Dear friend Dorian Foster

Dynamic light scattering (DLS) is a widely used technique for measuring the hydrodynamic diameter of nanoparticles in solution (DLS characterization of iron oxide.....). However, DLS measurements can be affected by many factors such as particle concentration, particle size distribution, and surface charge (Does anyone know how to......).

In your case, you mentioned that you have been trying different solvents (DMSO & water) and varying concentration but are consistently getting high HD values and bad readings in general. One possible reason for this could be that your particles are aggregating in solution. To avoid aggregation, you can try using surfactants such as Tween 20 or Triton X-100 (Does anyone know how to......).

Another suggestion is to use a separation technique such as field-flow fractionation (FFF) prior to DLS measurements. FFF can help separate particles based on their size and shape before DLS measurements are taken (DLS characterization of iron oxide.........).

I hope this helps! Let me know if you have any other questions.

Source:

(1) DLS characterization of iron oxide nanoparticles - any thoughts?. https://www.researchgate.net/post/DLS_characterization_of_iron_oxide_nanoparticles-any_thoughts.

(2) Highly Stable Amine Functionalized Iron Oxide Nanoparticles ... - PubMed. https://pubmed.ncbi.nlm.nih.gov/25554710/.

(3) Does anyone know how to interprete DLS measurements?. https://www.researchgate.net/post/Does_anyone_know_how_to_interprete_DLS_measurements.

(4) Is it possible to use DLS to determine the molecular ... - ResearchGate. https://www.researchgate.net/post/Is-it-possible-to-use-DLS-to-determine-the-molecular-weight-of-polymers-How-would-this-be-accurate-and-what-are-the-limitations.

Not too sure how is your result looks like. Here are some assumptions.

1. One sharp peak on high hydrodynamic size.

Possible reason 1: Your nanoparticles are aggregated. You may need to add some surfactant or adjust the pH of your solution. Alternatively, you may try to ultrasonicate your nanoparticle solution.

Possible reason 2: There might be dust/contaminants in your solution. As DLS is based on optical technique, the large molecules might be blocking your nanoparticles from the instrument to read, and hence disturbing the reading. In this case, you may try to use syringe filter to filter your solution multiple times before reading.

2. Multiple peaks, but one peak is fallen on the desired size

Possible reason: Same as the possible reason 2 mentioned above. You may need to filter your solution.

Also, you may need to take note on the morphology of your nanoparticle. The DLS technique assumes that all your particles are in spherical shape. If your nanoparticles are like rod shape, it might lead to inaccurate results, and generally having a high PDI value.

Let me know if you have any question. Hope it helps!

My suggestion: use SEM, not DLS to properly characterize the particles. They are most probably forming clusters of a higher diameter

Federico Svarc not sure if in this case electron microscopy (EM) is not the best choice since the particles are going to be modified and the difference before/after could be not detectable by microscopy because the sample have to be dried (not same configuration of the coating layer as in solution) and the contrast for the organic coating layer in general can be low.

Dorian Foster as mentioned earlier, for the preparation of your sample, you need to be sure that you do not introduce any inhomogenizes (like dust). I will recommend you to filter your solvent (medium) before dilution. If it is possible, you can also filter your suspension after dilution. Since you are going to compare your results before and after surface modification, it is better to use some buffer with low ionic strength, which will help you to fix the Debye screening length. Please note that DLS measure HD, meaning the diameter of the particle plus the layer which moves with it (it can be counter ions or attached polymer, for example). So in case you need to check if there is the increasing of HD after, for example, polymer grafting, the thickness of grafted polymer should be higher than Debye screening length. Since your particles are small the autocorelation function will be affected in case the presence of aggregates or big particles (intencity of sattered light is proportional to d^6). Also it is important to chose the good fitting function, since the standard Cumulant is assuming that there is only one main size of NPs with a specific Gaussian distribution. The SBL algorithm is more accurate in this case please check https://www.schaefer-tec.it/sites/default/files/01-%20Product%20Vasco%20series%20particle%20size.pdf

Jia Le Wee Thanks for your answer and sorry for the delay in response -- I pivoted to a different portion of my research. I am mostly seeing the first case in my previous DLS attempts and am definitely thinking the problem is primarily aggregation. Do you have any suggested solvents or papers you could refer me to on the subject?

Hello Dorian Foster

if you can add some data files it might help here.

Did the manufacturer state any suitable conditions for the material, then that might be a good starting point.

Colloidal Gold particles can be quite monodisperse, and are even available as a standard, see https://www.materials-talks.com/polydispersity-from-a-gold-standard/ and this poster Poster Investigation of Size and Polydispersity of a Nanoparticle R...

For more generic nanoparticle characterization topics you might like this "question & answer" collection at https://www.materials-talks.com/tips-tricks-for-nanoparticle-characterization/