ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2009, Vol. 45, No. 5, pp. 525–528. © Pleiades Publishing, Ltd., 2009. Original Russian Text © I.G. Efimova, M.A. Ziganshin, V.V. Gorbachuk, D.V. Soldatov, S.A. Ziganshina, A.P. Chuklanov, A.A. Bukharaev, 2009, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2009, Vol. 45, No. 5, pp. 474–477.
MOLECULAR AND SUPRAMOLECULAR STRUCTURES AT THE INTERFACES
Formation of Nanoislands on the Surface of Thin Dipeptide Films under the Effect of Vaporous Organic Compounds I. G. Efimovaa, M. A. Ziganshina, V. V. Gorbatchuka, D. V. Soldatovb, S. A. Ziganshinac, A. P. Chuklanovc, and A. A. Bukharaevc a
Butlerov Institute of Chemistry, Kazan State University, Kremlevskaya ul. 18, Kazan, 420008 Russia b University of Guelph, 50 Stone Road East, Guelph, Ontario, N1G 2W1 Canada c Zavoisky Physical-Technical Institute, Kazan Scientific Center, Russian Academy of Sciences, Sibirskii trakt 10/7, Kazan 420029 Russia e-mail:
[email protected] Received March 12, 2009
Abstract—Sorption properties of a thin L-alanyl-L-valine dipeptide film for vapors of organic compounds, i.e., methanol and toluene, were studied. Compositions of the inclusion compounds formed in the systems are determined using quartz microbalances. The surfaces morphology of of thin dipeptide films before and after the interaction with organic sorbate was studied with atomic force microscopy. The dipeptide was found to have a larger sorption capacity for methanol than for toluene. As a result of the interaction between a thin L-alanyl-Lvaline dipeptide layer with toluene vapor, nanoislets appear on the film surface, and the receptor ability of dipeptide inactivated. PACS numbers: 81.16.Rf DOI: 10.1134/S2070205109050049
INTRODUCTION Synthesis and investigation of the properties of novel materials with nano-size channels and pores suitable for recognition, binding, and storage gaseous compounds, as well as creating membranes and ionic channels, is one of the main problems of modern nanotechnology [1–5]. One of the approaches to the development of these materials is the use of structure elements capable for self-organization [4, 6]. Relatively small oligopeptide molecules, particularly L-alanyl-L-valine studied in this work, are promising in this respect [2, 4]. Depending on the nature of amino acids and the order of their binding in a dipeptide molecule, selforganization may lead to several kinds of nanostructures, including hydrophilic or hydrophobic helical porous nanocolumns or lamellar nanostructures with two- or three-dimensional networks of hydrogen bonds [2]. Relatively labile bonds in dipeptide-based nanomaterials enable one to obtain porous structures with diverse porosity, helicality, and geometry of molecular channels [3, 7, 8]. Nanoporous materials based on relatively small oligopeptides can bind effectively and in large amounts xenon [4] and carbon (IV) oxide [5] or encapsulate neutral organic molecules [9–11]. Due to the chirality of the inner part of channels in the dipeptide phase, it is possible to use such materials in molecular recognition and the separation of enantiomers [2, 12]. The most
important problem in developing novel nanomaterials based on oligopeptide molecules is the elaboration of techniques that ensure the controlled self-organization of the building elements to produce porous systems with the necessary topology and receptor properties [4]. In this work, the possibility of using L-alanyl-Lvaline dipeptide to recognize vaporous organic compounds was studied. For this purpose, the receptor’s properties of L-alanyl-L-valine, in a thin layer, for methanol and toluene vapors were investigated with the mass-sensitive piezoelectric quartz microbalances. In order to estimate the effect of an organic compound on the state of a thin dipeptide layer, the surface morphology of the latter before and after the interaction with the compound was determined with the use of atomic force microscopy. Compositions of the host–guest inclusion compounds formed were calculated, geometric parameters of the nanostructures were found, and their distribution functions determined. EXPERIMENTAL Materials L-Alanyl-L-valine dipeptide (Bachem) was used with no additional purification. Organic solvents were purified immediately before usage according to conventional techniques [13]. According to the data of gas chromatography, the content of the main substance in the organic compounds was no less than 99.5 %.
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Measuring Piezoelectric Sensor Responses (Quartz Microbalance Technique) As the working elements of sensors, 10 MHz resonators with 3.05-cm-diameter quartz plates and 0.51-cm-diameter polished gold electrodes (ICM Co., United States) were used. The design of the device is described in [14]. Two sides of three resonators were covered with thin layers of L-alanyl-L-valine dipeptide by dropping 1 µl dipeptide solution methanol in (0.69 mg/ml) to the surface and then drying it in a hot (45°ë) air flow. The fourth resonator was used as a reference. Upon removing the solvent, the applied coating caused an average decrease in the resonator frequency by 100 Hz. The mean thickness of the dipeptide layer was 40 nm as estimated from its weight, density ρ = 1.033 g/cm3 (calculated from the x-ray structure data [4, 11]), and the spot area. The sensor baseline noise did not exceed 1 Hz.
RESULTS AND DISCUSSION Composition of Clathrates According to the Data of Sensor Experiments Sensor responses of a thin L-alanyl-L-valine dipeptide layer to methanol and toluene vapors were determined. Based on the ∆Fsorbate sensor responses, the content of an organic sorbent in its complex with L-alanyl-Lvaline dipeptide (a mole of the bound organic compound per a mole of dipeptide) was estimated according to the following equation: S = (∆Fsorbate/∆Fdipeptide)(Mdipeptide/Msorbate),
The morphology of thin films of L-alanyl-L-valine dipeptide was studied with a Solver P47 scanning probe microscope (NT-MDT, Russia) in tapping mode. Standard silicon cantilevers with a force constant of 2.5 to 10 N/m and a resonance frequency of 115–190 kHz were used.
where ∆Fdipeptide is the change in the frequency of the quartz resonator upon coating its surface by a dipeptide layer and Mdipeptide and Msorbate are the molecular weights of L-alanyl-L-valine dipeptide (188 g/mol) and the organic compound, respectively. The calculated compositions of the dipeptide–methanol and dipeptide–toluene complexes were 0.67 ± 0.06 and 0.08 ± 0.01, respectively. The larger capacity of the dipeptide for methanol is probably related to the smaller molecular size of this adsorbate and the ability of methanol to form hydrogen bonds. As was found, the sensor response reaches the limiting value in 700 s when the dipeptide binds methanol and in about 3000 s when it binds toluene taking into account the experimental errors. After the first 200 s, 70 and 40% of the corresponding limiting amounts of methanol and toluene, respectively, are already bound. The differences in the binding kinetics of these compounds may be related to a substantial difference between the saturated vapor pressures of methanol (P0 = 16.95 kPa) and toluene (P0 = 3.79 kPa) and, accordingly, the smaller vaporization rate of toluene. The organic compounds considered were completely removed from a thin dipeptide layer by hot air. It was found that receptor properties of the coating, which was regenerated from methanol in this way, were completely restored. At the same time, the sorption capacity of the dipeptide layer upon removing toluene decreased by an order of magnitude compared to the capacity of the initial film.
Dipeptide films with mean thicknesses of 40 nm and diameters of 3 mm were prepared for AFM experiments on the surfaces of highly oriented pyrolytic graphite plates (5 × 5 mm) according to the same technique as that used for quartz resonators. Upon obtaining an AFM image of the original dipeptide film without solvent, the dipeptide on the surface of a pyrolytic-graphite plate was saturated with a vaporous organic compound. Then, the sorbate was removed according to the same technique as in the sensor experiment, and an AFM image of the resulting film was obtained. A mean square roughness of the surface (Rq) was determined from the AFM images according to the method outlined in [16].
Surface Morphology of a Thin Dipeptide Film The surface morphology of a thin film of L-alanylL-valine dipeptide deposited on the highly oriented pyrolytic graphite was studied both before and after the interaction with the vapors of organic compounds with the use of atomic force microscopy. The obtained threedimensional AFM images of the dipeptide film surface are shown in Fig. 2. The mean square roughness of the surface of the original film was Rq = 1.6 ± 0.2 nm. Upon the saturation of a thin L-alanyl-L-valine dipeptide film with vaporous methanol and subsequent removal of the latter by purging with hot air, the surface
The sensor experiment was carried out according to the technique outlined in [14]. During the experiment, upon dozing, the relative pressure of the solvent vapor increases and reaches a constant value under the following dynamic equilibrium conditions: P/P0 = 0.80 ± 0.05. This value was found using gas chromatography according to the procedure described in [15]. To regenerate the coating, resonators were blown with a hot (45°ë) air flow for two minutes. The blowing was repeated at least twice in order to reach a constant resonator’s frequency equal to the value typical of the original coating prepared from the methanol solution. An error in determining the sensor’s response was 2–3 Hz. Studying the Surface Morphology of a Thin Dipeptide Layer
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Fig. 2. Three-dimensional AFM images of the surface (a, c) of the original L-alanyl-L-valine dipeptide film deposited on highly oriented pyrolytic graphite from a methanol solution and dried in a hot (45°ë) air flow for 2 min; (b, d) saturated with (b) methanol vapors for 40 min and (d) toluene vapors for 2 h with subsequent drying in a hot (45°ë) air flow for 2 min.
morphology of the dipeptide layer remained nearly unchanged. At the crystallographic steps of pyrolytic graphite, small nano-spikes appeared (Fig. 2b). This may be related to the difference in the thickness of the dipeptide film at smooth sites of the highly oriented pyrolytic graphite and at crystallographic steps. The mean square roughness of the film surface at a site free of large defects was Rq = 1.5 ± 0.2 nm. Binding toluene vapors resulted in the surface formation of clearly pronounced nanoislets on the dipeptide film (Fig. 2d). A mean square roughness increased more than twofold compared to the original film and became Rq = 4.1 ± 0.2 nm. In order
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Fig. 3. (a, d) Two-dimensional AFM images of the surfaces of L-alanyl-L-valine dipeptide film saturated with (a) methanol for 40 min and (d) toluene for 2 h with the subsequent drying in a hot (45°ë) air flow for 2 min; and (b, e) histograms of the distributions of the nanostructure diameters on the film upon saturation with (b) methanol and (e) toluene; and (c, f) histograms of the distributions of particle heights upon saturation with (c) methanol and (f) toluene.
to characterize the observed changes in the surface morphology of a thin dipeptide film in more detail, we constructed histograms of the distributions of the effective diameters and heights of nanostructures (Fig. 3) using the previously developed program [17] and estimated the corresponding distribution densities. The results obtained show that, upon binding vaporous methanol, nano-spikes appear on the surface of an L-alanyl-L-valine film (Fig. 3a) with lateral sizes of 20–60 nm (Fig. 3b) and heights of about 2–5 nm (Fig. 3c). The most typical nano-spikes have diameters of 40 nm and heights
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of 3.2 nm. There are about 130 nanostructures of this kind per surface area of 25 µm2. The interaction between a dipeptide film and vaporous toluene results in the surface formation of nanoislets with lateral sizes of 80–180 nm (Fig. 3e) and heights of 2–15 nm (Fig. 3f). The most typical nanoislets have diameters of 130 nm and heights of 9.5 nm. There are about 280 nanostructures of this kind per a surface area of 25 µm2. The differences in the surface morphology of thin dipeptide films upon binding vaporous methanol and toluene can be related to the pseudo-polymorphous transition in the dipeptide phase from the channel to lamellar structure due to binding a larger toluene molecule. The absence of noticeable changes in the channel structure of L-alanyl-L-valine crystal upon binding methanol was shown in [11]. On the other hand, the crystallization of this dipeptide from a solution containing relatively large L-alanine molecules results in the formation of a lamellar structure, in which channels are absent [18]. The results obtained in this work indicate the possibility of using L-alanyl-L-valine dipeptide as a working material in mass-sensitive sensors, as well as in producing self-assembling nanostructured films with controlled surface morphology. ACKNOWLEDGMENTS The work was financially supported by the Russian Foundation for Basic Research (project nos. 08-0301107 and 09-03-97011_r_volga_region), as well as the Basic Research and Higher Education CRDF program and FAO (project no. REC007).
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