Liquid Cell Transmission Electron Microscopy (LC-TEM) is an imaging technique for observing materials and biological processes in their liquid environments at high resolution. It provides a direct, real-time view of nanoscale dynamics. The primary purpose is to overcome the limitations of traditional electron microscopy, which requires samples to be solid and dry. By keeping the sample in a hydrated state, LC-TEM allows researchers to visualize phenomena that were previously impossible to study directly, such as processes unfolding at the atomic and molecular levels.
Overcoming the Vacuum Barrier
A standard transmission electron microscope operates under a high vacuum. This environment is necessary for the electron beam to travel from its source to the detector without being scattered by air molecules, which would prevent the formation of a clear image.
Introducing a liquid into this high-vacuum chamber would cause it to evaporate instantly. This makes observing a specimen in its native, hydrated state impossible, as biological samples would be destroyed and chemical reactions in solution could not be studied.
This limitation historically restricted electron microscopy to fixed, dehydrated, and thinly sliced samples. While techniques like cryo-electron microscopy preserve hydration by flash-freezing, they only provide static snapshots. The need to visualize dynamic processes in a liquid, such as crystal growth or nanoparticle interactions, drove the development of a method to protect the sample. This led to the creation of a specialized sample holder that isolates the liquid from the vacuum.
The Liquid Cell Device
The solution is the liquid cell, a micro-fabricated sample holder that encapsulates a liquid sample inside the microscope. The cell is constructed from two small silicon chips placed on top of each other, sandwiching a small volume of liquid between them.
Each chip has a central window made from a thin membrane of silicon nitride. These windows are strong enough to withstand the pressure difference between the liquid and the vacuum, yet thin enough (around 50 nanometers) to be transparent to the electron beam. This transparency allows electrons to pass through the windows and liquid layer to form an image.
The distance between the chips defines the liquid layer’s thickness and is controlled by an integrated spacer. The assembly is hermetically sealed to prevent leakage. Advanced cells may include integrated electrodes for electrochemical experiments or heating elements to control the sample’s temperature, expanding the range of study.
Applications in Dynamic Processes
Recording real-time videos of nanoscale events provides insights across scientific disciplines. In materials science, LC-TEM is used to observe the synthesis of nanoparticles, tracking their formation, growth, and self-assembly in solution. This allows for the study of crystal growth mechanisms and dissolution pathways.
In energy storage, LC-TEM offers a look at the inner workings of batteries by visualizing electrochemical processes during charge and discharge cycles. For instance, the formation of lithium dendrites, which are needle-like structures that can cause battery failure, has been observed on electrode surfaces. This information aids in designing safer and more efficient batteries.
The technique is also transforming the understanding of biological systems. Researchers can watch protein assembly or observe interactions between viruses and antibodies in an environment that mimics physiological conditions. Other applications include monitoring drug delivery systems and studying the initial stages of biomineralization.
Interpreting Liquid-Phase Images
Analyzing LC-TEM data requires considering interactions between the electron beam, the liquid, and the sample. The high-energy electrons can induce unintended chemical reactions. This phenomenon, known as radiolysis, occurs when electrons split water molecules into reactive species, which can form hydrogen bubbles or alter the sample’s structure.
Distinguishing between authentic phenomena and beam-induced artifacts is a primary challenge. Researchers must perform control experiments to understand how the electron beam affects their system. This involves varying the electron dose, which is the number of electrons hitting the sample, to find the lowest flux that still yields a usable image.
By comparing results from different imaging conditions, scientists can confirm that observed dynamics are representative of the sample’s true behavior. For example, if a nanoparticle dissolves, researchers must confirm this is due to the chemical environment and not the electron beam. This validation is necessary to ensure conclusions accurately reflect the nanoscale processes.