Novel nanoparticles change color

Compiled by Photonics Spectra staff

Tiny polymeric containers stuffed with red and green quantum dots promise to provide continuous light for biomedical imaging.

Engineers at Ohio State University have invented nanoparticles that, somewhat unusually, glow red, green or yellow depending upon the state of the quantum dots contained within.

This series of photos depicts a novel nanoparticle changing from red to green to yellow over the course of 2 min. Courtesy of G. Ruan, Ohio State University.

Researchers routinely tag molecules with fluorescent materials to see them under the microscope. Unlike the more common fluorescent molecules, quantum dots shine brightly and could illuminate chemical reactions especially well, allowing researchers to see the inner workings of living cells. The new nanoparticles could be a useful addition to the arsenal of biomedical engineers trying to find the roots of diseases, scientists said.

“This work could be groundbreaking for the field of nanotechnology as a whole because it solves two seemingly irreconcilable problems with using quantum dots,” said research scientist Gang Ruan.

Because of their quantum mechanical effects, quantum dots “twinkle”: They randomly blink on and off. When many dots come together, however, the blinking becomes less noticeable – and when large clusters form, they appear to glow with a steady light. Researchers have found the blinking in quantum dots to be troublesome because it interrupts a tagged molecule’s trajectory when they are trying to follow it. But blinking is beneficial too because, when dots come together and the blinking disappears, they know that tagged molecules have aggregated.

The scientists discovered that they could use the “good” and avoid the “bad” aspects of blinking by grouping together a few quantum dots of different colors inside a micelle – a nanosize spherical container found in household detergents. Ruan and his team created micelles from polymers, with various combinations of red and green quantum dots inside.

Experiments confirmed that the micelles appeared to glow steadily. Those stuffed with only red quantum dots glowed red, and those stuffed with green glowed green. However, those stuffed with red and green dots changed from red to green to yellow. The color changes occurred when one dot or another blinked inside the micelle. Much as with the single-color-stuffed micelles, the dual-color-stuffed micelles could blink solely red or green – but if both lit up simultaneously, the micelle glowed yellow from the additive effect.

It’s not possible to control when color changes happen inside individual micelles, but the continuous glow allows uninterrupted tracking of tagged molecules, and color changes indicate when molecules have come together. The particles also could be used in fluid mechanics research, the scientists said.

The team also is working on developing magnetic particles to enhance medical imaging of cancer, which, combined with the quantum dot technology, could enable various types of imaging methods. Before the particles could be safe to use in the body, however, they would have to be made of biocompatible materials.

In the future, the scientists hope to introduce another color – blue – into the mix to see what happens.