Bottom-up fabrication: Self-assembly in biology

Self-assembly processes can occur spontaneously in nature, such as in cells and other biological systems, as well as in human-engineered systems. A self-assembly process usually results in an increase in internal organization of the system. Many biological systems use self-assembly to assemble various molecules and structures. Imitating these strategies and creating novel molecules with the ability to self-assemble into supramolecular assemblies is an important technique in nanotechnology. In self-assembly, the final (desired) structure is 'encoded' in the shape and properties of the molecules that are used, in contrast to traditional techniques, such as lithography, where the desired final structure must be carved out from a larger block of matter.

One example of a biology-based self-assembly process is the successful assembly of 3-D multi-component nanoscale structures by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. These structures incorporated light-absorbing and emitting particles that allowed tunable optical properties. In this work, illustrated in Figure 1, the scientists used DNA linkers with three binding sites (black “strings”) to connect gold nanoparticles (orange and red spheres) and fluorescent dye molecules (blue spheres) tagged with complementary DNA sequences. The DNA linker molecules had three binding sites. The two ends of the strands were designed to bind to complementary strands on plasmonic gold nanoparticles — particles in which a particular wavelength of light induces a collective oscillation of the conductive electrons, leading to strong absorption of light at that wavelength. The internal part of each DNA linker was coded to recognize a complementary strand chemically bound to a fluorescent dye molecule. This setup resulted in the self-assembly of 3-D body centered cubic crystalline structures with gold nanoparticles located at each corner of the cube and in the center, with dye molecules at defined positions in between.

Figure 1 - Multi-component nano-structures with tunable optical properties. Image courtesy of Brookhaven National Laboratory.

Following is short video that demonstrates the self-assembly of binary virus-gold nanoparticle superlattices.