Size limits of self-assembled colloidal structures made using specific interactions

2016

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Controlling motion at the microscopic scale is a fundamental goal in the development of biologically inspired systems. We show that the motion of active, self-propelled colloids can be sufficiently controlled for use as a tool to assemble complex structures such as braids and weaves out of microscopic filaments. Unlike typical self-assembly paradigms, these structures are held together by geometric constraints rather than adhesive bonds. The out-of-equilibrium assembly that we propose involves precisely controlling the 2D motion of active colloids so that their path has a nontrivial topology. We demonstrate with proof-ofprinciple Brownian dynamics simulations that, when the colloids are attached to long semiflexible filaments, this motion causes the filaments to braid. The ability of the active particles to provide sufficient force necessary to bend the filaments into a braid depends on a number of factors, including the self-propulsion mechanism, the properties of the filament, and the maximum curvature in the braid. Our work demonstrates that nonequilibrium assembly pathways can be designed using active particles.colloidal machines | active colloids | self-assembly | braids and weaves B iology has long been known to use a combination of programmable interactions with specific energy input through small molecules (ATP) to create remarkable machines. Because we have not yet developed the ability to emulate these machines artificially, understanding assembly pathways at the microscopic scale is a fundamental challenge in both biology and nanotechnology (1). However, recently there have been major advances in colloidal science, making it possible to program interactions between individual units. This has led to robust assembly of complex structures, approaching the complexity of biological systems. But biological systems also use specific energy input of small molecules to create machines. How can we do this with soft materials?A parallel recent development has been to use chemical reactions on the surface of colloids to create motility mechanisms. This has led to the growth of the field of active matter, which aims to design, control, and understand self-propelled particles (2-12). These active particles provide significant yet untapped potential: by combining this mechanism for energy input with controllable interactions, it becomes possible to imagine the creation of colloidal-scale machines that can do work and perform tasks.Here we present a proof-of-principle demonstration of how this could work. We show one way in which active colloids can be harnessed to drive the assembly of weaves and braids, turning long filaments into complex structures, using only active forces [e.g., hydrogen peroxide-propelled colloids (2-4)] and nonspecific short-range attractive forces (e.g., depletion forces). The typical self-assembly paradigm encodes information through the specific interactions between building blocks that in turn promote assembly. For example, a given amino acid sequence consistently folds into a specific pro...

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confidence: 99%

Using active colloids as machines to weave and braid on the micrometer scale

Goodrich

2016

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“…On the other hand, a cluster is characterized by the species of particles that comprise it, where the species of two particles determines the specific interaction between them. The number of different species of particles is not necessarily the same as N. It has been previously established how specific interactions can be chosen for any given geometry (8,16,24,25) and how self-assembly yield of clusters depends on the chosen interactions (8,24). In the present work, we do not consider self-assembly of clusters in a bath of particles (8,16,25,26); however, our ability to enumerate cluster geometries and choose interactions to stabilize any desired cluster is a critical ingredient for the discoveries reported here.…”

Section: Resultsmentioning

confidence: 99%

“…These stickers cause short-ranged, specific interactions between particles, with a programmable binding strength between pairs of particles. We have recently shown that this system has the potential of allowing high-yield equilibrium self-assembly of large structures, up to 1,000 particles (8). Classical experiments showed that DNA-based specific interactions allow assembly of crystals of nanoparticles (9)(10)(11)(12).…”

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confidence: 99%

Spontaneous emergence of catalytic cycles with colloidal spheres

Zeravcic

2017

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Colloidal particles endowed with specific time-dependent interactions are a promising route for realizing artificial materials that have the properties of living ones. Previous work has demonstrated how this system can give rise to self-replication. Here, we introduce the process of colloidal catalysis, in which clusters of particles catalyze the creation of other clusters through templating reactions. Surprisingly, we find that simple templating rules generically lead to the production of huge numbers of clusters. The templating reactions among this sea of clusters give rise to an exponentially growing catalytic cycle, a specific realization of Dyson's notion of an exponentially growing metabolism. We demonstrate this behavior with a fixed set of interactions between particles chosen to allow a catalysis of a specific sixparticle cluster from a specific seven-particle cluster, yet giving rise to the catalytic production of a sea of clusters of sizes between 2 and 11 particles. The fact that an exponentially growing cycle emerges naturally from such a simple scheme demonstrates that the emergence of exponentially growing metabolisms could be simpler than previously imagined.catalytic cycles | DNA-coated colloids | metabolism | templating T he origin of life is usually associated with self-replication, the ability of an entity to create copies of itself (1). Thus inspired, there have been many efforts in recent years aimed at creating artificial systems with self-replicative capacity. These are typically inspired by the linear chain mechanism of DNA. However, living systems are more than individual entities that can replicate themselves. Dyson (2) and Oparin (3) argued that a more critical aspect of living systems is the creation of a metabolism, a complex cascade of chemical reactions that together are able to accomplish more than any single chemical reaction can do on its own. Using a simple mathematical model, the so-called "garbage bag model," Dyson presented a scenario through which such a complex metabolism could arise spontaneously. His idea is that a random set of catalysts will catalyze arbitrary chemical reactions in a nonsynergistic fashion. However, if each catalyst is more likely to function when there are others that are synergistic with it, then there is a critical amount of cooperativity above which metabolic cycles spontaneously emerge. This idea has been explored through abstract simulations (4). In a similar spirit, Kauffman and coworkers have shown that if the probability of one species catalyzing the formation of another is above a threshold, then catalytic cycles naturally emerge (5-7).Whether this scenario can naturally occur in practice remains obscure. The fundamental question is to determine how likely it is for a soup of interacting catalysts to self-organize into an exponentially growing catalytic cycle. How special do the interactions between the different components need to be for spontaneous exponentially growing catalytic cycles to emerge? In this work, we present an explicit demon...

“…Such predictions are especially valuable, given the proliferation of different mechanisms for creating and combining distinct mechanisms of specificity: Mutual information provides an unbiased way of comparing their efficacy to each other. As programmable specificity continues to drive technological developments in self-assembly (41), understanding how the mutual information of paired components can be built up toward creating larger, multicomponent objects is a critical future direction of this work.…”

Section: Discussionmentioning

confidence: 99%

Information capacity of specific interactions

Huntley

Murugan

2016

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Specific interactions are a hallmark feature of self-assembly and signal-processing systems in both synthetic and biological settings. Specificity between components may arise from a wide variety of physical and chemical mechanisms in diverse contexts, from DNA hybridization to shape-sensitive depletion interactions. Despite this diversity, all systems that rely on interaction specificity operate under the constraint that increasing the number of distinct components inevitably increases off-target binding. Here we introduce "capacity," the maximal information encodable using specific interactions, to compare specificity across diverse experimental systems and to compute how specificity changes with physical parameters. Using this framework, we find that "shape" coding of interactions has higher capacity than chemical ("color") coding because the strength of off-target binding is strongly sublinear in bindingsite size for shapes while being linear for colors. We also find that different specificity mechanisms, such as shape and color, can be combined in a synergistic manner, giving a capacity greater than the sum of the parts.S pecific interactions between many species of components are the bedrock of biochemical function, allowing signal transduction along complex parallel pathways and self-assembly of multicomponent molecular machines. Inspired by their role in biology, engineered specific interactions have opened up tremendous opportunities in materials synthesis, achieving new morphologies of self-assembled structures with varied and designed functionality. The two major design approaches for programming specific interactions use either chemical specificity or shape complementarity.Chemical specificity is achieved by dividing binding sites into smaller regions, each of which can be given one of A "colors" or unique chemical identities. Sites bind to each other based on the sum of the interactions between corresponding regions. For example, a recent two-color system paints the flat surfaces of three-dimensional polyhedra with hydrophobic and hydrophilic patterns (1) or with a pattern of solder dots (2), allowing polyhedra to stick to each other based on the registry between their surface patterns. Another popular approach uses DNA hybridization, where specific matching of complementary sequences has been used to self-assemble structures purely from DNA strands (3, 4) and from nanoparticles coated with carefully chosen DNA strands (5-9).Shape complementarity uses the shapes of the component surfaces to achieve specific binding, even though the adhesion is via a nonspecific, typically short-range potential. In the synthetic context, shape-based modulation of attractive forces over a large dynamic range was first proposed and experimentally demonstrated for colloidal particles (10, 11), using tunable depletion forces (12, 13). Recent experiments have explored the range of possibilities opened up by such ideas, from lithographically designed planar particles (14) with undulating profile patterns to "Pacman" partic...