Nanoparticles Form Naturally on Silver and Copper Surfaces

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Nanoparticles have a diameter of few nanometers. Photo by Victorpuntes.

A study performed at the University of Oregon showed how silver nanoparticles can be formed spontaneously on the surface of either larger nanoparticles or bulk objects. A similar phenomenon was also observed for copper. This may indicate that we have been exposed to nanoparticles for a period of time much longer than previously thought.

Silver Nanoparticles: Their Properties and Use

Nanoparticles are particles whose diameter is in the order of nanometers, nm (10-9 m). As previously reported, nanoparticles, or in general other nanomaterials, show properties that are different from those of the corresponding bulk material.

Nanoparticles made of silver (Ag) are widely employed in several sectors, mainly due to their antibacterial and antifungal activity. They are used, for instance, as additives in devices such as catheters, or in bone implants, to prevent infections.

Health Risks of Silver Nanoparticles

Although Ag nanoparticles are extensively used, there is great concern about the possible health risks associated with them.

The release of silver particles into the human body from the devices in which they are employed, for instance, is one of the worrying aspects. Another issue is whether there is any danger associated specifically with their small dimensions; this is a problem that could be extended also to other nanomaterials currently employed in our society. Specific rules regarding the use of nanomaterials were introduced by the US Environmental Protection Agency (EPA).

Study of Silver Nanoparticles Behavior

A complete investigation into the behavior of silver nanoparticles was recently published in the American Chemical Society journal ACS Nano; it reports the work performed by professor Hutchison and his coworkers, from the Department of Chemistry and Materials Science Institute, of the University of Oregon.

This study focused on the reactivity of the surface of silver nanoparticles. The researchers observed that, in the presence of high level of humidity (more than 50%), newer and smaller nanoparticles were formed close to the parent particle.

The suggested mechanism for the formation of the new nanoparticle is the following:

  • Reaction of the silver from the surface of the particle with water and/or air oxygen, to form silver ion Ag+;
  • Surface diffusion of the ion Ag+;
  • Reduction of the ion Ag+, to form a smaller particle of metallic silver Ag.

These results were made possible with the use of a more advanced microscope technique, Smart Grid; this allowed the visualization of the nanoparticles and their changes with time.

Larger objests, such as silver spoons, were also tested. Photo by CremoPhotostream.

Tests on Greater Objects and on a Different Material

Analyses of the surface reactivity were also performed for bulk objects made of silver, and very commonly used in everyday life, such as jewelry and eating utensils; similar results were obtained, as the same process of nanoparticle formation was observed.

Bulk objects made from copper (Cu), such as wires, were also studied; again they gave comparable results – copper nanoparticles were formed in the presence of high air humidity, with the same mechanism of oxidation and reduction.

Naturally Formed Nanoparticles

The data reported in this study show that silver nanoparticles are not just artificially made in a laboratory; on the contrary, they can naturally form in a dynamic process on the silver surface. This process occurs for both silver nanoparticles and bulk silver objects.

The fact that nanoparticles were also detected on the surface of another metal, copper, could mean that this is a general behavior for materials which could get easily oxidized and reduced under standard atmospheric conditions. Hence, other materials may behave in a similar way and produce nanoparticles on their surfaces. This is surely a point that deserves more investigation.

Implications of Spontaneously-Forming Nanoparticles

These results show that it could be inappropriate to classify the hazard of a certain material based only on its size. This is because the surfaces of nanoparticles were shown to be very reactive; many nanoparticles of different, smaller, sizes may coexist on the surface of a “parent” nanoparticle.

Furthermore, the spontaneous formation of nanoparticles on the surface of bulk objects may indicate that humans have been in contact with, and exposed to, nanoparticles for a very long time, indeed much longer than expected.

Sources

R.D. Glover et al. Generation of Metal Nanoparticles from Silver and Copper Objects: Nanoparticle Dynamics on Surfaces and Potential Sources of Nanoparticles in the Environment.  ACS Nano, 5(11), 8950, 2011. Accessed December 6, 2011.

Environmental Protection Agency. Control of Nanoscale Materials under the Toxic Substances Control Act. Accessed December 6, 2011.

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