Semiconductor nanoparticles for biomedical applications have been researched for some time now. Not only are they millionths of a millimeter in size, they also offer great potential for cancer diagnostics and therapy, so-called theranostics. They enter cells, are activated by ultrasonic radiation and destroy the cells using the generated vibration.
Until now, the use of many semiconductor nanoparticles in the human body has failed due to their high toxicity. Yet this is different when it comes to silicon nanoparticles, says Dr. Vladimir Sivakov, Head of the Semiconductor Nanostructures Work Group at the Leibniz Institute of Photonic Technology (German: Leibniz-Institut für Photonische Technologien) in Jena.
Dr. Sivakov, what exactly are silicon nanoparticles?
Dr. Vladimir Sivakiv:Silicon is the second most abundant material in the earth’s crust. It is, therefore, available in nearly unlimited quantities. Thanks to special top-down or bottom-up processes, it is possible to produce silicon at the nanometer scale. The properties of such nanoscale objects differ in part significantly from the properties of the bulk material. We are able to produce highly porous silicon nanoparticles well under 100 nanometers in size with the processes we use. In addition, the thus produced silicon nanostructures emit orange-red light when they are irradiated with suitable wavelengths. This phenomenon called photoluminescence combined with the large surface area of the porous silicon nanoparticles makes them interesting as contrast agents but also as delivery vehicles for various therapeutic agents for cancer therapy.
Why is silicon so well suited for medicine? Is it possible to detect or rather treat every type of cancer with it?
Sivakov: Silicon is a material that has a non-toxic effect on the human body. Another significant advantage is the fact that compared to metallic or oxidic particles like gold or iron oxide, silicon is gradually broken down by the human body into silicic acid and can, therefore, be excreted without any problems after diagnostics and therapy have been completed. Unlike metallic nanoparticles which remain in the body and can cause further discomfort.
In principle, it is possible to diagnose and also treat any type of cancer with these particles. However, we are still in the early stages of our research in this case. Yet it is possible to equip silicon nanoparticles with a cancer-specific antigen on the surface. In doing so, the particles primarily accumulate in those regions of the body that are affected by cancer. This selectivity is just one of the many challenges we want to rise up to.
What does theranostics actually look like when using silicon?
Sivakov: Currently different approaches to this are being researched. A tumor diagnosis can take place based on the photoluminescent properties of the particles via light microscopic techniques for example. Examples of this include confocal microscopy or optical coherence tomography. The problem here is that visible light of the electromagnetic spectrum does not enter the human body very deeply, thereby limiting a diagnosis to primarily superficial tumors such as skin cancer for example. However, other physical effects of the silicon nanoparticles can be used to diagnose cancer: if particles are under radiofrequency irradiation, a strong photoacoustic signal can be detected. The advantage of this is that the penetration is considerably increased compared to the visible light of the electromagnetic spectrum, making it possible to also diagnose significantly deeper-set tumors.
Another objective of theranostics is to treat the tumor with the same particles used to make a diagnosis. Highly porous particles can be loaded with different cancer therapeutic agents to subsequently release them systematically at the tumor site. The advantage of this is that only the actual tumor is being attacked by the medicine while the rest of the body remains unaffected by the partially aggressive drugs. The particles begin to vibrate when irradiated with ultrasound. The vibration of the particles generates local heat which kills cells due to hyperthermia. At the same time, so-called cavitation takes place in an aqueous medium. In addition, the gas bubbles generated by this effect can also fight the tumor.
The IPHT Jena coordinated the creation of the "NanoPhoto" network in 2014. It is funded by the German Federal Ministry of Education and Research (German: Bundesministerium für Bildung und Forschung), BMBF. The goal is to promote the development of new materials for cancer diagnostics and therapy in Europe. What would you like to achieve with this Network in the future?
Sivakov: "NanoPhoto" was very positively received in Europe. The small Baltic Sea Network has now evolved from its initial five partners into a network that includes all of Europe. The "NanoPhoto" Network and the IPHT Jena co-organized the "Current Trends in Cancer Theranostics-CTCT-2015" conference which took place in June. This so far one-of-a-kind conference was so well received by the participants that the CTCT-2016 was already being set up during this event. The CTCT-2016 will take place in Druskininkai, Lithuania.
In the future, "NanoPhoto" is intended to promote even stronger networking between scientists of various disciplines – ranging from physicists and physicians to chemists and biologists. Only a high level of interdisciplinarity like this can advance theranostics in the future.