| Titanium dioxide (TiO2) is a biologically inert compound that has been studied for decades due to its unique photo-catalytic characteristics. It is present as particles in commercial paints, toothpastes, cosmetics, and sunscreens. Particles of TiO2 smaller than 20 nm have novel surface properties that permit covalent surface modification. These surface modifications allow the attachment of single stranded nucleic acid sequences directly to the nanoparticle surface creating TiO2-nucleic acid nanoconjugates. The nucleic acid component of the nanoconjugates maintains its ability to anneal to complementary sequences. Photo-excitation of the nanoconjugates causes a charge separation within the nanoparticle leading ultimately to the scission of the bound DNA molecule. Modification of the nanoparticle surface with gadolinium contrast agents permits detection of the nanoconjugates in cells by magnetic resonance imaging. Thus these metal-oxide-biomolecule nanoconjugates constitute a potential gene targeting and tumor imaging agent.Aside from the rationally designed aspects of nano-sized materials that provide beneficial attributes, nanoparticles also display novel properties that cannot be predicted based on the material's bulk form characteristics. In order for these potential nano-medicines to become a viable clinical therapy, it is imperative to understand the basic factors that govern their interaction with living cells. One of the drawbacks associated with use of TiO2 nanoparticles has been the lack of available intracellular imaging techniques. In the work described herein, new approaches to covalently modify the surface of the nanoparticles and TiO2-DNA nanoconjugates are described allowing fluorescent detection. These methods subsequently permitted (1) the analysis of the relative intracellular stability of the nanoconjugates in malignant cells and (2) the study of the endocytic mechanisms that govern the internalization of these particles into living cells. This understanding is vital to the functionality of the nanoconjugate, and to the ability to target specific cell types. Overall, these results provide a significant step in understanding the interactions between cells and TiO2 nanoparticles, while the techniques described open the door to intensive investigation of these nanoparticles for a variety of in vitro and in vivo work. |
