A growing trend in the design of biomedical nanomaterials is to combine imaging and therapeutic capabilities. However, nanoparticles for in vivo biomedical applications must be small enough to escape uptake by the liver and spleen and, continue to circulate within the blood system to reach their target site and perform the desired therapeutic function. Generating uniform, discrete multifunctional nanoparticles small enough for such applications is challenging.

Fig. 1: Transmission electron micrograph of magnetic core / silica shell nanoparticles.

Mesoporous silica has uniform pore sizes, large surface area and accessible internal volumes and is an appealing host material for fluorescent imaging molecules or therapeutic agents. Now researchers from Seoul National University1 have designed monodisperse biocompatible nanoparticles with a mesoporous silica shell and single nanocrystal magnetic core which can operate as both imaging agents and drug delivery vehicles and are small enough to survive in vivo.

Hyeon, Moon and colleagues coated iron oxide nanocrystals with a surfactant which enabled their dispersion in water. The surfactant also operated as the organic template to generate a mesoporous silica shell around the iron oxide nanoparticle core. The surfactant could be removed and the particles coated with a polymer which prevented non-specific adsorption of proteins. Electron microscopy showed the resulting core –shell nanoparticles to be uniform with diameters ranging from 45 to 105 nm. Notably, the diameters of the nanoparticles could be increased in a controlled way by varying the concentration of the iron oxide core nanoparticles in the original solution. Crystals other than iron oxide, and also 1-dimensional nanostructures could be uniformly coated with mesoporous silica in this way.

The core-shell nanoparticles exhibited physical characteristics suitable for magnetic resonance imaging. Fluorescence imaging and drug delivery capabilities were demonstrated in vitro by loading the mesoporous silica shell with the requisite molecules.

Furthermore, in preliminary in vivo experiments, intravenously injected fluorescently labelled nanoparticles survived long enough in the circulatory system to accumulate in the tumours of nude mice through a passive targeting mechanism. This was confirmed by magnetic resonance and fluorescence imaging of the tumour region compared to other tissues after introduction of the nanoparticles. The nanostructures thus show potential for combined in vivo imaging and therapeutic agents, say the researchers.