20.109(F09): Mod 3 Day 3 TEM

TEM

Introduction

The Transmission Electron Microscope (TEM) achieves its remarkable resolution by “illuminating” samples using an electron beam in a vacuum rather than using a conventional light source in air. Since the electron beam passes through the sample that is being examined, the sample must be sufficiently thin and sufficiently sturdy to be hit by electrons in a vacuum. It’s important to remember that many biological materials are damaged or destroyed by the incoming electrons and that the TEM can image only the species that survive this harsh treatment. The denser parts of the sample will absorb or scatter some of the electron beam, and it’s the scattered electrons or those that pass through the sample that are focused using an electromagnetic lens. This “electron shadow” then strikes a fluorescent screen, giving rise to an image that varies in darkness according to the sample's density. For samples that are amenable to TEM, this form of examination can allow observation of angstrom-sized objects and of cellular details down to near atomic levels.

Copper TEM grid with carbon mesh, photographed at 60X magnification

Samples are applied to a wafer-thin "grid" before being loaded into the TEM and placed under vacuum. The grid can be made of many kinds of materials. All have lines of a conductive metal, in our case copper, that disperse the electron beam and thereby help keep the sample from being blown to bits by the energy in the beam. A carbon mesh is strung between the metal lines. Once a sample has been applied to the grid, it's only the portions that come to rest on the carbon mesh can be visualized, along with any imperfections in the carbon mesh itself.

Today you'll wash the nanowires you synthesized last time and visualize a sample of them by TEM. The rest of the nanowires will be stored in a mortar until next time, when you'll grind them with some carbon and teflon for assembly into a battery.

Protocols

TEM sample preparation

1. Place 15 ul of your phage/gold solution on the shiny, bright side of the TEM grid that you have balanced in the specialized tweezers. Treat the grid with care and use the tweezers only on the edge to minimize damaging the delicate mesh.

   

   TEM grid balanced in tweezers

   
   
2. Allow the nanowires to settle onto the grid undisturbed for 30'.
3. Remove the droplet from the grid with your P200 set to 50 ul.
4. Wash the grid by adding 15 uL of sterile H2O onto the grid. Immediately remove the water.
5. Dry the grid by very gently touching the edge of the grid to a piece of blotting paper in a petri dish. Place the grid into the TEM grid holder to transport to the TEM facility (13-1012). A member of the teaching faculty will be in the facility to examine your samples with you.

Drying Samples after TEM

1. Centrifuge solutions at 10000 rpm, 15min.
2. Wash with water 3 times to get rid of remaining un-reacted ions Decant supernatant and add 30 mL water and re-suspend centrifuged materials in water. Centrifuge at 20000 rpm, 15min.
3. Groups that are adding silver or gold nanowires to their battery should add washed and resuspended silver/gold nanowires to washed and resuspended FePO4 •H2O at this step. Resuspend centrifuged FePO4 •H2O in 15 mL water and resuspend silver/gold nanowires in 15mL water. Mix two solutions. Centrifuge at 20000rpm, 15min.
4. Re-suspend centrifuged materials in 1~2mL water and dry at ~100oC under vacuum.

DONE!

For next time

1. You should be well on your way to preparing the presentation materials themselves and should be working on the wording you will use to describe your idea. Reconsult the specific directions for what you'll need as well as the more general guidelines for all oral presentations.