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Posted: Jun 13, 2013
Shedding a light on the cause for nanoparticle size distribution
(Nanowerk News) When buying shoes it does not matter how good-looking the shoes might be if the size does not fit. This is similar with nanoparticles, which are made by the so-called emulsion-solvent evaporation process. This process allows for the production of nanoparticles with high purity. Nevertheless they can still be improved: so far, their size distribution cannot be fully controlled. However, a defined size is of prime importance for future applications, whether it is for drug delivery or for intelligent coatings.
An interdisciplinary and international research collaboration at the Max Planck Institute for Polymer Research in Mainz was able to rule out coalescence as reason for the borad nanoparticle size distribution. Coalescence describes the tendency of colloidal droplets to melt together. For the first time, Daniel Crespy, who is group leader in the department of Katharina Landfester, was able to prove that the coalescence between droplets during the process is not significantly responsible for the broad size distribution of the particles.
“This study elucidates the mechanism of a common process used for the preparation of nanoparticles,“ says Daniel Crespy about his research results.
The chemist labeled the original materials prior to the preparation of the nanoparticles. Some polymers were labeled with red and others with blue dyes. During the synthesis, the polymers and a solventwere emulsified in water. After the evaporation of the solvent, solid nanoparticles are obtained. This is a common method to produce all types of nanoparticles. Crespy’s trick: Upon adding both red- and blue-labeled polymers to the solvent, nanoparticles with both colors were obtained. The so-called negative control shows that if red and blue particles are mixed, no aggregation occurs because species with both dyes were not detected.
What happens if a red emulsion from polymer and solvent is mixed with a blue emulsion? Less than every twelfth particle –around 8 percent – were labeled with both red and blue dyes, which means that coalescence does not play a significant role in the process.
For the first time, the scientists were able to directly quantify the occurrence of coalescence. Together with Kaloian Koynov, who is physicist and expert for spectroscopic methods at the MPI-P, Crespy could monitor the coalescence of nanometer sized droplets by fluorescence correlation spectroscopy.
The experimental results were finally confirmed by simulations based on Monte-Carlo algorithms performed by Davide Donadio, group leader of a Max Planck Research Group. Thanks to this study ("Particle Formation in the Emulsion-Solvent Evaporation Process"), the reason for the broad size distribution could be attributed to the process itself.