Chiral inorganic nanoparticles, i.e. those that cannot be superimposed on their own mirror image (like a screw due to its thread), dispersed in liquids have shown great potential in various technological applications. These include sensors, the creation of new biomimetic drugs and therapies, and nanorobotics. However, for all these applications, an open problem has been to characterise precisely the chirality of the particles experimentally.

This question was the focus of research by an international team of scientists from the University of Bath, University of Nebraska-Lincoln, Pennsylvania State University and the University of East Anglia, in which Sapienza's Department of Basic and Applied Sciences for Engineering also collaborated.

The study, led by Professor Ventsislav Valev of the University of Bath, and published in ACS Nano, explores the Tyndall effect, the phenomenon of light scattering that is easily detectable in everyday life, for example when a ray of sunlight passes through environments in which specks of dust or drops of water are suspended. This phenomenon is due to the presence in colloidal systems, suspensions or emulsions of particles of a size comparable to the wavelengths of incident light.

When particles, usually suspended in water, are illuminated, the way they scatter light contains information about both their size and geometry. Thanks to the linear Tyndall effect, it is therefore possible to perform these measurements, through various techniques based mainly on weak light sources in which the scattered light retains the same frequency as the light it illuminates. The non-linear Tyndall effect, on the other hand, occurs when laser light passes through tiny particles and is scattered at a frequency twice as high as the incident light, thus enabling the measurement of chirality.

The new study analysed this non-linear effect in silicon helices with a length of around 270 nm, which correspond in size to some viruses and large cell corpuscles. When these helices are illuminated by a circularly polarised laser source, the scattered light can tell us how the silicon helices wrap around. Ventsislav Valev said: "The importance of this application lies in the fact that silicon is the most abundant solid element on Earth, so each new property has potential for sustainable and cost-effective uses. Another reason is that measuring the winding direction is necessary for assembling inorganic materials from nanotechnology constituents. The importance is similar to that of making and then being able to measure the thread of a standardised screw.'

Emilija Petronijevic of the Department of Basic and Applied Sciences for Engineering at Sapienza and author of the publication, said: "This research takes chiro-optical characterisation to the level of "non-linearity", a level higher than the, albeit very useful, Tyndall length scale. It is very satisfying when experiments agree with the predictions of electromagnetic coupling in nanoscale chiral forms. This opens up new potential for optimising the effect and studying other material combinations. My collaboration in this study was supported by the PON Research and Innovation 2014-2020 project 'Nanofotonica a basso costo per un sensing chirale verde e sostenibile' (Low-cost nanophotonics for green and sustainable chiral sensing)".

The research was also funded by the Royal Society, the Leverhulme Trust and the Engineering and Physical Science Research Council (EPSRC).

References:

Chiroptical Second-Harmonic Tyndall Scattering from Silicon Nanohelices – B. J. Olohan, E. Petronijevic, Ufuk Kilic et al.

ACS NANO – DOI: 10.1021/acsnano.4c02006

Further Information

 Emilija Petronijevic

Department of Basic and Applied Sciences for Engineering - Sapienza Università di Roma

emilija.petronijevic@uniroma1.it