How does congo red work?

Congo red is a water soluble, secondary diazo acid dye that is used extensively in histological examinations, particularly in identifying amyloid. It works by binding to certain structures within cells and tissues allowing them to be visualized under a microscope. Congo red binds to amyloid fibrils due to their β-pleated sheet structure, forming hydrogen bonds along the pleats. This binding creates a regular, pseudo-crystal pattern, which under polarized light, exhibits green birefringence. The dye is primarily used for its selectivity in staining amyloid deposits, which aids in amyloidosis diagnosis.