Silica Passivation Efficiency Monitored By a Surface-Bound

Langmuir 2008, 24, 8541-8546

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Silica Passivation Efficiency Monitored By a Surface-Bound Fluorescent Dye Loretta L. Crowe and Laren M. Tolbert* Georgia Institute of Technology, School of Chemistry and Biochemistry, Atlanta, Georgia 30332-0400 ReceiVed August 14, 2007. ReVised Manuscript ReceiVed May 26, 2008 A new method for evaluating surface passivation of silicon surfaces using a strongly adsorbing and fluorescing perylenediimide (PDI) dye is reported. Silanes containing differing reactive groups delivering a trimethylsilyl moiety were investigated for their ability to passivate glass surfaces, both from solution and in the vapor phase, as a function of temperature and concentration. Among the silanizing agents used in this study were 1,1,1,3,3,3-hexamethyldisilazane (HMDS), allyltrimethylsilane (TMS-A), chlorotrimethylsilane (TMS-Cl), N-(trimethylsilyl)imidazole (TMS-Im), and N,N-dimethylaminotrimethylsilane (TMS-DMA). Surface coverage was determined using fluorescence intensity. The highest surface coverage films were obtained on glass treated with HMDS vapor at 280 °C or with solutions of TMS-Im or TMS-DMA at 105 °C. These studies provide important insight into the optimal methods for surface passivation.

Introduction The passivation and compatibilization of silicon and other metal oxide surfaces is an important constituent in a number of areas, including the production of efficient organic field effect transistors by providing efficient interfacial bonding,1 in gas and liquid chromatography stationary phases,2 and in preventing corrosion. The ideal passivation would result in a surface which would effectively repel any hydrophilic materials. On the one hand, such surfaces provide more compatible substrates for bonding organic materials. On the other hand, removal of “dangling” SiOH bonds may improve device efficiency by eliminating surface traps. Thus our goal in this study is both to examine methods for minimizing hydrophilicity and to develop methods for ascertaining the degree of such hydrophilicity. Our “ideal” surface is one which has no affinity for polar constituents. Silanization through attachment of trimethylsilyl end groups is the most widely used technique for modifying inorganic surfaces, particularly silica and the native oxide surface on silicon. Such passivation also reduces the activity of surfaces toward catalytic processes3 and provides compatibilization for hydrophobic4,5 and nanoscopic6 structures. Silanization with organosilanes can also be used to covalently block surface hydroxyl sites in order to prevent the adsorption of analyte molecules, as in chromatographic stationary phases;7,8 to enhance the activity and enatioselectivity of receptor units tethered to a surface;9–11 * To whom correspondence should be addressed. E-mail: laren.tolbert@ chemistry.gatech.edu. (1) See, for example Tang, Ming L.; Reichardt, Anna D. Miyaki; Nobuyuki, Stoltenberg; Randall M., Bao, Zhenan J. Am. Chem. Soc. 2008, 130, 6064–6065. (2) Hammers, W. E.; Janssen, R. H. A. M.; De Ligny, C. L. J. Chromatogr. 1978, 166, 9–20. (3) Fraile, Jose M.; Garcia, Jose I.; Mayoral, Jose A.; Royo, Ana J Tetrahedron: Asym. 1996, 7, 2263–2276. (4) Takach, N. E. J. Colloid Interface Sci. 1993, 162, 496–502. (5) Wei, M.; Bowman, R. S.; Wilson, J. L.; Morrow, N. R. J. Colloid Interface Sci. 1993, 157, 154–159. (6) Singh, Binay; Gandhi, Darshan D.; Singh, Amit P.; Moore, Richard; Ramanath, G. Appl. Phys. Lett. 2008, 92, 113516/1-113516/3. (7) Farwell, S. O.; Gluck, S. J. Anal. Chem. 1980, 52, 1968–1971. (8) Porsch, B. J. Liq. Chromatogr. 1991, 14, 71–78. (9) Bae, S. J.; Kim, S.-W.; Hyeon, T.; Kim, B. M. Chem. Commun. 2000, 31–32. (10) Rechavi, D.; Albela, B.; Bonneviot, L.; Lemaire, M. Tetrahedron 2005, 61, 6976–6981. (11) Rechavi, D.; Lemaire, M. Org. Lett. 2001, 3, 2493–2496.

to facilitate formation of effective PCR chips,12 and to modify an unfavorable surface to present specific functional groups for further reaction, interaction, and investigation.13–16 Most commonly used among functional groups for modifying and passivating silica surfaces are chlorosilanes17,18 and alkoxysilanes.19–21 More recently, nitrogen-bearing silanes (silazanes,4,22,23 imidazoles,8,24 and amines22 have displayed a much wider utility, as their byproducts (ammonia and substituted amines) can act as autocatalysts for the further reaction of the reagent with available surface silanols.4,22,25 Many other functionalities26,27 have also been investigated, including silyl halides (-I, -Br), silylcyanates, silylazides, silylisocyanates, silylisothiocyanates, silylsulfonates, silylacetamides, silylcarbodiimides, and allylsilanes,28 though their use is considerably less prevalent in the literature. The most general method for determination of surface hydrophobicity is the contact angle measurement.29 However, we are particularly interested in the high-passivation regime, where a high contact angle may not reveal the presence of isolated (12) Shoffner, Mann A.; Cheng, Jing; Hvichia, Georgi E.; Kricka, Larry J.; Wilding, Peter. Nucleic Acids Res. 1996, 24, 375–9. (13) Chen, X.; Tolbert, L. M.; Henderson, C. L.; Hess, D. W.; Ruhe, J. J. Vac. Sci. Technol., B 2001, 19, 2013–2019. (14) McArthur, E. A.; Ye, T.; Cross, J. P.; Petoud, S.; Borguet, E. J. Am. Chem. Soc. 2004, 126, 2260–2261. (15) Prucker, O.; Naumann, C. A.; Ruhe, J.; Knoll, W.; Frank, C. J. Am. Chem. Soc. 1999, 121, 8766–8770. (16) van der Boom, M. E.; Evmenenko, G.; Yu, C.; Dutta, P.; Marks, T. J. Langmuir 2003, 19, 10531–10537. (17) Pallandre, A.; Glinel, K.; Jonas, A. M.; Nysten, B. Nano Lett. 2004, 4, 365–371. (18) Ruckenstein, E.; Li, Z. F. AdV. Colloid Interface Sci. 2005, 113, 43–63. (19) Gan, D.; Lu, S.; Wang, Z. J. Colloid Interface Sci. 2001, 239, 272–277. (20) Vandenberg, E. T.; Bertilsson, L.; Liedberg, B.; Uvdal, K.; Erlandsson, R.; Elwing, H.; Lundstroem, I. J. Colloid Interface Sci. 1991, 147, 103–118. (21) Wang, W.; Gu, B. J. Phys. Chem. B 2005, 109, 22175–22180. (22) Deyhimi, F.; Coles, J. A. HelV. Chim. Acta 1982, 65, 1752–1759. (23) Kaul, F. A. R.; Puchta, G. T.; Schneider, H.; Bielert, F.; Mihalios, D.; Herrmann, W. A. Organometallics 2002, 21, 74–82. (24) McMurtrey, K. D. J. Liq. Chromatogr. 1988, 11, 3375–3384. (25) Kaas, R. L.; Kardos, J. L. Polym. Eng. Sci. 1971, 11, 11–18. (26) Soultani-Vigneron, S.; Dugas, V.; Rouillat, M. H.; Fedolliere, J.; Duclos, M. C.; Vnuk, E.; Phaner-Goutorbe, M.; Bulone, V.; Martin, J. R.; Wallach, J.; Cloarec, J. P. J. Chromatogr., B 2005, 822, 304–310. (27) Tertykh, V. A.; Belyakova, L. A.; Varvarin, A. M. React. Kinet. Catal. Lett. 1989, 40, 151–156. (28) Shimada, T.; Aoki, K.; Shinoda, Y.; Nakamura, T.; Tokunaga, N.; Inagaki, S.; Hayashi, T. J. Am. Chem. Soc. 2003, 125, 4688–4689. (29) Kumar, Girish; Prabhu, K. AdV. Colloid Interface Sci. 2007, 133, 61–89.

10.1021/la801343b CCC: $40.75  2008 American Chemical Society Published on Web 07/11/2008

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hydrophilic sites. Moreover, despite widespread interest in and general utility of surface modification with organofunctional silanes, descriptions of different experimental methods and quantitative comparisons of their efficacy remain incomplete, with no general comparison of a wider array of silanes or a broader selection of reaction conditions. In addition, the surface topography of the silicon substrate also contributes to significant hysteresis in the advancing and receding contact angles, complicated by the fact that contact angle is also a complicated function of surface composition.30 A more complex silanizing agent, tris(trimethylsiloxy)silyl chloride (tris-TMS) shows diminished hysteresis31 but was not a subject for this study. Our interest has been more in the field of organic field effect transistors32 and single molecule energy transfer, and we wanted a method that was more sensitive to the formation of individual hydrophilic sites. We were intrigued by our observation that the 4-fold hydrogen-bonding ureido-[2-(4-pyrimidone] complex developed by Meijer33 exhibits an extraordinarily robust affinity for silicon oxide surfaces.34 This moiety, coupled to a perylenediimide (PDI), a highly fluorescent dye, provided an extraordinarily sensitive monitor of surface hydrophilicity, and, presumably, the content of remaining surface OH groups. Thus one goal of this research was to provide a systematic investigation of organosilanes under a variety of reaction conditions, in order to better enable comparisons between silanizing reagents and reaction conditions and to make intelligent choice of silanizing agent. Coincidentally, we have developed a new method for examining surface coverage, useful in the range of high surface density where the common contact angle measurements provide little insight about individual surface sites. For this study we chose an array of commercially available monofunctional silanizing agents bearing an assortment of surface functional groups, namely chlorosilyl-, allylsilyl-, and several aminosilyl-species, and investigated their ability to passivate glass surfaces under a variety of conditions. The functionalized surfaces were evaluated by contact angle goniometry and a fluorescence-based variation of the methyl-red dye adsorption test for available silanols.35,36 This variation utilized a highly emissive perylenediimide derivative37,38 that had been coupled to a ureido-[2-(6-propyl-4-pyrimidone)] (UPy), which is known for its strong self-complementary hydrogen bonding (Kdim ≈ 107 M-1 in CHCl3),33,39,40 although it has also been shown to bind sturdily to glass in a nonspecific manner.34,41 Through binding to the remaining surface silanols after passivation, we were able to observe the effectiveness of a particular passivation method with fluorescence spectroscopy at very low concentrations. (30) Fadeev, Alexander Y.; McCarthy, Thomas J. Langmuir 1999, 15, 3759– 3766. (31) Fadeev, Alexander Y.; McCarthy, Thomas J. Langmuir 1999, 15, 7238– 7243. (32) Roberson, Luke B.; Kowalik, Janusz; Tolbert, Laren M.; Kloc, Christian; Zeis, Roswitha; Chi, Xiaoliu; Fleming, Richard; Wilkins, Charles J. Am. Chem. Soc. 2005, 127, 3069–3075. (33) Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 6761–6769. (34) Crowe, Loretta L.; Solntsev, Kyril M.; Tolbert, Laren M Langmuir 2007, 23, 6227–6232. (35) Shapiro, I.; Kolthoff, I. M. J. Am. Chem. Soc. 1950, 72, 776–782. (36) Heckel, A.; Seebach, D. Chem. Eur. J. 2002, 8, 559–572. (37) Gvishi, R.; Reisfeld, R.; Burshtein, Z. Chem. Phys. Lett. 1993, 213, 338– 344. (38) Kohl, C.; Weil, T.; Qu, J.; Mu¨llen, K. Chem. Eur. J. 2004, 10, 5297–5310. (39) Folmer, B. J. B.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1999, 121, 9001–9007. (40) Soentjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 7487–7493. (41) Yamauchi, K.; Lizotte, J. R.; Long, T. E. Macromolecules 2003, 36, 1083–1088.

Crowe and Tolbert

Experimental Section Methods and Materials. Chlorotrimethylsilane (TMS-Cl), N,Ndimethylaminotrimethylsilane (TMS-DMA), and triethylamine were purchased from Fisher Scientific. Allyltrimethylsilane (TMS-A), 1,1,1,3,3,3-hexamethyldisilazane (HMDS), and N-(trimethylsilyl)imidazole (TMS-Im) were purchased from Sigma-Aldrich. All solvents were purchased from VWR International. The synthesis and characterization of the PDI-functionalized hydrogen bonding tetraplex, 4-{4-[9-(2,6-diisopropyl-phenyl)-1,3,8,10-tetraoxo-5,6,12,13tetraphenoxy-3,8,9,10-tetrahydro-1H-anthra[2,1,9-def;6,5,10d′e′f′]diisoquinolin-2-yl]-phenyl}-butyric acid 6-{ureido-3-[2-(6propyl-4-pyrimidinyl)}-hexyl ester, (PDI-UPy) has been reported.34 4-{4-[9-(2,6-Diisopropyl-phenyl)-1,3,8,10-tetraoxo-5,6,12,13-tetraphenoxy-3,8,9,10-tetrahydro-1H-anthra[2,1,9-def;6,5,10d′e′f′]diisoquinolin-2-yl]-phenyl)butyric acid was generously provided by Dr. Neil Pschirer and Prof. Klaus Mu¨llen. All commercially available reagents and solvents were used as received without further purification, with the exception of toluene, which was freshly distilled before each use. Glass Surface Treatment. Glass slides (25 mm × 11 mm × 1 mm) were cleaned in piranha solution (75% H2SO4/25% H2O2) to remove all organic deposits, then rinsed well with distilled water, sonicated for 10 min in ethanol, dried under an argon stream, and stored in a Petri dish in a 75 °C. The cleaned slides were removed from the oven and immediately placed in the silanizing bath or chamber. For vapor studies, samples were sealed in a glass tube with 0.50 mL of the silanzing agent and capped with an O-ring joint, held tightly closed with a spring-clamp. Samples were prepared in quadruplicate, with the slides within the chamber separated by means of a small glass tube. The chamber was then placed into an oven held at a constant temperature (ranging from 25 to 280 °C) for times ranging from 5 min and 3 h. For solution studies, samples were placed in a clean glass vial filled with a 10.0 mM solution of the silanizing agent in freshly distilled toluene (xylene for the 130 °C experiment) and sealed with a phenolic screwcap containing a PTFE liner. Solutions containing chlorosilanes also included 20.0 µL of Et3N to act as a base/acid scavenger. The sealed vials were placed in an oil bath held at a constant temperature (75-130 °C) for 3 to 24 h. Dipcoated samples were immersed in the 10.0 mM silanizing (toluene) bath at room temperature, mechanically withdrawn at a 0.25 cm/s rate, and placed in a covered glass Petri dish in a 75 °C oven for three hours. After treatment, the slides were sonicated sequentially for 10 min in ethanol (three times) and CHCl3 (once), and then dried in air and placed in a 75 °C oven overnight. Contact angle goniometry measurements were made on an AST Products, Inc. VCA 2500XE, utilizing a 1.0 µL drop of deionized water, delivered by automated syringe pump. Drop profiles were captured and their angles were analyzed by the VCA Version 1.52 program. A total of four measurements at random locations on the slide were made for each treated sample. Emission data were collected on a SPEX Fluorolog spectrofluorometer at room temperature. Samples were excited at the 445 nm absorbance shoulder for perylenediimide. The slide was oriented in the same fashion for each measurement, such 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 a baseline for subtraction. After collection of the baseline, the samples were then treated with a 1.04 mM chloroform solution of the fluorescent dye-containing tetraplex, PDI-UPy, and each sample was rinsed with 10 mL fresh chloroform and air-dried, followed by immediate collection of the emission spectrum. All bar graphs displayed show the mean emission intensity recorded at the perylenediimide maximum, 602-608 nm.37

Results Experiments were performed using five different monofunctional silanizing agents, 1,1,1,3,3,3-hexamethyldisilazane

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Figure 2. Films prepared from 10 mM toluene solutions at 75 °C for varying periods of time. Columns represent the intensity of the PDIUPy emission maximum. Grey: bare glass, red: HMDS, green: TMS-A, blue: TMS-Cl, cyan: TMS-Im, and magenta: TMS-DMA. Respective contact angles (water) are given by the colored circles. Figure 1. Structures of molecules used in this study. Table 1. Emission Intensity (PDI-UPy) and Contact Angles of Films Prepared by Dipcoating from 10 mM Toluene Solutionsa

bare glass HMDS TMS-A TMS-Cl TMS-Im TMS-DMA a

emission, rel

emission (std dev)

contact angle

contact angle (std dev)

0.950 0.765 0.808 0.505 0.081 0.186

(0.039 (0.046 (0.100 (0.013 (0.015 (0.013

33.3° 72.5° 73.0° 57.0° 93.3° 77.3°

(1.71° (7.69° (7.94° (1.88° (1.77° (3.35°

Films were cured for 3 h at 75 °C.

(HMDS), allyltrimethylsilane (TMS-A), chlorotrimethylsilane (TMS-Cl), N-(trimethylsilyl)imidazole (TMS-Im), and N,Ndimethylaminotrimethylsilane (TMS-DMA) (Figure 1). The latter, in particular, has been shown to be an effective silanizing agent at lower temperatures.42 A hydrogen-bonding tetraplex array,33,39,40 functionalized with a fluorescent perylenediimide chromophore, 4-{4-[9-(2,6-diisopropyl-phenyl)-1,3,8,10-tetraoxo5,6,12,13-tetraphenoxy-3,8,9,10-tetrahydro-1H-anthra[2,1,9def;6,5,10-d′e′f′]diisoquinolin-2-yl]-phenyl}-butyric acid 6-{ureido3-[2-(6-propyl-4-pyrimidinyl)}hexyl ester, (PDI-UPy), which binds strongly to glass,34,41 was used to monitor the density of unblocked surface hydroxyls as a measure of the effectiveness of each individual passivation method. Static water contact angles and dye uptake (as monitored by fluorescence) on bare, unmodified glass were measured for each passivation method to provide a connection between experiments. Films prepared by dipcoating from 10 mM toluene solutions of the silanizing agents generally exhibited significant remaining hydrophilicity, with the exception of TMS-Im, which showed an elevated contact angle of 93.3° ( 1.8°, compared to bare glass (33.3° ( 1.7°), and a mean emission intensity of only 10% that of unmodified glass (Table 1). The contact angle and emission intensity results for most of the dipcoatedsamples (especially HMDS and TMS-A) were plagued by a (42) Szabo, Katalin; Ngoc Le Ha, Schneider; Philippe, Zeltner; Peter, Kovats; Ervin, S. HelV. Chim. Acta 1984, 67, 2128–42.

lack of reproducibility,43 as indicated by large standard deviations, both for measurements conducted on the same sample and measurements slide-to-slide. Nevertheless, even for dipcoated samples, there is a significant inverse correlation with contact angle and fluorescence intensity, except for TMS-Cl, which may reflect a residual concentration of ions from the initial treatment. The evaluation of the selected passivating agents at 75 °C was further expanded to solution depositions from 10 mM toluene solutions for a period of 3-24 h (Figure 2). TMS-A performed poorly, giving both low contact angles and high emission intensities. The best performing passivating candidates at 75 °C were TMS-Im and TMS-DMA, though no significant increase in coverage, as monitored by both contact angle and emission intensity, was noted with increasing treatment time. Three hours in solution appeared sufficient to exhaust the passivating potential of all of the selected reagents at 75 °C. The effect of elevated temperature on the blocking potential of 10 mM solutions was measured (Figure 3A). Depositions were carried out from solutions of toluene (75 °C, 105 °C) and xylene (130 °C). As before, TMS-A was the least effective in generating hydrophobicity, giving low contact angles and emission intensities within the same range as those of bare glass. Increasing temperature gave steady improvements on the passivating effect of HMDS, yielding moderately elevated contact angles and decreasing dye uptakes, however, even at its best performance (130 °C), the emission intensity was still roughly 15% that of bare glass. TMS-Cl, while showing acceptable results at 75 °C, failed to diminish dye uptake appreciably at 105 °C or even at 130 °C. At 105 °C, the emission intensity reached a low of approximately 5% that of bare glass. However large standard deviations in the emission intensity were observed for TMS-Cl at 130 °C. The best performance for deposition from the temperature study appeared to be treatments of glass with 10.0 mM toluene solutions of TMS-Im and TMS-DMA at 105 °C, which yielded contact angles of over 100° and emission intensities of approximately 1-2% of the bare glass control. (43) A referee has commented on the absence of advancing and receding contact angle measurements. Indeed, although this criticism is well taken, our main point is that such measurements will be less useful at high hydrophobicity levels.

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Figure 4. Emission intensity (PDI-UPy) and contact angle of films prepared from vapor deposition at 280 °C for 30 min in a sealed cell.

Figure 3. (A) Films prepared from 10 mM toluene and xylene solutions at varying temperatures for 3 h. (B) Films prepared from toluene solutions of varying concentrations at 105 °C for 3 h. Columns represent the intensity of the PDI-UPy emission maximum. Grey: bare glass, red: HMDS, green: TMS-A, blue: TMS-Cl, cyan: TMS-Im, and magenta: TMS-DMA. Respective contact angles (water) are given by the colored circles.

While all of the reagents studied showed an improvement in surface blocking at 105 °C over the results at 75 °C, only HMDS showed further improvement in 130 °C xylene solutions. The rest of the results at 130 °C more closely compared to the 75 °C results than those from 105 °C solutions, although this may also reflect the change to xylene solution. The effect of solution concentration was then examined in toluene solutions treated for three hours at 105 °C (Figure 3B). Again, HMDS gave steady improvement in both contact angle and dye blocking with increasing solution concentration, giving its best results in 100 mM solutions, with an emission intensity of 10% that of the bare glass control and a contact angle of 81.3° ( 3.9°. TMS-A proved consistent in its inability to effectively block the glass surface, with low contact angles and high emission intensities, comparable to that of unmodified glass. Somewhat surprising was the complete lack of passivation afforded by 1 mM solutions of TMS-Cl, while both 10 mM and 100 mM solutions provided high contact angles and low dye adsorption, as indicated by emission intensities of only 5% that of the glass controls. Low concentration (1 mM) solutions of TMS-Im and

TMS-DMA both showed significant ability to block hydroxyls on the glass surface; however, both emission intensities and contact angles showed significant standard deviations from the mean. More reliable results were obtained for 10 mM and 100 mM solutions, and while both showed reduced emission intensities (approximately 1% and 3% of bare glass) compared to 1 mM solutions (approximately 20% and 14% of bare glass), no significant advantage was seen to using a 100 mM solution over a 10 mM solution. Finally, the deposition of the selected passivating agents from vapor at 280 °C was investigated (Figure 4). Films were deposited from pure reagent vapor, held in a sealed container at 280 °C for 30 min. Due to the need for Et3N and the formation of the Et3N+Cl- salt byproduct, TMS-Cl was omitted from this study. All reagents performed well under these conditions, giving similar results, with emission intensities roughly 5-7% of bare glass with low standard deviations. Contact angles were also similar (ca. 104°) with the puzzling exception of TMS-A, which gave an emission intensity within the range of the other reagents, but a contact angle of fully 10° less. With respect to dye-blocking, HMDS and TMS-A performed slightly better than TMS-Im and TMS-DMA, which both tended to leave a visible light brown residue on the slides after treatment a such high temperatures. This residue was difficult to remove from the TMS-DMA samples and nearly impossible from the TMS-Im samples. Piranha solution removed the residues, but also destroyed any surface modification. Vapor deposition was further investigated using HMDS, which had proven to be the most effective reagent in the previous vapor experiment, despite its mediocre performance in the solution measurements. First, the effect of temperature was probed on glass samples treated for three hours in a sealed vial containing neat HMDS vapor (Figure 5). An increase in contact angle, coupled with a steady decrease in emission intensity was evident as the deposition temperature increased from 25 to 280 °C. The best results were seen at the maximum oven temperature of 280 °C, with a contact angle of 117.2° ( 3.6° and an emission intensity of 2% that of the unmodified glass control. This value of the contact angle is higher than those reported previously for a trimethylsilanized surface,30 except for the advancing contact

Silica PassiVation Efficiency

Figure 5. Emission intensity (PDI-UPy) and contact angle of HMDS films on glass prepared from vapor deposition at varying temperatures for 3 h in a sealed cell.

Langmuir, Vol. 24, No. 16, 2008 8545

Figure 7. Plot of contact angle vs PDI-UPy emission intensity for all experiments in this study.

Our goal in this study was to obtain the optimum conditions for passivation of silica surfaces by silanation, leading to

exhaustive blocking of the hydroxyl sites. Since remaining sites in the < 1% level might not have significant impact on contact angle measurements, we sought methodology for characterizing such surfaces. Figure 7 shows the combination of the results for all experiments as a plot of contact angle vs emission intensity. Curiously, the plot shows that the correlation is to one side of a logarithmic line relating the minimum contact angle with the minimum fluorescence intensity. Deviations from this limit may reflect variations in morphology of the silicate surface. Despite the significant number of points to the right of this limiting line, it is surprising that the parameters from treatment of surfaces with one gas-phase and two solution methods show such a strong correlation. It should also be noted that on samples such as unmodified glass and other samples with low surface coverage, there tends to be a much larger deviation in both contact angle and emission intensity, while more densely blocked surfaces tend to be more reproducible. The take-home message is that for examining densely passivated surfaces, fluorescent labeling methods may provide insight where contact angle measurements do not. TMS-A was chosen as one candidate for this study after Hayashi et al. reported the functionalization of silica gel by an allylorganosilane.45 For a 100 mM toluene solution of their selected allylsilane, 2-propenyl(3-chloropropyl)dimethylsilane, the researchers reported a loading density of 1.1 mmol/g after 15 h in refluxing toluene. Despite these promising results, for this study only vapor deposition at a temperature of 280 °C for 3 h was sufficient to show any significant surface coverage. HMDS is the most commonly used reagent, and works quite well as a surface passivating agent when deposited from the Vapor at 280 °C. In general, increasing the temperature also increases the surface coverage in both solution and vapor depositions. Increasing concentration also plays a role in boosting the amount of reagent which can react with the surface hydroxyls. Extended treatment time, in both solution and the vapor phase, does not offer much of an advantage, as HMDS samples are fully reacted after about three hours in 75 °C toluene and only 30 min in 280 °C vapor. TMS-Cl was ineffective as a passivating agent from dipcoating, presumably due to its high volatility and the need for extraneous

(44) Joung-Man, N; Park, J.-M. N.; Kim, J. H. J. Colloid Interface Sci. 1994, 168, 103–110.

(45) Shimada, T. A., K.; Shinoda, Y.; Nakamura, T.; Tokunaga, N.; Inagaki, S.; Hayashi, T. J. Am. Chem. Soc. 2003, 125, 4688–4689.

Figure 6. Emission intensity (PDI-UPy) and contact angle of HMDS films on glass prepared from vapor deposition at 280 °C for varying periods of time in a sealed cell.

angle reported by Park and Kim.44 We have no explanation for this observation, although we remark that the (static) contact angles were quite reproducible. Finally, the effect of vapor deposition time (280 °C, neat HMDS) was examined (Figure 6). Even after only 5 min at elevated temperature, the contact angle jumped to 99.7° ( 1.8° and the emission intensity plummeted to 3% of the bare glass control. Longer treatment increased the contact angle even further (ca. 116°), though the emission intensity only dropped slightly to 1% that of bare glass. Vapor exposure beyond 30 min did not yield any significant improvements in either contact angle or surface blocking.

Discussion

8546 Langmuir, Vol. 24, No. 16, 2008

base, Et3N. This high volatility could also be used to explain the total lack of surface coverage from 1 mM solutions, as such a small volume of regent most likely boiled out of the toluene solution and remained in the headspace above the heating solvent. With higher concentrations, however, TMS-Cl began to offer some surface coverage, though dye uptake results never fell below 5% of the emission intensity of the glass controls. Increasing temperature and deposition time also did little to improve the surface coverage, indicating that the silyl-chloride functionality reacts quickly and at very mild temperatures, though does not offer a very dense coverage layer. TMS-Im and TMS-DMA, both proved to be versatile silanizing agents with very similar results. They were the only reagents tested which offered moderate blocking of dye adsorption and mildly elevated contact angles from 1 mM solutions. At higher concentration, however, they both provided dense surface coverage with emission intensities falling to as low as 1-2% of that of the unmodified glass controls. Solution concentrations of 10 mM offered dense coverage with no significant advantage seen by utilizing a 10-fold higher concentration solution. Films prepared by solution deposition at 105 °C were more effective at blocking dye uptake than those prepared from 75 °C solutions; however a further improvement was not found in using a higher deposition temperature. In addition, TMS-Im was the only tested reagent which showed any appreciable blocking of dye uptake from films prepared by dip-coating, with emission intensities less than 10% that of the glass control. Deposition of TMS-Im and TMS-DMA from vapor at 280 °C proved problematic however, with the majority of the attempts using TMS-Im resulting in a brown film which remained impervious to all but the harshest cleaning methods. TMS-DMA was somewhat more favorable toward vapor deposition, though it too tended to form a brownish film. While also difficult to remove, this film did appear to be soluble in alcohols and water and could be at least visibly removed from the surface. It is suspected that these brown films result from the decomposition of TMS-Im (decomposes 212 °C) and TMS-DMA at such high temperatures, which creates uncertainty as to the exact nature of the resulting films which exhibit high contact angles and low dye uptake. Fortunately, TMS-DMA proves an efficient passivator at relatively modest temperatures. A curious observation from this study is the lack of improvement in surface coverage from any of the reagents tested when films were prepared from 10 mM xylene solutions at 130 °C, compared to toluene solutions at 105 °C. In most cases, the contact angles actually decreased and the emission intensities increased slightly compared to the lower temperature toluene solutions. At this time, the origin of this decrease in surface coverage is unclear. Overall, as the contact angle measured for films increased, the uptake of the hydrogen-bonding fluorophore decreased, yielding reduced emission intensity. However, with such large standard deviations, most likely due the amorphous nature of the glass surface and the increased surface roughness, relative to the native

Crowe and Tolbert

oxide layer on a silicon wafer, and the multitude of factors which affect contact angle, direct correlations between the values of the angle measurements and the surface densities could not be made, although restricting the data set to a common set of conditions (Figures 3 and 4), provided a satisfying logarithmic plot (see Table of Contents graphic). It could be shown, morever, that as the contact angle increased and the emission intensity due the adsorbed dye decreased, the standard deviations for both dropped, indicating a uniformity and reproducibility being imparted to the surface, undoubtedly from the deposition of a more dense organic layer, which blocked access of the fluorophore to the surface hydroxyls by a combination of steric hindrance and reaction to form siloxane bonds.

Conclusions Although HMDS is a common reagent for passivation of silica surfaces, high surface coverage films were only obtained on glass treated with HMDS vapor held at 280 °C for 30 min. When lower temperatures are in order, glass treated with 10 mM solutions of either TMS-Im or TMS-DMA at 105 °C for 3 h is more effective, in line with McCarthy’s observations.30 No significant advantage was observed for increasing deposition times, increasing solution concentrations by 10-fold, or raising solution deposition temperatures (up to 130 °C). Thus the choice of silanizing reagent must reflect a careful consideration of the accessible reaction conditions. In particular, surfaces for which even scattered hydrophilic defect sites can lead to delamination and long-range failure of electronic materials must be rigorously treated and characterized, and the logarithmic relationship seen in Figure 7 indicates a higher sensitivity of fluorescence methods in this coverage regime. A highly fluorescent perylenediimide fluorophore, linked to a strongly hydrogen-bonding ureido-[2-(4-pyrimidone)] moiety functions as a sensitive method to monitor the density of available, unblocked, surface hydroxyl sites at high passivation conditions. For such systems, static water contact angles are only marginally useful and require care in conclusions based upon such data, as the amorphous nature of the glass surface typically produces substantial deviations in the measurements, particularly on surfaces with low surface coverage of a passivating film. Acknowledgment. This research has been supported by the U.S. National Science Foundation through Grant No. CHE0456892. Janusz Kowalik and Mohan Srinivasarao are acknowledged for their many useful discussions and suggestions. The generous gift of 4-{4-[9-(2,6-diisopropyl-phenyl)-1,3,8,10-tetraoxo-5,6,12,13-tetraphenoxy-3,8,9,10-tetrahydro-1H-anthra[2,1,9def;6,5,10-d′e′f′]diisoquinolin-2-yl]-phenyl)butyric acid by Prof. Klaus Mu¨llen and Dr. Neil Pschirer is gratefully acknowledged. Supporting Information Available: Additional synthetic details. This material is available free of charge via the Internet at http://pubs.acs.org. LA801343B