Electrochemical Preparation of Pore Wall Modification Gradients

Mar 10, 2010 - represents a significant advance over gradients imposed across a flat .... ents18,22,23 can lead to pore wall modification gradients ac...
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Electrochemical Preparation of Pore Wall Modification Gradients across Thin Porous Silicon Layers Corrina M. Thompson,† Michel Nieuwoudt,† Anne M. Ruminski,‡ Michael J. Sailor,‡ and Gordon M. Miskelly*,† †

Department of Chemistry, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand, and Department of Chemistry and Biochemistry, The University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92039-0358

‡

Received November 20, 2009. Revised Manuscript Received February 25, 2010 Thin film porous silicon layers have been constructed in which the level of chemical modification to the pore walls is altered in a controlled gradient across the material. The gradient modification within such a nanoporous material represents a significant advance over gradients imposed across a flat surface. Gradients of methyl, pentyl acetate, and decyl groups are formed via electrochemical attachment of organohalides with an asymmetric electrode arrangement. The stability and hydrophobicity of the latter two systems have been improved through postprocess “end-capping” of the porous silicon with methyl groups. Two-dimensional mapping transmission FTIR microspectrophotometry and ATR-FTIR have been employed to characterize these new materials. Cleaving the surface-attached pentyl acetate groups to 5-hydroxypentyl groups leads to materials that can act as efficient visual indicators of the ethanol concentration in water over the range 1-10 vol %.

Introduction High-throughput screening and analysis of complex gas mixtures and biomolecules requires the development of new sensors with high inherent information density. A promising method to achieve this is to impose a well-characterized composition gradient across a sensor surface and then monitor the sensor response across that surface. This surface composition approach has been reported on flat surfaces1-5 but has been limited by the detection methods required to monitor changes at such surfaces. We aim to develop sensors based on gradients superimposed on porous silicon, for which several good detection strategies are available.6-12 Previous surface modifications of porous silicon have either been uniform across the sample or have been patterned,13-17 with *Corresponding author. E-mail: [email protected].

(1) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 256, 1539–1541. (2) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821–3827. (3) Ruardy, T. G.; Schakenraad, J. M.; van der Mei, H. C.; Busscher, H. J. Surf. Sci. Rep. 1997, 29, 1–30. (4) Riepl, M.; Ostblom, M.; Lundstrom, I.; Svensson, S. C. T.; Denier van der Gon, A. W.; Schaferling, M.; Liedberg, B. Langmuir 2005, 21, 1042–1050. (5) Meyyappan, S.; Shadnam, M. R.; Amirfazli, A. Langmuir 2008, 24, 2892– 2899. (6) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925–7930. (7) Janshoff, A.; Dancil, K.-P. S.; Steinem, C.; Greiner, D. P.; Lin, V.; Gurtner, C.; Motescharei, K.; Sailor, M. J.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 12108–12116. (8) Lin, V. S. Y.; Motesharei, K.; Dancil, K.-P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840–843. (9) Letant, S.; Sailor, M. J. Adv. Mater. 2000, 12, 355–359. (10) Sohn, H.; Letant, S. E.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399–5400. (11) Content, S.; Trogler, W. C.; Sailor, M. J. Chem.;Eur. J. 2000, 6, 2205– 2213. (12) Letant, S. E.; Content, S.; Tan, T. T.; Zenhausern, F.; Sailor, M. J. Sens. Actuators, B 2000, 69, 193–198. (13) Lee, E. J.; Ha, J. S.; Sailor, M. J. J. Am. Chem. Soc. 1995, 117, 8295–8296. (14) Lee, E. J.; Bitner, T. W.; Hall, A. P.; Sailor, M. J. J. Vac. Sci. Technol. B 1996, 14, 2850–2854. (15) Lee, E. J.; Bitner, T. W.; Ha, J. S.; Shane, M. J.; Sailor, M. J. J. Am. Chem. Soc. 1996, 118, 5375–5382. (16) Stewart, M. P.; Buriak, J. M. Angew. Chem., Int. Ed. 1998, 37, 3257–3260. (17) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 7821–7830.

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uniform modification within the patterned portions. Porosity gradients have been generated across Fabry-Perot18-22 and rugate porous silicon layers,23 either by asymmetric placement of the cathode during the preparation of the porous silicon18,22,23 or by using the porous silicon formed on the backside of a silicon electrode.19,20 Photochemical, chemical hydrosilylation, and electrochemical modification are all possible ways for creating chemical composition gradients across porous silicon by using changes in light intensity, concentration density, or current density across the porous silicon layer, respectively. The gradient so formed needs to be uniform through the porous silicon layer (1-100 μm), and this produces additional challenges compared to the preparation of surface gradients. The electrochemical reduction of organoiodides or bromides at porous silicon has been shown to lead to attachment of the organic groups to the porous silicon via Si-C bonds.24-26 The reaction most likely proceeds via generation of alkyl radicals which then have dual roles of abstracting H atoms from the surface and combining with the resulting silicon surface radicals, although it is also possible that organic or silicon surface anions are involved to some degree.24 The surface coverage achieved via this method on porous silicon may be as low as 20-30% for bulky groups but increases to about 80% of the surface Si-H for methyl (18) Collins, B. E.; Dancil, K.-P. S.; Abbi, G.; Sailor, M. J. Adv. Funct. Mater. 2002, 12, 187–191. (19) Karlsson, L. M.; Tengvall, P.; Lundstr€om, I.; Arwin, H. J. Electrochem. Soc. 2002, 149, C648–C652. (20) Karlsson, L. M.; Schubert, M.; Ashkenov, N.; Arwin, H. Thin Solid Films 2004, 455-456, 726–730. (21) Ilyas, S.; Gal, M. J. Mater. Sci.: Mater. Electron. 2007, 18, S61–S64. (22) Khung, Y. L.; Barritt, G.; Voelcker, N. H. Exp. Cell Res. 2008, 314, 789– 800. (23) Li, Y. Y.; Kim, P.; Sailor, M. J. Phys. Status Solidi A 2005, 202, 1616–1618. (24) Gurtner, C.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966–1968. (25) Lees, I. N.; Miskelly, G. M.; Lin, H.; Canaria, C. A.; Gurtner, C.; Sailor, M. J. Langmuir 2003, 19, 9812–9817. (26) Canaria, C. A.; Lees, I. N.; Wun, A. W.; Miskelly, G. M.; Sailor, M. J. Inorg. Chem. Commun. 2002, 5, 560–564.

Published on Web 03/10/2010

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groups.24,25 This high achievable density of methylation has been used to increase the stability of surfaces modified with bulkier groups by “end-capping” many of the remaining Si-H groups.25 These prior results show that electrochemical attachment is a mild modification method for porous silicon that is tolerant of many functional groups. We now report that electrochemical attachment of organohalides using the same asymmetric electrode placement used for forming porous silicon with porosity gradients18,22,23 can lead to pore wall modification gradients across thin porous silicon layers.

Experimental Section Materials. All reagents were obtained from commercial sources. Electrochemical solutions containing organohalides and LiI in the appropriate solvent (vide infra) were freezepump-thaw degassed three times to remove traces of oxygen and stored under nitrogen. Lithium iodide (99%, Acros Organics) was stored in a vacuum desiccator or was dried immediately before use. Preparation of Porous Si. Boron-doped, 1-5 Ω cm (p-type) polished, (100) orientiation, 500-550 μm thick Si wafers were obtained from Silicon Materials, Inc. (Carnegie, PA). Porous silicon (1.04 cm diameter) was prepared by anodically etching the silicon in a 1:1 aqueous 49% HF/ethanol solution at a current density of 28 mA cm-2 for 4 min, using a 16 mm diameter Pt gauze (100 mesh) as the counter electrode. The porous Si layers produced under these conditions are approximately 2-3 μm thick (determined by scanning electron microscopy) and sufficiently uniform that they display thin film interference effects. After the etching process, the samples were rinsed with ethanol and then dried under a stream of nitrogen.

Asymmetric Electrochemical Modification: General Procedure. Electrochemical modification was performed in the same Teflon electrochemical cell that was used to etch the porous silicon samples, but with asymmetric placement of the counter electrode (Figure 1). A glass adapter was fitted on top of the electrochemical cell to allow illumination of the sample while it was under an inert atmosphere. The electrolytic solution of 0.4 mol L-1 organohalide and 0.2 mol L-1 anhydrous LiI in acetonitrile (for 5-bromopentyl acetate or iodomethane) or acetonitrile/tetrahydrofuran (1:1) (for 1-bromodecane) was transferred into the electrochemical cell. A cathodic current of 1-8 mA cm-2 was passed for 45-60 s with white light (tungsten lamp) illumination (50 mW cm-2) from above. The wire counter electrodes were either Pt (0.25 mm diameter) or Mg (1.5 mm diameter, tapered to a 0.2 mm tip). The time-varying current waveforms were created with Tektronix ArbExpress-AXW100 Version 2.0.2005.30 software and applied to the PAR 173 potentiostat/galvanostat using a Tektronix AFG 3021 single channel arbitrary/function generator. The glass adapter was then removed and the sample was rinsed three times each with glacial acetic acid and acetonitrile and then dried with a stream of nitrogen. Before further modification the porous silicon sample was soaked in 1:1 49% aqueous HF/ethanol for 2 min, followed by an ethanol rinse.

Electrochemical End-Capping of Porous Si by Methylation. The porous silicon was modified by reduction of iodomethane using the same electrochemical procedure as described above, except that a 16 mm diameter Pt loop anode was used and a uniform cathodic current density of 7 mA cm-2 for 45 s with white light (tungsten lamp) illumination (50 mW cm-2) from above was applied. Deacetylation of Pentyl Acetate Groups. The deacetylation of pentyl acetate attached to porous silicon was carried out according to the procedure outlined by Hartman and Lago.27 A porous silicon sample that had been modified with pentyl acetate groups was heated at 60 °C for 25 min in a solution of ammonium (27) Hartman, L.; Lago, R. C. A. Lab. Pract. 1973, 22, 475–476.

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Figure 1. Diagram of the cell used for the asymmetric electrochemical modifications.18,23 chloride (0.64 mol L-1), concentrated sulfuric acid (0.3 mL), and methanol (6 mL) under an inert atmosphere. The sample was removed and rinsed with methanol (3  5 mL) and then ethanol (1  5 mL) and dried with a stream of nitrogen. Characterization. All transmission FT-IR spectra were acquired on a Thermo Electron Nicolet 8700 FTIR spectrometer with a Nicolet Continuμm Microscope equipped with a MCT/A detector. An average of 32 scans was obtained, at a spectral resolution of 4 cm-1 over the range of 650-4000 cm-1. The FTIR spectra were obtained in absorbance mode. A step size of 400 or 500 μm was used in 2-D mapping mode with an aperture size of 150  150 μm. All FT-IR spectra were processed using the Omnic Atlμs software. All ATR FT-IR spectra were acquired on a Perkin-Elmer Spectrum 400 FTIR spectrometer with a Universal ATR sampling accessory using a KRS5/diamond composite crystal. Spectra were averages of 32 scans with a resolution of 4 cm-1 over the range 400-4000 cm-1. Water contact angle measurements were obtained by imaging water drops on horizontal porous silicon samples using a Canon D-30 digital camera with 100 mm macro lens. Using a glass syringe, 3 μL drops of water were placed on the porous silicon surface. The contact angles formed at the interface of the droplets and the porous silicon surface were measured using Adobe Photoshop CS2 (Adobe Systems Inc.). A methyl-end-capped 5-hydroxypentyl gradient-modified porous silicon sample immersed in 20 mL of water was imaged with the same imaging system while ethanol was titrated into the solution in 100 μL aliquots.

Results and Discussion Preparation of Pore Wall Modification Gradients. Pore wall modification gradients were prepared electrochemically by using asymmetric placement of the anode as shown in Figure 1.18,23 One-directional pore wall modification gradient samples were created using 1-bromopentyl acetate or 1-bromodecane. This gradient modification was followed by electrochemical methylation with iodomethane using symmetric anode placement to “end-cap” the reactive Si-H groups remaining on the surface with methyl groups and so create a more stable material.25 The pentyl acetate-modified porous silicon samples prepared via electrochemical modification can subsequently be made more hydrophilic by removing the acetate group to give an alcohol-terminated surface (5-hydroxypentyl). The functional groups grafted to porous silicon in this study are shown in Figure 2. The effect of different current density-time profiles on the pore wall composition gradients formed across porous silicon were investigated, since applying a constant current density did not give a good pore wall modification gradient extending from low to high coverage. Increasing the current density linearly or exponentially with time lead to a larger magnitude change between the lowest and highest coverages obtained across a pore wall surface DOI: 10.1021/la904408h

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Figure 2. Functional groups that have been grafted onto porous silicon by asymmetric electrochemical reduction of organohalides: (a) decyl, (b) pentyl acetate, (c) 5-hydroxypentyl, and (d) methyl.

composition gradient. The most optimal conditions found for modification of the porous silicon layers were linear cathodic current ramps increasing from 1 to 5 mA cm-2 over 45 s (for creating pore wall modification gradients of pentyl acetate or decyl groups) or 1 to 8 mA cm-2 over 60 s (for creating pore wall modification gradients of methyl groups). The wire anode must be carefully positioned to provide an appropriate current density gradient across the porous silicon. For all pore wall modification gradients prepared using a Pt anode the tip of the vertical wire electrode was positioned ∼2 mm from the edge of the 1 cm diameter porous silicon cathode and ∼5 mm above the porous silicon. The height of the Pt electrode above the porous silicon is important in creating pore wall modification gradients: if the electrode is too close to the porous silicon surface, increased oxidation of the porous silicon occurs, as monitored by FTIR spectra, while if the electrode is placed further away from the porous silicon, a smaller current density gradient is imposed across the porous silicon and therefore a smaller composition gradient is formed. The oxidation of the porous silicon during electrochemical modification with a Pt anode is presumably mainly due to reaction of the iodine formed at the Pt anode with the porous silicon. The formation of iodine can be seen visually as a yellowing of the solution during the electrochemical process. The use of Ag or Mg wire anodes instead of Pt were investigated to see if these could improve the spatial extent and magnitude of the pore wall modification gradients formed, while diminishing the concomitant oxidation of the porous silicon. No yellowing of the electrochemical solution was observed when Ag or Mg wires were used as anodes, indicating that little or no net iodine production is occurring. In addition, the amount of oxidation of the porous silicon observed during electrochemical modification was greatly decreased when using Ag or Mg anodes. However, use of an Ag wire anode produced modified porous silicon samples that had a dull appearance, and SEM EDAX of these samples showed the presence of Ag-containing particulates on the outer surface of the porous silicon. Use of an Mg wire anode produced surface-modified porous silicon with no observable Mg on the surface by SEM EDAX, even though a fine white precipitate was observed to form in the solution near the Mg electrode during the electrochemistry. The absence of oxidation or deposition of particulates on the porous silicon allows the Mg wire anode to be placed closer to the porous silicon than is possible with a Pt anode. Thus, positioning the tip of the Mg wire anode ∼2 mm above the porous silicon 7600 DOI: 10.1021/la904408h

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surface led to larger pore wall modification gradients across porous silicon with lower oxidation levels than could be achieved with Pt wire anodes. The position of the wire anode across the porous silicon surface can also be varied to create gradients from one side to the other or concentric gradients. Pore wall compositional gradients in opposing directions on a given porous silicon layer were created to show that it is possible to create multiple compositional gradients using this electrochemical method. Thus, an initial pore wall modification gradient was prepared electrochemically using either 1-bromopentyl acetate or 1-bromodecane, and then a gradient methylation in the opposing direction was perfomed by moving the anode such that a current density gradient in the opposite direction was applied in the second electrochemical modification step. Characterization of Pore Wall Modification Gradients. The pore wall modification gradients have been characterized using two-dimensional FTIR microspectrophotometry and/or attenuated total reflectance (ATR) FTIR. One-directional gradients across porous silicon have been created via electrochemical modification with organohalides, using Pt or Mg wire as the anode. FTIR mapping of the ν(C-O) band at 1240 cm-1 observed for a pentyl acetatemodified and methyl-end-capped porous silicon sample prepared using a Pt wire anode positioned 5 mm above one edge of the cathode (Figure 3) showed formation of a pore wall modification gradient across the porous silicon. The highest level of pentyl acetate surface attachment occurs under the Pt wire anode, and comparison of the FTIR band intensities across the surface suggests that the surface density of pentyl acetate decreases by a factor of 3 over 8 mm. The ν(CdO) band at 1750 cm-1 shows a similar trend across the porous silicon surface, but detailed analysis is affected by the presence of water bands in this IR region. This sample had been methyl “endcapped” using a symmetrical anode placement after the initial gradient modification, and the methyl band at 770 cm-1 shows a reasonably constant level of modification across the porous silicon due to the large amount of “end-capping” that can occur. The ATR FTIR spectra of a decyl-gradient-modified (Mg anode) and then methyl-end-capped porous silicon sample (Figure 4) show clear evidence of the formation of a pore wall modification gradient across the porous silicon. Comparison of the sizes of the C-H stretching bands (2960, 2930, and 2860 cm-1) to the Si-Si and Si-Hx deformation modes (666, 622, and 614 cm-1) or the Si-H stretching bands (ca. 2100 cm-1) shows a large change from the highest level of decyl modification (under the anode) to almost no decyl attachment on the opposite side of the porous silicon (i.e., over a distance of ∼1 cm). Comparison of the FTIR spectra obtained for this sample and those obtained with uniformly modified porous silicon samples showed that the highest level of modification for the gradientmodified sample is similar to the best modification levels obtained during uniform modification of porous silicon with decyl groups. FTIR transmission microscope mapping of decyl-terminated porous silicon surfaces “end-capped” with methyl groups (Pt anode) using the height of the νC-H band at 2926 cm-1 (the band most representative of the decyl modification) also showed formation of a pore wall modification gradient, with the highest level of decyl modification under the Pt wire anode (Supporting Information). Porous silicon samples modified with a gradient of methyl groups can also be prepared, and ATR FTIR examination of these shows an intensity change of the Si-CH3 rocking mode at 770 cm-1 along the gradient (Supporting Information). Langmuir 2010, 26(10), 7598–7603

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Figure 4. ATR FTIR spectra of a decyl-modified Fabry-Perot porous silicon sample using an asymmetric Mg anode placement and a linear current ramp (cathodic current density of 1-5 mA cm-2 for 45 s) measured at different positions across the surface moving away from the edge of the porous silicon closest to the counter electrode. A background of a KRS5/diamond composite crystal in air was used.

Figure 3. (a) Transmission FTIR microscope map showing changes in the peak area intensity of the C-O stretching mode (at ∼1240 cm-1) across a porous silicon sample with a gradient of pentyl acetate groups and then “end-capped” with methyl groups (O, counter electrode position; cathodic current density of 1-5 mA cm-2 for 45 s). (b) Transmission FTIR line map extracted from area map showing the area of the C-O stretching mode (at 1240 cm-1) across the porous silicon sample (dashed line indicates the position of the counter electrode).

Opposing pore wall modification gradients can be created by placing the anode diametrically opposite to its original position prior to forming the second gradient. An FTIR transmission microscope map obtained for a porous silicon sample with a decyl-terminated pore wall composition gradient in one direction (Pt anode) and a methyl-terminated gradient in the opposite direction (Pt anode) is shown in Figure 5. The decyl CH stretching mode has been mapped as peak height intensity rather than area in this figure, since the methyl C-H stretches occur in the same frequency region. An FTIR transmission microscope map of a porous silicon sample with a pentyl acetate-terminated gradient in one direction and a methyl-terminated gradient in the opposite direction (Pt anode for both modifications) was also obtained (Supporting Information). These experiments clearly demonstrate the ability to impose two separate pore wall modification gradients across the porous silicon. However, the incomplete methylation occurring in the second step meant that these samples were not as stable toward subsequent oxidation as those that have been fully end-capped. The efficiency of converting surface-attached pentyl acetate groups to 5-hydroxypentyl groups is shown in the FTIR spectra in Figure 6, with an almost complete loss of the C-O and CdO stretching bands at 1240 and 1750 cm-1, respectively, and the Langmuir 2010, 26(10), 7598–7603

Figure 5. FTIR transmission microscope map showing changes in the (a) peak height intensity of the C-H stretching band (at ∼2926 cm-1) and (b) peak area intensity of the CH3 rocking band (at ∼770 cm-1) across a porous silicon sample with a decyl gradient in one direction and a methyl gradient in the opposite direction (O, counter electrode position).

Figure 6. Transmission FTIR spectra of porous silicon samples modified with (a) pentyl acetate surface and (b) 5-hydroxypentyl. Both samples have been end-capped with methyl groups.

appearance of a new broad band around 3300-3400 cm-1 associated with the O-H stretch indicative of an alcohol-terminated surface. No significant change in the thickness and porosity of the porous silicon film is observed, as determined by the DOI: 10.1021/la904408h

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Figure 7. Change in diffuse reflectivity observed when porous silicon with a methyl-end-capped 5-hydroxypentyl pore wall gradient is immersed in water containing different concentrations of ethanol. The gradient is horizontal in the images, with the maximum 5-hydroxypentyl surface concentration on the right-hand side. Stated values are the ethanol concentration (vol %). Grayscale images shown are the green channels of color images that were taken using an off-specular reflectance imaging geometry.

similarity of the interference fringes in the baseline of the FTIR spectra before and after ester hydrolysis, together with the lack of damage evident in SEM images of these samples. The constancy of the pore wall modification with depth into the porous silicon layer was investigated by comparing the intensities of the FTIR bands associated with the pore wall modifications and the porous silicon obtained using transmission and ATR measurement modes. Transmission FTIR spectra show the level of modification within the whole porous silicon layer because the IR radiation must penetrate the entire sample whereas ATR FTIR shows the level of modification near the outer surface of the porous silicon layer as the IR radiation only penetrates a limited distance into the porous silicon layer. The penetration depth of IR radiation into a material with a refractive index of ca. 1.5 is ∼2 μm for diamond ATR crystals, and the depth of the penetration into the porous silicon layer also depends on the contact between the crystal and the porous silicon. The similarity of the FTIR transmission and ATR spectra of electrochemically modified porous silicon samples indicates the level of modification is constant with depth for porous silicon layers that are 2-3 μm thick. Under conditions where incomplete electrochemical modification occurred (i.e., low current densities or shorter modification times), slightly thicker porous silicon layers (ca. 6 μm) gave ATRFTIR spectra that showed greater modification than did the transmission FTIR measurements. This suggests that electrochemical reduction of organohalides leads to pore wall modification that proceeds from the top down into the porous silicon layer. Therefore, for constancy of modification with depth longer electrolysis times are required for thicker porous silicon samples such as rugate reflectors. However, increasing the current density and/or longer modification time led to increased concomitant oxidation of the porous silicon, particularly with a Pt anode. Because of these factors, gradient modification of porous silicon layers thicker than about 10 μm for which modification was constant with depth could not be achieved via this electrochemical 7602 DOI: 10.1021/la904408h

method. Other possible methods for gradient modification were explored, including photochemical hydrosilylation and hydrosilylation of alkenes in the presence of diazonium salts.28 By imposing a light intensity gradient across porous silicon, a surface modification gradient can be formed by photochemical hydrosilylation of alkenes. However, comparison of ATR and transmission FTIR spectra showed most of the modification occurred near the outer surface of the porous silicon layer rather than being constant with depth. This could be due both to decreased light penetration into the porous silicon layer and because photochemical hydrosilylation only occurs with photoluminescent porous silicon. Fluorescence microscopic examination of the cross section of a freshly etched porous silicon sample prepared in the same way as the samples used for the modifications indicated that the fluorescence was only emitted near the outer surface of the porous silicon layer rather than from the whole layer. The nonuniformity of the photoluminescence within a porous silicon layer has also been noted by other authors.29-31 When a porous silicon sample with a methyl-end-capped 5-hydroxypentyl gradient was immersed in water, water only penetrated into the pores in a small region under where the counter electrode had been placed during the initial modification with 5-bromopentyl acetate (Figure 7). As ethanol was added to the water, the region in which the pores were wetted by the solution increased, with complete wetting only occurring above about 10 vol % ethanol. Thus, the porous silicon sample is responding with a spatial signal to a change in the ethanol concentration;a sensing modality which has not heretofore been reported for porous silicon. Such a spatial response means that the change can be detected by the human eye, rather than requiring instrumentation. (28) Wang, D.; Buriak, J. M. Langmuir 2006, 22, 6214–6221. (29) Prokes, S. M.; Freitas, J. A., Jr.; Searson, P. C. Appl. Phys. Lett. 1992, 60, 3295–3297. (30) Ookubo, N. J. Appl. Phys. 1993, 74, 6375–6382. (31) Sendova-Vassileva, M.; Dimova-Malinovska, D.; Kamenova, M.; Kakanakova-Georgieva, A.; Marinova, T. J. Lumin. 1999, 80, 179–182.

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Table 1. Water Contact Angles at the Outer Surface for All the Terminal Groups Attached to Fabry-Perot Porous Silicon Samples Modified Using Electrochemical Reduction of Organohalidesa terminal groups methyl decyl pentyl acetate 5-hydroxypentylb attached water contact 81 ( 5 80 ( 4 65 ( 13 41 ( 7 angle/deg a All samples were methyl end-capped. Results are mean ( standard deviation for measurements of 10 drops placed at different positions across sample surfaces. b Prepared by deacetylation of pentyl acetatemodified porous silicon.

Water contact angle measurements for porous silicon samples that had been uniformly modified with all the surface modification groups used in this study and then methyl end-capped show that the outer surface of a pentyl alcohol-modified porous silicon sample is more hydrophilic than the other samples prepared, with a substantial decrease in water contact angle being observed upon ester hydrolysis (Table 1). Therefore, a 5-hydroxypentyl gradientmodified porous silicon sample is expected to show a change in water contact angles across the outer surface from more hydrophilic where the 5-hydroxypentyl concentration is the highest to more hydrophobic where the 5-hydroxypentyl concentration is the lowest. However, if the methyl-end-capped 5-hydroxypentyl gradient is prepared using the method described in this paper (via cleavage of the pentyl acetate group in sulfuric acid/NH4Cl/ methanol), the contact angles of water droplets placed at any point on the outer surface of the porous silicon were close to 50°. This result is clearly in contrast with the bulk wetting phenomenon described in the previous paragraph, which shows a marked change in wettability of the pores across the gradient. The FTIR results (vide supra) also show that the pore wall modification within the bulk of the porous silicon changes along the gradient. Porous silicon samples with wettability differences between the outer surface and the interior pores have been reported previously,32,33 and this appears to be another example of this phenomenon.

Conclusions Electrochemical reduction of organohalides on porous silicon using either a Pt or Mg wire anode can give reproducible (32) Kilian, K. A.; Bocking, T.; Gaus, K.; Gooding, J. J. Angew. Chem., Int. Ed. 2008, 47, 2697–2699. (33) Kilian, K. A.; Bocking, T.; Gooding, J. J. Chem. Commun. 2009, 630–640.

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transverse gradients of pore wall modification. The best achievable pore wall modification gradient using electrochemistry was created using a Mg wire anode in close proximity to the porous silicon cathode, as this placement produced a large magnitude of pore wall modification gradient over ∼1 cm. Furthermore, use of Mg wire anodes led to minimal oxidation of the porous silicon during the modification step. The iodine produced in the anode reaction at Pt could be transported to the porous silicon surface and cause oxidation. Therefore, Pt anodes need to be kept at a greater distance from the porous silicon than do Mg anodes or else the electrochemical modification needs to be performed in a sufficiently short time that iodine does not reach the porous silicon. These factors limited the magnitude of surface chemical gradients obtainable with a Pt anode. The gradient-modified porous silicon samples have been characterized using FTIR (ATR, transmission, and transmission microscope mapping) to show the spatial extent of the pore wall compositional gradients. Thin porous silicon samples can be modified evenly with depth through the porous silicon layer using this electrochemical gradient modification technique. Such gradients allow us to vary the interaction of analytes across the porous silicon layer, and this can be monitored either at selected points across the surface or more globally using a CCD camera. This sensing principle is demonstrated by showing that a porous silicon sample with a methyl-end-capped 5-hydroxypentyl pore wall gradient can act as a sensor for the ethanol content of water. As the ethanol concentration increases, a greater proportion of the porous silicon can be fully wetted by the solution, and this leads to a change in reflectivity which is visible to the naked eye. Acknowledgment. The authors acknowledge the University of Auckland for supporting this research. A.M.R. acknowledges a Sigma Xi grant-in-aid of research award supporting travel. Supporting Information Available: FTIR transmission microscope map of methyl end-capped decyl-gradient-modified porous silicon and methyl-gradient porous silicon; FTIR transmission microscope map of porous silicon with a pentyl acetate-terminated gradient in one direction and a methyl-terminated gradient in the opposite direction. This material is available free of charge via the Internet at http:// pubs.acs.org.

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