Probing Carboxylic Acid Groups in Replaced and Mixed Self

Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, 55128 Mainz, ... carboxylic acid (COOH) group terminated self-assembled monolayer (SAM) o...
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Langmuir 2002, 18, 1801-1810

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Probing Carboxylic Acid Groups in Replaced and Mixed Self-Assembled Monolayers by Individual Ionized Dendrimer Molecules: An Atomic Force Microscopy Study Hua Zhang,† P. C. M. Grim,† Daojun Liu,† T. Vosch,† S. De Feyter,† U.-M. Wiesler,‡ A. J. Berresheim,‡ K. Mu¨llen,‡ C. Van Haesendonck,§ N. Vandamme,§ and F. C. De Schryver*,† Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium, Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany, and Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200 D, B-3001 Heverlee, Belgium Received July 10, 2001. In Final Form: November 26, 2001 Polyphenylene dendrimer molecules, G2Td(COOH)16, were ionized (G2 ions) and then adsorbed on a carboxylic acid (COOH) group terminated self-assembled monolayer (SAM) on a Au substrate through the linkage with Cu2+ ions. The individual G2 ions were observed by noncontact atomic force microscopy (NCAFM). The strong interaction between the G2 and Cu2+ ions, the latter of which are preadsorbed on the COOH SAM, led to a compression or deformation of the SAM and resulted in a decreased height of the G2 ions measured by NCAFM as compared to a theoretical model of the dendrimer molecules. The present method offers an approach to bind dendrimer molecules to a solid substrate and to study the nanoscale behavior of two kinds of thiol molecules in replaced and mixed SAMs.

Introduction As a new kind of functional material, dendrimers with a unique highly branched regular structure have recently attracted increasing interest.1 They can be used as catalysts when functionalized,2 as energy or charge transfer systems,3 as charge transport or light-emitting layers in organic light-emitting diodes (LEDs),4 as CO2selective molecular sensors,5 as biomimetic6 materials, and as templates for the preparation for monodisperse nanoparticles, such as Cu,7 Pd, and Pt.8 Based on * To whom correspondence should be addressed. Telephone: +32-16-327405. Fax: +32-16-327989. E-mail: Frans.DeSchryver@ chem.kuleuven.ac.be. † Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KULeuven). ‡ Max-Planck-Institut fu ¨ r Polymerforschung. § Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven (KULeuven). (1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 1996. (b) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (c) Fre´chet, J. M. J. Science 1994, 263, 1710. (d) Berresheim, A. J.; Mu¨ller, M.; Mu¨llen, K. Chem. Rev. 1999, 99, 1747. (2) (a) Reetz, M. T.; Lohmer, G.; Schwickardi, R. Angew. Chem., Int. Ed. 1997, 36, 1526. (b) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. (c) Chow, H.-F.; Mak, C. C. J. Org. Chem. 1997, 62, 5116. (3) (a) Jockusch, S.; Ramirez, J.; Sanghvi, K.; Nociti, R.; Turro, N. J.; Tomalia, D. A. Macromolecules 1999, 32, 4419. (b) Gilat, S. L.; Adronov, A.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. Engl. 1999, 38, 1422. (c) Hofkens, J.; Latterini, L.; De Belder, G.; Gensch, T.; Maus, M.; Vosch, T.; Karni, Y.; Schweitzer, G.; De Schryver, F. C.; Hermann, A.; Mu¨llen, K. Chem. Phys. Lett. 1999, 304, 1. (d) Devadoss, C.; Bharati, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635. (e) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26. (4) (a) Halim, M.; Pillow, J. N. G.; Samuel, I. D. W.; Burn, P. L. Adv. Mater. 1999, 11, 371. (b) Kraft, A. J. Chem. Soc., Chem. Commun. 1996, 77. (c) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv. Mater. 1996, 8, 237. (d) Shirota, Y.; Kuwabara, Y.; Inada, H.; Wakimoto, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.; Imai, K. Appl. Phys. Lett. 1994, 65, 807. (5) Kovvali, A. S.; Chen, H.; Sirkar, K. K. J. Am. Chem. Soc. 2000, 122, 7594.

dendrimer structures, monolayer or multilayer films can be obtained9 (onto which noble metal colloids can be deposited10) and resists for scanning probe lithography11 have been developed. Dendrimer molecules can be deposited on a solid substrate (e.g., mica, glass, Si, graphite, Au, etc.) by different techniques, such as film casting,12a spincoating,12b,13,14,15a,b rinsing the substrates with dendrimer solution,12c rubbing the dendrimer on substrates,13 dipping the substrate into dendrimer solutions,15c and electrostatic layer-by-layer deposition.9b,c Using atomic force microscopy (6) Kukowska-Latallo, J. F.; Bielinska, A. U.; Johnson, J.; Spindler, R.; Tomalia, D. A.; Baker, J. R. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 4897. (7) (a) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (b) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (8) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1999, 38, 364. (9) (a) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988. (b) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (c) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (10) (a) Rubin, S.; Bar, G.; Taylor, T. N.; Cutts, R. W.; Zawodzinski, T. A., Jr. J. Vac. Sci. Technol. 1996, 14, 1870. (b) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T. A., Jr. Langmuir 1996, 12, 1172. (11) (a) Tully, D. C.; Wilder, K.; Fre´chet, J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314. (b) Tully, D. C.; Trimble, A. R.; Fre´chet, J. M. J.; Wilder, K.; Quate, C. F. Chem. Mater. 1999, 11, 2892. (12) (a) Sheiko, S. S.; Eckert, G.; Ignat’eva, G.; Muzafarov, A. M.; Spickermann, J.; Ra¨der, H. J.; Mo¨ller, M. Macromol. Rapid Commun. 1996, 17, 283. (b) Huck, W. T. S.; van Veggel, F. C. J. M.; Sheiko, S. S.; Mo¨ller, M.; Reinhoudt, D. N. J. Phys. Org. Chem. 1998, 11, 540. (c) Hellmann, J.; Hamano, M.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Irie, M. Jpn. J. Appl. Phys. 1998, 37, L816. (13) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Magonov, S. N. Langmuir 2000, 16, 5487. (14) Loi, S.; Wiesler, U.-M.; Butt, H.-J.; Mu¨llen, K. Chem. Commun. 2000, 1169. (15) (a) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R.; Tomalia, D. A.; Meier, D. J. Langmuir 2000, 16, 5613. (b) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347. (c) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323.

10.1021/la011061t CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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(AFM),16 a powerful tool to visualize the topography of a surface, aggregated dendrimer molecules have been observed on a variety of surfaces such as mica,12 graphite,12a,b,17 glass,12a and a charged solid surface.9b,c Furthermore, self-organized structures of carbosilane liquid crystalline dendrimers on Si, glass, and mica surfaces,13 alkyl-substituted polyphenylene dendrimers on graphite,14 individual molecules of generations 4 and 8 (G4 and G8) polyamidoamine (PMMA) starburst dendrimers adsorbed on a Au(111) surface,15c as well as G5G10 PMMA dendrimers15a and four different types of coreshell tecto-(dendrimer)15b on mica have been observed. So far, studies concerning dendrimer monolayers, dendrimer-alkanethiol mixed monolayers,18 thiolated dendrimers19 adsorbed on Au substrates, and the insertion process of individual (metallo)dendrimer sulfide molecules into a SAM based on thiol-sulfide exchange20 have been reported. Self-assembled monolayers (SAMs),21 generated by the adsorption of organic molecules onto some specific solid substrates, have potential applications from nanotechnology to fundamental surface science, because of their stable and densely packed structures, their controllable surface functional groups and chemical properties, and their simple and rapid preparation. Employing selfassembly techniques, inhomogeneously functionalized surfaces can be fabricated which can be used as templates for the selective adsorption of proteins22 and the deposition of nanoparticles23 and polymers.24 Methods to create such inhomogeneously functionalized surfaces include microcontact printing (µCP),24,25 lithography,26 mixed SAMs,27 and replaced SAMs.28 Many techniques were used to study replaced and mixed SAMs. For example, voltammetry,28a scanning electron (16) (a) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (b) Takano, H.; Kenseth, J. R.; Wong, S.-S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845. (17) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (18) (a) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492. (b) Lackowski, W. M.; Campbell, J. K.; Edwards, G.; Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 7632. (19) Chechik, V.; Crooks, R. M. Langmuir 1999, 15, 6364. (20) (a) Friggeri, A.; Scho¨nherr, H.; van Manen, H.-J.; Huisman, B.H.; Vancso, G. J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 2000, 16, 7757. (b) Huisman, B.-H.; Scho¨nherr, H.; Huck, W. T. S.; Friggeri, A.; van Manen, H.-J.; Menozzi, E.; Vancso, G. J.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. Engl. 1999, 38, 2248. (21) (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (22) Wadu-Mesthrige, K.; Amro, N.; Xu, S.; Liu, G.-Y.Langmuir 1999, 15, 8580. (23) (a) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (b) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (24) Vaeth, K. M.; Jackman, R. J.; Black, A. J.; Whitesides, G. M.; Jensen, K. F. Langmuir 2000, 16, 8495. (25) (a) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (b) He, H. X.; Huang, W.; Zhang, H.; Li, Q. G.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 517. (c) He, H. X.; Li, Q. G.; Zhou, Z. Y.; Zhang, H.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 9683. (26) (a) Liu, G.-Y.; Xu, S.; Qian, Y. Acc. Chem. Res. 2000, 33, 457. (b) Sugimura, H.; Ushiyama, K.; Hozumi, A.; Takai, O. Langmuir 2000, 16, 885. (c) Zheng J. W.; Zhu, Z. H.; Chen, H. F.; Liu, Z. F. Langmuir 2000, 16, 4409. (d) Sugimura, H.; Nakagiri, N. J. Am. Chem. Soc. 1997, 119, 9226. (27) (a) Tamada, K.; Hara, M.; Sasabe, K.; Knoll, W. Langmuir 1997, 13, 1558. (b) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (c) Shon, Y.-S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (d) Imabayashi, S.-i.; Gon, N.; Sasaki, T.; Hobara, D.; Kakiuchi, T. Langmuir 1998, 14, 2348. (28) (a) Kakiuchi, T.; Sato, K.; Iida, M.; Hobara, D.; Imabayashi, S.-i.; Niki, K. Langmuir 2000, 16, 7238. (b) Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024. (c) Dishner, M. H.; Taborek, P.; Hemminger, J. C.; Feher, F. J. Langmuir 1998, 14, 6676.

Zhang et al.

microscopy (SEM),28a secondary ion mass spectrometry (SIMS),28b quartz crystal microbalance (QCM),28c and scanning tunneling microscopy (STM)28a,c,29 have been used to study the replacement of one SAM by immersing it into another thiol compound solution. As for mixed SAMs, some macroscopic methods such as ellipsometry,30a,31 X-ray photoelectron spectroscopy (XPS),30a,31 contact angle measurements,27d,30 infrared spectroscopy,27b,31 Auger electron spectroscopy (AES),27b and coulometry27b were used to characterize average chemical compositions and properties of the mixed SAMs, while microscopic techniques including AFM,27a,c lateral force microscopy (LFM),27b and STM31 have been used to observe nanometer scale molecular domains and phase separation. But in most of these studies, in order to characterize the composition and properties of replaced/mixed SAMs with scanning probe microscopic techniques (AFM, LFM, and STM), a substantial difference in properties between the thiol compounds in replaced and mixed SAMs is required. In our previous reports, individual and aggregated polyphenylene dendrimer molecules of G4-Td on mica32a were observed by noncontact AFM and their stiffness and adhesion properties were studied by pulsed force mode (PFM) AFM.33 Also, based on the different adhesive interaction with a silicon tip, two aggregates of polyphenylene-based dendrimers, G2Td(COOH)16 (Figure 1a) and G4-Td, could be easily discriminated by means of PFM-AFM.32b In this contribution, using self-assembly techniques, individual ionized G2Td(COOH)16 dendrimers (G2 ions) were adsorbed on the surface of a COOH SAM on Au through the linkage with Cu2+ ions, thereby forming a submonolayer structure. It is shown that individual G2 ions can be used to probe the presence of COOH groups in replaced/mixed SAMs. This paper also offers an approach to selectively assemble dendrimer molecules onto a substrate. Experimental Section 1. Materials. The polyphenylene dendrimer molecule used in this paper is denoted as G2Td(COOH)16 (structure shown in Figure 1a).32b Tetrahydrofuran (THF) (Spectroscan, Labscan Ltd., Dublin), ethanol (BDH Laboratory Suppliers, Poole, U.K.), potassium hydroxide (KOH) (Janssen Chimica, Geel, Belgium), copper acetate (Cu(OAc)2) (Leuven, Belgium), 11-mercaptoundecanoic acid (abbreviated as HOOC-C10-SH, Aldrich Chemical Co., Inc.), and 1-dodecanethiol (abbreviated as CH3-C11-SH, Aldrich Chemical Co., Inc.) were used as received. The concentrations of G2Td(COOH)16 in THF, HOOC-C10-SH, CH3C11-SH and Cu(OAc)2 in ethanol, and KOH in Milli-Q water were 6.8 × 10-6, 1.8 × 10-3, 1.8 × 10-3, 2.0 × 10-3, and 2.2 × 10-2 M, respectively. 2. Sample Preparation. 1. HOOC-C10-SH SAM (COOH SAM), HOOC-C10-SH/CH3-C11-SH Mixed SAMs (COOHCH3 Mixed SAMs), and CH3-C11-SH SAM Replaced by HOOC-C10-SH (COOH-CH3 Replaced SAMs). The COOH (29) (a) Lin, P.-H.; Guyot-Sionnest, P. Langmuir 1999, 15, 6825. (b) Ishida, T.; Mizutani, W.; Tokumoto, H.; Choi, N.; Akiba, U.; Fujihira, M. J. Vac. Sci. Technol., A 2000, 18, 1437. (c) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (30) (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (b) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (31) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (32) (a) Zhang, H.; Grim, P. C. M.; Foubert, P.; Vosch, T.; Vanoppen, P.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9009. (b) Zhang, H.; Grim, P. C. M.; Vosch, T.; Wiesler, U.-M.; Berresheim, A. J.; Mu¨llen, K.; De Schryver, F. C. Langmuir 2000, 16, 9294. (33) Rosa-Zeiser, A.; Weilandt, E.; Weilandt, H.; Marti, O. Meas. Sci. Technol. 1997, 8, 1333.

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Figure 1. (a) The molecular structure of G2Td(COOH)16. (b) A space filling view of a G2Td(COOH)16 dendrimer molecule which was built by the MMFF method in Spartan. (c) The tetrahedral presentation of G2Td(COOH)16. The distance between the top of one branch and the spot O, the center of the triangle at the base plane formed by the other three branches, is 3.1 nm. The distance between two apexes of any two branches equals 3.8 nm. SAM on Au was prepared according to a method described in the literature.34 In the present study, two kinds of gold substrates were used, made by evaporating Au on heated mica (hereafter referred to as Au-coated mica substrate)35 and sputtering Au on silicon (hereafter referred to as Au-coated Si substrate).34a-c The thickness of the Au layer on the mica substrate is about 100 nm.35 After removal from the vacuum, the Au-coated mica substrate was immediately immersed in a HOOC-C10-SH solution (1.8 × 10-3 M in ethanol) for 1 day to form the COOH SAM. To prepare the COOH-CH3 mixed SAMs, Au-coated mica substrates were immersed in the mixed HOOC-C10-SH/CH3C11-SH solution (both of 1.8 × 10-3 M in ethanol, at a volume ratio of 1:3, 1:1, or 3:1) for 1 day to form the various COOH-CH3 mixed SAMs. As for the preparation of the COOH-CH3 replaced SAM, first Au-coated mica substrates were immersed in the CH3-C11-SH solution (1.8 × 10-3 M in ethanol) for 1 day to form the CH3 SAMs. After rinsing with ethanol and drying with ultrapure N2 gas, the CH3 SAMs were immersed in a 1.8 × 10-3 M HOOC-C10-SH ethanol solution for time spans of 30 min, 4 h, 8 h, and 24 h to form the various COOH-CH3 replaced SAMs. After formation of the respective SAMs, the samples were sequentially rinsed with ethanol and Milli-Q water and then dried with ultrapure N2 gas. As for the Au-coated Si substrates,34a-c the silicon wafers were sputter-coated sequentially with a 10 nm Ti adhesion layer and (34) (a) Zhang, H.; He, H. X.; Wang, J.; Liu, Z. F. Langmuir 2000, 16, 4554. (b) Zhang, H.; He, H. X.; Mu, T.; Liu, Z. F. Thin Solid Films 1998, 329, 778. (c) Zhang, H.; Zhang, H. L.; He, H. X.; Zhu, T.; Liu, Z. F. Mater. Sci. Eng., C 1999, 8-9, 191. (d) Zhang, H.; He, H. X.; Wang, J.; Mu, T.; Liu, Z. F. Appl. Phys. A 1998, 66, S269. (35) Vandamme, N.; Verschoren, G.; Depuydt, A.; Cannaerts, M.; Bouwen, W.; Lievens, P.; Silverans, R. E.; Van Haesendonck, C. Appl. Phys. A 2001, 72, S177.

a 100 nm Au layer. After cleaning in 90 °C piranha solution (V(98% H2SO4)/V(30% H2O2) ) 7:3) (caution: piranha solution reacts violently with most organic materials and must be handled with extreme care) for 3-5 min and rinsing with Milli-Q water and ethanol, the Au-coated Si substrates were immersed in the corresponding thiol solutions to form the COOH replaced and mixed SAMs, under similar experimental conditions as for Aucoated mica substrates. 2. COOH SAM Decorated with Individual G2Td(COOH)16 Ions (G2 Ions) (Scheme 1). First, a COOH SAM was immersed in an aqueous KOH solution (2.2 × 10-2 M) for 5 min in order to ionize the COOH groups in the COOH SAM. After rinsing with ethanol and drying with ultrapure N2 gas, the sample was immersed in a 2.0 × 10-3 M Cu(OAc)2 ethanol solution for 5 min. The Cu2+ ions were bound onto the COO- anions36 to form a CuOOC SAM. After rinsing with ethanol and drying with ultrapure N2 gas again, the CuOOC SAM was immersed in a mixed solution (6.8 × 10-6 M G2Td(COOH)16 in THF and 2.2 × 10-2 M KOH in water with a molar ratio of 1:16, that is, [COOH]/[OH-] ) 1:1) for time spans of 1 s, 1 min, 5 min, 10 min, and 20 min, to adsorb the G2 ions (through their COO- anions present at the rim) to the Cu2+ ions of the CuOOC SAM. The obtained sample was rinsed with ethanol and dried with ultrapure N2 gas. Steps 1, 2, and 3, as depicted in Scheme 1, can be repeated to obtain multilayers of G2 ions adsorbed on the substrate. 3. Replaced/Mixed COOH-CH3 SAMs Decorated with G2 Ions. The preparation process to adsorb G2 ions on replaced/ mixed COOH-CH3 SAMs is similar to the previous step as (36) (a) Zhang, H.; Fu, D. G.; Ji, F.; Wang, G. X.; Yu, K. B.; Yao, T. Y. J. Chem. Soc., Dalton Trans. 1996, 3799. (b) Smith, D. R. Coord. Chem. Rev. 1998, 172, 457.

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Scheme 1. Preparation of Individual G2 Ions Adsorbed on the COOH SAM through the Linkage with Cu2+ Ions

Zhang et al. height of 26 ( 1.0 nm (Silicon MDT, Moscow, Russia). All images presented in this paper have not been processed other than leveling and contrast enhancement. The fast and slow scanning directions are horizontal and vertical, respectively. 4. Molecular Modeling of G2Td(COOH)16.37 The molecular model of G2Td(COOH)16 was built in a vacuum by a Merck Molecular Force Field (MMFF)37 method in Spartan (Wavefunction Inc., Irvine, CA) (as shown in Figure 1b). The MMFF, from Merck Pharmaceuticals, is limited in scope to organic systems and biopolymers (including some charged species) but allows the calculation of molecular geometry and conformation.

Results and Discussion

outlined above. Instead of the COOH SAM, now a replaced or mixed COOH-CH3 SAM is used. 3. Atomic Force Microscopy (AFM). After preparation, the samples were immediately imaged under ambient conditions with a Discoverer TMX 2010 AFM system (ThermoMicroscopes, San Francisco, CA), operated in the noncontact mode. Noncontact AFM (NCAFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NCAFM is on the order of tens to hundreds of angstroms. The probe is in the attractive force region, and the cantilever is pulled toward the sample. NCAFM provides a means for measuring sample topography with little or no contact between the tip and the sample. The total force between the tip and the sample in the noncontact regime is very low, generally about 10-12 N. This low force is advantageous for studying soft or elastic samples. For the NCAFM measurements, Si probes (ThermoMicroscopes) with spring constants of 11-16 N/m and resonance frequencies of 250-310 kHz were used. A piezoelectric tube scanner was used with a scan range of 7 µm × 7 µm in the XY-directions and 2.4 µm in the Z-direction. The z-scanner was calibrated using a silicon grating with a step

1. Visualization of Individual Ionized G2Td(COOH)16 Molecules (G2 Ions). In our previous publication, it was shown that upon adsorption on mica at a relatively low concentration of 3.2 × 10-8 M of G2Td(COOH)16 in THF, still dendrimer aggregates were observed,32b while at a similar concentration of 1.47 × 10-8 M, for G4-Td, individual dendrimer molecules were observed.32a This difference arises from the different rim functionality of the two different dendrimers. Apparently, the hydrogen bonds formed between the COOH groups at the rim of the G2Td(COOH)16 dendrimers easily lead to aggregations. To obtain individual G2Td(COOH)16 dendrimers, a KOH solution was added to the G2Td(COOH)16 solution to break the hydrogen bonds between the dendrimer molecules and to deprotonate the COOH groups of the G2Td(COOH)16 dendrimers (hereafter referred to as G2 ions). These G2 ions were then adsorbed on a CuOOC SAM (as described in the Experimental Section) and visualized in a step-by-step AFM analysis. Figure 2a shows a typical NCAFM image of a Au-coated mica substrate, whose surface roughness (root mean square (rms) value) equals 0.06 nm. On this Au-coated mica substrate, a COOH SAM was adsorbed and treated to obtain a CuOOC SAM (as described in the Experimental Section). Figure 2b shows the NCAFM image of such a surface, leading to a surface roughness (rms) of 0.09 nm. These images show that the Au-coated mica substrate is very flat and that adsorption of the CuOOC SAM does not alter the surface roughness appreciably. G2 ions were adsorbed onto the CuOOC SAM by binding their COO- groups at the rim to the Cu2+ ions. Figure 3a shows the NCAFM image of G2 ions adsorbed on a CuOOC SAM after immersion for 1 s. White spherical spots were observed on the CuOOC SAM, forming a submonolayer structure. It is reasonable to assume that the size of a G2 ion is similar to that of a G2Td(COOH)16 dendrimer molecule. The molecular model built by the MMFF method in Spartan37 and the tetrahedral presentation of the G2(COOH)16 dendrimer molecule are shown in parts b and c of Figure 1, respectively. Compared to the dimension of the molecule (Figure 1c), the apparent height of the spots measured by NCAFM is much lower. This height was assessed to be 1.0 ( 0.4 nm (for example, measured heights of 1.2 and 1.0 nm in Figure 3b), determined by fitting the histogram of the measured heights to a Gaussian curve. The difference between the theoretical and the experimentally observed value can be explained as follows. First, in Figure 3 the distance between neighboring spherical spots is less than 30 nm (distances c-f shown in Figure 3b), which is comparable with the radius of the AFM tip. This will result in an underestimation of the height of the spherical spots because some deeper valleys between the spots are inaccessible and the AFM tip does (37) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.

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Figure 2. A typical NCAFM image of a Au-coated mica substrate (a) and a CuOOC SAM on a Au-coated mica substrate (b).

Figure 3. (a) A NCAFM image of G2 ions adsorbed on a CuOOC SAM on a Au-coated mica substrate. The adsorption time of the G2 ions is 1 s. (b) The topography profile along the dotted line in (a), crossing the centers of individual G2 ions. Letters a-f represent the measured values for the dendrimer size or spacing between individual dendrimers, as indicated with arrows in the topography profile.

not probe the surface of the CuOOC SAM.38 Another reason for the reduced height of the G2 ions could be the compression or deformation of the CuOOC SAM due to the strong electrostatic attraction between the G2 ions and the CuOOC SAM (vide infra). Although it is impossible to measure the true height of the spherical spots because of their close proximity, on the basis of an apparent size of 10.6 ( 2.9 nm (full width at half-maximum, fwhm, see Figure 3b) which is much larger than the observed height of 1.0 ( 0.4 nm due to the geometrical tip-sample convolution effect,32a,39 we believe that the white spots in Figure 3a represent individual G2 ions (more direct evidence vide infra). Figure 4 shows the NCAFM images of G2 ions adsorbed on a CuOOC SAM for different adsorption times (1, 5, 10, and 20 min). The images show individual G2 ions at higher concentrations than in Figure 3a, arising from an increased adsorption time. Again, the adsorbed individual G2 ions form a submonolayer structure. By repeating steps 1, 2, and 3 depicted in Scheme 1, multilayers of G2 ions adsorbed on a substrate can be obtained. Figure 5 shows the topography of such a multilayer (consisting of five layers of G2 ions adsorbed on a Au-coated mica substrate), which is quite different from the images of individual G2 ions (Figures 3a and 4), a Au-coated substrate (Figure 2a), and a CuOOC SAM (Figure 2b). In Figure 5, individual G2 ions can no longer be observed, which suggests that the multilayer film is formed via random adsorption of G2 ions (as depicted in Scheme 1). (38) Koutsos, V.; van der Vegte, E. W.; Pelletier, E.; Stamouli, A.; Hadziioannou, G. Macromolecules 1997, 30, 4719. (39) (a) Yang, J.; Laurion, T.; Jao, T.-C.; Fendler, J. H. J. Phys. Chem. 1994, 98, 9391. (b) Markiewicz, P.; Goh, M. C. Langmuir 1994, 10, 5.

2. Visualization of COOH Groups in Replaced and Mixed SAMs. In the present study, we describe an indirect but effective method to probe the distribution of COOH groups in replaced and mixed SAMs by the adsorption of G2 ions. To demonstrate the effectiveness of this method, two thiol compounds with similar chain lengths but different molecular structures, COOH-C10-SH and CH3-C11-SH, were chosen. 1. Probing COOH Groups in Replaced COOH-CH3 SAMs. Figure 6 shows G2 ions adsorbed on the various COOH-CH3 replaced SAMs through the linkage with Cu2+ ions, where the replacement time of the CH3 SAM in the COOH-C10-SH ethanol solution equals 30 min, 4h , 8h , and 24 h. For G2 ions adsorbed on a 30-minreplaced COOH-CH3 SAM (Figure 6a), the number of G2 ions is much less than on a CuOOC SAM (Figures 3a and 4), leading to a larger separation between neighboring G2 ions. During the AFM imaging process, the AFM tip can access the surface of the SAM and the correct height and size of G2 ions can be measured (assuming the molecular lengths of CH3-C11-SH and CuOOC-C10SH SAMs to be identical). The height and width (fwhm) of the G2 ions on a 30-min-replaced HOOC-CH3 SAM equal 2.5 ( 0.4 and 13.6 ( 0.5 nm (Figure 6b), respectively. The height of 2.5 nm is somewhat smaller than the calculated value for the G2 ions (3.1 nm, Figure 1c), which probably arises from the compression or deformation of the SAM due to the strong electrostatic attraction between the G2 ions and the SAM. In our previous report,32a the measured height of a different polyphenylene dendrimer molecule, G4-Td, on mica was observed to be in very good agreement with the dimension obtained from the molecular model. Apparently, the very weak interaction between

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Figure 4. NCAFM images of G2 ions adsorbed on a CuOOC SAM on a Au-coated mica substrate. The adsorption time of the G2 ions is 1 min (a), 5 min (b), 10 min (c), and 20 min (d).

Figure 5. A NCAFM image of a five-layer film of G2 ions adsorbed on a CuOOC SAM on a Au-coated mica substrate by repeating steps 1, 2, and 3 in Scheme 1. The adsorption time of the G2 ions is 5 min.

the hydrophobic G4-Td dendrimers and the hydrophilic mica surface, together with the rigid molecular structure, resulted in nearly no deformation of the dendrimer molecules. In the present study, the G2 ions are also composed of connected rigid benzene moieties. Because of the high shape persistence and rigidity of the G2 ions, their deformation is not very likely. The measured width of the G2 ions (13.6 ( 0.5 nm), significantly larger than the observed height (2.5 ( 0.4 nm), can be ascribed to the geometrical tip-sample convolution effect.32a,39 This suggests that individual G2 ions, adsorbed on a replaced COOH/CH3 SAM and also a COOH SAM (as mentioned above), were observed by AFM. As reported, with an increasing replacement time of the CH3 SAM by COOH-C10-SH molecules, the COOH

content in the replaced SAM increases,29a,b which in turn leads to an increased adsorption of G2 ions. The NCAFM images displayed in Figure 6 give direct evidence of this fact. In Figure 6a, with a replacement time of 30 min, only a small number of individual G2 ions are present. Besides the small spots due to the adsorbed G2 ions, also somewhat larger white spots can be observed. They are probably caused by the adsorption of aggregates.40 Increasing the replacement time of the CH3 SAM by COOH-C10-SH indeed leads to an increased number of adsorbed G2 ions (Figure 6c-e). As a result, from the observed number of G2 ions adsorbed on the replaced CuOOC-CH3 SAM, the relative number of CH3 groups replaced by COOH groups can be qualitatively estimated. A larger quantity of adsorbed G2 ions reflects a larger number of COOH groups present. Theoretically, the largest contact area of a G2 ion on a COOH SAM is 6.25 nm2, which is the area of the triangle formed by the three branches of the dendrimer molecule (40) In Figure 6, a few large white spots are visible, which are expected to be dendrimer aggregates. As described in the Experimental Section, the solution of G2 ions was prepared by mixing an aqueous KOH solution and a G2(COOH)16 solution in THF ([OH-]/[COOH] ) 1/1). The solvent consists mainly of THF, and only a little H2O (V(THF)/V(H2O) ) 202:1) is added. Therefore, the deprotonation of the G2Td(COOH)16 dendrimers might be incomplete due to the reduced effectiveness of KOH in the solvent mixture with respect to H2O. The undeprotonated COOH groups at the rim of the G2Td(COOH)16 dendrimers can also form hydrogen bonds resulting in dendrimer aggregation. On a pure CuOOC SAM (Figures 3, 4, and 8a), dendrimer aggregates were seldomly observed, while on the replaced/mixed SAMs, a few dendrimer aggregates could be observed (Figures 6, 7, and 9). But this does not affect the main results. In this experiment, we cannot add an excess of KOH in order to be sure of complete deprotonation of the COOH groups of the G2(COOH)16 dendrimers, since the excess KOH in the G2 ion solution could react with the linkers of Cu2+ ions preadsorbed on a HOOC or replaced/mixed COOH/CH3 SAM. In this case, a reliable probing of the COOH groups in a SAM cannot take place because the G2 ions cannot be adsorbed on the COO- group terminated SAM.

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Figure 6. NCAFM images of the adsorption of G2 ions on replaced COOH-CH3 SAMs on Au-coated mica substrates through the linkage with Cu2+ ions. The time of replacement of the CH3 SAM by COOH-C10-SH is 30 min (a), 4 h (c), 8 h (d), and 24 h (e). (b) The topography profile across the centers of individual G2 ions along the dotted line in (a).

which are in contact with the base plane considering a tetrahedral structure (Figure 1c). The area per molecule in the SAM being 21.4 Å2,21a the number of molecules underneath an individual G2 ion is about 29. If we consider all molecules underneath the G2 ions in the replaced COOH-CH3 SAMs (Figure 6) to be COOH groups, the maximum number of CH3 groups replaced by COOH groups could be estimated by multiplying the number of G2 ions on the mixed SAM by 29. The lateral resolution of this method is, in principle, less than 3.8 nm, which is one lateral length of the tetrahedron (Figure 1c). However, the geometrical tipsample convolution effect32a,39 will decrease the practically attainable lateral resolution according to the measured width of the individual G2 ions (13.6 nm in this case). Nevertheless, it demonstrates the nanoscale spatial resolution of the current approach. 2. Probing COOH Groups in Mixed COOH-CH3 SAMs. G2 ions can also be used to probe the presence of COOH groups in mixed SAMs. To demonstrate this, three

different mixed COOH-CH3 SAMs have been prepared. The molar ratios of COOH-C10-SH and CH3-C11-SH in the mixed solutions are 1:3, 1:1, and 3:1 (abbreviated as 1/3, 1/1, and 3/1, respectively). After the adsorption of G2 ions through the linkage with Cu2+ ions, AFM images were again acquired. The NCAFM image in Figure 7a represents G2 ions adsorbed on the 1/3 COOH-CH3 mixed SAM. Only a few individual and aggregated40 G2 ions are observed. With increasing molar ratio of COOH-C10SH in the mixed solution, the adsorption of G2 ions increases, indicating that the number of COOH groups in the mixed SAMs also increases (Figure 7b,c). Except for a low number of aggregates,40 this image resembles the one in Figure 4 where G2 ions were adsorbed on a CuOOC SAM. The variation in the number of adsorbed G2 ions indicates that the surface properties of the various mixed COOH/CH3 SAMs are quite different. The formation dynamics of a mixed SAM is complicated, and the respective contribution of the components of the layer is not necessarily the same as in solution. This will

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Figure 7. NCAFM images of G2 ions adsorbed on mixed COOH-CH3 SAMs on Au-coated mica substrates through the linkage with Cu2+ ions. The molar ratios of COOH-C10-SH and CH3-C11-SH in the respective solutions are 1:3 (a), 1:1 (b), and 3:1 (c).

depend on the molar ratio of the different thiol molecules in solution, the chain lengths, the molecular structure, the tail groups, and the interaction of the thiol compounds in the SAM and the solvent. Whitesides et al.30 have studied mixed SAMs by contact angle measurements and proposed similar conclusions, here corroborated by direct AFM evidence. From the observed G2 ions adsorbed on the COOHCH3 mixed SAMs (Figure 7), no large domains or phase separation has been observed. This probably arises from the similarity in hydrocarbon chain length, chain properties, and intermolecular interactions of the two thiol compounds (HOOC-C10-SH and CH3-C11-SH). As for the replaced/mixed COOH/CH3 SAMs in this study, it is difficult to distinguish the two compounds in replaced/mixed COOH-CH3 SAMs by scanning probe microscopy (SPM), because the two thiol compounds, COOH-C10-SH and CH3-C11-SH, have similar molecular length and conductive properties, and the domain size in replaced/mixed SAMs is very small. But by using individual G2 ions as probes, it is easy to make the distinction between COOH and CH3 surface groups (see Figures 6 and 7). Of course, the spatial resolution of this approach is limited on one side by the dimension of the probe molecule and on the other side by the dimension of the AFM tip. A practical value for the present lateral resolution is 10-20 nm. 3. G2 Ions Adsorbed on SAMs on Au-Coated Si Substrates. In the present study, another Au film (coated on a Si substrate)34a-c was also used to form SAMs and subsequently adsorb the G2 ions. The main results obtained are similar to those on Au-coated mica substrates; however, some noteworthy differences are observed. Figure 8a shows the NCAFM image of G2 ions adsorbed on the surface of a CuOOC SAM which was immersed

into the solution of G2 ions for 1 s. G2 ions are observed on top of the sputtered Au islands on the substrate, forming a submonolayer structure. The roughness (rms value) of the CuOOC SAM on this Au-coated Si substrate (1 µm × 1 µm) is 1.1 nm (data not shown here). Compared to the Au-coated mica substrate (Figure 2a), the surface of this Au-sputtered substrate is relatively rough. The measured height of the G2 ions (Figure 8a) is 1.2 ( 0.3 nm, while their size (fwhm) ranges from 7.5 to 12.5 nm (Figure 8b), similar to those on a flat Au-coated mica substrate (Figure 3). Also here, the decreased height of the G2 ions can be explained by a compression or deformation of the SAM due to the strong electrostatic attraction, in combination with the close proximity of the G2 ions which prevents the AFM tip from touching the CuOOC SAM between the G2 ions. The relative large width of the G2 ions is due to the geometrical tip-sample convolution effect.32a,39 Figure 8c shows five layers of G2 ions adsorbed on a Au-coated Si substrate. Due to the random adsorption of the G2 ions, the morphology of the Au islands is somewhat obscured and individual G2 ions can no longer be discerned (as shown in Scheme 1). Through the linkage with Cu2+ ions, G2 ions were adsorbed on the different COOH-CH3 replaced SAMs on Au-coated Si substrates, where the replacement time of the CH3 SAM in the COOH-C10-SH ethanol solution is 5 min, 4 h, and 6 h (Figure 9). With increasing replacement time of the CH3 SAM by COOH-C10-SH molecules, the adsorption of G2 ions increases, providing evidence that the COOH content in the replaced SAMs increases. As known,29a,b the initial replacement process of a replaced SAM occurs at domain boundaries or at defect sites in the SAM. A direct visualization of this aspect is presented in Figure 9. In Figure 9a, on a 5-min-replaced

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Figure 8. (a) NCAFM images of G2 ions adsorbed on a CuOOC SAM on a Au-coated Si substrate. The adsorption time of G2 ions is 1 s. (b) The topography profile along the dotted line in (a), crossing the centers of individual G2 ions. (c) A NCAFM image of a five-layer film of G2 ions adsorbed on a CuOOC SAM on a Au-coated Si substrate by repeating steps 1, 2, and 3 in Scheme 1. The adsorption time of G2 ions is 5 min.

Figure 9. NCAFM images of G2 ions adsorbed on replaced COOH-CH3 SAMs on Au-coated Si substrates through the linkage with Cu2+ ions. The time of replacement of the CH3 SAM by COOH-C10-SH is 5 min (a), 4 h (b), and 6 h (c).

COOH/CH3 SAM, most of the G2 ions are adsorbed on the edges of the Au islands or the valleys between the Au

islands, where the domain boundaries or defect sites of a SAM are easily formed. With increasing replacement time,

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gradually the replacement process occurs everywhere, the G2 ions being adsorbed not only on the edges and the valleys but also on top of the Au islands (Figure 9b,c). Conclusions Individual ionized G2Td(COOH)16 molecules (G2 ions), adsorbed on a COOH SAM through the linkage with Cu2+ ions, were observed by NCAFM. The G2 ions formed a submonolayer structure on the CuOOC SAM. The strong interaction between the G2 ions and the CuOOC SAM, leading to a compression or deformation of the SAM, as well as the close proximity of the G2 ions, resulted in a decrease of the measured height of G2 ions compared to the calculated dimension of the dendrimer molecule. Using G2 ions as probes, the presence of COOH groups in a replaced/mixed COOH-CH3 SAM can be easily visualized. This technique shows that the initial replacement process of a replaced SAM preferentially occurs at

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domain boundaries or defect sites. Under the applied experimental conditions, no large-area domains were observed in the replaced/mixed SAMs. The attainable spatial resolution of this approach is limited by the dimension of the G2 ion as well as by the dimension of the AFM tip. At present, the attainable lateral resolution amounts to 13.6 nm, which equals the AFM tip convoluted size of the experimentally observed G2 ions. Acknowledgment. The authors thank the DWTC, through IUAP-IV-11, the FWO (Flemish Ministry of Education), ESF SMARTON and IWT through an STWW contract for financial support. The collaboration was made possible thanks to the EC-TMR project SISITOMAS. T.V. thanks IWT for a predoctoral fellowship. S.D.F. thanks the Fund for Scientific ResearchsFlanders for a postdoctoral fellowship. LA011061T