For the first time, researchers have shown that it is possible to recycle the nanoparticles used to make microscopic supraparticle lasers, addressing the need to sustainably manage these valuable materials. Supraparticle lasers exhibit an exceptional ability to control light at the nanoscale, enabling precise manipulation of properties like wavelength and intensity. This emerging technology works by confining light within a tiny sphere made from aggregated colloidal quantum dots, which absorb, emit and amplify light incredibly efficiently.

“Supraparticle lasers are already beginning to be used for targeted drug delivery and sensing applications, as well as for components in compact electronic systems,” said Dillon H. Downie at the University of Strathclyde. “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.” The researchers led by Nicolas Laurand at the University of Strathclyde show that quantum dots from supraparticle lasers can be recycled and reused to create new lasers with similar performance.

“Nanoparticle aggregates and supraparticle lasers are expected to play an increasingly prominent role in everything from wearable medical devices to ultrabright LEDs,” said Downie. “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.” To create supraparticle lasers, colloidal quantum dots are suspended in an oil-in-water emulsion stabilized with a surfactant. This forms microbubbles in which the colloidal quantum dots naturally aggregate. However, not all batches successfully create supraparticle lasers, and even those that do will degrade with time.

The idea to develop the new recycling method arose during a meeting when members of the research team expressed frustration over the costly loss of nanoparticle quantum dots in faulty supraparticle batches. In an off-the-cuff remark, Downie proposed a potential recycling technique. With Laurand’s support, they tried the method on a defective sample, aiming to recover and reuse the expensive quantum dots. “Our eureka moment came when we could very clearly see new, albeit crude, supraparticles under the microscope,” said Downie. “Encouraged by this success, we began refining the recovery method for producing and validating the quality of our recycled nanoparticles.”

To recycle the supraparticle lasers, the researchers first had to encourage them to disassemble. They did this by suspending them back into an oil phase, applying moderate heat and subjecting them to mechanical stress in the form of ultrasonic sound waves. Then, they combined the mixture with water to separate the oil with the quantum dots from the water with the impurities. The quantum dots in the oil phase were then filtered, treated with additional surface coating and tested to see if they could efficiently fluoresce before being reassembled into aggregates for use in a laser. The liquid separation coupled with filtration was carried out in an enclosed separating funnel system, maximizing the sample purity while minimizing solvent use and nanoparticle loss.

Using this recycling process, the researchers demonstrated recovery of 85 % of the quantum dots from initial supraparticles. The recycled quantum dots retained a photoluminescence quantum yield of 83 ± 16 %, compared to 86 ± 9 % for the initial batch. They successfully used the recycled nanoparticles to create lasers that performed similarly to their precursors. “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,” said Downie. “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 nanoparticle performance changes each time they are recycled. The supraparticles tested in this study were unmodified, but the researchers are exploring techniques for recycling functionalized or embedded supraparticles. (Source: U. Strathclyde)

Reference: D. H. Downie et al.: Recycling self-assembled colloidal quantum dot supraparticle lasers, Opt. Mat. Exp. 14,  2982 (2024); DOI: 10.1364/OME.537183

Link: Institute of Photonics, Dept. of Physics, SUPA, University of Strathclyde, Glasgow, Scotland, UK