Introduction
Hollow nanospheres are a class of special nanomaterials being named on the basis of their morphologies, which have an empty space inside and their dimensions are in the nanometer range. Because of the existence of a hollow cavity, the surface area of hollow structures is significantly larger while the density is much lower than that of their solid counterparts with the same composition and size.
Fig 1 SEM of hollow nanospheres
One of most effective synthesis methods for hollow nanospheres is hard templating synthesis (Fig 2). Briefly, hard templates with specific shapes were synthesized first, followed by coating the outer surface with a layer of desired materials, the core materials were then selectively removed to obtain the hollow structure.
Fig 2 The hard templating synthesis process
(Chem. Rev., 2016, 116, 10983−11060)
Hollow nanospheres have attracted a lot of interest due to their excellent chemical/physical properties which associated with their unique structures, such as large surface area, low density, good biocompatibility and high loading capacity. Because the void space can be used as the storage for different cargoes, they could serve as imaging contrast agents, drug delivery carriers, and anodes or cathodes for lithium ion batteries. In addition, they have a large variety of applications, including nanoreactors, catalysis, biosensors, environmental remediation and so forth.
Applications
Drug Delivery: Hollow nanospheres have been widely used as drug carriers because of cavity structure, large specific surface area, good biological safety and biocompatibility, and easy surface functional modification. The hollow structure makes them able to load medicines and other functional materials, which form significant advantages in drug delivery field.
Biomedical Imaging: Hollow nanospheres are excellent photoacoustic contrast agents, because they can effectively enhance the specific absorption of organizations and have much higher photoacoustic efficiency than blood.
Nanoreactors: Hollow nanospheres have naturally existed or synthesized void space inside the shell or on the porous shell, which could provide the “microenvironment” for some reactions, and thus could be treated as nanoreactors.
Energy Storage: The void space in hollow nanospheres can alleviate the structural strain and accommodate the volume variation during lithium insertion / deinsertion, significantly reducing the electrode pulverization and hence improving the cycling stability of Lithium-ion batteries.
