porous silicon sample was mounted in a purged sample chamber with the light focused normal to the surface. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis instrument, using monochromated Al KR (1486 eV) radiation with detection on the surface normal. The pressure during the analysis was ∼5 × 10-8 Torr. Scanning electron microscopy (SEM) images were obtained with a field emission Hitachi S4700 using an accelerating voltage of 5.0 keV. Top and cross-sectional views were obtained at varying magnifications to examine the pore morphology. Porous Silicon Etching Procedure. Heavily doped p-type silicon wafers (〈100〉, B-doped, 0.0018-0.0022 Ω‚cm) were used to prepare porous silicon samples by anodic etching in ethanolic HF solution (3:1 HF-EtOH (v/v)). Prior to the etching procedure, the wafers were cleaned with 3:1 (v/v) H2SO4-H2O2 for 30 min, rinsed with copious amounts of MilliQ water, and then immersed in 48% HF for 1 min. The backside of the wafer was contacted with In/Ga eutectic before mounting in a Teflon etching cell. Porous silicon layers were fabricated at a constant current density of 400 mA/cm2 for 10 s in the absence of light using a Pt mesh counter electrode to ensure a homogeneous electric field. The etched wafer was removed from the cell, rinsed with copious amounts of EtOH, and dried under a stream of nitrogen. Postoxidation of Porous Silicon. After being rinsed with ethanol, the freshly prepared PSi sample was anodically oxidized in 1 M H2SO4 for 5 min at a current density of 5 mA cm-2. The sample was rinsed with ethanol and dried under a stream of nitrogen. Under these conditions two types of Si-H stretches are clearly identified which can be assigned to (Si)3-xSi-Hx+1 centered at 2125 cm-1 and (SiO)3-xSi-Hx+1 centered at 2252 cm-1 (x ) 0-2). Under the conditions of the electrochemical anodization the ratio A2252/A2125 was found to be 0.6. Surface Modification with Activated Esters. Stock solutions (0.3 M, n-heptane) of the activated esters were made up in a 25 mL volumetric flask. The 5.0 mL aliquots of these solutions were degassed in a Schlenk tube with Ar for 30 min prior to the addition of the freshly prepared porous silicon surface. The Schlenk tube was heated to 120 °C for 18 h after which the silicon wafer was removed, rinsed with copious amounts of THF and TCE, and dried under a stream of nitrogen. These monolayers were characterized using DRIFT and XPS. Aminolysis Reactions. Three 100 mL stock solutions (1.0 mM) of n-butylamine, ethanolamine, Me-Ala, and Me-Cys were

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Figure 1. SEM images of a porous silicon layer etched at 400 mA/cm2 for 10 s in HF-EtOH (3:1 v/v): (a) plan view showing the approximate pore size of ca. 100 nm; (b) cross-sectional view showing the well-defined cylindrical pore morphology. made up in a phosphate-buffered saline (PBS) solution of pH ) 7.4. A control solution was made without any amine. These solutions were stored at 4 °C until use. 3-Nitrophenyl Undecylenic Ester. Porous silicon surfaces modified with the 3-nitrophenyl undecylenic ester were immersed in 5 mL of a butylamine or ethanolamine solution for up to 24 h in the dark and monitored by DRIFT at 2 h intervals. N-Succinimidyl Undecylenic Ester. Porous silicon surfaces with a monolayer of N-succinimidyl ester undecylenic ester were immersed in 5 mL of an ethanolamine or Me-Ala solution for up to 24 h in the dark and monitored by DRIFT at 2 h intervals. S-Thioethyl Undecylenic Ester. Porous silicon surfaces derivatized with S-ethyl undecylenic ester were immersed in the following: (a) 5 mL of the butylamine solution for up to 24 h and monitored by DRIFT at 2 h intervals; (b) 5 mL of the Me-Cys solution for 24 h in the dark monitored by DRIFT at 2 h intervals. The Me-Cys solution was pretreated with 0.1 mL of an aqueous 10 mM DTT solution.

Results and Discussion The shape, size, and orientation of the pores generated during the electrochemical etching of silicon wafers are dependent on the current density, electrolyte composition, and dopant type and level.13 We have used highly doped p-type Si(100) wafers to work at current densities sufficiently high to create pores >100 nm. Scanning electron microscopy (SEM) was used to characterize the porous layer morphology and thickness. A top view of the porous surface that has been etched at 400 mA for 10 s is shown in Figure 1a. At a magnification of ×500 000, the image clearly shows average pore sizes on the order of 100 nm. A cross-sectional view (Figure 1b) shows the cylindrical

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Figure 2. Diffuse reflectance FTIR spectrum of a p++-porous silicon layer freshly etched at 400 mA/cm2 for 10 s.

morphology of the pores and the well-defined parallel arrangement of the pores. Under the conditions that were used, the typical thickness of the porous layer was 5 µm. The porous silicon layer is transparent to wavelengths in the IR region, and spectra can be conveniently acquired using diffuse reflectance (DRIFT) or transmission FTIR spectroscopy. The DRIFT spectrum of a freshly etched porous silicon layer (Figure 2) clearly shows the νSi-Hx stretching modes at 2090 cm-1 (Si-H), 2115 cm-1 (SiH2), and 2140 cm-1 (Si-H3). An additional absorption present at 915 cm-1 corresponds to the δSiH2 scissor mode. Freshly etched, SiHx-terminated porous silicon is unstable in air and in aqueous solution and undergoes oxidative and hydrolytic corrosion. However, the hydrogen-terminated porous silicon reacts with alkenes under photochemical,31 thermal,30,32 or catalytic26,33 conditions to generate an alkyl monolayer on the surface. The formation of such an organic monolayer has been shown to be chemically robust.21,30 We have used the thermal route to derivatize the PSi surface with the activated esters I-III. FTIR Characterization. 3-Nitrophenyl Undecylenate (I). Figure 3a displays a transmission FTIR spectrum of a PSi surface modified with I at 120 °C for 18 h. The spectrum displays a number of additional bands that are characteristic of the ester (the carbonyl band at 1759 cm-1, the asymmetric and symmetric methylene νCsH stretching bands at 2926 and 2852 cm-1, the aromatic bands of νCsH at 3060 cm-1 and νCdC at 1601 cm-1, and the asymmetric and symmetric νNsO of the nitro group at 1532 and 1351 cm-1, respectively). There also is significant oxidation of the surface concurrent with the hydrosilylation reaction as seen from the large increase in the Si-O stretch near 1000 cm-1. Since it is known that these surfaces are stable in hydrocarbon solvents at the reaction temperature,40 the possibility that the oxidant was the nitroaryl group was tested by heating a freshly etched PSi sample in heptane containing 0.3 M nitrobenzene. The resulting surface was highly oxidized and contained some evidence for aromatic C-H stretch in the FTIR spectrum (Figure 3b). These observations are consistent with the recent reports by Sailor and co-workers40 of the nitroaromatic promoted oxidation of PSi in which the reaction of a nitroaromatic leads to the formation of silicon oxide and the nitrosoaromatic. While a detailed mechanism was not suggested, a plausible mechanism, based on the TEMPO(40) Content, S.; Trogler, W. C.; Sailor, M. J. Chem.sEur. J. 2000, 6, 2205-2213.

Figure 3. Transmission FTIR spectra of porous silicon (a) after thermal hydrosilylation with 3-nitrophenyl undecylenate (I) for 18 h at 120 °C and (b) after reaction with nitrobenzene for 18 h at 120 °C.

promoted oxidation of polysilanes proposed by Lucarini et al.,41 is shown below. In the present case the initiation step is not clear. There are at least two plausible reactions leading to the first proposed intermediate. The first is direct nucleophilic attack of the ArNO2 group leading to a hydrosilylation product. The other is electron transfer to form ArNO2•- followed by proton transfer. Both reactions lead to the formal hydrosilylation product, ArN(OH)OSi. There is some evidence for the formation of such a product in Figure 3b, which has small absorptions which can be assigned to aromatic C-H stretch and OH stretch. Although not ideal, studies of the reactions of the 3-nitrophenyl undecylenate modified PSi were carried out using the partially oxidized surfaces.

N-Succinimidyl Undecylenate (II). Derivatization of the porous silicon surface with N-succinimidyl undecylenate generates a surface-bound succinimidyl group. The FTIR (41) Lucarini, M.; Marchesi, E.; Peduli, G. F.; Chatgilialoglu, C. J. Org. Chem. 1998, 63, 1687-1693.

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Figure 5. Diffuse reflectance FT-IR spectra of (a) a PSi layer modified with S-thioethyl undecylenate (III) and (b) the activated thioester monolayer after immersion in a 0.1 mM PBS solution of cysteine methyl ester for 24 h.

Figure 4. Transmission FTIR spectra of (a) a PSi layer after reaction of N-succinimidyl undecylenate (II) with the hydrogenterminated PSi, (b) a PSi layer after reaction of N-succinimidyl undecylenate (II) with PSi partially oxidized by anodization in sulfuric acid for 5 min at a current density of 5 mA cm-2, and (c) the PSi film from (b) after immersion in a 0.1 mM PBS solution of ethanolamine for 15 h.

spectrum of the modified PSi exhibits the diagnostic features that correspond to the pendant activating group (Figure 4a). Three strong absorptions of νCdO at 1812, 1790, and 1740 cm-1 are associated with the cyclic imide and ester moieties, respectively. The aliphatic νCsH vibrational bands are also observed. In this case, it is clear from the FTIR results that the PSi does not oxidize measurably in competition with the hydrosilylation reaction. S-Thioethyl Undecylenate (III). Thermal hydrosilylation of a hydrogen-terminated porous silicon surface with S-thioethyl ester produces a surface-bound ester that is tethered with a decyl chain. The FTIR spectrum of the modified surfaces (Figure 5a) displays the characteristic aliphatic νCsH bands as well as a diagnostic carbonyl peak νCdO at 1695 cm-1 for the thioester. XPS Characterization. XP survey spectra (Figure 6) of the activated ester monolayers on PSi were acquired to complement the IR analysis and confirm the elemental composition of the surface-bound species. In the spectra for monolayers of I (Figure 6a) and II (Figure 6b) spectra, the 1s peaks of C, N, and O were detected at 286, 400, and 533 eV, respectively. The XPS analysis of the monolayer of III (Figure 6c) confirmed the presence of carbon (1s) and oxygen (1s) at 286 and 533 eV as well as sulfur at 164

Figure 6. XP survey spectra of activated undecylenate ester monolayers on porous silicon samples: (a) the as-anodized PSi films; PSi films modified with (b) I, (c) II and (d) III. Scheme 1

and 128 eV (corresponding to S1s and S2p). High-resolution XPS showed no detectable amounts of SiO2 on the PSi surfaces prepared from II or III. As expected, significant surface oxidation was evident by XPS on the surfaces prepared using I or from the reaction with nitrobenzene (see Supporting Information). Characterization of the activated ester monolayers by DRIFT and XPS analysis confirms that the bifunctional molecules are immobilized on the surface with the activating group remaining intact and, with the exception of I, without the formation of oxide. There is no evidence to suggest that the activated ester groups react with the hydride-terminated silicon. Thus, it is possible to create

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Langmuir, Vol. 18, No. 16, 2002

Wojtyk et al. Scheme 2

essentially oxide free PSi surfaces which are both passivated at the silicon organic interface by the covalently attached alkyl film and activated at the organic/solution interface for further chemical manipulation. Aminolysis Reactions. The ability of porous silicon samples modified with activated ester monolayers to react with primary amines in aqueous solution has been examined. To simplify the spectroscopic analysis the reactions of simple model amines have been studied. Immersion of PSi samples modified with I-III in a PBS solution but in the absence of amine did not produce any change in the IR spectrum suggesting that the hydrolysis of these esters is slow. Furthermore, no loss of material or oxide formation was observed which, again, suggested that these monolayers are stable and effectively passivate the silicon surface even in aqueous solution. 3-Nitrophenyl Undecylenate (I) Modified Surfaces. A PSi surface modified with I immersed in a PBS (pH 7.6 at 37 °C) solution of n-butylamine for 24 h did not appear to react. A likely explanation for the lack of reactivity is that the reaction of I does not actually lead to reaction with the alkene. Examination of Figure 3 shows some similarity between the IR spectrum from the reaction of I and that from the reaction of nitrobenzene. While the absorptions near 1530 and 1350 cm-1 can be assigned to the intact nitro group, absorptions at 1600 cm-1 and 1490 cm-1 as well as the OH absorption near 3300 cm-1 are common to both reactions. From this it may be inferred that a significant fraction of the reaction of I occurs at the nitro end. These esters also are expected to be photolabile at wavelengths