Gold nanoparticles create purple smoke

Researchers at the University of Bristol have confirmed that spherical gold nanoparticles are responsible for the purple smoke that appears when fulminating gold is detonated.

With this discovery, the scientists have solved a 400-year-old alchemy puzzle.

In a recent preprint, the researchers explain that fulminating gold, which was first discovered by alchemists in the 16th century, is a mixture of a number of different compounds, with ammonia providing the majority of the material’s explosive power.

German alchemist Sebald Schwaertzer noted the unusual purple smoke given off when fulminating gold was detonated in 1585, and the material was later studied by leading figures of chemistry in the 17th and 18th centuries, including Robert Hooke and Antoine Lavoisier.

But while the chemistry of the fulminating gold recipe has been understood for centuries, the question of what produced purple smoke remained unanswered.

It was long supposed, yet previously never proven that the rich purple colour of this cloud was due to it being formed of gold nanoparticles.

“I was delighted that our team has been able to help answer this question and further our understanding of this material,” Simon Hall, senior author of the study, said in a media statement. “Our experiment involved creating fulminating gold, then detonating 5mg samples on aluminum foil by heating it. We captured the smoke using copper meshes and then analyzed the smoke sample under a transmission electron microscope.”

Sure enough, the researchers found the smoke contained spherical gold nanoparticles, confirming the theory that the gold was playing a role in the mysterious smoke.

Having solved one historic scientific puzzle, Hall and his team plan to use this methodology to study the precise nature of clouds produced by other metal fulminates such as platinum, silver, lead, and mercury, which remains an open question.