Self-Assembly at All Scales PDF

S U P R A M O L E C U L A R C H E M I S T R Y A N D S E L F- A S S E M B L Y 8. C. Alexander, C. P. Jariwala, C. M. Lee, A. C. Griffin, 26. N. Mizoshita et al., Chem. Commun. 2002, 428 44. P. Bonhôte, A.-P. Dias, N. Papageorgiou, K....

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Eng. 40, 4425 (2001). Education, Culture, Sports, Science, and Technology. VIEWPOINT Self-Assembly at All Scales George M. Whitesides* and Bartosz Grzybowski Self-assembly is the autonomous organization of components into pat- pate energy. For example, molecular crystals terns or structures without human intervention. Self-assembling processes (4, 5) are formed by static self-assembly; so are common throughout nature and technology. They involve components are most folded, globular proteins. In static from the molecular (crystals) to the planetary (weather systems) scale and self-assembly, formation of the ordered struc- many different kinds of interactions. The concept of self-assembly is used ture may require energy (for example in the increasingly in many disciplines, with a different flavor and emphasis in form of stirring), but once it is formed, it is each. stable. Most research in self-assembly has focused on this static type In dynamic self-assembly (D) ( Table 1; There are several reasons for interest in self- ponents. It thereby connects reductionism to Fig. 2), the interactions responsible for the assembly (1, 2). First, humans are attracted complexity and emergence (3). formation of structures or patterns between by the appearance of order from disorder. components only occur if the system is Second, living cells self-assemble, and under- Is Anything Not Self-Assembly? dissipating energy. The patterns formed by standing life will therefore require under- “Self-assembly” is not a formalized subject, competition between reaction and diffusion standing self-assembly. The cell also offers and definitions of the term “self-assembly” in oscillating chemical reactions (6, 7 ) are countless examples of functional self-assem- seem to be limitlessly elastic. As a result, the simple examples; biological cells are much bly that stimulate the design of non-living term has been overused to the point of cliché. more complex ones. The study of dynamic systems. Third, self-assembly is one of the Processes ranging from the non-covalent as- self-assembly is in its infancy. few practical strategies for making ensembles sociation of organic molecules in solution to We define two further variants of self- of nanostructures. It will therefore be an es- the growth of semiconductor quantum dots assembly. In templated self-assembly (T), in- sential part of nanotechnology. Fourth, man- on solid substrates have been called self- teractions between the components and reg- ufacturing and robotics will benefit from ap- assembly. Here, we limit the term to process- ular features in their environment determine plications of self-assembly. Fifth, self-assem- es that involve pre-existing components (sep- the structures that form. Crystallization on bly is common to many dynamic, multicom- arate or distinct parts of a disordered struc- surfaces that determine the morphology of ponent systems, from smart materials and ture), are reversible, and can be controlled by the crystal is one example (8, 9); crystalliza- self-healing structures to netted sensors and proper design of the components. “Self-as- tion of colloids in three-dimensional optical computer networks. Finally, the focus on sembly” is thus not synonymous with fields is another (10). The characteristic of spontaneous development of patterns bridges “formation.” biological self-assembly (B) is the variety the study of distinct components and the and complexity of the functions that it study of systems with many interacting com- Types of Self-Assembly produces. Department of Chemistry and Chemical Biology, Har- There are two main kinds of self-assembly: vard University, Cambridge, MA 02138, USA. static and dynamic. Static self-assembly (S) Common Features of Self-Assembly *To whom correspondence should be addressed. E- (Table 1; Fig. 1) involves systems that are at Self-assembly reflects information coded (as mail: [email protected] global or local equilibrium and do not dissi- shape, surface properties, charge, polarizabil- 2418 29 MARCH 2002 VOL 295 SCIENCE www.sciencemag.org  S U P R A M O L E C U L A R C H E M I S T R Y A N D S E L F- A S S E M B L Y ity, magnetic dipole, mass, etc.) in individual teractions, hydrogen and coordination environment can modify the interactions be- components; these characteristics determine bonds). In the self-assembly of larger com- tween the components; the use of boundaries the interactions among them. The design of ponents—meso- or macroscopic objects—in- and other templates in self-assembly is par- components that organize themselves into de- teractions can often be selected and tailored, ticularly important, because templates can re- sired patterns and functions is the key to and can include interactions such as gravita- duce defects and control structures. applications of self-assembly. tional attraction, external electromagnetic Equilibration is usually required to reach The components must be able to move fields, and magnetic, capillary, and entropic ordered structures. If components stick to- with respect to one another. Their steady- interactions, which are not important in the gether irreversibly when they collide, they state positions balance attractions and repul- case of molecules. form a glass rather than a crystal or other sions. Molecular self-assembly involves non- Because self-assembly requires that the regular structure. Self-assembly requires that covalent or weak covalent interactions (van components be mobile, it usually takes place the components either equilibrate between der Waals, electrostatic, and hydrophobic in- in fluid phases or on smooth surfaces. The aggregated and non-aggregated states, or ad- just their positions relative to one another once in an aggregate. Fig. 1. Examples of static Dynamic Self-Assembly self-assembly. (A) Crystal structure of a ribosome. (B) Although much of current understanding Self-assembled peptide- of self-assembly comes from the examination amphiphile nanofibers. (C) of static systems, the greatest challenges, and An array of millimeter- opportunities, lie in studying dynamic sys- Downloaded from http://science.sciencemag.org/ on August 25, 2018 sized polymeric plates as- tems. Perhaps the most important justifica- sembled at a water/perflu- orodecalin interface by tion for studying self-assembly is its central capillary interactions. (D) role in life. The components of a cell replicate Thin film of a nematic liq- and assemble into another cell during mitosis; uid crystal on an isotropic bacteria swarm (11); fish school (12, 13). substrate. (E) Micrometer- Most efforts in biology have focused on static sized metallic polyhedra self-assembly. Life is, however, dynamic: folded from planar sub- strates. (F) A three-dimen- stop the flux of energy through the cell and it sional aggregate of micro- dies. meter plates assembled by We understand that the living cell is a capillary forces. [Image sack that contains a number of reacting chem- credits: (A) from (24); (B) icals, is studded with environmental sensors, from (25); (C) from (26); and allows heat and certain chemicals to pass (D) from (27); (E) from (28); (F) from (29)] across its walls. We also understand that the cell is a structure that is enclosed, self-repli- cating, energy dissipating, and adaptive. Yet we have little idea how to connect these two sets of characteristics. How does “life” emerge from a system of chemical reactions? Self-assembly may be one thread that con- nects the relative simplicity of chemical re- actions to the complexity of the dividing cell. At the molecular level, static self-assembly describes formation of the lipid bilayer, pair- ing of bases, and folding of some proteins. The behavior of critical structures in the Table 1. Examples of self-assembly (S, static, D, dynamic, T, templated, B, biological). System Type Applications/importance References Atomic, ionic, and molecular crystals S Materials, optoelectronics (1, 4, 5) Phase-separated and ionic layered polymers S (19) Self-assembled monolayers (SAMs) S, T Microfabrication, sensors, nanoelectronics (8) Lipid bilayers and black lipid films S Biomembranes, emulsions (20) Liquid crystals S Displays (21) Colloidal crystals S Band gap materials, molecular sieves (9, 18) Bubble rafts S Models of crack propagation (22) Macro- and mesoscopic structures (MESA) S or D, T Electronic circuits (14 –16) Fluidic self-assembly S, T Microfabrication (23) “Light matter” D, T (10) Oscillating and reaction-diffusion reactions D Biological oscillations (6, 7) Bacterial colonies D, B (11) Swarms (ants) and schools (fish) D, B New models for computation/optimization (12, 13) Weather patterns D (1) Solar systems D Galaxies D www.sciencemag.org SCIENCE VOL 295 29 MARCH 2002 2419  S U P R A M O L E C U L A R C H E M I S T R Y A N D S E L F- A S S E M B L Y cell—including actin filaments, histones and mitosis involve every type of self-assembly. nonliving systems, although these processes chromatin, and protein aggregates in signal- A hierarchy of self-assembling processes is are less studied, and less understood, than ing pathways—involves dynamic self-assem- thus fundamental to the operation of cell. those in living systems. Oscillating reactions bly. The complex processes that occur in Dynamic self-assembly is also common in in solution and on the surface of catalysts, Rayleigh-Bernard convection cells, patterns that form in fluidized beds of particles, and storm cells in the atmosphere are all exam- Fig. 2. Examples of dynamic self-assembly. (A) An optical ples; Table 1 lists others. micrograph of a cell with fluo- rescently labeled cytoskeleton Self-Assembly in Designed Systems and nucleus; microtubules A difficulty in studying self-assembly in liv- ("24 nm in diameter) are col- ing cells (and in many nonliving systems) is ored red. (B) Reaction-diffu- that it is impractical to change many of the sion waves in a Belousov-Zab- atinski reaction in a 3.5-inch parameters that determine the behavior of the Petri dish. (C) A simple aggre- systemthe components and the interactions gate of three millimeter-sized, among them—and thus difficult to test hy- rotating, magnetized disks in- potheses relating structures and properties of teracting with one another via these components and the aggregates that vortex-vortex interactions. (D) they form. We wished to have available a set A school of fish. (E) Concentric of self-assembling components in which Downloaded from http://science.sciencemag.org/ on August 25, 2018 rings formed by charged me- tallic beads 1 mm in diameter these parameters could be changed easily, in rolling in circular paths on a order to understand (and to be able to manip- dielectric support. (F) Convec- ulate) the processes by which components tion cells formed above a mi- self-assembled into aggregates. With this ob- cropatterned metallic support. jective, we have studied the self-assembly of The distance between the cen- ters of the cells is "2 mm. polyhedral plates or disks—a few millimeters [Image credits: (A) from (30); wide and a millimeter high—floating at the (B) from (26); (C) from (31)] interface between water and perfluorodecalin (14). These sizes are attractive because the components can be fabricated and observed easily, and because the interactions between these components are under precise experi- mental control. Static versions of this system depend on capillary interactions (15) be- tween menisci at the edges of the plates, and typically produce ordered aggregates with irregular edges. The processes observed at millimeter dimensions scale (with some mod- Fig. 3. Applications of self- ification) to submicron dimensions. Templat- assembly. (A) A 2 by 2 cross ing produces aggregates with defined shape. array made by sequential When drops of liquid are patterned on the assembly of n-type InP faces of components suspended in an immis- nanowires with orthogonal cible, isodense fluid, three-dimensional struc- flows. (B) Diffraction grat- ing formed on the surface tures can be generated. If this liquid is solder, of a poly(dimethylsiloxane) cooling forms interconnections that are me- sphere "1 mm in diameter. chanically strong and electrically conducting. The sphere was compressed This type of system points toward functional, between two glass slides, self-assembling microelectronic systems and its free surface was ex- (16). posed to oxygen plasma. Upon release of compres- An extension of these static systems illus- sion, the oxidized surface of trates dynamic self-assembly. Small ferro- the polymer buckled with a magnetic disks, floating at the liquid inter- uniform wavelength of "20 face, rotate under the influence of a rotating #m. (C) Three-dimensional external bar magnet. The average field of this electronic circuits self-as- magnet generates a central field that pulls the sembled from millimeter- sized polyhedra with elec- disks together. As they spin, they generate tronic components (LEDs) vortices in the fluid; the vortex-vortex inter- embossed on their faces. (D) actions are repulsive. The spinning disks as- An artificial, ferromagnetic semble into a variety of stable patterns (17). opal prepared by templated self-assembly of polymeric Learning from One Another microbeads. The optical properties of the aggregate Different fields of science take different can be adjusted by modify- roads to understanding; each brings some- ing external magnetic field. thing to self-assembly. Chemists and engi- [Image credits: (A) from (32); (B) from (16); (C) from (26); (D) from (33)] neers tend to solve problems by designing 2420 29 MARCH 2002 VOL 295 SCIENCE www.sciencemag.org  S U P R A M O L E C U L A R C H E M I S T R Y A N D S E L F- A S S E M B L Y and synthesizing (or fabricating, or building) puter of great interest—the brain—is three- using this understanding to design nonbio- new systems; physicists observe existing sys- dimensional. There are no clear strategic logical mimics of them, will offer many tems; biologists make modifications by mix- paths from two-dimensional to three-di- opportunities to build systems with new ing preexisting parts. Each style will be im- mensional technology (and, of course, no types of function. In the emerging area of portant in some aspect of self-assembly. absolute certainty that three-dimensional dynamic self-assembly, it is unclear wheth- For self-assembly to generate structures microelectronic devices will be useful, al- er the study of molecules, or of other types more complex than simple crystals, different though the brain is certainly a three-dimen- of components, will lead more efficiently to components in a mixture must come together sional system, and three dimensionality of- understanding. We understand very little in an ordered way. The selective recognition fers, in principle, the advantages of short about how dissipation of energy leads to of different molecular components in a mix- interconnects and efficient use of volume). the emergence of ordered structures from ture is the basis for much of molecular biol- Self-assembly offers a possible route to disordered components in these systems. ogy and medicinal chemistry. The parameters three-dimensional microsystems. But we know that they are vitally important that control molecular recognition (comple- (5) Netted Systems. At the outer limits in the cell. That knowledge, by itself, mentary shapes, complementary forces, and of self-assembly, at least as it is currently makes it worthwhile to study them. appropriate levels of plasticity) will also be defined in the physical and biological sci- broadly useful in the self-assembly of larger ences, are netted systems: computers, sen- References and Notes 1. D. Philip, J. F. Stoddart, Angew. Chem. Int. Ed. 35, systems. sors, and controllers that interact with one 1155 (1996). Dynamic systems are an important part of another only through the flow of bits and 2. P. Ball, The Self-Made Tapestry: Pattern Formation in the future of self-assembly. Biology is replete configure (or self-assemble) themselves Nature (Oxford Univ. 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DOI: 10.1126/science.1070821 Downloaded from http://science.sciencemag.org/ on August 25, 2018 ARTICLE TOOLS http://science.sciencemag.org/content/295/5564/2418 REFERENCES This article cites 26 articles, 6 of which you can access for free http://science.sciencemag.org/content/295/5564/2418#BIBL PERMISSIONS http://www.sciencemag.org/help/reprints-and-permissions Use of this article is subject to the Terms of Service Science (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. 2017 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. The title Science is a registered trademark of AAAS.