Infrared Microspectroscopy of Bionanomaterials (Diatoms) with Careful Evaluation of Void Effects

In order to characterize a representative natural bionanomaterial, present day centric diatom samples (diameter, 175–310 µm) have been analyzed and imaged by infrared (IR) micro-spectroscopy and scanning electron microscopy (SEM). Because diatom silica frustules have complex microscopic morphology, including many void areas such as micro- or nano-pores, the effects of voids on the spectral band shapes were first evaluated. With increasing void area percentage, 1220 cm⁻¹/1070 cm⁻¹ peak height ratio (Si–O polymerization index) increases and 950 cm⁻¹/800 cm⁻¹ peak height ratio (Si–OH/Si–O–Si) decreases, both approaching 1. Based on the void area percentage of representative diatom samples determined using SEM image analyses (51.5% to 20.5%) and spectral simulation, the 1220 cm⁻¹/1070 cm⁻¹ ratios of diatom samples are sometimes affected by the void effect, but the 950 cm⁻¹/800 cm⁻¹ ratios can indicate real structural information of silica. This void effect should be carefully evaluated for IR micro-spectroscopy of micro-nano-porous materials. Maturity of diatom specimens may be evaluated from: (1) void area percentages determined by SEM; (2) average thicknesses determined by optical microscope; and (3) average values of 1220 cm⁻¹/1070 cm⁻¹ peak height ratios (opposite trend to the void effect) determined by IR micro-spectroscopy. Microscopic heterogeneities of chemical structures of silica were obtained by IR micro-spectroscopic mapping of four representative diatoms. The 950 cm⁻¹/800 cm⁻¹ ratios show that large regions of some diatoms consist of hydrated amorphous immature silica. The successful analysis of diatoms by IR micro-spectroscopic data with careful void effect evaluation may be applied to physicochemical structures of many other bionanomaterials.