“We have shown that plate-shaped silver nanomaterials are comparatively more toxic than other shapes because of the presence of surface defects – a nano-specific property that results from defective crystal structures,” team member Saji George told nanotechweb.org.

The researchers, led by André Nel, used cells isolated from the gills of rainbow trout and zebrafish embryos as test models to assess how toxic silver nanoparticles are. They studied nanospheres, nanowires and nanoplates at incremental doses.

Nanoplates were observed to be the most toxic, while nanopsheres were only toxic at high concentrations. Nanowires were much less toxic.

Oxidative stress

The nanoplates induce oxidative stress in cells because they produce superoxides, which are toxic. Indeed, pre-treating the target cells with N-acetylcysteine – an antioxidant molecule – protects the cells from the effect of the nanoplates. “Interestingly, the nanoplates need to be in direct contact with the cells to be toxic – observations that were confirmed by the results in intact organisms (zebrafish embryos),” said George.

Surprisingly, among the tested shapes of silver nanomaterials, the nanoplates shed fewer silver ions into the culture medium, he adds. Until now, scientists believed that these ions were the main culprit behind the toxicity of silver nanoparticles. Closer examination of the silver nanoplates’ surfaces using high-resolution transmission electron microscopy (HRTEM) revealed crystal defects, such as stacking faults and point defects, which increase the reactivity of these particles. Coating the surface of nanoplates with cysteine covered the defects and so reduced toxicity.

The results should be interpreted with caution, however, because the assessments took place in incubation media that were devoid of proteins, which could facilitate plate contact with cell surfaces, says George.

“Our study shows that the shape of a nanoparticle is important when it comes to toxicity,” he said. “We therefore hope that our results will help in the development of ‘safe-by-design’ strategies to realize more sustainable nanotechnology applications.”

The team now plans to find out whether silver nanoplates are toxic to mammalian lungs if inhaled.

The current work is detailed in ACS Nano.