Langmuir 2003, 19, 205-207
Preparation of Ordered Arrays of Layer-by-Layer Modified Latex Particles Leonid M. Goldenberg,† Byung-Duk Jung, Ju¨rgen Wagner,† Joachim Stumpe,*,† Bernd-R. Paulke, and Eckhard Go¨rnitz Fraunhofer Institute for Applied Polymer Research, Geiselbergstr. 69, 14476 Golm, Germany Received September 12, 2002. In Final Form: October 31, 2002
A great deal of attention has been devoted to the fabrication of photonic crystals based on ordered arrays of colloidal particles.1 Among the recent developments in the preparation of colloidal arrays, one finds the flow cell method,2 the vertical deposition technique,3 and the slow lifting technique.4 Recently we succeeded in assembling latex particles of a broad range of 0.05-3 µm using vertical deposition;3d,e that is, the array formation is due to capillary forces during the evaporation of the dispersion medium at the meniscus on a substrate, such as hydrophilized glass, which is vertically placed in the latex suspension. This method is particularly attractive due to simplicity (no special equipment required) and reproducibility. However, the array quality depends on particle size, polydispersity, colloidal stability, material density and surface properties. Our experience with vertical deposition from aqueous suspensions revealed that the best quality of arrays was obtained with highly hydrophilic core-shell polystyrene-hydroxyethylmethacrylate (PS-HEMA) particles. This type of particles was also mentioned earlier for their good array-building properties.5 Thus, the equipment of particles during their synthesis with a hydrophilic shell is one of the ways of useful modification of particles. Another general way is the subsequent particle modification by a layer-by-layer (LbL) technique.6 LbL may also afford the control of array periodic distance and additional functionalization of arrays for example with photochemical properties. This was already applied to the fabrication of arrays of particles modified with semi* To whom correspondence should be addressed. E-mail:
[email protected]. † Also affiliated with the Institute of Thin Film Technology and Microsensorics, Teltow, Germany. (1) Xia, Y. N.; Gates, B.; Yin, Y. D.; Lu, Y. Adv. Mater. 2000, 12, 693. (2) (a) Park, S. H.; Xia, Y. N. Langmuir 1999, 15, 266. (b) Park, S. H.; Gates, B.; Xia, Y. N. Adv. Mater. 1999, 11, 693, 462. (c) Park, S. H.; Xia, Y. N. Chem. Mater. 1998, 10, 1745. (d) Park, S. H.; Qin, D.; Xia, Y. Adv. Mater. 1998, 10, 1028. (e) Yin, Y. D.; Gates, B.; Xia, Y. N. Adv. Mater. 2000, 12, 1426. (3) (a) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (b) Norris, D. J.; Vlasov, Y. A. Photonic Crystals and Light Localization in the 21st Century; Soukoulis, C. M., Ed.; NATO Science Series Vol. 563; Kluwer: Dordrecht, 2001; p 229. (c) Vlasov, Y. A.; Bo, X.-Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289. (d) Ye, Y. H.; LeBlanc, F.; Hache, A.; Truong, V. V. Appl. Phys. Lett. 2001, 78, 52. (d) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B.-R.; Go¨rnitz, E. Langmuir 2002, 18, 3319. (e) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B.-R.; Go¨rnitz, E. Mater. Sci. Eng., C 2003, 22, 233 and 405. (4) Gu, Z.-Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (5) Cardoso, A. H.; Leite, C. A. P.; Galembeck, F. Langmuir 1999, 15, 4447. (6) (a) Moehwald, H.; Lichtenfeld, H.; Moya, S.; Voigt, A.; Sukhorukov, G.; Leporatti, S.; Daehne, L.; Antipov, A.; Gao, C. Y.; Donath, E. Proceedings of the International Conference on Colloid and Surface Science: Tokyo, Japan, November 5-8, 2000; Studies in Surface Science and Catalysis Vol. 132; Elsevier: New York, 2001; p 485. (b) Sukhorukov, G. B.; Donath, E.; Moehwald, H. Polym. Adv. Technol. 1998, 9, 759. (c) Sukhorukov, G. B.; Donath, E.; Moehwald, H. Colloids Surf., A 1998, 137, 253. (d) Caruso, F. Adv. Mater. 2001, 13, 11.
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conductor quantum-dot nanoparticles.7a,b Stepwise modification was also achieved using silica nanoparticles,7c,8 and an increase of 70 nm in particle diameter was observed by scanning electron microscopy (SEM) in hexagonal close packed layers.7c Recently, the same group9a reported on colloidal array buildup using polyelectrolyte LbL-modified submicrometer particles. A relatively complicated flow cell technique was used for the fabrication, and the authors reported that only the use of so-called “tolerant” core-shell particles (modified with water-soluble poly(ethylene oxide) as the external layer) afforded good array quality. Later this procedure was further modified; another layer of stabilized gold nanoparticles was added,9b and particle arrays were formed by drying. In the last paper, the authors write9b that the arrays obtained from tolerant core-shell particles were comparable in quality to those of the original particles and standard LbLmodified particles afforded arrays of similar quality. A flow cell assembly technique was also used by us.3e However, the vertical deposition approach allows a faster and simpler test for the array-building properties of latex particles.3d,e In the present paper, a standard LbL technique is used with polyallylamine-hydrochloride (PAH, positively charged) and polystyrenesulfonate (PSS, negatively charged) as polyelectrolytes (without tedious particle purification as in ref 9). The consecutive LbL deposition (PAH/PSS sequence, a starting layer of PAH and PSS generally as the top layer) was controlled by the measurement of the zeta potential.11 The objective was focused to a fast and easy way of modification of particle surface properties in order to afford better arraybuilding properties as well as to introduce additional particle functionality. Generally, the improvement of the array quality, similar to the modification of particles with a hydrophilic HEMA shell in core-shell particles,3d is possible. Figure 1 exhibits the (111) Bragg peak measured by reflection Fourier transform infrared (FTIR) spectroscopy for three different batches of micrometer size particles modified by up to 5 bilayers (PAH/PSS) of polyelectrolyte. Modification with an odd number of monolayers (PAH on the top) was also possible. However, as the resulting particles were positively charged in that particular case a good array buildup was afforded only on a similarly modified positively charged glass substrate. When the LbL technique was applied to PSHEMA particles, no additional improvement could be achieved. For conventional hydrophobic PS particles, however, the arrays of LbL-modified particles show better developed Bragg peaks (Figure 1). The improvement of the deposition process can be seen by the naked eye. The original particles are relatively large (1.18 µm according to dynamic light scattering) and being hydrophobic tentatively do not have good adhesion to the glass substrate. Under conventional vertical deposition conditions (80 °C), they form films of only relatively small area (as a large part of the particles just sediment on the bottom of the vial). In a parallel experiment, modified particles (tentatively due to better attraction to the hydrophilic (7) (a) Rogach, A. L.; Sucha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; Moehwald, H.; Eychmueller, A.; Weller, H. Adv. Mater. 2000, 12, 333. (b) Rogach, A. L.; Kotov, N. A.; Koktysh, D. S.; Susha, A. S.; Caruso, F. Colloids Surf., A 2002, 202, 135. (c) Caruso, F.; Moehwald, H.; Langmuir 1999, 15, 8276. (8) Lvov, Y. M.; Price, R. R. Colloids Surf., B 2002, 23, 251. (9) (a) Kumaraswamy, G.; Dibaj, A. M.; Caruso, F. Langmuir 2002, 18, 4150. (b) Liang, Z.; Susha, A. S.; Caruso, F. Adv. Mater. 2002, 14, 1160.
10.1021/la026544r CCC: $25.00 © 2003 American Chemical Society Published on Web 12/05/2002
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Figure 1. Normalized FTIR reflectance spectra (angle, 30°; pinhole, ca. 3 mm diameter) of 1.18 µm PS particles modified with the LbL technique (black line, original particles; 2,4,5BL, 2,4,5 bilayers of PAH/PSS; 2AZO, 2 bilayers of PAH/PSS and 2 bilayers of PAZ-6/PSS; 1MC70, 1 bilayer of PAH/MC70) (a) and angle dependence of the (111) Bragg peak position with a nonlinear fit to Bragg conditions (b). D, particle diameter from nonlinear fit; in parentheses, expected particle diameter; n, effective refractive index from nonlinear fit.
Figure 2. Digital camera images of diffraction spots originating from white light illumination (ca. 3 mm beam) of arrays fabricated from 1.18 µm PS particles: (a) original particles; (b) particles modified with 4 bilayers of PAH/PSS.
Figure 3. Microscopic image (objective 100×) of an array of 1.18 µm PS particles: (a) original particles; (b) particles modified with 4 bilayers of PAH/PSS. Corresponding diffraction images with Bertrand lens (right insert) and 2D-FFT transformation (left insert) are shown.
glass substrate) form large uniform films similar in appearance to the samples of PS-HEMA particles.3d,e The superior array quality for modified particles is also demonstrated by white light diffraction with a beam of
ca. 3 mm diameter to suit the dimensions of the array of nonmodified particles (Figure 2). It is evident that diffraction spots for a modified particle array (Figure 2b) are sharper. A better array quality could also be seen on
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Figure 4. Fluorescence microscopic image of a particle array when 1.18 µm PS particles were modified with fluoresceine containing 39 nm nanoparticles of MC70 (a). The insert shows the fluorescence microscopic image of individual particles. Fluorescence spectra upon 390 nm excitation of MC70 nanoparticles (black), 1.18 µm modified PS particles in suspension (blue), and particle array (red) (b).
the microscopic level (Figure 3), where modified particle arrays show a sharper fast Fourier transform (FFT) image and diffraction pattern (Figure 3b) visualized by a Bertrand lens.3d,e,10 Thus, the improvement of array formation was detected by microscopic, spectroscopic, and diffractive techniques. Furthermore, to introduce additional array functionality original PS particles were modified with a main-chain azobenzene polyionene (PAZ-6 according to ref 10) and with 39 nm fluorescent latex nanoparticles (Figure 1). Azobenzene-modified particles could be useful to make photochemically active arrays, while using fluoresceinecontaining nanoparticles leads to fluorescence and a larger possibility to tune particle size.7 The modification of large particles by nanoparticles using the LbL technique has been attempted earlier (see discussion in review 6d). The strategy, although not as simple as standard LbL modification, seems to be an interesting way to introduce functionality and to tune particle size.6d However, colloidal array studies were only reported for very small CdTe nanoparticles.7 In the present work, it is shown (Figure 1b) that micrometer particles modified by relatively large (39 nm) nanoparticles could afford colloidal arrays of good quality. Individual particles and arrays (Figure 4a) could be imaged by fluorescence microscopy. Their fluorescence spectra show that the maximum of the dye (fluoresceine) emission is shifted to a longer wavelength probably due to dye aggregation after LbL treatment and array formation compared to the “normal” emission behavior of the nanoparticles (Figure 4b). To measure the change in particle diameter (or precisely the change in array lattice constant), the angle-dependent reflection spectra were monitored in a wide range. A nonlinear fit of these data to the Bragg condition yields the effective refractive index and the lattice constant.3d,e (10) (a) Jung, B. D.; Hong, J. D.; Voigt, A.; Leporatti, S.; Dahne, L.; Donath, E.; Moehwald, H. Colloids Surf., A 2002, 198-200, 483. (b) Hong, J.-D.; Jung, B.-D.; Kim, C. H.; Kim, K. Macromolecules 2000, 33, 7905. (11) Caruso, F.; Lichenfeld, H.; Donath, E.; Moehwald, H. Macromolecules 1999, 32, 2317. (12) Lauinger, N.; Pinnow, M.; Goernitz, E. J. Biol. Phys. 1997, 23, 73.
Figure 5. SEM image of 1.18 µm PS particles modified with 1 bilayer of PAH/MC70 (39 nm nanoparticles).
Figure 1b shows the fit data and also n, D, and theoretically expected D according to the polyelectrolyte monolayer thickness of 1-1.5 nm9 (in parentheses). The resulting considerably smaller D for the MC70 nanoparticle modified array could be explained by nonfull coverage of the large particle interface by nanoparticles and by interdigitation of nanoparticles in the array. A SEM study of individual modified particles (Figure 5) shows that generally particles are well covered by nanoparticles; however some particles are not covered or possibly only covered with the basic layer of PAH. In summary, an improved ordered array quality was afforded if the application of a standard LbL technique to modify micrometer-sized latex particles is combined with the vertical deposition technique. This strategy can also be exploited to achieve additional array functionalization (e.g., photoactivity and fluorescence), when a functionalized polyelectrolyte or functionalized nanoparticles are used. Acknowledgment. We thank the German Ministry for Science and Education (BMBF) for financial support. LA026544R
