Abstract
In order to understand how nanoparticles (NPs <100 nm) interact with cellular systems, potentially causing adverse effects, it is important to be able to detect and localize them within cells. Due to the small size of NPs, transmission electron microscopy (TEM) is an appropriate technique to use for visualizing NPs inside cells, since light microscopy fails to resolve them at a single particle level. However, the presence of other cellular and non-cellular nano-sized structures in TEM cell samples, which may resemble NPs in size, morphology and electron density, can obstruct the precise intracellular identification of NPs. Therefore, elemental analysis is recommended to confirm the presence of NPs inside the cell. The present study highlights the necessity to perform elemental analysis, specifically energy filtering TEM, to confirm intracellular NP localization using the example of quantum dots (QDs). Recently, QDs have gained increased attention due to their fluorescent characteristics, and possible applications for biomedical imaging have been suggested. Nevertheless, potential adverse effects cannot be excluded and some studies point to a correlation between intracellular particle localization and toxic effects.J774.A1 murine macrophage-like cells were exposed to NH2 polyethylene (PEG) QDs and elemental co-localization analysis of two elements present in the QDs (sulfur and cadmium) was performed on putative intracellular QDs with electron spectroscopic imaging (ESI). Both elements were shown on a single particle level and QDs were confirmed to be located inside intracellular vesicles. Nevertheless, ESI analysis showed that not all nano-sized structures, initially identified as QDs, were confirmed. This observation emphasizes the necessity to perform elemental analysis when investigating intracellular NP localization using TEM.
Highlights
The tremendous application potential of nano-sized particles (NPs 1-100 nm; ISO/TS 27687:2008) is in sharp contrast to a growing number of critical reports regarding their potential toxicity
This is predominantly due to quantum dots (QD) consisting of a heavy-metal core material, such as cadmiumtelluride (CdTe) or cadmium-selenide (CdSe), which is covered by a zinc sulfide (ZnS) shell
The QDs used in this study were coated with NH2 polyethylene glycol (PEG) and have previously been shown to cause no cytotoxicity [6] or pro-inflammatory cytokine stimulation in J774.A1 cells after 2 h [7]
Summary
The tremendous application potential of nano-sized particles (NPs 1-100 nm; ISO/TS 27687:2008) is in sharp contrast to a growing number of critical reports regarding their potential toxicity. Transmission electron microscopy (TEM) offers adequate resolution to visualize NPs at a single particle level as well as the ability to determine their localization in different cellular compartments. To confirm the presence of NPs and their localization inside cells, additional elemental analysis of the NP compositions is often required [1].
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