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Silver Nanoparticles Generated in Natural Environment

Silver nanoparticles (Univ. at Buffalo)

Transmission electron microscopy image of silver nanoparticles formed from silver ions in solution with humic acid. (Univ. at Buffalo)

A team of university and government chemists have found that given a source of silver ions, naturally occurring humic acid can synthesize stable silver nanoparticles. The researchers published their findings last month in the journal Environmental Science and Technology (paid subscription required).

Nanoscale silver particles — one nanometer equals one-billionth of a meter — are highly desirable materials in a variety of industries and products, such as  electronics, sensors, polymers, and textiles. Their anti-microbial and anti-fungal qualities make silver nanoparticles useful in health care applications, such as coatings and dressings.

Concerns have been raised, however, over environmental risks of man-made nanoparticles introduced into the environment, particularly long-term effects on microorganisms in soil or groundwater. The researchers — from Florida Institute of Technology, University at Buffalo, and National Institute of Standards and Technology — uncovered natural processes that generate silver nanoparticles, which suggest the nanoparticles can coexist in a natural environment.

The team found humic acid to be a friendly environment for generating silver nanoparticles. Humic acid is a mixture of organic acids formed during the decay of dead organic matter. Although its composition can vary depending on location and season, humic acid is nonetheless ubiquitous in the environment. It is also sold commercially as an additive for fertilizers.

The researchers mixed silver ions with humic acid from various sources at different temperatures and concentrations and found that acids from river water or sediments would form detectable silver nanoparticles at room temperature in as little as two to four days. The humic acid also appears to stabilize the nanoparticles by coating them and preventing the nanoparticles from clumping together into a larger mass of silver.

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