Science & Environment

Silver trumps gold in nano advance

Silver nanoparticle Bokwon Yoon
Image caption Silver nanoparticles have been used for their anti-bacterial and anti-fungal properties

Scientists have come up with a better way to make silver nanoparticles, which are used in textiles and cosmetics.

The tiny metallic particles are often used because of their anti-bacterial and anti-fungal properties - in wound dressings for example.

By using organic molecules to coat the particles, the researchers made them resistant to tarnishing, which has previously limited their use.

The work has been published in the journal Nature.

Previously, gold nanoparticles have been more widely used because they are more stable. Silver nanoparticles are cheaper, but tarnish too easily.

Now a team has developed a way to make stable silver nanoparticles on a larger scale than was possible before.

These nanoparticles are even more stable than their gold counterparts.

To manufacture the tiny particles, scientists created a mixture which combined silver atoms with organic molecules that formed an outer protective layer, and sulphur atoms that bridged the two.

"We had a big surprise when we looked at the structure of the different layers," said author Terry Bigioni from the University of Toledo in Ohio, US.

The nanoparticles were made up of 32 silver atoms arranged into a dodecahedral core. These were surrounded by what are known as "mounts": 3-D structures combining two silver atoms with five sulphur atoms.

The mounts held up the protective organic layer that surrounds the whole particle, Dr Bigioni explained.

It is this serendipitous, unique, and highly symmetrical structure that is responsible for the stability.

Nanoparticles are commonly grown using "seeds" - anchors to which the material clings in order for it to grow. This leads to many differently-sized particles being formed.

Another surprise from the research was that the seeds were not needed, and all the nanoparticles formed were the same size and identical in structure.

Once this unique structure has been created, the particle becomes an inert bystander that does not interfere with the growth of further nanoparticles. This means that large quantities of stable particles can be created.

"So far, we've only managed to destroy [the nanoparticles] at 400C," said Dr Bigioni.

Dr Rickard Arvidsson, from the Chalmers Institute of Technology in Sweden, highlighted potential environmental concerns for these ultra-stable particles.

"Generally, the persistence and stability of chemicals is associated with an environmental hazard, and there may be high environmental impacts when mining silver."

The stability and yield observed as a result of the new process has the potential to give silver nanoparticles the edge over those made from gold.

"We have an opportunity to study something very stable, and very unique. This will be big for materials science," Dr Bigioni explained.

The ultra-stable silver nanoparticles could find new uses in biomedical applications, the University of Toledo chemist said.

"As all the nanoparticles are the same size, testing their durability and effect in and on the human body is much easier than with conventional nanoparticles," he said.

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