Researchers at the University of St Andrews have produced the world’s first solid-state protein lasers, capable of record performance and some capable of self-assembly, by harnessing the optical engineering skills of bioluminescent jellyfish.
The findings, reported today (Monday 8 December 2014) in the international journal Nature Communications, have the potential to transform biomedical diagnosis of conditions such as cancer and advance the design of new materials.
The work was inspired by the discovery that nature may have optimized – with sub-nanometre precision – the size of the molecules driving the bioluminescence of jellyfish to allow them to shine as brightly as possible.
Professor Malte Gather from the University of St Andrews’ School of Physics and Astronomy, together with Dr Seok Hyun Yun at Harvard Medical School and Massachusetts General Hospital, calculated that the green fluorescent protein molecule – the molecule allowing certain jellyfish to emit bright green light – has just the right size to strike an optimal balance between not losing energy to unproductive quenching and being able to squeeze as many molecules as possible into the light-emitting cells of the animal.
Bioinspired by nature’s design, the researchers were able to make tiny solid-state lasers from these fluorescent proteins.
The green fluorescent protein, generally known as GFP, is found the pacific jellyfish Aequorea Victoria where it is involved as energy acceptor in the natural bioluminescence of the animal. Several years ago molecular biologists isolated the section of DNA that tells the cellular machinery of the jellyfish how to produce GFP. Using genetic engineering this DNA can be used to confer the bright green fluorescence to other species – to bacteria, fruit flies, even to mice – a method that is widely used today to visualize cells or structures within cells under the microscope. Such measurements require only relatively modest protein concentrations, usually in the micromolar range, which corresponds to about one million molecules per cell. In the light-emitting organ of the jellyfish, however, the protein concentration is believed to be more than thousand times higher.
“We wanted to know if natural evolution has optimized the structure of the protein molecules in jellyfish to allow such high concentrations,” said Professor Gather.
To do so, GFP was extracted from bacteria cultures and carefully purified. The brightness of these solutions was then measured as a function of GFP concentration and the results compared to data for an artificial fluorescent dye. Whilst the artificial dye stops to emit light if present at high concentration, the brightness of the protein solutions increased steadily with concentration, thus reaching higher absolute brightness levels than the artificial dye. Even a dry film of just the pure protein emitted bright green light.
To understand why GFP shines brighter than artificial materials, the researchers modelled how quickly energy is transferred between neighbouring protein molecules. Excessive energy transfer between molecules can reduce or completely quench the fluorescence of a material. While artificial dyes are composed of small molecules with a size of typically less than one nanometre, fluorescent proteins are somewhat larger barrel-shaped molecules, several nanometres in height and diameter.
Dr Yun said: “The research published today now shows that the dimensions of these molecules are just right to slow down energy transfer and thus supress quenching, whilst still allowing as high a number of protein molecules as possible to fit into any given volume.”
Intrigued by nature’s careful optical engineering, Gather and Yun contemplated whether GFP and other fluorescent proteins can be used in artificial optical devices. Professor Gather said:
“Lasers and optical sensors were particularly high our list as both require efficient emitters and because there is a growing demand to make them from biocompatible materials, in particular for use in biomedical applications.”
The world’s first solid-state protein lasers are the result of these efforts. The scientists developed a number of different laser configurations. A particularly efficient design began to emit laser light when the power provided to it was less than what can be achieved in lasers based on state-of-the-art synthetic dyes. Another design makes use of the concept of self-assembly and allows the structure of the laser to form by itself, simply by letting a drop of a GFP solution dry on a plain substrate.
Professor Gather believes that beyond using GFP and other fluorescent proteins, the study of their structure and their optical properties can bio-inspire improvements of artificial emitters. He said:
“We may learn how to further improve artificial emitter materials, such as colloidal quantum dots and organic semiconductors, by looking carefully at how the fluorescent protein molecule is designed.”
NOTES TO NEWS EDITORS
A full copy of the research paper is available from the Press Office. Contact 01334 462 108
Photos of the lasers are also available from the Press Office.
Professor Malte Gather is available for interview today (Monday 8 December) after 4pm and tomorrow (Tuesday 9 December) between 8am and 11am. Contact 01334 462 108 or email firstname.lastname@example.org to arrange.Research