BIG Discoveries from Tiny Particles – from Photonics to Pharmaceuticals, materials made with Polymer Nanoparticles hold promise for products of the future – U of Delaware

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In this illustration, arrows indicate the vibrational activity of particles studied by UD researchers, while the graph shows the frequencies of this vibration.
Credit: Illustration courtesy of Hojin Kim
Understanding the mechanical properties of nanoparticles are essential to realizing their promise in being used to create exciting new products. This new research has taken a significant step toward gaining the knowledge that can lead to better performance with products using polymer nanoparticles.

From photonics to pharmaceuticals, materials made with polymer nanoparticles hold promise for products of the future. However, there are still gaps in understanding the properties of these tiny plastic-like particles.

Now, Hojin Kim, a graduate student in chemical and biomolecular engineering at the University of Delaware, together with a team of collaborating scientists at the Max Planck Institute for Polymer Research in Germany, Princeton University and the University of Trento, has uncovered new insights about polymer nanoparticles. The team’s findings, including properties such as surface mobility, glass transition temperature and elastic modulus, were published in Nature Communications.

Under the direction of MPI Prof. George Fytas, the team used Brillouin light spectroscopy, a technique that spelunks the molecular properties of microscopic nanoparticles by examining how they vibrate.

“We analyzed the vibration between each nanoparticle to understand how their mechanical properties change at different temperatures,” Kim said. “We asked, ‘What does a vibration at different temperatures indicate? What does it physically mean?’ ”

The characteristics of polymer nanoparticles differ from those of larger particles of the same material. “Their nanostructure and small size provide different mechanical properties,” Kim said. “It’s really important to understand the thermal behavior of nanoparticles in order to improve the performance of a material.”

Take polystyrene, a material commonly used in nanotechnology. Larger particles of this material are used in plastic bottles, cups and packaging materials.

“Polymer nanoparticles can be more flexible or weaker at the glass transition temperature at which they soften from a stiff texture to a soft one, and it decreases as particle size decreases,” Kim said. That’s partly because polymer mobility at small particle surface can be activated easily. It’s important to know when and why this transition occurs, since some products, such as filter membranes, need to stay strong when exposed to a variety of conditions.

For example, a disposable plastic cup made with the polymer polystyrene might hold up in boiling water — but that cup doesn’t have nanoparticles. The research team found that polystyrene nanoparticles start to experience the thermal transition at 343 Kelvin (158 degrees F), known as the softening temperature, below a glass transition temperature of 372 K (210 F) of the nanoparticles, just short of the temperature of boiling water. When heated to this point, the nanoparticles don’t vibrate — they stand completely still.

This hadn’t been seen before, and the team found evidence to suggest that this temperature may activate a highly mobile surface layer in the nanoparticle, Kim said. As particles heated up between their softening temperature and glass transition temperature, the particles interacted with each other more and more. Other research groups have previously suspected that glass transition temperature drops with decreases in particle size decreases because of differences in particle mobility, but they could not observe it directly.

“Using different method and instruments, we analyzed our data at different temperatures and actually verified there is something on the polymer nanoparticle surface that is more mobile compared to its core,” he said.

By studying interactions between the nanoparticles, the team also uncovered their elastic modulus, or stiffness.

Next up, Kim plans to use this information to build a nanoparticle film that can govern the propagation of sound waves.

Eric Furst, professor and chair of the Department of Chemical and Biomolecular Engineering at UD, is also a corresponding author on the paper.

“Hojin took the lead on this project and achieved results beyond what I could have predicted,” said Furst. “He exemplifies excellence in doctoral engineering research at Delaware, and I can’t wait to see what he does next.”

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Materials provided by University of DelawareNote: Content may be edited for style and length.

Journal Reference:

  1. Hojin Kim, Yu Cang, Eunsoo Kang, Bartlomiej Graczykowski, Maria Secchi, Maurizio Montagna, Rodney D. Priestley, Eric M. Furst, George Fytas. Direct observation of polymer surface mobility via nanoparticle vibrationsNature Communications, 2018; 9 (1) DOI: 10.1038/s41467-018-04854-w

University of Delaware: Research Shows Promise for a Nanoparticle as “Nanofertilizer”

U of Delaware Nano Fertilizer 090315 id41225Researchers at the University of Delaware have discovered unique behaviors of hydroxyapatite nanoparticles (HANPs) that show promise as a phosphorus nanofertilizer and could be used to help slow the release of phosphorous in soils (“Effect of Size-Selective Retention on the Cotransport of Hydroxyapatite and Goethite Nanoparticles in Saturated Porous Media”).
This would both increase phosphorous uptake efficiencies in the growing of plants and also in protecting environmentally sensitive sites, including bodies of water, by reducing nutrient loading, which is important because phosphorous is a nonrenewable resource and an essential nutrient for agricultural production.
Funded by the United States Department of Agriculture (USDA), the research was conducted by Dengjun Wang, a postdoctoral researcher in the Department of Plant and Soil Sciences in UD’s College of Agriculture and Natural Resources; Yan Jin, professor of plant and soil sciences with a joint appointment in the Department of Civil and Environmental Engineering; and Deb Jaisi, assistant professor of plant and soil sciences with a joint appointment in the Department of Geological Sciences.
(from left) Deb Jaisi, Yan Jin and Dengjun Wang
UD researchers (from left) Deb Jaisi, Yan Jin and Dengjun Wang have discovered unique behaviors of hydroxyapatite nanoparticles that show promise as a phosphorus nanofertilizer and could be used to help slow the release of phosphorous in soils.
The HANPs are known as a strong sorbent for contaminants such as heavy metals and radionuclides and are already being used to remediate soils, sediments and ground waters. However, its potential as a better phosphorous fertilizer in agriculture has just started to be fully explored, the researchers said.
The nanoparticle-based fertilizer has three major advantages over conventional phosphorous fertilizers in that it does not release phosphorous as quickly as the conventional fertilizers, it does not change soil pH upon phosphorous release and the loss of phosphorous from soil is low. The slow and steady release of phosphorous allows plants to continuously take up the nutrient as they grow.
Jaisi said that the way phosphorous is currently applied to soils in fertilizer is like someone taking a glucose tablet as opposed to receiving it through an IV drip. While a commercial phosphorous fertilizer hits the soil all at once and does not allow sufficient time for plant uptake, resulting in phosphorous loss in runoff or by leaching, the HANPs provide a slow release of phosphorous for an extended period of time.
“When phosphorous is released from HANPs, it does not increase soil acidity,” said Jaisi. “There was an issue of global soil acidification after the Green (agriculture) Revolution, a direct consequence from the application of chemical fertilizers. The cost of reversing soil pH to optimal for crop production is extremely high.”
As the demand to provide food for a growing population has increased, so has the application of phosphorous fertilizers, which has led to phosphorous loss from agricultural soils to open waters and has caused eutrophication in environmentally sensitive areas like the Chesapeake Bay. With the ability of HANPs to release phosphorous slowly, the nanoparticles could prove to be environmentally beneficial by reducing phosphorous loss to open waters.
“You can minimize that risk and at the same time, increase the availability of phosphorous for a longer period of time during plant growth,” said Jin.
“I think the goal would be to explore whether this is a feasible form of phosphorous fertilizer to be used at large scales,” she added. “We’ve been applying a lot of phosphorous to soil for many years, and the available source is diminishing. We need to find new products and new ways of supplying the nutrient, while at the same time minimizing environmental impacts.”
“A major objective of this work,” Jaisi said, “was to look at the fate of these nanoparticles — if the nanoparticles themselves move away from the soil to open waters or if they remain in the soil, and how they interact with other nanoparticles in the soil. This is important because for the best utilization of phosphorous, HANPs have to remain in soil for an extended time and not be lost via runoff or by leaching.”
Wang said the HANPs have low mobility, and the presence of other nanoparticles in the soil, such as positively charged iron oxides that are ubiquitous in soil and other subsurface environments, would fix themselves to the negatively charged HANP particles and slow down their movement.
Jin explained that in order for plants to take up the phosphorous from HANPs, it needs to be released from the nanoparticles. “When plants grow, they continuously release different types of low molecular weight organic acids such as oxalic acid and citric acid. The acids that get into the soil will interact with those particles so that phosphorous can be released and be taken up by plants,” said Jin.
Wang said the process is very dynamic. “The plant continuously releases organic acids and these organic acids will dissolve the HANPs making phosphorous available for the plant. The release rate in the presence of these organic acids and the possibility of HANPs being a phosphorous fertilizer are currently being investigated by the research team.”
In reaching their conclusions, the team examined how HANPs interact with a naturally occurring goethite nanoparticles (GNPs), a common iron oxide in soils, to investigate the co-transport and retention of HANPs and GNPs in water-saturated sand columns under environmentally relevant transport conditions.
Wang said that the nanoparticle with which the group works is very small, ranging from one nanometer to 100 nanometers, with one nanometer being about 10,000 times smaller than the diameter of a human hair.
“These very tiny particles have large specific surface areas and high reactivity; they are quite fantastic to a variety of applications in various fields, including agriculture,” he said.
Source: By Adam Thomas, University of Delaware