David Mrozek

You have to look at your particulate system first – a liquid will not solve your basic issues. If you have your ZrO2 material in dry form then there are no independent, discrete, free, separate particles < 100 nm present in it. You simply can’t put enough energy in to separate out the fundamental particles. You have a set of fused sub- and post-micron aggregates and agglomerates in a powder. 'Nano powder' is basically an oxymoron. What's the Specific Surface Area (SSA) of your ZrO2? Powders are generally laser diffraction measurements, not DLS.

'The micron scale is volumetrically 109 times larger than the nanoscale. Confusing microtechnology with molecular technology is like confusing an elephant with a ladybug' K Eric Drexler quoted in Ed Regis Nano: the emerging science of nanotechnology' Little, Brown and Company, 1st Edition pp 207 - 208 (1995)

2 quotes from those much greater than I:

'I think dry nanotechnology is probably a dead-end' Rudy Rucker Transhumanity Magazine (August 2002)

‘If the particles are agglomerated and sub-micron it may be impossible to adequately disperse the particle… ‘The energy barrier to redispersion is greater if the particles have been dried. Therefore, the primary particles must remain dispersed in water...’ J H Adair, E. Suvaci, J Sindel, “Surface and Colloid Chemistry” Encyclopedia of materials: Science and Technology pp 8996 - 9006 Elsevier Science Ltd. 2001 ISBN 0-08-0431526

There are plenty of RG questions and answers dealing with this very topic. Keep the system wet and do not dry it to preserve the nano nature of the particulate system. Look at:

https://www.researchgate.net/post/How_can_I_overcome_dispersion_issues_faced_while_re-suspending_powder_nanoparticles

https://www.researchgate.net/post/How_the_agglomerated_green_nanoparticles_can_restored_into_nano_size_particles

One overall webinar (registration needed) is this one:

Dispersion and nanotechnology

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190528DispNanoMat

With the solvents you mention there’s the volatility issues giving rise to possible convection currents and interference with the DLS technique. Lower density liquids mean more settling. If your system is milky when suspended in water, then it’s sub-micron at best (> 100 nm). If it’s settling at all then it’s the micron plus to tens of microns region. A truly nano (100% < 100 nm) colloidal suspension of ZrO2 will appear as clear as water to the eye.

Qasim Qayyum Kashif

There are some comments I must make on your answer above:

• 'Generally, the solvent should have a refractive index that is similar to that of the nanoparticles, to minimize any scattering artifacts'. This is nonsense - the higher the RRI (relative refractive index) then the better the scattering for measurement. Indeed with an RRI of unity then the particles are completely invisible (index matched) and there is no scattering to measure in DLS
• 'Solvent should be.... free of contaminants'. I understand the reasons for this comment but probably the worst system to retain stability of colloidal systems is to dilute in DI water. Systems are kept stable by surfactants (wetting agents) and stabilizers (electrostatic/charge or steric)
• 'Generally, the concentration should be low enough to avoid any aggregation'. If the starting material is a powder then aggregation is a given. Seen the webinar referenced above
• Water is the best medium to transfer ultrasound energy. With volatile liquids then issues arise as most of the energy is converted into boiling the liquid and not separating (loose) agglomerates. Tightly (chemical) bound aggregates cannot be separated by sonication, by definition
• 'it is important to verify the results by comparing them to other characterization techniques, such as transmission electron microscopy (TEM) or scanning electron microscopy (SEM)'. I would agree with this comment but it's important to realize that the different techniques will not give the same result as different parameters of the system are measured and the ensemble and particle-by-particle techniques treat the raw data in different ways. The main difference in DLS and electron microscopy is the generation of intensity and number distributions which can be very different for a polydisperse system (the bane of all sizing techniques). However, differences in result are very useful in understanding the basics of the particulate system. Note in the entire history of electron microscopy not more than a few kilos of material in total have been measured...

David Mrozek

You can easily verify the performance of your DLS system with a latex standard. This eliminates the machine from the 3M variable budget (Machine, Method, Material).