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Spectroscopic Characterization of Sulfonyl Chloride Immobilization on Silica G . Alan Schick* and Ziqi Sun Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212 Received January 3, 1994. I n Final Form: July 8, 1994@ The immobilization reaction of benzenesulfonyl chloride (BSC) from the gas phase onto aminosilanemodified silica surfaces has been characterized by X-ray photoelectron spectroscopy ( X P S )and Fouriertransform infrared (FTIR)measurements. Porous (60-Adiameter pores) and nonporous silica substrates were investigated. The silane modification was accomplished by a gas-phase adsorption of 3-aminopropyltrimethoxysilane (APS). The nitrogen Is (Nls) XPS photopeaks of the modified substrates exhibit components at 398.8 and 401.0 eV, assignable respectively to free and protonated primary amine groups. Upon subsequent adsorption of BSC, a new signal from sulfur is observed at a binding energy of 168.8 eV, similar to values observed previously for sulfonamide compounds. Additionally, the 398.8-eV Nls photopeak decreases in intensity, and an additional component appears at 399.9 eV, consistent with the transformation of the primary amine to an amide functionality. FTIR spectra of APS-modified surfaces exhibit bands at 3298 and 3367 cm-', due to the symmetric and asymmetric N-H stretching modes, and at 3176 cm-l due to the -NH2 scissor overtone. Upon immobilization of BSC, the two bands associated with N-H stretch modes of the primary amine collapse to a single broad band at 3282 cm-l, consistent with the formation of an amide. Also, a number of bands associated with phenyl vibrations are observed, evincing the presence of BSC on the surface. From the FTIR and XPS data it is observed that varying extents of BSC coverage are obtained from similar experiments. Generally, BSC coverage is less on the porous silica, suggesting that the pore structure may hinder diffusion of BSC into the pores. Residual protonated amine groups are consistently observedfollowingthe reactions,which may also prevent complete BSC coverage.
Introduction The use of silane couplingagents for surface modification is an important and well-established field.1-3 As the foundation for silane-based self-assembly (SA) methods, these agents are useful for immobilizing specific chemical species onto surfaces in organized monolayer (ML) assemblies.6 Such ML's exhibit superior thermal and mechanical stabilities, owing to the covalent nature of the molecule-surface interactions. In recent years, highly organized monolayer (ML) films of organic dyes have become target components of photoconductors, optical actuators, and chemical sensor^.^^^ In this respect, SA methods have been used for immobilizing a number of electrochemically active dye molecules, such as porphyrin complexes, onto solid s u b ~ t r a t e s . ~ - l ~ The coupling chemistry for the immobilization of organic dyes is typically accomplished by using a condensation scheme, such as reactions between surface-bound primary amines and either carbonyl or sulfonyl chloride substitu-
* Corresponding author. Abstract published in Advance A C S Abstracts, September 1, 1994. (1)Plueddmann, E. P. Silane Coupling Agents; Plenum: New York, 1991. (2)Swalen, J. D.;Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Israelachvili, J.; McCarthy, T. J.; Murray, R. F.; Rabolt, J. F.; Wynne, K. J. Langmuir 1987,3,932. (3)Leyden, D.E., Ed. Silanes, Surfaces, and Interfaces: Proceedings of the Silanes, Surfaces, and Interfaces Held at Snowmass, Colorado, June 19-21, 1985; Gordon and Breach: New York, 1986. (4) Honeybourne, C. L. J . Phys. Chem. Solids 1987,48,109. (5)Tredgold, R. H.;Young, M. C. J.;Hoorfar, A. IEEProc. 1986,132, 151. (6)Ulman, A. An Introduction to Ultrathin Organic Films; Academic: New York, 1991;p 237. (7)Czolk, R.; Reichert, J.;Ache, H. J. Sens.Actuators B 1992,7,540. (8)Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.-N.; Schultz, F. A,; Murray, R. C. J . A m . Chem. SOC.1980,102,7629. (9)Rocklin, R. D.;Murray, R. C. J . Electroanal. Chem. 1979,100, 271. (10)Fujihira, M.; Kubota, T.; Osa, T. J . EZectroanaE. Chem. 1981, 119,379. @
ents which form amide or sulfonamide linkages.'JO These and similar immobilization strategies have been extensively developed and characterized for various protein labeling purposesll but have not been characterized in any quantitative fashion for densely-packed ML assemblies. Although qualitatively the reactions apparently proceed as expected, the high density of binding sites in organized ML's gives rise to questions regarding total coverage of the immobilized chromophores. Furthermore, organic dyes such as porphyrins are notorious for strong physisorption onto various types of silicate m a t e r i a l ~ , l ~ - ' ~ so simple detection of dye presence on surfaces following an adsorption process is not necessarily diagnostic. Thus, direct evidence for the expected immobilization scheme is necessary for characterizing the ultimate success of these types of binding strategies. In this work we characterize the coupling reaction between surface-bound primary amine and sulfonyl chloride substituents. A gas-phase adsorption procedure is used to modify both porous ( S o d diameter pores) and nonporous silica powders with 3-aminopropyltrimethoxysilane (APS).The reaction between the terminal amine groups of APS and sulfonyl chloride is effected by a subsequent gas-phase procedure using benzenesulfonyl chloride as a model dye molecule. This two-step immobilization scheme can be described as
where solid lines with hash marks are used to depict the (11)Haughland, R. P. Handbook ofFZuorescent Probes and Research Chemicals; Molecular Probes: Eugene, OR, 1992;p 5. (12)Schick, G. A.; Sun, Z. Thin Solid Films 1994,248,86. (13)Cady, S. S.;Pinnavaia, T. J. Znorg. Chem. 1978,17,1501.
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substrate surfaces. X-ray photoelectron spectroscopy (XPS)and Fourier-transform infrared (FTIR) spectroscopy are used to assess the coverage and success ofthe expected sulfonamide condensation.
Experimental Section 3-Aminopropyltrimethoxysilane(APS),benzenesulfonyl chloride (BSC), and N-methyl-p-toluenesulfonamide(TSA, purity 98%)were purchased from Aldrich and were used without further purification. A porous silica powder (Grace “Davisil”grade 633, 60-Apores,BETsurface area 520 mz/g)was used as the adsorption substrate for the XPS characterizations, and a nonporous fumed silica powder, Aerosil 380 (Degussa Co., BET area 375 m2/g), was used as the adsorption substrate for the FTIR characterizations. The powders were cleaned by the method described previously.12 Solvents used for surface cleaning were reagent grade. High-purity water was produced by a reverse-osmosis/ deionizatiodultrafiltration system described previously.12 The APS modificationof silica and subsequent immobilization of BSC were accomplished as gas-phase reactions by using the experimental scheme originally reported by Wikstrom et aZ.16 and later modified by Basiuk and Chuik0.l’ X-ray photoelectron spectra ( X P S ) were obtained a t room temperature (22.8 “C)by using a Perkin-Elmer PHI 5300 system with an operating pressure of 5 x lo-* Torr. Radiation was provided by a MgK(1253.6 ev) achromatic X-ray source operating at 15 keV and 400 W of power (incidence power 20 W/cm2).The photoelectrons were analyzed for a 45” take-off angle. Survey scans were collected for the binding energy range 0-1100 eV, and narrow scans of 20 eVranges were used to monitor individual elements. Resolution and reproducibility were both 0.1 eV for the narrow-region scans. The powder samples were mounted by using double-stick tape. Internal calibration was provided by referencing the spectra to the C l s photopeak a t 285.0 eV. Curve fitting of the narrow-region spectra was performed using the supplied Scienta ESCA3OO data-system software. Transmission FTIR spectra were recorded with a Nicolet Model 710 spectrometer equipped with a liquid nitrogen cooled mercury cadmium telluride detector sensitive over the range of 5004000 wavenumbers. The modified Aerosil380 substrate powders were pressed into KBr pellets (approximately 20 wt %) a t 15 000 psi under evacuation. The integrities ofthe ML’s in pellet samples were verified by XPS, which gave identical results for powder vs pressed pellet samples. All FTIR spectra were averages of 256 scans with 4-cm-1 resolution.
Results XPS survey spectra of clean, APS-modified, and APS+BSC-modified Davisil (porous) silica appear in Figure 1, and relative surface concentrations of the relevant elements are given in Table 1. For the clean sample, a trace amount of surface carbon is detected, which indicates some amount of sample contamination between cleaning and measurement. Upon APS modification, the carbon signal grows substantially and a nitrogen signal is observed. Following the subsequent treatment with BSC, additional intensity is seen for carbon, and new signals are observed for sulfur and chlorine. These data are consistent with previous observations and constitute the expected results for the immobilization of BSC.12 Identical results were obtained from Aerosil substrates. Beyond simply detecting the presence of the expected compounds on the surfaces, narrow-region XPS scans can help to quantify the chemistry of the adsorption process. Here, the sensitivity ofXPS to the chemical environments of the specific elements can lend insight into the nature (14) Van Damme, H.; Crespin, M.; Obrecht, F.; Cruz, M. I.; Fripiat, J . J. J. Colloid Znterface Sci. 1978, 66,43. (15)Bergaya, F.; Van Damme, H. Geochim. Cosmochim.Acta 1982, 46,349. (16) Wikstrom, P.; Mandenius, C. F.; Larsson, P.-0. J. Chromatogr. 1988,455, 105. (17) Basiuk, V. A,; Chuiko, A. A. J. Chromatogr. 1990, 521, 29.
SW
400 3W 200 Binding Energy (eV)
100
Figure 1. XPS survey spectra of Davisil (porous) silica powders: (a, bottom) clean, (b, middle) APS-modified, and (c, top) APS+BSC-modified. The traces have been displaced vertically for clarity. The ordinate scale is the same for the three traces. The photopeaks are labeled according to the element and orbital of their origin. Table 1. Relative XPS Elemental Surface Conditions for Clean and Modified Porous Silica Powders APS ASPSBSC elementa clean (%) modified (%) modified (%) Si2p 31.8 29.7 23.1 01s 61.4 52.0 47.1 Cls 6.8 14.9 24.7 Nls n.0.b 3.4 2.7 2.5 S2P n.0. n.0. c12p n.0. n.0.
