Application of low-energy scanning transmission electron microscopy for the study of Pt-nanoparticle uptake in human colon carcinoma cells

Platinum Nanoparticles in Biomedicine: Preparation, Anti-Cancer Activity, and Drug Delivery Vehicles

Cancer is the main cause of morbidity and mortality worldwide, excluding infectious disease. Because of their lack of specificity in chemotherapy agents are used for cancer treatment, these agents have severe systemic side effects, and gradually lose their therapeutic effects because most cancers become multidrug resistant. Platinum nanoparticles (PtNPs) are relatively new agents that are being tested in cancer therapy. This review covers the various methods for the preparation and physicochemical characterization of PtNPs. PtNPs have been shown to possess some intrinsic anticancer activity, probably due to their antioxidant action, which slows tumor growth. Targeting ligands can be attached to functionalized metal PtNPs to improve their tumor targeting ability. PtNPs-based therapeutic systems can enable the controlled release of drugs, to improve the efficiency and reduce the side effects of cancer therapy. Pt-based materials play a key role in clinical research. Thus, the diagnostic and medical industries are exploring the possibility of using PtNPs as a next-generation anticancer therapeutic agent. Although, biologically prepared nanomaterials exhibit high efficacy with low concentrations, several factors still need to be considered for clinical use of PtNPs such as the source of raw materials, stability, solubility, the method of production, biodistribution, accumulation, controlled release, cell-specific targeting, and toxicological issues to human beings. The development of PtNPs as an anticancer agent is one of the most valuable approaches for cancer treatment. The future of PtNPs in biomedical applications holds great promise, especially in the area of disease diagnosis, early detection, cellular and deep tissue imaging, drug/gene delivery, as well as multifunctional therapeutics.

Differential electron scattering cross-sections at low electron energies: The influence of screening parameter

For quantitative electron microscopy the comparison of measured and simulated data is essential. Monte Carlo (MC) simulations are well established to calculate the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) intensities on a non-atomic scale. In this work we focus on the importance of the screening parameter in differential screened Rutherford cross-sections for MC simulations and on the contribution of the screening parameter to the atomic-number dependence of the HAADF-STEM intensity at electron energies ≤ 30 keV. Materials investigated were chosen to cover a wide range of atomic numbers Z to study the Z dependence of the screening parameter. Comparison of measured and simulated HAADF-STEM intensities with different screening parameters known from the literature were tested and failed to generally describe the experimental data. Hence, the screening parameter was adapted to obtain the best match between experimental and MC-simulated HAADF-STEM intensities. The Z dependence of the HAADF-STEM intensity was derived.

Quantitative STEM imaging of electron beam induced mass loss of epoxy resin sections

In sample preparation of biological samples for electron microscopy, many types of embedding media are widely used. Unfortunately, none of them is perfectly resistant to beam induced damage. The article is focused on mass loss measuring of pure epoxy resin EMbed 812 that replaced Epon - the most widely used embedding resin for biological electron microscopy, in a form of ultrathin sections with thicknesses ranging from 30 to 100 nm. The STEM imaging was performed in a quantitative way which allowed us to estimate the mass loss directly up to the total dose of 3000 e^-/nm². For data acquisition we used SEM equipped with a commercial STEM detector working at a relatively low acceleration voltage of 30 kV. In this study we estimated the influence of various factors which can affect the endurance of the epoxy resin EMbed 812 ultrathin sections under an electron beam, such as the sample aging, differences between storing the samples in forms of ultrathin sections and whole blocks, ultrathin sections thicknesses, temperature of the sample, probe current, and one or two-sided carbon coating of ultrathin sections. The aim of this work is to investigate beam induced mass loss at electron energies of SEM and find out how to reduce the mass loss.

Extracellular Vesicles as Vehicles for the Delivery of Food Bioactives

The nutritional value of food can be improved by the addition of bioactive compounds. However, most of these favorable food additives demonstrate low bioavailability because of their limited stability, solubility, and structural transformations upon digestion and absorption. One strategy to combat these limitations is to integrate bioactives into nanoparticles, although the mostly used artificial materials may result in immune system activation and fast clearing times. Therefore, novel, more biocompatible delivery systems are required. Extracellular vesicles are communication tools designed by evolution to transfer information between cells, organs, and whole organisms. Hence, these vesicles offer enormous potential for targeted bioactive compound delivery.

Assessment of in vitro particle dosimetry models at the single cell and particle level by scanning electron microscopy

Background

Particokinetic models are important to predict the effective cellular dose, which is key to understanding the interactions of particles with biological systems. For the reliable establishment of dose–response curves in, e.g., the field of pharmacology and toxicology, mostly the In vitro Sedimentation, Diffusion and Dosimetry (ISDD) and Distorted Grid (DG) models have been employed. Here, we used high resolution scanning electron microscopy to quantify deposited numbers of particles on cellular and intercellular surfaces and compare experimental findings with results predicted by the ISDD and DG models.

Results

Exposure of human lung epithelial A549 cells to various concentrations of differently sized silica particles (100, 200 and 500 nm) revealed a remarkably higher dose deposited on intercellular regions compared to cellular surfaces. The ISDD and DG models correctly predicted the areal densities of particles in the intercellular space when a high adsorption (“stickiness”) to the surface was emulated. In contrast, the lower dose on cells was accurately inferred by the DG model in the case of “non-sticky” boundary conditions. Finally, the presence of cells seemed to enhance particle deposition, as aerial densities on cell-free substrates were clearly reduced.

Conclusions

Our results further validate the use of particokinetic models but also demonstrate their limitations, specifically, with respect to the spatial distribution of particles on heterogeneous surfaces. Consideration of surface properties with respect to adhesion and desorption should advance modelling approaches to ultimately predict the cellular dose with higher precision.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0426-2) contains supplementary material, which is available to authorized users.

On the Progress of Scanning Transmission Electron Microscopy (STEM) Imaging in a Scanning Electron Microscope

Transmission electron microscopy (TEM) with low-energy electrons has been recognized as an important addition to the family of electron microscopies as it may avoid knock-on damage and increase the contrast of weakly scattering objects. Scanning electron microscopes (SEMs) are well suited for low-energy electron microscopy with maximum electron energies of 30 keV, but they are mainly used for topography imaging of bulk samples. Implementation of a scanning transmission electron microscopy (STEM) detector and a charge-coupled-device camera for the acquisition of on-axis transmission electron diffraction (TED) patterns, in combination with recent resolution improvements, make SEMs highly interesting for structure analysis of some electron-transparent specimens which are traditionally investigated by TEM. A new aspect is correlative SEM, STEM, and TED imaging from the same specimen region in a SEM which leads to a wealth of information. Simultaneous image acquisition gives information on surface topography, inner structure including crystal defects and qualitative material contrast. Lattice-fringe resolution is obtained in bright-field STEM imaging. The benefits of correlative SEM/STEM/TED imaging in a SEM are exemplified by structure analyses from representative sample classes such as nanoparticulates and bulk materials.

Dark-field image contrast in transmission scanning electron microscopy: Effects of substrate thickness and detector collection angle

An annular dark field (ADF) detector was placed beneath a specimen in a field emission scanning electron microscope operated at 30kV to calibrate detector response to incident beam current, and to create transmission images of gold nanoparticles on silicon nitride (SiN) substrates of various thicknesses. Based on the linear response of the ADF detector diodes to beam current, we developed a method that allowed for direct determination of the percentage of that beam current forward scattered to the ADF detector from the sample, i.e. the transmitted electron (TE) yield. Collection angles for the ADF detector region were defined using a masking aperture above the detector and were systematically varied by changing the sample to detector distance. We found the contrast of the nanoparticles, relative to the SiN substrate, decreased monotonically with decreasing inner exclusion angle and increasing substrate thickness. We also performed Monte Carlo electron scattering simulations, which showed quantitative agreement with experimental contrast associated with the nanoparticles. Together, the experiments and Monte Carlo simulations revealed that the decrease in contrast with decreasing inner exclusion angle was due to a rapid increase in the TE yield of the low atomic number substrate. Nanoparticles imaged at low inner exclusion angles (<150mrad) and on thick substrates (>50nm) showed low image contrast in their centers surrounded by a bright high-contrast halo on their edges. This complex image contrast was predicted by Monte Carlo simulations, which we interpreted in terms of mixing of the nominally bright field (BF) and ADF electron signals. Our systematic investigation of inner exclusion angle and substrate thickness effects on ADF t-SEM imaging provides fundamental understanding of the contrast mechanisms for image formation, which in turn suggest practical limitations and optimal imaging conditions for different substrate thicknesses.

Clift MJ, Blank F, Petri-Fink A, Rothen-Rutishauser B. Quantification of nanoparticles at the single-cell level: an overview about state-of-the-art techniques and their limitations

With the increasing production and use of engineered nanoparticles it is crucial that their interaction with biological systems is understood. Due to the small size of nanoparticles, their identification and localization within single cells is extremely challenging. Therefore, various cutting-edge techniques are required to detect and to quantify metals, metal oxides, magnetic, fluorescent, as well as electron-dense nanoparticles. Several techniques will be discussed in detail, such as inductively coupled plasma atomic emission spectroscopy, flow cytometry, laser scanning microscopy combined with digital image restoration, as well as quantitative analysis by means of stereology on transmission electron microscopy images. An overview will be given regarding the advantages of those visualization/quantification systems, including a thorough discussion about limitations and pitfalls.