Back in 1873, the German physicist Ernst Abbe discovered a fundamental limit in the performance of imaging systems such as microscopes or camera lenses. These systems simply cannot resolve features smaller than a critical size determined by the wavelength of light.
For visible light, this resolution limit is about 200 nanometers; anything smaller cannot be resolved. That includes viruses, features inside cells such as microtubules and DNA molecules, even the grooves on a standard Blu-ray DVD disc.
But in recent years, physicists have discovered a way around Abbe’s limit. Whenever light bounces off an object, it diffracts and interferes, causing any fine details to be lost. For visible light, this process takes place in the first few nanometers from the surface.
The way round Abbe’s limit is to record the pattern of reflected light before it interferes. This so-called near-field, or evanescent, light contains all the fine detail. The trick is to find a way to transmit this near field light beyond its usual range.
And that’s exactly what physicists have done. They’ve discovered various exotic substances that can transmit near-field light. Place one of these in contact with the surface to be imaged and it can convey the light to a conventional imaging system. Such a material is known as a superlens.
These superlenses are obviously small but also often delicate and tricky to make. What’s more, they tend to work only at specific frequencies of light. So finding new robust ones that work with white light is an important task.
Today, James Monks and pals from Bangor University in Wales show that spider silk is capable of resolving details in white light smaller than Abbe’s resolution limit. Their work is the first demonstration of a biological superlens.
The technique is straightforward. The team begins with silk spun by the Nephila edulis, a large spider better known as the Australian Golden Orb Weaver. This produces silk about 6,800 nanometers in diameter which it weaves into a web about one meter across.
Silk is transparent and cylindrical in structure, a shape that allows it to focus light. And because it is tiny, it does the focusing on the nanometer scale which matches that of near-field light.
Monks and co simply lay a strand of this spider silk across a Blu-ray DVD disc, illuminate it with white light, and photograph it through a standard 100x microscope objective.
The surface of this disc is made up of channels that are 200 nanometers wide, each separated by a distance of 100 nanometers.
That’s smaller than an optical microscope can ordinarily resolve using white light. So any detail showing these channels is clear evidence that the spider silk is acting as a superlens.
The images show exactly these details. “This provides evidence of the super-resolution capability of spider silk to overcome the optical diffraction limit,” say Monks and co. “This is the first biological superlens system that has successfully overcome the diffraction limit.”
That’s interesting work, not least because spider silk is easy to come by, wonderfully flexible, and hugely robust. That means it could be used in a wide range of situations.
Monks and co suggest running the silk back and forth to create a two-dimensional array, which could be encapsulated in a transparent medium such as a tape of some kind. This could then be placed on any sample that needs to be resolved.
Clearly biological superlenses have significant potential for the future.
Ref: arxiv.org/abs/1604.08119: Spider Silk: The Mother Nature’s Biological Superlens