Imaging and Quantitative Analysis on the Etching of Diatom Frustules via Digital Holographic Microscopy

Frustules, whose length spans from a few micrometers to more than a hundred micrometers, have been the subject of various modifications to improve their physical properties because of their complex porous silica structure. However, three-dimensional measurements of these changes can be challenging because of the complex 3D architecture and limitations of known methods. In this study, we present a new method that applies digital holographic microscopy (DHM) to analyze controlled etched frustules and observe real-time degradation of frustules at the single-cell level. Frustules obtained from Craspedostauros sp. diatoms were etched in 1 N NaOH for 5 min at 25 and 60 °C, respectively, and the frustule's valve was analyzed using DHM. DHM uses a combination of holography and tomography to reconstruct a 3D refractive index image of the frustule. Measurements of the width, volume, and surface area are achieved. Results showed that at 60 °C of etching, a significant difference with the unetched frustule was observed for all measurements but with high fluctuation values. Finally, real-time observation of the degradation of the frustule is observed when immersed in a high concentration of NaOH. This is the first time the real-time etching of the frustule is observed at the single-cell level. This research provides an easy estimation of the 3D measurements of frustules that may provide new fundamental information and applications.