Which technique requires the sample to be dried and coated with metal?

The technique that requires the sample to be dried and coated with metal is Scanning Electron Microscopy (SEM). SEM operates by using focused beams of electrons to scan the surface of a sample. The interactions between the electrons and the sample generate various signals that can be detected to form high-resolution images of the surface structure.

To achieve clear images in SEM, biological samples often need to be dried because the electron beam would otherwise interact with the water molecules in the sample, leading to distortions and achieving poor images. Coating the sample with a thin layer of metal, usually gold or palladium, enhances the conductivity of the surface and improves the quality of the electron signal. This process prevents charging effects that can occur when electrons accumulate on non-conductive surfaces.

In contrast, other techniques mentioned do not require such preparation steps. For instance, Transmission Electron Microscopy (TEM) requires ultrathin sections of samples but does not necessitate coating with metal. Fluorescent Microscopy can visualize samples in their natural states using fluorescence without drying or coating. Stereomicroscopy, often used for lower magnification and dissection purposes, typically examines samples without the need for extensive preparation like drying and coating.