Formation of metal nanoparticles

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2012

New approach available for studying how nanoparticles move through the environment.

KEY CONCEPTS
• Concern about the adverse effect of nanomaterials on human health and safety led to a study determining how nanoparticles are released from a source and move through the environment.
• In working with silver nanoparticles, relative humidities above 50% were needed to generate daughter particles.
• The results from this study suggest that silver containing objects (such as jewelry) may readily form nanoparticles when in contact with skin and may have been in contact with humans for a long period of time.

NEW DISCOVERIES IN THE REALM OF NANOTECHNOLOGY HAVE LED TO A BETTER UNDERSTANDING of how friction and wear occur on the atomic scale, as well as the development of new nanomaterials that show great promise in boosting the performance of lubricants. A second area that has benefited is the preparation of new catalysts that can further facilitate chemical processes and also more effectively reduce emissions.

An example of the latter comes from a previous TLT article as researchers developed a technique for impregnating iron nanoparticles into porous carbon spheres (1). The resulting iron impregnated carbon microspheres were able to efficiently reduce aqueous hexavalent chromium. Potentially this technology could be used in multipollutant control applications, including reducing NO_x from gas streams.

A second example was the development of a new anode material useful in lithium-ion battery applications that consisted of a gradient of silicon, aluminum and carbon (2). This material has the appearance of a scoop of ice cream and is known as a nanoscoop. Testing of the nanoscoop showed it can better handle the continuous charge/discharge cycles required in the operation of high performance batteries.

One underlining concern with new types of nanomaterials is their potential adverse affect on human health and safety. James Hutchison, Lokey- Harrington Chair in Chemistry and Founding Director of the ONAMI Safer Nanomaterials and Nanomanufacturing Institute at the University of Oregon in Eugene, Ore., says, “People have looked at nanomaterials as being an absolutely new species that have never been seen before, which means a very precautionary approach needs to be taken. But we believe that nanomaterials have already been present in the environment. There is a large background of naturally occurring nanoparticles that have been present in the environment for many years, and some of these are analogous to the newly engineered nanomaterials.”

Of particular concern is the origin of nanoparticles. Hutchison asks, “How easily are nanoparticles released from macroscopic products and what is the fate of these nanoparticles?” The answers to these questions are not easy.

Hutchison explains, “Trying to trace the origin and fate of a nanoparticle is analogous to looking for a needle in a haystack. Nanoparticles are released from a source and become highly diluted in the environment. This makes them very difficult to capture, and then determine the chain of custody.”

Not many techniques have been available for studying how nanoparticles move through the environment. Hutchison says, “There have been occasional microscopy studies and indirect techniques used to trace the movement of nanoparticles, but all have produced limited results.”

One material that has become of growing interest is silver. Hutchison explains, “Silver is worthy of study because its nanoparticles are used more widely in antimicrobial applications and the known toxicity of the silver cation.”

A new approach has now been developed to understand the fate of silver nanoparticles in the environment.

DAUGHTER PARTICLES
Hutchison led a research effort that involved the development of a new strategy to monitor nanoparticles under different types of environmental conditions. The researchers took polyvinylpyrrolidone stabilized silver nanoparticles and bound them to a positively-charged silicon dioxide analysis grid.

Under exposure to ambient laboratory conditions for four weeks, the nanoparticles were found to form smaller, daughter particles as seen by transmission electron microscopy. Hutchison says, “This observation was surprising because aqueous solutions of silver nanoparticles kept in a dark, cool environment are stable for several months. Yet, we have observed the formation of these daughter particles within only a few hours.”

The researchers changed the environmental conditions to see how this affected the formation of smaller silver nanoparticles. Hutchison says, “The most interesting parameter we found was humidity. We found that 50% relative humidity was the threshold that determined whether daughter particles were formed. Below that figure, no daughter particles were seen.”

Hutchison indicates that an adsorbed water layer dissolves some of the silver cations and then those cations diffuse across the surface. He explains, “At a relative humidity below 50%, the adsorbed water is very ice-like due to the presence of hydrogen bonding, and no free water is available to dissolve or transport silver cations. But at relative humidities above 50%, there is more free water on top of the ice-like water. This free water will dissolve silver cations and then diffuse in a gradient away from the parent particles.”

Hutchison proposes a three-stage mechanism to explain this process. As noted in Figure 2, oxidation of silver produces cations in the presence of oxygen and water. Diffusion of the silver cations in the water is facilitated by the strong concentration gradient around the parent particles. Silver is then formed through reduction of the cations by a variety of mild reductants present in the environment.


Figure 2. The formation of smaller daughter nanoparticles can be explained by a three-stage mechanism. Water is a particularly important element because this process requires a relative humidity of at least 50%. (Courtesy of the University of Oregon)

All of the studies were done in the dark because the silver cations and nanoparticles are photoactive. As an interesting follow-up experiment, silver wire and a sterling silver earring were placed in the silicon dioxide grid and placed in a humid environment. Transmission electron microscopy showed that silver nanoparticles were generated near each of these macroscopic objects.

Hutchison says, “The result from the experiments with macroscopic objects suggests that silver containing objects (such as jewelry) in frequent contact with human skin may readily form nanoparticles.”

Hutchison goes further and speculates that humans have been in contact with silver nanoparticles for a long period of time. The results mean that background levels of nanoparticles are probably present in the environment. This should be recognized when governmental agencies determine how to regulate these materials.

One other unanswered question raised by Hutchison is what other incidental nanoparticles might be present in our environment that we have not been able to identify?

Additional information can be found in a recent article (3) or by contacting Hutchison at hutch@uoregon.edu.

REFERENCES
1. Canter, N. (2011), “Iron-Based Nanocatalyst,” TLT, 67 (4), pp. 12-13.
2. Canter, N. (2011), “New Lithium-Ion Battery Technology,” TLT, 67 (3), pp. 12-13.
3. Glover, R., Miller, J. and Hutchison, J. (2011), “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), pp. 8950-8957.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.