Lanthanides — the rare earth elements one can find listed outside the periodic table — have become indispensable in the development of modern technologies, such as smartphone batteries, lamps, and magnets, because of their strong electromagnetic and light properties.
Their photoluminescent properties also make them valuable for several nanotechnological applications, including imaging probes and microoptics. And although such luminescent, nanoscale devices are expected to transform the technology industry, there exists a lack of cost-effective and environment friendly approaches to scale up production.
Unicellular algae, although more well known as sustainable sources of organic molecules, are capable of producing extremely intricate, mineralized structures. 3D-nanostructured materials generated by living organisms have many advantages over those achieved via chemical or physical synthesis routes; inorganic intricate biominerals are produced with low environmental impact and production costs, have complex structures, and are biocompatible.
“We are working with the microalgae species Emiliania huxleyi, which covers itself with mesoporous, nano-, and microstructured 3D plates. These so-called coccoliths are biomineralized inside the microalga cell under genetic control and are highly reproducible. Thus, the coccoliths are interesting candidates for use as functional materials, but are, up to now, not luminescent,” admits Giulia Santomauro, researcher at the University of Stuttgart.
The calcite coccoliths naturally contain other elements besides calcium. Strontium, barium, magnesium, and boron are taken up by the microalgae out of the surrounding environment and incorporated in trace amounts in the calcite lattice. The ions serve as micronutrients and are essential in limited quantities for their growth. But even though lanthanides are nonessential elements, they can be taken up by microalgae and can have stimulatory effects.
The research team, which also included scientists from the Swiss Federal Laboratories for Materials Science and Technology, investigated the possibility of producing luminescent coccoliths via the biomineralization mechanism of living Emiliania huxleyi cells by growing them in media containing terbium, a luminescent, rare earth metal.
Doping of the coccoliths with Tb3+, which emits green light upon excitation, yielded green, luminescent, 3D-mesoporous microstructures. These coccoliths are estimated to be superior to conventionally produced terbium-doped materials owing to their unique hierarchical structure. Moreover, by incorporating the Tb3+ into the coccoliths via biomineralization and not just labeling them resulted in permanent alteration of the coccoliths and thus prolonged luminescence.
“These results offer a new alternative route for the production of mesoporous, nanostructured, and green luminescent functional materials, using the natural biomineralization processes of living microalgae.” concluded Giulia Santomauro.
This method could be used to produce coccoliths luminescent in other colors too, for example, by incorporation of thulium (blue luminescence) or europium (red luminescence). Moreover, since the coccoliths consist of biocompatible calcite, they could be used as drug delivery systems, where they can easily be tracked due to their luminescent properties.
Research article found at G. Santomauro, et al. Advanced BioSystems, 2020, doi/10.1002/adbi.201900301