Self-Assembled Monolayers on Polymer Surfaces: Kinetics

Langmuir 1999, 15, 5323-5328

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Self-Assembled Monolayers on Polymer Surfaces: Kinetics, Functionalization, and Photopatterning Peter Bo¨hme,† Ganesh Vedantham,‡ Todd Przybycien,‡ and Georges Belfort* Howard P. Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 Received November 2, 1998. In Final Form: April 20, 1999 Monolayers of bifunctional bolaamphiphiles were self-assembled on a polymeric substrate (spin-coated film of poly(allylamine hydrochloride)). Various features of the novel bifunctional, well-ordered, and relatively stiff monolayer and the subsequent reactions of the exposed benzoyl azide groups were investigated. The characterization of the monolayers and the kinetics of self-assembly was determined via reflectionabsorption infrared spectroscopy and ellipsometry. Atomic force microscopy and ellipsometry were used to measure roughness and thickness, respectively. From molecular modeling and ellipsometry, the bolaaphiphiles in the monolayers were found to be inclined at an angle of about 68° to the normal. In addition, the monomolecular layers appeared to be fairly crystalline. Serial functionalization of the exposed benzoyl azide groups with dansyl-cadaverine in the liquid phase followed by propylamine in the gas phase illustrated a stable, dense, well-ordered, two-dimensional monomolecular layer of bolaamphiphiles. Photopatterning, via UV irradiation of exposed benzoyl azide groups, was also demonstrated. The novelty of the surface modification procedure described here is the extraordinary ease with which selfassembly took place and the stability of the resulting monolayers which formed a highly reactive new surface suitable for further modification. Arranging molecules in a tidy ordered array on a polymeric surface is a main result of this work.

Introduction Organization of molecules in two dimensions has long been recognized as a method to provide well-defined, ultrathin and ordered materials with novel barrier or microstructural properties and well-controlled surface characteristics.1,2 Langmuir-Blodgett (LB) films were the first systems with artificial monolayer organization3 and have been used extensively for studying biological surfaces, for applications in material science,2 and recently for studying cooperative behavior of catalytically active metal complexes.4 However, they suffer from low stability, are sensitive to contamination, are labor intensive, and require special know-how and specialized equipment.5 Molecular self-assemblies are a superior path for forming stable monomolecular films. The assembly process is thermodynamically driven and requires less special attention or equipment than the preparation of LB monolayers. Self-assembled monolayers (SAMs) have, most often, been formed over metalic supports.1,2,6-11 They have become important in studies of surface-dependent processes such as wetting, adhesion, surface forces, friction, and adsorption.12,13 It has recently been shown * Corresponding author. Fax: 518-276-4030. E-mail: belfog@ rpi.edu. † Present address: BASF Schwarzheide, Research Department, 01561 Schwarzheide, Germany. ‡ Present address: Department of Chemical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213. (1) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (2) Ulman, A. Introduction to Thin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (3) Kuhn, H. Thin Solid Films 1983, 99, 1. (4) To¨llner, K.; Popovitz-Biro, R.; Lahav, M.; Milstein, D. Science 1997, 278, 2100. (5) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH Publishers: New York, 1994; Chapter 4. (6) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513.

that dense monomolecular layers with functional surfaces could be obtained using R,ω-disubstituted amphiphiles for the preparation of SAMs.14-17 In principle, the constitution of a monolayer surface depends on the molecular structure of the amphiphile. For example, Whitesides and co-workers have prepared SAMs of ω-substituted alkanethiols on gold surfaces in order to use the chemically defined surfaces for comparison of contact angle measurements.18 The preparations of ω-substituted SAMs are expected to have significant applications in biosensors and biomaterials.19 Lo¨fås20 has used SAMs for the functionalization of optical sensor surfaces intended for biospecific interaction analysis. This mass-sensitive, realtime analysis technique with label-free detection has been used by various investigators for analysis of qualitative and quantitative biospecific interactions, including affinity and kinetic rate constants.21,22 Pritchard and co-workers23 described the use of SAMs prepared from a nitrophenyl (7) Haller, I. J. Am. Chem. Soc. 1978, 100, 8050. (8) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (9) Maoz, R.; Sagiv, J. J. Colloids Interface Sci. 1984, 100, 465. (10) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (11) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (12) Ulman, A. CHEMTECH 1995, 3, 22. (13) Ulman, A. Chem. Rev. 1996, 96, 1533. (14) Wollman, E. W.; Frisbie, C. D.; Wrighton, M. S. Langmuir 1993, 9, 1517. (15) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (16) Rozsnyai, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993. (17) Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996. (18) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 1990. (19) Mrksich, M.; Whitesides, G. M. TIBTECH 1995, 13, 228. (20) Lo¨fås, S. Pure Appl. Chem 1995, 67, 829. (21) Cooper, T. M.; Campbell, A. L.; Crane, R. M. Langmuir 1995, 11, 2713. (22) Jordan, C. E.; Corn, R. M. Anal. Chem. 1997, 69, 1449. (23) Pritchard, D. J.; Morgan, H.; Cooper, J. M. Angew. Chem. 1995, 34, 91.

10.1021/la981548a CCC: $18.00 © 1999 American Chemical Society Published on Web 06/10/1999

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Figure 1. (a) Schematic of 1,12-bis(benzoic acid azide) amphiphile consisting of two benzoic acid azide headgroups linked to an alkyl chain through amide bonds, designated as DBAzCn, where n ) 10 is the number of -(CH2)- groups in the alkyl chain. (b) Filled molecular ball-and-stick model of two DBAzC10 molecules covalently bound to poly(allylamine hydrochloride) (PAAm) film spin-coated on a gold coated slide at an angle of 68° to the normal. In addition to π-π and hydrophobic interactions, stabilization through hydrogen-bonding between at least one amide group(s) of neighboring molecules is shown. The dark atoms are the carbonyl groups. These structures were drawn and optimized using SYBYL 6.01 (Tripos Associates, Inc., St. Louis, MO). (c) Relevant dipoles and their orientation with respect to the horizontal gold surface of a DBAzC10 molecule during self-assembly on PAAm surface shown schematically: a, ν(CdO)azide; b, ν(CdC)aromatic; c, ν(C-N)amide; d, ν(CdO)amide; e, ν(N-H)amide. See Table 1 for wavenumber assignments for the DBAzC10 molecules as a pure substance and in a self-assembled monolayer.

azide terminated amphiphile for the immobilization of immunoglobulin G on gold and glass surfaces. Tirrell and co-workers24,25 have studied the formation, interaction, structure, and stability of photoreactive bolaform amphiphile multilayers. The tailored modification of polymeric surfaces has engendered wide interest with respect to applications for these materials.26,27 To date, relatively little has been published on the use of SAMs for modifying the surface of polymeric materials. Whitesides, Chaudhury, and others investigated the self-assembly behavior of ω-substituted trichlorosilanes onto chemically oxidized polyethylene film surfaces.28-32 They showed that dense and ordered monolayers were formed with the alkyl chains in an all-trans configuration. Chemical modification of the exposed surface of the SAM was possible without destroying the layer structure. McGarvey and co-workers33 have shown that ω-substituted alkyl acid azide amphiphiles are also able to form adsorbed monolayers on the surface of polymer films. Fuhrhop and co-workers have synthesized a new class of amphiphiles consisting of one or two benzoic acid azide headgroups linked to an alkyl chain through amide bonds.34,35 Bo¨hme et al. have successfully used bolaamphiphiles, amphiphiles containing two identical head(24) Tsao, Y.-H.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461. (25) Mao, G.; Tsao, Y.-H.; Tirrell, M.; Davis, H. T. Langmuir 1995, 11, 942. (26) Ratner, B. D. J. Biomed. Mater. Res. 1993, 27, 837. (27) Ulbricht, M.; Belfort, G. J. Appl. Polym. Sci. 1995, 56, 325. (28) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921. (29) Chaudhury, M. K.; Whitesides, G. M. Langmuir 1991, 7, 1013. (30) Ferguson, G. S.; Chaudhury, M. K.; Biebuyck, H. A.; Whitesides, G. M. Macromolecules 1993, 26, 5870. (31) Chaudhury, M. K.; Owen, M. J. J. Phys. Chem. 1993, 97, 5722. (32) Chaudhury, M. K. Biosens. Bioelectron. 1995, 10, 785. (33) McGarvey, C. E.; Holden, D. A.; Tchir, M. F. Langmuir 1991, 7, 2669. (34) Fuhrhop, J.-H.; Ko¨ning, J. In Molecular Assemblies and Membranes: Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; Royal Soc. Chem.: London, 1994; p 1.

groups, for the preparation of Langmuir-Blodgett monolayers on the surface of poly(acrylonitrile) films.36 The chemical structure of this kind of bolaamphiphile is shown in Figure 1a. Two self-assembled molecules bound to poly(allylamine hydrochloride) (PAAm) surface (using a balland-stick model) are shown in Figure 1b. In Figure 1c, the relevant dipoles and their orientation with respect to the horizontal gold surface of a particular bolaamphiphile molecule during self-assembly on PAAm surface are shown schematically. In this paper, we report on a process of creating SAMs of symmetrically substituted bis(benzoic acid azide) amphiphiles over a polymeric surface, on the characterization of this process and on the functionalization of the exposed reactive groups on the SAMs. The new interface formed after self-assembly was composed of reactive benzoyl azide groups that were then used for grafting various functionalities and for demonstrating photopatterning with UV irradiation. Materials and Methods Materials. All the chemicals were used as received from Aldrich Chemicals (Milwaukee, WI). The chemicals required for synthesis of 1,12-dodecanediacid dibenzoyl azide amide (see Figure 1a, n ) 10, DBAzC10) and formation and characterization of SAMs were the following: N,N-dimethylformamide (DMF); tetrahydrofuran (THF); p-amino benzoic acid; triethylamine (TEA); diphenylphosphoryl azide; poly(allylamine hydrochloride) (PAAm, molecular weight 50-65 kDa); dodecanedioyl dichloride. Details of the synthesis and the purification procedures of the bolaamphiphiles and analytical methods and data can be found in the literature.34-36 Dansylcadaverine was obtained from Fluka Chemicals (Basel, Switzerland). In addition, various solutions used for cleaning the metallic surface such as Chromerge (chromic-sulfuric acid), methanol, hexane, and dichloroethane were also purchased from Aldrich Chemicals. Deionized (DI) (35) Bo¨hme, P.; Hicke, H.-G.; Ulbricht, M.; Fuhrhop, J.-H. J. Appl. Polym. Sci. 1995, 55, 1495. (36) Bo¨hme, P.; Hicke, H.-G.; Boettcher, C.; Fuhrhop, J.-H. J Am. Chem. Soc. 1995, 117, 5824.

Self-Assembled Monolayers on Polymer Surfaces water was of ultrahigh purity and high resistivity (16-18 MΩ cm). Gold-coated glass slides were purchased from Spectra Tech (Shelton, CT). Substrate Preparation. Thin films of PAAm were spincoated (Photoresist spinner, Headway Co., Garland, TX) on goldcoated glass slides, and reflection absorption infrared (RAIR) spectroscopy was used for the kinetic study. Silicon wafers were used for surface characterization with ellipsometry and atomic force microscopy (AFM). A 0.5% solution of PAAm in water was applied to the substrate and spun at 3000 rpm for 40 s and then dried at 50 °C under vacuum for 12 h. Prior to spin coating, the gold-coated glass slides and the silicon wafers were thoroughly cleaned with Chromerge and treated in an ultrasonic bath with three different solvents: methanol, hexane, and dichloroethane. SAM Preparation. SAMs were prepared on spin-coated polymeric substrates by placing the substrate in either 0.1% (w/ v) or 0.05% (w/v) solution of DBAzC10 dissolved in THF; the solution was incubated in an ultrasonic bath for 60 s to achieve complete dissolution. To investigate the kinetics of self-assembly, different samples were treated under identical conditions for different time periods. After removal from the DBAzC10 solution, the samples were immediately washed three times with pure THF and placed into a beaker with fresh, pure THF for 10 min. After drying, the samples were analyzed using RAIR or ellipsometry. The time period between preparation and analysis was shorter than 60 min in all cases in order to avoid or reduce interference from molecular rearrangement processes at the surface. The time-dependent stability of immobilized SAMs on PAAm was determined by storing samples for a longer time period in different environments (solvents and air). Infrared Measurements. To follow the kinetics of selfassembly and its chemical nature, samples were studied by RAIR spectroscopy. Spectra were taken on a Nicolet Magna-IR 550 Series II spectrometer (Madison, WI) with a MCT/A detector at 4 cm-1 resolution. Typically, 1000 background spectra were collected before each sample measurement. For RAIR, a reflection attachment equipped with a double diamond polarizer was used at an angle of 84°. Transmission spectra of the pure compound were taken with the sample mixed in a potassium bromide pellet. Original measured spectra are shown without baseline corrections or other modifications. The peaks of the intense CO2 spectra (2270-2370 cm-1) were, however, removed since none of the relevant peaks of the compounds studied here were in this range. AFM Measurements. AFM measurements were obtained with an AutoProbe CP (Park Scientific Instruments, Sunnyvale, CA) instrument. A 5 µm × 5 µm scanning head with a 100 Å Si3N4 tip was used in a contact-scanning mode. To eliminate external vibrational noise, the AFM was mounted on a vibrationtable (Technical Manufacturing Corp., Peabody, MA). Multiple images at different spots with various magnifications were obtained to ensure reproducibility. Ellipsometry. Ellipsometric measurements were made on Model II-004 Rudolph Instrument (Fairfield, NJ) using a HeNe laser (wavelength of 6328 Å) and an incident angle of 70°. Measurements were taken at three different spots on each sample. The ellipsometer and spin-coater were located in a Class 100 clean room at the Center for Industrial Innovation at Rensselaer Polytechnic Institute.

Results and Discussion Surface Characterization Using AFM/Ellipsometry. Figure 2 shows a typical AFM image of the PAAm film surface obtained after spin coating on a silicon wafer. The resulting film surface was smooth and flat with a median height difference of 12 Å and an average roughness of 3.2 Å. At least 5 different images were taken for each sample, and inhomogeneities such as pores, scratches, or bulbs were not observed. Also, exposure of the PAAm film to pure THF did not result in any changes in the surface morphology as observed with ellipsometry. No indications of swelling or other changes in the film structure were obtained after THF exposure for 24 h. The thickness of the film on the silicon wafers was 142 ( 0.2 Å, and that on gold-coated glass slides was 91 ( 6.3 Å. The refractive

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Figure 2. Atomic force microscopy image of the surface of a spin-coated film of poly(allylamine hydrochloride) (PAAm) taken in contact mode. The probe tip was Si3N4 with a diameter of 100 Å. The RMS roughness of the film was 3.4 Å. The average roughness was 3.2 Å and the median height was 12 Å.

Figure 3. Kinetics of self-assembly. A series of RAIR spectra is shown illustrating the evolution of the surface monolayer indicated by the increase in azide peaks [ν (N3)as at 2140 cm-1 and ν(N3)sy at 1270 cm-1] and the amide peaks [ν(C-N)amide I at 1680 cm-1 and ν(C-N)amide II at 1570 cm-1] for (a) a spincoated poly(allylamine hydrochloride) (PAAm) substrate film on a gold-coated slide and during the self-assembly of DBAzC10 molecules on PAAm after (b) 90 min, (c) 180 min, and (d) 600 min.

index of silicon was 3.882 + 0.019i whereas that for gold was 0.1 + 3.1i; silicon wafers were used for all ellipsometric measurements since the error in computing the film thickness is proportional to the complex part of the substrate refractive index.37 Kinetics of Self-Assembly. Reflection adsorption infrared spectroscopy (RAIR) has been successfully used to investigate the formation and the molecular structure of surface monolayers.38,39 Figure 3 shows a series of RAIR spectra illustrating the evolution of the azide peaks [ν(N3)as at 2140 cm-1 and ν(N3)sy at 1270 cm-1] and the amide peaks [ν(C-N)amide I at 1680 cm-1 and ν(C-N)amide II at (37) Tompking, H. G. A User’s Guide to Ellipsometry; Academic Press Inc.: Boston, MA, 1993. (38) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946. (39) Allara, D. L. In Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heinemann: Boston, MA 1995.

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Figure 4. Kinetics of self-assembly of DBAzC10 molecules in THF onto glass-coated slides using RAIR from a solution concentration of 0.05% (w/v) ([) and 0.10% (w/v) (b) and onto a silicon wafer using elipsometry from a solution concentration of 0.05% (w/v) (9).

1570 cm-1] during self-assembly of DBAzC10 on PAAm. Figure 4 shows the kinetics of self-assembly onto glass coated slides of 0.05 and 0.1% (w/v) DBAzC10 in THF by RAIR (at 2140 cm-1) and onto a silicon wafer of 0.05 (w/v) DBAzC10 in THF by ellipsometry. It can be seen that, for the 0.05% (w/v) concentration during the formation of the monolayer DBAzCl0, the azide peak intensity reaches a plateau within 13.3 h. The shape and the slope of the curves are very similar for both the techniques. From the thickness of the monolayer (11.6 ( 0.1 Å), using ellipsometry, and from calculations of the structure of the bolaamphiphile using SYBYL 6.01 (Tripos Associates, Inc., St. Louis, MO), it was found that the molecules forming the SAM were inclined at an angle of about 68° to the normal. This orientation might be the result of a combination of stabilization through hydrogen bond interactions between at least one amide group of each molecule and its nearest neighbor, π-π interactions between neighboring benzene rings, and/or hydrophobic interactions between the -(CH2)- strand of one molecule and its neighbor (Figure 1b). In this ball-and-stick model, the dark atoms are the carbonyl groups. Increasing the DBAzC10 concentration to 0.1% (w/v) in THF resulted in faster reaction kinetics (Figure 4). Monolayer Characterization. The RAIR difference spectra of a PAAm film before and after immersion in a 0.05% (w/v) solution of DBAzC10 in THF is depicted in Figure 5 b,c, respectively. Also shown in Figure 5a is the transmission spectrum of neat DBAzC10 powder mixed in a KBr pellet. The peak positions of the spectra seen in Figure 5 are summarized in Table 1 for both the selfassembled monolayer and the pure substance. All of these characteristic peaks were detected in the RAIR spectrum after self-assembly. Of significance is the appearance of the azide peaks at 2140 and 1263 cm-1, as this is the first indication of the formation of monolayers with a stretched molecular structure and an upright orientation of the molecules.34 For a parallel orientation or U-shaped molecular structure on the surface, most of the azide groups would react with the surface resulting in the disappearance of the characteristic azide peaks. This was clearly not observed here. Also of interest is the occurrence of a peak at 1570 cm-1, which is assigned to the amide II peak of the newly formed amide bond between the polymer and the amphiphile headgroup. This peak is shifted by about +25 cm-1 as compared to a monomeric model compound synthesized from the bolaamphiphile and ethylenediamine (not shown). The shift is probably caused by strong

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Figure 5. Comparison of infrared spectra: (a) transmission spectra of neat DBAzC10 powder mixed in a KBr pellet; (b) reflection absorption infrared spectra (RAIR) of poly(allylamine hydrochloride) (PAAm) film spin-coated on a gold coated slide; (c) reflection-absorption infrared spectra (RAIR) after selfassembly of a monolayer of DBAzC10 molecules on a PAAm film. Table 1. Peak Positions of IR Spectra of Self-Assembled Monolayers of DBAzC10 and Neat DBAzC1040 wavenumbers (cm-1) assgnt

self-assembled monolayersa (RAIR)

ν(NH) ν(CH2)asym ν(CH2)sym ν(N3)asym ν(CdO, azide) ν(CdO, amide I)bola ν(CdO, amide I)new ν(CdC)aromatic ν(NsH, amide II)new ν(NsH, amide II)bola ν(CH2) ν(N3)sym

3280 2917 2851 2140 1668 1669 1657 1605 1570 1529 1410 1263

pure substance (transm IR)b 3315 2921 2857 2135 1690 1665 1603, 1595 1535 1413 1272

a Reflection absorption infrared (RAIR) spectrum of DBAzC10 self-assembled over PAAm film. b As neat DBAzC10 powder mixed in KBr pellet.

intermolecular hydrogen bonding between adjacent amino proton and carbonyl constituents of the amide linkages.41 Also significant is the difference in the ratio of peak intensities between the RAIR spectrum and the transmission spectrum, particularly in the carbonyl region. Both carbonyl bands, ν(CdO)azide at 1690 cm-1 and ν(CdO)amide at 1665 cm-1, are less intense in the RAIR spectrum as compared to that of the aromatic ring ν(CdC)aromatic at ∼1605 cm-1. This observation may be explained with the help of selection rules for RAIR spectroscopy.38,42 Vibrations with a dipole moment parallel to the metallic surface are less intense, and it seems most likely that both the carbonyl groups are oriented approximately parallel to the surface (Figure 1c). Finally, previous studies have shown that the C-H asymmetric (νa) and symmetric (νs) stretching modes are sensitive indicators for the extent of the lateral interactions between long polymethylene (40) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, F. G. The Handbook of Infrared and Raman Characteristics Frequencies of Organic Molecules; Academic Press Inc.: San Diego, CA, 1991. (41) Mirkin, N. G.; Krimm, S. J. Am. Chem. Soc. 1991, 113, 9742. (42) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62.

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chains.43-45 For example, crystalline polymethylene chains have a peak position for the νa(-CH2-) of 2920 cm-1, while in the liquid state the peak is found at 2928 cm-1.45 The symmetric stretching modes for νs(-CH2-) were 2850 and 2856 cm-1 for the crystalline and liquid phases, respectively. The difference here was only 6 cm-1. Porter et al.45 list the peak positions for crystalline and liquid states of n-alkanethiols as 2918 and 2924 cm-1 for νa(-CH2-) and 2851 and 2855 cm-1 for νs(-CH2-), respectively. We obtained 2917 ( 2 and 2851 ( 2 cm-1 for νa and νs, respectively (Figure 5(c)) for the bolaamphiphile SAMs. Hence, these SAMs of bolaamphiphiles are likely fairly crystalline. Stability of the Aroyl Azide Group on the SAMs. The long-term stability of the aroyl azide groups on the outer surface of the self-assembled bolaamphiphiles was measured in various environments by observing the azide peak intensity in the RAIR spectra. The different environments chosen for this study are those required for storage after self-assembly (air) or those required for further modification of benzoyl azide group (9:1 THF/DMF; 99:1 THF/TEA; water buffered with Borax at pH 8.0; 7:3 THF/DMF and 1:1 MeOH/water). The percent decrease in azide intensity at 2140 cm-1 per day for the different environments listed above was 1, 3.9, 6.3, 7.5, 7.7, and 10, respectively. Clearly, the reactive benzoic azide is stable in air as well as in nonnucleophilic solvents for times sufficiently long to enable subsequent chemical modifications. Chemical Functionalization. Bo¨hme and co-workers transferred a Langmuir-Blodgett (LB) monolayer of DBAzC10 onto the surface of a polyacrylonitrile (PAN) film and found it to be well-ordered.36 Due to the stretched structure of the bolaamphiphiles within the monolayer, one benzoic azide headgroup per molecule was exposed to the gas phase allowing gas-phase reactions with different amine molecules to be performed. However, due to their poor stability, deposited LB monolayers of bolaamphiphile molecules on polymer PAN films were delaminated in the presence of liquids. SAMs, on the other hand, covalently attached to a substrate remained unchanged in a variety of fluid environments. To test the reactivity of the bolaamphiphile SAMs, two different systems were used: gas-phase reaction with propylamine and liquid-phase reaction with an amine-substituted fluorescence labeling agent (dansylcadaverine, DCV, solution in THF). The azide peak intensity (at 2140 cm-1) of bolaamphiphile SAM samples was measured by RAIR spectroscopy during the reaction. As clearly seen in Figure 6c, the nucleophilc substitution reaction of propylamine from the gas phase was very fast. Over 95% of the aroyl azide groups were completely replaced by propyl amide in less than 120 s (not shown). The gas-phase behavior was similar for the LB monolayers indicating similar molecular structure and orientation of the interface.36 However, the situation was different with the dansyl derivative where the reaction was slower and incomplete. The reaction was essentially over after 60 h, reaching a yield of about 50% of the initial azide peak intensity. Thereafter, no further changes were observed (see b in Figure 6). Whereas the area per bolaamphiphile molecule was about 0.25 nm2 in a wellordered dense monolayer,34-36 the area needed for the dansyl derivative was about twice this value (see cartoons (43) Snyder, R. G.; Strausss, H. L. Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (44) Snyder, R. G.; Maroncelli, M.; Strausss, H. L. Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623. (45) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.

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Figure 6. Surface reaction of self-assembled DBAzC10 molecules with 0.0167 M dansylcadaverine (in THF) (DCV) followed by reaction with propylamine (gas phase): (a) initial azide peak (at 2140 cm-1) of self-assembled DBAzC10 molecules; (b) decay of azide peak intensity due to reaction with DCV; (c) disappearance of the azide peak intensity due to fast reaction of remaining azide groups with propylamine.

in Figure 6). Therefore, the remaining unreacted aroyl azide groups were sterically hindered from reacting with DCV molecules. However, after placement of this partially reacted monolayer in a desiccator containing gaseous propylamine, a fast and complete conversion from azide to amide groups occurred. Thus, the relatively small propylamine molecules were able to penetrate the DCVsurface layer easily and react with the residual aroyl azide groups. Photopatterning. As mentioned above, the exposed surface of the self-assembled bolaamphiphile film has reactive azide groups which can be utilized for further functionalization. After nonionizing photochemical (UV) irradiation, highly reactive nitrene radicals are formed.46 Typical nitrene radical reactions are insertion reactions, convenient for immobilization via nitrene insertion in -NH-, -OH, and -CH- bonds. Therefore, bolaamphiphiles of bis(benzoic acid azide) can also be used for photopatterning. UV irradiation of the DBAzC10 SAMs results in the photochemical degradation of the azide moiety as evidenced by the disappearance of the azide stretching peak at 2140 cm-1 in Figure 7. It is not surprising that there are no traces of photoproducts since there are a wide variety of possible reactions during photolysis of acid azides. The irradiation conditions were 300 nm UV radiation for 120 s at 21 °C. A high-pressure mercury lamp (3.0 mJ/(cm2 min) using a compact radiometer, UV Process Supply Inc., Chicago, IL) was used for the irradiation, and aluminum foil was used to cover half of the DBAzC10 SAM sample. The environmental conditions were not special in any way; i.e., an inert gas atmosphere or special light filters were unnecessary. Conclusions A polymer surface, PAAm, was modified with selfassembled monolayers of bis(benzoic acid azide) bolaamphiphiles to create a new, highly reactive surface.47 The kinetics of self-assembly of the bolaamphiphiles was measured using RAIR and ellipsometry. Furthermore, immobilization reactions on the newly formed benzoyl acid azide surface were possible via nucleophilic substitution reactions from gas and solution phases and by UV-initiated photopatterning. A wide variety of modifications are (46) Schuster, G. B.; Autrey, T. J. Am. Chem. Soc. 1987, 109, 5814. (47) Bo¨hme, P.; Belfort, G. U.S. Patent 5,852,127, Dec. 22, 1998.

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by photochemical oxidation and low-temperature plasma techniques to prepare them for covalent attachment of self-assembled monolayers is being pursued.48,49 Further work on attaching biologically active enzymes onto the SAMs and on conducting reactions at the interface is currently being investigated.

Figure 7. Photopatterning of the DBAzC10 self-assembled monolayer. RAIR spectra are shown (a) before and (b) after UV irradiation. Irradiation conditions: 300 nm; 120 s; 21 °C. Notice the disappearance of the azide group at 2140 cm-1 on exposure to UV irradiation.

possible in order to tailor the interfacial chemistry of a polymeric material. Pretreatment of polymeric surfaces

Acknowledgment. This work was funded by the U.S. Department of Energy, Basic Energy Sciences, Chemical Sciences Division (Grant No. DE-FG02-90ER14114), and National Science Foundation, Division of Chemical and Thermal Systems (Grant CTS-9400610). We thank Dr. Srikanteswara Dakshina Murthy and Brian Frank for their assistance with ellipsometry and AFM, respectively. G.V. thanks Howard P. Isermann for a fellowship and the North American Membrane Society for a 1998 Graduate Student Fellowship. P.B. acknowledges the Deutsche Forschungsgemeinschaft for their support of a postdoctoral fellowship (Grant No. Bo1386/I). LA981548A (48) Pieracci, J.; Crivello, J. V.; Belfort, G J. Membr. Sci. 1999, 156, 223. (49) Chen, H.; Belfort, G. J. Appl. Polym. Sci. 1999, 72, 1699.