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Rice Scientists Replace Pricy Solvents With Cheap Processing Fluids
In an important advance toward the large-scale
manufacture of fluorescent quantum dots, scientists at Rice University have
developed a new method of replacing the pricy solvents used in quantum dot
synthesis with cheaper oils that are commonplace at industrial chemical
Rice's study, which was conducted under the auspices of the Center for Biological and Environmental Nanotechnology (CBEN), is published online and slated to appear in the October issue of the journal Nanotechnology.
"CBEN started to undertake some exploratory work more than a year ago on the scale-up issues of quantum dot manufacture, but the solvents turned out to be so expensive that we just couldn't afford to run more than a few large-reactor experiments," said the study's lead author, Michael Wong, assistant professor of chemical and biomolecular engineering and of chemistry. "That was a great reality check, and it made us look at the problem of solvent cost sooner rather than later."
Quantum dots typically cost more than $2,000 per gram from commercial sources, and pricy solvents like octadecene, or ODE - the least expensive solvent used in quantum dot preparation today - account for about 90 percent costs of raw materials.
Heat transfer fluids - stable, heat-resistant oils that are used to move heat between processing units at chemical plants - can cost up to seven times less than ODE. Replacing ODE with the heat-transfer fluid Dowtherm A, for example, reduces the overall materials cost of making quantum dots by about 80 percent.
Quantum dots are tiny crystals of semiconducting materials - cadmium selenide or CdSe is the most popular flavor - that measure just a few nanometers in diameter. Most of the commercial possibilities discussed for quantum dots - bioimaging, color displays, lasers, etc. - relate to their size-controlled fluorescence. For example, CdSe quantum dots have the ability to absorb high-energy photons of ultraviolet light and re-emit them as photons of visible light. They glow different colors depending on the size, shifting from the red to the blue end of the spectrum as the crystals get smaller.
The reproducible synthesis of high-quality quantum dots became a reality in the early 1990s when researchers at MIT pioneered a new method of producing quantum dots with uniform sizes and well-defined optical signatures. The basic recipe for making quantum dots hasn't changed much since it was first developed. A solvent is heated to almost 500 degrees Fahrenheit and solutions containing cadmium and selenium compounds are injected. They chemically decompose and recombine as pure CdSe nanoparticles. Once these nanocrystals form, scientists can adjust their optical properties by growing them to precisely the size they want by adjusting the cooking time.
The solvent originally used for this process was trioctylphosphine oxide, or TOPO, which costs more than $150 per liter. Later, other scientists introduced a new recipe by replacing TOPO with a mixture of ODE and oleic acid.
Wong said the CBEN research team, which included CBEN Director Vicki Colvin, professor of chemistry, and Nikos Mantzaris, assistant professor of chemical and biomolecular engineering and of bioengineering, had some initial doubts about whether heat-transfer fluids could be substituted for ODE. "They were cheap and they didn't break down at high temperatures, but no one uses these compounds for chemical reactions," said Wong.
In addition to finding that other quantum dot nanostructures could be made in heat transfer fluids, the team concluded that any solvent could be used to replace ODE. Thanks to a mathematical modeling approach developed by Mantzaris, the team now has a method for predicting the particle size and growth behavior of quantum dots based on only three physical properties of a given solvent: viscosity, surface free energy, and solubility of bulk cadmium selenide powder.
The research was funded by the National Science Foundation.
Other co-authors include graduate students Sabashini Asokan, Karl Krueger and Zuze Mu; post doctoral research associate Ammar Alkhawaldeh; and undergraduate researcher Alessandra Carreon.
The Center for Biological and Environmental Nanotechnology is a National Science Foundation Nanoscale Science and Engineering Center dedicated to developing sustainable nanotechnologies that improve human health and the environment. Located at Rice University in Houston, CBEN is a leader in ensuring that nanotechnology develops responsibly and with strong public support.
For more information, please visit cben.rice.edu
About Rice University:
Rice University is consistently ranked one of America's best teaching and research universities. It is distinguished by its: size - 2,850 undergraduates and 1,950 graduate students; selectivity - 10 applicants for each place in the freshman class; resources - an undergraduate student-to-faculty ratio of 6-to-1, and the fifth largest endowment per student among American universities; residential college system, which builds communities that are both close-knit and diverse; and collaborative culture, which crosses disciplines, integrates teaching and research, and intermingles undergraduate and graduate work. Rice's wooded campus is located in the nation's fourth largest city and on America's South Coast.
For more information, please visit www.rice.eduContact:
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