Gold has a reputation for being chemically untouchable. But according to Western University chemist Yolanda Hedberg, that reputation is a little misleading - gold is not unreactive so much as selective about what it reacts with.
"Every material has their own enemy. Its own kryptonite," said Hedberg, Canada Research Chair in Corrosion Science. "For gold, one of those enemies is sulphur."
That distinction helps explain what Hedberg and her collaborators at Surface Science Western and the University of Copenhagen found when they tested five pairs of 10K and 14K gold earrings purchased from a jeweller.
After a week in cysteine-modified artificial sweat, all five pairs released measurable amounts of gold. Cysteine is an amino acid, or protein building block, made naturally by the body that contains sulphur, which binds to gold.
The amounts were tiny, and the findings do not show that the released gold is enough to cause an allergic reaction. But the study, published by the journal Contact Dermatitis, demonstrates gold in jewelry can become chemically available under the right conditions - a finding that helps explain how exposure to gold can occur at the skin.
In the study, none of five earrings purchased from professional jewellers released detectable nickel, distinguishing them from the inexpensive gold-plated earrings bought from e-commerce sites like Temu that were examined in a previous study. (Christopher Kindratsky/Western Communications)
Gold, silver and platinum are known as noble metals because they do not readily react with oxygen. That resistance to oxidation is a big part of gold's reputation for stability. But, Hedberg says, it does not mean the metal is universally unreactive.
"Gold reacts to different things," said Hedberg, a chemistry professor in Western's Faculty of Science. "Gold doesn't react much with oxygen, but it can react with sulphur and some amino groups."
And for some people, that reaction poses a problem.
"Gold allergy is real, although it is less common than nickel allergy. But for a metal to trigger an allergic response, some of that metal has to become chemically available to the body," said Hedberg. "So we wanted to understand how that might happen when gold jewelry is in contact with skin."
Sweat contains amino acids and other compounds that can interact with metals. The researchers added cysteine, an amino acid containing sulphur, to an artificial sweat solution and exposed the earrings to it for 168 hours at about 30 C. The team then measured the metals released into the solution using highly sensitive mass spectrometry, a scientific technique used to weigh molecules.
There is no standard artificial-sweat test specifically designed to capture how gold behaves in the presence of sulphur, so Hedberg added cysteine to the solution to create conditions that would reveal whether gold could become soluble.
"The experiment was designed around a simple chemical principle. For a metal to cause an allergic response, it first has to become chemically available," said Hedberg.
Finding gold in all the wrong places
Gold was detected in the release solution from all five pairs of earrings.
Only tiny amounts of gold were released over the week-long test, with a median of 0.057 micrograms per square centimetre. The amount varied among the earrings, ranging from 0.016 to 0.15 micrograms.
Copper was released in much greater amounts, exceeding gold in every sample. The median copper release was 0.53 micrograms per square centimetre per week. Copper is generally considered harmless for humans and has beneficial antibacterial properties. Its higher release was expected, since the earrings were alloys, meaning gold was combined with other metals. Surface analysis showed the materials consisted mainly of gold, copper, silver and zinc, with only small amounts of nickel detected at the surface.
But unlike the inexpensive gold-plated earrings purchased from businesses like e-commerce site Temu, which Hedberg and colleagues examined in an earlier study, none of the five jeweller-purchased earrings released detectable nickel under the conditions of the new experiment.
Gold-plated jewelry can have a thin gold coating over one or more underlying layers that may contain nickel. In the earlier study on inexpensive earrings, nickel was detected in the artificial sweat in which all 10 inexpensive gold-plated earrings were tested, often at levels that exceeded European regulatory limits for jewelry sold for pierced body parts.
Element-specific spectroscopy showed gold throughout the material, rather than just a thin surface coating for the jewelry-store earrings, while the Temu earrings, if they contained any gold, had only a thin-plated layer.
"The chemistry and composition can vary widely, and price alone doesn't tell you whether a material will cause an allergic reaction. In this particular comparison, however, the distinction between thin-plated earrings and the more expensive gold-alloy earrings from the jewelry store helps explain why nickel behaved so differently," said Hedberg.
'Wouldn't take a chance'
The study measured how much gold became soluble (able to dissolve in a liquid) under specific laboratory conditions. It did not measure how much gold reaches or remains on a person's skin during actual wear, nor does it establish a dose at which gold jewelry triggers dermatitis.
Hedberg says the very small amounts of gold detected in the study would probably not be enough to trigger an allergic reaction in most circumstances. But that does not make the result irrelevant to someone who already knows they are allergic to gold.
"If I had a gold allergy, I still wouldn't take the chance," said Hedberg. "Jewelry moves against your skin, sweat chemistry varies and friction can increase the release."
For most people, however, nickel remains a more immediate concern with inexpensive jewelry, rather than gold. In the earlier study of cheap gold-plated earrings, nickel was detected in every release solution. In the new study of jeweller-purchased 10K and 14K gold earrings, nickel was below the detection limit in every sample.
"This study doesn't show that gold jewelry is causing widespread allergic reactions. What it shows is something narrower, but chemically important. Gold can be released from jewelry under conditions that make the metal soluble, so there is a potential pathway for exposure," said Hedberg.
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