The world's first afterglow-magnetic nanoparticles
Researchers at the University of Siegen have developed the world's first nanoparticles that feature both magnetic and afterglow properties. The potential for applications is enormous: For example, the novel particles could be used to discover cancer cells in patients or detect the concrete distribution of fine particulates in organs, providing evidence of their toxicity.
Nanoparticles are miniscule particles with diameters of less than 0.0000001 metres, or 100 nanometres. That makes them invisible to the human eye without powerful microscopes. Their size offers significant advantages: For example, in medicine they can be injected into the bloodstream as biomarkers. If they were larger, the particles would be unable to penetrate the cell walls and would immediately sink to the floor of the blood vessels.
But nanoparticles are different: their tiny size allows them to "float" in the bloodstream. Fine particulates are also a similar size. "Nano-materials R&D is an increasingly important field globally. The novel properties of these materials will in future be used in lots of fields such as energy conversion and sensor technology," says Dr. Claudia Wickleder, the Chair of Anorganic Chemistry and a member of the Center for Micro- and Nanochemistry and Engineering (Cµ) at the University of Siegen.
nanopartikel_webLuminescent, magnetic nanoparticles have already been around for a few years. They glow after irradiation with light. However, as soon as the light source is removed - e.g. when they are injected into the body - they stop glowing. In contrast, afterglow magnets have the advantage that they continue to glow even in the dark. They work just like the stars much loved by children, which glow in the dark for a while even after the bedroom light has been switched off. The researchers at the University of Siegen, Dr. Huayna Terraschke (a former PhD student at the University of Siegen and now a junior professor at the University of Kiel) and Dr. Claudia Wickleder discovered that afterglow materials are ideally suited to applications in medical sensor technology.
The problem with luminescent sensors without afterglow is that they have to be activated by UV light, which doesn't penetrate through skin. Persistently luminescent materials can be activated by UV light before being injected, after which they continue to glow. Their magnetic properties are necessary for use in medical procedures such as magnetic resonance tomography (MRT).
The challenge of this special multi-functionality is that magnetic substances usually cancel out the luminescence. Therefore, nanoparticles like these simply did not exist until now. The research group in Siegen led by Dr. Claudia Wickleder has solved this problem. The team introduced an intermediate layer between the magnetic core and the luminescent outer shell. "This layer as well as the core and the shell had to consist entirely of non-toxic substances", says Dr. Wickleder. "Otherwise, medical applications would have been out of the question." The researchers discovered ideal materials: iron oxide (magnetite) for the core, silicon dioxide (SiO2) for the intermediate layer and strontium aluminate doped with rare earth elements for the shell.
The next challenge was keeping the nanoparticles as small as possible. Each of the three layers - core, intermediate layer and shell - had to be as thin as possible. The iron-oxide core alone is 15 nanometres in diameter. To put this into perspective: A 1-cent coin is ten million times bigger. With the intermediate layer, the diameter increases to 25, and with the shell to 67 nanometers. "But for practical use in medicine or biosensory technology, the particles need to be even smaller and above all the luminescence has to be more efficient," says Wickleder.
In biosensing, such particles could be used to attach themselves onto cancer cells. However, a single afterglow nanoparticle would be too small to be visible inside the body. This is where the magnetic property comes in. External magnets can concentrate the luminous nanoparticles inside the body so that they are easy to trace.
With their cooperation partners Dr. Stefan Lienenklaus and Dr. Siegfried Weiß from the Medical University of Hanover (MHH), the Siegen-based researchers successfully tested the novel nanoparticles with an experiment on mice using fine particulates. The nanoparticles were modelled to imitate fine particulates in air. The mice were exposed to these particulates. By means of the luminescent magnets, the researchers were able to precisely detect which organs collected the fine particulates. Imaging examinations made the afterglow visible. At the same time, the researchers also showed that the nanoparticles used, unlike other materials, are completely non-toxic for the mice and therefore also uphold animal rights.
The novel nanoparticles also offer potential in non-medical applications. For example in quality control for steel, where they could reveal tiny, invisible cracks which significantly impair the steel's stability. This may be of interest to companies in the local steel-processing industry, who could initiate cooperation programmes with the University of Siegen.
Source: University of Siegen