Main

The trusty sphere remains the preferred form for nanoparticles but this shape leaves only one surface for modification, complicating the generation of multifunctional particles. In 2002, Charles Martin's laboratory at the University of Florida decided to try a different approach. “We needed a technology that could be modified differentially between the inner and outer surface,” explains Sang Bok Lee, a former postdoc in Martin's lab. The solution they arrived at was to use silica nanotubes—easy to synthesize, soluble in aqueous solutions, and offering two easy-to-modify surfaces (Mitchell et al., 2002).

Lee, now at the University of Maryland, has recently expanded on this work, demonstrating a modification that allows the generation of magnetic versions of these nanotubes (Son et al., 2005). By simply layering the inner surface of the tubes with magnetite, the tubes become much more useful for in vivo applications. “The magnetic properties will give you imaging capabilities using MRI, so that you can simply trace the nanoparticles inside the body,” says Lee. “And one more potential advantage with using these magnetic nanotubes is with magnetically assisted biointeraction.... If you can use magnetic fields, then you may be able to hold those nanoparticles at specific spots inside the body, to give enough time for those particles to interact with cancer cells or [other targets].”

They also have promise for in vitro applications. In one experiment, Lee's group functionalized the inner surfaces with molecules capable of binding a specific dye; when the tubes were added to a dye solution and then magnetically isolated, nearly 95% of the dye was removed. Likewise, nanotubes with inner surfaces coated with antigen proved capable of highly specific magnetic separation of antibodies recognizing that protein.

Lee's primary interest, however, remains in optimizing these magnetic nanotubes for use in drug delivery. Although drugs can readily be loaded into the tubes, preventing early drug release remains an obstacle, and the group is investigating potential solutions. “Ideally,” says Lee, “we are hoping to modify the inner surface with drug molecules through strong chemical interactions such as ionic or chemical bonds... [after which] we can use enzymatic activity or another treatment to cleave those chemical bonds so that drug molecules can easily be released after a short time.”