Colloids, freezing colloidal gold

The first report of this type was by Zavala and Brandon [48] who sectioned root tips at low temperature in order to try and prevent the redistribution of hormones. Their tissue sections were then probed with dihydrozeatin antibodies labelled with rhodamine. For electron microscopy, samples were freeze-substituted in ethanol or acetone and then embedded in polymers prior to sectioning and probing with colloidal gold-labelled antibodies. Although apparent specific labelling was reported in this work, it has been criticised [147] because dihydrozeatin is a relatively rare cytokinin, that had not been... 

The hydrosols are very stable, almost optically clear and may be kept for years without any change taking place in the appearance. That the appearance is deceptive, however, is shown by the fact that the protective effect on colloidal gold gradually lessens. On freezing leaf-like crystals are formed that are no longer soluble in water. 

The freeze fracture method has been used to study the structure of colloidal particles in water-oil mixtures stabilized by polymer emulsifiers. Microemulsions consisting of water, toluene and graft copolymer composed of a polystyrene backbone and a poly(ethylene oxide) graft were deposited onto a small gold plate, quenched in liquid nitrogen in equilibrium with its own solid phase [436]. Replicas of the fractured surfaces were washed with tetrahydrofuran, which showed the micellar structure of the copolymers. A similar method was used for the preparation of polystyrene polymer latexes for TEM study of the size distribution [437]. In this case, the frozen droplet was microtomed, with a cold knife at -100 to -120°C, etched for up to 90 s and then a platinum-carbon replica was prepared. Etching was found to be unnecessary and a potential cause of error. The remaining latex was dissolved away before examination of the replica. Such replicas can reveal the size distribution and structure of the latex particles. 

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