One-Step Formation of Straight Nanostripes from a Mammal Lipid

Nov 27, 2008 - in the cerebrospinal fluid of mammal animals and being an additive for medical use ... straight nanostripes from mammal lipid by AFM wi...
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Langmuir 2009, 25, 2290-2293

One-Step Formation of Straight Nanostripes from a Mammal Lipid-Oleamide Directly on Highly Oriented Pyrolytic Graphite Renjie Zhang,*,†,‡ Helmuth Mo¨hwald,‡ and Dirk G. Kurth‡,§,| Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education, Shandong UniVersity, 250100 Jinan, China, Department of Interfaces, Max Planck Institute of Colloids and Interfaces Sciences, 14424 Potsdam, Germany, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and Chemical Technology of Materials Synthesis, Julius-Maximilians-UniVersity Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany ReceiVed October 24, 2008. ReVised Manuscript ReceiVed NoVember 27, 2008 Hierarchical nanostructures are obtained directly on highly oriented pyrolytic graphite (HOPG) by spin coating of dilute chloroform solution of 9-Z-octadecenamide (oleamide), a natural lipid with cis-CdC- conformation, existing in the cerebrospinal fluid of mammal animals and being an additive for medical use and food packaging. Straight separated nanostripes with a length of 70-300 nm exist in the topmost layer and compact nanostripes in the bottom layer contacting HOPG. Compact nanostripes have a periodicity spacing of 3.8 nm, indicating H-bonding between two rows of oleamide molecules. The orientation of the hierarchical nanostructures differs by n60° ( 8° (n ) 1 or 2), reflecting the epitaxial ordering along the HOPG substrate. The nanostripes are stable against annealing. A molecular packing scheme for the nanostructures is proposed, where the -CdC bond angle in oleamide is 120° and the plane of the carbon skeleton lies parallel to the HOPG substrate. Nanostripes in the topmost layer are formed from separated rows of oleamide molecules, due to the short-range surface potential of the substrate. The scheme involves direct influence of HOPG on the orientation of oleamide molecules to form nanostripes without any purposely added saturated alkanes and H-bonds between amide groups in adjacent two rows of oleamide molecules.

Introduction Oleamide (9-Z-octadecenamide), a natural lipid existing in the cerebrospinal fluid of mammal animals, 1 induces physiological sleep2 and transfers signals in biochemical reactions.3-5 It is an additive for medical use and food packaging in polymer products without dosage restriction.6 Besides biocompatibility, oleamide is important as an amphiphile with a cis-unsaturated chain, whose molecular arrangement is largely unknown. In comparison, molecules with saturated all trans alkyl chains have been systematically studied by scanning probe microscopy concerning molecular orientation and self-assembly.7 Studies on oleamide are limited mainly to its biochemical reactions such as hydrolysis4 and lubrication due to diffusion to the product surface after manufacture.8 To study molecular recognition and self-assembly,9-11 as well as to assemble molecules in a predictable way on a surface to * To whom correspondence should be addressed. E-mail: [email protected]. † Shandong University. ‡ Max Planck Institute of Colloids and Interfaces Sciences. § National Institute for Materials Science. | Julius-Maximilians-University Wu¨rzburg. (1) Lerner, R. A.; Siuzdak, G.; Prospero-Garcia, O.; Henriksen, S. J.; Boger, D. L.; Cravatt, B. F Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 9505. (2) Cravatt, B. F.; Prospero-Garcia, O.; Siuzdak, G.; Gilula, N. B.; Henriksen, S. J.; Boger, D. L.; Lerner, R. A. Science 1995, 268, 1506. (3) Huidobro-Toro, J. P.; Harris, R. A Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 8078. (4) Thomas, E. A.; Carson, M. J.; Neal, M. J.; Sutcliffe, J. G. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 14115. (5) Facci, L.; Toso, R. D.; Romanello, S.; Buriani, A.; Skaper, S. D.; Leon, A. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 3376. (6) Tice, P. Packaging Materials: 4. Polyethylene for Food Packaging Applications; ILSI Europe: Brussels, Belgium, 2003; http://europe.ilsi.org/ publications/Report+Series/foodpkgapps4.htm. (7) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544. (8) Ga¨chter, R.; Mu¨ller, H. Plastics AdditiVes, 4th ed.; Carl Hanser Verlag: Munich, Germany, 1993. (9) Lehn, J. M. Science 1993, 260, 1762. (10) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705.

achieve the fabrication and manipulation of functional devices individually at the nanoscopic level,12 atomic force microscopy (AFM) with atomic resolution and less disturbance of nanostructures from organic molecules is often used. There are nearly no reported straight organic nanostripes even from shape-persistent DNAs, which generally present only shortrange ordering in surface-confined architectures.13 Only with assistanceoflong-chainalkanescannanostripesoforganic-inorganic complexes be obtained.14 This work addresses the hierarchy of straight nanostripes from mammal lipid by AFM without purposeful addition of any saturated alkanes on highly oriented pyrolytic graphite (HOPG) with ordered planar carbon lattices. It provides insight into understanding a novel route of obtaining hierarchical structures from biological molecules on substrates with symmetric characteristics. The chains and head groups of the oleamide molecules in this work are simultaneously considered with respect to hierarchical nanostructures. Unsaturated chains satisfy not only hydrophobicity but also flexibility compared to saturated ones like octadecyl, providing a longer time to form nanostructures. The -CONH2 head groups forming intermolecular H-bonds between adjacent oleamide molecules further stabilize these structures in an aqueous biological environment. In addition, the double bond causes a distortion of the symmetry, potentially breaking the commensurability with the HOPG substrate.

Experimental Section 9-Z-Octadecenamide (oleamide), chloroform, ethyl acetate, and hexane were used as received (Sigma Inc., Deisenhofen, Germany). Five microliters of dilute chloroform solutions of oleamide (9.59 × (11) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (12) Gimzewski, J. K.; Joachim, C. Science 1999, 283, 1683. (13) Sheiko, S. S. AdV. Polym. Sci. 2000, 151, 61. (14) Kurth, D. G.; Severin, N.; Rabe, J. P. Angew. Chem., Int. Ed. 2002, 41, 3681.

10.1021/la8035506 CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

Straight Nanostripes from a Mammal Lipid-Oleamide

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10-5 to 6.14 × 10-3 mol/L) were spin-coated onto HOPG (quality ZYB, Advanced Ceramics Co., Cleveland, OH) at 40 revolutions per second. The samples were air-dried before AFM investigation for 10 min at room temperature, 25 °C. The AFM was a commercially available Nanoscope III (Digital Instruments Inc., Santa Barbara, CA). Normal Si cantilevers (length 125 µm and width 30 µm) with a spring constant of about 40 N · m-1 and a resonance frequency in the range of 200-400 kHz were used. The measurements were carried out in air with tapping mode (TM). One should pay close attention to the operation parameters in AFM measurement, especially on the pairwise selection of a big setpoint value and small driving amplitude. Generally attractive interaction between tip and sample sets in at a finite tip-sample separation before repulsive interaction sets in.15 So the tip-sample distance is purposely lowered to reach the repulsive force region, where the imaging signal is stable.15 A typical amplitude setpoint value was 70-80% of the maximum resonance amplitude. The typical tapping amplitude of the cantilever was about 5.0 nm for decreasing the distance between the tip of the cantilever and the sample surface to increase the lateral resolution. Dynamic laser scattering (DLS) measurements were carried out with the ALV goniometer and the ALV-5000 digital correlator at 25 °C (ALV GmbH, Langen, Germany) with a frequency-doubled Neodym-YAG laser (Coherent DPSS532, 532.0 nm, 300 mW) as light source.

Results and Discussion Separated nanostripes distribute homogeneously and compactly on HOPG after spin coating of 5 µL of 1.32 × 10-3 mol/L oleamide solution in chloroform. Stripes with a length from 20.0 to 300.0 nm can be clearly distinguished at a scan size of 2 µm to 780 nm (Figure 1a-d). Hierarchical nanostructures of oleamide are resolved at different scanning sizes and separated stripes appear in the topmost layer when large areas are scanned, while the underlying layer contacting HOPG is composed of compact ones, when small areas are scanned. A detailed observation at larger magnification shows well-aligned compact nanostripes with a periodicity spacing of 3.9 ( 0.3 nm. The compact nanostripes have similar lengths as separated nanostripes. Here we successfully obtained nanostripes from oleamide without assistance of any purposely added long-chain saturated alkanes, which is the case for nanostripes from polyelectrolyte-amphiphile complexes (PAC),14 where alkanes align on the substrate and induce orientation of nanostripes. The orientation of the oleamide nanostripes follows three preferential directions, enclosing an angle of n60 ( 8° (n ) 1 or 2), as indicated by arrows oa’, ob’, and oc’ in Figure 1b and oa”, ob”, and oc” in Figure 1f. Separated and compact nanostripes formed from the 1.32 × 10-3 mol/L oleamide chloroform solution persist even after thermal treatment. As shown in Figure 2, after annealing at 60 °C for 30 min, a large number of nanopatterns are still present with preferential directions enclosing an angle of 60 ( 8° on HOPG. The phase images provide more information than the height images. As shown in Figure 2, compact nanostripes welldistinguished in the phase image are not clearly discernible in the height image. Similar phenomena have been discussed by other researchers for biomolecules.16 The periodicity and length of the compact nanostripes calculated from Figure 2b are about 3.9 ( 0.3 nm and 20-300.0 nm, respectively. The same periodicity as before annealing indicates the same molecular packing of oleamide molecules. (15) Ku¨hle, A.; Sørensen, A. H.; Zandbergen, J. B.; Bohr, J. Appl. Phys. A 1998, 66, S329. (16) Argaman, M.; Golan, R.; Thomson, N. H.; Hansma, H. G. Nucleic Acids Res. 1997, 25, 4379.

Figure 1. AFM images of oleamide nanostripes on HOPG formed by spin coating a 1.32 × 10-3 mol/L solution in chloroform. Scan range: (a, b) 2.00 µm; (c, d) 783.0 nm; (e, f) 500.0 nm. a, c, and e are height images; b, d, and f are phase images. Arrows of oa’, ob’, and oc’ in Figure 1b enclose angles of 120 ( 8°; ob” encloses angles of 60 ( 8° with oa” and oc” in Figure 1f, respectively.

Figure 2. AFM images of oleamide nanopatterns, after annealing at 60 °C for 30 min, where the oleamide concentration in chloroform is 1.32 × 10-3 mol/L. Scan range, 488.0 nm. Arrows of od’ and oe’ enclose an angle of 60°.

Rodlike microstructures have been observed in solution, at the air/water interface,17,18 and at the gas/solid interface.19 It is necessary to explain the formation process of nanopatterns from oleamide. Nanostructures of oleamide are not detected by DLS in chloroform solution as well as in ethyl acetate and hexane in the concentration range of 9.59 × 10-5 to 6.14 × 10-3 mol/L. Chloroform is a solvent favorable for H-bond formation between oleamide molecules since it does not compete with the -NH2 group to form H-bonds. However, good solubility and the low concentration disfavor the formation of nanostructures based on (17) Grunze, M.; Haest, C. W.; Deuticke, B. Biochim. Biophys. Acta 1982, 693, 237. (18) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1991. (19) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173.

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H-bonds. Thus, the nanopatterns form during the spin-coating process. The oleamide molecules are mobile immediately after spin coating; even traces of remaining solvent facilitate molecular diffusion. Chloroform as a very volatile solvent evaporates quickly; the distance between the oleamide molecules with unsaturated chains decreases gradually to facilitate the formation of hierarchical nanopatterns on HOPG. The straight separated nanostripes in the topmost layer and compact nanostripes contacting the HOPG experience epitaxial ordering of the HOPG and orient with preferential orientation enclosing an angle of 120°, indicating the direct induction by HOPG (arrows in Figures 1 and 2). This is a driving force for the one-step formation of stable nanopatterns from cis-unsaturated oleamide directly on HOPG. The effect of HOPG on saturated long-chain alkanes to induce the formation of nanostripes of PAC has been revealed previously.14 Oleamide molecules with the CdC group are different from saturated ones. The chains with CdC bonds decrease the hydrophobic interaction between the oleamide molecules, indicated by the lower melting point, 70-78 °C, of oleamide with unsaturated CdC bonds in the 18-C chain, compared to 96-104 °C of stearamide without CdC. Although weaker, the hydrophobic interaction between the chains of oleamide with CdC is strong enough to form compact nanostripes under the direct induction of HOPG. In the few papers published recently on unsaturated molecules studied by scanning tunneling microscopy (STM),20 registry between the adsorbate and substrate as well as the orientation of the plane of the carbon skeleton relative to the graphite surface was adopted. However, the different bond length and angle of -C- in HOPG and in the alkyl chain were assumed to be the same. The bond angle of paraffin should be 105.44° rather than 110° to perfectly fit the backbone of the carbon lattice of HOPG21 considering the C-C bond length difference between HOPG and aliphatic chains. A recent study claimed that a mismatch rather than a match did exist and could be resolved for a molecule with 26 methylene groups on HOPG.22 In addition, the commensurability between the carbon skeletons of the molecules and the HOPG carbon lattice does not exist if an alkyl derivative has a head group requiring more space. An example is the 10% mismatch between the flat-lying alkyl skeletons of stearic acid and the substrate lattice. Oleamide has the -CONH2 head group, which should also induce a mismatch.22 So the two-dimensional lattice of the HOPG substrate and the structure of the oleamide molecules are constructed in this work with the data in crystalline organic compounds to illustrate the compact nanostripes on HOPG. Oleamide molecules are superimposed onto the HOPG by parallelizing the plane of the carbon skeleton of oleamide molecules with the carbon lattice of the underlying HOPG. Bond length values are as follows: 1.42 Å for C-C in HOPG; 1.54 Å and 1.33 Å for C-C and CdC in oleamide, respectively. A -C- bond angle of 110° for aliphatic chains in oleamide different from 120° for -C- in HOPG is used. The distance between the -CONH2 head groups is regulated by the hydrogen bonds to be about 1.8 Å since H-bonds form between -CONH2 groups. The existence of H-bonds is deduced from the fact that a larger head group interaction exists between oleamide than between oleic acid molecules. Therefore, the former is solid while the latter is liquid at room temperature, although they have the same cis-unsaturated chains. In addition, H-bonds between two in-plane oleic acid molecules on HOPG were experimentally confirmed by STM.20 For the packing of the two (20) Tao, F.; Goswami, J.; Bernasek, S. L. J. Phys. Chem. B 2006, 110, 4199. (21) Groszek, A. J. Proc. R. Soc. London A 1970, 314, 473. (22) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424.

Zhang et al.

Figure 3. Scheme of the formation of compact oleamide nanostripes in the bottom layer.

saturated aliphatic chains connected by the CdC bond, the distance between alkane chains of about 5.0 Å from the crystal data of aliphatic chains is expected from literature data.23 The -CdC bond angle is taken to be 120°, based on theoretical prediction for the sp2 orbital hybridization of the carbon atom and on the STM measurement.20 A part of a compact nanostripe drawn using the above parameters is illustrated in Figure 3, where oleamide molecules in adjacent rows rotate 180° around the substrate normal. To reduce steric repulsion between head groups,20 the -CONH2 group of one molecule shifts from the like group of its adjacent molecule in the same row. The compact nanostripe has a width of 3.9 nm, consistent with the experimental observation. The chain-compact nanostripe angle is 60°, similar to those of other amphiphiles,20 reflecting the epitaxial symmetry of HOPG. This molecular packing scheme shows that HOPG can induce well the formation of compact nanostripes, although the commensurability or match is not concerning for the mammal lipid, cis-unsaturated oleamide. Large-scale observation is important for studying surface morphology including the topmost layer, which is the advantage of AFM against STM, where the latter requires a short separation of several angstroms between tip and sample to achieve satisfactory imaging tunneling currents. Figure 1a-d shows the hierarchical structure of nanorods on the topmost layer, which are associated with the closely packed nanostripes of oleamide

(23) The spacing between two nearest aliphatic chains in the unit cell is 5.0 Å for the herringbone packing mode. Kuzmenko, I.; Kaganer, V. M.; Leiserowitz, L. Langmuir 1998, 14, 3882 The projections of the carbon skeleton of such molecules are not coplanar, although they are parallel. On the HOPG, the coplanar aliphatic chains of oleamide should have a larger spacing due to the repulsion of hydrogen atoms in adjacent molecules. However, there exists also one center molecule in the unit cell, which might show repulsion. Without such a center molecule, the spacing should be smaller. Concerning the compensation effect of the above two factors, we use 5.0 Å as the spacing between two neighboring aliphatic chains. .

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There should exist a critical thickness above which the templating effect of HOPG disappears. However, such a value cannot be measured since no surface structures can be imaged by AFM for samples from solutions above the concentration of 1.32 × 10-3 mol/L, due to the strong surface adhesion to the cantilever tips, even after repetitive tries with different vibration amplitudes and damping of the cantilever.

Conclusions

Figure 4. Scheme of the formation of the separated nanostripes in the topmost layer.

in the bottom layer.24 The nanostripes are governed by the balance of oleamide-HOPG interaction and oleamide-oleamide interaction. The molecular adsorption energy within the bottom layer is 37.3 kcal · mol-1 on HOPG; due to the short range of the surface potential, it decreases by ten times to -3 kcal · mol-1 within the second layer and decreases by forty times to -1 kcal · mol-1 in the third layer. 25 As a result, taking two rows of molecules H-bonded together as a unit (which is called lamellae in the STM image), these units pack closely in the bottom layer due to the strong substrate attraction. However, in the topmost layer, they separate due to decreased attraction potential from the substrate and the shortage of molecules for forming a topmost layer with high coverage, as shown in Figure 4. The width of the nanostripes is about 7.0 nm, nearly twice the width of the nanostripes in the bottom layer. Considering the tip convolution effect and the strong head group interaction, we deduce that the nanostripes are formed by two rows of oleamide molecules H-bonded together. (24) Magonov, S. N.; Wawkuschewski, A.; Cantow, H. J.; Liang, W.; Whangbo, M. H. Appl. Phys. A 1994, 59, 119. (25) Hentschke, R.; Schu¨rmann, B. L.; Rabe, J. P. J. Chem. Phys. 1992, 96, 6213.

We have obtained hierarchical straight separated nanostripes and compact nanostripes from a mammal lipid-oleamide without adding any alkanes. The HOPG directly induces the formation of nanopatterns, which do not form in solution. The stiff separated nanostripes in the topmost layer experience the symmetry of HOPG. Lipids with similar characteristics of chains and head groups like naturally existing oleamide in mammals are expected to form such homogeneous nanostructures instead of large patches of separated fast-frozen domains like those from amides with saturated chains. This work helps to understand the selfrecognition and self-assembly of mammal molecules under induction by substrates with defined symmetry and enables organization of hierarchical structures of biomolecules at interfaces in predictable ways, which is also practically essential to improve surface properties like lubrication in an aqueous environment. Thus, it is important in life sciences, supramolecular devices, and advanced materials. Acknowledgment. Renjie Zhang thanks the Department of Science and Technology, Shandong Province, the National Natural Science Foundation of China (No. 20103005, 20533050), the Huo Yingdong Education Foundation, MOE, China, for financial support (81012), and the Max Planck Society and the Alexander von Humboldt Foundation for research fellowships. LA8035506