Particle size. The particle size of complexes can be characterized using dynamic light scattering, which is a well-established technique for measurement of particle size over a scale range of nanometers to micrometers. In suspension, particles scatter light, which fluctuates depending on size. Therefore the intensity of fluctuations in scattered light (due to Brownian motion of particles in solution) are analysed by an autocorrelation method to yield a diffusion constant and ultimately a particle size expressed as the mean hydrodynamic diameter (Xxxx, S.T. et al., 1994). Although the polydispersity of cationic lipid/nucleic acid complexes make the analysis of the resulting data difficult, a mean diameter can still be obtained. The particle size (diameter) of a lipid/ nucleic acid complex has been reported to influence the deposition of the complexes after administration as well as their uptake by target cells (Xxxxxxxx, X.X. xx al., 2004, Xxxxxx, X. et al., 2004). Because large molecules are poorly endocytosed, it has been suggested that transfection complexes with small particle size (≤100 nm) are more efficient for transfection and provide better extravasation to target cells. In contrast, more recent reports have shown that larger particles (micron range) are preferred for in vitro transfection (Xxxxxxx, R.I. et al., 2002, Xxxxxxxx, C. et al., 2006, Xxxxxxxx, X.X. et al., 2007). This was associated with quicker sedimentation at the cell surface compared to smaller complexes and a suggestion that large complexes reach the lysosomes relatively slowly compared to smaller particles. Cationic lipid/siRNA complexes are usually smaller than cationic lipid/pDNA complexes (nano range, from ~50 to 800 nm), however, transfection efficiency has not been restricted to small or large particle size as complexes of small size (<200 nm) or larger aggregates (500-800 nm) have both been reported to be efficient in delivering functional siRNA (Spagnou, S. et al., 2004, Xxxxx, Y. et al., 2010).
Particle size. See appropriate SOP "Use of Micron Air-jet sieve".
