A wide range of fields have shown considerable interest in imaging wet specimens using electron microscopy. Understanding the mechanisms controlling dynamic processes like nucleation, growth, and self-assembly is helpful for customizing nanomaterial properties to suit specific applications. In order to understand these mechanisms, imaging nanoparticles in liquid using electron microscopy is an essential step.
In this study, Protochips’ Poseidon liquid TEM holder was used to image different sizes of citrate stabilized gold nanoparticles in liquid. In the electron microscope, a self-contained, maintainable liquid environment is provided by Poseidon to enable liquid imaging.
A 0.5 µL droplet of solution consisting of 1.4 nm and 10 nm gold nanoparticles was dispensed on the silicon nitride surface to deposit the gold nanoparticles on an E-chip™. After the solvent was left to evaporate, scanning TEM was used to confirm the presence of gold nanoparticles on the silicon nitride window.
The liquid imaging was performed by placing the E-chip in the Poseidon holder and dispensing a 0.8 µL droplet of 10% phosphate buffered saline onto the E-chip.
On top of the first E-chip, a second E-chip consisting of a spacer was placed and the chamber was closed completely to create a vacuum tight seal. Buffer solution was continuously applied to the sample at a flow rate of 50 nL/s.
An FEI CM200FEG TEM operating at 200 kV was used to image the aqueous nanoparticles. The thickness of the liquid was found to be ~ 0.8 µm.
Figures 1 and 2 show the TEM bright field images of the gold nanoparticles. Against the bright liquid background, the gold nanoparticles appear dark, as can be seen at lower magnification (Figure 1).
The nanoparticles show both diffraction contrast and amplitude (mass-thickness).
Figure 1. TEM bright field image of the gold nanoparticles appear dark against the bright background of the liquid.
Diffraction contrast is noticeable in the appearance of intensity variations, especially in large crystalline particles. These variations are not in agreement with the smooth, symmetric variation that would be obtained from an incoherent signal.
Figures 2 and 3 show these variations in the gold particles as a bright field image and the corresponding inverted intensity profile, respectively. The resolution obtained was found to be ~ 5 nm.
Figure 2. Bright field image showing intensity variations.
Figure 3. Inverted intensity profile of the intensity variations
The Poseidon TEM holder is offered in 2 or 3 port configurations. Using the 2 port configuration, a constant liquid flow can be achieved across the sample, while reagents can be introduced and mixed during imaging using the 3 port configuration.
Various dynamic processes, including nucleation, growth, particle-particle interactions and self-assembly, can be studied using the Poseidon TEM holder.
This information has been sourced, reviewed and adapted from materials provided by Protochips.
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