Nanoparticle hollowing method promises medical advances

Hollowed nanoparticles
Image caption Electron transmission microscopy images show the nanoboxes with different interior structures

A process to "carve" highly complicated shapes into nanoparticles has been unveiled by a team of researchers.

It involves a chemical process which hollows out the particles into shapes such as double-walled boxes and multi-chambered tubes.

The researchers said this would aid the creation of more complex nano-objects.

They said these could ultimately be used to revolutionise medical tests and aid drugs treatments.

Ion attack

The research was carried out by the Catalan Institute of Nanotechnology in Bellaterra, Spain and is published inthe latest issue of Science.

To deliver their results the scientists refined a series of existing corrosion techniques including the "galvanic effect".

This involved treating tiny silver cubes with cationic gold - a type of gold that had had some electrons removed from its atoms, turning it into an ion.

When brought together at room temperature the cationic gold "attacked" the silver, stealing its electrons.

The loss of the electrons turned the affected silver atoms into ions which dissolved into a provided solution.

Meanwhile, by gaining electrons the cationic gold was transformed into "normal" metallic gold which was then deposited onto the top of the silver cube.

"This protects the silver - and as the cube's surface becomes covered, the reaction becomes more aggressive in other parts of the cube that have not been coated," said Prof Victor Puntes, the team's principal researcher.

"In the end you end up with a single hole on the surface of the silver which is not covered by gold where the reaction advances and then enters the cube from inside."

The professor said this prompted a second process known as the Kirkendall effect where silver atoms from inside the cube started "migrating" to the gold outside "offering themselves up" thus creating a void inside the cube.

"We can control the process to make different holes resulting in different structures," Prof Puntes added.

Although both the galvanic effect and the Kirkendall have been used for years, the scientists said that previous efforts to combine them in this way had failed because the galvanic effect was too aggressive.

They said their innovation was to introduce a range of factors which made the silver more resistant, the cationic gold less aggressive and dissolved by-products of the process which would have otherwise interfered with the structure's development.

Image caption The technique allows a range of hollowed shapes to be created in metallic nanoparticles

Nano-device drugs

Although the process has only just been outlined, the scientists are excited by its potential uses for the medical industry.

They said particles could be hollowed out so that they absorbed different energy wavelengths, helping to create body scanners that would be more accurate than current magnetic resonance imaging (MRI) equipment.

The researchers added that the technique could also aid drug delivery.

"It's a wonderful molecular suitcase," said Prof Puntes.

"You can have different sizes of cavity meaning that different-sized molecules enter different rooms of a structure. So you can have complex and controlled relief on the nanoscale - like cell dosing - dosing with a mixture of drugs that would otherwise be difficult to carry out."

Other examples given included the creation of components for nanoscale robots and new techniques to remove pollution from the environment.

However, the professor acknowledged that at this early stage he could only guess at the eventual uses such nanomaterials would have.

"When people first invented plastic they didn't know what to do with it, we knew electricity was around for over a thousand years before we learned how to do something useful with it," Prof Puntes said.

"This creates different materials so they will probably have lots of different properties."

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