Supraparticles are used as catalysts under UV light. [Image: Dillon H. Downie, University of Strathclyde]
Microscopic lasers made from quantum dots hold great promise for future roles in targeted drug delivery and optoelectronics. But when their job is done, what happens to the often rare and expensive semiconductors that go into these tiny devices?
Researchers at a university in the UK have figured out how to disassemble the nanoscale lasers and recycle the quantum dots inside them (Opt. Mater. Express, doi: 10.1364/OME.537183). The method recovers 85% of the dots with enough of a photoluminescence quantum yield to form new lasers.
Quantum dot supraparticles
To make the tiny lasers, scientists suspend colloidal quantum dots in a mixture of oil, water and a surfactant. Just as dish detergent and water make suds in a sink full of greasy dishes, the dot-filled mixture makes small bubbles around which the dots congregate. Researchers have dubbed these agglomerations “supraparticles.”
According to lead author Dillon H. Downie, University of Strathclyde, supraparticles are a recent field of study, as Daniel Vanmaekelbergh’s group at Utrecht University in the Netherlands first described them only a decade ago.
Each colloidal quantum dot is about 5 to 10 nm in diameter, Downie says, and the supraparticle assemblies are about three orders of magnitude bigger: 5 to 10 μm across. A single supraparticle consists of anywhere between hundreds of thousands and a few million quantum dots.
Researchers were able to recover quantum dots from supraparticle lasers and use them to create new lasers that performed similarly to their precursors. These scanning electron microscopy images show the original lasers and the recycled lasers. [Image: Dillon H. Downie, University of Strathclyde]
Likewise, the number of supraparticles that practical applications would require varies, from many thousands in drug-delivery systems to a few dozen in LED lasers, and to a single supraparticle tailored to serve as a digital barcode or microscale sensor.
“One sample of colloidal quantum dots can go upwards of US$100 per milligram, which will produce the same weight value of supraparticles,” Downie says. “These costs depend on the nanoparticle species, but it can get very expensive very quickly. The cost of fabricating the supraparticles themselves, however, is comparatively cheap—all being within a one-pot and room temperature reaction.”
Recycling while maintaining purity
To recycle the colloidal quantum dots, the Strathclyde team first had to thoroughly remove all contaminants from the equipment using organic solvents followed by a blast of pressurized nitrogen. These steps helped to maintain the purity of the dots and prevent them from aggregating in the wrong places. Next, the researchers dissolved the supraparticles in toluene subjected to alternating cycles of ultrasonic vibrations and heat, followed by a procedure to reattach ligands to the quantum dots.
“The biggest innovation the study produced was simply by proving that colloidal quantum dots can still produce lasers even after recycling,” Downie says. “This method provides ample justification to those who wish to incorporate this supraparticle technology into their devices through proving its reusability. This is seen as especially relevant towards their use as targeted drug-delivery systems, which would otherwise be limited to single-use.”
According to Downie, the team’s biggest challenge was achieving a high level of purity and functionality among the recycled dots and the supraparticles formed from them. “The recycled quantum dot samples were often contaminated with dissolved salts, undissolved aggregates and dust particles,” he says. “This would lead to supraparticles that were not of sufficient quality to become lasers. The quantum dots themselves often lost their protective ligands in the process, which also reduced their quantum yields. Overcoming this challenge required the gradual development of a method that combined filtration, separation with oil and water, and subsequent ligand reattachment cycles. This ensured that the colloidal quantum dots were both pure and high performing prior to their reassembly into new supraparticle lasers.”
Any lab that “has access to clean glassware, ultrasonic baths, moderate heating elements and commonly available solvents” can recycle supraparticles and their constituent dots, Downie says. Regardless of the potential for shortages of nanoparticles and restrictions on exports of rare-earth elements, he adds, scientists should recycle these elements for economic and environmental reasons.