The Magic of Self-Assembly at the Nanoscale

Sketches show the self-assembly of octapodal nanoparticles.
(Left) Sketch of two approaching octapods. For each octapod, the tip-to-tip length is 2L, and the pod diameter is Dp. (Right) I. The two lowest energy configurations of an octapod–octapod dimer in bulk solution, as determined from theoretical calculations of the vdW interactions: (I-1) “interlocked” configuration and (I-2) pod–pod parallel configuration. Both configurations are involved in the experimentally observed formation of 3D octapod superstructures in solution. II. The experimentally observed (and calculated) 2D configurations of octapods on a flat substrate after solvent evaporation: Square-lattice (II-1) and binary square lattice (II-2).

Have you ever thought a little magic could come in handy? For instance, picking up after a children’s play date would be so much easier with the “reparo” spell from the Harry Porter movies!

The next best thing might be self-assembly, which is when disordered components spontaneously form an organized structure, using interactions among themselves without external force. Self-assembly is especially helpful on the nanoscale, with objects that are way too small for meaningful forced organization.

Dynamic molecular self-assembly is a favorite technique of Mother Nature, starting with reversible assembly and dissociation of DNA structures during replication, transcription, and translation — in fact, it’s used in the majority of the processes in a living organism. Inspired by nature, nanotechnology uses the principles and components of molecular self-assembly to build “bigger” nanostructures. Along with nanoparticles, which are used as building blocks, polymers are often used as a glue to convert noncovalent, hydrophobic, hydrogen, ionic, and and Van der Waals forces into a strong glue to hold together structures according the polymer structure.

Possibilities Almost Endless

Using different types of nanoparticles of different shapes and materials already offers numerous possibilities, but when you factor in the variety of polymers that can be used either to modify the surface of nanoparticles or to interact with them in an engineered way, the possibilities become almost endless. Currently, the nanoscience of self-assembly is about 10 years old and is experiencing rapid growth, with almost 500 articles published in 2013 alone.

With advances of nanotechnology in medicine and the use of nanoparticles for drug delivery, understanding and controlling the processes of self-assembly could open ways to manipulate nanoscale cargo in the body with high precision. In addition, with advances in nanolithography and the variety of available or easy-to-synthesize nanoparticles, controlled self-assembly could enable many revolutionary colorimetric sensing and nanoelectronic applications.

Some recent examples of nanoparticle polymer-mediated self-assembly include:

  1. Gold and silver spherical and triangular nanoparticles spontaneously assembled in discrete, bifurcated, and looped nano chains in the presence of polyamine ligands in solution: Because these  assemblies convey a characteristic color to the nanoparticle solution, the process could be used for heavy metal sensing.
  2. CdSe/CdS octapod-shape nanoparticles (imagine a 3D snowflake with eight legs): When dried in the presence of polymethyl methacrylate, they assemble into beautiful, orderly hexagonal “ballerina networks” on the air-surface interface.
  3. Amazing nano chainmail structures formed according to theoretical design from ligand-stabilized platinum and gold nanoparticles and poly(isoprene-block-styrene-block-(N,N-dimethylamino ethyl methacrylate) polymer as structure-directing agent.

Adding theory to nanoparticle self-assembly transforms the process from passive observation into an active stage of design. Anne Ju from Cornell University News Office describes:

For close to two decades, Cornell University scientists have developed processes for using polymers to self-assemble inorganic nanoparticles into porous structures that could revolutionize electronics, energy and more. This process has now been driven to an unprecedented level of precision using metal nanoparticles, and is supported by rigorous analysis of the theoretical details behind why and how these particles assemble with polymers. Such a deep understanding of the complex interplay between the chemistry and physics that drive complex self-assembly paves the way for these new materials to enter many applications, from electrocatalysis in fuel cells to voltage conductance in circuits.

We can agree with Arthur C. Clarke, a British science fiction writer, who said: “Any sufficiently advanced technology is indistinguishable from magic.”

Image from “Self-Assembly of Octapod-Shaped Colloidal Nanocrystals into a Hexagonal Ballerina Network Embedded in a Thin Polymer Film,” via Creative Commons license.
Source: “Nanoparticle Networks’ Design Enhanced by Theory,” by Anne Ju, Cornell University News Office, labmanager.com, March 3, 2014.
Source: “Linking Experiment and Theory for Three-dimensional Networked Binary Metal Nanoparticle-triblock Terpolymer Superstructures,” by Zihui Li, Kahyun Hur, Hiroaki Sai, Takeshi Higuchi, Atsushi Takahara, Hiroshi Jinnai, Sol M. Gruner, and Ulrich Wiesner, Nature Communications 5, 3247 (2014), doi:10.1038/ncomms4247.
Source: “Polyamine Ligand-Mediated Self-Assembly of Gold and Silver Nanoparticles into Chainlike Structures in Aqueous Solution: Towards New Nanostructured Chemosensors,” by Adrián Fernández-Lodeiro, Javier Fernández-Lodeiro, Cristina Núñez, Rufina Bastida, José Luis Capelo, and Carlos Lodeiro, ChemistryOpen. Dec 2013; 2(5-6): 200–207, doi: 10.1002/open.201300023, Aug 2, 2013.
Source: “Self-Assembly of Octapod-Shaped Colloidal Nanocrystals into a Hexagonal Ballerina Network Embedded in a Thin Polymer Film,” by Milena P. Arciniegas, Mee R. Kim, Joost De Graaf, Rosaria Brescia, Sergio Marras, Karol Miszta, Marjolein Dijkstra, René van Roij, and Liberato Manna, Nano Letters, 2014, 14 (2), pp 1056–1063, DOI: 10.1021/nl404732m, January 21, 2014.

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