Dear Ainin Razak,

The SPR peak at 520 nm shows indeed you have Gold Nanoparticles (NPs). To learn more about calculating (in my opinion I would rather call it estimating) Gold NP sizes based on UV-VIS spectra please refer to this article, it does a great job at explaining how this can be done; https://www2.chemistry.msu.edu/courses/cem434/ac0702084_UVVis_CS.pdf

Next, when comparing size data on NPs using different techniques, you will always see differences. Don't forget that the color is in dispersion and SEM you do in a dry state. Drying NPs dispersions can lead to undesired agglomeration and hence larger sizes. Furthermore in using microscopic techniques (which use a number-based size distribution), one is prone to find the outliers, meaning it is easy to spot a few large particles in a sea of smaller ones. Whereas scattering is seen as intensity meaning giving an intensity-based distribution which is diameter to the power 6 different from number-based.

Finally, it is not that the nanoparticle range all of a sudden stops at 100 nm... In my opinion, the 100 nm value stems from legislation rather than science! Legislators and manufacturers were trying to get clear borders on what to call a nanomaterial. Scientifically speaking the 100 nm border makes no sense. I would focus on what is important for your work and if needed remove the larger particles if they hinder their application.

Kind regards, Leo

Adding to the excellent answer by Leo Vleugels , I would also draw your attention to the fact that PLAL almost always results in charge-stabilized NPs.

This means that upon drying the NPs as part of the sample preparation for electronic imaging (TEM/SEM), electrical double-layers (EDLs) get effectively compressed and electrostatic interactions become effectively screened as the liquid water content drops and the concentration of ionic species increases.

It is almost certain that the end result will be agglomeration and aggregation of many of your primary particles.

Thank you for the very detail answer Leo Vleugels and Omar Z. Sharaf . This means that, the process of agglomeration still could happen even after dry it on carbon tape? Before this, I thought that by drying it on carbon tape, it will prevent the agglomeration of nanoparticles.

Ainin Razak

There are good comments above from Leo Vleugels and Omar Z. Sharaf . I would add the following:

• Gustav Mie based his interpretation of light interacting with matter, via Maxwell's equations, on the color of gold sols. He has plots within his classic 1908 paper for maxima in the VIS region based on the size of the gold nanoparticles. See (registration required): The life of Gustav Mie and the development of the Lorenz-Mie solution to Maxwell’s equations https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W140225Gustav-mie-maxwell.html Slide 17 is relevant
• Carry out a Stokes' Law equation based on the size and (high) density (19.3 g/cm3) for gold. I think you'll find that 100 nm particles cannot stay in stable suspension and will settle/sediment

Based on the above 2 bullet points, and if you have a transparent (no turbidity or evidence of settling) purple sol then the likely size for your material is in the ~ 5 - 50 nm region and your particles of > 100 nm shown in SEM are a result of agglomeration and aggregation in the SEM preparation stage (drying , coating etc). Take a look at:

Dispersion and nanotechnology

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