Researchers have found a promising technology for clearing water of pollutants: a new nanoparticle that converts light to heat.
Trace amounts of contaminants such as pesticides, pharmaceuticals, and perfluorooctanoic acid in sources of drinking water have posed significant health risks to humans in recent years. These micropollutants have eluded conventional treatment processes to remove them.
Certain chemical treatment processes - typically involving ozone, hydrogen peroxide, or UV light - have proven effective. These processes, however, can be expensive and energy-intensive.
A new nanoparticle, created as part of an effort of the Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), could lead to technologies that get around those limitations. NEWT is a joint effort by researchers at Yale, Rice University, Arizona State University, and the University of Texas-El Paso.
A study of the new nanoparticle - led by the lab of Yale's Jaehong Kim, the Henry P. Becton Sr. Professor and chair of chemical & environmental engineering, in collaboration with Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering at Rice University - was published June 22 in Proceedings of the National Academy of Sciences.
Researchers in several fields have shown interest in gold nanoparticles for their photothermal and photocatalytic properties, which have proven effective in cancer therapy and other uses. Photothermal properties convert light to heat; photocatalytic properties use light to activate a chemical process.
Yet the nanoparticles haven't figured heavily in water purification efforts, researchers said, partly because of the difficulty of dispersing nanoparticles in water without stabilizing agents that aren't good for water purification applications.
The NEWT researchers solved the stabilization problem by designing and synthesizing "Janus" gold nanorods. These nanoparticles, hundreds of times smaller than the width of a human hair, are half-coated with silica. This design element is critical, researchers said, since the silica-coated half allows each nanorod to remain separate from the others and suspended in the water.
"We started with gold nanoparticles and then explored a way to stabilize them," Kim said. "With this partial coating, the nanorods get dispersed in water really well, and that's very useful for this kind of application."
The nanorods absorb intense levels of light and convert the light to heat localized on the surfaces - a process far more efficient than heating the entire volume of water. The method is low-cost and sustainable, the researchers said, because it uses sunlight. The same part of the nanorod also acts as an electron-transfer catalyst to promote destruction of micropollutants such as pesticides and pharmaceuticals.
"It achieves various functions - in particular by using the solar radiation to produce highly localized heat," Kim said. "This is the first demonstration of using that particular phenomena for pollutant destruction."
Naomi Halas, director of the Laboratory for Nanophotonics at Rice University, played the key role of elucidating the complex mechanisms behind how the photothermal and photocatalytic reactions occur on this unique nanoparticle.
"This is really nanoengineering at its best, a novel nanoparticle designed to solve an important problem in what would otherwise be an impossible environment," Halas said.
Kim said more work is needed to scale up the nanoparticle for real-world application, including finding a material less expensive than gold.
Pedro Alvarez, the George R. Brown Professor of Civil and Environmental Engineering at Rice University and founding director of NEWT, called the study "a great example of how forefront advances in nanotechnology can pave a new way to solve water challenges."
NEWT's nanosystems-engineering research center, headquartered at Rice University, applies nanotechnology to develop off-grid water treatment systems that both protect human lives and support sustainable economic development. In addition to its academic partners, NEWT's leadership includes more than 30 industry and government partners.
