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Langmuir 1996,11, 2322-2324
Sulfonate-Functionalized,Siloxane-Anchored, Self-AssembledMonolayers Rochael J. Collins and Chaim N. Sukenik* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 Received January 19, 1995. In Final Form: March 30, 1995 Functionalized, siloxane-anchored, self-assembled monolayers (SAMs)have been shown to provide uniform, stable, and versatile modification of oxidehydroxide-bearing surfaces.1-6 In cases where the desired surface functionality ( C H Z ) can be incorporated into a surface-reactive alkylsilane (e.g., a n alkyltrichlorosilane), the direct deposition of compounds of the general structure C13SiCHz(CH2),CHzX ( 1 )from organic solution onto a clean solid substrate can effectively generate uniformly functionalized surfaces. In instances where the desired X group cannot be directly deposited, it can often be installed by a n in situ transformation of a suitable precursor. Sulfonated surfaces have found extensive application in b i o m a t e r i a l ~and ~ in various surface-controlled processes; hence, the efficient creation of CHzSOaH-bearing surfaces is a particularly important goal.1,6 We have previouslyreported2 the in situ creation of surfaces bearing CH2S03H groups based on the oxidation of the CHzSH group (which was in turn derived from the hydrolysis or reduction of either CHzSCN or CHzSCOCH3)using H20d CH3COOH (a reagent also used for the in situ oxidation of a SAM containing sulfide to one containing sulfone5). Calvert and co-workers have shown in a closely related (shorter chain) system that this procedure yields incomplete oxidatioa6 As a n alternative (superior) oxidation procedure, they have reported618 the photo-oxidation of siloxane-anchored surface thiols (directly deposited as a trialkoxysilane monomer). However, the fact that such a photochemical approach is limited to line-of-sight surface treatment, coupled with the fact that none of the reported transformations provide surfaces which show the complete wetting that would be expected for a truly uniform, hydrophilic, sulfonated surface, has prompted the search for alternative surface preparations. When compared to procedures based on thiol oxidation, using a thioacetate (CH2SCOCH3) as the precursor functionality has the attraction of ease of synthesis and film formation, good compatibility with the reactive Sic13 group, long-term stability under ambient conditions, and good W chromophoric properties of the thioacetate moiety. On the basis of reports of the solution oxidation of thioacetates to sulfonic acids, we have explored the direct oxidation of thioacetate-functionalized SAMs to the (l)Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A,; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994,264, 48-55. (2)Balachander, N.; Sukenik, C. N. Langmuir 1990,6,1621-1627. (3)Lee, Y.W.;Reed-Mundell,J.; Sukenik, C. N.;Zull, J. E. Langmuir 1993,9, 3009. (4)(a) Netzer, L.; Sagiv, J. J. Am. Chem. SOC.1983, 105, 674.(b) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989,5, 1074-1087. ( 5 ) Tillman, N.; Ulman, A,; Elman, J. F. Langmuir 1989,5, 1020. (6)Bhatia, S.K.;Teixeira, J. L.; Anderson, M.; Shriver-Lake, L. C.; Calvert, J. M.; Georger, J. H.; Hickman, J. J.; Dulcey, C. S.; Schoen, P. E.; Ligler, F. S . Anal. Biochem. 1993,208,197-205. (7)See, for example: Okkema, A. Z.; Giroux, T. A,; Grasel, T. G.; Cooper, S . L. Mater. Res. SOC.Symp. Proc. 1989,110, 91-96. ( 8 ) (a) Bhatia, S. K.; Hickman, J. J.; Ligler, F. S. J.Am. Chem. SOC. 1992,114,4432-4433.(b) For related photo-oxidations of alkanethiols on gold, see: Huang, J.;Hemminger, J. C. J . Am. Chem.SOC.1993,115, 3342-3343. Tarlov, M. J.; Burgess, D. R. F.; Gillen, G. J.Am Chem. SOC.1993,115, 5305-5306.
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desired sulfonic acid bearing films. Solutions of HzOd CH&OOH,g oxone (potassium peroxymonosulfate)/water,1° and HzOdHCOOHll were compared, along with thioacetate photo-oxidation. The goal was to identify a procedure that does not undermine the integrity of the monolayer film, but does quantitatively oxidize the sulfur. The initially deposited thioacetate SAM and each of the oxidized surfaces were evaluated using XPS, wetting, and ellipsometry measurements. The thioacetate group emerges as a n excellent oxidation substrate. Analysis of the effectiveness and relative merits of these various oxidants is reported herein.
Experimental Section Preparationand Characterizationof Monolayer Films.
Silicon wafers (Silicon Sense Inc., prime grade Si, p-typeboron doped, orientation (loo), thickness of 525 & 25 pm) were cut and then cleaned by sequential sonication in acetone, CH30H, and CHzClz. Treatment ofthe Si with piranha solution (HzS04/HzOz, 70130 vlv, 80 "C; CAUTION PIRANHA solution is a strong oxidant and will react violently with many organics! Handle with care!) yielded an oxide layer which was measured by ellipsometry to be 15 & 3 A thick and a surface that was totally wetted by water. These wafers were coated with thioacetate S A M s using a solution of 1(n= 14, X = SCOCH3)in dicyclohexyl (Aldrich, distilled and passed through activity 2 alumina) as described previously.2 Briefly, the thioacetate alkyltrichlorosilane was prepared by hydrosilylation of the correspondingolefin (preparative HPLC purified),which was obtained by nucleophilic displacement of bromide from u-hexadecenylbromide by potassium thioacetate. Hydrosilylation of u-hexadecenyl thioacetate was typically accomplished by 2-3 days' reaction with HSiC13 in C c 4 at 45-50 "C, using chloroplatinic acid catalyst. The final purification was achieved by bulb-to-bulb distillation at reduced pressure. Deposition times for monolayer formation were often 5-10 h (depending on the ambient humidity). Extensive cleaning of substrates after deposition with CHzClz (sonication and wiping)was needed. The thickness and wetting behavior of the thioacetate S A M were measured. XPS showed the expected sulfur and carbon signals. In situ Oxidation Procedures. Solutions of oxidants and the conditions for their use were as follows: 30%HzOz in 99.7% glacial acetic acid ( 1 5 v/v), 42-45 "C; oxone (Aldrich)in water (saturated, pH = 1.81, ambient temperature; 30%HzOz in 98% formic acid (Fluka, 1:lOvlv, pH = O . l ) , 0 "C. Thioacetate-bearing substrates were treated by immersing them into a beaker of the indicated oxidant (typically for 1h), withdrawing them from the solution, rinsing with doubly distilled water, and drying with a stream of nitrogen. Photo-oxidation involved irradiation for 1 h intervals (ambient temperature, oxygen purge) using a UVP Model W G - 5 4 lamp at 254 nm (intensity 2200pW at 3 in.) held 2 in. from the surface of the flat sample. In some instances initially oxidized surfaces were soaked in ethanol (60-90 min, ambient temperature), rinsed with doubly distilled water, 5% HC1, and a second portion of water, and dried with nitrogen. They were then characterized andor further oxidized as described above. The surface of all in situ modified wafers was characterized by wetting,XPS,and ellipsometrymeasurements. Reference UV spectroscopy of solution samples (Varian Cary 2300 Wlvis spectrophotometerwith a DS-15 data station) was done in quartz cuvettes (1 mm path length). Wetting. A Rame-Hart Model 100 contact angle goniometer was used for wettability measurements. Advancing contact angles were determined by placing a drop (approximatevolume 3 pL) of water on the sample with a microsyringe and advancing the volume (adding approximately 2 pL), keeping the area in contact with the substrate constant and leaving the syringe in the drop. Receding contact angles were determined by withdrawing the water until the lowest angle was achieved, without changing the area of the drop in contact with the substrate. (9)Showell, J. S.;Russell, J. R.; Swern, D. J. Org. Chem. 1962,27, 2853-2858. (10)Reddie, R. N. Synth. Commun. 1987,17 (9),1129-1139. (11) Higashiura, K.;Ienaga, K. J. 0rg.Chem. 1992,57,764-766.
0 1995 American Chemical Society
Notes
Langmuir, Vol. 11, No. 6, 1995 2323
Thickness. Ellipsometric measurements were made using a variable angle spectroscopic ellipsometer (J.A. Woollam Co.) with a xenon source and a 1mm spot. The instrument was always calibrated against 250ASiOz on Si. Data was collected at takeoff angles of 75-77" (1" increments) and at wavelengths of 30008000 A (100 A increments). Each data point was averaged over five revolutions of the analyzer. Data were processed using V.A.S.E. software version 1.1 (J. A. Woollam). Thickness measurement accuracy was f1A. Ellipsometrically determined
thicknesseswere compared to theoretical values calculated using the simplifying assumptions of the monolayer being perpendicular to the substrate and alkyl chains in an extended, alltrans conformation, with standard bond angles and lengths. X P S . XPS measurements were done on a Perkin-ElmerESCA Torr and a 5400 using an Al Ka source at a vacuum of takeoff angle of 20". High-resolution multiplex spectra were collected on a 1 mm spot, using 50 eV pass energy and an acquisition time of 20 min. Peak positions were referenced to C l s at 284.7 eV.
Table 1. Oxidation of Thioacetate-Bearing Monolayer Filmsa contact angle XPS s 2p3'21 (advhec) 2P"Z (% S) treatment of S A M film 72" f l"165" f 2" 164.81163.6 eV (100) thioacetate S A M (unoxidized) HzOz/CH&OOH
12" & 2"1
