Researchers in Spain have invented a new technique for making hollow nanoparticles with sophisticated shapes and compositions. The method, which combines two well-known corrosion processes into a single step, modifies the shape of tiny nanoparticles after they have been created. The resulting nanostructures could find use in drug delivery, catalysis and even as structural components for nanorobots.

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 colleagues at the Institut Català de Nanotecnologia have overcome this so-called sticky nanofinger problem using chemistry. The researchers have shown that corrosion processes such as galvanic replacement and the Kirkendall effect can be used to attack and pit nanoparticles from the "inside out". The result being complex geometric interconnected multicavity hollow nanostructures. Corrosion is much more aggressive for nanoparticles – compared with larger structures – because the tiny particles have larger surface areas relative to their volume.

A variety of nano-objects

The structures produced by the team range in shape from molecular labyrinths or nanomazes (made from silver and gold or platinum) to gold fullerenes. Other structures, such as nanoboxes, porous nanotubes and nanoframes, can also be fashioned from silver and gold nanoparticles (see figure).

The Kirkendall effect occurs when there is a movement of vacancies in a metal that is in the opposite direction to that of natural atom diffusion. 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 the thicknesses of which 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 says. "The nanoworld is a billion times smaller than the ordinary world, and phenomena occurring there seem like pure miracles when compared with 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 in the body – 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, adds Puntes.

Details of the work can be found in Science.