Nanoscale Hollow Spheres

 Nanoscale hollow spheres are intensely discussed nowadays with regard to their functionality (e.g., large specific surface, low specific weight, container-type morphology) and technical application (e.g., catalysis, gas storage, low-weight building materials, drug delivery). Most commonly hollow spheres are prepared by so-called hard-template methods, including the precipitation of a shell - representing the sphere wall later on - on a solid template. Accordingly, monodisperse, non-agglomerated and nanoscaled templates such as silica, elemental metals (e.g., Fe, Ag, Au, Pt), quantum dots (e.g. CdSe) or organic polymers (e.g. polymer latices) are prerequisite. Moreover, the template has to be removed completely without any destruction of the sphere wall.

 

As an alternative approach to nanoscale hollow sphers, we have introduced microemulsion techniques for the first time. In contrast to standard microemulsion techniques, the reaction is here not performed inside a micelle, but at its phase boundary (Figure 1). To this concern, one reactant is added to the polar phase; the second reactant is added to the non-polar phase. By this measure the reaction is indeed restricted to the phase boundary resulting in various types of hollow spheres (Figure 2). Typically, the as-preapred nanoscale hollow spheres exhibit outer diameters of 20 to 30 nm, a wall thickness of 3 to 10 nm and an inner diameter of 10 to 20 nm.

Figure 1: Scheme illustrating the synthesis of nanoscale hollow spheres via microemulsion techniques with one reactant present in the dispersant phase (here: dodecane) and another reactant present in the micelle phase (here: H₂O).

Figure 2: HRTEM of selected nanoscale hollow spheres as prepared via microemulsion techniques.

 

In addition to various types of hollow spheres, the microemulsion strategy allows an independent modification of the inner and outer surface of the hollow spheres. As an example, we have prepared core@shell nanocomposites with Pd⁰ encapsulated by SnO₂ shells (Pd@SnO₂) and SnO₂ shells covered with Pd⁰ (SnO₂@Pd) (Figure 3). Such structures are highly relevant for sensing of reductive gases, such as H₂, CO, CH₄, ethanol. Here, both structures show very different sensor performance. Notably, the SnO₂@Pd nanocomposite shows a very promising performance under CO exposure, and especially, under real-life conditions in humid air and at low temperatures (Figure 9). Such behaviour of core@shell nanostructures can be interesting for various kinds of sensors as well as for catalysis, in general.

Figure 3: Different sensing behavior of Pd@SnO₂ and SnO₂@Pd core@shell nanostructures for temperature-dependent sensing of CO (Pd-free SnO₂ shells as a reference).

 

Besides sensing, nanoscale hollow spheres can be used for gas sorption and separation. As an example, AlO(OH) hollow spheres with a specific surface area of 530 m^2/g show a CO₂ uptake of 260 mg/g. As the sorption of nitrogen is lower (30 mg/g), a separation effect can be observed by performing certain temperature-pressure cycles (Figure 4).

 

Figure 4: CO₂ and N₂ sorption of nanoscale AlO(OH) hollow spheres performing temperature-pressure cycles.

 

An additional aspect of nanoscale hollow spheres is related to their container-type functionality. To this concern, we realized doxorubicin-filled AlO(OH) hollow spheres (DOX@AlO(OH)) as drug-delivery system for cancer therapy (Figure 5). Based on lung adenocarcinoma cells (in-vitro model) and an orthotopic breast cancer BALB/c mouse model (in-vitro model), a promising anti-tumor as well as anti-metastatic effect is observed at low side effects. This concept of drug encapsulation in nanoscale AlO(OH) hollow spheres can become interesting on a much more general scope, including additional pharmaceuticals as well as alternative hollow sphere materials.

  

Figure 5: Scheme illustrating the encapsulation doxorubicin-filled AlO(OH) nanocontainers and its use in an orthotopic breast cancer BALB/c mouse model.

   

For more information see:

C. Zimmermann, C. Feldmann*, M. Wanner, D. Gerthsen, Nanoscale Gold Hollow Spheres via Microemulsion Approach, Small 2007, 3, 1347–1349.

D. H. M. Buchold, C. Feldmann*, Nanoscale g-AlO(OH) Hollow Spheres: Synthesis and Container-Type Functionality, Nano Lett. 2007, 7, 3489–3492.

H. Gröger, F. Gyger, P. Leidinger, C. Zurmühl, C. Feldmann*, Microemulsion Approach to Nanocontainers and its Variability in Composition and Load, Adv. Mater. 2009, 21, 1586–1590.

S. Simonato, H. Gröger, J. Möllmer, R. Staudt, A. Puls, F. Dreisbach, C. Feldmann*, Reversible Sorption and Storage of CO₂ with Nanoscale γ-AlO(OH) Hollow Spheres, Chem. Commun. 2012, 48, 844–846.

F. Gyger, A. Sackmann, M. Hübner, P. Bockstaller, D. Gerthsen, H. Lichtenberg, J.-D. Grunwaldt, N. Barsan*, U. Weimar*, C. Feldmann*, Pd@SnO₂ and SnO₂@Pd Core@Shell Nanocomposite Sensors, Particle 2013, in press.

H. Goesmann, C. Feldmann*, Nanoparticulate Functional Materials (Review), Angew. Chem. Int. Ed. 2010, 49, 1362−1395.