Collodion support films

Put fixed bacteria on a 400-mesh grid coated with a collodion support film. 

Dip grids with replica film in acetone (dissolves the collodion support film). 

Insoluble surface films can be studied by electron microscopy. The films are transferred from the substrate on to a collodion support and shadow-cast by a beam of metal atoms directed at an angle a (about 15°) to the surface (Figure 4.21). If the width x of the uncoated region is measured, the thickness of the film, x tan a, can be calculated for example, a /i-C36H73COOH film has been shown to be about 5 nm thick - i.e. consistent with a vertically orientated monomolecular layer. The technique has also been used for following the state of the surface as a film is compressed. 

Electron microscopy represents the only direct method that permits to see with our own eyes the interior of a sample with a resolution of a few nanometers. To characterize the texture of hypercrosslinked polystyrenes both transmission and scanning electron microscopy have been appfied. In the former case, ultrathin sections with a thickness of about 600 A were cut firom a sample fixed in epoxy resin, and then directly examined in transmission mode. Alternatively, two-step replicas have been prepared firom the cleavage face. To prepare the repfica the surface was first coated with a collodion film appfied from amyl acetate solution, and then with a carbon—platinum film. Finally, the collodion support was dissolved and the free carbon—platinum repfica examined under a transmission microscope. 

Powders are suspended in a fluid, mixed and dropped onto the plastic or carbon coated support grid. If the powders do not disperse by this method, the powder can be rolled onto a plastic support film on a glass slide, and cut, scored, floated off and picked up. Another variation is that powders may be mixed with the collodion or formvar and a film cast with the material held in the support film. 

In 1855, the moldable features of collodion were exploited by the British inventor and chemist Alexander Parkes, who marketed the material as Parkesine. Combs, earrings, buttons, bracelets, billiard balls, and even false teeth were manufactured in his factories. Parkes chose to focus more on quantity than on quality, however. Because he used low-grade cotton and cheap but unsuitable solvents, many of his products lacked durability, which led to commercial failure. In 1870, John Hyatt, a young inventor from Albany, New York, discovered that collodions moldable properties were vastly improved by using camphor as a solvent. Hyatts brother Isaiah named this camphor-based nitrocellulose material celluloid. Because of its greater workability, celluloid became the plastic of choice for the manufacture of many household items. In addition, thin transparent films of celluloid made excellent supports for photosensitive emulsions, a boon to the photography industry and a first step in the development of motion pictures. 

The brominated sticks are embedded in Spurr epoxy resin (Spurr 1969) and sectioned with a diamond knife or glass knife mounted on an ultramicrotome to give cross sections of 0.15//m thickness for TEM-EDXA or 0.5//m thickness for SEM-EDXA. The sections are then placed on a carbon-coated collodion film on the specimen support grid. The specimens are carbon-coated again to avoid charging. 

Unstained RTT fibril, greatly stretched by accidental breaking of supporting collodion film. Reprinted from Schmitt, Hall and Jakus. 

Plastic, carbon and metal films (Section 4.7) are used as specimen supports on TEM grids. There are two plastic support materials in use collodion, 0.5% solution of nitrocellulose in amyl acetate and formvar, 0.25% polyvinyl formal in ethylene dichloride. These polymers are available as powders, solutions or prepared films on TEM grids. Formvar films, especially holey ones, are used as substrates for the formation of holey carbon films. Today, collodion is not used too often, as it is not as stable in the electron beam as formvar or carbon films. 

Premixed solutions of formvar and collodion are recommended for high quality film supports as the powders take several days to dissolve and are less uniform. Collodion is generally film cast on a water surface. A large Petri dish filled... 

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