Light-Induced In Situ Transmission Electron Microscopy for Observation of the Liquid-Soft Matter Interaction

Summary

The present protocol describes transmission electron microscope (TEM) modifications with a light illumination system, the fabrication of liquid cells, and in situ TEM observations of light-induced interactions between bacterial cells and a photosensitizer. The sample preparation methods, electron beam damage, and imaging are also discussed.

Abstract

The current protocol describes the modifications of the transmission electron microscope (TEM) setup for in situ light-induced observations. A glass optical fiber inserted into the electron column above the objective lens polepiece, and a laser, an adjustable light source, was used to fabricate the device. After the illuminator has been calibrated using an external measuring system, it allows one to adjust the intensity of the lighting to the needs of the observed process. This lighting system was utilized to image antimicrobial photodynamic therapy phenomena, which are currently the subject of intense research. The sample was prepared by spotting a suspension of bacteria on a carbon, graphene, or silicon nitride substrate, blotting the excess solution, spotting the photosensitizer solution, blotting the excess liquid again, and then assembling the liquid cell with a second substrate or graphene film. The process of the imaging experiment itself includes choosing the right place for observation with the use of low magnification and a minimum dose of electrons, and then cyclical activation of the light source to capture subsequent images at specified intervals with the minimum amount of electrons necessary. The electron dose of each exposure and the time and intensity of lighting used need to be carefully recorded due to the complexity of the observed phenomena as, at the same time, the process is both light- and electron-driven. After the actual experiment is performed, additional control observations must be made, in which the same doses of electrons are used but without additional light influence and smaller doses of electrons are used for higher doses of light. This makes it possible to distinguish light-induced microstructural effects from those caused by electrons in both the fields of life and materials science.

Introduction

Light-induced phenomena in high resolution are interesting in many fields such as nanoengineering^1,2,3, catalysis^4,5, and biophotonics⁶. Some original designs that allow such experiments can be found in the literature,including modifications of the sample holders^1,4,7,8,9 and the optical fiber attached to the microscope

Protocol

1. Transmission electron microscope modification

1. Modify the transmission electron microscope by connecting the optical fiber (see Table of Materials) to the microscope column at the top of the objective lens. Allow a few centimeters of space between the objective lens and the condenser lens, plus a free slot for accessories.
   NOTE: A dedicated sample holder⁴ or a holder for cathodoluminescent observations in reverse configuration

Discussion

The installation and commissioning of the illuminator require basic service knowledge and can damage the microscope. The simplest way to introduce the light to the microscope is to connect the optical fiber from the top of the objective lens, where there is usually room for the TEM deflection coils and additional detectors. More free space can also be expected from older top-entry devices, where the same location houses the sample vacuum lock and the sample installation mechanism. This configuration is widely described i.......

Materials

Name	Company	Catalog Number	Comments
Carbon film on 200 mesh copper grid	Agar Scientific	AGS160	The standard TEM grids for observations and liquid cell preparation
Crossover Tweezers	Dumont	N5	The tweezers are neecesarry for liquid cell preparation
Photodiode Power Sensor	ThorLabs	S130C	The sensor used for light intensity measurement
Polyimide-Coated Multimode Fiber	Thorlabs	FG400UEP	Must be built into the microscope using the on-site built adapter, according to the 10.1016/j.ultramic.2021.113388
Transmission Electron Microscope	Hitachi	H-800	Can be replaced with any side-entry microscope, available for modification
Tuneable Diode Laser	CNI	MRL-III-660D	The light wavelength must be chosen basing on photosensitizer's absorption spectrum