To make quantum dot technology more sustainable, researchers at the University of Strathclyde developed a practical, inexpensive way to recycle the colloidal quantum dots (CQDs) used to make microscopic supraparticle (SP) lasers. The researchers created new lasers from the recycled CQDs and demonstrated that the performance of these lasers was comparable to their predecessors. Recovery and reuse of CQDs would improve their economic and environmental sustainability. Additionally, the method developed by the team could be used to recycle a variety of nanoparticles. “Our method reduces costs and environmental impact by minimizing the need for new nanoparticles and the disposal of old ones, and it should be applicable to any colloidal nanoparticle species, especially rare-earth ones,” researcher Dillon H. Downie said. Researchers developed a way to recycle the quantum dots used to make microscopic supraparticle (SP) lasers. A sample of the SPs recycled in the experiment is shown. Courtesy of Dillon H. Downie, University of Strathclyde. CQDs are the building blocks of SP lasers. The laser light is confined within a tiny sphere made from aggregated CQDs that efficiently absorb, emit, and amplify the light. SP lasers are assembled by suspending and stabilizing CQDs in an oil-in-water emulsion. During this process, microbubbles are formed in which the CQDs aggregate. Not all CQD batches can create an SP laser, and even the CQD batches that are successful degrade over time. To prevent the costly loss of CQDs in faulty SP batches, Downie proposed an approach to recycling them. The research team, led by senior research fellow Nicolas Laurand, tested the technique on a defective sample. First, the researchers disassembled SP lasers by suspending the lasers in an oil phase, applying moderate heat, and subjecting the lasers to mechanical stress from ultrasonic sound waves. They then combined the oil mixture with water to separate the oil containing the CQDs from the water containing the impurities. They filtered the CQDs, treated them with an additional surface coating, and tested them to see if they could fluoresce efficiently. The CQDs that passed the test were reassembled into aggregates for creating SP lasers. To maximize the purity of the CQDs and minimize the use of solvents and loss of nanoparticles, the researchers used an enclosed separating funnel system to divide the liquids and filter the CQDs. The goal was to create a sustainable recycling method that was nontoxic and did not require extreme conditions or specialized equipment. “Our ‘eureka moment’ came when we could very clearly see new, albeit crude, supraparticles under the microscope,” Downie said. “Encouraged by this success, we began refining the recovery method for producing and validating the quality of our recycled nanoparticles.” Using this technique, the researchers demonstrated a CQD recovery yield of 85%. The recycled CQDs retained a photoluminescence quantum yield of 83 ± 16%, compared to the 86 ± 9% yield demonstrated by the initial batch. Use of the recycled nanoparticles to synthesize SPs via self-assembly enabled lasing SPs to be re-created with thresholds comparable to their precursors — that is, 32.8 ± 8.2 millijoules per square centimeter (mJ·cm-2) and 34.8 ± 8.6 mJ·cm-2, respectively. The SPs created with the recovered CQDs retained whispering gallery mode lasing characteristics and could be repurposed for various photonic applications. SP lasers can control light at the nanoscale, enabling precise manipulation of wavelength, intensity, and other properties. These microscopic lasers can be used for photocatalysis, biological and environmental sensing, integrated photonics, and medicine. A SP laser confines and amplifies light through whispering gallery modes — resonant light waves circulating along a spherical boundary — inside a tiny sphere made from aggregated CQDs. Courtesy of Dillon H. Downie, University of Strathclyde. “Supraparticle lasers are already beginning to be used for targeted drug delivery and sensing applications, as well as for components in compact electronic systems,” Downie said. “Nanoparticle aggregates and supraparticle lasers are expected to play an increasingly prominent role in everything from wearable medical devices to ultrabright LEDs.” The CQD recycling technique is an open-bench method. It does not require extreme conditions, specialized equipment, or extensive in-house synthesis. “The discoveries about quantum dot behavior over the past 40 years, culminating in last year’s Nobel Prize in Chemistry, allowed us to understand each stage of the recycling process at a fundamental level,” Downie said. “This enabled us to develop a simple method that is practical even for labs that lack specialized equipment like centrifuges, scrubbers, or magnetic field generators.” The researchers plan to perform additional testing to better understand how the performance of the nanoparticles changes each time they are recycled. The SPs tested in this study were unmodified, but the team is exploring ways to recycle functionalized or embedded SPs. The new CQD recycling method opens the potential for continued investigation into the recycling of other nanoparticle species, including rare-earth up-converting nanoparticles, and is expected to be fully scalable. The CQD recycling technique could enhance the manufacturing of SP lasers and contribute to the overall recycling efforts of a broad range of colloidal nanoparticle species. “We envision this method being used to extend the life cycle of supraparticles, which could be repurposed for various applications such as medical biosensors, representing a significant advance toward sustainable nanoengineering,” Downie said. The research was published in Optical Materials Express (www.doi.org/10.1364/OME.537183).
