9812
Langmuir 2003, 19, 9812-9817
Chemical Stability of Porous Silicon Surfaces Electrochemically Modified with Functional Alkyl Species Inez N. Lees,† Haohao Lin,‡ Christie A. Canaria,‡ Christian Gurtner,‡ Michael J. Sailor,*,‡ and Gordon M. Miskelly*,† Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand, and Department of Chemistry and Biochemistry, The University of California, San Diego, 9500 Gillman Drive, La Jolla, California 92039-0358 Received July 3, 2003 The chemical stability of electrochemically alkylated porous silicon is studied. The hydride-terminated surface of p-type or p++-type porous silicon is stabilized by electrochemical reduction of organohalides in acetonitrile solutions. Reduction of 6-iodo-ethylhexanoate, 1-iodo-6-(trifluoroacetylamino)hexane, iodomethane, 1-bromohexane, or ethyl 4-bromobutyrate at a porous Si cathode results in removal of the halogen and attachment of the organic fragment to the porous Si surface via a Si-C bond. A two-step procedure involving attachment of the functional group of interest followed by attachment of methyl groups (by reduction of iodomethane) to residual, more sterically inaccessible sites on the porous Si surface is found to yield a more stable material. Three tests of the chemical stability of the modified surfaces are performed: treatment with dimethyl sulfoxide (a chemical oxidant for porous Si), treatment with aqueous Cu2+, and exposure to 10% ethanol in a solution of phosphate buffered (pH ) 7.4) aqueous saline. The reactions are characterized by atomic force microscopy, Fourier transform infrared (FT-IR) and optical reflectivity spectroscopies. The data indicate that electrochemical alkylation greatly improves the stability of porous Si against oxidation and corrosion, and that the methyl capping procedure provides the most stable material.
Introduction Hydrogen-terminated porous silicon has been shown to be sufficiently stable that studies can be performed on this material in inert atmospheres or in air for short periods of time. However, this surface is subject to oxidation by atmospheric oxygen and by certain organic solutions, or hydrolysis by aqueous solutions, to give a silicon oxide surface. This leads to instability of the porous silicon layer that is unacceptable for many applications. One strategy to avoid such corrosion has been to deliberately oxidize the surface under controlled conditions. Such thermally (or ozone) oxidized surfaces show increased stability toward further oxidation.1-8 An alternative strategy to protect the silicon surface from corrosion has been to form Si-C bonds on the surface9 via such routes as (i) hydrosilylation of alkenes or alkynes,10-14 (ii) reaction * Corresponding authors. † The University of Auckland. ‡ The University of California. (1) Bsiesy, A.; Vial, J. C.; Gaspard, F.; He´rino, R.; Ligeon, M.; Muller, F.; Romestain, R.; Wasiela, A.; Halimaoui, A.; Bomchil, G. Surf. Sci. 1991, 254, 195-200. (2) Unagami, T. Jpn. J. Appl. Phys. 1980, 19, 231-241. (3) Petrova-Koch, V.; Muschik, T.; Kux, A.; Meyer, B. K.; Koch, F.; Lehmann, V. Appl. Phys. Lett. 1992, 61, 943-945. (4) Li, K.-H.; Tsai, C.; Campbell, J. C.; Hance, B. K.; White, J. M. Appl. Phys. Lett. 1993, 62, 3501-3503. (5) Munder, H.; Berger, M. G.; Frohnhoff, S.; Thonissen, M.; Luth, H.; Jeske, M.; Schultze, J. W. J. Luminesc. 1993, 57, 223-226. (6) Harper, J.; Sailor, M. J. Langmuir 1997, 13, 4652-4658. (7) van Noort, D.; Welin-Klintstrom, S.; Arwin, H.; Zangooie, S.; Lundstrom, I.; Mandenius, C.-F. Biosens. Bioelectron. 1998, 13, 439449. (8) Zangooie, S.; Bjorklund, R.; Arwin, H. Thin Solid Films 1998, 313-314, 825-830. (9) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (10) Boukherroub, R.; Morin, S.; Wayner, D. D. M.; Lockwood, D. J. Phys. Status Solidi A 2000, 182, 117-121. (11) Boukherroub, R.; Wojtyk, J. T. C.; Wayner, D. D. M.; Lockwood, D. J. J. Electrochem. Soc. 2002, 149, 59-63.
of the surface with alkyllithium or Grignard reagents,15-18 electrochemical reductive or oxidative coupling of the surface with alkynes,19 (iv) electrochemical reductive coupling of the surface with alkyl halides,20 or (v) electrochemical oxidative coupling of the surface with diazonium salts, alkyllithium reagents, or Grignard reagents.21-23 Most of the above methods attach aliphatic or aromatic hydrocarbons to the porous silicon surface to give hydrophobic surfaces with increased resistance to hydrolysis. The electrochemical alkyl halide coupling route has also been used to incorporate organic moieties with functional groups such as esters and nitriles onto the surface.20 Such functional groups provide the opportunity for further modification of the silicon surface using standard organic chemistry strategies. One drawback to the use of such modified surfaces is that surface derivatization is not complete, with only 20-80% of the Si-H bonds being replaced depending on the derivatization method and the (12) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683-2685. (13) Buriak, J. M. Chem. Rev. 2002, 102, 1272-1308. (14) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001. (15) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1998, 120, 45164517. (16) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1999, 121, 71627163. (17) Song, J. H.; Sailor, M. J. J. Am. Chem. Soc. 1998, 120, 23762381. (18) Song, J. H.; Sailor, M. J. Inorg. Chem. 1999, 38, 1498-1503. (19) Robins, E. G.; Stewart, M. P.; Buriak, J. M. Chem. Commun. 1999, 2479-2480. (20) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966-1968. (21) Vieillard, C.; Warntjes, M.; Ozanam, F.; Chazalviel, J.-N. In Proceedings of the Electrochemical Society; Lockwood, D. J., Fauchet, P. M., Koshida, N., Brueck, S. R. J., Eds.; Chicago: Illinois, 1996; Vol. 95-25, pp 250-258. (22) Allongue, P.; deVilleneuve, C. H.; Pinson, J.; Ozanam, F.; Chazalviel, J. N.; Wallart, X. Electrochim. Acta 1998, 43, 2791-2798. (23) deVilleneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. Phys. Chem. B 1997, 101, 2415-2420.
10.1021/la035197y CCC: $25.00 © 2003 American Chemical Society Published on Web 10/07/2003
Chemical Stability of Porous Silicon Surfaces
steric bulk of the molecule used. The surfaces so formed have been shown to be more resistant toward oxidation or hydrolysis than freshly prepared surfaces,13,20,22 but the Si-H groups remaining can still be subject to attack by nucleophilic or oxidizing reagents. In this work we show that the chemical stability of porous Si can be increased by replacing residual Si-H species on the surface with methyl groups. Reacting a previously modified surface with a smaller reagent to “cap off” sites left unreacted by larger moieties has been used successfully in the synthesis of silica-based packings for reversed-phase chromatography.24 Here we report that functionalized porous Si surfaces can be reductively coupled with iodomethane (CH3I), resulting in replacement of most of the remaining Si-H bonds. The high degree of surface coverage obtained by methylation has been reported previously,20,22 but the extension to mixed surfaces has not been explored. Several experiments designed to quantify the stability of the modified porous Si samples are performed, involving the use of chemical oxidants and solutions that mimic those used in bioassay applications or that might be encountered in other sensor applications. Experimental Section Materials. Acetonitrile (99.8%, EM Science, Inc.), and dimethyl sulfoxide (AR Fisher, Inc.), were distilled from CaH2 under N2 before use. 1-Iodo-6-(trifluoroacetylamino)hexane was prepared using a literature method and the purity confirmed by NMR.25 Iodomethane (99.5%, Aldrich Chemicals, Inc.), 1-bromohexane (98%, Aldrich Chemicals, Inc.), and ethyl 4-bromobutyrate (95%, Aldrich Chemicals, Inc.) were degassed before use. Ethyl 6-bromohexanoate was prepared from 6-bromohexanoic acid after the method Furniss, et al., used in the preparation of ethyl bromoacetate26 and characterized by NMR. Dulbecco’s phosphate-buffered saline (PBS) aqueous solution (Invitrogen, Inc. cat # 14190) was mixed with 10% ethanol (v/v). Lithium iodide (ACROS Organics) was stored in an inertatmosphere glovebox or was dried immediately before use. Deionized water was filtered through a Nanopure system (Barnstead, DuBuque, IA). Preparation of Porous Si. Two types of Si wafers were used in this study: boron-doped, with a resistivity of 1-2 Ω‚cm (ptype) and boron-doped with a resistivity of 0.6-1.0 mΩ‚cm (p++type). Both types of samples were polished, 〈100〉 oriented, 300550 µm-thick wafers obtained from International Wafer Service. For the p-type wafers, porous silicon was prepared by anodically etching the silicon in a 1:1 aqueous 48% HF/ethanol solution at a current density of 40 mA cm-2 and with 100 mW cm-2 white light irradiation for 2 min. For the p++-type wafers, porous silicon was prepared by anodically etching the silicon in a 3:1 aqueous 48% HF/ethanol solution at a current density of 200 mA cm-2 for 30 s. The porous Si layers produced under these conditions are approximately 2-3 µm thick and sufficiently uniform that they display thin film interference effects. Electrochemical Modification General Procedure. Electrochemical modification was performed in the same Teflon electrochemical cell that was used to etch the porous silicon samples, after removal of the HF-containing electrolyte and thorough rinsing of the cell with pure ethanol. The cell was fitted with a glass cap and fittings to allow it to be connected to an inert atmosphere Schlenk line. Using standard Schlenk techniques,27 a solution of 0.2 M organohalide and 0.2 M anhydrous LiI in dry, distilled acetonitrile was transferred into the electrochemical cell. A cathodic current of 10 mA/cm2 (for p-type samples) or 5 (24) Kirkland, J. J.; Henderson, J. W.; DeStefano, J. J.; Straten, M. A. v.; Claessens, H. A. J. Chromatogr., A 1997, 762, 97-112. (25) Chipowsky, S.; Lee, Y. C. Carbohydr. Res. 1973, 31, 339-346. (26) Furniss, B. S.; Hannaford, A. J.; Rogers, V.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 4th ed.; Longman: London, 1979; pp 508-509. (27) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-Sensitive Compounds, 2nd ed.; John Wiley and Sons: New York, 1986.
Langmuir, Vol. 19, No. 23, 2003 9813 mA/cm2 (for p2+-type samples) was passed for 30-45 s. Because the liquid junction generated with p-type Si acts like a reversebiased diode under these conditions,28 illumination with a 100 mW/cm2 white light source (tungsten lamp) was required to generate photocurrent sufficient to support the current densities needed for the reaction. The dopant level is high enough in degenerately doped p++ Si such that no illumination should be required to support the current in these samples. Indeed, FTIR measurements indicated that the coverage of alkyl species on porous Si derived from p++ Si was not dependent on the intensity of illumination. However, these samples were also illuminated during modification for consistency. After disassembly of the Schlenk cell, the sample was washed with acetic acid, acetonitrile, and then ethanol, and dried under a stream of nitrogen. If the infrared spectrum indicated the presence of Si oxide on the material, it was removed by soaking the porous silicon sample in 1:1 48% aqueous HF/ethanol for 2 min, followed by a brief ethanol rinse. In most cases this step was not necessary. Electrochemical Endcapping of Porous Si by Methylation. The porous silicon was reductively coupled with iodomethane using the same electrochemistry as described above. For p-type samples, a solution of 0.5 M iodomethane and 0.2 M LiI in acetonitrile was used. For p++-type samples, a solution of 0.2 M iodomethane and 0.2 M LiI in acetonitrile was used. FT-IR Spectroscopy. All FT-IR spectra were acquired on a Nicolet model 550 Magna Series II FT-IR instrument or on a BIORAD Digilab FTS-60, using an N2 purged sample chamber. An average of 64 scans were acquired, at a spectral resolution of 4 cm-1. A background scan obtained from an unetched piece of Si that had been washed with HF was subtracted from each sample spectrum. The FT-IR spectra of p-type porous silicon were obtained in absorbance mode, while the spectra of p++-type porous silicon were obtained in diffuse reflectance mode, using a Spectra-Pro diffuse reflectance attachment. AFM Characterization of Porous Silicon Morphology. Atomic force microscopy (AFM) images were obtained under ambient conditions using a Nanoscope IIIa multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA) operating in tapping mode to investigate porous silicon morphology. Interferometric Reflectance Spectra. Interferometric reflectance spectra of porous silicon were obtained using an S 2000 fiber optic spectrometer (Ocean Optics, FL). A tungsten light source was focused to a spot size of approximately 1 mm2 on the porous silicon surface. A CCD detector operating in the range of 400-1000 nm acquired spectra every 30 s. The illumination and detection of the reflected light was performed using an incident angle of 0° to the surface normal. Values of effective optical thickness (EOT) were obtained directly from a fast Fourier transform of the reflectance spectra.29
Results and Discussion Preparation and Characterization of Modified Porous Si Samples. The general procedure used to modify porous Si surfaces in this work is outlined in eq 1. A functional group is first attached to porous Si by
electrochemical reduction of an alkyl halide. Subsequent reduction of iodomethane at the porous Si cathode results in endcapping of residual Si-H species with a methyl (28) Lewis, N. S. Electrochem. Soc. Interface 1996, 5, 28-31. (29) Le´tant, S.; Sailor, M. J. Adv. Mater. 2000, 12, 355-359.
9814
Langmuir, Vol. 19, No. 23, 2003
Lees et al.
group. Both steps result in modification of the silicon surface via Si-C bond formation. In this work, two different functional groups were used: trifluoroacetylamino (2 and 3, eq 2) and an ethyl ester (4, 5, 6, and 7, eqs 3 and 4). These molecules were chosen as potential routes into subsequent functionalization of the porous Si surface with a variety of organic groups or biomolecules.30 The ethyl ester functionality was tethered to the porous Si surface via either a 3-carbon aliphatic chain (4 and 5, eq 3) or a 5-carbon aliphatic chain (6 and 7, eq 4).
Figure 1. Transmission-mode FTIR spectrum of 2, a p-type porous silicon sample modified with 6-trifluoroacetamidohexyl groups, (dashed trace) and 3, the material after subsequent endcapping with methyl groups, (solid trace). The gradually rolling baseline in this spectrum arises from thin film optical interference in the porous silicon layer (see text).
Surface modification reactions were followed by Fourier transform infrared spectroscopy (FTIR). A decrease in intensity of both the Si-H stretching vibration and the Si-H wag mode in the infrared spectrum indicates the degree to which the surface Si-H species have been replaced by alkyl groups. The addition of larger molecules, such as the hexane derivatives used here, leads to a 20 to 40% decrease in the area of the Si-H stretching band in the FTIR spectrum. Electrochemical methylation of a
porous silicon surface has been shown to provide higher coverage, based on the decrease in the Si-H stretching band, presumably due to the smaller size of a methyl group.20,31 The data in this work indicate that when alkylation with one of the larger organic groups is followed by electrochemical methylation, the Si-H stretching band is reduced in area by approximately 80% and the SiH2 scissors mode is no longer visible in the FTIR spectrum (Figure 1). Reduction of 1-iodo-6-(trifluoroacetylamino)hexane at a p-type porous silicon cathode leads to attachment of the trifluoroacetamidohexyl group (eq 2, compound 2), as shown by a decrease in the bands assigned to νSi-H (2100 cm-1) and δSiH2 (910 cm-1) vibrations and the appearance of new bands corresponding to νN-H (3360 cm-1), νCH (2936, 2860 cm-1), amide I (1710 cm-1), amide II (1560 cm-1), and νC-F (1190 cm-1) stretching vibrations. Notably, this method leads to minimal oxidation of the surface, as shown by the small or nonexistent oxide band at ca. 1100 cm-1, Figure 1. The trifluoroacetamidohexyl-modified porous silicon can then be methylated via electrochemical reduction of iodomethane (eq 2, compound 3). This treatment leads to a large reduction of the FTIR features associated with surface Si-H species at 2100 and 650 cm-1, and almost complete loss of the feature at 912 cm-1, while retaining the features of the trifluoroacetamidohexyl group, Figure 1. As noted previously, the major new feature observed in the FTIR spectrum upon methylation is the rocking vibration of the methyl group at 766 cm-1.20,31,32 This Si-C rocking band has been conclusively assigned by 13C and 2 H isotopic substitution experiments.32 The Si-C stretching vibration near 680 cm-1 appears in the same spectral region as Si-Si and Si-H bands and so is more difficult to observe.32 The rolling baseline observed in the infrared spectrum of Figure 1 arises from Fabry-Pe´rot interference in the thin porous Si film. The position of these fringes is sensitive to the net refractive index of the film,29,33 and as such provides a measure of the amount of silicon that is oxidized or removed in the electrochemical processing steps. The near identity of the interference pattern in the (30) Hermanson, G. T. Bioconjugate Techniques; Academic Press: San Diego, 1996. (31) Ozanam, F.; Vieillard, C.; Warntjes, M.; Dubois, T.; Pauly, M.; Chazalviel, J. N. Can. J. Chem. Eng. 1998, 76, 1020-1026. (32) Canaria, C. A.; Lees, I. N.; Wun, A. W.; Miskelly, G. M.; Sailor, M. J. Inorg. Chem. Commun. 2002, 5, 560-564. (33) Curtis, C. L.; Doan, V. V.; Credo, G. M.; Sailor, M. J. J. Electrochem. Soc. 1993, 140, 3492-3494.
Chemical Stability of Porous Silicon Surfaces
Langmuir, Vol. 19, No. 23, 2003 9815
baseline of both FTIR spectra of Figure 1 indicates that the methylation treatment causes no significant change in thickness or porosity of the porous silicon film. The pore morphology does not change appreciably upon electrochemical modification. Atomic force microscope (AFM) images obtained from freshly etched porous silicon (1), porous silicon electrochemically modified with ethyl 4-bromobutyrate (4, eq 3), porous silicon electrochemically modified with iodomethane (8, eq 5), and porous silicon electrochemically modified first with ethyl 4-bromobutyrate and then iodomethane (5, eq 3) are presented in Figure S1 (Supporting Information). All four AFM images reveal features on the order of 50 nm and no significant difference in surface roughness. The similarity of these images indicates that no significant change in the pore morphology occurs upon electrochemical modification.
Oxidative Stability of Modified Porous Si toward Dimethyl Sulfoxide. Dimethyl sulfoxide (DMSO) has been found to act as a mild oxidant of H-terminated porous Si.17 The reaction is thought to proceed by attack of the oxygen atom of DMSO at a Si-Si bond and elimination of dimethyl sulfide.17 Thus the DMSO reaction is a good probe of the susceptibility of the modified porous Si surface toward nucleophilic attack at Si-Si bonds uncomplicated by issues of relative hydrophobicity. The stability of the endcapped surfaces was examined using ethyl hexanoate as the functional species. Exposure of freshly etched, H-terminated porous Si, 1, to DMSO results in a rapid increase in the Si-O stretching feature at 1060 cm-1 observed in the infrared spectrum (Figure 2A). In contrast, treatment of methylated porous Si, 8, with DMSO leads to little oxidation over 2 h (Figure 2B), despite the fact that 15-20% of the original intensity of the Si-H stretching band remains in the FTIR spectrum (not shown). DMSO oxidizes porous Si modified with ethylhexanoate, 6, to a greater extent than the methylated material (Figure 2C), as can be expected given the limited surface coverage and higher density of residual Si-H species in 6. Replacement of these residual Si-H species with CH3, 7, provides the most stable material, and no oxidation is detectable by FTIR even after exposure to DMSO for 2 h (Figure 2D). These results indicate that even though methyl endcapping does not replace all the surface Si-H groups, the surface of porous Si is either inaccessible to the DMSO molecule or it is unreactive on a time scale of 2 h. The procedure leads to a greater than 40-fold decrease in the rate of oxidation by DMSO based on the relative rates of growth of the Si-O band in the IR spectra. Chemical Stability of Modified Porous Si toward Cu2+(aq). While DMSO oxidizes porous Si by O atom transfer, reducible transition metal ions or complexes can oxidize porous Si by electron or H atom transfer, generating metallic deposits within the pores. For example, exposure of porous silicon to solutions containing Ag+, Au+, Pd2+, or Cu2+ leads to oxidation of the silicon and deposition of the corresponding metal.34-38 The stability (34) Andsager, D.; Hillard, J.; Hetrick, J. M.; AbuHassan, L. H.; Plisch, M.; Nayfeh, M. H. J. Appl. Phys. 1993, 74, 4783-4785. (35) Andsager, D.; Hilliard, J.; Nayfeh, M. H. Appl. Phys. Lett. 1994, 64, 1141-1143.
Figure 2. Transmission-mode FTIR spectra of modified porous Si samples upon treatment with dimethyl sulfoxide (DMSO). (A) H-terminated porous silicon, compound 1, measured after 0, 5, 10, 20, 40 min of exposure to DMSO. (B) Methylated porous silicon, compound 8, measured after 0 and 2 h of exposure. (C) Porous silicon modified with 6-ethylhexanoate, compound 6, measured after 0, 5, 20, 40 min of exposure. (D) Porous silicon modified with 6-ethylhexanoate groups and then endcapped with methyl groups, compound 7, after 0 and 2 h of exposure. The sharp bands observed in spectra C and D arise from the ethylhexanoate species.
of modified porous Si toward electron-transfer induced oxidation was probed by treatment with aqueous CuCl2. A solution containing 1.0 × 10-2 M CuCl2 in 1:1 H2O/ ethanol was used because earlier studies34,38 reported that this concentration of copper oxidizes porous silicon at a reasonable rate. The mixed solvent system was used in this study to improve wetting of the pores.39 Infrared spectroscopic measurements indicate that porous Si modified with ethyl hexanoate, 6, is still susceptible to oxidation by Cu2+ (Figure 3A), whereas ethyl hexanoatemodified material that is endcapped with methyl groups, 7, shows no evidence of oxidation even after 2 h of exposure (Figure 3B). The nonendcapped porous Si surface 6 reacts with the copper ion solution at a rate similar to that of the original H-terminated porous Si surface 1. Chemical Stability of Modified Porous Si in Phosphate-Buffered Saline (PBS) Solution. For biosensor and many in-vivo or in-vitro applications, porous Si must be stable in buffered aqueous media. Preoxidation of porous Si (by either thermal or ozone routes) has been shown to provide improved stability relative to the freshly etched material,7,8,40,41 although sensors made with oxidized porous Si are still subject to drift due to dissolution (36) Coulthard, I.; Jiang, D.-T.; Lorimer, J. W.; Sham, T. K.; Feng, X.-H. Langmuir 1993, 9, 3441-3445. (37) Coulthard, I.; Sham, T. K. In Mater. Res. Soc. Symp. Proc.; 1997; Vol. 457, pp 161-165. (38) Tsuboi, T.; Sakka, T.; Ogata, Y. H. J. Appl. Phys. 1998, 83, 45014506. (39) Canaria, C. A.; Huang, M.; Cho, Y.; Heinrich, J. L.; Lee, L. I.; Shane, M. J.; Smith, R. C.; Sailor, M. J.; Miskelly, G. M. Adv. Funct. Mater. 2002, 12, 495-500. (40) Lin, V. S.; Motesharei, K.; Dancil, K. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840-843. (41) 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, 12108-12116.
9816
Langmuir, Vol. 19, No. 23, 2003
Figure 3. Transmission-mode FTIR spectra showing the effect of exposure of porous Si to Cu2+(aq). (A) porous Si modified with 6-ethylhexanoate groups, compound 6, measured after 0, 10, 20, 30, and 60 min of immersion in a 5 × 10-3 M solution of CuCl2 in 1:1 H2O/ethanol. (B) Porous Si modified with 6-ethylhexanoate groups and then endcapped with methyl groups, compound 7, measured after 0 and 90 min of exposure to the Cu2+(aq) solution. The dashed trace in each spectrum indicates the initial (time ) 0) spectrum.
of the oxide.42 The Si atoms at the surface of oxidized porous Si are susceptible to attack by nucleophiles due to the electron withdrawing power of neighboring oxygen atoms. Because carbon is less electronegative than oxygen, porous Si modified via Si-C bonds is more stable, and porous Si alkylated by hydrosilylation has demonstrated remarkable stability in biological media.43 In the present work, the stability of electrochemically alkylated porous Si was tested in phosphate buffered saline (PBS) aqueous solutions. For these studies, p++-type Si was used because it can be prepared with a pore structure that is large enough to admit antibodies and other biomolecules while still retaining the optical properties used to transduce the binding signal.42,44 Ethanol (10%, v/v) was included in the PBS solution to reduce the surface tension sufficient to ensure buffer penetration into the pores. Stability of the electrochemically modified samples was monitored using optical reflectivity. When prepared under the appropriate conditions, porous Si films and multilayers display optical reflectivity spectra characteristic of FabryPe´rot layers or photonic crystals. The position of the spectral peaks depends on the average refractive index of the layers, and shifts in these photonic features have been shown to provide a very sensitive transduction modality for sensing of condensable vapors,45,46 proteins,44,47 DNA, and other molecules.44,45,47-49 Oxidation or dissolution of the porous Si matrix causes a decrease in the refractive (42) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930. (43) Canham, L. T.; Stewart, M. P.; Buriak, J. M.; Reeves, C. L.; Anderson, M.; Squire, E. K.; Allcock, P.; Snow, P. A. Phys. Status Solidi A 2000, 182, 521-525. (44) Collins, B. E.; Dancil, K.-P.; Abbi, G.; Sailor, M. J. Adv. Func. Mater. 2002, 12, 187-191. (45) Snow, P. A.; Squire, E. K.; Russell, P. S. J.; Canham, L. T. J. Appl. Phys. 1999, 86, 1781-1784. (46) Gao, T.; Gao, J.; Sailor, M. J. Langmuir 2002, 18, 9953-9957. (47) Arwin, H.; Gavutis, M.; Gustafsson, J.; Schultzberg, M.; Zangooie, S.; Tengvall, P. Phys. Status Solidi A 2000, 182, 515-520.
Lees et al.
Figure 4. Stability of various surface-modified p++ porous Si samples in 10% (v/v) ethanol in aqueous PBS, pH 7.4, presented as the relative effective optical thickness change (EOT) as a function of time. (A) Sample modified with methyl, 8. (B) Sample modified first with ethyl butyrate, then with methyl, 5. (C) Sample modified with hexyl, 9. (D) Ozone-oxidized porous Si. (E) Sample modified with ethyl 4-bromobutyrate, 4. (F) Freshly etched porous Si, 1. EOT is obtained from the Fourier transform of the reflectivity intensity versus energy plots, and represents the product of the film’s average refractive index and its thickness. The initial value of effective optical thickness (EOT) for each of the samples A-F is (A) 14,327 nm, (B) 13,484 nm, (C) 14,187 nm, (D) 13,669 nm, (E) 13,652 nm, and (F) 12,519 nm. Slopes of each of these curves are: (A) 0.032 nm/min, (B) 0.28 nm/min, (C) 0.85 nm/min, (D) 0.87 nm/min, (E) 1.7 nm/ min, and (F) 4.1 nm/min.
index of the film and a characteristic blue shift in the spectrum.29 While hydrolysis or oxidation of porous Si may lead to gaseous or soluble species that are lost from the surface and therefore not observable by FTIR measurements, loss or oxidation of porous Si leads to a change in composition that is readily detected by the interferometric reflectance measurement. Therefore the optical measurement provides a quick, reliable and quantitative comparison of stability of different porous Si samples. In the present study, changes in refractive index of the film are monitored by Fourier transform of the reflected intensity versus energy spectrum. A value of the effective optical thickness (EOT), representing the product of the film’s average refractive index and its thickness is obtained directly from the Fourier transfom.29 The porous Si samples were mounted in a flow cell fitted with a transparent window, with the buffer solution flowing through at a rate of 0.3 mL/min. Stability was monitored by measurement of the relative change in index, ∆EOT/ EOT of the thin porous silicon layer as a function of time (Figure 4). Each of these experiments was reproduced three times on separate samples. Freshly etched, H-terminated porous Si (1) is unstable in aqueous solutions and readily suffers oxidative and hydrolytic corrosion (Figure 4, trace F). This results in a decrease in the EOT by about 1.5% in 45 min, or 4.1 nm/ min. The ethyl butyrate surface, 4, shows improved stability in PBS solution such that there is approximately a 0.6% decrease in EOT in 45 min, or 1.7 nm/min (Figure 4, trace E). Surface 4 is less stable than a surface obtained by ozone oxidation (Figure 4, trace D). The ozone-oxidation treatment has been used previously for biosensor ap(48) Chan, S.; Horner, S. R.; Miller, B. L.; Fauchet, P. M. J. Am. Chem. Soc. 2001, 123, 11797-11798. (49) Sailor, M. J. In Properties of Porous Silicon; Canham, L., Ed.; Short Run Press Ltd.: London, 1997; Vol. 18, pp 364-370.
Chemical Stability of Porous Silicon Surfaces
plications.41 The change in EOT of the ozone oxidized material is found to be 0.4% in 45 min, or 0.87 nm/min. Consistent with the DMSO and Cu2+ oxidation results discussed above, endcapping of a functionalized surface by methylation improves its stability significantly. Thus methyl endcapped ethyl butyrate, 5, displays a significant improvement in stability (0.1% in 45 min, or 0.28 nm/ min) over ethyl butyrate-modified material 4 (Figure 4 traces B and E, respectively). The greatest stability in this aqueous test is observed with the sample modified solely with methyl species, 8, displaying a relative decrease in EOT of only 0.01% in 45 min, or 0.032 nm/min. The importance of the low steric requirements of the methyl group in determining stability is underscored by results of experiments using the hexyl group as a capping reagent, eq 6. A surface modified with hexyl groups, 9, is expected to present a hydrophobic surface similar to one modified with methyl groups, 8, although the degree of coverage is expected to be lower for the bulkier hexyl species. Indeed, the methyl group 8 provides approximately 30 times greater stability relative to the hexyl group 9. The hexyl-terminated surface 9 is found to display a relative decay in EOT of 0.3% in 45 min, or 0.85 nm/min (Figure 4, trace C), significantly less stable than a methylated surface 8 (Figure 4, trace A) and comparable to ozone-oxidized material (Figure 4, trace D). The hexylmodified surface 9 is somewhat more stable than the estermodified material 4 (Figure 4, traces C and E, respectively). This difference may be attributed to the greater hydrophobic nature of an aliphatic group versus an ester. Presumably the more hydrophilic ester allows more efficient wetting of the near-surface region on porous Si, leading to an increased rate of oxidation by water. Since the extent of surface coverage by these two species is not expected to be identical, the small difference in stability
Langmuir, Vol. 19, No. 23, 2003 9817
between 9 and 4 may also be attributable to a higher surface coverage of the hexyl species relative to the ester. Similar improvements in stability toward aqueous media are encountered with species 3 relative to 2. Thus, the methyl endcapped trifluoroacetamidohexyl modified surface 3 is stable for at least an hour in 0.2 M sodium acetate in ethanol-water and only displays a small amount of hydrolysis after 30 min in 0.2 M Na2CO3/0.2 M NaHCO3 in EtOH/H2O, while the nonmethylated surface 2 is hydrolyzed in both these solutions. Conclusions When attached to porous Si, functional organic groups such as amides or esters provide less protection against corrosion or oxidation than aliphatic species. Introduction of such groups is necessary if the surface is to be modified with further chemistry. Addition of an electrochemical methylation step after attachment of the ester or amide provides improved stability while preserving the integrity of the functional group. Although demonstrated for electrochemical reduction and grafting of functional organic halides, the endcapping procedure described in this work should be compatible with most reported surface modification reactions for porous Si. Alternative strategies to obtain high coverage of methyl species at the porous Si surface such as electrochemical oxidation of methyl Grignard or methyl lithium involve harsh reagents that require more care to avoid destruction of the functional species. The greater stability imparted to porous Si by this method is ascribed to the ability of a small methyl group to cap off reactive surface species that are not accessible to larger functional organic reagents. Stability was probed using reagents that test susceptibility to oxidation by electron transfer, oxygen atom transfer, and nucleophilic attack mechanisms. Acknowledgment. The authors thank the National Science Foundation, Division of Materials Research and the Air Force Office of Scientific Research (grant # F4962002-1-0288) for funding. Supporting Information Available: Tapping mode AFM images of the modified porous silicon surfaces. This material is available free of charge via the Internet at http://pubs.ac.org. LA035197Y
