Graphic representation of the interplay of chirality and magnetism.
Graphic representation of the interplay of chirality and magnetism.

Nanomaterials whose optical properties change in response to a magnetic field could have a central role in spintronics, magnetooptics, magnetochemisty, and chiral catalysis. But, until recently, materials combining optical properties and chiral asymmetry – structures that are mirror images of each other like left and right hands – have been based on transition metal complexes. Now researchers have come up with a simple alternative: ceramic Co3O4 nanoparticles coated with amino acids [Yeom et al., Science 359 (2018) 309].

“Chiral inorganic materials are a fast-developing area of science for chiral photonics,” explains Nicholas A. Kotov, who led the research at the University of Michigan and the Federal University of São Carlos in Brazil. “However, there is a problem with chiral inorganic nanostructures because they can only dynamically alter polarized light beams by chemically changing the geometry of their nanoscale structure or surrounding media.”

To get around this difficulty, the researchers coated tiny (2 nm diameter) Co3O4 nanoparticles with l- and d-forms of the chiral amino acid cysteine. The amino acid attaches to the surface of the crystalline nanoparticles, twisting the crystal lattice. This distortion in the crystal lattice, which depends on the chirality of the amino acid coating, gives rise to magnetochiral properties in the nanoparticles.

When paramagnetic Co3O4 nanoparticles are dispersed in a solvent or formed into a gel, the material exhibits much stronger chiroptical activity in the UV part of the light spectrum than nonparamagnetic nanoparticles. When a magnetic field is applied to the material, the magnetic moments of the Co ions, which have been displaced from their usual lattice positions, line up. In practice, this means that circularly polarized light can travel more readily through Co3O4 nanoparticle gels when the magnetic field is ‘on’. The transparency of Co3O4 nanoparticle gels to circularly polarized light can, in this way, be switched on and off repeatedly using a magnetic field.

“We demonstrate that field modulation of light beams by chiral inorganic nanostructures is possible using external magnetic field,” says Kotov. “Moreover, it is achieved at room temperature, with high fidelity, and using inexpensive, common materials.”

This is the first time that such robust chiromagnetic nanoparticles with room temperature operation have been reported, point out the researchers, and could form the basis for many photonic technologies.

“Circularly polarized light can be used in three-dimensional displays, holography, fiber-optic networks, anti-counterfeiting tags, and other areas,” Kotov says. “Field modulation also makes it possible to use the new materials in biosensing.”

Alexander Govorov of Ohio University agrees that the newly discovered combination of chiral and magnetic properties in a single nanoparticle could be useful in chiral bio-recognition and sensing that is controlled by an external magnetic field.

“The interesting finding here is that these bi-functional nanocrystals are both chiral and magnetic,” he points out. “The chirality comes from the small attached biomolecules (or ligands) while the magnetism is a property of the crystal nanoparticles. This combination of bio- and solid-state materials in one nanocrystal is appealing.”

Phenomena arising from the subtle interplay of chirality and magnetism are attracting much interest at the moment, points out Laurence D. Barron FRS, Emeritus Gardiner Professor of Chemistry at the University of Glasgow.

“Unlike existing chiromagnetic systems, this combination of paramagnetism and chirality in inorganic nanoparticles and gels has the potential to provide magneto-optical devices operating at low fields and ambient temperatures, which opens up new possibilities for both fundamental studies and practical applications,” he comments.

The researchers now want to improve light modulation in the visible part of the spectrum and investigate the field modulation effect in the infrared.

“At the moment we are using hydrogels but we want to replace them with tough, chiromagnetic glasses in the future,” adds Kotov.

This article was first published in Nano Today 19 (2018) 5-6.