In an important advance toward the large-scale manufacture offluorescent quantum dots, scientists at Rice University have developeda new method of replacing the pricey solvents used in quantum dotsynthesis with cheaper oils that are commonplace at industrial chemicalplants.
Rice's study, which was conducted under the auspices of the Center forBiological and Environmental Nanotechnology (CBEN), is published onlineand slated to appear in the October issue of the journal Nanotechnology.
"CBEN started to undertake some exploratory work more than a year agoon the scale-up issues of quantum dot manufacture, but the solventsturned out to be so expensive that we just couldn't afford to run morethan a few large-reactor experiments," said the study's lead author,Michael Wong, assistant professor of chemical and biomolecularengineering and of chemistry. "That was a great reality check, and itmade us look at the problem of solvent cost sooner rather than later."
Quantum dots typically cost more than $2,000 per gram from commercialsources, and pricey solvents like octadecene, or ODE - the leastexpensive solvent used in quantum dot preparation today - account forabout 90 percent costs of raw materials.
Heat-transfer fluids - stable, heat-resistant oils that are used tomove heat between processing units at chemical plants - can cost up toseven times less than ODE. Replacing ODE with the heat-transfer fluidDowtherm A, for example, reduces the overall materials cost of makingquantum dots by about 80 percent.
Quantum dots are tiny crystals of semiconducting materials - cadmiumselenide or CdSe is the most popular flavor - that measure just a fewnanometers in diameter. Most of the commercial possibilities discussedfor quantum dots - bioimaging, color displays, lasers, etc. - relate totheir size-controlled fluorescence. For example, CdSe quantum dots havethe ability to absorb high-energy photons of ultraviolet light andre-emit them as photons of visible light. They glow different colors,depending on the size, shifting from the red to the blue end of thespectrum as the crystals get smaller.
The reproducible synthesis of high-quality quantum dots became areality in the early 1990s when researchers at MIT pioneered a newmethod of producing quantum dots with uniform sizes and well-definedoptical signatures. The basic recipe for making quantum dots hasn'tchanged much since it was first developed. A solvent is heated toalmost 500 degrees Fahrenheit, and solutions containing cadmium andselenium compounds are injected. They chemically decompose andrecombine as pure CdSe nanoparticles. Once these nanocrystals form,scientists can adjust their optical properties by growing them toprecisely the size they want by adjusting the cooking time.
The solvent originally used for this process was trioctylphosphineoxide, or TOPO, which costs more than $150 per liter. Later, otherscientists introduced a new recipe by replacing TOPO with a mixture ofODE and oleic acid.
Wong said the CBEN research team, which included CBEN Director VickiColvin, professor of chemistry, and Nikos Mantzaris, assistantprofessor of chemical and biomolecular engineering and ofbioengineering, had some initial doubts about whether heat-transferfluids could be substituted for ODE.
"They were cheap and they didn't break down at high temperatures, butno one uses these compounds for chemical reactions," said Wong. Inaddition to finding that other quantum dot nanostructures could be madein heat- transfer fluids, the team concluded that any solvent could beused to replace ODE. Thanks to a mathematical modeling approachdeveloped by Mantzaris, the team now has a method for predicting theparticle size and growth behavior of quantum dots based on only threephysical properties of a given solvent: viscosity, surface free energyand solubility of bulk cadmium selenide powder.
The research was funded by the National Science Foundation.
Other co-authors include graduate students Sabashini Asokan, KarlKrueger and Zuze Mu; postdoctoral research associate Ammar Alkhawaldeh;and undergraduate researcher Alessandra Carreon.
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