A Nonviral DNA Delivery System Based on Surface Modified Silica-Nanoparticles Can Efficiently Transfect Cells in Vitro

Diverse polycationic polymers have been used as nonviral transfection agents. Here we report the ability of colloidal silica particles with covalently attached cationic surface modifications to transfect plasmid DNA in vitro and make an attempt to describe the structure of the resulting transfection complexes. In analogy to the terms lipoplex and polyplex, we propose to describe the nanoparticle−DNA complexes by the term “nanoplex”. Three batches, Si10E, Si100E, and Si26H, sized between 10 and 100 nm and with ζ potentials ranging from +7 to +31 mV at pH 7.4 were evaluated. The galactosidase expression plasmid DNA pCMVβ was immobilized on the particle surface and efficiently transfected Cos-1 cells. The transfection activity was accompanied by very low cytotoxicity, with LD50 values in the milligrams per milliliter range. The most active batch, Si26H, was produced by modification of commercially available silica particles with N-(6-aminohexyl)-3-aminopropyltrimethoxysilane, yielding spherical nanoparticles with a mean diameter of 26 nm and a ζ potential of +31 mV at pH 7.4. Complexes of Si26H and pCMVβ plasmid DNA formed at w/w ratios of 10 were most effective in promoting transfection of Cos-1 cells in the absence of serum. At this ratio, >90% of the DNA was associated with the particles, yielding nanoplexes with a net negative surface charge. When the transfection medium was supplemented with 10% serum, maximum gene expression was observed at a w/w ratio of 30, at which the resulting particle−DNA complexes possessed a positive surface charge. Transfection was strongly increased in the presence of 100 μM chloroquine in the incubation medium and reached approximately 30% of the efficiency of a 60 kDa polyethylenimine. In contrast to polyethylenimine, no toxicity was observed at the concentrations required. Atomic force microscopy of Si26H−DNA complexes revealed a spaghetti-meatball-like structure. The surface of complexes prepared at a w/w ratio of 30 was dominated by particles half-spheres. Complex sizes correlated well with those determined previously by dynamic light scattering.