Describing and controlling the material’s structure-function relationship has been the Holy Grail of materials science through the years. Nanoparticles are an appealing group of components to be used in functional materials because they display size-dependent properties, such as superparamagnetism and plasmonic absorption of light.
Moreover, manipulating the arrangement of nanoparticles can result in unexpected properties, but such studies are hard to conduct because of limited efficient methods to create well-defined 3D nanostructures.
According to Researchers from the Biohybrid Materials Group, led by Professor Mauri Kostiainen, nature’s own charged nanoparticles – protein cages and viruses – can be exploited to define the structure of composite nanomaterials.
Proteins and viruses are perfect model particles to be used in materials science, as they are genetically encoded and possess an atomically precise structure. These distinct biological particles can be used to control the arrangement of other nanoparticles in an aqueous solution. In the current study, the team shows that joining native Tobacco Mosaic Virus with gold nanoparticles in a controlled manner results in metal-protein superlattice wires.
“We initially studied geometrical aspects of nanoparticle superlattice engineering. We hypothesized that the size-ratio of oppositely charged nanorods (TMV viruses) and nanospheres (gold nanoparticles) could efficiently be used to control the two-dimensional superlattice geometry. We were actually able to demonstrate this. Even more interestingly, our structural characterization revealed details about the cooperative assembly mechanisms that proceeds in a zipper-like manner, leading to high-aspect-ratio superlattice wires,” Kostiainen says. “Controlling the macroscopic habit of self-assembled nanomaterials is far from trivial,” he adds.
Superlattice wires potential to form new materials
The results revealed that nanoscale interactions truly control the macroscopic habit of the created superlattice wires. The Researchers noticed that the created macroscopic wires experience a right-handed helical twist that was explained by the electrostatic pull between the asymmetrically patterned TMV virus and the oppositely charged spherical nanoparticles. As plasmonic nanostructures efficiently impact the propagation of light, the helical twisting caused asymmetric optical properties (plasmonic circular dichroism) of the material.
This result is ground breaking in the sense that it demonstrates that macroscopic structures and physical properties can be determined by the detailed nanostructure, i.e. the amino acid sequence of the virus particles. Genetical engineering routinely deals with designing the amino acid sequence of proteins, and it is a matter of time when similar or even more sophisticated macroscopic habit and structure-function properties are demonstrated for ab-initio designed protein cages.
Dr. Ville Liljeström, involved in the project
The research team showed a proof-of-concept revealing that the superlattice wires can be used to create materials with physical properties manipulated by external fields. The wires could be aligned by a magnetic field by functionalizing the superlattice wires with magnetic nanoparticles. In this manner they created plasmonic polarizing films. The purpose of the demonstration was to exhibit that electrostatic self-assembly of nanoparticles can possibly be used to create processable materials for future applications.
Research article: Liljeström, V.; Ora, A.; Hassinen, J.; Rekola, H.; Nonappa; Heilala, M.; Hynninen, V.; Joensuu, J.; Ras, R. H. A.; Törmä, P.; Ikkala, O.; Kostiainen, M. A. Cooperative Colloidal Self-Assembly of Metal-Protein Superlattice Wires. Nature Communications 8, 2017. DOI: 10.1038/s41467-017-00697-z.