Gold particles are typically used in:
Immunoelectron microscopy localizes antigens at ultrastructural level by conjugating antibodies to electron-dense colloidal gold particles of 5-20 nm diameter. Gold strongly scatters electrons, appearing as black dots in TEM, pinpointing protein distribution within organelles. Unlike fluorophores used in confocal or fluorescence microscopy which bleach and lack electron contrast, gold provides stable, quantifiable label for high-resolution mapping. Antibodies linked directly or via protein A-gold complexes enable double labeling with different sizes, essential for studying trafficking, vesicular transport, and cytoskeletal-associated proteins with nanometer context.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.