I want to measure the hydrodynamic radii of PNIPAM microgel particles dispersed in DI water using DLS method in HORIBA nanoparticle analyzer. I tried to measure with two concentrations 0.001 wt% and 0.0001 wt% of the dispersion but the data is showing some arbitrary high values with high PI. I could see individual particles using the optical microscope after drop casting on glass substrates. Particle size would be around 800 nm. I used two different refractive index 1.52 and 1.33 for the PNIPAM particles to check but that did not help. How can I get proper data?
Dear Sanjib Majumder, please reger to the following documents. My Regards
https://link.springer.com/article/10.1007/s003960000350
https://doi.org/10.1016/S0032-3861(01)00099-4
My answer does not directly address the question but does address the point that you are making measurements in DI water. Do not do that for DLS. Always have some background electrolyte present (e.g., 0.1mM KCl(aq).
The reason is that if your particles have a significant zeta potential, the double layer will be very large in DI water. As particles diffuse, their double layers will interact and slow down the motion of the particles. Hence, the diffusion rate will appear slower. Since DLS measures diffusion, it will lead to artificially high diameters. Adding some background electrolyte will significantly reduce the thickness of the double layer and prevent this problem.
Which fitting algorithm do you use to fit your autocorrelation function? The presence of small amount of aggregates can dramatically increase your PI and shift the Zav. Check https://www.cordouan-tech.com/nanoparticle-size-dynamic-light-scattering/