Preparation and characterization of nanoparticles
The results of DLS indicated that there was no significant difference between the particle size of NPsA, NPsB and NPsAB (the hydrodynamic diameter of NPsA, NPsB and NPsAB was about 82, 94 and 88 nm respectively). However, the hydrodynamic diameter of the nanoparticles increased after being conjugated with siRNA and PTX (Fig. 4). For example, the particle size of the NPsA increased from 88 nm to 120 nm after being encapsulated with siRNA and PTX. The surface charge of NPsA, NPsB and NPsAB was significantly reduced after being encapsulated with siRNA and PTX. It seems that the negative charge of phosphate backbone of the siRNA leads to decrease in the charge of the nanoparticles. A previous study found that the nanoparticles with a slightly negative surface charge and average particle sizes ranging between 100 and 150 nm have much higher transfer efficiency than larger nanoparticles (Bala, Hariharan, & Kumar, 2004; C. He, Hu, Yin, Tang, & Yin, 2010; Panyam & Labhasetwar, 2003). Although nanoparticles smaller than 100 may have higher transfection efficiency than nanoparticles larger than 100 nm but the high nonspecific uptake of these nanoparticles could increase the side effects on healthy tissue (Puppo et al., 2014).
The transmission electron microscopy (TEM) of the nanoparticles showed that the NPsA/siRNA/PTX, NPsB/siRNA/PTX and NPsAB/siRNA/PTX nanoparticles have spherical shape with uniform size distribution and smooth surface (Fig. 5). There was no significant difference between the morphology of the nanoparticles. The results also showed that the average size of the nanoparticles was about 100 nm. These results were consistent with DLS data (Fig. 4).