Different nanoparticle shapes and sizes are needed for different applications. For example, large surface areas are needed for catalysts and long chains are required for antennae.  

In the laboratory, nanoparticle shape and size has been generally controlled via chemical synthesis, a process that is based on a nucleation step followed by crystal growth. This process can lead to irregular shapes and sizes, and can be difficult to control.

Nature is much better at chemistry than people, and perhaps unsurprisingly some species of bacteria are known to produce carefully designed iron oxide nanoparticles.

One such species is called Magnetospirillum magneticum. They control the size of the nanoparticles by seeding and building them inside membrane bound “nanoreactors”. Through precise control over the crystal growth process, these membrane-bound nanoparticles become strongly magnetic, and as a result, the organisms can orient themselves with respect to earth’s magnetic field.

For the last two decades, researchers have been interested in manipulating the genetic code of these bacteria to exert control over the types of nanoparticles that are produced. Recent work by an American team looked at a new technique for gene expression manipulation in this context.

Most work on altering the genetics of Magnetospirillum magneticum has focused on gene knockouts. In contrast, this work introduced copies of relevant genes into the bacteria in the form of plasmids. These plasmids also contained promoter sequences that could be activated by small molecules, allowing gene expression to be easily up up-regulated.

The team identified a range of genes of interest. Over-expression of most of these genes led to little impact on nanoparticle dimensions. However, one gene was found that had a substantial effect. Over-expression of MamC lead to a mixture of spheres and cubes, without any elongated particles.

In such a complex system, the scope for further genetic tweaks to Magnetospirillum magneticum is massive. Expression timing and checkpoint control are crucial parts of the biological nanoparticle design process, and as such, one area of particular interest going forward with this research will be the design of temporal circuits that could control the order of gene expression.

This work offers a tantalizing glimpse into the capability of new genetic engineering techniques. As the field continues to grow and evolve, the fine control over these types of biological mechanisms becomes ever more precise. The application of these techniques to problems like this will be a fascinating area to keep an eye on for years to come.

Reference: M. Furubayashi et. al, Genetic Tuning of Iron Oxide Nanoparticle Size, Shape, and Surface Properties in Magnetospirillum magneticum, Advanced Functional Materials, (2020). DOI: 10.1002/adfm.202004813