The application of nanomaterials such as nanoparticles, nanotubes, nanorods, and nanowires in biological systems has attracted great interest in the fields of materials science and biochemistry.[1] Because of their dimensions, which make them suitable for application in biological systems, the potential of nanomaterials for biolabeling, [2] biodetection, [3] bioseparation, [4] and biomolecule delivery [5] has been explored. In particular, the use of some inorganic nanomaterials as biomolecule carriers has been shown; gold nanoparticles and nanorods, for example, have been employed in DNA delivery.[5]Unlike nanoparticles or nanorods, nanotubes have a unique, hollow structure, which allows the modification of their inner surface and filling with specific biomolecules. [6] In addition, the tube structure may act as a physical shield for the inserted biomolecules and provide advantages for biomolecule delivery. However, the applications of nanotubes as biomolecule carriers are still very rare. In this work, fluorescent silica nanotubes with an inner diameter of hundreds of nanometers are synthesized by a sol±gel reaction using an anodic aluminum oxide (AAO) membrane as a template. The green-and red-fluorescent silica nanotubes are obtained by incorporating CdSe/ZnS core±shell semiconductor nanocrystals with diameters of about 4 nm and 8 nm, respectively. The fluorescent nature of the nanotubes allowed us to visualize their localization in living cells. The inner surfaces of the nanotubes were coated with positive charges to provide efficient DNA loading. We found that nanotubes filled with the gene encoding green fluorescence protein (GFP) entered monkey kidney COS-7 cells and that these cells exhibited GFP expression. These results demonstrate a novel application of nanotubes in biomolecule delivery. Figure 1 describes briefly the preparation of fluorescent silica nanotubes (fNTs) and their use for gene delivery. A commercial AAO membrane with an average pore diameter of about 200 nm was employed as the template for silicananotube preparation. A layer of silica was coated on the entire surface of the membrane by a sol±gel process using tetraethyl orthosilicate as reactant.[7] The silica layer was then passivated with a monolayer of 3-(aminopropyl)trimethoxysilane (APTMS) to generate a polycationic surface.[8] The resulting membrane was placed in a CdSe/ZnS core±shell nanocrystal solution [9] to form a nanocrystal layer on the silica surface due to the electrostatic forces. An additional silica layer was then coated onto the membrane to protect the nanocrystals from oxidation. In order for the nanotubes to be usable in DNA delivery, the silica surface was further passivated with a polycationic layer of APTMS to facilitate DNA loading. The membrane was then mechanically polished to remove the silica from the top and bottom surfaces of the membrane until the resulting membrane had a thickness of approximately 1±3 lm. The membrane was subsequently removed to release the nanotubes. The resulting silica nanotubes were w...

Supplemental Digital Content is available in the text