Nanobjects can be assembled from the bottom up, atom by atom or molecule by molecule, but this is usually a tedious process that generally involves picking up individual atoms or molecules with the tip of a scanning electron or atomic force microscope. The technique is also fiddly because the microscope tip has a tendency to “stick” to the nano-objects.

Now, Edgar Gonzàlez and co-workers have overcome this so-called sticky nanofinger problem thanks to colloidal chemistry. The researchers have shown that corrosion processes (like galvanic replacement and the Kirkendall effect) can be used to attack and pit nanoparticles from the “inside-out” to produce complex geometric interconnected multi-cavity hollow nanostructures. Corrosion is much more aggressive at the nanoscale than on the macroscale because of the larger surface area of nanoparticles exposed to attack.

A variety of nano-objects
The structures produced range in shape from molecular labyrinths or nano-mazes (made from silver and gold or platinum) to gold fullerenes. Other structures, such as nano-boxes, porous nanotubes and nanoframes, can also be fashioned from silver and gold nanoparticles (see figure).

The Kirkendall effect occurs when there is a flux of vacancies in the opposite direction to that of natural atom diffusion in a metal. This flux leads to voids being produced in the material. Galvanic replacement is also a simple way to make hollow nanostructures of noble metals when silver nanostructures are used as sacrificial templates. It is a one-step process that dissolves metallic nanostructures to produce constructs that are enclosed by continuous or porous walls whose thickness can be controlled.

Corroding objects in such a way would be impossible on the macroscale, says team leader Victor Puntes. “In the nanoworld, however, the effect occurs spontaneously if the corrosion ingredients and nanoparticles are mixed together properly,” he told nanotechweb.org. “The nanoworld is a billion times smaller than the ordinary world, and phenomena occurring here seem like pure miracles when compared to those happening on our everyday scale.”

Carrying cargo
The hollow nanoparticle capsules or cages can protect and carry different types of payload. They could be used to safely transport a drug to a target – for instance, to treat a tumour – or carry a specific catalyst to a reaction site. The capsules can also be open or closed, heated and manipulated by electromagnetic fields.

The researchers observed the structures they made using high-resolution transmission electron microscopy, which allowed them to analyse and visualize different shapes atom by atom.

The technique can also be readily adapted to industrial-scale production levels, added Puntes.

Details of the work can be found in Science.