Yes, it is possible that the diluent can contribute to your result and make the apparent size smaller than expected. The signal from just the poloxamer buffer shows the presence of something that appears like 150nm particles. When you add just a few 500nm particles to that, it may not be resolved as a second peak butt instead become one broader peak. http://www.materials-talks.com/blog/2016/08/16/why-does-dls-say-that-my-particles-are-too-small/ and http://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/ may be of interest to help explain this.

500 nm is quite large and settling may occur complicating the measurement. Why did you choose this size? It's ideal for laser diffraction, by the way. Do you have the ability to compare with laser diffraction as another light scattering technique? Also the scattering from 500 nm particles is enormous in relation to say 5 nm (d^6 dependence in the Rayleigh region) and will swamp the scattering of smaller material. What are you trying to measure? I confused when you say you measured your diluent and it's 150 nm. To me a diluent would be a pure liquid (with dissolved solids) such as PBS buffer.

I agree that 500nm is really big for DLS measurement based on Rayleigh diffusion. Have you measured the viscosity of the buffer with Pluronics? Viscosity has a huge effect on measured size as seen in the Stockes-Einstein equation: D = kT/3 pi . h . d

with:

D : diffusion coefficient

k : Boltzmann constant

h : viscosity of diluent

d : diameter

Thanks everyone. We determined that our viscosity value was significantly off. When we repeated using the correct viscosity, the results are more in-line with what we expected.