I'm producing ~200 nm siRNA PLGA/DOTAP nanoparticles using the double-emulsion solvent evaporation technique. When I sized the particles using DLS the majority (95%) were of the correct size range, however 5% of particles had a peak size of 5 um. This came as a surprise and I'm curious if anybody has ever come across this and could speculate on the reasons?
It is normal that particles are multimodal. during the preparation some particles may be large or aggregate. you can centrifuge at low speed/time to remove them then collect your particles at high speed/time.
No problem with that, it can be that the formulation is aggregating or you may have some "dirt" in the cell. Also, see whether a visualization as volume instead of intensity gets rid of the peak. Measuring intensity often carries along some "artifacts" which are intrinsic in the DLS, as the contribution of just a few huge particles (as is probably your case) is extremely high (bigger particles scatters more light, higher intensity is recorded).
In any case, do not worry.
Most likely aggregation of particles, it is common in all nanoparticle samples, filtering will remove those if you get small enough filter. You can also look at this sample in detail with the use of NTA microscope. There is one in Trinity (IMM) that will tell you exactly the size distribution of your particles as DLS is more of an average size technique.
Many thank all for your answers. I might give centrifugation a try to see if that solves it. Filtering sounds like a good idea (thanks Tanya ;-)), but my sample size would be too small. For my next batch I've taken some other steps too if the particles are something else than aggregates. These include degaussing the equpiment (dust) and filtering the PVA with 0.2 um filter instead of 0.44 um. Will let you know what helps.
You can get small volume spin filters that might work. Hope it works out:)
It is very common. You can try decantation technique to remove those fine partilces and freeze dry.
Filtering the PVA with a 0.2 um filter seems to have done the job. Thanks for all the tips.
Just want to add that according to your data, filtering 0.2 µm will result in eliminating your aggregates and your dirt, but probably you have also separated quite a good portion of your sample (your size distribution is spot on 0.2µm). Your yield may be affected.
The data do not look as "bad" as you make them sound. The ~5micron peak could be from air bubbles, and centrifugation or filtration (0.45micrometer) of your final sample could help there. In addition, even though it is 5% by intensity, in terms of concentration the contribution is very likely much smaller. (You can look at the volume distribution of your same data set). For polymer filtration, the Anotop 20nm filters are quite good, if you want to filter any monomer components before your process.
http://www.materials-talks.com/blog/2014/01/23/intensity-volume-number-which-size-is-correct/
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