News | February 1, 2018

New, Safe Quantum Dots Of Zinc Oxide

Quantum dots of zinc oxide produced by a new method involving organometallic precursors are safe for human cells - demonstrated by chemists from Warsaw. The Institute of Physical Chemistry of the PAS informed PAP about the new method of producing ZnO quantum dots.

Due to the lack of heavy metals, zinc oxide nanoparticles (ZnO) are today one of the most commonly used nanomaterials. Although they seem to be safe for humans, there are still no standards for their toxicity.

Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) in Warsaw and the Faculty of Chemistry of the Warsaw University of Technology (PW) have developed a method of producing ZnO quantum dots with particularly interesting and invariable physicochemical properties - informs the IPC PAS in a press release sent to PAP.

Among the properties of the quantum dots obtained, the researchers mention: monodispersity, relatively high quantum efficiency, record-long luminescence lifetime, and absence of paramagnetic defects in the core structure. The features of the stabilizing surface of the organic layer make the new ZnO quantum dots resistant to both chemical and biological environments.

"The zinc oxide nanocrystals obtained by us are characterized by definitely better chemical and physical properties than their counterparts currently produced by the most popular sol-gel method with the participation of inorganic precursors" - emphasizes quoted in the statement of prof. dr hab. Eng. Janusz Lewiński from the IPC PAS and PW.

"The lifetime of luminescence, or luminescence, in our case of quantum dots is much longer - and up to several orders of magnitude! Moreover, until now only short decay times of ZnO luminescence have been observed, on the order of a few-dozen picoseconds and characteristic for nanoparticles obtained by sol-gel method, or slightly longer, nanosecond, typical only for ZnO monocrystals. We have a luminescent material that can be used as a new generation optical marker for biomedical applications, "adds the researcher.

In combination with biologically active molecules, new nanoparticles could be used in biology or medicine, e.g. for imaging cells and tissues. This would allow monitoring the development of the disease or the effectiveness of the therapy applied much more accurately than before.

In the latest publication in the scientific journal "Chemistry - A European Journal" (DOI: 10.1002 / chem.201704207), Warsaw scientists in collaboration with a group from the Jagiellonian University showed that their zinc oxide nanoparticles are indeed safe. As they explain in the IChF PAN communication, these studies - which were financed thanks to the TEAM Grants of the Foundation for Polish Science and the OPUS of the National Science Center - allow realistically to think about the rapid introduction of new ZnO quantum dots, among others for biological and medical laboratories.

Classically produced Zn-Zn nanoparticles are not well stabilized and isolated from the environment. This means that, for example, interactions that take place at the interface between the inorganic ZnO core and the biological environment can lead to the formation of reactive oxygen species or the dissolution and release of potentially toxic zinc cations.

"Zinc oxide is generally considered to be a relatively safe, biocompatible and biocompatible material, but many ZnO toxicity studies involve heterogeneous nanoparticles, and too large to penetrate into the cells," Dr Eng. Małgorzata Wolska-Pietkiewicz from PW.

"We were also aware that in practice many of the nanoparticles' properties depend not only on their size, but also on the surface properties of the nanocrystalline ZnO and the organic stabilizing shell, so we decided to modify our synthesis method so that the resulting ZnO nanoparticles behave in the interiors cells in the most neutral way "- he explains.

The team of prof. Lewinski produces quantum dots of zinc oxide from organometallic compounds (precursors). When the purpose is biological applications, the end result is stable nanoparticles with a spherical shape, made of a crystalline ZnO core with a diameter of 4-5 nanometers and surrounded by a cladding of organic compounds. This coat increases the size of nanoparticles (their hydrodynamic diameter is about 12 nm) and performs protective functions: on the one hand it protects the inorganic core from degradation due to interaction with often very reactive biological environment, on the other - eliminates the influence of zinc oxide itself on this environment.

"Nanoparticles with core sizes below 10 nm are particularly easily penetrated inside the cells." Such particles are considered potentially dangerous, "explains Dr Wolska-Pietkiewicz." Interestingly, we created ZnO nanoparticles, contrary to popular opinion indicating that the smaller systems, the greater their toxicity, showed extremely low adverse effects in in-vitro model tests, and the results obtained, as well as studies conducted in parallel in the parent team, provided further evidence of the unique character of the nanocrystalline ZnO obtained as a result of the transformation of organometallic molecular precursors, "notes the researcher.

Despite the fact that studies on zinc oxide quantum dots give hope for numerous applications, there are concerns about the effects of their impact on people and the environment. Because nanoparticles can get into the body including by inhalation, healthy and cancerous human lung cells were selected for toxicological tests. They showed that the organic layer surrounding the improved nanoparticles is indeed impermeable: zinc ions are not released into the environment, and reactive oxygen species are not formed. The toxicity of new zinc oxide nanoparticles turned out to be negligible even at high concentrations.

"Our + recipe + production of quantum dots of zinc oxide causes that they simply do not interact with the biological environment - says Professor Lewiński, quoted in the statement - We have strong foundations to start working on applications, not only in medical imaging, but also in other areas in which nanoparticles could potentially interact with the human body, eg as one of the components of paints. We also develop new technologies for the synthesis of ZnO quantum dots and in the framework of the NANOXO start-up we are looking for new applications "- sums up the researcher.

SOURCE: Polish Academy Of Science