Growing metals on cotton
Researchers at the Pacific Northwest National Laboratory (PNNL) have created a new form of metal crystals grown on cotton. They've used acid-treated cellulose fibers from cotton to crystallize them. Then, they grew all kinds of metal nanocrystals measuring between 2 and 200 nanometers on what they call "a cotton assembly line." They successfully built nanocrystals of gold, silver, palladium, platinum, copper or nickel. And they think that this technology could be used in a wide range of applications, including biosensors, biological imaging, drug delivery and catalytic converters.
Before going further, below are two images showing some precious metal crystals obtained with this process. An electron micrograph (TEM) of a metal, in this case platinum, deposited on cellulose, is shown on the left. The crystalline cellulose without metal is shown as an inset. And on the right, you can see another TEM showing the pattern of platinum clustering along hydroxyl sites on the cellulose surface. (Credit: PNNL)
This research effort has been led by Yongsoon Shin and Gregory Exarhos, who both work at the PNNL's Fundamental Science Directorate. But how did they conduct their experiments?
Using acid-treated cellulose fibers from cotton as a natural template, the PNNL team has been able to grow gold, silver, palladium, platinum, copper, nickel and other metal and metal-oxide nanocrystals quickly and of uniform size, Shin said. The metals display catalytic, electrical and optical that would not be present in larger or odd-sized crystals.
The acid converts the cellulose to a large, stable crystallized molecule rich in oxygen-hydrogen, or hydroxyl, groups, predictably spaced along the long chemical chains, or polymers, that comprise the cellulose molecule's backbone. When most metal salts dissolved in solution are added in a pressurized oven and heated 70 to 200 degrees centigrade or warmer for four to 16 hours, uniform metal crystals form at the hydroxyl sites.
This research work has been presented on Monday at the 233rd National Meeting & Exposition of the American Chemical Society (March 25-29, 2007, Chicago, IL) in one of the sessions focused on Nanotechnology: A Fiber Perspective. The title of the presentation was "The use of cellulose nanocrystal for the preparation of inorganic nanocrystals" and here is the beginning of the abstract written in plain English, but in scientific 'jargon.'.
Cellulose nanocrystal (CNXL), which is separated from cotton cellulose by acid hydrolysis, has been utilized for the synthesis of various kinds of metal and metal oxides. The surface hydroxyl groups serve to reduce metal ions such as Ag(I), Pt(IV), Pd(II), and Se(IV) to corresponding nanocrystalline metals at 160-200°C in air without adding any reducing agents. The original crystalline structure of the CNXL is maintained in the temperature range and the hydroxyl groups reduce metal ions to metal nanocrystals on the CNXL surface.
Now, let's look at a previous research project from Yongsoon Shin, who already turned instant petrified wood into super ceramics (PNNL news release, May 19, 2005). With his team, he developed a process "to create two new ceramic materials that are laboratory versions of petrified wood. These materials combine the hardness of metal with the high surface area of carbon to form metal carbides that are stronger than steel and can withstand temperatures to 1,400 degrees Celsius."
Below is an electron microscopic image showing "a cross section of wood that was artificially petrified in days, mimicking a natural process that takes millions of years" (Credit: PNNL). Here is a link to larger versions of this image.
So what will be Shin's next project? I guess we'll discover it in a couple of years.
Sources: DOE/Pacific Northwest National Laboratory news release, March 26, 2007; and various websites
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