They're everywhere and they can also treat cancer.
They're tiny, they reflect and absorb light in useful ways, and they're increasingly found in our homes: quantum dots. These fancy-sounding nanoparticles, which are called “quantum” because of their tiny size, have a number of commercial applications, from sensors and solar cells to TVs and cell phone chargers. They're also used in medicine, where—among other things—they help doctors see cancer inside bodies and deliver drugs where they're needed, hopefully without harming surrounding cells.
But though quantum dots sound like magic, the process of making them—and the materials they're made from—do real harm to workers and the world around them. One prevalent way to make the dots is to synthesize them in a separate medium using a method called “colloidal synthesis.” Think muddy water: the dots are the dirt particles, and the stabilizing medium is the water, although the method here has many more steps than making mud. That medium is usually composed of hazardous biochemicals.
New research from a team of scientists in the UK and India demonstrates that it's possible to make quantum dots using waste from the tea industry. About one third of each tea crop produced around the world isn't used to make drinkable tea, says Sudhagar Pitchaimuthu, the study's corresponding author. Pitchaimuthu, who is a research fellow at Swansea University, spends most of his time looking for novel compounds that are currently going to waste, like these tea leaves, and seeing how they can be used to make new products rather than just heading to a landfill.
The team used tea leaf extract as their stabilizing medium when making cadmium quantum dots. “After the reaction, we didn't remove the tea leaf extract,” says Pitchaimuthu, so some beneficial parts of the tea extract remained as part of the quantum dots. They then tested their tea dots to ascertain their antibacterial properties, their use for bioimaging, and whether they could be used to inhibit the growth of cancer cells.
What they found was promising: the “green” quantum dots, as they're described in the paper, have potential, says Pitchaimuthu. The research suggests that they are suited for uses inside living bodies, such as antibacterial coatings on joint implants, as well as being able to provide better-than-average images when used in scans and, unexpectedly, being able to damage and even kill cancer cells. This last one is because the dots are able to penetrate into tiny pores on the cells, known as nanopores. Pitchaimuthu speculates this ability may be related to the tea extract particles.
The extract might also be the reason that these quantum dots give off light for scans better than others, by about 200 nanometers, he says. But why the tea extract seems to do some of these things remains poorly understood, and will be the focus of the collaborators' next research: for the moment, this publication discusses how to use the tea extract to produce quantum dots and what beneficial properties it seems to have.
This work is part of a larger push to make more environmentally friendly quantum dots. Although they're made using tea extract, these quantum dots, like most others, consist of heavy metals, prominently cadmium, which are toxic to all living cells. That means the process of making them is bad for workers and bad for the environment. “It would be great if we could go in the cadmium-free dot direction,” Pitchaimuthu says—particularly if we could make the dots out of materials that are currently wasted, like the surplus tea.
The biggest toxicity problems for the dots “come not from the method of production but from the materials themselves,” says chemist Dmitri Talapin of the University of Illinois, who was not involved with the study. He says that producing quantum dots using tea extract isn't revolutionary, although it is an interesting step toward making more environmentally responsible dots.
Quantum dots, with their wide industrial and medical applications, are “arguably the first commercially successful application of nanotechnology,” he says. It remains to be seen if industry can make them environmentally friendly.