Silica Nanospheres in a Photonic Display
Scientists from University of California at Berkeley have recently discovered that a strange naturally occurring bright display of the ‘disco’ or ‘electric’ clam Ctenoides ales is actually a photonic display created by a layer of silica nanospheres. The display functions solely by reflecting light.
Their article was published in the Journal of The Royal Society titled “A Dynamic Broadband Reflector Built from Microscopic Silica Spheres in the ‘Disco’ Clam Ctenoides Ales,” where the researchers shared their findings.(( Dougherty Lindsey F., Johnsen Sönke, Caldwell Roy L. and Marshall N. Justin, A dynamic broadband reflector built from microscopic silica spheres in the ‘disco’ clam Ctenoides ales, J. R. Soc. Interface.1120140407))
The ‘disco’ or ‘electric’ clam Ctenoides ales (Limidae) is the only species of bivalve known to have a behaviourally mediated photic display. This display is so vivid that it has been repeatedly confused for bioluminescence, but it is actually the result of scattered light. The flashing occurs on the mantle lip, where electron microscopy revealed two distinct tissue sides: one highly scattering side that contains dense aggregations of spheres composed of silica, and one highly absorbing side that does not.(( Dougherty Lindsey F., Johnsen Sönke, Caldwell Roy L. and Marshall N. Justin, A dynamic broadband reflector built from microscopic silica spheres in the ‘disco’ clam Ctenoides ales, J. R. Soc. Interface.1120140407))
Laboratory elemental analysis of the reflective nanospheres showed that the particles are indeed composed of amorphous silica, including both the outer shells and the inner cores. It appears that silica nanospheres are secreted by the animal and used as a light scattering structure in a behavior modulated reflective photonic display.
The measurements show that the diameter of the silica nanospheres is at around 300 nanometers, an optimal particle size for scattering visible light, especially the shorter blue-green wavelengths of 400-500nm that predominate at 3-50 meters underwater, which is typically the clam’s habitat. In addition to the diameter, the highly organized packing structure of the nanospheres aids in the scattering of the visible light at the shorter wavelengths.
The display is so bright that it has been mistakenly thought of as bioluminescent, but the findings show that it is actually based on light scattered by photonic nanostructures and not the light produced by a chemical reaction within a living organism.
This study is interesting to scientists in many different fields because it opens their minds up to many creative uses of silica nanospheres that have not been known before.
The findings show a practical way to manipulate light in low light situations.
About Silica Nanospheres:
Highly precise and monodisperse silica nanospheres with narrow size distribution, diameters ranging from 166nanometers to 9.2micron and sphericity of greater than 99% are commercially available.
Silica microspheres and nanospheres are composed of synthetic amorphous silica made in a laboratory batch environment. These nanoparticles are inherently hydrophillic and negatively charged.
Additional advantage of silica nanospheres is glass transition temperature of over 1000 degrees Celsius, which makes them suitable for high temperature applications. These particles provide higher opacity than glass microspheres and are often used as contrast particles. Similar to glass, silica is a very durable ceramic material that can withstand solvents and extreme high pressure high temperature environments.
Precision spherical micro and nano spherical particles are chemically-stable, inert, safe materials, frequently used as fillers and spacers for biotechnology, sintering, microfluidics, electronics, optical coatings, medical devices and other high tech applications.
Silica has a high index of refraction at n=1.43 at 589nm.
Mechanism for Light Scattering of Silica Nanospheres
In an atricle published in Nature Communications, Yuang Hsing Fu and co-authors investigated directional visible light scattering by silicon nanoparticles.((Fu, Y., Kuznetsov, A., Miroshnichenko, A. et al. Directional visible light scattering by silicon nanoparticles. Nat Commun 4, 1527 (2013). https://doi.org/10.1038/ncomms2538)) In their work they experimentally demonstrated that silicon nanoparticles of spherical shape exhibit strongly anisotropic scattering in the visible spectral range.
This unique scattering behaviour results from interference of electric and magnetic dipoles, excited by external illumination. As a result in a broad spectral range these nanoparticles can scatter the most of energy in the forward direction acting like ‘Huygens’ sources. For a different spectral range the most of the energy is scattered in the backward direction.
Control and fine tuning of these spectral regions is possible because of almost linear dependence of the scattering resonances on the nanoparticle size. It is also shown that by squeezing the nanoparticles in the direction of light propagation, it is possible to further overlap electric and magnetic dipole resonances. This results in a noticeable increase of their forward scattering.((Fu, Y., Kuznetsov, A., Miroshnichenko, A. et al. Directional visible light scattering by silicon nanoparticles. Nat Commun 4, 1527 (2013). https://doi.org/10.1038/ncomms2538))
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