Abstract
Diatoms are in focus as biological materials for a range of photonic applications. Many of these applications would require embedding a multitude of diatoms in a matrix (e.g. paint, crème or lacquer); however, most studies on the photonic and spectral properties of diatoms frustules (silica walls) have been carried out on single cells. In this study, for the first time, we test the spectral properties of layers of frustules of three diatom species (Coscinodiscus granii, Thalassiosira punctifera and Thalassiosira pseudonana), with special focus on transmission and reflectance in the UV range. The transmittance efficiency in the UV A and B range was: T. pseudonana (56–59%) >C. granii (53–54%) >T. punctifera (18–21%) for the rinsed frustules. To investigate the underlying cause of these differences, we performed X-ray scattering analysis, measurement of layer thickness and microscopic determination of frustule nanostructures. We further tested dried intact cells in the same experimental setup. Based on these data we discuss the relative importance of crystal structure properties, nanostructure and quantity of material on the spectral properties of diatom layers. Characterization of the UV protection performance of layers of diatom frustules is of central relevance for their potential use as innovative bio-based UV filters.
Highlights
The pursuit of new materials for protecting against UV irradiance is ongoing due to the high demand of UV protection accompanying the continuous increase of UV exposure on the earth[1] and the limitations of conventional UV blockers[2,3], which are synthesized compounds, including organic UV absorbers containing phenolic groups with intramolecular hydrogen bonds[4] and inorganic metal oxide nanoparticles such as TiO2, ZnO and CeO2 with UV light scattering and absorption properties[5]
Spectra from 280 to 700 nm of layers of rinsed frustule of each of the three tested diatom species are presented in Fig. 1, which shows the different patterns in transmittance, reflectance and absorption
Transmission efficiency increased with increasing wavelength, steeper for T. pseudonana and T. punctifera than C. granii (Fig. 1)
Summary
The pursuit of new materials for protecting against UV irradiance is ongoing due to the high demand of UV protection accompanying the continuous increase of UV exposure on the earth[1] and the limitations of conventional UV blockers[2,3], which are synthesized compounds, including organic UV absorbers containing phenolic groups with intramolecular hydrogen bonds[4] and inorganic metal oxide nanoparticles such as TiO2, ZnO and CeO2 with UV light scattering and absorption properties[5]. Three centric diatom species, Coscinodiscus granii, Thalassiosira pseudonana and Thalassiosira punctifera, were selected due to their range of different morphological characteristics as reported in literature[17] These characteristics, in particular the size and periodicity of the larger pores (foramen) in the frustule, have been hypothesized to be central for the photonic characteristics of the frustule[11] and we wanted to see if these differences in morphology were central to optical properties of a random layer. We tested the performance of both intact cells with cell content and separated valves, rinsed of organic material, and discuss the respective properties of these These results offer new information to expand the properties of single frustule valves to possible applications requiring layers of frustules (e.g. in a matrix), with particular emphasis on potential for UV protection
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