Practical strategies to reduce aggregation of silica coated QDs from microemulsion method?

26 September 2023 2 2K Report

Hi all,

I have silica coated QDs using the standard microemulsion method (in cyclohexane with Igepal +TEOS, followed by repeated washing by centrifuge in ethanol). However my particles are often lost to a 200nm PTFE filter. Ive tried this method on a few different QDs and am generally getting a good fitting correlation function on DLS with a hydrodynamic radius of 250nm and PDI of ~0.15. I sonicate my samples before DLS and run the measurements under low (0- 0.5 mM NaCl) salt concentrations.

On TEM my coated particles are ~30 nm and I cannot see any fusing between silica shells.

I have coated gold nanoparticles using the Stober process (in ethanol) without issue.

Does anyone have practical suggestions to redisperse or prevent these aggregates from occurring?

Yuri Mirgorod

Leshy Patchett

You have nanoparticles coated with silica. They were obtained by hydrolysis of TEOS. After hydrolysis, non-hydrolyzed TEOS may remain on the surface of nanoparticles; silica is well hydrated and forms various structures with water and with each other. You don’t write anything about cleaning after hydrolysis. Besides Brownian motion, which would be fundamental for gold nanoparticles, your nanoparticles have other fluctuations: density fluctuations and even quantum fluctuations. See articles

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The device measures the total fitting correlation function of all fluctuations and from it the total diffusion of nanoparticles is obtained, and then, using the Einstein-Smoluchowski formula, the diameter of the nanoparticles is determined. Therefore, you may not have aggregates . The DLS is misleading you.

In the TEM you see a solid nanoparticle without hydration by water. There is no water in the vacuum TEM; it evaporates.

Ulf Nobbmann

Hello Leshy Patchett

without the data file this is slightly tricky to answer. You indicate that DLS shows a radius of 250nm with a PDI of 0.15 (which is somewhat polydisperse, certainly not monodisperse). Then these particles are also clogging a 200nm filter, confirming the presence of large particles.

Both seem to support the presence of these larger particles. The observed smaller size by TEM may be an issue with a number versus intensity based distribution, and/or a different preparation.

In order to optimize, you may have to adjust solvent, additives, concentration, temperature,...

For a more general discussion on distributions this blog might be of interest https://www.materials-talks.com/intensity-volume-number-which-size-is-correct/

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