Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites Gero Decher, Et Al

Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites Gero Decher, et al. Science 277, 1232 (1997); DOI: 10.1126/science.277.5330.1232

This copy is for your personal, non-commercial use only.

If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.

Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.

The following resources related to this article are available online at www.sciencemag.org (this information is current as of February 24, 2010 ):

Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/cgi/content/full/277/5330/1232

This article has been cited by 30 articles hosted by HighWire Press; see: http://www.sciencemag.org/cgi/content/full/277/5330/1232#otherarticles

This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci on February 24, 2010 www.sciencemag.org Downloaded from

Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 1997 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. Physics (Cornell Univ. Press, Ithaca, NY, 1979); F. W. 27. J. T. Chen, E. L. Thomas, C. K. Ober, G.-P. Mao, 32. We have benefited from interactions with many stu- Wiegel, Conformational Phase Transitions in a Mac- ibid. 273, 343 (1996); J. T. Chen et al., Macromole- dents and co-workers and would like to thank former romolecule: Exactly Solvable Models in Phase Tran- cules 28, 1688 (1995). members of our research groups. In particular, sitions and Critical Phenomena, C. Domb and J. L. 28. G.-P. Mao et al., Macromolecules 30, 2556 (1997). C.K.O. thanks G.-P. Mao and J.-G. Wang and E.L.T. Lebowitz, Eds. (Academic Press, New York, 1983), 29. A. Omenat, R. A. M. Hikmet, J. Lub, P. Van der Sluis, thanks J. T. Chen. Funding from NSF (DMR-970527) vol. 7. ibid. 29, 6730 (1996); W. Y. Zheng and P. T. Ham- (to M.M., C.K.O., and E.L.T.) and both NSF (DMR- 4. F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. mond, Macromol. Rapid Commun. 17, 813 (1996). 9201845) and the Air Force Office of Sponsored Chem. 41, 525 (1990). 30. M. Brehmer, R. Zentel, G. Mao, C. K. Ober, Macro- Research (MURI PC 218975) (to C.K.O. and E.L.T.) 5. For a discussion of several of the processes leading mol. Symp. 117, 245 (1997). is gratefully acknowledged. C.K.O. thanks the Office to self-organization, see special issue on the 1996 31. J.-G. Wang, G.-P. Mao, C. K. Ober, E. J. Kramer, of Naval Research for support of the fluoropolymer NATO Workshop on Tandem Molecular Interactions, Macromolecules 30, 1906 (1997). work. in R. Zentel, G. Galli, C. K. Ober, Eds., Macromol. Symp. 117 (1997). 6. E. Dessipri, D. A. Tirrell, E. D. T. Atkins, Macromole- cules 29, 3545 (1996). 7. M. Muthukumar, J. Chem. Phys. 103, 4723 (1995); Comput. Mater. Sci. 4, 370 (1995). Fuzzy Nanoassemblies: 8. For the specific example of diblock copolymers, the topological connectivity of various monomers in every chain appears as a density-density correlation Toward Layered Polymeric at a distance r being proportional to 1/r so that ␣ϭ 2 (the Fourier transform of 1/r is 1/k 2 in three dimensions). The long-range interaction can origi- Multicomposites nate from such entropic correlations or electrostatic interactions (as in the case of surfactant and polyelectrolyte solutions) or chemical reactions (as Gero Decher in the case of simultaneous occurrence of mac- rophase separation and chemical reactions or hydrogen bonding). Multilayer films of organic compounds on solid surfaces have been studied for more than 9. The scheme requires consideration of shape and 60 years because they allow fabrication of multicomposite molecular assemblies of packing at the atomic and molecular level as well as tailored architecture. However, both the Langmuir-Blodgett technique and chemisorp- at higher levels of scale leading to the three dimen- sional structure of the overall aggregate. Bringing the tion from solution can be used only with certain classes of molecules. An alternative set of components together into the requisite array in approach—fabrication of multilayers by consecutive adsorption of polyanions and poly- a single process step (for example, by cooling the cations—is far more general and has been extended to other materials such as proteins mixture) is extremely difficult, particularly when the basic units have conformational diversity under the or colloids. Because polymers are typically flexible molecules, the resulting superlattice

conditions of the assembly process. Indeed, this sit- architectures are somewhat fuzzy structures, but the absence of crystallinity in these on February 24, 2010 uation is a critical issue, for if the system or parents of films is expected to be beneficial for many potential applications. the system adopt wrong shapes and packings, then the desired structures may not even form. 10. A. H. Simmons, C. A. Michal, L. W. Jelinski, Science 271, 84 (1996). 11. T. Gulik-Krzywicki, C. Fouquey, J.-M. Lehn, Proc. n the last two to three decades, materials structures, although it is also clear that struc Natl. Acad. Sci. U.S.A. 90, 163 (1993); M. Kotera, I - J.-M. Lehn, J.-P. Vigneron, Tetrahedron 51 (No. 7), science has developed into an interdiscipli- tures as complex as those found in the bio- 1953 (1995). nary field that encompasses organic, poly- logical world, such as the flagellar motor, 12. M. Mu¨ ller, F. Kremer, R. Stadler, E. W. Fischer, U. meric, and even biological components in cannot yet be fabricated. Seidel, Colloid Polym. Sci. 273, 38 (1995).

13. T. Kato, H. Kihara, U. Kumar, T. Uryu, J. M. J. Fre´- addition to the classic metals and inorgan- It is, however, possible to consecutively www.sciencemag.org chet, Angew. Chem. Int. Ed. Engl. 33, 1644 (1994); ics. Although carbon-based molecules offer deposit single molecular layers onto planar T. Kato, M. Nakano, T. Moteki, T. Uryu, S. Ujiie, an enormous structural diversity and tun- solid supports and to form multilayers in Macromolecules 28, 8875 (1995). 14. C. G. Bazuin, F. A. Brandys, T. M. Eve, M. Plante, ability in terms of potential properties or which nanoscale arrangements of organic Macromol. Symp. 84, 183 (1994); C. G. Bazuin and processability, they also typically suffer from molecules can be controlled at least in one A. Tork, Macromolecules 28, 8877 (1995); R. a lack of stability when exposed to heat, dimension (along the layer normal). This Tal’roze, S. Kuptsov, T. Sycheva, V. Bezborodov, N. Plate, ibid., p. 8689. oxidizing agents, electromagnetic radiation, approach also fulfills another prerequisite 15. J. Ruokalainen et al., Macromolecules 28, 7779 or (as in the case of complex biomolecules) for functional macroscopic devices: a fixed

(1995); J. Ruokalainen, G. ten Brinke, O. Ikkala, M. dehydration. Multicomposites make it pos- relation between nanoscopic order and Downloaded from Torkkeli, R. Serimaa, ibid. 29, 3409 (1996). sible to combine two or more desirable prop macroscopic orientation. To fully exploit an 16. A. Pron, J. Lostra, J.-E. O¨ sterholm, P. J. Smith, - Polymer 34, 4235 (1993); T. Vikki et al., Macromol- erties, as in the classic reinforced plastics, or assembled structure, it is necessary to know ecules 29, 2945 (1996). to provide additional stability for otherwise the location or orientation (or both) of 17. Peter R. Ashton et al., Angew. Int. Ed. Engl. 36, 735 highly labile functional biomolecules or bio- every molecule, not only with respect to (1997). 18. C. Gong and H. Gibson, Macromolecules 29, 7029 molecular assemblies. Even higher device each other (as in ordered or phase-separated (1996); G. Wenz, Angew. Chem. Int. Ed. Engl. 33, functionality will arise from a combination bulk systems at the nanometer scale, such as 803 (1994). of physical and chemical processes (such as liquid crystals, copolymers, or zeolites), but 19. C. K. Ober and G. Wegner, Adv. Mater. 9, 17 (1997). 20. V. Percec and G. Johansson, Macromol. Symp. 95, electron or energy transfer) and chemical also with respect to a macroscopic coordi- 173 (1995). transformations found in nature (such as nate. Only materials that have such struc- 21. J. M. Park, B. B. Muhoberac, P. L. Dubin, J. Xia, photochemical energy conversion). Such tural hierarchy (1) allow molecular proper- Macromolecules 25, 290 (1992). 22. E. L. Thomas, D. M. Anderson, C. J. Henkee, D. devices require control of molecular orien- ties to be fully exploited in macroscopic Hoffman, Nature 334, 598 (1988). tation and organization on the nanoscale, as devices, as, for example, in organic 23. J. M. Seddon, Biochim. Biophys. Acta 1031,1 their function strongly depends on the local waveguides for second-order nonlinear op- (1990); T. Hashimoto, M. Takenaka, T. Izumitani, J. chemical environment. It is therefore highly tics or in biosensors. Chem. Phys. 97, 697 (1992); J. Lauger, R. Lay, W. Gronski, ibid. 101, 7181 (1994). desirable to develop methods for the con- In simple multilayer systems, this de- 24. J.-P. Billot, A. Douy, B. Gallot, Makromol. Chem. trolled assembly of multicomponent nano- mand is reduced to the sequence of layers 178, 1641 (1977). and to the orientation of molecules with 25. J. Adams and W. Gronski, Makromol. Chem. Rapid Universite´ Louis Pasteur and CNRS, Institut Charles Sad- Commun. 10, 553 (1989). ron, 6, rue Boussingault, F-67083 Strasbourg Cedex, respect to the layer normal. For about 60 26. S. I. Stupp et al., Science 276, 384 (1997). France. E-mail: [email protected] years, the molecularly controlled fabrica- 1232 SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 ⅐ www.sciencemag.org ARTICLES tion of nanostructured films has been dom- than one equal charge allows for charge lyte addition has important consequences inated by the so-called Langmuir-Blodgett reversal on the surface, which has two im- for flocculation and is therefore of interest (LB) technique, in which monolayers are portant consequences: (i) repulsion of equal- in large-scale industrial processes such as formed on a water surface and then trans- ly charged molecules and thus self-regula- sewage dewatering or paper making, and ferred onto a solid support (2, 3). Indeed, tion of the adsorption and restriction to a the two-step treatment of colloids or cel- the pioneering work on synthetic nanoscale single layer, and (ii) the ability of an oppo- lulosic fibers with polycations and polya- heterostructures of organic molecules was sitely charged molecule to be adsorbed in a nions has been studied for many years (19, carried out by Kuhn and co-workers in the second step on top of the first one. Cyclic 20). However, the process is considered late 1960s using the LB technique (4). repetition of both adsorption steps leads to difficult and the resulting structures are Their experiments with donor and acceptor the formation of multilayer structures. not well understood; therefore, existing dyes in different layers of LB films provided The crucial factor of charge reversal of a industrial applications may benefit from a direct proof of distance-dependent Fo¨rster surface upon adsorption of an oppositely better understanding of polyelectrolyte energy transfer on the nanoscale. These charged polyelectrolyte has long been multilayer films as model systems, as these experiments were also the first true nano- known for the case of polyion adsorption on can be well characterized by a wide variety manipulations, as they allowed for the me- colloids, but has also been observed on mac- of physical techniques. chanical handling of individual molecular roscopic surfaces (14, 15). The consecutive layers (such as separation and contact for- adsorption of cationic colloids composed of Fabrication of Polyelectrolyte and mation) with angstrom precision (5). a heparin-hexadecylamine complex and of Related Multilayers The LB technique requires special pure heparin on polyethylene surfaces that equipment and has severe limitations with were oxidized or sulfated (or both) leads to Multilayer structures composed of polyions respect to substrate size and topology as well films with interesting nonthrombogenic or other charged molecular or colloidal ob- as film quality and stability. Since the early properties (16). However, these films were jects (or both) are fabricated as schemati- 1980s, self-assembly techniques based main- reported to be homogeneous monolayers cally outlined in Fig. 1. Because the process ly on silane-SiO2 (6) and metal phospho- that arise from submonolayer coverage after only involves adsorption from solution, nate chemistry (7) were developed as an the first deposition cycle and subsequent there are in principle no restrictions with alternative to LB films. However, self-as- completion of surface coverage in cycles 2 respect to substrate size and topology; mul- sembled films based on covalent or coordi- to 5; additional deposition cycles lead to tilayers have been prepared on colloids and nation chemistry are restricted to certain surface flocculation and destruction of layer on objects with dimensions of several tens classes of organics, and high-quality multi- uniformity. This report was rather discour- of centimeters. Film deposition on a glass on February 24, 2010 layer films cannot be reliably obtained. aging given the early and promising exper- slide from ordinary beakers can be carried These problems are most likely caused by iments of Iler on the fabrication of multi- out either manually or by an automated the high steric demand of covalent chem- layers of charged inorganic colloids by con- device (21) (Fig. 1A). A representation of istry and the severely limited number of secutive adsorption (17), which were never the buildup of a multilayer film at the mo- reactions with exactly 100% yield, which is proven to be layered structures or the pro- lecular level (Fig. 1B) shows a positively a prerequisite for the preservation of func- tein-polyelectrolyte multilayers proposed by charged substrate adsorbing a polyanion tional group density in each layer. Fromherz in 1980 (18). and a polycation consecutively; in this ex- It was therefore desirable to develop a Sequential cationic-anionic polyelectro- ample, the counterions have been omitted

simple approach that would yield nanoar- www.sciencemag.org chitecture films with good positioning of individual layers, but whose fabrication Fig. 1. (A) Schematic of the A would be largely independent on the nature, film deposition process using slides and beakers. size, and topology of the substrate. The elec- Steps 1 and 3 represent the trostatic attraction between oppositely adsorption of a polyanion charged molecules seemed to be a good can- and polycation, respectively, 1234 didate as a driving force for multilayer build- and steps 2 and 4 are wash- B up, because it has the least steric demand of ing steps. The four steps are Downloaded from all chemical bonds. Since the early 1990s, the basic buildup sequence our group has developed a technique for the for the simplest film archi- construction of multicomposite films of rod- tecture, (A/B)n. The construction of more complex like molecules equipped with ionic groups at 1. Polyanion 3. Polycation both ends (8), polyelectrolytes (9), or other film architectures requires only additional beakers and 2. Wash 4. Wash charged materials through layer-by-layer ad- a different deposition se- Substrate sorption from aqueous solution (10, 11). quence. (B) Simplified mo- The process, which is extremely simple, is lecular picture of the first two depicted in Fig. 1 for the case of polyanion- adsorption steps, depicting polycation deposition on a positively film deposition starting with a positively charged substrate. charged surface. Strong electrostatic attrac- C tion occurs between a charged surface and Counterions are omitted for an oppositely charged molecule in solution; clarity. The polyion conformation and layer interpene- this phenomenon has long been known to + Ð tration are an idealization of NH3 CI be a factor in the adsorption of small organ- the surface charge reversal ics and polyelectrolytes (12), but it has rare- with each adsorption step. Ð + SO3 Na ly been studied with respect to the molecular (C) Chemical structures of details of layer formation (13). In principle, two typical polyions, the so- the adsorption of molecules carrying more dium salt of poly(styrene sulfonate) and poly(allylamine hydrochloride). www.sciencemag.org ⅐ SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 1233 and the stoichiometry of charged groups small, the first layer binds to the surface their reformation in a more equilibrated between polyions and between the substrate with only a few groups and exposes a larger conformation of the polymer chains. and polyanion is arbitrary (see below). Two number of oppositely charged groups to the Films are typically deposited from adsor- typical polyelectrolytes, sodium poly(sty- solution. This effective “multiplication of bate concentrations of several milligrams rene sulfonate) and poly(allylamine hydro- surface functionality” often continues over per milliliter. These concentrations are chloride), are shown in Fig. 1C. a few layers before a linear deposition re- much greater than that required to reach The use of polyelectrolytes rather than gime is reached (22–26). the plateau of the adsorption isotherm, but small molecules is advantageous mainly be- Similar to this self-regulation of thick- this excess ensures that the solutions do not cause good adhesion of a layer to the un- ness increments per layer, there is a tenden- become depleted during the fabrication of derlying substrate or film requires a certain cy toward a certain value of the interfacial films composed of several hundred layers. number of ionic bonds. Therefore, the over- overlap between a polyanion layer and a One or more washing steps are usually used compensation of the surface charge by the polycation layer and a certain roughness at after the adsorption of each layer to avoid incoming layer is more a property of the the film-air interface; these attributes are contamination of the next adsorption solu- polymer than a property of the surface. This probably a property of the polyanion-poly- tion by liquid adhering to the substrate from is because polymers can simply bridge over cation pair rather than a property of the the previous adsorption step. The washing underlying defects; their conformation at substrate. We have observed that polyelec- step also helps to stabilize weakly adsorbed the surface (and thus also the newly created trolyte multilayers have similar surface polymer layers (24). Typical adsorption film surface) is mostly dependent on the roughness, regardless of the roughness of the times per layer range from minutes in the chosen polyelectrolytes and adsorption con- underlying substrates. One possible expla- case of polyelectrolytes (24, 25, 28)to ditions and much less dependent on the nation is that the surface roughness of hours in the case of gold colloids (29, 30), substrate or the substrate charge density rough polyelectrolyte films can be “an- depending on molar masses, concentra- (10, 22). The linear increase of film thick- nealed” to smaller values by consecutively tions, and agitation of the solutions. ness with the number of deposited layers is immersing films in solutions of salt and pure The major advantages of layer-by-layer often similar even if different substrates are water (27). Presumably, in this post-prepa- adsorption from solution are that many dif- used, which makes the film properties rath- ration treatment of the films, the salt breaks ferent materials can be incorporated in in- er independent of the substrate. In cases some of the anion-cation bonds, and its dividual multilayer films and that the film where substrate charge densities are very removal by washing in pure water leads to architectures are completely determined by on February 24, 2010 Fig. 2. (A) Curves XR-1, XR-2, and XR-3 are specular x-ray reflectivity scans of multilayer films composed of poly(styrene sulfonate) and poly(allylamine hydrochloride). Only partial curves for XR-1 and XR-2 are shown, to keep the same range of the scattering vector q for all scans and for better comparison with (B). The film

thicknesses increase from 16.5 nm to www.sciencemag.org 120.5 nm because of different numbers of layers and deposition from solution containing different amounts of salt. All curves can be modeled quantitatively with a film of homogeneous thickness and no internal structure. Scan XR-3 was taken from the same film specimen as curve NR-6 in (B); however, its internal layer structure Downloaded from is only detected by neutron reflectivity. XR-4 is a reflectivity scan of a multilayer with the architecture

((A/B)3A/G)4, where G consists of negatively charged gold colloids of 13.5 nm diameter (30). The superlattice formed by the layers of the gold colloids is clearly seen; the high in- tensity of the (001) Bragg peak aris- es from the large electron density difference between the polymers and the and NR-3 were approximately matched by depositing NR-2 from 3 M NaCl gold. Scans are shifted in the y direction to preserve clarity. (B) Curves and NR-3 from 2 M NaCl; therefore, Bragg peaks, if present, should appear NR-1 to NR-6 are specular neutron reflectivity scans of multilayer films; all at similar q values. This result rules out the hypothesis that low count rates except NR-5 were obtained from films composed of sodium poly(styrene at high Q values are responsible for the absence of Bragg peaks in NR-1 sulfonate) and poly(allylamine). NR-1 and NR-2 are scans of films in which and NR-2 and shows that the deuteron concentration along the layer every second layer was deuterated [(A/Bd)n film architecture]. NR-3 has an normal in (A/Bd)n films is constant with respect to experimental resolution. (A/B/A/Bd)n architecture and was measured from the same film specimen NR-5 is a reflectivity scan of a multilayer originally fabricated from poly(sty- as NR-4 after 11 months of storage in a regular laboratory, demonstrating rene sulfonate) and the tetrahydrothiophenium precursor polyelectrolyte of the good stability of multilayer films over time. Scan NR-6 has ((A/B)2A/ PPV; every fourth poly(styrene sulfonate) layer is perdeuterated. After 32 Bd)n architecture and was measured from the same specimen that does min of elimination at 120°C, the conversion to PPV is almost complete but not show Bragg peaks in x-ray reflectivity [(A), scan XR-3]. Although differ- the layer structure remains intact, as evidenced from the Bragg peak. ent in architecture, the positions of the deuterated layers in samples NR-2 Scans are shifted in the y direction to preserve clarity.

1234 SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 ⅐ www.sciencemag.org ARTICLES the deposition sequence. The most remark- specific layer positions in a multilayer film such a homogenous distribution of charged able current examples of multicomposite were Bragg peaks observed by neutron re- ionic groups in the film contradicts the notion films include proteins (31–33), clay plate- flectometry [((A/B)mA/Bd)n film architec- of defined individual layers of polyions within lets (31, 34–37), virus particles (38), and tures, m ϭ 1, 2, . . . ; Fig. 2B, traces NR-3to such multilayer assemblies, but this apparent gold colloids (30, 39). Nanostructured sur- NR-6], which clearly demonstrated an in- discrepancy is easily explained. In a simplified face-confined films do, of course, have bulk ternal layer structure (50, 51). A single polyelectrolyte multilayer structure composed analogs; there are similarities between poly- Bragg peak was also observed by x-ray re- of 10 layers (Fig. 3), each layer is represented electrolyte multilayers and their bulk coun- flectivity in films in which every fourth by an arbitrarily chosen sinusoidal concentra- terparts (40). Polyelectrolyte-clay multilay- layer was a polyanion containing side tion profile. The 50% overlap of layers of ers and bulk organoclay nanocomposites groups of azo dyes (52). Thus, the absence equal charge has the consequence that at any (41–46), which have interesting materials of Bragg peaks in (A/B)n-type films does not point inside the film (the substrate-film and properties themselves, may be structurally arise from small density differences between film-air interfaces are different), the sum of even closer. However, no straightforward different layers, but rather from large over- the concentration of equal ionic groups is strategy exists to prepare bulk multicompos- laps between adjacent layers. unity in both the cationic and anionic case, as ites with more than two components in On the basis of this result, together with represented by the lines composed of blue dots which the distance of all constituents or our inability to detect significant amounts of (concentration profile of anionic groups) and their orientation (or both) can be molecu- small counterions in the film, polyelectrolyte red dots (concentration profile of cationic larly controlled. Template approaches such multilayers should have a 1:1 stoichiometry of groups). The line composed of green dots as layer-by-layer assembly are much more anionic and cationic groups. In this case, ev- represents the concentration profile for a label promising in this respect. Another differ- ery anionic group of a polyanion is bound to a applied to every fourth layer and shows that ence between bulk systems and surface-con- cationic group of a polycation, which is also chemical functional groups (or labels) can be fined multilayers is that bulk nanocompos- the predominant case in bulk polyion com- precisely positioned at certain distances from ites are often turbid materials, whereas lay- plexes of flexible polyelectrolytes of high the substrate or with respect to each other. er-by-layer assembled films can be applied charge density and similar molecular weights Thus, Fig. 3 represents a film model in which as wavelength-thick transparent coatings (53). Note that for weak polyelectrolytes, not the high overlap of layers of equal charge on, for example, optical devices. all of the monomers need to be charged, so allows for a 1:1 stoichiometry of anionic and that the overall stoichiometry may deviate cationic groups within the film and provides Structure of Polyelectrolyte from 1:1 (54). This has the interesting con- the base for a true layer structure.

Multilayers sequence that within the resolution of the The deuterium concentration of such an on February 24, 2010 charge-to-charge distance along the polyelec- (A/B/A/Bd)n film, which is the most impor- Reflectivity techniques, especially neutron trolyte backbone (typically 0.25 to 0.5 nm), tant contributor to the scattering length and x-ray reflectometry, are well suited for the concentration of anionic and cationic density profile, is described by the line of the characterization of multilayer films, as groups must be identical throughout the poly- green dots in Fig. 3. The profile is sinusoi- they allow the determination of concentra- electrolyte multilayer film and constant along dal, which agrees with the observation of a tion gradients along the layer normal. In the layer normal. At first, it would seem that single Bragg peak for this architecture (Fig. many experiments on multilayer films composed of flexible, strong polyelectrolytes of

approximately equal charge-to-charge dis- Fig. 3. Schematic of a www.sciencemag.org polyelectrolyte multilayer tances (polyanions and polycations with 1.0 one charged group per monomer unit), x- composed of 10 layers, ray reflectograms have only exhibited so- each represented by an arbitrarily chosen sinu- called Kiessig fringes that arise from the soidal concentration pro- 0.8 interference of x-ray beams reflected at the file (black lines). For a substrate-film and film-air interfaces (9, positively charged sub- 47–49). Typical reflectivity curves are strate, the five blue lay- shown in Fig. 2A (traces XR-1toXR-3). In ers and five red layers Downloaded from 0.6 these x-ray scans, different numbers of os- represent polyanion and cillations arise from the different film thick- polycation layers, re- nesses that were obtained either by chang- spectively. The spread of each layer and the dis- Substrate ing the total number of layers or by depos- 0.4 iting from polyelectrolyte solutions of dif- tance between them were chosen such that ferent ionic strength (10, 47). They show a every two layers of equal Relative concentration large number of well-resolved Kiessig fring- charge start to overlap at es that were originally believed to be caused a relative concentration 0.2 by the electron densities of two consecutive of 50%. The overlap of layers being too close to yield enough con- blue and red layers (pur- ple) has no physical trast. However, neutron reflectograms of 0.0 films in which all polyanion layers were meaning. The lines composed of blue dots (an- labeled with deuterium [(A/Bd)n film architecture, where A is a polycation, B is a ionic groups) and red 12 345 678 910 dots (cationic groups) Layer number polyanion, Bd is a perdeuterated polyanion, represent the sum of and n is the number of deposition cycles] concentrations from all layers within the film. A positional shift of red layers with respect to blue layers also showed Kiessig fringes as the only char- causes changes in charge concentration only at the two interfaces, not in the center of the film. The line acteristic feature (Fig. 2B, traces NR-1 and composed of green dots represents the concentration profile for a label applied to every fourth layer

NR-2). Only when we started to deuterate [(A/B/A/Bd)n architecture ϭ deuterium labels in layers 3 and 7]. www.sciencemag.org ⅐ SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 1235 2B, scan NR-4). Evaluation of the position dinitrophenyl units that can form charge- tion that a water-soluble polyelectrolyte of such Bragg peaks has yielded a distance of transfer complexes (67). Even covalent precursor of the intractable electrolumines- ϳ10 nm between the deuterated layers, chemistry can successfully be used for mul- cent polymer poly(p-phenylene vinylene) which depends on the details of film prep- tilayer fabrication, both in purely covalent (PPV) can be incorporated into polyelec- aration, such as salt concentration in the and in mixed covalent-ionic film architec- trolyte films and subsequently thermally adsorption solution. If the deuterium labels tures using polyamines both as covalent and converted to PPV (93). The neutron reflec- are placed in every sixth (Fig. 2B, scan as ionic layer-forming reagents. Again, the tivity scan NR-5 in Fig. 2B shows that the NR-6) or eighth layer (concentration pro- polyfunctionality of the macromolecules multilayer structure of such films, which files not indicated in Fig. 3), the deuterium makes it possible to avoid the problem of contain hydrophobic PPV layers after elim- concentration profile deviates from a sinu- the required 100% yield in every chemi- ination, remains intact (104). soidal curve, and, as a consequence of the sorbed layer, which limits film buildup with anharmonic distortion of the profile, up to small bifunctional organics. Conclusions and Outlook four Bragg peaks are observed. These data Fullerenes such as C60 can be used as have been evaluated quantitatively (55). covalent building blocks in alternation with Layer-by-layer assembly by adsorption from Bragg peaks were also observed in films with polyamines (68), because their spherical solution is a general approach for the fabri- (A/B)n architecture (22, 34, 56), but in shape and sixfold reactivity toward amines cation of multicomponent films on solid these cases the B layers were composed of will always lead to a covalent attachment supports. Materials can be selected from a inorganic platelets or polymers with stabi- with the underlying layer (69) of poly- pool of small organic molecules, polymers, lized conformations that inhibit overlap amine, in which some of the amino-reactive natural proteins, inorganic clusters, clay with adjacent layers. sites of the C60 are exposed toward the film particles, and colloids. Although we have The film model as depicted in Fig. 3 is surface and thus allow covalent attachment only begun to explore useful combinations only valid for films composed of simple to the next polyamine layer. Another ad- of materials, the organization of different flexible polyelectrolytes that can form 1:1 vantage of fullerenes is that they provide elementary units in an ordered nanoscopic complexes. Films composed of more exotic scattering length density contrast in neu- device may lead to a kind of nanomachin- materials will exhibit more complicated tron reflectivity, and we have shown that ery like that envisioned by Feynman in the structures. A detailed neutron reflectivity covalent multilayers containing C60 are 1960s (105). One potential property of such study of interfacial roughness differences true superlattice structures (70). However, devices is a simple dynamic structure in between poly(styrene sulfonate) and sulfo- ionic interactions are the most versatile; which the distance between two layers of nated poly(aniline) has recently been car- they permit the use of water as a solvent, “hard” objects (colloids or proteins) is ad- on February 24, 2010 ried out (57). which is both environmentally attractive justed by controlling the degree of swelling The apparent macroscopic homogeneity and allows the use of charged biopolymers in an intermediate “soft” layer (polyelectro- of polyelectrolyte films can actually be an such as DNA (71, 72) as well as polyelec- lyte) simply by changing, for example, hu- advantage; for example, in optical applica- trolytes, proteins (31–33, 73), colloids (30, midity. In contrast to bulk systems, where tions, “fuzzy layered assemblies” do not have 31, 34, 37, 39, 74–77), and many other swelling could ruin the mechanical proper- any of the defects present in truly crystal- charged or chargeable materials. ties of a material, such dynamic structure line films. This film model of 50% overlap The incorporation of proteins in mul- control could lead to tunable diffraction or of layers of equal charge also explains the tilayer films may lead to the application of optical properties in nanofilm devices.

high conductivities found in multilayers polyelectrolyte multilayers as biosensors www.sciencemag.org composed of conducting polymers (58)be 78 79, 80 REFERENCES - ( ) or in biotechnology ( ); the ______cause the conducting layers are not isolated latter may even provide the base for new 1. R. Lakes, Nature 361, 511 (1993). from each other. developments in multistep chemical catal- 2. K. B. Blodgett, J. Am. Chem. Soc. 56, 495 (1934). ysis. A crucial point in this type of appli- 3. ࿜࿜࿜࿜ and I. Langmuir, Phys. Rev. 51, 964 (1937). Similar Systems and Potential cation will be the control of transport in 4. H. Kuhn and D. Mo¨ bius, Angew. Chem. Int. Ed. Engl. 10, 620 (1971). Applications multilayer films (72, 81, 82). Multilayer 5. O. Inacker, H. Kuhn, D. Mo¨ bius, G. Debuch, Z. microcapsules (83) may have biomedical Phys. Chem. Neue Folge 101, 337 (1976).

The concept of electrostatically driven as- applications as well. Multilayer films can 6. L. Netzer and J. Sagiv, J. Am. Chem. Soc. 105, 674 Downloaded from (1983). sembly of multilayer structures allows for also be fabricated on colloids, which may 7. G. Cao, H.-G. Hong, T. E. Mallouk, Acc. Chem. the incorporation of a wealth of different have implications for photovoltaics (84). Res. 25, 420 (1992). materials (59). However, polymers as mul- Other possible applications include the 8. G. Decher and J.-D. Hong, Makromol. Chem. Mac- tifunctional materials also offer the choice incorporation of dye molecules to tailor romol. Symp. 46, 321 (1991). 9. ࿜࿜࿜࿜, J. Schmitt, Thin Solid Films 210–211, 831 of building up layered structures through the optical properties of polyion films (1992). other types of interaction. A biologically (85–91). In the case of rodlike amphi- 10. G. Decher, in Templating, Self-Assembly and Self- interesting interaction is the one between philes carrying hydrophilic head groups at Organization,vol.9ofComprehensive Supramo- lecular Chemistry, J.-P. Sauvage and M. W. Hos- biotin and avidin, and we have used this for both ends and a central diacetylene group, seini, Eds. (Pergamon, Oxford, 1996), 507–528. the formation of multilayers composed of multilayer systems may also have high in- 11. W. Knoll, Curr. Opin. Colloid Interface Sci. 1, 137 streptavidin and biotinylated poly(L-lysine) plane order, because the topochemical po- (1996). 12. M. A. C. Stuart, G. J. Fleer, J. Lyklema, W. Norde, (60), which were patterned by photoabla- lymerization of the diacetylene group only J. M. H. M. Scheutjens, Adv. Colloid Interface Sci. tion (61); similar systems were later used for works in a single crystal (56). 34, 477 (1991). the electrochemical sensing of glucose (62). The use of multilayers as gas separation 13. I. Haller, J. Am. Chem. Soc. 100, 8050 (1978). Polyelectrolyte multilayers have also been membranes is a currently developing tech 14. T. Afsharrad, A. I. Bailey, P. F. Luckham, W. Mac- - naughton, D. Chapman, Colloids Surf. 25, 263 fabricated on patterned surfaces (63–65). nology (92). As of today, the most ad- (1987). Film deposition is also possible with poly- vanced development of polyion-based films 15. P. Berndt, K. Kurihara, T. Kunitake, Langmuir 8, mer pairs that can form strong hydrogen- is probably their potential for the fabrica- 2486 (1992). 16. C.-G. Go¨ lander, H. Arwin, J. C. Eriksson, I. Lund- bond bridges (66) or by using polymer pairs tion of light-emitting diodes (49, 93–103). strom, R. Larsson, Colloids Surf. 5, 1 (1982). containing side groups with carbazole and This interest was kindled by the demonstra- 17. R. K. Iler, J. Colloid Interface Sci. 21, 569 (1966). 1236 SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 ⅐ www.sciencemag.org ARTICLES

18. P. Fromherz, in Electron Microscopy at Molecular 61. G. Decher, B. Lehr, K. Lowack, Y. Lvov, J. Schmitt, 12879 (1995). Dimensions, W. Baumeister and W. Vogell, Eds. Biosensors Bioelectronics 9, 677 (1994). 85. X. Zhang, M. Gao, X. Kong, Y. Sun, J. Shen, J. (Springer-Verlag, Berlin, 1980), 338–349. 62. P.-G. He et al., Mater. Sci. Eng. C2, 103 (1994). Chem. Soc. Chem. Commun. 1994, 1055 (1994). 19. H. E. Ries and B. L. Meyers, J. Appl. Polym. Sci. 15, 63. P. T. Hammond and G. M. Whitesides, Macromol- 86. T. M. Cooper, A. L. Campbell, R. L. Crane, Lang- 2023 (1971). ecules 28, 7569 (1995). muir 11, 2713 (1995). ¨ 20. R. Aksberg and L. Odberg, Nord. Pulp Pap. Res. J. 64. T. G. Vargo et al., Supramol. Sci. 2, 169 (1996). 87. A. Laschewsky et al., Thin Solid Films 284–285, 5, 168 (1990). 65. S. L. Clark, M. Montague, P. T. Hammond, ibid. 4, 334 (1996). 21. Self-assembly deposition device (Riegler & Kirstein, 141 (1997). 88. J. K. Lee, D. S. Yoo, E. S. Handy, M. F. Rubner, Wiesbaden, Germany). 66. W. B. Stockton and M. F. Rubner, Macromolecules Appl. Phys. Lett. 69, 1686 (1996). 22. A. Laschewsky, Eur. Chem. Chron. 2, 13 (1997). 30, 2717 (1997). 89. Y. Sun et al., J. Chem. Soc. Chem. Commun. 1996, 23. Y. M. Lvov and G. Decher, Crystallogr. Rep. 39, 67. Y. Shimazaki, M. Mitsuishi, S. Ito, M. Yamamoto, 2379 (1996). 628 (1994). Langmuir 13, 1385 (1997). 90. F. Saremi, G. Lange, B. Tieke, Adv. Mater. 8, 923 24. N. G. Hoogeveen, M. A. C. Stuart, G. Fleer, M. R. 68. G. Decher, Nachr. Chem. Tech. Lab. 41, 793 (1996). Bo¨ hmer, Langmuir 12, 3675 (1996). (1993). 25. R. Advincula, E. Aust, W. Meyer, W. Knoll, ibid.,p. 69. K. Chen, W. B. Caldwell, C. A. Mirkin, J. Am. Chem. 91. K. Ariga, Y. Lvov, T. Kunitake, J. Am. Chem. Soc. 3536. Soc. 115, 1193 (1993). 119, 2224 (1997). 26. D. Laurent and J. B. Schlenoff, ibid. 13, 1552 70. J. Schmitt, thesis, Johannes Gutenberg-Universi- 92. P. Stroeve, V. Vasques, M. A. N. Coelho, J. F. (1997). ta¨ t, Mainz, Germany (1996). Rabolt, Thin Solid Films 284–285, 708 (1996). 27. G. B. Sukhorukov, J. Schmitt, G. Decher, Ber. Bun- 71. Y. Lvov, G. Decher, G. Sukhorukov, Macromole- 93. M. Ferreira, M. F. Rubner, B. R. Hsieh, Mater. Res. senges. Phys. Chem. 100, 948 (1996). cules 26, 5396 (1993). Soc. Symp. Proc. 328, 119 (1994). 28. J. J. Ramsden, Y. M. Lvov, G. Decher, Thin Solid 72. G. B. Sukhorukov, H. Mo¨ hwald, G. Decher, Y. M. 94. J. Tian et al., Polym. Prepr. Am. Chem. Soc. Div. Films 254, 246 (1995). Lvov, Thin Solid Films 284–285, 220 (1996). Polym. Chem. 35, 761 (1994). 29. R. G. Freeman et al., Science 267, 1629 (1995). 73. Y. Lvov, K. Ariga, T. Kunitake, Chem. Lett. 1994, 95. A. C. Fou, O. Onitsuka, M. Ferreira, M. F. Rubner, 30. J. Schmitt et al., Adv. Mater. 9, 61 (1997). 2323 (1994). B. R. Hsieh, Mater. Res. Soc. Symp. Proc. 369, 31. S. W. Keller, H.-N. Kim, T. E. Mallouk, J. Am. Chem. 74. M. Gao et al., J. Chem. Soc. Chem. Commun. 575 (1995). Soc. 116, 8817 (1994). 1994, 2777 (1994). 96. M. Onoda and K. Yoshino, J. Appl. Phys. 78, 4456 32. W. Kong et al., Makromol. Chem. Rapid Commun. 75. N. A. Kotov, I. De´ka´ ny, J. H. Fendler, J. Phys. (1995). 15, 405 (1994). Chem. 99, 13065 (1995). 97. J. Tian et al., Adv. Mater. 7, 395 (1995). 33. Y. Lvov, K. Ariga, T. Kunitake, J. Am. Chem. Soc. 76. ࿜࿜࿜࿜, Adv. Mater. 8, 637 (1996). 98. J. Tian, C. C. Wu, M. E. Thompson, J. C. Sturm, 117, 6117 (1995). 77. K. Ariga, Y. Lvov, M. Onda, I. Ichinose, T. Kunitake, R. A. Register, Chem. Mater. 7, 2190 (1995). 34. E. R. Kleinfeld and G. S. Ferguson, Science 265, Chem. Lett. 1997, 125 (1997). 99. A. C. Fou, O. Onitsuka, M. Ferreira, M. F. Rubner, 370 (1994). 78. Y. Sun, X. Zhang, C. Sun, B. Wang, J. Shen, Mac- B. R. Hsieh, J. Appl. Phys. 79, 7501 (1996). 35. G. S. Ferguson and E. R. Kleinfeld, Adv. Mater. 7, romol. Chem. Phys. 197, 147 (1996). 100. H. Hong et al., ibid., p. 3082. 414 (1995). 79. M. Onda, Y. Lvov, K. Ariga, T. Kunitake, Biotechnol. 36. E. R. Kleinfeld and G. S. Ferguson, Chem. Mater. 7, Bioeng. 51, 163 (1996). 101. B. Lehr, M. Seufert, G. Wenz, G. Decher, Su- 2327 (1995). 80. ࿜࿜࿜࿜, J. Ferment. Bioeng. 82, 502 (1996). pramol. Sci. 2, 199 (1996). 37. Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, Langmuir 81. R. von Klitzing and H. Mo¨ hwald, Thin Solid Films 102. O. Onitsuka, A. C. Fou, M. Ferreira, B. R. Hsieh,

12, 3038 (1996). 284–285, 352 (1996). M. F. Rubner, J. Appl. Phys. 80, 4067 (1996). on February 24, 2010 38. Y. Lvov et al., ibid. 10, 4232 (1994). 82. ࿜࿜࿜࿜, Macromolecules 29, 6901 (1996). 103. H. Hong et al., Supramol. Sci. 4, 67 (1997). 39. D. L. Feldheim, K. C. Grabar, M. J. Natan, T. E. 83. R. Pommersheim, J. Schrenzenmeir, W. Vogt, 104. B. Lehr, J. Schmitt, R. Oeser, G. Decher, in Mallouk, J. Am. Chem. Soc. 118, 7640 (1996). Macromol. Chem. Phys. 195, 1557 (1994). preparation. 40. B. Philipp, H. Dautzenberg, K.-J. Linow, J. Ko¨ tz, W. 84. S. W. Keller, S. A. Johnson, E. S. Brigham, E. H. 105. R. P. Feynman, in Miniaturization, H. D. Gilbert, Ed. Dawydoff, Prog. Polym. Sci. 14, 91 (1989). Yonemoto, T. E. Mallouk, J. Am. Chem. Soc. 117, (Reinhold, New York, 1961), pp. 282–296. 41. Z. Wang, T. Lan, T. J. Pinnavaia, Chem. Mater. 8, 2200 (1996). 42. W. L. Ijdo, T. Lee, T. J. Pinnavaia, Adv. Mater. 8,79 (1996). 43. T. J. Pinnavaia, T. Lan, Z. Wang, H. Shi, P. D. Computational Design of Kaviratna, in Nanotechnology: Molecularly De- signed Materials, vol. 622 of ACS Symposium Se- www.sciencemag.org ries (American Chemical Society, Washington, DC, Hierarchically Structured 1996), pp. 250–261. 44. T. Lan and T. J. Pinnavaia, Mater. Res. Soc. Symp. Proc. 435, 79 (1996). Materials 45. R. Krishnamoorti, R. A. Vaia, E. P. Giannelis, Chem. Mater. 8, 1728 (1996). 46. E. P. Giannelis, Adv. Mater. 8, 29 (1996). G. B. Olson 47. G. Decher and J. Schmitt, Prog. Colloid Polym. Sci. 89, 160 (1992).

48. Y. Lvov, G. Decher, H. Mo¨ hwald, Langmuir 9, 481 A systems approach that integrates processing, structure, property, and performance Downloaded from (1993). relations has been used in the conceptual design of multilevel-structured materials. For 49. H. Hong et al., Adv. Mater. 7, 846 (1995). 50. J. Schmitt et al., Macromolecules 26, 7058 (1993). high-performance alloy steels, numerical implementation of materials science principles 51. D. Korneev, Y. Lvov, G. Decher, J. Schmitt, S. provides a hierarchy of computational models defining subsystem design parameters Yaradaikin, Physica B 213–214, 954 (1995). that are integrated, through computational thermodynamics, in the comprehensive de- 52. G. Decher, Y. Lvov, J. Schmitt, Thin Solid Films 244, 772 (1994). sign of materials as interactive systems. Designed properties combine strength, tough- 53. H. Dautzenberg, J. Hartmann, S. Grunewald, F. ness, and resistance to impurity embrittlement. The methods have also been applied to Brand, Ber. Bunsenges. Phys. Chem. 100, 1024 nonferrous metals, ceramics, and polymers. (1996). 54. W. Chen and T. J. McCarthy, Macromolecules 30, 78 (1997). 55. M. Lo¨ sche, J. Schmitt, G. Decher, W. G. Bouw- man, K. Kjær, in preparation. 56. F. Saremi, E. Maassen, B. Tieke, G. Jordan, W. For millennia, materials have been devel- marily been to explain the products of em- Rammensee, Langmuir 11, 1068 (1995). oped through the empirical correlation of piricism after their development. In the past 57. G. J. Kellog et al., ibid. 12, 5109 (1996). processing and properties. The past century decade, the numerical implementation of 58. A. C. Fou and M. F. Rubner, Macromolecules 28, has seen the formation of a science of ma- materials science principles and the integra- 7115 (1995). 59. G. Decher, in The Polymeric Materials Encyclope- terials that has defined the structural basis tion of resulting computational capabilities dia: Synthesis, Properties, and Applications,J.C. of materials behavior, but its role has pri- within a systems engineering framework has Salamone, Ed. (CRC Press, Boca Raton, FL, 1996), given birth to a revolutionary approach (1) vol. 6, 4540. The author is in the Department of Materials Science and 60. J.-D. Hong, K. Lowack, J. Schmitt, G. Decher, Engineering, Northwestern University, 2225 North Cam- in the form of quantitative conceptual de- Prog. Colloid Polym. Sci. 93, 98 (1993). pus Drive, Evanston, IL 60208–3108, USA. sign of materials. www.sciencemag.org ⅐ SCIENCE ⅐ VOL. 277 ⅐ 29 AUGUST 1997 1237