Dear Sirwan Zare:

 You rotate the supposedly  soft sphere. Under the influence of centrifugal force sphere flattened. Therefore, the hydrodynamic radius increases. The relaxation time of the system is large. Maybe it will take after settling previous size.

Regards.

Hello,

i would advise you to carry out centrifugation at lesser RPM and later try sonicating the particles for 5min after the coating. This step can separate out the particles if they are aggregated. 

You could try sonication to break up the aggregations. Try and see if that works.

In my opinion it is not necessarily the problem of your system but rather of the duration of your centrifugation step. Derived from the Stokes equation you obtain the following expression for the settling velocity of your particles:

V = d^2 (p_p - p_f)*g/(18*n)

where V is the settling velocity, d is the particle diameter, p_p is the particle density (for chitosan-TPP particles ~1.3 g/cm^3), p_f is the fluid density (~1 g/cm^3 for aqueous solutions), g is the gravitational force during centrifugation and n is the viscosity of the fluid (~1 mPa*s for aqueous solutions).

You can repeat the math, but to isolate particles in a small reaction tube (1 cm) with 250 nm you need to centrifuge ~16 min at 10000g. However, if you also want smaller particles from your distribution, say all particles up to 150 nm, you need to centrifuge for ~45 min. I also attach an excel sheet with the calculations.

If you now measure your isolated particles, they will always be bigger than the original population since you loose small particles during the process. If after this, your particles are still too big, then there is probably a problem with the stability of your system.

Hi

I have the same issue with increasing size of PLGA after centrifugation step. Have you found any way to avoid this issue.

Thank you

ALI

Sirwan Zare did u find a solution to this problem?

if yes would u plz tell me how?

TIA