Electron Microscopy Comprehensive Notes

Electron Microscopy Notes

Introduction to Electron Microscopy

  • Electron Microscopy (EM): A technique that uses electrons to illuminate samples, providing higher resolution than light microscopy due to shorter wavelength of electrons.
Types of Electron Microscopes
  • Transmission Electron Microscope (TEM): Used for observing internal structures of samples.
  • Scanning Electron Microscope (SEM): Analyzes surface structures by detecting secondary electrons emitted from the surface.
  • Reflection Electron Microscope (REM): Uses reflected electrons to obtain information about surface topography.
  • Scanning Transmission Electron Microscope (STEM): Combines features of both SEM and TEM.
  • Low Voltage Electron Microscope (LVEM): Operates at lower voltages, suitable for specific applications.
  • High Voltage Electron Microscope (HVEM): Operates at high voltages, used for more precise applications.
Resolution Limitations
  • Ordinary light microscopes have a resolution limit of 200 nm due to light diffraction.
  • TEM resolution can reach 50 pm (picometers) using high-resolution techniques (HRTEM).

Components of TEM

  • Electron Source: Usually a tungsten filament or lanthanum/cerium hexaboride crystal heated by a circuit.
  • Vacuum System: Maintains a vacuum to prevent electron scattering.
  • Electromagnetic Lenses: Focus and direct the electron beam.
  • Sample Holder: Where samples are placed for observation.
  • Beam Alignment Control Panel: Adjusts the electron beam's focus and position.
  • Monitor: Displays the images captured by a CCD (charge-coupled device) camera.
  • Observation Space: Contains a fluorescent screen for viewing.

Operating Principle of TEM

  • An electron cannon generates an electron beam accelerated through a high voltage (20 kV - 120 kV).
  • Electrons are focused by electromagnetic lenses and penetrate the sample, forming an image that is captured and displayed.
  • Applications: Widely used for biological samples, metallurgical analysis, and semiconductor industry studies.

Scanning Electron Microscope (SEM)

General Principles
  • Analyzes secondary electrons emitted from the sample due to primary electron bombardment.
  • The primary electrons interact with the sample, generating secondary, backscattered, and Auger electrons, as well as X-rays.
SEM Operating Principle
  1. Electron Beam Generation: An electron cannon produces the beam.
  2. Beam Focus: Focused by electromagnetic lenses through a series of openings that filter out scattered electrons.
  3. Surface Mapping: The focused beam scans the sample in a well-defined pattern (left to right, top to bottom).
  4. Signal Detection: Secondary electrons are detected to form a composite image.
  5. Image Formation: Takes 30-60 seconds per image.

Atomic Force Microscope (AFM)

  • Uses a cantilever with a sharp tip to scan the surface of the sample.
  • Tip-sample interaction causes the cantilever to deflect, allowing topographic mapping at the nanoscale.

Fluorescence Microscopy

Overview
  • Fluorescence: Light emission resulting from the absorption of shorter wavelength light, typically UV, and emission of longer wavelength light.
  • Common fluorophores include FITC for proteins and DAPI for DNA.
  • Used extensively in cellular biology for visualizing nucleic acids and proteins in living cells.
Components of Fluorescence Microscope
  • Light Source: Usually a xenon or mercury arc lamp.
  • Excitation Filter: Allows only excitation wavelengths to pass.
  • Dichroic Mirror: Reflects excitation and transmits emission light.
  • Emission Filter: Blocks excitation light and transmits the emission.
  • Objective Lens: Forms the image after light passes through filters.
Applications
  • Used in studies involving cells, viruses, nucleic acids, and for immunofluorescence techniques, where antibodies tagged with fluorophores detect specific antigens.

Phase Contrast Microscopy

Definition
  • Amplifies differences in refractive indices to enhance image contrast without staining, allowing live observation of cells.
  • Developed by Frederik Zernike, who received a Nobel Prize for this innovation in 1953.
Applications
  • Observing living cells and their internal structures such as mitochondria and nuclei without fixation or staining.

Polarized Light Microscopy

Principle
  • Enhances contrast in specimens with birefringent properties, where optical properties depend on light propagation direction.
  • Birefringence: The optical property where a material has different refractive indices based on light orientation.
Applications
  • Widely used in geology and biology for studying crystalline structures, minerals, and biological fibers.

Special Techniques in Cell and Molecular Biology

Cell Cultures
  • Techniques that allow for laboratory growth of cells for various applications.
  • Includes primary cultures and immortalized cell lines for extended use in research.
Chromatography and Electrophoresis
  • Separation techniques used to isolate and analyze proteins and nucleic acids based on properties like size and charge.
  • Electrophoresis allows for separation of macromolecules through gels, providing insights into molecular characteristics.