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Reversible Attachment of Perylenediimide Fluorophore to Glass Surfaces via Strong Hydrogen-Bonding Loretta L. Crowe, Kyril M. Solntsev, and Laren M. Tolbert* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed February 15, 2007. In Final Form: March 19, 2007 Chemical immobilization of a triethoxysilyl-functionalized hydrogen-bonding ureido-[2-(4-pyrimidone)] tetraplex produced interaction sites on a glass substrate that allowed association with a perylenediimide-functionalized tetraplex, providing noncovalent links of the fluorophore to the surface. The association between the self-complementary molecules was exceptionally strong, both in solution and at the surface, such that effective hydrogen-bonding was retained after repeated solvent washes.
Introduction The covalent attachment of active molecules to surfaces is now an important item in the surface scientist’s toolbox. Such tools include thiol-modified DNA probes on gold,1,2 alkane thiols on gold,3,4 silanes on silica5-10 and on metal oxides,11,12 diazonium salts on glassy carbon,13 alkylation of chlorinated silicon surfaces,14 and the modification of hydrogen-terminated silicon surfaces.15-17 The majority of these methods suffer from the irreversibility of the surface attachment, although alkane thiols on gold do exhibit some limited exchange with sluggish diffusion of the attached molecule. We were intrigued by the possibility of using the strong tetra-hydrogen-bonding group, ureido-[2(4-pyrimidone)], developed by Meijer and co-workers18-20 as a reversible point of attachment. This study demonstrates the feasibility of such attachment by use of a fluorescent dye to a glass surface via the specific hydrogen-bonding of the ureidopyrmidone, similar to a study reported for tetraplexes on gold.21 Previous research involving linking fluorescent chromophores to a surface has focused primarily on physisorption, either of the * To whom correspondence should
[email protected].
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addressed.
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(1) Sauthier, M. L.; Carroll, R. L.; Gorman, C. B.; Franzen, S. Langmuir 2002, 18, 1825-1830. (2) del Campo, A.; Bruce, I. J. Top. Curr. Chem. 2005, 260, 77-111. (3) Li, X. M.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2004, 14, 29542971. (4) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (5) Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1848-1851. (6) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstroem, I. J. Colloid Interface Sci. 1991, 147, 103-118. (7) Deyhimi, F.; Coles, J. A. HelV. Chim. Acta 1982, 65, 1752-1759. (8) Wang, W.; Gu, B. J. Phys. Chem. B 2005, 109, 22175-22180. (9) Hicks, J. C.; Jones, C. W. Langmuir 2006, 22, 2676-2681. (10) Porsch, B. J. Liq. Chromatogr. 1991, 14, 71-78. (11) Dire, S.; Babonneau, F.; Sanchez, C.; Livage, J. J. Mater. Chem. 1992, 2, 239-244. (12) Holzinger, D.; Kickelbick, G. J. Mater. Chem. 2004, 14, 2017-2023. (13) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201-207. (14) Bansal, A.; Li, X.; Yi, S. I.; Weinberg, W. H.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 10266-10277. (15) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc. Perkin Trans. 2 2002, 23-24. (16) Buriak, J. M. Chem ReV. 2002, 102, 1271-1308. (17) Hamers, R. J. Surf. Sci. 2006, 600, 3361-3362. (18) Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 6761-6769. (19) Folmer, B. J. B.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1999, 121, 9001-9007. (20) Soentjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 7487-7493. (21) Zou, S.; Zhang, Z.; Foerch, R.; Knoll, W.; Schoenherr, H.; Vancso, G. J. Langmuir 2003, 19, 8618-8621.
fluorophore directly or of another molecule, frequently a polymer, possessing a fluorescent tag.22 Fewer investigations have attached a linker covalently to the surface, followed by chemical attachment of the linker to a fluorophore.23-25 The method described here has the advantage of reversible, yet robust, binding in which the receptor is covalently bound to the glass surface. Hydrogen-bonding ureido-[2-(4-pyrimidones)] are currently being investigated mostly for their usefulness in supramolecular polymeric architectures, either as noncovalent linker units in the main chain of the polymer26-29 or as reversible crosslinkers pendent to the backbone of polyacrylates30 and polyolefins.31 The tetraplex array has also been used to join fullerenes for organic photovoltaic devices,32-34 and as the connection between porphyrins units in a bimolecular receptor for recognition of dipyridyl molecules.35 The tetraplex is generated by the reaction of an isocyanate with the primary amine group on isocytosine (Scheme 1). An isocytosine 2 must be used, rather than a generic amine, in order to provide the molecule hydrogen-bonding capabilities exceeding that of a generic urea.29 Dimerization constants in chloroform of greater than 106 M-1 have been reported for these hydrogenbonding tetraplexes, much stronger than those for hydrogenbonding in urea, which possesses an association constant of ∼150 M-1.19,29 An intramolecular hydrogen bond is formed between the hydrogen of the secondary isocytosine amine group and the (22) Hoshi, T.; Anzai, J.-i.; Osa, T. Anal. Chem. 1995, 67, 770-774. (23) Hayashi, Y.; Ichimura, K. Langmuir 1996, 12, 831-835. (24) Strashnikova, N.; Papper, V.; Parkhomyuk, P.; Likhtenshtein, G. I.; Ratner, V.; Marks, R. J. Photochem. Photobiol. A 1999, 122, 133-142. (25) Wang, H.; Fang, Y.; Cui, Y.; Hu, D.; Gao, G. Mater. Chem. Phys. 2003, 77, 185-191. (26) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. ReV. 2001, 101, 4071-4097. (27) Folmer, B. J. B.; Cavini, E. Chem. Commun. 1998, 1847-1848. (28) Folmer, B. J. B.; Sijbesma, R. P.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 2093-2094. (29) Lange, R. F. M.; Van Gurp, M.; Meijer, E. W. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3657-3670. (30) Yamauchi, K.; Lizotte, J. R.; Long, T. E. Macromolecules 2003, 36, 1083-1088. (31) Rieth, L. R.; Eaton, R. F.; Coates, G. W. Angew. Chem., Int. Ed. 2001, 40, 2153-2156. (32) Beckers, E. H. A.; van Hal, P. A.; Schenning, A. P. H. J.; El-Ghayoury, A.; Peeters, E.; Rispens, M. T.; Hummelen, J. C.; Meijer, E. W.; Janssen, R. A. J. J. Mater. Chem. 2002, 12, 2054-2060. (33) Gonzalez, J. J.; de Mendoza, J.; Gonzalez, S.; Priego, E. M.; Martin, N.; Luo, C.; Guldi, D. M. Chem. Commun. 2001, 163-164. (34) Rispens, M. T.; Sanchez, L.; Knol, J.; Hummelen, J. C. Chem. Commun. 2001, 161-162. (35) Shao, X.-B.; Jiang, X. K.; Zhao, X.; Zhao, C.-X.; Chen, Y.; Li, Z.-T. J. Org. Chem. 2004, 69, 899-907.
10.1021/la700455b CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007
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Scheme 1. Synthetic Route to Functionalized Tetraplexes
Figure 1. Emission of PDI-UPy in CHCl3 and on a glass surface.
carbonyl oxygen from the newly formed urea linkage, adding rigidity to the tetraplex molecule and allowing self-complementary in the hydrogen-bonding linker. Through this binding motif, a molecule immobilized on a surface can self-assemble with a complementary molecule containing a fluorescent dye, linking the dye to the surface (Scheme 2). In this study, a ureido-[2-(4-pyrimidone)] was functionalized with a triethoxysilylpropyl group (i.e., TEOS-UPy, 3a) and was capable of chemically binding to a glass surface, while a second ureido-[2-(6-propyl-4-pyrimidone)] containing a strongly fluorescent perylenediimide36 dye (PDI-UPy, 3c) was synthesized via Scheme 1. To investigate the reversibility of the hydrogenbonding at the surface, the nonfluorescent 1-hexylureido-3-[2(6-methyl-4-pyrimidone)] (Hex-UPy, 3b) was prepared. As tetraplex binding sites on the dye-treated surface were replaced by this new molecule, the perylenediimide emission decreased in intensity until only the baseline scattering from the surface was seen. Experimental Section Methods and Materials. Diphenylphosphoryl azide, triethylamine, 2-amino-4-hydroxy-6-methylpyrimidine, guanidine carbonate, ethyl butyrylacetate, N,N-dimethylaminotrimethylsilane, and all solvents were purchased through Fisher Scientific. Boron trichloride in hexane, 4-dimethylaminopyridine, 1,1′-carbonyldiimidazole, isocytosine, and triethylamine were purchased through Sigma-Aldrich, heptanoic acid was obtained from Eastman-Kodak, and (3-isocyanatopropyl)triethoxysilane was purchased from Gelest, Inc. 4-{4[9-(2,6-Diisopropyl-phenyl)-1,3,8,10-tetraoxo-5,6,12,13-tetraphenoxy3,8,9,10-tetrahydro-1H-anthra[2,1,9-def;6,5,10-d′e′f′]diisoquinolin2-yl]-phenyl}butyric acid37 was generously provided by Klaus Mu¨llen and Neil Pschirer of Max-Planck Institut fu¨r Polymerforschung. All commercially available reagents and solvents were used as received without further purification, with the exception of toluene, triethylamine, and ethyl acetate, which were freshly distilled before each use. (36) Gvishi, R.; Reisfeld, R.; Burshtein, Z. Chem. Phys. Lett. 1993, 213, 338344. (37) Qu, J.; Kohl, C.; Pottek, M.; Muellen, K. Angew. Chem., Int. Ed. 2004, 43, 1528-1531.
Figure 2. Surface modification of glass slides. Effect of hydrogenbonding strength on fluorescence after washing. Details of the synthesis and characterization of the final compounds (Figure 1), 1-(3-triethoxysilylpropyl)-ureido-3-[2-(4-pyrimidone)] (TEOS-UPy, 3a), 1-hexylureido-3-[2-(6-methyl-4-pyrimidone)] (Hex-UPy, 3b), 4-{4-[9-(2,6-diisopropyl-phenyl)-1,3,8,10-tetraoxo5,6,12,13-tetraphenoxy-3,8,9,10-tetrahydro-1H-anthra[2,1,9-def;6,5,10-d′e′f′]diisoquinolin-2-yl]-phenyl}-butyric acid 6-{ureido-3-[2(6-propyl-4-pyrimidinyl)}-hexyl ester (PDI-UPy, 3c), and 4-{4[9-(2,6-diisopropyl-phenyl)-1,3,8,10-tetraoxo-5,6,12,13-tetraphenoxy3,8,9,10-tetrahydro-1H-anthra[2,1,9-def;6,5,10-d′e′f′]diisoquinolin2-yl]-phenyl}-methylbutyrate (PDI-Me) are reported in the Supporting Information. After the tetraplex deposition, film thickness was determined by X-ray reflectivity (XRR) on a Siemens D5000 X-ray diffractometer equipped with a sample stage for reflectivity measurements and a ceramic X-ray source (Cu KR), and by ellipsometry on an EL X-1 ellipsometer, utilizing a 3mW class IIIA laser at 632.8 nm, and the results fit to a monolayer silane model. XRR and ellipsometry measurements were carried out in the labs of Dr. Ju¨rgen Ru¨he in the Institute for Microsystems Technology at the University of
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Scheme 2. Attachment of a Fluorophore to Silica Surface via Tetraplex Interaction
Freiburg in Freiburg, Germany. Films were characterized by X-ray photoelectron spectroscopy on a Physical Electronics (PHI) Model 1600 XPS system equipped with a monochromator, and atomic force microscopy on a Veeco AFM with a Dimension 3100 scanning probe microscope in the Microelectronics Research Center at Georgia Tech. Film characterization results are reported in the Supporting Information. Absorption spectra were recorded on a Perkin-Elmer Lambda 19 UV/vis/NIR spectrometer. All emission data were collected on a SPEX Fluorolog spectrofluorometer at room temperature and corrected to the emission of a NIST tungsten lamp. All samples were excited at the 445 nm absorbance shoulder for perylenediimide. All bar graphs displayed show the mean emission intensity recorded at the perylenediimide maximum, 602-608 nm.36 Glass Surface Treatment. Glass microscope slides (25 mm × 11 mm × 1 mm) were cleaned in piranha solution (75% H2SO4/25% H2O2 by volume) to remove all organics, then rinsed well with distilled water, sonicated for 10 min in ethanol, dried under an argon stream, and stored at 75 °C to prevent water adsorption until use. The cleaned slides were removed from the oven and then immediately dip-coated (25 °C, 0.25 cm/s dip rate) in a freshly prepared bath of 1-(3-triethoxysilylpropyl)ureido-3-[2-(4-pyrimidone)] (TEOS-UPy) (0.5 mM) in ethyl acetate, containing a 1.5molar excess of 1-hexylureido-3-[2-(6-methyl-4-pyrimidone)] (Hex-UPy). The coated slides were then placed on a watchglass in a 130 °C oven overnight to cure, initiating the covalent linkage to the silica surface. After curing, the slides were rinsed with distilled water, ethanol, and CHCl3 to remove any non-immobilized tetraplex molecules and dried in air. The washed slides were treated in a 10 mM bath of N,N-dimethylaminotrimethylsilane (DMAS) in freshly distilled toluene at 105 °C for 12 h to passivate the remaining surface hydroxyls with trimethylsilyl groups. After passivation, the slides were washed in ethanol and chloroform and dried in air. Each slide was oriented in the same fashion for each measurement, so that the emission change could be monitored at a consistent point on the surface and fluorescence emission spectra were collected for each sample, before treatment with a fluorophore, to provide baseline correction.38 After baseline collection, the samples were treated with a 1.04 mM chloroform solution of the fluorescent tetraplex, PDIUPy, then each sample was rinsed with 10 mL of chloroform and air-dried. In the exchange experiment, the fluorophore-tagged slide was further introduced to three treatments in a 3.1 mM chloroform solution of Hex-UPy, with fluorescent measurements taken after each treatment. (38) It should be noted that the background fluorescence is taken from the glass slide, after modification with TEOS-UPy and DMAS, and thus contains UPy monomers and trimethylsilyl groups at the surface.
Results To determine the stability of these hydrogen-bonded dimers on surfaces, slides were prepared by attaching the hetero-dimer of Hex-UPy and TEOS-UPy to glass. Introduction of treated slides to a chloroform solution of PDI-UPy produced films exhibiting broadened perylenediimide emission peaks, which displayed a slight blue-shift with decreasing concentration of the fluorophore on the glass surface (Figure 2). When PDI-UPy was introduced to an untreated glass surface, perylenediimide emission peaks were observed, indicating significant physisorption of the tetraplex moiety to the hydrophilic silica surface. As a control to eliminate the adsorption of the perylenediimide itself, the bare glass and the TMOS-UPymodified glass were treated in the same manner with PDI-Me, resulting in greatly reduced dye retention, as monitored by fluorescence emission (Figure 3). Treating a bare silica surface with DMAS to block all available surface hydroxyls virtually eliminated the uptake of PDI-UPy, as monitored via fluorescence spectroscopy. Thus, passivation of available surface hydroxyl sites with DMAS after UPy deposition has been incorporated into the general preparation for functionalized slides. The reversibility of surface binding was investigated by concentration-driven displacement of PDI-UPy with the nonemissive Hex-UPy in chloroform. The first treatment resulted in a significant drop in fluorescence intensity of the perylenediimide monomer emission (ca. 50%), with subsequent treatments lowering the emission intensity even further to less than 25% after three exchanges (Figure 4A). An examination of the exchange process, showed that the use of Hex-UPy reduced the fluorescence intensity from the dye molecules hydrogen-bonded to both bare glass and the UPy-modified glass surface (Figure 4B); although a paired t-test indicated that the emission decrease on UPy-modified films did not occur in a statistically more effective manner with Hex-UPy than with pure solvent alone. Interestingly, while pure solvent washes did not remove the PDIUPy from bare glass, they were more than sufficient to remove the dye from a UPy-functionalized glass surface. The exchange did not appear to be dependent upon concentration of the HexUPy in the 1-3 mM regime; although in this case, the t-test also indicated no difference in the CHCl3 wash of PDI-UPy on bare glass from the other Hex-UPy washed samples. Not only was the UPy bond capable of being exchanged from solution with UPy molecules containing differing functional groups, but washing the surface with a solvent which competed
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Figure 4. Three cycles of fluorophores association/dissociation on glass surface. Surfaces investigated are (a) TMS-passivated (by DMAS) glass modified with UPy sites, (b) TMS-passivated glass, and (c) bare, unmodified glass. PDI-UPy was bound to the surface from ethyl acetate solutions and dissociated with DMSO washes. Surface was rinsed with chloroform and air-dried before each recomplexation.
Figure 3. (A) Concentration-driven displacements of PDI-UPy on TEOS-UPy base layer by sequential treatments with Hex-UPy/ CHCl3 solution. (B) Differences in exchange statistics between pure solvent washes and UPy-loaded exhanges on bare glass and UPymodified glass. The lines serve to guide the eyes.
for hydrogen bonds, such as water, methanol, or DMSO, which has been shown to switch the UPy to a non self-complementary tautomer,18 was extremely effective in removing the PDI-UPy from the surface (Figure 5). The fluorophore could be readily recomplexed to the surface (both via attached UPy sites and to available surface hydroxyls) by simply rinsing the sample and immersing it in an ethyl acetate solution of PDI-UPy. This association/dissociation was carried out for three cycles without significant loss of emission recovery; although it was observed that over successive removal cycles, a gradual decrease in the effectiveness of the DMSO washes occurred. These perceived differences were confirmed by a t-test. It must also be noted that the manner in which the glass surface was treated before contact with the UPy molecules was very important. Oven-dried surfaces provided the highest uptake of hydrogen-bonding molecules (Figure 6), while simply air drying the surfaces after water washes led to a substantial reduction in the amount of PDI-UPy which was able to adsorb onto the surface. Removal of the dye with
Figure 5. Surface pretreatment effect on ability of unmodified glass to adsorb PDI-UPy molecules. Heated samples were placed in a 130 °C oven, while unheated samples were dried in air.
fresh, dry DMSO was the most effective method and did not require additional drying to prevent a decrease in the reuptake of dye-functionalized tetraplexes from solution. In order to probe the capacity for UPy recovery from modified and unmodified glass surfaces, a stock solution of PDI-UPy (0.137 mM in CHCl3) was prepared and divided into 3.5 mL aliquots. To each aliquot was added 77 mg of silica modified in one of three ways: (1) commercial reversed-phase silica (C18 passivated), (2) silica gel extensively passivated with DMAS, and (3) silica gel modified to contain UPy binding sites, followed by passivation of unreacted silica surface with DMAS. Unmodified silica gel was used as a control. In the silica gel control, nearly all of the UPy-tagged dye was adsorbed from solution (Figure 6A), while the UPy-modified silica approximately half of the dye. The passivated controls each permitted far less PDI-
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Figure 6. Absorption of PDI-UPy from 0.137 mM CHCl3 solution by modified silica gels. The picture shows chloroform solutions after the addition of 77 mg of silica gels, PDI-UPy/CHCl3 supernatant, and CHCl3 resolvation of PDI-UPy recovered from silica gel by 20 mL of DMSO. (i) PDI-UPy stock solution, (ii) unmodified silica, (iii) commercial C18-silica, (iv) silica treated with DMAS, and (v) silica treated with TEOS-UPy and DMAS. Images of the supernatant and recover solutions are taken under irradiation at 366 nm. (A) Absorbances of CHCl3 solutions from silica gel dye uptake experiment, showing supernatants (sn, solid lines) and dye recoveries (recov, dashed lines). (B) Sum of supernatant and dye recovery spectra, illustrating ability to fully recover PDI-UPy.
UPy binding from solution, with the TMS-modified gel showing a greater optical density of PDI in the supernatant than the commercially available C18-modified silica. Once the dye-loaded silica gels were filtered off from their supernatants, they were washed with 20 mL of DMSO and the solvent removed under vacuum. The recovered PDI-UPy was then redissolved in CHCl3 and the absorbance spectra of the solutions were collected. Combining the absorbance spectra of the recovered dye solutions and their respective non-adsorbed supernatants resulted in optical densities closely approaching that of the initial stock solution (Figure 6B), with the TMS-modified and UPy/TMS-modified silica gels allowing for a 98% total recovery of the stock solution. Commercial C18-modified silica gel produced a 97% recovery, while unmodified silica gel only allowed for an 89% total dye recovery.
Discussion In a significant impediment to formation of unique of UPy interactions on glass, it appears that the tetraplex, while indeed strongly bonding, is not exclusively self-complementary, and in fact will bind quite sturdily to the available surface sites on silica,30 presumably because of their inherently acidic nature. Exchange studies indicate no significant preference for tetraplex surface sites over nonspecific silica sites, thus requiring passivation of the silica surface after the deposition of the initial tetraplex dimer in order to isolate the effects due to the UPyUPy interaction from those due to the UPy-surface interaction. The binding of UPy molecules to bare glass, however, does appear to be somewhat stronger than the self-complementary binding seen in UPy-functionalized glass, as evidenced by the ability of pure solvent washes of chloroform to remove hydrogen-
bound dye-functionalized tetraplex molecules from UPy-modified glass, whereas the same washes performed on an unmodified glass surface produced no loss of emission intensity. The significantly higher perylenediimide emission seen from surfaces (both UPy-glass and unmodified glass) treated with PDI-UPy over those treated with PDI-Me indicates that it is in fact the tetraplex array which is responsible for the adsorption of the fluorescent dye to the surface, rather than the dye itself. The initial surface structure and treatment of the glass surfaces used plays a very significant role in the emission intensity levels obtained, with a wide deviation seen among samples treated in the same manner. This also appears to be true in the unmodified glass samples, with surface roughness and surface water both greatly affecting the amount of fluorophore which will adsorb. Etching the glass in a basic ethanol solution greatly increases the number of available surface sites, and thus the adsorption of dye-functionalized tetraplexes. Pretreating a surface with water without rigorous oven-drying, on the other hand, effectively blocks the uptake of hydrogen-bonding molecules, presumably by creating a thin layer of water molecules at the silica surface. The reversible nature of the surface binding is supported by the sequential reduction of the perylenediimide emission intensity through concentration-driven displacements (see Figure 4). Somewhat surprisingly, the perylenediimide emission is not completely removed after one or two treatments with a HexUPy-saturated solution, even though the concentration of HexUPy in chloroform is at the millimolar level, while the concentration of PDI-UPy on the surface is far less. One likely explanation is that the PDI-UPy, once returned to solution, merely seizes the opportunity to hydrogen-bond to any available surface hydroxyl sites which remain unblocked. Washing the surface
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with DMSO, however, allows for the efficient removal of the dye from the surface, through the rearrangement of the UPy moiety to reversibly form a non-self-complementary conformation which has a much reduced capacity for hydrogen-bonding. After removal of the noncovalently bound PDI-UPy, it can be readily and repeatedly reassociated and removed, indicating that the surface-bound UPy is stable in its self-complementary tautomer even after heating to 130 °C. This reversibility needs further investigations to determine whether it is a kinetically or thermodynamically driven process. The DMSO wash does appear to lose its effectiveness at removing the hydrogen-bound dyes from the surface over successive cycles. This may indicate a disruption of the initially silanized surfaces or a more robust binding mechanism which is not disrupted by the solvent washes. The recovery of PDI-UPy adsorbed by bare and modified silicas also supports the use of DMSO as an effective, yet nondestructive removal agent for UPy molecules hydrogen-bound to both surface silanols and surface UPy sites (see Figure 7). Nearly full recovery of PDI-UPy was seen for all silica gels tested, though the reduced recovery in unmodified silica, compared to its modified counterparts, likely resulted more from the large amount of dye that it adsorbed than any real differences in retention strength between the UPy-UPy and UPy-silanol interactions. The experimentally prepared TMS-modified silica performed slightly better than the commercially available C18 silica gel, allowing for less UPy uptake and proving that the method of passivation using DMAS is sufficient to block almost all surface silanols, thus confirming that the interaction seen in the UPy-modified silica gel and glass is almost entirely due to the UPy-UPy interaction. This technique was a modification of a method in the literature which uses methyl red to evaluate the passivation effectiveness of modified silica gels;39,40 however, the stronger binding of the UPy array allowed for more effective uptake of the dye from solution. (39) Heckel, A.; Seebach, D. Chem. Eur. J. 2002, 8, 559-572.
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Conclusions TEOS-UPy provides a convenient scaffold for reversible attachment of functional groups to glass surfaces, though care must be taken to adequately passivate the surface afterward to limit nonspecific interactions between the hydrogen-bonding tetraplexes and the surface hydroxyl sites. The retention of fluorescent emission from the perylenediimide-modified surfaces, even after solvent washes, suggests that the attachment is robust, allowing for further functionalization of surfaces, yet easily reversed by concentration-driven displacements, and washes utilizing solvents which interact with the tetraplex moiety, allowing it to rearrange to a non-self-complementary conformation, or are themselves hydrogen-bonding. Use of such reversible binding should have many applications in supported catalysis and temporary immobilization of other functional structures, as long as solvents which do not disrupt hydrogen-bonding are not employed. Acknowledgment. This research has been supported by the U.S. National Science Foundation (CHE-0456892). Janusz Kowalik, Ju¨rgen Ru¨he, and Ossie Prucker are gratefully acknowledged for their many useful discussions and suggestions. Jonas Jarvholm and Ashwini Sinha are also credited for their work in collecting the AFM and XPS data, respectively. Prof. Klaus Mu¨llen and Neil Pschirer provided a generous supply of PDI acid. Supporting Information Available: Experimental synthesis and characterization details along with XPS, XRR, and AFM characterization of TEOS-UPy films. This material is available free of charge via the Internet at http://pubs.acs.org. LA700455B (40) Shapiro, I.; Kolthoff, I. M. J. Am. Chem. Soc. 1950, 72, 776-782.
