Gold Nanoparticles Boost QD Glow

A DNA template that controls the distance between gold nanoparticles and quantum dots can predictably increase or decrease the intensity of a quantum dot’s fluorescence. The breakthrough could lead to the use of quantum dots as a component in better chemical sensors, photodetectors and nanoscale lasers.

Those who have tried to tune a radio know that moving their hands toward or away from the antenna can improve or ruin the reception, but that controlling this strange effect is difficult. Similarly, nanotechnology researchers have been frustrated trying to control the light emitted from quantum dots, which brighten or dim with the proximity of other particles.

Researchers at the National Institute of Standards and Technology (NIST) have developed ways to accurately and precisely place different kinds of nanoparticles near each other and to measure the behavior of the resulting nanoscale constructs. Because nanoparticle-based inventions require multiple types of particles to work together, it is crucial to have reliable methods to assemble them and to understand how they interact.

The NIST team explored the behavior of quantum dots and gold nanoparticles placed in different configurations on small rectangular constructs made of self-assembled DNA (see inset for photograph). Laser light (green) allowed the team to explore changes in the fluorescence lifetime of the quantum dots when close to gold particles of different sizes. Courtesy of NIST.

Two types of nanoparticles were observed: quantum dots, which glow with fluorescent light when illuminated, and gold nanoparticles, which have long been known to enhance the intensity of light around them. The two could work together to make nanoscale sensors from rectangles of woven DNA strands formed using a DNA origami technique.

The DNA rectangles can be engineered to capture various types of nanoparticles at specific locations with a precision of about 1 nm. Minute changes in the distance between a quantum dot in close proximity with a gold nanoparticle on the rectangle cause the quantum dot to glow more or less brightly as it moves away from or toward the gold.

Because these small movements can be easily detected by tracking the changes in the quantum dot’s brightness, they can be used to reveal, for example, the presence of a particular chemical that is selectively attached to the DNA rectangle. However, getting it to work properly is difficult, said Alex Liddle, a scientist with NIST’s Center for Nanoscale Science and Technology.

“A quantum dot is highly sensitive to the distance between it and the gold, as well as the size, number and arrangement of the gold particles,” he said. “These factors can boost its fluorescence, mask it or change how long its glow lasts. We wanted a way to measure these effects, which had never been done before.”

Several groups of DNA rectangles were made, each with a different configuration of quantum dots and gold particles in a solution. Using a laser as a spotlight, the team followed the movement of individual DNA rectangles in the liquid, detecting changes in the quantum dots’ fluorescence lifetime when they were close to gold particles of different sizes. The scientists also demonstrated that they could exactly predict the fluorescence lifetime, depending on the size of the nearby gold nanoparticles.

Although the tracking technique was time-consuming, Liddle said that the strength of their results will enable them to engineer the quantum dots to have a specific desired lifetime and could even lead to better measurement methods.

“Our main goals for the future are to build better nanoscale sensors using this approach and to develop the metrology necessary to measure their performance,” he said.

The findings were reported in Angewandte Chemie (doi: 10.1002/anie.201206253).

For more information, visit: www.nist.gov