Tallinn University of Technology

Energy efficiency is a value that is becoming more and more important in the world. This leads to an increasing use of ecologically friendly building materials and, together with it, the need for environmentally friendly repellents. But what could these be? The answer lies in a doctoral thesis recently defended at Tallinn University of Technology.

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The solution to this issue might be plant mediated synthesis of silver-based nanoparticles and their use as a antimicrobial agent in environmentally friendly applications. In a doctoral thesis with the same title, author Siim Küünal admits that, due to greenhouse gas emissions and an increasing energy demand, energy efficiency has become an important keyword.

"The energy demand of the construction sector alone is estimated to account for 40% of the total energy demand of the world. To counteract this, people have started to find solutions to introduce an energy-saving mindset, including the development of environmentally friendly green construction, which has been gaining momentum year by year,” Küünal explains in the summary of his thesis.

The protection of straw bale houses can be environmentally friendly

One of the trends, he says, is the use of ecologically friendly building materials – for example, straw bales left over from farming. At the same time, the type of straw bale used in construction is a fertile ground for bacteria and moulds, which creates a need for an eco-friendly repellent: both for straw and for building and finishing materials in general.

"Green-synthesised silver nanoparticles fall into this category. The antimicrobial properties of silver have been known for centuries, and silver nanoparticles and their effects on bacteria and fungi have been widely studied over the past two decades," Küünal notes.

How were the results achieved?

According to the author, pure silver nanoparticles were tested on microorganisms that are most commonly found in straw. The first stage of the doctoral thesis focused on antimicrobial tests against bacteria and fungi isolated from straw.

The experiments were carried out on the surface of the material, on bacteria and fungi in separate media in Petri dishes, and on isolated cultures in laboratory conditions. The results of the tests provided the basis for focusing the thesis upon the development of synthesis options for ecologically friendly silver nanoparticles, in order to use them similarly in the protection of eco-materials. Among the so-called green synthesis options, the use of plant extracts for the synthesis of silver nanoparticles from silver nitrate was one of the most notable due to its simplicity and scalability.

“Because we were using a common weed, Plantago major, the process was also economical,” Küünal stresses. "A number of plants widely regarded as weeds were able to synthesise the particles, but the synthesis process of Plantago major was developed further."

However, according to the author, the novelty of the thesis lies in the discovery that not only does a specific plant influence the parameters and composition of the synthesised nanoparticles, but these are also influenced by different ways of preparing the extract of the same plant and by the parameters of the nanoparticle synthesis. Depending on how the Plantago major extract was prepared and how the synthesis was carried out, the composition of the nanoparticles could be checked: whether the main phase was silver or silver chloride.

It is important to note that both types of silver nanoparticles were also successfully tested on model organisms, in order to assess their suitability for use as antimicrobial agents.

Research proved to be more exciting than expected

Siim Küünal admits that the research was of great interest to him personally, as this type of nanoparticle synthesis using local plants had not previously been conducted in the region. "The selection was very wide and a lot of different plants and crops were tested."

What also came as a surprise to him was the realisation that it is possible to synthesise a wide variety of nanoparticles using the same plant. "In conclusion, the research side of the thesis was much more interesting and fascinating than I could have ever have imagined before I started my doctoral studies. On the other hand, the structuring of the data, writing the thesis and publishing it was somewhat more complicated than I expected," Küünal states.