Luminescent nanoparticles and a low-power laser for super-resolution microscopy

Australian and Chinese researchers have used a low-power laser beam to switch luminescent nanoparticles on and off, in a breakthrough that will lead to super-resolution microscopy for the detailed study of live cells and organisms. Their work has been published in the journal Nature.

The use of nanoparticles for bio-imaging is a relatively recent development which has attracted widespread attention. Typically, these nanoparticles are placed in biological samples and then excited by the light of a confocal microscope. The nanoparticles act as tiny ‘lamps’ which show where they are located, while any unwanted luminescence is suppressed.

However, fundamental limitations of light restrict the minimum size of images to about 200 nm — about half of one excitation wavelength and insufficient to visualise many biological structures of interest.

“A particular problem of current ‘stimulated emission depletion’ microscopy is that high laser power is required to suppress emission from normal dyes and this can damage the biological samples that we are trying to look at — obviously not ideal when trying to make a diagnosis,” said Professor Professor Jim Piper, leader of the research team at Macquarie University and the ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP).

Another problem, said Professor Dayong Jin from the University of Technology Sydney (UTS), is cost. “Currently, in order to switch each individual pixel on and off for super-resolution imaging, you need a bulky laser with lots of power,” Professor Jin said.

“The high-powered laser means you end up with very expensive equipment — typically over $1 million.”

The research overcame these issues by using luminescent nanocrystals with the chemical element thulium added at high concentration. This creates an excitation condition whereby the signals can be optically modulated via either spontaneous emission pathway or stimulated emission pathway, decreasing the requirement for a high-power laser by 2–3 orders of magnitude.

“Our nanoparticles are unique in that luminescence can be amplified and modulated with commonly available low-power semiconductor lasers,” said Professor Piper. This has enabled the researchers to achieve images with a super resolution of 28 nm — about 1/36 of a wavelength of light.

Professor Jin said super-resolution imaging opens a lot of opportunities to understand how the life machine works, hopefully leading to a better understanding of antibiotic-resistant pathogens and diseases, as well as the immune system.

“These allow researchers to see well beyond normal limits of standard microscopes,” Professor Piper added. “It will let you see deeper and more clearly at the cellular and intracellular level — where proteins, antibodies and enzymes ultimately run the machinery of life.”

Image caption: Here we see a nanoparticle giving off blue luminescence (top left), whereas the blue luminescence of other excited nanoparticles (eg, top right) is being switched off by the low-power infrared laser (shown in pink as a semi-sphere in the middle of the image). This allows us to see images at super resolution. Image courtesy of Xianlin Zheng (via CNBP) under CC BY 2.0