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Discrimination of Dendrimer Aggregates on Mica Based on Adhesion Force: A Pulsed Force Mode Atomic Force Microscopy Study Hua Zhang,† P. C. M. Grim,† T. Vosch,† U.-M. Wiesler,‡ A. J. Berresheim,‡ K. Mu¨llen,‡ and F. C. De Schryver*,† Laboratory for Molecular Dynamics and Spectroscopy, Department of Chemistry, Katholieke Universiteit Leuven (KU Leuven), Celestijnenlaan 200F, B-3001 Heverlee, Belgium, and Max-Planck-Institut fu¨ r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany Received June 15, 2000. In Final Form: September 7, 2000 Two different types of aggregated polyphenylene dendrimer molecules, G2Td(COOH)16 and G4-Td, obtained by spin coating a dilute solution onto freshly cleaved mica surfaces, and their adhesion properties were measured by pulsed force mode (PFM) AFM. The adhesion properties could be related to the chemical nature of the outer surface of the dendrimers and the mica surface, and the thin film of water adsorbed on mica when imaged under ambient conditions. Most importantly, from the adhesion image of mixed aggregates of G2Td(COOH)16 and G4-Td, as measured by PFM-AFM, the two different aggregated dendrimers can be easily discriminated.
Introduction Dendrimers, a new kind of functional materials, with an unique highly branched regular structure, have attracted increasing interest.1 Using atomic force microscopy (AFM),2 a powerful tool to visualize the topography of a surface, aggregated dendrimer molecules have been observed on a variety of surfaces, such as mica,3 graphite,3b,c,4 glass,3b a charged solid surface,5 and hydrophobically functionalized Au.6 Recently, individual molecules of four different types of core-shell tecto(dendrimer) were observed on a mica surface.7 To increase the possibilities of AFM, the pulsed force mode (PFM) AFM technique8 has been developed to obtain information on the local stiffness and adhesion of a sample, simultaneously with the topographic image. It is especially useful for imaging soft samples, since it can reduce the lateral force between the tip and the sample. For example, * To whom correspondence should be addressed. Telephone: +32-16-327405. Fax: +32-16-327989. E-mail: Frans.DeSchryver@ chem.kuleuven.ac.be. † Katholieke Universiteit Leuven. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. (1) (a) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 1996. (b) Fre´chet, J. M. J. Science 1994, 263, 1710. (c) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (2) (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. (3) (a) Hellmann, J.; Hamano, M.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Irie, M. Jpn. J. Appl. Phys. 1998, 37, L816. (b) 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. (c) 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. (4) Stocker, W.; Karakaya, B.; Schu¨rmann, B. L.; Rabe, J. P.; Schlu¨ter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (5) (a) Bliznyuk, V. N.; Rinderspacher, F.; Tsukruk, V. V. Polymer 1998, 39, 5249. (b) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (6) Iyer, J.; Hammond, P. T. Langmuir 1999, 15, 1299. (7) Li, J.; Swanson, D. R.; Qin, D.; Brothers, H. M.; Piehler, L. T.; Tomalia, D.; Meier, D. J. Langmuir 1999, 15, 7347. (8) Rosa-Zeiser, A.; Weilandt, E.; Weilandt, H.; Marti, O. Meas. Sci. Technol. 1997, 8, 1333.
the adhesion,9a,b,d,f the stiffness9d and electrostatic properties8 of the polymers, the adhesion of self-assembled monolayers (SAMs) on silicon,9b the adhesion of the highly oriented pyrolytic graphite (HOPG) in the electrolyte solution,8,9b and the surface charge of the vapor-deposited Al on a quartz plate9c,e and a gold-covered glass substrate9b have been successfully measured. In a previous report,10 we have observed individual and aggregated polyphenylene dendrimer molecules of G4-Td on mica by NCAFM and studied their stiffness and adhesion properties by PFM-AFM. In this contribution, we demonstrate that from a mixture of two aggregates of polyphenylene-based dendrimers, G2Td(COOH)16 and G4-Td, each component can be discriminated on the basis of the different adhesion interaction with the AFM tip. Experimental Section Materials. The polyphenylene dendrimer molecules11 used in this paper are denoted as G2Td(COOH)16 and G4-Td (as shown in Figure 1a and b, respectively). CH2Cl2 and tetrahydrofuran (THF) (Spectroscan, Labscan Ltd., Dublin) were used as received. The concentrations of G2Td(COOH)16 in THF and of G4-Td in CH2Cl2 were 3.2 × 10-8 and 7.4 × 10-7 M, respectively. Sample Preparation. Immediately before spin coating, the mica surface was cleaved with Scotch tape. The dendrimer solution was spin-coated under ambient conditions at a speed of (9) (a) Marti, O.; Stifter, T.; Waschipky, H.; Quintus, M.; Hild, S. Colloids Surf., A 1999, 154, 65. (b) Krotil, H.-U.; Stifter, T.; Waschipky, H.; Weishaupt, K.; Hild, S.; Marti, O. Surf. Interface Anal. 1999, 27, 336. (c) Miyatani, T.; Okamoto, S.; Rosa, A.; Marti, O.; Fujihira, M. Appl. Phys. A 1998, 66, S349. (d) Leijala, A.; Hautoja¨rvi, J. Textile Res. J. 1998, 68, 193. (e) Miyatani, T.; Horii, M.; Rosa, A.; Fujihira, M.; Marti, O. Appl. Phys. Lett. 1997, 71, 2632. (f) Luzinov, I.; Minko, S.; Senkovsky, V.; Voronov, A.; Hild, S.; Marti, O.; Wilke, W. Macromolecules 1998, 31, 3945. (10) 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, in press. (11) (a) Morgenroth, F.; Reuther, E.; Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 631; Angew. Chem. 1997, 109 (6), 647. (b) Morgenroth, F.; Ku¨bel, C.; Mu¨llen, K. J. Mater. Chem. 1997, 7, 1207. (c) Morgenroth, F.; Berresheim, A. J.; Wagner, M.; Mu¨llen, K. J. Chem. Soc., Chem. Commun. 1998, 1139. (d) Wiesler, U.-M.; Mu¨llen, K. J. Chem. Soc., Chem. Commun. 1999, 2293. (e) Wiesler, U.-M.; Berresheim, A. J.; Morgenroth, F.; Mu¨llen, K. Macromolecules, submitted.
10.1021/la0008378 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/01/2000
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Figure 1. Molecular structure of G2Td(COOH)16 (a) and G4-Td (b) dendrimers. A space filling view of a G2Td(COOH)16 (c) and G4-Td (d) dendrimer molecule which were built by a Merck molecular force field (MMFF) method in Spartan. 5000 rpm for 90 s for the G2Td(COOH)16 solution and 2000 rpm for 20 s for the G4-Td solution. The sample of mixed G2Td(COOH)16 and G4-Td aggregates was obtained by sequentially spin coating the 3.2 × 10-8 M G2Td(COOH)16 THF solution and the 7.4 × 10-7 M G4-Td CH2Cl2 solution on a mica surface. Atomic Force Microscopy (AFM). After spin coating, the samples were imaged under ambient conditions with a Discoverer TMX 2010 AFM system (ThermoMicroscopes, San Francisco, CA), operated in the pulsed force mode (PFM) (Wissenschaftliche Instrumente und Technologie GmbH (WITec), Germany). Local adhesion, simultaneously imaged with the sample topography, was investigated. The PFM was added to the AFM system as an external module. The PFM electronics introduces a sinusoidal modulation of the z-piezo of the AFM with an amplitude between 10 and 500 nm at a user-selectable frequency between 100 Hz and 2 kHz. Due to this rather large amplitude, a full force curve
is measured during each period of the oscillation. From the force curve, the adhesion can be directly obtained by setting the appropriate electronic triggers of the PFM module. For the PFMAFM measurements, Si probes (ThermoMicroscopes, San Francisco, CA) with a spring constant of 0.08-0.20 N/m and a resonance frequency of 8-14 kHz were used. A piezoelectric tube scanner was used with a scan range of 7 µm × 7 µm in the XY-direction and 2.4 µm in the Z-direction. The z-scanner was calibrated using a silicon grating with a step height of 25.5 ( 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. Merck Molecular Force Field.12 The molecular models of G2Td(COOH)16 and G4-Td were built in a vacuum by a Merck molecular force field (MMFF)12 method in Spartan (Wavefunction
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Figure 2. Topography (a) and adhesion (b) images of G2Td(COOH)16 aggregates on mica observed by PFM-AFM. The height scales are 13.90 nm, and from 0.396 V (dark) to 1.217 V (bright), respectively. Topography (c) and adhesion (d) images of G4-Td aggregates on mica observed by PFM-AFM. The height scales are 9.86 nm, and from 1.350 V (dark) to 2.168 V (bright), respectively. Inc., Irvine, CA) (as shown in Figure 1c and d, respectively). 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 Using PFM-AFM,8 the sample topographies and the adhesion properties of G2Td(COOH)16 and G4-Td dendrimers adsorbed on mica by spin coating a 3.2 × 10-8 M THF solution at a speed of 5000 rpm for 90 s and a 7.4 × 10-7 M THF solution at a speed of 2000 rpm for 20 s, respectively, have been measured simultaneously. The white spherical spots in the topography image of G2Td(12) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.
(COOH)16 dendrimers on mica (Figure 2a) are observed with their height ranging from 8.7 to 23.8 nm and their size (full width at half-height) ranging from 53.5 to 69.6 nm. Compared to the dimension of the conformation of a single G2Td(COOH)16 molecule (Figure 1c), the G2Td(COOH)16 dendrimer molecules in Figure 2a must be aggregated. Aggregates on the order of three to eight molecular layers are present on the mica surface, as the dimension of a single dendrimer molecule was assessed to be 3.1 nm.10 This is the distance between the top of one branch and the center of the triangle at the base plane formed by the other three branches considering a tetrahedral structure. In our previous work,10 we have observed individual G4-Td dendrimers (structure shown in Figure 1b) at a
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Figure 3. Topography (a) and adhesion (b) images of the mixed aggregated dendrimers (G2Td(COOH)16 and G4-Td) on mica observed by PFM-AFM. The height scales are 13.94 nm, and from 0.732 V (dark) to 1.557 V (bright), respectively. A and B represent G2Td(COOH)16 and G4-Td aggregates, respectively.
concentration of 1.47 × 10-8 M, whereas for G2Td(COOH)16 at a concentration of 3.2 × 10-8 M aggregation already occurs. This difference probably arises from the different rim functionality of the two kinds of dendrimer molecules. The outer rim of the G2Td(COOH)16 dendrimer is composed of 16 COOH groups, resulting in the fact that these G2Td(COOH)16 dendrimer molecules can easily aggregate through the formation of hydrogen bonds. When the concentration of G4-Td dendrimers in CH2Cl2 is increased to 7.4 × 10-7 M, G4-Td dendrimer aggregates are also observed.10 Figure 2c is a topography image of the G4-Td dendrimer aggregates adsorbed on mica. The height of the white spots was determined to be in the range 7.6-13 nm, leading to the conclusion that also in this case the dendrimer molecules must be aggregated, since the dimension of a single molecule equals 4.9 nm (Figure 1d).10 For the adhesion images (Figure 2b and d), dark image contrast means a low adhesion signal while bright image contrast means a high adhesion signal. Figure 2b and d shows that the adhesion signals of both the G2Td(COOH)16 and G4-Td aggregates are lower than that of the mica substrate. This can be explained by considering the nature of the surface groups of the mica, the dendrimer molecules, and the tip. As for the G2Td(COOH)16 aggregates (Figure 2b), although there are some COOH groups at the rim of the G2Td(COOH)16 dendrimers, still their surface hydrophobicity is stronger than their surface hydrophilicity because most of the outer groups are hydrophobic. The mica surface and the silicon tip are both hydrophilic in nature, leading to a relatively high adhesion force, whereas the adhesion force between the tip and the dendrimer molecules is relatively low. As is known,13 when a sample is imaged by AFM under ambient conditions, a thin film of water is present on the mica surface (about 0.3 nm) and the hydrophilic mica is more susceptible to moisture in air. There is a capillary interaction between the tip and (13) Homola, A. M.; Israelachvili, J. N.; McGuiggan, P. M.; Gee, M. L. Wear 1990, 136, 65.
the thin film of water on the mica surface,14 which will increase the adhesion force. This explains why the G2Td(COOH)16 aggregates have a dark image contrast in the adhesion image. The adhesion image of the G4-Td aggregates (Figure 2d) also exhibits dark areas in those positions where the aggregates are located. As the outer rim of the G4-Td aggregate is completely hydrophobic, the adhesion between the tip and the aggregates is low compared to the adhesion between the hydrophilic tip and the hydrophilic mica surface.10 A sample of mixed G2Td(COOH)16 and G4-Td aggregated dendrimers on a mica surface was obtained by sequentially spin coating a 3.2 × 10-8 M G2Td(COOH)16 THF solution at a speed of 5000 rpm for 90 s and 7.4 × 10-7 M G4-Td CH2Cl2 solution at a speed of 2000 rpm for 20 s. The topography and adhesion images, as simultaneously obtained by PFM-AFM, are presented in Figure 3a and b, respectively. From the topography image (Figure 3a), it is impossible to discriminate the two different dendrimers, but the adhesion image (Figure 3b) shows a large contrast between the two different types of dendrimers. There are obviously two different adhesion signals present, marked as A and B, where A exhibits a higher adhesion than B and both A and B show a lower adhesion force than the mica surface. Considering the adhesion interaction between the silicon tip and the dendrimers, it is easy to attribute A and B to G2Td(COOH)16 and G4-Td aggregates, respectively. Compared to the completely hydrophobic surface of G4-Td aggregates (spots of type B), those of the G2Td(COOH)16 aggregates (spots of type A) are relatively hydrophilic, although they are both hydrophobic relative to the mica surface. The G2Td(COOH)16 aggregates therefore exhibit a stronger adhesion interaction with the hydrophilic silicon tip and consequently show a higher adhesion signal in the adhesion image as compared to the G4-Td aggregates. This result demonstrates that PFM-AFM also has the (14) Scandella, L.; Schumacher, A.; Kruse, N.; Prins, R.; Meyer, E.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J. Thin Solid Films 1994, 240, 101.
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ability to distinguish differences in physical properties based on chemical functionality, similar to chemical force microscopy (CFM).15 Conclusions Round shaped aggregates of G2Td(COOH)16 and G4Td have been observed on a mica substrate, and their adhesion properties have been determined by the recently developed PFM-AFM technique. Compared to the mica substrate, both the G2Td(COOH)16 and the G4-Td aggregates show a lower adhesion than the mica substrate. This can be explained by the relative hydrophobicity of the outer rim of both dendrimers, relative to the completely hydrophilic mica surface, and a thin film of water adsorbed on the mica surface under ambient conditions. The G2Td(COOH)16 dendrimer is less hydrophobic in nature than the G4-Td dendrimer. This chemical property (15) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (b) Noy, A.; Frisbie, C. D.; Rozsnyai, L. F.; Wrighton, M. S.; Lieber, C. M. J. Am. Chem. Soc. 1995, 117, 7943.
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clearly shows up in the adhesion image of the mixed dendrimer sample, where a distinct contrast is present between the two different types of dendrimers. On the basis of their different adhesion properties, the dendrimers can be easily distinguished, whereas this is impossible from the topography image. Compared to AFM, PFMAFM is more sensitive to the surface physical and chemical properties of materials, and therefore it can be used to discriminate between different materials and/or different surface properties. This work shows that PFM-AFM has a high spatial resolution, which can be employed to distinguish different surface functional groups, leading to differences in adhesion. Acknowledgment. The authors thank the DWTC, through IUAP-IV-11, the FWO (Flemish Ministry of Education), and ESF SMARTON for financial support. T.V. thanks the IWT for a predoctoral scholarship. The collaboration was made possible thanks to the TMR project SISITOMAS. LA0008378
