Transmission Electron Microscopy and Scanning Transmission X-Ray Microscopy Studies on the Bioaccumulation and Tissue Level Absorption of TiO2 Nanoparticles in Daphnia magna.

In this study, bioaccumulation and tissue-level absorption of TiO2 nanoparticles (NPs) in freshwater invertebrates were investigated using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). The TiO2 NPs were used to test impacts of core sizes (i.e., 5 ± 2 nm and 23 ± 7 nm for TiO2(SYN) and TiO2(P25), respectively) and agglomerations (i.e., well dispersed vs. highly agglomerated) on the uptake of TiO2 NPs in Daphnia magna (D. magna). Highly agglomerated TiO2 NPs, regardless of their core sizes, were heavily taken up into the digestive tract of D. magna and no detectable penetration of both TiO2 NPs into the gut epithelial cells of D. magna was observed in TEM and STXM images. However, significant damages involving morphological changes in the microvilli and gut epithelial cells (e.g., irregular shaped microvilli, epithelial cell protrusion, and dilatation of cytoplasmic inclusion) were observed only with the commercial TiO2 NPs (TiO2(P25)) with larger core size and mixed crystalline phase, while the laboratory synthesized TiO2 NPs (TiO2(Syn)) with smaller core size and single crystalline phase showed slight morphological changes in the gut microvilli and epithelial cells. In the case of D. magna exposed to the well dispersed synthetic TiO2 NP ((Cit)TiO2(Syn)), only a negligible amount of TiO2 NPs were found within the digestive tract of the D. magna without any significant damages in the gut microvilli and epithelial cells and any detectable penetrations of TiO2 NPs into epithelial cells of D. magna gut. These TEM and STXM observations confirmed us that uptake of NP into D. magna are strongly dependent on their agglomeration (i.e., hydrodynamic sizes), rather than their core sizes, while direct penetration of NPs into tissues of digestive tract seems unlikely without significant morphological changes (e.g., collapse of the epithelial tissue) caused by high toxicity of NPs or released metal ions.

Scanning transmission X-ray microscopy probe for in situ mechanism study of graphene-oxide-based resistive random access memory.

Here, an in situ probe for scanning transmission X-ray microscopy (STXM) has been developed and applied to the study of the bipolar resistive switching (BRS) mechanism in an Al/graphene oxide (GO)/Al resistive random access memory (RRAM) device. To perform in situ STXM studies at the C K- and O K-edges, both the RRAM junctions and the I0 junction were fabricated on a single Si3N4 membrane to obtain local XANES spectra at these absorption edges with more delicate I0 normalization. Using this probe combined with the synchrotron-based STXM technique, it was possible to observe unique chemical changes involved in the BRS process of the Al/GO/Al RRAM device. Reversible oxidation and reduction of GO induced by the externally applied bias voltages were observed at the O K-edge XANES feature located at 538.2 eV, which strongly supported the oxygen ion drift model that was recently proposed from ex situ transmission electron microscope studies.

Structural colour of unary and binary colloidal crystals probed by scanning transmission X-ray microscopy and optical microscopy.

Colloidal crystals composed of micro- or nano- colloids have been investigated in various fields such as photonics due to their unique optical properties. Binary colloidal crystals have an outstanding potential for fine-tuning material properties by changing the components, concentration, or size of colloids. Because of their tunable optical, electrical, magnetic, and mechanical properties, those materials attracted great attention. However, it has been hard to elucidate internal structures without fluorescent labelling or cross-sectioning. Here, we demonstrate the structural analysis of not only unary but also binary colloidal crystals using scanning transmission x-ray microscopy and compare the results with colloidal structures and optical properties observed by optical microscopy. Based on the comparison of images obtained by these two methods, the domains of colloidal crystals consisting of different structures and colours were directly identified without any additional sample preparation. Therefore, it was possible to investigate the structural colours of local domains of unary and binary colloidal crystals such as the face centred cubic (FCC) structure with different orientations, that is FCC (111) and FCC (001), and hexagonal close-packed structure, HCP (0001).

Nanoscale characterization of local structures and defects in photonic crystals using synchrotron-based transmission soft X-ray microscopy.

For the structural characterization of the polystyrene (PS)-based photonic crystals (PCs), fast and direct imaging capabilities of full field transmission X-ray microscopy (TXM) were demonstrated at soft X-ray energy. PS-based PCs were prepared on an O2-plasma treated Si3N4 window and their local structures and defects were investigated using this label-free TXM technique with an image acquisition speed of ~10 sec/frame and marginal radiation damage. Micro-domains of face-centered cubic (FCC (111)) and hexagonal close-packed (HCP (0001)) structures were dominantly found in PS-based PCs, while point and line defects, FCC (100), and 12-fold symmetry structures were also identified as minor components. Additionally, in situ observation capability for hydrated samples and 3D tomographic reconstruction of TXM images were also demonstrated. This soft X-ray full field TXM technique with faster image acquisition speed, in situ observation, and 3D tomography capability can be complementally used with the other X-ray microscopic techniques (i.e., scanning transmission X-ray microscopy, STXM) as well as conventional characterization methods (e.g., electron microscopic and optical/fluorescence microscopic techniques) for clearer structure identification of self-assembled PCs and better understanding of the relationship between their structures and resultant optical properties.

X-ray and electron microscopy studies on the biodistribution and biomodification of iron oxide nanoparticles in Daphnia magna.

Biodistribution and biomodification of iron oxide (Fe3O4 and α-Fe2O3) nanoparticles (NPs) in a well-known toxicity test organism, Daphnia magna (D. magna), were investigated using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). In addition to the morphological changes in the gut tissues of D. magna, biodistribution and biomodification of iron oxide NPs in the digestive tract of D. magna were also monitored in this study. Upon exposures to both iron oxide NPs, unique morphological changes (e.g., irregular shaped microvilli, epithelial cell protrusion, and dilatation of cytoplasmic inclusion) in the gut tissues of D. magna were observed along with bacterial colonization of the gut lumen. However, despite their heavy accumulations in the digesitive tract, TEM and STXM images confirmed us that both Fe3O4 and α-Fe2O3 NPs were not penetrating into the gut tissues of D. magna. Moreover, for the Fe3O4 NPs in direct contact with the gut microvilli of D. magna, slight but significant spectral changes were observed in their Fe L-edge X-ray absorption near edge structure (XANES) spectra, which indicated that there were biomodifications of Fe3O4 NPs, probably involving oxidative dissolution of Fe3O4 NPs followed by rapid precipitation of ferric oxide or hydroxide. However, no significant changes were observed in the Fe L-edge XANES spectra of the α-Fe2O3 NPs present in the gut lumen of D. magna. These X-ray and electron microscopic observations confirmed us that, despite similarities in core sizes and chemical compositions, NPs with different crystalline phase and dissolution rates can interact quite differently with their local environment, may result in different biodistribution and cause completely dissimilar toxicities.

Enhanced separation of colloidal particles in an AsPFF device with a tilted sidewall and vertical focusing channels (t-AsPFF-v).

Here, we report an enhanced separation of colloidal particles using a novel type AsPFF (asymmetric pinched flow fractionation) device with two additional features (t-AsPFF-v). Particularly, by adding a tilted sidewall and vertical focusing channels to the conventional AsPFF device, a significant enhancement in the separation resolution (R(m,n)) has been achieved, which was estimated to be 10.4 for 6 and 10 µm PS particles and found to be 11.6 times better than that of a conventional AsPFF.