Synthesis and biomedical applications of hollow nanostructures

Hollow nanostructures have attracted tremendous attention from researchers in various disciplines because their high surface to volume ratio and large pore volume are highly desirable for many technological applications including drug delivery system. Several colloidal synthetic methods have been used to synthesize various hollow nanostructures. These synthetic approaches are mainly categorized into four main classes according to how the hollow structure is formed: the Kirkendall effect, chemical etching, galvanic replacement, and template-mediated approach. The large pores inside the hollow nanostructures can encapsulate and release various drugs and biomolecules, while the surface of the nanostructure can be functionalized for drug targeting or bio-labeling. These features make the hollow nanostructures a unique and promising candidate as multifunctional drug delivery vehicles. This review article covers recent progress concerning the synthesis of hollow nanostructures with their sizes smaller than 200 nm and their biomedical applications including specific targeting, imaging, and controlled release of therapeutics for simultaneous diagnosis and therapy.