Chirality is a property that is present in nature in which an object cannot be superimposed on its mirror image. For example, the right hand cannot be superimposed on its mirror image, the left hand. Even if the molecular formula is identical, the three-dimensional orientation is different.
The identification of chiral structures through spectroscopy is fundamental in chemistry, biology and the pharmaceutical industry, as it provides important information on protein secondary structures, electron transitions and the conformation of small molecules. However, detecting molecular chirality is extremely complicated, as the signal that appears in the spectrometer is so weak that it is difficult to identify these differences in the molecular structure.
To advance in this field, the research group Polyfunctional Polymeric Materials (POLY2) of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC) will work on the project CELICOIDS - Nanohelicoid metamaterials templated by cellulose nanocrystals with end-tethered polymers. As part of this project, it will develop new metamaterials −materials designed to have a property that is not found in nature – that can increase the spectroscopic sensitivity to distinguish molecular chirality. These helical metamaterials are obtained from cellulose, a polysaccharide extracted from paper, cotton or other plant fibres. The cellulose will be treated by hydrolysis to extract cellulose nanocrystals, to which functional synthetic polymers will be end-tethered. These modified nanocrystals will then be used as a template to manufacture metal metamaterials.
When the liquid crystals of cellulose dry, they can self-assemble to form helical structures. The innovation lies in the fact that now the ends of these nanometric crystals will be modified to attach functional synthetic polymers that will guide the formation of metallic nanohelicoids.
As a result, a film will be obtained that can be impregnated with metals such as gold, for example, which will be immobilised at the ends, where these functional polymers are. After crystallising the metal and removing the modified cellulose template, the ultimate goal is to achieve a new metal helical structure, which is similar to a snail or Archimedes screw on nanoscale. This is a pioneering procedure as, to date, researchers have not been able to produce such complex metal structures through self-assembly.
The resulting structure will also be chiral and will have electromagnetic properties that are not observed in nature, which will improve chiral molecule detection in several fields of research. When combined in a solution of stereoisomers, they will make it act as a communication vehicle between the light and the small chiral molecules, which will amplify the overall signal for detection in spectroscopy, such as circular dichroism.
The final applications of these new metamaterials range from chiral optics to their use in nanodevices, machines and chiral sensors, and applications in the fields of biology and the pharmaceutical industry. They could also be the basis for future invisibility cloaking devices and lenses for super-resolution imaging in natural sciences and the health sector.
In addition, the proposed engineering process, based only on self-assembly, represents a paradigm shift in the research and manufacture of metamaterials as it simplifies the manufacture of chiral metamaterials. This manufacture currently relies on complex and costly vacuum vapour deposition processes.
CELICOIDS has funding of close to two million euros, due to grants from the European Research Council (ERC), as part of the Horizon Europe programme. The project will last five years (March 2023 – February 2028).