Langmuir 1992,8, 3145-3149
3146
An Atomic Force Microscope Study of Langmuir-Blodgett Films of a B4a4-HelicalPentadecavaline Albrecht L. Weisenhorn,'!+ Daniel U. Riimer: and Gian P. Lorenzit Institut fiir Histologie, Universitiit, Pkrolles, CH-1700 Fribourg, Switzerland, and Institut fiir Polymere, ETH Zentrum, CH-8092 Ziirich, Switzerland Received June 9,1992. In Final Form: September 21,1992 An atomic force microscope (AFM) has given "molecular" resolution of a multilayerLangmuir-Blodgett A monolayer film of (LB) film consisting of the P4-helicaloligopeptide Boc-D-Val-(L-Val-D-Val),-OMe. the oligopeptide was compressed at the air-water interface of a Langmuir trough to about 45 mN/m and then transferredmonolayer by monolayerto a previously silanizedultraflat silicon wafer by vertical dipping. AF'M images in water as well as in air have revealed ordered arrays of the P4-helicaloligopeptide with a repeat distance of about 1.1 nm. This is consistent with an area-filling arrangement of P4-helices oriented with their axis perpendicularto the substrate. We have measured the height of a four-layer film in water to be about 10 nm, which is in agreement with the expected height of four P4-helices of the oligopeptide. In air, the apparent height of the LB film that was measured in the AF'M image was only about 5 nm because the high capillary forces cause deformationof the film and therefore a reduction of the height in the AFM images. We have also seen a domain boundary of two ordered arrays with different crystal orientation. And finally, we have observed the film structure at the micrometer scale. 1. Introduction
Langmuir are two-dimensional ordered arrays of molecules that can be spread and compressed at an interface, e.g., an air-water interface; conventionally, the molecules are amphiphilic to avoid clustering. Only fairly recently, also hydrophobic molecules were successfully The Langmuir films can be forced used to make into different phases by changing the area per molecule with a trough barrier. The phases are monitored by measuring the lateral surface pressure as a function of the area per molecule in the surface pressure-area diagram or isotherm. These Langmuir filmscan then be transferred by different techniques4J to different substrates such as mica, silicon wafers, glass, or gold surfaces and are then called Langmuir-Blodgett (LB) films. The structure of Langmuir or LB films has been extensively studied by various spectroscopies,6-8 by X - r a ~ , ~by- ~electron ~ d i f f r a c t i ~ n , ~by ~ -surface '~ plasmons spectroscopy,lsJ6by fluorescencemicroscopy,le18 by Brewster angle microscopy + Institut fih Histologie. t Institut fih Polymere. (1) Gaines,G. L., Jr., In Insoluble Monolayersat Liquid-GasInterfaces; Prigogine, I., Ed.; John Wiley and Sone: New York, 1966. (2) Aganval, V. K. Phys. Today 1988, June, 40-46. (3) Orthmann, E.; Wegener,G. Angew. Chem.,Int.Ed. Engl. 1986,25, 1105. (4) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937,51, 964. (5) Langmuir, I.; Schaefer, V. J. J. Am. Chem. SOC.1938,60, 1351. (6) Rothberg, L.; Higashi, G.S.; Allara, D. L.; Garoff, S. Chem. Phys. Lett. 1987, 133, 67. (7) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1987,86, 1601. (8)Vandevyver, M.; Barraud, A. J. Mol. Electron. 1988, 4, 207. (9) Kjaer, K.; Ala-Nieleen, J.; Helm, C. A.; Tippman-Krayer, P.; Mbhwald, H. J. Phys. Chem. 1989,93, 3200. (10) Sukhorukov, G.B.; Lobyehev, V. I. Mol. Mater. 1992,1,91. (11) Yanueova, L.; Klechkovekaya, V.; Sveehnikova,L.; Stiopina, N.; Kruchinin, V. Presented a t the Liquid Crystal Conference, Pisa, Italy, 1992. (12) Fischer, A.; Sackmann, E. J. Phys. (Paris)1985,45,517. (13) Fischer, A.; Sackmann, E. Nature 1986,313,299. (14) Garoff,S.;Deckman,H. W.;Dunemuir,J. H.;Alvarez,M.S.;Bloch, J. M.J. Phys. (Paris) 1986,47,701. (15) Hickel, W.; Kamp, D.; Knoll, W. Nature 1989,339, 186. (16)Schmitt, F.-J.; Weisenhom, A. L.; Hansma, P. K.; Knoll, W. Makromol. Chem., Macromol. Symp. 1991,46, 133. (17) Heyn, S.-P.; Tillmann, R. W.; Egger, M.; Gaub, H. E. J.Biochem. Biophys. Meth. 1991,22,145. (18) Goettgens, B. M.; Tillmann, R. W.; Radmacher, M.;Gaub, H.E. Langmurr 1992,8, 1768.
(BAM),lSz1by scanningtunneling microscopy (STM),22-u and by atomic force microscopy (AFM). The AFM2627 has successfully obtained molecular resolution of different LB films on flat substrates,18*2ggg revealed defects in these f i l m ~ ~ ~and 9 ~ 'domain boundaries,38 measured friction on and observed binding reactions to them.16p42*43 In the study of binding reactions, a LB film with active sites was observed before and after adding molecules to the solution. Molecules binding to the film were clearly seen. Other methode to monitor binding reactions of LB filmsuse surface plaemone spectroscopy,16microgravimetryy4*Gand measurement of photoinduced charge transfer.4 This might open an exciting future for molecular films (19) Hbnig, D.; Mijbiw, D. J. Phya. Chem. 1991,95,4690. (20) Hbnon, S.; Meunier, J. Reu. Sci. Instrum. 1991,62,936. (21) Hbnig, D.; Mbbiue, D. Thin Solid Film 1992,210/211,64. (22) Smith, D. P. E.; Bryant, A.;Quate,C. F.; Rabe, J. P.; Gerber, Ch.; Swalen. J. D. R o c . Natl. Acad. Scr. U.S.A. 1987.84.969. (23)Lang, C. A.; Hbrber, J. K. H.; Hbsch, T.W.; Heckl, W. M.; Mbhwald, H. J. Vac. Sci. Technol. A 1988,6,368. (24)Hbrber, J. K. H.; Lang, C. A.; Hbsch, T. W.; Heckl, W. M.; Mbhwald, H. Chem. Phys. Lett. 1988,145,151. (26) Binnig, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986,56, 930. (26) Wickramasinghe, H. K. Sci. Am. 1989,260,98. (27) Rugar, D.; Hansma, P. K. Phys. Today 1990, October, 23. (28) Egger, M.; Ohneeorge, F.; Weisenhorn, A. L.; Heyn, S.-P.; Drake, B.; Prater, C. B.; Gould, S. A. C.; Hanema, P. K.; Gaub, H. E. J.S t r u t . Biol. 1990, 103, 89. (29)Weisenhom,A.L.;Drake,B.;Prater,C.B.;Gould,S.A.C.;Hansma, P. K.; Ohnesorge, F.; Egger, M.; Heyn, S.-P.; Gaub, H. E. Biophys. J. 1990,58,1251. (30) Weisenhorn, A. L.; Egger, M.; Ohnesorge,F.; Gould, S. A.C.; Heyn, S.-P.; Hansma,H. G.; Sinsheimer, R. L.; Gaub, H. E.; Hansma, P. K. Langmuir 1991, 7, 8. (31) Hansma, H. G.;Weisenhorn, A. L.; Edmundaon, A. B.; Gaub, H. E.; Hamma, P. K. Clin. Chem. 1991,37,1497. (32) Zesadzineki, J. A. N.; Helm, C. A.;Longo, M. L.; Weisenhorn, A. L.; Gould, S. A. C.; Hansma, P. K. Biophys. J. 1991,69,766. (33)Meyer, E.; Howald, L.; Ovemey,R. M.; H e h l m n n u ,H.; Frommer, J.; Gbtherodt, H.-J.; Wagner, T.; Schier, H.; Roth, S.Nature 1991,349, 398. (34)Radmacher, M.; Eberle, K.; Gaub, H. E. Ultramicroscopy 1992, 42-44,968. (35) Schwartz, D. K.; Garnaee, J.; Viswanathan, R.; Zasadzineki, J. A. N. Science 1992, 257, 508. (36)Hansma, H. G.;Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo, M. L.; Zaeadzinski, J. A. N. Langmuir 1991, 7,1061. (37) V i e w a n a h , R.; Schwartz, D. K.; Gamaes, J.; Zasadzhki, J. A. N. Langmuir 1992,8, 1603. (38) Gamaes, J.; Schwartz, D. K.; Viewanathan, R.; Zasadzinski, J. A. N. Nature 1992,357,M.
0 1992 American Chemical Society
3146 Langmuir, Vol. 8, No. 12, 1992
in the application of biosensors, design of channel structures (e.g., ion channels included in model membranes), and bioelectronics.4B-48 Oligopeptides made out Of D and L residues in alternating sequence can form channel structures of hollow helices (@-helices).49The intemal diameter of ab-helix is between 0.2 and 0.8 nm (dependingon the type of the helix), which allows metal ions and small neutral molecules to pass through. Therefore, these @-helicaloligopeptides are good candidates for the design of artificialchannels of controlled permeability. As an approach, we are investigating the possibility to obtain ultrathin films of them (mono- or multilayers) with the helices oriented perpendicular to the substrate. We report here on an AFM study of LB films that we have obtained from one such D,L-dternating oligopeptide, namely the pentadecavaline Boc-D-V~~-(LVal-D-Val).r-OMe.This pentadecavaline, as shown earlier in solution studies,50has a strong tendencyto give /?-helices with about 4.4 residues per turn (#P44-helices)and a lefthanded sense of twist. 2. Materials and Methods 2.1. Boc-D-Val-(cVal-oVal),-OMe. The preparation has of the oligopeptide Boc-D-Val-(L-Val-D-Val),-OMe been described e l s e ~ h e r e .A~model ~ ~ ~ of ita P4-helical structure can be seen in Figure la. 2.2. Silanized Silicon Wafer Substrate. The silicon wafers (Si100)s2with an averaged range of height of
