Chiral and chiroptical materials are routinely found in the animal world, with some well-known examples being butterfly wings and beetle shells. These sophisticated materials are biocomposites with helical structures arranged in a hierarchical fashion and self-assemble from primitive nanoscale building blocks. Such a process is simpler and much more energy efficient than many top-down industrial techniques in producing similarly sized 3D materials.

Inorganic nanoparticles modified with amino acids can mimic such biological materials, and these particles can also be chiral – that is, their mirror image cannot be superimposed on the original. Chirality is ubiquitous in living organisms and is important for virtually all biological functions.

A team of researchers led by Nicholas Kotov of the University of Michigan in the US has now made helical supraparticles by self-assembling chiral CdTe nanoparticles coated with cysteine around Te cores. Like their biological equivalents, these structures are chiroptical in the visible part of the electromagnetic spectrum.

Significant chiral bias

“There are multiple attractive forces between the nanoparticles that include van der Waal interactions and hydrogen bonding,” explains Kotov. “These forces are counteracted by electrostatic attractions, which leads to the particle agglomerates being self-limited to the size they can reach and to a specific diameter of the final rod-like assembly. When the particles come closer to each other, there is a significant chiral bias in their spatial orientation with respect to each other, depending on the chirality of the surface ligands – in our case, whether the ligand is L- or D-cysteine. This bias makes them come together in a helical fashion, rather like the way protein units assemble in a tobacco mosaic capsid.”

The parallels in the behaviour between inorganic nanoparticles and proteins opens up a number of venues for further research, he tells “One example will be to replicate the function of so-called enantiomeric catalysts in chiral nanoparticles for lowering the costs of drug production.”

The team, which includes researchers from the Wenzhou Institute of Biomaterials and Engineering in China, the Lawrence Berkeley National Laboratory in the US and Myongji University in South Korea, says it will now try to use CdTe or other chiral nanoparticles that strongly absorb light to drive the enantioselective transformation of biologically active molecules.

“The ability of the nanoparticles to assemble into specific patterns depending on the chirality of their surface ligands indicates that there could be similar differences when it comes to their interactions with biological entities, like cells,” explains Kotov. “If this is true then we could look into using this effect to improve drug-delivery mechanisms as well as to develop new antibacterial and antiviral drugs. In these cases, CdTe will certainly be out of the question, because of its toxicity, and other types of nanoparticles will need to be used.”

The work is detailed in ACS Nano DOI: 10.1021/acsnano.5b05983.