Gold nanoparticles riddle solved – offering medical hope
Products based on nanoparticles are not new, with gold nanoparticles used by the Ancient Egyptians to make paints lasting for millennia - but they are promising to deliver a wide range of new medical imaging and treatment technologies.
The question is, how to make them to have the properties one wants. A variety of recipes are known, but why different approaches give different products, and the chemical details of what happens during nanoparticle formation, had not been rationalised.
An international group of researchers including the University of Sydney have solved the riddle in work published today in The Proceedings of the National Academy of Science of the United States of America. It is all in the glue that binds the surface of the gold nanoparticles to keep potentially destructive chemicals out of contact range.
University of Sydney Theoretical Chemistry Emeritus Professor Noel Hush said the research showed that for nanoparticles coated by sulfur-linked species, the attached molecules are held to gold by the same forces that hold together the gene-defining molecules in our DNA.
Typically sulfur compounds have been used for these glues as this binds by an ordinary chemical bond more strongly than most common chemical linkers. Gold is referred to as a noble metal because it generally does not react with the things around it. What makes sulfur special turned out to be the key to understanding how nanoparticle grow.
For 30 years, people have believed that this is the way that sulfur glues stick to and protect gold nanoparticles. However, this research shows that such bonds cannot be made to noble metallic gold, only to isolated gold atom.
Instead, it is the van der Waals force responsible for secondary biological structure that binds sulfur to gold metal and nanoparticles. The strength of such forces to gold is legendary, and of all common elements that bind to gold, sulfur experiences the largest. Chemical reactions can then be classified as to whether they favour or disfavour Au(I)-thiolate production, and the reaction products classified accordingly, providing the key missing link to understanding nanoparticle synthesis.
Armed with this insight, the researchers were able to predict intermediate chemicals that must have been present during the Brust-Schiffrin nanoparticle synthesis. These completely unexpected intermediates were then subsequently observed, confirming that the nature of the sulfur glue had been correctly determined.
“Knowing how processes occur allow them to be controlled and new products produced,” Professor Hush explained. “These discoveries will pave the way for a new generation of gold nanotechnologies.”
Gold nanoparticles are used very extensively in medicine and industry. In electron microscopic examination of cells, nanoparticles are inserted into the cell to track its movements. This is very widely used in medical research and practice to track cell growth in cancer tumours for example. They are also used in antibody research particularly on cell surfaces.
“In industry, they are of very great importance in catalysis and our research will give impetus to rational design of nanoparticles to meet specific requirements, with immediate effect,” Emeritus Professor Hush said.
Expert in nanotechnology and commercialisation, Professor Thomas Maschmeyer said: “There are many peculiarities in the reactivity of gold at such small scales – this work opens a new way to think about these exciting nano-engineered systems.”
Emeritus Professor of Biochemistry Philip Kuchel commented that with these findings, it would be possible to bring about new sorts of atomic and molecular interactions with gold.
“Tiny gold particles, even around 100 atoms, will be able to be modified with antibodies using new forms of linkage chemistry, to target them to the surface of particular cell types,” Emeritus Professor Kuchel said. “Such cells might be cancerous ones, or those performing specialised functions such as insulin secretion in diabetic patients.
“The cellular or even sub-celluar location of the targeted nanoparticle might be determined and used to answer fundamental questions in molecular cell biology, and potentially in targeting gold-linked drugs to a particular tissue/cell-type in vivo.
“The findings may carry over to nano-electronics and the new generation of mini-devices.”