Nanoscale Res Lett (2009) 4:982–992 gene transport. Although PLA-PEG nanoparticles has several good qualities, such as good biodegradability, biocompatibility, and nonimmunogenicity, it takes on negative surface charges because of the carboxylic end groups of PLA. To prepare PLA-PEG mediated DNA complex, the cationic surface-active agent CTAB was used to modify the surface of PLA-PEG nanoparticles to make the surface take on positive charges. However, it was found that nanoparticles prepared with sole CTAB as surfactant (CTAB-NPs) revealed highly aggregated structures and heterogeneous size in TEM imaging (Fig. 1a), which was in accord with the other report [29]. In contrast, nanopar- ticles formulated with the popular non-ionic surfactant, Tween 80 (Tween 80-NPs) exhibited a uniform and smaller particle size without aggregation (Fig. 1b), which might be due to the superior emulsifying capacity of Tween 80. Moreover, it has some interesting characteristics useful for gene therapy because of the presence of poly(ethylene- glycol) (PEG) chains in its structure. It has been hypothe- sized that Tween 80 may have a similar fusogenic property with DOPE (Dioleylphosphatidylethanolamine) [30]. When a comparable concentration of Tween 80 is added to the formulation of nanoparticles, their transfection capacity will be improved signiﬁcantly [31]. Besides, Tween 80 has another important characteristic for the transfection of these systems in vivo. It creates a steric barrier [32] which shields the excess of positive charges of the systems and reduces the interaction with blood components, such as serum proteins, which could limit the arrival of the gene therapy system to the cell surface. Taking these into con- sideration, Tween 80 was applied as a cosurfactant together with the cationic surfactant CTAB to formulate PLA-PEG nanoparticles in the present study. 987 Formation of DNA-PLA-PEG-NPs Agarose gel electrophoresis and Picogreen-ﬂuorometry method were, respectively, performed to investigate the optimal ratio of NPs to DNA for the binding efﬁciency qualitatively and quantitatively. It was observed that sole plasmid DNA could migrate to the positive electrode under the electric ﬁeld such as lane 1 and 8 in Fig. 2. Once DNA was associated with the nanoparticles which was too large to diffuse through the agarose matrix, the mobility of DNA was hindered and it was detained in the well of the agarose gel. In addition, the part of DNA which did not bind with PLA-PEG-NPs migrated to the positive electrode in the same manner with the control DNA. The results of agarose gel electrophoresis (Fig. 2) showed that PLA-PEG-NPs may bind with DNA to various degrees with different mass ratios of NPs to DNA in the complex. There was decreasing free DNA present in the lane with the increase of the ratio of NPs/DNA. When the mass ratio of NPs to DNA reached 100:1 or above, almost all DNA was com- bined with NPs without free DNA bands in the lane visible. Meanwhile, the results obtained by Picogreen-ﬂuorometry analysis were consistent with that of agarose gel electro- phoresis. As shown in Fig. 3, the binding efﬁciency was increasingly raised with the mass ratio of NPs to DNA from 10:1 to 100:1, and it was 95.36% when the ratio reached 100:1. Further increase of the ratio did not sig- niﬁcantly improve the binding efﬁciency but will decrease the DNA loading efﬁciency of nanoparticles. Thus 100:1 was selected as the optimal mass ratio of NPs to DNA for further study. Fig 1 TEM imaging of CTAB-NPs (a) and Tween 80-NPs (b) Fig. 2 Agarose gel electrophoresis analysis of PLA-PEG-NPs com- bining with DNA at different mass ratio. Lane 1, 8: DNA Control; Lane 2–7: the mass ratio of NPs/DNA was 10:1, 50:1, 100:1, 150:1, 200:1, 500:1, respectively 123