pubs.acs.org/Langmuir © 2010 American Chemical Society
Self-Assembled Nanofibers of Fluorescent Zeolite L Crystals and Conjugated Polymer Varun Vohra,*,†,‡ Alberto Bolognesi,†,‡ Gion Calzaferri,§ and Chiara Botta†,‡ †
‡
Istituto per lo Studio delle Macromolecole (ISMac-CNR), Via Bassini 15, Milan 20133, Italy, Polo Scientifico Tecnologico, CNR, Via Fantoli 16, Milan 20133, Italy, and §Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Received November 24, 2009. Revised Manuscript Received December 22, 2009
Through this work, we present self-assembled structures which can be obtained by mixing surface modified dye loaded zeolite L crystals and cationic precursors of a conjugated polymer. The zeolite crystals are modified with anionic end groups which give the former a polyanionic character and allow a polyelectrolytic assembly. Microfluidic forces, introduced during the drying of a drop of water containing both polyelectrolytes casted on a clean glass substrate, and localized adsorption on the single zeolite crystal lead to the formation of micro- and nanofibers of highly ordered zeolite nanocrystals. As a result, the fibers display a very well polarized emission from the organic dye included into the nanochannels of the inorganic crystal. Playing on the relative concentrations of polycation and zeolites, and after thermal conversion of the polymer precursor, rigid and insoluble fibers with diameters ranging from less than 200 nm to a few micrometers are obtained.
Introduction Zeolite L crystals1,2 are aluminosilicates which can be the base for host-guest organic inorganic compounds with peculiar optical properties.3-7 The anionic framework of the zeolite crystals allow organic cations to be exchanged with the counterions present inside the channels. The one-dimensional channels of the crystals present some major advantages regarding fluorescence properties of the included, emissive organic dye. The high fluorescence quantum yield of the inclusion compounds is obtained by avoiding the aggregation of the dye molecules inside the channels and therefore avoiding the quenching of the fluorescence.8 The size of the channel entrances will allow the chosen molecules to enter only one at a time. Once inside the channel, the molecules will remain separated one from the other. The dye inside the channel can be addressed through resonant energy transfer.9,10 Polyelectrolytic assembly is a well-known phenomenon in the world of thin films of polymers. Polyelectrolytes are polymers *To whom correspondence should be addressed. E-mail: varun.vohra@ ismac.cnr.it. (1) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (2) Gu, Z.-Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894–897. (3) Gfeller, N.; Calzaferri, G. J. Phys. Chem. B 1997, 101, 1396–1408. (4) Huber, S.; Calzaferri, G. Angew. Chem. 2004, 43, 6738–6742. (5) Maas, H.; Calzaferri, G. Angew. Chem., Int. Ed. 2002, 41, 2284–2288. (6) Calzaferri, G.; Lutkouskaya, K. Photochem. Photobiol. Sci. 2008, 7, 879–910. (7) Schultz-Ekloff, G.; W€ohrle, D.; van Duffel, B.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91–138. (8) Gfeller, N.; Megelski, S.; Calzaferri, G. J. Phys. Chem. B 1998, 102, 928–932. (9) Vohra, V.; Devaux, A.; Dieu, L.-Q.; Scavia, G.; Catellani, M.; Calzaferri, G.; Botta, C. Adv. Mater. 2009, 21, 1146–1150. (10) Vohra, V.; Pasini, M.; Porzio, W.; Destri, S.; Calzaferri, G.; Botta, C. ACS Nano, submitted. (11) Decher, G. Science 1997, 277, 1232–1237. (12) Lee, G. S.; Lee, Y. J.; Yoon, K. B. J. Am. Chem. Soc. 2001, 123, 9769–9779. (13) Jessel, N.; Oulad-Abdelghani, M.; Meyer, F.; Lavalle, P.; Haikel, Y.; Schaaf, P.; Voegel, J.-C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 8618–8621. (14) Etienne, O.; Picart, C.; Taddei, C.; Keller, P.; Hubsch, E.; Schaaf, P.; Voegel, J.-C.; Haikel, Y.; Ogier, J. A.; Egles, C. J. Dent. Res. 2006, 85, 44–48. (15) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531–12535.
1590 DOI: 10.1021/la904450e
having ionic pendant groups present along the backbone. The ionic pending groups can be either anionic or cationic.11-15 Simply by dip coating alternatively in polyanionic and polycationic solutions, a multilayer thin film of alternating polyelectrolytes can be deposited. The driving force of this assembly is electrostatic: the charges present on the polyanion are compensated by the charges present on the polycation and vice versa. Such polyelectrolytic properties can lead to very original assemblies. In this work, we study the assemblies obtained using poly[pxylene-R-(dialkyl-sulfonium halide)], a precursor of poly(p-phenylene-vinylene) (PPV) which exhibits polycationic properties, and modified zeolite L crystals having anions on their surface. A syn elimination of the sulfonium halide groups present along the PPV precursor chains when annealed at 150 °C results in the obtention of unsubstituted PPV. Therefore, through a proper thermal conversion of the PPV precursors, insoluble structures of great interest for multilayered solution processes are obtained.16 The converted polymer is an unsubstituted conjugated material with interesting optical and electronical properties.17,18 We show the possibility of creating hybrid nanofibers of less than 200 nm in diameter containing dye loaded zeolite L crystals. Such hybrid insoluble nanofibers contain a high concentration of very well oriented zeolite L crystals and display polarized emission from the dye included in the inorganic crystal’s channels. Using the simple and cost-effective drop-casting technique, we prepare optically active nanofibers in which the problem of solid state zeolite aggregation is overcome providing, in the meantime, a link between the organic dye included in the nanochannels and the macroworld through the conjugated polymer.9,10 These nanofibers therefore have a great potential to be used in light emitting devices or nanodevices. (16) Vohra, V.; Yunus, S.; Attout, A.; Giovanella, U.; Scavia, G.; Tubino, R.; Botta, C.; Bolognesi, A. Soft Matter 2009, 5, 1656–1661. (17) Bolognesi, A.; Botta, C.; Facchinetti, D.; Jandke, M.; Kreger, K.; Strohriegl, P.; Relini, A.; Rolandi, R.; Blumstengel, S. Adv. Mater. 2001, 13, 1072–1075. (18) Mulazzi, E.; Botta, C.; Facchinetti, D.; Bolognesi, A. Synth. Met. 2004, 142, 85–89.
Published on Web 01/04/2010
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Experimental Section Materials. Fumaric acid and aminopropyltriethoxysilane (APTES) were bought from Sigma-Aldrich. The oxonine molecules were synthesized as described previously.19 Zeolite L nanocrystals of about 30 nm size were used if not otherwise stated. Zeolite L microcrystals were used only for optical microscopy analysis of single zeolites. Both the zeolite L nano- and microcrystals have been synthesized and characterized as described previously.20 The PPV precursors were synthesized following the procedure described in literature.17 Optical Characterization. Confocal and fluorescence microscopy images were collected with a Nikon Eclipse TE2000-U inverted confocal microscope with a long working distance and using a Plan Apo VC objective (magnification 100, N.A. 1.4) and a Plan Fluor objective (magnification 40, N.A. 0.75). The confocal measurements were done with three simultaneous excitations using a DAPI diode laser at 407 nm, an Arþ-ion laser at 488 nm and a He-Ne laser at 543 nm. UV/vis transmission spectra were obtained with a Lambda-9 spectrometer. The photoluminescence spectra were recorded by using a 270 M SPEX spectrometer equipped with a N2-cooled CCD by exciting with a monochromatized xenon lamp. The spectra were corrected for the instrument response. The fluorescence micrograph was obtained through excitation with a 100 W Hg lamp, using excitation filtered between 450 and 490 nm and recording the emission between 505 and 520 nm. Preparation of Oxonine Loaded Carboxylate Functionalized Zeolite Crystals. The oxonine loading and the initial functionalization steps were done as described in a previous work.21 By grafting APTES and consequently fumaric acid on the zeolite surface, we obtain carboxylic acid functionalized crystals. The resulting modified zeolite L crystals are suspended in an aqueous solution of NaOH to transform the carboxylic acid groups into carboxylate salts giving the material polyanionic properties. The suspension is then centrifuged and washed four times in 5 mL of double distilled water. The centrifuged crystals are then places in an oven at 75 °C overnight.
Preparation of Core-Shell Polymer-Zeolite Structures. To obtain the polyelectrolytic assembly of zeolites and PPV precursors, typically 25 mg of carboxylate functionalized zeolites is suspended in water. An amount of 100 μL of an aqueous solution of PPV precursors (10 wt % with respect to water) is then added to the zeolite suspension which is consequently stirred at 90 °C for 1 day in an inert atmosphere. After PPV conversion, the PPV encapsulated zeolites precipitate and can be collected by centrifugation at 5000 rpm for 20 min. The encapsuled crystals are then dried overnight at 75 °C.
Preparation of the Self-Assembled Polymer-Zeolite Nanofibers. Typically, 25 mg of carboxylate functionalized zeolites
are suspended in water. An amount of 300 μL of an aqueous solution of PPV precursors (10 wt % with respect to water) is then added to the zeolite suspension which is consequently drop-casted on a clean glass substrate. During the water evaporation, nanofibers form, and after PPV thermal conversion (150 °C for 30 min under inert atmosphere) insoluble nanofibers are formed. Increasing the quantity of PPV precursors added to the suspension, the diameters of the formed fibers increase up to a few micrometers.
Results and Discussion The zeolites were first loaded with an organic molecule: oxonine. The latter is a cationic dye which can be exchanged with the counterions present in the anionic framework of the (19) Maas, H.; Khatyr, A.; Calzaferri, G. Microporous Mesoporous Mater. 2003, 65, 233–242. (20) Zabala Ruiz, A.; Br€uhwiler, D.; Ban, T.; Calzaferri, G. Monatsh. Chem. 2005, 136, 77–89. (21) Vohra, V.; Bolognesi, A.; Calzaferri, G.; Botta, C. Langmuir 2009, 25, 12019–12023.
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zeolite L crystals. Oxonine molecules occupy two repeating units of the crystals. The loading of the zeolite L was done with a ratio of oxonine to repeating unit of 1:10. Depending on their size and shape, organic dyes included in the zeolite channels arrange differently. For instance, oxonine molecules form an angle of 72° with the channel axis.22 The first step to obtain zeolites encapsuled in PPV is to modify the zeolite surface. The silanol groups present on the external surface of the crystals react with APTES through the formation of Si-O-Si bridges.23,24 The resulting zeolite L crystals have amine groups on their external surface. These amine groups are then further modified with fumaric acid to present carboxylic acid groups on the zeolite surface. The carboxylic acid groups are then put in the presence of NaOH and transformed into anions. After these modifications, the dye loaded zeolite L crystals present the same assembly properties as those of a polyanion. PPV precursors are polycations that can be converted into PPV through thermal conversion in an inert atmosphere. The precursors can be used for electrolytic self-assembly with the zeolite L crystals. To visualize the morphologies obtained, a careful confocal microscopy study was conducted on modified oxonine loaded zeolite crystals which have a mean diameter of 1 μm and a mean length of 5 μm. The base and the coat of the zeolite crystals have different reactivities due to the presence of the channel entrances. Although the functionalization of the zeolite also occurs on the bases of the crystal, at the channel entrances, the presence of cationic species (counterions or exchanged oxonine molecules) screen the anionic properties of the carboxylate groups, leading to a preferential adsorption of the PPV precursors on the coat of the zeolite (Figure 1). After selective adsorption and thermal conversion of the precursors, cylindrical core-shell insoluble microcrystals are formed. Adding PPV precursors to a suspension of zeolite L nanocrystals (30 nm 30 nm) in water and casting the solution on a glass substrate can lead to different morphologies. The PPV precursors can be consequently thermally converted into PPV to make those structures insoluble in most solvents (polar and organic). As we previously saw in Figure 1, adding a small amount of PPV precursors allows one to fabricate core-shell polymer zeolite structures where the polymer envelops a single zeolite crystal. A higher PPV precursor concentration can give rise to the formation of self-assembled nanofibers by simply drop-casting the polymer-zeolite solution on a clean glass substrate. Playing on the relative concentrations of PPV and zeolites, we can obtain fibers of different sizes, aspect ratios, and population densities (Figure 2). For instance, nanofibers of a mean diameter of 200 nm and a mean length of 20 μm are obtained by dropcasting a mixture of 300 μL of PPV precursor solution and 25 mg of oxonine loaded nanozeolites suspended in 1.7 μL of water. The population for this first concentration is around 12 800 nanofibers/cm2. On the other hand, under the same drop drying conditions, PPV-oxonine loaded nanocrystal assemblies obtained by adding 900 μL of precursor solution to a suspension of 25 mg of zeolites in 1.1 μL of water lead to larger fibers with a smaller aspect ratio. Such fibers have a mean diameter of 5 μm, a mean length of 50 μm, and a population of around 2100 fibers/cm2. In Figure 3, the polarized fluorescence images obtained from larger fibers allow us to understand better the arrangement of the zeolites inside the fibers. The zeolite nanocrystals are loaded with a dye molecule which stays almost perpendicular (72°) to the (22) Megelski, S.; Lieb, A.; Pauchard, M.; Drechsler, A.; Glaus, S.; Debus, C.; Meixner, A. J.; Calzaferri, G. J. Phys. Chem. B 2001, 105, 25–35. (23) Sun, W.; Ji, J.; Shen, J. Langmuir 2008, 24, 11338–11341. (24) Busby, M.; Kerschbaumer, H.; Calzaferri, G.; De Cola, L. Adv. Mater. 2008, 20, 1614–1618.
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Figure 1. Excitation profiles (dashed line) and photoluminescence spectra (solid line) of (a) thermally converted PPV and (b) oxonine; (c) schematic representation of the modified zeolite and (d) typical confocal image of a 5 μm long oxonine loaded zeolite crystal having PPV adsorbed on its coat after thermal treatment. The confocal image is typical of what can be seen on most of the crystals.
Figure 2. Top: Optical (left) and fluorescence (right, blue wavelength range of excitation) microscope images of 200 nm large nanofibers obtained by drop-casting a mixture of 300 μL of PPV precursor solution and 25 mg of oxonine loaded nanozeolites suspended in water. Bottom: Confocal microscope images of PPV-oxonine loaded nanocrystal assemblies obtained by adding 900 μL of precursor solution to a suspension of 25 mg of zeolites in water (simultaneous excitation wavelengths: 405, 488, and 543 nm).
zeolite channel axis, and through this fluorescent microscopy study it can be concluded that the emission from the dye is also polarized at an angle of around 72° with respect to the fiber axis. The zeolites are therefore oriented with their axis along the fiber axis, as schematized in Figure 3. The nanofibers displayed in Figures 2 and 3 result from the combination of different phenomena. In previous works, it has been demonstrated that drop-casting a colloidal solution of spherical silicalite-1 from a nonvolatile solvent leads to such an (25) Song, W.; Grassian, V. H.; Larsen, S. C. Microporous Mesoporous Mater. 2006, 88, 77–83.
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Figure 3. Fluorescence microscopy images of horizontally (top) and vertically (middle) polarized emission of almost horizontal hybrid fibers and (bottom) schematic representation of the selfassembled structure.
arrangement with high aspect ratio nanotubes.25 The mechanism of such nanofiber formation is linked to the microfluidic forces created during solvent evaporation. Several groups have studied and modeled the drying of drops of colloidal dispersions.26-29 (26) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature 1997, 389, 827–829. (27) Deegan, R. D. Phys. Rev. E. 2000, 61, 475–485. (28) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. Rev. E 2003, 62, 756–765. (29) Pauchard, L.; Allain, C. C. R. Phys. 2003, 4, 231–239.
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Although it has been shown that such phenomena can occur with spherical silicalite-1, we could not obtain such an arrangement by simply drop-casting zeolite L nanocrystals (with or without surface modification) on their own from water. We therefore believe that the mechanism leading to the nanofiber formation is related to the rod-coil nature of the PPV precursor dressed zeolite crystals. In literature, many examples of rod-coil block polymers which selfassemble can be found.30-32 The polymer-zeolite core-shell structure can be considered as a similar case in which the central inorganic crystal acts as the rod and the unconverted polymer surrounding the crystal would be the coil. By increasing the amount of PPV precursors, fibers become larger but also more ordered (Figure 3). Self-assembly phenomena involving rod-coil block copolymers depend on the relative lengths of the different blocks. The case of our core-shell structures is somehow similar: a larger amount of PPV precursors induces a higher quantity of adsorbed polymer on the zeolite surface which will consequently increase the self-assembly properties of the core shell structure. Once the structure is obtained, thermal annealing of the conjugated polymer precursors into unsubstituted PPV will lead to insoluble nanofibers encapsulating highly ordered dye loaded zeolite crystals. (30) Sary, N.; Brochon, C.; Hadziioannou, G.; Mezzenga, R. Eur. Phys. J. E 2007, 24, 379–384. (31) Sary, N.; Rubatat, L.; Brochon, C.; Hadziioannou, G.; Mezzenga, R. Macromol. Symp. 2008, 268, 28–32. (32) Tu, Y.; Graham, M. J.; Van Horn, R. M.; Chen, E.; Fan, X.; Chen, X.; Zhou, Q.; Wan, X.; Harris, F. W.; Cheng, S. Z. D. Polymer 2009, 50, 5170–5174.
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Conclusions Functionalization of the loaded zeolite crystals with an anion was obtained. The resulting zeolites are used as polyanions to self-assemble with PPV precursors having cations along the polymer chain. Playing on the relative concentration of PPV precursors and zeolites, different self-assembly morphologies are obtained. A higher precursor concentration allows us to obtain self-assembled core-shell nanofibers formed by stacked zeolite L nanocrystals surrounded by the polymer. The highly ordered zeolite L crystals display polarized emission almost perpendicular to the fiber axis. The zeolites’ axis is therefore parallel to the axis of the fiber. After thermal treatment, the polymeric shell of the fiber is converted into an insoluble conjugated polymer. As they are insoluble, such hybrid fibers could be the base of innovative hybrid light emitting diodes. Furthermore, since the dye inside the zeolite crystals can be addressed through energy transfer from the electroluminescent polymer, the hybrid self-assembled nanofibers open new perspectives in the fabrication of nanodevices. Acknowledgment. The work was supported by the European Commission through the Human Potential Program (MarieCurie RTN “Nanomatch” Contract No. MRTN-CT-2006035884; website: www.nanomatch.eu). We thank the group of Dominik Br€uhwiler (University of Z€urich) for providing us with the zeolite crystals.
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