Light interaction with nano-structured diatom frustule, from UV-A to NIR
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Light interaction with nano-structured diatom frustule, from UV-A to NIR Christian Maibohm1, Josefine H. Nielsen1 and Karsten Rottwitt1 1 Fotonik, Technical University of Denmark, Ørsteds plads, 2800 Kgs. Lyngby, Denmark ABSTRACT Diatoms are found in nearly every aqueous environment and play a vital part of the global primary production system contributing with up to 25 % and are efficient light harvesting organisms. Unique to diatoms are the hard cell wall, called the frustule surrounding the single cell. The frustule is made from bio-synthesized silicate, perforated by wavelength sized features where the morphology of the nano-structured “greenhouse” is species dependent. Diatoms would therefore make for one of the most interesting “green” resources since it has not only potential as a biomass production system but also for nano-structured inorganic material. To understand the biological significance and to integrate diatomic frustules as active material in devices a fundamental understanding of how light interacts with the frustule is needed. In this study we focus on centric diatoms, i.e. having rotational symmetry where morphological parameters vary between the different investigated species. We report how light interacts with the frustule in the wavelength range from UV-A (320-380 nm) to NIR (900 nm). High resolution spectroscopy and CCD images are used to identify photoluminescence (PL) and variations in the transmitted light caused by the nano-structured frustule. Furthermore we show, by placing the frustule on a quartz half sphere how light transmission is a function of the angle of incidence and wavelength. INTRODUCTION Diatoms are a class of unicellular photosynthetic microorganisms and are among the most common phytoplankton in aquatic environments. They have a colossal impact on the primary production of the world contributing with an amount equaling that of the rainforests [1, 2]. Diatoms are unique compared to other phytoplankton because of the nano-structured silica shell surrounding the protoplasm, called the frustule. Even though well investigated, the full functionality of the frustule is not yet understood but it provides the diatom with several properties; protection from mechanical and biotic stress, efficient nutrition uptake and light harvesting properties [3-9]. Optical investigation of the frustule carried out so far has mostly been with the organic material removed and with illumination at normal or close to normal incidence [6-12]. However marine diatoms are not motile in the ocean water column and are therefore also illuminated from angles very different from normal incidence depending on its relative orientation to the sun. In this work the frustule is illuminated at angles from normal up to 60˚, we show how the light intensity and spectra changes as a function of angle. In addition we show how the nano-structured frustule is able to scatter light towards the cell interior even for large angles. Furthermore we show that intense UV illumination at 325 nm induces PL in some frustules whil
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