The first biological superlenses

Spiders are helping nanotechnologists again

O. Alekseeva, Finger, volume 24, No. 1/2-2017

Back in 1873, the German optical physicist Ernst Abbe established that the resolution of an optical microscope (the "diffraction limit") is defined as d=l/(2n sina), where l is the wavelength of light, n is the refractive index of the medium, and a is the angular aperture of the lens. This famous formula is even carved on the monument of Abbe, installed near the University of Jena.

Until recently, it was impossible to study structures with a distance between elements less than 200 nm using an optical microscope. It was only thanks to the development of nanophotonics, plasmonics, and the development of metamaterials that the diffraction limit was overcome. Based on several different scientific approaches, so-called nanoscopes were created. (E. Betzig (USA), W. Merner (USA) and S. Hell (Germany) were even awarded the Nobel Prize in Chemistry in 2014 for the development of ultra-high-resolution fluorescence microscopy methods.) However, studies using such nanoscopes require narrow-band laser radiation.

It has recently been discovered that transparent microspheres and cylinders are capable of acting as superlens providing a resolution of 50-100 nm when using visible light. First, the authors of the theoretical work [1] showed that when light is scattered on a dielectric microcylinder, the focusing area is abnormally elongated, forming a "jet" of light with a very small size in the transverse direction. Photonic nanojet (photonic nanostream) – this term was introduced by the authors. Then, based on this effect, the first nanoscopes with superlens-microspheres were developed [2].

Now in many countries of the world, researchers are developing and improving this technique, offering various options for superlens (microspheres, microfibers). Here is one recent example. Scientists from China and the UK have developed a method for producing a three-dimensional dielectric metamaterial consisting of TiO ₂ (15 nm) anatase nanoparticles with a high refractive index (n=2.55) [3].

Solid immersion superlenses made of such material provide a resolution of ~45 nm in the entire visible light range. First, the researchers prepared an aqueous suspension of nanoparticles, obtained a precipitate, then the water above the precipitate was replaced with organic solvents (tetrachloroethylene + hexane) that do not mix with water. A drop of a new mixture ("nano-solid fluid" in the terminology of the authors) from a syringe was applied to the object under study (Fig.1, left). An organic solvent containing tetrachloroethylene (boiling point 121.1 ^o C) formed a protective outer layer that does not allow the water remaining in the pores between the TiO ₂ nanoparticles to evaporate. Thus, a new plastic "nano-solid fluid" material was obtained, from which 3D structures can be formed. After additional evaporation of solvents and water, a "solid-like" 3D structure with an even denser packing of nanoparticles was formed (Fig.1, in the center). Through the super lens, you can see the small details of the chip (Fig.1, right).

Fig. 1. The scheme of obtaining a hemispherical lens from a metamaterial. A drop of a mixture of anatase TiO ₂ nanoparticles, residual water and organic solvent is applied to the object under study. After evaporation of the solvent, a structure is formed with an even denser packing of nanoparticles – micro (semi)spherical lens. On the right – through the super lens, the details of the object under study are visible.

It is clear that the manufacturing process of such superlens is quite complicated. It would be desirable to find a suitable material in nature. The authors of [3] and other papers on microlenses from Bangor University (UK) attracted zoologists from the University of Oxford (UK) to their research, and as a result, for the first time demonstrated a biological superlens for visible light [4]. A cylindrical fragment of transparent spider silk was used, which was pulled from the small ampulloid gland of the Nephila edulis spider widespread in Australia, the nephila-goldworm (this gland produces a thread for moving the spider). Figure 2 shows a spider and a scheme for using a biological lens.

Fig. 2. a – The spider Nephila edulis and the web. b is the scheme of using biological superlens. A cylindrical fragment of spider silk (diameter 6.8 microns, refractive index 1.55) was placed on the surface of the sample (Blu-ray disc) and fixed with tape.

The researchers tested the quality of the lens on two types of samples – a microchip and an optical Blu-ray disc with even smaller elements of the surface structure. The optical microscope did not make it possible to distinguish grooves with a width of 100 nm on the disk due to the diffraction limit equal in this case to 333.3 nm (the wavelength of the light used is ~ 600 nm), but the transparent biological superlens provided high resolution. Figure 3 shows the SEM image of the disk surface and the image obtained using an optical nanoscope.

Fig. 3. SEM image of the disk surface (left). An image obtained in the reflected light mode using an optical nanoscope with a superlens made of spider silk.

Cylindrical superlens made of spider silk has one clear advantage over microspherical lenses – it can provide a large field of view in the direction of the fiber (up to several centimeters). Such durable and economical lenses can be widely used, especially in the study of biological systems.

Many people experience unpleasant emotions at the sight of spiders, and even fear. But gradually the attitude towards these living beings is changing. Beautiful large blue spiders have become popular pets. There are interesting sites dedicated to the world of spiders. In the Spanish city of Bilbao at the entrance to the Guggenheim Museum there is a huge sculpture of a black spider (see photo). It is said that this spider (more precisely, the spider) has healing energy. The author of the sculpture, the famous Louise Bourgeois (1911-2010), embodied the image of her mother in spiders (who, by the way, was engaged in weaving): "my mother was wise, patient, irreplaceable, neat and useful, like a spider."

Spiders are really useful, and not only because they destroy flies and mosquitoes. Finger told how studies of the nanostructure of the web led to the discovery (and explanation) of many interesting properties, for example, unusually high thermal conductivity, strength, elasticity [5]. The results help in the development of new biomimetic fibrous materials. The study of biophotonic nanostructures responsible for the bright blue coloration of bird-eating spiders is important for creating improved screens of phones, televisions and other devices, to reduce the likelihood of glare and dimming [6]. Now the spiders have helped to make superlenses.

New successes are ahead. Professor Fritz Vollrath from Oxford Silk Group, one of the authors [4], has been studying spiders for more than 40 years. They live in a special greenhouse on the roof of Oxford University. Volrat is currently working on the creation of spider silk-based implants (perhaps they will be available to doctors by 2018) [7]. "We can still learn a lot from spiders!", the professor is sure.

Literature:

1. Z.Chen et al., Opt. Express 12, 1214 (2004).
2. Z.Wang et al., Nature Commun. 2, 218 (2011).
3. W.Fan et al., Sci. Adv. 2, e1600901 (2016).
4. N.Monks et al., NanoLett. 16, 5842 (2016).
5. Finger 19, issue 17, p. 2 (2012).
6. Finger 23, issue 3/4, p. 4 (2016).
7. oxfordsilkgroup.com

Portal "Eternal youth" http://vechnayamolodost.ru  20.02.2017