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July 2019

Catalysts at the Nanoscale

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During times of war, technology tends to develop at an accelerated pace. World War II was no exception. Using newly invented incendiary devices, Army Rangers from D company of the 2nd Battalion disabled five 155mm guns aimed at the US fleet off of Utah Beach during the invasion of Normandy. These thermite grenades silently fused the guns’ elevating and traversing mechanisms by burning at 3000o C. Now, over 60 years later, thermites are receiving a facelift with the help of nanotechnology. And while the military is keenly interested in nanotechnology, hundreds of other organizations are seeking various applications within the nascent field.

The exciting prospect of nanotechnology is its potential use in almost any conceivable domain. Every field from medicine and electronics to manufacturing and fashion stand to benefit from advances in nanotechnology. And while nano-scale technology is multifaceted in its application, the use of nanocrystals as catalysts is perhaps the most intriguing

ystals as catalysts is perhaps the most intriguing. In order to understand why nanocrystal catalysts are so promising, one first needs to understand some basic principles of Chemistry. For any chemical reaction to occur, two reactive species must come in contact with each other. The term reactive does not imply unstable; wood is relatively inert unless it is heated. However, if it is warmed to approximately 451o Fahrenheit, the hydrogen and carbon composition of the wood reacts with oxygen in the air to form carbon dioxide and water. While this paper will not address how or why this reaction occurs, it is important to understand that it will cease if the reactive species are not in close proximity to each other. Fire will never originate in the core of burning logs because oxygen is not able to penetrate far beyond the surface. Anyone who has built a campfire knows that the fire only burns on the wood’s exterior.

Another key concept to understanding nanocrystal catalysis involves the ratio of surface area and volume. As an object gets larger, its surface area increases less in relation to its volume. A good way to visualize this is to imagine a cube (Figure 1). Volume is measured by multiplying the cube’s length, width, and height. Surface area is determined by finding the area of one face of the cube (length times height) and multiplying it by the number of faces: in this case, six

Figure 1 shows that halving the length of each side results in an eightfold decrease in volume, but only a fourfold decrease in surface area. Therefore, smaller objects have more surface area with respect to their volume. This has important implications for chemical reactions. High surface area-tovolume ratios are favorable for chemical reactions. Going back to the campfire example, kindling is used to start the fire. The small pieces of wood have a greater surface area with respect to their volume than larger logs. Lighting the kindling therefore results in a quicker combustion. Additionally, if one throws a handful of sawdust onto a burning fire, a giant flare results. This reaction is chemically identical to ordinary wood burning, but it occurs at a much faster rate.

The general purpose of catalysts is to increase the speed of a given reaction. This is achieved through kinetic means and does not directly affect the thermodynamic properties of a chemical system. Introducing a catalyst increases the speed of a reaction in one of three ways; it can lower the activation energy for the reaction, act as a facilitator and bring the reactive species together more effectively, or create a higher yield of one species when two or more products are formed. Depending on the application, nano-catalysts can be used in all the ways listed above.

Nano-materials are more effective than conventional catalysts for two reasons. First, their extremely small size (typically 10-80 nanometers) yields a tremendous surface area-to-volume ratio. Also, when materials are fabricated on the nanoscale, they achieve properties not found within their macroscopic counterparts. Both of these reasons account for the versatility and effectiveness of nanocatalysts

Perhaps no one is more interested in studying nano-catalysts than the military. Always looking to gain an edge over other nations, the United States military has invested hundreds of millions of dollars in nanotechnology. One of its more significant discoveries deals with superthermites. Traditionally, thermites are a class of powders that produce metal and an immense amount of heat when reacted. But when thermites are infused with nanometals, specifically nano-aluminum and molybdenum, they produce reactions that progress thousands of times faster than their ordinary analogs

[1]. The increased kinetics of the reaction releases heat more quickly, and therefore makes the reaction more intense. Although the military has classified much of the research, it is believed that superthermites will replace conventional organic explosives like TNT for use in a variety of applications

[2]. Nanometals also have military applications as propellants. Adding nanoaluminum to missiles, torpedoes, and other munitions can theoretically launch projectiles further and faster than conventional charges. Burn rates with nanoaluminum can be over ten times higher than contemporary propellants

[1]. Equipped with nanometals, rockets could potentially reach their targets before any evasive action could be taken.

Every year, military munitions plants crank out hundreds of millions of bullets for armed conflicts and training. In 1997, scientists began to develop alternatives to toxic lead used in standard rounds

[1]. Fabricating bullets using nanoaluminum considerably increases their range. In addition, since nanomaterials provide a higher concentration of energy with fewer raw materials, manufacturing munitions with nano-catalysts would actually decrease the overall cost while increasing their effectiveness

One of the biggest draws of implementing nanotechnology within weapons lies within its control. Varying the mixture of different nanometals yields a wide range of energy release rates

[3]. These rates can span from conventional explosives to superthermites. This fine-tuning can also be accomplished by altering the size of the catalysts on the nanoscale.

While the military seeks to destroy the world (or, at least, small parts of it) by utilizing nanotechnology, various groups are hoping nanocatalysts can save it. Several California-based companies are currently developing nanomaterials for improved catalytic converters

[4]. With over 500 million cars in existence worldwide

[5], improving fuel efficiency can have drastic effects: both environmentally and economically. Improving the rates of catalytic combustion occurs through product selectivity. In a common automobile, hydrocarbons in the gasoline ignite to create water, carbon dioxide, and the force necessary to drive the engine’s pistons. However, impurities in gasoline coupled with inefficient reactions yield harmful byproducts such as carbon monoxide, nitrogen dioxide, and sulfur hexafluoride. These compounds can lead to ozone depletion, acid rain, and other environmental concerns. But nanotech catalysts have the potential to make automobile combustion up to 100% selective and therefore produce little or no toxic byproducts. And since the automobile catalyst market alone is a $5-7 billion industry, companies stand to save $4.5 billion annually by implementing nanocatalysts into their converters while saving the environment as well

[6]. The limited quantity of fossil fuels on the planet has led researchers towards finding alternative sources of energy. Fuel cells have the ability to generate vast amounts of energy in an environmentally friendly way. Combining oxygen and hydrogen gas within a fuel cell creates enough energy to power a car and other large machines while producing water as the only byproduct. The efficiency of fuel cells is linked to their rate-limiting reaction, or in this case, oxygen reduction. Conventionally, the promotion of the oxidation reaction is accomplished through the use of expensive platinum catalysts. Recently, nano-nickel has been found to promote reactions traditionally catalyzed by platinum

[7]. In addition to costing four times less than platinum, nano-nickel has a greater catalytic activity. And newly developed manufacturing processes stand to drop the price on nano-nickel even further while allowing it to be used within large-scale industrial applications.

Physicist Richard Feynman once said that, “at the atomic level, we have new kinds of forces and new kinds of possibilities, new kinds of effects.” [8] Contemporary technologies have proved his predictions 45 years after the fact. Nanotechnology exists as an intriguing dichotomy; the fundamental principles of nano-catalysis can be harnessed to destroy the world or to help heal its wounds. It is only a matter of time before the potential of nanocrystal catalysts are realized and implemented on a widespread scale

This student-produced report is part of a larger pamphlet on nanotechnologies circa 2005, the partial output of a course on “Nanotechnology and Society” (Science and Technology Studies, Section 84405, by C. Tahan) which was taught in the spring semester of 2005 at the University of Wisconsin-Madison. Visit http://tahan.com/charlie/nanosociety/course201/ for the other reports and more information.


[1] Gartner, John. “Military Reloads with Nanotech.” January 21, 2005. http://www.techreview.com/articles/05/01/wo/wo_gartner012105.asp

[2] Buechler, Karen. “Atomic Layer Deposition of Oxidizer Coatings on Aluminum Nanoparticles to Fabricate Superthermite Explosives.” July 29, 2002. http://www.dodsbir.net/awardlist/sttr1_02.htm

[3] Lowe, Terry. “Nanometals Revolution is Crucial in Sustaining the Metals Industry.” February 6, 2002. http://www.smalltimes.com/document_display.cfm?document_id=3045

[4] Choi, Charles. “Nano World: Nanocatalysts for oil, drugs.” March 25, 2005. http://www.wpherald.com/storyview.php?StoryID=20050325-123219-4868r

[5] http://www.clearair.com/html/emissions_problem.htm

[6] Helsel, Sandra. “President and CEO Michael Pak in NYC at Nano Business Conference to Explain How Nano Catalysts deliver Clean Energy and Clean Air Solutions.” May 14, 2004. http://www.prweb.com/releases/2004/5/prweb125879.htm

[7] Mitchell, Peter. “Nano-nickel catalysts.” February 7, 2005. http://www.qsinano.com/pdf/pp22&23.pdf

[8] Feynman, Richard. “There’s Plenty of Room at the Bottom.” December 29, 1959. http://www.zyvex.com/nanotech/feynman.html

EPA Grants 1st Approval For Nanopesticide

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EPA Grants 1st Approval For Nanopesticide

More than a year after floating the idea, the U.S. Environmental Protection Agency has granted the first approval for a pesticide that’s based on a nanoscale material—a Swiss-made antimicrobial nanosilver product used in fabrics.

The EPA announced Thursday that it is moving forward with a four-year “conditional registration” for HeiQ Materials’ AGS-20 product, which is essentially a composite of nanosilver and nanoscale silica. According to HeiQ’s application—first filed in 2008—AGS-20 will be incorporated into textiles.

Nanosilver is being used more and more in workout clothing, touted as a way to fight that gym-rat smell.

The agency proposed the conditional registration—which comes with a laundry list of required tests—in August 2010, and opened a public comment period. Then there was radio silence, except for an EPA announcement last summer that it planned to seek more information from manufacturers using nanomaterials, potentially including nano-enabled products that got the green light before the agency sharpened its focus on ultra-tiny substances.

Nano-watchers have been waiting eagerly for the agency’s decision, wondering whether the move would signal a larger shift in how nanomaterials, including silver, are monitored and regulated.

Nanotechnology leverages the often-unique properties of super-small particles to create products with amazing qualities. These materials can make better batteries or lighter and stronger bike frames, as well as new medical instruments and medicines that can save lives. They’re increasingly common in consumer products, from “mineral-based” sunscreens to stain-repellent pants to boat paints that resist algae growth.

Nanomaterials are believed to hold great promise for a wide variety of applications. But shrinking these substances can change their properties, and scientists are struggling to figure out whether that shift can make them dangerous in the process, and how and why it happens.

HeiQ CEO Carlo Centonze said in a statement that the company is “pleased that that EPA has recognized the potential benefits provided by HeiQ AGS-20.” He said the EPA’s approval came “in part because it could lead to less silver released in the environment while providing longer protection against the growth of odor- and stain-causing bacteria.”

Lynn Bergeson, a lawyer at the Washington firm Bergeson & Campbell who works with nanotechnology companies on navigating the regulatory process, said she was “thrilled” by the EPA’s move, but wasn’t sure it portends additional action by the agency.

“It really is very difficult to speculate as to whether this means anything other than the decision was made early on to approve this and the agency has carefully considered the comments and is sticking by its decision,” said Bergeson, who doesn’t represent HeiQ.

The EPA considers nanosilver, and its larger-sized counterpart, a pesticide, and evaluates it under the Federal Insecticide, Fungicide and Rodenticide Act, or FIFRA. According to that law, pesticides must be registered before going on the market.

EPA officials weren’t available for comment Thursday, aside from a brief press release announcing the move. (Click here to read the rules it proposed in 2010; it’s unclear whether there have been significant changes.)

Nanosilver has drawn a lot of attention from scientists and regulators. It’s widely used as an antimicrobial, in toothbrushes and other products as well as sweat socks. Studies have shown nanosilver turns up in sewage sludge—and also sloughing off of larger silver objects, raising questions about how much silver has been all around us, and whether these new products might have an additional impact.

Environmental groups and other advocates have warned that nanosilver could be a problem for the environment, by building up in water, soil and marine life or by disrupting the natural bacteria that are all around us. Others have questioned whether the widening use of nanosilver might create problems with resistance, either in people or in bacteria.

The silver industry maintains that the metal, which has been used for centuries to fight germs, is safe in any size.

In his statement, Centonze said HeiQ will comply with all the EPA’s requirements: “It is our constant effort to produce and communicate scientific findings to EPA and other regulators in their efforts to adapt risk assessment to new policies and environmental findings.”

Jaydee Hanson, policy director at the International Center for Technology Assessment, said he was disappointed that the EPA decided to move ahead with HeiQ’s application before completing work on broader guidelines for nanoscale pesticides. The ICTA, in concert with other advocacy groups, has petitioned both the FDA and EPA to start regulating products that contain nanomaterials.

The company should get credit for going through the registration process, he said, noting that other manufacturers seem to be cloaking their nanopesticide ingredients in order to avoid EPA scrutiny.

But the agency should have waited, Hanson said, at least until it gets the results of the toxicology tests required under the conditional registration.

“It’s a little bit like, ‘OK, let your horses out, and maybe we can recall them afterwards,’‘’ Hanson said.

“There are serious potential environmental problems, if we get every fabric out there impregnated with nanosilver,” he said, adding that he expects to see more companies apply to register nanosilver pesticides in the wake of the EPA’s decision.

HeiQ’s application, and the EPA proposal for the conditional registration, drew a variety of comments from industry, environmental and consumer advocates and the public (click here to see all of the background information). Many were negative, and it’s unclear how—or if—they affected the EPA’s decision.

“I think it’s going to be when we see their new regulations before we know whether they really paid attention to the comments or not,” Hanson said.

Bergeson said she also anticipates more applications for nanopesticides. The conditional nature of the registration is not ideal for a commercial product, since there’s the potential that after the four-year trial period, the EPA could pull it from the market. But it’s better than the stop sign companies have been getting, she said.

The wait for this decision “was a signal to pesticide registrants that the agency had now to find its sea legs” on nano-enabled products, she said. “Now, with the issuance of the final registration, I think you can expect EPA to be more confident and for pesticide registrants to be more hopeful.”

Antimicrobial Properties of a Novel Silver-Silica Nanocomposite Material

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Nanotechnology enables development and production of novel silver-based composite materials. We used in vitro tests to demonstrate the antimicrobial activity of a silver-silica nanocomposite compared to the activities of conventional materials, such as silver nitrate and silver zeolite. A silver-silica-containing polystyrene material was manufactured and shown to possess strong antimicrobial properties.

Many applications, including medicine and food production and storage, would benefit greatly from incorporation of safe and inexpensive long-lasting biocides into polymers, paints, or textiles (). The antimicrobial effect of silver additives is broadly used in various injection-molded plastic products, in textiles (), and in coating-based applications, including air ducts, countertops, and food preparation areas (). Some important advantages of silver-based antimicrobials are their excellent thermal stability and their health and environmental safety (). However, like the use of all biocide products, the use of silver is strictly controlled by various national laws and control agencies. In the United States, the Environmental Protection Agency has regulated the use of silver as a biocide since 1954 () under the Federal Insecticide Fungicide and Rodenticide Act. In the European Union, a European biocide product directive (EU/BPD/98) imposes regulatory requirements on the use and claims associated with all biocide products ().

In the past few years, there has been a tremendous push for development of inorganic nanoparticles with structures that exhibit novel physical, chemical, and biological properties (). In particular, the potential benefits of nano-silver materials have been recognized by many industries due to the strong antimicrobial activity of silver against a broad spectrum of bacteria, viruses, and fungi and the low frequency of development of resistance ().

Generally, silver-based antimicrobial additives consist of silver ions integrated into inert matrices consisting of ceramic, glass, or zeolite. Other silver additives based on silver salts or metallic silver may be readily incorporated into thermoplastic polymers, such as polyethylene, polypropylene, polystyrene, or nylon (). The bactericidal efficacy of silver-containing polymers is based on the release of silver ions (Ag+) through interaction with a liquid watery phase (). Although the antimicrobial effects of silver ions and salts have been intensively studied, the mechanism of the inhibitory action of silver on microbes is still not fully understood. It has been proposed that silver ions interact with disulfide or sulfhydryl groups of enzymes, causing structural changes that lead to disruption of metabolic processes followed by cell death (). The inhibitory action of silver nanoparticles is also based on the release of Ag+ (). Exposure of microorganisms to silver nanoparticles was shown to result in strong antimicrobial activity (). In addition to the increased surface area and associated increased potential for the release of Ag+, when dispersed in liquid suspensions, silver nanoparticles may accumulate in the bacterial cytoplasmic membrane, causing a significant increase in permeability and cell death (), and penetrate bacterial cells (). Recently, it has been suggested that the antimicrobial mechanism of silver nanoparticles may also be related to membrane damage due to free radicals that are derived from the surface of the nanoparticles (). This bactericidal activity also appears to be dependent on the size and shape of the silver nanoparticles ().

In this study, we evaluated the properties of a novel silver-silica nanocomposite material (HeiQ AGS-20; HeiQ Materials, Bad Zurzach, Switzerland) used as an antimicrobial additive and compared its efficacy to the efficacies of the conventional silver additives silver nitrate (AgNO3; 63.5% Ag) and silver zeolite (38% Ag bound to type A zeolite; Sigma-Aldrich, Buchs, Switzerland). The novel silver-silica nanocomposite material was produced using an industrial flame spray pyrolysis process. This process involves combustion of a flammable solvent containing homogeneously dissolved compounds as the source of components for the synthesis of the material (). A representative transmission electron micrograph of the silver-silica material is shown in Fig. Fig.1.1. The nanocomposite consists of silver nanoparticles embedded in a matrix of amorphous silicon dioxide (SiO2). The SiO2 fine structure consists of aggregate matrix particles with an average diameter of approximately 1 μm (Fig. (Fig.1A).1A). Silver metal particles are located on the surface of the silica and are also embedded within the matrix (Fig. (Fig.1B).1B). High-magnification scanning transmission electron microscopy imaging of a localized region of a nanoparticle indicated that each silica particle contains many small silver metal particles with a typical diameter between 1 and 10 nm (Fig. (Fig.1C).1C). The specific surface area of the nanocomposite powder, as measured by nitrogen adsorption (), is typically about 250 m2/g, a value which is consistent with the open structure of the silica aggregate shown in Fig. Fig.1.1. It can be concluded that, upon contact with moisture, the pure silver particles act as a source that releases silver ions, which represent the active antimicrobial principle (). Some key advantages of the novel nanocomposite are the dispersion of the discrete silver particles throughout the silica (which prevents agglomeration of the silver particles), the small diameter of the silver particles (which results in a large surface area and release of a large amount of Ag+, which results in high antimicrobial efficiency), and the small size of the silver-silica composite (ca. 1 μm) (which allows the material to be uniformly dispersed and readily incorporated into a variety of substrates, including synthetic fibers, plastics, and other thin or delicate materials). The silica structure acts as a convenient carrier for incorporating the fine silver particles into plastics, textiles, and coatings. A further advantage is that the immobilization of silver nanoparticles within the silica structure limits the potential for release and disposal of the nanoparticles themselves. This property may be highly desirable because of the possible abilities of nanoparticles to cross biological membranes and other barriers ().

An external file that holds a picture, illustration, etc. Object name is zam0090998820001.jpg

(A) Transmission electron micrograph showing an amorphous silicon dioxide aggregate particle (gray structure) together with numerous supported silver metal particles (dark spots). (B) Scanning transmission electron micrograph of the structure shown in panel A, providing better contrast between the silica structure (gray) and the silver metal particles (bright spots). (C) Higher magnification of the region in panel B enclosed in a box. The silver metal particles are typically between 1 and 10 nm in diameter. Transmission electron microscope and scanning transmission electron microscope images were obtained using an FEI Tecnai F30 FEG microscope operated at 300 kV.

The microorganisms and growth conditions used for antimicrobial testing are shown in Table Table1.1. The MICs for all combinations of silver materials and microorganisms were determined by preparing twofold serial dilutions of the additives in an appropriate growth medium (Table (Table1).1). The tubes were then inoculated with 107 CFU/ml from overnight cultures of the bacteria or 106 CFU/ml for Candida albicans and incubated on a shaker (180 rpm) for 24 h. The MIC was defined as the lowest concentration of the silver additive at which no visual turbidity of the growth medium developed. The minimal bactericidal concentration (MBC) was determined by surface plating 0.2-ml aliquots from the nonturbid tubes, followed by incubation at 37°C for 24 h. The MBC was defined as the lowest concentration of silver additive resulting in less than 200 colonies per plate (corresponding to a killing rate of more than 4 logs). Aspergillus niger spores were harvested by floating the spores in densely grown lawns on malt extract agar plates in an extraction buffer (0.1% [vol/vol] Tween 20, 145 mM sodium chloride, 20 mM sodium phosphate; pH 7.4) and removing them. The MIC was determined by spreading approximately 200 spores on malt extract agar plates containing serial dilutions of the silver additives (Table (Table1)1) and was defined as the lowest concentration that prevented visible growth after 72 h of incubation at 30°C.


Comparison of the antimicrobial activities of silver nanocomposite powder, silver nitrate, and silver zeolite

Microorganism Silver nanocomposite

Silver nitrate

Silver zeolite

MIC (μg/ml)a MBC or MFC (μg/ml)b MIC (μg/ml)a MBC or MFC (μg/ml)b MIC (μg/ml)a MBC or MFC (μg/ml)b
Escherichia coli ATCC 2732c 62.5 125 7.8 15.6 3.9 15.6
Klebsiella pneumoniae ATCC 4352c 62.5 125 3.9 7.8 7.8 31.2
Pseudomonas fluorescens LME 2333d 62.5 250 7.8 7.8 15.6 31.2
Salmonella enterica serovar Enteritidis D1c 62.5 250 3.9 7.8 15.6 62.5
Salmonella enterica serovar Typhimurium DB 7155c 62.5 250 3.9 15.6 15.6 31.2
Enterococcus faecalis ATCC 19433e 62.5 250 3.9 7.8 7.8 7.8
Bacillus cereus ATCC 14579e 250 500 31.2 31.2 62.5 250
Listeria monocytogenes Scott Af 500 1,000 31.2 31.2 31.2 62.5
Staphylococcus aureus ATCC 29213f 250 1,000 15.6 15.6 15.6 125
Candida albicans ATCC 10259g 125 2,000 31.2 250 62.5 250
Aspergillus niger ATCC 9642h 2,000 NDi 15.6 ND 125 ND
aThe MIC was determined at least in duplicate.
bThe MBC or minimum fungicidal concentration (MFC) was determined at least in duplicate.
cCultured in Luria-Bertani broth (10 g/liter peptone from casein, 5 g/liter yeast extract, 10 g/liter NaCl; Merck, Darmstadt, Germany) at 37°C.
dCultured in Biotone tryptose broth (Biolife, Milan, Italy) at 30°C.
eCultured in Biotone tryptose broth (Biolife, Milan, Italy) at 37°C.
fCultured in half-strength brain heart infusion broth (Biolife, Milan, Italy) at 37°C.
gCultured in malt extract broth (Merck, Darmstadt, Germany) at 37°C.
hCultured on malt extract agar (Merck, Darmstadt, Germany) at 30°C. The MIC for A. niger was defined as the lowest concentration not associated with visible growth on malt extract agar after 72 h of incubation at 30°C.
iND, not determined.

MICs and MBCs for the silver additives tested are shown in Table Table1.1. All experiments were performed at least in duplicate. For bacteria, the MICs of the nanocomposite material ranged from 62.5 to 500 μg/ml, corresponding to 12.5 to 100 μg pure Ag/ml. The MICs of silver nitrate varied from 3.9 to 31.2 μg/ml (corresponding to 2.4 to 19.8 μg Ag/ml), and the MICs of silver zeolite ranged from 3.9 to 31.2 μg/ml (corresponding to 2 to 12 μg Ag/ml). The MBCs determined were in the ranges from 125 to 1,000 μg/ml for the nanocomposite powder, from 7.8 to 31.2 μg/ml for silver nitrate, and from 7.8 to 125 μg/ml for silver zeolite. Growth of C. albicans was inhibited by 125 μg/ml silver nanocomposite, and the minimal fungicidal concentration was 2 mg/ml. Development of visible colonies of A. niger on agar plates was also completely inhibited by 2 mg silver nanocomposite per ml agar.

In general, gram-positive bacteria appeared to be more tolerant to silver than gram-negative cells (Table (Table1),1), except for Enterococcus faecalis, for which the MICs and MBCs were similar to those for gram-negative bacteria. It has previously been reported that gram-positive bacteria are less susceptible to the antimicrobial activity of silver (). It was speculated that this may be due to differences in the cell wall structure (). The cell wall of gram-positive bacteria contains multiple layers of peptidoglycan compared to the cell wall of gram-negative bacteria. Peptidoglycan is a complex structure and often contains teichoic acids or lipoteichoic acids which have a strong negative charge, which may contribute to sequestration of free Ag+ions. Thus, gram-positive bacteria may allow less Ag+ to reach the cytoplasmic membrane than gram-negative bacteria allow () and may therefore be less susceptible.

Susceptibility tests using different silver compounds in previous studies revealed that the MICs of silver particles for Escherichia coli ranged from 2 to 75 μg/ml (). However, because corresponding silver concentrations were not specified, it is not possible to compare these values to our results. For silver zeolite containing 1.9% (wt/wt) Ag, the previously reported MICs determined by using a similar protocol ranged from 256 to 2,048 μg/ml, corresponding to 4.8 to 38.4 μg/ml of Ag (). Here we used silver zeolite containing 38% (wt/wt) Ag to determine the MIC for E. coli. The MIC determined (1.9 to 3.9 μg/ml) was much lower than the previously reported MICs of the silver zeolite containing 1.9% Ag. The nanocomposite material had an MIC of 62.5 μg/ml (12.5 μg Ag/ml) for E. coli. Not considering the relative Ag content, silver nitrate and silver zeolite (38% Ag) resulted in inhibition that was approximately 10 times more effective than the inhibition observed with the nanocomposite. This can be explained by the fact that in aqueous systems silver nitrate dissolves completely and the silver is completely available in its biologically active ionic form. The silver ions held in the zeolite structure are also relatively rapidly released into solution. In contrast, the silver nanoparticles embedded within the silica matrix release Ag+ in a more gradual, controlled manner and at a much lower rate (). Thus, although silver nitrate and silver zeolite are more effective in applications where high Ag+ concentrations are required immediately, the effect is only short lived. In contrast, the nanocomposite powder allows slow and controlled release of Ag+, resulting in long-term antimicrobial activity. This should be a clear advantage in any long-term antimicrobial applications (e.g., contact surfaces, fibers, plastics, medical devices, food-manufacturing equipment, cutting boards, etc.).

To examine the antimicrobial properties of a typical application product, silver-containing polystyrene plates were manufactured from commercially available polystyrene polymer (clear, unfilled) using a thermoplastic injection-molding process (). Test coupons that were 50 by 50 by 1.5 mm and contained the nanocomposite material (0.25% [wt/wt], corresponding to approximately 500 ppm Ag) were produced by dry blending the polystyrene polymer with the required amount of polymer concentrate containing the silver nanocomposite additive, which was followed by injection molding. The antimicrobial activity was determined by using the Japanese industrial standard test (JIS Z 2801:2000) (). In brief, the test samples were placed in petri dishes and inoculated with 0.4 ml of a bacterial culture containing 105 to 106 CFU/ml. The inoculum was covered with a polyester film (X-131 transparent copier film; Folex Imaging), and the petri dishes were incubated at 37°C for 24 h in a humid chamber to prevent desiccation. After the incubation period 20 ml of extraction solution (0.1% [vol/vol] Tween 20, 145 mM sodium chloride, 20.5 mM sodium phosphate; pH 7.4) was added to the petri dishes and shaken for 2 min. Subsequently, serial dilutions of the extraction solution were spread on agar plates in triplicate and incubated at 37°C overnight. Colonies were counted visually, and the numbers of CFU per sample were determined. The activity value was calculated from the mean value for the individual samples by subtraction of the log value determined for the test sample from the log value determined for the control. The results for viable counts determined for the control and the silver nanocomposite-containing samples are shown in Table Table2.2. The activity values determined by the JIS Z 2801:2000 method () were 4.4 for E. coli and 2.1 for Staphylococcus aureus (P < 0.05, Student’s t test [n = 3]). The results demonstrate that the silver-silica nancomposite-containing polystyrene material has significant antibacterial activity against both E. coli and S. aureus.


Antimicrobial activity of silver nanocomposite-containing polystyrene platesa

Organism No. of cells (CFU/sample)b

Polystyrene control after inoculation Polystyrene control after 24 h Polystyrene with silver after 24 h
Escherichia coli 1.7 × 105 ± 4.9 × 104 2.6 × 106 ± 6.5 × 104 <100 4.4
Staphylococcus aureus 1.7 × 105 ± 7.9 × 104 1.8 × 105 ± 6.9 × 104 1.4 × 103 ± 9.0 × 102 2.1
aActivity was tested by using Japanese industrial standard JIS Z 2801:2000 ().
bThe values are means ± standard deviations for measurements obtained for three independent sample pieces. The values for the polystyrene control after 24 h and polystyrene with silver after 24 h were significantly different according to the Student t test (P < 0.05; n = 3).

In this study, a silver-silica nanocomposite material with a novel structure and composition was investigated to determine its antimicrobial properties. The material exhibited very good antimicrobial activity against a wide range of microorganisms. The inhibition of microbial growth due to surface contact with the silver-silica nanocomposite-containing polystyrene demonstrated that materials functionalized with the silver nanocomposite have excellent antimicrobial properties. Further studies of the mode of action of the silver-silica nanocomposite material with gram-positive and gram-negative bacteria and also with yeasts and molds are required to fully evaluate its potential for use as an antimicrobial additive in various materials.


We are grateful to Joos Kiener for excellent technical assistance and to Elisabeth Müller Gubler from the Electron Microscopy Center of ETH Zürich for the electron microscopy images. M.J.H. thanks S. E. Pratsinis for his suggestions and ideas concerning synthesis of the material and the Particle Technology Laboratory, Department of Mechanical and Process Engineering at the Swiss Federal Institute of Technology (ETH Zurich), for supporting initial development of the material.


Published ahead of print on 6 March 2009.


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2. Anonymous. 1993. EPA reregistration eligibility document for silver, case 4082. U.S. Environmental Protection Agency, Washington, DC. http://www.epa.gov/oppsrrd1/REDs/old_reds/silver-pdf.
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Are Nanoparticles Safe?

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The rapid development of nanotechnology has increased fears about the health risks of nano-objects. Are these fears justified? Do we need a new discipline, nanotoxicology, to evaluate the risks? Harald F. Krug and Peter Wick of the Swiss Federal Laboratories for Materials Science and Technology discuss these questions in the journal Angewandte Chemie.

“Research into the safety of nanotechnology combines biology, chemistry, and physics with workplace hygiene, materials science, and engineering to create a truly interdisciplinary research field,” explain Krug and Wick. “There are several factors to take into account in the interaction of nano-objects with organisms,” they add.

The term nanotoxicology is fully justified. “Nanoscale particles can enter into cells by other means of transport than larger particles.” Another critical feature is the large surface area of nano-objects relative to their volume. If a similar amount of substance is absorbed, an organism comes into contact with a significantly larger number of molecules with nanoparticles than with larger particles.