Freshwater plants break down gold nanoparticles

Gold nanoparticles, which are promising for applications from electronics to biomedicine because of their useful combination of properties, might not be as stable in the environment as commonly thought, according to new research [Avellan et al.Nature Nanotechnology (2018),].

As nanoparticles become more widely used in consumer products, these engineered particles are starting to find their way in soils and aquatic systems as items degrade and are discarded. Scientists had generally assumed that metal nanoparticles, particularly gold, are sufficiently inert to remain stable in the environment. Now, however, Gregory V. Lowry and his colleagues from the Center for the Environmental Implications of NanoTechnology, Carnegie Mellon University, Duke University, and the University of Kentucky are challenging this assumption.

Using a replica of a natural outdoor freshwater wetland environment, known as a ‘mesocosm’, the team found that small doses of gold nanoparticles (Au-NPs) introduced gradually into the system do not remain unchanged but are broken down by aquatic plants over a period of months (Fig. 1). Native to Brazil, Egeria densa is a type of macrophyte that grows in fresh water (Fig. 2). Over a timespan of nine months, E. densa accumulated and transformed nearly 70% of the Au-NPs introduced into the system into cyanide, hydroxide, and thiol complexes. Once associated with the plants, all the gold is transformed into complexes, leaving no purely metallic gold.

“We were taken completely by surprise,” says Mark R. Wiesner, the James B. Duke Professor and chair of civil and environmental engineering at Duke. “The nanoparticles that were supposed to be the most stable turned out to be the least stable of all.”

The researchers had not been looking for the effect at all. The study had, in fact, set out to investigate the environmental behaviour of nitrogen, phosphorus, and copper hydroxide NPs from commercial pesticides and fertilisers.

“The Au-NPs were used as a tracer for NP behaviours,” explains Lowry. “We employed a long-term, low-concentration input of NPs that allowed us to observe them entering the natural biogeochemical cycle. This could not have been observed any other way, and had not been observed prior to our study.”

A clue to the surprising result came when the researchers examined colonies of bacteria known as biofilms, which grow on the shoots and leaves of macrophytes. These biofilms appear able to dissolve Au-NPs within a matter of days. The secret to this capability, the researchers believe, is that the microorganisms making up the biofilm secrete cyanide as a by-product, which acts as a catalyst for the biodissolution of metal particles.

“Our finding highlights that NPs, even if very stable thermodynamically, can be transformed at high rates when they enter biological complex aquatic environments,” says Lowry. “Since these transformations influence NP mobility, persistence, and toxicity to the environment, this is a very important finding for future research predicting NP behaviour to consider.”

Aquatic plants and ecosystems, particularly those that include bacterial species ill-adapted to metallic environments, could become sinks for metal nanoparticles. Similar processes could affect many other metal nanoparticles and their breakdown in aquatic environments. The team is now urging that studies assessing the long-term fate of metal nanoparticles in the environment take into accounta these biological transformations.

“It is increasingly recognised that studies of the fate and behaviour of nanomaterials need to consider chemical and biotic interactions over longer time-scales than those commonly used in laboratory tests,” explains David Spurgeon of the UK’s Centre for Ecology & Hydrology.

Mesocosm systems are important tools for such evaluations, he adds, because they allow different species to interact under natural conditions over longer time periods than typical laboratory tests.

“[This work] provides just the kind of new insight that can arise from a mesocosm experiment,” he says. “Contrary to previous assumptions that Au-NPs would be stable in freshwater environments, their studies identify that Au-NPs are instead biotransformed and taken up by the dominant plant species in the system (in this case, E. densa).”

Spurgeon believes that tests would not have revealed the biotransformation of Au-NPs in a less biologically complex system, or over shorter time periods, or if unrealistically high concentrations of Au-NPs had been introduced into the system. Further studies are now needed to get a full understanding of the functional mechanisms underpinning Au-NP transformation, their role in geochemical cycling, and fate in aquatic systems, he says.