The use and extent of nanoparticles have increased dramatically the past years, and it is expected to increase further. Silver nanoparticles are the most commonly used nanoparticles, where their antibacterial properties are used to enhance commercial products, such as in wound dressings, cosmetics, textiles, and food packaging.
The high degree of leaching of silver nanoparticles from consumer products results in accumulations in landfills and the terrestrial environment.
“Despite the fact that silver nanoparticles are amongst the most extensively studied nanomaterial to date, there is still a certain amount of controversy about their toxicity,” PhD candidate Lisa Magdalena Rossbach says.
How toxic is it really?
Results from toxicity tests on silver nanoparticles are hard to compare, due to differences in exposure media, organisms, particle characteristics and the endpoints studied. Toxicity studies primarily focus on the exposure of a specific life stage or a single generation of an organism. The lack of multigenerational studies could mean a large uncertainty about long-term effects.
Toxicity and antioxidant defenses
Rossbach has studied the toxicity of silver nanoparticles on the nematode Caenorhabditis elegans.
Her project was designed so test the effects of silver nanoparticles on nematodes over generations. She compared the toxicity of Ag NP NM300K in comparison to AgNO3.
“I have also examined the nematode’s antioxidant defenses following exposure, and changes in toxic responses over generations,” she comments.
Rossbach measured growth, fertility, and reproduction, as well as the uptake and retention of the nanoparticles by the nematodes.
“There was a consistent and comparable dose response relationship for all measured parameters,” she says.
The most sensitive one was reproduction, where concentrations for both AgNO3 and NM300K Ag NP of were of similar magnitude.
“My results indicate that the toxic response might be governed by the exposing agent.”
Oxidation in the intestinal lumen
She has assessed changes in the intracellular redox state as well as links to toxicity test endpoints in the nematode C. elegans, following the exposure to either AgNO3 or Ag NPs.
“Toxicity test effects showed a dose response comparable to the previous experiment,” Rossbach comments.
She observed distinct differences in tissue distribution between the two forms of Ag. Analyses showed significantly higher oxidation levels in the tissues lining the intestinal lumen from the Ag NP exposure, compared to controls and AgNO3 exposed nematodes.
“I found a 60 % increase in oxidation levels located in the epithelial cells surrounding the lumen at high levels of Ag NP compared to lower levels of AgNO3.”
Changes in growth and fertility
Six generations of nematodes were exposed to solutions of AgNO3 or Ag NP. Toxicity tests at the end of each generation were used to monitor changes in growth, fertility, and reproduction.
Rossbach’s results showed that the nematodes were significantly more sensitive towards the Ag NP population, in terms of growth and fertility, than towards AgNO3 exposure.
Increased sensitivity towards cerium
Exposure to one stressor has been shown to have the potential to change the toxic response to a second stressor. Following the multigenerational exposure to either AgNO3 or Ag NPs, nematodes were exposed to cerium (Ce3+), cerium dioxide nanoparticles (CeO2 NPs), cadmium (Cd2+), copper (Cu2+) and paraquat, in standard toxicity tests.
Following the multigenerational exposure towards silver nanoparticles, either AgNO3 or Ag NPs, nematodes showed no changes in sensitivity when exposed to cadmium, copper, and CeO2 NPs.
Both Ag NP and AgNO3 population nematodes exhibited increased sensitivity towards Ce ions, compared to controls. The six generational exposure towards Ag NPs however, rendered nematodes less sensitive towards paraquat in terms of fertility.
Probability of multi-generational exposure
"Multigenerational exposure studies are of high relevance," Rossbach says.
"Because it is very likely that organisms exposed to toxicants in the environment will be exposed for a prolonged time scale, and across generations."
Also Rossbach studies showed that that the characterization of the AgNO3 exposure in the media and presence of organisms contains a high aggregated silver fraction. She comments that many studies refer to the AgNO3 exposure as an IONIC control, which might be a bit misleading considering the high aggregation found in these studies. It should rather be referred to as a positive control.