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Tuning Intermolecular Interactions in a Rodlike Polymer Assembled at Surfaces and in Solution Paolo Samorı`,*,†,‡ Jack J. J. M. Donners,§ Nikolai Severin,† Matthijs B. J. Otten,†,| Ju¨rgen P. Rabe,*,† Roeland J. M. Nolte,§,| and Nico A. J. M. Sommerdijk*,§ Department of Physics, Humboldt University Berlin, Newtonstrasse 15, D-12489 Berlin, Germany, ISOF-C.N.R., via Gobetti 101, 40129 Bologna, Italy, Nanochemistry Laboratory, ISIS-ULP, 8 Alle´ e Gaspard Monge, 67083 Strasbourg, France, Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology, Post Office Box 513, 5600 MB Eindhoven, The Netherlands, and Department of Organic Chemistry, NSR Center, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands Received June 19, 2004. In Final Form: August 12, 2004 The isolation of single polyelectrolyte chains of water-soluble poly(isocyanodipeptide)s (PICs) bearing carboxylic acid terminated side chains occurring both at surfaces and in solution was accomplished by reducing the intermolecular interactions through complexation with cations or positively charged surfactants. Scanning force microscopy and viscosity analyses revealed that this method allows to tune the conformation of the macromolecule, which is of importance for tailoring the physicochemical properties of the material. This is particularly significant for the use of these polymer chains as seed for biomineralization processes.
The pioneering work of Lehn, Pedersen, and Cram has created much research activity focused on achieving full control over weak intermolecular interactions, which is the essence of supramolecular chemistry.1 The use of noncovalent interactions,2 including ionic ones,3 allows a rational design of tailor-made structures with interesting electrical, magnetic, or optical properties.4 In this context, the possibility to tune intra- and interchain interactions in functionalized macromolecules is of interest for different applications. For example, in molecular electronics this makes it possible to reduce the photoluminescence efficiency and the energy gap in films based on rotaxinated rodlike conjugated polymers.5 Similarly, encapsulation can be exploited to reduce the interactions, thereby tuning the reactivity and stability of chemical species.6 This has been demonstrated to be practicable to facilitate the transfection of DNA.7 Hydrophobic poly(isocyanodipeptide)s (PICs) are rodlike macromolecules with a helical conformation that can be stabilized and extraordinarily rigidified by secondary interactions, for example, hydrogen bonding, between the * To whom correspondence should be addressed. E-mail: samori@ isof.cnr.it,
[email protected],
[email protected]. † Humboldt University Berlin. ‡ ISOF-C.N.R. and ISIS-ULP. § Eindhoven University of Technology. | University of Nijmegen. (1) (a) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89-112. (b) Pedersen, C. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1021-1027. (c) Cram, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1009-1020. (2) (a) Stupp, S. I.; LeBonheur, V.; Walker, K.; Li, L. S.; Huggins, K. E.; Keser, M.; Amstutz, A. Science 1997, 276, 384-389. (b) Bong, D. T.; Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int Ed. 2001, 40, 988-1011. (c) Lehn, J. M. Science 2002, 295, 2400-2403. (d) Ikkala, O.; ten Brinke, G. Science 2002, 295, 2407-2409. (3) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673-683. (4) Alivisatos, A. P.; Barbara, P. F.; Castleman, A. W.; Chang, J.; Dixon, D. A.; Klein, M. L.; McLendon, G. L.; Miller, J. S.; Ratner, M. A.; Rossky, P. J.; Stupp, S. I.; Thompson, M. E. Adv. Mater. 1998, 10, 1297-1336. (5) Cacialli, F.; Wilson, J. S.; Michels, J. J.; Daniel, C.; Silva, C.; Friend, R. H.; Severin, N.; Samorı`, P.; Rabe, J. P.; O’Connell, M. J.; Taylor, P. N.; Anderson, H. L. Nat. Mater. 2002, 1, 160-164. (6) Hof, F.; Craig, S. L.; Nuckolls, C.; Rebek, J., Jr. Angew. Chem., Int. Ed. 2002, 41, 1488-1508.
Chart 1. Chemical Formula of 1
alanine units in the side chains.8,9 The helical structure in the hydrophilic L-alanyl-D-alanine-derived poly(isocyanide) (1, see Chart 1) persists even in aqueous media, due to retention of the hydrogen bonds, as shown by variable temperature (VT) 1H NMR. The exchange of the NH protons for deuterium in D2O was slow as expected for protons participating in strong hydrogen bonds. Moreover, VT circular dichroism (CD) spectroscopy showed that while a high structural order is present up to room temperature, this order gets lost at elevated temperatures.8 We report here a tapping mode scanning force microscopy (TM-SFM) study on the aggregation of the hydrophilic PIC 1 on surfaces and a viscosity analysis of this aggregation process in aqueous solutions in the presence of cations. Submonolayer thick films of 1 were processed on highly oriented pyrolytic graphite (HOPG) from water solutions at pH ) 7 following two methodologies. In the first set of experiments, neat ultrathin films of 1 were prepared either by drop-casting or by spin-coating (at 35 rps). In the second set of experiments, 1 was co-deposited on HOPG with (7) (a) Ra¨dler, J. O.; Koltover, I.; Salditt, T.; Safinya, C. R. Science 1997, 275, 810-814. (b) Koltover, I.; Salditt, T.; Ra¨dler, J. O.; Safinya, C. R. Science 1998, 281, 78-81. (c) Kirby, A. J.; Camilleri, P.; Engberts, J. B. F. N.; Feiters, M. C.; Nolte, R. J. M.; So¨derman, O.; Bergsma, M.; Bell, P. C.; Fielden, M. L.; Garcı´a Rodrı´guez, C. L.; Gue´dat, P.; Kremer, A.; McGregor, C.; Perrin, C.; Ronsin, G.; van Eijk, M. C. P. Angew. Chem., Int. Ed. 2003, 42, 1448-1457. (8) Cornelissen, J. J. L. M.; Donners, J. J. J. M.; Metselaar, G.; de Gelder, R.; Graswinckel, W. S.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001, 293, 676-680. (9) Samorı´, P.; Ecker, C.; Go¨ssl, I.; de Witte, P. A. J.; Cornelissen, J. J. L. M.; Metselaar, G. A.; Otten, M. B. J.; Rowan, A. E.; Nolte, R. J. M.; Rabe, J. P. Macromolecules 2002, 35 , 5290-5294.
10.1021/la048485i CCC: $27.50 © 2004 American Chemical Society Published on Web 09/14/2004
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Figure 1. TM-SFM image of 1 spin-coated on graphite: (a) survey picture with arrows denoting graphite steps; (b) zoom-in. The Z range in both cases is h ) 2 nm.
either C12H25NH2 or C18H37NH2. This was accomplished in four steps: (1) spin-coating a very diluted solution (roughly 10-2 g/L) of C12H25NH2 or C18H37NH2 in CHCl3 on HOPG; (2) drying the sample for 10 min in air at 40 °C; (3) spin-coating on the C12H25NH2 or C18H37NH2 layer an aqueous solution of 1; (4) drying the sample in air for 20 min at room temperature. The resulting ultrathin films were studied by SFM,10 using a Nanoscope IIIa setup (Digital Instruments, Santa Barbara, CA) operating at room temperature in air environment. TM-SFM investigation of ultrathin films prepared by either spin-coating or solution-casting of neat aqueous PIC solutions on graphite revealed a submonolayer of nanometer scale objects with a thickness between 0.7 and 1.8 nm and a variety of lateral shapes with dimensions up to some hundreds of nanometers (Figure 1). These features are attributed to clusters of 1 aggregated in a rather uncontrolled fashion, most probably originating from strong interstrand interactions or from long-range intrastrand interactions, due to the occurrence of Hbondings between the COOH groups in the periphery of the side arms, which are only partially deprotonated at pH ) 7. The presence of interstrand interactions between the carboxyl groups of 1 was found to depend on the specific states of the carboxylic acid groups of the polymers. Titration experiments indicated an isoelectric point of 6.0, whereas the polymers were found to precipitate at pH 5.0. This precipitation is probably due to the formation of intermacromolecular hydrogen bonds leading to aggregation of the polymers.11 Our first approach to prevent the aggregation of the chains of 1 in solution was the addition of monovalent ions that can interact with the carboxylic moieties.12 Viscosity measurements of 1 in pure water revealed an intrinsic viscosity of 0.24 dL/g. The reduced viscosity increased upon dilution, most probably due to the breaking of the aggregates. Upon addition of NaCl or CaCl2 (up to 9 mM), a drop in intrinsic viscosity to 0.10 dL/g was observed which can be explained by the screening of the (10) (a) Bustamante, C.; Keller, D. Phys. Today 1995, 48, 32-38. (b) Takano, H.; Kenseth, J. R.; Wong, S. S.; O’Brien, J. C.; Porter, M. D. Chem. Rev. 1999, 99, 2845-2890. (c) Sheiko, S. S.; Mo¨ller, M. Chem. Rev. 2001, 101, 4099-4124. (11) Donners, J. J. J. M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. J. Am. Chem. Soc. 2002, 124, 9700-9701. (12) Weissbuch, I.; Guo, S.; Edgar, R.; Cohen, S.; Howes, P.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Adv. Mater. 1998, 10, 117-121.
charged side groups in the polymer chains and the concomitant breaking of the aggregates, as is typically observed for polyelectrolytes. The complexation of calcium ions may also lead to the screening of the peptide bonds in the side chains of the polymer thereby prohibiting the slow but gradual uncoiling of the helix, which occurs in neat PIC.11 However, when calcium ions were used the reduced viscosity showed a small increase upon dilution indicating that in this case some aggregates were still present. Under the conditions used, small-angle X-ray scattering also indicated that in the presence of Ca2+ ions (9 mM) small aggregates with a lamellar cross section of 40 Å were present, which corresponds to 3-4 polymer chains. These polymacromolecular architectures, consisting of 3-4 polymer chains, are attributed to interstrand calcium complexation. To more effectively isolate the single chains, we aimed at reducing the interactions between the carboxyl moieties by embedding the molecules in a surfactant monolayer. We have chosen amino alkanes since in the protonated state, existing at pH ) 7, their polar NH3+ heads are likely to interact with the carboxylic acid groups while the apolar aliphatic tails are known to crystallize very well on HOPG forming lamellae.13 In this way, the surfactant can act as a template for the immobilization of PICs as individual chains on the HOPG surface.14 The TM-SFM image in Figure 2a displays single chains of 1 co-deposited with C12H25NH2. Topographical profiles provided evidence of single chains with lengths up to 80 nm. The charged chains, which exhibit heights of ca. 0.7 nm, adopt a coiled conformation which is markedly different from the rigid conformations that were observed and characterized for the uncharged poly(isocyano-L-alanyl-D-alanine methyl ester).9 This provides further evidence of a strong tendency of the water-soluble PIC chains to interact via intermolecular interactions or long-range intramolecular interactions, inducing the coiled conformation. It is important to point out that the surfactant we used for the isolation of the macromolecules, that is, C12H25NH2, has a short alkyl chain causing two main disadvantages: (i) it does not order very well on HOPG compared to longer hydrocarbon chains due to the thermodynamics of the physisorption at surfaces,15 and (13) Severin, N.; Barner, J.; Kalachev, A. A.; Rabe, J. P. Nano Lett. 2004, 4, 577-580. (14) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681-3683.
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presence of C18H37NH2 at surfaces is shown in Figure 2b. Here the single chains of 1 appear extremely straight and exhibit heights of approximately 0.7 nm. They are aligned along preferential directions according to the underlying lamella of the surfactant template, thus following the 3-fold symmetry of the substrate underneath (see the Supporting Information).16 The visualized lamellae have an interlamellar spacing of (5.0 ( 0.3) nm, and they are oriented with the main axes along the two directions at 60° from each other as indicated by the arrows. It has been difficult in the past to obtain molecular weight data for polyelectrolytes using conventional analytical methods such as size exclusion chromatography, light scattering, or matrix-assisted laser desorption-ionization mass spectrometry (MALDI-MS). Following the approach presented here, the perfect isolation and stiffening of such polymer chains might in the future allow one to determine with a high degree of precision their molecular weight distribution,17 which so far has only been possible for the hydrophobic PIC.8,9 In our case, this information is particularly important due to the difficulties in gaining absolute determination of molecular weights on these macromolecules with any of the above-mentioned techniques. The film preparation carried out skipping step 4, that is, the drying of the final sample, gave rise to liquidlike films that exhibit smeared out features which change their shape under the SFM tip (not shown here). In summary, the isolation of single negatively charged polymer chains with the help of cations and positively charged surfactants both in solution and on surfaces opened a way to gain better control over their intermolecular interactions. This makes it possible to modulate the apparent stiffness of the chain as well as the excluded volume effects which are both of importance for tailoring the physicochemical properties of the material. Moreover, this method offers an approach to quantify the molecular weight distribution of polymers that tend to aggregate into clusters due to strong intermolecular interactions or which possess a coiled conformation and hence cannot be visualized by SFM with molecular resolution. Finally, the capability of tuning the intermolecular interactions in our system can be of help in improving the templating effect of the PIC chains, for example, in the biomimetic nucleation of minerals.11,18 Acknowledgment. This work was supported by the European Science Foundation through the programs SMARTON and SONS-BIONICS.
Figure 2. SFM image of 1 co-deposited with (a) C12H25NH2 and (b) C18H37NH2. The Z scale bar in both cases is h ) 4 nm.
(ii) it melts at 25 °C which is a very unfavorable temperature since the experiments were performed in a noncontrolled room-temperature environment, so one could easily pass from one to the other side of this transition, leading to structural instabilities. Thus, we have extended our studies to the use of C18H37NH2 as the surfactant. The molecular ordering of 1 in the (15) Samorı´, P.; Severin, N.; Mu¨llen, K.; Rabe, J. P. Adv. Mater. 2000, 12, 579-582.
Supporting Information Available: Cartoon of a monolayer of surfactants consisting of a polar headgroup and an aliphatic chain self-assembled on HOPG. This material is available free of charge via the Internet at http://pubs.acs.org. LA048485I (16) The absence of rodlike nanostructures in films prepared by physisorbing the neat C18H37NH2 on HOPG confirmed that the observed features are extended PIC chains rather than the second overlayer of the C18H37NH2. (17) Sheiko, S. S.; da Silva, M.; Shirvaniants, D.; LaRue, I.; Prokhorova, S.; Mo¨ller, M.; Beers, K.; Matyjaszewski, K. J. Am. Chem. Soc. 2003, 125, 6725-6728. (b) Severin, N.; Rabe, J. P.; Kurth, D. G. J. Am. Chem. Soc. 2004, 126, 3696-3697. (18) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684-1688.
