Gold may be flashier, but now chemists are giving silver a chance to shine. They’ve figured out how to make silver nanoparticles that are even more stable than gold nanoparticles.
Durable and easy to handle, gold nanoparticles are used for drug delivery, cell imaging and many other applications. Because silver is cheaper and more abundant than gold, it may seem like a more convenient nanomaterial. But unlike gold, silver degrades easily, which makes building stable silver nanoparticles extremely difficult.
Now scientists at the University of Toledo in Ohio and other institutions have solved the problem and created silver nanoparticles with staying power. The secret lies in the particles’ symmetrical, compact structure and neatly arranged electrons, the team reported Wednesday in the journal Nature.
The researchers synthesized the particles by accident. They were originally investigating how silver interacts with light. To measure the metal’s light-absorbing properties, they attached sulfur-containing molecules to silver nanoparticles. Then they used mass spectrometry to examine the structure of the new particles.
That’s when they noticed something odd.
Because silver degrades so easily, silver nanoparticles normally come in a wide range of sizes, each containing different numbers of atoms. But this time, all the nanoparticles were the same size. Each time the researchers repeated the process, they ended up with the same-sized nanoparticles.
In other words, this version of the silver nanoparticle was so stable that no others formed.
“It was just so superior in stability to everything else,” said Terry Bigioni, a chemist at the University of Toledo who helped lead the study.
But what made this nanoparticle so stable? Mass spectrometry and X-ray crystallography revealed that it had a highly ordered, compact structure. The particle’s center was a soccer-ball-shaped “cage” consisting of 12 silver atoms, surrounded by 20 silver atoms arranged in a similar fashion. Six sulfur-containing molecules, or “mounts,” were bonded to this silver core, protecting it from being attacked by other reactive molecules.
“The curvature of that core is just right for these mounts to nestle on the surface where they bind nice and strongly,” Bigioni said. It’s possible that silver nanoparticles that were bigger or smaller would have resulted in cores with a less ideal shape.
The team’s silver nanoparticle also had the perfect number of electrons in just the right locations. The outer shell of silver atoms normally contains one lone electron, which makes them highly reactive. But all the electrons in the new nanoparticle were paired. What’s more, they occupied regions where energy levels were low, making them even less likely to participate in chemical reactions.
“The electrons are really happy,” Bigioni said. “They don’t want to move around.”
The researchers then stumbled on an even bigger surprise. Not only were the new silver nanoparticles extremely stable, but they were even more stable than gold nanoparticles.
“It took a while for that to sink in,” Bigioni said. “No one predicted anything like this.”
But some think it’s still too early for the researchers to draw that conclusion, because they used only one type of gold nanoparticle for comparison.
“To make that claim really robust, it would be better to use a broader palette” of gold nanoparticles, said Christopher Ackerson, a chemist at Colorado State University, who wasn’t involved in the study.
But if the researchers are right, silver nanoparticles may one day replace gold nanoparticles for some applications.
In the meantime, scientists still need to work out some key details. For example, although the new nanoparticles remained stable in the solution where they were produced, it’s not clear whether they’ll be stable in the human body.
“It’s just the start,” Bigioni said. “There’s definitely more work to do.”