Researchers at the University of Strathclyde, UK, have developed a new method to recycle the valuable semiconductor colloidal quantum dots used to fabricate supraparticle lasers. The recovered particles can be reused to build new lasers with a photoluminescence quantum yield almost as high as lasers made from new particles.

Supraparticle lasers are a relatively new class of micro-scale lasers that show much promise in applications such as photocatalysis, environmental sensing, integrated photonics and biomedicine. The active media in these lasers – the supraparticles – are made by assembling and densely packing colloidal quantum dots (CQDs) in the microbubbles formed in a surfactant-stabilized oil-and-water emulsion. The underlying mechanism is similar to the way that dish soap, cooking oil and water mix when we do the washing up, explains Dillon H Downie, a physics PhD student at Strathclyde and a member of the research team led by Nicolas Laurand.

Supraparticles have a high refractive index compared to their surrounding medium. Thanks to this difference, light at the interface between them experiences total internal reflection. This means that when the diameter of the supraparticles is an integer multiple of the wavelength of the incident light, so-called whispering gallery modes (resonant light waves that travel around a concave boundary) form within the supraparticles.

“The supraparticles are therefore microresonators made of an optical gain material (the quantum dots),” explains Downie, “and individual supraparticles can be made to lase by optically pumping them.”

The problem is that many CQDs are made from expensive and sometimes toxic elements. Demand for these increasingly scarce elements will likely outstrip supply before the end of this decade, but at present, only 2% of quantum dots made from these rare-earth elements are recycled. While researchers have been exploring ways of recovering them from electronic waste, the techniques employed often require specialized instruments, complex bio-metallurgical absorbents and hazardous acid-leaching processes. A more environmentally friendly approach is thus sorely needed.

Exceptional recycling potential

In the new work, Laurand, Downie and colleagues recycled supraparticle lasers by first disassembling the CQDs in them. They did this by suspending the dots in an oil phase and applying ultrasonic high-frequency sound waves and heat. They then added water to separate out the dots. Finally, they filtered and purified the disassembled CQDs and tested their fluorescence efficiency before reassembling them into a new laser configuration.

Using this process, the researchers were able to recover 85% of the quantum dots from the initial supraparticle batch. They also found that the recycled quantum dots boasted a photoluminescence quantum yield of 83 ± 16%, which is comparable to the 86 ± 9% for the original particles.

“By testing the lasers’ performance both before and after this process we confirmed their exceptional recycling potential,” Downie says.

Simple, practical technique

Downie describes the team’s technique as simple and practical even for research labs that lack specialized equipment such as centrifuges and scrubbers. He adds that it could also be applied to other self-assembled nanocomposites.

“As we expect nanoparticle aggregates in everything from wearable medical devices to ultrabright LEDs in the future, it is, therefore, not inconceivable that some of these could be sent back for specialized recycling in the same way we do with commercial batteries today,” he tells Physics World. “We may even see a future where rare-earth or some semiconductor elements become critically scarce, necessitating the recycling for any and all devices containing such valuable nanoparticles.”

By proving that supraparticles are reusable, Downie adds, the team’s method provides “ample justification” to anyone wishing to incorporate supraparticle technology into their devices. “This is seen as especially relevant if they are to be used in biomedical applications such as targeted drug delivery systems, which would otherwise be limited to single-use,” he says.

With work on colloidal quantum dots and supraparticle lasers maturing at an incredible rate, Downie adds that it is “fantastic to be able to mature the process of their recycling alongside this progress, especially at such an early stage in the field”.

The study is detailed in Optical Materials Express.

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